Water-based Acrylics Dispersions: Applications in Architectural Coatings 9783748604914

Table of contents :
Foreword
Acknowledgments
Contents
1 Introduction and basic principles
2 Formulation of water-based architectural paints
3 Primers
4 Exterior paints on mineral substrates
5 Wood coatings
6 Gloss emulsion paints or trim paints
7 Interior paints
8 Plasters and renders
Appendix
Recommended literature
List of abbreviations
Authors
Index

Citation preview

Roland Baumstark Roelof Balk

Water-based Acrylic Dispersions Applications in Architectural Coatings

2nd Revised Edition

Cover: studali – stock.adobe.com

Bibliographische Information der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliographie; detaillierte bibliographische Daten sind im Internet über http://dnb.ddb.de abrufbar.

Roland Baumstark and Roelof Balk Water-based Acrylic Dispersions: Applications in Architectural Coatings, 2nd Revised Edition Based on 1st Edition Waterbased Acylates for Decorative Coatings Hanover: Vincentz Network, 2022 European Coatings Library ISBN 3-7486-0489-X ISBN 978-3-7486-0489-1 © 2022 Vincentz Network GmbH & Co. KG, Hanover Vincentz Network GmbH & Co. KG, Plathnerstr. 4c, 30175 Hanover, Germany This work is copyrighted, including the individual contributions and figures. Any usage outside the strict limits of copyright law without the consent of the publisher is prohibited and punishable by law. This especially pertains to reproduction, translation, microfilming and the storage and processing in electronic systems. The information on formulations is based on testing performed to the best of our knowledge. Please ask for our book catalogue Vincentz Network, Plathnerstr. 4c, 30175 Hanover, Germany T +49 511 9910-033, F +49 511 9910-029 [email protected], www.european-coatings.com Layout: Vincentz Network, Hanover, Germany Printed by: Gutenberg Beuys Feindruckerei GmbH, Hanover, Germany

European Coatings Library

Roland Baumstark Roelof Balk

Water-based Acrylic Dispersions Applications in Architectural Coatings

2nd Revised Edition

Foreword

Foreword Water-based polyacrylates, as emulsion binders, dispersing resins or thickening polymers, are nowadays impossible to do without as raw materials in the paint and coatings industry. Since their introduction in the fifties and sixties of the last century pure acrylic and styrene-acrylic dispersions have established themselves as environmentally friendly high-performance alternatives to solvent-borne, air-drying alkyd resin binders mainly used before. They show a particularly wide performance spectrum, excellent levels of durability and pigment binding ability combined with a superior chemical robustness. In addition, their structural variability allows their still ongoing development as binder class for coating materials. This book gives an overview of the preparation and properties of water-based acrylic dispersions and the special features pertaining to their use in the architectural coatings sector. This completely revised 2nd edition also includes innovations in these fields of the last 20 years. Following a general introduction to acrylics and polymer dispersions, a separate chapter on the basics of emulsion paint formulation is included. In the following chapters, deeper insight is given into the diverse types of paints, such as primers, masonry paints, including the special topics dispersion-silicate paints, silicone-resin paints and elastomeric crack-bridging paints, interior paints, wood coatings, trim paints and textured finishes or plasters. The aim of the book is to present an easy to read, up-to-date picture of the preparation of acrylic binders and their formulation to diverse types of architectural paints. Owing to the breadth of the applications of acrylic dispersions and the diversity of present-day knowledge concerning colloid chemistry, it is impossible for this book to deal with every theory and aspect of dispersions and paints manufacture. The reader is therefore referred to the quoted literature, which offer more in-depth views on the various topics. In addition, the architectural coatings market differs strongly between the regions. This cannot fully be covered by this small book. The focus in the formulation chapters is therefore on the European paint market. The book is aimed both at students and newcomers in the field of paints and coatings, but it also gives the experienced dispersion user and paint formulator in the industry relevant information and specific hints. The provided information gives assistance with the selection and daily use of modern polymer dispersions.

4

Foreword

Acknowledgments The authors wish to thank the co-author of the first edition Manfred Schwartz for his initial contribution. Our thanks also goes to Konrad Roschmann and Bastiaan Lohmeijer and their lab teams for their support, among others with graphics, and to diverse other colleagues and experts such as Ivan Cabrera, Heribert Kossmann, Alan Smith, Oliver Wagner and Chen-Le Zhao, all of them present or former employees of BASF, without whose previous work, their knowledge and publications this book would not have been possible to write. We would also like to thank Jürgen Kaczun for his contribution to the wood coatings chapter. Finally, thanks are due to BASF SE, Ludwigshafen for approving the manuscript for publication. Ludwigshafen, September 2022 Roland Baumstark and Roelof Balk

SF_Sponsored-by_big.indd 2

Sponsored by:

5 07.11.22 11:34

Contents

Contents 1 Introduction and basic principles 1.1 Architectural coatings and binders 1.1.1 Polymer dispersions 1.1.2 Binder classes, polymerization and polyacrylates 1.1.3 Free-radical polymerization and emulsion polymerization 1.2 Polyacrylates, pure acrylics and styrene-acrylic copolymers  1.3 Film formation of polymer dispersions 1.3.1 Mechanism and minimum film forming temperature 1.3.2 Parameters determining the minimum film forming temperature 1.3.3 Coalescents/solvents and plasticizers 1.3.4 Environmental aspects  1.4 Parameters and properties of architectural coatings binders 1.4.1 Total solids content 1.4.2 pH value 1.4.3 Viscosity and rheology 1.4.4 Coagulum 1.4.5 Particle size/particle size distribution 1.4.6 Surface tension 1.4.7 Stability 1.4.8 Residual monomers and volatiles 1.4.9 Molecular weight/molecular weight distribution 1.4.10 Mechanical properties of polymer and coating films 1.5 Parameters influencing binder properties during their preparation 1.5.1 Monomer selection 1.5.2 Emulsifiers and protective colloids 1.5.3 Initiators and chain transfer agents 1.5.4 Buffer substances and neutralizing agents 1.5.5 Preservatives 1.5.6 Defoamers 1.5.7 Polymerization control 1.5.8 Multi-phase systems 1.5.9 Seed polymerization 1.5.10 Hybrids 1.6 References

11 11 12 13 14 18 24 24 25 26 32 32 33 33 34 36 37 39 40 40 41 42 43 44 44 46 47 48 49 49 49 51 52 54

2 Formulation of water-based architectural paints 2.1 Requirements of paints 2.2 Composition of architectural paints 2.2.1 Pigment volume concentration

59 59 59 60

6

Contents 2.2.2 Interior and exterior paints 2.3 Formulating constituents of an emulsion paint 2.3.1 Paint binders 2.3.2 Pigments 2.3.3 Fillers  2.3.4 Additives  2.4 References

63 65 65 76 80 82 98

3 Primers 3.1 Definitions and requirements 3.2 Aqueous primers based on acrylic dispersions 3.3 Formulation of primers 3.4 Test methods 3.5 References

101 101 103 106 107 108

4 Exterior paints on mineral substrates 4.1 Introduction 4.2 Masonry paints 4.2.1 Basics on binders, standards and different types of masonry paints 4.2.2 Durability of masonry paints 4.2.3 Masonry paints based on polymer dispersions – binders and formulation parameters  4.2.4 Masonry paints formulated at high PVC 4.2.5 Solvent-free exterior paints: influence of the Tg of binders and of titanium dioxide  4.2.6 Important other exterior paint functionalities and influence parameters 4.2.7 General formulation hints for masonry paints  4.2.8 House paints 4.2.9 Tinting colours and full colour or deep-tone paints 4.2.10 References 4.3 Polymer dispersions in silicate systems 4.3.1 Resistance to hydrolysis 4.3.2 Water absorption 4.3.3 Interactions between dispersion and water glass 4.3.4 Demands imposed on an optimum dispersion 4.3.5 Silicate dispersion system 4.3.6 Silicate dispersion plasters and renders 4.3.7 Formulation for a dispersion silicate or silicate emulsion paint 4.3.8 Formulation for a silicate dispersion plaster 4.3.9 Properties of dispersion silicate paints and renders 4.3.10 References

109 109 111 111 116 128 155 166 169 189 190 194 195 197 199 200 201 203 204 206 207 207 210 211

7

Contents 4.4 Polymer dispersions as binders in silicone resin systems 4.4.1 Polymer dispersions in silicone resin systems 4.4.2 Pigment binding capacity 4.4.3 Silicone resin emulsion and CPVC 4.4.4 Weathering behaviour 4.4.5 Demands imposed on an appropriate dispersion 4.4.6 Formulation of silicone resin paints 4.4.7 Silicone resin renders 4.4.8 References 4.5 Elastic coating systems 4.5.1 Principal requirements of coating systems for renovating façades 4.5.2 Mechanical properties of dispersion films 4.5.3 Dirt pick-up resistance 4.5.4 Summary 4.5.6 References 4.6 Comparison of different masonry paint systems  4.6.1 References 4.7 Outlook

211 213 215 216 216 217 218 222 223 224 225 226 232 242 244 244 246 246

5 Wood coatings 5.1 Special features of wood as a building material 5.2 Classification of wood coatings and market 5.2.1 Primers/impregnating stains  5.2.2 Stain blocking primers 5.2.3 Exterior topcoats 5.2.4 Binders for wood coatings and properties 5.2.5 Interior wood finishes 5.3 References

249 249 252 253 253 256 266 277 282

6 Gloss emulsion paints or trim paints 6.1 Introduction and requirements 6.2 Gloss and haze 6.3 Binders for gloss emulsion paints 6.3.1 Binders and gloss 6.3.2 Titanium dioxide and gloss; effect of the dispersing operation 6.4 Properties of acrylic gloss emulsion paints 6.5 Interaction with associative thickeners  6.6 References

287 287 289 295 296 301 302 305 307

7 Interior paints 7.1 Introduction and definition

309 309

8

Contents 7.2 Base technical requirements for interior paints 7.3 Pigment binding capacity and critical pigment volume concentration 7.3.1 Factors influencing the CPVC 7.4 Wet and dry hiding power 7.4.1 Influencing parameters and possibilities to reduce TiO2 7.5 Processing/application properties 7.6 High-volume application in one operation (one-coat paint) 7.7 Open time 7.8 Mud cracking 7.9 Tinting 7.10 Wet abrasion resistance or wet scrub resistance 7.11 Emission and solvent-free interior paints (low VOC) 7.12 Renewables in binders and modern interior paints 7.13 Biocide-free and anti-allergic interior paints 7.14 Formulation scheme for interior paints 7.15 Semi-gloss interior paints – latex paints and stain resistance 7.16 References

310 311 312 316 319 320 322 322 323 324 326 333 335 335 337 344 349

8 Plasters and renders 8.1 Introduction and definition 8.2 Classification of pasty plasters and renders, technical requirements 8.2.1 Requirements imposed on the binder 8.2.2 Binding power 8.2.3 Water absorption and moisture protection 8.2.4 Thermoplasticity, alkali resistance and burning behaviour 8.2.5 Processing properties 8.3 Surface structures 8.3.1 Coloured aggregates or marble stone finishes 8.4 Exterior insulation, finish systems (EIFS or ETICS) and fire testing 8.5 Formulation scheme for dispersion-based, pasty renders/plasters  8.6 “Winter quality” – quick drying formulations 8.7 Typical binders for dispersion-based, pasty plasters and renders 8.7.1 Marble stone finishes 8.8 References

351 351 353 355 356 356 357 359 360 360 362 371 372 373 374 374



Appendix

376



Recommended literature

376



List of abbreviations

377

Authors

381

9

Architectural coatings and binders

1 Introduction and basic principles 1.1 Architectural coatings and binders Architectural coatings have a dual function: on the one hand, they make a significant contribution to the aesthetics of the building or the structural components, for instance by colouring or accentuating their structure. On the other one they protect the building material against external influences, such as moisture, sunlight, or mechanical or chemical damage. The majority of water-based architectural coatings are complex mixtures of a wide variety of chemical components, as shown by the following compilation: Main components – Water – Binders – Pigments – Fillers Additives/auxiliaries – Dispersants and wetting agents – Thickeners/rheology modifiers – Defoamers – Preservatives/biocides – Solvents/film-forming auxiliaries It is not uncommon for aqueous architectural coatings to contain between 10 and 20 different ingredients. The function of the binder is to give the coating the necessary cohesion, durability, weathering stability, good mechanical properties such as flexibility or hardness, and to give the paint its advantageous processing properties. The binder embeds the colouring pigments and fillers in a stable matrix and joins them to the substrate. This distinguishes the finished coating from, say, classroom chalk, which is based on pressed chalk, acts without a binder and is therefore easily washed off. Besides the pure inorganic material water-glass, which has already been in use for a long time in silicate systems, today the binders used for water-based architectural coatings are predominantly polymer dispersions. Water-based Acrylic Dispersions © Copyright 2022 by Vincentz Network, Hanover, Germany

11

Introduction and basic principles According to Chem Research GmbH, in 2019 ca. 42 million tonnes of coatings material (coatings and lacquers) were produced worldwide, the largest part of which were architectural coatings (21.4 million tonnes). In Europe alone, that year the demand for architectural coatings was approx. 5.2 million tonnes [1]. IRL reported for 2017 even a market size of 7.6 million tonnes for Europe. According to that study [2], today already more than 80 % of architectural coatings are water-based and formulated with water-based polymer dispersions. This would mean that 800,000 to more than 1 million tonnes of polymer dispersions per year are used for this application in Europe today. The solvent-borne alkyd resin systems which formerly dominated the sector are increasingly being replaced by more environment-friendly, aqueous coating systems bound with polymer dispersions; they are still available and have a certain place, especially in applications such as wood coatings and trim paints. Moreover, also the alkyd technologies developed towards either water-based or high solid systems to meet the present-day requirements [3]. In Germany, the production statistics of the paint and printing ink industry association (VDL) [4] for 2018 record a total of 683,296 metric tons of aqueous emulsion paints for interior and masonry application alone, whereas of high solids and aqueous alkyd systems only 63,930 metric tons are produced.

1.1.1 Polymer dispersions [5–8] Generally, a dispersion is a multi-phase system in which at least one phase is present in a state of microscopically fine distribution (the disperse phase: liquid or solid, for example) within a continuous phase (e.g. liquid or gas). In polymer dispersions, the disperse phase

Figure 1.1:  A polymer dispersion at various magnification levels

12

Architectural coatings and binders consists of (spherical) polymer particles with a diameter which is usually less than 1 µm; the continuous phase is water. Water-based polymer dispersions are usually milky white liquids whose viscosity varies from low, like water, to high, like whipped cream. In analogy to the milky sap of the plants which provide natural rubber, they are also often referred to as latex, and the polymer particles as latex particles (Figure 1.1). A single millilitre of polymer dispersion contains on average approximately 1015 particles. In turn, from 1 to 10,000 macromolecules are present per particle, each of these macromolecules being composed of about 50 to 106 building blocks (monomers). Polymer dispersions are not thermodynamically stable per se. The polymer system has the tendency to minimize its large internal surface area by agglomeration of the particles, coming together in lumps (coagulation), followed by settling or creaming (dependent on relative density of polymer and medium). By adding charge carriers (charge or Coulomb stabilization) or uncharged spacers of medium to high molecular weight (steric or entropic stabilization) to the surface of the polymer particles, however, the disperse state can be stabilized [9,10]. Nevertheless, external influences, such as shearing (as a result of shaking, pumping or stirring, for example), freezing, pressure or salt exposure, may in adverse circumstances cause the stabilization to fail; the dispersion then coagulates. Among polymer dispersions a distinction is made between primary dispersions, prepared by polymerizing the basic building blocks (monomers) directly in the liquid phase (via emulsion polymerization in water, for example), and secondary dispersions, for which a preformed polymer, such as a solution polymer or film-forming resin, is distributed or dispersed in the medium in a second step, usually involving input of mechanical energy [11]. The primary dispersions possess the greatest importance; they are industrially readily obtainable by emulsion polymerization and show good cost performance properties. Among the secondary dispersions, the last 15 years alkyd emulsions have gained an important position besides the class of the polyurethane dispersions [3, 4, 12]. These last ones are primarily used in the industrial coatings sector, for wood coatings and for trim paints.

1.1.2 Binder classes, polymerization and polyacrylates The chemistry of the polymers in the field of aqueous architectural coatings is very diverse. The most important classes of binders in the coating sector are: – Acrylate copolymers (pure or straight acrylics), – Styrene-acrylate copolymers (styrene-acrylics), – Alkyd and alkyd-acrylate polymers, – Vinyl acetate homopolymers and copolymers (vinyl acetate-ethylene, vinyl acetateethylene-vinyl chloride terpolymers, vinyl acetate-versatic acid vinyl ester, vinyl acetate-maleate, vinyl acetate-acrylate).

13

Introduction and basic principles Table 1.1: List of the most frequently used monomers in architectural dispersions Acrylates/ acrylic esters n-Butyl acrylate (n-BA) 2-Ethylhexyl acrylate (2-EHA) Ethyl acrylate (EA) Acrylic acid (AA) Acrylamide (AM)

Methacrylates/ methacrylic esters Methyl methacrylate (MMA) n-Butyl methacrylate (n-BMA) Methacrylic acid (MAA) Methacrylamide (MAM)

Other monomers Styrene (S) Vinyl acetate (VAc) Acrylonitrile (AN) Vinyl chloride (VCl) Vinyl versatate (“VeoVa” a) Ethylene (E)

a registered trademark of Hexion, Columbus, Ohio, USA

Other dispersions, such as styrene-butadiene copolymers or polyurethane dispersions play only a minor role in the coatings field. The reason for this is the poor weathering stability and/or strong yellowing tendency of the styrene-butadiene dispersions and the high price of the (secondary) polyurethane dispersions (PUD’s), respectively. Consequently, the use of the very hydrophobic styrene-butadiene dispersions is restricted to non-topcoat applications such as primers, especially anti-corrosion primers, and the use of polyurethane dispersions to high-performance wood coatings for especially parquet and furniture because of their excellent mechanical properties and chemical resistances.

1.1.3 Free-radical polymerization [13–20] and emulsion polymerization [10, 21–34] Apart from vinyl acetate homopolymers, the most important binder types are exclusively copolymers, where the fundamental properties are brought about by free-radical poly­merization of a specific combination of different α,β-unsaturated organic building blocks (monomers).

Reaction scheme

Free-radical polymerization is a chain reaction initiated by the decomposition of an initiator molecule (I2) to form fragments having a reactive, unpaired electron. These initiator radicals (I∙) then attack the double bond of a monomer molecule (M), forming chain radicals (I-M∙). These radicals can react with a further monomer molecule to produce extended chain radicals (I-M-M∙). The chain reaction continues to propagate until the growth of the chains (I-MnM∙) is terminated by recombination (e.g. dimerization) or disproportionation (hydrogen transfer). The resulting polymer chain length can be controlled by introducing so-called chain transfer agents (R-X). These compounds have a labile carbon-hydrogen, carbon-halogen or sulphur-hydrogen bond (e.g. mercaptans), which stop the growing chain by hydrogen or halogen atom transfer. The residual chain transfer agent radical R∙

14

Architectural coatings and binders starts a new chain. The primary effect of the chain transfer agent is a controlled reduction of the degree of polymerization. A free-radical polymerization is characterized by its rapid, exothermic course. The high-molecular polymers formed are referred to as addition polymers. The overall mechanism of the reaction is represented by the following scheme: Initiator Decomposition: I2 

2 I∙

(I = Initiator)

Chain Initiation:

I-M∙

(M = Monomer)

I∙ + M 

Chain Growth: I-M∙ + M  I-M-M∙ I-MnM∙ + M  I -Mn+1M∙ Chain Termination Reactions:

(∙ = Radical)

Recombination

(e.g. dimerization)

  I-MnM∙ + I-MmM∙

    I-M(m+n+2)-I

Disproportionation I-Mn-CH2-CHX ∙ + I-Mm-CH2-CHX ∙  Chain Transfer

  I-MmM∙ + R-X



I-Mn-CH=CHX + I-Mm-CH2-CH2X I-MmMX + R∙

The monomer building blocks most frequently used for architectural dispersions are given in Table 1.1; the basic structures of the principal classes of monomer are shown in Figure 1.2.

Mechanism of emulsion polymerization

As already described the industrial preparation of the (primary) water-based dispersions takes place via a specific form of free-radical polymerization known as emulsion polymerization. In this process, the monomers react in water in the presence of surface-active compounds of low molecular mass (emulsifiers) or oligomers/polymers (protective colloids) by adding a water-soluble free-radical initiator and heating. The emulsified monomers undergo polymerization to form the dispersed macromolecules. The micellar mechanism of the emulsion polymerization of styrene was first described by Harkins [35] and by Smith and Ewart [36] (see Figure 1.3). According to them, before the addition of initiator the monomers in the polymerization reactor are distributed between emulsifier-stabilized monomer droplets (having Figure 1.2:  Structures of four important a diameter of 1 to 10 µm) and so-called monomer classes

15

Introduction and basic principles micelles, i.e. aggregates of 20 to 100 emulsifier molecules (with a diameter of 5 to 15 nm). The fraction of monomer present in molecular solution in the water is very small. On heating, the initiator breaks down in the water phase to form radicals which initially grow by attachment to the water-dissolved monomers to form oligomeric radicals. At a certain chain length, depending on type of attached monomers, these radicals are no longer soluble in water and they start to precipitate at available surfaces. Since the number of micelles in the reactor per unit volume (approximately 1018 per cm3) is significantly higher than that of the monomer droplets (approximately 1010 per cm3), and since the total surface area of the micelles is substantially greater than that of the monomer droplets, the oligomeric radicals enter almost exclusively the micelles. There, the chains continue to grow, as a result of which the micelles should in fact become increasingly depleted of monomer. This does not occur as long as the transport of monomer molecules from the monomer droplets via the water phase to the micelles is sufficient. Consequently, the monomer concentration in the water phase remains a constant as long as there are monomer droplets present in the reactor. Meanwhile, in the micelles, the poly­mer chains grow to build up latex particles until all monomer droplets have disappeared. In the course of the polymerization, therefore, the growing, polymer-filled micelles turn into emulsifier-stabilized latex particles. To supplement the theory of micellar particle formation as described above, Fitch and Tsai  [37] postulated the principle of “homogeneous nucleation”, later further developed by Figure 1.3:  Mechanism of emulsion polymerization Ugelstad and Hansen [38]. According A: monomer droplet with monomer (E) and to this principle, initiation by a wasurfactant molecules (F) ter-soluble charged peroxide radical B: micelle with monomer molecules which adds to monomer units in the C: polymer particle, stabilized with surfactant water phase is the trigger for the formolecules, containing several macromolecules, one of these with reactive radical chain end (x), mation of oligomeric macroradicals. and monomer molecules (A) Above a chain length defined for D: water soluble initiator radical (x) each monomer (2 to 100 units), the E: monomer in the water phase (dissolved) limit of solubility is exceeded and F: molecularly dissolved surfactant molecule G: water molecules primary particles are formed. These

16

Architectural coatings and binders primary particles are usually unstable and undergo agglomeration until they reach a state of colloidal stability: secondary particles. The diameter of the secondary particles is limited by the amount of emulsifier and the polarity of the polymer. The mechanisms discussed above for the formation of the particles are limiting cases [39]. Depending on the choice of monomers and how the polymerization is performed, both mechanisms come into play to a certain extent. For instance, for monomers with higher solubilities in water (such as vinyl acetate, ethyl acrylate and methyl methacrylate) initiated with peroxides, homogeneous nucleation is important and can even become dominant. Irrespective of the mechanism discussed, a prerequisite for the emulsion polymerization is that the monomers used are at least slightly soluble in water. Consequently, while monomers such as styrene or 2-ethylhexylacrylate will still readily undergo emulsion polymerization, polymer dispersions of very hydrophobic, long-chain and hence virtually water-insoluble (meth)acrylates, such as lauryl (meth)acrylate or stearyl acrylate, are not obtainable by conventional emulsion polymerization. To polymerize this kind of monomers in an aqueous environment, a so-called mini-emulsion polymerization process can be used, in which the polymerization takes place in the droplets of a preformed mini-emulsion of the monomer(s). These droplets must be sufficiently small, i.e. from 50 to 300 nm, and stable in order to get an appropriate reaction rate and good control over the process. To get the small droplet size, a macro-emulsion of monomer(s) plus emulsifier(s)/stabilizer(s) has to be subjected to high shear (e.g. by using an ultrasonic device or (high-pressure) mechanical homogenizer), and to gain the necessary stability often a very hydrophobic costabilizer is used that is soluble in the monomer, but very insoluble in water (e.g. hexadecane) [40]. For the emulsion polymerization process on the industrial scale, the monomers are usually pre-emulsified in water (i.e. to give a macro-emulsion). The emulsion thus prepared, and the initiator solution are then metered separately into the polymerization reactor over a defined period of time. With this semi-continuous or feed process, the instantaneous conversion of the monomers is very high (usually >90 %), so that a randomly assembled copolymer is formed, largely irrespective of differences in reactivity and copolymerization parameters. Moreover, the semi-continuous process offers the advantage over the formerly used batch process, that the heat of polymerization produced can be regulated by way of the metering time and dissipated in a controlled fashion by external cooling. Before starting the emulsion polymerization process, often a small amount of a smallsized seed latex is added to the reactor. This results in a better control of the overall poly­ merization process, and of the final particle size of the dispersion. In comparison to solution polymerization, emulsion polymerization, in which the resulting polymer particles are finely distributed in water, has the additional advantage that even high molecular weights (up to more than 1 million Dalton) can be obtained with a low system viscosity. The industrial polymer dispersions therefore usually have high poly­ mer contents of from 40 to 60 % by weight.

17

Introduction and basic principles Table 1.2: Solubilities of the principal monomers for acrylic dispersions in the architectural coatings sector and glass transition temperatures of their homopolymers [41] Monomer building blocks Acrylates

Water solubility at 25 °C in g/100 cm3

Glass transition temperature (Tg) of the homopolymer [°C]

Methyl acrylate (MA)

5.2

+22

Ethyl acrylate (EA)

1.6

-8 (or -17 [16])

n-Butyl acrylate (n-BA)

0.15

-43

iso-Butyl acrylate (i-BA)

0.18

-17

t-Butyl acrylate (t-BA)

0.15

+55

2-Ethylhexyl acrylate (2-EHA)

0.04

-58

Lauryl acrylate (LA)

250 up to 400 °C, such as the mono-ester type “Texanol” d, di-esters of dicarboxylic acids, such as “Loxanol” CA 5308 c and 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (TXIB d) or tripropylene glycol mono-n-butylether (“Solvenon” TPnB c), for various solvents see Table 1.8. The reason for this is that according to provisions of the European Union and of the German paint industry association, the term solvent is used only when the boiling point is 250 °C at

26

Film formation of polymer dispersions Table 1.7: List of solvents/plasticizers of high boiling point Solvents/plasticizers Dibutyl phthalate

Example of trademarks

Dioctyl phthalate Diethylenglycol hexyl ether, hexylene glycol Polypropylene glycol alkyl phenyl ether

“Loxanol” PL 5060 c

Tributoxyethyl phosphate 2,2,4-Trimethyl-1,3-pentanediol di-isobutyrate (TXIB d)

“Optifilm Enhancer” 300 d

Triethylene glycol--bis-(2-ethyl hexanoate)

“Optifilm Enhancer” 400 d

Tripropylene glycol mono-butylether

most (at 1 atm) [68, 69]; in this case it contributes to the VOC content of the paint (according to the EU-directive 2004/42/EG “Decopaint”) [70]. All film-forming auxiliaries having boiling points above 250 °C are by definition plasticizers. They may be used without restriction in emulsion paints labelled as “solvent-free”, despite the fact that they do not remain permanently in the coating: they belong to the so-called SVOCs (semi volatile organic compounds) [68]. In general, modern coalescents should enable excellent film formation of the paints by softening the polymer particles and helping them to coalesce to a continuous film. They should preferably be easy in use and compatible with a broad range of polymer dispersions. In addition, they must be non-toxic, mild in odour, non-VOC, improving the overall coating performance and, today, should be even bio-based or biodegradable. The film-forming auxiliary particularly affects the position of the MFFT. There are three major aspects influencing the effectiveness of a coalescent: its distribution between polymer particle and water phase, its softening ability of the polymer and the rate it evaporates during drying of the film. An important part is played by the compatibility and solvation capacity of the coalescent with respect to the latex particles [71]. Its efficiency can be estimated by determining its Tg and using this value in theoretical Tg calculations of binder plus coalescent. This was done by Taylor and Klots [72] – they found values from -67 down to -129 °C for the Tg’s of the coalescents they studied. Hydrophobic solvents, such as the ester types “Loxanol” CA 5308c and “Texanol” d or the classical paraffin based white spirits, are highly compatible with hydrophobic polymers. Accordingly, depending on their affinity to the latex particles, they swell and plasticize them already in the wet state to a greater extent than hydrophilic solvents such as the glycol ethers like butyl glycol or butyl diglycol. These last ones are then predominantly

27

Introduction and basic principles Table 1.8: List of commonly used solvents/film-forming auxiliaries in water-borne architectural paints Type and commercial name

BP [°C]

Evaporation rate (vs n-BuAc = 100)

n-Butyl acetate (n-BuAc)

127

100*

100**

Solubility [g/100 ml] 0.43

170

7.8*

8.2/6**

∞

0.3**

Butyl glycol (BG), 2-butoxy-1-ethanol, butyl “Cellosolve”e “Carbitol” e

Butyl diglycol (BDG); butyl (diethylene glycol) monobutyl ether)

231

 

184–195

 

1.5

Butyl diglycol acetate

246

 

soluble

Ethyl diglycol; diethylene glycol methyl ether (EDG)

200

 

1.7**

∞

2,2,4-Trimethyl-1,3-pentanediol monoisobutyrate (“Texanol” d; TMB)

254

0.165*

0.2**

0.09

Diisobutyl esters of long-chain (C4-C6) dicarboxylic acids (e.g. “Loxanol” CA 5308 c)

>260

0.2*

0.2**

insoluble

Dipropylene glycol methyl ether (“Dowanol” DPM e/ “Solvenon” DPM c)

175

3.7*

3/2**

∞

Dipropylene glycol dimethyl ether (“Proglyde” DMM e)

162

 

10.3**

53 (35)

Dipropylene glycol n-propyl ether (“Dowanol” DPnP e)

213

0.64*

1.4**

17.2

Dipropylene glycol n-butyl ether (“Dowanol” DPnB e/“Solvenon” DPnB c)

230

0.5*

1**

5

Tripropylene glycol n-butyl ether (“Dowanol” TPnB e)

274

 

9) or nucleophilic groups of the auxiliaries (e.g. sulphinate or sulphite units from reducing agents) and/or of the polymer (e.g. mercapto groups from chain transfer agents) attack the chloroisothiazolinone active substance and lead to a rapid loss of action. Benzisothiazolinone is substantially more stable to nucleophilic attack than chloroisothiazolinone but will undergo oxidative breakdown in the presence of excess peroxide. In a number of applications, FA is abandoned and not allowed to use; even FA-depot and potential FA-forming components such as bronopol have to be omitted. But, in addition, the isothiazolinones get more and more under pressure by their potential sensitizing activity causing allergic skin or asthmatic reactions to sensitive people. Today, a lot of research is done in order to reduce or even omit MIT and CIT in the binders still maintaining the stability against microorganism attack. In certain applications, the allowed levels are so low (max. levels without labelling H 317 are 15 ppm of CIT/MIT 3:1 and/or 15 ppm of sole MIT) that this stability is no longer guaranteed or only given if a high load of up to 500 ppm BIT is used in addition. As alternatives to the isothiazolinones the last years sodium and zinc pyrithione containing biocide formulations are offered; however, zinc pyrithione is meanwhile classified as reprotoxic category 1b (leading to H 360 labelling in future) and aqua toxic (Aquatic Acute 1 with M-factor of 1000), which makes its application in coatings problematic. In addition, sodium pyrithione is less active and can cause a bluish discoloration in case traces of iron ions are present. Moreover, also the pyrithiones cannot be used without isothiazolinones and must be combined with a certain dosage of BIT to get enough microbiocidal activity. Additional biocide types such 2-butyl-1,2-benzisothiazolin-3-one/butyl-benzisothiazolinone(BBIT) or methyl-benzisothiazolinone (MBIT) are also already regulated, or lack in performance. One of the latest measures is therefore the application of high pH values in a range of 10 to 11.5. This makes it possible to produce completely biocide-free dispersions and paints. However, care must be taken to guarantee stability against hydrolysis of the acrylic moieties and to avoid a resulting larger pH drop and loss of stability against microorganisms during storage. Some of these new biocide-free binders based on styrene-acrylic chemistry are already available in the market.

48

Parameters influencing binder properties during their preparation Alternatively, also the sensitivity of the CIT molecule to alkaline conditions can be used to formulate biocide-free paints in a safer way: starting with sole CIT containing (with small amounts of MIT), robustly preserved dispersions (preferably also hydrolytic stable styrene-acrylic types), later on, during paint formulation, a base is added. This allows increasing the pH in the paints above 10 for preservation and at the same time triggering CIT degradation by the alkalinity. This concept is described in more detail in Chapter 7 for interior paints.

1.5.6 Defoamers Defoamers are generally added in small amounts to dispersions if the foaming tendency of the latter is strong, in order to prevent formation of excessive surface foam or micro-foam during preparation, handling and transport. Such foam often leads to grit and coagulum formation. In general, the transport defoamer dosage in polymer dispersions is quite low (12). Therefore, polymer dispersions for such coatings as well as their films must be highly alkali resistant and very stable to saponification. A lack of saponification resistance in the dispersion may severely curtail the useful life of a coating, as a result of chalking, cracking or loss of adhesion. In general, the ester groups present in polyacrylate and poly­ vinyl ester dispersions make them potentially saponifiable (hydrolysable). As a criterion for the hydrolysis resistance it is possible to employ the measurement of the so-called saponification test number. In this measurement, 10 g of a dispersion (for a solids content of 50 % by weight) are diluted with 30 ml of water, adjusted to a pH of 7 and reacted with 50 ml of 1 N sodium hydroxide solution at 50 °C for 24 hours. The consumption of alkali is determined by back titration with 1 N hydrochloric acid. A value of 50 (ml of hydrochloric acid) represents a perfectly saponification-stable polymer (for example, the styrene-acrylic dispersion “Acronal” S 790 a), where no sodium hydroxide solution is consumed; lower values point to a certain tendency towards hydrolysis. With complete saponification, the value is 0. Wagner tested the saponification numbers of different polymer dispersion types (see also the Chapter 4.3 on silicate paints), and reacted customary commercial polymer dispersions, as described, with sodium hydroxide solution [16]. From his results (see Figure 2.2) it is very clear that styrene-acrylic and pure acrylic copolymers possess the best saponification stabilities. An essential condition here is that the copolymers have been prepared with long-chain acrylic esters which are difficult to saponify, such as n-butyl acrylate or even better the longer side chained and branched 2-ethylhexyl acrylate. Polyvinyl esters based on vinyl acetate and vinyl propionate are less stable with respect to saponification than the acrylic systems, even after copolymerizing with ethylene, vinyl chloride or expensive, sterically bulky monomers, such as the versatic acid esters or tert-butyl acrylate. a “Acronal” registered trademark of BASF SE, Ludwigshafen, Germany

67

Formulation of water-based architectural paints In terms of saponification resistance, styrene-acrylic copolymers prove to be increasingly superior to the pure acrylics as the styrene fraction goes up – this is due to styrene’s fundamental hydrolysis stable chemical structure. In general, the sensitivity to saponification increases as the particle size decreases, owing to the increase in specific surface area.

Water resistance of the polymer film

When films of polymer dispersions are stored in water, they absorb water, swell and undergo blushing or whitening [17]. The water resistance of a dispersion film can be measured both by the rate of water absorption and by the amount of water absorbed after a fixed time period (usually 24 hours). Water penetration usually has a plasticizing effect and increases the stretchability (reflected by the elongation at break) of the films but lowers the mechanical strength and in many cases causes loss of adhesion to pigment and substrate. The reason for this is the reduction in the adhesion forces of the polymer as a function of the amount of

Figure 2.2:  Saponification numbers of polymer dispersions with different copolymer composition (market products) [16]

68

Formulating constituents of an emulsion paint water absorbed. In case of high water uptake by the coating, this is also accompanied by dimensional changes of the coating, which can easily result in formation of blisters. The aim is therefore to minimize the water swellability or water absorption of a coating binder.

Water absorption (WA)

The level of the water absorption of a dispersion film is subject to a variety of influencing factors: 1. chemical composition and polarity of the polymer, 2. type and amount of water-soluble salts and emulsifiers (which, enclosed between the particles, produce an osmotic pressure), 3. type and amount of water-swellable auxiliaries, e.g. protective colloids, 4. particle size, 5. film quality, drying conditions (incl. used type and amount of coalescents), 6. film thickness, 7. glass transition temperature, 8. temperature, 9. salt content and pH of the water. Above all, the water absorption of the polymer is determined by the polarity of the monomers used. Hydrophilic functional groups (e.g. carboxyl groups), which are solvated by water, increase the water absorption. The general rule is that the more hydrophilic the basic polymer itself is, the higher the water absorption under otherwise identical conditions. This can be seen from the water absorption values of a series of polyacrylate dispersions with virtually identical glass transition temperature. The dispersion films absorb significantly more water as the chain length goes down and thus as the polarity of the soft acrylate monomer goes up (in the series EHA < BA < EA), and, also, when there is a change from the hard monomer styrene to the more hydrophilic alternative MMA [18] (see Figure 2.3).

Effects of auxiliaries

Emulsifiers and other water-soluble materials and salts (for example, sodium- or potassium sulphate as decomposition products of sodium- or potassium peroxodisulphate, classical polymerization initiators) may form a network structure in the dispersion film and in part also accumulate at the film surface. This improves the wettability by water. In addition, the water-soluble materials pass into solution, and an osmotic pressure is produced. This leads to water penetration into the film causing whitening or blushing. Depending on the elasticity and glass transition temperature of the polymer, the film gives way to the pressure and thereby creates space for newly penetrating water. Channels are then formed through which there is further leaching of soluble constituents. The water-whitening derives from refractive index inhomogeneities caused by the penetration of the water into the interstitial

69

Formulation of water-based architectural paints

Figure  2.3:  Water uptake of different pure acrylic and styrene-acrylic dispersion films with same Tg and stabilization (values from [18])

Figure 2.4:  Comparison of long-time water uptake of films from dialysed and non-dialysed latexes [19]

70

Formulating constituents of an emulsion paint phase. As a defect, it affects transparent and semi-transparent coatings, such as clear varnishes, marble stone finishes and wood stains. In dark tinted masonry paints, it may also cause some negative colour lightening at water contact. However, most often it disappears again after a (sufficiently long) redrying period. As the amount of stabilizer increases, there is normally an increase in the water absorption and in the tendency of the films to blush. The relationship between emulsifier content and water resistance of a dispersion has been clearly demonstrated by Lamprecht [17] and Snuparek [19]. Snuparek was able to reduce the water absorption significantly (see Figure 2.4) by using dialysis methods to effect subsequent removal of the emulsifier from a model dispersion. By dialysis using semi-permeable membranes also the water whitening resistance can be strongly improved, which proves again that water sensitivity is strongly affected by water soluble salts and compounds in the dispersion film. The general rule when deciding on the amount of stabilizer for binder preparation is to find a good compromise between the water resistance of the film on the one hand, and a correct particles size control plus enough colloidal stability of the dispersion, on the other.

Effect of particle size

The water absorption (WA) level of the pure dispersion film after 24 hours is a first indicator for assessing the water resistance of the binder. From the effect of particle size on the long-term water resistance (see example in Figure 2.5), however, it becomes clear that the state reached in terms of water absorption after 24 hours is still not usually one of equilibrium. Coarse dispersions, which usually exhibit relatively poor filming, display rapid water penetration. Fine-sized polymer dispersions, which lead to a more coherent film, absorb water more slowly but, since there are higher barriers to the leaching of the water-soluble constituents, frequently attain higher final values after long water storage.

Test method

Water absorption levels increase as the film thickness drops, the water temperature rises, and the salt content of the water falls. In addition, in

Figure 2.5:  Influence of the particle size on long time water uptake [19]

71

Formulation of water-based architectural paints case of carboxylate polymers, also the pH of the water comes into play: the higher the pH, the higher the water absorption. Consequently, standardized test conditions (e.g. DIN EN ISO 62, formerly DIN 53 495; Method A) and use of deionized water with a fixed pH are vital if one wishes to compare water absorption values. Wagner likewise addressed the water absorption of different types of polymer dispersions [16]. He used commercial polymer dispersions for his study. His results (see Figure 2.6) reveal that pure acrylics and styrene-acrylic copolymers generally have lower water swellabilities/water absorption levels in comparison to polyvinyl esters. Here again, however, depending on the styrene content, styrene-acrylic copolymers are superior to the pure acrylics, since styrene is significantly more hydrophobic than the alternative hard monomer, methyl methacrylate, which is used in pure acrylic systems. When dispersion films are exposed to water several times with drying in between, the general observation is of increasing water repellence expressed in a lowering of water absorption values [15, 18, 20]. This can be attributed to leaching of water-soluble fractions

Figure 2.6:  Water absorption of polymer dispersion films with different copolymer compositions [16]

72

Formulating constituents of an emulsion paint and an improvement in film quality because of the onward progress of film formation during water contact and each redrying step. Figure 2.7 shows a graphic representation of the water absorption levels of the films prepared from two styrene-acrylic dispersions (AS1 and AS2), a pure acrylic dispersion (RA), and a very hydrophobic styrene-butadiene dispersion (SB), as a function of the number of water exposure cycles. The test cycles comprised 24 hours of water storage followed by 48 hours of restorative drying at 50 °C. The films had been dried at room temperature (to constant weight) for several days beforehand. The dry film thickness was approximately 500 µm. For all the dispersions, the water absorption decreased with the number of cycles. The greatest decrease was observed for the first two to three cycles. For the dispersions AS2, RA and SB, which had a high starting level of water absorption (initial WA values >20 %), the increase of water repellence was much more pronounced than in the case of dispersion AS1, in which the initial water absorption level was already low and the repellence concomitantly high.

Figure 2.7:  Amount of water absorbed by dispersion films after various water immersion/ drying cycles [20]

73

Formulation of water-based architectural paints

Water vapour permeability of the polymer film

According to Künzel, water absorption and water vapour permeability must be in balanced proportion [21]. As a priority, the penetration of water into the substrate must be prevented by virtue of the good water resistance of the coating and water barrier formed. If, how­ever, moisture does enter the substrate, then adequate permeability of the coating to water vapour must be in place to guarantee rapid restorative drying. Accordingly, the water vapour permeability of the dispersion film (measured in accordance with pr EN 1062-2, ISO 7783 or DIN 52 615) in interplay with its water resistance is of decisive significance. The water vapour transmission rate V is the amount of water vapour in gram per square meter which passes a coating in 24 h at defined water vapour pressure difference. From V the equivalent air layer thickness Sd of the coating can be calculated. The higher the V value and the lower the Sd value at defined film layer thickness the easier water vapour can pass the dispersion or paint film. The measurement can be done on free coating films (if defect free and not too brittle) or after application of dispersion or paint in a defined layer thickness on a porous sintered glass or other porous support with V >240 g/m2/day. In EN ISO 1062-1 for exterior paints three different categories of water vapour permeability of paint films are defined, V1 to V3 (see Chapter 4). The water vapour transmission permeability of polymer films was comparatively determined by Kossmann and Schwartz [15] as a function of the number of moisture cycles using

15

Water vapour transmission rate [g/m².d]

without water immersion after 1 cycle after 3 cycles after 5 cycles 10

5

0

pure acrylic

styrene-acrylic Binder

Figure 2.8:  Water vapour transmission rates of polymer dispersion films [15]

74

Internal

#

Formulating constituents of an emulsion paint a styrene-acrylic dispersion (AS) and a pure acrylic dispersion (RA) as examples. The graphic representation of the results is shown in Figure 2.8. Over all cycles, the film of the AS dispersion is always less permeable to water vapour than that of the RA dispersion. As the number of cycles goes up, there is a decrease in the water vapour permeability of the RA dispersion, whereas in the case of the AS dispersion there is virtually no change. Wagner [16] investigated the difference between films of pure acrylic and styrene-acrylic copolymers too, but into somewhat more depth; in contrast to the investigations indicated earlier, which used market products having different systems of auxiliaries and different preparation conditions, he considered model dispersions prepared especially for the purpose. These dispersions differed only in the nature and amount of the principal monomers – emulsifiers, functional monomers, auxiliaries and preparation processes were identical in all cases. Care was taken to ensure that all polymers had approximately the same minimum film formation temperature. Figure 2.9 compares the water absorption levels (after 48 h water contact time) and water vapour permeabilities, the latter being expressed in the paper [16] as NGL (= Normierte Gleichwertige Luftschichtdicke; which can be translated in

60 water vapour transmission NGL water absorption after 48 h

1.5

45

1.0

30

0.5

15

0.0

S/EHA

MMA/EHA

S/BA

MMA/BA S/EA MA/EA Copolymer composition

S/MA

MMA/MA

Water absorption [%]

Normalized equivalent air layer thickness [10-2.m3/g]

2.0

0

Figure 2.9:  Water uptake (in weight percent after 48 h) and water vapour transmission NGL Internal of styrene-acrylic and pure acrylic dispersions with same Tg; NGL = normalized equivalent air layer thickness (the equivalent air layer or Sd value [in m] per 100 g/m2 of the coating material) [16] #

75

Formulation of water-based architectural paints “normalized equivalent air layer thickness”) which is nothing else than the Sd value [in m] per 1 g/m2 of the coating material based on these various acrylic dispersions. This means, the lower the NGL value is, the better the water vapour permeability of the dispersion film. These data reveal that the water vapour diffusion resistance increases but the water absorption decreases as the length of the side chain of the acrylic (main) monomer increases. Both the water absorption levels and the water vapour permeabilities of the pure acrylic copolymers are higher than those of the corresponding styrene-acrylic copolymers [16]. As the copolymer becomes more hydrophobic, therefore, there is a drop in the water vapour permeability in parallel with the water absorption. This result was confirmed by later work of Baumstark, Costa and Schwartz [18]; see also Chapter 4. Attempts to reduce the water absorption therefore usually also result in a deterioration of the water vapour permeability.

Pigment binding power

The pigment binding power of polymer dispersion is another important parameter, which unfortunately can only be tested in test paint formulations (as other important binder influences on paint performance criteria, e.g. on durability, dirt pick-up resistance, or hiding power). Typical criteria for the pigment binding power of a polymer dispersion are the position of the CPVC for a defined pigment/filler blend or, in practice most often the wet scrub resistance according DIN EN ISO 11 998 at defined binder content in a fixed high PVC paint formulation. Important parameters on pigment binding power of acrylic dispersion are the particle size, the Tg of the polymer, the polarity, and the polymer architecture. More details are described in the Chapter 7 on interior paints.

2.3.2 Pigments [22, 23] Pigments are solid, mostly fine-sized white or coloured organic or inorganic particles. They are as powder materials, contrary to coloured dyes, insoluble in water. Pigments are used to give the coating or paint good hiding power and the desired colour. Moreover, they may enhance the weathering stability of the coating, preferably by shielding the binder against UV exposure (see section on masonry paints). The most important white pigment is titanium dioxide, owing to its high refractive index of ca. 2.6 to 2.7. The alternative white pigments, zinc oxide, zinc sulphide or lithopone have lower refractive indices (see Table 2.3). They are used only to a minor extent, owing to the accompanying poorer whiteness, the lower hiding power, and the greater chalking tendency of the coatings. At best, they are used for their specific antifungal activity in masonry paints. Colour pigments used may include both organic and inorganic materials. By virtue of their greater light fastness, better chemical stability and greater ease of dispersion, how-

76

Formulating constituents of an emulsion paint ever, in practice water-based paints contain predominantly inorganic pigments. Typical inorganic types are iron oxides (yellow and red), chromium oxide (green), molybdates (red), bismuth vanadate (yellow) or carbon black. Organic pigments, e.g. azo-, or polycyclic-types (phthalocyanines, diketo-pyrrolopyrroles etc.), are normally used in the paint industry in the form of ready prepared pigment pastes (often called tinting pastes) and usually only for shading the colours. The organic pigments deliver more brilliant colours than the inorganics but lack often in UVand alkaline stability. Some of them are therefore only in use for interior paints. In general, all pigments, independent of their nature being organic or inorganic, are defined by their “Colour Index” (short C.I.) which is published by the British Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists.

Organic white pigments

Organic white pigments, so called opacifiers, have meanwhile captured an important part of the market of TiO2. This is driven by the high costs of the rutile pigment, the negative effect of using rutile on carbon footprint and the potential carcinogenic character of its powder form. Organic opacifiers are non-film-forming polymer dispersions rich in styrene and carboxy groups having a particle size between 300 and 400 nm. The polymer particles are core-shell type and contain cavities which are filled with water in the wet state, but after drying they are retained and then contain air. Because of the difference in refractive index between polymer and air (i.e. ca. 1.55 versus 1.0, respectively), light is scattered and so the coating gets opacity during drying. In addition, owing to their favourable size, the opaque particles have a spacer function for the titanium dioxide particles and thus ensure a higher degree of dispersion of the TiO2 primary particles in the film when the paint dries. Opaque particles are produced using a complex multi-stage emulsion polymerization process. In fact, core-shell particles are produced in this process. Both core and shell are synthesized in various stages. The core comprises a carboxy-rich polymer whereas the shell is a virtually acid-free, styrene-rich polymer of high glass transition temperature. After the polymerization of the first shell, which must be very ductile, the carboxylic acids of the core are deprotonated by addition of a base, resulting in strong swelling of the particles. Then next shell stages are polymerized to fix and strengthen the swollen particles, so that they don’t collapse during drying. The use of organic white pigments is getting more and more popular, especially in low PVC formulations. Of course, they mainly contribute to the dry hiding of the coating, not, or only very limited to wet hiding. Their activity also depends on the type and amount of the coalescent agents used; the shell of the particles should not be destroyed during the drying and film forming process. Thus, care should be taken by the choice of the cosolvents. The organic white pigments are marketed as partial replacement for titanium dioxide, which is expensive. The produced volumes strongly depend on the fluctuating TiO2 price

77

Formulation of water-based architectural paints level: the higher the price of TiO2, the more it is replaced by organic opacifiers in paint formulations. In contrast to TiO2 the efficiency of opacifiers does not show an asymptotic saturation with increasing dosage. There is normally a linear relationship between the dosage level of opacifier and the hiding power of the paints. Therefore, it is most efficient and economical to replace titanium dioxide by organic opacifier in formulations of high TiO2 PVC. In general, replacement of up to 20 % of the TiO2 load by opacifier is possible.

Titanium dioxide [24]

The particle size of the white pigment titanium dioxide, as applied in most paint formulations ranges from 200 to 300 nm. Depending on production process, titanium dioxide exists in two modifications, anatase and rutile, with different crystal forms and, as a result, markedly different refractive indices. The refractive index of rutile is 2.70 while that of anatase is “only” 2.55. Though anatase is less expensive, because of its lower refractive index, its hiding power is poorer. Its principal disadvantage for exterior applications, however, is the fact that because of its more pronounced photocatalytic activity and its lower shielding effect, it is less beneficial to the degradation behaviour of the binder and chalking of the coating. The anatase surface reacts with UV light, moisture and elemental oxygen to form hydroxyl radicals which attack and degrade the binder. Because of this process, particles of pigment and filler are exposed on the surface of the coating, leading to the phenomenon of chalking and colour fading. Accordingly, anatase is suitable only for interior applications. In the last years however fine-sized anatase types found some new interest as special photocatalytic pigment in specific low PVC exterior paints to achieve some NOx-reduction or to improve dirt pick-up resistance. However, the positive effects seen on dirt pick-up resistance had been mainly caused by stronger binder degradation and consequent stronger chalking and thereby self-cleaning tendency. The more expensive rutile, on the other hand, leads to greater hiding power and at the same time displays a substantially reduced UV activity. Rutile is therefore the white pigment of choice for an exterior coating. By surface treatment with ZrO2, Al2O3, SiO2, or in certain rare cases with ZnO, it is possible to further reduce the already lower photocatalytic activity of rutile. Consequently, there are in some cases considerable differences in the outdoor weathering characteristics of paints containing rutiles originating from different post treatment and preparation procedures (sulphate or chloride process). It is therefore advisable to use specific preliminary experiments to verify the suitability of the type of rutile intended for use in a specific application. As already pointed out, one of the possibilities to reduce formulation costs, is to partly replace TiO2 by organic opacifiers. In addition, the last years new binder technologies are developed to improve the utilization of TiO2 in the paint by improving its dispersion and avoiding so-called crowding [25]. These new technologies are referred to as adsorbing bind-

78

Formulating constituents of an emulsion paint er technologies. In these technologies, the surface of the particles of the binder is (partly) modified with functional groups which are TiO2-affine, like specific carboxylic or phosphate ones. If this affinity is strong enough, the binder particles adsorb already in the wet paint to the TiO2 particles enabling a very efficient dispersion and spacing of them. As a result, improved opacity and hiding power of the coating is obtained. Thus, to get a similar hiding power as a paint based on a conventional binder, with such a new binder type significantly less TiO2 can be used in the formulation. In addition, better tint strength with colour pigments is achieved. In Figure 2.10 is shown how the adsorbing functionality of a binder influences the distribution of both the TiO2 and a blue colour pigment in a positive way by means of Confocal Laser Scanning Microscopy (CLSM). With this technique an object is microscoped using a focussed laser beam; the focus of this beam is imaged while scanning the sample. Often, the fluorescence of the sample is measured, enhanced by selective colouring with fluorescence markers. In the shown figures the binder is selectively coloured by a red marker, whereas TiO2 is not coloured, it appears white. The formulation was diluted and then dried. The only difference between the formulations giving the left and right picture is the binder – both have the same overall composition, but the one used for the left picture is functionalized. As a result, the distribution of both TiO2 and blue pigment is much more homogeneous, resulting in better opacity, hiding and tint strength of the final coating. In addition to a different pigment distribution, also the rheology of both formulations is different. Moreover, to really get the improved titanium dioxide utilization in the formulation,

Figure 2.10:  CLSM pictures of two same formulations, left with adsorbing binder, right with a binder with same composition but without adsorbing functionality. The formulations comprise TiO2 and a blue pigment

79

Formulation of water-based architectural paints care should be taken by the choice of pigment dispersant and thickener package to not impede the intended binder – TiO2 interaction.

Other pigments [26,27]

In addition to white pigments, a very wide variety of coloured pigments are used to colour the coating. The less expensive inorganic pigments (e.g. iron, cadmium, chromium or lead oxides or sulphides, lead molybdate, cobalt blue, carbon black) possess much better UV stability (with the exception of carbon black) then the expensive organic pigments (e.g. phthalocyanine, azo pigments, quinacridones, perylenes, carbazoles). However, they generally do not lead to the same brightness of the colours. For exterior applications, therefore, only the metal oxides are suitable, which in many cases result in good alkali resistance of the coating as well. Because of environmental reasons, iron oxides are the major inorganic colour pigments used nowadays. Furthermore, in some cases also bismuth vanadate can be used to replace the toxic lead and cadmium compounds. Specific points relating to coloured paints are discussed in the section on tinting and deep tone colours of exterior paints.

2.3.3 Fillers [23, 28, 29] Fillers are inorganic materials having a lower refractive index than the true white pigments (according to DIN 55 943 and 55 945, refractive index values for fillers are 100 µm ≤ 200 µm – E4 > 200 µm ≤ 400 µm – E5 > 400 µm Grain size/particle size (S): based on biggest particles present in sufficient amount to influence the surface structure – S1 fine < 100 µm according to EN 1524 – S2 middle < 300 µm according to ISO 787-7 or EN ISO 787-18 – S3 coarse < 1500 µm according to ISO 787-7 or EN ISO 787-18 – S4 very coarse > 1500 µm according to ISO 787-7 or EN ISO 787-18 Water vapour permeability/water vapour diffusion coefficient (V): – V0 no requirement – V1 high > 150 g/(m2 x d) or Sd < 0.14 m (according to EN ISO 7783-2) – V2 middle ≤ 150 g/(m2 x d) or Sd ≥ 0.14 and < 1.4 m (according to EN ISO 7783-2) – V3 low ≤ 15 g/(m2 x d) or Sd ≥ 1.4 m (according to EN ISO 7783-2) Liquid water permeability/capillary water absorption (W): – W0 no requirement – W1 high > 0.5 kg/(m2 x h0.5) – W2 middle ≤ 0.5 ≥ 0.1 kg/(m2 x h0.5) – W3 low ≤ 0.1 kg/(m2 x h0.5) Crack bridging properties (A): – A0 no requirement – A1 > 100 µm, no specific test speed – A2 > 250 µm at 0.05 mm/min – A3 > 500 µm at 0.05 mm/min – A4 > 1250 µm at 0.05 mm/min – A5 > 2500 µm at 0.05 mm/min

115

Exterior paints on mineral substrates Test temperature for A1 is 23 °C; for A2 to A5 recommended -10 °C [other temperatures are possible but must be indicated, e.g. A4 (-20 °C)] Carbon dioxide permeability (C): – C0 no requirement – C1 < 5 g/(m2 x d) or Sd CO2 > 50 m (according to EN 1062-6) Categorization and declaration: In the product data sheets, the producers should describe their exterior paints according to this concept. A classic low PVC masonry paint could for instance be described as: EN 1062-1; G1 (or G2), E2, S1, V2, W2 (or W3), A0, C0 (or C1).

4.2.2 Durability of masonry paints Natural and artificial weathering tests

The aim of weathering tests of exterior coatings is to get information about their lifetime. The major problem with outdoor exposure – i.e. natural weathering – is that in practice it takes too much time. To shorten this time, accelerated artificial weathering test methods are available. They involve exposing test specimens to an artificial light source in a cabinet in which for instance temperature, humidity and water spray are controlled. However, the problem with this approach is the interaction between the different parameters in the weathering process. The key parameter in all accelerated weathering apparatus is the UV light source, which should ideally simulate solar radiation. Most of the commercialized devices are using gas-discharge lamp (e.g. xenon arc lamps; ASTM D 2565; DIN EN ISO 16 474-2), electric arc (e.g. carbon arc; ASTM D 1499) or fluorescent lamps (ASTM D 4329; DIN EN ISO 16 474-3) to simulate and accelerate the effect of sunlight and its UV part. These methods of artificial weathering are also generally described in EN ISO 4892 and 4892-2 (Xenon arc). UV light can be divided into three wavelengths regions: UV-A (315 – 400 nm), which is the least harmful to polymers, forming 6 % of the total solar radiation reaching earth, UV-B (280 – 315 nm) which is more damaging to polymers, forming 0.1 % of the total solar radiation reaching earth, and UV-C ( RT), to very soft, sometimes tacky, almost cold-flowing (Tg/MFFT < RT). Fixing free polymer films on a carrier or substrate for extended periods is therefore problematic. Sometimes also simple clear coats can be formulated and applied on alumina plates. This is mainly done for hard lacquers. These methods are in complete contrast to an interesting and more practical method already developed in the 1990s by Kossmann [2] referred to as the “Whitestone test”.

Whitestone test for binder quality

The formulation used for the “Whitestone test” is like the one for a coloured aggregate or marble stone finish, except that only white marble chippings are used instead of other coloured particles. The marble chippings coating can be formulated with most polymer dispersions used for coating binders. For testing, the formulation shown in Table 4.1 can be used as modification of a former recipe published in [2].

119

Exterior paints on mineral substrates Table 4.1: Whitestone test formulation Component Polymer dispersion, 50 %

Parts by weight 195

Hydroxyethyl cellulose, 4 % solution Butyloxyethyl acetate (or today “Solvenon” DPnB

35 a)

15

White marble chippings, 2 mm

750

Defoamer

2–3

Low/mid shear HEUR or water (to adjust viscosity) Total

x 1000

a registered trademark of BASF SE, Ludwigshafen, Germany

The solids content and viscosity of the coating must be adapted in the formulation. To obtain a coating with an optimal working viscosity for trowel application, it may be necessary to add either an extra thickener (preferably a low/mid-shear HEUR type), or extra water. Although the required amount of thickener (hydroxyethyl cellulose) is determined by the viscosity of the polymer dispersion, the amount added should be kept as low as possible. The type of aggregate used is important. There are several reasons not to use quartz aggregate. Quartz usually contains yellowish inclusions of iron compounds. Moreover, when used as an extender in masonry paints, quartz can promote binder degradation and chalking [3]. By contrast, pure marble produces a striking whiteness once the thin layer of binder has been weathered away. The point at which the polymer film starts to decompose can be made more visible by tinting the binder matrix with a small amount of a tinting paste based on phthalocyanine blue pigment (using 0.03 to max. 0.05 % of a pigment preparation, such as “Luconyl” NG Blue 6900 a, with respect to the whole formulation is sufficient). The thickener solution and the pigment preparation are premixed together to achieve a more accurate mixing. Kossmann showed that a colour pigment concentration at such low level has a negligible effect on the overall UV absorption and does not extra protect the binder against sunlight caused photo-oxidation. The blue pigment greatly improves the contrast between the blue polymer film and the white marble chippings, making it easier to distinguish the two materials and to photo-document the exposure test results. To achieve efficient film formation, a suitable coalescing agent must be added to all polymer dispersions with a minimum film forming temperature (MFFT) above 5 °C. To avoid different drying times and film qualities in a test series, the use of a coalescent such as “Solvenon” DPnBoa or butyl diglycol to each formulation is recommended. a registered trademark of BASF SE, Ludwigshafen, Germany

120

Masonry paints Pot preservation is not necessary if the coating material is applied within a few days after its preparation. The amount of defoamer dosed should be so low, that not all the foam is destroyed, as some foam is desirable to prevent settling of the marble chippings. This makes it easier to accurately apply the material by trowel and to achieve a consistent film. The Whitestone coating is applied to primed fibre-cement board at a thickness of ca. 4 mm – about twice the (mean) particle size (2 mm) of the marble aggregate. The jaggedness of the marble chips leads to a considerable variation in the thickness of the polymer film on their surface; it can be well below 100 µm. To make it easier to assess the effects of weathering, it is recommended to expose the samples both at 45 and 90° (vertical) facing south. Experience shows that also in this case a surface placed at 45° weathers about twice as fast as a vertical one. However, also vertical exposure clearly demonstrates the durability trends between different binders as shown in Figure 4.4. When exposed to the weather, the white marble coatings may become dirty. The extent depends on type and softness of the binder. Moreover, it is known that chlorinated binders (e.g. acrylics containing vinylidene chloride (VDC) or terpolymers based on VAc with vinyl chloride (VCl)) are prone to yellowing. There is also some yellowing tendency of polymers containing styrene during exposure to UV light, but this is only noticeable when directly compared against coatings based on robust pure acrylic binders. Dirt pick-up and yellowing will become noticeable after 4 to 8 weeks, but it will take several months for the degradation of VCl and VDC containing binders to become noticeable. The duration will finally depend on the VCl and VDC content. As the binder degrades and the film weathers away with the consequence of exposing the underlying marble, the coating will appear much brighter because of the whiteness of the naked marble stones.

pure acrylic A 1 MFFT = 14 °C

pure acrylic A 2 MFFT = 20 °C

styrene-acrylic SA 1 MFFT = 20 °C

VAc/E/VCl MFFT = 12 °C

Figure 4.4:  Whitestone test result with different polymers (after 5 years vertical exposure); best durability with the least visibility of “naked” white marble particles for a pure acrylic (left); lowest durability for a terpolymer VAc/E/VCl (right)

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Exterior paints on mineral substrates Table 4.2: Some properties of the investigated binders MFFT [°C] 13

Water absorption after 24 h [%] 12

n-Butyl acrylate, styrene

20

8

Vinyl acetate, vinyl chloride, ethylene

12

27

Binder Dispersion RA 3

Monomers Methyl methacrylate, n-butyl acrylate

Dispersion AS 1 Dispersion VCE 1

Comparison of outdoor exposure in Whitestone test with artificially weathered coatings To answer the question how natural and artificial weathering results can be compared, three different commercial polymer dispersions (see Table 4.2) were used to formulate blue-tinted Whitestone coatings. They were weathered naturally at 45° south in the southwest of Germany and artificially in “Suntest” and “Xenotest” 1200 weathering machines [2]. As mentioned earlier, accelerated and natural weathering tests of unpigmented binders cannot be expected to yield comparable results, particularly if the accelerated test does not involve exposure to moisture. This is the case with the “Suntest” weathering machine (from Atlas), which is used only with a Xenon-arc light source exposure device to see damage by light and temperature increases. Here, a power density of 1000 W/m² of light with wavelengths in the range 270 to 800 nm only caused the terpolymer VCE 1 to yellow – no destruction of the binder film on the marble chips occurred. A “Xenotest” 1200 apparatus (from Heraeus Instruments) was used for a further test, in which the coatings were exposed to both light and moisture. The test panels, irradiated permanently by a Xenon-arc lamp, were subjected every 20 minutes to three minutes of water spray. Surprisingly, in the “Xenotest” the formulation based on the styrene-acrylic copolymer dispersion AS 1 was found to degrade more Figure 4.5:  Whitestone coating tinted with phthalocyanine blue – results of “Suntest” and” Xenotest” 1200 [2] than the formulation based on the

122

Masonry paints Table 4.3: Classification of the binder types Group Type of binder A Acrylic copolymers (pure acrylics) B

Styrene-acrylic copolymers

C

Vinyl ester dispersions (homo- and copolymers)

D

Copolymers with vinyl chloride (VCl) and vinylidene chloride (VDC)

terpolymer VCE 1, which is opposed to the result after nearly two years outdoor exposure (Figure 4.5). This is a typical example of how quickly unsound assessment of the performance of a coating can be done if only based on the results of an accelerated weathering test. This misleading result discriminates the styrene-acrylic dispersions in general and may lead to the wrong thinking that they are less suitable for exterior coatings. Investigations by Schwartz and Kossmann [4] highlighted the importance of natural weathering trials for assessing the real performance of coatings. If the “Xenotest” results are compared with those after 22 months of natural weathering (45 and 90°, facing south), it is apparent that the outdoor exposure of Whitestone coatings produces more realistic results in also quite short time. In the outdoor test there is no big difference in appearance between the vertical specimens coated with formulations of RA 3 and AS 1, whereas destruction of the surface of the VCE 1 coating is clearly noticeable. At 45° exposure, the entire polymer surface of the VCE 1 coating has been weathered off, whereas in case of the AS 1 dispersion, only the first exposed marble chips can be detected (Figure 4.6). These comparative tests thus prove that artificial with harsh conditions and natural weathering under “normal” Western European conditions can produce very different results in durability ranking [5].

Comparison of different binder chemistries in Whitestone test Although the findings of the Whitestone test are paramount, other results from the test series are also of interest. For comparison purposes, it is useful to group different dispersion binders according to their chemical composition, which should exhibit similar behaviour during outdoor exposure (see groups A to D in Table 4.3). The polymers within each group A to D have different glass transition temperatures (Tg). They differ in base copolymer composition and may contain self-crosslinking technology. This is the case with dispersions in the groups A and B of pure acrylics and styrene-acrylics. Group C comprises copolymers of vinyl acetate and vinyl propionate, but without vinyl chloride. Group D comprises chlorinated binders, whose chlorine contents can vary widely. VCl on the one hand is used therein in combination with vinyl

123

Exterior paints on mineral substrates ester and acrylic monomers; VDC on the other hand is usually copolymerised with acrylates and VCl. Group A dispersions, all pure acrylics, solely comprise esters of acrylic and methacrylic acid as main monomers. Depending on the proportion of the various monomers, their glass transition temperatures vary. They were produced without and with self-crosslinking technology. Formulations based on the classic BA/MMA dispersion RA 3 were found to have the highest photochemical stability (Figure 4.7; panel Kss 7014). Slight decomposition of the polymer films was observed with the self-crosslinking pure acrylic dispersions (panels Kss 7015 and 7016). One interesting comparison within Group B, the styrene-acrylic copolymers, is between panels Kss 7031 (dispersion AS 1), Kss 7032 and Kss 7033 (Figure 4.8 ), all of which com-

Figure 4.6:  Outdoor exposure result (after 22 months) of blue tinted Whitestone test formulations with different binder types (from left to right: pure acrylic RA 3, styrene-acrylic AS 1, terpolymer VCE 1)

124

Masonry paints prise n-butyl acrylate and styrene. Kss 7032, owing to its higher proportion of BA and lower MFFT/Tg, yields a softer film, which makes it more prone to dirt pick-up, but less susceptible to light attack, due to the lower styrene content and the protective film of dirt (see panel Kss 6991). Kss 7033 has the same monomer composition as Kss 7031 but is slightly modified to make it self-crosslinking. Like for the pure acrylics, self-crosslinking somewhat lowers the photochemical stability. Astonishingly, for the vinyl ester copolymers without VCl (Figure 4.9) the photochemical stability approaches that of the pure acrylates. This was confirmed later by Wojciechowski et al. [6] in studies on the photo-degradation of thin polymer films.

Figure 4.7:  Pure acrylics in Whitestone test formulations after 30 months outdoor exposure [2] Top/middle: blue-tinted Whitestone finishes; below: non-tinted, clear Whitestone finishes

125

Exterior paints on mineral substrates The formulations Kss 7044 and Kss 7045 consist of vinyl acetate/ethylene copolymers. They showed low degradation but strong dirt pick-up. Films of binders containing VCl as hard component are prone to rapid decomposition during natural weathering, as already indicated. Nonetheless, it should be kept in mind that, apart from their better UV stability, the main reasons for preferring acrylic and styrene-acrylic copolymers are their better alkali resistance, lower water absorption and, as a result, better long-term mineral substrate protection.

Figure 4.8:  Styrene-acrylics in Whitestone test formulations after 30 months outdoor exposure [2] Top/middle: blue-tinted Whitestone finishes; below: non-tinted, clear Whitestone finishes

126

Masonry paints Some remarks on VCl and VDC containing vinyl acetate or acrylic copolymer dispersions: there is no chlorinated binder capable of withstanding more than 30 months of outdoor weathering without significant degradation – especially VDC containing binders perform poorly. It is surprising that this polymer type was once recommended for fire-resistant renders and water resistant exterior anticorrosive coatings: low water absorption and long-term durability were back then obviously not the major criteria for choosing this binder type.

Figure 4.9:  Vinylester-copolymers (no terpolymers, i.e. without VCl or VDC) in Whitestone test formulations after 30 months outdoor exposure [2] Top/middle: blue-tinted Whitestone finishes; below: non-tinted, clear Whitestone finishes

127

Exterior paints on mineral substrates Summary Whitestone test results on durability of different binders When different polymers are subjected to natural weathering, various degrees of changes are observed after a certain exposure time. In case of pure acrylics, the changes are only small, but in other cases, especially if using VCl as comonomer in vinyl esters, degradation is tremendous. In an extreme case, the film is destroyed until exposition of the individual (white) marble chips, i.e. complete degradation of top and deeper layers of the polymeric binder, so that the marble chips became loose at the surface. Although not being a substitute for tests of fully formulated pigmented systems, the “Whitestone test” provides a quick method – four to five times faster than usual – for assessing and comparing the photochemical stability of polymer binders in exterior coatings under natural weathering conditions. Initial film degradation can be made better visible by slightly tinting the binder with phthalocyanine blue pigment. The Whitestone test provides information on possible applications for different types of binders. A binder that shows visible signs of degradation after only six months should not be used in binder-rich systems like gloss paints or unpigmented (i.e. colourless) clear coatings. In general, pure acrylics showed the best durability in the Whitestone test, followed by vinyl ester copolymers without VCl or VDC. Self-crosslinking seems to weaken the durability of acrylics. In clear coats, styrene-acrylics are lower in durability than pure acrylics, but still far better than the VAE-based terpolymers containing the halogen monomers VCl or VDC. Overall, artificial weathering with harsh UV radiation is negative for styrene-acrylates and indicates a worse performance than found in practice. Since paint formulations usually differ in their pigmentation and filler package and consequently in the shielding of the binder, the real outdoor performance should always be tested in parallel. Of course, also within a specific binder category, differences in durability are possible. This will be described in more detail for pure acrylics and styrene-acrylics of different monomer composition in the following sections.

4.2.3 Masonry paints based on polymer dispersions – binders and formulation parameters Influence of binder chemistry, paint PVC and extenders Against vinyl acetate-based and pure acrylic binders, styrene-acrylic dispersions were developed in the mid-1960s. The market demanded cheaper binders, yet with all the advantages of pure acrylics, like low water absorption, high alkali resistance and low dirt pick-up. These requirements led to the development of the first generation of styrene-acrylic dispersions based on n-butyl acrylate (n-BA) and styrene. In the masonry paints segment they gained in popularity over the years, as accelerated and long-term

128

Masonry paints weathering tests confirmed their excellent cost-performance profile. The first commercial and most popular type, “Acronal” 290 D oa, was more than 50 years the universal market standard for architectural coatings. Later, this dispersion was replaced by APEO-free versions such as “Acronal” S 790 a and “Acronal ECO” 6716 a, both with an MFFT of ca. 20 °C. An additional advantage of these fine-sized anionic styrene-acrylic dispersions is their high pigment-binding capacity. Even today most masonry paints are still formulated with relatively hard binders (mostly MFFT = 15 to 25 °C) and coalescing agents. Therefore, most fundamental investigations described in this book are based on such conventional systems. In a later part (Chapter 4.2.5), analogous findings of masonry paints formulated with softer binders without need for additional solvents will be discussed. Figure 2.1 (see Chapter 2.2.1) shows the types of coatings for which the two binder classes of pure acrylics and styrene-acrylics are in use today and the typical pigment vol-

Figure 4.10:  Water vapour transmission rates WVTR in dependence of the masonry paint PVC (based on a pure acrylate (Ak) and styrene-acrylate (Ak/S)) [7]

a registered trademark of BASF SE, Ludwigshafen, Germany

129

Exterior paints on mineral substrates Table 4.4: Some properties of the two binders used in the study [4] Product Dispersion Ak

Monomers MMA, n-BA

Dispersion Ak/S

S, n-BA

MFFT [°C] 13

Water absorption after 24 h [%] 12

20

8

ume concentrations of the formulations. In the following, differences and similarities between both types of acrylic dispersion will be discussed in more detail.

Laboratory tests on polymer dispersions for masonry A large variety of polymer dispersions is available for the formulation of masonry paints. To assess their potential, comparative laboratory investigations were carried out on paint films based on two typical commercial dispersions (see Table 4.4) of the types of pure acrylate (Ak) and styrene-acrylate (Ak/S) in the PVC range of 15 to 55 %. The pure binder films differ slightly in water absorption with certain disadvantage for Ak versus Ak/S, but formulated paint films with both copolymers deliver quite comparable

Figure 4.11:  Results of tensile strength measurements of paint films in dependence of the PVC, untreated versus after 4 cycles of water immersion and redrying [7]

130

Masonry paints water absorption values. However, the Ak/S-system leads, independently of the PVC, to a water vapour transmission rate which is inferior to that of the Ak-system (Figure 4.10) [7]. The mechanics of the paint films are, as expected, strongly influenced by the PVC, but also by water immersion and drying processes. Remarkable is the U-shape of the tensile strength curves (Figure 4.11). Untreated Ak/S paint films (i.e. those not immersed in water) were found to have the lowest strength, irrespective of PVC. The tensile stresses of the Ak and Ak/S control specimens (not immersed in water) pass through a minimum at a PVC of 30 and 35 %, respectively. To explain the shape of the curves, it is necessary to consider two opposing effects. Firstly, emulsifiers in the binder, and thus in the paint, can have a plasticizing effect, which decreases with increasing PVC. Secondly, the “hard” components in the formulation – pigments and extenders – reinforce the polymer matrix (the higher the PVC the more), thereby increasing the stiffness [8] and tensile strength of the film. Immersion in water was found to increase film strength, especially during the first wash/dry cycle (Figure 4.12). This effect is more pronounced in the Ak/S system than in the Ak system. This increase in film strength also depends on PVC [7]. Increasing the PVC of the paint formulation from 15 to 55 % causes a drastic fall in the extensibility expressed in a drop of elongation-at-break of the paint films. This is a general trend (see Figure 4.12); it worsens (mainly for the AK/S-based paints) by repeated immersion in water. All these experiments demonstrate that by washing out the water-soluble components of the formulation, the mechanical properties of the paint films change – the films become more brittle and partly lose their extensibility. This phenomenon was also described in a study by Bradac and Novak [9]. In general, it is assumed that during film formation of a polymer dispersion the water-soluble constituents accumulate at the boundaries of the original polymer particles. By washing out these water-soluble components, they leave small voids and defects in the film. Although these voids and defects are small in terms of volume, they can exert a considerable influence on the film mechanics. In summary, the observed effects of emulsifiers (and other water-soluble components) can be ascribed to, on one hand, plasticization [10] helping film formation, and, on the other one to an impeding effect on film formation by hampering the coalescence of the particles [11]. By forced drying of the (wet) film at 50 °C – i.e. well above the glass transition temperature of the binder – the film forming process can be continued leading to a more cohesive film. In principle, the results of the tests on Ak paints are comparable to those on the Ak/S ones. One difference worth noting is that the action of water does not increase the tensile strength and decrease the elongation-at-break of Ak films as much as of the Ak/S ones, particularly at low pigment loadings.

131

Exterior paints on mineral substrates The extensibility of both types of films worsens with increasing pigment content (see again Figure 4.12). But there are differences, too. In contrast to the behaviour of Ak/S films, the extensibility loss of Ak films is largely independent of cyclic water immersion and redrying. Another feature is the rate of extensibility loss with PVC. For Ak films, this rate is mostly gradual up to a PVC of 25 %; after that, it becomes larger, as seen by the change in the slope of the curves. Water strengthens Ak/S paint films and decreases their extensibility more than Ak paint films. Outdoors, where paint films are subjected to action of water in the form of dew and rain, the improvement of the tensile properties of Ak/S paint films is advantageous. In artificial weathering tests, only little colour change was observed in the styrene-acrylate paints, but it was better visible than in pure acrylic formulations. However, the differences were not so big. Moreover, no significant difference was found regarding the yellowing of the two types of paint. The yellowing Δb* is, for the styrene-acrylic as well as for the pure acrylic paints, in the same range and practically independent of PVC. These findings agree with those of Stevens [12], who concluded that, at PVC of 20 %, the presence of styrene had no effect on the yellowing tendency of styrene-acrylic paints.

Figure 4.12:  Elongation-at-break/extensibility of paint films in dependence of the PVC; untreated films versus films after 4 cycles of water immersion and redrying [7]

132

Masonry paints Table 4.5: Some properties of the pigments in use [4] Pigment Density [g/cm³] Rutile titanium dioxide 4.1

Oil absorption [g/100 g] 18

Yellow iron oxide

4.1

65

Red iron oxide

5.0

26

The large total colour difference values (ΔE*), occurring in the styrene-acrylic formulations, particularly at PVC = 55 %, therefore result from changes in the values of L* (the lightness of the shade) and a* (the redness or greenness of the shade) and not of b* [7].

Differences between styrene-acrylates and pure acrylates in natural weathering The two typical polymer dispersions – the pure acrylate (Ak) and the styrene-acrylate (Ak/S) – were also used in a following natural exposure study [4] (for the binders see again Table 4.4). Tests were carried out on films of the pure dispersions and of various masonry paints in which the dispersions were used as binder. The paints were applied to a substrate and allowed to weather naturally; they were examined for evidence of chalking and colour change at defined intervals. As already discussed, laboratory experiments cannot tell the whole story, but they can be used to support long-term weathering tests. The following variables were considered: a) type of dispersion (acrylate versus styrene-acrylate), b) type of pigment (titanium dioxide versus iron oxide), c) pigment volume concentration, and d) type of extender. The paints used in the tests were produced in two colours: the first a titanium dioxide containing formulation tinted in a cream tone by additional yellow iron oxide (for recipe see Table 4.12), the second in a reddish-brown coloured tone solely obtained by a mixture of red and yellow iron oxide. Some important data of the pigments is listed in Table 4.5. An 83 :17 blend of calcite (calcium carbonate) and talc was fixed as extender mix in those tests where the type of extender was not the variable (i.e. the variables a to c) [4]. Fibre-cement board was chosen as the alkaline substrate on which the paints were tested. The board was first primed with a water-based emulsion primer (to even out any differences in absorbency and to prevent efflorescence due to leaching of alkali salts from the substrate), followed by two coats of the test paint with a total coat weight of 300 g/m2. The panels were placed in a south-facing exposure rack inclined at 45° at Limburgerhof, southern Germany (equivalent to the weathering of coatings on vertical surfaces, but 2 to 2½ times faster, see above). At set intervals, the coatings were photographed and examined for chalking (DIN 53 159: 2010-08) and colour changes (DIN EN ISO 11 664-4).

133

Exterior paints on mineral substrates Table 4.6: Chalking of cream-coloured masonry paints due to exposure  (DIN 53 159; 1 = strong chalking, 10 = no chalking) Type of binder Pure acrylate Ak Styrene-acrylate Ak/S Years 1 2 9 1 2 9 PVC = 35 % 9 9 6 9 9 6 PVC = 45 %

9

8

5

9

9

6

PVC = 55 %

9

7

5

9

7

5

Effect of binder type at low pigment volume concentration The effect of binder type and PVC was investigated in matt masonry paints. A useful measure of weathering resistance is the extent of chalking. Weathering is the process whereby UV radiation and moisture destroy the binder at the surface of the coating, thereby exposing the pigment and filler particles. Chalking was assessed on a scale of 1 (heavy chalking) to 10 (no chalking). Normally, no colour changes occur at ratings between 6 and 10. Paints were formulated with the pure acrylic and styrene-acrylic binders at PVC of 35, 45 and 55 %. Coated test panels were kept outdoors for a nine-year period. At PVC of 35 and 45 %, the effect of the binder type is small: after 2 years exposure, chalking still got a rating of 9 to 8 (i.e. low chalking); there was also no noticeable colour difference between both formulations. The PVC has a greater influence than the binder type. Comparing chalking of paints with different PVC, a difference is observed after two years (the change is roughly the same for each binder type). After this period, chalking develops to a rating of 9 for a PVC of 35 % and to 7 for a PVC of 55 % (see Table 4.6). Chalking, and with it colour change, becomes more pronounced after 9 years. Ratings for each value of PVC are 5 or 6; the pigment loading therefore makes little difference. Again, the effect of the binder is small. Similar results were observed with the red-brown, full-shade paints. A paint with high PVC applied to a vertical substrate (a wall in practice) chalks and its colour fades earlier than when applied to a south-facing inclined surface. The reason is that the inclined surface easier collects airborne dirt. Since the dirt protects the binder from UV radiation, the coating exhibits less chalking in the initial stages of weathering. However, this effect evens out after many years of weathering.

Effect of pigment/extender ratio on weathering properties Weathering tests were carried out on paints with the same binders and colour shades, but with PVC of 45 and 55 %. In comparison with the paints mentioned above, different ratios of pigment and extender were used.

134

Masonry paints Table 4.7: Total colour difference ΔE* of cream-coloured paints after 5 years outdoor exposure Binder type Pure acrylate Ak Styrene-acrylate Ak/S PVC [%] Pigment/ 45 55 45 55 extender ratio 10:90 6.6 5.7 7.0 6.0 30:70

3.0

2.7

2.5

1.9

50:50

1.6

2.8

2.1

2.0

Coloured pigments with good hiding power (e.g. iron oxides) are often mixed with a high proportion of extender to reduce the formulation costs. Masonry paints of higher PVC that contain much extender are prone to early colour fading. This, however, does not necessary mean that strong chalking occurs in the initial stages of photochemical degradation of the binder: the exposed pigment and extender particles may still be firmly anchored by the remaining binder. Here, only fading was investigated by photometrical determination of the total colour difference ΔE* after 5 years of weathering. The results in dependence of PVC and pigment/extender ratio are given in Table 4.7. Paints with the higher proportion of extender (90 %) showed the strongest shift in ΔE* and faded the most. This confirms what Kresse [13] suggested: the extender provokes fading in formulations containing less than 30 % of pigment. The findings were independent of the type of binder.

Effect of the extender type on durability Most masonry paints contain fillers/extenders. These raw materials not only make the formulation more economic, but also impart desirable properties – for example, they improve rheology, film mechanics, abrasion resistance and hiding power by spacing of titanium dioxide particles and as already mentioned, by controlling film porosity the resulting dry hiding effect at high PVC. Furthermore, they provide surface texture. Most extenders are mineral substances with a wide variety of chemical composition. Table 4.8 lists some common minerals used as extenders; see also Chapter 2.3.3 and Table 4.5. In the exposure tests, the only variable was the type of extender: the binders, PVC and colour shades were adopted from the previous trials; a pigment/extender ratio of 30 : 70 was chosen. The results after 3.5 years are summarised in Table 4.9 and Figure 4.13 to Figure 4.16. Except for calcite, dolomite and barytes, which caused low chalking (ratings of just 8 and 9), the rest of the extenders produced in most cases significant chalking. There was also a slight

135

Exterior paints on mineral substrates Table 4.8: Types of fillers for exterior masonry and their properties Specific Oil pH [of 10 % Silica surface absorption slurry in SiO2 [%] area [m2/g] [g/100g] water] Calcite 1000

617

289

700

496

750

Cycle 1

[%]

>1000

631

273

686

482

717

Cycle 2

[%]

>1000

664

289

729

506

650

Cycle 3

[%]

>1000

619

405

610

501

642

Cycle 4

[%]

>1000

744

389

663

583

740

[g/(m².d)]

111.9

76.5

36.2

52.0

22.5

63.2

[m]

0.2

1.2

0.5

0.4

0.9

0.3

WVP * Sd * Of a film of 150 µm dry thickness

bling the substrate to dry out. More on this topic can be found in the relevant literature [19] and Chapter 2. The water vapour permeability (WVP) or water vapour transmission rate (WVTR) of the styrene-acrylate films is lower than that of the corresponding pure acrylic films of the same layer thickness (see also Table 4.10). The EA/MMA copolymer (M1) was found to be the most permeable to water vapour, M3 and S3 with 2-EHA-based copolymers were the least permeable. Thus, the WVTR decreases with increasing length of the alkanol side-chain. The results show, that the more hydrophilic the polymer is, the higher both water absorption and water vapour permeability are, as expected.

142

Masonry paints Table 4.11: (contiuned) Dispersion Colour 0 hrs changes (SUN weathering) after 24 hrs

after 96 hrs

after 144 hrs

L*

M1 94.1

M2 96.0

M3 95.9

S2 95.8

S3 95.8

SM2 95.4

a*

-1.0

-0.9

-1.0

-1.0

-0.9

-1.0

b*

0.1

0.1

0.0

0.2

0.2

0.1

L*

95.7

96.0

95.9

95.7

9.,5

95.9

a*

-1.2

-1.3

-1.3

-1.9

-2.3

-1.4

b*

0.6

0.7

0.7

2.6

4.0

1.2

∆ b*

0.5

0.6

0.7

2.4

3.8

1.1

∆ E*

1.7

0.7

0.8

2.6

4.1

1.3

L*

96.0

96.3

96.1

95.4

94.4

96.0

a*

-1.5

-1.5

-1.5

-3.5

-4.4

-2.0

b*

0.4

0.4

0.5

7.2

13.2

2.0

∆ b*

0.3

0.3

0.5

7.0

13.0

1.9

∆ E*

2.0

0.7

0.7

7.5

16.5

2.2

L*

96.1

96.3

96.1

94.9

93.5

96.0

a*

-1.5

-1.5

-1.5

-4.2

-4,7

-2.2

b*

0.4

0.3

0.4

10.8

17.3

2.4

∆ b*

0.3

0.2

0.4

10.6

17.1

2.3

∆ E*

2.1

0.7

0.7

11.1

17.7

2.7

Tensile tests (tensile strength/elongation-at-break) of free dispersion films Although a comparison is influenced by the small differences in the glass transition temperatures of the polymers, it was found that the tensile strength of the polymer film increases with increasing length of the alkanol chain, while the corresponding strain-(elongation)-at-break decreases (see also Table 4.11). Except for the EA/MMA copolymer (M1), repeated washing and drying causes the tensile strength of the films to increase – that of the pure acrylates more than that of the styrene-acrylates. The elongation-at-break of the pure acrylates (except M1) and of S3 also increases; on the other hand, S2 and SM2 become less elastic. Repeated washing and drying strengthens the pure acrylic films, while the tensile properties of the styrene-acrylate films remain largely unchanged. The M1 copolymer, which is hydrophilic, exhibits remarkable behaviour: while the repeated action

143

Exterior paints on mineral substrates

Figure 4.17:  UV-absorption spectra of binder M1 before/after irradiation

Figure 4.18:  UV-absorption spectra of binder M2 before/after irradiation

144

Masonry paints of water led to a drop in its tensile strength, the film was found to remain very elastic; in other words, the M1 films suffer a considerable loss in cohesion (intrinsic strength). Effect of artificial weathering of the dispersion films Dry films of the model dispersions on a quartz plate were irradiated in an Atlas CPS+ apparatus. The wavelengths were in the range of 270 to 800 nm. The irradiation was unfiltered with a power of 1000 W/m2. No flooding occurred. The absorption spectra of the dispersion films were measured after 0, 24, 96 and 144 hours of irradiation. Some typical spectra are shown in Figure 4.17 to Figure 4.21. The absorbance A (formerly optical density or extinction) of a sample (at a certain wavelength) is defined by the equation

A = - log (I/l0)

with l = intensity of transmitted light and l0 = intensity of incoming light. An absorbance of 1 corresponds therefore to absorption of the incident light by the polymer film of 90 %, an absorbance of 2 to 99 %. There was almost no change in the absorption spectrum of M2 by irradiation (see Figure 4.18). On the other hand, the presence of ethyl acrylate in M1 (see Figure 4.17) and

Figure 4.19:  UV-absorption spectra of binder M3 before/after irradiation

145

Exterior paints on mineral substrates

Figure 4.20:  UV-absorption spectra of binder S2 before/after irradiation

Figure 4.21:  UV-absorption spectra of binder S3 before/after irradiation

146

Masonry paints 2-ethylhexyl acrylate in M3 (see Figure 4.19) resulted in a slight increase of the absorbance below 300 nm as a function of irradiation time. Much larger initial absorption in the UVrange of 240 to 280 nm is caused by the aromatic styrene groups and strong changes in the spectra were found in the styrene-acrylate versions S2 and S3 (see Figure 4.20 and Figure 4.21). Comparison of the absorption spectra of S2 with S3 shows that the absorbance of S3 (the EHA copolymer) is larger over the whole spectrum, irrespective of exposure duration. This is due to the higher styrene content of S3 (54 % for S3 compared to 45 % for S2). UV-induced changes in the EHA/S copolymer are therefore more pronounced than those in a BA/S copolymer of the same glass temperature. Figure 4.22 shows the absorption spectra of SM2. The absorbencies are larger than those of M2, but smaller than those of S2, it represents the intermediate case, as expected at basis of its styrene content. In summary, it can be said that the styrene-acrylates show pronounced absorption already before and strongly pronounced after irradiation in the studied UV wavelength range, contrary to the pure acrylates. The absorption intensities are correlated to their styrene content. The intensity increase due to irradiation is much larger that for the acrylates and is also correlated to the styrene content. The changes for the pure acrylates are small, the smallest for M2 containing BA, and the largest for M3 containing EHA. This means that the BA copolymers exhibit the best photostability.

Figure 4.22:  UV-absorption spectra of binder SM2 before/after irradiation

147

Exterior paints on mineral substrates In addition, the colour changes of the polymer films were determined. The results are also summarized in Table 4.11. ΔE* values for the styrene-acrylates S2, S3 and SM2 increase with exposure duration, as they do for the pure acrylate M1, but at a much lower level. The pure acrylates M2 and M3 showed the smallest colour changes after 144 h exposure, the styrene-acrylates S2 and S3 the largest. The yellowing of the films, expressed as a change in Δb* along the blue-yellow axis, is quite strong for the styrene-acrylates, in strong contrast to the pure acrylates. The colour change of SM2, which contains the hard monomers MMA and styrene, is surprisingly small. This underlines the fact that a polymer must contain well above 20 % of styrene to show significant yellowing [12]. S3, the styrene-richest of all samples, exhibits the strongest yellowing. Comparing the pure acrylates M1, M2 and M3 with respect to yellowing tendency, there is little difference between the constituent "soft" monomers EA, n-BA and EHA. This shows that the poor performance of S3 is indeed due to its high styrene content. Whitestone test result The presence of pigments and extenders in the paint formulation has a considerable effect when it comes to weathering resistance, as these substances protect the binder from the destructive action of UV radiation. Therefore, to rule out the protective effects of pigments on the binder, only pigment-free formulations were used in weathering trials. The formulation used for the Whitestone test is like that described before. By tinting the formulation with a small amount of phthalocyanine blue (see discussion above [5]), the point at which the polymer degradation starts was made better visible. Destruction and weathering-off of the blue tinted binder matrix exposes the marble chips at the surface and let the finishes appear whiter. The extent of photochemical destruction of the binder was assessed after 12 months of outdoor exposure. The results are documented photographically in Figure 4.23.

Figure 4.23:  Influence of monomer composition in Whitestone test coatings after 12 months exposure (45° south)

148

Masonry paints Table 4.12: Formulation of the cream-coloured test paints (pigment/filler ratio = 30 : 70) Exterior model paint formulation Parts by Weight Kind of raw material Deionized water 54 “Natrosol” 250 HR b

1

Thickener, hydroxyethyl cellulose, HEC

“Calgon” N c, 10 % solution

7

Dispersing agent 1, sodium hexametaphosphate

“Dispex” AA 4030 a

2

Dispersing agent 2, ammonium salt of poly acrylic acid

Ammonia, conc.

1

Neutralizing agent

White spirit K 60

7

Coalescent 1

7

Coalescent 2

“Loxanol” CA 5308

a a

2

Defoamer, mineral oil based

Polymer dispersion, 50 %

70

Acrylic model dispersion M1, M2, M3, S2, S3, SM2

“Kronos” 2043 d

100

White pigment, titanium dioxide

Iron oxide yellow 930 M

20

Yellow pigment, ferrous oxide

227

Filler 1, calcium carbonate

45

Filler 2, fine talc

2

Pot preservative

1

Defoamer; mineral oil based

“Foamaster” MO 2134

“Omyacarb” 5

e

Talc AT 1 “Acticide” FI

f

“Foamaster” MO 2134

a

Polymer dispersion, 50 %

454

Total

1000

Acrylic model dispersion M1, M2, M3, S2, S3, SM2

Characteristics: PVC [%] ca. 35, Solids by weight [%] ca. 65 a b c d e f

registered trademark of BASF SE, Ludwigshafen, Germany registered trademark of Ashland, Wilmington, Delaware, USA registered trademark of ICL Phosphate Specialty, Creve Coeur, USA registered trademark of Kronos International, Dallas, Texas, USA registered trademark of Omya AG, Oftringen, Switzerland registered trademark of Thor Chemie, Speyer, Germany

A comparative assessment was difficult because of a varying dirt pick-up of the coatings. Nevertheless, considerable polymer degradation was found in M1, followed by M3. The differences between M2 and S2 on the other hand are very small. In general, the pure acrylates M1, M2 and M3, due to their stronger dirt pick-up, appear worse than S2 and S3. Noticeable is the very extensive dirt pick-up of the hydrophilic pure acrylates M1 and M2, as well as the terpolymer SM2. The 2-EHA containing copolymers M3 and S3, which

149

Exterior paints on mineral substrates Table 4.13: Test results for the formulated cream-coloured model paints (according to the base formulation in Table 4.12) Cream paint Unit M1 M2 M3 S2 S3 SM2 Viscosity (100 s-1) [mPas] 471 476 495 419 446 616 Water absorption

[%]

32.2

13.4

12.6

14.4

10.7

11.0

Wash-out losses/ extracts

[%]

0.9

0.8

0.8

1.1

1.1

0.8

[N/mm²]

1.9

3

5.2

1.7

2.0

2.3

[%]

490

170

60

290

250

160

[g/(m²∙d)]

32.5

17.3

9.4

14.2

8.0

14.8

[m]

0.6

1.1

2.1

1.4

2.5

1.3

Tensile strength at RT Elongation-at-break at RT WVTR (on cardboard *) Sd

* WVTR of uncoated cardboard = 260 g/(m².d)

are more hydrophobic, exhibited the least soiling. This agrees with earlier findings of Smith and Wagner [20], whose experiments also showed that the least dirt pick-up of exterior coatings was of those based on hydrophobic binders. The weathering stability and resistance to dirt pick-up of the polymers therefore not only depend on the type of “hard” monomers present (here S or MMA), but also to a large extent on the “soft” acrylic co-monomers.

Tests on formulated paints Water absorption and water vapour permeability Apart from binder type, other water-soluble constituents such as thickeners, dispersants and wetting agents influence the water absorption of fully formulated paints. In earlier work [4, 7], the influence of PVC on the paint properties was discussed. Based on commercial binders of the types of pure acrylate (Ak) and styrene-acrylate (Ak/S) having similar Tg, WA and WVTR were determined for coloured paints in the range of PVC = 15 to 55 %. For the whole PVC range, paint films based on the Ak/S binder give lower WVTR and WA than the ones based on Ak (see Figure 4.10). For the tests with the model binders, the same paint formulation was used as in earlier work with PVC of 35 %, which is well below the CPVC. The paint formulation is given in Table 4.12. The light cream colour shade obtained by using an excess of titanium dioxide and a small amount of a yellow ferrous oxide was used to determine differences in dirt pick-up resistance and colour changes more precisely. The water absorption values of the pure binders are given in Table 4.10 and of the formulated paints in Table 4.13. Styrene-acrylate paints absorb less water than their pure

150

Masonry paints acrylic counterparts. The M1-based paint (EA/MMA) was found to absorb the most water. The same relationship between water absorption and hydrophilicity of the binder found in the dispersion films is therefore present in the paint films. The WVTR values of the paints are also listed in Table 4.13. As was the case with the dispersion films, the M1-based paints were found to be the most permeable to water vapour. As for the water absorption, the WVTR of the paints decreased with decreasing polarity of the polymer. Tensile tests (tensile strength/elongation-at-break) Like for the polymer films, it was found that the longer the alkanol chain of the binder polymer is, the higher the tensile strength of the paint film, but the lower the elongation-at-break (see Table 4.13). For binders containing the same type of alkanol chain, the tensile strength of a pure acrylate was higher than that of the corresponding styrene-acrylate (i.e. M2 vs. S2 and M3 vs. S3); the reverse is true for the elongation-at-break. When it comes to outdoor applications, the higher elasticity of the Ak/S systems – and therefore their potentially better crack-bridging properties – is more advantageous than the higher tensile strength of the pure Ak systems. Weathering of the paint films: artificial weathering The results of the weathering trials are given in Table 4.14. The total colour difference ΔE* increases with exposure duration in all the systems tested. The values for M2 and M3 are, however, somewhat lower than those of the corresponding styrene-acrylates S2 and S3. In contrast, the pure acrylic M1, which is hydrophilic, underwent the greatest colour change (ΔE* = 8.7). The differences in ΔE* values are mainly due to the bigger shifts in b* value of the styrene-acrylate paints along the blue-yellow axis away from yellow in the direction of blue (i.e. negative Δb*).This is surprising since Δb* of the non-formulated styrene-acrylate dispersion films moved into the positive direction, i.e. towards yellow. Disregarding the M1 paint, it can be said that the colour changes of the tested paints is small. This clearly demonstrates the difference between pigmented systems and pigment-free polymer films. The screening or shielding effect of the pigments and extenders largely equalizes the differences in photostability of the various binders. Much clearer differences can be observed considering the resistance to chalking of the paints. The strongest chalking was found in the M1-based paint; the S2 and S3 paints also chalk, but to a lesser extent. No difference in final chalking due to the different acrylic monomer was found for the styrene-acrylate paints. In the case of the pure acrylates, however, M2 chalks slightly less than M3. M2 and M3 are both less prone to chalking than their styrene-acrylate counterparts S2 and S3. The performance of SM2 lies between that of M2 and S2 and after 1000 h Xeno-treatment still at the level of M3.

151

Exterior paints on mineral substrates Table 4.14: Results of accelerated weathering of formulated model paints (according to the base formulation in Table 4.12) Cream paint M1 M2 M3 S2 S3 SM2 0 h at L* 79.24 79.01 78.79 78.57 78.04 78.78 begin a* 4.43 4.60 4.65 4.86 5.04 4.80 after 250 h

after 500 h Colour changes (Xeno 1200) after 750 h

after 1000 h

Chalking * (Xeno 1200)

b*

33.61

33.57

33.73

33.47

33.84

33.60

L*

75.20

75.32

74.88

74.96

74.70

75.16

a*

4.26

4.20

4.31

4.17

4.09

4.20

b*

29.59

30.06

30.45

27.60

27.02

28.60

Δb*

-4.02

-3.51

-3.28

-5.87

-6.82

-5.00

ΔE*

5.70

5.11

5.11

6.93

7.65

6.20

L*

74.93

75.24

75.46

75.41

74.99

75.66

a*

4.12

3.88

3.86

3.88

3.97

3.82

b*

29.20

29.45

28.90

25.91

26.07

26.45

Δb*

-4.41

-4.12

-4.83

-7.56

-7.77

-7.15

ΔE*

6.17

5.63

5.92

8.25

8.42

7.86

L*

75.27

75.95

75.50

75.21

74.91

75.55

a*

4.08

3.58

3.77

3.91

4.17

3.99

b*

27.17

27.58

28.02

26.61

26.66

27.20

Δb*

-6.44

-5.99

-5.71

-6.86

-7.18

-6.40

ΔE*

7.57

6.80

6.65

7.70

7.88

7.21

L*

75.62

75.84

75.40

75.05

74.79

75.21

a*

3.79

3.67

3.83

3.82

3.95

3.96

b*

25.73

28.07

28.48

27.45

26.98

28.50

Δb*

-7.88

-5.50

-5.25

-6.02

-6.86

-5.10

ΔE*

8.70

6.42

6.30

7.05

7.67

6.28

after 250 h

0

0

0

0-

0-

0

after 500 h

0

0

0-

1

2

0-

after 750 h

1

0-

1-

2

3

2

after 1000 h

5

2

3

4

4

3

* 0 = no chalking, 5 = strong chalking , if a - sign is appended, the value is worse than the full value

152

Masonry paints Natural weathering Washed fibre-cement boards were painted and weathered outdoors for one year (45°, south facing). As in the Whitestone test, the pure acrylic paints were found to pick-up most dirt. Of these, the EA/MMA copolymer paint showed the strongest dirt pick-up. M2 picked up somewhat less dirt and M3 even less. The less hydrophilic the binder of the pure acrylic paint was, the less dirt was found on its surface. The SM2 terpolymer exhibited the same high level of dirt pick-up as M2. In general, S2 and S3 picked up less dirt than the corresponding pure acrylics. Among the styrene-acrylates, S3 was better than S2. Paints based on the hydrophobic EHA-containing polymers S3 and M3 exhibited both the lowest dirt pick-up after 1 year of natural weathering (see Figure 4.24). The lower dirt pick-up of these two paints was certainly promoted by the somewhat higher glass transition temperatures of both binders. It may derive also from the fact that the branched side-chain of 2-EHA is more sensitive to photochemical reactions which can lead to slight surface crosslinking. Colorimetric measurements showed that the total colour difference ΔE* decreased with increasing alkanol chain length (see Figure 4.25). The values of ΔL*, the change in lightness, supported the dirt pick-up characteristics found by visual assessment. Thus, both ΔL* and Δb* became smaller upon increasing length of the alkanol side-chain of the binder. Comparing M2 with S2 and M3 with S3 – i.e. each pair with the same “soft” but different “hard” monomer – shows that there is only a small difference between the ΔE*, ΔL* and Δb* values.

Summary of the results on choice of monomers

The styrene-acrylates, both as dispersion and paint films, proved to be more hydrophobic and less permeable to water vapour than the corresponding pure acrylic formulations. Furthermore, water absorption and water vapour transmission decrease as the length of the copolymer side-chain increases (EA < BA < EHA). In all cases, except for M1 with EA, repeated water immersion and drying caused the tensile strength of the dispersion films to increase, although the effect was not so pro-

Figure 4.24:  Cream coloured exterior model paints - panels after 12 months outdoor exposure

153

Exterior paints on mineral substrates nounced for styrene-acrylates. The tensile strength of the paint films increased with alkanol chain length; their elongation-at-break on the other hand decreased. In general, the tensile strength of the pure acrylic paint films was higher and their elongation-at-break lower, than in case of their styrene-acrylate counterparts. Due to the high absorbance of the phenyl group in the viewed wavelength range, styrene-acrylic films transmit less UV radiation than pure acrylic ones. Irradiation by a “Suntest” equipment generally causes the absorbance of the polymer films to increase, but the increase is considerably less in the case of Ak films. S3, the EHA/S copolymer, the binder richest in styrene (54 %), exhibited the largest increase in UV absorbance. Colorimetric measurements following artificial weathering of the dispersion films showed that, overall, Ak films were much more resistant to yellowing than Ak/S films. The colour differences became insignificant in fully formulated paints due to the photo-stabilizing effect of the pigments and extenders – values of ΔE* and Δb* for the Ak/S-based paints were only slightly larger than for the Ak systems. During one year of natural weathering, formulated Whitestone aggregate coatings and masonry paints containing the Ak binders M1 and M2 were found to pick-up most dirt. In addition, the strongest polymer degradation was observed for M1. Thus, weathering sta-

15

10 ΔE* 5

Δa*

Δb*

0

ΔL* -5

-10

M1

M2

M3

S2

S3

SM 2

Fig. 7 CIELab-values afterdata 1 year exposure of paintsmodel basedpaints on model Internal Figure 4.25 :  Colorimetric of outdoor colour changes of exterior afterdispersions 12 months outdoor exposure #

154

Masonry paints bility and resistance to soiling not only depend on the difference between the hard monomers S and MMA, but also to a large extent on the soft acrylic monomers, in other words on the overall hydrophobicity of the coating system. M3 and S3, the most hydrophobic of the polymers, showed the lowest dirt pick-up. It can be concluded that, despite clear differences in UV-stability (with advantage for the BA-containing types) and resistance to water, the pure acrylic dispersions M2 (BA/ MMA) and M3 (EHA/MMA), the styrene-acrylic dispersions S2 (BA/S) and S3 (EHA/S), as well as the terpolymer SM2 (BA/MMA/S), are in principle all suitable for use as binders in masonry paints. The EHA copolymers M3 and S3, the most hydrophobic of the group, have advantages in terms of resistance to water and dirt pick-up, but seem to be weaker in UV-stability. Interesting also is the BA/MMA/S terpolymer SM2, which, compared with M2, is more water resistant, and compared with S2, more resistant to yellowing. The EA/ MMA copolymer M1, due to its pronounced hydrophilicity and dirt pick-up tendency (plus due to its limited alkaline stability), is unsuitable for use in today’s exterior coatings on mineral, alkaline substrates. Dirt pick-up and weatherability are a function of both “hard” (styrene or MMA), and “soft” monomer, because both influence the hydrophilicity and durability of the copolymer.

4.2.4 Masonry paints formulated at high PVC In some regions (mainly Germany and neighbour countries), for special applications and as economic solutions, selected masonry paints are formulated nearby or slightly above the critical PVC (CPVC). These are often called “Sil-paints”, as different brands in the German market are ending with the suffix “-sil” (e.g. “Herbosil”og, “Amphisil”ph , “Jumbosil”qi, etc.). Especially for renovation of old buildings as well as for coatings applied on lime-based mortar, the masonry paint should have an excellent water vapour transmission rate (WVTR). To demonstrate the influence of the PVC on performance a series of paints were formulated at PVC of 40, 50 and 60 % with 5 different binders. For this purpose, again typical commercial binders from the group styrene-acrylates (Ak/S) and pure acrylates (Ak) were used to formulate masonry paints in the range above CPVC. Some characteristic data of the five chosen products are shown in the following Table 4.15. In this study all binders were based on the sole use of n-BA as soft monomer in combination with styrene or MMA as hard monomers. Binders Ak1 and Ak2 mainly differ in the glass transition temperature; they have about the same small particle size, the same stabilization package plus neutralization with ammonia. g registered trademark of Herbol, Akzo Nobel Deco GmbH, Cologne, Germany h registered trademark of Caparol, Deutsche Amphibolin Werke, Oberramstadt/ German i registered trademark of Sto SE & Co. KGaA, Stühlingen, Germany

155

Exterior paints on mineral substrates Table 4.15: Some characteristics of the commercial binders Tg Binder % n-BA %S [°C] Disp Ak/S1 55 45 12

MFFT [°C] 7

Neutralizing agent KOH

Disp Ak/S2a

50

50

25

18

NH3

Disp Ak/S2b

50

50

25

20

NaOH

Binder Disp Ak1

% n-BA 55

% MMA 45

Tg [°C] 12

MFFT [°C] 7

Neutralizing agent NH3

Disp Ak2

50

50

20

13

NH3

Binders Ak/S2a and Ak/S2b are based on the same main monomers n-butyl acrylate and styrene. They were produced with different emulsifiers as well as with different neutralizing agents after polymerisation. Dispersion Ak/S2a was neutralised with ammonia, whereas AK/S1 and Ak/S2b were neutralised with potassium and sodium hydroxide, respectively. The experiments were carried out with matt masonry paints in the PVC range of 40 to 60 %, i.e. with paints well below and nearby the critical PVC. They contained from 22 to 40 % of polymer dispersion (at 50 % solids). Pigment/filler pastes were prepared in accordance with the formulation shown in Table 4.16; afterwards the dispersions were added. Independently of the MFFT of the binders, all paints were formulated with coalescent to avoid problems with film formation and differences in drying speed. The laboratory tests were supplemented with outdoor exposure, which was evaluated by assessment of chalking, colour fading and dirt pick-up according to a defined time schedule.

Results of laboratory tests

The water absorption (WA) of free paint films decreases with increasing MFFT of the binder. This is independent of the binder type being pure acrylic Ak or styrene-acrylic Ak/S (see Table 4.17 to Table 4.19). The WA values of Ak-based paints are higher than those of corresponding Ak/S-based paints. Surprisingly, paints based on the dispersion Ak/S2b, which is neutralised by non-evaporable sodium hydroxide, have a lower WA and capillary water uptake than the Ak/S2a based paints with ammonia. This holds for both PVC = 40 and 50 % and shows that, independently of a higher water sensitivity caused by the permanent sodium ions in the pure binder film, the ranking of the water absorption performance of the paint films may differ. Furthermore, the WVTR of paints based on the binder Ak/S2b is better than that of paints based on binder Ak/S2a. The values for WVTR and for the capillary water uptake of paints based on “soft” dispersions are better than those of paints based on “harder” dispersions. For paints

156

Masonry paints Table 4.16: Paint formations of different PVC (all at 59 % solids content) PVC 40 % 103

PVC 50 % 116

PVC 60 % 127

“Dispex” AA 4040 a, dispersing agent 1, polyacrylic acid type

2

2

2

Ammonia, conc.

2

2

2

Ingredients Water

In can preservative

3

3

4

Sodium polyphosphate, “Calgon N” c, dispersing agent 2, as 25 wt.-% solution

4

5

5

Cellulose ether thickener, “Natrosol” 250 HHR b, as 2 wt.-% solution

50

56

62

Mid-shear HEUR-thickener, “Rheovis” PU 1280 a

4

5

5

Defoamer, “Agitan” 280 j

1

1

1

Solvent 1, White spirit K 60 (BP = 180 – 210 °C), coalescent 1

12

14

15

Solvent 2, Butyl diglycol, coalescent 2

12

14

15

Solvent 3, Propylene glycol, open time prolonger

16

18

20

Titanium dioxide/rutile, “Kronos” 2043 d

155

175

192

Filler 1, “Omyacarb” 5 GU e, calcite 5 µm

175

198

216

Filler 2, Talkum AT 1, fine talc

55

62

68

Defoamer, “Agitan” 280 j

2

2

2

Total Paste

596

673

737

Dispersion AK or AK/S-type (50 % solids)

404

304

221

23

42

1000

1000

1000

59

59

59

Water Total Solids by weight [%] a registered trademark of BASF SE, Ludwigshafen, Germany b registered trademark of Ashland, Wilmington, Delaware, USA c registered trademark of ICL Phosphate Specialty, Creve Coeur, USA

d registered trademark of Kronos International, Dallas, Texas, USA e registered trademark of Omya AG, Oftringen, Switzerland j registered trademark of Münzing Chemie, Abstatt, Germany

based on the “soft” binder Ak/S1 formulated at low PVC (40 and 50 %), there are surprisingly low WA and high WVTR values in comparison to the corresponding paints based on the “harder” binders Ak/S2a and 2b. This could be due to the different stabilization packages in use and/or the lower acid number in combination with the ammonia-free neutralization.

157

Exterior paints on mineral substrates Table 4.17: Results for paints with the different binder types at PVC = 40 % Dispersion Feature Unit Ak1 Ak2 Ak/S1 Ak/S2a Ak/S2b Water uptake after [%] 14.3 12.0 11.0 11.9 9.7 24 h, cycle 1 Water uptake after 24 h, cycle 2 Tensile strength Elongation-at-break

[%]

10.5

9.5

8.8

6.3

3.9

[N/ mm²]

4.8

5.7

4.3

5.2

5.4

[%]

30

20

150

80

50

Amount 300 g/m2

WVTR

[g/(m²∙d)]

41.5

36.4

41.7

24.1

29.0

Sd

[m]

0.5

0.5

0.5

0.8

0.7

Amount 500 g/m2

WVTR

[g/(m²∙d)]

28.8

27.3

35.9

18.8

18.2

Sd

[m]

0.7

0.7

0.5

1.1

1.1

[kg/(m²∙h1/2)]

0.02

0.03

0.03

0.02

0.01

Capillary water up-take

Table 4.18: Results for paints with the different binder types at PVC = 50 % Dispersion Feature Unit Ak1 Ak2 Ak/S1 Ak/S2a Ak/S2b Water uptake after 24 [%] 12.0 9.6 8.1 9.2 8.2 h, cycle 1 Water uptake after 24 [%] 8.1 7.3 6.0 5.0 3.7 h, cycle 2 Tensile strength [N/ mm2] 6.6 7.8 5.7 6.8 6.6 Elongation-at-break [%] 20 8 30 20 10 Amount 300 g/m2

WVTR Sd

[g/(m2.d)] [m]

40.5 0.5

37.7 0.5

47.2 0.4

28.9 0.7

27.7 0.7

Amount 500 g/m2

WVTR Sd

[g/(m2.d)] [m]

27.0 0.7

29.8 0.7

33.2 0.6

17.9 1.1

19.4 1.0

[kg/(m2∙h1/2)]

0.04

0.05

0.01

0.03

0.01

Capillary water up-take

In the case of pure acrylic binders, a decrease of MFFT of the dispersions leads to an increase of the WA of free films of the paints. For paints applied on mineral substrates, this rule changes due to fact that the determination methods differ. Normally, free films are used for measuring the water absorption (“normal” WA). In practice, the water absorption by swelling of the paint film will never happen in the same

158

Masonry paints Table 4.19: Results for paints with the different binder types at PVC = 60 % Dispersion Amount 300 g/m2 Amount 500 g/m2

WVTR Sd WVTR Sd

Capillary water up-take

Ak1

Ak2

Ak/S1

Ak/S2a

Ak/S2b

[g/(m2.d)]

164

145

190

119

111

[m]

0.1

0.1

0.1

0.2

0.2

[g/(m2.d)]

117

104

135

77.1

82.3

[m]

0.2

0.2

0.1

0.3

0.2

[kg/(m2.h1/2)]

0.4

0.32

0.31

0.06

0.05

way, because it is applied on a substrate. Thus, the (“normal”) WA just gives a hint on the hydrophilicity and water sensitivity of the paint film in general. Unfortunately, there is no real correlation with the capillary water absorption according to EN 1062-3. For the determination of the capillary water uptake paints are applied on a limestone as mineral substrate and the water barrier properties of the painted surface are checked. For the WVP tests are done on porous paper card, or better porous sintered glass. The film formation on the limestone simulates particularly well the application on a wall on laboratory scale. The water tightness of paints based on pure acrylic binders decreases as the PVC increases from 40 to 50 %. As the values for capillary water uptake and WA show, in the tested styrene-acrylic binders such a decrease in water tightness upon increasing PVC to 50 % was not observed. Films of paints with PVC = 60 % were too brittle to be handled as free films for mechanical testing and for the determination of WA. This indicates that for both binder classes the critcal PVC was approached or already exceeded. The water vapour permeability data delivered for the paints confirm it. Sd values of 0.1 to 0.2 m at 300 g/m2 and 0.1 to 0.3 m at 500 g/m2 wet-applied material demonstrate the porosity of the paint films. Only the styrene-acrylic based paints with Ak/S2a and AK/S2b are still category V2 and borderline to achieve a uniform porosity. Here, the CPVC values seem to be slighly higher than for the other three binders. Paints based on Ak binders show, independently of PVC, a higher WVTR than the corresponding paints based on Ak/S binders. This was also found in the other discussed studies. It is interesting, that with Ak/S-containing paints, there is only a slight increase in capillary WA when the PVC increases from 50 to 60 %, whereas the capillary WA for the Ak-based paints increases tremendously. For such systems, being low in binder content, the capillary water uptake decreased with increasing MFFT of the binder. Especially

159

Exterior paints on mineral substrates for masonry paints formulated above the CPVC ”harder” Ak/S binders should be preferred due to a better relation of WVTR to capillary WA. The differences between the tensile strength values for various binder classes are small for binders having the same MFFT. The flexibility of the free paint films, expressed by the elongation-at-break, is better for binders based on Ak/S than for those based on Ak. Unfortunately, the real position of the CPVC for the different binders in this formulation was not further studied; it seems to be nearby to the 60 % and must be slightly higher for the Ak/S2a and Ak/2b grades than for the other binders.

Outdoor weathering results For the outdoor weathering tests primed fibre-cement boards were used, as described above. The panels were placed in a south-facing exposure rack inclined at 45° (as discussed, weathering is then 2 to 2½ times faster than for vertical surfaces). At fixed intervals of 12 months the coatings were photographed and examined for evidence of chalking (DIN 53 159: 2010-8), dirt pick-up and colour change (old DIN 6174; actual EN ISO 11 664-4). Colour changes due to outdoor exposure The total colour changes ΔE*, as a function of outdoor exposure time for paints with PVC = 50 %, are shown in Figure 4.26. The tendencies are quite similar for all three PVCs. Therefore, only the paints of PVC = 50 % will be discussed. The ΔE* values decrease with increasing MFFT of the binder. This effect is more pronounced for the styrene-acrylates. It confirms former results with unpigmented polymers films [22]. Colour changes of paints based on pure acrylic binders are higher than those of paints based on corresponding styrene-acrylic ones. For pure acrylic paints, the ΔE* decreases slightly over time. The graphs for the colour changes of the styrene-acrylate paints are U-shaped. For the first and second year ΔE* drops and in the third year it increases again. In addition, in the third year, the value of ΔE* of AS2b based paints is lower than that of AS2a based ones. The total colour difference ΔE* is derived from difference values along the three axes, L*, a* and b* of the CIELAB colour coordinate system. The L* value describes changes in brightness while changes on the red-green or yellow-blue axis are expressed by Δa* or Δb*, respectively. Figure 4.27 shows the L* value and its changes with outdoor exposure time as function of the binders in use. After one year of outdoor exposure, the change (decrease) of the L* value for pure acrylic-based paints is larger than that for styrene-acrylic ones. This correlates with the somewhat stronger dirt pick-up of the pure acrylic-based paints (see Table 4.20). With ongoing outdoor exposure, the L* values of the Ak paints

160

Masonry paints further increase, whereas those of the Ak/S paints, after an initial drop, nearly stay constant after 2 years. With respect to dirt pick-up, as an optical defect, and the corresponding change of the L* values during the first period of exposure, Ak/S-based binders are favoured. The comparison between the two binders Ak/S2a and Ak/S2b is interesting. Although they have the same monomer composition, after 3 years of exposure both ΔE* and ΔL* are smaller for Ak/S2b. These results indicate a (somewhat) lower dirt pick-up for Ak/S2b, although the visual judgement did not really confirm this (Table 4.20). The change of the second component of ΔE*, the Δb* value, is a measure for the yellowing of the coating. The graphs of the measured b* values are shown in Figure 4.28. Against all odds, no increasing yellowing is observed with increasing MFFT of the Ak/S binders. The higher MFFT is a consequence of the increasing styrene content of the binder. In contrast to what could be expected, paints based on binders Ak/S2a or Ak/S2b show less yellowing than paints based on Ak/S1. The comparison with the paints based on Ak is also surprising. At corresponding MFFT, the yellowing of Ak-paints is not better than that of Ak/S based paints. The graph of changes for the third component of ΔE*, a*, is shown in Figure 4.29. With long exposure time, all paints shift towards green, as shown by the decreasing a*

Figure 4.26:  Colour changes ΔE* of masonry paints at PVC = 50 % in dependence of the binder type after 1, 2 and 3 years of outdoor exposure.

161

Exterior paints on mineral substrates Table 4.20: Dirt pick-up and polymer degradation/chalking of the masonry paints in  dependence of binder type and PVC Binder Ak1 Ak2 Ak/S1 Ak/S2a Ak/S2b PVC = 40 % 1 year dirt pick-up

2

1.5

1.5

1.5

1.5

polymer degradation/chalking

0

0

0

0

0

2 years dirt pick-up

3

2

2

2

2

polymer degradation/chalking

1

1

2

1

2

3

2.5

2

2

2

1.5

2

3 PVC = 50 %

3

3

2.5

2

2.5

2

1.5

0

0

0

0

0

2.5

2

2

2

2

3

1

3

3

3

dirt pick-up

3.5

3

3.5

3.5

3.5

polymer degradation/chalking

3.5

2

4 PVC = 60 %

4

4

dirt pick-up

2.5

2.5

2

2

2

polymer degradation/chalking

0.5

0.5

0.5

0.5

0.5

3 3

2.5 3

2.5 4

2.5 4

2.5 4

3 years dirt pick-up

4

3.5

3.5

3.5

3.5

polymer degradation/chalking

4

3

5

4.5

5

3 years dirt pick-up polymer degradation/chalking 1 year dirt pick-up polymer degradation/chalking 2 years dirt pick-up polymer degradation/chalking 3 years

1 year

2 years dirt pick-up polymer degradation/chalking

ranking: 0 = excellent, no dirt pick-up/no chalking, 5 = bad, strong dirt pick-up/strong chalking

162

Masonry paints values (after 3 years exposure). This shift can be explained by a growing number of microorganisms on the panels with time, because the paints were only made with a pot preservation additive, no additional film protecting fungicides or algicides were used.

Chalking due to outdoor exposure The chalking tendency of the paints increases with their PVC (see Table 4.20). After one year of outdoor exposure, paints with a PVC of 60 % already show starting chalking, whereas for those with PVC = 40 and 50 % there is no indication of binder degradation after such a short exposure time. Chalking worsens with increasing exposure time, reaching already a value of 3 to 5, depending on the binder, at a PVC of 60 % after 3 years at 45° south. Paints based on pure acrylic binders exhibit less chalking than those based on styrene-acrylic binder types. In all cases, paints based on the harder binder Ak2 show slightly more chalking than those based on the softer Ak1. For the corresponding styrene-acrylic binder pair with different Tg/MFFT, Ak/S1 and Ak/S2, there are not such

95

93

before exposure 91 L* value

after 1 year

after 2 years

89

after 3 years 87

85

Ak1

Ak2

Ak/S1

Ak/S2a

Ak/S2b

Binder

Figure 4.27:  L* values (as measure for greying and dirt pick-up) of masonry paints (at PVC = 50 %) and their change after 3 years in outdoor exposure #

163

Exterior paints on mineral substrates differences. The outdoor behaviour of both binders Ak/S2a and Ak/S2b is also very similar. This means, that the hardness (i.e. styrene content) and the type of neutralizing agent does not play an important role in the behaviour of the styrene-acrylics.

Summary on influence of PVC for different acrylic binders

Masonry paints based on acrylic emulsions with different minimum film forming temperature and neutralisation were formulated in the PVC-range of 40 to 60 % and submitted to artificial as well as to outdoor weathering. Free paint films absorb less water with increasing MFFT of the binder, independent of the binder type. For applied paints and their capillary water barrier this does not hold true. The observed colour changes in the outdoor experiments point to an influence of the type of binder. The pure acrylic binders gave higher ΔE* values than the corresponding styrene-acrylates, i.e. their dirt pick-up is higher. An effect due to neutralisation of the binder could not be found. No clear correlation could be found between the hardness of the tested binders and their dirt pick-up, chalking and degradation behaviour [21]. This shows that the complete

Figure 4.28:  b* values (as measure for yellowing tendency) of masonry paints (at PVC = 50 %) and their change after 3 years in outdoor exposure

164

Masonry paints binder composition must be considered and not just the category of polymer and its Tg/ MFFT. The WVP values of all tested paints (also at PVC = 60 %) indicate that they are still closed-porous. But that the paints of PVC = 60 % are already nearby the CPVC is obvious, as all films of these paints were already too brittle to be handled, e.g. for stress-strain measurements and water absorption tests. The CO2 permeability of high-PVC masonry coatings is normally much higher in comparison to paints formulated below the CPVC. Therefore, high-PVC coatings are better suitable for old lime-concrete based walls and for renovation of old lime-containing painted surfaces. However, because of their lack in CO2 barrier properties, they are unsuitable for concrete protection paints. They are not able to hinder carbonation of the concrete, which is needed to avoid corrosion of the reinforcing steel. Thanks to their porosity and their good water vapour transmission rate, such high-PVC “Sil-paints” can be described as open for gas transfer processes in general. Liquid water and moisture, possibly present in the substrate, can evaporate through the paint film, without building-up extra vapour pressure, which may result in blistering or peeling-off of the paint film.

Figure 4.29:  a* values (green colour trend as measure for algae/microorganism growth) of masonry paints (at PVC = 50 %) and their change after 3 years in outdoor exposure

165

Exterior paints on mineral substrates An advantage over the also open-porous silicate paints is the fact, that supercritical masonry paints are only neutral to slightly alkaline, whereas silicate paints (as function of the used water glass) are strongly alkaline. This alkalinity, induced by the water glass in dispersion silicate paints, can lead to efflorescence and surface whitening, which does not occur in the same way with sole polymer dispersion-based masonry paints formulated above the CPVC. Furthermore, masonry paints, formulated above CPVC, lead to better hiding power due to the “dry hiding” effect – despite a low amount of binder and pigment (for more explanation, see Chapter 7 on interior paints). Especially in South America and in southern regions of Europe, masonry paints are often formulated at high PVC, some even above CPVC, sometimes because of simple cost reasons, sometimes because of technical requirements. In all these regions, such masonry “Sil-paints” are applied on lime-containing substrates. It must be emphasised, however, that the durability, chalking resistance and colour retention of such low binder containing paints is not the best. But not everywhere there is a desire to apply an exterior coating for a very long time. It always depends on a cost/benefit analysis which PVC range of the paint is most favourable. For old walls, physical paint properties are clearly more important. In humid and colder regions, the water household of paints rich in binder and formulated below the CPVC may become a problem due to their reduced water vapour transmission rate. This can be better solved by using a masonry paint formulated above CPVC.

4.2.5 Solvent-free exterior paints: influence of the Tg of binders and of titanium dioxide [23] In masonry paints, the weatherability is strongly influenced, as shown, by the binder chemistry and specific copolymer composition. However, weathering performance and fading behaviour of paints is also a function of other paint ingredients such as extenders and pigments. As described before, investigations on the weatherability of acrylic based paints led to differences between formulations containing titanium dioxide and those without titanium dioxide. There are comparative studies in the literature in which acrylic copolymers are described with different film forming temperatures formulated with titanium dioxide and subjected to different treatments [4, 23]. As in many applications, the principal question for masonry is, which performance profile the right one is for a copolymer dispersion based on styrene and acrylic monomers to achieve a long-lasting paint. Furthermore, the market is looking for binders, which permit the formulation of solvent-free architectural coatings. This requirement can be met by using a binder with an MFFT below 5 °C. In the following, a study of Schwartz and Zhao

166

Masonry paints Table 4.21: Some characteristics of the used binders [23] Binder % n-BA %S Ak/S soft 60 40

Tg [°C] 8

MFFT [°C] 400 µm)

V2

W2

A5

I3

Impermeability/ crack-bridging

14

mine the actual dirt pick-up, such as weather, site, nature, and degree of air pollution, for example, are highly complex, irregular and in fact unpredictable. Many attempts have, and still are, being made to elaborate corresponding lab tests, but to date there is still no test which, within acceptable periods of time, reliably simulates the actual dirt pick-up behaviour over years.

Binder polymers on the market For the formulation of crack-bridging coatings, the following dispersions are found in the market: – styrene-acrylic types, and – pure acrylics. These binders are today plasticizer-free with very good long-term extensibility, having glass transition temperatures (Tg) in the range from 0 to −30 °C by using high amounts (60 to 90 %) of soft monomer building blocks such as n-butyl acrylate or 2-ethyl hexylacrylate. As

239

Exterior paints on mineral substrates hard comonomers styrene, MMA or acrylonitrile (AN) can be used. To an increasing extent, dual (surface and bulk) crosslinked styrene-acrylic and pure acrylic dispersions are on the market, having low glass transition temperatures and reduced dirt pick-up. Furthermore, to promote adhesion to mineral substrates, silane-modification of the binders by copolymerization or by post-addition of silane-functional compounds is possible.[8] More recent generations of binders for these applications are ammonia free and as a consequence distinguished by their mild odour. They often operate with diverse crosslinking systems (bulk crosslinking by multivalent ions or alternative ways and surface crosslinking induced by sunlight and photoinitiators) to control the film mechanics and surface tack. They may be characterized as follows: – free of ammonia and APEOs, – Tg = approx. −30 °C, – solids content = 50 to 60 %, – high elongation-at-break values (also at low temperature), better than first generation products, but with virtually the same tensile strength, – low water absorption and good water vapour permeability, – low dirt pick-up. In general, the pure acrylic binder types of very low Tg certainly lead to higher water vapour permeability than their styrene-acrylic alternatives (see Chapters 2 and 4.2). This is of specific importance for elastomeric coatings: as a consequence of the low PVC plus high layer thickness there is a higher risk of an unwanted water vapour barrier.

Formulations of elastomeric coatings

Also, the formulations and viscosities of elastomeric coatings are different to standard exterior paints. To increase the coating elasticity the binder content in elastomeric coatings is quite high (normally 20 to 30 weight-% on formulation) and the PVC is low (25 to max. 45 %), which is far below the critical PVC. In addition, the low and especially the high shear viscosities and solids contents (60 % or higher) are very high to realize a sufficient layer thickness (mostly 600 up to 1000 µm wet) in two coats with classical paint roller application. The high shear viscosity of elastomeric paints can easily be achieved in these binder-rich formulations by using high-shear HEUR or polyether types of rheology modifiers. The pot viscosity and film structure (e.g. by using special structuring rollers) of the applied elastomeric coating is normally regulated by acrylic (ASE or HASE types) or hydroxyethyl cellulose thickener types. To enable high solid content formulations, modern acrylic binders for elastomeric coatings are mostly sufficiently robust to be used in the grind (i.e. to withstand the high shear forces experienced during the grinding process). In addition, they often have high solids content (up to 60 %), possible because of their multi-modal particle size distribution.

240

Elastic coating systems Table 4.38: Economic crack-bridging paint based on a soft styrene-acrylic binder Component Parts by weight Water 90 Dispersing agent, “Dispex” AA 4030 a

5

In-can preservative

2

Defoamer, e.g. “Foamaster” MO 2114 a

2

Ammonia, conc.

2

Base thickener, “Natrosol” 250 HR

b

3

Propylene glycol

20

Butyl diglycol

15

High shear rheology modifier, “Rheovis” PE 1330

a

4

Binder, “Acronal” S 562 a (50 % solids)

100

Titanium dioxide, rutile, e.g. “Kronos” 2190 d

125

Filler calcite, “Omyacarb” 5 GU

e

240 Disperse at high speed

Binder, “Acronal” S 562 a (50 % solids)

370

Film preservative

10

Defoamer, “Foamaster” MO 2114 a

4

Water

8 Add in the stated sequence at slow stirring

Total

1000

Solids content: approx. 60 %, PVC: 35 % Stormer viscosity: ca. 130 KU; ICI viscosity: ca. 2.0 Poise Paint film elasticity (for free film prepared from 1000 µm wet, after 28 drying at RT, S2 bone shape specimen, test speed 400 mm/min): Elongation-at-break: 240 % at 23 °C, 40 % at 0 °C, 20 % at −5 °C Tensile strength: 4.8 N/mm2 at 23 °C, 13.4 N/mm2 at 0 °C, 19 N/mm2 at −5 °C a b d e

registered trademark of BASF SE, Ludwigshafen, Germany registered trademark of Ashland, Wilmington, Delaware, USA registered trademark of Kronos, Lowell, Massachusetts, USA registered trademark of Omya, Oftringen, Switzerland

As consequence of the high film build, the amount of expensive titanium dioxide in the formulations is normally quite low (10 to 15 weight-%). In addition, sometimes titanium poor undercoats are used as 1st coat, to further reduce total raw material costs. As fillers most often classical calcite of low oil absorption, sometimes combined with a lower amount of talc is in use.

241

Exterior paints on mineral substrates An example of an economic crack-bridging façade paint as used in Southern Europe based on the styrene-acrylic binder “Acronal” S 562 a with Tg approx. −8 °C is given in Table 4.38. In addition, an example of a modern high-quality, low-temperature elastic, crack-bridging paint formulation based on the self-crosslinking, pure acrylic binder “Acronal” PLUS 6257 a with Tg approx. −28 °C is given in Table 4.39. Especially in Southern Europe (Spain, Portugal, Italy, Greece) low temperature elasticity is less needed then in the colder French or German climate. In addition, the price sensitivity is higher and more often elastomeric coatings are applied at new buildings with fresh cementitious substrates (exterior walls and terraces). That is why in these countries alkaline stable, cost effective styrene-acrylic binders with a Tg between 0 and −15 °C are preferred, sometimes in blends with harder standard architectural coatings binders with a Tg in the range 10 to 20 °C. However, in the last years the renovation of old cracked façades with high quality and labour-intensive elastomeric wall coating systems (of A4 and A5 category with reinforcement mesh, e.g. in France) is more and more replaced by the application of EIFS (exterior insulation finishing system) or ETICS (external thermal insulation composite system). With somewhat more work and effort they deliver an additional thermal insulation effect while often also financial support by the local authorities is obtained. On the other hand, flexible roof coatings replacing bitumen sheet protection achieve increasing importance for efficient water sealing of e.g. flat roofs, terraces or balconies. Quite often, these very thick layer (up to 2 mm dry layer) flat roof coatings are based on the same type of self-crosslinking elastic acrylic binders and the base formulation concepts as used for vertical application in crack-bridging façade coatings.

4.5.4 Summary The decisive criteria for binders for crack-bridging systems are as follows: – appropriate mechanical properties even at low temperatures, – high dirt pick-up resistance, and – effective moisture protection for the façades. The mechanical properties are met by low-Tg binders crosslinked inter-particularly e.g. with metal salts. Light-induced surface crosslinking promoted by photoinitiators (e.g. of benzophenone type) improves the inherently low dirt pick-up resistance without adversely affecting the mechanical properties. The tests of the crack-bridging system in accordance with French and European standards by independent institutes relate exclusively to moisture protection and mechanical properties (before and after artificial weathering). In the absence of suitable artificial testing methods, the critical dirt pick-up tendency of elastomeric coatings currently remains still uncontrolled by the test institutes.

242

Elastic coating systems Table 4.39: High quality, crack-bridging paint with low temperature elasticity based on a self-crosslinking pure acrylic, low Tg binder Component Parts by weight Water 198 Dispersing agent, “Dispex” CX 4320 a

10

In-can preservative

2

Defoamer, e.g. “Foamaster” MO 2114

a

3

Ammonia, conc.

2

Base thickener, “Natrosol” 250 HHR b

4

Propylene glycol

20

“Texanol” l

10

High shear rheology modifier, “Rheovis” PE 1330 a Titanium dioxide, rutile, e.g. “Kronos” 2190

d

Filler calcite, e.g. “Omyacarb” 5 GU e

5 125 288

Disperse at high speed Binder, “Acronal” PLUS 6257 (60 % solids) a

300

Film preservative

10

Defoamer, “Foamaster” MO 2114

a

Water

4 19

Add in the stated sequence at slow stirring Total

1000

Solids content: approx. 60 %, PVC: 45 % Stormer Viscosity: >140 KU; ICI Viscosity: ca. 3.1 Poise Paint film elasticity (for free film prepared from 1000 µm wet, after 28 drying at RT, S2 bone shape specimen, test speed 400 mm/min): Elongation-at-break: 140 % at 23 °C, 90 % at 0 °C, 60 % at −10 °C Tensile strength: 2.5 N/mm2 at 23 °C, 4.6 N/mm2 at 0 °C, 6.7 N/mm2 at -10 °C a b d e l

registered trademark of BASF SE, Ludwigshafen, Germany registered trademark of Ashland, Wilmington, Delaware, USA registered trademark of Kronos, Lowell, Massachusetts, USA registered trademark of Omya, Oftringen, Switzerland registered trademark of Eastman, Kingsport, Tennessee, USA

243

Exterior paints on mineral substrates

4.5.6 References [1] P. Pföhler, A. Zosel, R. Baumstark, Elastic coating systems for the renovation of façades, Surf. Coat. Austr. 1995, 18 – 23. [2] H. Künzel, Anforderungen an Außenanstriche und Beschichtungen aus Kunstharz-dispersionen, Kunststoffe Bau Heft 12, 26–32 (1968) [3] H. Künzel, Beurteilung des Regenschutzes von Außenbeschichtungen, Institut für Bauphysik der Fraunhofer Gesellschaft, Mitteilung 18, 1978 [4] P. Gieler, Dissertation „Überlegungen und Versuche zur Rissüberbrückungsfähigkeit spezieller Beschichtungssysteme an Fassaden“, Universität Dortmund, 1989

[5] P. Peyser, Glass transition temperatures of polymers, VI/S. 209–277 in J. Brandrup, E.H. Immergut, Polymer Handbook, Interscience Publ., New York, 3rd ed., 1989 [6] H. Kossmann, M. Schwartz, Using natural weathering to assess polymer dispersion performance, Paint and Coatings Industry Magazine, März 1999, 48–56, Troy, Michigan (USA) [7] A. Smith, O. Wagner, Factors affecting dirt pick-up in latex coatings, J. Coatings Technology, 68, No. 862, Nov. 1996, 37–41 [8] S. Cavalho, I. Fernandes, J. Bortado, Staying on the tiles, European Coatings J. 09 (2005) 42–47.

4.6 Comparison of different masonry paint systems After the detailed introduction of the different emulsion-based masonry paint and binder concepts, Table 4.40 summarizes the typical performance profile in strengths and weaknesses of the four paint types listed below. 1. Paints based on polymer dispersion (of different PVC), 2. Elastomeric/crack-bridging paints based on special polymer dispersions, 3. Paints based on water glass plus polymer dispersion, 4. Paints based on silicone resin plus polymer dispersion. Not taken up into this comparison are the textured finishes or renders, which also could be viewed as paints of high solids with coarse fillers at low binder content and very high viscosity. Masonry coatings of PVC < CPVC based on polymer dispersions can be applied on all substrates with exception of mortar group 1 (lime mortar). They can be formulated from glossy to matt and show as function of the chosen binder a pronounced elasticity. They have a low water up-take and lie with an Sd value of normally 0.5 to 1.5 m in category V2, which means still in a good, but not yet perfect range of water vapour transmission rates. Silicate paints based on water glass plus polymer dispersion need mineral surfaces with reactive Si-O functions for acceptable adhesion. This alkaline type of paint, however, permits neither glossy, nor elastic coatings and is limited in the choice of polymer binders and pigments. These products are characterised by a high water vapour transmission rate

244

Comparison of different masonry paint systems Table 4.40: Typical properties of diverse types of water-based exterior paints Silicone DispersionPaint Dispersion Elastomeric "Sil" resin silicate PVC low very low high high very high Properties Gloss grade

semi-gloss to matt

matt to semi-gloss

very matt

very matt

very matt

Water barrier properties

+++

+++

+(+)

++(+)

+/-

Water vapour permeability

++

+/-

+++

+++

+++

Chalking resistance/ colour retention

+++

++(+)

+/-

++

+/-

Elasticity

+(+)

+++

-

-

--

Adhesion

++(+)

+(+)

++

++

+

Dirt pick-up resistance

++

-

++

+++

+++

Variety of colours

+++

+++

++

++

-

and a water absorption coefficient below 0.3 kg/m2.h0.5, provided a suitable formulation was used. In the system based on silicone resin emulsion plus polymer dispersion, a remarkably perfect balance of low water absorption coefficient and high water vapour transmission rate is obtained. However, this porous type of paint system cannot be formulated to coatings with glossy surface, high elasticity and intense colours. The paints of the lowest quality are the “Sil” paints with sole dispersion binder (most often a styrene-acrylic type) at PVC ≥ 60 %. They are less watertight than real silicone resin paints, but open-porous and often additionally hydrophobized with a small amount of silicone oil, amino siloxane or wax emulsion. Electron microscopic photographs reveal the differences between the three major coating systems. Whereas paints based on polymer dispersions as sole binder at PVC of 50 % have a practically closed film structure, porosity can be seen for silicone resin paints with PVC >60 %, as well as for polymer-poor silicate paints [1] (see Figure 4.48). Specific aspects for silicate, silicone and elastomeric/crack-bridging paints are discussed in Chapter 4.3, 4.4 and 4.5, respectively.

245

Exterior paints on mineral substrates When looking at these differences, it must be kept in mind that within each category strong differences in quality and, as a matter of fact, also in costs can be found in the market.

4.6.1 References [1] H. Kossmann, Fassadenbeschichtungen auf Basis von Acrylat-Copolymerdispersionen und Siliconemulsionen, Farbe & Lack 97, 05/1991, 412–415.

4.7 Outlook Unfortunately, the masonry paint fulfilling all requirements in perfection, is still not available. Nevertheless, the façade renovation cycles are meanwhile up to more than 20 years, which confirms that already today exterior paint quality is quite good. The high-PVC porous paint grades are brittle and tend more to crack and to chalk. In addition, there is a lack in choice and brilliance of initial colours, often in link with limited colour retention. Nevertheless, they show excellent breathability and dirt pick-up resistance. The lower PVC grades, especially if based on pure acrylics, are more elastic (depending on binder Tg), deliver the best durability and colour retention, but lack in water vapour permeability, especially if applied in thicker layers. The specific binder-rich elastomeric paints, based on soft low-Tg binders, fail in dirt pick-up resistance. Thus, new binder and formulation concepts are needed to allow low-PVC paints of controlled thermoplasticity with a “flexible and breathable” membrane effect (like known for textiles) to come nearer to the perfect masonry paint. In addition, the elimination of fungus and algae growth on the coating surface preferably without, or at low dosage of film preservatives, is getting in stronger focus. A better control of biocide leaching by careful pre-selection of paint and binder ingredients and the limitation of surface humidity by good spreading and distribution of rainwater and quick surface drying behaviour are potential concepts to get better control of infestation by microorganisms. In addition, the seasonality of exterior painting with water-based paints must be extended, both in the direction of wetter and colder conditions, where drying is restricted, and in the direction of higher temperatures, where drying is too fast. This will help to further replace the still remaining solvent-borne solutions. To promote the film formation of the paint at low temperatures, so-called “winter additives” are already in use for a longer period of time. They are mostly polyamines (e.g. polyethylene imine) or cationic polymers (e.g. polydiallyldimethyl ammonium chloride =

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Outlook poly-DADMAC), which accelerate the film formation by controlled agglomeration of the anionic paint composition. Unfortunately, the pH of such formulations has to be rather high to avoid incompatibility and gelation, and their use is only possible at cold and humid conditions, i.e. at temperatures between 0 and 15 °C. In case of higher temperatures, storage stability is insufficient (e.g. due to decreasing pH) and open time suffers too much. Furthermore, in the past few years, a new generation of functionalized “all-weather type” acrylic binders appeared on the market, without the need for high pH formulations and without too many restrictions in open time if painting in summer. They allow to speedup the early-rain resistance of the paints with broader and more secure workability in all seasons. However, working at temperatures below 5 °C and light frost is still a problem for water-based solutions and is not solved now. Likewise, there is currently no solution for painting at high temperatures, especially during summer in the southern European countries, because water-based systems still dry too fast. Improvements into these directions are highly welcome and still subject of various research topics both in the paint and raw materials industry.

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Special features of wood as a building material

5 Wood coatings Coatings for the natural “living” material wood (Figure 5.1 ) and binders designed for them are subjected to a number of specific requirements which differ markedly from those for coatings on mineral substrates, such as interior wall paints or masonry paints [1, 2].

5.1 Special features of wood as a building material In order to gain a better understanding of the properties generally required for a wood coating, it is appropriate to give a brief outline of the special features of wood as a substance and as a building material first [2, 3]. These are some important ones: – variability in basic structure and chemical composition, – anisotropy of the wood structure, – water swelling power and hygroscopicity, – lack of dimensional stability (swelling and shrinking with changes in moisture content, tendency to form cracks), – instability to UV exposure (discolouration, greying, lignin breakdown), – susceptibility to microorganism attack (moulds, blue stain fungi, etc.) at wood moisture levels >20 %.

Figure 5.1:  Anatomy of wood; Top: cuts in three different directions.[1] ; B: bark; R: inner bark/wood rays; K: cambium; E: early wood; L: late wood cells (year rings); MC: mark channel; G: resin channels; H: pores; Bottom: wood cell structure (1 mm3) for coniferous wood [2]

Water-based Acrylic Dispersions © Copyright 2022 by Vincentz Network, Hanover, Germany

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Wood coatings Table 5.1: Wood consists chemically of the following constituents Chemical basis Amount in weight-% Cellulose 40 to 60  Hemicellulose

20 to 40 

Lignin

15 to 30 

Residual organic (sugar, starch, peptides)

1 to 10 

Inorganic constituents (mineral substances)

0.2 to 1 

Wood is chemically composed of cellulose, lignin plus hemicellulose as main materials and diverse additional ingredients of lower concentration (see Table 5.1). The cellulose polymers give the wood its high tensile strength (reinforcement), the lignin represents the connection (cement or matrix) between the cellulose chains, and the hemicellulose molecules strengthen the chemical bonds between the cellulose and lignin molecules. The properties of wood are not only determined by the three main constituents but also by accompanying ingredients, such as alkaloids, dyes, tannins, fats, oils, and starches. They are partly responsible for the colour, aroma, and durability of the wood. Difficulties with the surface treatment of wood can also often be tracked back to these ingredients. As a hygroscopic material, wood absorbs moisture from its surroundings and can also release it again. The amount of moisture bound by the cellulose is in equilibrium with the humidity of the surrounding atmosphere. Changes in wood moisture content cause swelling and shrinkage of the wood. These volume changes (”movement” of the wood) are different in the three sectional directions of the wood (radial, tangential and longitudinal). This means that wood is an anisotropic and complex building material. Depending on the type of wood and the change in moisture content, wood swells and shrinks radially (in the direction of the wood rays) up to 10 times and tangentially (along the annual rings) up to 25 times as strongly as in the longitudinal direction (in the fibre direction or direction of lengthwise growth); see Table 5.2. Because of the anisotropic swelling and drying characteristics, cracks in the wood and the coating are especially likely at the annual rings and at the boundary between heartwood and sapwood. Additionally, excessive wood moisture (>20 %) promotes fungal infestation and rotting. Fungi, which destroy or discolour wood, such as moulds or the blue stain particularly prevalent in coniferous woods, detract from the aesthetic appearance and from the durability of wood and wood coatings. This becomes a particular problem if the wood is used as a construction material outdoors, where it sometimes is exposed to very high moisture levels (driving rain, snow, dew, condensation, etc.).

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Special features of wood as a building material Table 5.2: Volume changes of wood with moisture change Shrinkage or swelling [%] Wood variety radial tangential longitudinal Pine 4.0 7.7 0.4 Beech

5.8

11.8

0.3

Oak

4.6

10.0

0.4

Mahogany

1.2

5.7

0.2

Wood also absorbs UV light. Exposure to UV radiation from sunlight and the absorption of this radiation by the aromatic chromophoric groups, cause the photochemical breakdown (depolymerization) of certain wood constituents, principally lignin. This degradation starts with discolouration/darkening in the case of light-coloured woods, later on followed by bleaching, greying and destroying phenomena; it is accompanied by cracking at the surface and loss of adhesion of the coating to the loosening cellulose framework. Moreover, this situation is worsened because of leaching of the water-soluble lignin breakdown products under weathering (see Figure 5.2). The remaining loose cellulose fibres are responsible for the grey to silver surface appearance of the weathered wood.

Figure 5.2:  CLSM pictures of crosscuts of pinewood panels; left picture: new and right one: weathered wood. Due to degradation, the wood shows auto-fluorescence, its structure is lost, and the surface gets very rough

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Wood coatings Because of the special features outlined above, it is important to maintain the integrity and functional reliability of the wooden structures by using coating systems tailored to the variety and quality of the wood and to the stresses to which it will be exposed under the various ambient conditions.

5.2 Classification of wood coatings and market Water-based wood coating materials are characterized according to whether they are used outdoors or indoors, according to their function, or according to the degree of transparency [2, 4]. For outdoor use there are low-build wood preservative coatings or impregnating stains, which contain antifungal additives and are able to penetrate well into the wood, and also waterproofing, film-forming, mid- to high-build compounds, which contain no active substances but instead offer merely physical surface protection of the wood. These include not just decorative or classical architectural wood coatings, but also factory coatings for exterior claddings and for wooden windows and doors, also called joinery. For interior application, the main types are coatings for furniture and flooring/parquet. According to their function, wood coatings can generally be differentiated into primers, fillers, and topcoats. Topcoats can in turn be subdivided into transparent or semi-transparent systems (clear varnishes and semi-transparent stains) and pigmented systems (waterproofing paints and emulsion gloss or trim paints). A further distinction can be made between high-performance or high-build coatings for dimensionally stable components, such as windows and doors, with particular requirements in respect of freedom from tack, blocking resistance and moisture protection, and simple low- to mid-build coating materials for components where dimensional stability is not so important, such as panelling, cladding, balustrades, fences or pergolas. A fundamental requirement for decorative wood coatings and especially for industrial applied types is to use primers, fillers and topcoats having properties attuned to one another. The industrial wood coatings market in Europe for furniture, flooring and joinery was already in 2008 estimated to be 450,000 tonnes/year. Furniture is by far the most important segment of this market, representing about 70 % of the total volume. Most of the remaining volume is either in flooring or joinery. The wood care part is hard to estimate independently of joinery but represents more than 10 % in value of the global architectural coatings market. The biggest wood coatings markets in Europe are by tradition Germany, Italy, and Scandinavia, but also France and today Poland play important roles [5]. The German market for lacquers and stains in 2019 was described by the association of paint producers VDL at a volume of 71,000 tonnes and a value of 521 Mio €. The ad-

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Classification of wood coatings and market ditional market for wood lacquers for furniture was listed with another 50,000 tonnes at a value of 242 Mio € [6]. In general, wood coatings are a relatively small percentage of the total coatings and architectural coatings market but represent still a significant volume and over proportional value.

5.2.1 Primers/impregnating stains A blue stain protection primer or impregnating stain is intended to establish the connection between wood and topcoat and also to guarantee a deep-acting protection against fungal attack and blue staining by the introduction of a biocide (e.g. copper salts, borates or organic fungicides). In former times also chromium salts were used, however, due to legislation these are no longer allowed. It is intended to provide an effective and firm foundation so that the subsequent coatings adhere well and do not flake off. Because the wood fibres rise strongly on contact with water, the primer must be ready to be sanded soon after drying. To avoid cracking, however, the dried coating should not be too hard and brittle. Characteristics of wood primers/impregnating agents are the low solids content (usually