Functional fillers : chemical composition, morphology, performance, applications 9781523113576, 152311357X, 9781927885376, 192788537X, 9781927885390, 1927885396

Table of contents :
Content: Cover image
Title page
Table of Contents
Copyright
Chapter 1: INTRODUCTION
Chapter 2: CHEMICAL COMPOSITION OF CLASSICAL FILLERS
2.1 Fillers --
element concentration
2.2 Functional groups
2.3 Trace elements
Chapter 3: FUNCTIONAL FILLERS SPECIAL PHYSICAL PROPERTIES
3.1 SUPERLIGHT
3.2 HIGH DENSITY
3.3 THERMAL INSULATION AND CONDUCTIVITY
3.4 THERMAL ENERGY STORAGE
Chapter 4: FUNCTIONAL FILLERS ELECTRICAL AND MAGNETIC PROPERTIES
4.1 CONDUCTIVE
4.2 INSULATING
4.3 INSULATING/CONDUCTIVE MIXTURES
4.4 DIELECTRIC
4.5 MAGNETIC
4.6 MAGNETODIELECTRIC
4.7 EMI SHIELDING 4.8 MICROWAVE ABSORPTION4.9 PIEZORESISTIVE
4.10 ELECTROSTATIC DISCHARGE PREVENTION
Chapter 5: FUNCTIONAL FILLERS --
STRUCTURE
5.1 MOLECULAR
5.2 CARBON DOTS
5.3 NANOFILLERS
5.4 NANOWIRES
5.5 NANORODS
5.6 NANOSHEETS
5.7 NANODIAMONDS
5.8 HIGH ASPECT RATIO
5.9 LAYERED DOUBLE HYDROXIDES
5.10 FUNCTIONALIZED FILLERS
5.11 ENCAPSULATED FILLERS
5.12 HYBRID
Chapter 6: FUNCTIONAL FILLERS --
APPLICATIONS
6.1 LUBRICANT
6.2 ANTI-CORROSION
6.3 MEMBRANES
6.4 OSTEOCONDUCTIVE AND OTHER BONE TISSUE ENGINEERING FILLERS
6.5 SOFT TISSUE FILLERS
6.6 ANTIMICROBIAL Chapter 7: FUNCTIONAL FILLERS --
RENEWABLE AND RECYCLING7.1 BIOFILLERS
7.2 BIOSORBENTS
7.3 GEOPOLYMERS
7.4 RECYCLED
INDEX

Citation preview

Functional Fillers

Chemical composition, morphology, performance, applications

George Wypych

Toronto 2018

Published by ChemTec Publishing 38 Earswick Drive, Toronto, Ontario M1E 1C6, Canada © ChemTec Publishing, 2018 ISBN 978-1-927885-37-6 (hardcover); ISBN 978-1-927885-39-0 (PDF) Cover design: Anita Wypych

All rights reserved. No part of this publication may be reproduced, stored or transmitted in any form or by any means without written permission of copyright owner. No responsibility is assumed by the Author and the Publisher for any injury or/and damage to persons or properties as a matter of products liability, negligence, use, or operation of any methods, product ideas, or instructions published or suggested in this book.

Library and Archives Canada Cataloguing in Publication Wypych, George, author Functional fillers : chemical composition, morphology, performance, applications / George Wypych. Includes bibliographical references and index. Issued in print and electronic formats. ISBN 978-1-927885-37-6 (hardcover).--ISBN 978-1-927885-39-0 (PDF) 1. Fillers (Materials). I. Title. TP159.F47W97 2018

668.4'11

C2017-905287-X C2017-905288-8

Printed in Australia, United Kingdom and United States of America

1

Introduction The decades of developments of fillers’ structure and composition completely changed their potential applications from the inexpensive filling material used for a price reduction to performance additives which control properties of advanced materials. This became possible because of modifications in their composition and morphological structure. This book has two sections: • analysis of the chemical composition and morphology of classical fillers (100 groups of fillers as listed in Handbook of Fillers, 4th Edition) which contribute to the exceptional enhancement in their properties and applications • presentation of fillers of a new generation which provide designers with special properties not typical of classical fillers presently used by industry The first section is designed for the analysis of the effects of chemical composition and morphology on filler performance and application to assist in the selection of the most efficient materials for application. In this section, materials are grouped according to their composition. The classical fillers are composed of more than thirty chemical elements (not counting the trace elements). The classification in the section is based on the composition of chemical elements to explore the effects of elemental composition on properties, function, and application of fillers as it relates to their composition. Many fillers are composed of a multitude of elements (frequently in comparable levels of concentrations) and such fillers will be repeated in the major tables for different elements and their position on the concentration scale will be used for generalization. The first section contains three more chapters, including functional groups, trace elements, and morphology. Many functional groups are either present in the typical filler structure or purposely inserted to foster filler reactivity with the surrounding materials. These are frequently very important modifications which need to be properly understood and selected for the development of high-performance products. On the other hand, the trace elements are likely to create problems in the processing of polymeric materials, lower their thermal and UV stability, affect the safety of products in application and processing, or may cause discoloration. For these reasons, they have to be properly accounted for in the selection for the product. Morphological features of the classical fillers are at least as important as composition considering they affect on filler distribution, reinforcement ability, color, density, physical properties, biocompatibility, etc. In this chapter, the typical morphological features of classical fillers will be presented with explanation of how they have impacted performance of a particular filler. The second section includes discussion of emerging materials. It is difficult to find universal classification for these materials, therefore, a slightly loose qualification to

2

Introduction

groups is used. The additives are grouped by structure, physical properties, electrical and magnetic properties, applications, and the use of renewable or recycled materials for their production. The following subgroups are included in this part: • Structure Molecular Carbon dots Nano Nanowires Nanorods Nanosheets Nanodiamonds High aspect ratio Layered double hydroxides Functionalized Encapsulated Hybrid • Physical properties Superlight Dense Thermally conductive Thermal energy storage • Electrical and magnetic properties Conductive Insulating/conductive mixtures Dielectric Magnetic Magnetodielectric EMI shielding Microwave absorption Piezoresistive Electrostatic discharge prevention • Applications Lubricant Anti-corrosion 4 Membrane Osteoconductive and other bone engineering fillers Tissue fillers Antimicrobial • Renewable and recycled Biofillers Biosorbents Geopolymers Recycled materials

3

The evaluation of the potential of these fillers and their performance is based on the most current research papers and patents. The illustration of the results obtained with these materials shows their potential impact on modern industrial applications. Application of the new generation materials has a very essential impact on the new developments in medicine, dentistry, and pharmacy.

2

Chemical Composition of Classical Fillers Classical fillers are grouped in this sections according to their elemental composition. The sections of this chapter are listed according to the element name and arranged in the sequence according to the element symbol. Each section begins with a table containing the concentration of an element in a filler, its name, its chemical formula, typical features, and major applications. The table is followed by a discussion on how the concentration of a particular element affects properties and application. The following subsections include discussion of the functional groups and the trace elements present in the classical fillers.

2.1 Fillers − element concentration 2.1.1 Ag (silver)

Conc., %

Name

99.3-99.9 Silpower, Silsphere, Silflakes

Formula Ag

Feature

Application

high conductivity, high density (10.5)

inks, adhesives, coatings, powder metallurgy, pharmaceuticals, medicine

30-40

Conduct-O-Fil Ag coated glass high conductivity flakes low loading

at inks, adhesives, EMI shielding

20-33

Conduct-O-Fil Ag coated hollow very low density and conductive adhesives, glass spheres high conductivity paints and coatings

8-20

Conduct-O-Fil Ag coated fibers

4-16

Conduct-O-Fil Ag coated solid high conductivity glass spheres

low loading, improved EMI shielding, adhetensile properties sives, tapes, coatings EMI shielding, films, adhesives

The above list indicates one major application of silver and silver-containing fillers which uses high electrical conductivity of silver. Pure silver has here some disadvantages such as a high price and a high density. Because conductivity is proportional to the volumetric concentration, the high density of silver, compared to glass coated with silver, further increases the quantity of the filler to be added to formulation in order to obtain the desired effect. The shape of the particle also matters. Flakes and fibers are more efficient in conductive and EMI shielding applications. The most efficient (regarding utilization of

6

Chemical Composition of Classical Fillers

silver) are hollow glass spheres coated with silver because they have very low density but, at the same time, they are breakable and cannot be used in some applications (e.g., EMI shielding) which require consistency and high impact resistance. Anti-microbial applications are not represented in this table because usual application method does not include mixing filler with polymer matrix since silver has to be available on (or close to) the surface to perform. Below are some examples of such applications. Kevlar fabric used in the airspace applications requires microbiological protection to eliminate the risk of microbial contamination on the board of spacecraft or orbital station.1 Kevlar fabric has been coated with antibacterial silver nanocluster/silica composite layer with different thicknesses (60-300 nm), by means of radio frequency co-sputtering technique.1 The antimicrobial finishing of textiles intended for food processing industry was accomplished by plasma enhanced chemical vapor deposition of Ag-SiOCH composites coated with AlxOy or SiOCH encapsulation layers.2 The encapsulation was used to increase abrasion resistance of the coating to washing cycles and oxidation resistance.2 The encapsulation did not affect antimicrobial activity.2 Treating textiles with ionic silver after washing reduced bacterial contamination.3 The textiles were treated with SilvaClean in accordance with Environmental Protection Agency label directions for use to provide 1.3-1.4 mg silver per kilogram textile.3 REFERENCES 1 2 3

Balagna, C; Irfan, M; Perero, S; Miola, M; Maina, G; Santella, D; Simone, A, Surf. Coat., Technol., 321, 438-47, 2017. Brunon, C; Chadeau, E; Oulahal, N; Grossiord, C; Dubost, L; Simon, F; Bessueille, F; Degraeve, P; Leonard, D, Thin Solid Films, 628, 132-41, 2017. Openshaw, JJ; Morris, WM; Lowry, GV; Nazmi, A, Amer. J. Infection Control, 44, 12, 1705-8, 2016.

2.1.2 Al (aluminum)

7

2.1.2 Al (aluminum) Conc., %

Name

Formula

Feature

Application

95-99.98

Al flakes

Al

brilliance, reflectivity, coatings, EMI shielding, barrier properties, thermal conductivity heat sinks

65.83

Al nitride

AlN

hardness, thermal conductivity

52.92

Al oxide

Al2O3

antiblocking, catalyst, abrasives, abrasive antiagglomeration, anticaking, paper

45.95

AlB whisker

(Al2O3)9(B2O3)2

high aspect ratio

automotive parts, precision parts, cams, relays

34.59

Al hydroxide

Al(OH)3

water release

flame retardation

19.62

halloysite

Al2Si2O5(OH)4 x nH2O

tubular shape, low density in plastics

controlled release, reinforcement, flame retardation

13-20

ceramic beads variable

12.6-16.9 ball clay

composites, electronic packaging

light weight (hollow), adhesives, paints, chemical resistance asphalt, sealants, flooring, coatings

Al2O32SiO22H2O acid/base interaction

adhesives, cables, tires

12-23.1

kaolin

Al2O32SiO22H2O fineness, platelet structure, brightness

paints, wire & cable, flooring, tires, sealants

7.3-9.8

feldspar

(Na or K or Ca) refractive index compa- adhesives, coatings, Al1-2 Si3-2O8 rable to plastics sealants, rubber

6.8-10.9

bentonite

(Na,Ca)(Al,Mg)6 (Si4O10)3(OH)6 nH2O

swelling, platelet struc- filtration, rheological ture, intercalation, modification, drilling exfoliation muds

6.4-14.6

zeolite

variable

micropores, ion exchange

molecular sieves, adsorption, removal of moisture and odors

6.21-8.3

glass fibers

variable

high aspect ratio

composites composites

fly ash

variable

low density, low cost

4.7-19.8

5.7-10

mica

variable

high aspect ratio, high electrical, composites, resistivity, low coeffi- cosmetics, insulation, cient of thermal exp. paints

4.7-6.2

attapulgite

(MgAl)2Si4O10 (OH)4(H2O)

high aspect ratio, rod-shaped

absorbents, antidiarrheal, drilling mud, remediation

Aluminum is a very popular element in numerous fillers, especially in the fillers obtained from natural sources. The concentration of aluminum plays a dominant role in distinguishing between the properties of filler based on a pure metal (shiny, reflective) and all other fillers. Properties of other than metal flake fillers are derived mostly from its crystalline structure and morphology which depend on conditions of formation and pro-

8

Chemical Composition of Classical Fillers

cessing. Numerous features (and thus applications) are typical of these fillers as partially listed in the above table.

2.1.3 Au (gold)

9

2.1.3 Au (gold) Conc., % 99.96

Name gold powder

Formula Au

Feature high electrical conductivity

Application adhesives, coatings, inks

The gold powder was used as a pigment for coating a large lightning fixture constructed from a long aluminum tube.1 The pigment was deposited by a powder coating and it was rated for 15-years durability.1 Attractive cellular gold structures were obtained via two metallurgical approaches: sintering of loosely packed gold fibers and replication of polymeric templates.2 Application of gold powder in antistatic applications was discussed elsewhere.3 REFERENCES 1 2 3

Anon. Focus Powder Coat., 2015, 4, 5-6, 2015. Andersen, O; Göhler, H; Kostmann, C; Quadbeck, P; Diologent, F; Colas, D; Kieback, B, Metal Powder Rep., in press, 2017. Wypych, G, Databook of Antistatics, ChemTec Publishing, Toronto, 2014, pp. 288-298.

10

Chemical Composition of Classical Fillers

2.1.4 B (boron) Conc., %

Name

41.4-43.5 boron nitride

Formula

Feature

Application

BN

low friction, prevents adhesives, dental melt fracture, lubrica- cement, electronics, tion, high resistivity mold release

20.9

zinc borate

2ZnO 3B2O3 3.5H2O

flame retardant

6.5-8.7

barium metaborate

BaB2O4 H2O

corrosion inhibitor, coatings, paints, mold growth inhibitor sealants, adhesives flame retardant

4.09

AlB whisker

(Al2O3)9 (B2O3)2

high aspect ratio, high reinforcement of tensile modulus precision parts

3.75

E-glass

aspect ratio, strength

flame retardant, biocide

composites

Boron increases strength of glass and it is a mild anti-mold and a good flame retarding element. Boron is a clear example of an element which affects properties of fillers to perform many important tasks listed in the table above. Its hexagonal crystalline form puts boron nitride in the range of the most efficient solid lubricants.

2.1.5 Ba (barium)

11

2.1.5 Ba (barium) Conc., %

Name

Formula

Feature

Application

58.9

Ba titanate

BaTiO3

thermal stability, ferro- electrical, pyro electric properties, and piezo electric reflective properties composites

58.8

Ba sulfate

BaSO4

high density, absorption

57.0

Ba metaborate BaB2O4 H2O

corrosion inhibitor, coatings, paints, mold growth inhibitor sealants, adhesives flame retardant

43.8

Ba ferrite

BaOFe2O3

absorption of radiation, energy attenuating magnetic compounds, catalyst

41.5

lithopone

ZnS BaSO4

reflective, anti-algae

paints, paper, pigment

29.6

BaSr sulfate

BaSO4 & SrSO4

inexpensive, reflectivity

foams, paints, plastics, rubber

x-ray medical, x-ray, insulation, acoustical

Barium titanate has tetragonal crystalline structure. It can be produced in a form of fibers. Barium is toxic to some microorganisms and in a water-soluble form it is very toxic to humans. For this reason, insoluble sulfate is commonly used. The presence of a heavy metal such as barium imposes various absorption properties and a high reflectivity.

12

Chemical Composition of Classical Fillers

2.1.6 Be (beryllium) Conc., % 36.0

Name Be oxide

Formula BeO

Feature

Application

high melting (2500), high thermal conductivity, dielectric properties

products which require high thermal conductivity and dielectric properties

Beryllium oxide, similar to graphene, can form a single atomic layer.1 Beryllium oxide, as a member of alkali earth oxides group, crystallized in wurtzite phase with sp3 hybridization interaction between components.1 Also, BeO nanotubes can be formed.1 REFERENCES 1

Jalilian, J; Safari, M; Naderizadeh, S, Computational Mater. Sci., 117, 120-6, 2016.

2.1.7 C (carbon)

13

2.1.7 C (carbon) Conc., % 97

Name

Formula

Feature

Application

graphene

C

sheet form, conductivity

solar, coatings, electronics, membranes

95-99

carbon black

C

conductivity, color, reinforcement

pigment, tires, UV protection

90-99

carbon nanotubes

C

reinforcement, conductivity

composites, electronics, batteries, shielding

88-99.9

graphite

C

conductivity, platelet batteries, brakes, structure, lubrication coatings, self-lubricating parts

84.3-99

carbon fibers

C

reinforcement, conductivity

brakes, composites, electronics

anthracite

C

conductivity, cheap, chemical resistance

battery cases, liners

70.5

aramid fiber

(C14H10N2O2)n

wear resistance, impact automotive, composites

51.6

cellulose

(C8H10O5)n

different morphologies nanocomposites, possible, renewable reinforcement

51.3-75

graphene oxide

44.7

chitosan

77

brakes,

sheet form, insulating, reinforcement, chips, compatibility cancer treatment, batteries, membranes (C6H11NO4)n

antibacterial, reinforcement

medical, pharmaceutical, composites

Carbon element imparts thermal and electric conductivity, reinforces (mostly due to the high aspect ratio), and can be used as a black pigment (and UV protection). The purer the carbon the better electrical and mechanical properties (as a general trend).

14

Chemical Composition of Classical Fillers

2.1.8 Ca (calcium) Conc., %

Name

Formula

69.8

eggshell filler

54.1

Ca hydroxide

Ca(OH)2

51.3

Ca fluoride

CaF2

Feature waste

Application composites, mineral supplement neutralization, paper, paints

refractive

optical, dental

41.3

apatite

Ca5(PO4)3(OH,F,Cl) biocompatibility

medical, paper

38.8

Ca phosphate

Ca3(PO4)2

medical, dietary supplement

34.5

Ca silicate

CaSiO3

34-40

Ca carbonate CaCO3

30.7-33.9 wollastonite

biocompatibility

medical, anticaking brightness, cheap

paint, PVC, pharmaceutical, paper, toothpaste, rubber

CaSiO3

asbestos replacement, abrasives, adhesives, scratch resistance insulation

29.4

Ca sulfate

CaSO4

non-toxic

construction, bonefiller, pharmaceutical

22

dolomite

CaMg(CO3)2

cheap, non-toxic

animal feed, paints, paper, adhesives, sealants, mastics

11.3

huntite

Mg3Ca(CO3)4

2.9-9

glass

flame retardant

composites

component of glass

dolomite mainly used

Calcium-based fillers are the most common fillers used in numerous applications. Calcium carbonates dominate filler market and PVC applications. The most important reason for this widespread use is a low price and a low toxicity (many fillers are used as animal feed and as a dietary supplement for human consumption). Calcium fillers are biocompatible and find applications in bone restoration. Finally, toothpaste is a common product based on calcium fillers.

2.1.9 Co (cobalt)

15

2.1.9 Co (cobalt) Conc., %

Name

99.8-99.9 cobalt powder Co

Formula

Feature conductivity

Application conductive products, surge protection, static charge dissipation

Co is a precious transition metal used in cathode materials.1 A study investigated fabrication of nanosized cobalt powder from cobalt hydroxide Co(OH)2 recovered from spent lithium ion battery.1 The nanosized cobalt powder was fabricated with a mean particle size of 100-500 nm and purity of 99.21 wt%.1 REFERENCES 1

Yun, J-Y; Park, D; Jung, S-S; Wang, J-P, Appl. Surf. Sci., 415, 80-4, 2017.

16

Chemical Composition of Classical Fillers

2.1.10 Cu (copper) Conc., %

Name

Formula

98.5-99.9 copper powder Cu 88 70-90

Feature

Application

conductivity

plastics and paints

bronze powder Cu, Sn

conductivity

plastics and paints

brass powder

conductivity

plastics and paints

Cu, Zn

High electrical conductivity of copper is the key to its applications. Copper is one of the three (Ag, Cu, Ni) used powders in EMI shielding applications. Its morphological form (spherical, porous, flake, or elongated) further influences conductivity and efficiency. Gold-plated copper spheres and silver-plated flakes are also used to prevent copper oxidation which decreases its conductivity. Copper and brass fibers are used in brake pads applications. Copper oxide has antimicrobial properties and it is used in wood preservation and marine antifouling products.

2.1.11 F (fluorine)

17

2.1.11 F (fluorine) Conc., %

Name

46.3-48.2 Ca fluoride

Formula CaF2

Feature refractive

Application optical, dental

18

Chemical Composition of Classical Fillers

2.1.12 Fe (iron) Conc., % 99

Name iron powder

97.8-99.8 carbonyl iron powder

Formula

Feature

Fe

Application sintering

Fe

electrical conductivity

EMI/RFI shielding, nutritional supplement

stainless steel powder

Fe, Cr, Ni, Mo

magnetic properties

electronics, magnets, ferromagnetic carriers

69-72

magnetite

Fe3O4

magnetic properties

ferromagnetic compounds, pipe coating, radiation shielding

56-70

iron oxide

Fe2O3

magnetic properties, anti-corrosive

magnetic composites, paints, medical

6.3-7.7

phlogopite mica

high aspect ratio

composites

70

The use of iron powder in plastics is hampered by staining by its corrosion products. For this reasons, the stainless steel is likely to be used to change magnetic and conductive properties of plastic materials.

2.1.13 Gd (gadolinium)

19

2.1.13 Gd (gadolinium) Conc., % 86.7

Name gadolinium oxide

Formula Gd2O3

Feature

Application

MRI contrast enhance- MRI imaging, fluoresment, magnetocaloric, cent materials, plasma neutron poisoning display panels, additives, scintillators, xray shield, screen printing inks, nuclear reaction control

The magnetocaloric effect (the principle of magnetic refrigeration) is environmentfriendly energy-efficient refrigeration mechanism.1 It is anticipated that it may become the future cooling technology.1 Gadolinium oxide nanotubes of diameter ~200 nm were synthesized through electrochemical technique to be used for their magnetocaloric effect.1 REFERENCES 1

Paul, R; Paramanik, T; Das, K; Sen, P; Satpati, S; Das, I, J. Magnet. Magnet. Mater., 417, 182-8, 2016.

20

Chemical Composition of Classical Fillers

2.1.14 K (potassium) Conc., % 12.3

Name potassium hexatitanate whisker

Formula

Feature

Application

K2O 6TiO2

high aspect ratio

composites, brake pads, lithium batteries

3.1-3.7

pumice

hardness, flatting

abrasives, paint, fillers

2.4-9.1

mica

KAl2(AlSi3O10) (OH)2

high aspect ratio, barrier properties

reinforcement

zeolite

variable

exchange cation

membranes

0-6

2.1.15 Mg (magnesium)

21

2.1.15 Mg (magnesium) Conc., % 55-59

40-41.5

Name Mg oxide

Formula MgO

Mg hydroxide Mg(OH)2

Feature bactericide, flame retardant, acid scavenger

Application construction products, pharmaceutical, wire & cable, pigment

water release

flame retarding

20.6

huntite

Mg3Ca(CO3)4

flame retardant

reinforcement, paints

14.8

sepiolite

Mg4Si6O15(OH)2 6H2O

absorption, high aspect asbestos replacement, ratio compatibilizer

12

dolomite

CaMg(CO3)2

inexpensive

talc

Mg3Si4O10(OH)

low hardness, platelet plastics, insulation, structure caulk, paper

1.8-7.2

attapulgite

(MgAl)2Si4O10 (OH)4(H2O)

absorption

1-1.8

bentonite

(Na,Ca)(Al,Mg)6 (Si4O10)3(OH)6 nH2O

absorption, rheological adhesives, caulks, cosproperties metics, paper, pharmaceuticals, wine, paint

9-20

grouts, animal feed, paint, paper, plaster

adsorbents, medication, remediation materials, paint, sealants

Magnesium is a common element which participates in many minerals contributing to their complex morphological structures and thus a variety of properties and applications.

22

Chemical Composition of Classical Fillers

2.1.16 Mo (molybdenum) Conc., % 99-99.99 66.7

Name

Formula

Mo powder

Mo

Mo oxide

MoO3

Feature

Application aerospace, electronics

flame retardant

corrosion inhibitor, wire & cable

61

ammonium (NH4)4Mo8O26 octamolybdate

flame retardant

PVC, textiles

60

Mo disulfide

lubrication

aerosols, metalworking compounds, piston rings

MoS2

Molybdenum-containing products are renown for their lubricating and flame retarding properties.

2.1.17 N (nitrogen)

23

2.1.17 N (nitrogen) Conc., %

Name

Formula

Feature

Application

56.4

boron nitride

BN

low friction

greases, mold release, space, aerosols, dental

40

silicon nitride

Si3N4

thermal conductivity, abrasion resistance

abrasive materials

34.1

Al nitride

AlN

thermal conductivity

composites, electronics packaging

11.8

aramid fiber

(C14H10N2O2)n

wear resistance

automotive, brake pads, composites, paper, office equipment

8.7

chitosan

(C6H11NO4)n

antibacterial

medical, pharmaceutical, paper, composites

4.5

ammonium (NH4)4Mo8O26 octamolybdate

flame retardant

PVC, textiles

A low friction, good thermal conductivity, abrasion resistance, wear resistance, flame retardancy, and antibacterial properties benefit from the use of the compounds containing nitrogen.

24

Chemical Composition of Classical Fillers

2.1.18 Na (sodium) Conc., % 11.9

Name

Formula

Feature

Application

Na antimonate NaSbO3

flame retardancy

plastics, paints, textiles

9.8-10.6

A-glass

chemical resistance

glass beads

3.5-4.8

feldspar

inexpensive

adhesives, paints, plastics, rubber, sealants

2.4-3

pumice

abrasiveness

cleaning, polishing

Sodium is a common admixture in many fillers but does not seem to have any considerable influence on their characteristics (except for effects related to its water solubility).

2.1.19 Ni (nickel)

25

2.1.19 Ni (nickel) Conc., % 99

Name nickel powder Ni and flake

70.9-78.6 nickel oxide 12

Formula

NiZn ferrite

Feature electrical conductivity

NiO, Ni2O3 Ni1-xZnxFe2O4

Application antistatic applications, EMI shielding, decorative lacquers, conductive inks catalysts, pigments, ferrites

ferromagnetic, microwave absorbing properties

electromagnetic materials, shape memory polymers

Nickel-coated carbon and graphite fibers, nickel-coated mica, silver-coated nickel granules and flakes, nickel foams give the advantage of a high conductivity of nickel and its relatively low price as compared with silver and gold.

26

Chemical Composition of Classical Fillers

2.1.19 P (phosphorus) Conc., %

Name

Formula

Feature

Application

20.0

Ca phosphate

Ca3(PO4)2

biocompatibility

medical, dietary supplement, composites

19.1

apatite

Ca5(PO4)3 (OH,F,Cl)

biocompatibility

dermal filler, medical, paper

The combination of phosphorus and calcium gives biocompatible fillers, frequently used in bone tissue engineering.

2.1.20 Pb (lead)

27

2.1.20 Pb (lead) Conc., % 92.8

60 47.9

Name

Formula

Feature

Application

lead oxide

PbO

magnetic resonance, magnetic data storage, radiation absorption capacitor, ferromagnetic material, pigment, x-ray photoconductor

BaPb ferrite

BaPb

magnetic properties

ferrite

magnetic properties

ferrite

BaSrPb ferrite BaSrPb

Lead gives ferromagnetic properties to combinations with other metal and non-metal compounds.

28

Chemical Composition of Classical Fillers

2.1.21 S (sulfur) Conc., %

Name

Formula

Feature

Application

32.9

zinc sulfide

ZnS

high refractive index

pigment

23.6

Ca sulfate

CaSO4

non-toxic

bone-void filling, toothpaste, construction, food, paints, tofu production

15.6

BaSr sulfate

BaSO4/SrSO4

inexpensive

foams, paints, plastics, rubber, thermoluminescence

13.7

Ba sulfate

BaSO4

water insoluble

x-ray examination, insulating materials, coatings, bowling balls

5.5-9.9

lithopone

ZnS BaSO4

weathering and algae coatings, paints, paper, protection mastics

Sulfates are water insoluble which makes them durable in outdoor applications and nontoxic in the case of barium-containing products. Zinc sulfide has one of the highest refractive indices of all inorganic materials which makes it an efficient pigment.

2.1.22 Sb (antimony)

29

2.1.22 Sb (antimony) Conc., %

Name

Formula

Feature

Application

83.5

Sb trioxide

Sb2O3

flame retarding properties

coatings, electronics, paints, paper, plastics, textiles, rubber, UV resistant pigments

75.3

Sb pentoxide

Sb2O5

flame retarding properties

automotive, coatings, appliances, office products, wallcoverings

63.2

Na antimonate NaSbO3

flame retardant properties

chemical intermediate, plastics, paints, textiles

Antimony is strong in one application − flame retardancy. As such it is used in numerous polymers and products.

30

Chemical Composition of Classical Fillers

2.1.23 Si (silicon) Conc., %

Name

Formula

Feature

Application

56-70

silicon carbide SiC

hardness

abrasives, clutch, disc brakes

46.7

cristobalite

SiO2

inert

abrasives, anticorrosive coatings, automotive, dental, stucco, plastering, road marks

46.7

silica gel

SiO2

inert, morphology

drying, odor absorption, pharmaceuticals, window spacer

SiO2

inert, hard

abrasive materials, casting and potting compounds, electrostatic coatings

SiO2

inert

electronics, optical fibers

45.5-46.6 sand

SiO2

inert, inexpensive

adhesives, coatings, mortars, construction, sealants, stucco

45.5-46.4 precipitated silica

SiO2

small particle size

adhesives, aircraft coating, battery separators, coil coating, tires

Si3N4

hardness

abrasives, thermally conductive materials

44.8-46.6 fumed silica

SiO2

specific surface area, rheology modifier, reinbulk density forcement, toothpaste

39.9-42.9 diatomaceous earth

SiO2

porous, absorbent

antiblock, anticaking, catalyst support, paper, drilling mud, stucco

variable

porous, adsorbent

agrochemicals, thermal insulation, filtering

33.1-34.6 pumice

variable

morphology

abrasives, antiblocking, chemical carrier, cleaning, cosmetics

31.9-35.9 feldspar

(Na or K or Ca) Al1-2Si3-2O8

inexpensive

adhesives, coatings, plastics, rubber

AlSi2O5OH

platelet

caulks, cosmetics, paints, paper, rubber

variable

adsorption, ment

Mg4Si6O15(OH)2 6H2O

fibrous structure

46.2-46.4 quartz

46-46.2

45-59-5

33.1-35

30.8-35

fused silica

silicon nitride

perlite

pyrophyllite

26.2-34.6 bentonite

26.2

sepiolite

reinforce- rheological modifier, coatings, cosmetics, sealants, mastics asbestos replacement, reinforcement

2.1.23 Si (silicon)

Conc., %

Name

31

Formula

Feature

Application

24.9-28.6 ball clay

Al2O32SiO22H2O inexpensive

cables, footwear, joint compounds, protective coatings

24.5-34.1 glass beads

SiO2

inert

aerospace, automotive, composites, foam, golf balls, paints, sealants

aspect ratio

24.2-29

glass fibers

SiO2

24.2

Ca silicate

CaSiO3

reinforcement fiber, medical, anticaking

23.4-37.4 attapulgite

(MgAl)2Si4O10 (OH)4(H2O)

adsorption

environmental remediation, absorbents, medical, drilling mud

21.7-31.8 talc

Mg3Si4O10(OH)2

soft, platelet

agricultural film, anticaking, animal feed reinforcement, caulking

21.6

kaolin

Al2O32SiO22H2O fineness

sealant, coated fabrics, cosmetics, crack fillers

20.3

halloysite

Al2Si2O5(OH)4 nH2O

morphology

drug delivery, nucleation, reinforcement

wollastonite

CaSiO3

morphology

reinforcement, insulation, coatings

18.7-22.4 mica

variable

morphology

reinforcement, insulation, cosmetics

17.7-21.5 vermiculite

variable

morphology

adsorption, reinforcement, construction

slate flour

variable

inexpensive

filler

fly ash

variable

waste product

construction, composites, polyester mortar

20-24.3

16.3-29 14-28

Silicon is the most popular element among filler products. Its oxides are cocrystalized with other oxides forming complex morphological arrangements with a variety of properties and thus applications. Chemical inertness contributes to its popularity in commercial applications.

32

Chemical Composition of Classical Fillers

2.1.24 Sn (tin) Conc., % 51.1 10

Name zinc stannate bronze powder

Formula ZnSnO3

Feature

Application

flame retardant

numerous polymers

conductivity

plastics, paints

2.1.25 Sr (strontium)

33

2.1.25 Sr (strontium) Conc., %

Name

30

Ba/Sr carbonate

Formula

Feature

BaCO3 SrCO3

25

ferrite

SrO6Fe2O3

21

Ba, Sr sulfate

BaSO4 & SrSO4

Application ferrite magnetic filler

inexpensive

foams, paints, plastics, rubber, thermoluminescence

34

Chemical Composition of Classical Fillers

2.1.26 Ti (titanium) Conc., %

Name

Formula

Feature

Application

59.9

Ti dioxide

TiO2

refractive index

pigment, UV stabilizer

20.5

Ba titanate

BaTiO3

ferroelectric

capacitors, ferroelectric ceramics, filler for ferroelectric polymers, optics, pyro and piezoelectric composites, thermistors

18.8

K hexatitanate whisker K2O6TiO2

morphology

automotive parts, brake pads, capacitors, catalysts, coatings, composites

Titanium dioxide is the most efficient white pigment because of its very high refractive index (2.55-2.73). Nano grades are light-transparent but not for UV radiation, therefore, they can be used as UV absorbers.

2.1.27 W (tungsten)

35

2.1.27 W (tungsten) Conc., %

Name

99.5-99.9 tungsten

Formula W

Feature

Application airbag deployment system, composites, weights (lead replacement), x-ray targets

36

Chemical Composition of Classical Fillers

2.1.28 Zn (zinc) Conc., %

Name

Formula

Feature

Application

67.1

Zn sulfide

ZnS

brightness

pigment

65.3

Zn oxide

ZnO

brightness

pigment, UV stabilizer

52.7

NiZn ferrite

NiZn

ferroelectric

energy attenuating powders

Zn borate

2ZnO3B2O3 3.5H2O

flame retardant, biocide numerous polymers

28.2

Zn stannate

ZnSnO3

flame retardant

numerous polymers

19-39

lithopone

ZnS BaSO4

brightness

pigment

13.8

NiZn ferrite

Ni1-xZnxFe2O4

ferromagnetic

electromagnetic materials

10-30

brass powder

conductivity

plastics and paints

30

Zinc imparts three major properties, including high brightness, conductivity, and flame retardancy.

2.1.28 Zn (zinc)

37

2.2 Functional groups Numerous functional groups can be found in fillers, including Cl, COOH, epoxy, F, H, H2O, NH2, NH, OH, OMe, ONa, S, silane rests, and SO3. Covalent bonding and interaction between the polymer matrix and the filler are discussed in detail in two chapters (6 & 7) of Handbook of Fillers.1 Here are discussed some examples of exceptional changes illustrating the effects of functional groups. Figure 2.1. Two-dimensional SAXS pattern of texModification of synthetic, high aspect tured films of PVAl-6-aminocaprohydroxamic ratio clay ([Na0.5]inter[Mg2.5Li0.5]oct[Si4]tetO10F2) acid. [Adapted, by permission from Tsurko, ES; Feicht, P; Habel, C; Schilling, T; Daab, M; Rosen- with 6-aminocaprohydroxamic acid hydrochlofeldt, S; Breu, J, J. Membrane Sci., 540, 212-8, ride changed the interaction between the poly2017.] vinylalcohol matrix and the filler.2 The waterborne nanocomposite became insensitive to swelling, even at an elevated relative humidity.2 The modifier formed a strong hydrogen bond with the hydroxyl group of PVAl via the hydroxamic acid functional group.2 This prevented the swelling of crystalline PVAl domains.2 The layered silicates not only reduce permeability in a nonselective manner via the tortuous path mechanism, but hydrophobized water-soluble polymer matrix changing waterborne formulation to a barrier film.2 The mean angle of the orientation distribution of the stacks is 1.45°.2 This parallel orientation of clay platelets is the key factor of formation of tortuous pathway and it contributes significantly to the superior barrier properties.2 Three modifications of silica nanoparticles having different surface functional groups (amino, epoxide, and alkyl chain groups) were used as fillers in the amine-cured epoxy resin systems to investigate the relationship between the interfacial interaction and

Figure 2.2. The effect of attractive and repulsive interfacial interactions of the silica nanofillers with the amineepoxy matrix on viscosity and viscosity stability. [Adapted, by permission from Guo, Q; Zhu, P; Li, G; Wen, J; Wang, T; Lu, D; Sun, R; Wong, C, Compos. Part B: Eng., 116, 388-97, 2017.]

38

Chemical Composition of Classical Fillers

Figure 2.3. Generalized representation for halloysite silane grafting using either ethanol or toluene. [Adapted, by permission from Bischoff, E; Daitx, T; Simon, DA; Schrekker, HS; Liberman, SA; Mauler, RS, Appl. Clay Sci., 112-113, 68-74, 2015.]

the rheological and thermal properties of silica nanoparticles reinforced epoxy nanocomposites.3 It has been found that the attractive interfacial interaction significantly contributed to the reduction of the viscosity, enhanced the viscosity stability, and improved the thermal stability of epoxy nanocomposites.3 The repulsive interfacial interaction had a negative effect on both the viscosity and viscosity stability.3 Figure 2.2 shows the nature of attractive and repulsive forces for different modifications and the resultant effect on viscosity.3 The modification with amino group results in enhancement of attractive forces in the system and thus keeps viscosity low.3 Figure 2.3 illustrates that the conditions under which modification occurs also affect the outcome.4 The direct grafting occurs between the hydrolyzed silane with the hydroxyl groups located at the edges, lumen, and surface defects, while the oligomerization occurs when there is a higher water content physically adsorbed on the filler surface for further silane hydrolysis.4 Hydrolysis can be promoted by the use of polar protic solvents due to the effect of solvation and the high dielectric constant.4 When dry toluene was used, higher levels of grafting were achieved, reducing the silane hydrolysis, which slowed down silane oligomerization.4

2.1.28 Zn (zinc)

39

Dispersion of filler in the polymer matrix is the conditio sine qua non of the interaction between filler and polymer and all its benefits. Altering the surface chemistry of carbon black is sometimes necessary to disperse it in a medium.5 Mercaptopropyltrimethoxysilane was used for the modification of carbon black to disperse it in some solvents and epoxy monomer.5 The modification led to the formation of a surface decorated by sulfur and silicon functionalities which changed the surface composition of the particles.5 The particles after modification assumed a higher negative surface charge and showed a larger surface area.5 The surface modification helped to disperse carbon black in any kind of solvent.5 Good distribution of filler and its purposely selected functionality may lead to a covalent reaction with matrix polymer as was the case poly(hydroxybutyrate-co-hydroxyvalerate) reinforced with halloysite modified with aminosilane.6 The amino group reacted with the carbonyl group of PHBV, changing its chemical structure and thus its final properties.6 The above few examples underline the importance and the broadness of scope of chemical modifications which permit tailoring functions of fillers to the requirements of improvement of materials. REFERENCES 1 2 3 4 5 6

Wypych G, Handbook of Fillers, 4th Ed, ChemTec Publishing, Toronto, 2016. Tsurko, ES; Feicht, P; Habel, C; Schilling, T; Daab, M; Rosenfeldt, S; Breu, J, J. Membrane Sci., 540, 212-8, 2017. Guo, Q; Zhu, P; Li, G; Wen, J; Wang, T; Lu, D; Sun, R; Wong, C, Compos. Part B: Eng., 116, 388-97, 2017. Bischoff, E; Daitx, T; Simon, DA; Schrekker, HS; Liberman, SA; Mauler, RS, Appl. Clay Sci., 112-113, 68-74, 2015. Atif, M; Bongiovanni, R; Giorcelli, M; Celasco, E; Tagliaferro, A, Appl. Surf. Sci., 286, 142-8, 2013. Carli, LN; Daitx, TS; Soares, GV; Crespo, JS; Mauler, RS, Appl. Clay Sci., 87, 311-9, 2014.

40

Chemical Composition of Classical Fillers

2.3 Trace elements A large number of elements is listed for individual fillers, including Ag, Al, As, Au, Ba, Ca, Cd, Cl, Co, Cr, Cu, F, Fe, Hg, In, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Pb, S, Sb, Sc, Se, Si, Sn, Sr, Ta, Th, Ti, U, V, and Zn (37 in total were found). The trace element concentration value assessment depends on a context, for example: • in geochemistry, the presence of up to 1000 ppm (0.1%) of any element is considered as a trace element • in analytical chemistry, where trace element is usually a contamination which may affect results of testing, this concentration is lowered to 100 ppm Several agencies gave guidelines regarding the presence of trace elements in various products. European medicines agency issued guidelines regarding the maximum acceptable limits of residues of some metals in drug substances and excipients, which are summarized in Table 2.1. Table 2.1. Maximum acceptable limits of some metals in drug substances and excipients1 PDE, μg/day

RfD, μg/kg/day

250

1.5 (CrIII), 0.003 (CrIV)

copper

2,500

50

iron

13,000

Element chromium

manganese

250

140

molybdenum

300

5

nickel

300

20

vanadium

300

1

1,300

300

zinc

TD, μg/kg/day

160 5 300-1,000

PDE − permitted daily exposure (50 kg body weight); EPA RfD − reference dose; TD − WHO tolerable daily intake

Table 2.2 gives trace element risk assessment for 7 elements based on data from US Food and Nutrition Board, FDA, and WHO. Table 2.2. Trace element risk assessment2 RDA, μg/day

Element chromium

FDA, μg/day

WHO PM μg/day

120

copper

900

2,000

35,000

iron

8,000

18,000

56,000

manganese molybdenum

2,000 45

75

selenium

55

70

zinc

11,000

15,000

2.1.28 Zn (zinc)

41

Table 2.3 gives a human body concentration and daily intake of selected elements from food sources based on the suggestions of The Food and Nutrition Board, Institute of Medicine, and National Academy of Sciences. 2.3. Human adult body content and adequate or typical daily dietary intakes3 Total concentration in human body

Adequate daily intake (ADI) or typical daily intake (TDI)

30-50 mg

2-10 mg (TDI)

arsenic

1-2 mg

12-15 μg (ADI)

cadmium

5-20 mg

10-20 μg (TDI)

lead

120 mg

15-100 μg (TDI)

molybdenum

10 mg

45 μg (ADI)

nickel

1-2 mg

100 μg (ADI)

silicon

2-3 g

5-20 mg (ADI)

7-14 mg

1-40 mg (TDI)

Element aluminum

tin

Filler manufacturers usually state that the concentration of trace elements is below the limits established by Pharmacopeia and frequently give information on actual concentration. In addition to the concentration of trace elements, some crystalline forms are a matter of concern. This regards asbestiforms present in some grades of talc and concentration of crystalline silica in some fillers. Both admixtures are considered carcinogenic and, if they appear at high concentration, they need to be mentioned on product labels. These concerns are addressed in Handbook of Fillers.4 REFERENCES 1 2 3 4

Guideline on the specification limits for residues of metal catalyst or metal reagents. European Medicine Agency. London, 21 February, 2008. Goldhaber, SB, Regulatory Toxicology Pharm., 38, 2, 232-42, 2003. Nielsen, FH, Trace elements. Encyclopedia of Food Science and Nutrition, 2nd Ed., Academic Press, 2003, pp. 5820-28. Wypych G, Handbook of Fillers, 4th Ed, ChemTec Publishing, Toronto, 2016.

3

Functional Fillers Special Physical Properties 3.1 SUPERLIGHT Table 3.1 shows density and Mohs hardness of fillers. Table 3.1. Density and Mohs hardness of selected fillers. Density, g/cm3

Mohs hardness

gold

18.8

2.5-3

lead oxide

9.3

copper

8.92

2.5-3

nickel

8.9

4

Filler

gadolinium oxide

7.1-7.5

iron oxide

4.5-5.8

3.8-5.1

4-4.5

3-4

3.5

10

barium sulfate diamond aluminum oxide aluminum talc

3.4-3.95

9

2.7

2-2.9

2.7-2.85

1-1.5

quartz

2.65

7

sand

2.65

7

clay

2.6

2-2.5

calcium carbonate

2.6-2.9

3-4

2.3

5.5-6

glass beads, solid

2.23-2.54

6.5

silica, precipitated

1.9-2.1

1

carbon black

1.7-1.96

2-2.9

fused silica

1.5-2.2

6.5-7

anthracite

1.3-1.8

2.2-3.8

IncoFoam (nickel foam)

0.2-2.9

glass beads, hollow

0.12-1.1

pumice

6-6.5

44

Functional Fillers Special Physical Properties

Table 3.1. Density and Mohs hardness of selected fillers. Filler ceramic beads

Density, g/cm3

Mohs hardness

0.08-2.5

5-7

There are several lessons which can be learned from this table. First of all, it is apparent that density and Mohs hardness do not correlate for the entire set of fillers as presented in this table. Metals included in this table have usually high (exception aluminum) density and are quite soft in comparison with some inorganic filler materials. Diamond, carbon black, and anthracite have the same chemical formula (C) but very different density and Mohs hardness. Here, density and Mohs hardness correlate with crystallinity. Similar is the case of silica grades, such as quartz, sand, and precipitated silica. Their density and hardness depend on crystallinity (precipitated silica is amorphous and its density and hardness dramatically drop down as compared with quartz and sand). Fused silica is another interesting example because it has lower density and still high hardness. The density is lower because it is more porous than quartz and sand but Mohs hardness is still high because of its crystalline form. We can also note that fillers such as talc and clay have relatively high density and low Mohs hardness because they have platelet structure which peels off under a pressure and lowers the result of hardness testing. The average density of fillers at 2.6-2.9 (calcium carbonate) is substantially higher than the majority of polymers. Thus, in most polymeric applications, the disadvantage of fillers is their high density because we usually need to produce materials which are strong and light. It is apparent that it is quite easy to design fillers which are light by the formation of hollow structures which substantially decrease density (typically 15-30 times) but it is essential to create such fillers which are both light and resistant to compression. This can be accomplished by selection of strong materials, right thickness of walls, crystallinity and morphology preventing breakage. IncoFoam is one of such materials. It is constructed of nickel which has relatively high Mohs hardness (4; similar to Figure 3.1. IncoFoam. [Adapted, by permission, from steel and iron). Nickel has high density but Paserin, V; Marcuson, S; Shu, J; Wilkinson, DS, Adv. it can be formed into sufficiently thin walls Eng. Mater., 6, 6, 454-9, 2004.] which can withstand compression forces. The morphology of IncoFoam is given in Figure 3.1. IncoFoam is a complete material which can be used for many purposes such as battery walls and solar cells.1 It can be selected as an example of a material structure which can be manufactured from a combination of particles of hollow filler and the polymeric matrix. The balance between compression strength and material density can be obtained by a selection of wall thickness of

3.2 High density

45

hollow particle of filler, structure of hollow spaces and proportion between the volume of filler and volume occupied by the polymer matrix. The lightweight Extendospheres® maintain low specific gravity without sacrificing strength. XOL 200 is a unique super-light filler, which consists of small stony glass microspheres. It gives 8-10 times larger volume for the same weight.2 It improves impact strength and gives fire protection, dimensional stability, and thermal insulation properties to a product. Application of light fillers is important for materials which can work under submerged conditions and maintain light weight.3 Expanded clay materials used in an invention can withstand pressure of 225 bar corresponding to sea depth of 2250 m.3 Development of light magnetic products is complicated by the fact that magnetic fillers have high density.4 This was overcome by the use of technology in which hybrid magnetic nanocomposites were obtained from highly concentrated emulsion polymerization.4 In solid foams made out of divinylbenzene–polystyrene matrix, the nanoparticles of iron oxide were deposited on foam walls.4 The density of the solid foams was 50-70 kg m-3 (20 times lighter than that of the non-porous monoliths) and nanocomposite had superparamagnetic properties.4 There are some products, such as sandable fillers used in construction, which do not require mechanical strength but should be lightweight and contain easy breakable filler to assist sanding process. These products can use beneficially the lightest hollow glass beads.

3.2 HIGH DENSITY Weighting, balancing, vibration dampening, and radiation shielding require high-density materials. Previously used metals are replaced with high-density composite materials which have specific gravity in the range of 2-11 g/cm3.5 Tungsten has superhigh density of 19.35 (other potential applicable metals have the following densities: osmium − 22.48, iridium − 22.42, platinum − 21.45, rhenium − 20.53, gold − 18.88, bismuth − 9.80, copper − 8.92). The replacement for lead in applications where the high density of lead is important, but where the toxicity of lead is undesirable, a high-density material comprising tungsten, metal fiber (steel fiber to improve mechanical properties) and binder (polyamide) has been used.6 Typical weight ratio is tungsten powder : polyamide : stainless steel fiber = 88 : 4 : 8.6 Bismuth-containing products have also been patented although bismuth has much lower density than tungsten but its oxide has a higher density than the tungsten oxide (8.90 compared with 7.16). Typical composite has at least 66 wt% of bismuth oxide which may be used in a combination with other filler such as, for example, barium sulfate.7 Many polymeric materials can be used as binders but novolac resins were found suitable.7 These composites are used for radiation shielding.7 Bismuth oxide was also used in vulcanized rubber compounds as a high-density filler to improve wet traction and rolling resistance of tires.8 Chesapeake’s HDM® is a cost-effective product made out of a non-toxic blend of iron, steel, oxides, and cement used in various applications such as radiation shielding blocks, protective blast walls and barriers, ship ballast, counter balance weights for lift bridges or industrial machinery. It has a density of 3.6 g/cm3.

46

Functional Fillers Special Physical Properties

3.3 THERMAL INSULATION AND CONDUCTIVITY Table 3.2 gives the values of thermal conductivity and specific heat for fillers, metals, and selected polymers. Table 3.2 Thermal conductivity and specific heat of fillers, metals, and polymers. Material

Thermal conductivity, W/mK

Specific heat, J/gK

Metals silver

450

0.188

copper

400

0.376

gold

315

0.13

aluminum

237

nickel

158

0.44

molybdenum

138

0.25

tungsten

2.35

0.088

Fillers (particulate and fibers) graphene diamond carbon nanotubes

1050-5150

2.1

1000 50-3500

carbon fibers

9-1000

0.71

beryllium oxide

250-265

1.03

aluminum nitrite

140-320

silicon carbide graphite zinc oxide

120

0.75

110-600

0.7

60

boron nitride

20-300

0.794

silicon nitride

16-33

0.17

magnesium oxide

8-60

1.03

aluminum oxide

20.5-29.3

titanium dioxide

9-13

0.71

calcium fluoride

9.7

0.85

silica

1.1-13.6

0.8

5.1

0.67

2.4-3

0.83

2.1

0.82

aluminum flakes

2.04

0.9

barium sulfate

1.31

magnetite calcium carbonate talc

glass fiber wollastonite

1 0.82

0.83

3.3 Thermal insulation and conductivity

47

Table 3.2 Thermal conductivity and specific heat of fillers, metals, and polymers. Material carbonyl iron powder

Thermal conductivity, W/mK

Specific heat, J/gK

0.80

mica

0.71

carbon black

0.45

pumice

0.34

ceramic beads

0.23

molybdenum disulfide

0.13-0.19

vermiculite

0.128-0.2

0.5 0.87

0.84-1.08

halloysite

0.092

0.92

calcium silicate

0.053

0.84

glass beads

0.039-0.2

1.17

perlite

0.04-0.06

0.84

aramid fiber

0.04-0.05

1.42

Polymers polyethylene, high density

0.45-0.52

1.9

polyethylene, low density

0.33

1.9-2.3

polytetrafluoroethylene

0.25

1.0

polyamide-6

0.24-0.28

1.7

cellulose acetate

0.16-0.36

1.2-1.9

polycarbonate

0.19-0.22

1.2

polymethylenemethacrylate

0.17-0.19

1.4-1.5

polyethylene terephthalate

0.15-0.4

1.2-1.35

polyvinylchloride

0.12-0.25

1.0-1.5

polypropylene

0.1-0.22

1.7-1.9

polystyrene

0.1-0.13

1.2

Thermal insulator should have a very low thermal conductivity and a high specific heat and thermal conductor should have a high thermal conductivity and a low specific heat. Based on this definitions, metals are thermal conductors of variable efficiency but they are inferior in comparison to some fillers such as graphene, carbon nanotubes, diamond, graphite, beryllium oxide, aluminum nitride, silicon carbide, and boron nitride. Polymers are insulating materials and the addition of metals and just mentioned fillers improves their conductivity. There is also a large number of fillers which are capable of decreasing the thermal conductivity of polymers, including especially aramid fibers, perlite, glass beads, calcium silicate, halloysite, vermiculite, and ceramic beads. Many common fillers used in polymers, such as, for example, calcium carbonate, talc, clay, silica or glass fibers increase the thermal conductivity of polymers. Thermal insulation improvement of polymeric material is a subject of numerous research − the results of which are summarized below. The key property of a thermal

48

Functional Fillers Special Physical Properties

building insulation material is the thermal conductivity.9 The goal in building insulation is to achieve thermal conductivity lower than 0.065 W/mK.9 Cellular materials such as calcium silicate, bonded perlite, vermiculite, and ceramic products used for insulation do not contain polymers.9 Also, fibrous materials such as glass wool are used for insulation (typical thermal conductivity is 0.03-0.04 W/mK).9 Many organic materials are also used for insulation, including polymers such as expanded polystyrene (0.03-0.04 W/mK) and polyurethane foam (0.02-0.3 W/mK).9 Insulation property is achieved by incorporation of gas bubbles. So far the insulating fillers are not in use.9 Only reflective fillers are used for metallized reflective membranes.9 These insulating materials compete or are used together with other typical materials used in buildings, such as wood (0.1-0.2 W/mK), concrete (0.15-2.5 W/mK), plaster (0.9 W/mK), brick (0.4-1 W/mK), stone (1-2 W/mK) and glass (0.8 W/mK).9 From this comparison, it is evident that polymers and their composites have favorable properties to improve insulation, make building lighter, and easier to remodel.9 Waterborne matrix (styrene-acrylic resin) was modified with γ-alumina, aerogel, and hollow glass beads to form an inorganic-organic composite paint.9 The alumina use had surface area of 155 m2/g and pore size of 5.8 nm.9 The paints contained the following proportion of water/ resin/filler/additives = 45/10/35/10 wt%.9 Coatings containing hollow glass microspheres, hollow ceramic microspheres, aerogel, and γ-alumina had conductivities of 0.146, 0.046, 0.039, 0.049 W/mK, Figure 3.2. Thermal conductivity of silicone composite respectively.9 containing hollow glass microspheres versus the perFigure 3.2 shows that the thermal concentage of broken spheres. [Adapted, by permission, ductivity of composites containing hollow from Hu, Y; Mei, R; An, Z; Zhang, J, Compos. Sci. Technol., 79, 64-9, 2013.] glass microspheres, HGM, depends on a percentage of broken spheres.10 The presence of broken HGM enhanced the mechanical property but weakened the heat insulation of composite materials.10 The materials used for thermal insulation of pipes have been discussed in detail in a book chapter regarding their composition, size, dimensions, physical properties, inspection, packaging, and marking.11 These products have the following thermal conductivities in W/mK:11 • mineral fiber preformed insulation − 0.065 (200oC) and 0.073 (450oC) • mineral fiber block and board − 0.065 (200oC) • mineral fiber blanket − 0.065 (200oC) • calcium silicate preformed block − 0.079 (type I) and 0.039 (type II) at 204oC • cellular glass/foam glass − 0.045 (20oC), 0.042 (0oC), 0.038 (-20oC) • cork board and pipe − 0.042 (20oC) • rigid-cellular PUR and PIR spray applied − 0.023 (180 days, 21oC) • preformed rigid-cellular PUR and PIR − 0.023 (180 days, 21oC) • rock wool/glass wool − 0.034 (25oC)

3.3 Thermal insulation and conductivity

49

Microencapsulated phase change material (Micronal DS 5040X) was used as a substitute for sand (20 vol%) to increase a heat capacity of concrete.12 The heat capacity and thermal mass of mortar mixes with different amount of PCM are presented in Figure 3.3.12 The PCM addition considerably increases the heat capacity of the mortar. PCM delays the entry of heat into the building and the microencapsulated PCM Figure 3.3. DSC curves (endothermic) for mortar sam- creates a thermal gradient in the speciples with different microencapsulated PCM proportions men.12 A reduction in the thermal conducin 28 days of curing time. [Adapted, by permission, tivity with the increase of PCM amount was from Jayalath, A; San Nicolas, R; Sofi, M; Shanks, R; observed in both concrete and mortar Ngo, T; Aye, L; Mendis, P, Constr. Build. Mater., 120, 408-17, 2016.] mixes.12 The aerogel/fibrous ceramic composite, inspired by the bird’s nest structure, was synthesized using mullite fibers as a matrix and ZrO2-SiO2 aerogels as a filler through vacuum impregnation.13 The macropores of mullite fibers are filled by aerogels to maintain high porosity (85%).13 ZrO2-SiO2 composite aerogel was prepared by prehydrolysis using tetraethyl orthosilicate and ZrOCl2 as precursors.13 1,2-epoxypropane was used as gelation promoter.13 Nitric acid was a catalyst.13 Polyethylene glycol 600 and formamide were used as a dispersant and a dry control chemical agent.13 The aerogel/fibrous ceramic composite had a thermal conductivity of 0.0524 at room temperature and 0.082–0.182 W/ mK in the temperature range of 500-1200 °C indicating its excellent thermal insulation property in a wide temperature range.13 A substantial reduction of attic temperature (13oC) was achieved by a combination of moving-air-cavity (aluminum tube) and thermal insulation coating.14 The coating was designed with eggshell used for the production of insulating filler.14 The thermal conductivity of the resultant insulating coating was 0.107 W/mK.14 Addition of 30% of tire crumbs to the light-weight concrete resulted in the reduction of its thermal conductivity from 0.2 to 0.165 W/mK.15 A reflective heat insulation coating was prepared using a geopolymer.16 The geopolymer was made of sodium silicate solutions and metakaolin, as the primary film forming a material containing sericite powder, talcum powder, titanium dioxide, and hollow glass microspheres as fillers.16 With titanium dioxide content of 12% and 6% hollow glass microspheres, the reflective heat insulation coating had a reflectivity over 90% and thermal insulation performance (internal and external surface temperature difference) of up to 24°C.16 The energy-saving building insulation material, steel structure coating, industrial and civil reflective insulation, which is radiation-resistant and fire preventive, was obtained from fluorocarbon resin containing 10% “space suit” ceramic hollow microspheres.17 Heat sink additives undergo an endothermic reaction to absorb heat when exposed to significant temperature increases.18 Heat sink additives are able to consume 685-1400 Joules of energy per gram.18 Alumina trihydrate is one of the preferred heat sink addi-

50

Functional Fillers Special Physical Properties

tives.18 It is stable at room temperature but releases water between 180-205oC.18 Heat sink additive is added to the paper layer of gypsum boards.18 A composition for the preparation of a fire protection mortar comprises19 • 45-70 wt% cement binder • 8-20 wt% calcite • 8-20 wt% mica • 0-5 wt% xonolite (Ca6Si6O17(OH)2) • 0.1-20 wt% expanded perlite • 0.1-10 wt% fibers • 0.01-2 wt% air entrainer and foaming agent • 0.01-2 wt% processing aids. Xonolite and perlite improve thermal insulation properties.19 The thermal conductivity of materials can be improved by a large number of fillers. Leading trends are discussed in the applications below. Because of integration and functionalization of electronics and applications such as light emitting diodes and flexible electronics, thermal dissipation is a challenging problem.20 The heat transfer rate is given by the following equation: ΔT q = kA ------L

[3.1]

where: k A ΔT L

thermal conductivity cross-section of transfer area temperature difference conduction path length.

This simple equation shows that for a given product, conductivity is the only parameter which may influence cooling rate which has to, at least, balance the heating rate or otherwise, the product will increase in temperature. Polymers have low thermal conductivities, therefore fillers having a high thermal conductivity (such as those included at the top of Table 3.3) have to be used to increase it. In applications which require a high thermal conductivity and a low electrical conductivity, some fillers are added which have a medium-high thermal conductivity and a low electrical conductivity (e.g., aluminum oxide, beryllium oxide, boron nitride, silicon nitride, or aluminum nitride). If filler particles are anisotropic, their orientation does matter for directional conductivity. The concentration of filler particles is important for conductivity but also for mechanical properties and frequently a compromise must be found to maintain both properties on a required level. In some solutions, an additional (reinforcing) filler has to be added to improve failing mechanical performance with increased concentration of conductive filler. A good polymer-filler interaction helps to improve thermal conductivity because a conduction may be severely hampered by conductivity barriers caused by inferior interface properties between polymer and filler particles. Finally, a uniform dispersion of fillers is essential for the performance of the conductive composite. Considering all the above influences other than the rated performance of the filler, it is wise to compare experimental performance with results of modelling, which may be easier and more effective than the evaluation of filler orientation, distribution, polymerfiller interaction, interface formation, and related phenomena. Hashin-Shtrikman modified

3.3 Thermal insulation and conductivity

51

model gives good predictions of thermal conductivity of hybrid filler-polymer composites.21 The model is expressed by the following equations:21

where:

1 + 2λM * k eff = --------------------1 – λM

[3.2]

κ2 – 1 κ1 – 1 - φ 1 + --------------φ M = -------------κ1 + 2 κ2 + 2 2

[3.3]

k*eff λ κ φ 1,2

non-dimensional effective thermal conductivity correction factor thermal conductivity ratio volume fraction particle 1 and 2

Silica coated flake graphite is a thermally conductive and electrically insulating filler for epoxy resin composites.22 Coating increases volume resistivity of graphite flakes by 35 times.22 The thermal conductivity of a coated flake is very similar to the uncoated flake.22 Addition of 50 wt% of a coated graphite flake to epoxy resin increases its thermal conductivity by 15.2 times.22 A synergistic effect in the thermal conductivity enhancement of poly(3-hydroxybutyrate) composites was achieved using BN/Al2O3 fillers.23 The thermally conductive fillers have formed conductive pathways.23 A more efficient thermally conductive pathways originated from the alignment of BN on the Al2O3 surfaces.23 They were formed because of lowering the viscosity on addition of Al2O3 which led to a decreased thermal interface resistance.23 Figure 3.4 illustrates the nature of synergism.23 In PHB/Al2O3 composite, the spherical Al2O3 particles form thermally conductive pathways.23 They connect with each other by point-to-point contact (Figure 3.4d) which results in a thermal interface resistance with the surrounding polymer chains because of the phonon mismatch between Al2O3 and

Figure 3.4. Schematic illustration of the morphological variation of PHB composites, a. PHB/BN composite; b. PHB/BN/Al2O3 composite with synergistic effect; c. PHB/BN/Al2O3 composite with lower content of BN and d. PHB/Al2O3 composite. [Adapted, by permission, from Li, Z; Ju, D; Han, L; Dong, L, Thermochim. Acta, 652, 9-16, 2017.]

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Functional Fillers Special Physical Properties

Figure 3.5. Composite microstructure with volume ratio of LDPE to epoxy gradually increasing from (a) to (d). [Adapted, by permission, from Zhou, BL; Wang, J; Zheng, H; Ma, J; Wu, J; Gan, ZH; Liu, J, J. Alloys Compounds, 701, 499-507, 2017.]

PHB. BN particles are more efficient in enhancing the thermal conductivity of PHB as compared with Al2O3 particles because their flat surface favors the interaction between the filler and the polymer matrix.23 The point-to-point contact of BN particles is replaced by covering the surface of Al2O3 (Figure 3.4b), significantly increasing the contact area at the filler/filler interface within the hybrid filler network.23 Anisotropic aluminum nitride whiskers and isotropic spheres were used as hybrid fillers in epoxy and polyvinylidene fluoride matrices to enhance the thermal conductivity of polymer composites.26 The epoxy composite reached a maximum thermal conductivity of 4.321 W/mK at the volume ratio of AlN whiskers to spheres of 1:1.26 When the surface of hybrid fillers was modified by silane coupling agent, the thermal conductivity of the epoxy composite increased significantly to 5.232 W/mK.26 This high thermal conductivity could be attributed to the formation of efficient three dimensional network.26 Graphite powder (0.3 wt%) was added to polyvinylalcohol-acrylic acid solution and a graphite foam was produced.24 Both mechanical and thermal properties were enhanced due to the interaction of graphite with matrix which resulted in an increase of crystalline thickness by 11.2% (the apparent density increased by 4.05%).24 This resulted in thermal conductivity increase by 59.3% and compression strength increase by 120%.24

3.3 Thermal insulation and conductivity

53

The hybrid filler particles are mostly distributed in the epoxy phase in the LDPE/ epoxy composites.25 The thermally conductive networks of filler particles can be easily constructed when a suitable volume ratio of LDPE to epoxy and the platelet BN particles is established.25 Figure 3.5 illustrates this case.25 In epoxy matrix (Figure 3.5a), the filler particles are uniformly distributed in the matrix.25 They are too distant to form contacts and thus the thermal conductivity of the composites is low.25 On addition of LDPE (Figure 3.5b), the volume fraction of the filler particles in epoxy is increased because particles are only distributed in the epoxy.25 Some of these particles are in contact which increases the thermal conductivity but it is still not sufficient to obtain conductive composite.25 With further addition of LDPE (Figure 3.5c), more nitride particles are in the contact with each other, which results in the formation of thermally conductive networks and greatly increased thermal conductivity of the composite.25 On further addition of LDPE (Figure 3.5d), the epoxy phase becomes discontinuous and the thermally conductive network is destroyed, causing a decrease in the thermal conductivity of the composite.25 The optimum proportion of LDPE to epoxy was at the ratio of 3:7.25 With addition of 15 vol% of BN, the thermal conductivity of the blend was 0.7 W/mK.25 Silane coupling improved thermal conductivity effect of hexagonal boron nitride dispersed in polytetrafluoroethylene.27 The surface treatment improved the interfacial adhesion between filler platelets and PTFE matrix.27 Also, the in-plane orientation degree of platelets in PTFE matrix decreased, which effectively improved the thermal conductivity of the composites.27 The thermal conductivity of composite with 30 vol% filler content was 0.722 W/mK, which was 2.7 folds higher than for pure PTFE.27 Superconducting test coils impregnated with epoxy composites containing cubic boron nitride (cBN) particles, hexagonal boron nitride (hBN) particles, and a mixture of cBN/hBN particles were fabricated.28 Micro-voids (acting as major obstacles to the formation of thermally conductive passages) were observed in the epoxy composites containing each of the particle fillers alone but were absent when the cBN/hBN particle mixture was used (Figure 3.6).28 The epoxy containing the cBN/ hBN particle mixture exhibited superior cooling performance, Figure 3.6. SEM images of epoxy/cBN (a), epoxy/hBN (b), and epoxy/cBN–hBN composites (c). [Adapted, by permission, from indicating that this composite Jeong, S-H; Song, J-B; Kim, KL; Choi, YH; Lee, H, Composites effectively facilitated heat transfer Part B: Eng., 107, 22-8, 2016.] between the coil and liquid nitrogen.28 Graphene oxide was treated with two types of surfactants, i.e., silane coupling agent and 4,4’-diphenylmethane diisocyanate, incorporated into phenyl silicone rubber at a low

54

Functional Fillers Special Physical Properties

concentration (=0.2 wt%), and cured by the room temperature vulcanization.29 The composites filled with silane-modified graphene oxide had better thermal conductivity.29 Aluminum nitride, AlN, particles of different sizes, graphite, graphene oxide were used alone or in a synergistic manner to prepare thermally conductive adhesives.30 Adhesives filled with larger AIN particles possess higher thermal conductivFigure 3.7. Schematic diagram of graphene oxide, GO, ity than those filled with smaller particles at 30 acting as bridge in the matrix filled with AlN particles. a given filling content. Graphene oxide [Adapted, by permission, from Yuan, W; Xiao, Q; Li, L; was found to be better than graphite regardXu, T, Appl. Thermal Eng., 106, 1057-67, 2016.] ing improvement of the thermal conductivity of the epoxy resin.30 With 8 wt% graphene oxide or 70 wt% 5 µm-AlN particles, the thermal conductivities of adhesives were 6.42 times and 11.8 times that of pure epoxy, respectively.30 To further improve the thermal conductivity, the mixture of 50 wt% 5 µmAlN particles and 6 wt% graphene oxide were used, and the final thermal conductivity

Figure 3.8. SEM images of thermal adhesives cross-sections. (a) Pure epoxy resin, adhesive with (b) ZnO, (c) BN, (d) Al2O3, (e) graphite, (f) Al, (g) Cu, (h) diamond and (i) Ag. [Adapted, by permission, from Fu, Y-X; He, Z-X; Mo, D-C; Lu, S-S, Appl. Therm. Eng., 66, 1-2, 493-8, 2014.]

3.3 Thermal insulation and conductivity

55

Figure 3.9. Upper left − Inductively coupled plasma-treated expanded graphite (a) before treatment, (b) at 400°C, (c) at 600°C and (d) at 800°C. Bottom left − Inductively coupled plasma system. Middle − Micro-CT images including 3D filler network sizes for the composites for composites having different volume expansion. Right − Effect of different concentration and expansion volume on thermal conductivity of composite. [Adapted, by permission, from Kim, HS; Kim, JH; Kim, WY; Lee, HS; Kim, SY; Khil, M-S, Carbon, 119, 40-6, 2017.]

reached 2.77 W/mK, that was 14.6 times of that of neat epoxy.30 Figure 3.7 shows the arrangement of filler particles in the matrix.30 The composite containing binary filler (aggregated and elongated (whisker) boron nitride with volume ratio of 7:3 ratio) showed maximum values of thermal conductivity of 3.62 W/mK, which was 19.1 times larger than that of the pristine epoxy.31 It is probable that this ratio is a suitable composition because of the formation of 3D interparticle structures, which correlate with the heat flow pathways and interfacial phonon scattering.31 The thermal adhesives were prepared from epoxy resin containing natural graphite, copper, aluminum, zinc oxide, boron nitride, aluminum oxide, diamond and silver powders.32 Figure 3.8 shows dispersion and morphology of fillers used in the conductive adhesives.32 The layer-shaped filler (e.g., graphite) is more favorable than the ball-shaped filler and the sharp-corner-shaped filler in enhancing the thermal conductivity of epoxy resin.32 The natural graphite/epoxy adhesive had the best thermal conductivity of 1.68 W/mK.32 The relationship between the volume expansion (interlayer spacing) of expanded graphite and thermal conductivity of filled composites was studied (Figure 3.9).33 The higher the treatment temperature of inductively coupled plasma process the greater the extent of the volume expansion of the expanded graphite.33 The thermal conductivity of the composites was increased with the expanded volume of the filler, at the same filler content. 33 Excellent thermal conductivity (10.77 W/mK) and heat dissipation characteristics of the composite were obtained when the 3D thermally conductive EG filler network was generated at the highest temperature.33 Thermally conductive plastic composition contains polymer (polycarbonate or polyamide) and a blend of boron nitride (platelets 0.3x200 μm), metal oxide (e.g., ZnO or MgO), and silane (e.g., epoxy, vinyl, methacryloxy, or mercapto).34 The thermally conductive composition has an in-plane thermal conductivity of about 10 W/mK or greater.34 The thermal interface materials are used for heat removal in electronics, optoelectronics, photonics, and battery technology.35 The thermal interface material can include a

56

Functional Fillers Special Physical Properties

matrix and a filler.35 The filler can be graphene or multilayer graphene dispersed within the matrix.35 Alternatively, the thermal interface material can also include a matrix, a metallic filler, and a graphene filler.35 The material had a thermal conductivity within the range of 2-10 W/mK at room temperature.35 Thermoplastic composite contains nanodiamond as a conductive filler.36 The nanodiamond particles have sizes of 4-6 nm.36 Nanodiamond can be used in combination with boron nitride.36 Polyamide and polyphenylene sulfide were used for the development of conductive composites.36 Boehmite has high thermal conductivity, flame retardancy, and high filling characteristics.37 Boehmite is subjected to heat treatment at 320-430oC under elevated pressure and in the presence of steam.37

3.4 THERMAL ENERGY STORAGE The generated solar energy needs methods to store heat to meet the needs when solar light is not available.38 Rock bed uses air as heat transfer fluid for thermal energy storage in the concentrated solar power plants.38 The gabbro rock was found to be the best candidate material for the solar power plants.38 Smart gypsum composite contains microcapsules containing phase change materials for building materials with high thermal energy storage capacity useful in high comfort construction systems.39 A commercial product (BASF DS 5000) consists of a dispersion of Micronal®PCM having a solid content of 42 wt%.39 Micronal®DS 5001X contains nheptadecane as the core material with melting point of 26°C and a shell from polymethylmethacrylate.39 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Paserin, V; Marcuson, S; Shu, J; Wilkinson, DS, Adv. Eng. Mater., 6, 6, 454-9, 2004. Endospheres XOL 200. SphereOne Inc. Echtermeyer, A; Lippe, K, EP2614114, Compbuoy AS, Jul 17, 2013. Ghosh, G; Vilchez, A; Esquena, J; Solans, C; Rodríguez-Abreu, C, Mater. Chem. Phys., 130, 1-2, 786-93, 2011. Durkee, RR, High Density Composites Replace Lead. Ecomass Technologies. Bray, AV; Muskopf, BA; Dingus, ML, US6517774, Ideas to Market, L.P., Feb. 11, 2003. McCord, S, US8308986, Nov. 13, 2012. Baranek, TM, US6734245, Bridgestone/Firestone North American Tire, Llc, May 11, 2004. Comite, A; Cozza, ES; Di Tanna, G; Mandolfino, C; Milella, F; Vicini, S, Prog. Org. Coat., 78, 124-32, 2015. Hu, Y; Mei, R; An, Z; Zhang, J, Compos. Sci. Technol., 79, 64-9, 2013. Bahadori, A, Thermal Insulation Handbook for the Oil, Gas, and Petrochemical Industries. Chapter Three − Material Selection for Thermal Insulation. Elsevier, 2014, pp. 239-301. Jayalath, A; San Nicolas, R; Sofi, M; Shanks, R; Ngo, T; Aye, L; Mendis, P, Constr. Build. Mater., 120, 408-17, 2016. He, J; Li, X; Su, D; Ji, H; Wang, XJ, J. Eur. Ceramic Soc., 36, 6, 1487-93, 2016. Yew, MC; Ramli Sulong, NH; Chong, WT; Poh, SC; Ang, BC; Tan, KH, Energy Conversion Mangem., 75, 241-8, 2013. Kashani, A; Ngo, TD; Mendis, P; Black, JR; Hajimohammadi, A, J. Cleaner Prod., 149, 925-35, 2017. Zhang, Z; Wang, K; Mo, B; Li, X; Cui, X, Energ. Build., 87, 220-5, 2015. CN103725116, Apr. 16, 2014. Chan, C; Song, WD; Cao, B; Rosenthal, G; Yu, Q; Veeramasuneni, S, US20130216762, United States Gypsum Company, Aug. 22, 2013. Wu, X; Opsommer, A, CA2840343, Promat Research & Technology Centre N.V., Aug. 5, 2014. Chen, H; Ginzburg, VV; Yang, J; Yang, Y; Liu, W; Huang, Y; Du, L; Chen, B, Prog. Polym. Sci., 69, 41-85, 2016.

3.4 Thermal energy storage

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

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Ngo, IL; Vattikuti, SVP; Byon, C, Int. J. Heat Mass Transf., 114, 727-34, 2017. Li, W; Feng, W; Huang, H, J. Mater. Sci.: Mater. Electron., 27, 6364-70, 2016. Li, Z; Ju, D; Han, L; Dong, L, Thermochim. Acta, 652, 9-16, 2017. Kim, J-H; Lee, H-I; Lee, Y-S, Mater. Sci. Eng., 696A, 174-81, 2017. Zhou, BL; Wang, J; Zheng, H; Ma, J; Wu, J; Gan, ZH; Liu, J, J. Alloys Compounds, 701, 499-507, 2017. Dang, TML, Kim, C-Y; Zhang, Y; Yang, J-F; Masaki, T; Yoon, D-H, Composites, Part B; Eng., 114, 237-46, 2017. Pan, C; Kou, K; Jia, Q; Zhang, Y; Wu, G; Ji, T, Composites Part B: Eng., 111, 83-90, 2017. Jeong, S-H; Song, J-B; Kim, KL; Choi, YH; Lee, H, Composites Part B: Eng., 107, 22-8, 2016. Xu, Y; Gao, Q; Liang, H; Zheng, K, Polym. Testing, 54, 168-75, 2016. Yuan, W; Xiao, Q; Li, L; Xu, T, Appl. Thermal Eng., 106, 1057-67, 2016. Kim, K; Kim, M; Kim, J, Compos. Sci. Technol., 103, 72-7, 2014. Fu, Y-X; He, Z-X; Mo, D-C; Lu, S-S, Appl. Therm. Eng., 66, 1-2, 493-8, 2014. Kim, HS; Kim, JH; Kim, WY; Lee, HS; Kim, SY; Khil, M-S, Carbon, 119, 40-6, 2017. Raman, C; Xiang, B; Murugaiah, A, US8946333, Momentive Performance Materials Inc., Feb. 3, 2015. Balandin, AA, US20140120399, The Regents of the University of California, May 1, 2014. Myllymaki, V; Syren, J, US9085723, Carbodeon Ltd Oy, Jul. 21, 2015. Ota, Y; Mizutani, M; Kido, K; Kawai, T; Kihou, H, EP2918546, Kawai Lime Industry Co., Ltd., Sep. 16, 2015. Tiskatine, Aharoune, A; Bouirden, L; Ihlal, A, Appl. Therm. Eng., 117, 591-608, 2017. http://www2.basf.us/corporate/080204_micronal.htm

4

Functional Fillers Electrical and Magnetic Properties 4.1 CONDUCTIVE The cementitious composites containing nano-SiO2 (NS), nano-TiO2 (NT), carbon nanotubes (CNTs), carbon nanofibers (CNFs) and carbon microfibers (CFs) were studied.1 The resistivity of composites containing CNTs and CNFs decreased with the increase of dosage. CNTs was more conductive than CNFs (resistivity of a composite containing 1 wt% of these fillers decreased by 99.1 and 92.0%, respectively as compared with concrete without fillers).1 The particulate fillers do not perform as well (resistivity of a composite containing 1 wt% of NT and NS increased by 13% and decreased by 53%, respectively).1 The extremely high aspect ratio of carbon nanotubes and nanofibers is instrumental for the formation of a conductive network at a level as low as 0.1 wt%.1 With longer carbon fibers (3 and 6 mm), the resistivity decreased more (81.5% compared with 90.3% at the filler content of 0.5%, respectively).1 The resistivity of composites containing microfibers was larger than that of the composites with CNFs because nanofibers have a smaller diameter and can form conductive networks at a lower dosage.1 The data discussed here are not only typical of the addition of conductive fillers to concrete but they illustrate the general effect of filler morphology on other materials, as well. The alternating current electrical resistivity of the concrete composites containing variable amounts of carbon nanotube/carbon black (40:60) is shown in Figure 4.1.2 When the filler content ranges Figure 4.1. AC electrical resistivity of the concrete com- from 0.39 to 1.52 vol%, the AC electrical posites with different content of filler (CTN/CB). resistivity of the composites dramatically [Adapted, by permission, from Zhang, L; Han, B; Quy2 ang, J; Yu, X; Sun, S; Ou, J; Archives Civil Mech. Eng., decreases due to the filler addition. This 17, 2, 354-64, 2017.] range is known as the percolation threshold

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Functional Fillers Electrical and Magnetic

Figure 4.2. Illustration of CNT dispersion in polymer blend: left − initial dispersion, middle, right − CNT dispersion by “filler-transfer-induced dispersion” in nanocomposite. [Adapted, by permission, from Lee, CJ; Salehiyan, R; Ham, DS; Cho, SK; Lee, S-J; Kim, KJ; Yoo, Y; Hyun, K; Lee, JH; Choi, WJ, Polymer, 84, 198208, 2016.]

zone of the electrical conductivity of the composite.2 It is frequently used in the analysis of the results of conductivity studies, especially because effectiveness of conductive fillers can be easily compared.2 Figure 4.2 illustrates the effect of filler affinity on its dispersion in the polymer blend.3 Initially, the carbon nanotubes were dispersed in PS/PPE component which was subsequently melt mixed with polyamide-66.3 The CNTs were found to be dispersed preferentially in the polyamide-66 phase and not in the miscible polystyrene/poly(phenylene ether) blend phase because CNTs have a higher affinity to PA-66.3 The CNTs transfer occurred through the blend interface from the poor wetting phase (PS/PPE phase) to the better wetting phase (PA-66 phase).3 The electrical conductivity of the PA-66/(PS/PPE)/ CNTs nanocomposites was significantly increased compared to the PA-66/CNTs nanocomposites at the same CNT loading due to the fact that polyamide formed continuous conductive pathways within the blend with highly concentrated conductive filler.4 The electrical conductivity of composite can be improved several orders of magnitude (depending on the carbon-based particle and its concentration) by annealing the composites at its melt state.4 Also, the addition of 1 vol% of CNT to PP composites containing 2 vol% of thermally reduced graphene oxide gives electrical conductivity of ~10-4 S/m, which permits, in consequence, to save ~50% of CNT.4 The conductivity of MWNT/carbon black 50/50 composite is six orders of magnitude larger than the conductivity of a composite (Figure 4.3) containing only CB (the viscosity increases only by a factor of two).5 Figure 4.4 shows that the change in resistivity with the increase in MWCNT concentration is only marginal because of improper dispersion of MWCNT in matrix polymercontaining graphene nanoplatelets.6 A gradual incorporation of carbon black in NBR-graphene nanoplatelets composite progressively reduces the resistivity.6 At only 10 phr carbon black addition, the resistivity drops down by two decades.6 This reduction is attributed to the formation of a continuous conductive network of graphene nanoplateletscarbon black in the insulating polymer matrix.6 Figure 4.5 shows a structural arrangement

4.1 Conductive

Figure 4.3. DC conductivity of different filler concentrations for MWNT, CB and MWNT/CB with wt% ratio of 50/50 at room temperature. [Adapted, by permission, from Hilarius, K; Lellinger, D; Alig, I; Villmow, T; Pegel, S; Pötschke, P, Polymer, 54, 21, 5865-74, 2013.]

61

Figure 4.4. Change in DC resistivity with the introduction of CB and MWCNT into NBR-GNP composites. [Adapted, by permission, from Mondal, S; Khastgir, D; Composites Part A; Appl. Sci. Manuf., in press, 2017.]

Figure 4.5. Schematic representation of network formation in GNP-CB and GNP-MWCNT hybrid system. [Adapted, by permission, from Mondal, S; Khastgir, D; Composites Part A; Appl. Sci. Manuf., 102, 154, 2017.]

of conductive fillers which explains why the addition of MWCNT does not improve conductivity as does the presence of carbon black.6 Figure 4.6 shows that the SCFNA process involves three steps:7 • a homogeneous dispersion of mixture of conductive fillers and polymer achieved by melt compounding or solution mixing • the homogeneous dispersed mixture is compressed in a pressing mold to initiate and accomplish the networking of the conductive fillers by their self-assembly mechanisms • the sample is further compressed after forming a self-assembled network to a thickness smaller than the average diameter of the conductive network threads, dm.

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Functional Fillers Electrical and Magnetic

Figure 4.6. Scheme of technological pathway of spatial confining forced network assembly, SCFNA, versus conventional compounding approach. [Adapted, by permission, from Wu, D; Gao, X; Sun, J; Wu, D; Liu, Y; Kormakov, S; Zheng, X; Wu, L; Huang, Y; Guo, Z, Composites Part A: Appl. Sci. Manuf., 102, 88-95, 2017.]

Figure 4.7. Proposed models of filler network formations in the NR matrix with various filler loadings of CCB (A and B), CNT (C and D), or CNT/CCB (E and F). [Adapted, by permission, from Nakaramontri, Y; Pichaiyut, S; Wisunthorn, S; Nakason, C, Eur. Polym. J., 90, 467-84, 2017.]

The network must be compressed to be more compacted in a confining space.7 The thickness of the sample is critical. If the thickness of the sample is much larger than dm, the compression could only cause the self-assembled network wiggling in the polymer matrix without densification of the network threads.7 The electrical conductivity of the polypropylene/short carbon fibers made by this method was increased by 4 orders of magnitude as compared to the material obtained by an ordinary compounding technology.7 The natural rubber vulcanizates were prepared with carbon nanotubes (CNT), conductive carbon black (CCB), and CNT/CCB hybrid filler.8 A significant increase in conductivity resulted when 5 phr CNT and 7.5 phr CCB was used which might be attributed to the improved electron tunneling when the CNT encapsulates are bridged by CCB aggregates (Figure 4.7F).8 The CNT/CCB combination significantly reduced the inter-particle gaps from ~6 to ~2 nm.8 Bound rubber was rapidly increased (by up to a factor of 3)

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Figure 4.9. Experimental and theoretical results of electrical conductivity of the composite films. [Adapted, by permission, from Yu, J; Cha, JE; Kim, SY, Composites Part B: Eng., 110, 171-7, 2017.]

when a mixture of fillers was used (as compared with carbon black alone).8 The increase in bound rubber might be related Figure 4.8. Electrical conductivities of the NR vulcanito the improved dispersion of CCB, which zates filled with CCB (A), CNT (B), or CNT/CCB hybrid filler (C), as functions of frequency. [Adapted, bridged the CNT network in the NR by permission, from Nakaramontri, Y; Pichaiyut, S; matrix.8 Figure 4.8 shows that use of a comWisunthorn, S; Nakason, C, Eur. Polym. J., 90, 467-84, bination of fillers had a dramatic effect on 2017.] the increase of electrical conductivity of composite, especially when right proportion between both fillers was selected.8 CCB conductivity increased by 9 orders of magnitude when combination of fillers was used and by at least two orders of magnitude with CTN.8 At the same time, the bound rubber was increased and the rubber layer thickness was substantially decreased (6.19 nm for 5 phr CTN, 7.68 nm for 7.5 phr CCB, and 2.52 for their combination).8 A solvent-free melting process was used for fabrication of conductive polymer composite films filled with well-dispersed graphene nanoplatelet fillers, GPN.9 The excellent dispersion of GNP fillers was confirmed by comparing the experimentally determined electrical and thermal conductivity values of the composite films with the theoretical calculations obtained using a Mori-Tanaka method (Figure 4.9) which were in a good agreement.9 The composite films exhibited an electrical conductivity of the order of 101 S/m and the in-plane thermal conductivity of 7.1 W/m·K with 20 wt% GNP fillers.9 An unsupported glove has two layers and the outer layer has electrostatic discharge properties.10 A combination of fillers is used to reduce the surface resistance and volume resistance of glove which helps to dissipate static charge building up resulting from the triboelectric action.10 The elastomeric latex consists of nitrile latex which is a copolymer of butadiene and acrylonitrile.10 The conductive filler is used in a quantity of 2.1 to 7.4 wt% of the standard mixture of nitrile rubber. Typical carbon and carbonaceous materials which can be used alone or in combination are carbon composites and graphite.10 A deformable elastomeric conductor is designed to transmit electrical signals.11 It comprises an elastomeric polymer matrix and conductive filler material uniformly dispersed to render the electrically conductive material.11 The particles should have an aspect

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ratio sufficiently large to enable the particles to substantially remain in contact and/or in close proximity with adjacent particles so as to maintain conductive pathways in the material when the material is subjected to the deformations of 10% or more strain.11 Over a transmission distance of an electrical signal through the conductor, the transmission should not suffer greater than about 3 dB of signal attenuation when subjected to the deformation.11 Nickel coated carbon fiber can be used in the application to a large number of suitable polymers.11 The 3D printing composite polymer material contains a combination of carbon nanotubes with other nanofiller to produce uniform and smooth surface.12 In addition to carbon nanotubes, graphene nanoplatelets and graphite powder are suggested.12 A rubber compound for the manufacture of electrostatic dissipative products comprises a rubber component derived from poly(butadiene-co-acrylonitrile) and an electrically conductive filler component derived from sulfonic acid doped polyaniline.13 A flexible conductive ink composition comprises a resin binder (phenoxy resin), silver-plated core conductive particles (copper), and conductive particles having a surface area at least 1.0 m2/g.14 3D diamond printing is performed using a pre-ceramic polymer with a nanoparticle filler.15 The pre-ceramic polymer is poly(hydridocarbyne) or polymethylsilyne.15 The solvent comprises acetone, tetrahydrofuran, toluene, or acetonitrile.15 The pre-ceramic powder is a nanodiamond having a particle size of 30 nm or less.15

4.2 INSULATING Silica coated graphite flake is a thermally conductive and electrically insulating filler for epoxy resin composites which can be fabricated with surfactants and acid-base catalysis by a flexible sol-gel method.16 The volume resistivity of the silica coated graphite increased from 184 Ωm for the original graphite to 6391 Ωm because the silica coating provides an electrically insulating property.16 The silica coated graphite/epoxy resin composite had a high volume resistivity of 3.11x109 Ωm and thermal conductivity of 3.08 W/mK at 50 wt% filler loading.16 The light-emitting diodes, highpower-density communication devices, and energy-storage systems take advantage of such fillers.16 Expanded graphite, EG, was coated with an insulating ceramic shell (SiO2 or boron nitride (BN)/SiO2) using a simple sol-gel process.17 The epoxy composites were prepared with these hybrid fillers.17 The surface resistivity of the EG-BN-epoxy Figure 4.10. Surface resistivity of (a) EG-BN-epoxy composite decreased rapidly to 100 Ω/sq composite, (b) SiO2/EG-BN-epoxy composite and (c) BN/SiO2/EG-BN-epoxy composite. [Adapted, by per- with filler content greater than 5 wt% (Figmission, from Mun, SY; Lim, HM; Lee, S-H, Mater. ure 4.10).17 The surface resistivity of

Res. Bull., 97, 19-23, 2018.]

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Figure 4.11. Heat transfer and electrical conductivity mechanism of (a) CF and (b) CF-MgO fillers in polymer composite. [Adapted, by permission, from Zhang, J; Du, Z; Zou, W; Li, H; Zhang, C, Compos. Sci. Technol., 148, 1-8, 2017.]

1013 Ω/sq was recorded for epoxy composites with SiO2/EG or BN/ SiO2/EG content greater than 5 wt%.17 Although coating of SiO2 has low thermal conductivity, the thermal conductivity of epoxy composites with low SiO2/EG or BN/SiO2/EG filler content was greater than that of the BN-epoxy composite due to the improved adhesion between SiO2 and polymer resin interface.17 A magnesium oxide (MgO) nanoparticles-decorated carbon fiber hybrid (CF-MgO) were used in polyamide-6 to develop comFigure 4.12. SEM images of (a) pristine CF, (b) CF-COOH, (c) CF- posite with thermal conductivity KH550, and (d) CF-MgO. [Adapted, by permission, from Zhang, J; and electrical insulation.18 The Du, Z; Zou, W; Li, H; Zhang, C, Compos. Sci. Technol., 148, 1-8, carbon fibers and MgO nanoparti2017.] cles were first treated with coupling agents having amine and epoxy groups, respectively.18 Then CF-MgO was produced by grafting the nanoparticles onto the surface of the fiber.18 The strong interfacial interac-

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Figure 4.13. Morphology evolution of fillers during the extrusion and the construction process of high efficiency thermally conductive and electrically insulating pathways. [Adapted, by permission, from Zhang, X; Jiajia Zhang, J; Zhang, X; Li, C; Wang, J; Li, H; Xia, L; Wu, H; Guo, S, Compos. Sci. Technol., 150, 217-26, 2017.]

tion between the filler and the matrix was attributed to the presence of the coupling agents and the rough surface of nanoparticles-decorated CF.18 Figure 4.11 illustrates principles of thermal and electrical conductivity.18 Figure 4.12 shows morphological features of pristine fibers, with coupling agents, and decorated with MgO. Figure 4.12d shows the surface of fiber insulated by electrically non-conductive particles of MgO.18 The heat dissipation of the thermally conductive and electrically insulating composites can be enhanced by constructing high efficiency thermally conductive pathways.19 Graphite having excellent thermal conductivity was highly oriented and uniformly dispersed in polymer matrix to keep the electrical resistivity of the composite at a high level, even when the content of graphite was 33 wt%.19 Silicon carbide particles were added into the composites as junctions to release the thermally conductive potential of graphite via formation of phonon transport channels.19 The uniformly dispersed structure was constructed to increase the percolation threshold of fillers by a consecutive and powerful shear flow fields.19 There was a series of laminating multiplying elements employed which was named as the multistage stretching extrusion.19 During the extrusion process, in each of the laminating multiplying elements, polymer melts were sliced into two left and right parts by a divider, and then they flew through two up and down, thinner and wider channels, respectively (Figure 4.13).19 Then, they vertically recombined.19 Because the graphite platelets were uniformly dispersed and oriented in parallel there was no connection between platelets.19 Addition of silicon carbide improved contact for better thermal conductivity but did not affect electrical insulation characteristics of the material.19

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Figure 4.14. Schematic illustration of the development of 1-D and 2-D fillers in polymers induced by various rotation sources: step 1) Orientation parallel to the longitudinal direction, step 2) End-to-end attachment of fillers (linear structure), step 3) Denser localization of fillers to form a linear structure, and step 4) A longer route for thermal conduction. [Adapted, by permission, from Cho, H-B; Nakayama, T; Suematsu, H; Suzuki, T; Jiang, W; Niihara, K; Song, E; Eom, NSA; Kim, K; Choa, Y-H, Compos. Sci. Technol., 129, 205-13, 2016.]

The thermally conductive and electrically insulative polyphenylene sulfide composite contains boron nitride and multiwalled carbon nanotubes.20 The hexagonal boron nitride coating of multiwalled carbon nanotubes reduced electrical conductivity but improved thermal conductivity of the composite.20 The addition of 1 wt% of boron nitride coated carbon nanotubes reduced electrical conductivity by 7 orders of magnitude.20 Electrically insulating polymeric nanocomposites with high thermal conductivity are applicable as thermal-management materials in high-power-density electronics and optoelectronics.21 Such polymer nanocomposites can be obtained using up to 15 vol% hexagonal boron nitride nanosheets.21 The combination of electric-field switching and the application of fillers with various aspect ratios enables the rearrangement of the BN nanofillers into linear densely packed BN structures.21 The flexible nanocomposite films exhibit electrical resistivity greater than 1.50×10-6 MΩcm and a thermal conductivity of 1.56 W/mK.21 Reorienting and relocating filler particles such that end-to-end attachment occurs in the polymer matrix is an effective means of enhancing the thermal conductivity using a relatively small amount of thermally conducting filler.21 Figure 4.14 shows a fourstep process of product formation.21 Orienting thermally conducting filler particles parallel to the direction of the heat flux can effectively increase the thermal conductivity of a composite (step 1).21 The linear structure produced by the end-to-end attachment of filler particles constitutes an easier heat-conduction route through the filler particles, avoiding the route through the polymer (step 2).21 This structure can be further developed into a

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more dense arrangement of the filler particles (step 3) through the application of ON-OFF cycles of an applied electric field.21 The linear densely packed BN structures anchored to the composite surfaces were fabricated when the coordinated effects of polarization, dipole-dipole moments, electrophoresis, Coulombic attraction, and the structural and thickness variations of the assembly were controlled by varying the applied electric field and the BN content.21 Anisotropic insulating materials were obtained by formation of multilayered structures containing from 2 to 32 layers.22 The thermally conductive and electrically insulating composites had a significant anisotropic electrical resistivity; for example, the in-plane electrical resistivities (parallel to the layer direction) were below 117 Ωcm, while the through-plane electrical resistivities were over 5×1013 Ωcm.22 The exfoliated graphite nanoplatelets enhanced paraffin’s thermal conductivity. The electrical resistivity of paraffin was reduced from above 1012 Ω-cm to below 10 Ω-cm due to the high electrical conductivity of the added carbon filler.23 When exfoliated graphite nanoplatelets were modified by an acid treatment, which strongly oxidized the surface, the electrical resistivity of the phase change material was raised by up to 11 orders of magnitude.23 Graphene can significantly improve the thermal conductivity of polymers because of excellent thermal conductance, but it also causes serious reduction in electrical insulation and thus limits the applications of its polymer composites in the thermal management of electronics and systems.24 To solve this problem, electrically insulating Al2O3 is used to decorate high quality (defect-free) graphene nanoplatelets.24 Numerous Al2O3 nanoparticles can be formed on the inert GNP surfaces by a fast nucleation and hydrolysis of Al(NO3)3 precursor followed by calcination at 600°C.24 Chemical exfoliation of boron nitride nanosheets was carried out by microfluidization, followed by functionalization with vinyl-trimethoxy silane to make them compatible with polymers.25 The filler obtained was used in manufacturing thermally conductive and electrically insulating polymer nanocomposites.25 An electrically insulating sheet contains a sheet-like filler layer and an aromatic resin.26 An aromatic polysulfide resin was the aromatic resin having a sulfide bond in a molecule and inorganic filler was magnesium hydroxide which was treated with N-2(aminoethyl)-3-amino propyl methyl dimethoxy silane.26

4.3 INSULATING/CONDUCTIVE MIXTURES Carbon/polymer based flexible electrothermal films were produced with pyrolysis of carbon black N991 and multiwalled carbon nanotubes in polyethyleneoxide matrix with addition of non-conductive SiO2 filler.27 The 5 wt% SiO2 gives a stable electrical resistivity and excellent electrothermal reproducibility.27 The technology was used for fabrication of a flexible electrothermal film with a reversible thermochromic temperature-indicating function (changes color after reaching certain temperature).27 The conductivity of an immiscible polymer blend system (microfibrillar conductive poly(ethylene terephthalate)/polyethylene) containing carbon black was changed by the addition of insulating CaCO3 nanoparticles.28 The insulating CaCO3 nanoparticles substitute some of the conductive CB particles and obstruct the electron pathways, resulting in

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the changes in resistivity of the composites.28 The changes can be tailored depending on the insulating filler content.28

4.4 DIELECTRIC Materials which have the relative permittivity below that of silicon dioxide (3.9) are considered as low dielectric constant materials.29 The materials which have relative permittivity larger than silicon nitride are considered to be materials having high dielectric properties. Air has the lowest dielectric constant (1) and relaxor ferroelectric the highest (24,700).29 In the context of fillers, we will discuss below only materials with high relative permittivity. The Ni0.5Ti0.5NbO4 filler was modified using γ-aminopropyl triethoxysilane for use in cyanate ester resin composite designed for ultra-low-loss printed circuit board applications.30 The relative permittivity of filler was 56.8 and the relative permittivity of the composite could be regulated in the range of 4.48-14.8 by a volume fraction of filler treated with silane in the range of 0.1-0.55, respectively.30 Miniaturization of the smart electronics requires materials with improved volume efficiency and energy storage density.31 High electrical breakdown strength and dielectric constant are two key parameters to achieve high energy storage density.31 Barium titanate was used as a filler and thermoplastic polyurethane as a matrix of highly flexible composite with a high relative permittivity.31 With 30 vol% filler, the dielectric constant of the composite was 31.31 The TPU based composites can be used as an alternative to PVDF based composites for energy storage applications.31 A combination of a high relative permittivity of an inorganic filler and a high breakdown strength of the polymeric material increases the energy density of organic dielectric capacitor films.32 The inorganic fillers with higher relative permittivity include BaTiO3, BaxSryTiO3, multiwalled carbon nanotubes, graphene, or SiO2 which can be combined with PVDF which also has a relatively high relative permittivity.32 A combination of BaTiO3 and MWCNT in PVDF gave the highest relative permittivity.32 The TiO2 nanowires modified with BaTiO3 were used in the gradient dielectric composite consisting of the poly(vinylidene fluoride-co-hexafluopropylene) matrix.33 The permittivity of the com-

Figure 4.15. SEM image of the TiO2 nanowires modified BT particles: (a) Agglomerated particles. (b) A single particle with part of BT particle surface exposed outside. [Adapted, by permission, from Huang, Q; Luo, H; Chen, C; Zhou, X; Zhou, K; Zhang, D, J. Alloys Compounds, 696, 1220-7, 2017.]

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Figure 4.17. Relative permittivity of epoxy thin film composites with variable amount of MWCNTs. [Adapted, by permission, from Poh, CL; Mariatti, M; Noor, AFM; Sidek, O; Chuah, TP; Chow, SC, Composites Part B: Eng., 85, 50-8, 2016.]

posites increased as a result of large interfacial polarization induced by the large specific surface area of the nanowires as compared to the composite with randomly mixed TiO2/BaTiO3 fillers (Figure 4.15).33 Dielectric permittivity of composite containing 5-20 vol% filler at frequency of 100 Hz was in the range of 10-15.33 The dielectric properties of the MWCNT-filled epoxy composites with filler loading in the range from 0 to The 1.5 vol% were investigated.34 MWCNT filler was treated using polyoxyethylene octyl phenyl ether (Triton X-100), sodium dodecyl sulfate (SDS), and 3-aminopropyltriethoxysilane (AMPTES).34 Figure 4.16 shows the morphology of treated and untreated multiwalled carbon nanotubes.34 The average wall thickness of Figure 4.16. TEM images of (a) and (b) untreated MWCNT, (c) and (d) Triton X-100 treated MWCNT, untreated MWCNT was 5.4 nm whereas (e) and (f) SDS treated MWCNT, (g) and (h) Triton X- those Triton X-100, SDS and Triton X-100 100 with AMPTES treated MWCNT (magnification of 38000x for (a), (c), (e) and (g) and 450000x for (b), (d), with AMPTES treated MWCNT were 8.75, (f), (h)). [Adapted, by permission, from Poh, CL; 9.2, and 10.9 nm, respectively.34 This was Mariatti, M; Noor, AFM; Sidek, O; Chuah, TP; because the surfactant and silane coupling Chow, SC, Composites Part B: Eng., 85, 50-8, 2016.] agent attached to the surface of MWCNT and made the MWCNT surface thicker.34 Figure 4.17 shows that the larger the amount of added filler the higher the relative permittivity of the composite.34 Not presented here graph shows that surface treatment increased relative permittivity by about 30-50 times depending on treatment and frequency.34 A better dispersion of MWCNTs creates more minicapacitors and improves the relative permittivity of the composites.34 An interesting account of the effect of filler concentration on the elasticity of polymer and relative permittivity is illustrated in Figure 4.18.35 At low filler loading, nanoin-

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clusions are apart from each other and their mutual interactions are weak, therefore the particle-polymer interaction dominates.35 The dipolar group orientation is restricted which leads to a decreased permittivity.35 On the other hand, the strong adhesion between nanoinclusions and macromolecules increases glass transition temperature.35 At intermediate filler loading, the interparticle distance decreases and nanoinclusions interact with each other reducing the number of particles which are tightly bound to macroWhen polymer molecules.35 chains regain their flexibility, the Figure 4.18. Representation of the three filler load zone model. [Adapted, by permission, from Vryonis, O; Anastassopoulos, DL; permittivity raises, and Tg Vradis, AA; Psarras, GC, Polymer, 95, 82-90, 2016.] decreases.35 At higher filler content, the number of nanoparticles interacting with polymer chains increases causing spatial obstructions in segmental and polar side groups mobility which increases Tg values.35 The permittivity of nanocomposites increases further, as a consequence of the high permittivity value of the BaSrTiO3 nanoparticles.35 Figure 4.19 shows the results of studies of the effect of core-shell polyaniline/carbon nanotubes on the electrical properties of composites with polyurethane developed for the thin-film capacitor.36 A thin film of PANI is wrapped around the surface of the carbon

Figure 4.19. Chemical structure, morphology, and the effect of PANI/CNT fillers on electrical properties of its composites with polyurethane. [Adapted, by permission, from Xu, W; Ding, Y; Yu, Y; Jiang, S; Chen. L; Hou, H, Mater. Lett., 192, 25-8, 2017.]

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Figure 4.20. Dielectric properties of pure PU and PU composites. [Adapted, by permission, from Chen, T; Pan, L; Lin, M; Wang, B; Liu, L; Li, Y; Qiu, J; Zhu, K, Polym. Testing, 47, 4-11, 2015.]

nanotube.36 Without this coating, it is difficult to disperse nanotubes because they have very strong van der Waals interactions and become easily agglomerated and PANI weakens intermolecular forces.36 Coating and increased concentration of filler improved dielectric properties of composite.36 Composite containing 10 wt% filler had the highest relative permittivity (171.4 @ 100 Hz).36 Figure 4.20 shows the effect of carbon nanotubes, CNT, graphene nanosheets, GRN, and their mixtures on dielectric constants of polyurethane composites.37 It is evident that

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Figure 4.21. (a) XRD patterns of Bi2S3 and Bi2S3@SiO2 nanorods with the insert showing the morphology of Bi2S3 nanorods. TEM and HRTEM images of Bi2S3@SiO2 nanorods with changing shell thickness: (b) 4-5 nm (S-1), (c) 7-8 nm (S-2) and (d) 10-11 nm (S-3). (e) EDS analysis on the elements content of O, Si, S, Bi in Bi2S3@SiO2 nanorods with various shell thicknesses. [Adapted, by permission, from He, D; Wang, Y; Song, S; Liu, S; Luo, Y; Deng, Y, Compos. Sci. Technol., 151, 25-33, 2017.]

both fillers form synergic mixtures giving the highest increase of dielectric constant when the ratio of both fillers is 1:1 and concentration is 2.5%.37 The construction of core-shell architectures has been proven to be a powerful strategy to dramatically enhance the dielectric and energy storage performances.38 The core-

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shell structured amorphous SiO2 encapsulating Bi2S3 nanorods with varying shell thickness were employed as the fillers in poly(vinylidene fluoride).38 The core-shell architecture is instrumental in suppressing dielectric loss yet maintaining the high dielectric constant of the polymer-based composites, providing a promising route to achieve highperformance dielectrics.38 XRD patterns (Figure 4.21a) show that all reflections can be indexed to a pure orthorhombic phase of Bi2S3, and SiO2 coating does not bring in a new crystalline phase, indicating its amorphous state.38 Depending on the conditions of process, different thicknesses of coating are obtained.38 The ceramic filled fluoropolymer compositions are designed for electrical circuits with plural conductive pathways.39 The ceramic filler was selected from a group consisting of calcium strontium nanotitanate and calcium lanthanum titanate.39 The dielectric constant of the filler was in the range of 112-158.39

4.5 MAGNETIC Multiferroic materials show two (or all three) “ferroic” properties (ferroelectricity, ferromagnetism, and ferroelasticity).40 The magnetoelectric response of a material is described by the magnetoelectric voltage coefficient (αME) given by the following equation:40 δP α ME = ------δH

[4.1]

where: P H

electrical polarization magnetic field

The most interesting magnetoelectric composites are constructed from a piezoelectric polymer and magnetostrictive particles.40 A combination of piezoelectric poly(vinylidene fluoride) and magnetostrictive CoFe2O4 particles is one example.40 The magnetoelectric response of the microspheres was influenced by the magnetic field intensity, sphere diameter, and the content of microspheres.40 The highest magnetoelectric response was achieved when composite contained 90 wt% microspheres having average size of 1.2 μm.40 Ferrites have strong magnetocrystalline anisotropy and high values of magnetic characteristics, such as coercivity and remanent magnetic induction.41 They also have a good chemical stability (a high resistance to solvents, salts, lubricants, alkalis, harmful gases, soft acids and atmospheric effects).41 The incorporation of magnetic polycrystalline ferrite into rubber matrix permits production of rubber magnets.41 The rubber magnetic composites were prepared using strontium ferrite, natural rubber, and acrylonitrile-butadiene rubber using crosslinking by sulfur/peroxide curing.41 The magnetic properties of composite were not influenced by the curing system composition.41 The lithium ferrite shows better shielding properties in acrylonitrile-butadiene rubber matrix than manganese-zinc ferrite but all composites containing at least 200 phr ferrite had sufficient shielding properties of electromagnetic radiation.42 The unmodified and PVAl surface-modified strontium ferrite were incorporated into rubber matrices based on NBR and NBR/PVC in concentrations up to 100 phr.43 The modification did not have influence on coercivity of composites.43

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The magnetorheological fluid contains hyperbranched polycarbosilane as a carrier medium and micron-sized carbonyl iron particles as a filler.44 The fluid containing up to 72 wt% filler had Newtonian properties at zero magnetic field but a strong magnetorheological response was noted in a magnetic field, causing five orders of magnitude increase in viscosity (Figure 4.22).44 Magnetic elastomers contained a silicone matrix and magnetically hard NdFeB particles.45 The magnetic particles can Figure 4.22. Viscosity vs. shear rate of magnetorheologrotate in a soft polymer matrix under ical fluids (MR-500, MR-260, MR-120, PCS-Bu contain 45 83, 72, 55, 0 wt% of carbonyl iron, respectively) in the applied magnetic field. Long relaxation absence of magnetic field (filled symbols) and at the times result from the restructuring of the maximum magnetic field (empty symbols). [Adapted, magnetic filler under the influence of an by permission, from Vasiliev, VG; Sheremetyeva, NA; applied mechanical force and magnetic Buzin, MI; Turenko, DV; Papkov, VS; Klepikov, IA; Razumovskaya, IV; Muzafarov, AM; Kramarenko, EYu; interactions.45 Due to the particle rotation, Smart Mater. Struct., 25, 055016, 2016.] the loss factor increased abruptly when the magnetic field was turned on in the opposite direction to the sample magnetization, further decreasing with time.45 Anisotropic magnetorheological elastomers containing iron powder were prepared under an external magnetic field.46 Columns formed at a mass content of ~5% iron powder, tubular structures at ~14% and a densely packed structure when the iron mass content was in the range of 23-33%.46 Poly(2,6-dimethyl-1,4-phenylene oxide), ethylcellulose, and magnetic neodymium powder were used in a magnetic membranes.47 The addition of magnetic powder enhanced gas diffusivity in the membranes.47 Soft actuators can be designed from ferrogels by combining the elastic behavior of a polymer matrix with the magnetic properties of a magnetic filler.48 The length and aspect ratio of filler and distance from the magnetic field source can be used to tune the performance of ferrogel.48 Ferrogels can be used in artificial tissues, drug carriers, and cancer therapeutics, active clothing, robotics, damping components, vibration/shock absorbers, and stiffness tunable mounts.48 The magnetic polymer composites can be used as radio-absorbing materials for protection of people against the action of electromagnetic fields.49 Via a selection of the mixture of components, it is possible to substantially lower the reflection coefficient and extend the frequency regions.48 Polyurethane matrix was filled with MnZn ferrite and various types of conductive fillers (carbon black, carbon fibers, aluminum powder, and polypyrrole).50 The ferrite particles were surrounded by a conductive medium forming core-shell structure.50 The concentration of the conductive filler is important because the enhancement effect is observed only at a certain concentration of the filler, which is close to the electrical percolation

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Figure 4.23. Aberration corrected scanning transmission electron microscopy images of the FeCo/CG after heating up to high temperature. (a) Low magnification high-angle annular dark field micrograph. Inset: size distribution histogram. (b) Atomic resolution image and (c) Bright filed image showing several graphitic layers. [Adapted, by permission, from Castrillon, M; Mayoral, A; Urtizberea, A; Marquina, C; Irusta, S; Meier, JG; Santamaria, J, Nanotechnology, 24, 505702, 2013.]

threshold of the filler component in a matrix.50 The microwave magnetic field induces a current in the conductive shell and thus increases the magnetic losses.50 Microwave absorbing coatings were prepared by using nanosized nickel-zinc nanoferrite/polyaniline as a filler and epoxy polymer as a binder.51 Nanosized PANI filler is much more effective than nanosized ferrite fillers in electromagnetic wave absorption.51 FeCo-alloy graphite-coated nanoparticles with mean particle diameter under 8 nm were developed for application as a magnetic filler (Figure 4.23).52 Because of the protective effect of the graphite shell, FeCo nanoparticles are stable under oxygen atmosphere up to 200oC.52

Figure 4.24. Schematic diagram of the technique for graphene and few-layer-graphene functionalization with magnetic nanoparticles. The steps include: addition of poly-sodium-4-styrene-sulfonate, PSS, to graphene solution resulting in graphene surface coating with the “primer”; addition of polyelectrolyte poly-dimethyl-diallylammonium chloride, PDDA, which sticks to PSS “primer” via electrostatic interactions and provides distribution of positive charges on graphene fillers; addition of the solution of magnetic nanoparticles, which attach to PDDA layer via electrostatic interaction during the stirring; mixing of the magnetically functionalized graphene fillers with the matrix material resulting in the composite, which is ready for filler alignment with an external magnetic field. [Adapted, by permission, from Renteria, J; Legedza, S; Salgado, R; Balandin, MP; Ramirez, S; Saadah, M; Kargar, F; Balandin, AA, Mater. Design, 88, 214-21, 2015.]

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Functionalization of graphene with Fe3O4 nanoparticles permits alignment of the fillers in an external magnetic field during dispersion (Figure 4.24).53 The self-aligning “magnetic graphene” fillers improve heat conduction in composites. This may be used as a new method of thermal management in advanced electronics.53 Thermally conductive BN/ SiC binary filler and epoxy composite materials were fabricated via magnetic alignment.54 The magnetic iron oxide particles on Figure 4.25. SEM images of diatomite particles after electroplating the surface of the filler allowed at different vibration frequency: (a) 10 Hz, (b) 20 Hz, (c) 30 Hz, particle re-orientation under the and (d) 40 Hz. [Adapted, by permission, from Lan, M; Li, H; external magnetic field.54 AnisotHuang, W; Xu, G; Li, Y, J. Magnetism Magnetic Mater., 377, ropy was achieved and the verti243-51, 2015.] cally aligned BN composite had a high thermal conductivity and generated a vertical heat flow path.54 The SiC nanoparticles hindered BN-particle aggregation and led to the formation of a threedimensional heat conduction path essential for increased thermal conductivity.54 The amorphous flakes coated by NiZnCu ferrite nanoparticles with core-shell-like structure were fabricated using mechanical ballmilling.55 The core-shell-like structure greatly decreased permittivity and improved the absorption properties.55 Flaky-shaped carbonyl iron fillers were fabricated using ball56 Figure 4.26. TEM images of Fe3O4 powders, obtained at different milling, followed by annealing. precursor-to-solvent ratio: respectively 0.04 (a), 0.09 (b), 0.18 (c) The composites with a ball-mill and 0.27 (d) mol/mol. [Adapted, by permission, from Barrera, G; time of 48–96 h had the optimum Sciancalepore, C; Messori, M; Allia, P; Tiberto, P; Bondioli, F, high-frequency properties.56 Eur. Polym. J., 94, 354-65, 2017.] The flake-shaped diatomite particles were electroplated Ni-Fe alloy.57 The core-shell flake-shaped diatomite particles with a high content of Ni-Fe alloy and good surface qualities of the coating can be

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Figure 4.27. A schematic diagram of continuous process of fiber catalysis and metal deposition by electroless plating. [Adapted, by permission, from Lee, J; Jung, BM; Lee, SB; Lee, SK; Kim, KH, Appl. Surf. Sci., 415, 99-103, 2017.]

obtained by adjusting cathode vibration frequency (Figure 4.25).57 When the vibration frequency was increased, the micropores on the surface of diatomite became clearly visible, and the quality of the surface coating was significantly improved.57 Magnetite nanoparticles were prepared by a non-hydrolytic sol-gel process in the presence of iron(III)-acetylacetonate as precursor and 2-ethyl-1,3-hexanediol as a reactive solvent.58 The nanoparticle size was affected by the precursor-to-solvent ratio (Figure 4.26).58 The magnetite nanoparticles formed aggregates whose size and shape depended on concentration.58 FeCoNi coated glass fibers were used in PC composite sheets for electromagnetic absorption and shielding.59 Absorption power increased by 95% and inter-decoupling of this composite showed maximum 30 dB at around 5.3 GHz, which is comparable to that of a conductive Cu foil.59 Figure 4.27 shows production line for continuous coating of glass fiber.59 A series of silicone-rubber elastomers were mixed with a ferrofluid and polymerized with and without external magnetic field.60 Pure rubber and the magnetic elastomers synthesized without field had a substantial number of large polymer coils (blobs) which were vertically prolate.60 In the magnetic elastomers polymerized under the magnetic field, the blobs were preferably elongated in the direction normal to the field.60 A soft magnetic composite was formed from atomized ferromagnetic particles.61 The particles of a predetermined size range were formed and were coated by at least one layer of electrically insulating nano-sized inorganic filler (e.g., halloysite, FePO4, SiO2, kaolin) to form insulated ferromagnetic powder.61

4.6 MAGNETODIELECTRIC The magnetodielectric materials are composite materials made out of magnetic particles and dielectric materials that serve as electrical insulators of magnetic particles.62 Magnetic properties of magnetodielectric materials depend on constituent particle properties, their shape and volume.62 Mechanical and thermal characteristics depend mainly on the ratio of magnetic material, and dielectric material.62 The magnetodielectric (or magnetocapacitance) effect refers to a phenomenon where dielectric constant is controlled by magnetic field.63 The effect is observed in magnetoelectric materials since the dielectric constant is intrinsically related to the electric polarization and indirectly related to the magnetic order.63

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A strong magnetodielectric effect of up to 150% increase in permittivity in the magnetic field of 10 kOe was obtained for magnetorheological elastomers based on silicone rubber and magnetic fillers of various chemical nature (Fe, NdFeB and Fe3O4) and particle sizes.64 The Fe and Fe3O4 fillers are magnetically soft, while the NdFeB particles have a large remanent magnetization.64 The size and concentration of the magnetic particles and a conductivity of magnetic materials had a significant influence on the magnetodielectric effect.64 NdFeB gave the higher magnetic response out of tested fillers.64 The magnetodielectric composites with attenuation as high as 33 dB and wider bandwidth of operation have been developed.65 The composition of fillers used included CoFe2O4 (needle-like particles), CaTiO3 (hollow cuboids) and commercial carbonyl iron powder.65 Functionalized iron particles were used as fillers for magnetodielectrics.66 Surfactant-assisted ball milling was employed for preparation of metal powders for magnetodielectric composites.66 The iron powder was mechanically milled in various organic media (paraffin, polystyrene, polyethylene) with and without surfactant additives (perfluorononanoic acid and stearic acid).66 A compromise between the permeability and magnetic loss tangent can be found by variation of milling conditions (milling without surfactants), which leads to the development of a magnetodielectric material meeting the requirements for the magnetic antennas.66 Fe particles for magnetodielectrics were obtained by co-milling of Fe and paraffin.67 The superparamagnetic magneto-dielectric polymer nanocomposites were synthesized for 3D electromagnetic structures/devices manufactured by 3D printing, compression molding or injection molding.68 A single-domain magnetic Fe3O4 CoFe2O4 nanoparticles having a mean size of 8±1 nm coated with a surfactant and uniformly dispersed in the solvent-free low-loss polymer (butadiene resin and a copolymer) at a concentration between 30 and 80 wt% formed a superparamagnetic polymer nanocomposite material.68

4.7 EMI SHIELDING Electronic equipment should be designed in such a way that it can minimize the amount of emitted electromagnetic radiation (interference). It should also tolerate electromagnetic interference from the environment due to adequate electromagnetic shielding. Faraday’s cage known in the first half of 19th century is ideal technical solution but not practical in common applications. These are the reasons for a wide-spread interest in EMI shielding composites. Below, the applications of modern fillers in this field are discussed and suggested. Various forms of SiC materials, including SiC powders, fibers, foams, and matrix composites, have gained attention in the development of EMI shielding materials due to their outstanding physical and chemical properties.69 SiC fiber bundles were used as reinforcement in SiCf/SiC composites which were fabricated by the combined chemical vapor infiltration and precursor infiltration pyrolysis.69 In these composites, the pyrocarbon interphase improves interaction between fibers and matrix, and the addition of Al2O3 filler compensates for matrix shrinkage and makes composite stronger.69 The composite has high absorption and reflection shielding effectiveness.69

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Figure 4.28. Atomic force micrographs of Ketjen carbon black-ethylene methyl acrylate copolymer composites (a) 5 wt%, (b) 10 wt%, and (c) 20 wt% K-CB loading, respectively. [Adapted, by permission, from Mondal, S; Ganguly, S; Das, P; Khastgir, D; Das, NC, Compos. Part B: Eng., 119, 41-56, 2017.]

Ketjen carbon black-ethylene methyl acrylate copolymer composites have low percolation threshold and efficient EMI shielding efficiency dominated by a conductive dissipation.70 These composites have a low percolation threshold (8.6 wt%) and high EMI shielding value of 33.9 dB with 20 wt% Ketjen carbon black, K-CB. Below percolation threshold point at very low filler content (5 wt%), its particles remain isolated (Figure 4.28a).70 With 10 wt% loading, partial agglomeration of K-CB particles forms conductive network (Figure 4.28b).70 Above the percolation threshold point (20 wt%), the K-CB inclusions are very close to each other generating continuous conductive network (Figure 4.28c).70 Figure 4.29 explains the reasons for a good performance of Ketjen black composite.70 The high structure carbon black blended with polymer needs more incorporation energy but, once it is incorporated, a minimum energy is required for a good dispersion.70 A combination of multiwalled carbon nanotubes and manganese zinc ferrite was used in epoxide for EMI shielding composite.71 The composite with a filler ratio of MWNCTs to MnZn ferrite=3:1 gives the best EMI shielding performance dominated by absorption.71 Composite having a filler loading of 4 vol% and thickness of 2 mm achieved a shielding effectiveness of 44 dB at 10 GHz with the assistance of conductive silver backing.71

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Figure 4.29. Schematic representation of entanglement of the polymer chains on the surface of carbon black particles. [Adapted, by permission, from Mondal, S; Ganguly, S; Das, P; Khastgir, D; Das, NC, Compos. Part B: Eng., 119, 41-56, 2017.]

The segregated structure of poly(vinylidene fluoride)/multiwalled carbon nanotubes composites forms a group of highly efficient EMI shielding materials.72 In a segregated system, conductive fillers are present at the polymer particle interfaces rather than being dispersed over the whole matrix.72 This creates conductive pathways with a lesser amount of conductive filler.72 The superior conductive networks are needed to reach 20 dB with volume conductivity of 0.01 S/cm.72 Due the homogeneous dispersion structure, filler

Figure 4.30. Schematic diagram of the fabrication of the segregated PVDF/MWCNTs composites. [Adapted, by permission, from Wang, H; Zheng, K; Zhang, X; Du, T; Xiao, C; Ding, X; Bao, C; Chen, L; Tian, X, Compos. Part A: Appl.Sci. Manuf., 90, 606-13, 2016.]

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Figure 4.32. Schematic representation of EMI shielding mechanism in PVDF/graphite composite. [Adapted, by permission, from Joseph, N; Varghese, J; Sebastian, MT, Composites Part B: Eng., 123, 271-8, 2017.]

loadings much higher than the electrical percolation threshold are required to construct superior conductive interconnected networks.72 Previous attempts with such Figure 4.31. (a) SEM image of PVDF granules coated networks based on the homogeneous diswith MWCNTs particles; (b) TEM image of the MWCNTs layers between PVDF regions. [Adapted, by persion required 8 wt% graphene oxide to permission, from Wang, H; Zheng, K; Zhang, X; Du, T; reach 30 dB or 10-20 wt% carbon nanoXiao, C; Ding, X; Bao, C; Chen, L; Tian, X, Compos. tubes to reach 30 and 20 dB, respectively.72 Part A: Appl.Sci. Manuf., 90, 606-13, 2016.] The segregated PVDF/carbon nanotube system with 7 wt% filler content gave high electrical conductivity and high EMI shielding effectiveness, reaching 0.06 S/cm and 30.89 dB.72 Figure 4.30 shows the method of fabrication of the composite.72 The hot compaction occurs at 170oC. Figure 4.31 shows SEM images of PVDF granules coated with carbon nanofibers and TEM of layers of nanofiber which are deposited between PVDF granules.72 This effective method is simple, low-cost, and environmentally-friendly.72 The graphite/PVDF composite having sample thickness of 1 mm exhibited good shielding properties of 55–57 dB at the frequency range of 8.2–18 GHz at the highest filler loading.73 Absorption was the primary mechanism for EMI shielding (Figure 4.32).73 The two-dimensional structure of the filler enhanced the absorption shielding efficiency. The EMI shielding effect improved with an increase in sample thickness.73 Red mud, RM, is an insoluble industrial waste used for EMI shielding applications.74 Composites of PANI/RM have been prepared by in situ chemical oxidative polymerization.74 The composites had shielding effectiveness dominated by absorption of 33-41 dB (>99.99% attenuation) in 8.2-12.4 GHz frequency range at a thickness of 3 mm.74 Highly anisotropic Cu oblate ellipsoids incorporated into polystyrene revealed excellent broadband electromagnetic shielding effectiveness of 80.0 to 62.1 dB at a frequency region between 300.0 kHz and 12.0 GHz, at low Cu contents.75 Hollow polystyrene beads

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Figure 4.33. (a) Schematic illustration of fabrication process of polymer composites with highly anisotropic Cu oblate ellipsoids. Optical microscope images of: (b) expandable polymer beads, (c) expanded polymer beads, (d) Cu-coated EBs. Insets show highlighted SEM micrographs. SEM images of the cross-section view (e) and top view (f) of CuEB7/PS composite. Inset shows magnified cross-section image of highly deformed Cu oblate ellipsoid in the composite. [Adapted, by permission, from Lee, SH; Yu, S; Shahzad, F; Hong, JP; Kim, WN; Park, C; Hong, SM; Koo, CM, Compos. Sci. Technol., 144, 57-62, 2017.]

were Cu-coated using electroless plating.75 The hollow polymer beads (Expancel 461 DU 40) were formed by thermal expansion of acrylonitrile-based polymer beads containing a blowing agent.75 Figure 4.33 shows technological process and related morphological changes.75 Because of anisotropic structure, Cu particles had percolation threshold at 7 vol%.75 The multiwalled carbon nanotubes were synthesized over Fe catalyst at a broad range of temperatures from 550 to 950°C (at 100°C intervals) by chemical vapor deposition and compounded with poly(vinylidene fluoride), PVDF.76 The nanocomposites made with CNT synthesized at 650°C had significantly lower percolation threshold (around 0.4 wt%) and higher electromagnetic interference shielding effectiveness of 20.3 dB over the X-band for 3.5 wt% CNT and 1.1 mm thickness than those produced at any other temperature.76 Superior EMI shielding of CNT650°C was attributed to a combination of high carbon purity, aspect ratio, crystallinity, and moderate powder conductivity. Figure 4.34 shows that CNT650°C and CNT750°C are composed of entangled low-diameter CNTs over the catalyst particles.76 Further elevation of temperature (850 and 950°C) results in CNTs having much larger diameters.76 Graphene oxide (TRGO) was covalently modified by grafting 2-aminoethyl methacrylate on its surface and used for production nanocomposites with TPU by solution mixing.77 The modification enhanced interfacial interactions and mechanical damping. The resultant electromagnetic interference shielding effectiveness was 25 dB (99.7% attenuation).77 The dielectric damping and shielding via absorption mechanism were influenced by the viscoelastic energy dissipation.77 A graphene-based coating grown on an outer lens/window/dome of an electro-optical system to enhance the transmittance of light with wavelength in NIR and MWIR spectrum and to suppress EM radiation between 100 MHz to 20 GHz range.78 An EMI shielding contains graphene as the layer itself or as a filler in the metal-oxide matrix.78

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Figure 4.34. SEM micrographs of CNT powders synthesized at various temperatures. [Adapted, by permission, from Mirkhani, SA; Arjmand, M; Sadeghi, S; Krause, B; Pötschke, P; Sundararaj, U, Synthetic Metals, 230, 39-50, 2017.]

An EMI shielding composite film for use in printed circuit boards has at least two layers, a top layer is electrically conductive in all directions (isotropic), and a bottom layer is electrically conductive only in the Z (thickness) direction (anisotropic).79 The bottom layer is in contact with the grounding pads of the circuitry of the electronic device to be shielded.79 The conductive top layer functions similarly to the metallic boxes to prevent the electromagnetic radiation from both entering the boxes and escaping into the environment.79 The top layer conductive filler particles are selected from the group consisting of silver, nickel, copper, graphite, carbon nanotubes, and core/shell particles in the amount of at least 15 vol%.79 An electromagnetic interference shielding film has a conductive adhesive layer including a binder resin and a conductive filler.80 The binder resin comprises a reaction product obtained by a curing reaction between at least one selected from the group consisting of polyester and polyurethane and an epoxy group-containing curing agent.80 The conductive filler is either of a dendrite, flake, or spherically shaped metal particle.80

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4.8 MICROWAVE ABSORPTION The electromagnetic wave energy use grows at the rate by 7-14% per year, potentially causing serious damages to information security and human health.81 Microwave equipment is used in military and civil applications.81 The FeCrMoNiPBCSi amorphous alloy system has good soft-magnetic property and high glass forming ability.81 It was used for electromagnetic wave absorbing composite based on silicone matrix.81 It provides strong electromagnetic wave attenuation with reflection loss of 60.3 dB at 7.08 GHz for the composite containing 30 vol% filler (the bandwidth reaches 2.30 GHz for RL < 10 dB).81 Magnetic particle-filled elastomeric composites are used in microwave shielding applications.82 The microwave reflection properties of such composites was enhanced by a magnetic filler concentration gradient along the thickness in the iron powder-filled elastomeric composite.82 The variation in the loading along thickness results in a broad-spectrum absorption of the microwaves.82 With equal sample thickness and filler volume fraction, the functionally graded elastomeric composite exhibits 2.5 times higher microwave reflection over a broader frequency range than the uniformly dispersed elastomeric composite.82 The maximum microwave reflection (power absorption >95%) in a broadband frequency range (approximately 9.5 GHz) by a comparatively thin, flexible and conformal the functionally graded elastomeric composite sheet was attained.82 High-energy mechanical milling was used for fabrication of magnetic filler particles for metal-polymer composites applied in different microwave absorption devices.83 The milling was combined with wet mechanochemical synthesis resulting in a thin layer formed by perfluorononanoic acid molecules on iron powder.83 The molecules were partially defluorinated under mechanochemical synthesis facilitating linkage between polystyrene fragments and iron which improved chemical compatibility.83 High-performance microwave absorbing materials should meet the following requirements:84 • the impedance matching condition in order to reduce the reflection of EM wave on the surface • the incident EM wave should be attenuated rapidly by the absorber • the applied materials should exhibit strong dielectric loss. Excellent microwave absorbing materials should be composed of an electrically insulating matrix (e.g., the amorphous phase containing pores) and an electrically absorbing phase (e.g., turbostratic C and nanocrystalline SiC).84 Silicon carbide, as a wide band gap semiconductor, possesses good microwave absorption properties.84 The microwave performance of a composite is determined by a carrier concentration, crystallization structure, grain/particle size, and size distribution.84 The insulating polymer matrix (epoxy) was embedded with NiCuZn ferrite.85 The frequency response of the microwave absorption of ferrite composite was attributed to its macroscopic magnetic loss and dielectric loss correlated with domain-wall motion, spin resonance, and dipolar relaxation.85 The maximum return loss of single-layer absorber having the thickness of 7.2 mm was estimated to be -53.9 dB with an absorption bandwidth of 1.72 GHz.85 A nanocomposite with broadband absorption less than -30 dB was prepared using a combination of perovskite (SrTiO3) and magnetic inverse spinel structures such as magnetite, cobalt ferrite, manganese ferrite, and polyaniline.86 The optimal composition con-

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Figure 4.35. Schematic diagram of the microwave shielding mechanism of the composite films. [Adapted, by permission, from Kar, E; Bose, N; Dutta, B; Mukherjee, N; Mukherjee, S, Eur. Polym. J., 90, 442-55, 2017.]

tained MnFe2O4 as a magnetic component.86 The composite was a strong and contained a high-performance absorber to remove microwaves in the X band range.86 The poly(vinylidene fluoride)/submicron graphite platelet composite was used for microwave shielding.87 The 400 µm thin film can efficiently block 97.5% (at 8.2 GHz) of incident EM radiation.87 A five times higher dielectric constant was achieved for the composite film having a thickness of 300 µm with a low tangent loss.87 Figure 4.35 shows a schematic diagram of the microwave shielding mechanism of the composite films.87 The shielding by absorption (~ 68.75%) is the dominant mechanism for PVDF/graphite composite films rather than the shielding by reflection (~ 31.25%).87 FeNi3 nanoalloy decorated on the 3D architecture of reduced graphene oxide/molybdenum disulfide has excellent electromagnetic wave absorption properties for microwave absorption.88 The maximum absorption bandwidth was 4.72 GHz with the thickness of 2 mm and the corresponding reflection loss value is -30.39 dB when the filler loading of was 40%.88 The conductive carbon black/magnetite hybrid filler was used in microwave absorbing composites based on natural rubber.89 The composite comprising a hybrid filler whose carbon:magnetite phase ratio is 90:10 gives the best performance.89 The presence of Fe3O4 in the hybrid filler has a crucial influence on decreasing the reflection of microwaves.89 The introduction of silane coupling agent optimizes the structure and microwave dielectric properties of PTFE-based composites containing Ba(Mg1/3Nb2/3)O3.90 The silane coupling agent has no influence on the crystallinity of filler but improves its compatibility with the matrix polymer.90 The relative permittivity (εr) of vinyltrimethoxysilane-modified filler/PTFE composite was 5.84 while the loss tangent reached 1.5×10-3 at microwave frequencies (around 10 GHz).90

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High temperature electromagnetic and microwave absorbing properties of polyimide/ multiwalled carbon nanotubes nanocomposites have been studied.91 The complex permittivity of the nanocomposite increased with increasing the temperature because of shorter relaxation time and enhanced electrical conductivity at elevated temperature.91 When the content of absorbent was 5%, the -10 dB absorption bandwidth could reach 1.3 GHz, with the thickness of 2.1 mm, while the bandwidth below -10 dB Figure 4.36. Schematic illustration of carbon coated reduced grawas 2.04 GHz with the thickness phene oxide/magnetic CoFe2O4 hollow particles modified multiof 1.7 mm at 373 K.91 On further wall carbon nanotube composites. [Adapted, by permission, from Zhang, K; Gao, X; Zhang, Q; Li, T; Chen, H; Chen, X, J. Alloys temperature increase, the absorpCompounds, 723, 912-21, 2017.] tion becomes worse due to the poor impedance matching.91 The ferrite nanoparticles and ferrite/multiwall carbon nanotubes (Cu0.25Ni0.25Zn0.5Fe2O4/MWCNTs) nanocomposites were synthesized using co-precipitation and solution-mixing techniques.92 Carbon nanotubes were functionalized with acid to introduce carboxylic groups for effectively attaching ferrite nanoparticles and epoxy.92 The excellent microwave absorption with wide bandwidth was attributed to interfacial electric polarization, electromagnetic impedance matching, and multiple scattering network structure of ferrite/MWCNTs nanocomposites.92 The electromagnetic properties of Ti3C2 nanosheets were investigated in the frequency range of 12.4–18 GHz.93 Reflection loss values of the Ti3C2 nanosheets filled composites exceeded -11 dB (more than 92% absorption) in the frequency range of 12.4–18 GHz with a thickness of 1.4 mm.93 Lightweight 1D-2D magnetic-modified composites based on asphalt carbon coating reduced graphene oxide/magnetic hollow CoFe2O4 modified MWCNTs improved microwave absorption.94 The maximum reflection loss reached -46.8 dB with a thickness of 1.6 mm at 11.6 GHz at 20 wt% loading.94 Figure 4.36 shows method of production of lightweight filler.94 The microwire/carbon nanotube/rubber multiscale hybrid composite was produced to be used for microwave absorption.95 The hybrid composite had enhanced conductivity and permittivity and decreased intrinsic impedance, providing significant improvement of absorption with the increase of reflection loss.95 The counter-influence of wires and CNTs plays an important role in formulating the resulting electromagnetic responses.95 Silicone rubber composites filled with Ba(Zn1/3Ta2/3)O3 were prepared by hot pressing.96 The composites were flexible and stretchable.96 The coefficient of thermal expansion and specific heat capacity decreased whereas thermal conductivity, thermal

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diffusivity, and the moisture absorption increased with increase in filler loading.96 Fe/BaFeO2.5 composite particles were prepared by hydrogenthermal reduction of BaFe12O19 particles with the average diameter of Fe/BaFeO2.5 composite particles of 1 µm and the lamellar thickness of 100 nm.97 Due to the unique 2-dimension shape characteristic, the permittivity and permeability of Fe/BaFeO2.5 laminated composite particles Figure 4.37. (a) and (b) SEM images of β-MnO2 nanorods; (c) were higher than that of Fe/ TEM image of β-MnO2 nanorods. Inset shows a lattice resolved BaFeO2.5 composite particles HRTEM image and SAED image of a single nanorod; (d) FESEM without lamellar structure.97 The image of the fractured surface of β-MnO2/PVDF membrane. [Adapted, by permission, from Niu, Y; Li, X-P, Inorg. Chem. maximum reflection loss was Commun., 55, 25-9, 2015.] 29.94 dB for a coating having thickness of 1.36 mm.97 Carbon nanotube reinforced concrete composites were characterized for microwave bands currently employed in wireless telecommunication systems.98 The 3 cm of the concrete material enriched with 3 wt% of nanotube had shielding effectiveness values of up to 12 dB in the frequency range around 2.6 GHz.98 When the material thickness was increased to 10 cm, the elecFigure 4.38. Mechanism of shielding by silver-coated absorption was further carbonyl iron powders. [Adapted, by permission, from tromagnetic Cao, XG; Ren, H; Zhang, HY, J. Alloys Compounds, enhanced.98 The shielding effectiveness 631, 133-7, 2015.] increased to 80 dB at 2.6 GHz for a 30 cm thick material.98 The shielding effectiveness values of naked concrete walls are typically in the range 20-30 dB at such frequencies.98 Highly porous polydimethylsiloxane composites containing cellular-structured microscale graphene foams and conductive carbon nanotubes were fabricated.99 The three-dimensional, multi-scale hybrid composites with inherent percolation and a high porosity of 90.8% have a remarkable electromagnetic interference shielding effectiveness of ~75 dB, a 200% enhancement compared with 25 dB of the composites made from graphene alone with the same content and porosity.99 The β-manganese dioxide nanorods have been fabricated on a large scale by a simple hydrothermal process.100 The β-MnO2/PVDF nanocomposites exhibited enhanced wave absorption properties with the minimum reflection loss of the β-MnO2/PVDF nanocomposite reaching -30.1 dB (> 99.9% attenuation) at 8.16 GHz with a filler loading of 40 wt%.100 Figure 4.37 shows the morphological features of nanorods.100 The main micro-

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wave absorbing mechanism includes electronic polarization, orientation polarization, interface polarization and the synergic effect between β-MnO2 nanorods and PVDF.100 Electroless silver coated carbonyl iron powder was used in electroconductive adhesive.101 The reflection of silver coating and absorption of carbonyl iron powder dominate the shielding mechanism of the silver-coated carbonyl iron powder.101 The shielding effectiveness increased with the increase of the thickness of electroconductive adhesive.101 Figure 4.38 shows the mechanism of action of the filler.101 The SiO2 coated carbonyl iron was used as a filler for polyimide in heat-resistant microwave absorbing material.102 When the filler content was 60 wt%, the value of minimum reflection loss decreased from -25 dB to -33 dB with the thickness increased from 1.5 mm to 2.1 mm.102

4.9 PIEZORESISTIVE Piezoelectric composites with 0–3 connectivity are used for sensors, actuators, and transducers in a variety of fields, such as flexible soft composite materials for the generation and detection of underwater acoustic signals (sonar), medical diagnostic systems (e.g. micropumps in micro-/nano-fluidic devices), and tactile sensors (energy scavenger for aerospace/automotive/domestic devices or touch-base switches for the consumer market).103 The ferroelectric lead zirconate titanate (Pb(ZrxTi1-x)O3 is the most widely used ceramic for such composite materials after dispersion in polydimethysiloxane.103 This combination gives excellent piezoelectric properties, such as permittivity up to 40, piezoelectric charge constant up to 25 pC/N, piezoelectric voltage coefficient up to 75 mV m/N, and electrical conductivity of 106 S/m at 1000 Hz.103 A highly stretchable elastomeric nanocomposites were obtained by mixing conductive multiwall carbon nanotubes with poly(glycerol sebacate) prepolymer and curing it at 120oC.104 The percolation threshold for the electrical conductivity was ~1 wt% for MWCNTs.104 The piezoresistive behavior shows a negative gauge factor which was between -0.5 and -0.8 under uniaxial tensile stress.104 The composite is suitable for piezoresistive sensor in biomedical engineering applications and flexible and wearable devices.104 Addition of insulating filler with different sizes and surface characteristics was used in tunable piezoresistive conductive polymer composites.105 In principle, the morphology of conductive network and interfacial interactions with polymer matrix should affect the pressure/strain sensing behavior of conductive polymer composites.105 Therefore, the surface characteristics and size of the insulating fillers should foster this behavior because it affects both conductive network and interfacial interaction.105 The insulating silicon dioxide having different particle sizes and surface characteristics was incorporated into carbon black/silicone rubber composites to modify their piezoresistive behavior.105 A high-performance piezoresistive sensors technology can use this method to produce materials with reversible piezoresistivity for large pressure application (below 2500 kPa) and tunable piezoresistive sensitivity.105 A composite of natural rubber and multiwall carbon nanotubes was evaluated for application as a piezoresistive tensile sensor.106 The conductivity of the unstrained NR/ MWCNT nanocomposite was described by the percolation theory with the critical exponent of ~2.31.106 A nonlinear electrical response to tensile stress-induced strains was

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obtained.106 The mechanisms of the conductivity (e.g., constriction, tunnelling, and hopping) were dominant in the identified domains of strain.106 The developed quantitative understanding of the nonlinear resistance of the MWCNT/NR composite as a function of strain can be used in sensor applications.106 The nickel/silicone composites (co-filled with micron nickel and submicron nickel particles) with low filler content have high Figure 4.39. SEM images of fracture surfaces of Aerographite piezoresistive sensitivity.107 The epoxy composite, a) overview of a facetted fracture surface of a composite with 0.48 wt% Aerographite, b) close up of a single tet- composites with mass ratio of rapod with inner polymer filling and fringed rupture of graphite nickel/silicone rubber under 1.8:1 layers, c)/d) SEM images of fracture surfaces of Aerographite had a high pressure sensitivity.107 epoxy composite with 0.71 wt% Aerographite, c) side A; d) oppoThe conductivity changed 9 site side B. [Adapted, by permission, from Garlof, S; Fukuda, T; Mecklenburg, M; Smazna, D; Mishra, YK; Adelung, R; Schulte, K; orders of magnitude under a low Fiedler, B, Compos. Sci. Technol., 134, 226-33, 2016.] pressure of 0.096 MPa.107 The composites show good repeatability of piezoresistivity.107 The gauge factor of the composites was strain-controlled.107 Aerographite is a carbon aerogel consisting of three-dimensionally interconnected graphitic microtubes.108 An infiltration with epoxy leads to Aerographite/epoxy composites with filler contents in the range of 0.26–1.24 wt% having electrical conductivity in the range of 2–13.6 S/m (orders of magnitude larger than CNT-based composites).108 Figure 4.39 shows the morphology of the composites.108 The Aerographite/epoxy composites show a piezoresistive behavior comparable with carbon nanotubes or graphene composites.108 The aspect ratio, content, geometry, and orientation affect piezoresistivity of composites.109 Piezoresistive behavior is adjustable and the filler content near the percolation threshold leads to the highest sensitivity whereas the alignment of particles leads to the higher linearity.109 The higher aspect ratios and the higher filler contents shift the maximum to a lower strain level, but the sensitivity is reduced.109 In the composite films, the alignment of particles in the load direction enhanced the strain sensing capabilities.109 Poor sensitivity at low pressure regimes (90%) and covered a frequency range of 8.411.6 GHz.68 The polyaniline nanorods were used as a filler in nanocomposite electrolyte studied for applications in rechargeable batteries.69 The ionic conductivity increased with increasing nanorod content to a maximum of 2.3×10-3 S cm-1 at 5 wt% of nanorod content.69 The conductivity mechanism follows jump relaxation model.69 The nanocomposite electrolyte containing 5 wt% of dedoped PANI nanorods offers the electrochemical stability of 5.3 V which is adequate for practical applications.69 The electrolyte system is thermally stable up to 260°C.69 Antibacterial SnO2 nanorods found applications as efficient fillers for poly(propylene fumarate-co-ethylene glycol) biomaterials.70 They exhibit biocide action against Gram-positive and Gram-negative bacteria.70 The filler also improves friction coefficient and wear rate.70 The biomaterials are candidates for soft tissue engineering.70 Nano-crystallites of diphenyl aluminum phosphate having a nanorod shape were incorporated as an orientation-enhancing agent into an epoxy matrix.71 The composite was oriented using an external magnetic field during its cure.71 The anisotropic nanorod crystallites become oriented and highly enhanced the ordering of the liquid crystalline matrix in the magnetic Figure 5.32. The morphology of hydroxyapatite nanofield.71 The ordering effect of the field was rods. [Adapted, by permission, from Taheri, MM; stronger if it was perpendicular to the main Kadir, MRA; Shokuhfar, T; Hamlekhan, A; Shirdar, MR; Naghizadeh, F, Mater. Design, 82, 119-25, 2015.] surface of the flat samples (rather than par-

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allel).71 Figure 5.31 shows major effects of orientation on the properties of the nanocomposite.71 Fluoridated hydroxyapatite nanorods were used as fillers to improve mechanical properties of dental composite.72 The most favorable mechanical properties were achieved with 0.2 wt% filler.72 The composite has the advantage of preventing the formation of secondary caries due to the release of fluoride.72 Figure 5.32 shows the morphology of the filler.72 The composite materials comprising a thiol-ene polymer resin in a continuous phase and a filler in a discontinuous phase with the filler comprising fluorapatite crystals and silica was designed.73 It is considered a “smart composite” because, when the shrinkage stress of the composite is greater than a programmable or pre-selected threshold level, it will elicit visual clues for the dentist.73 The bundles of nanorod-like calcium hydroxyapatite crystals are arranged roughly parallel to each other to improve physico-chemical properties of enamel.73 The calcium carbonate nanorods were obtained by mixing an aqueous solution of NaHCO3 and an aqueous solution of CaCl2, then atomizing them in a pre-heated air flow.74 The calcium carbonate obtained is composed of nanorods having a width of 50 nm and length of 200 nm.74

5.6 NANOSHEETS Electrically insulating polymeric nanocomposites with high thermal conductivity are required for thermal-management in highpower-density electronics and optoelectronics.75 To design material which has thermal conductivity in the range of 1–5 W/mK, a large amount (over 70 vol%) of Figure 5.33. Schematic illustration of the development of 1D and 2D filler arrangement induced by various rotation sources: step 1) electrically conducting fillers such Orientation parallel to the longitudinal direction, step 2) End-to-end as carbon allotropes is required attachment of fillers (linear structure), step 3) Denser localization but such material also has electriof fillers to form a linear structure, and step 4) A longer route for cal conductivity.75 The application thermal conduction. [Adapted, by permission, from Cho, H-B; Nakayama, T; Suematsu, H; Suzuki, T; Jiang, W; Niihara, K; of fillers with various aspect ratios Song, E; Eom, NSA; Kim, S; Choa, Y-H, Compos. Sci. Technol., enables the rearrangement of the 129, 205-13, 2016.] boron nitride nanofillers into linear densely packed boron nitride structures.75 A composite containing these structures has electrical resistivity greater than 1.5×10-6 MΩ cm and a thermal conductivity of 1.56 W/ mK.75 Figure 5.33 shows steps leading to the most effective spacial arrangement which increases thermal conductivity.75 The fabrication of filament-like linear structures of boron nitride nanosheets (steps 2 & 3) was accomplished by controlling the polymer viscosity and the electric field.75 The combination of electrostatic and Coulombic attraction during assembling of boron nitride nanosheets can drive the end-to-end assembly to assume the linear bundle structures.75

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Nickel oxide nanosheets were used in the fabrication of composite with poly(vinylidene fluoride).76 The crystallinity, βphase content, and morphology were directly related to the NiO filler concentration because of a heterogeneous nucleation.76 Electrical measurements revealed dielectric constants up to 45 at 1 kHz and an increase in conductivity by more than two orders of magnitude, with a percolation threshold of about 4.0 vol% of NiO.76 1D fiber-like carbon nanoFigure 5.34. TEM images of carbon nanotubes (a, b) and graphene nanosheets (c, d). [Adapted, by permission, from Chen, T; Pan, L; tubes and 2D platelet-like graLin, M; Wang, B; Liu, L; Li, Y; Qiu, J; Zhu, K, Polym. Testing, 47, phene nanosheets were used as the 4-11, 2015.] conducting fillers to improve the mechanical, thermal and electrical properties of polyurethane nanocomposites at very low loading.77 Figure 5.34 shows the flakelike individual transparent graphene nanosheets with wrinkles and rolled sheets.77 The graphene nanosheets were found to adhere to the multiwalled carbon nanoFigure 5.35. Illustration of the “tortuous pathway” created by tube bundles.77 The dielectric conincorporation of exfoliated nanoplatelets into a polymer matrix stants of the composite films film. (a) In a polymer film, diffusing gas molecules migrate via a pathway that is perpendicular to the film orientation. (b) In a nano- containing both fillers were higher composite, diffusing molecules navigate around impenetrable than for the polyurethane composplatelets and through interfacial zones, which have permeability ites with any of the fillers alone.77 characteristics different from those of the pure polymer. [Adapted The titanium carbide (Ti3C2) by permission, from Cui, Y; Kundalwal, SI; Kumar, S, Carbon, 98, 313-33, 2016.] nanosheets were found to be excellent microwave absorbers.78 Reflection loss values of the Ti3C2 nanosheets filled composites exceeded 11 dB (more than 92% absorption) in the frequency range of 12.4–18 GHz with a thickness of 1.4 mm.78 To improve the anticorrosive performance of aluminum powder, graphene oxide was modified with aluminum powder using 3-aminopropylphosphonic acid as “link” agent.79 The graphene oxide nanosheets were functionalized with 3-aminopropylphosphonic acid reacting with the epoxy groups of graphene oxide.79 Subsequently, a layer of GO nanosheets uniformly and tightly covered the surface of flaky aluminum particles.79 The anti-

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Figure 5.36. The preparation of graphene/TiO2 hybrid. [Adapted, by permission, from Feng, X; Xing, W; Song, L; Hu, Y; Liew, KM, Chem. Eng. J., 260, 524-31, 2015.]

corrosive performance of the modified aluminum powder was enhanced via the barrier effect of the attached graphene oxide.79 Graphene layers in the polymer matrix are capable of producing a tortuous path, which acts as a barrier for gases.80 A high tortuosity leads to superior barrier properties and lower permeability of polymer nanocomposites (Figure 5.35).80 The magnetite nanoparticles-graphene nanosheet/polyurethane system is potentially usable as an actuator material which can respond to both the electric field and magnetic field with high reversibility.81 The 0.5 vol% of the hybrid material system provided the highest storage modulus sensitivity of 3.97 under the applied external electric field of 2 kV/mm, and showed the maximum bending distance of 13.65 mm and the dielectrophoresis force of 284 µN under the applied electric field of 500 V/mm.81 Titanium dioxide nanoparticles were homogeneously anchored onto the graphene oxide nanosheets and the reduction of graphene oxide occurred simultaneously during the one-pot solvothermal process (Figure 5.36).82 With the incorporation of 2.0 wt% graphene/TiO2 hybrids, a 3.5°C increment in the glass transition temperature and a 50%

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increase in the storage modulus were observed as compared to the neat PVC.82 The HCl release rate was reduced by the addition of graphene/TiO2 hybrid to PVC.82 Hexagonal boron nitride, hBN, is a thermally stable material.83 It does not occur naturally, but it is manufactured at high temperatures from boron sources such as boron oxide or boric acid and nitrogen sources such as melamine, urea, or ammonia.83 In some ways hBN resembles graphite; both consist of stacked sheets with the component atoms arranged in a honeycomb pattern, the boron/nitrogen pair of atoms is isoelectric to a pair of atoms in graphite, and both are good thermal conductors.83 Unlike graphite, however, hBN is an electrical insulator with a band gap of about 5.2 eV.83 It also has a much higher thermal stability than graphite, with a melting temperature near 3,000°C.83 The cubic form of boron nitride, cBN, is analogous to diamond, both in structure and hardness, with the hardness of cBN second only to diamond.83 The chemical vapor deposition can be used to produce single sheet hBN.83 However, for large-scale applications, such as nanofillers in polymer composites, exfoliation was a more economically attractive route to single sheet hBN.83 Exfoliation is performed by thermal treatment of boron nitride at 950 to 1000°C in air for a predetermined period of time.83 The resultant material after further treatment is transparent.83 The exfoliated boron nitride is functionalized by reaction with phenyl isocyanate.83 A thermally conductive filler composition comprises a blend of a boron nitride, a metal oxide (e.g., zinc oxide), and a silane for production of a thermally conductive plastic.84 The silane is chosen from a thiocarboxylate silane, a blocked mercapto silane, or their combination.84

5.7 NANODIAMONDS Nanodiamond is produced by detonation synthesis in large commercial quantities.85 It has a hardness of 10 on Mohs scale, the thermal conductivity of 2000 W/mK (one of the highest of any material) and a high refractive index.85,86 Nanodiamond is composed of small nearly spherical ~5 nm particles with a specific surface area of ~350 m2/g and a narrow particle size distribution.85 Nanodiamond fillers are functionalized to improve their interaction with the matrix.85 The most frequent groups introduced onto the filler surface are amino, hydroxyl carboxyl, and carbonyl.85 Biocompatibility is one of the important features of these fillers which found applications in medicine.85 Nanodiamonds were first produced in the 1960s but only by the end of the 1990s attracted interest in the application as a filler for various purposes.86 Among diamond-based nanomaterials, detonation nanodiamond, also known as ultradispersed diamond or ultrananocrystalline diamond, is most attractive for nanotechnological applications. Detonation nanodiamond can be easily suspended in water, has moderate cost, and is non-cytotoxic and biocompatible.87 Nanodiamonds in concentrations from 0.1 to 1 wt% were compounded with poly(vinylidene fluoride) for tissue engineering applications.86 The solvent cast porous composites mainly crystallize in the electroactive γ-phase of PVDF.86 The pre-osteoblast culture tests revealed that the inclusion of nanodiamond particles did not induce cytotoxicity.86 Because of the potential of the nanodiamond filler for protein functionalization and drug delivery, the composites are a suitable platform for biomedical applications.86 When nanodiamond is mixed with a polymer matrix, the non-functionalized nanoparticles exhibit poor dispersion and tend to aggregate.87 The major issue in the composite

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manufacturing is to improve filler dispersion by high shear mixing, extrusion, in situ polymerization, wrapping the nanoparticles with surfactants, or covalent chemical functionalization.87 Finnish producer of functionalized superhard materials − Carbodeon can achieve a 20% increase in polymer thermal performance by using as little as 0.03 wt% of its nanodiamond filler at an overall 45% thermal filler loading.88 For more demanding applications, the conductivity increase by 100% can be achieved using 1.5% nanodiamond materials at 20% thermal filler loading.88 The increase in the thermal conductivity is achieved without affecting the electrical insulation or mechanical properties of material, making the nanodiamond filler “an ideal choice” for a wide range of electronics and LED applications, such as polymer heat sinks.88 Polyamide-66 reference material containing 45 wt% of boron nitride as the thermal filler, and material developed by Carbodeon having 44.9% boron nitride and 0.1% of its uDiamond® nanodiamond powder were compared for their thermoconductivity.89 The thermal conductivity increased by 25% compared to that of the reference material.89 Application of nanodiamonds to thermoplastic composites for thermal applications was patented by Carbodeon.92 Styrene-butadiene rubber nanodiamond-filled elastomer composite has a double increase in rupture stress with an increase in rupture strain at the filler volume fraction equal to 2%.90 Nanodiamond (RayND, Ray technics Ltd., Israel) particles were prepared via pulsed laser ablation of a carbon target. It has higher purity (no admixtures) in comparison with nanodiamonds produced by the detonation synthesis.90 The particles have a spherical shape and a complex structure comprising several layers. The diamond core is 45 nm in size with high density in the range of 3.25-3.4 g/cm3.90 Above the core, there are graphitic surface structures followed by an active shell containing functional groups with unpaired electrons.90 X-ray photoelectron spectroscopy confirmed a chemical bonding between the (3aminopropyl)triethoxysilane-functionalized nanodiamond and (3-glycidyloxypropyl)trimethoxysilane-functionalized graphene oxide.91 The nanodiamond nanoclusters with an average diameter of 50-100 nm were uniformly grown on the graphene oxide surface.91 The nanodiamond nanoclusters played crack pinning role in the polymer matrix composite.91 The shoe sole composition includes nanodiamond produced by the shock wave method as an inorganic reinforcing filler.93 The nanodiamond increases the life of the shoe sole while reducing its total weight.93 Carbon black, silicon carbide, and alumina can be used in the shoe sole rubber composition in addition to nanodiamond.93 The nanodiamond used was produced by an explosive shock wave method with a particle size of 2-100 nm and a specific surface area of 200-450 m2/g (BET).93 3D diamond printing was done using a pre-ceramic polymer (poly(hydridocarbyne) or poly(methylsilyne)) with a nanoparticle filler (detonation nanodiamond powder).94 Poly(methylsilyne) may pyrolyze to form silicon carbide when heated to a decomposition temperature.94 At its pyrolysis temperature, poly(methylsilyne) may react with the surface of ceramic powder and bond to it.94 After heating, the 3D printed object may be composed of cubic crystal structure detonation nanodiamonds within silicon carbide, with the silicon carbide bonded to the surface of the detonation nanodiamonds.94

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Figure 5.37. Illustration of various composites demonstrating for multiwalled carbon nanotubes, MWNTs (black), an example of conducting path (dashed red line) and capacitance between the conducting path and nearby MWNTs (white). (a) High aspect ratio MWNTs in bundled form in PDMS. (b) Well dispersed high aspect ratio MWNTs aligned in the y-direction in PDMS. (c) Evenly dispersed and randomly oriented high aspect ratio MWNTs in PDMS. (d) Lower aspect ratio CNTs in bundled form within a polymer. (e) Lower aspect ratio CNTs evenly dispersed and aligned in the y-direction within a polymer. (f) Lower aspect ratio CNTs evenly dispersed and randomly oriented within a polymer. [Adapted, by permission, from Theilmann, P; Yun, D-J; Asbeck, P; Park, S-H, Organic Electronics, 14, 6, 1531-7, 2013.]

5.8 HIGH ASPECT RATIO A superior electromagnetic interference shielding and dielectric properties of carbon nanotube polydimethylsiloxane composites were achieved using high aspect ratio nanotubes.95 Very high aspect ratio carbon nanotubes combined with an effective fabrication process result in a low percolation threshold (~0.06 vol%) and excellent electromagnetic interference shielding efficiency.95 The high aspect ratio fillers, which reinforce the polymer and lower volume fractions of the percolation threshold, are the most desirable in these applications.95 Among various elongated fillers, carbon nanotubes are most attractive because their aspect ratio can reach 106.95 In addition to a high aspect ratio and good conductivity, fillers may be able to react or interact with the matrix.95 Carbon nanotubes can be functionalized with a variety of coupling agents which permits tailoring interaction suitable for any given matrix polymer.95 It was calculated that the single-walled carbon nanotubes with an aspect ratio of ~5000 (length ~ 5 µm, diameter ~ 1 nm), enable a conducting pathway at a volume fraction of ~0.01% when dispersed uniformly within a non-conducting polymer.95 But the key in this is good dispersion without aggregation and bundling.95 This means that the technology of dispersion must be well designed to take full advantage of properties of high aspect ratio fillers.95 Figure 5.37 shows various arrangements of high aspect ratio filler in PDMS.95 High aspect ratio MWNTs exist in bundle form (up to 15 µm diameter) where each MWNT is tied to hundreds of others by the van der Waals interaction.95

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Three simulation methods were used for prediction of the electrical conductivity and the electrical percolation threshold of fiber composites, including finite element modelling, percolation threshold modelling, and electrical network modelling.96 The best results in prediction of experimental data were obtained using electrical network modelling softcore simulations, without adjustable parameters.96 The predictions were excellent for the longest fibers but poor for the shortest.96 The shapes of the electrical network modelling curves were however always close to that of an empirical conductivity formula.96 Several physical transport mechanisms can simultaneously contribute to the conduction.96 These mechanisms include hopping, tunnelling, Schottky barrier transport, and Poole-Frenkel emission.96 The hopping mechanism usually dominates.96 Several factors may influence the conduction of the composite, such as96 • pressure leads to an increase in conductivity since the contact area between the fillers becomes larger, especially for powders • small air pockets, which can arise in the composite due to the mixing process, may decrease the conductivity • absorption of moisture significantly increases the conductivity • the conductivity increases with increasing voltage • the conductivity usually decreases with time. Fiber-reinforced composites are widely used in dentistry.97 The most common fibers include galls, carbon/graphite, polyethylene, and aramid fibers.97 The reinforcing efficiency of fibers is related to their orientation, length, and volume fraction.97 Fibers should have high surface free energy and they are frequently silanized for adhesion promotion to the polymer matrix.97 Fibers must be adhered durably to the polymer matrix and have good hydrolytic stability.97 The phosphate glass melt blended with poly(ethylene terephthalate) in the presence of a sulfonated PET copolymer compatibilizer provided an effective dispersion of the filler in the polymer matrix.98 Compression molded films were then stretched to a draw ratio of 3 × 3 at the polymer glass transition temperature causing phosphate glass droplets to be transformed into platelets with high aspect ratio.98 A 20-fold reduction in the oxygen permeability was obtained with 20 vol% of the oriented platelets.98 The mechanical properties of commodity plastics with immiscible soft inclusions can be enhanced, simply by pressure induced flow processing in the solid state.99 The shape and orientation of the soft fillers were changed by ABS processing, resulting in an array of aligned and oriented nanosize deformed rubber domains (Figure 5.38).99 These deformed domains effectively controlled the propagation of cracks inside the solid matrix and were responsible for a multifold increase in tensile and impact toughness.99 The key toughening Figure 5.38. Change of mechanisms of toughening due mechanism is related to the nucleation of to soft inclusions of high aspect ratio. [Adapted, by permission, Zhang, S; Zhu, S; Han, K; Feng, X; Ma, Y; crazes which grow in the direction normal Yu, M; Reiter, G, Polymer, 54, 21, 6019-25, 2013.] to the rubber inclusions.99 Propagation of cracks is restricted by the nearby rubber

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Figure 5.39. (a) Illustration of BaTiO3 nanowires, a PVDF matrix, and their composites. (b) Flexible BaTiO3 nanowire filled PVDF composites. [Adapted, by permission, from Choi, W; Choi, K; Yang, G; Kim, JC; Yu, C, Polym. Test., 53, 143-8, 2016.]

inclusions.99 These observations and mechanisms were similar with other styrenic polymers, such as SAN, HIPS, and SBS.99 Thermoset nanocomposite films were prepared from a two-component waterborne polyurethane and high aspect ratio nanofibrillated cellulose.100 The enhancement in modulus and tensile strength were attributed to a strong interaction resulting from the formation of hydrogen bonds and chemical grafting between nanofiller and PU matrix.100 Combination of two platy nano-additives with high aspect ratio (a layered double hydroxide and graphene oxide) drastically changed the flame retardancy of polystyrene nanocomposite.101 The effect is stronger than that of any of the two nanocomposites made with either filler.101 A stable suspension of the two nano-additives was achieved by surface modifications of layered double hydroxide and graphene oxide by 3,4-dihydroxybenzophenone and 1-dodecylamine, respectively.101 After formation of a protective layer, combustion occurred at a much-reduced heat release rate, and consequently, the related burn out time was prolonged by 154%.101 Flexible piezoelectric nanocomposites were synthesized with BaTiO3 nanowires and poly(vinylidene fluoride).102 The piezoelectric performance of the composite was studied by varying the aspect ratio and volume fraction of the nanowire.102 A high aspect ratio significantly increased the dielectric constant up to 64, which is over 800% improvement compared to those from the composites containing spheroid shape BaTiO3 nanoparticles.102 Figure 5.39 shows morphology and distribution of nanowires in a flexible matrix of PVDF.102 The concept of “double percolation” (conductive fillers are selectively located in one phase of a co-continuous polymer blend to form a percolated network in the selected phase) is widely used to reduce the percolation thresholds of conductive polymer composites to a fraction of their original values.103 Multiwalled carbon nanotubes with a very high aspect ratio (~1000) are selectively distributed at a continuous interface of a co-continuous

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immiscible poly(lactic acid)/poly(ε-caprolactone) blend.103 Compared to the PLA/ PCL/MWCNTs composites obtained by the traditional double percolation method (percolation threshold was ~0.97 wt%), the percolation threshold of PLA/MWCNTs/ PCL composites (~0.025 wt%) dropped 2 orders of magnitude due to controlling the Figure 5.40 Schematic illustration of the advantage of MWCNTs at the continuous interface Ag/N-GNS hybrid filler ECAs. (a) Ag particles alone between the PLA and PCL phases.103 The cannot form a conductive network at low Ag loading; (b) N-GNSs can easily connect separated Ag particles. PLA/MWCNTs/PCL composites were Rt (tunneling resistance) >> Rc (contact resistance) or obtained by mixing MWCNTs in PLA and RGNS (resistance of GNS). Nitrogen doping can effec103 tively reduce RGNS, leading to a lower resistivity of the then addition of PCl. hybrid filler. [Adapted, by permission, from Pu, N-W; Nitrogen-doped graphene nanosheets Peng, Y-Y; Wang, P-C; Chen, C-Y; Shi, J-N; Liu, Y-M; were used as a conductive filler for a polyGer, M-D; Chang, C-L, Carbon, 67, 449-56, 2014.] mer resin adhesive and as a performance improver for a silver-filled electrically conductive adhesive.104 With 30 wt% of Ag filler, the polymer resin was still non-conducting, while a resistivity of 4.4×10-2 Ω-cm was obtained using an Ag/nitrogen-doped graphene nanosheet filler fortified with only 1 wt%104 of nitrogen-doped graphene nanosheet due to a large specific surface area, high aspect ratio, and good electrical conductivity of the doped graphene.104 The resistivity is much lower for combination filler than that for any of its components, indicating a synergistic effect.104 This effect is illustrated schematically in Figure 5.40.104 There are two types of mechanisms for the conduction between neighboring conductive fillers embedded in the nonconductive resin matrix: tunneling or direct contact.104 The tunneling resistance (Rt) increases exponentially with the separation and it is in general much greater than the contact resistance (Rc).104 When the Ag loading is low, the Ag particles are too sparse to form an interconnected conductive network, and most of the conduction can only rely on the inefficient tunneling through thick resin barriers.104 When a small amount of nitrogendoped graphene nanosheets are added, the originally separated Ag particles have good chances to form contacts with these highly-conductive two-dimensional sheets because of their high specific surface area and excellent aspect ratio.104 The resistance of graphene nanosheets (RGNS) is effectively reduced by nitrogen doping.104 The overall resistance of conductive pathways is significantly reduced.104 A filamentary structure of ink extruded from a nozzle during 3D printing comprised a continuous filament including filler particles dispersed therein.105 A fraction of the filler particles in the continuous filament comprised high aspect ratio particles having a predetermined orientation with respect to a longitudinal axis of the continuous filament.105 The high aspect ratio particles were aligned along the longitudinal axis of the continuous filament.105 Carbon fibers and SiC whiskers were preferred high aspect ratio fillers, clay platelets were filler particles and epoxy resin was a binder.105 Structural acrylic adhesive contained acrylic functional monomer, toughening agents, adhesion promoter, a high aspect ratio filler (ratio in the range of 20-60) selected from fibrillated filler and a halloysite clay filler.106

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Thermally conductive plastic composition comprised thermoplastic polymer (e.g. polycarbonate or polyamide) and the filler composition of a boron nitride, a metal oxide (e.g., ZnO or TiO2), and a silane (a thiocarboxylate silane or a blocked mercapto silane).107 Deformable elastomeric conductors for differential electronic signal transmission comprised an elastomeric polymer matrix and conductive filler material (nickel coated carbon fiber having aspect ratio of 50) uniformly dispersed in the elastomeric polymer matrix sufficient to render the material electrically conductive.108 The conductive filler material may include non-entangled particles having an aspect ratio sufficiently large to enable the particles to remain in contact and/or in close proximity with adjacent particles so as to maintain conductive pathways in the material when the material is subjected to deformation up to and exceeding 10% strain.108

5.9 LAYERED DOUBLE HYDROXIDES The reinforcement of polymer matrices by exfoliation and microdispersion of layered clays is a common concept.109 The traditional clays used in such reinforcements are silicates (typically montmorillonite) which have a cation-exchanging capability and therefore the organic modifiers of the interlayer structure are usually alkyl ammonium cations.109 In contrast, layered double hydroxides form a family of lamellar inorganic compounds with exchangeable anions in the interlayer space and they are also a candidate for polymer-reinforcing fillers.109 Their anion exchange capacities are generally 2-4 times larger than for the silicate clays.109 Fatty acids derived from natural lipids are employable to replace the initial interlayer anions.109 A number of hydroxyl groups are present on the surface planes of the delamination of layered double hydroxide, differing from montmorillonite.109 The layered double hydroxide in concentration of 3-3.5 wt% were used to improve the thermo-mechanical property of the miscible poly(acryloyl morpholine)/cellulose acetate blend.109 They also contributed to the reinforcement of the blend.109 Layered double hydroxides found many uses in environmental protection, agriculture, heterogeneous catalysis, polymer nanocomposites, biotechnology, and pharmaceutical technology and cosmetics.110 Findings were discussed in a book chapter.110 Edible active coatings based on pectins filled with layered double hydroxides-salicylate were prepared.111 The hybrid filler loading was 5 wt%.111 Barrier properties (sorption, diffusion and permeability) to water vapor indicated that a composite plasticized with 4 vol% of glycerol gave the best barrier properties, meaning that the interaction degree of pectin-active filler and glycerol plays a significant role in determining the transport phenomena.111 Thermal-oxidative aging behavior of nitrile-butadiene rubber was improved with layered double hydroxides.112 A sodium p-styrenesulfonate hydrate was used to modify layered double hydroxide through ion exchange.112 Metal corrosion causes serious environmental pollution and waste of resources.113 The effective way to delay corrosion is the coating screen method but the surface of a coating is covered with micropores, which can't effectively block the corrosive medium, especially in the case of waterborne resins.113 Waterborne epoxy resins are widely used in coatings because of their excellent adhesion, good chemical stability, and good corrosion resistance.113 But, the waterborne epoxy resins also produce a large number of micropores

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Figure 5.41. Illustration of the preparation procedure of composite with addition of filler, M-rGO-ZnAl-LDH. rGO-ZnAl-LDH − reduced graphene oxide-zinc-aluminum layered double hydroxide. [Adapted, by permission, from Yu, D; Wen, S; Yang, J; Wang, J; Chen, Y; Luo, J; Wu, Y, Surf. Coat. Technol., 326A, 207-15, 2017.]

Figure 5.42. Schematic representation of the corrosion inhibition properties of M-rGO-ZnAl-LDH/EP composite coating. [Adapted, by permission, from Yu, D; Wen, S; Yang, J; Wang, J; Chen, Y; Luo, J; Wu, Y, Surf. Coat. Technol., 326A, 207-15, 2017.]

during the high temperature curing process.113 Situation is farther exacerbated by addition of inorganic fillers which, being incompatible with matrix, form even more openings for water and other environmental stressors.113 In order to improve the compatibility of the inorganic material with the polymer system, the inorganic material usually needs to be modified.113 Graphene and its derivatives have excellent properties in protection of metals against corrosion but their water dispersibility is still a challenge, especially its re-stacking, which seriously affects its applications.113 Combination of layered double hydroxides and graphene derivatives seems to make technological sense.113 The reduced graphene oxide-zinc-aluminum layered double hydroxides micro-nano fillers were prepared via a one-step process.113 They were modified by 3-aminopropyl triethoxysilane and incorporated into a waterborne epoxy matrix to produce composite coating.113 The corrosion resistance of the coating was remarkably improved when the ratio of reduced graphene oxide:oxide-zinc-aluminum layered double hydroxide was 2:1 and its amount in coating was 0.5 wt%.113 The corrosion current density was only 0.0733 µA/cm2 and the coating

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resistance was 2.77E4 Ω cm2 for such modified coating as compared with 0.469 µA/cm2 and 2.10E3 Ω cm2, respectively, for a pure epoxy.113 Figure 5.41 shows steps of the technological process of production of filler and composite.113 Figure 5.42 illustrates the effect of micropores on corrosion and its prevention by the filler.113 3-(4-Hydroxyphenyl)propionic acid, a biobased hydroxy acid, has been used as organic modifier in layered double hydroxides based on ZnAl and MgAl cations.114 Poly(butylene succinate) bionanocomposites have been prepared via in situ polymerization and melt blending leading to the completely green and potentially fully biodegradable materials.114 The addition of filler decreases the gas and solvent permeability and has potential antibacterial effect.114 Polysulfone filled with the layered double hydroxide (Mg0.62Al0.38(OH)2(Cl)0.38·0.6H2O) was used for preparation of composite anion exchange membranes.115 The composite membranes have a distinctly lower water uptake and swelling.115 Their mechanical properties under fully humidified conditions are clearly enhanced (3-fold increase of Young’s modulus).115 The conductivity of the composite membranes is comparable with that of the pristine ionomers (2-4 mS cm-1).115 The type of polymer, dispersion, polymer affinity, the preparation technique, and content are the main factors which can affect the structure and the properties of composites.116 The main advantages of layered double hydroxides are their effect on improvement of mechanical, thermal, and barrier properties, as well as flame retarding properties.116 Poly(l-lactide), with a glass transition temperature around 329 K, is a relatively stiff and brittle polymer with a low deformation at break.117 One way to improve the properties of PLA is to incorporate nanoparticles to form polymer-based nanocomposites.117 Nanocomposites based on poly(l-lactide) and organically modified MgAl layered double hydroxides were prepared by melt blending.117 The MgAl-LDH is homogeneously distributed in the matrix as stacks of 6 layers and/or partly exfoliated layers.117 A limiting concentration of LDH was 21 wt% at which the crystallization of PLA was completely suppressed by the nanofiller.117 Polymer nanocomposite based on poly(l-lactide) and MgAl layered double hydroxide nanofiller were prepared.118 Due to the relatively long crystallization half-time of PLA, its crystallization can be completely suppressed by sufficiently high cooling rate.118 To prepare a completely amorphous sample, a cooling rate of 100 K/min was required.118 The layered double hydroxide organoclays are assembled through coprecipitation with ligninosulfonate interleaved inorganic host structure sheets.119 The biopolymer was found in the interlayer space adopting a bilayer molecular arrangement resulting in a basal spacing of 2.54 nm.119 Hydrophilic and hydrophobic ZnAl layered double hydroxides were synthesized and modified using sodium dodecyl benzene sulfonate and used to prepare polystyrene composites.120 The hydrophilic filler had a better exfoliation effect than the hydrophobic layered double hydroxide in the hydrophobic polymer structure.120 The orientation of surfactant was perpendicular in the intercalated LDH, while almost parallel in nanocomposites. The filler enhanced the thermal stability of the nanocomposites.120 Layered double hydroxides were incorporated into Nafion (sulfonated tetrafluoroethylene based fluoropolymer-copolymer) for the development of hybrid nanocomposites useful in the high temperature polymer electrolyte membrane fuel cells.121 The water

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Figure 5.43. Schematic representations of LDH structure (on the left) and octahedral units M(OH)6 of LDH's layers (on the right). [Adapted, by permission, from Nicotera, I; Angjeli, K; Coppola, L; Enotiadis, A; Pedicini, R; Carbone, A; Gournis, D, Solid State Ionics, 276, 40-46, 2015.]

absorption and diffusion in the hybrid membranes was increased at high temperature, proving the exceptional water retention property together with superior mechanical properties.121 Figure 5.43 shows the structure of layered double hydroxides.121 These LDHs have an anion exchange capacity, which depends on the isomorphic substitution of Mg2+ ions by higher valence cations.121 As a consequence, the layers have a fixed positive charge and neutrality.121 The positive charge of the layers affects many fundamental properties of the LDHs, including anion exchange capacity, anion fixation, swelling ability, water holding, and high specific surface areas.121 These materials have the natural ability to absorb organic or inorganic guest anionic species (and even neutral polar molecules).121 They can be used in a wide range of applications, such as catalysis photochemistry, electrochemistry, polymerization, magnetization, biomedical sciences, and environmental applications.121 These composites have the ability to maintain a certain amount of “mobile water” for several hours without any external humidification.121

Figure 5.44. Schematic illustration of the LDH in-situ growth mechanism in various CNTs aqueous suspension. [Adapted, by permission, from Du, M; Ye, W; Fu, H; Lv, W; Zheng, Q, Compos. Sci. Technol., 105, 28-36, 2014.]

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Composites with 3D structure that combine 1D carbon nanotubes and 2D lamellar flakes were obtained by in situ growth of layered double hydroxides in CNTs aqueous suspensions.122 The morphology of the composites can be easily tuned by changing the CNTs concentration and the LDH growth time.122 The 3D composites with interlayer dodecylbenzenesulfonate anions are efficient fillers, significantly improving the mechanical properties and thermal stability of polypropylene.122 Figure 5.44 illustrates mechanisms of growth of layered double hydroxides in various aqueous suspensions of carbon nanotubes.122 Similar to the critical overlap concentration in polymer solution, the CNTs come in contact with each other when their concentration reaches a certain value.122 This critical concentration is defined as ϕ*.122 A single carbon nanotube can be viewed as a macromolecular chain in a solvent due to its large length/diameter ratio.122 The average diameter and length of the CNTs used were around 30 nm and 10 µm, respectively.122 The wall thickness of a nanotube was about 8 nm according to a TEM image.122 At CNTs concentration < ϕ*, CNTs disperse individually in aqueous suspension (Figure 5.44a).122 There are few heterogeneous nuclei on the surface of the CNTs, and the homogeneous nucleation in water becomes dominant.122 The LDH crystals then grew from these nuclei giving rise to a broad size distribution of LDH flakes.122 At CNTs concentration close to ϕ*, CNTs in aqueous suspension can overlap with each other, which favors the generation of Al(OH)3 seeds on their surface (Figure 5.44b).122 Heterogeneous nucleation dominates in such system and the LDH crystals grow at almost the same rate.122 At CNTs concentration > ϕ*, more CNTs overlap and even become severely entangled (Figure 5.44c), which results in the formation of numerous micropores.122 The LDH crystals are likely to grow outward perpendicular rather than parallel to the external surface of the CNTs, due to spatial competition.122 Eventually, many imperfect and smaller-sized LDHs flakes are produced.122 A low-smoke, non-halogenated flame retardant composition was made from polypropylene, magnesium dihydroxide, and aluminum magnesium layered double hydroxide modified with a hydrogenated fatty acid.123 The additive was designed with application in power cable jacket in mind.123 The materials for sorption and gas storage were made in a form of aerogels and xerogels containing graphene or graphene oxide and layered double hydroxides.124 One of its applications is an absorption of carbon dioxide from atmosphere.124

5.10 FUNCTIONALIZED FILLERS Functionalization of fillers have been discussed in many sections of this book. Here are some new examples to emphasize importance of functionalization of fillers in building the required properties of materials. In fuel cell applications, Nafion is used as a polymer.125 The main disadvantage of the process is that the conductivity declines above 90°C because anisotropic swelling causes high resistance and poor adhesion between the membrane and electrodes.125 This can be improved by the use of fillers.125 Titanium dioxide was selected as the filler because of its particle size, better interaction with polymeric matrix, greater influence on the original characteristics of the electrolyte.125 The titanium dioxide was functionalized with (3-mercaptopropyl)trimethoxysilane (Figure 5.45) which resulted in the grafting onto the ceramic oxide surface of silylpropyl moieties bearing terminal sulfonic acid groups, a procedure previously used to enhance the acidity of filler.125 The highest conductivity

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Figure 5.45. Synthetic pathway for the surface functionalization of TiO2 nanoparticles. [Adapted, by permission, from Cozzi, D; de Bonis, C; D'Epifanio, A; Mecheri, B; Tavares, AC; Licoccia, S, J. Power Sources, 248, 1127-32, 2014.]

5.46. Morphology of three foams. Symbols explained in the text. [Adapted, by permission, from Mondal, T; Basak, S; Bhowmick, AK, Polymer, 127, 106-18, 2017.]

value was obtained for the composite membrane containing 10 wt% of functionalized filler (s = 0.08 S cm-1 at 140°C).125 The presence of the filler resulted in both higher power density delivered and lower methanol crossover with respect to unfilled Nafion membrane.125 These results show that the propylsulfonic-functionalized titania is an effective proton-conducting filler for use in Nafion-based composite membranes operating at temperatures higher than 100°C.125 Mixed matrix membranes for separation of carbon dioxide from nitrogen and methane were developed by adding MIL-125(Ti) which was amine-functionalized as filler to Matrimid® polyimide.126 The membranes had excellent performance in gas separation, a significant increase in mixed gas selectivity and permeability.126 Fabrication of polymeric foam with controlled porosity, good mechanical properties, and electrical conductivity is a difficult task.127 The 1-methyl imidazole chloride ionic liquid grafted graphene oxide, ILF, was utilized as an effective agent for controlling the strength, distribution and dimensions of the pores of polyurethane foam because the polymeric foams fabricated with pristine expandable graphite, GF, and the neat polymer, PUF, had a distorted cell structure and an uneven distribution of the pores.127 Figure 5.46 shows the difference between these three foams.127 The distorted cell structure results from foaming neat polymer and polymer containing graphite whereas regular cells are formed

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in the foam containing 1-methyl imidazole chloride ionic liquid grafted graphene oxide.127 The functionalized graphene oxide modified foam had improved specific electrical conductivity, higher dielectric constant, and improved thermal stability.127 The reactive rubber nanoparticles and organically modified nanoclay were used in an epoxy matrix with the aim of obtaining an improved material with higher toughness without compromising the other desired mechanical properties.128 Incorporation of reactive rubber nanoparticles resulted in a softer nanocomposite with lower stiffness and improved toughness whereas the incorporation of organically modified nanoclay in the cured epoxy increased the stiffness and lowered the toughness of the epoxy/nanocomposites compared to those of the neat resin.128 Combining the two fillers in optimal concentrations (5 wt% rubber particles and 6 wt% clay) enhanced the flexural stress, modulus, and toughness.128 A process for producing polymer-functionalized filler particles can be used for fabrication of additives for rubber mixtures.129 The polymers for functionalizing the filler particles are obtained in-situ starting from filler particles having a surface-bound reversible addition fragmentation chain transfer, RAFT, agent.129 The binding of silyl-bearing RAFT agents and of Diels-Alder RAFT agents occurs via the R-group (a functional group on RAFT agent) and the subsequent RAFT polymerization proceeds via a reactive site of the RAFT agent.129 The polymer composition is a dispersion capable of forming a water-resistant film upon drying (or curing), based on polymer (styrene-butadiene copolymer), a silane crosslinking agent (3-glycidoxypropyl methyl diethoxysilane) and a filler coated with a silane crosslinking agent (epoxy silane).130 The polymer composition according to the invention has excellent properties in terms of water tightness with pressurized water, elongation at break and tensile strength.130

5.11 ENCAPSULATED FILLERS Magnetic and conducting polypropylene nanocomposites were fabricated with iron encapsulated in carbon nanotubes.131 Prevention of iron oxidation is the reason for encapsulation.131 The addition of 3.8 wt% of the filler changes the insulating PP matrix to a semiconductor.131 The addition of 0.8 wt% CNTs results in ferromagnetic behavior in the diamagnetic polymer matrix.131 Hybrid nanofillers representing graphene encapsulated alumina nanofibers were selected as an additive to develop toughened electroconductive partially stabilized zirconia.132 Alumina nanofibers of several nanometers in diameter were encapsulated with multi-layered graphene shells.132 The fibers of 50 mm in length and an average single fiber diameter of 7±2 nm were encapsulated by several layers of graphene.132 Composites containing just 0.6 vol% of graphene corresponding to 3 vol% of hybrid nanofibers exhibited high electroconductivity of 58 S/m without deterioration of mechanical properties.132 Figure 5.47 shows the morphology of fiber covered with a 2-3 nm thick graphene layer.132 A high dielectric constant and low loss tangent poly(vinylidene fluoride) composites were developed using graphene-encapsulated barium titanate hybrid fillers.133 Barium titanate particles encapsulated with graphene oxide were prepared via electrostatic selfassembly and subsequent chemical reduction.133 The hybrid fillers have two advantages for tuning dielectric properties: the extremely low content of reduced graphene oxide, RGO (it can be exactly controlled) and individual RGO sheets segregated by barium titan-

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Figure 5.47. SEM micrograph of the nanofibers network (a); SEM image of the mixture of zirconia nanoparticles and fibres (b); TEM image of alumina nanofibers after chemical vapor deposition treatment (c); schematic representation of the single nanofiller (d). [Adapted, by permission, from Hussainova, I; Drozdova, M; Pérez-Coll, D; Rubio-Marcos, F; Jasiuk, I; Soares, JANT; Rodríguez, MA, J. Eur. Ceramic Soc., 37, 2, 3713-9, 2017.]

Figure 5.48. SEM images of (a) BT, (b) BT-GO, (c) BT-RGO and (d) TEM image of BT-GO. [Adapted, by permission, from Li, Y; Shi, Y; Cai, F; Xue, J; Chen, F; Fu, Q, Compos. Part A: Appl. Sci. Manuf., 78, 318-26, 2015.]

ate particles prevent leakage current.133 The improvement of relative permittivity constant is attributable to the formation of microcapacitors by highly conductive RGO sheets segregated by barium titanate particles.133 The distance between adjacent RGO sheets is large enough to prevent leakage current from tunneling Figure 5.48 conductance.133 shows morphology of filler transformation involved in the process of production of a hybrid filler.133 RGO sheets obtained by a chemical reduction have less negative charges.133 So, that the electrostatic attraction with barium titan-

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ate particles is diminished which otherwise might have brought about the detachment of wrapping RGO sheets.133 Zinc ferrite has been synthesized with the diameter of 50 nm and encapsulated with polyethylene glycol, PEG-6000.134 The encapsulated filler was used to prepare composite in which poly(vinylidene fluoride) was a matrix polymer.134 The robust PEG layer acted as the coupling agent on the interface between organic and inorganic phases and improved Maxwell-Wagner-Sillars interfacial polarization by forming an interaction zone with Gouy-Chapman diffuse layer of polyglycolated zinc ferrite.134 The induced polar phase and dielectric permittivity of the composite were increased after surface modification and the dielectric loss was decreased at a low level of filler.134 Polymers filled with ferroelectric powders are endowed with high dielectric constant, low dielectric loss and good processability, thus they have potential applications in capacitors, energy-storing devices, smart actuators, etc.135 Barium titanate was coated with PATP polymer and used in preparation of composite with epoxy resin.135 The nanoparticles were encapsulated with a PATP shell of 5-10 nm, Figure 5.49. TEM images of (a) raw barium titanate, BT and (b) BT–PATP nanoparticles. [Adapted, by per- which improved the dispersion of nanoparmission, from Wei, X; Xing, R; Zhang, B; Zhang, Q; ticles in the epoxy resin and suppressed the Bulin, C, Ceramics Intl., 41, Suppl. 1, S492-7, 2015.] dielectric loss of composite.135 The coating is clearly visible in Figure 5.49.135 A functional nanoreservoir based on multiwalled carbon nanotubes and β-cyclodextrin was synthesized.136 The functional nanoreservoir loaded with a “green” corrosion inhibitor benzimidazole was compounded with epoxy as a filler to enhance its anticorrosion performance.136 The addition of functional nanoreservoir loaded with the corrosion inhibitor significantly improved steel resistance to corrosion, which was attributed to the release of the encapsulated corrosion inhibitor.136 The encapsulation of corrosion inhibitors within responsive nanocontainers is used for self-healing protective coatings.136 The sandwich-like silica encapsulated graphene oxide hybrids (SiO2@GO) were fabricated by a sol-gel method to enhance the dielectric properties of polyimide.137 A dielectric constant of 73 (40 Hz) was obtained for SiO2@GO/PI composites when the fraction of SiO2@GO/PI was 20 wt%.137 Phase change materials must be encapsulated or contained to avoid leakage when phase change occurs.138 A phase change materials are used for thermal energy storage in building applications for passive systems or heating and cooling usages.138 A phase change materials (e.g., paraffins) are encapsulated in polypropylene.138 A phase change material lowers mechanical strength of polypropylene because paraffin acts as plasticizer.138 Triclosan and indomethacin-loaded nanocapsules were incorporated into adhesive system.139 The goal of this research was to provide fillers for adhesive systems to act as antimicrobial and anti-inflammatory agents with continuous action.139 The nanocontainers were made from polymer (MMA-co-MAA), Eudragit® S100.139 The spherical and bio-

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compatible nanocontainers had a mean size of 159 nm.139 Indomethacin and triclosanloaded nanocapsules were incorporated into primer and adhesive, promoting controlled drugs release, indomethacin diffusion through dentin and antimicrobial effect without compromising its physicochemical properties.139 Bovine serum albumin was encapsulated in microspheres to enhance bioactivity of the scaffold.140 Bovine serum albumin loaded with chitosan-based microspheres having a diameter of 3.8-61.6 µm was fabricated by an emulsion crosslinking method, followed by embedding into carboxymethyl chitosan-oxidized chondroitin sulfate hydrogels to produce a composite chitosan-based microspheres/gel scaffold.140 Chitosan microspheres improved mechanical properties of the scaffold.140 The cumulative release of bovine serum albumin during 2 weeks from chitosan-based microspheres embedded hydrogel was 30%, which was significantly lower than for chitosan-based microspheres and hydrogels.140 The synthetic aggregate for use in concrete is a composite material of recycled plastic having a filler encapsulated in the plastic.141 The synthetic aggregate includes between 30 and 50% recycled shredded plastic, the balance being filler.141 Heating the aerosol droplets to evaporate water from the aerosol droplets and the resulting compression induces the formation of capsules having an average size of less than 1 nm.142 The capsules comprise crumpled graphene oxide shells with the graphene oxide sheets and silicon nanostructures encapsulated within the crumpled graphene oxide shells.142 The anode materials for lithium-ion batteries comprise the capsules, wherein the nanostructures are composed of an electrochemically active material, such as silicon.142 The method for preparing a microcapsule comprising carbon nanotube-exfoliated graphene or CNT-exfoliated graphene comprising a uniformly adsorbed nano-metal is described, as well as a method for preparing a conductive thermoplastic resin composition comprising a conductive polymer filler comprising the capsule.143 Polymer used for encapsulation is poly(ethylene oxide-b-propylene oxide).143 Polymerization of monomers occurs in the presence of exfoliated graphene resulting in microencapsulation.143 The articles of clothing including composite phase change materials for temperature regulation contain phase change material (e.g., paraffin, octadecane) encapsulated in crosslinked polyethylene glycol matrix.144 The composite phase change material has a latent heat of enthalpy greater than about 50 kJ/kg.144 A phase change occurs from 10 to 60°C.144

5.12 HYBRID Traditional monolithic fillers are utilized in many applications of composites that become limited in the terms of property enhancement.145 For example, epoxy/clay composites improve thermal properties, alumina powders enhance fracture toughness and fracture energy, kaolin improves tensile and impact strength.145 All these fillers enhance different properties of epoxy composites.145 This and perhaps limiting capability in a property enhancement by a single filler prompted researchers to look at hybrid fillers. These hybrid fillers may be as simple as a mixture of two different traditional fillers or may be a product of chemical synthesis which permits decoration of a filler surface with nanoparticles of another filler or wrap around fiber a two-dimensional platelet such as, for example, graphene. Many of these hybrid fillers were discussed in different contexts of properties

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Figure 5.50. Synthetic procedure of production of hybrid filler. [Adapted, by permission, from Hu, D; Zhong, B; Jia, Z; Lin, J; Liu, M; Luo, Y; Jia, D, Mater. Lett., 188, 327-30, 2017.]

throughout this book. Here, we will look at numerous examples of fillers which were not discussed before. Hybrid technology is one of the most researched topics and as such, it deserves close monitoring. Various defects (or imperfections) occur on carbon nanotubes such as single- or twoatom vacancies and the Stone-Wales defects (Stone-Wales defect creates pentagon and heptagon pairs by rearrangement of the bonds) that cause tube distortion, tube disorder, and so on.145 The mechanical strength and electrical conductivity of CNTs can be affected accordingly.145 For these reasons, the control of the process which creates these defects is essential because too many defects can deteriorate important property.145 In many instances defects are internally introduced by oxidation, bombarding with argon or hydrogen, or a variety of chemical reactions. In such a case, we rather call them modifications. The defects and modifications are important in the formation of hybrid fillers because fillers can be modified by doping into defects which creates the formation of functional groups further used for hybridization. A hybrid filler of halloysite nanotubes/silica was fabricated by electrostatic selfassembly process.146 Figure 5.50 shows the process.146 Silica is synthesized by sol-gel process and modified by the addition of tetraethoxysilane and 3-triethoxysilylpropylamine.146 The suspension of modified silica was gradually added into the suspension of halloysite nanotubes and ultrasonicated for 30 min.146 The hybrid filler was formed by coassembly between negatively charged halloysite nanotubes and positively charged modified silica.146 The modification of halloysite nanotubes can suppress their aggregation.146 A composite based on poly(ethylene terephthalate) and nano-hybrid filler composed of calcium ferrite and carbon nanotubes was obtained by a direct growth of carbon nanotubes on calcium ferrite-based iron catalysts.147 The catalyst (calcium ferrite) was placed in a quartz boat inside the reactor, located in a horizontal electric furnace.147 Temperature was then raised to 700°C and He was replaced with required amount isobutane.147 After 2 h, the reaction was stopped and the product was cooled down to a room temperature under hydrogen+helium flow.147 The hybridization improved dispersion of carbon nano-

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Figure 5.51. High resolution TEM micrographs of epoxy/hybrid composite. [Adapted, by permission, from Zakaria, MR; Akil, HM; Kudus, MHA; Kadarman, AH, Compos. Struct., 132, 50-64, 2015.]

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Figure 5.52. High resolution TEM micrographs of epoxy/filler mixture composite. [Adapted, by permission, from Zakaria, MR; Akil, HM; Kudus, MHA; Kadarman, AH, Compos. Struct., 132, 50-64, 2015.]

tubes which had very low percolation threshold based on the electrical conductivity (0.76 wt%).147 The carbon nanotube-alumina hybrid filler was chemically synthesized via chemical vapor deposition and compounded with epoxide.148 The composite properties were compared with a composite containing simple mixture of carbon nanotubes and alumina. The hybrid filler produced better dispersion and had higher flexural modulus and dielectric constant.148 Figures 5.51 and 5.52 show clear difference between distribution of hybrid filler and mixed filler in the composite.148 The carbon nanotubes surround alumina particles in hybrid filler whereas they disperse separately in composite containing physical mixture of fillers.148 A lignin/silica hybrid filler was compounded with natural rubber as a replacement of silica filler.149 Addition of hybrid filler improved the processability, anti-aging resistance and anti-flex cracking of composites.149 The vulcanizate containing 20 phr lignin and 30 phr of silica in the hybrid filler exhibited the optimal overall mechanical properties.149 The rubber conductive composite loaded with carbon-silica hybrid filler was used as a single director and a resonant half-wave dipole in antenna.150 The conductive composite loaded with carbon-silica hybrid filler can effectively reduce maximum specific absorption rate by 70%, increase the radiation efficiency by 67%, and the bandwidth by 110%).150 A hybrid filler composed of hollow glass microspheres and aluminum and boron nitride particles was compounded with a low-density polyethylene via powder mixing and hot pressing technology to obtain composites.151 The surface modification of filler had a beneficial effect on the thermal conductivity and dielectric properties of the composites due to a good interfacial adhesion between the filler and matrix.151 The optimal volume ratio of both fillers was 1:1.151 The thermal conductivity increased with filler volume frac-

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Figure 5.53. Schematic sketch of microstructure of the composites prepared with different compositions: (a) volume ratio of nitride particles to HGMs is lower; (b) total filler volume fraction of filler is lower; (c) both volume ratio of nitride particle to HGM and total filler volume fraction of filler are high enough. [Adapted, by permission, from Zhu, BL; Wang, J; Zheng, H; Ma, J; Wu, J; Wu, R, Compos. Part B: Eng., 69, 496-506, 2015.]

Figure 5.54. A schematic representation of the structure of NR/BR blend rubber vulcanizate containing precipitated silica/a sodium salt of rubber seed oil modified kaolin hybrid filler. [Adapted, by permission, from Vijay, VR; Anitha, AM; Menon, ARR, Polymer, 89, 135-42, 2016.]

tion and it was mainly related to the type of nitride particles.151 SEM micrographs show that the hybrid filler particles are agglomerated around the LDPE matrix particles.151 Within the agglomerates, the smaller-sized nitride particles in the hybrid filler can easily form thermally conductive networks to produce the composites with high thermal conductivity.151 Figure 5.53 shows the structure of conductive pathways at different concentrations of fillers.151 Precipitated silica and kaolin modified with a sodium salt of rubber seed oil were blended with natural and polybutadiene rubbers.152 The addition of hybrid filler lowered Mooney viscosity, increased cure rate, increased chemical crosslink density index and bound rubber content, increased rubber-rubber and rubber-filler interactions, lowered abrasion loss and compression set and increased tensile strength, elongation at break, and tear strength indicating its potential for application as tire tread composition.152 The lower value of loss factor (tanδ) of these compositions implies lower heat build-up in tire tread applications.152 Figure 5.54 shows a structure of filler in the rubber vulcanizate.152 Hybrids of multiwalled carbon nanotubes and graphene oxide were developed to improve the mechanical and tribological performances of hybrid polytetrafluoroethylene/

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Nomex fabric/phenolic composite.153 The wear rates of hybrid fillers composites were significantly reduced when hybrid filler contained 1 wt% multiwalled carbon nanotubes and 2 wt% graphene oxide.153 Figure 5.55 shows morphology of individual fillers and hybrid.153 Most of the oxidized multiwalled carbon nanotubes were attached to graphene oxide surfaces by hydrogen bonding.153 The roughness of the graphene oxide nanosheets was increased, which enhanced interface stability between graphene oxide nanosheets and the phenolic resin matrix by mechanical interlocking.153 But, the functionality of graphene oxide was also sheltered by interaction with multiwalled carbon nanotubes, leading to a worse distribution of graphene oxide and weakened interfacial bonding.153 To sum up, the performance of the interfacial stability was decided by the competition of such negative and positive effects introduced by the multiwalled carbon nanotubes.153 This implies the Figure 5.55. TEM images of graphene oxide (a), MWCNTs (b), importance of proper proportion of and their hybrid (c). [Adapted, by permission, from Ren, G; Zhang, Z; Song, Y; Li, X; Yan, J; Wang, Y; Zhu, X, Compos. Sci. Technol., multiwalled carbon nanotubes and graphene oxide.153 146, 155-60, 2017.] The polymer composites with much improved dielectric constant and ultra-low dielectric loss can be achieved using hybrid filler and controlling the dispersion of the conductive filler in the polymer matrix.154 The graphene oxide was immobilized on the surface of large-sized insulating hexagonal boron nitride via electrostatic self-assembly and Figure 5.56. Schematic representation of morphology and distribu- subjected to reduction of gration of hybrid filler (left) and mixture of components (right) in epoxy matrix. [Adapted, by permission, from Wu, K; Lei, C; Yang, phene oxide using 4, 4-diamino W; Chai, S; Chen, F; Fu, Q, Compos. Sci. Technol., 134, 191-200, diphenyl methane.154 Since the 2016. reduced graphene oxide sheets were fixed on the surface of boron nitride, they were well separated from each other even at high loading.154 Figure 5.56 shows the difference in distribution of hybrid filler and mixture of its components. The low dielectric loss observed in the composite was ascribed to both embedded insulating network of boron nitride which inhibited the mobility of charge carrier and to well-separated reduced graphene oxide sheets via immobilization.154 In addition to improved dielectric properties, the nanocomposites exhibited a good thermal conductivity.154

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Mokhireva, KA; Svistkov, AL; Solod'ko, VN; Komar, LA; Stöckelhuber, KW, Polym. Test., 59, 46-54, 2017. Zhang, Y; Rhee, KY; Park, S-J, Composites Part B: Eng., 114, 111-20, 2017. Myllymaki, V; Syren, J, US9085723, Carbodeon Ltd Oy, Jul. 21, 2015. Tolchinsky, GP, US20150203651, PGT International LLC, Jul. 23, 2015. Findley, DG, US9302945, Lockheed Martin Corporation, Apr. 5, 2016. Theilmann, P; Yun, D-J; Asbeck, P; Park, S-H, Organic Electronics, 14, 6, 1531-7, 2013. Nilsson, F; Krückel, J; Schubert, DW; Chen, F; Unge, M; Gedde, MU; Hedenqvist, MS, Compos. Sci. Technol., 132, 16-23, 2016. Vallittu, P; Matinlinna, J, Chapter 2. Types of FRCs used in dentistry. A Clinical Guide to Fibre Reinforced Composites (FRCs) in Dentistry. Woodhead Publishing, 2017, 1pp. 1-34. Lin, Y; Tyler, R; Sun, H; Shi, K; Schiraldi, DA, Polymer, 127, 236-40, 2017. Zhang, S; Zhu, S; Han, K; Feng, X; Ma, Y; Yu, M; Reiter, G, Polymer, 54, 21, 6019-25, 2013. Wu, G-m; Liu, G-f; Chen, J; Kong, Z-w, Prog. Org. Coat., 106, 170-6, 2017. Edenharter, A; Feicht, P; Diar-Bakerly, B; Beyer, G; Breu, J, Polymer, 91, 41-9, 2016. Choi, W; Choi, K; Yang, G; Kim, JC; Yu, C, Polym. Test., 53, 143-8, 2016. Huang, J; Cui Mao, C; Yutian Zhu, Y; Wei Jiang, W; Xiaodong Yang, X, Carbon, 73, 267-74, 2014. Pu, N-W; Peng, Y-Y; Wang, P-C; Chen, C-Y; Shi, J-N; Liu, Y-M; Ger, M-D; Chang, C-L, Carbon, 67, 449-56, 2014. Lewis, JA; Compton, BG; Raney, JR; Ober, TJ, WO2015120429, President and Fellows of Harvard College, Aug. 13, 2015. Kropp, MA; Thompson, ZJ, WO2013126377, 3M Innovative Properties Company, Aug. 29, 2013. Raman, C; Xiang, B; Murugaiah, A, US8946333, Momentive Performance Materials Inc., Feb. 3, 2015. Shumaker, JL; Slipher, GA; Mrozek, RA, US20150129276, U.S. Army Research Laboratory, May 14, 2015. Yoshitake, S; Suzuki, T; Miyashita, Y; Aoki, D; Teramoto, Y; Nishio, Y, Carbohydrate Polym., 98, 1, 331-8, 2013. Costantino, U; Leroux, F; Nocchetti, M; Mousty, C, Chapter 6 - LDH in Physical, Chemical, Biochemical, and Life Sciences. Developments in Clay Science. Elsevier, 2013, pp. 765-91. Gorrasi, G; Bugatti, V, LWT - Food Sci. Technol., 69, 139-48, 2016. He, X; Li, T; Shi, Z; Wang, X; Xue, F; Wu, Z; Chen, Q, Polym. Deg. Stab., 133, 219-26, 2016. Yu, D; Wen, S; Yang, J; Wang, J; Chen, Y; Luo, J; Wu, Y, Surf. Coat. Technol., 326A, 207-15, 2017. Totaro, G; Sisti, L; Celli, A; Askanian, H; Hennous, M; Verney, V; Leroux, F, Eur. Polym. J., 94, 20-32, 2017. Di Vona, ML; Casciola, M; Donnadio, A; Nocchetti, M; Pasquini, L; Narducci, R; Knauth, P, Intl. J. Hydrogen Ener., 42, 5, 3197-3205, 2017. Mallakpour, S; Khadem, E, Chapter 9 - Opportunities and challenges in the use of layered double hydroxide to produce hybrid polymer composites. Hybrid Polymer Composite Materials. Woodhead Publishing, 2017, pp. 235-61. Leng, J; Purohit, PJ; Kang, N; Wang, D-Y; Falkenhagen, J; Emmerling, F; Thünemann, AF; Schönhals, A, Eur. Polym. J., 68, 338-54, 2015. Leng, J; Kang, N; Wang, D-Y; Wurm, A; Schick, C; Schönhals, A, Polymer, 108, 257-64, 2017. Hennous, M; Derriche, Z; Privas, E; Navard, P; Verney, V; Leroux, F, Appl. Clay Sci., 71, 42-8, 2013. Youssef, AM; Bujdosó, T; Hornok, V; Papp, S; Hakim, AFA; Dékány, I, Appl. Clay Sci., 77-78, 46-51, 2013. Nicotera, I; Angjeli, K; Coppola, L; Enotiadis,A; Pedicini, R; Carbone, A; Gournis, D, Solid State Ionics, 276, 40-46, 2015. Du, M; Ye, W; Fu, H; Lv, W; Zheng, Q, Compos. Sci. Technol., 105, 28-36, 2014. Flenniken, C; Lee, CD, US20150064465, Equistar Chemicals, Lp, Mar. 5, 2015. Gallastegui, AG; Shaffer, M; Alyoubi, AO; Basahel, S, WO2013132259, Bio Nano Consulting, King Abdulaziz University, Sep. 12, 2013. Cozzi, D; de Bonis, C; D'Epifanio, A; Mecheri, B; Tavares, AC; Licoccia, S, J. Power Sources, 248, 1127-32, 2014. Anjum, MW; Bueken, B; De Vos, D; Vankelecom, IFJ, J. Membrane Sci., 502, 21-8, 2016. Mondal, T; Basak, S; Bhowmick, AK, Polymer, 127, 106-18, 2017. Ahmed, MA; Kandil, UF; Shaker, NO; Hashem, AI, J. Rad. Res. Appl. Sci., 8, 549-61, 2015. Herzog, K; Vana, P; Mueller, L; Springer, B; Recker, C, US20130261272, Continental Reifen Deutschland Gmbh, Oct. 3, 2013. Hara, M; Hermans, S; Seppinen, A, EP2873687, GVK Coating Technology Oy, May 20, 2015. Nisar, M; Pérez Bergmann, C; Geshev, J; Quijada, R; Barrera Galland, G, Polymer, 118, 68-74, 2017.

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132 Hussainova, I; Drozdova, M; Pérez-Coll, D; Rubio-Marcos, F; Jasiuk, I; Soares, JANT; Rodríguez, MA, J. Eur. Ceramic Soc., 37, 2, 3713-9, 2017. 133 Li, Y; Shi, Y; Cai, F; Xue, J; Chen, F; Fu, Q, Compos. Part A: Appl. Sci. Manuf., 78, 318-26, 2015. 134 Chinya, I; Pal, A; Sen, S, J. Alloys Compounds, 722, 829-38, 2017. 135 Wei, X; Xing, R; Zhang, B; Zhang, Q; Bulin, C, Ceramics Intl., 41, Supl. 1, S492-7, 2015. 136 He, Y; Zhang, C; Wu, F; Xu, Z, Synth. Metals, 212, 186-94, 2016. 137 Liu, L; Lv, F; Zhang, Y; Li, P; Tong, W; Ding, L; Zhang, G, Compos. Part A: Appl. Sci. Manuf., 99, 41-7, 2017. 138 Giro-Paloma, J; Rayón, E; Roa, JJ; Martínez, M; Fernández, AI, Eur. Polym. J., 63, 29-36, 2015. 139 Genari, B; Leitune, VCB; Jornada, DS; Camassola, M; Arthur, RA; Pohlmann, AR; Stanisçuaski Guterres, S; Collares, FM; Werner Samuel, SM, Dental Mater., 33, 6, 735-42, 2017. 140 Fan, M; Ma, Y; Tan, H; Jia, Y; Zou, S; Guo, S; Zhao, M; Huang, H; Ling, Z; Chen, Y; Hu, X, Mater. Sci. Eng.: C, 71, 67-74, 2017. 141 Alqahtani, FK; Khan, MI; Ghataora, G, US8921463, King Saud University, Dec. 30, 2014. 142 Huang, J; Jang, HD; Luo, J, US20130344392, Northwestern University, Dec. 26, 2013. 143 Kim, SH, WO2014038851, Hanbat National University Industry-Academic Cooperation Foundation, Mar. 13, 2014. 144 Han, J; Wang, WJ, US20150197678, Honeywell International Inc., Jul. 16, 2015. 145 Saleh, SSM; Akil, HM; Kudus, MHA, Fillers and Reinforcements for Advanced Nanocomposites, Elsevier, 2015, pp. 81-98. 146 Hu, D; Zhong, B; Jia, Z; Lin, J; Liu, M; Luo, Y; Jia, D, Mater. Lett., 188, 327-30, 2017. 147 Gorrasi, G; Milone, C; Piperopoulos, E; Pantani, R, Composites Part B: Eng., 81, 44-52, 2015. 148 Zakaria, MR; Akil, HM; Kudus, MHA; Kadarman, AH, Compos. Struct., 132, 50-64, 2015. 149 Yu, P; He, H; Jia, Y; Tian, S; Chen, J; Jia, D; Luo, Y, Polym. Test., 54, 176-85, 2016. 150 Al-Ghamdi, AA; Al-Hartomy, OA; Al-Solamy, FR; Dishovsky, NT; Atanasov, NT; Atanasova, GL, AEU - Intl. J. Electr. Commun., 72, 184-91, 2017. 151 Zhu, BL; Wang, J; Zheng, H; Ma, J; Wu, J; Wu, R, Compos. Part B: Eng., 69, 496-506, 2016. 152 Vijay, VR; Anitha, AM; Menon, ARR, Polymer, 89, 135-42, 2016. 153 Ren, G; Zhang, Z; Song, Y; Li, X; Yan, J; Wang,Y; Zhu, X, Compos. Sci. Technol., 146, 155-60, 2017. 154 Wu, K; Lei, C; Yang, W; Chai, S; Chen, F; Fu, Q, Compos. Sci. Technol., 134, 191-200, 2016.

6

Functional fillers - Applications 6.1 LUBRICANT Fillers usually have more than one role in their applications. One of such roles may be lubrication. Boron nitride can be used as a lubricant, insulating thermoconductive filler, UV-light emitter and can capture large amounts of hydrocarbons and gaseous molecules, provided that it has a porous structure.1 Hexagonal boron nitride was used in metalworking fluids.2 The addition of 0.5 wt% filler gave the best performance measured by tapping torque and thrust force.2 Polytetrafluroethylene and hexagonal boron nitride had synergistic effect enhancing tribo-performance of oils.3 Both additives have antifriction and antiwear properties.3 PTFE is also used as an extreme-pressure additive.3 Graphite, PTFE, hexagonal boron nitride, mica, boric acid, and potassium titanate were studied as solid lubricants in polyaryletherketone composites.4 The tribo-performance was evaluated in adhesive wear mode on tribometer.4 Composites exhibited very low specific wear rates of 2-8×10-16 m3/Nm and very low coefficient of friction of 0.040.08, which decreased with load and velocity.4 The boron nitride performed well with exception of severe operating conditions. Potassium titanate was the best performer.4 The balanced and more powerful propellants lead to an accelerated gun barrel erosion and markedly shortened useful barrel life.5 Boron nitride has a potential to reduce gun wear effects in advanced propellants.5 Samples fired with boron nitride had a boron oxide surface coating of flat platelets and seemed to lack of significant quantities of iron oxide (less than 10%).5 Samples fired without boron nitride were covered with pits, bumps, and octahedral crystals indicative of Fe3O4.5 The polyester powder coatings were filled with graphite or hexagonal boron nitride.6 Graphite had a higher antiwear ability as compared to hexagonal boron nitride but the best friction and scratch resistance was observed with 5 wt% of hexagonal boron nitride.6 Hexagonal boron nitride is a promising “green” lubrication additive for water.7 The sheets consisting of 300 nm wide and sub-30 nm thick flakes were synthesized without surfactants.7 The nano-sheets remained stably dispersed in water for 30 days.7 Even small amounts of hexagonal boron nitride nano-sheets could enhance wear resistance and reduce friction coefficient.7 Polymer gears are generally operated without lubrication, however, high running temperatures in medium to high power transmission applications shorten operating life of these gears.8 Molybdenum disulfide, graphite flake, boron nitride, and PTFE were used as lubricating coatings on unreinforced polyetheretherketone.8 PTFE provided the greatest

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Figure 6.1. Scaled schematic diagram of boron nitride and canola oil particulate mixtures with size: (a) 70 nm, (b) 0.5 µm, (c) 1.5 µm, and (d) 5.0 µm at the tribo-interface. [Adapted, by permission, from Reeves, CJ; Menezes, PL; Lovell, MR; Jen, T-C, Tribol. Intl., 88, 40-55, 2015.]

reduction in frictional forces and failure mechanisms were delamination of the coatings and abrasive wear.8 For rough surfaces, the tribological performance increases when larger particles are used in the lubricants.9 For smooth surfaces, the tribological performance increases when smaller particles are incorporated.9 The combined effect of roughness and particle size is attributed to the load carrying ability of particles, coalescence of particles in the asperity valleys, and thin particulate transfer film that occurs at the interface.9 Figure 6.1 explains some reasons for this behavior.9 The smaller spherical-shaped particles (0.07 µm) have the ability to coalesce in the asperity valleys due to their small size and create thin lubricious transfer films which lower friction and wear as illustrated in Figure 6.1a.9 The larger, more plate-shaped particles minimize the friction and wear caused by rough surfaces by supporting more contact load in the tribo-interface as depicted in Figure 6.1d.9 Figures 6.1b

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Figure 6.2. The spray drying synthesis of boron nitride precursors. [Adapted, by permission, from Han, W; Ma, Z; Liu, S; Ge, C; Wang, L; Zhang, X, Ceramics Intl., 43, 13, 10192-10200, 2017.]

and 6.1c represent the particle sizes that are decreasing in sphericity and transitioning between the two competing tribological phenomena.9 Highly-dispersible boron nitride nanoparticles were obtained by spray drying and pyrolysis synthesis.10 The boron nitride nanoparticles have a small Figure 6.3. Pristine (c) and surface modified (d) boron nitride. size of 20 nm, high specific surface [Adapted, by permission, from Xie, B-H; Huang, X; Zhang, G-J, area (145 m2 g-1), and large pore Ceramics Intl., 39, 7, 8543-8, 2013.] volume (0.41 cm3 g-1).10 The particles give outstanding antifriction properties at the content of 1.5 wt%.10 Figure 6.2 shows spray drying of boric acid and urea which were selected as reactants.10 An ultra-flexible hexagonal boron nitride/polymethylmethacrylate composite having an extra-large elongation at rupture was prepared by introducing micron-sized surfacemodified hexagonal boron nitride platelets into a PMMA matrix.11 At 32 wt% loading, the composite can be folded freely and has an elongation at rupture of 68%, which is over 30 times larger than that of PMMA.11 This results from the lubricant nature and the sliding properties of hexagonal boron nitride platelets.11 Figure 6.3 shows the morphology of pristine and modified boron nitride.11 Before surface modification, the boron nitride shows perfect platelet morphology.11 The edges of the platelets are smooth and intact (Figure 6.3c).11 After surface modification, the edges of many platelets show layered structures (Figure 6.3d). The stacked layers slide to some extent against each other.11 PTFE nanoparticles gave increasingly improved performance up to 3 wt% concentration in lubricating oils.12 Above this concentration, the effect deteriorated which was explained by the formation of agglomerates which had a size larger than the gaps between the matching surfaces.12

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γ-radiation exposure significantly improves the tribological properties of graphite/ PTFE dry lubricating system due to a decrease in coefficient of friction and wear properties.13 γ-radiation can be used to fluorinate graphite in the presence of fluoropolymer such as PTFE, resulting in improved lubrication properties.13 The specific wear rates of composites containing hydrogen plasma treated PTFE and nitrogen plasma treated PTFE were reduced by 33% (0.7×10-6 mm3/N m) and 50% (0.6×10-6 mm3/N m), respectively, as compared to pristine PTFE-α-olefin PA66 composites (1.1×10-6 mm3/N m); while showing the similar coefficient of friction values.14 Organosilane modification of graphitic nanoplatelets improved by 97% wear resistance of high-density polyethylene composite.15 The organosilane-modification minimized the influence of sliding velocity on wear resistance of the composite maintaining excellent wear resistance in a broad range of sliding velocities up to 2.0 m s-1.15 The nano-scale frictional properties of multi-layered graphene films can be used as solid lubricants to achieve superlow friction and wear in processing of micro-electromechanical systems.16 The frictional force of a high-purity multilayered graphene (7–9 layers) is significantly lower than that of highly ordered pyrolytic graphite.16 The quality and purity of multilayered graphene plays an important role in reducing lateral forces.16 The oxidation of graphene results in dramatically increased friction values.16 Graphite-filled epoxy resin has improved stiffness when high content is added.17 With at least 30 wt% graphite, the wear performance is improved.17 Graphene was modified with molybdenum disulfide using click chemistry resulting in reduction of friction coefficient.18 Potassium titanate is used as a filler in a good quality non-asbestos organic friction materials.19 It is known to reduce friction-fluctuations, fade and wear.19 Nano-potassium titanate particles significantly improved friction coefficient and wear resistance compared to micro-potassium titanate.19 A synthetic mica was intercalated with organic cation.20 A d-spacing of a synthetic mica increased due to intercalation reaction.20 An intercalated synthetic mica shows lubricity equivalent to molybdenum sulfide.20

6.2 ANTI-CORROSION The anti-corrosion properties of epoxy composite coatings were improved by addition of functionalized fullerene C60 and graphene.21 Fullerene C60 has the shape of an icosahedron.21 It is built out of carbon atoms located at the nodes of 20 hexagons and 12 pentagons arranged in a cage lattice (diameter 0.7 nm) defined by alternating single and double bonds.21 The nanofillers strongly self-associate into ropes and other structures that are extremely difficult to disperse in polymers, especially graphene which forms irreversible agglomerates due to π−π stacking and van der Waals interactions.21 The functional groups have been grafted on the surface of fullerene and graphene using 3-aminopropyltriethoxysilane.21 Figure 6.4 shows that the tortuosity of pathway prevents diffusion of corrosive substances.21 The significance of surface grafted groups is not restricted to the improvements in dispersion but also reduces porosity of coating and improves adhesion to steel.21 The anti-corrosion properties of graphene/EP coatings are superior to FC60/EP coatings because of the higher surface area of graphene which makes the diffusion path of permeating corrosive solutions more tortuous.21 Also, excellent electrical conductivity of gra-

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Figure 6.4. Performance of epoxy composite coatings with appropriate content of fullerene (a) and graphene (b) during corrosion process. [Adapted, by permission, from Liu, D; Zhao, W; Liu, S; Cen, Q; Xue, Q, Surf. Coat. Technol., 286, 354-64, 2016.]

Figure 6.5. Multiwalled carbon nanotubes decorated with titanium dioxide nanoparticles. [Adapted, by permission, from Kumar, A; Kumar, K; Ghosh, PK; Yadav, KL, Ultrasonics Sonochemistry, 41, 37-46, 2018.]

phene causes that the electrons are not able to reach a cathodic site.21 There is a limit of filler concentration which is at 0.5 wt%, above which anti-corrosive performance is not improved − most likely because of the aggregation of nanofillers which causes formation of nanocracks assisting diffusion of corrosive substances.21

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Graphite, graphene, hybrid filler containing carbon nanotubes were used to improve the electrical conductivity and anti-corrosion properties of polyurethane coatings.22 At the same filler loading, the electrical conductivity of hybrid filler system was significantly higher than that of the single filler system (0.77 S/m at 5 wt% while single filler system was not conductive).22 Hybrid filler system had the best electrical conductivity and acceptable anticorrosion capacity.22 Multiwalled carbon nanotubes were decorated with TiO2 nanoparticles to form a new hybrid structure of filler which was then used in the epoxy composite.23 The blend of both fillers was sonicated in acetone followed by magnetic stirring and drying in vacuum oven.23 The hybrid filler/ epoxy nanocomposite exhibited superior anti-corrosion and mechanical performance as compared with the nanocomposite produced by loading of only MWCNTs, TiO2 nanoparticles, or neat epoxy.23 The composite coating reduced corrosion rate on mild steel to 0.87×10-3 from 16.81 miliinches per year.23 Titanium and its alloys are wildly and successfully used in producing implants for their good mechanical properties, bioactivity, and corrosion resistance.24 To achieve good bioactivity and anti-corrosion properties, the surface of titanium often needs modifications, such as an alkali treatment, anodic oxidation of TiO2 and coatings.24 Graphene oxide and cross-linked gelatin were used in hydroxyapatite coatings preventing corrosion of titanium.24 The coating acted as a barrier that prevented the electrolyte from reaching the metal surface.24 These coatings had better bond strength and Figure 6.6. (a) Crack formation in primer, (b) a 3D corrosion resistance than hydroxyapatite reconstruction of a section of the specimen. [Adapted, 24 by permission, from Trueman, A; Knight, S; Colwell, J; coatings. Hashimoto, T; Carr, J; Skeldon, P; Thompson, G, Graphene can accelerate metal corroCorrosion Sci., 75, 376-85, 2013.] sion because of its thermodynamic stability

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and high conductivity.25 A few-layer fluorographene was prepared by a liquid-phase exfoliation method.25 Fluorographene was incorporated into poly(vinyl butyral) coatings to enhance its corrosion protection performances.25 The coating had enhanced barrier property preventing the penetration of aggressive species.25 Unlike graphene, fluorographene cannot promote metal corrosion. Because of its insulating nature, it impedes the formation of metal-filler galvanic corrosion cells.25 The effects of carbon nanofillers morphology (namely carbon black, multiwall carbon nanotubes, and graphene) on the anticorrosive and physicomechanical properties of hyperbranched alkyd resin-based coatings were studied.26 Graphene filler gave the best corrosion resistance.26 3D tomography by automated in situ block face ultramicrotome imaging using an field emission gun-environmental scanning electron microscope was used to study complex corrosion protective paint coatings.27 The method permits 3D observation of paint microstructure, crack formation in coating, morphology and distribution of paint additives, and corrosion inhibitor depletion.27 For the photo-aged and damaged paint sample, a crack was evident that passed through the primer approximately parallel to the substrate surface (Figure 6.6a).27 There was a sharp microcracking (less than 1 µm wide) at the crack-tip within the epoxy matrix.27 The crack was guided along the silica/epoxy interface. Some silica particles were cracked the entire way through.27 The image in Figure 6.6b shows movement of some of the material around the crack, which was evident from the curved particles which should be straight if no movement occurred.27 To entrap a corrosion inhibitor agent into a host matrix and avoid its possible weakening/plasticizing toward an organic coating Figure 6.7. Mechanisms of corrosion prevention of aluminum alloy and enable its progressive release by incorporation of ethylenediaminetetraacetic acid and layered double hydroxide. [Adapted, by permission, from Stimpfling, T; under stimuli, the layered double Leroux, F; Hintze-Bruening, H, Appl. Clay Sci., 83-84, 32-41, hydroxide framework was 2013.] selected.28 The layered double hydroxide reservoirs loaded with ethylenediaminetetraacetic acid as well as with chromate, carbonate and chloride anions were dispersed into the epoxy primer coating.28 A deleterious effect of ethylenediaminetetraacetic acid anions was observed when it was free in solution while a prevention of corrosion phenomenon was observed when the same anion was intercalated into layered double hydroxide nanoreservoir (Figure 6.7).28 Such behavior could be attributed to the buffering effect occurring for a large range of pH values thus preventing the copper replating.28 The possible corrosion mechanisms involves diadochy, buffering, and possible complexing reaction against electrolyte salt concentration versus exposure time.28 An anticorrosive pigment is incorporated in the topcoat of an anticorrosion coating system which greatly reduces the corrosion rate of the substrate metal in the environments of aggressive ions.29 The inorganic cation exchange pigment is selected from the group

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consisting of a metal ion-exchanged silica, metal ion exchanged alumina, synthesized zeolites, natural zeolites, and natural cation exchangers.29 The coating composition for protecting iron and steel structures contains particulate zinc, conductive pigments, and hollow glass microspheres.30 A conductive pigment is selected from the group consisting of graphite, carbon black, aluminium pigments, black iron oxide, antimony-doped tin oxide, mica coated with antimony-doped tin oxide, carbon nanotubes, and carbon fibers.30 Zinc acts as a sacrificial anodic material and protects the steel substrate, which becomes the cathode.30 Addition of microspheres and conductive pigments reduces microcracking.30 A coating comprising functionalized graphene and polymer protects roll steel, galvanized roll steel, equipment, automobiles, ships, construction and marine structures from corrosion, fouling and UV deterioration.31 The functionalized graphene has 1-10 sheets.31 The functionalized graphene contains a chemical group selected from amino, cyano, carboxylic acid, hydroxyl, isocyanate, aldehyde, epoxide, urea, or anhydride.31 The suitable resin is a phenolic resin, a polyester resin, a polyurethane, or an epoxy resin.31

6.3 MEMBRANES Sulfonated polysulfone/zirconium hydrogen phosphate composite membranes have been studied for application in direct methanol fuel cell technology.32 Composite membranes with sulfonation degrees of 35 and 42% had excellent proton conductivity, water uptake, thermal resistance, oxidative stability, and methanol suppression.32 The maximum power density obtained for membrane with 42% sulfonation degree (119 mW cm-2) was 13% higher than that obtained for Nafion® 115 (105 mW cm-2).32 Filled elastomeric membranes were used for separation of toluene-methanol mixtures by pervaporation.33 The hybrid membranes were made by incorporating filler, such as carbon black, clay and zeolite, in natural rubbers.33 The carbon black/natural rubber membrane was the most efficient in respect to toluene flux.33 Nanocomposite membranes were produced from iso-oriented graphite nanoplatelets (5 wt%) in polyethylene matrix.34 Inclusion of filler reduced the permeability of gases by factor of 2.34 The reduced permeability could not be explained by a tortuous migration path in the polymeric matrix but was instead attributed to the formation of rigidified polymer layers around the filler particles.34 Halloysite nanotubes were used as fillers for mixed matrix To membranes synthesis.35 improve the filler dispersion and filler-matrix interface affinity, the raw halloysite were modified by either alkali etching or 3-aminopropyltriethoxysilane grafting 35 As a result of sur(Figure 6.8). Figure 6.8. Schematic diagram of surface modification process to face etching, the defect holes were obtain surface etched or silane (APTES) grafted halloysite. [Adapted, by permission, from Ge, L; Lin, R; Wang, L; Rufford, formed on the surfaces of etchedTE; Villacorta, B; Liu, S; Liu, LX; Zhu, Z, Separation Purification halloysite, forming a rougher halTechnol., 173, 63-71, 2017.]

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Figure 6.9. Comparison of structure of graphene oxide and its modification. [Adapted, by permission, from Filice, S; D’Angelo, D; Libertino, S; Nicotera, I; Kosma, V; Privitera, V; Scalese, S, Carbon, 82, 489-99, 2015.]

loysite walls, increased surface area, and CO2 adsorption capacity.35 Surface etching significantly improved the halloysite/polymer interfacial affinity (0.06% voids in membrane containing raw halloysite and 0.02% in membrane containing etched halloysite).35 The surface etching improved interfacial morphology and membrane separation performance.35 Nanocomposite membranes were produced by dispersion of a few layer graphene platelets in epoxy matrix.36 Gas permeability decreased with increased filler content.36 The increased filler content does not change the free volume structure but the fractional free volume decreases.36 The gas permeability of the membrane decreased as a consequence of formation of a rigidified polymer regions surrounding the filler.36 Porous poly(vinylidene fluoride-co-trifluoroethylene) based composite membranes were filled with clay (montmorillonite), zeolites (Y zeolite), ceramics (barium titanate) and carbonaceous (multiwalled-carbon nanotubes) fillers to find the best performance as a separator membranes in lithium-ion batteries.37 The overall electrochemical behavior of the separator membranes improves with the inclusion of fillers as compared to pure polymer, as demonstrated by the increase of the room temperature ionic conductivity.37 The montmorillonite-filled polymer gave the best performance.37 Microporous polymer membranes based on poly(vinylidene fluoride-co-trifluoroethylene) containing barium titanate were studied in respect of the effects of particles size and concentration.40 The average pore size increased from ~25 µm for the pristine polymer to 90 µm for the membrane with 32 wt% of BaTiO3.40 The porosity remained nearly constant at ~72%.40 The ionic conductivity increased with the addition of BaTiO3.40 The most suitable membrane for battery separator applications contained 16 wt% of filler having particles of 500 nm in diameter.40

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Figure 6.10. Structure of core-shell silica particles having acidic (S) and basic (P) polymer shells. [Adapted, by permission, from Ko, T; Kim, K; Kim, S-K; Lee, J-C, Polymer, 71, 70-81, 2015.]

Polyvinylpyrrolidone-free polyethersulfone ultrafiltration membrane was synthesized by incorporation of high amounts of inorganic nanoparticles into the polymeric matrix.38 Best results were obtained with a membrane containing 50 wt% titania nanoparticles.38 Hybrid Nafion membranes were prepared using anatase-type TiO2 nanoparticles, graphene oxide, and organo-modified graphene oxide (Figure 6.9) for use in water purification applications.39 The membrane containing organo-modified graphene oxide gave the best performance.39 The phenylsulfonic-functionalized nanometric titania was synthesized to be used as a filler in Nafion-based composite membranes for direct methanol fuel cell applications.43 The modified filler gave higher ion exchange capacity and proton conductivity values than non-modified titanium dioxide.43 The composite membrane containing 10 wt% filler increased crystallinity and had the highest conductivity of 0.11 S cm-1 at 140°C.43 Organic/inorganic composite membranes were prepared from sulfonated poly(arylene ether sulfone) and core-shell silica particles having acidic and basic polymer shells (Figure 6.10).41 Incorporation of S–Si and P–Si increased dimensional stability, mechanical strength, and proton conductivity of the membranes.41 P–Si was found to be more effective filler material in improving these properties by formation of well-connected hydrophilic channels having sulfonate/pyridinium structures formed around the silica particles through the acid-base interaction between the pyridine groups of filler shell and the sulfonic acid groups in polymer.41 Sulfonated, organosilane-functionalized graphene oxide synthesized through the grafting of graphene oxide with 3-mercaptopropyl trimethoxysilane and subsequent oxidation have been used as a filler in sulfonated polyetheretherketone membrane.42 The incorporation of filler increases the ion-exchange capacity, water uptake, and proton

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Figure 6.11. Gas molecules permeate via the interfacial voids between MWCNTs and polystyrene: (a) MWCNTs are aggregated and randomly dispersed; (b) MWCNTs are vertically aligned and uniformly dispersed. [Adapted, by permission, from Wu, B; Li, X; An, D; Zhao, S; Wang, Y, J. Membrane Sci., 462, 62-8, 2014.]

conductivity of the membrane.42 These membranes are attractive in applications as proton exchange membranes for direct methanol fuel cells.42 Multiwalled carbon nanotubes/polystyrene membranes were cast in the presence of an alternating electric field.44 Multiwalled carbon nanotubes in the electrocast membranes were vertically aligned and better dispersed.44 The electrocast membranes had improved gas permeability and selectivity (Figure 6.11).44 The gas permeability of both oxygen and nitrogen increased with the content of MWCNTs embedded in the membranes.44 The electrocast membranes exhibited higher oxygen permeabilities and improved selectivities of oxygen over nitrogen, as compared to the membranes cast without the assistance of electric field.44 Composite anion exchange membranes based on polysulfone grafted with 1,4-diazabicyclo[2.2.2]octane and surface functionalized TiO2 nanoparticles were prepared.45 Tri(hydroxymethyl)propane was used for hydrophilic and polymethylhydrosiloxane for hydrophobic surface functionalization.45 Microscopic observations showed a more homogeneous dispersion of hydrophobic TiO2 nanoparticles, which were also well embedded in the polymer, whereas agglomeration was evident for hydrophilic surface treatment.45 The ionic conductivity was higher for hydrophobic TiO2, probably due to more homogeneous filler distribution with lower agglomeration.45 Positron annihilation lifetime technique was applied to study the size and density of free volume in Nafion/TiO2-nanoparticles composite membrane.46 The proton-transporting ability was correlated with free volume of membrane.46 The composite membrane containing 5 wt% of TiO2 nanofillers exhibited good electrochemical performance under reduced humidity.46 It can be saturated with water at relative humidity of 50%, under which ionic clusters and proton transporting channels are formed, indicating that this membrane is effective for fuel cells operated at reduced humidification levels.46 The reverse osmosis membranes containing cationic (montmorillonite) and anionic (layered double hydroxide) clay nanosheets were fabricated on polysulfone support.47 Membranes filled with both clays showed increased hydrophilicity and improved desalination performance as well as improved antifouling performance towards protein, cationic surfactant, and natural organic matter foulants.47

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Figure 6.12. Cross-sectional overview of membrane M21 indicating two distinct regions of the membrane, top layer and bulk; b) schematic representation of membrane cross-section with distinct two microstructures c) detailed cross-sectional view on top layer; d) bottom surface of the membrane M21; e) top surface of membrane M21. [Adapted, by permission, from Burnat, D; Schlupp, M; Wichser, A; Lothenbach, B; Gorbar, M; Züttel, A; Vog, UF, J. Power Sources, 291, 163-72, 2015.]

Membranes containing mineral filler for high temperature alkaline electrolysis were developed by a phase inversion process with polysulfone as binder.48 Utility of wollastonite, forsterite, and barite was assessed by 8000 h-long leaching experiments.48 Barite released only 6.22x10-4 M of Ba ions into the electrolyte and was selected as promising filler material, due to its excellent stability.48 Developed membranes provided hydrogen purity of 99.83 at 200 mA cm-2, which was comparable to previously used chrysotile membranes and higher than the commercial state-of-the-art Zirfon 500utp membrane.48 Figure 6.12 shows details of membrane morphology.48 Considering porosity, two membrane regions can be identified: (I) a thin top layer and (II, IIa) a bulk part.48 The bulk part can again be separated into a region with finger-like pores (towards the top, IIa) and a region with more regular pores (towards the bottom (II).48 The top layer (I) has a thickness between 1 and 5 µm and it is polymer-rich with very low porosity, while the bulk part, which represents the largest volume of the membrane, features significantly higher porosity.48 The relatively dense, polymer-rich top layer is formed on the surface due to a direct exposure to the non-solvent during the coagulation stage.48

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Figure 6.13. SEM images of membrane cross-sections: a) pure PSF, b) PSF0/1 (TiO2/MWCNT), c) PSF 0.2/0.8, d) PSF 0.5/0.5, e) PSF 0.8/0.2 and f) PSF 0/1. [Adapted, by permission, from Esfahani, MR; Tyler, JL; Stretz, HA; Wells, MJM, Desalination, 372, 47-56, 2015.]

Polysulfone/nano-TiO2/multiwalled carbon nanotube ultrafiltration membranes with variable nanoparticle ratios (total filler content at 1 wt%) were fabricated by the phase inversion method.49 Addition of any of the two nanofillers resulted in the finger-like interconnected pores and increased numbers of pores in the surface layer of the membrane.49 Membranes having a greater amount of MWCNTs had an increased pore size, and therefore greater pure water flux (Figure 6.13).49 The proportion of the fillers permits tailoring the membrane morphology to the requirements.49 Membranes containing both fillers

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exhibited an optimal balance of performance and synergism in terms of increased flux combined with increased total organic carbon rejection.49 Composite membranes were prepared by incorporating different percentages of activated carbon (up to 40 wt%) in PVC matrix introduced as the selective sorbent fillers for toluene.50 The pervaporation toluene flux was seven times higher, whereas the toluene selectivity was only slightly decreased (the permeate enrichment decreased from 89 to 83 wt% at 74°C).50 A significant enhancement of hydrophilicity was reported for polysulfone membranes after modification with graphene oxide followed by surface imprinting of composite.51 The surface imprinting was employed to enhance membrane selectivity towards polycyclic aromatic hydrocarbons present in water.51 The composite imprinted membranes showed improvement in flux from 8.56 for unmodified polysulfone membrane to 15.3 LM-2 h-1 for the composite imprinted membrane.51 The salt rejection increased from 57.2±4.2 for polysulfone membrane to 76±4.5% for composite imprinted membrane.51 These results show that it is possible to improve the hydrophilicity of the membranes without affecting their performance.51 The graphene oxide sheets were functionalized with histidine molecules and used in sulfonated polyetheretherketone matrix to fabricate hybrid polymer electrolyte membranes for direct methanol fuel cells.52 The acidic −SO3H groups (proton donors) in matrix and basic imidazole groups (proton acceptors) in histidine molecules form acid-base pairs which transport protons synergistically, yielding efficient proton channels inside the hybrid membranes.52 The proton conductivity (at 100% RH) of hybrid membranes was increased by 30.2% as compared with the polyetheretherketone membrane.52 Figure 6.14 explains proton conduction mechanism.52 The proton interacts with −SO3H group through hydrogen bonding, transfers along a cluster of hydrogen-bonded H2O molecules to arrive at the basic imidazole group.52 Then the proton triggers Figure 6.14. Mechanism of proton conduction in the hybrid mem- the protonation of imidazole and branes. [Adapted, by permission, from Yin, Y; Wang, H; Cao, L; finally moves out of the imidazoLi, Z; Li, Z; Gang, M; Wang, C; Wu, H; Jiang, Z; Zhang, P, lium through hydrogen bonding Electrochim. Acta, 203, 178-88, 2016.] with another water molecule.52 In this manner, the protons are transferred continuously by acid-base pairs via Grotthus mechanism, in which the hydrogen-bonding network is formed and broken alternately.52 Some protons interact with free water molecules in the membrane to generate hydronium ions which further facilitate the diffusion of protons through continuous nanochannels via vehicular mechanism.52 Reclaimed tire rubber can serve as the precursor for preparation of gas separation membrane.53 It is a promising membrane material for CO2/N2 and O2/N2 separation.53 The

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Figure 6.15. The preparation process of aminated titania nanotubes. [Adapted, by permission, from Xin, Q; Gao, Y; Wu, X; Li, C; Liu, T; Shi, Y; Li, Y; Jiang, Z; Wu, H; Cao, X, J. Membrane Sci., 488, 13-29, 2015.]

Figure 6.16. Preparation of functionalized glass microspheres. [Adapted, by permission, from Ahn, K; Kim, M; Kim, K; Ju, H; Oh, I; Kim, J, J. Power Sources, 276, 309-19, 2015.]

gas separation performance depends on the reclaiming process.53 Reclaimed tire rubber obtained by a cryo-mechanical reclaiming process demonstrated greater gas separation performance than reclaimed tire rubber obtained by a mechanical-chemical reclaiming process.53 Abundant amine groups were introduced onto titania nanotubes by a facile method (Figure 6.15).54 The modified titania nanotubes were used for preparation of membrane for enhanced CO2 separation using sulfonated polyetheretherketone as matrix.54 Aminated titania nanotubes improved interface compatibility resulting in formation of high performance membrane.54 A crosslinked composition comprising polymer functionalized with silane and a silane coated filler is useful in preparing water resistant membranes for wet rooms.55 Polymer is a styrene-butadiene copolymer and filler is a surface treated wollastonite.55 Polymer and filler were functionalized with epoxy silane (e.g., 3-glycidoxypropylmethyldiethoxysilane or 3-glycidoxypropyltriethoxysilane).55 A low-methanol-permeability sulfonated poly(phenylene oxide) membranes with hollow glass microspheres were developed for direct methanol fuel cells.56 Figure 6.16 shows functionalization steps of glass microspheres.56 The sulfonated poly(phenylene oxide) having 38.2% sulfonation has the best properties for membrane application.56 The composite membranes exhibit proton conductivities ranging from 0.0350 to 0.0212 S cm-1 and low methanol permeability ranging from 1.02×10-6 to 3.4×10-7 cm2 s-1 at 20°C.56

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Figure 6.17. Schematic route of synthesis of hybrid filler. [Adapted, by permission, from Kumar, M; Gholamvand, Z; Morrissey, A; Nolan, K; Ulbricht, M; Lawler, J, J. Membrane Sci., 506, 38-49, 2016.]

A non-invasive technique based on confocal laser scanning microscopy that allows the visualization of penetrant diffusion permitted to show that the crosslink ratio and filler content affect the sorption performance of polymer films.57 Graphene oxide-TiO2 hybrid filler was synthesized by in situ sol-gel reaction at pH=2 using graphene oxide nanosheets suspension and titanium isopropoxide precursor.58 Figure 6.17 shows the structure of hybrid filler and its method of synthesis.58 Membranes were fabricated from polymer blend solutions containing polysulfone and hybrid filler.58 Membranes were efficient in the removal of humic acid from 10 ppm solution.58 Membrane containing 5 wt% hybrid filler had the best performance.58 Hollow silicalite spheres (1 µm diameter) covered with a shell (thickness 30 nm) of silicalite crystals were prepared for fabrication of pervaporation membrane using polydimethylsiloxane matrix.59 The micro- and mesoporous spherical shells had a BET surface area of over 800 m2/g.59 The zeolitic shell improves the ethanol selectivity through its specific pore structure and hydrophobicity.59

6.4 OSTEOCONDUCTIVE AND OTHER BONE TISSUE ENGINEERING FILLERS Osteogenesis is the process of development and formation of bone. Osteoconduction is the ability of bone-forming cells in the grafting area to move across a scaffold and slowly replace it with new bone over time. Osteoconductive materials serve as a scaffold onto which bone cells (osteoblasts and osteoclasts) can attach, migrate, grow and/or divide. The osteoblasts are cells with a single nucleus that synthesize bone which in the process of bone formation function in groups of connected cells. The osteoclast is a large multinucleate bone cell that absorbs bone tissue during growth and healing. Hydroxyapatite (Ca10(PO4)6(OH)2) is the major component of animal bones attracting increasing interest for use in bone grafting or scaffolding in bone tissue engineering.60 Synthetic structures for temporary replacement and promotion of bone healing consist of a biodegradable polymer and natural or synthetic hydroxyapatite.60 Poor adhesion between hydroxyapatite and polymeric matrix is a major limitation which requires modification of hydroxyapatite.60 The plasma polymerization technology modifies surface of filler particles by creating a film with functional groups at nanoscale thickness keeping unaltered their bulk properties.60 The surface functionalization was achieved by plasma polymeriza-

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Figure 6.18. PLLA electrospun fibers. a) dense fibers b) surface porous fibers. PLLA micro-fillers c) smooth micro-fillers d) surface porous micro-fillers. 10,000× magnification. [Adapted, by permission, from Castro, AGB; Polini, A; Azami, Z; Leeuwenburgh, SCG; Jansen, JA; Yang, F; van den Beucken, JJJP, J. Mech. Beh. Biomed. Mater., 71, 286-94, 2017.]

tion using ε-caprolactone and acrylic acid monomers.60 Modification of the hydroxyapatite surface with ε-caprolactone increased its hydrophobicity as compared with neat hydroxyapatite surfaces.60 Pullulan hydrogels were reinforced with nanocrystalline hydroxyapatite (5 wt%) and poly(3-hydroxybutyrate) fibers (3 wt%) containing nanocrystalline hydroxyapatite (3 wt%).61 With these fillers, pullulan hydrogel had improved compressive modulus of the scaffold by 10 fold.61 The hydrophilic nature of pullulan did not support adhesion and spreading of cells therefore porous composite scaffolds were modified using a double diffusion method to deposit hydroxyapatite on pore walls.61 Uniform coating of hydroxyapatite was rapidly formed throughout the three-dimensional scaffolds which rendered them osteoconductive and improved their compressive modulus.61 The temporal relationship between in situ generated calcium content (mineralization) and the mechanical properties of an injectable orthobiologic bone-filler material were studied.62 The injectable scaffolds provide an opportunity to introduce cells and other biological cues to a defect site to promote bone regeneration.62 In situ mineralization by encapsulated cells increased the mechanical properties of the injectable orthobiologic scaffold by several folds within three weeks.62

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Calcium phosphate cement is biocompatible, resorbable, injectable and osteoconductive which renders it suitable for bone repair and regeneration.63 At the same time, their brittle nature limits their application to non-load-bearing applications.63 Incorporation of long polymeric fibers can improve the mechanical properties of calcium phosphate cements but they aggregate.63 The short polymeric fillers can be easily dispersed in the cement matrix and poly(L-lactic) fibers and they were studied for this application.63 The fiber can be obtained with a variety of morphologies (Figure 6.18).63 These fibers were homogeneously dispersed throughout the cement but still did not result in reinforcement most likely because they require adhesion promoters.63 The monetite calcium phosphate cement is degradable, osteoconductive and it improves bone healing.64 The osteoclastic resorption of monetite cement was studied using primary mouse bone marrow macrophages.64 The monetite samples with differentiated osteoclasts had a 1.4 fold elevated calcium ion concentration compared to monetite samples with undifferentiated cells.64 This indicates active resorption of monetite in the presence of osteoclasts.64 Reticulated vitreous carbon foams are of interest in orthopedic applications due to their porous, honeycomb-like structure, biocompatibility, and bio-inert nature but they lack the strength required in orthopedic applications.65 A hydroxyapatite-poly(D,L-lactide-co-glycolide) infiltrated carbon foam was developed for orthopedic applications.65 This material system allows for robust osteoblast adhesion.65 The composite scaffold was not cytotoxic to cells and permitted the attachment and spreading of osteoblasts.65 It can be used in the repair of bone defects and as a bone fixation plates and screws.65 Magnesium-based bone graft substitute was studied for potential orthopedic applications.66 The mechanical properties of MgSr alloys were close to those of cortical bone, and the compressive strength could reach 300 MPa, suggesting its potential application in load-bearing bone as a bone defect filler.66 MgSr alloys exhibit good cytocompatibility and antibacterial properties.66 The three-dimensional composite scaffolds for bone tissue engineering can be fabricated by solvent casting, thermally induced phase separation, rapid prototyping, melt molding, gas foaming, and electrospinning, etc.67 A composite material used for scaffolds contains at least two different components: the “reinforcement” phase, which is incorporated into the second component known as a “matrix”.67 The reinforcement has a form of particles, whiskers, fibers, lamellae or a mesh and it is microscopically different. For structural integrity, the interface between the reinforcement and the matrix may frequently need reinforcement by the use of adhesion promoters.67 The most common inorganic materials include hydroxyapatite, other calcium phosphate phases, apatite-wollastonite glass-ceramics, and bioactive glasses (e.g., borate glasses).67 The main physical and mechanical properties to be considered include biocompatibility, porosity, pore size and pore interconnectivity, surface characteristics, osteoinductivity, mechanical properties matching native extracellular matrix, and bioresorbability.67 When the pulp tissue of a young tooth has been damaged, an advanced endodontic therapy must be initiated.68 A routine treatment requires removal of the diseased pulp tissue and replacement by calcium hydroxide paste or mineral trioxide aggregate cement to allow a thin hard tissue formation, a procedure known as apexification.68 A more efficient approach promotes in-growth of healthy tissue into the void space of a tooth created dur-

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ing the removal of damaged or diseased pulp tissue.68 Such an approach comprises a method of removing diseased pulp tissue from a tooth, disinfecting the tooth, and implanting a collagen matrix coupled with calcium phosphate mineral composite material into the tooth.68 The calcium mineral used in this procedure is based on carbonate apatite.68 A high strength synthetic bone for bone replacement increasing compressive strength and facilitating blood circulation contains calcium sulfate hemihydrate which penetrates into the pores of a porous inorganic material such as β-tricalcium phosphate.69 The calcium sulfate hemihydrate forms a hydrated crystal of calcium sulfate dihydrate to expand the volume in the pores, preventing the removal of a filler by a physical force.69 A method for producing bone grafts using 3D printing is employed using a 3D image of a graft location to produce a 3D model of the graft.70 This is printed using a 3D printer and an ink that produces a porous, biocompatible, biodegradable material that is conducive to osteoinduction.70 This is porous polymethylmethacrylate made osteoinductive by demineralized bone.70 The ink is provided as a precursor powder and liquid.70 The powder contains demineralized bone, sucrose crystals, and a polymerization initiator.70 The liquid contains methylmethacrylate.70 Optional compounds include antibiotics, radio-pacifiers, and compounds to increase biodegradability.70 Once mixed, the methylmethacrylate polymerizes to polymethylmethacrylate.70 Once the graft is placed, natural bone gradually replaces the graft.70 A biodegradable and biocompatible T-plate nanocomposite implant with stem cells for treating and repairing broken bones, damaged tissues and torn ligaments comprises a polymeric matrix based on poly(lactic glycolic acids), a bioceramic part comprising hydroxyapatite nanoparticles, and an endometrial stem cell.71 The casting of the poly(lactic glycolic acids) and hydroxyapatite nanocomposite are done in a mold to obtain a Tplate nanocomposite.71 A composite bone graft material can be made from biocompatible poly(D,L-lacticco-glycolic acid) and bioceramic particles exposed on its surface using a gas foaming particle leaching method and infused with collagen.72 The bioceramic material is selected from the group consisting of hydroxyapatite, tricalcium phosphate, bioglass, calcium phosphate or bone.72 A method for producing a porous structure of calcium polyphosphate, comprising steps of mixing monocalcium phosphate with silicic acid and sintering the mixture produced a porous calcium polyphosphate.73 This method can be used to obtain a porous biomaterial having an adjustable porosity, which can also activate the platelets of a plateletrich plasma and generate the release of platelet growth factors.73

6.5 SOFT TISSUE FILLERS The soft tissue fillers have been used to repair the contours especially in cosmetic surgeries.74 Hyaluronic acid is widely used as a soft tissue filler.74 Hyaluronic acid, a high molecular weight biopolysaccharide, is widely distributed in the intercellular matrix and extracellular matrix of the connective tissue of animals and human.74 Because of its rapid degradation and absorption by the host’s body, frequent refilling of hyaluronic acid is required.74 A porous structure of tricalcium phosphate ceramic particles can potentially be used to absorb the gel-like filler of hyaluronic acid and the fat cells to produce microchannels in the filler which permits the body fluid to circulate so the fat cells can sur-

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vive.74 The degradable ceramic particles are gradually absorbed and the growing fat cells will replace their positions to achieve the effect of long-term self-filling effect.74 It was found that tricalcium phosphate ceramic particles were biocompatible and the pore size between 100 and 300 μm was conducive for cell growth.74 Porous biodegradable ceramic particles with hyaluronic acid hydrogel was found useful as the carrier of the cell, and subcutaneous filling and shaping could be performed after mixing the composite material with the patient's own adipocytes.74 The review of hyaluronic acid applications, types, durability of treatment effect and other related aspects can be found in review papers.75,76 Calcium hydroxyapatite is a versatile semi-permanent filler with a high elastic modulus for composite lifting.77 It was first approved as an injectable implant by the US Food and Drug Administration as a soft tissue radiographic marker in 2001 before quickly expanding its indications to include vocal fold augmentation, repair of oromaxillofacial defects, and soft tissue augmentation for stress urinary incontinence.77 Later, commercial product Radiesse was approved as a filler for augmentation of nasolabial folds, facial lipoatrophy, and for hand rejuvenation.77 Radiesse is composed of nonimmunogenic synthetic bone with microspheres 25 to 45 µm in diameter within a 70% carboxymethylcellulose carrier gel.77 Several weeks after injection, the carrier gel is absorbed and net neutral volume replacement occurs through neocollagenesis.77 Subsequently, it degrades into calcium and phosphate ions over time and it is excreted slowly from the body, creating lasting volume for an average of 12 to 18 months.77 Calcium hydroxylapatite and poly(L-lactic acid), the so-called collagen stimulators, offer a unique and effective way to address this issue with natural-appearing results with long duration.78 According to the American Society of Plastic Surgeons, more than 1.7 million injections of soft tissue fillers were performed in the United States in 2014 alone.78 Most of these injections used hyaluronic acid-based products, however close to a quarter of a million of these injections were carried out over the year using calcium hydroxylapatite (Radiesse) and poly(L-lactic acid) (Sculptra, Galderma).78 The applications of facial fillers and cements formulated with hydroxyapatite require that it has a form of spheroidal particles, with size greater than 40 microns to reduce the in situ inflammation of connective tissue.79 The method of reprocessing the natural hydroxyapatite particles, obtained from fresh bovine bone, which is subjected to chemical and thermal treatments (pyrolysis in an oxidizing atmosphere at 900oC for 2 h), grinding and sieving (particles less than 200 µm) was developed.79 X-ray diffraction analysis revealed the partial decomposition of the hydroxyapatite to other calcium phosphates: α-Ca3(PO4)2 and Ca4(PO4)2O, essential phases of the formulation of bone cements, which accelerate the absorption of the biological particles in the biological medium, requirement important for facial fillers.79 Polydimethylsiloxane beads were prepared using a simple fluidic device and modified with polydopamine to improve the cell attachment.80 The polydopamine layer at the surface of the polydimethylsiloxane beads provided a favorable environment for cell culture.80 The polydopamine-modified PDMS beads can potentially be employed as filler materials for tissue engineering.80 Particle size, stiffness, and surface functionality are important determinants of safety and efficacy of injectable soft-tissue fillers.81 Suspension photopolymerization and semiinterpenetrating network were used to synthesize soft, functionalizable, spherical hydrogel

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microparticles of tunable size and stiffness.81 Microparticles were prepared from acrylated forms of polyethylene glycol, gelatin, and hyaluronic acid.81 The process parameters were varied to produce microparticles with size ranging from 115 to 515 µm.81 An overview of infections associated with soft tissue facial fillers, their prevention and treatment and surgeon’s guide to facial soft tissue filler injections are discussed elsewhere.82,83 The dermal fillers based on silk attached to hyaluronic acid by crosslinker (1,4butanediol diglycidyl ether) are considered useful in treatment of facial imperfections, facial defects, facial augmentations, breast imperfections, breast augmentations, or breast reconstructions.84

6.6 ANTIMICROBIAL The Databook of Preservatives includes a number of additives which are of inorganic or organic/inorganic origin, such as ammonium sulfate, boric acid, calcium hypochlorite, chlorine, chlorine dioxide, copper hydroxide, copper oxide, copper carbonate/copperhydroxide, cyclohexylhydroxydiazene 1-oxide, potassium salt, diboron trioxide, disodium octaborate tetrahydrate, disodium tetraborate, anhydrous, disodium tetraborate, decahydrate, disodium tetraborate, pentahydrate, granulated copper, hydrogen peroxide, metamsodium, potassium (E,E)-hexa-2,4-dienoate, potassium 2-biphenylate, potassium dimethyldithiocarbamate, pyrithione zinc, reaction mass of N,N-didecyl-N,N-dimethylammonium carbonate and N,N-didecyl-N,N-dimethylammonium bicarbonate, reaction mass of silicon dioxide and silver, reaction mass of titanium dioxide and silver chloride, reaction products of 5,5-dimethylhydantoin, 5-ethyl-5-methylhydantoin with bromine and chlorine, reaction products of 5,5-dimethylhydantoin, 5-ethyl-5-methylhydantoin with chlorine, silver, silver adsorbed on silicon dioxide (as a nanomaterial in the form of a stable aggregate with primary particles in the nanoscale), silver copper zeolite, silver nitrate, silver phosphate glass, silver sodium hydrogen zirconium phosphate, silver zeolite, silver zinc zeolite, sodium 2-biphenylate, sodium bromide, sodium dichloroisocyanurate dihydrate, sodium dimethyldithiocarbamate, sodium hypochlorite, sodium N-(hydroxymethyl)glycinate, sodium p-chloro-m-cresolate, sulfuryl difluoride, tetrakis(hydroxymethyl)phosphonium sulfate, and triclosene sodium.85 This list contains substances included in European Regulation No 528/2012 of the European Parliament and the Council which outlines the important aspects of the use of biocidal products and governs principles of introduction of new products to the market and in Annex V of this regulation which contains main groups and product types which belong to biocidal products.85 This long list shows that inorganic materials are commonly used as antimicrobial products, and the list only includes materials which are permitted for commercial use because they are effective and relatively less harmful to humans and environment. There are numerous products under current studies and these are discussed below. The polymeric films based on polyvinylalcohol, chitosan, and lignin nanoparticles were produced to test antibacterial activity.86 Antimicrobial assays revealed inhibition of the bacterial growth of Gram-negative Erwinia carotovora subsp. and Xanthomonas arboricola pv. pruni, suggesting innovative opportunities against bacterial plant/fruit pathogens in food packaging applications.86

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Bacterial colonization and biofilm formation on dental composites shorten the service life of dental restorative materials.87 The commercial barium borosilicate based glass powders, fillers widely used in dental composites, were covalently immobilized with silver sulfadiazine to provide an antimicrobial effect.87 The resultant glass powders had strong antimicrobial effects against Streptococcus mutans in the concentration of 2-10% of the resin.87 Complex mixture of microorganisms, toxins, allergens, volatile microbial organic compounds affect the health of people living or working in dwellings and buildings.88 A modified diatomaceous earth was studied for use as antimicrobial filler material.88 Zeta potential measurements revealed a bilayer arrangement of the ammonium groups on the functionalized filler and the bioassays showed their antimicrobial activity.88 The hydrophilic, fungicidal, and bactericidal properties of quaternary ammonium salts make them a good candidate to modify polydimethylsiloxane for use in biocidal surfaces.89 The quaternary ammonium salts compounds belong to the class of cationic disinfectants.89 An electrostatic interaction of quaternary ammonium salt molecules with the cell wall is the reason for their biocidal activity.89 The lipophilic moiety present in the ammonium salts destabilizes the cytoplasmic membrane, leading to the death of the bacterial cells.89 The polydimethylsiloxane-quaternary ammonium salts polymers are used for finishing of textiles, in cosmetics and personal care.89 The polydimethylsiloxane-quaternary ammonium salts materials with a longer hydrocarbon chain at nitrogen exhibit biocidal properties destroying bacteria, fungi, and algae.89 The biocidal activity against both Gram-positive and -negative bacteria showed that the coatings eliminated up to 99.9% of pathogenic bacteria on the surface.89 Self-decontamination occurred and there was no risk of leaching antimicrobial moieties because the quaternary ammonium salt sequences are incorporated into the backbone of the polymer.89 Gelatin-based nanocomposite films blended with organic fillers and nanometals, such as nanosilver, nanocopper, zinc oxide nanoparticles, and titanium dioxide nanoparticles exhibited strong antimicrobial activity against foodborne pathogenic microorganisms.90 The antimicrobial gelatin-based nanocomposite films have a high potential for application in the active food packaging industry.90 The macroporous Ag/TiO2 composite foams were produced via particle-stabilized Pickering emulsion for bactericidal and photocatalytic activities.91 The Ag/TiO2 particles consisted of Ag core surrounded by mesoporous TiO2 shell.91 The composite foams render a synergistic antimicrobial activity against Escherichia coli with the efficiency of 99% under dark conditions in 3 h.91 ZnO nanoneedles were synthesized employing a co-precipitation method, followed by preparation of white cement composites containing ZnO filler.92 The resultant cement composite showed photocatalytic degradation of pollutant (Rhodamine 6G) under ultraviolet irradiation, enhanced hydrophobic nature, and the antimicrobial properties.92 Antimicrobial studies were performed using bacterial strains Escherichia coli, Bacillus subtilis, and fungal strain Aspergillus niger.92 A significant improvement in bacterial and fungal degradation was observed in ZnO modified cement.92 TiO2-CuO were used as nanofillers for epoxy coatings to protect the surface of steel against corrosion and bacterial growth.93 The nanocomposite coatings had strong antimicrobial activity against Escherichia coli.93

6.6 Antimicrobial

175

Figure 6.19. (a, b) STEM and (c, d, e) TEM images of ZnO deposited on halloysite. [Adapted, by permission, from De Silva, RT; Pasbakhsh, P; Lee, SM; Kit, AY, Appl. Clay Sci., 111, 10-20, 2015.]

Graphene nanoplatelets (filler) and ciprofloxacin (biocide) were incorporated via melt-compounding into poly(lactic acid) to obtain biopolymer-based nanocomposite with antimicrobial properties.94 The incorporation of graphene nanoplatelets affected the release of ciprofloxacin without hindering the antimicrobial activity.94 The presence of graphene nanoplatelets reduced the burst release effect thus suggesting the potential ability of graphene nanoplatelets for controlled drug release applications.94

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ZnO nanoparticles were deposited on the outer and inner surfaces of halloysite nanotubes using a novel solvothermal method (Figure 6.19).95 The hybrid filler was incorporated into the poly(lactic acid) matrix.95 The hybrid filler performs well as an antimicrobial agent against bacteria such as Escherichia coli and Staphylococcus aureus.95 The bacteria count was reduced by more than 99%.95 Small quantities of Ag-TiO2 have a bactericidal effect on Streptococcus mutans under visible light conditions.96 Although reduced, the bactericidal effect was still observed when Ag-TiO2 was embedded in the polymer.96 Nanosilver/diatomite nanocomposites were developed by in situ reduction method.97 The nanosilver/diatomite nanocomposites demonstrated excellent antibacterial properties towards Gram-positive and Gram-negative bacteria (0.5 g of the nanosilver diatomite could kills >99.999% of Escherichia coli within half an hour).97 The nanosilver/diatomite nanocomposites have potential application in water purification industry.97 Polymethylmethacrylate-TiO2 nanocomposite material with improved antibacterial characteristics was used for manufacturing 3D printed dental prosthesis.98 TiO2 nanoparticles inhibited the growth of Candida scotti strain. Increasing quantity of nanotitania has resulted in the formation of aggregates instead of the homogenous dispersion of nanoparticles.98 A good dispersion of the TiO2 nanoparticles was obtained for 0.4 wt%, therefore it was used for stereolithographic denture prototyping.98 Rice straw of agricultural waste was utilized as a source material to synthesize wollastonite. The wollastonite was then copper doped.99 The size of the wollastonite particles increased from 900 to 1184 nm after dopîng.99 The growth inhibition was more significant against S. aureus than E. coli.99 Ammonium sulfate, potassium sulfate, magnesium sulfate, zinc sulfate and/or sodium sulfate, metal salts such as silver sulfate and/or copper sulfate are added to plastics with fillers or directly to fillers to make them antimicrobial and prevent odor formation.100 Bentonite particles were mixed with the hydroxyethyl cellulose matrix by mechanical dispersion and composite films were obtained.101 The hydroxyethyl cellulose/bentonite composite films had an antimicrobial effect against Escherichia coli, Staphylococcus aureus, and fungi.101 Nano hydroxyapatite doped with zinc (0.2 wt%), silver (0.25 wt%), or gold (0.025 wt%) has been freeze-dried and calcined at 650°C.102 Antibacterial activities of these composites were investigated against pathogenic species, such as Escherichia coli, Staphylococcus aureus, Staphylococcus spp., Bacillus cereus, and Candida albicans.102 Silver-containing composites are promising antimicrobial materials for coating of orthopedic and dental implants or used as bone cements in surgical applications.102 REFERENCES 1 2 3 4 5 6 7 8 9

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13-29, 2015. Hara, M; Hermans, S; Seppinen, A, EP2873687, GVK Coating Technology Oy, May 20, 2015. Ahn, K; Kim, M; Kim, K; Ju, H; Oh, I; Kim, J, J. Power Sources, 276, 309-19, 2015. Wong, JSP; Hu, M; Shi, D; Li, RKY; Wong, JSS, Polymer, 58, 67-75, 2015. Kumar, M; Gholamvand, Z; Morrissey, A; Nolan, K; Ulbricht, M; Lawler, J, J. Membrane Sci., 506, 38-49, 2016. Naik, PV; Kerkhofs, S; Martens, JA; Vankelecom, IFJ, J. Membrane Sci., 502, 48-56, 2016. Petisco-Ferrero, S; Sánchez-Ilárduya, MB; Díez, A; Martín, L Arrate, EM; Sarasua, JR, Appl. Surf. Sci., 386, 327-36, 2016. Arora, AA; Sharma, P; Katti, DS, Carbohydrate Polym., 123, 180-9, 2015. Bialorucki, C; Subramanian, G; Elsaadany, M; Yildirim-Ayan, E, J. Mech. Beh. Biomed. Mater., 38, 143-53, 2014. Castro, AGB; Polini, A; Azami, Z; Leeuwenburgh, SCG; Jansen, JA; Yang, F; van den Beucken, JJJP, J. Mech. Beh. Biomed. Mater., 71, 286-94, 2017. Montazerolghaem, M; Karlsson Ott, M; Engqvist, H; Melhus, H; Rasmusson, AJ, Mater. Sci. Eng. C, 52, 212-8, 2015. Rodriguez, DE; Guiza-Arguello, V; Ochoa, OO; Gharat, T; Sue, HJ; Lafdi, K; Hahn, MS, Carbon, 99, 106-14, 2016. Liu, C; Wan, P; Tan, LL; Wan, K; Yang, K, J. Orthopaedic Transl., 2, 139-48, 2014. Liverani, L; Roether, JA; Boccaccini, AR, Nanofiber composites in bone tissue engineering. Woodhead Publishing, 2014. Nevins, A, US20140272803, Sep. 18, 2014. Park, KJ; Park, SB; Shin, JO, WO2015129972, Ossein Co. Ltd., Sep. 3, 2015. Greyf, A, US20150054195, Feb. 26, 2015. Ai, J; Elyasifar, N; Azami, M; Bahrami, N; Tavakol, S, US20140356410, Dec. 4, 2014. Giorno, T, US20130218291, Aug. 22, 2013. Anitua, AE, WO2014091036, Biotechnology Institute, I Mas D, S.L., Jun. 1‘9, 2014. Yang, Y-C; Chen, C-C; Wu, W-C; Hsu, W-L; Tseng, S-C, Ceramics Intl., in press, 2017. Gutowski, KA, Clinics Plast. Surgery, 43, 3, 489-96, 2016. Greene, JJ; Sidle, DM, Facial Plast. Surgery Clinics North America, 23, 4, 423-32, 2015. Lee, JC; Lorenc, ZP, Clinics Plast. Surgery, 43, 3, 497-503, 2016. Breithaupt, A; Fitzgerald, R, Facial Plast. Surgery Clinics North America, 23, 4, 459-69, 2015. Martinez, C; La Gattina, G; Garrido, L; Gilabert, U; Ozols, A, Procedia Mater. Sci., 8, 319-23, 2015. Jun, D-R; Moon, S-K; Choi, S-W; Colloids Surf. B: Biointerfaces, 121, 395-9, 2014. Chan, KMC; Li, RH; Chapman, JW; Trac, EM; Kobler, JB; Zeitels, SM; Langer, R; Karajanagi, SS, Acta Biomater., 10, 6, 2563-73, 2014. Ferneini, EM; Daniel Beauvais, D; Steven I. Aronin, SI, J. Oral Maxillofacial Surgery, 75, 1, 160-6, 2017. Ferneini, EM; Hapelas, S; Watras, J; Ferneini, AM; Weyman, D; Fewins, J, J. Oral Maxillofacial Surgery, in press, 2017. Pavlovic, E; Serban, MA; Yu, X; Manesis, NJ, US20140315828, Allergan, Inc., Oct. 23, 2014. Wypych, A; Wypych, G, Databook of Preservatives, ChemTec Publishing, Toronto, 2015. Yang, W; Owczarek, JS; Fortunati, E; Kozanecki, M; Mazzaglia, A; Balestra, GM; Kenny, JM; Torre, L; Puglia, D, Ind. Crops Prod., 94, 800-11, 2016. Srivastava, R; SunY, Mater. Sci. Eng. C, 75, 524-34, 2017. Fernández, MA; Bellotti, N, Mater. Lett., 194, 130-4, 2017. Chrusciel, JJ; Lesniak, E, Prog. Polym. Sci., 41, 67-121, 2015. Shankar, S; Jaiswal,L; Rhim, J-W, Antimicrobial Food Packaging. Chapter 27 − Gelatin-Based Nanocomposite Films: Potential Use in Antimicrobial Active Packaging. Academic Press, 2016, pp. 339-48. Wang, Y-W; Chen, C-W; Hsieh, JH; Tseng, WJ, Ceramics Intl., 43, supl.1, 5797-5801, 2017. Singh, VP; Sandeep, K; Kushwaha, HS; Powar, S; Vaish, R, Constr. Build. Mater., 158, 285-94, 2018. Kumar, AM; Khan, A; Suleiman, R; Qamar, M; Saravanan, S; Dafalla, H, Prog. Org. Coat., 114, 9-18, 2018. Scaffaro, R; Botta, L; Maio, A; Gallo, G, Composites Part B: Eng., 109, 138-46, 2017. De Silva, RT; Pasbakhsh, P; Lee, SM; Kit, AY, Appl. Clay Sci., 111, 10-20, 2015. Chambers, C; Stewart, SB; Su, B; Jenkinson, HF; Sandy, JR; Ireland, AJ, Dental Mater., 33, 3, e115-e123, 2017. Xia, Y; Jiang, X; Zhang, J; Lin, M; Tang, X; Zhang, J, Appl. Surf. Sci., 396, 1760-4, 2017. Totu, EE; Nechifor, AC; Nechifor, G; Aboul-Enein, HY; Cristache, CM, J. Dentistry, 59, 68-77, 2017. Azeena, S; Subhapradha, N; Selvamurugan, N; Narayan, S; Srinivasan, N; Murugesan, R; Chung, TW;

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Moorthi, A, Mater. Sci. Eng. C, 71, 1156-65, 2017. 100 Creasey, DH; Cummins, BW; Becker, T; Creasey, JH, US20140171545, Contact Marketing Solutions Innovative Technologies, Jun. 1/9, 2014. 101 Alekseeva, OV; Rodionova, AN; Bagrovskaya, NA; Agafonov, AV; Noskov, AV, Arabian J. Chem., in press, 2017. 102 Mocanu, A; Furtos, G; Rapuntean, S; Horovitz, O; Flore, C; Garbo, C; Danisteanu, A; Rapuntean, G; Prejmerean, C; Tomoaia-Cotisel, M, Appl. Surf. Sci., 298, 225-35, 2014.

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7

Functional Fillers − Renewable and Recycling 7.1 BIOFILLERS Several biofillers were already discussed in Handbook of Fillers including cellulose particles, chitosan, clamshell powder, corn cob powder, eggshell filler, nutshell powder, wood flour, cellulose fiber, and cellulose nanofibrils.1 These fillers play increasingly important role in polymer processing formulations. Below, some emerging biofillers are discussed including lignocellulose derivatives, agricultural waste, wool keratin, seaweed, and sucrose palmitate. Cellulosic nanofibrils in the form of nanocellulose can be extracted from the lignocellulosic bioresources by using appropriate chemical, mechanical, enzymatic or combination methods.2 Nanocellulosic materials have many interesting features such as nano dimension (higher surface area to volume ratio), nontoxicity, biodegradability, and biocompatibility.2 They can find applications in the biomedical field, reinforcement of polymer matrix, energy, and environment.2 Lignocellulosic biomass consists of cellulose, hemicellulose, and lignin.2 Cellulose consists of amorphous and crystalline phases.2 The amorphous phase can be broken, removed, or modified by mechanical, chemical or enzymatic means.2 The nanostructured crystalline phase of cellulose is substantially more resistant because of the presence of hydrogen bonding and compact structure difficult to penetrate by small molecular weight substances.2 The extraction of nanocellulose from the cellulosic biomass includes two major steps: pretreatment and removal of amorphous phase (prior to the mechanical, chemical or enzymatic treatment, alkali treatment, and bleaching are involved).2 The prime objective of the pretreatment is to remove lignin, hemicelluloses, wax, and oils which cover the external surface of the fiber cell wall.2 Alkali treatment depolymerizes the native cellulose structure, defibrillates the external cellulose microfibrils and exposes short length crystallites.2 The bleaching treatment removes the cementing material from the fiber.2 Lignocellulosic materials can be also used in their original composition as relative to their source.3 The coconut shell, rice husk, and teakwood were studied as an alternative reinforcement to replace the non-renewable fillers used in the conventional composites.3 The filler size, volume content, filler type (chemical composition and shape), and dispersion influenced properties of composites.3 In general, a high lignin content increased the composite stiffness whereas a high cellulose content favored deformability.3 The high cellulose content and surface roughness supported a better adhesion due to a large number of

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Figure 7.1. SEM micrograph of alkali lignins (a) and lignin-rich residue (b) fillers. [Adapted, by permission, from Angelini, S; Cerruti, P; Immirzi, B; Santagata, G; Scarinzi, G; Malinconico, M, Intl. J. Biol. Macromol., 71, 163-73, 2014.]

hydrogen bonds and improved mechanical interlocking. These three sources varied widely in their composition, as follows:3

cellulose, % hemicellulose, % lignin, %

coconut shell 14-19 31-34 38-46

rice husk 37-45 18-21 26-31

teakwood 45-49 23-29 31-32

Composites of natural rubber and barley, corn, or wheat straw used as biofillers were studied.4 Straw is an agricultural waste and as such its utilization solves the problem of accumulation of materials which do not have direct use in consumption.4 The rubber mixtures containing lignocellulosic materials had a favorable kinetics of crosslinking.4 The addition of filler improving mechanical and barrier properties of composites as well as filler did not affect the thermal stability of composite.4 A lignin-rich residue obtained as a by-product from the fermentative bioethanol production process and commercial alkali lignins were used as fillers for poly(3-hydroxybutyrate).5 Addition of suitable amount of lignin-rich filler dramatically affected the rheological behavior of the polymer melt.5 The filler also acted as a heterogeneous nucleating agent.5 Resilience and elongation at break values were negatively impacted due to a poor interfacial adhesion between filler and matrix.5 Figure 7.1 shows the morphological features of both fillers.5 The alkali lignins featured spherical particles with large size dispersity, ranging from a few to hundreds of micrometers.5 Particles contained free volume enclosures.5 A heterogeneous morphology with many irregularly shaped lignin particles along with some fibrous material of likely polysaccharidic nature was typical of the lignin-rich residue.5 A lignocellulosic biomass was processed to isolate its main components: holocellulose and acid-insoluble lignin.6 The lignocellulosic biomass and its derivatives were used as fillers in poly(3-hydroxybutyrate).6 The addition of acid-insoluble lignin to PHB

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affected its rheological properties, significantly enhancing melt stability due to the formation of a percolated filler network within the matrix.6 The SEM analysis demonstrated homogeneous dispersion of the filler and a good interfacial adhesion with the matrix.6 The lignocellulosic biomass acted as a pro-degrading agent toward PHB matrix causing a decrease in molecular weight of the polymer during the thermal treatment.6 A pretreatment of lignocellulosic material was found important to increase compatibility between matrix polymers and lignocellulosic fillers.7 Choline acetate (a cholinium ionic liquid) was used in the pretreatment of bagasse powder for polypropylene composite.7 The tensile strength of the composites was 20 MPa, 35 MPa, and 40 MPa for polypropylene only, untreated bagasse powder, and choline acetate-pretreated bagasse powder, respectively.7 The elastic modulus was 0.7 GPa, 2.0 GPa, and 2.6 GPa for polypropylene only, untreated bagasse powder, and choline acetate-pretreated bagasse powder, respectively.7 Thus, choline acetate pretreatment of bagasse powder enhanced properties of polypropylene composites.7 Lignocellulosic fibers obtained by dry grinding of three agro-residues including wheat straw, brewing spent grains, and olive mills were compared as fillers in poly(3hydroxybutyrate-co-valerate) for food packaging applications.8 Ground fillers had different sizes of 109 µm, 148 µm, and 46 µm, respectively.8 Poor fiber/matrix adhesion, degradation of PHBV polymer chains, and a decrease of PHBV’s crystallinity were noticeable. The mechanical properties were degraded in the presence of all fibers.8 The water vapor transfer rate of composites was increased with wheat straw but it was decreased for olive mills-based materials.8 Regarding the food packaging applications, PHBV/wheat straw fibers composites are promising materials for respiring food products, whereas PHBV/ olive mills composites were preferred for water sensitive products.8 A polypropylene composition for injection molded parts includes biobased filler such as coconut shell powder, wood fiber, or agave fiber.9 Biobased fillers such as wood fiber, ground coconut shells, and agave fiber have densities of less than 1.2 that are less than half of the density of talc.9 One biobased filler material that is similar to talc as a reinforcing agent is a ground coconut shell having a particle size of 150 μm.9 Wool fibers were subjected to a green hydrolysis with superheated water in a microwave reactor to reprocess keratin-based wastes.10 The keratin hydrolyzates containing free amino acids, peptides, and low molecular weight proteins were exploited as a biofiller of polypropylene composite.10 Maleic anhydride grafted polypropylene was used as a compatibilizer.10 Keratin preserved the molecular weight of the polymer matrix during the processing and gave good overall mechanical properties of composites.10 The increase of polypropylene crystallization rate and the enhancement of thermal stability were observed as a function of the keratin amount in the composite.10 Keratins extracted from Merino wool and Brown Alpaca fibers by sulfitolysis and commercial hydrolyzed keratins were used as fillers in poly(l-lactic acid)-based biocomposites.11 The biocomposites had round-like surface topography of microsized keratin particles.11 The adhesion depended on the keratin source and its interaction with the matrix as were transparency and thermal responses.11 The biocomposites could be useful materials for medical applications.11 The combination of human bone-marrow mesenchymal stem cells with a biocomposite could open new perspectives for the treatment of skin wounds.11

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The thermoplastic sugar palm starch/agar blend contained Eucheuma cottonii seaweed waste as biofiller.12 Incorporation of seaweed significantly improved the tensile, flexural, and impact properties of the composites.12 The thermal stability of the composites was enhanced with the addition of seaweed.12 The biodegradation of the composites was also improved by incorporation of seaweed.12 The composite was suitable for products such as trays, plates, etc.12 During extraction in the production of alginate, insoluble seaweed residuals are formed, which have to be separated from carbohydrate solution.13 Diatomaceous earth is frequently used as a filtering agent.13 There is thus a huge amount of organic-inorganic byproduct produced.13 To use this matter as a filler in polymer composites would be very attractive from the economic and environFigure 7.2. The mechanism for thermal degradation of poly(lactic mental points of view.13 The addiacid) bionanocomposites containing low and high content of tion of filler at 40 wt% resulted in sucrose palmitate. [Adapted, by permission, from Valapa, R; Pugazhenthi, G; Katiyar, V, Int. J. Biol. Macromol., 65, 275-83, 2014.] the increase of Young’s modulus by 20% compared to the neat poly(lactic acid).13 The presence of filler particles improved stress distribution and led to stronger composites.13 The results suggest the use of a huge stock of biobased by-product for making polymer composites which can find application as packaging materials.13 The influence of sucrose palmitate on the hydrolytic degradation behavior of poly(lactic acid) nanocomposites was studied.14 Sucrose palmitate is eco-friendly reinforcement for fabrication of poly(lactic acid) bionanocomposites.14 The bionanocomposites demonstrated ~69% reduction in the oxygen permeation as compared to neat poly(lactic acid).14 The thermogravimetric analysis confirms the loss of thermal stability of the neat poly(lactic acid) as well as composites after hydrolytic degradation.14 Transparency measurements showed the enhancement in opacity of both the poly(lactic acid) and poly(lactic acid) nanocomposites with progress in hydrolytic degradation.14 The presence of sucrose palmitate in the poly(lactic acid) matrix greatly influences the hydrolytic degradation of poly(lactic acid).14 Considering that after the disposal of packaging mate-

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rial, it is highly desirable that it would be degraded at a rapid pace, the filler fits the expectations.14 The thermal degradation behavior of poly(lactic acid) biocomposites containing sucrose palmitate was studied to determine suitability for food packaging application.15 The sucrose palmitate acts as a protective barrier by decelerating the thermal degradation rate of poly(lactic acid).15 Figure 7.2 outlines thermal degradation mechanism of poly(lactic acid) biocomposite with sucrose palmitate.15 The thermal degradation process at higher sucrose palmitate loading is accelerated by relatively high concentration of H+ released during the thermal decomposition of sucrose which accelerates the hydrolysis of ester bonds in the poly(lactic acid) matrix.15

7.2 BIOSORBENTS Contamination of aquatic systems by synthetic organic contaminants and pollutants poses significant public and environmental health risks in industrial and developing countries.16 The pollution originates from textile, agrochemical, and pharmaceutical industries. In developing countries, pollutants removal is less likely because of high costs and lack of advanced cleaning methods.16 Biosorption for removal of organic contaminants in developing countries is attractive because of 16 • large quantities of biomaterials are available for use as biosorbents • the technology is relatively cheap compared to the advanced methods. On the other hand spent biosorbents present an environmental risk because of the potential for surface and ground water contamination.16 Possible disposal methods include road surfacing, a soil amendment, and phyto-remediation.16 All of these aspects require studies which is one of the reasons for a large body of research currently available including several hundred publications per year. Information from some of these numerous publications is included in the analysis below. Calabrian pine sawdust was employed in biosorption of an industrial hetero-bioreactive dye.17 The biosorption of dye was rapid and the dye removal efficiency increased as the dye concentration increased.17 The pseudo-second-order model best represented the kinetics of dye biosorption.17 The negative value of standard Gibbs free energy change suggested that physical forces were involved in the spontaneous dye biosorption.17 Biosorbent for Blue Dye 113 removal was synthesized by NaOH and surfactant treatment of dead leaves of Prunus dulcis. Almost all dye was removed.18 Maximum biosorption capacity was estimated as 97.09 mg g-1 for surfactant modified biosorbent.18 The adsorption kinetics was well described by pseudo-second-order equation and LangmuirTemkin isotherms best-fitted equilibrium data.18 The adsorption was exothermic and spontaneous in nature.18 Acid violet 17 dye was removed from wastewater by biosorbent obtained from NaOH and H2SO4 activation of fallen leaves of Ficus racemosa.19 Elution studies confirmed reusability of biosorbent for up to 5 cycles with only marginal loss of efficiency.19 The maximum Langmuir biosorption capacities were 45.25, 61.35 and 119.05 mg/g for raw biosorbent, H2SO4 activated biosorbent, and NaOH activated biosorbent, respectively.19 The activation energy of the bioabsorption process was 7.07 kJ/mol.19 The azo dye was removed from aqueous solution by a biosorbent prepared with Aspergillus nidulans cultured in tobacco wastewater.20 The kinetics and equilibrium bio-

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sorption were well-described by pseudo-second-order kinetic and Langmuir isotherm model, respectively.20 According to the Langmuir model, the maximum biosorption capacity of Congo red was 357.14 mg/g at 30°C and pH 6.8 − well above other biosorbents reported.20 Malachite green was removed from water by forestry waste mixture composed of sawdust biomasses.21 Cetyltrimethylammonium bromide was used for biosorbent modification.21 The Sips isotherm and the logistic model fitted well the dye biosorption data.21 The maximum biosorption capacity was 52.610 mg g-1 at the optimized conditions.21 Anadara inaequivalvis shells were used for removal of Pb(II) and Cu(II) from aqueous solutions.22 The maximum biosorption capacities of Cu(II) and Pb(II) were 330.2 and 621.1 mg/g, respectively (seashell particle size 250 µm, 100 mg/L metal ion solution, 2.5 mg biosorbent, 25°C).22 Langmuir isotherm fitted the equilibrium data better than the Freundlich and Temkin isotherms.22 The biosorption was spontaneous and exothermic. The kinetic parameters were of pseudo-second-order.22 Patulin (mycotoxin) contaminated juice is an important food safety issue throughout the world.23 The magnetic Fe3O4 nanoparticles were coated with inactivated Candida utilis CICC1769 cells and chitosan were used as a biosorbent for the removal of patulin in fruit juice.23 The biosorbent was found to reduce patulin by over 90% in the orange juice without any significant negative impacts on the quality parameters of the juice.23 Basic red 46 and Basic violet 3 were removed by green composite biosorbent made from a combination of Spirogyra sp. and Rhizoclonium sp. filamentous algal biomasses.24 The best fit was achieved with Sips isotherm model.24 The maximum values of biosorption were 53.303 and 37.734 mg g-1 for Br46 and Bv3, respectively.24 The co-biosorption process of dyes was described by the pseudo-second-order model.24 Marula seed husk (Sclerocarya birrea) biomass was used as a low-cost biosorbent for removal of Pb(II) and Cu(II) from aqueous solutions.25 The biosorption follows Langmuir isotherm.25 The equilibrium sorption capacities of the seed husk were 20 for Pb(II) and 10.20 mg g-1 for Cu(II).25 The thermodynamic parameters showed that the biosorption was spontaneous and endothermic.25 Cadmium (II) biosorption onto composite chitosan biosorbent in which ceramic alumina was coated on chitosan.26 Pseudo-second-order model and Elovich equation represented the experimental data.26 The maximum monolayer biosorption capacity was 108.7 mg g-1 at 318 K.26 Cadmium and lead were biosorbed by dried cactus (Opuntia ficus indica) cladodes.27 The percentage of biosorption increased with an increase in the biosorbent dosage and the decrease of particle size.27 The maximum biosorption occurred at pH of 5.8 and 3.5, respectively for cadmium (II) and lead (II) ions.27 Biosorption kinetic data were fitted with the pseudo-second-order kinetic model.27 The Langmuir model described a maximum monolayer biosorption capacity of 30.42 and 98.62 mg/g, respectively for cadmium (II) and lead (II) ions.27 The biosorption yield decreased with an increase in solution temperature.27 The FTIR analysis indicated the involvement of C=O, O−C and C−O−C groups in metal binding.27 A magnetic biosorbent from peach gum polysaccharide was used for selective and efficient removal of cationic dyes.28 The biosorbent was fabricated by a simple one-step reaction based on the simultaneous formation of magnetic nanoparticles and crosslinking

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of natural peach gum polysaccharide.28 The maximum adsorption capacity was 231 mg/ g.28 The kinetic data showed good correlation with pseudo-second-order model.28 Agricultural and industrial wastes can be used as biosorbents with the limited processing required.29 The biosorption of copper(II) ion from aqueous solutions by hydrochloric acid treated tomato factory waste was investigated.29 The maximum metal removal of 92.08% was achieved at pH=8.29 The biosorption of copper(II) ion on tomato waste biosorbent was exothermic at 293-313 K.29 Chestnut shells were evaluated as a low-cost biosorbent for pesticide removal.30 The citric acid pretreatment increased the removal efficiency of chestnut shells by 15%.30 The adsorption process was well-represented by the pseudo-second-order kinetic model.30 Freundlich isotherm described the adsorption equilibrium.30 A coastal seaweed community biomass was used for the bioremoval of zinc ions.31 The biosorption capacity of biosorbent was 115.198 mg g-1.31 The seaweed was composed of Chaetomorpha sp., Polysiphonia sp., Ulva sp. and Cystoseira sp. species harvested from the north coast of Turkey.31 It was treated with sodium hydroxide and then used as a natural biosorbent material for the bioremediation.31 The biosorption of Ni(II) ions on Macauba (Acrocomia aculeata) oil extraction residue was optimized using fixed-bed column.32 Ni(II) biosorption can be related to hydroxyl and phenolic groups.32 Breadnut peel (Artocarpus camansi) was found to be a highly effective low-cost biosorbent for methylene blue.33 There are over 1×105 types of dyes available and more than 7×105 tones of dyestuff are annually produced for textile, cosmetics, food and paper industries.33 The large amounts of dyes are disposed into wastewater or natural ecosystem. The breadnut peel biosorbent had a maximum biosorption capacity of 409 mg g-1.33 The biosorption process was both spontaneous and exothermic.33 Mussel-inspired synthesis of magnetic polydopamine-chitosan nanoparticles as biosorbent for dyes and metals removal.34 The high surface area of nanoparticles and highlevel active sites from polydopamine and chitosan creates a multiple interactions to absorb pollutants.34 The maximum adsorption capacity was up to 245.6, 47, 151.6, 204, and 61 mg g-1 and the removal percentages reached 98.4, 92, 95.8, 96.9, and 92.5% for Hg(II), Pb(II), Cr(IV), Methylene blue and Malachite green, respectively.34 The percentage of branched-chain amino acid to aromatic amino acids in protein enzymatic hydrolyzates containing peptides is known as Fischer’s ratio.35 The protein enzymatic hydrolyzates with a Fischer’s ratio higher than 20 are used in specific medical diets for treatment of patients with liver diseases, including hepatic encephalopathy in order to avoid the adverse effects of aromatic amino acids.35 A chitosan-based biosorbent crosslinked with phenethylamine was synthesized and used for adsorption of aromatic amino acids from rice protein enzymatic hydrolyzates.35 The particle size of biosorbent was 500-1000 µm with pore diameter of 50-100 µm.35 The Fisher’s ratio of 21.2 was achieved using this biosorbent.35 A high-capacity adsorption of aniline was achieved using surface modified lignocellulose-biomass jute fibers.36 Pyromellitic dianhydride-modified jute fibers were prepared by microwave treatment to generate a biosorbent for aniline removal.36 The maximum adsorption capacity was observed at pH=7.0 and the adsorption process was spontaneous and endothermic.36 The aniline adsorption followed the pseudo-second-order kinetic

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model and Langmuir isotherm model.36 The biosorbent was regenerated by the desorption of aniline using 0.5 M HCl solution.36 The adsorption capacity after regeneration was even higher than that of virgin modified jute fibers.36 A biosorbent, comprising at least one of watermelon rind, sugarcane bagasse, and garden grass in synergistic quantities, was used for heavy metal removal.37 Cu, Zn, and Pb were removed with high efficiency.37 An amine-grafted chitosan nanofiber was used in heavy metal adsorption.38 The nanofibers had extremely small diameters, large surface-to-volume ratio, high porosity, and superior mechanical performance.38 The maximum adsorption capacity of amine grafted chitosan nanofibers for Cu(II) was 166.67 mg g-1.38 This adsorption capacity was much higher than that for the existing conventional and chitosan adsorbents.38 The combination of processing and amine grafting significantly increased the adsorption capacity of nanofiber membranes.38

7.3 GEOPOLYMERS Alkali-activated cements − inorganic polymers (geopolymers) − are a possible alternative for manufacturing structural materials for environmentally sustainable construction and building products.39 They can be synthesized from artificial or natural aluminosilicate materials at the relatively mild temperatures permitting incorporation of organic additives.39 The products have a much smaller CO2 footprint than traditional Portland cements, good chemical resistance, and mechanical performance.39 Geopolymer production involves dissolution of aluminosilicates in a strong basic medium, followed by polymerization to a solid geopolymer structure.39 Thermo-mechanical properties of geopolymers obtained from Cameroonian volcanic ash were studied.39 The test samples were cured at 90oC and the mechanical strength increased for up to 21 days. In NaOH solutions, the strength development was faster than in KOH solutions.39 A dry and wet compressive strength was respectively around 40 MPa and 20 MPa after 21 days.39 The largest particles acted as a reactive filler while smaller particles dissolved in the activating solution.39 Pure volcanic ash and synthesized materials consisted of mostly amorphous material with some newly formed crystalline phases.39 KOH specimens were more thermally stable, shrinking less than 3% after heating up to 1000oC.39 Volcanic ashes can be found in Colombia, Cameroon, the Democratic Republic of Congo, Indonesia, Italy, Russia, and the USA.40 They can be considered as natural pozzolan due to their high silica, alumina, and iron oxide contents.40 Volcanic ashes with large amount of amorphous phase (18.2-42.5 wt%) led to geopolymers with compressive strengths of 3.1-12.6 MPa.40 The volcanic ashes with small amount of amorphous phase (10.2-12.5 wt%) led to geopolymers with compressive strength of 1.0-4.1 MPa.40 The compressive strength of geopolymers increased with increasing amount of amorphous phase and decreased with increasing SiO2/Al2O3 molar ratio of amorphous phase.40 The volcanic ashes could be used either for geopolymer synthesis or as filler which depends on SiO2/Al2O3 molar ratio of amorphous phase.40 The functional geopolymers based on local resources such as kaolinitic soil and zeolitic tuff were studied for applications such as the construction of water storage containers, water transfer channels, and water filtration systems.41 The optimum ratio of water

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was 28% of clay fraction, giving improvements of physical, mechanical, and adsorption properties of the geopolymeric products with the highest compressive strength, density, and maximum adsorption capacity toward cadmium.41 The phase evolution in metakaolin geopolymers containing different activators and aluminosilicate fillers has been studied.42 Calcination of kaolinite formed more reactive metakaolin containing highly reactive amorphous phase which influenced geopolymer synthesis process.42 Activator composition (NaOH, sodium silicate, or their mixtures) determined the phase development and microstructure.42 The non-reactive fillers increased the compressive strength of the geopolymers.42 Metakaolin geopolymers need high water content to have appropriate rheology in some applications.43 At the same time, an extensive drying shrinkage and cracking occurs with high water content in metakaolin geopolymers.43 Addition of more than 10 vol% sand eliminates cracking on heating to 110°C because sand particles limit linear shrinkage by forming a network.43 At higher sand additions microcracks form around non-shrinking sand particles.43 The compressive strength of metakaolin geopolymers with silicate, silicate+quartz sand and silicate+rutile sand was 31.2, 52.2 and 41.5 MPa, respectively.45 The addition of alumina and wollastonite fillers reduced thermal shrinkage of fly ash-based geopolymers.44 Samples containing filler exhibited 30-35% less volume shrinkage at 1000°C than the control.44 Wollastonite addition improved the flexural strength which was attributed to its acicular shape imparting fibrelike qualities.44 The composites exhibited potential for fire and thermal resistant applications.44 Geopolymers have quasibrittle behavior because of their ceramic-like characteristics.46 Graphene can be used as an additive to improve the mechanical properties of composites.46 The compressive and flexural strengths were improved by 1.44 and 2.16 times, respectively, with Figure 7.3. Characteristic toughening mechanisms of graphene addition of 1% graphene.46 Even reinforced geopolymer-based composites. [Adapted, by permission, from Ranjbar, N; Mehrali, M; Mehrali, M; Johnson Alenga- at low filler weight fractions, graram, U; Jumaat, MZ, Cement Concrete Res., 76, 222-31, 2015.] phene increased the toughness,

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stress and strain at the first crack, and rigidity (Figure 7.3).46 At the same time, the wettability decreased with an increase in graphene content.46 Natural zeolite was selected as a filler for metakaolin-based geopolymers.47 Geopolymer containing 50% zeolite had the highest compressive strength.47 Geopolymers adsorbed heavy metals in the order Pb2+ > Cd2+ > Zn2+, Cu2+ > Cr3+.47 The maximum adsorption capacity of Cu2+ and Cr3+ was the highest for geopolymer with 100% of metakaolin, while for Pb2+, Cd2+ and Zn2+ the highest adsorption capacity was for geopolymers containing 75% of metakaolin, indicating that 25% zeolite addition to geopolymers had efficiently improved the adsorption capacity.47 Fluidized-bed combustion ash can be used as a partially reactive filler for metakaolin geopolymer.48 The composite material has good thermal performance and compressive strength (~30 MPa) suitable for the building sector.48 The addition of 2 wt% glass microfibers to ash geopolymer gave the highest levels of fracture toughness, compressive strength, Young's modulus, and hardness.49 The glass microfibers acted as a filler for voids within the matrix, making a dense geopolymer and improving the microstructure of the binder.49 Possible applications include roofing sheets, tiles, cladding panels, and some interior applications in the construction industry.49 An alkaline solution of thermally activated kaolinite clay was filled with calcium carbonate.50 The incorporation of up to 12 wt% calcium carbonate did not affect the setting mechanisms or the performance of the matrix.50 Calcite acted as an inert filler.50 Geopolymer cements were obtained using sodium waterglass and phosphoric acid as hardeners. Geopolymer cement from phosphoric acid indicates the formation of a new crystalline phase berlinite.51 Berlinite dispersed in the matrix, acts as a filler and reinforces the geopolymer network.51 The compressive strength of phosphate-based geopolymer cement was 93.8 MPa and the alkali-based geopolymer cement was 63.8 MPa.51 The difference of the compressive strength could be related to the formation of berlinite (AlPO4) which acted as a filler and reinforced the structure and therefore the compressive strength of the specimen.51 Carbon nanotubes acted as filler of geopolymer minimizing the air gaps content in the matrix.52 The denser structure improved the physical and mechanical properties of the composite. The electrical capacity, relative permittivity, and real and imaginary impedance component of composite were improved.52 A higher content of fine ceramic particles under 90 µm improved dimensional stability of potassium-based geopolymer.53 The geopolymer reinforced with fine ceramic particles revealed a constant flexural strength of ~12 MPa and a compressive strength of ~90 MPa, both in initial state and after exposure at 1000°C.53 The Bayer process liquors were used as a primary source of caustic sodium aluminate in combination with fly ash as a source of reactive silica and additional alumina.54 Geopolymers with a Si/Al ratio of 2.3 and a Na/Al ratio of 0.8 were targeted.54 When synthetic plant liquor was used as the alkali activator, geopolymers with a mean compressive strength of 33 MPa were synthesized, while use of processed plant liquor resulted in compressive strengths of 43 MPa.54 A high strength, low-density geopolymer composite cellular concrete compositions were patented including Class F fly ash, gelation enhancer, and hardening enhancer.55 Enhancers were selected from the group consisting of: metakaolin, meta-halloysite,

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micron and nanoparticles of silica and alumina, and any pozzolanic aluminosilicate material that had a low alkali-earth oxide content and a high dissolution rate in alkaline solution.55 The hardening enhancers were selected from a group consisting of: ground granulated blast furnace slag, Class C fly ash, vitreous calcium aluminosilicate, kiln dust, CaO-rich pozzolanic aluminosilicates, or their combinations.55 The mineral polymer based on a metakaolin having a porous or non-porous structure can be used for reducing pollutants such as NOx (e.g, NO2), SOx (e.g., SO2) and/or CO2, for absorbing polluting volatile organic compounds such as volatile organic hydrocarbons and for capturing particulate pollutants, such as those produced by diesel engines.56 A composition for use in the manufacturing of wood-based panels comprised a mineral binder composition and wooden particles.57 The mineral binder composition comprised water glass material (selected from sodium silicate, potassium silicate, lithium silicate), a chemical base (e.g., sodium hydroxide or potassium hydroxide), water, and aluminosilicate powder (e.g, slag micro-powder, fly ash, paper sludge ash, silica fume, kaolin based minerals (e.g. metakaolin), calcium silicate minerals, calcium aluminate based minerals, plasticized and crushed clay, or oil shale ash).57 The wooden particles were saturated with the mineral binder composition by spraying.57

7.4 RECYCLED Red Mud is a by-product produced during the Bayer process of refining of bauxite to produce aluminum (2 tons of red mud remains after production of 1 ton of aluminum).58 It contains metallic oxides.58 The red color results from iron oxide (~60% of its mass).58 The massive quantities generated trigger efforts of finding suitable uses for red mud. Some applications include the production of building materials, ceramics, coloring agents for paint works, micro-fertilizer and a neutralizer of pesticides in agriculture, pH modifier in the heap leaching of gold ores, and a pigment in anticorrosive marine paints.58 Red mud can also be utilized to control electromagnetic pollution by using it as an electromagnetic shielding material in composites with polyaniline.58 The maximum shielding effectiveness observed was 41.38 dB for a composite containing 25% PANI at 12.4 GHz which was absorption dominated.58 A reduced graphite oxide was prepared by exfoliation of graphite waste from the metal smelting industry to be used in natural rubber composites.59 The graphite oxide was reduced by L-ascorbic acid and subjected to thermal reduction.59 The residual oxygencontaining groups were almost completely removed by the thermal reduction and the conjugated graphene networks were restored.59 The electrical conductivity of natural rubber was increased by the inclusion of such prepared graphite at a percolation threshold of 5 phr, with an electrical conductivity of 8.71x106 S/m.59 Local marine litter such as Kelp brown algae (Eklonia spp.) and Bivalve mollusk shells (Veneridae spp.) were examined as combined reinforcement hybrid wood/waste filler/polypropylene composite.60 The biocomposites achieved an enhanced performance suitable for high-moisture environment applications.60 The repurposing of waste glass into a construction material reduces the consumption of natural resources, minimizes greenhouse emissions, and alleviates landfill scarcity.61 The workability of concrete mixes containing crushed waste glass as a partial replacement for fine aggregates was lower than for the mixtures containing natural aggregates.61 Up to

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30% of rock in subbase of asphalt can be replaced with crushed waste glass (