Intumescent Coatings for Fire Protection of Building Structures and Materials [1st ed.] 9783030594213, 9783030594220

Table of contents :
Front Matter ....Pages i-xi
Basic Ingredients of Intumescent Compositions (Olga Zybina, Marina Gravit)....Pages 1-51
Technology Basis of the Thermolytic Synthesis of Char Formation Polymeric System (Olga Zybina, Marina Gravit)....Pages 53-90
Polymer Binders of Flame-Retardant Intumescent Coatings (Olga Zybina, Marina Gravit)....Pages 91-115
Intumescent Nanocoatings for Fire Safety (Olga Zybina, Marina Gravit)....Pages 117-141
Problematic Issues of Applying and Using Intumescent Coatings (Olga Zybina, Marina Gravit)....Pages 143-189
Aspects of Tests of Intumescent Coatings for Woods, Fabrics, Plastics and Composite Materials (Olga Zybina, Marina Gravit)....Pages 191-207
Back Matter ....Pages 209-210

Citation preview

Springer Series on Polymer and Composite Materials

Olga Zybina Marina Gravit

Intumescent Coatings for Fire Protection of Building Structures and Materials

Springer Series on Polymer and Composite Materials Series Editor Susheel Kalia, Army Cadet College Wing, Indian Military Academy, Dehradun, India

The “Springer Series on Polymer and Composite Materials” publishes monographs and edited works in the areas of Polymer Science and Composite Materials. These compound classes form the basis for the development of many new materials for various applications. The series covers biomaterials, nanomaterials, polymeric nanofibers, and electrospun materials, polymer hybrids, composite materials from macro- to nano-scale, and many more; from fundamentals, over the synthesis and development of the new materials, to their applications. The authored or edited books in this series address researchers and professionals, academic and industrial chemists involved in the areas of Polymer Science and the development of new Materials. They cover aspects such as the chemistry, physics, characterization, and material science of Polymers, and Polymer and Composite Materials. The books in this series can serve a growing demand for concise and comprehensive treatments of specific topics in this rapidly growing field. The series will be interesting for researchers working in this field and cover the latest advances in polymers and composite materials. Potential topics include, but are not limited to: Fibers and Polymers: • • • • • • • • • • •

Lignocellulosic biomass and natural fibers Polymer nanofibers Polysaccharides and their derivatives Conducting polymers Surface functionalization of polymers Bio-inspired and stimuli-responsive polymers Shape-memory and self-healing polymers Hydrogels Rubber Polymeric foams Biodegradation and recycling of polymers

Bio- and Nano- Composites:• • • • • • • • •

Fiber-reinforced composites including both long and short fibers Wood-based composites Polymer blends Hybrid materials (organic-inorganic) Nanocomposite hydrogels Mechanical behavior of composites The Interface and Interphase in polymer composites Biodegradation and recycling of polymer composites Applications of composite materials

More information about this series at http://www.springer.com/series/13173

Olga Zybina Marina Gravit •

Intumescent Coatings for Fire Protection of Building Structures and Materials

123

Olga Zybina Peter the Great St. Petersburg Polytechnic University St. Petersburg, Russia

Marina Gravit Peter the Great St. Petersburg Polytechnic University St. Petersburg, Russia

ISSN 2364-1878 ISSN 2364-1886 (electronic) Springer Series on Polymer and Composite Materials ISBN 978-3-030-59421-3 ISBN 978-3-030-59422-0 (eBook) https://doi.org/10.1007/978-3-030-59422-0 © Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Introduction

Fire casualties are one of the major preventable tragedies in modern civilization. Reducing losses from fires through the development and improvement of fire protection of building structures and materials is an urgent task. Great attention to fire safety is paid at the stage of buildings and constructions design as all the structural materials used in the construction process lose their design strength when exposed to fire: concrete cracks [1–5], wood burns out [6] and steel loses strength [7–9]. To ensure the required values of fire resistance of building structures, passive fire protection is widely used, which means protection of building elements with special insulating materials. Insufficient efficiency of fire protection means can turn into a tragedy in case of fire, as, for example, happened with the towers of the World Trade Center in New York on September 11, 2001, when the cause for building destruction was metal structures bearing capacity loss [10] as a result of their rapid heating to critical temperatures [11]. Such a fate can befall any structure poorly protected from fire [12]. To prevent tragic scenarios, new methods and materials are being continuously developed to improve the fire protection of buildings [13]. One of the widely used ways to achieve the required protection of building materials and structures is the use of thin-layer fire-retardant coatings of intumescent type, which form in the fire on the protected surface of the foam-oxide shielding layer many times thicker than the original coating. Intumescent fire-protection coatings, also called intumescent coatings, are known and used for a relatively long time. The first data are revealed in the patent of H. Tramm [14] and the co-authors published in 1938, later [15–20] the structure of components of the intumescent systems from the point of view of their functions was described. The first extensive review article was published in the early 1970s by H. Vandersall [16] and it formulated the fundamental principles of thermolytic synthesis of fire-protection intumescent coatings, which have not changed fundamentally since then. In recent decades, the study of intumescent systems and the possible mechanism of their fire-protection action has been carried out in the works of foreign and domestic research teams of G. Camino, S. Bourbigot, F. A. Levites, L. N. Myskovsy, S. A. Nenakhov, I. S. Reshetnikov. The peculiarities of coking during the combustion of polymeric materials in the presence of flame retardants v

vi

Introduction

were studied by G. E. Zaikov, N. A. Khalturinsky, A. A. Berlin and others. As a result, the existing approaches to the design of fire-protection intumescent compositions with specified properties [21], as practice shows, contain a lot of gaps and contradictions, that is why, they remain the method of “trial and error”. Technological practice requires their development and improvement, which, in fact, is the subject of this work. Of course, during the significant period of application of intumescent coatings, the relevant scientific industry has developed a technical complex for obtaining such materials that provide effective fire protection [22]. This complex, which includes several variants of superpositions of ingredients, is purely empirical, although some researchers are trying to systematize experimental data and present them in a more or less slender, understandable way from the point of view of modern science. These attempts are very fragmented, which does not contribute to the adoption of optimal technological solutions. The creation of a sound scientific and technical concept that would describe the phenomenology of the intumescent process—physical–chemical processes of thermolytic synthesis of the heat-insulating foam-char layer, as well as the fire-protection effect of intumescent coatings, at least in general, will allow regulating the properties of intumescent composite materials in accordance with each specific task of fire protection. A large number of foreign and domestic publications [23–29] concerning the topic of increasing the fire resistance of building structures and materials confirms the fact that in the field of fire-protection coatings, continuous research is conducted to improve the compositions of the intumescent type. Thus, the theme of the monograph meets the needs of technological practice of creation and application of fire-protection intumescent compositions with high performance properties and, of course, is relevant. The authors express their sincere gratitude to everyone who helped in the work of the monograph. Special gratitude should be expressed to professors Mnatsakanov Suren Sarkisovich and Babkin Oleg Eduardovich for their support and great personal contribution to the creation of the monograph. They are co-authors of a number of publications included in the book. Many scientific and technical aspects of the monograph were discussed with them. This publication could not take place without the participation and invaluable support of the Chairman of the Board of Directors of the “Gefest” Enterprise group, Head of the Department of “Fire safety” of Peter the Great St. Petersburg Polytechnic University (SPbPU) Prof. Leonid Tanklevsky and Prof. of Peter the Great St. Petersburg Polytechnic University Dr. Nikolay Vatin. We are thankful, for the help in carrying out experiments, valuable tips and advice when working on the material, to Prof. A. Y. Snegirev, Associate Professor I. E. Yakunina, engineers S. V. Tikhomirova, A. A. Ustinov, А. S. Tomakhova, M. A. Shakhova and S. I. Pavlov. We are thankful to our colleagues from SPbPU, editors and all employees of the publishing house who worked on the publication of the monograph.

Introduction

vii

We express our gratitude to the family and friends for their support and understanding of the importance of this work. We will be grateful to all readers for their comments and suggestions on the materials of the book. Olga Zybina Marina Gravit

Contents

1 Basic Ingredients of Intumescent Compositions . . . . . . . 1.1 Flame-Retardant Mechanism of Intumescent Materials 1.2 Key Ingredients of Intumescent Compositions . . . . . . 1.3 Carbon Sources for Intumescent Systems . . . . . . . . . . 1.3.1 Polyfunctional Alcohols . . . . . . . . . . . . . . . . . 1.3.2 Cellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Acid Donors of Intumescent Compositions . . . . . . . . . 1.5 Gas-Forming Ingredients . . . . . . . . . . . . . . . . . . . . . . 1.5.1 Urea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 Guanidine . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.3 Melamine . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.4 Chloroparaffin . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Titanium Dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

2 Technology Basis of the Thermolytic Synthesis of Char Formation Polymeric System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Analyzing the Existing Views of the Intumescent Layer Forming Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Physicochemical Transformations of Char-Forming Ingredients in Intumescent Compositions During Thermolysis . . . . . . . . . . 2.3 Thermolytic Synthesis Mechanism of a Heat-Insulating Intumescent Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

1 4 6 9 9 14 18 28 30 32 33 39 39 42 44

..

53

..

54

..

64

.. .. ..

81 86 88

. . . . . . . . . . . . . . .

ix

x

Contents

3 Polymer Binders of Flame-Retardant Intumescent Coatings . 3.1 Synthetic Polymer Water Dispersions as Binders of Flame-Retardant Compositions . . . . . . . . . . . . . . . . . . 3.2 Organic Polymer Solutions as Binders of Intumescent Compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Applicability Conditions of Monoammonium Phosphate in Organic Solvent-Based Intumescent Compositions . . . . 3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Intumescent Nanocoatings for Fire Safety . . . . . . . . . . 4.1 Influence of Graphene Structures on the Properties of Intumescent Compositions . . . . . . . . . . . . . . . . 4.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

......

91

......

95

. . . . . . 108 . . . . . . 111 . . . . . . 113 . . . . . . 113

. . . . . . . . . . . 117 . . . . . . . . . . . 129 . . . . . . . . . . . 137 . . . . . . . . . . . 138

5 Problematic Issues of Applying and Using Intumescent Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Practical Experience of Flame-Retardant Intumescent Compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1 Intumescent Paints in Coating System . . . . . . . . . . . . . . 5.1.2 Issues of Diagnosing the Condition of FRICT . . . . . . . . 5.2 Developing Methods for the Timely Assessment of the Operational Efficiency of Intumescent Coatings . . . . . . . 5.2.1 Developing Express Methods for Determining the Adhesion of a Flame-Retardant Coating . . . . . . . . . 5.2.2 Developing an Express Method for Determining the Intumescent Indicators of a Coating . . . . . . . . . . . . 5.2.3 Assessing the Pore Spacing of the Charred Layer of Intumescent Coatings . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Effects of Finishing Coats on the Intumescent Properties of Flame-Retardant Materials . . . . . . . . . . . . . . . . . . . . 5.3 Using Flame-Retardant Char-Forming Coatings to Protect Facilities of an Oil and Gas Complex . . . . . . . . . . . . . . . . . . . 5.3.1 Methodology for Testing Facilities When Burning Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Industry Standards of Flame-Retardant Paint-Related Materials for Protecting Structures of Oil and Gas Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . 143 . . 144 . . 144 . . 147 . . 150 . . 152 . . 159 . . 161 . . 164 . . 173 . . 173

. . 178 . . 184 . . 186

Contents

6 Aspects of Tests of Intumescent Coatings for Woods, Fabrics, Plastics and Composite Materials . . . . . . . . . . . . . . . . . . . . . . 6.1 Fireproofing Wood with Intumescent Paint Materials . . . . . 6.2 Fireproofing Plastics and Composite Materials (Cable Lines and Cable Coatings) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xi

. . . . . 191 . . . . . 191 . . . . . 199 . . . . . 206 . . . . . 206

Summary of the Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

Chapter 1

Basic Ingredients of Intumescent Compositions

Abstract This chapter overviews the typology and evolution of flame retardants for building structures, their chemical composition and the functional contribution of the main components of the intumescent system to the fire-retardant mechanism. Approaches to the development of the most widely used materials based on the intumescent triad, including ammonium polyphosphate (APP), melamine (MA) and pentaerythritol (PE), are considered. The main groups of the required ingredients of these intumescent compositions are classified in detail, in accordance with the ideas about the functional contribution of each of them to the process of thermolytic synthesis of heat-insulating charring coatings: acid donors (ammonium phosphates), char formers (pentaerythritol, cellulose) and porophores (melamine, urea, guanidine and chloroparaffins). On the basis of a critical analysis of literary sources and experimental data obtained by the authors on the properties of substances that traditionally constitute intumescent materials, a hypothesis has been put forward that established ideas about the functional contribution of the discussed components of intumescent systems and the nature of the processes are inaccurate. The results of authors’ own research are presented, which allow to review and clarify the role of the main ingredients in the synthesis of charred layer. The authors experimentally confirmed and theoretically substantiated the new data on the behavior of pentaerythritol during thermolysis of the triad intumescent system of melamine–pentaerythritol–ammonium polyphosphate, which exclude pentaerythritols esterification by phosphoric acids under the considered conditions. It is shown that today there is no coherent concept that could be the scientific and technological foundation for creating fire-retardant intumescent compositions and could unambiguously describe the physicochemical nature of the processes of thermolytic synthesis of charred heat-insulating layers of intumescent coatings. Keywords Building construction · Fires · Fire resistance · Flame retardants · Fire-retardant coatings · Intumescent flame-retardant materials · Intumescent coatings · Melamine · Ammonium polyphosphate · Pentaerythritol · Char · Charred layer · Thermolysis · Intumescent char · Intumescent layer

© Springer Nature Switzerland AG 2020 O. Zybina and M. Gravit, Intumescent Coatings for Fire Protection of Building Structures and Materials, Springer Series on Polymer and Composite Materials, https://doi.org/10.1007/978-3-030-59422-0_1

1

2

1 Basic Ingredients of Intumescent Compositions

Most of the widely used construction materials have to be protected from fire, since a majority of them are unstable in high temperatures and flame [1–5]. Some products and structures burn down completely in fire—these are wooden [6] and polymeric ones [7, 8] while non-combustible, reinforced concrete [9–11] and metal ones lose their bearing capacity several dozens of minutes after they have heated up to critical temperatures [12–18]. One of the ways to increase fire resistance of building structures is to create insulating coatings on their surface [19]. Such coatings are divided into thin-layer (paints) and thick-layer (coatings, mastics and plasters) ones [20– 22]. The flame-retardant efficiency of coatings based on thick-layer compositions is achieved by a low heat conductivity coefficient and their unchanged structure during flame exposure [23–26]. The practical, functional meaning of thin-layer coatings is that a charred layer with very low heat conductivity is thermolytically formed on the surface of the protected structure. In contrast to the previously existing concept of fire protection, which implied using self-extinguishing materials, the concept that exists today is more knowledge-intensive and involves screening the surface with comprehensively functioning intumescent thin-layer paint-like systems that are easy to apply to the surface of building structures [27–30]. Since early times wooden buildings had commonly been protected from fire by clay treatment [31]. However, it is Gay-Lussac [32] who is believed to be the founder of the scientific approach to creating flame-retardant compositions after he proposed in 1821 to use a mixture of ammonium phosphate with borax for fire protection of cellulose materials. He was also the first to use the term “intumescence” to describe the observed flameretardant effect of the composition he developed. Then, in 1934, a German patent [32] claimed an increase in the flame resistance of wood in case it was treated with a mixture of diammonium phosphate and formaldehyde. The authors reported that the carbonized layer increased in volume in case the substrate was heated, but since the word “intumescence” was not used in the text of the patent, the first patent for an intumescent composition is considered to be the one published in the USA in 1938 by H. Tramm and co-authors [33]. American scientists have proposed a composition based on diammonium phosphate, dicyndiamide and formaldehyde. Thanks to this combination of ingredients, an intumescent carbon layer emerged upon exposure to high temperature. The next famous work on the chemistry of intumescent compounds was an article published in 1971 by Vandersall [34]. Later, other researchers made attempts [35–39] to describe the components of intumescent systems in terms of their functions. In the USSR, employees of the chemical department of the Central Scientific Research Laboratory (CSRL of the NKVD of the USSR) started to work on the creation of flame-retardant coatings. The research studies were conducted under the supervision of Taubkin [40] and were initially aimed at designing flame retardants based on such available components as superphosphate, clay, lime, sulfite-alcohol stillage, and waste products from the production of chlorinated products. The compositions developed by the CSRL of the NKVD of the USSR were successfully used to protect wooden structures during the Great Patriotic War of 1941–1945. In the postwar years, a method was developed for flame-retardant treatment of wood materials under pressure (in autoclaves), but surface treatment of wood was also widely used. In the 1950–1960s, the following flame retardants were developed in the USSR: DSK-P

1 Basic Ingredients of Intumescent Compositions

3

(based on diammonium phosphate, ammonium sulfate and kerosene contact), PPL (based on potash and organochlorine compounds with the addition of a plasticizer), FAM (based on lithopone monomer and vermiculite) and so on. Also, compositions containing phenol-formaldehyde resins (S-1, S-35), urea-formaldehyde resins (M60, M-48), urotropine, ammonium chloride and technical diammonium phosphate flame retardants were used on wood to protect it from fire. Since the 1960–1970s, intumescent flame retardants were produced for fire protection of wood and metal structures (Pirolan-64, Albert DS, DS-463). The materials were based on urea-formaldehyde resins, which included ammonium phosphate and dicyandiamide, and inorganic compounds of silicon, aluminum, titanium and iron. A little later, the widest spread ones were intumescent compositions having the names of VPM (for metal surfaces), VPD (wooden surfaces) and OPK (for electrical cables) and their subsequent modifications developed by employees of the VNIIPO: Kolganova, Levites, Moscovskaya and others [41, 42]. Heat-resistant fibrous fillers and stabilizers were introduced into the composition of coating VPM-2. Although they made the coating to look like putty and have a consistency of it, they were the ones to find the first solution to the problem of adhesion of the charred layer to a hot metal surface. The composition of VPM-3 included a new flame-retardant factor [40], due to which the flame-retardant efficiency of the coating increased, which made it possible to reduce the consumption and thickness of the coating layer. However, the problem of new flame retardants available in the 1980s remained acute. Its roots lay in the fact that the intumescent compositions contained a substance called melem (triaminoheptazine), which was mandatory and served as heat-resistant filler. Also, the melem was distinguished by high gas release when exposed to flame, which had a positive effect on the intumescence of coatings in general. However, melem was produced in limited amounts, which slowed down the outputs of flame retardants. An attempt was made to use intumescent (intercalated) graphite as an analog of melem. It was obtained from ordinary graphite treated with strong acids. Under temperature influence, intercalated graphite increases significantly in volume and so it is still used in many intumescent compositions as an intumescent agent [43–48]. Today, they keep on designing and producing the intumescent coatings with improved properties because the rates of construction are going up [49–51], cities are growing [52–55] and large public projects are carried out [56]. According to marketing research “Intumescent Coatings Market by Type (Thin-Film, Thick Film), Substrates (Structural Steel and Cast Iron, Wood), Application Technique (Spray, Brush and Roller), End-use Industry (Building and Construction, Industrial), and Region—Global Forecast to 2023” [57], the intumescent coatings market was valued at USD 863.2 million in 2017 and is projected to reach USD 1,110.0 million by 2023, at a CAGR of 4.3%. Today, intumescent compositions based on solutions of synthetic polymers in organic solvents, water dispersions of polymers and solvent-free epoxy binder-based compositions are the ones most widely represented on the market. Recently, UVcurable intumescent coatings have been developed [58]. Two types of materials prevail by the type of intumescent composition based on: intercalated graphite or

4

1 Basic Ingredients of Intumescent Compositions

classical intumescent triad, which includes melamine (MA), pentaerythritol (PA) and ammonium polyphosphate (APP). Mixed formulations are less common.

1.1 Flame-Retardant Mechanism of Intumescent Materials The pyrolysis of flame-retardant intumescent compositions (FRICs) is accompanied by complex physical and chemical processes. Phase and other structural transformations occur under the influence of thermal effects. From many parallel and sequential chemical reactions, a variety of products form and, eventually, a polymer-oligomeric substrate-isolating layer—charred layer—appears. Despite the long and, on the whole, quite successful application of intumescent materials used for fire protection of building structures, thermal destruction of FRIC is one of the phenomena that has not been studied sufficiently [38]. Various components in intumescent systems introduce additional difficulties in studying the laws of its thermal decomposition. Extensive empirical material has been accumulated concerning application of flame-retardant intumescent compositions as a result of numerous studies of their thermolysis, and some common laws and facts have been established, but, in most cases, they are difficult to interpret due to the complexity and variety of the processes that take place. Attempts to present the thermal synthesis of the charred layer in the form of specific chemical reactions are accompanied by major difficulties. As usual, in such cases, there are various hypotheses and points of view that the authors are trying to confirm with experimental data [34, 36, 59, 60]. Today, general principles have been developed to create effective intumescent compositions, ensure their resistance to heat effects and regulate targeted thermal transformations [32, 59, 61, 62]. A specific feature of flame chemistry is that temperature and concentrations of the source and intermediate substances and products are distributed in space in a complex way, and the fact that a lot of various degradation products are there both in the condensed and gas pre-flame regions. All of it makes experimental studies extremely complicated, as well as seriously impedes rigorous quantitative theories of processes that occur during burning of flame-retardant compositions, which would take into account all the chemical and other specifics of actual systems. Nevertheless, some common qualitative regularities are characteristic for the burning of most of these materials [63, 64]. In the pyrolysis of FRIC materials, the oxidizing agent is atmospheric oxygen, and the burning agents are hydrocarbon and carbohydrate degradation products of the polymer binder and other organic fillers, which, as a result of oxidation, gradually turn into water and carbon dioxide or, in case of incomplete oxidation, into carbon monoxide. An important circumstance affecting all stages of burning is the formation of a foamed char when FRICs are exposed to flame. The first important consequence of the forming of the charred layer is a drop in the release of combustible products into the gas phase, and reduction in the flow of combustible gases to the flame. Owing to the fact that a significant amount of char is formed in the condensed phase, it does

1.1 Flame-Retardant Mechanism of Intumescent Materials

5

not get into the gas flame, and the latter is depleted in carbon and, accordingly, the ratio significantly changes between combustible gases (hydrocarbons) and ammonia released during thermal decomposition of ammonium and other nitrogen-containing compounds. In this case, the effect of thermal dilution of the reduced amount of combustible gases with a large amount of NH3 becomes very significant. The carbon remaining in the solid phase of the intumescent layer is gradually oxidized to carbon oxides causing a relatively large heat effect. Ultimately, the case will end in complete burnout of carbon, a decrease in the density of the foamed layer and its collapse from the surface of the protected structure. But the time within which the flame-retardant coating will provide fire protection before the fire brigade arrives should be sufficient [64]. Thus, the burning of intumescent materials is a very sophisticated physicochemical process, and involves both chemical reactions of the destruction of polymer-analogous transformations, including crosslinking and carbonization in the condensed phase (as well as chemical reactions causing gas products to transform and oxidize), and physical processes, accompanying intensive heat and mass transfer [65]. Reactions in the condensed phase actually result in two main types of products [66]: (1) gaseous substances (combustible and non-combustible), and (2) solid products (carbon-containing and mineral). When the reaction occurs in the gas phase of the pre-flame region, fuel for flame, soot, and so on is formed. The least studied are the synthesis reactions taking place under the influence of high temperatures, as a result of which, as we believe, molecular structuring occurs with the formation of oligomeric, polymeric structures, including char-forming structures which are spatially best crosslinked. The researchers still have different opinions about the nature and morphology of these structures [32, 59, 60, 67]. In our research studies discussed below, we made quite determined attempts to clarify the chemistry of reactions the ingredients of fire protection char-forming compositions undergo in thermolysis. Several main points are highlighted in the assumed protective mechanism [34, 64, 68]: the first is about the substrate-isolating layer, which is formed due to a combination of structuring processes—synthesis of polymer oligomeric products during burning—char formation, carbonization and intumescence of the surface of the material on fire. The formed charred layer acts as a physical barrier, which reduces heat and mass transfer from the gas phase to the condensed one. The intumescent layer makes it difficult for gaseous fuel to get to the flame zone and limits the flow of atmospheric oxygen to the protected layer. In addition, due to the various phase transformations that fire-protection char-forming compositions undergo during thermal degradation, part of the supplied heat energy is absorbed. The released gaseous products, diffusing into the environment, cool the heated layers of the material, and thus additionally absorb a certain amount of thermal energy. An important effect on the values of absorbed heat is made by the composition and amount of gaseous degradation products. Volatile products that contain a significant amount of hydrogen in the molecules have the highest heat-absorbing ability. The next possible factor, as a result of which some part of the heat energy is absorbed, is heat absorption due to re-emission by the carbonized surface. In this case, thermal

6

1 Basic Ingredients of Intumescent Compositions

radiation depends mainly on the degree to which the surface of the material is heated and is determined by the Stefan–Boltzmann equation as a function of the surface temperature raised to the fourth power. Hence the materials whose ablation processes are accompanied by higher heating of the surface (i.e., various carbonized materials, materials containing inorganic fillers) should have the greatest emission rate [41]. Thus, a stable intumescent material limits gaseous “fuel” from forming and heat from spreading, and leads to self-extinguishing under standard conditions. Some authors [34, 68] indicate that in case an intumescent flame-retardant layer is formed, heat transferred toward the substrate decreases by about 100 times. The chemical make-up of flame-retardant intumescent compositions (FRICM) by itself largely determines the direction of its destruction in the conditions of fire and high temperatures. According to the researchers [63], the more condensed aromatic or heteroaromatic groups are contained in the formed intumescent layer, the higher is the char yield. Let us discuss in more detail the conditions necessary for the processes of foaming and char formation, which accompany the transformation of intumescent systems, and on the criteria for choosing the main components of the latter ones. It is obvious that steady foaming of intumescent coatings implies that gases must release after the intumescent mass has melted, but before it has begun to solidify, that is, before the carbonized layer has formed. So, when a composition is designed, the components must be selected in such a way that they have specific melting and destruction temperatures so that they will react in a given sequence, creating conditions necessary for targeted transformations of coating ingredients [68].

1.2 Key Ingredients of Intumescent Compositions Intumescent fire-retardant materials are multicomponent systems. Traditionally, the special components frequently used in intumescent coatings are divided into three main groups [32, 37, 38, 69–71]: (a) coking agents, which, as a rule, are polyatomic alcohols or polyols—organic hydroxyl-containing compounds with a high content of carbon; (b) catalysts (acidic components), which are inorganic acids or substances releasing acid at 100–250 °C; (c) foaming agents, which are organic amines or amides; some inorganic salts are capable of releasing significant amounts of noncombustible gases upon thermal decomposition; as a rule, these are alkali metal carbonates and ammonium salts. The principles of devising recipes for flame-retardant intumescent compositions imply superposition of the above-mentioned “mandatory” ingredients. A group of such compounds, which is most widely discussed in the scientific and technical literature, includes, in particular, ammonium phosphates, mainly polyphosphates, pentaerythritol and melamine [72–93]. It is natural that to obtain a homogeneous composition these ingredients should be put together by a certain binder, which is most often water dispersion or a polymer solution.

1.2 Key Ingredients of Intumescent Compositions Table 1.1 Recipe sample of water dispersion flame-retardant paint [156]

7

Components

Content (%) (mass.)

Polyvinyl acetate dispersion Mowilith DM 230

25.0

Ammonium polyphosphate Exolit PFA 422

22.0

Melamine Melafine

7.5

Pentaerythritol Charmor PM40

9.0

Titanium dioxide Kronos 2063

6.0

Aerosil 200

1.0

Cellulose thickener Natrosol Hr 250, 2% 4.0 solution Coalescer NX 795

1.3

Wetting agent Disperbyk 190

1.0

Sodium polyphosphate, 10% water solution

0.75

Water

Up to 100

Along with these key components, intumescent compositions have additional components, for example, pigments, in particular, titanium dioxide; fillers of various natures: fiberglass, mineral fibers, kaolin, talc, aluminum oxide, aluminum hydroxide, magnesium hydroxide, precipitated silica, silicates, hollow microspheres, crushed cellulose; plasticizers, thickeners, wetting agents, preservatives and other target additives [72–100]. Flame-retardant intumescent compositions (Table 1.1) for protecting steel structures, which are common today, do not differ a lot from the ones used 40 years ago [34]. The recommended, empirically verified ratio of ammonium polyphosphate, pentaerythritol, melamine and titanium dioxide is 3:1:1:1. It is believed that this is the very ratio which ensures that an effective intumescent coating is created [61, 64, 67, 68]. It should be noted that in an effort to improve and increase the effectiveness of fireprotection char-forming compositions, many authors [101–104] use the principles of various analogies trying to replace each of the above ingredients. It is clear that the simplest analogies are due to the presence of functional groups in the substances suggested. They are either the same as in the source materials, or similar in their reaction capacity. Therefore, for example, cellulose and starch are recommended instead of pentaerythritol; urea instead of melamine [32, 59, 68]; and ammonium phosphates are probably the only ones instead of which virtually nothing is recommended. Our experience of replacing classical components with functional analogs has shown unsatisfactory results. In particular, we conducted an experiment on the sequential replacement of one of the ingredients of the intumescent triad in the fire-retardant composition (APP–MA–PE as 3:1:1) with functional analogs recommended in patent and scientific literature (Table 1.2). An aqueous dispersion of polyvinyl acetate was used as the film former. The coatings formed by intumescent

8

1 Basic Ingredients of Intumescent Compositions

Table 1.2 Results replacement of basic intumescent components with their functional analogs (photos made by Zybina O.A.) No. of The replaced sample component

Substitute

Swelling Form of the sample in comparison with the coefficient initial one (after annealing at 600°)

Initial – sample

–

47

1

Pentaerythritol Galactose

2

Pentaerythritol D-Fructose –

3

Melamine

Urea

13

4

Ammonium polyphosphate

Potassium phosphate

15

–

–

compositions were annealed in a furnace (at 600 °C) and the volume of intumescent char was measured and compared with the thickness of initial coating. The test results showed that pentaerythritol is an irreplaceable component of intumescent compounds, since in its absence the composition leads to the loss of its intumescent properties. And the replacement of melamine and ammonium polyphosphate by their “functional analogs” leads to a significant deterioration in volume and structure of the forming char, which in turn affects the flame-retardant properties of the tested materials. Significant discrepancies in the theoretical ideas about the possibility of replacing the components of the intumescent composition with the realities of technological practice gave us the idea of the need to revise and clarify the existing ideas about the mechanism of synthesis of the polymer base of intumescent char, which are controversial nowadays [105, 106]. As noted above, the ingredients of fire protection char-forming compositions are classified according to their intended functional role. These include: acid sources, char formers and porophores—the substances that produce foaming gases as they decompose. To justify the role and significance of each of the above-mentioned

1.2 Key Ingredients of Intumescent Compositions

9

“mandatory ingredients”, we will conduct an analysis and compare the research results of various authors, including our own ones, for each of these ingredients.

1.3 Carbon Sources for Intumescent Systems Carbon sources are usually selected among various hydroxyl-containing hydrocarbons [32, 59, 67]. Intumescent systems most commonly have the following components: starch, dextrin, polyfunctional alcohols, in particular mono-, di-, and tripentaerythritol or their mixtures [72], sorbitol, resorcinol, trimethylolmelamine, triethylene glycol, phenol-formaldehydes and phenols. Other hydroxyl-containing components may include certain oils, cellulose, starch, proteins, glucose, maltose, mannitol, liquid polyols with a linear chain C2 –C5 and compounds of a more complex structure [32, 59, 72–93]. The effectiveness of potential carbon sources depends on their carbon content and the number of reactive hydroxyl groups [107]. It is believed [32, 59, 67] that the mass of ash which can be obtained depends on the amount of carbon, while the content of hydroxyl groups determines the rate of dehydration, and hence the rate of foaming. It is also highlighted [41] that polyethyleneterephthalate and starches are the most frequently used char formers. Additional char-forming agents [72] include expandable graphite, which, in our opinion, is true and not true at the same time. This contradiction will be considered later.

1.3.1 Polyfunctional Alcohols As noted above, it is pentaerythritol derivatives that are the most effective char formers due to their multifunctionality and high carbon content [32, 68, 107]. By their carbon content, pentaerythritol reaches 44%, di-pentaerythritol 50% and tripentaerythritol 53%. It should be highlighted immediately that if an ingredient is selected by these parameters, then it is worth considering polyvinyl alcohol in addition to the above-mentioned pentaerythritol, starch and cellulose. It contains many OH groups available for reactions, and its carbon content exceeds 50% of weight. So, there is a question: why pentaerythritol, rather than PVA? However, it is important that pentaerythritol is almost completely crystalline, while the crystallinity of PVA does not exceed 50%. When flame-retardant intumescent materials are used, the spatial structure must appear in the process of the most representative manifestation of their protective parameters, that is, in the process of burning or exposure to high temperatures. That is why, it is pentaerythritol that has become the most widely used among polyatomic alcohols: firstly, because it is multifunctional, and secondly, because it is not a polymer, different from PVA, and can earlier and easier, that is, in due time, “participate” in the synthesis of organophosphate structure of the intumescent layer, and from the very beginning does not prevent gas from releasing

10

1 Basic Ingredients of Intumescent Compositions

when intumescent compositions foam [61]. The same is fully true with regard to the advantage that pentaerythritol has over such recognized carbonization agents as starch and cellulose. Pentaerythritol [108]—2,2-bis-(hydroxymethyl)-1,3-propanediol, tetrahydroxy neopentane, tetramethylolmethane—is a colorless crystalline substance with a melting point of 268–269 °C. It exists in tetragonal crystalline modification, transforming into cubic one at 180 °C. Solubility in water (mass%) at 20 °C is 5.56; solubility (mass%) at 100 °C in ethylene glycol is 12.9, in glycerin is 10.3, in formamide is 21, in pyridine is 3.7, and in other organic solvents it is poorly soluble. It is important to note that a lot of chemical interactions of PE with various substances entail formation of cyclic products [108]. So, for example, products such as pentaerythritol disulfite can form with SOCl2 (Scheme 1.1). For example, with dibutyltin oxide, PE produces a cyclic alcoholate (Scheme 1.2). In an acidic environment, it forms mono- and bicyclic acetals with aldehydes and ketones, for instance (Scheme 1.3). Apparently, the reactions common for PE have made the researchers [34, 39, 60, 109], as shown below, come up with an idea that similar cyclic esters are most likely to form with phosphoric acids. However, it is also known [110, 111] that when PE is heated up to 270–280 °C with activated Al or Cu powder, it transforms into 2-methyl acrolein, methanol and formaldehyde. A.N. Chistyakov when dehydrating pentaerythritol obtained acrolein, methyl alcohol, carbon oxides and a crystalline substance which had a melting point of 49 °C, unknown structure, and composition C7 H14 O4 . Dobryansky and Markina [111] studied the thermocatalytic transformation of pentaerythritol on an artificial aluminosilicate catalyst at a temperature of 260–280 °C. The derivatives found included acrolein, methyl alcohol, formal pentaerythritol with a melting point of 50–51 °C, of composition C7 H12 O4 and gases. As it is known, acrolein and 2-methyl acrolein are the products of interaction of acetaldehyde and formaldehyde. It is by condensation of these aldehydes

Scheme 1.1 Structural formula of pentaerythritol disulfite

Scheme 1.2 Formation of cyclic alcoholate from dibutyltin oxide and pentaerythritol

1.3 Carbon Sources for Intumescent Systems

11

Scheme 1.3 Formation of mono- and bicyclic acetals in reaction between pentaerythritol and aldehydes/ketones

that acrolein and some of its derivatives are obtained in production industry. In its turn, acrolein is a highly reactive compound, which can form pyridine bases by the Chichibabin reaction with ammonia and amines, and behaves as dienophile in diene synthesis reactions making cyclic products to form [112]. One of the ways to obtain acrolein is through glycerol dehydration, which occurs according to the scheme (Scheme 1.4). This is interesting because glycerin, being a polyfunctional alcohol, is also quite widely used in flame-retardant compositions as a carbonization agent. By dehydrating one more polyol, ethylene glycol over a natural activated aluminosilicate catalyst at a temperature of 250–300 °C, Chistyakov [110] obtained acetic aldehyde, dioxane, diethylene glycol, ethylene glycol acetal, acetic acid and gases. This reaction has also been studied on Al2 O3 i TiO2 . On Al2 O3 at 365 °C, ethylene glycol was dehydrated and acetaldehyde is formed, which, however, continued to decompose and form gases. Only 1.5% of acetaldehyde was detected in the reaction products, while gas release amounted to 8% as expressed by the source material. At 345 °C, ethylene glycol is completely decomposed by the catalyst TiO2 . The process goes deeper with the formation of acetaldehyde and other gaseous reaction products. The authors noted a huge difference in side reactions on these catalysts. Judging from the above, we reckon that under topochemical conditions (high temperature and presence of dehydration catalysts—phosphoric acids and TiO2 ), common for thermolytic synthesis, the most likely processes should be dehydration of polyols, mainly pentaerythritol, with the formation of aldehydes. If the method of pentaerythritol synthesis is examined, this conclusion seems obvious.

Scheme 1.4 Formation of acrolein through glycerol dehydration

12

1 Basic Ingredients of Intumescent Compositions

Pentaerythritol was obtained in the late nineteenth century by condensation of acetaldehyde with formaldehyde. It began to be manufactured commercially in the 1930s [112]. The condensation of acetaldehyde with formaldehyde is carried out in an alkaline medium. The process occurs according to the following stages (Scheme 1.5). The resulting intermediate product—trioxyaldehyde—can only be detected by the polarographic method, since it easily reacts further with aldehydes present in the reaction system. Various by-products can form apart from pentaerythritol. In order to obtain technical pentaerythritol, it is extracted in a mixture with a small amount of dipentaerythritol by evaporating the extraction solution. To obtain pure pentaerythritol, some operations of evaporation and fractional crystallization are conducted [113]. Pentaerythritol is a tetrahydric alcohol. In this respect, it is not hard to compare it; for example, with such a homolog as glycerin—a trihydric alcohol. On the other hand, pentaerythritol, in terms of the quantity of hydroxyl groups contained in the molecule and the mass content of carbon, can be compared with a large number of other chemical compounds, commonly called polyols in scientific literature on flameretardant materials. These are various sugars, and are generally hydroxyl-containing oligomeric and polymer materials. For example, those with polyvinyl alcohol, where the OH-groups and the mass of carbon exceed the same ones in pentaerythritol. At the same time, it is important to take into consideration that, unlike all the above compounds, only pentaerythritol is almost purely crystalline, which largely determines its reaction capacity. Based on all the well-known physicochemical concepts that determine the reaction capacity of reagents that have a crystalline structure, it follows that in order to make their reaction capacity independent of diffusion processes, the crystals should either be dissolved or melted. The DTA and TGA curves of pentaerythritol (Fig. 1.1) show the data which indicate the temperature region of the morphological transition of pentaerythritol [114]. The transition starts at a temperature of 185 °C and finishes in the region of 250 °C. These values are reasonably consistent with a lot of literature data [67], which differ only slightly from each other. It should be noted that, in accordance with the TGA curves, the onset of the destruction is marked by a decrease in mass at a temperature above the one indicated as an endothermic melting peak on the DTA curve. Thus, the destruction starts after morphological rearrangement. Consequently, pentaerythritol should be heated to a temperature not lower than 18 °C, at which it starts rearranging morphologically. Otherwise, the reaction should be conducted in a polar medium of the solvent that is thermodynamically similar to pentaerythritol.

Scheme 1.5 Condensation of acetaldehyde with formaldehyde

1.3 Carbon Sources for Intumescent Systems Fig. 1.1 View of thermoanalytical curves of pentaerythritol [114]

13

TG, mg 35 30 25 20 15 10 5

DTA

290

270

250

230

210

190

170

150

130

90

110

70

50

30

10

0

TºC

35 30 25 20

∆H

15 10 5

80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280

0

T 0C

Polymer binder-based flame-retardant intumescent compositions, applied on protected surface, represent a solid in a fairly thin layer—up to 3 mm. It is quite obvious that no dissolution can occur during burning and/or thermolysis, but the co-ingredients of pentaerythritol have no affinity toward it, so it can be expected that pentaerythritol does not begin to react before it melts—at a temperature not lower than 190 °C, that is, at the moment when morphological rearrangement of the crystal lattice starts and when structural rearrangements of the substance itself can occur. What does it mean? If pentaerythritol is obtained from acetic aldehyde and formaldehyde, the reaction is thermodynamically triggered, and so is the reverse reaction, accordingly. This means that when pentaerythritol is heated in the presence of a dehydrating agent—ammonium phosphate—which is a mandatory component of flame-retardant compositions, pentaerythritol can also decompose on the surface of titanium dioxide to form aldehydes from which it was synthesized. It should be emphasized once again the fact that under the conditions we are studying, that is during thermolysis of fire-retardant compositions, pentaerythritol can in no way be considered a polyatomic alcohol with reactions characteristic of alcohol and, first of all, that of esterification. According to our own experience, if other char-forming ingredients are used, such as starch and cellulose derivatives,

14

1 Basic Ingredients of Intumescent Compositions

to partially or completely replace pentaerythritol, not only do they fail to bring advantages over pentaerythritol but are also much inferior in comparison with it in terms of the efficiency of expansion. So, the question is why these materials do work? Hypothetically, the following answer can be given to this question: during thermolysis, hydroxyl-containing polymers go through the stage when hydroxy groups transform to aldehyde, which can interact with melamine. Naturally, in this case, spatial difficulties in the reactions of polymer molecules can reduce the efficiency of formation of melamine aldehyde resin. However, to be more specific, we should consider in detail the existing ideas about thermal destruction of cellulose.

1.3.2 Cellulose There have been attempts to use cellulose or starch in flame-retardant intumescent compositions [115]. Their role in intumescent compositions is considered obvious; these substances, along with pentaerythritol, are referred to [32, 59, 67, 68] as carbonization agents, although no description of their “behavior” during thermolysis of intumescent systems has been found in the reviewed literature. However, experience suggests that cellulose and starch are much less effective in comparison with pentaerythritol. As a rule, these substances are not used in materials for fire protection of metal structures. What is more, impregnating compositions for fire protection of wood, including intumescent compositions include an acid-donating phosphorus-containing agent and a nitrogen-containing component, which plays the role of “porophore”, but often lack a char-forming additive, since it is believed that the wood itself will carry out this function. This assumption is frequently true. Cellulose has been used for a long time to make carbon fibers, and to answer the question why it is inefficient in intumescent compositions, let us consider the specifics of the pyrolysis of cellulose. Thermal destruction of cellulose is accompanied by a large number of parallel and sequential reactions. Due to these processes, cellulose experiences profound transformations, and intermediate products are formed with some of them turning to carbon within a certain temperature interval. The thermal destruction is affected by the structure of the macromolecule and supramolecular stages of cellulose, the medium in which the heat treatment occurs, impurities and catalysts, heating conditions, and so on. The main processes of cellulose degradation are completed at a temperature of about 350 °C. At this stage, numerous compounds are formed, the greatest mass loss of cellulose occurs, and a coal residue containing up to 60–70% of carbon is formed. This stage of thermal destruction is called the pyrolysis of cellulose. The products resulting from the thermal destruction of cellulose can be divided into three groups: compounds that are volatile at low temperatures, resins and a solid residue, which, depending on the final temperature of treatment, contains various amounts of carbon [116]. Byrne et al. [117] subjected cotton cellulose to pyrolysis in the absence of catalysts and analyzed volatile products. More than 19 compounds containing a

1.3 Carbon Sources for Intumescent Systems

15

carbonyl group were found in the pyrolysis products, including glyoxals, glycolaldehyde, 5-(hydroxymethyl)-furfural, formaldehyde, acetic aldehyde, n-butylaldehyde, acrolein, acetone, methylethyltone, mesoxyaldehyde, glyceraldehyde, dihydroxyacetone, furfural, hydroxymethyl-2-furylketone, grape aldehyde and acids (glyoxal, levulin, grape). They also studied vacuum pyrolysis of various cotton fabrics treated with flame retardants containing bromoform, phosphorus compounds, urea and a mixture of borax and boric acid (B/BA) in the ratio 7:3 and found out that the amount of flame retardant used and the duration of heat treatment affect the composition of pyrolysis products. If the duration of heating is short and the content of flame retardant is large, the yield of resins reduces while the coal residue goes up. With a longer duration of heating, even if the content of flame retardant is large, the yield of coal residue decreases. In the presence of flame retardants, the dehydration process becomes more intensive. The release of water in this case is approximately two times greater than that of untreated cellulose. At the same time, if flame retardants are present, the formation of levoglucosan, one of the characteristic products of thermal degradation of cellulose, sharply reduces or does not happen at all. If just 1.05% of B/BA is added, the yield of resins and levoglucosan goes down dramatically and the release of solid residue increases, and if 8% is added, the yield of levoglucosan drops to zero. Any further increase of B/BA has little effect on the release ratio of these components. An important conclusion follows from these data: when dehydration predominantly occurs during thermal decomposition at the initial stages, the reaction of levoglucosan formation is suppressed. Reference [118] used the TGA method to study the kinetics of thermal destruction of the source cellulose and cellulose treated with flame retardants. Sodium tetraborate Na2 B4 O7 •10H2 O, potassium bicarbonate KHCO3 , monoammonium phosphate NH4 H2 PO4 and hexahydric aluminum chloride AlCl3 •6H2 O were used as flame retardants. The mechanism of action of these salts is different, but a common thing is that the yield of resins reduces during thermal destruction. 2% of salts (from the mass of cellulose) were added to cellulose, that is, much less than in case of flameretardant treatment. Heating was carried out in vacuum at a rate of 3 °C/min with a residual pressure of 0.3 mm Hg or in air at a rate of 12 °C/7 min and with an air flow rate of 30 ml/min. Whatman paper with a content of 99.3% of a-cellulose served as the source cellulose. TGA showed that flame retardants increase the yield of coal residue, with NH4 H2 PO4 being the most efficient. Cellulose treated with other salts (final treatment temperature was 360 °C) has an intermediate position between the source cellulose and cellulose treated with NH4 H2 PO4 . The minimum yield of coal residue is documented for the source cellulose. Added flame retardants shift the onset of intense thermal destruction and, consequently, mass loss of cellulose to a region of lower temperatures. In the presence of NH4 H2 PO4 , cellulose begins to decompose at 230 °C, while the source cellulose begins to decompose at about 330 °C, that is, by 100 °C higher. In case other salts are present, decomposition begins between 230 and 330 °C. Fire retardants affect the maximum rate of mass loss and the temperature interval within which it occurs (Table 1.3). For cellulose treated with AlCl3 •6H2 O, the maximum rate of mass loss is close to that for the source cellulose, but it is also

16

1 Basic Ingredients of Intumescent Compositions

Table 1.3 Rates of mass loss and respective temperatures during decomposition of cellulose treated and untreated by flame retardants [118] TGA data

Material Untreated cellulose

Treated cellulose NH4 H2 PO4

KHCO3

Na2 B4 O7 •10H2 O

AlCl3 •6H2 O

Rate of mass loss (mg/min)

7.4

5.2

5.6

7.0

7.5

Temperature (°C)

335

280

305

325

300

shifted to a region of lower temperatures. Although the maximum rate of mass loss decreases, the total rate of thermal destruction of cellulose treated with flame retardants is much greater than that of untreated one. In the presence of flame retardants, exothermic and endothermic effects shift to a region of lower temperatures, which is quite consistent with the shift in mass losses and the reactions occurring in the presence of flame retardants in a region of lower temperatures. In addition, they cause a decrease in the quantitative values of endo- and exothermic effects and make the exothermic effect spread to a wider temperature interval. It is worth paying attention to Lewis acids and bases used as char formation catalysts in the pyrolysis of cellulose [116]. Such compounds include phosphorus pentachloride, phosphorus tribromide, ammonium chloride, ammonium bromide, phosphoric anhydride, phosphate trichloride, sulfur oxychloride and hydrochloric acid. Typical flame retardants were used at the first stage. One of the works [119] recommends using a mixture of borax and diammonium phosphate as catalysts. Vohler and Sperk [120] suggested ammonium phosphate, a mixture of borax and boric acid, vaporous iron compounds, transition metal halides of the periodic system. It is also proposed [116] to use salts of strong acids and an ammonium base—ammonium sulfate, diammonium phosphate and ammonium chloride. Hydrocellulose material in the presence of these compounds was treated up to a temperature of 400 °C with further carbonization and graphitization in an inert medium. It is noted that in the presence of catalysts, carbon yield increases significantly and approaches the theoretical value. The specific effect of Lewis acids and bases is that under their influence, cellulose dehydrates intensively and thereby other reactions are suppressed, making carbon yield go down. In addition, they form a protective gas environment, which prevents the material from inflaming, and make transition forms of carbon form quicker. An effect similar to that of Lewis acids and bases is caused by phosphoric, sulfuric, nitric acids and salts of these acids [116]. They can be used as aqueous solutions or solutions in organic substances. The first stage of carbonization is carried out in the presence of these catalysts at a temperature of up to 350 °C in air or in an inert medium. Subsequent carbonization is carried out in an inert medium. In the presence of acids, the first stage of carbonization sharply decreases in duration (Table 1.4). To obtain carbon fiber, a series of catalysts is suggested [64]. They contain phosphorus, in particular, diammonium phosphate and more complex compounds, such as

1.3 Carbon Sources for Intumescent Systems Table 1.4 Effect of acid treatment of hydrocellulose material on the duration of heat treatment [116]

17

Untreated

Treated

Temperature (°C)

Time (h)

Temperature (°C)

Time (h)

180

>48

180

~4

250

~10

230

~2

350

–

250

~6 (min)

350

>2 (min)

phosphorothiotriamide, cyclic trimmer of phosphoronitriamide, dimethylphosphoramide, methylphosphordiamide, trimethylphosphate and diethylphosphoramide. An interesting observation on how chloroparaffins affect char formation during polymer thermolysis is given by Berlin [115–120]. If heated chloroparaffin evaporates from the polymer matrix before it decomposes and undergoes other transformations in the gas phase, the flame-retardant effect of chlorine is weak and confined to a slight dilution of combustible gases with a small amount of hydrogen chloride, which, as noted above, acts as a Lewis compound. The oxygen index (OI) remains at the level of 17–19. If chloroparaffin decomposes in the condensed phase, then the OI becomes very high (40–45). In this case, the composition of gases entering the flame changes significantly. For low molecular weight chloroparaffins, the composition of gases by the amount of carbon and hydrogen coincides with the chemical composition of the source material. When high molecular weight chloroparaffin decomposes in the condensed phase, a significant amount of char which does not enter the gas flame is formed. The latter is depleted by carbon and, accordingly, the ratio between combustible gases (hydrocarbons) and inert hydrogen chloride changes in it substantially. In this case, the same effect of thermal dilution, but now of a small amount of combustible gases by a large amount of hydrogen chloride, becomes highly significant. The presence of HCl in the pyrolysis zone can shift the destruction temperatures of a polymer material to lower ones. Thus, there is every reason to believe that chloroparaffin, as well as the above-mentioned compounds, functions as a catalyst for char formation due to the release of a dehydrating agent such as HCl during decomposition. Konkin [116] makes the following conclusions about the effect of flame retardants on thermal destruction of cellulose. 1. Flame retardants increase the yield of coal residue and gaseous products and reduce the yield of resins and levoglucosan. 2. In the presence of flame retardants, cellulose begins to decompose at lower temperatures and within a wider temperature interval. 3. Compared to untreated cellulose, the maximum rate of mass loss of cellulose treated with flame retardants is reduced and shifted to a region of lower temperatures, but the total rate of destruction goes up. 4. Flame retardants become effective when their content is relatively small of about 1–2%.

18

1 Basic Ingredients of Intumescent Compositions

5. The specific effect of flame retardants is that in their presence, at the initial stages of heat treatment, cellulose dehydrates more intensively, and, as a result, reactions that make resin-like products appear are suppressed. 6. Flame retardants can be considered specific catalysts for the pyrolysis of cellulose. Summarizing the data on the pyrolysis mechanism, Konkin et al. came to the following conclusions: 1. The pyrolysis process includes three main types of reactions: dehydration, depolymerization and more profound decomposition of cellulose with the formation of various decomposition products. 2. Dehydration and depolymerization are competing reactions. Dehydration inhibits the depolymerization reaction, leading to a high yield of resin and levoglucosan, while making carbon yield go down. 3. Catalysts (flame retardants, Lewis acids and bases) accelerate the dehydration reaction and thereby make carbon yield increase and resin yield go down. With regard to intumescent systems, the data help us to answer the question about the functional value of cellulose in intumescent compositions—similar to pentaerythritol, cellulose, in the presence of a dehydrating agent, is a source of aldehydes. However, due to its structure, cellulose decomposes with the formation of aldehydes in a more chaotic way: with a wider variety, but, probably, with a lower concentration of products reactive with melamine, as well as at temperatures different from those usual for pentaerythritol. As a result, the foam char formed during the destruction of the fire-retardant coating based on cellulose is inferior in its characteristics to the material based on pentaerythritol. Nevertheless, when fire-retardant compositions are developed for wood, the “aldehyde” contribution of cellulose to the intumescent process should be taken into account. When individual problems are solved related to increasing the flame resistance of cellulose-containing materials (wood, paper, fabric), this contribution may be quite sufficient. Furthermore, it becomes clear why cellulosic materials are preferred as thickening agents in water-dispersible intumescent paints. Even though acrylic and urethane thickeners can also be used to achieve the desired rheological properties, most of the published recipes include modified cellulose. It is empirically suggested that the latter, even if its content in the composition is relatively small, can significantly affect the process of thermolytic synthesis of the charred layer by supplying the abovementioned reactive products, including furfural derivatives, into the intumescent “pot”.

1.4 Acid Donors of Intumescent Compositions In flame-retardant intumescent compositions, inorganic and organic phosphorus compounds are widely used as an acid source. The flame-retardant properties of phosphorus-containing compounds have been known for a long time. Phosphoruscontaining fragments are introduced into coating systems not only to reduce their

1.4 Acid Donors of Intumescent Compositions

19

flammability but also because they often increase adhesion, corrosion resistance, and have other useful properties. Moreover, phosphorus-containing additives are practically the only ones that prevent smoldering [68]. The greatest importance is given to polyphosphates. Vandersall [34] at the “rise” of the widespread use of flame-retardant intumescent systems defined three types of materials. These are compositions based on watersoluble monoammonium phosphate (MAP), on relatively water-insoluble ammonium polyphosphate, or on insoluble melamine phosphate. MAP is easily soluble in water, and reacts with other components of water-dispersible flame retardant, which makes it impossible to get a composition that has stable flame retardant and performance characteristics. Now, typical acidic ingredients of fire protection charforming compositions include orthophosphoric acid, its esters, salts (in particular, ammonium, amine and amide salts), melamine phosphate and ammonium polyphosphate [32, 34–38, 59, 67, 68]. Salts of sulfuric and boric acids are also used, including p-nitroalanine disulfate, ammonium sulfate and alkali metal borates [4–25]. Yet, the most preferred are ammonium salts of phosphoric acid or polyphosphoric acids, in particular, ammonium polyphosphates having formula (NH4 PO3 )n , where n is a number from 10 to 1500 [72]. It should be noted that ammonium polyphosphate “leaves behind” other known acid sources in terms of elemental phosphorus content, which is about 32%. Other acid donors contain less phosphorus, for example, 26.9% in monoammonium phosphate; 14.2% in melamine phosphate and 19.6% in urea phosphate. The best known among the esters of phosphoric acids are the one-, two- and three-substituted orthophosphates, ROP(O)(OH)2 , (RO)2 P(O)OH and (RO)3 PO, respectively, (where R is an alkyl, aryl or heterocyclic residue). Let us consider orthophosphates and polymer (or condensed) phosphates in more detail. Orthophosphates—salts of orthophosphoric acid H3 PO4 —are known as mono-, di- and tri-substituted. When one- and two-substituted orthophosphates are heated, they dehydrate, release structural water and form polymer (linear or ring) phosphates according to the scheme (Scheme 1.6). A typical representative of orthophosphates is monoammonium phosphate (NH4 H2 PO4 ), which represents colorless crystals with a tetragonal lattice. It decomposes releasing NH3 at 190 °C and is soluble in water. Diammonium phosphate is a crystalline substance, dissolves well in water and melts with decomposition at 70 °C. Triammonium phosphate is a crystalline substance that decomposes at room temperature. Condensed phosphates are divided into three main types: linear phosphates (polyphosphates) form an infinite number of salts, whose anions consist of PO4 − tetrahedrons connected to each other by oxygen atoms in unbranched chains; cyclic

Scheme 1.6 Formation of polymer phosphates through heating of ortophosphates

20

1 Basic Ingredients of Intumescent Compositions

phosphates (metaphosphates) have a ring structure; ultraphosphates are branched phosphates that are combinations of rings and chains. It is believed [121] that linear polyphosphates are prone to form cyclic and branched structures in thermolysis of intumescent compositions. Polymer phosphates of various structural types can be described by the formulas: linear polyphosphates Men+2 Pn O3n+1 and ring metaphosphates Men Pn O3n (where n is the polymerization degree). The properties of polymer phosphates depend on the kind of cations, the structure of phosphate anions, the polymerization degree, the structure of phosphate, and so on. For example, the solubility of linear polyphosphates, as a rule, decreases if the polymerization degree grows, but can be increased by modifying polyphosphates, in particular, by changing the cooling rate of the melt. Polymer phosphates (linear and ring) are obtained mainly by thermal dehydration of monoand bi-substituted orthophosphates or by neutralizing of the corresponding poly- or meta- (cyclic) phosphoric acids (Scheme 1.7). Sometimes these processes are combined, for example, during high-temperature ammonization of phosphoric acid so that ammonium polyphosphates can be obtained. Condensed phosphates contain P atoms in a completely oxidized state, so they are chemically quite stable. However, it is well known that they are hydrolytically unstable, and under certain conditions all P–O–P bonds in the structure can be broken. The final products of hydrolysis are discrete orthophosphate ions, although the course and rate of hydrolysis depend on certain condensed anions and the prevailing conditions. Phosphates containing more than one cation are called double, triple, and so on (in the general case, multicationic), and in case of a more complex anionic composition— mixed phosphates. Condensed phosphates are a unique class of inorganic compounds, which, similar to organic substances, form homologous series of oligomeric and polymeric derivatives, stable not only in the solid state but also in aqueous solutions. Owing to a wide range of useful properties pre-conditioned by the polymer structure, this class of phosphates has attracted the attention of many researchers for a long while [122]. Ammonium polyphosphate was first tested in flame-retardant materials in 1965, and the recipe of water-dispersed material presented by Vandesall has been used to this day [34, 67]. Ammonium polyphosphate (NH4 PO4 )n [108] is a colorless crystalline substance. It is most widely used as flame-retardant additive in the manufacturing of flameretardant paints, varnishes, impregnations, mastics, plastics, electric cable sheathing, and so on. According to the reference data, APP melts at a temperature of 180– 185 °C. Intensive decomposition into ammonia and polyphosphoric acid occurs at a temperature of about 300 °C. There are two types of ammonium polyphosphate used as a flame retardant: with crystalline phase I with the polymerization degree

Scheme 1.7 Formation of polymer phosphates through neutralizing of phosphoric acids

1.4 Acid Donors of Intumescent Compositions

21

n < 1000, and highly condensed polyphosphate with crystalline phase II with the polymerization degree n > 1000. Ammonium polyphosphates with crystalline phase I have a linear structure with a varying chain length, decompose at lower temperatures and have a higher water solubility compared to ammonium polyphosphates with crystalline phase II. Ammonium polyphosphates with crystalline phase II have a similar but more sophisticated and branched structure, with greater molecular weight, high thermal stability and lower solubility in water than ammonium polyphosphates with crystalline phase I. One of the ways to make ammonium polyphosphate highly condensed is through heating it in the reaction zone in the presence of ammonium polyphosphate and through mixing ammonium orthophosphate and urea in 1:1 molar ratio in gaseous ammonia atmosphere with the formation of a low melting eutectic point. Then, the melt is heated at a speed of 160–200°/min up to 230–240 °C with crystallization of the melt and up to 280–300 °C with exposure for dehydration of the reaction product by maintaining rarefaction equal to 0.52–1.29 kPa in the reaction zone, after which the reaction product is cooled and ground. The result is a highly condensed ammonium polyphosphate with a high temperature of thermal decomposition onset (275–285 °C). In this case, the mass loss of the product is 0.22–0.05%. Highly condensed polyphosphate is thermally stable up to 324 °C (profound endo effect on the derivatogram). Mass loss at 324 °C is 1.0% The fire-retardant efficiency of ammonium polyphosphates is pre-conditioned, among other things, by their thermal stability. The temperature of the onset of mass loss should not be lower than 210–220 °C, and the mass loss should also be minimal (no more than 0.2%). The thermal stability of polyphosphates depends on the proportion of low molecular weight polyphosphates in the composition of the product, reducing the temperature of the beginning of decomposition. The lower the proportion of low molecular weight polyphosphates is in the product, the higher the temperature at which decomposition of the product begins, and the more effective is its flame retardancy [122]. Review by Nenakhov [67] presents the results of a detailed study conducted by S. Drevel et al. into the physical properties of widely known commercial products (APP in form I—Antiblase MC with water solubility of 2.8% of mass. of Rhodia; APP in form II—Exolit AP422 with a solubility in water of less than 1.0% of mass. of Clariant). IR spectra are presented for both products (Fig. 1.2). The authors draw attention to the absorption peaks common for both forms of APP: 1250 cm−1 for the P=O bond, 1010 and 1070 cm−1 for the P–O bond, and the peaks present only in form I at a frequency of 760, 660 and 602 cm−1 . The peak at 800 cm−1 is identified as the P–O–P bond. The work also contains X-ray diffraction patterns showing that the product in form I contains impurities of ammonium carbonate and hydrogenated ammonium phosphate. Both products have an orthorhombic structure with rather different parameters of the crystal lattice. The particles of APP in form I are 10–20 µm in size, which is typical for this form; particles in form II are large: 10–40 µm or more.

22

1 Basic Ingredients of Intumescent Compositions

Fig. 1.2 IR spectra of APP in form I (above) and in form II (below)

Ammonium polyphosphate in water should behave as a polyelectrolyte with all the logical consequences; for example, it should actively bind water and, therefore, increase viscosity. This fact is confirmed by the authors of [123], where they note that hygroscopicity, partial solubility in water and hydrolytic instability of ammonium polyphosphate have a negative impact on the technological and flame-retardant properties of the intumescent material. Thus, ammonium polyphosphate is an inorganic polymer, and its synthesis predetermines its fairly regular structure. Chemical regularity determines steric regularity and therefore the ability to crystallize. Ammonium salts of polyphosphoric acid are responsible to the utmost for the protective effect due to the multiple intumescence of the flame-retardant material and good preservation of the carbonized charred layer on the surface of the protected material. However, the literature completely lacks data on the role, and physical and chemical nature, which pre-determine a special “phenomenon” of APP. Only the most common and obvious phenomena that are inherent in all the ingredients of intumescent systems are known: about the ability to synthesize poly- and oligomeric compounds at elevated temperatures and the release of large volumes of relatively non-toxic gases resulting from the destruction of the ingredients of flame-retardant compositions. We have studied various APP samples. In the first series of research [114], APP was characterized by the following parameters: solubility in water—the higher it is, the lower is the molecular mass; P2 O5 content—the higher this indicator, the lower is the substitution degree. The very same indicator is controlled by pH parameter.

1.4 Acid Donors of Intumescent Compositions Table 1.5 Characteristics of APP samples of different brands [114]

23

Sample

P2 O5 content (%)

pH

Solubility in water (g/100 ml)

1

69.0

5–6

4.0

2

70.0

5–6

3.0

3

72.0

5–6

0.5

4

71.0

6–7

0.1

Table 1.5 makes it clear that if there is approximate coincidence of the substitution parameters of polyphosphoric acid, an increase in the average molecular mass is observed in the direction from sample 1 to sample 4. Figure 1.3 shows that when solubility in water goes down, crystallinity increases— the endothermic peak on the DTA curve and the temperature of the beginning of DTA melting process and destruction process (TGA curves). As it can be seen from the comparison of the DTA curves, the endothermic peaks (melting of crystalline structures) characterizing the presented polyphosphates are quite close by temperature (320–370 °C). The beginning of intensive mass loss coincides with this region. An exception is the sample illustrated in Fig. 1.3a. A small melting peak on the DTA curve is visible in the region 180–200 °C, where destruction begins (the TGA curve). It is natural that the loss of mass should occur, first of all, because of the release of ammonia, which is an agent swelling the intumescent material. Apparently, sample 1 is actually ammonium salt, while the rest are highly condensed ammonium polyphosphates that have surface treatment. Patent literature [84, 124–126] contains information that ammonium polyphosphate with crystalline phase II is modified with its particles being additionally treated with various additives (melamine formaldehyde resin, melamine, urea phosphate, melamine phosphate, dicyandiamide, silicone, etc.), which makes it suitable for use in specialized sectors of industry. This fact is confirmed by the IR spectroscopy (which will be discussed below) of an APP sample belonging to a Chinese brand. Having established a noticeable difference in the behavior of APP samples in thermolysis, we decided to carry out further thermoanalytical studies of APP brands, which (according to the manufacturers and suppliers) are consumed in the production of fire-protection char-forming compositions [127]. The test products were APP with crystalline phase II and polymerization degree of at least 1000. Additional specifications are given in Table 1.6. The first stage of the research study included analysis of an FR Cros 484 sample of ammonium polyphosphate. The thermogram of this sample (Fig. 1.4a) shows that the substance within the studied temperature interval (up to 800 °C) begins to decompose at 95 °C, and the maximum effects of mass loss and thermal transformations are recorded at 399 °C both on the DTG and the DTA curve. This sample, unlike the others, has two DTA peaks. The first peak, logged at 95 °C, indicates that there is crystalline hydrate water in this sample, while all other samples (Fig. 1.4b–e) do not lack this peak.

24

TG, mg 35 30 25 20 15 10 5 0

20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 660 700

Fig. 1.3 View of thermoanalytical curves of ammonium polyphosphate: a sample 1; b sample 2; c sample 3; d sample 4 [114]

1 Basic Ingredients of Intumescent Compositions

TºC DTA 35 30 25

∆H

20 15 10 5 0 20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 660 700

T 0C

a

TG, mg 35 30 25 20 15 10 5 0 20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 660 700

T 0C DTA 35 30 25

∆H 20 15 10 5 0 20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 660 700

T 0C

b

1.4 Acid Donors of Intumescent Compositions TG, mg 35 30 25 20 15 10 5 0 20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 660 700

T 0C DTA 35 30 25

∆H 20 15 10 5 0 20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 660 700

TG, mg

T 0C

c

35 30 25 20 15 10 5

20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 660 700

0

TºC

DTA 35 30 25 20 15 10 5 0

20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 660 700

Fig. 1.3 (continued)

25

d

TºC

26

1 Basic Ingredients of Intumescent Compositions

Table 1.6 Specifications of APP brands used fire protection char-forming compositions (by the manufacturers’ data) No.

Brand of sample

P (%)

N (%)

Viscosity (MPa*c)

P2 O5 (%)

pH (10% solution)

Temperature of decomposition (°C)

1

FR Cros 484 (Budenheim)

32

14

19

72

5.5

300

2

RN-A20 (Roshal Group)

31

14

*

*

6–8

275

3

SF APP 201 (Shifang)

31–32

14–15

≤100

*

6

275

4

JLS-APP-Special (JLS Chemical)

31–32

14–15

≤5

72

5.5

*

5

JLS-APP 101 (JLS Chemical)

28–30

17–20

≤20

68

5.5–7.0

*

It is obvious that the above endothermic effect can seriously affect the fireretardant properties of an intumescent material containing FR Cros 484 polyphosphate due to heat absorption at the initial stage of thermolysis of the coating and release of vapor-phase water, which reduces the concentration of combustible substances and oxidizing agent because of dilution. The second endothermic peak, corresponding to the final decomposition of APP into ammonia and polyphosphoric acid, covers a significant temperature interval of 320–400 °C. The curves obtained in the analysis of the remaining ammonium polyphosphate samples (Fig. 1.4b–e) are similar to each other. Thus, the maximum values of endoeffects on the DTA curves of samples 2–5 (Table 1.6) are within the temperature interval of 350–370 °C and correspond to the minima of the DTG curves, which means that the substance has decomposed and ammonia has released. These temperatures are much higher than the reference values, which can be explained by the surface treatment of the particles and a high polymerization degree of the presented samples. From experience it is known that polyphosphate brands with increased heat resistance are specially produced for use in fire-protection char-forming compositions to ensure the release of ammonia and acids at a given time—when amino aldehyde resin, which has to foam and harden, has already formed. The IR spectrophotometry of sample 3 (Table 1.5) confirms this assumption (Fig. 1.5). There is a wide intensive absorption band (AB) in the region (in cm−1 ) of 3600–2600, with maxima in the region of 3240, 3070, and refraction in the region of 2840, which indicates that the chemical composition contains hydroxyl groups and amino groups; a series of small peaks combined into one peak in the region of 2450 indicates the presence of amino groups; wide intensive ABs of 1000, 1100 indicate there are phosphate ions in the composition; a set of ABs of 1450, 1050, 1250 is evidence of 1,3,5-triazine (structural fragment of melamine); a wide intensive AB of 3030–3300 indicates the presence of an ammonium ion.

1.4 Acid Donors of Intumescent Compositions

27

Fig. 1.4 Results of thermal analysis of ammonium polyphosphate used in intumescent compositions: a FR Cros 484; b RN-A20; c SF APP 201; d JLS-APP-Special; e JLS-APP 101 [127]

28

1 Basic Ingredients of Intumescent Compositions

Fig. 1.5 IR-spectrum of sample 3 of APP [61]

Another proof that such an assumption is logical is that there are melting peaks of melamine, close by temperature intervals (Fig. 1.6). Reference [128] presents the results of study of APP thermolysis using the method of differential scanning calorimetry (DSC) in air and nitrogen flows. A test in air showed that decomposition begins at a temperature similar to the sublimation temperature of pentaerythritol, and ends later at 380 °C. The array of the DSC data shows the complexity of the reactions, which include various stages of crosslinking, the cause for the release of water and ammonia and result in forming phosphate polymer structures (cyclic and ultraphosphates) and polyphosphoric acids. Thus, the “phenomenon” of protective efficiency of APP is pre-conditioned by the high decomposition temperature of the compound as a whole and its modifying additive, for example, melamine. All samples of ammonium polyphosphate decompose with ammonia release in the region of 340 °C, which does not correspond to the reference data and, obviously, happens because of the special surface treatment of polyphosphate aimed at increasing its decomposition temperature.

1.5 Gas-Forming Ingredients In most of the known flame-retardant intumescent coatings, an important role is played by the ingredients, which are classified as porophores—the gas-forming agents that make char foam. It is known [32, 34, 38, 59, 68] that gas forms due to the thermal decomposition of substances and the evaporation of low molecular weight

1.5 Gas-Forming Ingredients Fig. 1.6 DTA and TGA of melamine [114]

29 TG, mg 35 30 25 20 15 10 5 0

20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 660 700

T 0C

DTA 35 30 25

∆H 20 15 10 5 0

20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 660 700

T 0C

components. In relation to ammonium phosphate coking compositions, melamine has been considered a traditional gas-forming agent “for ages” [32, 59]. Foaming agents must decompose at the right temperature and release a large amount of gas [129]. Today, in the intumescent compositions the following gas-forming agents are used: organic amines and amides such as urea, butylurea, dicyandiamide, casein, urotropine, guanidine, sulfamides, polyamide and aminoformaldehyde oligomers, melamine and its derivatives, including melamine phosphate, melamine cyanurate, melamine borate, melamine polyphosphate and so on [72–93]. The temperatures and products of destruction of some of the best-known foaming agents are given in Table 1.7. As noted above, the most effective recipes of flame-retardant compositions have been experimentally selected. Along with ammonium pentaerythritol and polyphosphate, they include melamine as a foaming agent [130]. Why is the composition precisely like this? The logic of the researchers was the following: in the process of foaming, several reactions should take place almost simultaneously but in a certain sequence, so the process of gas formation should begin when the polymer oligomeric skeleton has already formed. It has to be foamed, “lifted” while the intumescent layer is cooled by diffusing gaseous decomposition products until it is finally hardened. The temperatures are from 280 °C. Within this temperature range, gases are produced

30 Table 1.7 Decomposition temperature of foaming compounds [68]

1 Basic Ingredients of Intumescent Compositions Material

Gaseous product

Decomposition temperature (°C)

Melamine

NH3 , CO2 , H2 O

300

Ganidine (NH2 )2 C=NH

NH3 , CO2 , H2 O

160

Glycine NH2 –CH2 –COOH

NH3 , CO2 , H2 O

~233

Urea CH2 –CO–NH2

NH3 , CO2 , H2 O

~130

Chloroparaffin Cm Hn –Cln

HCl, CO2 , H2 O

160–350

by melamine, not considering the contribution of polyphosphate and binder; therefore, it is melamine which is the most suitable foaming agent for flame-retardant compositions. However, there have been attempts to use other gas-forming agents, but they are usually less successful. Melamine still cannot be replaced without worsening flame-retardant properties. Let us try to figure out why and consider the most popular porophores described in the literature.

1.5.1 Urea Urea [108], carbamide or carbonic diamide are white crystals soluble in polar solvents. When heated up to 150 °C and above, urea transforms subsequently into NH4 NCO, NH3 , CO2 , biuret, cyanuric acid; in a closed vessel, especially in ammonia, amination products of cyanuric acid, such as melamine, are formed. Urea was discovered by Rouelle in 1773 and first obtained by Wöhler, who evaporated it from an aqueous solution of ammonium cyanate in 1828 [76]. This transformation is the first synthesis of an organic compound from an inorganic one. In industry, urea is synthesized by the Bazarov reaction from ammonia and carbon dioxide. For this reason, urea production facilities are combined with ammonia ones. Urea is a bulk product used mainly as a nitrogen fertilizer (nitrogen content is 46%). Another important industrial application of urea is in the synthesis of urea-formaldehyde resins, which are widely used as adhesives in the manufacture of fiberboards and furniture production. Thermal decomposition of urea [131] occurs in two stages with the maxima of the rate of mass loss at 230 and 350 °C. Urea almost completely decomposes (the residue does not exceed 3% of mass) at a temperature of about 400 °C. At the first stage, the decomposition products are carbon dioxide, ammonia and water; at the second stage, they are carbamylurea and cyanamide compounds. Urea can be considered an amino acid amide [132]. Thus, two amino groups in urea are not identical to each other—one of them is amide, and the other one is amine.

1.5 Gas-Forming Ingredients

31

It is believed that the amide group NH2 reacts with formaldehyde to form methyl urea [110] (Scheme 1.8). Such compounds are intermediate products when crosslinked urea-formaldehyde resins are obtained. Monomethylol urea is trimerized with water loss. Then, formaldehyde reacts with the CO–NH2 groups of trimers to form a crosslinked product. The stoichiometry of the mechanism calls for a urea:formaldehyde ratio of 1:1.5 (Scheme 1.9). However, cyclic groupings are not necessary for a crosslinked polymer to form. Linear and branched sections are known in accordance with the structure shown below [133], if there is a small number or no cycles at all (Scheme 1.10). Crosslinking is accelerated by either acidic products or substances with functional groups capable of playing the role of an acid at high temperatures. The authors of [131] considered the behavior of urea mixed with APP and pentaerythritol (1:3:1 ratio, respectively) and concluded that urea, emitting gaseous

Scheme 1.8 Formation of methyl urea from urea

Scheme 1.9 Iteraction between formaldehyde and CO–NH2 groups of trimerized monomethynol urea

32

1 Basic Ingredients of Intumescent Compositions

Scheme 1.10 Linear and branched sections of a crosslinked polymer—product of reaction in Scheme 1.9

products up to 70% of its initial mass, does not make a significant contribution to the foaming of the decomposing ternary mixture, since these gases are released at a temperature that is too low (most at 175–300 °C, least at 300–400 °C). The ternary mixture foams at a temperature above 375 °C. We believe that it may be explained in the following way. If the gases formed at relatively low temperatures are byproducts in the synthesis of cyclic nitrogen-containing compounds like melamine, which later are formed by resins with aldehydes, then it is clear why foaming does not occur during copolymerization at the initial stage of thermolysis—the material to be expanded has not been synthesized yet. Presumably, at temperatures above 375 °C, the melamine-aldehyde resin formed from melamine becomes crosslinked under the effect of actively decomposing polyphosphoric acid, and as it hardens, it foams the gases that are still being released.

1.5.2 Guanidine Guanidine [108] is a colorless crystalline substance with the melting point of 50 °C. It has very intense basic properties: similar to sodium hydroxide, it absorbs air and turns into guanidinium carbonate. If heated guanidine turns into urea, and the reaction is accompanied by ammonia release [132] (Scheme 1.11). Guanidine in water is hydrolyzed to urea (50% in 20 days at 29 °C). With acids it forms salts that are resistant to hydrolysis. It is easily alkylated, and acylated. Condensation of guanidine with polyfunctional compounds (diesters, diketones, diamines, etc.) results in heterocyclic compounds (pyrimidines, piperazines, triazines, etc.) or polymers. For example [108] (Scheme 1.12). In manufacturing industry, guanidine and its salts are obtained by fusing the corresponding ammonium salts with urea or cyanguanidine (dicyandiamide), or

1.5 Gas-Forming Ingredients

33

Scheme 1.11 Formation of urea from guanidine

Scheme 1.12 Formation of heterocyclic compounds through condensation of guanidine with polyfunctional compounds

NH4NO3 + 2(NH2)2CO → (H2N)2C=NH + CO2 + 2NH3

NH H2N

H2,Ni

NHCN

(H2N)2C

NH + HCN

Scheme 1.13 Formation of guanidine by hydrogenolysis of cyanguanidine in an aqueous solution over Raney nickel

by hydrogenolysis of cyanguanidine in an aqueous solution over Raney nickel, for example (Scheme 1.13). The polycondensation products of guanidine with formalin or hexamethylenediamine are strongly basic ion-exchange resins. All of the above suggest that guanidine in intumescent systems can take part in forming nitrogen-containing heterocyclic structures.

1.5.3 Melamine Melamine is considered to be one of the most effective gas-forming agents in foaming and coking compositions. At the same time, many aspects of behavior of melamine in fire-protection char-forming compositions are still not clear [130]. These are issues related to the nature of gas formation, the degree of contribution melamine that makes to the general endothermic process, characteristic of foaming compositions, and the

34

1 Basic Ingredients of Intumescent Compositions

possibility and the way for melamine or its decomposition products to participate in the “construction” of the charred layer. Melamine [108] (2,4,6-triamino-1,3,5-triazine) is an amino derivative of symmetric triazine, cyanamide trimer or cyanuric acid triamide, and an organic base. Chemically pure melamine represents colorless prismatic and odorless crystals, which are practically insoluble in cold water (5% at 100 °C) and in the majority of organic solvents. It is soluble in glycerol and pyridine. Melamine belongs to the class of cyclic cyanamides and has a set of properties that are a consequence of energy stability of triazine nuclei. These include heat, light and chemical resistance, and an ability to form multifunctional reactive chemical compounds. That is why melamine is a valuable raw material for producing a whole lot of polymer compounds. Melamine is a base. It forms complex salts with acids, which decompose when heated. When heated above 354 °C, melamine decomposes with splitting off ammonia (NH3 ) and forming melem. Melamine easily reacts with aldehydes, condenses with cellulose, sugars, glycols, and other organic compounds containing OH-groups. All melamine reactions go by amino groups. In 1834, when fusing potassium thiocyanate with ammonium chloride, J. Liebig was the first to receive melamine. Later, in 1913, scientists Stolle and Krauch produced it in another way—from dicyandiamide (Scheme 1.14). The first production units based on this method appeared much later, in the late 1930s. After World War II was over, the technology for producing melamine from carbamide started to develop. Melamine was synthesized from urea for the first time in the early 1940s, and only in the 1960s it attracted the attention of industrialists. Until the 1970s, two types of melamine production units continued to be used: from dicyandiamide and from carbamide. The growing popularity of the method to synthesize melamine from urea coincided with the growing popularity of the latter as an effective fertilizer. As carbamide production was growing widespread and the technology was improving, it became more efficient and economically preferable to produce urea-based melamine. Currently, all over the world melamine is produced from carbamide. Melamine is obtained from urea at 350–450 °C and at a pressure of 50–200 MPa. Two variants of the carbamide pyrolysis process have become widely spread and industrialized: at low pressure in the presence of a catalyst called aluminum oxide and at high pressure without a catalyst. In the synthesis of melamine from urea, one mole of the product is accompanied by the production of three moles of carbon dioxide and six moles of ammonia (these amounts of by-product gases are the reason why urea is referred to porophores in

Scheme 1.14 Formation of melamine from dicyandiamide

1.5 Gas-Forming Ingredients

35

APP). At the first stage, urea breaks off ammonia to form cyanic acid, which then, with the removal of carbon dioxide, forms melamine [46] (Scheme 1.15). Melamine is used for the production of melamine-aldehyde polymers, varnishes and adhesives, which have high mechanical strength, low electrical conductivity, good water and heat resistance. The polycondensation of melamine with formaldehyde is less studied than urea-formaldehyde one. However, the mechanism of the condensation reaction and the basic structure of building resins seem to be similar to the schemes for urea-formaldehyde condensation products [113, 133]. The same as in the polycondensation of urea with formaldehyde, methylol derivatives of melamine are initially formed. However, the distinguishing feature is that in this case, in the presence of a respective excess of formaldehyde, all six hydrogens of the melamine amide groups can react and form hexamethylolmelamine (as well as di-, tri-, tetra- and penta-methylol derivatives) according to the scheme [79] (Scheme 1.16). Then methylol products (in an acidic environment) react with each other and with melamine, forming methylene and, partially, ester bridges between the rings. According to the works of Kiselev [134], hexamethylolmelamine is obtained from melamine interacting with a double excess of formaldehyde (12 mol of formaldehyde per 1 mol of melamine) in a neutral environment (pH = 7–7.5) at moderate temperatures (60–80 °C); pentamethylolmelamine is formed under the same conditions if 8 mol of formaldehyde are taken per 1 mol of melamine. Further processes resulting in the formation of spatial polymers occur in a weak acidic environment (pH = 4.5–5) and at temperatures of 80–90 °C. The higher functionality of melamine compared to urea pre-conditions higher density of methylene intermolecular bonds and, therefore, greater strength, heat resistance and hydrophobic behavior of the final polycondensation product. The structure of this polymer, by analogy with urea-formaldehyde resins, can be described as a

Scheme 1.15 Formation of melamine from urea with the release of ammonia and carbon dioxide

Scheme 1.16 Formation of hexamethylolmelamine through polycondensation of urea with formaldehyde

36

1 Basic Ingredients of Intumescent Compositions

spatial conglomerate of rings connected in all directions by –CH2 – and –CH2 –O– CH2 – with groups having openings (“holes”) remaining in some places because of the presence of methylol groups that have not reacted: Tri- and hexa-substituted methylol derivatives (I and II) are easily obtained and can be polymerized into crosslinked products. The structure of the final product can be illustrated given the release of water as a result of the interaction of N-methylol groups and the remaining –NH groups. In the case of hexamethylolmethamine, the reaction can occur due to intermolecular esterification of methylol groups or because some of the formaldehyde molecules split off with subsequent condensation, the same as in the first case. By analogy with the reaction between formaldehyde and urea, it can be assumed that there are cyclic structures when interaction with melamine is observed [135] (Scheme 1.17). If strong bases, such as guanidine, are included in the composition of the melamine-formaldehyde resin, the resin obtained can exchange anions due to the basic groups, whose formation can be explained by the following scheme [135] (Scheme 1.18). The mixtures of melamine with urea or phenol can condense with formaldehyde. Such compositions in individual sections of the net structure contain structural elements of homopolymers. The structure of the three-dimensional resin made of

Scheme 1.17 Cyclic structure of a product of iteraction between formaldehyde and melamine

Scheme 1.18 Formation of melamine-formaldehyde resin through iteraction between melamine, formaldehyde and guanidine

1.5 Gas-Forming Ingredients

37

melamine and phenol with formaldehyde can be represented by the following scheme [135] (Scheme 1.19). For further discussion it is also important to note that under certain topochemical conditions melamine, guanidine, and some other compounds form graphite-like carbon nitride structures of the form (Scheme 1.20). The literature seems scarce of data on the behavior of melamine in intumescent compositions. The most common are the data of DTA, DSC, IR-spectroscopy of individual melamine, which confirm each other [61, 130]. The intumescent process with different content of melamine is described in detail in [130]. Figure 1.6 shows the DTA and TGA curves of melamine, which imply that this is also a crystalline product with Tmelt = 364 °C. Melting occurs with negative enthalpy and much later morphological rearrangement and melting of pentaerythritol. References [67, 114] describe the thermal phenomena occurring when melamine is annealed in air and nitrogen with differential scanning calorimetry and thermogravimetry methods. The main sublimation peak of melamine starts at 330 °C and ends at 380 °C with a very clear peak. The second smaller peak appears immediately after that. The authors note that it is expected that melamine will sublimate, decompose in the gas phase and polymerize in this temperature range with the release of crosslinked carbon–nitrogen polymers, similar to how it occurs in melem or melon. The temperature at which sublimation begins is very close to the temperatures obtained for APP and pentaerythritol, and in general, transformations for these substances develop in similar temperature intervals (Fig. 1.7). This fact means that there are optimal transformation conditions for these three active components to combine in intumescent reactions. We believe that when the morphological structure

Scheme 1.19 The structure of the three-dimensional resin made of melamine and phenol with formaldehyde

38

1 Basic Ingredients of Intumescent Compositions

Scheme 1.20 Graphite-like carbon nitride structure formed from melamine and guanidine

of pentaerythritol is rearranged with an increase in the lability of chemical bonds, “constituent” molecules of pentaerythritol—formaldehyde and acetaldehyde—are released. Melamine reacts with them, forming an excellent structure, which cannot be anything but melamine aldehyde resin. With the participation of polyphosphoric acid that breaks from ammonia, the resin gets crosslinked in the volume, which makes the intumescent layer harden.

0,007

pentaerythritol

0,005 ammonium polyphosphate

0,003 0,001

melamine -0,001

490

460

430

400

370

340

310

280

250

220

190

160

-0,005

130

-0,003

100

Heat flow, relative units

0,009

Temperature, 0C Fig. 1.7 DSC curves for individual components: from top to bottom: melamine, APP, pentaerythritol [67]

1.5 Gas-Forming Ingredients

39

1.5.4 Chloroparaffin Literatures [32, 59, 67, 68] also attribute to chloroparaffins (CP) role of porophores, given that these compounds are also considered as flame retardants and plasticizing additives. Our observations prove that indeed this component contributes to an increase in the ratio of the charred layer. The researchers, obviously, assume that the case is in the gaseous products of destruction, in particular, in the released hydrogen chloride. However, as shown above, gaseous products below a temperature of 370 °C do not contribute significantly to foaming, and the destruction temperatures of CP used in flame-retardant compositions do not exceed 350 °C. Therefore, case is different: chloroparaffins, as will be shown below, can shift the destruction of polymer binders in compositions to a region of lower temperatures, preventing the intumescence from being suppressed by later thermolysis of the polymer film.

1.6 Titanium Dioxide For a long time, it was believed [60] that titanium dioxide was just an inert pigment in flame-retardant intumescent compositions, which was responsible for covering power and color. However, we and other researchers obtained data [135–137] which confirm that titanium dioxide plays a more important role in the thermolytic synthesis of the intumescent layer, determining the structure of the charred layer. Titanium dioxide is presented on the market mainly in two crystalline forms: rutile and anatase. The rutile modification is more capable of scattering light and more stable than the anatase one. Therefore, it is most widely used in polymer compositions, including paints and varnishes [138]. The physicochemical characteristics of titanium dioxide are determined mainly by the size of the pigment particles and the type of chemical treatment of surface, which can be inorganic and organic (Fig. 1.7). Surface treatment is designed to improve dispersibility (in water and in a number of organic solvents), covering power and weather resistance. Inorganic treatment is carried out mainly by depositing the compounds of aluminum and silicon, or, less frequently, of zirconium on the surface. The ratio of these compounds determines the acid–base balance of surface of titanium dioxide particles, which varies within a very wide range. It can affect the character of the ORP thermolysis. Organic treatment is more diverse. The surface of particles may have an optical brightener, or a modifier of hydrophilic–lipophilic properties. This will determine different distribution of titanium dioxide particles in the composition, which can also affect the character of thermolysis of intumescent coatings [139]. Therefore, each brand of titanium dioxide has specific characteristics due to a different surface structure [140]. It is known [135, 136] that the fire-retardant efficiency of the final coating is affected by the quantitative content of titanium dioxide, in particular, such characteristics as the ratio of the charred layer, its adhesion to hot metal and thermal stability. So, as a result of a relatively long exposure to fire of intumescent coatings containing

40

1 Basic Ingredients of Intumescent Compositions

titanium dioxide, the upper intumescent layers burn out to form white foam, which is mainly titanium pyrophosphate and is the result of the interaction between APP and TiO2 . There are data [141] that titanium pyrophosphate is formed because P2 O5 interacts with TiO2 . It was the fire resistance of a coating containing rutile-TiO2 , which is much greater in comparison with the one containing anatase-TiO2 [142]. The researchers think that the difference may be explained by the size distribution and crystalline forms of TiO2 . Anatase also turns into rutile at temperatures above 900 °C [143]. It is obvious that the chemical nature and surface structure of titanium dioxide should also affect the fire-retardant efficiency of intumescent coatings [143–146]. In order to examine this hypothesis, it was decided to conduct a comparative thermal analysis and evaluate the expansion ratio of a line of intumescent coatings obtained on the basis of a single water-dispersive intumescent composition, and differing only in terms of the titanium dioxide brand included in the composition. Various types of titanium dioxide— A1, R1, R2 (in accordance with DIN EN ISO 591) for various intended use were deliberately selected for testing. The specifications of titanium dioxide samples of different brands are presented in Table 1.8. Table 1.8 Specifications of titanium dioxide samples in fire-protection char-forming compositions. Reproduced with permission from [136]. Copyright Trans Tech Publications, Ltd. 2019 Specifications of TiO2

Sample of the fire-protection char-forming composition 1

2

3

4

5

Producer

Du Pont

Scott Chemicals

Scott Chemicals

Cristal Global

Cristal Global

Modification

Rutile

Anatase

Rutile

Anatase

Rutile

Surface treatment

Al2 O3 , SiO2, organic

Al2 O3 , SiO2

Al2 O3 + ZrO2 , organic

none

Al2 O3 + ZrO2 , organic

TiO2 content (%)

93

99

92.5

98.5

94

Oil absorption (g/100 g)

13.9

20

20

20

21

Specifications of TiO2

Sample of the fire-protection char-forming composition 6

7

8

9

Producer

Billions

Cristal Global

Billions

Sichuan Lomon Titanium Industry CO., LTD

Modification

Rutile

Rutile

Rutile

Rutile

Surface treatment Organic

Al2 O3 + ZrO2 , organic

Al2 O3 , ZrO2 , amphiphilically modified

Al2 O3 , SiO2 , organic

TiO2 content (%) 98

95

94

93.9

Oil absorption (g/100 g)

19

19

22

19

1.6 Titanium Dioxide

41

Thermal analysis of fire-retardant coating samples was carried out using the appliance Derivatograph Q-1500D of the system J. Paulik, P. Paulik, I. Erdey made by the company MOM, Hungary with an automated console designed for processing the obtained data. The experiments were conducted in an oxidizing atmosphere of air with a heating rate being constant. The sample weight was 50 mg. The crucibles were platinum. The heating rate was 10 °C/min. The reference substance was aluminum oxide. The expansion ratio was determined in accordance with the well-known methodology [147]. Figure 1.8 and Table 1.9 illustrated the test results of fire-protection char-forming compositions containing various titanium dioxide brands. The results of thermal analysis and measurement of the expansion ratio confirmed the hypothesis that the brand of titanium dioxide, and hence its crystalline form, as

Fig. 1.8 View of TGA curves of samples 1–9, containing titanium dioxide of different brands. Reproduced with permission from [136]. Copyright Trans Tech Publications, Ltd. 2019

Table 1.9 Results of comparative tests of fire-protection char-forming compositions containing various titanium dioxide brands. Reproduced with permission from [136]. Copyright Trans Tech Publications, Ltd. 2019 Parameter

Sample of FPCFC 1

2

3

4

5

6

7

8

9

T 5% mass loss (°C)

187

210

190

200

214

179

198

221

184

T 50% mass loss (°C)

364

345

364

351

372

355

360

363

365

T Total mass loss (°C)

774

773

864

757

779

841

750

763

780

Ash residue (%)

33.2

34.6

34.0

41.0

40.4

33.0

32.6

40.6

40.2

Expansion ratio

69

39

55

53

70

45

59

48

53

42

1 Basic Ingredients of Intumescent Compositions

well as the type of surface treatment, significantly affect the thermolysis of intumescent compositions: according to the DTA data [137] the temperatures of the main stages of thermolysis are the closest for samples 3, 5 and 7 which have the same surface treatment from a chemical point of view (Al2 O3 + ZrO2 + organic treatment) [148, 149]. However, the heat resistance (according to the TGA data) of these samples is completely different: sample 3 has the highest value of temperature of total mass loss among all the samples, and sample 7 has the lowest one. Samples 2 and 4 (anatase modification) have average heat resistance results; sample 2 has the smallest expansion ratio of all; and sample 7 has the smallest ash residue. The largest ash residue was recorded for samples (4, 5, 8, 9), which are characterized by a relatively large oil absorption, that is, for samples with a more developed surface. The results of the study do not let us establish a clear correlation between the type of surface treatment and the properties of the intumescent coating, which is only natural, because, as noted above, the properties of the surface are different for all samples and has to be studied further. We cannot but note that there is connection between the photocatalytic activity of the brands of titanium dioxide and the expansion ratio, and the latter is the greater, the lower is the photocatalytic activity. However, it has been perfectly established that the brand of titanium dioxide affects the properties of the intumescent composition and the course of physicochemical processes occurring in thermolysis, and this effect is created by the surface properties of TiO2 samples, presumably by the number and strength of acid centers. It is known that aluminophosphates [150], molybdate phosphates [151], titanium dioxide [152] and zirconium dioxide [153] have a dehydrating ability with respect to alcohols. The data obtained allow us to conclude that in the intumescent system, titanium dioxide, including the case when it has a modified surface, functions as a catalytically active agent, whose highly developed surface [154, 155] reduces the activation energy of some ongoing reactions, for example, morphological rearrangement of pentaerythritol with its subsequent conversion to aldehydes. Hence titanium dioxide can act as a nucleating agent of carbonized foam [136, 137].

1.7 Conclusions Based on the critical analysis of the data on properties of the substances that conventionally make up intumescent materials, a hypothesis has been put forward that traditional ideas about the functional contribution of the discussed components of intumescent systems and the nature of the processes are inaccurate. Review of literature and patent descriptions clearly shows that, despite a huge range of diverse, theoretically suitable components, like acid agents, char formers and porophores, in the vast majority of cases the most effective intumescent compositions are the ones based on melamine, pentaerythritol and ammonium phosphate. However, there is

1.7 Conclusions

43

very little data on the substances themselves and on the role they play in the intumescent process. Given the existing research studies, it is extremely difficult to formulate a coherent theory of thermolysis of intumescent materials and, especially, to convey the processes in the form of specific chemical equations. It seems that new studies only bring more questions, but do not answer the main ones: Why these very components? How is the charred layer formed? How can the properties of flame retardants be adjusted to approach specific problems? An impression forms that the researchers “go around in circles”, intuitively sensing that “the truth is out there”, but they fail to uncover it. In our opinion, everyone is misled by the idea that pentaerythritol is a polyatomic alcohol, which must form esters with phosphoric acids according to all indications. What naturally occurs if topochemical conditions are more “favorable” for this reaction? With regard to the interpretation of pentaerythritol in flame-retardant compositions in Refs. [32, 34, 59, 60, 67], the critical remarks are not about the fact that the authors of these literature works incorrectly understand the resulting picture of the totality of reactions of pentaerythritol in the intumescent process, but rather that this interpretation lacks one main thing: how does the spatial structure of the resin which forms the charred layer after carbonization appear? Pentaerythritol is an exceptionally essential ingredient in intumescent compositions. It is known that there have been successful attempts to replace it with dipentaerythritis, but there is no big difference in the behavior of these products. As for the attempts to replace pentaerythritol with starch and cellulose, they result in significant deterioration of the flame-retardant properties of intumescent materials, and PVA is not used at all for the well-known reasons. This makes us suggest that PE is an irreplaceable component if it is used in a composition with melamine and APP. Pentaerythritol is exclusive, in our opinion, because it releases aldehydes in due time to obtain melamine aldehyde resin during interaction with melamine. The “timeliness” is ensured because it begins to react at temperatures of about 280 °C. This is exactly the temperature at which aldehydes must bind to oligomer-polymer resins and subsequently harden with releasing phosphoric acid participating in the process. We are quite sure that in the compositions we consider, substantially crystalline pentaerythritol displays “non-alcohol” behavior, that is it is not as much prone to form esters with acids, but rather to self-rearrange and form aldehydes, which is thermodynamically more favorable under the described conditions. If nothing prevents pentaerythritol in this process from heating to the temperature of the morphological transition, crystalline rearrangement becomes “conflict-free”. But if there are molecules in an accessible proximity which can perceive rearrangement fragments (and these fragments are bound to form), polyol does not have time to complete its thermodynamic transition and breaks down into the products from which it was synthesized: aldehydes and water. This process can be facilitated, along with the dehydrating agents of the flame-retardant composition, by ingredients such as aluminosilicates, titanium dioxide and aluminum oxide, on the surface of which, this process, probably, generates. Based on the afore-mentioned, it can be assumed that cellulose in the

44

1 Basic Ingredients of Intumescent Compositions

intumescent composition is also a source of aldehydes; they are simply less specific than in the case of pentaerythritol. According to the researchers [116], the furfural scheme of cellulose thermolysis deserves special attention. Melamine, urea and their derivatives can form relatively heat-resistant resins with aldehydes, including furfural. The role of gas-forming agents ascribed to them in the intumescent process, as will be shown below, is one-dimensional. These components are no more porophores than ammonium polyphosphate, and no less carbonization agents than pentaerythritol. So the existing classification division of components of intumescent systems, apparently, is incorrect and should be revised.

References 1. Bushmanova AV, Barabanshchikov YuG, Semenov KV et al (2017) Thermal cracking resistance in massive foundation slabs in the building period. Mag Civ Eng 8:193–200. https:// doi.org/10.18720/MCE.76.17 2. Lazovskaya T, Tarkhov D, Vasiliev A (2018) Multilayer solution of the heat equation. Comput Intell Res 736:17–22. https://doi.org/10.1007/978-3-319-66604-4_3 3. Rudskoy AI, Kozhaspirov GE, Cliber J et al (2016) Modern metal materials and processes. Mater Phys Mech 25(1):1–8 4. Parshikov RA, Rudskoy AI, Zolotov AM et al (2016) Analysis of the features of plastic flow of a sample under severe plastic deformation. Rev Adv Mater 45(1–2):67–75 5. Zaborova D, Vieira G, Musorina T et al (2018) Experimental study of the thermal stability of building materials. Adv Intell Syst Comput 692:482–489. https://doi.org/10.1007/978-3319-70987-1_51 6. Rasha IK, Pertseva ON, Lazareva AYu et al (2017) Computational modeling of random stress distribution for wooden structures. Civ Eng J 69(1):23–33. https://doi.org/10.18720/MCE. 69.2 7. Al Ali M, Bajzecerova V, Kvocak V (2017) Design methods of timber-concrete composite ceiling structure. Mag Civ Eng 5:88–95. https://doi.org/10.18720/MCE.73.8 8. Sainov MP, Zverev AO (2017) Workability of high rockfill dam with a polymer face. Mag Civ Eng 7:76–83. https://doi.org/10.18720/MCE.75.7 9. Struchkova AY, Barabanshchikov YuG, Semenov KS et al (2018) Heat dissipation of cement and calculation of crack resistance of concrete massifs. Mag Civ Eng 2:128–135. https://doi. org/10.18720/MCE.78.10 10. Rybakov VA, Ananeva IA, Rodicheva AO et al (2017) Stress-strain state of composite reinforced concretes lab elements under fire activity. Mag Civ Eng 6:161–174. https://doi.org/10. 18720/MCE.74.13 11. Benin A, Semenov S, Semenov A et al (2017) Identification of parameters for the model of elastoplastic damage for superheated concrete. In: The MATEEK 107 conference network. https://doi.org/10.1051/matecconf/201710700042 12. Indemitsev DA, Semenov BN, Skubov DYu et al (2018) Structural transformations of the material under dynamic loading. Adv Struct Mater 87:185–195. https://doi.org/10.1007/9783-319-73694-5_11 13. Vavilov DS, Dendetsev DA, Semenov BN et al (2017) On structural transformations in a material under nonstationary actions. Mech Solids 52(4):391–396. https://doi.org/10.3103/ S0025654417040057

References

45

14. Bardin A, Korotkov A, Kazarnovsky V (2017) Restriction of fire resistance of steel transport structures. Simplified calculation methods. IOP Conf Ser Earth Environ Sci 90(1):012207. https://doi.org/10.1088/1755-1315/90/1/012207 15. Matveev MA, Kolbasnikov NG, Kononov AA (2017) Causes of high-temperature permeability of single-crystal steels. Proc Indian Inst Metals 70(8):2193–2204. https://doi.org/10. 1007/s12666-017-1042-9 16. Kazakov A, Kiselev D, Kazakova E et al (2017) Quantitative description of microstructural strapping in steels. Charact Charact Mater 6(3):224–236. https://doi.org/10.1520/MPC201 60009 17. Zhukov VA (2017) Dislocation-phonon mechanism and interpolation dependence of fatiguedamaged structural steel. Lect Notes Mech Eng 155–164. https://doi.org/10.1007/978-3-31953363-6_16 18. Apostolopoulos S, Drakakaki A, Apostolopoulos A et al (2017) Characteristic defects— corrosion damage and mechanical behavior of two-phase reinforcement. Mater Phys Mech 30(1):1–19 19. Kuzkin VA, Krivtsov AM (2017) Fast and slow thermal processes in harmonic scalar lattices. J Phys Condens Matter 29(50):505401. https://doi.org/10.1088/1361-648X/aa98eb 20. Loganina VI, Pyshkina IS, Martyashin GV (2017) Effect of the supplement based on calcium hydrosilicates on the resistance of lime coatings. Mag Civ Eng 4:20–27. https://doi.org/10. 18720/MCE.72.3 21. Cherkashin AV, Pykhtin KA, Begich YE et al (2017) Mechanical properties of nanocarbon modified cement. Mag Civ Eng 4:54–61. https://doi.org/10.18720/MCE.72.7 22. Nizina TA, Balykov AS, Volodin VV et al (2017) Fiber fine-grained concretes with polyfunctional modifying additives. Mag Civ Eng 4:73–83. https://doi.org/10.18720/MCE. 72.9 23. Bulanov PE, Mavliev LF, Vdovin EA et al (2017) The interaction between the kaolinite or bentonite clay and plasticizing surface-active agents. Mag Civ Eng 7:171–179. https://doi. org/10.18720/MCE.75.17 24. Barabanshchikov YuG, Belyaeva SV, Arkhipov IE et al (2017) Influence of superplasticizers on the concrete mix properties. Mag Civ Eng 6:140–146. https://doi.org/10.18720/MCE.74.11 25. Benin AV, Semenov AS, Semenov SG et al (2017) Methods of identification of concrete elastic-plastic-damage models. Mag Civ Eng 8:279–297. https://doi.org/10.18720/MCE. 76.24 26. Kuz’min MP, Larionov LM, Kondratiev VV et al (2017) Burnt rock of the coal deposits in the concrete products manufacturing. Mag Civ Eng 8:169–180. https://doi.org/10.18720/MCE. 76.15 27. Gammeltoft P, Hobdari B (2017) Multinational corporations with emerging markets, international knowledge flows and innovations. Int J Technol Manag 74(1–4):1–22. https://doi.org/ 10.1504/IJTM.2017.083619 28. Galyachev AV, Alkhimenko AI (2017) Features of structural elements of facade cassettes made of fine ceramics. Civ Eng J 69(1):64–76. https://doi.org/10.18720/MCE.69.6 29. Korotchenko IA, Ivanov EN, Manovitsky SS et al (2017) Deformation of concrete creep in calculating the state of thermal stresses of massive concrete and reinforced concrete structures. Civ Eng J 69(1):56–63. https://doi.org/10.18720/MCE.69.5 30. Denisov AV (2017) The impact of superplasticizers on the radiation changes in Portland cement stone and concretes. Mag Civ Eng 5:70–87. https://doi.org/10.18720/mce.73.7 31. Fryanova KO, Perminov VA (2017) Impact of forest fires on buildings and structures. Mag Civ Eng 7:15–22. https://doi.org/10.18720/MCE.75.2

46

1 Basic Ingredients of Intumescent Compositions

32. Thirumal M (2016) Recent developments of intumescent fire protection coatings for structural steel: a review. J Fire Sci 34(2):120–163 33. Tramm H, Clar C, Kuhnel P et al (1938) US Patent 2,106,938, Feb 1938 34. Vandersall HL (1971) Intumescent coating systems. Their development and chemistry. J Fire Flammabl 2:97–140 35. Wang JQ, Chow WK (2005) A brief review on fire retardants for polymeric foams. J Appl Polym Sci 97:366–376 36. Camino C, Costa L, Martinasso G (1989) Intumescent fireretardant systems. Polym Degrad Stab 23:359–376 37. Troitzsch JH (1983) Methods for the fire protection of plastics and coatings by flame retardant and intumescent systems. Prog Organic Coat 11:41–69 38. Olivera RBRS, Moreno ALJ, Vieira LCM (2017) Intumescent paint as fire protection coating. Struct Mater J 10(1):220–231 39. Horrocks AR, Price D (2001) Fire retardant materials. Woodhead Publishing Limited, Cambridge 40. Sobur SV (2014) Fire protection of materials and structures: course and reference book, 5-th edn, revised. Moscow, Russia 41. Taubkin SN, Kolganov MN, Levites FA (2003) Flame retardant intumescent paints. In: Low flammability polymer materials: materials of the 5th international conference. Volgograd, Russia 42. Smirnov NV, Bulgakov VV, Etumyan AS et al (2012) The results and prospects of research on assessing fire risk of building, textile materials and the effectiveness of fire protection means. Anniversary collected papers of FSBI VNIIPO MChS of Russia. Moscow, Russia 43. Ignatenkova EB, Chernova NS, Zybina OA (2010) Fire protection of cable products with intercalated graphite-based intumescent materials. Problems of film and television development, a collection of scientific papers of St. Petersburg State University of Film and Television 22:202–205 44. Zavyalov DE, Zybina OA, Mnatsakanov SS et al (2009) Flame retardant intumescent compositions based on intercalated graphite. Chem Ind 86(8):414–417 45. Mnatsakanov CS, Chernova NS, Zybina OA et al (2012) A method for obtaining a vibrationabsorbing flame retardant composition. RF Patent 2470966, 27 Dec 2012 46. Chiang CL, Hsu SW (2010) Novel epoxy/expandable graphite halogen-free flame retardant composites−preparation, characterization, and properties. J Polym Res 17(3):315–323 47. Saidaminov MI, Maksimova NV, Kuznetsov NG et al (2012) Fire protection performance of oxidized graphite modified with boric acid. Inorganic Mater 48(3):258–262 48. Gravit MV (2016) Basic requirements for flame retardant coatings of buildings, structures, outdoor installations: a course book. Saint Petersburg, Russia 49. Bolodian I, Melikhov A, Tanklevskiy L (2017) Fire safety arrangement of inhabited pressurized compartments of manned spacecraft. Acta Astronaut 135:92–99. https://doi.org/10. 1016/j.actaastro.2016.10.003 50. Efremov SV, Romantsova OV, Ulybin VB (2018) Safety of the monopropellant’s production at the purification stage. Acta Astronaut. https://doi.org/10.1016/j.actaastro.2018.01.018 51. Bolodyan IA, Melikhov AS, Tanklevskiy LT (2017) Analysis of statistical data about design of fire-safe materials in oxygen-rich atmosphere of inhabited pressurized compartments of Russian manned spacecraft. Acta Astronaut. https://doi.org/10.1016/j.actaastro.2017.10.019 52. Bolodian I, Melikhov A, Tanklevskiy L (2017) Automatic fire-extinguishing system for inhabited pressurized compartments of manned spacecraft. Acta Astronaut. https://doi.org/10.1016/ j.actaastro.2016.10.002

References

47

53. Tanklevskiy L, Tsoy A, Snegirev A (2017) Electrically controlled dynamic sprinkler activation: Computational assessment of potential efficiency. Fire Saf J. https://doi.org/10.1016/j. firesaf.2017.04.019 54. Vilken V, Kalinina O, Dubgorn A (2018) Specificity of multi-storey construction and real estate markets in the regional economy: analysis of Russian practice. E3S Web Conf 33:03012. https://doi.org/10.1051/e3sconf/20183303012 55. Vasilyev A, Tarkhov D, Bolgov I et al (2016) Multilayer neural network models based on experimental data for processes of sample deformation and destruction. In: CEUR workshop proceedings 56. Burlov VG, Grobitski AM, Grobitskaya AM (2016) Construction management in terms of indicator of the successfully fulfilled production task. Mag Civ Eng. https://doi.org/10.5862/ MCE.63.5 57. Frost & Sullivan (2013) Global intumescent coatings market to expand steadily. http:// www.frost.com/srch/catalog-search.do?searchType=sub&rd_submit=Go&queryText=int umescent+coating+market&sortBy=R. Accessed 12 Feb 2020 58. Tomakhova A, Zybina O, Suprun V et al (2019) Development of led-curable intumescent polymer coatings for fire protection of building constructions. IOP Conf Ser Mater Sci Eng (IOP Publishing). https://doi.org/10.1088/1757-899X/666/1/012089 59. Puri RG, Khanna AS, Ravindra G (2017) Intumescent coatings: a review on recent progress. J Coat Technol Res 14(1):1–20 60. Khalturinsky NA, Rudakova TA (2013) On the formation mechanism of flame retardant intumescent coatings. Bull South Federal Univ 8:215–220 61. Zybina OA, Varlamov AV, Mnatsakanov SS (2010) Problems of the technology of charforming flame retardant compositions. Center for Development of Scientific Cooperation. Novosibirsk, Russia 62. Zybina OA, Babkin OE (2018) On making recipes of efficient flame retardant paints for building structures. Paintwork Mater Appl 3:36–39 63. Aseeva RM, Zaikov GE (1981) Burning of polymer materials. Moscow, Russia 64. Berlin AA (1996) Burning of polymers and low flammability polymer materials. Soros Educ J 9:57–69 65. Golod VM, Sufiarov VS (2017) Evolution of structural and chemical heterogeneity by rapid solidification by spraying gas. Ser Rep IOP: Mater Sci Technol 192(1):012009. https://doi. org/10.1088/1757-899X/192/1/012009 66. Bourbigot S, Le Bras M, Duquesne S et al (2004) Recent advances for intumescent polymers. Macromol Mater Eng 289:499–511 67. Nenakhov SA, Pimenova VP (2010) Physics and chemistry of ammonium polyphosphatebased foaming fire-retardant coatings. Literature review. Fire Explos Saf 8:11–58 68. Mashlyakovsky LN, Lykov AD, Repkin VYu (1989) Low flammability organic coatings. Leningrad, Russia 69. Zobacheva AYu, Nemov AS, Borovkov AI et al (2017) Designing and modeling of structural materials of a three-component composite material. Mater Phys Mech 34(1):51–58. https:// doi.org/10.18720/MPM.3412017_6 70. Shymchenko AV, Tereshchenko VV, Ryabov YuA et al (2017) Review of computational approaches to the modeling of modern materials in accordance with modern advanced production trends. Mater Phys Mech 32(3):328–352 71. Jimenez M, Duquesne S, Bourbigot S (2006) Intumescent fire protective coating: toward a better understanding of their mechanism of action. Thermochim Acta 449:16–26

48

1 Basic Ingredients of Intumescent Compositions

72. Thewes V (2014) Composition for an intumescent fire protection coating, fire protection coating, its use and manufacturing process for an intumescent fire protection coating. USA Patent 20140005298, 02 Jan 2014 73. Wierzbicki M, Fernando J, Packard K (2014) Intumescent material for fire protection. USA Patent 8729155, 20 May 2014 74. Bilbija D (2014) Fire resistant coatings. USA Patent 20140094539, 03 May 2014 75. Kreh R (2013) Intumescent fireproofing systems and methods. USA Patent, 27 Aug 2013 76. Taylor A, Butterfield S, Darryl Green J et al (2013) Intumescent coating compositions. USA Patent 8461244, 03 Jun 2013 77. Kreh R (2013) Intumescent fireproofing systems and methods. USA Patent 20130090410, 11 Apr 2013 78. Kotzev D, Diakoumakos C (2013) Flame retardant polymer compositions. USA Patent 8372899, 12 Feb 2013 79. Winterowd J, Robak G (2013) Fire-resistant wood product. USA Patent 20130000239, 03 Jan 2013 80. Kasowski R (2012) Protective barrier composition comprising reaction of phosphorous acid with amines applied to a substrate. USA Patent 8212073, 03 Jul 2012 81. Schmitt G, Neugebauer P (2012) Intumescent coating composition with enhanced metal adhesion. USA Patent 20120164462 28 Jun 2012 82. Wade R (2011) Intumescent composition. USA Patent 20110311830, 22 Dec 2011 83. Wierzbicki M, Fernando J, Packard K et al (2011) Intumescent material for fire protection. USA Patent 20110136937, 09 Jun 2011 84. Brown G, Eaton R (2011) Flame-retardant polyolefin/thermoplastic polyurethane composition. USA Patent 20110011616, 20 Feb 2011 85. Aslin D (2011) Fire resistant materials. USA Patent 7863342, 04 Jan 2011 86. Reinheimer A (2010) Intumescing, multi-component epoxide resin-coating composition for fire protection and its use. USA Patent 7820736, 26 Oct 2010 87. Breen C, Thompson S (2010) Water based intumescent coating formulation especially suitable for structural steel components in civil engineering. USA Patent 20100209645, 19 Aug 2010 88. Aslin D (2011) Fire resistant materials. USA Patent 7772294, 10 Aug 2011 89. Schmitt G, Neugebauer P, Scholl S et al (2010) Resin system for intumescent coating with enhanced metal adhesion. USA Patent 20100190886, 29 Jul 2010 90. Reyes J (2010) Fire resistant thermoplastic or thermoset compositions containing an intumescent specialty chemical. USA Patent 20100086268, 08 May 2010 91. Zavyalov DE, Zybina OA, Manatsakanov SS (2014) Catalytic effect of intercalated graphite on fire retardant intumescent composition. Abstract at the international scientific and technical conference “High-Tech Technologies of Functional Materials” 92. Gardelle B, Duquesne S, Vandereecken P et al (2013) Resistance to fire of curable silicone/expandable graphite based coating: effect of the catalyst. Eur Polym J 49(8):2031–2041 93. Yakunina IE, Nechaev KV, Zybina OA et al (2011) Developing flame retardant intumescent compositions for metal structures. Abstracts at the international scientific-practical conference “Multiscale modeling of structures and nanotechnology” 94. Osipov IA, Zybina OA (2014) Developing fire-retardant sealing composition for sealing expansion joints of building structures. J Civ Eng 8(52):20–24 95. Makeenko AV, Larionova TV, Klimova-Korsmik OG et al (2017) Synthesis of complex oxides with a garnet structure by spray drying an aqueous solution of a salt. Tech Phys 62(4):613–618. https://doi.org/10.1134/S1063784217040168 96. Bobrynina E, Alkhalaf AA, Shamshurin A et al (2017) Synthesis of composite powders FeZrO2 by the thermochemical method. Key Tech Mater 721 KEM. https://doi.org/10.4028/ www.scientific.net/KEM.721.285 97. Kurapova OYu, Glumov OV, Pivovarov MM et al (2017) Structure and conductivity of ceramics from calcium stabilized zirconium, made from lyophilized nanopowder. Rev Adv Mater Sci 52(1–2):134–141

References

49

98. Bogdanov SP, Garshin AP (2017) Production of composite materials from refractory powders with surface nanofilms. Refract Ind Ceram 58(2):202–207. https://doi.org/10.1007/s11148017-0081-4 99. Bogdanov SP, Garshin AP (2017) Refractory core powders for the additive industry. Solid State Phenom 265 SSP. https://doi.org/10.4028/www.scientific.net/SSP.265.529 100. Lisovenko DS, Baimova YuA, Rysaeva LK et al (2016) Equilibrium diamond-like carbon nanostructures with cubic anisotropy: elastic properties. Phys Status Solidi (B) Basic Res 253(7):1295–1302. https://doi.org/10.1002/pssb.201600049 101. Alalykina LV, Fedoreeva LA, Chernaya IC et al (1995) Raw material mixture for flame retardant coating. RF Patent 2034816, 10 May 1995 102. Shuklin CG et al (2001) Flame retardant polymer composition for coatings. RF Patent 2176258, 27 Nov 2001 103. Sakharov AM, Krukovsky SP, Yarosh AA et al (2006) A method for obtaining phosphoruscontaining triaminotoluene formaldehyde resin. RF Patent 2285015, 10 Oct 2006 104. Sakharov AM, Yarosh AA, Krukovsky SP et al (2008) A method for obtaining triaminotoluene phosphate urea-formaldehyde resin. RF Patent 2328507, 10 Jul 2008 105. Ustinov A, Zybina O, Tanklevsky L et al (2018) Intumescent coatings with improved properties for high-rise construction. E3S Web Conf 106. Ustinov A, Zybina O, Tomakhova A et al (2018) The enhancement of operating properties of intumescent fire-protective compositions. MATEC Web Conf 107. Bhatnagar VM, Vergnaud JM (1983) Fire retardant paints. Paint India 7–9:15–28 108. Porokhov AM, Knunyants IL (1988) Chemical encyclopedia. Moscow, Russia 109. Antonov AV, Reshetnikov IS, Khalturinsky NA (1999) The burning of char-forming polymer systems. Success Chem 68(7)663–673 110. Dobryansky AF, Tishchenko VV, Gavrilov BG (1967) Transformation of oil hydrocarbons. Leningrad, Russia 111. Dobryansky AF, Markin GV, Khimtseva MA (1962) Thermocatalytic conversion of 2,2dihydroxymethylbutanol-1 on aluminosilicate catalyst. Petrochemicals 2:45–56 112. Kabanov VA, Kargin VA (1972) Encyclopedia of polymers. Moscow, Russia 113. Barg EI (1954) Technology of synthetic plastics. Leningrad, Russia 114. Oleinikov KV, Trotsenko PA, Zybina OA et al (2008) The main components of flame retardant intumescent materials and their role in forming protective charred layers. Chem Ind 85(1):49– 52 115. Levites FA, Barabanova LP (1979) Flame retardant intumescent compounds. Review of patent descriptions. VNIIPO 6:21–25 116. Konkin AA (1974) Carbon and other heat-resistant fiber materials. Moscow, Russia 117. Byrne GA, Gardiner D, Holmes FH (1966) The pyrolysis of cellulose and the action of flameretardants. III. Further analysis and investigations of products. J Appl Chem 16(3):81–88 118. Tang BK, Neill WK (1964) Thermal analysis of high polymers. J Polym Sci 6:65–81 119. Schmidt D, Jones W (1962) Chemical engineering progress 120. Vohler O, Sperk E (1966) Berichte der Deutschen Keramischen Gesellschaft 121. Zhdanov YuV (1979) Chemistry and technology of polyphosphates. Moscow, Russia 122. Avdeev VV, Godunov IA, Shkirov VA et al (2002) A method for obtaining a highly condensed ammonium polyphosphate. RF Patent 2180890, 27 Mar 2002 123. Wade CA, Callaghan SJ, Strickland GS et al (2001) Investigation of methods and protocols for regulating the fire performance of materials with applied fire retardant surface coatings. Fire Code Research Re-form, FCRC Project 2 B-3 124. Fukumura C, Inoue K, Iwata M et al (1993) A melamine-coated ammonium polyphosphate and a process for producing the same. EU Patent 19940103258, 07 Mar 1993

50

1 Basic Ingredients of Intumescent Compositions

125. Wu K, Wang Z, Hu Y (2008) Microencapsulated ammonium polyphosphate with urea– melamine–formaldehyde shell: preparation, characterization, and its flame retardance in polypropylene. Polym Adv Technol 19(8):1118–1125 126. Tang Q, Wang B, Shi Y et al (2013) Microencapsulated ammonium polyphosphate with glycidyl methacrylate shell: application to flame retardant epoxy resin. Ind Eng Chem Res 52(16):5640–5647 127. Zybina OA, Silnikov MV, Gravit MV (2016) Thermoanalytical research study of various grades of ammonium polyphosphate for intumescent flame retardant compositions. Iss Def Technol 9–10(99–100):76–79 128. Pagella C, Raffaghello F, De Favery DM (1998) Differential scanning calorimetry of intumescent coatings. Polym Paint Colour J 188(4402):16–18 129. Nenakhov SA (2010) Influence of gas-forming agent concentration on the development patterns of the charred layer of flame retardants. Fire Explos Saf 19(3):14–26 130. Camino G, Costa L, Trossarelly L (1984) Study of mechanism of intumescence in fire retardant polymers. Part III: Effect of urea on ammonium polyphosphate-pentaerythritol system. Polym Degrad Stab 7:221–229 131. Berezin BD, Berezin DB (1999) Course in modern organic chemistry. Moscow, Russia 132. Belov PS (1965) Fundamentals of petrochemical synthesis technology. Moscow, Russia 133. Kiselev VS, Sorokin MF (1947) On the mechanism of the melamine formaldehyde reaction. The works of D. Mendeleev University of Chemical Technology of Moscow 12:25–34 134. Sorenson W, Campbell T (1963) Preparative methods of polymer chemistry. Moscow, Russia 135. Polyakova VI, Zybina OA, Mnatsakanov SS (2015) The functional contribution of titanium dioxide to the thermolytic synthesis of intumescent coatings. In: Science-driven technology of functional materials: proceedings of the international scientific and technical conference. Saint Petersburg, Russia 136. Zybina OA, Ustinov AA, Andreev AV (2019) On the impact caused by titanium dioxide of different trademarks on the properties of intumescent fire-protective coatings. Mater Sci Forum 945:212–217 137. Ti-pure titanium dioxide from chemours. https://www.chemours.com/DTT/ru_RU/Coatings/ more_about_tipure.html. Accessed 12 Feb 2020 138. Drevelle C, Lefebvre J, Duquesne S et al (2005) Thermal and fire behaviour of ammonium polyphosphate/acrylic coated cotton. Polym Degrad Stab 88:130–137 139. Shugurov SM, Kurapova OYu, Lopatin SI et al (2017) Thermodynamic properties of the La2 O3 -ZrO2 system by the Knudsen effusion mass spectrometry at high temperature. Rapid Commun Mass Spectrom 31(23):2021–2029. https://doi.org/10.1002/rcm.7997 140. Chemours. On surface treatment of DuPont brand. https://www.chemours.com/DTT/ru_RU/ assets/downloads/Surface_coatings.pdf. Accessed 12 Feb 2020 141. Frim A, Zhukov R (2010) Thin film flame retardant intumescent coatings for structural metal. Paintwork Mater Appl 10:41–47 142. Weil E (2011) Fire-protective and flame-retardant coatings—a state-of-the-art review. J Fire Sci 29:259–295 143. Aziz H, Ahmad F, Zia-ul-Mustafa M (2014) Effect of titanium oxide on fire performance of intumescent fire retardant coating. Adv Mater Res 935:224–228 144. Zybina OA, Varlamov AV, Chernova NS et al (2009) On the role and transformations of the components of fire-retardant intumescent paint compositions in thermolysis. J Appl Chem 82(4):1445–1449 145. Amir N, Ahmad F, Hazwan M et al (2017) Synergistic effects of titanium dioxide and zinc borate on thermal degradation and water resistance of epoxy based intumescent fire retardant coatings. Key Eng Mater 740:41–47

References

51

146. Hongfei L, Zhongwu H, Zhang S et al (2015) Effects of titanium dioxide on the flammability and char formation of water-based coatings containing intumescent flame retardants. Prog Organic Coat 78:318–324 147. Smirnov NV, Bulaga SN, Duderov NG (2011) Assessing the quality of fire protection and using the type of flame retardant coatings at facilities: guidelines of FGU VNIIPO. Moscow, Russia 148. Ilatovskaya M, Savinykh G, Fabrichnaya O (2017) Thermodynamic description of the ZrO2 TiO2 -Al2 O3 system based on experimental data. J Eur Ceram Soc 37(10):3461–3469. https:// doi.org/10.1016/j.jeurceramsoc.2017.03.064 149. Sayenko I, Ilatovskaya M, Savinykh G et al (2018) Experimental study of phase relations and thermodynamic properties in the system ZrO2 -TiO2 . J Am Ceram Soc 101(1):386–399. https://doi.org/10.1111/jace.15176 150. Antonyuk SN (2005) Catalytic transformations of methanol aimed at obtaining methyl formate, dimethyl ester, carbon monoxide and hydrogen. Moscow, Russia 151. Sukhanov MV, Schelokov IA, Ermilova MM et al (2008) The catalytic properties of sodium zirconium molybdate phosphate in transformation reactions of methanol. J Appl Chem 81(1):19–24 152. Xu M, Lunsford JH, Goodman DW et al (1997) Synthesis of dimethyl ether (DME) from methanol over solid-acid catalysts. Appl Catal 149:289–301 153. Manriquez ME, Lopez T, Gomez R et al (2004) Preparation of TiO2 -ZrO2 mixed oxides with controlled acid-basic properties. J Mol Catal 220:229–237 154. Konakov VG, Kurapova OY, Borisova NV et al (2017) Synthesis and phase formation in MxOy-ZrO2 nanosized precursors (M = Zn2+, Cd2+, Pb2+, Bi3+). J Sol-Gel Sci Technol 82(1):214–223. https://doi.org/10.1007/s10971-016-4278-7 155. Kurapova OYu, Golubev SN, Ushakov VM et al (2017) Stabilization of solid solutions based on cubic zirconium dioxide obtained by cryochemical methods: thermodynamic and kinetic factors. Rev Adv Mater Sci 48(2):147–155 156. Charmor for Intumescent Coatings (2010) Promotional materials. http://www.chemcam.it/ Charmor.pdf. Accessed 12 Feb 2020

Chapter 2

Technology Basis of the Thermolytic Synthesis of Char Formation Polymeric System

Abstract This chapter overviews the sequence of physicochemical transformations during combustion of intumescent compositions and the principle of their fireretardant action. The results of studying the structure and composition of the products of thermolysis of intumescent components using the Fourier transform infrared spectroscopy, chromatography–mass spectrometry and scanning electron microscopy are summarized. As a result of the studies conducted by the authors, the functions of the main ingredients of intumescent fire-retardant compositions based on MA, PE, APP were revised and refined. Evidence is provided for the fact that pentaerythritol does not manifest itself as a tetrahydric alcohol outside the environment of polar solvents, and therefore, as was assumed in earlier studies, it does not form ether resins with ammonium polyphosphate, which are swollen by gaseous products of melamines thermolysis. Instrumental methods showed that pentaerythritol at temperatures above 190 °C, against the background of crystalline structural rearrangement in the presence of active co-reagents such as melamine, decomposes into formic and acetic aldehydes used in its synthesis. Those, at the indicated temperatures, in the presence of polyphosphoric acid (in the form of an incompletely substituted ammonium phosphate) effectively form a three-dimensional melamine aldehyde structure with melamine. The mechanism of intumescence as a superposition of physicochemical processes determines the necessity and sufficiency of the contributions of each of them to the thermolytic synthesis of the forming char: the formation of a melamine aldehyde polymer, and subsequent gas evolution, expansion and carbonization, has been experimentally proved and theoretically justified. Based on the data on the ability of pentaerythritol to form aldehydes under certain conditions, the authors refine the scientific concept that describes the mechanism of synthesis of pentaphthalic resins. It was shown that along with the reactions of formation of pentaphthalic esters, reactions of phthalic anhydride with aldehydes should be carried out, providing a more branched and crosslinked structure of the resulting resin. Keywords Intumescent coatings · Char · Intumescent char · Intumescent layer · Fire-retardant compositions · Mechanism of char formation · Melamine · Ammonium polyphosphate · Pentaerythritol · Thermal analysis · Infrared

© Springer Nature Switzerland AG 2020 O. Zybina and M. Gravit, Intumescent Coatings for Fire Protection of Building Structures and Materials, Springer Series on Polymer and Composite Materials, https://doi.org/10.1007/978-3-030-59422-0_2

53

54

2 Technology Basis of the Thermolytic Synthesis …

spectroscopy · Chromatography–mass spectrometry · Thermolysis · Aminoaldehyde resin A lot of physical–chemical processes must occur in a certain sequence for a heat-insulating intumescent layer to form during thermolysis. According to some researchers, as noted above, first the phosphorus compounds have to decompose with formation of a dehydrating agent—an acid, which then reacts with the carbon source. Then, simultaneously with the resin forming from the carbon source when the former decomposes, gases have to build up and make the melted composition intumesce. As in case of all chemical reactions, the temperature at which this sequence of reactions occurs is a very important factor. From experience it is known that if just one of the above processes does not take place, there is no intumescence [1].

2.1 Analyzing the Existing Views of the Intumescent Layer Forming Mechanism According to paper [2], dedicated to studying the regularities of thermal-oxidative destruction of polymer materials, including heterochain thermoplastic fiber-forming polymers, in the presence of phosphorus-containing flame retardants and fireretardant compositions, it is found out that when a flame retardant interacts with polymers, the processes of structuring and forming thermally stable systems prevail over destruction reactions, which contributes to a reduction in the release rate of volatile compounds, lowers toxicity of pyrolysis products, forms a carbonized layer enriched in graphite-like structures, have low oxidability and good thermal properties. The conducted research studies made it possible to find out the regularities of the reduced fire hazards of polymers in case they are treated by flame retardants (Fig. 2.1). The authors of [3–6] describe the intumescent process in more detail. According to them, for stable intumescence of typical foaming coatings, the following conditions have to be complied with: release at a temperature of 150–215 °C of non-organic acid, capable of polyol esterification; melting of the composition during esterification or immediately prior to it; rearrangement of the product that originated due to the interaction of polyol and non-organic acid as a result of dehydration with the formation of a carbon non-organic (usually, carbon-phosphorus) residue; foaming of the carbonaceous mass by the generated gases and vapors; gelatinization and further hardening of the foamed layer. According to [3, 4, 6, 7], the carbonization processes start with the rearrangement of phosphates, are accompanied by polyol esterification and end up in the formation of hard carbon-phosphate gel at a temperature of about 360 °C.

2.1 Analyzing the Existing Views of the Intumescent …

55

Carbonaceous polymers +flame retardants (interaction with polymer matrix)

Decomposition temperature reduces Decomposition rate reduces

Quantity of gaseous compounds reduces. Toxicity of pyrolysis products reduces

Melt viscosity during thermolysis increases

Carbonization process intensifies

Oxidation rate of CR reduces Oxidation temperature of CR

Exothermic effect of CR oxidation reduces process i ifi

Formation of dense CR rich in graphite-like structures

The surface temperature of carbonized layer grows

Thermal cost for CR heating increases

Heat and temperature conductivity reduce Transmission coefficient reduces Reflection coefficient grows

Heat retention properties increase

Fire resistance of material increases

Fig. 2.1 The factors reducing the fire hazard of polymers in case they are treated by flame retardants. CR is carbonaceous residue [2]

Since there is no clarity about the mechanism of char formation, numerous attempts have been made to investigate the thermal chemistry of intumescent compositions. When studying the structural transformations of ammonium polyphosphatepentaerythritol (APP–PE) binary systems, the authors of [8] came to a conclusion that as a result of the reaction, ester is formed with release of ammonia and water and simultaneously partial destruction of the high-molecular chain structure of APP. The first possible way (I) is ammonia elimination from APP with further esterification and water release (Scheme 2.1). However, the question about a possible way of the reaction with ester formation remains open, as the authors recognize. So, we would like to note that the generation of such kinds of esters does not contribute to the formation of spatial polymeric structures. The second way (II) is the reaction of pentaerythritol phosphorylation (or alcohol hydrolysis of APP) wherein ammonia does not necessarily have to release (Scheme 2.2).

56

2 Technology Basis of the Thermolytic Synthesis …

Scheme 2.1 Transformation of ammonium polyphosphate in the heating conditions

Scheme 2.2 Pentaerythritol phosphorylation

In case of reaction (I), ammonia and water have to release to develop an ester bond, but the chain structure of APP is preserved. In case of reaction (II), ester is formed with interruption of the APP macrochain, and ammonia and water can release as a secondary process. According to the authors’ assumption, both reactions take place and more detailed research have to be conducted to define the dominant reaction. The authors of work [9] proposed a scheme for ammonium polyphosphate pentaerythritol phosphorylation in a binary mixture too, wherein the main polyphosphoric chain of APP gets interrupted and phosphate ester groups—P(O)OCH2 — are formed. This process takes place without volatile products being formed at a temperature of 210 °C (Scheme 2.3). Then polyphosphoric acid cyclic esters are formed with ammonia and water release (Scheme 2.4). The repetition of these reactions results in structures of the following type (Scheme 2.5). Since the pyrolysis products of the APP and PE mixture have paramagnetic properties, the authors assume that such structures are obtained in the intermediate phase of carbene formation (Scheme 2.6).

+

P

H 4NO

O

O

O O

n

C( C H 2 O H )4

O

P

H 4 NO

OH n

+ ( HO CH2)3 CC H2

O

P

H4N O

Scheme 2.3 Ammonium polyphosphate + pentaerythritol phosphorylation in a binary mixture

2.1 Analyzing the Existing Views of the Intumescent … CH 2 OH ( HOCH 2)2 C CH 2 O

O P

57

O O

P

H 4 NO H 4 N O

HOCH 2

O

HOCH 2

O

O P ( O)

- H 2O, -NH3

O

P

H4 N O

n

n

Scheme 2.4 Formation of polyphosphoric acid cyclic esters

O

O P (O) O

O

O P(O)

(O)P O

O

O O

( O )P O

Scheme 2.5 Structural formula of a product obtained by repetition of reaction in 2.4

2

 + 2&+   &

 + 2

 + 2&+   &&+

2

3

2

2 2+

2  + 2&+   &&+  23

2

Scheme 2.6 Formation of product from Scheme 2.5 in the intermediate phase of carbene formation

The formation of cyclic structures as intermediates in the thermal transformations of the APP and PE mixture allowed the authors to use pentaerythritol diphosphate (PDP) as a model compound for studying the mechanism of intumescence. It is shown [9] that the first phase in PDP decomposition is condensation accompanied by water release and formation of the following type of structures (Scheme 2.7). Based on the NMR spectra of 31 P and 13 C, the mechanism of char formation is suggested (Scheme 2.8). In the first phase, as a result of protonation and C-bond breaking, carbenium ion 1 is obtained. It can be subjected to regrouping, giving stable tertiary carbocation 2, with a proton abstracting from it, which results in the formation of a double bond. Further pyrolysis of ester 3 can occur with phosphoric acid elimination and lead to diene-type derivatives 4, capable of reacting by the Diels–Alder reaction with the formation of polycyclic ring structures of type 5. If this sequence of reactions repeats, it results in conjugated aromatic structures. Reactions can be complicated by condensation of phosphoric acids and polymerization, so the forming structures are most probably disordered.

Q

2 2 3 +2 2

2 2 3 2 2+

 Q + 2

Scheme 2.7 First phase in PDP decomposition

+2

2 2 3 2

2 2 3 2 2

Q

+

58

2 Technology Basis of the Thermolytic Synthesis …

Scheme 2.8 Mechanism of char formation suggested by the authors

Such cyclic pentaerythritol phosphate structures were found [10] when they studied the products of thermolysis of a mixture of APP Exolit 263 and PE manufactured by Montedison using the methods of acidimetric titration of acid groups and obtaining NMR spectra. For further discussion it is important to consider the statement [11] that the study of the alteration products of the APP–PE mixture in the ratio 3:1 after it has been annealed for 12 h within a temperature range of 190–560 °C using the method of Raman microspectroscopy and NMR spectroscopy showed the lack of significant transformations at a temperature of 190 °C. In the selected temperature range of the annealing temperature rise, significant changes were recorded starting from a temperature of 280 °C. It is noted that the binary APP–pentaerythritol system has a certain weak fire-retardant effect in the absence of melamine. Interest in such systems implies that these systems have a certain practical significance. As it seems to us, this pair can be effective in reducing the combustibility of polymeric materials where it is the polymer itself that is chard. When binary systems are used as independent intumescent compositions for protecting structures, they are ineffective for fire protection of most materials except for wood. Melamine added to the binary APP–pentaerythritol system provides this triple system (triad) with some new effects. For example, the yield of charred material and its expansion ratio increase, while the behavior of the system becomes more endothermic. The data on the study of thermochemical transformations of ternary mixtures is very limited. The assumed reactions are described in most detail in [12]. According to Z. Wang et al. at a temperature of 260 °C, ammonium polyphosphate undergoes thermolysis accompanied by a release of phosphoric acid, ammonia and water (Scheme 2.9). Pentaerythritol is dehydrated and affected by the acid; esterification occurs at a temperature of 320–360 °C (Scheme 2.10).

2.1 Analyzing the Existing Views of the Intumescent …

59

Scheme 2.9 Thermolysis of ammonium polyphosphate (at 260 °C) accompanied by a release of phosphoric acid, ammonia and water

Scheme 2.10 Esterification of pentaerythritol at 320–360 °C

Melamine starts to degrade at a temperature of 280–350 °C, forming a large amount of non-combustible gases (Scheme 2.11).

Scheme 2.11 Thermal degradation of melamine at 280–350 °C

60

2 Technology Basis of the Thermolytic Synthesis …

Khalturinsky puts forward the following mechanism of char formation [6] (Scheme 2.12). References [13, 14] represent the synthesis mechanism by the following scheme of the processes which take place sequentially. • Formation of polyphosphoric acids (Scheme 2.13). • Pentaerythritol carbonization (Scheme 2.14). • Formation of intumescent gases due to decompounding porophore (Scheme 2.15). As seen from the above data, the structure of the charred frame resins formed during thermolysis is not understood clearly, and therefore the researchers cannot answer the question of how the intumesced melt hardens. The specifics of how melamine behaves within a wide range of its concentrations in phosphate ammonium-spirit compositions in the presence of high molecular weight hydrocarbon and in its absence are described in the work of Nenakhov [15]. It is O O P

O

O O

O

P O

ONH4

ONH4

ONH4

O

O

O

O P

O

O

P

P

O

O

H 2C

+

HOH2C HOH2C

OH

CH2

O P O

P O

O O OCH O CH2O 2 O P C P O P O CH2O

OH

O O

CH2OH

OCH2

H CH2

O

- NH3 , OH2

P O

C 2C

CH2OH C

O

P O O

Scheme 2.12 Mechanism of char formation suggested by Khalturinsky [6]

Scheme 2.13 Formation of polyphosphoric acids

Scheme 2.14 Pentaerythritol carbonization

O

2.1 Analyzing the Existing Views of the Intumescent …

61

Scheme 2.15 Formation of intumescent gases due to decompounding porophore

shown that, apart from the well-known function of a gas-forming agent, melamine also provides the “Stefan” character of flow of the endothermic transformation front through the thickness of the coating; its decomposition products directly participate in the “construction” of char and contribute to the development of thermal oxidation in the presence of high molecular weight hydrocarbon. However, the author does not explain what reactions are responsible for this “construction”, even though, among other assumptions, an idea is put forward that a rise in the residual mass of the compositions with an increasing concentration of melamine may have something to do with a non-volatile residue that is formed because the decomposition products of melamine interact with the decomposition products of other components. Then S. A. Nenakhov notes that as the melamine concentration is rising within the concentration range being studied, such properties of the compositions as the foaming coefficient, the limiting time of substrate heating under foaming coatings change by s-shaped curves, that is, two critical melamine concentrations can be distinguished. Within the concentration range between these critical concentrations, the properties of the charred layer change considerably. The change in properties below the first critical concentration is only slight. Above the second critical concentration of melamine in the foaming, coking composition, the foaming coefficient and the thermal resistance of char stop increasing. According to the author this is because the defectiveness of the charred layer is growing and excess gaseous products, which cannot be absorbed by the system, are removed from it. What we believe is not that the system cannot “consume” the excess gases, but that it “runs out” of the products that can react with melamine if its content in the system goes up. Melamine turns out to be in excess relative to these products, in particular, to the decomposition products of pentaerythritol. At the end of the detailed review [16], the authors are right to highlight that with the understanding of the thermolytic synthesis of the intumescent layer that we have today, the most poorly studied process in the formation of organophosphate ammonium compositions-based charred layer is the hardening of the porous structure of char after foaming. Why and how does the charred layer gelate and harden? How can this process be regulated? In our opinion, the answers lie on the surface. The MA, PE, APP-based intumescent compositions form aminoaldehyde resins. On the one hand, that is obvious, and on the other hand, the researchers are confused by the lack of aldehydes in the intumescent system. Understanding pentaerythritol as a spirit that must undergo

62

2 Technology Basis of the Thermolytic Synthesis …

phosphorylation with the formation of ester is not even questioned. Moreover, a lot of the published recipe data can be considered as indirect evidence of what these systems are [13, 14, 17–37], where intumescent flame-retardant materials are obtained directly on the basis of aminoaldehyde oligomers and ammonium phosphates. For example, in his review [38] V. M. Balakin et al. report that back in 1954 K. Christianson patented a carbamide-formaldehyde resin-based flame retardant. In order to impart flame-retardant properties to the resin, the patent proposed using compounds comprising ammonia or ammonium salts of inorganic acids could act as dehydrating agents. Methylamine, dimethylamine, ethylamine, ethylenediamine, urea, melamine, morpholine and other similar nitrogen-containing compounds were recommended to use as the cationic part. As for the anionic part, phosphates were suggested as preferred substances while radicals including sulfates and borates were also possible. In the 1950s, the possibilities were described to obtain fire-retardant intumescent coatings for metal. The US patents [30, 31] proposed carbamide-formaldehyde oligomer-based intumescent systems obtained by joint condensation of paraform, monoammonium or diammonium phosphate, urea, starch or dextrin and aldehyde. After the introduction of the aldehyde, the composition had to be used within 1.5 h. Even though these coatings were difficult to use and virtually non-waterproof, they were the first industrial intumescent paints. It is also said that flame retardants that were obtained before the material was applied to the surface to be protected through mixing amine aldehyde oligomers with phosphorus-containing compounds (polyphosphates, ammonium phosphates) were produced on an industrial scale: for example, in the GDR—DS-324 composition, in Finland—Vineter composition, in the SFRY and the FRG—Pyromors composition, and so on. In 1984, Pearson patented [33] as a fire-retardant intumescent coating, the reaction product of urea, formaldehyde, phosphoric acid and triethanolamine in the ratio of 1:2.7:1.33:0.2, respectively. In Russia, the authors of paper [36] obtained a flame-retardant intumescent coating, which has to be prepared directly before it is applied on the surface to be protected. This coating comprises a filming agent (urea-formaldehyde resin), a charforming mixture (a mixture of polyatomic alcohols and ammonium polyphosphate), an antiseptic, a surfactant, a plasticizer and a defoaming agent. The application life of the flame-retardant depends on its dilution ratio and on the type of solvent used. Ambartsumyan [22] developed a flame-retardant intumescent composition for wooden structures, which include amine formaldehyde resin, ammonium phosphate, pentaerythritol, kaolin, p-tert-butylphenol-formaldehyde resin and water. Water-soluble melamine-formaldehyde resin, carbamidelamino-formaldehyde resin or urea-formaldehyde resin were used as aminoformaldehyde resin. The life of the finished composition is very short, so ammonium phosphate has to be introduced right before wooden structures are treated (the life of the composition without ammonium phosphate is 6 months). Pimenova [21] had a flame-retardant intumescent composition patented. It contains water-soluble urea-formaldehyde or urea-melamine-formaldehyde resin, ammonium polyphosphate, dicyandiamide, alcohol, thermosetting resin, an inert

2.1 Analyzing the Existing Views of the Intumescent …

63

filler and water. A mixture of esters and dioxane alcohol formals is used as alcohol in the composition while epoxy resin is used as thermosetting resin, and aerosil and titanium dioxide are used as inert filler. Additionally, hexamethoxymethylmelamine resin, polyvinyl acetate dispersion, melem, trichloroethyl phosphate or trichloropropyl phosphate, sodium carboxymethyl cellulose or methyl hydroxyethyl cellulose are included in the composition. A greater interest is attracted by developments [30–35, 39], which involve methods for modifying urea-formaldehyde resins using phosphorus-containing compounds in synthesis phase. The patent [30] specifies the methods by which oligomers are obtained through mixing formaldehyde, catalytic acid, triethanolamine and urea. Inorganic acids such as hydrochloric, sulfuric and phosphoric acids can be used as catalytic acid, with phosphoric acid being the most preferable, according to the authors. Being a catalytic acid, not only does it allow us to achieve the desired result in a quick way but also to control the reaction. Moreover, the obtained product has better flame-retardant properties. Most of the above aminoaldehyde oligomer-based flame-retardant compositions have some disadvantages compared to conventional intumescent paints. These include a short lifetime of the working composition, and, consequently, the need to make the material two-packed so that it is prepared directly before it is applied on the structures, insufficient effectiveness in case metal has to be flame-proofed, high costs and so on. In our opinion, the main advantage of PE-, MA-, and APP-based intumescent compositions is the fact that, during thermolysis, the process of synthesis of aminoaldehyde resin takes place simultaneously with the process of intumescence of its melt and is accompanied by a significant endothermic effect. On the other hand, in the considered aminoaldehyde oligomer-based compositions, the transformations mostly entail coking, so, such materials work with a much weaker endothermic effect. Their intumescent performance is also significantly worse. This is because the processes of resin melting (provided that the resin is not crosslinked in volume, which is unlikely, given the origin of synthesis in the presence of phosphates) and gas release are asynchronous. If the resin is spatially cross-linked, it will char at once and intumescence will be insignificant. Thus, experience suggests that intumescent materials that are based on the classical intumescent triad are the best today in terms of their flame-retardant effectiveness and application technology. Research aimed at creating aminoaldehyde systembased flame-retardant intumescent coatings is nothing more than the developers’ gut feeling that intumescing aminoaldehyde resins should be effective in flame-retardant coatings. However, the challenge is to find suitable aldehydes, which would be nontoxic and inert toward other ingredients in the original flame-retardant composition. The source of aldehydes—pentaerythritol—was found empirically long ago. However, its role has not been really understood yet. Let us consider the behavior of the components of intumescent compositions and their mixtures during thermolysis in order to eliminate this contradiction.

64

2 Technology Basis of the Thermolytic Synthesis …

2.2 Physicochemical Transformations of Char-Forming Ingredients in Intumescent Compositions During Thermolysis We used chromatography–mass spectrometry [40] to conduct analytical research of the charred layer [41]. The work consisted of two main stages, including the obtaining and thermolysis of a flame-retardant coating, and chromatography–mass spectrometric study of the mixtures of substances extracted from the charred layer. Based on literature review, patent descriptions of recipes and our own experimental data, we developed a recipe of water-dispersible flame-retardant paint (Table 2.1). The coating based on it was subsequently studied using the above methods. The composition included the basic ingredients of intumescent systems: melamine, pentaerythritol, ammonium polyphosphate. Dicyandiamide was used as an additional component to prove the hypothesis that it works not so much as porophore, as stated in the literatures [3–6], but as a structure-forming agent of the polymeroligomeric system of the intumescent layer. A water dispersion of vinyl acetateethylene copolymer was used as a binder since, hypothetically, all vinyl acetate-based polymers can form cyclic condensed structures during thermolysis. Five tested samples, which represented a flame-retardant coating formed by an intumescent composition applied to metal plates, were placed in a muffle furnace for 10 min at a temperature of 200, 300, 400 and 500 °C, respectively. Then, the products of thermal degradation of the samples subjected to the different temperatures were placed in the cartridges of a Soxhlet extractor and extracted sequentially with hexane, benzene, dichloromethane, acetone, ethanol and acetic acid until no substances were detected in the extractor samples according to data of thin-layer chromatography. Using a vacuum rotary evaporator, the solvents were removed from the extractive fractions. The obtained mixtures were studied using chromatography–mass spectrometry. Chromatography–mass spectrometric research was conducted on a system including an Agilent 6890 gas chromatograph, which has an interface with an Agilent 5973 N high molecular mass-selective detector. The chromatograph is equipped with Table 2.1 Proportions of the intumescent flame-retardant composition [41]

Components

Weight fraction

Water dispersion of the copolymer of ethylene 23 and vinyl acetate Ammonium polyphosphate Melamine

27 9

Pentaerythritol

11

Dicyandiamide

2

Titanium dioxide Water

5 23

2.2 Physicochemical Transformations of Char-Forming …

65

a 30 m long quartz capillary column, being 0.25 mm in diameter and having an HP5MS phase. Helium with a flow rate of 1 ml/min served as a carrier gas. The evaporator temperature was 320 °C. The temperature was programmed to rise from 100 to 300 °C at a speed of 6 °C per min. The ionizing voltage of the source was 70 eV. The total ion current (TIC) mode was used. Individual substances were identified using computer search in the NIST library of mass spectra of organic compounds and the literature data. We obtained the chromatographic profiles of the mixtures from hexane, benzene, methylene chloride, acetone, ethanol and acetic acid extract fractions of carbonizate formed at 200, 300, 400 and 500 °C. Since the scope of this work does not allow us to include the analysis and images of all the profiles, we present the most significant ones in terms of the problem that is considered. At all of the thermolysis temperatures of the coating listed in Table 2.2, we found acetic acid in hexanoic and other extractive fractions (Fig. 2.2), which initially forms upon destructive saponification of the binder polymer at temperatures up to 250 °C, and then upon destruction of pentaerythritol if temperatures are higher. In the latter case, it is nothing more than oxidized acetaldehyde. In addition, at all temperature modes of the treatment (Table 2.2), another acetaldehyde derivative is traced. Precisely speaking, it is the product of interaction of acetic acid and ammonia—N, N-dimethylacetamide (Fig. 2.3). This compound can act as a substance that has catalytic properties in cyclization reactions [42] and (or) take part directly in the formation of aromatic structures similar to the ones found in the acetic acid extract of the sample subjected to thermolysis at 400 °C (Fig. 2.4). Melamine phosphate (Fig. 2.5) found in the analysis of acetic acid extract of carbonizate (obtained at 400 °C) proves our hypothesis that melamine does not decompose into NH3 , H2 O, CO in the intumescent process, acting as a porophore, but retains its structural integrity and undergoes chemical reactions in which the above gases are formed naturally as by-products. It is because of the fact that as soon as phosphoric acid is released, it reacts with melamine, which exhibits its basic properties and forms addition products—salts. Our ideas are quite consistent with the observations of some researchers who suggest that melamine and (or) its decomposition products participate in the “construction” of the charred layer, for example, through carbon and nitrogen condensation [15]. The thermogravimetric research conducted by the authors [15] showed that the residual mass of carbonizate increases when melamine concentration goes up. It means that part of melamine incorporates actively into char rather than get decomposed entirely into foaming gases. It was shown above that the temperature at which thermal destruction of melamine in a free state starts is 354 °C. However, in a melting intumescent composition, melamine can also be in a salt form, which, as it is known, is more heat-resistant and can remain unchanged up to 386 °C which is confirmed by the CMS data (Fig. 2.5). Before the decomposition temperature is reached, melamine will interact with various substances contained in the hot reaction mixture of the intumescent system, for example, with aldehydes formed in plenty when pentaerythritol decomposes. During these reactions, a significant amount of by-products is released:

66

2 Technology Basis of the Thermolytic Synthesis …

Table 2.2 Substances identified by the chromatography–mass spectrometry in the composition of the charred layer [41] Thermolysis temperature of the coating, ° C

200

300

400

O

O

O

OH

N

O N NH3+

OH

N

N

O O

O

O

O

O

O

O

O

N

N

OH

O

O

O

O

O

O

O

Thermolysis products

O

O

O

3-

PO4

O

O

O

*

NH3+

O

O

N

NH3+

N

O

O

O

O

N

O

OH

OH

O

O

O

OH

O

500

N

HN H

H N

O

O 2

NH2

N

H N O

O

H O

N

O

O

H O

H

H

H N

O

N

OH 2

N

O N

N

NH2

NH N

NH2

O H

O O P O O

NH2

NH2 N N

NH2

ammonia, water vapor and carbon oxides. Moreover, melamine can also react with other substances. In particular, it can interact with substances that have a carboxyl group. Scheme 2.16 illustrates an example of such interaction between melamine and benzoic acid (Table 2.2), whose derivatives are also found in the products of thermolytic treatment of the flame-retardant material (Fig. 2.4). The quantum chemical calculations made by Tarasenko and Zhuravsky when they considered the mechanism of this reaction [43] showed that amide II (Scheme 2.16) is a non-planar system because of the steric difficulties that arise from the proximity of the hydrogen atoms of the benzene ring and the nitrogen atoms of the triazine ring. In such a system (especially at higher temperatures), two processes are possible:

2.2 Physicochemical Transformations of Char-Forming …

67

Fig. 2.2 Mass-fragmentogram of acetaldehyde oxidized to acetic acid

Fig. 2.3 Mass-fragmentogram of N, N-dimethylacetamide in methylene chloride extractive phase of a sample heated up to 200 °C

a hydrogen atom can transfer from the benzene ring to the primary amino group of melamine with breaking of ammonia, which is typical for reactions involving the closure of heterocycles and forming complex (III), or an oxidative closure of a heterocycle can occur with breaking of two hydrogen atoms (complex IV). The presence of a significant amount of ammonia in the off gases of the heat treatment of oxidized AU impregnated with melamine indicates that the process takes place predominantly with the formation of complex (III).

68

2 Technology Basis of the Thermolytic Synthesis …

Fig. 2.4 Mass-fragmentogram of 6-methyl-4-(2-oxo-2-phenylethyl)-2H-1.4-benzoxazine-3(4H)-1 in the acetic acid extractive phase of a sample heated up to 200 °C

Fig. 2.5 Mass-fragmentogram of the salt form of melamine in the acetic acid extractive phase of a sample heated up to 400 °C

Since other primary amino groups are present in the triazine ring of melamine, another process can also take place: breaking of NH3 and closure of another (now fivemembered) heterocycle, that is complex (IV) can convert to complex (V). According to the calculations, melamine can react with carboxyl groups producing not only saltlike products and amide, but also thermally stable nitrogen-containing heterocyclic compounds that form during heating. Furthermore, nitrogen-containing polycyclic compounds can form according to the Scheme 2.17. In the course of the reactions of acetamide dehydration under the action of phosphorus oxide and acetic acid ammonolysis, acetonitrile and its derivatives, for example, amino-acetonitrile are formed in the excess of ammonia. When they interact with formamide (at 250 °C), they can form 4,5-diaminopyrimidine. As the latter

2.2 Physicochemical Transformations of Char-Forming …

69

Scheme 2.16 Interaction between melamine and benzoic acid

Scheme 2.17 Interaction between melamine and formamide that leads to formation of polycyclic compounds

cyclizes, purine and similar compounds are formed. Such compounds (Figs. 2.6, 2.7 and 2.8) are detected in the samples of the charred layer starting at a temperature of 300 °C, for example, glycosylamines adenosine and guanosine, whose molecules consist of purine base residue bound through a nitrogen atom to ribose residue in a furanose form. The latter is formed because pentaerythritol restructures into aldehydes which self-condense. Formaldehyde self-condenses in sugars with catalytic participation of tertiary amines [44]. As shown above, dicyandiamide and urea are traditional ingredients used in the synthesis of melamine in production industry. The same gases are formed as byproducts in the synthesis of melamine: ammonia, carbon oxides and water vapor. In flame-retardant compositions where urea and dicyandiamide (but no melamine) are used, carbonizate is still very likely to be produced due to the synthesis of melamine,

70

2 Technology Basis of the Thermolytic Synthesis …

Fig. 2.6 Mass-fragmentogram of adenosine in the acetic extractive phase of a sample heated up to 400 °C

Fig. 2.7 Mass-fragmentogram of guanosine in the acetic extractive phase of a sample heated up to 500 °C

after which spatially crosslinked resins and aldehydes are formed from it. Apparently, in addition to the production of melamine, dicyandiamide also takes part in the thermolytic synthesis of nitrogen-containing heterocyclic products (Figs. 2.6, 2.7, 2.8 and 2.9), and contributes to the crosslinking of the resulting resins. Eventually, as a result of the above transformations, graphite-like carbon nitride (g-C3 N4 ) can form. It represents condensed nitrogen-containing fragments included in the general graphene π-conjugation system, which significantly improves the thermal stability of the charred layer [44–46]. Strong heterocyclic fragments allow nitrogen atoms to stay in the carbon matrix in a form of various functional nitrogencontaining groups during exposure to high temperatures (Scheme 2.18) [43]. When we analyze the spectrograms (Table 2.2), we find that pentaerythritol (PE) (Fig. 2.10) is unchanged in acetic extracts of the samples heated to 300 °C. It is not

2.2 Physicochemical Transformations of Char-Forming …

71

Fig. 2.8 Mass-fragmentogram of aspidofractinin-3-yl-methanol (copsinol) in the methylene chloride extractive phase of a sample heated up to 400 °C

Fig. 2.9 Mass-fragmentogram of N -[(E)-(2-nitrophenyl) methylidene] carbonohydrosonicated diamide in the acetone extractive phase of a sample heated up to 500 °C

Scheme 2.18 Fragment of intumescent char structure

72

2 Technology Basis of the Thermolytic Synthesis …

Fig. 2.10 Mass-fragmentogram of pentaerythritol in the acetic extractive phase of a sample heated up to 300 °C

detected at higher treatment temperatures. This is the evidence of complete destruction of pentaerythritol. However, at a temperature of 400 °C, a compound, which was not there before, is spotted (Fig. 2.11). It can be considered with a high degree of probability, an intermediate product of pentaerythritol decomposition. Repeatedly, pentaerythritol in intumescent systems is traditionally regarded as a product forming poly- and oligoesters with phosphoric acids. We believe that this is not the matter. In fact, alcohols are nothing but a source of oxidative conversion to aldehydes. This is common both for aliphatic alcohols, and for cellulose, starch, and so on. As shown above, the melting point of PE is within the range of 256–262 °C. Pentaerythritol is

Fig. 2.11 Mass-fragmentogram of 1,2,3-propanetriol in the acetic extractive phase of a sample heated up to 300 °C

2.2 Physicochemical Transformations of Char-Forming …

73

a solid, poorly soluble in water and organic solvents. These properties are related to the presence of four hydroxyl groups in the molecule. They form a large number of intermolecular hydrogen interactions with the molecules of the water of crystallization and make the pentaerythritol structure be more like the crystalline one, which is additionally “densified” by hydrogen bonds similar to polyvinyl alcohol. Crystalline pentaerythritol can undergo a reaction only after its crystalline phase is destroyed. Otherwise its functional groups are inaccessible, not least because there are many hydrogen bonds between the hydroxyls of methylol groups in the crystal folds due to the regularity of the mutual positions of molecular fragments. The destruction of the crystalline phase starts at about 185 °C (Fig. 2.12b), and finishes at about of 260 °C as a result of morphological rearrangements. On the other hand, ammonium polyphosphate, being also an acid due to unsubstituted groups, is crystalline to a large extent. Therefore, the acidic functional groups are occluded (closed) and reaction-inaccessible. According to the DTA data (Fig. 2.12a), the reaction becomes possible at about 370 °C. The data provided suggest that there is no pentaerythritol any more at this temperature. However, there is reactive melamine capable of “perceiving” the forming aldehydes. An intensive process of PE degradation with the formation of

Fig. 2.12 The results of thermal analysis: a DTA (1) and TGA (2) ammonium polyphosphate curves; b DTA (1) and TGA (2) pentaerythritol curves; c DTA (1) and DTG (2) melamine curves; d DTA (1) and TGA (2) curves of the binary melamine-pentaerythritol mixture. Reproduced with permission from [45]. Copyright Springer Nature 2009

74

2 Technology Basis of the Thermolytic Synthesis …

aldehydes (form- and acetaldehyde) starts after the melting process finishes within the fundamental temperature interval of 190–220 °C. If the DTA and TGA curves [45] (Fig. 2.12d) of a mixture of melamine and pentaerythritol are considered, it is seen that the second endothermic peak characteristic of PE shifts to the region of lower temperatures and is represented very weakly. This means that the crystal lattice, which formed during morphological rearrangement, has structural defects. The loss of the mixture mass by the start of the transition of the TGA curves begins at about 240 °C and virtually stops at 360 °C. This occurs despite the existing ideas about the gas-forming role of melamine, which imply that it is precisely in the temperature interval above 350 °C where a significant loss of the mass should be. But it does not happen because, most probably, melamine binds aldehydes into oligomer-polymer resins. At a temperature of about 300 °C, the mass loss reaches almost 100%. It should be expected that aminoaldehyde resin should form within this temperature interval. The melting point of melamine is marked by an endothermic peak occurring at 340 °C (Fig. 2.12c). After the melting within the temperature interval of 350–430 °C, individual melamine converts to melem, and at 430–500 °C, melamine converts to melon. Melon is resistant to heating up to 740 °C. So, melamine and the products of its conversions exist for quite a while and, correspondingly, can react with aldehydes and form melamine formaldehyde and melamine acetaldehyde oligomers. A high temperature of synthesis and the presence of phosphoric acids should contribute to the development of spatially crosslinked structures. This reasoning is sound, and provides indirect evidence that pentaerythritol cannot form ester bonds in the conditions described above [45]. As for the technological practice, PE is mainly an ingredient for synthesizing pentaphthalic resins, to which it has given a name in some sense. The main synthesis process of alkyd pentaphthalic resins—an interesterification reaction—takes place at temperatures that are obviously higher than the melting point of PE, that is higher than 260 °C. In the literature it is noted [47] that the products of interesterification of castor oil and pentaerythritol are largely comprised of formaldehyde. The authors do not clarify where it can come from. Given the above, it is reasonable to assume that the socalled pentaphthalic resins are, in effect, the same glyptal ones, being additionally crosslinked by the aldehydes formed during partial destruction of pentaerythritol, mainly acetaldehyde (Scheme 2.19). Since there is no specific qualitative reaction to formaldehyde and acetaldehyde, and in the technological practice of coating compositions the most toxic one—formaldehyde—is looked for, acetaldehyde is often mistaken for its “younger” homolog. The attempts to attribute to PE the ability to form esters at temperatures below the melting point are not sound unless the reaction takes place in a solution. Since PE is obtained by the interaction of formaldehyde and acetaldehyde in the presence of calcium hydroxide, the acidic environment (flame-retardant compositions always contain phosphoric acid fragments) and high temperature will lead to a

2.2 Physicochemical Transformations of Char-Forming …

75

Scheme 2.19 Formation of pentaphthalic resins

reverse process, that is formation of acetaldehyde and formaldehyde. It is the aldehyde molecules that subsequently cause the formation of macromolecules condensed from aldehydes, amines and amides. Starting at a temperature of 300 °C, furfural appears in the intumescent system (Fig. 2.13). Presumably, it can be a product of thermolytic rearrangement of pentaerythritol at given temperatures. It is a well-known fact that when pentoses are heated in the presence of mineral acids, they form furfural according to the Scheme 2.20 [47]. It is possible that in similar topochemical conditions (at higher temperatures and in the presence of polyphosphoric acid), aldehydes formed from pentaerythritol enter into a self-condensation reaction, which produces pentoses, and then furfural. In its turn, furfural can undergo some transformations in the presence of strong acids, which entail formation of both high molecular weight and low molecular weight compounds that have relatively high heat resistance. It should be noted that a considerable amount of furfural and its derivatives is released during the pyrolysis of wood, in particular, as a result of dehydration of hexoses and pentoses, formed due to hydrolysis of some

76

2 Technology Basis of the Thermolytic Synthesis …

Fig. 2.13 Mass-fragmentogram of furfural in the acetic acid extractive phase of a sample heated up to 300 °C

OH

OH O

HO

H

H

+

-3 H 2O

CHO

OH Scheme 2.20 Formation of furfural from pentose

part of holocellulose. This fact allows us to answer the question why intumescent compositions that contain no pentaerythritol are effective at fire protection of wood and ineffective at protecting metal structures, for example. This is because wood, due to its chemical structure, as shown above, is a source of aldehydes, including furfural. In the samples obtained at a temperature of 400 °C, we find derivatives of ammonium polyphosphate–triethyl phosphate (Fig. 2.14). As in the case of melamine phosphate (Fig. 2.5), the destruction of the polymer chain of polyphosphate is very illustrative. When it is heated, low molecular weight fragments are formed. These are, primarily, trifunctional orthophosphoric acid, which is embedded in the carbonized structure of the charred layer due to the interaction of all acidic groups with the reactive components of the mixture. In this case, we could not find any esters of phosphoric acids in any of the samples we examined. Figure 2.15 shows the results of the studied carbonized product of pyrolysis of the flame-retardant material using the method of IR spectroscopy. The IR spectrum shows the curves of the fire-retardant material treated in a muffle furnace at a temperature of 500 °C for 40 s. A group of bands in the range of 2,882– 3,000 cm is caused by the vibrations of the C–H bond in the carbon structure. The double of 3,164–3,120 corresponds to the vibrations—NH2. The absorption band at

2.2 Physicochemical Transformations of Char-Forming …

77

Fig. 2.14 Mass-fragmentogram of triethyl phosphate in the acetic extractive phase of a sample heated up to 400 °C

Fig. 2.15 IR spectrum of the carbonized residue. Reproduced with permission from [53]. Copyright Springer Nature 2012

1,631 cm−1 should be attributed to the valence vibrations of the C=N azomethine bond, which, most probably, represents an azomethine bridge, being typical, for example, for the interaction products of amines and aldehydes. There is clear evidence of the appearance of amide groups (1,404 cm−1 ). The absorption bands of 988 cm−1 correspond to the C–N–H groups, which is the evidence that nitrogen-containing heterocyclic compounds are formed. The absorption band of 1,007 cm−1 corresponds

78

2 Technology Basis of the Thermolytic Synthesis …

to the phosphate ion. The appearance of an intense wide band of 1,240 cm−1 corresponds to vibrations of the P–O bonds; the P=O bond is, most likely, screened. The O–P–O group with a corresponding band of 489 cm−1 is recorded too. Thus, the IR spectroscopy helped to find the functional groups and fragments of molecules of substances detected during the study of the carbonized residue by chromatography–mass spectrometry, mostly, nitrogen-containing heterocyclic and organophosphate products. To confirm all the results listed before, a thermolytic synthesis of swollen resin was conducted using a mixture of melamine and pentaerythritol located on molten ammonium polyphosphate’s surface (Fig. 2.16). To conduct the synthesis, melamine and pentaerythritol (in 1:1 proportion) were mixed and adjusted to molten solid mass of ammonium polyphosphate. As that system was exposed to annealing in a furnace at >350 °C, the forming and swelling of sand-colored resin were detecting. A sensitive

Fig. 2.16 Images of a foamed polymer layer—a product of thermolytic synthesis from melamine and pentaerythritol: a in general; b in volume. The microstructure of char according to scanning electron microscopy: c (×80); d (×220) [48]. Photos made by Zybina O.A.

2.2 Physicochemical Transformations of Char-Forming …

79

test of releasing gases by resorcinol dissolved in water has shown the presence of aldehydes in those gases [48]. A swelling coefficient of resin is close to such of a charred layer made from melamine–pentaerythritol–ammonium polyphosphate (3:1:1) mixture (Fig. 2.17a), whereas a mixture of pentaerythritol and ammonium polyphosphate did not form a resin (Fig. 2.17b). There are groups of bands on the spectrum which characterize melamine-aldehyde resin (Fig. 2.18). Bands at the intervals (1560 and 1450) cm−1 are caused by triazine ring’s vibrations; a band at 1630 cm−1 is related to –C=N– bond vibrations. A band at 1448 cm−1 defines deformational vibrations of –CH2 – bonds. A bond at 1230 cm−1 is caused by C–N vibrations. A band at 981 cm−1 is related to –C–N–H. IR spectra of this product were obtained using Fourier-transform IR spectrometer IRPrestige-21 (SHIMADZU, Japan) and Fourier-transform IR-microscope AIM-8800 in transmission regime. Figure 2.18 shows an area (150 × 200 μm) from which the IR spectrum was obtained; spectra were the same in other areas of the material [48].

Fig. 2.17 Presentation of binary mixtures exposed to annealing in comparison with the primal sample (on the background): a pentaerythritol + ammonium polyphosphate (no swelling); b pentaerythritol + melamine (thermolytic synthesis of a resin). Photos made by Zybina O.A.

Fig. 2.18 An IR spectrum (a) of the synthesized resin (at 350 °C) (b) [48]

80

2 Technology Basis of the Thermolytic Synthesis …

Fig. 2.19 IR spectrum of a binary mixture of MA and PE in the initial (1) and foamed state (2)

To approve the results which were described earlier, IR spectra of initial melaminepentaerythritol binary mixture, a foamed polymer formed from it without additives, a foamed polymer formed from mixture with metal phthalocyanines, and the sample of melamine-aldehyde resin were recorded. Spectra were obtained using IR spectrometer Tensor 37 (by Bruker) with the prefix of the disturbed total internal reflection MIRacle (by Pike) with ZnSe crystal and diamond spraying in area 4000–600 cm−1 , resolution 2 cm−1 and averaging over 32 scans. When comparing the IR spectra of the initial binary mixture of MA and PE (Fig. 2.19, curve 1) and the same mixture, which underwent thermolysis in a muffle furnace (Fig. 2.19, curve 2), it is seen that no absorption bands, which are typical for pentaerythritol, can be detected in a foamed sample (the contribution of stretching vibrations of –OH groups decreased by 3310 cm−1 , CH bond intensities decreased by 2950, 2870 cm−1 , 1125 cm−1 , 1010, 995 cm−1 , deformational –OH by 653 cm−1 ), therefore pentaerythritol decomposed and the mixture no longer contains compounds with hydroxyl groups. Thus, the spectrum of foamed samples is melamine, which has condensed with the decomposition products of pentaerythritol. When superimposing IR spectra (Fig. 2.20) of melamine-aldehyde resin (4), a foamed polymer—a product of thermolysis of a binary mixture of MA and PE without additives (1) of foamed polymers modified with iron (2) and copper (3) phthalocyanines, it can be seen that all the samples are products of melamines condensation and are also close in composition to the melamine-aldehyde resin with the characteristical absorption bands discussed above. Additives in the composition of samples (2.3) lead to an increase in the absorption intensity characteristic of stretching vibrations of the –NH2 group by 3600–3100 cm−1 and the intensity of absorption of CH bonds by 2950, 2870 cm−1 compared to the sample without additives (1). This may indicate the effect of phthalocyanine metal complexes on the structure of the foamed polymer formed from melamine and pentaerythritol.

2.2 Physicochemical Transformations of Char-Forming …

81

Fig. 2.20 Comparison of IR spectra of melamine aldehyde resin (curve 4) and products of thermolytic synthesis of a binary mixture of melamine and pentaerythritol without additives (curve 1) and with additives (curves 2, 3)

Thus, on the basis of a combination of experimental data, it is proved that melamine during the thermolytic synthesis of the intumescent layer forms three-crosslinked polymer-oligomeric structures with aldehydes formed during the decomposition of pentaerythritol [49].

2.3 Thermolytic Synthesis Mechanism of a Heat-Insulating Intumescent Layer The complex processes of thermal destruction of a multicomponent intumescent composition can hardly be specified by just one scheme. However, it is important to understand, at least generally, how the intumescent process goes on during thermolysis of a fire-retardant coating so that the properties of the created materials can be regulated intendedly. Based on the above data, it is possible to assume some key interrelated reactions, which eventually end up in forming a polymer-oligomeric skeleton of the intumescent layer. At the temperature interval of 200–260 °C, pentaerythritol starts decomposing on the active surface of titanium dioxide and gaseous formic and acetic aldehydes form. Under their “pressure” a cavity opens around a filler particle—a cell forms. When melamine interacts with aldehydes, polymer-oligomeric structures of amino resins are formed. The resins harden and crosslink mainly due to the reactions of their functional groups. The hardening reactions are caused by the interaction of free methylol groups. They are accompanied by splitting off water. The reaction is accelerated as a result of the catalyzing effect of nitrogen of amide groups and the

82

2 Technology Basis of the Thermolytic Synthesis …

influence of protons (acid catalysis). Phosphoric acids are most frequently used in fire-retardant compositions. Protons of the former polarize methylol and methylol ester groups (Scheme 2.21). As a rule, stronger acids are not used in intumescent materials because they can undermine the stability of the source composition during shelf life. What is more, they remain in paint coating and, similarly to any hydrophilic compounds, reduce their moisture resistance. To improve stability of organo- and water-base compositions during storage, the so-called “blocked” catalysts are used. The “blocking” is based on the use of ammonium salts of phosphoric acids [50, 51]. However, not all salts are suitable. For instance, it is not recommended to use monoammonium phosphate in water systems—water dispersion compositions. Its hydrolysis products react with melamine and make the composition very thick. Moreover, its fire-retardant effectiveness is impaired substantially since melamine less actively participates in forming melamine aldehyde resins while bound monoammonium phosphate does not contribute to creating structures of melamine aldehyde

Scheme 2.21 Hardening and crosslinking of an intumescent resin

2.3 Thermolytic Synthesis Mechanism of a Heat-Insulating …

83

resins which are located spatially regularly relative to the surface. APP is most preferable. It contains few active protons at normal temperature, so there is no interaction with melamine. When heated, ammonium polyphosphate begins to decompose, ammonia evaporates, and the released polyphosphoric acid, adding melamine to the available ammonia vacancies, begins to have a structuring impact on the hardening process of the formed melamine-acetaldehyde resins. Apart from the processes described, there is also hardening of amine resin components. The self-hardening process, as Scheme 2.22 illustrates, is a continuation of the molecular chain growth. Free NH-groups can react with methylol groups making water split off and methylene bridges form, and with methylol ester groups making alcohol split off and methylene bridges form. When methylol groups react with each other, it results in dimethylene oxide bridges forming [51]. The condensation of melamine derivatives is completed with the formation of branched thermoset oligomers (Scheme 2.23). The nature of the reactions taking place when amino resins are harder can determine the properties of the charred layer. Interaction with the functional groups of polyphosphoric acid during the hardening process results in obtaining an elastic intumescent layer. Self-hardening makes the foamed layers more firm and brittle. Hardening increases the thermal stability of the resin. Thermal decomposition of amino resins, determined by mass losses, starts at rather high temperatures. The thermal properties of melamine aldehyde resins are determined by high thermal stability of triazine nuclei and methylene groups associated with triazine nuclei. The most distinctive characteristic of the forming resins is that mass losses are uniform and relatively slow in a wide temperature interval. The phenomenon is pre-conditioned by the fact that products with increasing heat resistance are formed at each stage of thermal decomposition, and they become thermally self-stabilized. For example, –CH2 –O–CH2 – bridges release formaldehyde and the crosslinks are “compacted” to form –NH–CH2 –NH–. As the temperature rises, as it will be shown below, the aromatic planes in the carbonized layer grow in number and size, and graphite-like carbon nitride (g-C3 N4 ) is formed. These planes are located chaotically in the mass of the charred layer material.

Scheme 2.22 Self-hardening of amine resin

84

2 Technology Basis of the Thermolytic Synthesis …

Scheme 2.23 Formation of branched thermoset oligomers by condensation of melamine derivatives

The coking process can be represented in a simplified way as a series of competitive reactions, with dehydration, cyclization and polymerization being of primary importance. Some phosphorus compounds have a catalytic effect in polymerization and polycondensation reactions. The following ones are commonly used for this purpose: phosphorus and ammonium halides, phosphoric anhydride and acid chlorides of phosphorus acids, that is Lewis acids, capable of forming active catalytic systems (donor–acceptor complexes). Polymeric materials treated with these compounds tend to carbonize at lower temperatures. These compounds also contribute to dehydration and dehydrogenation reactions. It is known [52] that polyphosphoric acid is used as a catalyst in dehydrocondensation of aromatic ketones and amines. As a result, a high yield of graphite-like substances was obtained already at 170 °C. It was found [52] that insoluble and flame-resistant carbon substances form during thermolysis of aromatic hydrocarbons in the temperature interval of 200–250 °C. The process was carried out in the presence of methyl or phenylphosphonic acids. Obviously, ammonium polyphosphate also catalyzes the coking and graphitization processes of the formed carbonizate. Besides, when considering the system with melamine as an ingredient in the charred layer synthesis, it is interesting to consider the possibility of using another substance instead of melamine—urea. Urea degradation products can also react with aldehydes. Obviously, it is reasonable to use urea in case the requirements for the fire-retardant effectiveness of charred layers are significantly lower than in case of metal fire protection. Moreover, resins that form from the interaction of melamine and formaldehyde are more char-forming than urea-formaldehyde resin. It has something to do with the cyclic structure of melamine, and the presence of three reactive amino groups.

2.3 Thermolytic Synthesis Mechanism of a Heat-Insulating …

85

Urea-formaldehyde resins have insufficient branching of chains and a small number of crosslinks (as evidenced by the low char number of hardened products and their rapid destruction when they are heated without access to air) (Scheme 2.24) [38]. Given the above assumptions, urea can be suitable for using, for example, in case intumescent materials are created to be applied on wood. By and large, the fire protection mechanism, or rather, the intumescence mechanism of fire-protection char-forming compositions under the effect of high temperatures, is the following: at the initial stage when intumescent materials form, PE decomposes, and formaldehyde and acetaldehyde are released. Melamine aldehyde resins start synthesizing. The spatial regularity of formation of these resins is supported by ammonium polyphosphate. In case a flame-retardant material is applied to a metal surface, APP forms salt bonds with the metal with its unsubstituted acid groups. Another part of these groups attaches melamine, which starts the spatial synthesis of resins. The process of resin synthesis is followed by a parallel decomposition of ingredients emitting ammonia, carbon oxides and vaporous water. We believe that the maximum synchronism is ensured by the decomposition of APP. The gaseous products of its destruction rush to the surface and foam the resin mass, which is at a temperature above the flow point. The released acid “triggers” rapid crosslinking of the resin in volume. The outcome is a process of mass hardening and carbonization. This is the protective charred layer, which reduces the thermal conductivity and restrains the heating rate or the start of burning of the protected surface. Having fairly obvious data on the nature and mechanism of formation of charred layers, one more fundamental issue has to be clarified concerning the formation of foamed flame-retardant structures, namely the nature of their adhesion on the surface of protected metal objects. Its roots, of course, lie at the use of polyphosphoric acid. The main agent of the adhesive interaction of the initial intumescent coating is a polymer binder, which ensures uniform and effective adhesion of the material to the substrate. High temperatures and flame cause rapid transformations, both in the structure of flame-retardant coating and metal. Thermal degradation of the polymer entails catastrophic disappearance of most of the points of adhesive interaction of the coating with the substrate. In case a carbonized fire-retardant layer forms on hot metal, obviously, the forming charred layer adheres to it due to various kinds

Scheme 2.24 Structure of urea-formaldehyde resin

86

2 Technology Basis of the Thermolytic Synthesis …

of chemical interactions. Free P–OH groups of polyphosphoric acid play a decisive role in the adhesive contact. Chemical bonds can also appear between the metal surface, usually covered in oxide film that forms on virtually any metal surface, and the reactive groups of polyphosphoric acid. It is the length of the ammonium polyphosphate macromolecule that, on the one hand, determines the total energy of chemical bonds of polyphosphoric acid (remaining after ammonia release) with surface functional groups of the protected metal, and, on the other hand, creates the conditions for the formation of a relatively regular carbonized polymer layer spatially adjacent to the protected surface. Since the melamine molecules involved in the formation of spatial structures of melamine aldehyde resins fix on the available acid groups of polyphosphate, targeted spatial synthesis of the polymer matrix is stimulated and it subsequently bonds as a result of three-dimensional crosslinking. Ammonium polyphosphate as partially substituted polyphosphoric acid appears to be an elongated and fixed on the protected surface element of the 3D assembly of growing in volume melamine aldehyde resins.

2.4 Conclusions According to the literature review, the existing beliefs about the mechanism of thermolytic synthesis of the heat-insulating charred layer do not provide an answer to the main questions: how is the spatial structure of the polymer skeleton formed and what makes the foamed melt of the obtained resins harden fast and in due time? This fact, together with some other indirect proofs offered in the text, provides evidence that the mechanisms described do not reflect the ongoing processes in an accurate way. Many years of technological practice in creating and using intumescent flameretardant materials have allowed us to develop an effective empirical complex of ingredients to solve standard problems. When there is need to develop materials with new properties, the lack of a precise concept describing the patterns of the intumescent process during the “work” of flame-retardant coatings makes researchers, who implement a “rule of thumb” strategy, waste a lot of material resources and time. The analysis of patent descriptions shows that many solutions that are proposed are not the best in terms of their recipes, which is due to (among other things) the studies of the mechanism of thermolytic synthesis of the charred layer with high flame-retardant effectiveness. The reason is the fact that none of the sources doubt the concept that charred layer-forming polymer oligomeric resins are produced as a process of pentaerythritol esterification with ammonium polyphosphate accompanied by formation of ester resins. However, the literature lacks any unambiguous concept of the structure of these resins. As a result of our research, we reconsidered and clarified the functions of the main ingredients of intumescent flame-retardant compositions and proposed a better substantiated mechanism for synthesizing the intumescent layer during thermolysis of a flame-retardant coating. It is shown that pentaerythritol is a source of aldehydes

2.4 Conclusions

87

under the described conditions of thermolysis. Many authors consider pentaerythritol a typical alcohol—a carbonization agent that must form esters with phosphoric acid, so it is suggested that PE should be replaced with other polyhydroxyl compounds, which, as a rule, does not result in any effective outcomes. PE is interesting because, under appropriate topochemical conditions, it exhibits a behavior that is more common to carbohydrates. In particular, according to a retroaldol decomposition reaction, it forms primary aldol condensation products—formaldehyde and acetaldehyde. So far PE is an irreplaceable ingredient in intumescent materials if aldehydes are not used at all in compositions to comply with, among other things, the environmental safety requirements of the production technology. In addition, the most reactive aldehydes in terms of temperature parameters cannot be preserved due to their volatility during the active stage when a three-dimensional structure is formed and then carbonized. Getting off the subject, it should be noted that research dedicated to the behavior of pentaerythritol in intumescent systems made us hit upon an idea that pentaerythritol is also very likely to be destroyed in the synthesis of alkyd resins under high temperatures. Whether the decomposition is complete—into aldehydes, or partially into glycerin and aldehyde—is the subject of further research. However, there is every reason to suppose that a material that is considered to be, for example, pentaphthalic resin (a complex polyester of pentaerythritol and phthalic anhydride) is not pentaphthalic resin in the truest sense of the word. Therefore, the mechanism of obtaining pentaphthalic resins has to be clarified. Going back to the ingredients of flame-retardant compositions, it is shown that melamine, urea and dicyandiamide are not porophores, in a sense. They directly participate in the construction of the intumescent layer, interacting with aldehydes with the formation of resins that have a complex composition. Thus, the role of these compounds is in the combination of the abilities of intense gas release, which is a consequence of the formed three-dimensional aminoaldehyde polymer structure— the reinforcing matrix of the carbonized charred layer. Ammonium polyphosphate is the most important “multi-tasking operator” of the intumescent process. It makes a significant, and, possibly, most important contribution to gas release, by freeing, in due time, water and ammonia in the course of chemical transformations. After that it ensures quick hardening of the foamed melt of melamine aldehyde resin. It also catalyzes the carbonization process, followed by graphitization of the resulting spatially cross-linked polymer-oligomeric skeleton. It holds the forming charred layer on the surface of the protected metal structure due to chemisorption, and is, in fact, the main adhesive agent of the flame-retardant composition during thermolysis process. We believe that titanium dioxide is a nucleating agent of the cellular structure because its particles are catalytic centers, where the intumescent process originates and around which significant volumes of gaseous products are formed from various chemical transformations. In addition, titanium dioxide as infusible filler gives additional performance properties to the emerging charred layer, in particular, greater heat resistance and strength.

88

2 Technology Basis of the Thermolytic Synthesis …

Thus, in the thermolysis of an intumescent system based on pentaerythritol, melamine and ammonium phosphates, the key process is the synthesis of threedimensional aminoaldehyde polymer-oligomeric resin which has a complex composition and structure, is foamed by gaseous by-products of the ongoing transformations and hardens with participation of phosphoric acids. As a result, the charred layer that is formed is voluminous and relatively strong and has good heat-insulating properties. The fire-retardant effect of intumescent compositions is determined not only by the barrier properties of the carbonized layer, but also by high endothermic effects due to phase and chemical transformations, and evaporation of decomposition products—water and ammonia.

References 1. Wade CA, Callaghan SJ, Strickland GS et al (2001) Investigation of methods and protocols for regulating the fire performance of materials with applied fire retardant surface coatings. http:// www.firesciencereviews.com/content/2/1/4. Accessed 12 Feb 2020 2. Zubkova NS, Antonov YuS (2002) Reducing the flammability of textile materials: solving environmental and socioeconomic problems. Russ J Chem 46(1):96–102 3. Vandersall HL (1971) Intumescent coating systems. Their development and chemistry. J Fire Flammabl 2:97–140 4. Mashlyakovsky LN, Lykov AD, Vyu Repkin (1989) Low flammability organic coatings. Leningrad, Russia 5. Troitzsch JH (1983) Methods for the fire protection of plastics and coatings by flame retardant and intumescent systems. Prog Org Coat 11:41–69 6. Khalturinsky NA, Rudakova TA (2013) On the formation mechanism of flame retardant intumescent coatings. Bull South Federal Univ 8:215–220 7. Breen C (2001) Intumescent coatings. Passive fire protection for steel. USA Patent 7652087, 13 Jul 2011 8. Camino G, Costa L, Trossarelly L (1985) Study of the mechanism of intumescence in fire retardant polymers. Part VI: Mechanism of ester formation in ammonium polyphosphatepentaerythritol mixtures. Polym Degrad Stab 12:213–228 9. Antonov AV, Reshetnikov IS, Khalturinsky NA (1999) The burning of char-forming polymer systems. Success Chem 68(7):663–673 10. Bourbigot S, LeBras M, Delobel R (1993) Carbonization mechanisms resulting from intumescence association with the ammonium polyphosphate-pentaerythritol fire retardant system. Carbon 31(8):1219–1294 11. Wang Z, Han F, Ke W (2005) Influence of nano-LDHs on char formation and fire-resistant properties of flame-retardant coating. Prog Org Coat 53:29–37 12. Ambartsumyan PG, Kutko SD (1995) Flame retardant intumescent composition. RU Patent 2028348, 09 Feb 1995 13. Steel Construction.info (2019) Fire-resistant structural steel. http://www.steelconstruction. info/Fire_protecting_structural_steelwork. Accessed 12 Feb 2020 14. Ruban L, Zaikov G (2001) Importance of intumescence in polymers fire retardancy. Int J Polym Mater 48:295–310 15. Nenakhov SA, Pimenova VP (2010) The impact of the gas-forming agent concentration on the development regularities of the charred layer of flame retardant compositions. Fire Explos Saf 19(3):14–26 16. Quintiere JG, Williams FA (2014) Comments on the national institute of standards and technology investigation of the 2001 World Trade Center fires. J Fire Sci 32:281–291

References

89

17. Alalykina LV, Fedoreeva LA, Chernaya IS et al (1995) Raw mix for a flame retardant coating, RU Patent 2034816, 10 May 1995 18. Shuklin SG et al (2001) Flame retardant polymer composition for coatings. RU Patent 2176258, 27 Nov 2001 19. Sakharov AM, Krukovsky SP, Yarosh AA et al (2006) A method for obtaining a phosphoruscontaining triaminotoluene formaldehyde resin. RU Patent 2285015, 10 Oct 2006 20. Sakharov AM, Yarosh AA, Krukovsky SP et al (2008) The method for obtaining triaminotoluene-phosphate-urea-formaldehyde resin. RU Patent 2328507, 10 Jul 2008 21. Pimenova VP (2002) Flame retardant intumescent composition. RU Patent 2185409, 20 Jul 2002 22. Ambartsumyan RG (1998) Fire retardant intumescent composition for wooden surfaces. RU Patent 2119516, 27 Sept 1998 23. Tychino NA, Leonovich AA (1999) Film-forming flame retardant binder for flake boards. Wood boards: theory and practice: 2nd scientific and practical seminar. St. Petersburg, Russia, 17–18 March 1999 24. Avdeev VV, Godunov IA, Shkirov VA et al (2002) A method for obtaining a highly condensed ammonium polyphosphate. RU Patent 2180890, 27 Mar 2002 25. Afanasyev SB, Makhlai VN, Mikhailin MP (2006) A method for obtaining a flame retardant. RU Patent 2270751, 27 Feb 2006 26. Afanasyev SB, Makhlai VN, Korotkoye RV (2006) A method for obtaining a flame retardant. RU Patent 2284263, 27 Sept 2006 27. Afanasyev SB, Makhlai VN (2006) A method for obtaining a flame retardant. RU Patent 2290299, 27 May 2006 28. Afanasyev SV, Lisovskaya LV, Tripolitsin AA (2007) Production and consumption of FCC in Russia. Wood. RU 1:9–10 29. Makhlai VN, Afanasyev SB, Tripolitsin AA (2008) Intumescent flame retardant composition. RU Patent 2339671, 27 Nov 2008 30. Pearson GA (1975) Novel resinous compositions comprising sequential reaction product of formaldehyde, inorganic acid, trietanolamine and urea. USA Patent 3883462, 13 May 1975 31. Pearson GA (1978) Fire retardant ureaformaldehyde composition. USA Patent 4119598, 10 Oct 1978 32. Pearson GA (1983) Fire retardant composition. USA Patent 4370442, 25 Jan 1983 33. Pearson GA (1984) Novel fire retardant composition and methods. USA Patent 4427745, 24 Jan 1984 34. Pearson GA (1987) Fire retardant composition. USA Patent 4663239, 05 May 1987 35. Pearson GA (1980) Novel resinous coating composition. USA Patent 4215172, 29 Jul 1980 36. Medvedev YuN, Poedintsev IF, Boytsov VF et al (1994) Flame retardant composition for coating. RU Patent 2017778, 15 Aug 1994 37. Alalykina LV, Fedoreeva LA (995) Raw mixture for flame retardant. RU Patent 2034806, 10 May 1995 38. Balakin VM, Seleznev AM, Belonogov KV (2010) Initial assessment of the fire-retardant properties of intumescent coatings based on various water dispersions. Fire Explos Saf 19(6):14–19 39. Egorov VV, Grigoryev YuA, Khalturinsky NA (2003) Flame retardant intumescent coating. Low flammability polymer materials: Abstracts of the 5th international conference. Volgograd, Russia, 1–2 October 2003 40. Khmelnitsky RA, Brodsky ES (1984) Chromato-mass spectrometry. Moscow, Russia 41. Zybina OA, Yakunina IE, Babkin OE et al (2014) Voynolovich specific reactions of ingredients in flame retardant intumescent paint compositions. Paintwork Mater Appl 12:30–33 42. Porokhov AM, Knunyants IL (1988) Chemical encyclopedia. Moscow, Russia 43. Tarasenko YuA, Zhuravsky SV et al (2010) Modeling the interaction of melamine with the surface of active carbons. Bull Kharkov Natl Univ 932:129–138 44. Khomenko TK, Sakharov MM, Golovin OA (1980) Synthesis of carbohydrates from formaldehyde. Success Chem 49(6):1079–1105

90

2 Technology Basis of the Thermolytic Synthesis …

45. Zybina OA, Varlamov AV, Chernova NS et al (2009) On the role and transformations of the components of fire-retardant intumescent paint compositions in thermolysis. J Appl Chem 82(4):1445–1449 46. Korsunsky BL, Pepekin VI (1997) On the way to carbon nitride. Success Chem 66(11):1003– 1014 47. Kuharsky M, Ya Lindeman, Ya Malchevsky et al (1965) Laboratory work in chemistry and technology of polymeric materials. Moscow, Russia 48. Ustinov A, Zybina O, Tomakhova A et al (2018) The enhancement of operating properties of intumescent fire-protective compositions. MATEC Web Conf 245:110008. https://doi.org/10. 1051/matecconf/201824511008 49. Ustinov A, Zybina O, Tanklevsky L (2019) The chemical mechanism of thermolytic synthesis in charring intumescent coatings for passive fire protection. In: Proceedings of the ninth international seminar on fire and explosion hazards, vol 2, pp 991–1000 50. Kiselev VS, Sorokin MF (1947) On the mechanism of the melamine formaldehyde reaction. The works of D. Mendeleev University of Chemical Technology of Moscow 12:25–34 51. Konkin AA (1974) Carbon and other heat-resistant fiber materials. Moscow, Russia 52. Kobykhno I, Tolochko O, Vasilieva E et al (2017) The influence of meta-and parasubstance aromatic diamines on the properties of poly (amidoimidourethane). Key Tech Mater 721:23–27. http://doi.org/10.4028/www.scientific.net/KEM.721.23 53. Zavyalov DE, Zybina OA, Mnatsakanov SS (2012) Comparative study of the behavior of ammonium phosphate in fireproofing intumescent formulations. Russ J Appl Chem 85:150–152

Chapter 3

Polymer Binders of Flame-Retardant Intumescent Coatings

Abstract This chapter presents the results of review and comparison of contributions made by film formers of intumescent composition, which initially forms a fire-retardant coating. Based on a critical analysis of literature and experimental data, it is shown that the nature of polymer binder has a direct effect on the fireretardant efficiency of the intumescent coating. It has been shown that it performs a number of significant functions: the binder is a matrix in which the components of flame-retardant composition and target additives are uniformly distributed; it ensures adhesion of initial coating to protected substrate, and is involved in the formation of structures catalytically active in intumescent process. In this chapter, the authors answer the question of why homopolymers and copolymers of vinyl acetate with ethylene, dibutyl maleate, vinyl chloride, esters of branched carboxylic acids and others are most widely used as binders for intumescent compositions. As part of consideration of conditions of applicability of film formers of various nature to create effective flame-retardant compositions, the results of comparative thermal tests of various intumescent coatings are presented. It is shown that the most suitable for intumescent systems are film formers that are sensitive to heat and capable of forming carbocyclic structures upon thermolysis. The occurrence of catalytically active polymer carbon clusters with a graphite-like structure during the thermal degradation of certain polymer binder has been experimentally proved. The expediency of using polyvinyl alcohol and its polymer analogs as binders in intumescent compositions, which have optimal decomposition temperatures and form thermolysis regions of local ordering of aromatic structures of polymer carbon, is substantiated. Keywords Flame retardant · Intumescent coatings · Polymers · Polymer binder · Binders in intumescent compositions · Thermal destruction polymers · Adhesion · Polyvinyl acetate · Copolymers · Fire test · Thermolysis · Polymer aromatic carbon As in the case with antipyrene composition components, the range of polymer binders used in intumescent compounds is quite limited despite the vast array of film formers present on the global market today [1, 2]. In terms of fire rating, the most preferable

© Springer Nature Switzerland AG 2020 O. Zybina and M. Gravit, Intumescent Coatings for Fire Protection of Building Structures and Materials, Springer Series on Polymer and Composite Materials, https://doi.org/10.1007/978-3-030-59422-0_3

91

92

3 Polymer Binders of Flame-Retardant Intumescent Coatings

Fig. 3.1 Structure of the carbonized residue of: a an effective intumescent coating; b an ineffective intumescent coating [9]

Fig. 3.2 Fire tests of epoxy-based intumescent coating samples: a test bench; b placing a metal sample with an applied coating; c the appearance of the sample after testing—the intumescent layer does not cover the protected surface [10]

coatings are [3–8]: vinyl acetate homopolymers; copolymers of vinyl acetate, ethylene and vinyl chloride; branched-chain copolymers based on vinyl acetate and vinyl ester of one or more carboxylic acids; copolymers of vinyl acetate and maleic acid dibutyl ether; copolymers of vinyl acetate and acrylic ester; copolymers of styrene and acrylic ester; acrylic ester copolymers; copolymers of vinyl toluene and acrylic ester. Epoxy resins are being increasingly used as a polymer base. However, as a rule, due to the relatively high temperatures of thermal destruction and the spatial structure, these binders inhibit the intumescent process and do not let isotropic finemeshed chard cellular material to form (Fig. 3.1a). Instead, they contribute to the formation of coarse bubble carbonizate (Fig. 3.1b) with a low thermal insulation capacity. In addition to the above, it has to be noted that the polymer matrices, which form a spatial structure after hardening, not only form coarse bubble chard cellular material but also fail to completely cover the protected surface (Fig. 3.2c) [10]. In order to overcome this problem, intercalated graphite is often added to the composition of fire retardants. It expands independent of the nature of the polymer matrix where it is distributed. This is caused by the initial structure of intercalated graphite (IG). Between its layers there are introduction substances that turn into gaseous products when heated, increase considerably in volume and force apart the graphite layers. Most recipes for foreign epoxy fire-protection char-forming compositions include IG. However, IG-based intumescent compositions have a number of drawbacks, including insufficient adhesion to the substrate and resistance to the impact

3 Polymer Binders of Flame-Retardant Intumescent Coatings

93

Scheme 3.1 Structural formulas of the most common polymer binders used in intumescent compositions

forces of turbulent gas torrents of chard cellular material formed during burning; the coarse particles of graphite in the composition reduce its application performance and aesthetic qualities. The binder is selected depending on the operation environment of the structure covered by the coating and its expected durability [11]. Water-based compositions are mostly used indoors. Acrylic organo-soluble fire-protection char-forming compositions are designed to protect structures indoors or outdoors (provided there is additional protection), while epoxide systems are used in conditions where high resistance to atmospheric effects and other safety features are expected. The literature review showed that the binders, most widely used in fire-protection char-forming compositions, are mainly acrylic, styrene-acrylic, polyvinyl acetate (PVAD) dispersions or their copolymers, as well as epoxy and organosilicon copolymers (Scheme 3.1). Epoxies are most frequently diane. Organosilicon polymers have a branched, partially crosslinked and cyclic structure with silicon atoms containing OH-groups. As they harden, a spatial structure is formed because new Si–O–Si bonds are created due to further condensation of residual OH-groups, and partial detachment and oxidation of organic radicals. Organosilicon polymers combine high elasticity and thermoplasticity with extra mechanical strength. However, they are almost never used independently in intumescent materials because the mechanical characteristics of the charred layer are unsatisfactory [12]. The resins listed in Table 3.1 can be used to make intumescent compositions both for indoor and outdoor applications, while the coatings they form can ensure up to 2 h of flame protection of a metal structure, depending on the size of the steel profile. The above polymers are usually highly compatible with polysiloxanes and can be used in combination with them. No doubt, these film-forming groups were selected empirically. The publications include few studies on how certain binders influence the flame-retardant properties of intumescent compositions. For example, MacNair and Stepler [13] concluded

94

3 Polymer Binders of Flame-Retardant Intumescent Coatings

Table 3.1 Film-forming polymers for organic solvents-based flame-retardant compositions Name

Monomer system

Vitrification temperature average value, °C

Pliolite AC3-H

Vinyltoluene/acrylate

61

Pliolite AC4

Styrene/acrylate

55

Pliolite AC5G

Styrene/acrylate

52

Pliolite AC80

Styrene/acrylate

54

Pliolite S5E

Butadiene/styrene

57

Pliolite Ultra 100

Styrene/acrylate

61

Pliolite VTAC-L

Vinyltoluene/acrylate

61

PLIOWAY EC1

Vinyl/acrylate

56

PLIOWAY ECT

Vinyl/acrylate

56

PLIOWAY Ultra 200

Vinyl/acrylate

57

PLIOWAY Ultra 350LV

Vinyl/acrylate

60

PLIOWAY Ultra G20

Vinyl/acrylate

49

that, with regard to flame-retardant paints, the binder plays an important role in the development of the foaming process, which they confirmed with an experiment with various binders (cellulose nitrate, polyurethane) in combination with a single flameretardant phase. Polymeric binders can inhibit foaming, so their content should be maintained at a minimum level. It was found that the degree of oxidation of alkyd resins is in reverse proportion to the height of foam and flame-retardant efficiency. Vandersall [14] presents data according to which if the degree of oxidation grows, flame-retardant efficiency increases by about three times, while foaming reduces by five times. It should be noted that this statement about the improvement of flameretardant efficiency with a sharp decrease in the amount of foaming is surprising. In our opinion, the cause of such an unusual, even paradoxical phenomenon is the chemical nature of the structures obtained. Extensive foam is relatively easy to achieve, but if, for example, the structure of the foaming polymer is not spatially crosslinked, its heat resistance will be relatively low. Reference [15] stipulates the requirements to be considered when film former is selected for a fire-retardant composition. It is noted that binder, in addition to providing the properties mandatory for a majority of decorative paint coatings, should degrade within the same temperature interval in which the foam-forming ingredients are destroyed so as not to interfere with the chemical reactions leading to the formation of a substrate-insulating intumescent layer. Moreover, the melt viscosity of film former cannot be too low (to prevent the foaming mass from dripping from the vertical surface of the protected structure) or too high (so as not to block foaming) [16, 17]. The molten polymer must have a certain “viscoelastic” state comparable to that of chewing gum so that the gases generated in the intumescent process can foam the melt. We believe that things can be somewhat more complicated than just the rheology of film former melt. As shown in [11], the “drainability” of the charred layer is affected by the content of other components of intumescent mixture, in

3 Polymer Binders of Flame-Retardant Intumescent Coatings

95

particular titanium dioxide. Ibidem also noted that some film-forming agents, as a result of a synergy effect with phosphorus-containing modifiers, including ammonium polyphosphate, form more heat stable charred layers [18, 19]. This phenomenon was also noted by us [6] and its nature will be explored below. The results of research into the behavior of intumescent compositions based on various water dispersions of polymers were found in the work of Balakin et al. [20]. The authors of this work note the most preferable film formers for intumescent flame retardants, but do not explain why one polymer material is advantageous in comparison to another one. No doubt it is difficult to make any assumptions on this subject when there is no clearly reasoned scenario of the intumescent process, especially since polymer degradation in its essence is an extremely complex process, accompanied by many sequential and parallel reactions, which depend on many factors. However, for our part, we consider it possible to express certain general assumptions based on the results of the considered and compared contributions that are made by polymer binders, initially forming protective compositions.

3.1 Synthetic Polymer Water Dispersions as Binders of Flame-Retardant Compositions Most of the flame-retardant intumescent compositions described in the patent and scientific literature are based on polymer water dispersions used as binders of the “key” ingredients. The latter ones when exposed to heat from the outside form, first, a polymer-oligomeric structure, which during carbonization turns into a skeleton of the charred layer, and secondly, non-combustible foaming gases (primarily, ammonia and carbon oxides). It is natural that the main ingredients, which were established long ago—pentaerythritol, ammonium polyphosphate and melamine—should be waterinsoluble. And, in fact, they are. Water dispersions of vinyl acetate homo- and copolymers are the most common film-forming agents for water-dispersed flame-retardant paints. Until lately, vinyl acetate homopolymer dispersion, plasticized with low molecular weight plasticizers, was most commonly used. In recent years, a growing importance has been attached to dispersions of copolymers of vinyl acetate with ethylene, butene and esters of acrylic, maleic, fumaric acids and other monomers, including branched vinyl carboxylic acid esters. In this case the role of plasticizer is played by comonomer units distributed in the molecular chain of the copolymer. The most effective comonomers for vinyl acetate are ethylene and isobutylene. The plasticizing ability is, though, not the only criterion used when the effectiveness of comonomers for vinyl acetate is evaluated. The performance properties of films and coatings are essential, too. Thus, copolymers of vinyl acetate with ethylene, maleates, acrylates and vinyl esters of higher isomeric carboxylic acids have the greatest resistance to alkaline hydrolysis. Copolymers of vinyl acetate with the

96

3 Polymer Binders of Flame-Retardant Intumescent Coatings

above-mentioned acid esters form films that are strongly resistant to UV radiation, water and have good physical and mechanical properties. They are used as film formers in water-dispersion paints intended for indoor and outdoor use, paints for asbestos, wood, paper and cardboard, and in paints with high dry residue [21]. In copolymers of vinyl acetate with ethylene, the nature of the plasticizing effect of ethylene determines considerable advantages of these copolymers over other copolymers: they have the same molecular weight, but greater tensile strength, bigger elasticity and are highly resistant to UV radiation due to the fact that the number of tertiary carbon atoms which are usually responsible for the destruction of polymers goes down in macromolecules. In addition, vinyl acetate–ethylene copolymer-based coatings are resistant to saponification, fading and cracking, and remain elastic when aging. They also have high alkali resistance, and are resistant to the action of mineral oils and liquid fuels. Paints can be used to paint masonry, and storage tanks for liquid fuel. Historically, the market of weather-resistant binders has included acrylate and styrene-acrylate latexes, with the share of the latter ones being much bigger. However, there is an alternative to this type of binder. These are copolymers of vinyl acetate and vinyl versatate (VA\VV). Vinyl versatate monomer (VV) is a versatic acid vinyl ester, which is a saturated monocarboxylic acid having a branched structure with ten carbon atoms and is a low viscosity liquid monomer with a typical ester odor. VV is a very good intermediate which is used to produce polymers through vinyl group reactions. It is mainly applied to modify comonomer in polymer latexes used in the production of emulsion paints, cement additives and so on. Copolymers of vinyl acetate with vinyl versatate have a number of considerable advantages compared to the traditionally used copolymers of VA with other “ester” comonomers, namely: high resistance to alkaline hydrolysis. This is explained by the fact that VV, as a versatic acid derivative, protects vinyl acetate fragments of polymer chains from alkaline hydrolysis due to hydrophobic properties and steric hindrances proven by the quaternary alkyl group of versatic acid, which ensures that these copolymers are widely used in architectural paints, primers and spackling pastes. Binders (VA\VV) have much greater weather resistance, which is attributed to the structure of the polymer. Thus, styrene-acrylate latexes give coatings 4–6 years of service life under atmospheric conditions, while VA/VV latexes are operative for 8–10 years, since vinyl acetate and vinyl versatate copolymers have greater resistance to UV radiation because there are absolutely no aromatic fragments in the polymer chain [22]. In addition to the above, VA\VV copolymers are highly water resistant and have good pigment intensity. As for film-forming ability, it is preferable to use copolymers containing 10– 50% of the VV mass, and the optimum content of versatate is about 25% of the mass. VV/VA/butyl acrylate triple copolymers are known to be increasingly used to produce anti-corrosion paints. Copolymers of vinyl acetate with diacetone acrylamide have great hardness, gloss, moisture resistance, adhesion and elasticity; the ones with photosensitive unsaturated monomers such as p-benzylphenoxyacetate for high weather resistance and trichloropropylene copolymers are recommended for low flammability paints.

3.1 Synthetic Polymer Water Dispersions as Binders …

97

Triple copolymers of vinyl acetate with 5–40% of ethylene and a small amount of triallyl cyanurate can be used in paints intended for construction purposes. Such coatings are highly resistant to temperature differences and have good adhesion to various substrates, including wood that has not been primed. Five-component copolymer dispersions, including 20–70% of vinyl acetate, 20–70% of alkyl acrylate, 0.5–3% of α, β-unsaturated amide (N-methyl or N-methoxymethyl acrylamide, hydroxymethyl diacetone acrylamide derivatives), 0.25–5% of α, β-unsaturated acids (acrylic, methacrylic, itaconic, maleic or fumaric acid) and 5–30% of acrylonitrile can be used as film formers for paints that have high storage stability and high film-forming ability at low temperatures. Coatings have great scratch resistance, gloss retention during operation, and dirt can be easily removed from their surface. Water dispersions of homo- and vinyl acetate copolymers are the most commonly used film-forming agents for water-dispersible flame-retardant paints. We think that this choice is not accidental, although, according to the published research studies, it is not fully understood. In [20] the results of comparative assessment of flame-retardant properties of intumescent coatings obtained on the basis of various water dispersions are presented. Polyvinyl acetate, acrylic and styrene acrylic dispersions were selected as water dispersions. The coatings contained the following components: pentaerythritol, titanium dioxide, ammonium polyphosphate, stabilizer, melamine, pigment dispersant, cellulose thickener, ethylene glycol and water. To make a comparative assessment of fire-retardant properties of the coatings obtained, tests were carried out on treated wooden samples on a “flame tube” setup. Table 3.2 contains brief test results of the intumescent materials. The samples that were believed most effective by the authors underwent further tests. Studies were carried out to determine the expansion ratio and heat insulation efficiency of flame-retardant intumescent coatings by measuring the temperature inside a hollow metal cylinder heated by a gas burner. According to the test results (Table 3.3), compositions 2 and 10 based on Polyvinyl acetate and acrylic copolymer dispersion, respectively, have the best intumescent coefficient. The graphs presented in Fig. 3.3 show that the control cylinder without coating reaches a temperature of 500 °C in 12 min. At the same time, cylinders 2, 3, 8 and 10, which have flame-retardant coatings, reach a temperature of 285, 379, 336 and 339 °C, respectively, which is 1.47–1.75 times lower than the temperature of the control cylinder. The constant temperature achieved in the tests was 285, 418, 353 and 370 °C, respectively. Summarizing the test results, the researchers conclude that compositions 2 (based on homopolymer Polyvinyl acetate dispersion of brand DF 51/10C) and 10 (based on the acrylic dispersion of brand Primal AC-261 K, which is a copolymer of methyl methacrylate, butyl acrylate and acrylic acid) have the highest flame-retardant efficiency. The data presented are largely consistent with ours, given below. Yet, it is a pity that the authors make no assumptions about why intumescent compositions based on these dispersions are most efficient for fire protection.

98

3 Polymer Binders of Flame-Retardant Intumescent Coatings

Table 3.2 Fire tests results of samples of intumescent coatings based on various types of binders [20] Composition no.

Type of polymer dispersion

Brand

Average mass loss of samples (with the consumption of FRIC being 200 g/m2 ) (%)

1

Polyvinyl acetate

D 51C

7.3

DF 51/10C

5.6

2 3

BC-4363

8.9

4

Styrene acrylic

DS-960

12.8

5

Akratam AS-011

10.3

6

XZ-94790

16.7

7

DM-109

17.8

8

Primal WL-100

9.7

Primal CL 3371

12.7

10

Primal AC-261 K

8.1

11

Mainkote HG-86 ER

17.5

12

Avance MV-100

124

13

Acrylic 6430

7.5

9

Acrylic

Table 3.3 Results of determined expansion ratio [20] Composition no.

Type of polymer dispersion

Coating thickness (mm)

Thickness of the charred layer (mm)

Intumescent coefficient (k)

2

DF51/10C (Polyvinyl acetate)

0.49

34.0

69.4

0.39

33.0

84.6

3

BC-4363 (styrene 0.41 acrylic) 0.47

8.3

20.2

12.4

26.4

8

Primal WL-100 (styrene acrylic)

0.39

5.2

13.3

0.35

6.6

18.9

Primal AC-261K (acrylic)

0.39

17.2

44.1

0.38

17.5

46.1

10

We also studied the nature of film-forming agents affecting the fire-retardant characteristics of intumescent coatings. In particular, we determined the intumescent coefficient and the time within which the limit states of the samples are achieved under the temperature conditions corresponding to the standard curve of cellulose fire [23]. Flame-retardant efficiency of paints for metal structures is determined using standardized comprehensive thermophysical tests, which are maximally close to the conditions of real fire [24]. These methods are time-consuming and expensive. They

3.1 Synthetic Polymer Water Dispersions as Binders …

99

Fig. 3.3 Dependence of the temperature inside the cylinder on the heating time in the flame of a gas burner for samples 2, 3, 8, 10 and the control sample without coating

are reasonable to use at the stage when material is certified, so, in the conditions of a chemical laboratory, researchers apply semi-quantitative comparative methods to assess the properties of intumescent compositions, which they develop and substantiate by themselves in accordance with the set scientific and practical objectives. According to the publications, the expansion ratio and adhesion-strength properties of the charred layer are the characteristics that are determined most generally [25]. The expansion ratio of intumescent coatings can be considered as a function of flame-retardant efficiency. Yet, it is natural that it should be considered in combination with other methods, since there is no data about what thickness the carbonized layer should be, other things being equal, to achieve the time necessary to resist negative fire factors. For example, it is known that intumescence of epoxy intumescent coatings is worse in comparison with vinyl acetate coatings, which is why they are applied in thicker layers—about 4 mm, and even so they cannot “catch up” with water-dispersion compositions in terms of the charred layer expansion ratio. However, they cannot be blamed for being inefficient. They are most widely used in the protection of oil and gas facilities, and they are the ones which are recommended for protection against hydrocarbon fire. However, of course, search is being carried out to increase the expansion ratio of epoxy compositions. For example, there are descriptions [26] of hybrid epoxy vinyl materials, where the epoxy part is responsible for high climatic resistance, and the vinyl part for lowering the destruction temperature and, as a result, increasing the char expansion ratio. The expansion ratio was determined as the ratio of the thickness of the intumescent carbonized layer to the thickness of the initial coating layer [27]. To determine the time of resistance to heating (the time of the limit state), the experts of FNPP “Gefest” ltd. assembled a laboratory setup on a testing area, as shown in Fig. 3.4. The method

100

3 Polymer Binders of Flame-Retardant Intumescent Coatings

Fig. 3.4 Laboratory setup for thermophysical testing of fire-retardant coatings; types: a general; b longitudinal section; c cross-section

implied exerting thermal effect on a test sample and determining the time from the beginning of the thermal effect until the limit state of the sample was reached. The samples were tested in the mode of “standard fire”, in which the average furnace temperature, measured by installed thermocouples, was monitored and adjusted according to dependence [23]: Tfurnace = To + 345 log10 (8t + 1), where To is the initial furnace temperature; t is the time, min. Figure 3.5 shows the results of the calibration start-up of the furnace. Steel plates 200 × 200 × 4 mm in size covered in flame retardant were used as samples. Fire-retardant compositions were applied to cleaned, degreased, primed surface. The aimed thickness of the dry layer was 1 mm, not including the primer layer (0.3 mm). Control measurements of the actual thicknesses of the coating were made before the tests at least at nine points. The arithmetic mean value of all measurements was taken as a result. The temperature on the surface of the test plates was measured using thermoelectric converters (TECs), which were installed on the unheated surface of the samples at a rate of three pieces using a coupling method. The unheated surface of the test sample was isolated with 100 mm thick mineral wool board ROCKWOOL. The temperature of the metal of the test sample was determined as the arithmetic mean of the readings of the TECs located in the designated places. The tests were carried out until the limit state of the test sample was achieved. The limit state is the time within which a temperature of 500 °C was achieved by the steel of the test samples (average temperature by three TECs). Even though we did not establish any direct correlation between the standardized field tests and our own ones, in our opinion, comparative laboratory methods for determining the given indicators of the flame-retardant coating can definitely characterize the change in the flame-retardant qualities of the intumescent material. However, it is obvious that a compact electric-heated laboratory setup lacks, for

3.1 Synthetic Polymer Water Dispersions as Binders …

101

Fig. 3.5 Temperature change during calibration start-up of the laboratory setup for thermophysical testing of flame-retardant coatings

example, some important factors typical for fire, such as the impact force of turbulent flows of hot gases. Therefore, the time when the samples achieve the limiting state during field tests is shorter. Compositions that were identical in terms of their content and based on ammonium polyphosphate, melamine, and pentaerythritol were tested. Only the type of film former varied. The binder content was 22% (mass.) with respect to the remaining mass of the ingredients. The results of our research in the flame-retardant characteristics of the fire protection char-forming compositions based on various types of waterdispersion binders are presented in Table 3.4. As expected in accordance with the published studies, vinyl acetate copolymers showed the best results. The best in all respects proved to be VA\VV copolymer. An important feature of such dispersions is their sensitivity to heat. At a temperature of 120 °C or higher, even high molecular weight samples develop an irreversible plastic flow and saponification to polyvinyl alcohol. If they are heated up to 170 °C or higher, destruction occurs, followed by the formation of carbonized residue. The thermal destruction of polyvinyl alcohol (PVA) occurs in two stages [28]. The main process at the initial stage is dehydration. The remaining polymer product mostly comprises conjugated unsaturated polyene-type structures. At the second stage (450 °C), polyene structures undergo further decomposition, which results in forming a large number of hydrocarbons (Table 3.5). Diels–Alder addition reactions of conjugated and double bonds in various polymer chains take place. This leads to intermolecular crosslinking, carbonization and graphitization.

102

3 Polymer Binders of Flame-Retardant Intumescent Coatings

Table 3.4 Flame-retardant indicators of intumescent compositions based on various water dispersions of binder polymers Brand of binder

Type of binder

Expansion ratio (k) Onset time of limit state (min)

PVAD DF/15C

Polyvinyl acetate dispersion

48.5

68

Emulex 523

Polyvinyl acetate dispersion

46.4

63

DC55B-TS

Copolymer of vinyl acetate and vinyl versatate

47.1

69

DPM 5035V

Copolymer of vinyl acetate with dibutyl maleate

38.5

57

AC C5003

Styrene-acrylic dispersion

14.1

37

Mowilith LDM 1780 Copolymer of vinyl acetate with ethylene

34.1

56

Primal AC-337

Acrylic dispersion

28.8

51

Vinnapas CEZ 18

Copolymer of vinyl 38.7 acetate with ethylene and vinyl chloride

60

Table 3.5 PVA thermal degradation products (240 °C) [25]

Products

Weight (%)

Water

33.400

CO

0.120

CO2

0.180

Hydrocarbons (C1–C2)

0.010

Acetaldehyde

1.170

Acetone

0.380

Ethanol

0.290

Benzene

0.060

Mesityl oxide

0.760

3-pentanone-2

0.190

3,5-heptadienone-2

0.099

2,4-hexadieneol

0.550

Benzaldehyde

0.022

Acetophenone

0.021

2,4,6-octatrienol

0.110

3,5,7-nonatrienol

0.020

Unidentified

0.082

3.1 Synthetic Polymer Water Dispersions as Binders …

103

Table 3.6 Condensed structures determined by chromatography-mass spectrometry in the composition of carbonizate obtained at given temperatures

Thermolysis temperature of the coang (°C)

200

300

400

500

O O

OH

Thermolysis products

Table 3.6 shows the cyclic condensed compounds detected as a result of the chromatography–mass spectrometric analysis described in the second chapter. These compounds are already formed at the early stages of heating of the intumescent coating. We should remind that water dispersion of vinyl acetate–ethylene copolymer was used as a film-forming agent, and the complete formulation of the coating is shown in Table 2.1. There are grounds to believe that these substances are formed predominantly when the polymer binder destroys. For instance, six-membered condensed structures can form due to the above-mentioned diene condensation. More complex structures can be the products of condensation and recombination of molecule fragments that are produced at early stages of thermolysis. In particular, this process can involve aldehydes, ketones, and other highly reactive compounds, which are in excess during thermolysis of the intumescent coating. In the final stage of the process, aromatic structures of the condensed phase (as thermodynamically more stable) are expected to form as a result of thermal activation, which leads to the production of carbon networks. A comparative review [28, 29] of the thermochemical transformations of hydroxyl-containing fibers, including polyvinyl alcohol fibers, in the presence of APP, presented in the literature, shows that carbonized products are systems containing hybrid varieties of carbon atoms sp2 and sp3 , where an ordered graphitelike carbon phase is formed. These hybrid varieties confirm the similarity of carbon obtained by thermolysis to the structure of intercalated graphite due to alternation of simple and double bonds. The authors believe that this is because H3 PO4 can esterify part of polyvinyl alcohol hydroxyl groups A with the formation of both acidic esters and cross-links in the polymer having the following structure (Scheme 3.2).

104

3 Polymer Binders of Flame-Retardant Intumescent Coatings

Scheme 3.2 Esterification of polyvinyl alcohol hydroxyl groups by phosphoric acid

Chemical analysis of the thermal oxidation products shows that thermochemical transformations are accompanied by accumulated double bonds in the polymer, and their content goes up by almost an order of magnitude, thanks to a dehydration reaction, which takes place according to an intramolecular mechanism, and formation of chain fragments of macromolecules with a system of polyconjugated bonds. Along with the formation of double bonds, the groups of an acidic nature increase in the polymer, with the ambient temperature having a dominant effect on the oxidation process. A significant change in the functional composition and structural transformations of polyvinyl alcohol stimulate condensation processes, and, thereby, formation of more heat-resistant intermediate pyrolysis products. Owing to this, the temperature range of polymer expands by about 100 °C. Accordingly, the yield of char residue also grows. The data presented and the advantages of using vinyl acetate-based polymers as binders in flame-retardant compositions allowed us to assume (and this assumption, as shown below, proved to be true) that it is polyvinyl alcohol derivatives that are most preferable as binders, as they can form graphite structures in the presence of no more than 5% of APP during thermolysis. The proof of such thermolytic transformations was obtained, thanks to a comprehensive thermal analysis of polyvinyl alcohol and its derivatives using the derivatograph Q-1500D (F. Paulik, J. Paulik, L. Erdey) and an automated thermal analysis method using the software program “Thermo”. From the total number of the mass change curves (TGA), mass change rate (DTG) and thermal effects in the course of polymer transformation with increasing temperature (DTA), we select and present the TGA curves as most illustrative. Let us compare the TGA curves of polymers without the pyrolytic polyvinyl alcohol additive and with the additive (Fig. 3.6). First, it should be noted that the TGA curves (Fig. 3.6a) of all the three curves at the initial stage of thermolysis—approximately up to 400 °C—practically coincide. The curves characterizing the PVA and the initial PVAD coincide along the entire length, that is, up to 550 °C. PVB loses weight before 630–650 °C. The evidence is that it has a stronger carbon skeleton, which, of course, is pre-determined by the

3.1 Synthetic Polymer Water Dispersions as Binders …

105

Fig. 3.6 Thermogravimetric curves of PVA (O), PVAD (×), PVB (D): a without APP; b with the addition of APP [6]

binding in acetal cycles of a significant part—up to half—of the vinyl groups of macromolecules of the source polyvinyl alcohol. Figure 3.6b shows that all the three curves practically coincide. The effects of the final decomposition move to the region of higher temperatures of 750–800 °C, while thermolysis occurs at a lower rate (TGA curves are smoother), which is the evidence of the process of carbon structure perfection of the carbonizing residue, that is, the formation of graphite-like structures. An X-ray structural analysis of the carbonized residue of the intumescent material (Fig. 3.7), consisting of ammonium polyphosphate, melamine, pentaerythritol and water dispersion of vinyl acetate–vinyl versatate copolymer shows that there is a crystalline phase. This is evidenced by a diffraction peak in the region of 22°, which is approximately in an angular position in the place of the corresponding three-dimensional reflection of graphite. The peak obtained has no clear maximum,

Fig. 3.7 Diffractogram of the carbonized residue of an intumescent composition based on water dispersion of vinyl acetate and vinyl versatate

106

3 Polymer Binders of Flame-Retardant Intumescent Coatings

which indicates that carbonizate samples lack three-dimensional order. One of the explanations for the asymmetry of profile of the lines is that there is amorphous carbon in the analyzed samples. In accordance with today’s understandings [30, 31], the three-dimensional structure of the carbon material represents randomly located crystallites connected to each other by small (short) aliphatic groups, as evidenced by Carom /Caliph , which is 18–30 for ordinary (natural) coals, and 14–20 for activated coal and charcoal. The crystallites themselves are some fragments with a structure similar to that attributed to a monocrystal of graphite, with the only difference being that the sizes of such formations are insignificant. For instance, for natural carbon materials they equal La = 34A, Lc = 19A, while for activated coals La = 17.8–37.0A (a network containing 7–15 aromatic rings in a row), Lc = 7.5–18.2A (3–6 layers of networks). Thus, the surface of a carbon body can be formed as a fragment of an aliphatic bridge (no more than 2–3%), or a crystalline structure, with two limiting cases being possible for the latter one: 1. A surface formed by the lateral part of the crystallite, that is, it represents the exterior group of parallel condensed carbon networks forming crystallite; 2. A surface formed by a basic condensed carbon network of crystallite. If we consider the very random arrangement of crystallites in the bulk of the coal material itself, we can assume that such randomness is also inherent in the surface, that is, the location of a particular crystallite fragment on the surface is probabilistic in nature. Moreover, two groups of carbon atoms (A and B) can be distinguished for the basal plane. They differ in electronic structure and properties and are carbon atoms located in the central part of the flat network (group A), and carbon atoms located on the periphery of the flat network (group B), which are bonded to hydrogen atoms or another group, for example, a bonding aliphatic carbon radical. In turn, each group is divided into two more kinds. One of them is characterized by the presence of a carbon atom in the adjacent layer (A1 and B1 ) while there is no “neighbor” like this for the other one (A2 and B2 ). It can be noted that the number of atoms having structures of types A1 and A2 is equal, respectively, to the number of atoms having structures of types B1 and B2 and are also equal inter se. The carbon atoms of group A are in sp2 -hybrid state and form only σ- and πcarbon–carbon bonds with relatively equivalent carbon atoms. The carbon atoms of group B are also in sp2 -hybrid state and form three bonds similar to those of carbon atoms of the first group, while the fourth σ-bond is formed with an atom of the functional group, and even if this bond is with a carbon atom, the electronic structure of the latter is different from all other carbon atoms. Obviously, B2 and B1 have a different ability to form various functional groups. For a lateral fragment, from a third to a quarter of all carbon atoms belong to group A, and the rest (group B) are quite active and may contain a functional group. As for the base surface, only peripheral atoms (group B) demonstrate activity and may

3.1 Synthetic Polymer Water Dispersions as Binders …

107

contain a functional group. Thus, the surface formed by the lateral crystallite fragment is quite active while the surface formed by the basal plane is less active. Functional groups may include aliphatic radicals, hydrogen atoms, and various oxygencontaining (hydroxyl, carboxyl, carbonyl, lactone, quinoid, etc.) groups (Scheme 3.3, groups 1–5, respectively) [31]. Each of the functional groups is attached to one carbon atom, the lactone group is closed to two adjacent carbon atoms and the quinoid group changes the electronic structure of the carbon atom bound to it. These features account for a multiplicity of options for the realization of the surface properties for the same original structure. Depending on the direction and way the surface transforms, various functional groups of the surface are formed and, accordingly, the surface can acquire directional specificity when absorbing certain molecules. The deposition of phosphorus groups on the carbon surface by molecular layering [31] with subsequent vapor-phase hydrolysis results in groupings of the form as follows (Scheme 3.4). Attached to the surface by chemical bonds C–O–P (where P is a fragment of the surface carbon structure). Such modification of the surface due to the formation of surface elemental oxide compounds occurs at the most active centers and causes significant change in the properties of both the surface and the entire carbon material as a whole. For example, during thermo-oxidative processes, modified graphites have greater heat resistance. This passivating effect of P, as well as Br, Si, Al on the oxidation of graphite is used [32, 33] when methods are developed to protect carbon–carbon composites from oxidation. Scheme 3.3 Different functional groups in the structure of char residue

Scheme 3.4 Structures that form as a result of deposition of phosphorus groups on carbon surface of intumescent char

108

3 Polymer Binders of Flame-Retardant Intumescent Coatings

Given the above, it can be assumed that the above-mentioned synergy of APP and hydrocarbon binder in intumescent systems is manifested in the formation of more thermostable graphite-like structures passivated by fragments of phosphoric acids.

3.2 Organic Polymer Solutions as Binders of Intumescent Compositions In addition to water dispersions of polymers and synthetic latexes, there is great interest in film-forming agents based on organic polymer solutions. Coatings based on them have some important specifics. They have high adhesion to substrate, great resistance to UV rays, weak solutions of alkalis and acids, better performance compared to coatings on water polymer solutions, high moisture and weather resistance. They need less time for drying and, finally, can be applied at lower temperatures. Various solutions of acrylic copolymers (e.g., methyl methacrylate and butyl methacrylate) seem especially promising if used as film formers because they help to obtain coatings with good strength characteristics, good adhesion to the protected surface and high weather resistance. Research on fire-retardant indicators of intumescent compositions based on solvent 15% solutions of binder polymers, similarly to those described above, show (Table 3.7) that acrylic film formers are inferior to the copolymer of vinyl acetate and vinyl chloride in this case, too. It is believed that acrylates suppress the intumescent process due to higher temperatures of thermal destruction. Moreover, the diffractogram of the carbonized residue of an intumescent composition consisting of ammonium polyphosphate, melamine, pentaerythritol and solution of acrylic resin in xylene Degalan 64/12, lacks the crystalline phase (Fig. 3.8). Table 3.7 Fire-retardant indicators of intumescent compositions based on various solvent solutions of binder polymers Brand of binder

Type of binder

Expansion ratio (k)

Time of limit state (min)

Degalan LP 64/12

Copolymer of butyl methacrylate with methyl methacrylate

26.5

50

Degalan LP 64/12 with CP-470

Copolymer butyl methacrylate with methyl methacrylate

45.5

71

Degalan P 675

Isobutyl methacrylate homopolymer

24.4

51

VINNOL H15/42

Copolymer of vinyl acetate with vinyl chloride

43.2

62

3.2 Organic Polymer Solutions as Binders …

109

Fig. 3.8 Diffractogram of the carbonized residue of an intumescent composition based on the acrylic copolymer solution Degalan 64/12

The situation changes when chloroparaffins (CP-470) in amounts of 10% (mass.) are added to the composition. The char ratio in thermolysis of a material containing CP-470 goes up, which, as we believe, make some researchers refer chloroparaffins to porophores [34], whereas in reality it only lowers the destruction temperature of the acrylic binder due to the dehydrating action of the resulting hydrogen chloride and directs the process toward the formation of graphite-like structures, which we can see in a form of crystalline phases in Fig. 3.9. In addition, as shown below, it has a stabilizing effect on ammonium phosphates, preventing them from premature (before resin is formed) decomposition with release of foaming gases. Fire-protection char-forming compositions (FPCFC) based on a solution of copolymer of vinyl acetate with vinyl chloride showed good results in the tests of

Fig. 3.9 Diffractogram of the carbonized residue of the intumescent composition based on a solution of the acrylic copolymer Degalan 64/12 plasticized with CP-470

110

3 Polymer Binders of Flame-Retardant Intumescent Coatings

flame-retardant efficiency (Table). Let us consider the possible contribution of vinyl chloride monomer to the thermolysis of the copolymer with vinyl acetate using the example of thermal destruction of polyvinyl chloride (PVC) described in the literature [29]. In the air, polyvinyl chloride starts to decompose releasing hydrogen chloride at 150 °C. During burning, all the chlorine is released from polyvinyl chloride at the early stages. Intermolecular bonds form, possibly because radicals recombine (Scheme 3.5). In the process of carbonization, further OH splitting off and increase in conjugated systems is observed. At the same time, structuring occurs following the Diels–Alder reaction, which, as it is known for polyvinyl chloride, is accompanied by HCl release (Scheme 3.6). Aromatic planes are formed and aggregated into packets (002 reflexes in X-ray photos) for vinylidene chloride beginning at 400 °C. In order to study the structure of the pitch [29], it was oxidized with HNO3, and fractional dissolution was carried out using various solvents [35]. Individual fractions were examined using the method of IR spectroscopy. The results of these studies made the authors to conclude that pitch consists of a set of 3–4 aromatic rings framed by alkyl radicals and has the following structure (Scheme 3.7).

Scheme 3.5 Formation of intermolecular bonds in the process of PVC thermal destruction

Scheme 3.6 Chemical transformations of PVC in the process of carbonization

3.2 Organic Polymer Solutions as Binders …

111

Scheme 3.7 Chemical structure of intumescent pitch

The formula gives only approximate information on the composition of the main product. In fact, pitch is a mixture of various compounds. Thus, the role of vinyl chloride in the binder of the intumescent composition is ambivalent (the same as in case of chloroparaffin, though). The drawback is that it releases toxic hydrogen chloride during decomposition, and the advantage is that the same hydrogen chloride acts as an additional inhibitor of chain radical burning processes in the gas phase and as a catalyst of carbonization and graphitization.

3.3 Applicability Conditions of Monoammonium Phosphate in Organic Solvent-Based Intumescent Compositions When we consider organic solutions of binders, we have to remember that we can use ingredients in the composition that are sufficiently efficient in terms of fire protection, and more affordable, but not stable in water systems. The difference between organic intumescent systems and water systems primarily lies in the fact that the most polar components (from the one mentioned above) in solutions of organic polymers are oriented by their polar groups not outward (as is in case when water is used as diluent), but inward, that is, its less polar components are outward-oriented. So, in the case of ammonium polyphosphate, in which not all acidic protons are replaced by ammonium groups, the functional—NH4 + —and H+ -groups are occluded, that is, “buried” in the mass of macromolecules. As it was shown, APP fulfills a diverse function in charred layer formation: firstly, to form salts with amino groups of melamine, and, secondly, to attach a protective charred layer with its acid H+ to the metal (together with several other functions mentioned above). So, it was suggested [36]

112

3 Polymer Binders of Flame-Retardant Intumescent Coatings

that in organically-based compositions APP should be replaced with monosubstituted monoammonium phosphate, which is insoluble in organic matter but does not “hide” its acid groups. The result turned out to be very convincing, but later it was found out that monoammonium phosphate, which was not specifically stabilized (and is most often produced as fertilizer, where stabilization is not required) “flattens” in 2–3 days, losing its ammonium groups—ammonia evaporates both from the coating, and from the mass of the composition: the coating significantly reduces the efficiency of gas release. Using stabilizers it was possible to achieve intumescence effects, similar in their quantitative values and the structure of the charred layer to those obtained when water-dispersion-based compositions are used, being completely or partially replaced with ordinary phosphates, for example, monoammonium phosphate NH4 H2 PO4 . A method of constant “acidifying” of the thermolizable mass of the composition to stabilize the monoammonium phosphate that it contained was found unexpectedly by introducing in the composition 4 mass% of polymers or oligomers containing chlorine: polyvinyl chloride (PVC), perchlorovinyl resin (PCV) and chloroparaffins. During thermal degradation, chlorine-containing polymers first begin to release hydrogen chloride—HCl. It stabilizes the system. Some difficulties arise only when pure PVC is used: in this case, tetrahydrofuran or an ethyl acetate and cyclohexanone mixture have to be used as solvent for the system. But if PVC is replaced with its copolymer with vinyl acetate or vinyl alcohol, these difficulties can be easily overcome. As noted above, ammonium polyphosphates are essentially crystalline inorganic polymers, which are insoluble to any considerable degree either in water (for the lowest molecular weight products, solubility is 5% of mass) or in organic solvents. Therefore, to distribute them evenly in the entire volume of compositions such as paints, polyphosphates are dispersed by milling and represent a suspension that is well stabilized with surfactants. Ammonium phosphates in water systems are a true water solution. That is why, it easily reacts with melamine, forming salt products. Since melamine is trifunctional by –NH2 – group, and monoammonium phosphate has two unsubstituted acid groups (in addition, it should be taken into account that free phosphoric acid can always be present in such a salt of a weak base), the composition, almost immediately after adding monoammonium phosphate, significantly increases in viscosity, and often becomes irreversibly gel-like. Furthermore, we should consider the fact that part of the recipe component—melamine—turns out to be bound and will not be able to participate in the synthesis of the frame resin of the charred layer subsequently in due time. The comparative data presented earlier is evidence that in water-dispersion systems when salt of polyphosphoric acid is replaced with monophosphate, the expansion ratio decreases by 1.5–2 times. Monoammonium phosphate (stabilized by chloroparaffin), which is not soluble in organic media, exists in flame-retardant compositions based on organic polymer solutions in the form of suspension and therefore “works” as its polymer homolog. In this case, the expansion ratio does not go down when polymer salt is replaced with monomeric one. Using only APP in organic systems is not reasonable due to its higher cost, and also because the charred layer formed on APP binds to the metal later and, often, to

3.3 Applicability Conditions of Monoammonium Phosphate …

113

a weaker degree (due to the fact that H+ are occluded) as a result of which, it peels off the protected substrate during thermolysis.

3.4 Conclusions Thus, polymer binders in flame-retardant coatings perform the following functions: (1) it is a matrix where the remaining components of the composition are evenly distributed; (2) it provides adhesion of the original coating to the substrate; (3) gaseous products formed during thermal decomposition of the polymer contribute to the process of foaming of the flame-retardant intumescent composition melt; (4) it takes part in the carbonization process with the formation of graphite-like structures, which, as shown in the fourth chapter, are catalytically active in the intumescent process. During thermal degradation of the most widely used polymeric binders of fireprotection char-forming compositions, Diels–Alder reactions take place resulting in adding conjugated and double bonds, condensation and recombination of low molecular weight products of the initial stage of thermal destruction of the material, leading to intermolecular crosslinking, carbonization and graftization. The formed graphite-like crystalline structures catalyze the chemical reactions that occur during thermolytic synthesis of the charred layer. Chloroparaffin is not a porophore in the proper sense of the word. It improves the expansion of intumescent coatings based on acrylic binders due to the lowering of the temperature of their thermal destruction, which occurs under the effect of the released dehydrating agent—hydrogen chloride. The latter one, the same as APP, catalyzes the processes of carbonization and graphitization of binders.

References 1. Ogorodov LI, Lustina OV (2017) Mechanical characteristics of polyethylene. Mag Civ Eng 6:17–32. https://doi.org/10.18720/MCE.74.2 2. Vakulenko DA, Turusov RA (2017) Water resistance of polymer compounds. Mag Civ Eng 7:106–113. https://doi.org/10.18720/MCE.75.10 3. Thirumal M (2016) Recent developments of intumescent fire protection coatings for structural steel: a review. J Fire Sci 34(2):120–163 4. Puri RG, Khanna AS (2017) Intumescent coatings: a review on recent progress. J Coat Technol Res 14(1):1–20 5. Zybina OA, Varlamov AV, Mnatsakanov SS (2010) Problems of the technology of char-forming flame retardant compositions. Novosibirsk, Russia 6. Shatalin SS, Varlamov AV, Zybina OA et al (2014) On binders in flame retardant intumescent compositions. Des Mater Technol 4(34):37–40 7. Zybina O, Gravit M, Pizhurin A (2017) The research of influence polymeric compounds on the effectiveness of intumescent coatings for the fire-protection of construction structures. IOP Conf Ser Earth Environ Sci (IOP Publishing) 90(1):012206

114

3 Polymer Binders of Flame-Retardant Intumescent Coatings

8. Shumilov SA, Varlamov AV, Zybina OA (2016) Specifics of using polyvinyl alcohol and polyvinyl chloride in flame retardant intumescent paints. Paintwork Mater Appl 1–2:54–57 9. Mashlyakovsky LN, Lykov AD, Repkin VYu (1989) Low flammability organic coatings. Leningrad, Russia 10. Shumilov A, Zybina OA, Mnatsakanov SS (2017) Thermolytic behavior of divinyl polymers. Success Mod Sci 8(3):207–211 11. Gorshkov NI, Bezrukova MA, Kipper AI et al (2017) Investigation of complexation between perrhenate ion and N-vinylpyrrolidone/N-vinylamine copolymers. Int J Anal Anal Polym 22(4):330–337. https://doi.org/10.1080/1023666X.2017.1295521 12. Garustovich IV, Shishilov ON. Specifics of the application of flame retardant intumescent coatings for industrial use. http://www.o3-e.ru/innovations. Accessed 12 Feb 2020 13. MacNair RN, Stepler JT (1970) Investigation of intumescent fabric coatings for protection against thermal radiation and flam. Am Dyest Rep 27–36 14. Vandersall HL (1971) Intumescent coating systems. Their development and chemistry. J Fire Flammabl 2:97–140 15. Wade CA, Callaghan SJ, Strickland GS et al (2001) Investigation of methods and protocols for regulating the fire performance of materials with applied fire retardant surface coatings. http:// www.firesciencereviews.com/content/2/1/4. Accessed 12 Feb 2020 16. Zybina OA, Voinolovich D, Babkin OE (2015) UV-hardening technology-based intumescent polymer coatings. Paintwork Mater Appl 1–2:76–79 17. Yew MC, Ramli Sulong NH, Yew MK et al (2014) Investigation on solventborne intumescent flame-retardant coatings for steel. Mater Res Innov 18:384–388 18. Le Bras M, Bourbigot S, Revel B (1999) Comprehensive study of the degradation of an intumescent EVA-based material during combustion. J Mater Sci 34(23):5777–5782 19. Wang G, Yang J (2012) Influences of molecular weight of epoxy binder on fire protection of waterborne intumescent fire resistive coating. Surf Coat Technol 206(8–9):2146–2151 20. Balakin VM, Seleznev AM, Belonogov KV (2010) Initial assessment of the fire-retardant properties of intumescent coatings based on various water dispersions. Fire Explos Saf 19(6):14–19 21. Tolmachev IA, Verkholantsev VV (1979) New water dispersion paints. Leningrad, Russia 22. Lebedev D, Okunev A, Aleshin M et al (1947) Applicability of polymer composite materials in development. Mater Phys Mech 34(1):90–96. https://doi.org/10.18720/MPM.3412017_11 23. International Organization for Standardization (2012) Fire resistance tests—elements of building construction (ISO Standard No. 834–2012) 24. State Standard R 53295-2009 (2009) Fire retardant compositions for steel constructions. General requirements. Method for determining fire retardant efficiency. Standartinform, Moscow, 15 pp 25. Lomakin SM, Zaikov GE (1995) A new method for reducing flammability of polymeric materials. Text Chem 2:20–33 26. Reinheimer A (2010) Intumescing, multi-component epoxide resin-coating composition for fire protection and its use. USA Patent 7820736, 26 Nov 2010 27. Smirnov NV, Bulaga SN, Duderov NG et al (2011) Assessing the quality of fire protection and using the type of flame retardant coatings at facilities: guidelines of FGU VNIIPO. Moscow, Russia 28. Kharchenko IM (2006) Thermochemical transformations of polyvinyl alcohol fiber as affected by pyrolytic additives in the production of carbon fiber sorbents. Moscow, Russia 29. Konkin AA (1974) Carbon and other heat-resistant fiber materials. Moscow, Russia 30. Aleskovsky BD (1990) A course in the chemistry of supramolecular compounds. Leningrad, Russia 31. Soldatov AI (2001) Structure and properties of the surface of carbon materials. Bull Chelyabinsk State Univ Ser 4 Chem 1(2):155–163 32. Malygin AA, Postnova AM, Shevchenko GK (1996) Adsorption properties and thermal stability of carbon fibers modified with boron and phosphorus compounds. Bull Univ Ser Chem Chem Technol 39(4–5):133–135

References

115

33. Rogers N (2011) Seal with expandable graphite. USA Patent 7935420, 03 May 2011 34. Surov AV, Subbotin DI, Popov VE et al (2017) Thermal steam plasma decomposition of organochlorine compounds. J Phys 927(1):012060. https://doi.org/10.1088/1742-6596/927/1/ 012060 35. Lebedeva IO, Zhulina EB, Frans AM et al (2017) Dendron and hyperbranched polymer brushes in good and poor solvents. Langmuir 33(5):1315–1325. https://doi.org/10.1021/acs.langmuir. 6b04285 36. Zavyalov DE, Zybina OA, Mitrofanov VV et al (2012) A comparative study of the behavior of ammonium phosphates in flame retardant intumescent compositions. J Appl Chem 85(1):157– 159

Chapter 4

Intumescent Nanocoatings for Fire Safety

Abstract This chapter presents the results of modification of flame-retardant intumescent compositions with additives in order to increase their operational characteristics. Carbon nanostructures and their precursors: monolayer and multilayer nanotubes, fullerenes and their endohedral metal complexes, graphenes, including thermally expanded graphite, and tetraazate tetrabenzoporphyrin complexes were considered as modifying additives that increase the operational characteristics of flame-retardant charring compositions. The results of instrumental studies of the physicochemical behavior of intumescent compositions upon thermolysis in the presence of modifying additives are presented. For the laboratory study of fireretardant effectiveness of coatings based on modified intumescent compositions, a device was designed to set the temperature regime of a cellulose fire, with the function of controlling the temperature in a furnace and on unheated surface of a sample. The char microstructure was studied by scanning electron microscopy in the secondary electron mode. The surface structure of char was determined using an atomic force microscope. The behavior of the modified intumescent compositions during thermolysis was studied using differential thermal analysis, as well as by oxidative microcalorimetry, according to ASTM D7309 (method A) (“standard test method for determining the flammability characteristics of plastics and other solid materials using oxidative microcalorimetry”). Field fire tests were carried out in specialized accredited laboratories in accordance with the requirements of national standards. The catalytic effect of carbon nanostructures of various morphologies and their precursors on the synthesis of polymer basis of char is established and theoretically justified, leading to a change in its microstructure and, as a result, to a significant increase in the fire-retardant efficiency of intumescent coatings. Methods of directional regulation of structure and properties of polymer-based intumescent char-forming materials are considered with the aim of increasing their fire-retardant efficiency, taking into account the application technology and operating conditions. Keywords Flame retardant · Intumescent coating · Nanotubes · Carbon nanotubes · Graphene · Fullerene · Intercalated graphite · Oxidized graphite · Thermal analysis · Microcalorimetry · Fire test · Char · Intumescent char · Intumescent layer © Springer Nature Switzerland AG 2020 O. Zybina and M. Gravit, Intumescent Coatings for Fire Protection of Building Structures and Materials, Springer Series on Polymer and Composite Materials, https://doi.org/10.1007/978-3-030-59422-0_4

117

118

4 Intumescent Nanocoatings for Fire Safety

Preserving the protective intumescent coating under effects of adverse factors, the worst of which being turbulent flows of hot gases occurring during a fire, is a relevant issue since it is easy to imagine that when forming charred layer heat-insulating coatings, all practical sense is lost if they cannot be preserved on the surface of the protected material and, instead, collapse quickly. The solution to this problem is mainly found by protecting metal surfaces from heating up since any of the wellknown theories on adhesion show that metals that are heated to certain temperatures are not capable of exhibiting their original physical and chemical properties, the initial appearance of which determines certain values of the adhesive parameters. It is crucial to extend the time the carbonized layer of the heat-insulating substrate is preserved to as long as possible. In order to do this, the char layer must be made, first of all, durable enough, that is, resistant to the effects of the air and gas flows unavoidable during a fire, and, secondly, adhesive to the protected metal which continuously rises in temperature. The only solution to this situation is to achieve a structure of the charred layer which would integrate the adhesive effect of the individual points of adhesion of the carbonated residue with the hot metal; in other words, to achieve a high cohesive strength of the charred layer with a maximum possible area of adhesive contact. Introducing various dispersed or fibrous fillers into the polymer-oligomeric matrix of the charred layer [1, 2] is theoretically aimed at improving the deformation durability and thermal-insulating characteristics of the intumescent coating and increasing its “viability” during a fire. However, various types of non-organic fillers included in the composition of fire-protection char-forming compositions (FPCFC) lead to significant differences in the coatings regarding swelling, the morphology of the charred layer and the thermal protective characteristics. It was discovered that the high melt viscosity of the char-forming resin, which occurs as a result of including non-organic fillers, prevents the polymer chains from rotating and relaxing and affects the intumescent properties of the coating, reducing the expansion coefficient and fire resistance [3]. Attempts [4, 5] to use various kinds of mineral fibers as a reinforcing material turned out to be ineffective since these additives not only reduce the technological effectiveness of using FPCFC but also suppress the expansion of intumescent coatings, resulting in a reduction of their flame-retardant properties. Some fillers (zeolite, aluminum hydroxide and others) interfere with the formation of the optimal isotropic cellular structure of the charred layer, reducing its physical and mechanical properties [6]. There is also opposing data. For example, compounds with aluminum oxide, carbon, fiberglass and aramid fibers increase the durability of the charred layer [7]. Wollastonite, kaolin clay, alumina and crushed eggshells are also reported to be used as fillers of the intumescent composition. The inclusion of the last filler into the composition leads to increased water resistance, heat resistance and adhesive strength [8–12]. In recent years, the production of flame-retardant compositions containing nanosized fillers (the particles of which range in size from 1 to 100 nm in at least one dimension) has been of great interest. Various nano-sized additives have also been

4 Intumescent Nanocoatings for Fire Safety

119

widely used in recent years in order to increase the lifecycle of intumescent coatings. These include montmorillonite, nanosilica, nanorutile, polyhedral oligomeric silsesquioxanes, metal oxides and oxides of rare-earth elements [13–28]. Carbon frameworks—nanotubes (CNT), fullerenes, graphenes, astralenes and their derivatives [29–40]—are currently being considered as promising nano-sized fillers for flame-retardant intumescent compositions. Fullerenes [41] refer to the class of molecules which contain C atoms and form membranes with 12 pentagonal rings and two or more hexagonal rings. Every C atom in the fullerenes is joined with three adjacent atoms. The total number of atoms is always even [42]. Each fullerene contains 2(10 + n) carbon atoms, where n is the number of hexagons (n cannot equal 1). Pentagons in a classic C60 fullerene do not fuse with each other and are separated from each other by no more than one hexagon (the pentagon isolation rule). In fullerenes with more than 60 atoms, the pentagons can be separated by a larger number of hexagons. Carbon nanotubes (CNT) [43] are whisker nanoparticles of carbon atoms or other elements containing an extended internal cavity. According to their chemical composition, CNT can be regarded as simple substances, binary, triple or more complex bonds, and can have a layered structure of two or more substances [44]. Carbon nanotubes, unlike fullerenes, are not molecules in the full sense of the word. The common definition of nanotubes as molecules should not be accepted as fully accurate [45]. They do not have a strictly defined molecular mass; every tube contains its own number of C atoms. They cannot be classified as common polymers. It is unclear what should be considered the monomer molecule: C atom, C6 hexagon or a ring of hexagons. Individual CNTs are difficult to identify as a separate modification of carbon since they are not crystals [46]. After all, they are not classic clusters (groups of closely spaced, tightly bound atoms, sometimes ultrafine particles; metal atoms bound to each other, surrounded by ligands). Rather, they can be considered as unique clusters [47]. The most widespread and studied carbon nanotubes are formed during the folding of graphene planes [48, 49]. Graphenes [50] are flat grids of carbon atoms located in the corners of regular hexagons at a distance of 0.1418 nm (Fig. 4.13). Each carbon atom in the graphene is bound with three adjacent atoms [51]. Graphenes form layers in the graphite crystals, as well as more complex forms of carbon structures [52]. Individual flat graphene sheets were first able to be achieved by splitting graphite in 2004 (Institute of Microelectronics Technology and High Purity Materials RAS, Chernogolovka, Russia, and the University of Manchester, Great Britain) [45, 53–67]. Carbon is known for the formation of three main types of bonds: sp, sp2 and sp3 . Graphite has a flat structure with sp2 -hybridization for σ-bonds and p- for π-bonds. Characteristic for graphite, as with benzene, is for the σ-bonds to be localized, while the π-electrons form a delocalized system [68]. Each bond in the graphite is 1/3 double and 2/3 single. Fullerenes and CNT are characterized by the presence of hybrid orbitals, intermediate between sp2 and sp3 , where each fullerene has its own strictly determined share of sp3 -bonds [69]. In this respect, carbon nanotubes are closer to graphite.

120

4 Intumescent Nanocoatings for Fire Safety

Fig. 4.1 C60 fullerenes in an emerging structure of the charred layer [71]

Fullerenes and CNT are known for an exceptionally high degree of anisotropy and durability characteristics, superior in these areas to other well-known types of dispersed and fibrous fillers [70]. They can contribute to an increased “lifetime” of the charred layer in a fire. Molecules of the CFW, unique in their properties, which appear “captured” by the carbonized structure of the charred layer, are capable of giving an extra hardness to the polymer chains of the foam layer due to the latter being able to “wrap” around relatively large particles (Fig. 4.1) with a relatively big radius of curvature (Fig. 4.1) [71]. In order to determine the flame-retardant effectiveness of intumescent paints, standardized complex thermo-physical tests are conducted in conditions as close to a real fire as possible. These methods are labor-intensive and costly. Their use is advisable in the certification process of a material. Thus, during the initial stage, in order to obtain comparative data, lab methods for evaluating individual indicators of the intumescent coating, which can definitely characterize the change in its flameretardant properties, were used. The following indicators were chosen for this role: multiplicity of the intumescent layer, the adhesive–cohesive indicators of the charred layer, as well as important technical indicators of the coating such as its adhesion to the protected substrate, microhardness and its UV-resistance. A certified intumescent water-dispersive flame-retardant paint was used as the source material during the experiment. This paint was used as a base to make samples of flame-retardant intumescent coatings with various nanobody fullerenes and CNT. The research carried out shows that introducing nanoadditives into the makeup of a flame-retardant composition contributes to the increased stability of the material to

4 Intumescent Nanocoatings for Fire Safety

121

Fig. 4.2 Change of intumescent parameters of FRICM from fullerene content [4]

Fig. 4.3 Change in the strength parameters of the charred layer from fullerene content (the adhesion coefficient reflects the amount of collapsed foam layer from the substrate surface after a specific mechanical impact) [4]

mechanical and physical impacts in comparison to similar materials not containing carbon frameworks (CFW) [4]. Thus, when adding carbon black containing a mixture of C60 –C70 fullerenes into the contents of a flame-retardant material, there is an increase in the strength and intumescent indicators (Figs. 4.2 and 4.3). A maximum increase in the intumescent (char rate) and strength indicators (resistance to collapsing from the protected surface during a flashback) of the charred layer is seen when the content of mixed carbon black fullerenes is around 0.6–0.9% by weight. Likewise, the hardness, light-resistance and the separation force values of the initial coating from the metal substrate increase when evaluating the adhesion of the latter in comparison to the control sample. An increase in the amount of this component in the composition up to 2% has shown no significant improvements; at a higher content, the given indicators worsen.

122

4 Intumescent Nanocoatings for Fire Safety

Fig. 4.4 Change in the strength parameters of the charred layer from CNT content [4]

A similar picture is seen when adding single-wall nanotubes to the flame-retardant composition (Figs. 4.4 and 4.15). However, in the case of carbon tubes, all the indicators mentioned above are slightly better than when using fullerenes. It has been established that with an increase in the amount of CFW carbon black, the time within which the charred layer is capable of resisting destruction during the “burn out” (oxidation to gaseous products) also increases. The time the CFW-containing charred layer burns out increases, in all likelihood, due to the formation of a denser and more uniform structure of the charred layer. This happens as a result of the phenomenon of adsorption on the highly developed surface of the cluster structures of oligopolymer molecules of the carbonaceous layer (resins). As a result, the degree of crosslinking of the polymer and, consequently, of the molecular mass of the carbonized intumescent layer increases. When introducing CFW into the flame-retardant composition, the problem of their aggregation to microparticles must be dealt with. The aggregation process displays the van der Waals force between the individual carbon nanomolecules [72]. When adding them into the flame-retardant composition during the initial state, an uneven distribution in the matrix of the coating takes place. This can cause the physical and mechanical properties of the original coating to deteriorate since agglomerates of fullerenes and CNT are stress raisers [73]. Thus, identifying means for dispersing the agglomerated CFW and distributing them evenly throughout the flame-retardant composition is an important task. The term “dispersion” usually implies the comminution of solids in a liquid environment. Looking at a comparative analysis of various methods for dispersing CFW, some researchers [70] propose the most promising to be the ultrasonic grinding of CFW in a liquid environment, since exposure to ultrasound makes it possible to obtain highly dispersed homogenous mixtures. Dispersion of the suspensions occurs when CFW aggregates, joined together as a result of sticking, sintering or cleaving, are exposed to ultrasound. During the ultrasonic dispersion of such suspensions, the dispersity of the product increases by several orders in comparison to traditional mechanical grinding. In

4 Intumescent Nanocoatings for Fire Safety

123

Fig. 4.5 Change in the intumescent parameters of the flame-retardant material depending on how the CFW is distributed in the composition [34]

general, dispersion occurs due to cavitation erosion. Cavitation is when cavities (“bubbles” or “voids”), filled with gas, steam or a combination thereof, form in a fluid. Microcracks and surface irregularities can be found on the surface of the CFW aggregates and are distinguished by a heightened stress concentration and are where the nucleation of cavitation bubbles occurs. Under effect from intense microflows, liquid penetrates into the pores and cracks, where the cavitation bubbles can collapse and generate a powerful shockwave, destroying the aggregate. Surfactants allow the resulting suspension to stabilize. The use of dimethylformamide (DMF) as a surfactant is well established [70]. The properties of compositions containing a mixture of C60–70 fullerenes and nanotubes were compared. These were mixed directly in the flame-retardant composition by hand and using ultrasound in an ultrasound machine with a 1 kW capacity (CNT and fullerenes were added as aqueous suspensions in the presence of DMF). All the nanoadditives in the composition made up 0.7%. It was established that the intumescent and strength indicators (Figs. 4.5 and 4.6) of flame-retardant materials developed with ultrasound increase in comparison to the control samples. The atomic force microscopy (AFM) of the charred layer surface (Fig. 4.7) shows [74] that the fullerenes introduced into the composition not only retain their structural integrity after exposure to high temperatures, they also change the structure of the carbonaceous layer. This likely has to do with the effect of the self-organization of the organophosphate polymer structures of the carbonized remains in the presence of nanobodies, which includes the spatial structuring and sizing of polymer fragments due to the specific adsorption of macromolecules of hybrid chains on the nanocluster core. This effect causes the encapsulation of nanoparticles into a 3-D oligomericpolymer screen of the carbonaceous layer, which also appears on images. When using CFW to reinforce the charred layer, spatial structures providing the directed anisotropy of the properties of the final material are presumedly formed. The carbon phosphate matrix takes part in creating the limit load of the intumescent layer, transferring the forces onto the nanobodies. The physical and mechanical properties

124

4 Intumescent Nanocoatings for Fire Safety

Fig. 4.6 Change in the strength parameters of the charred layer according to how the CFW is distributed in the composition (The percentage of the charred layer remaining on the substrate surface after the mechanical impact) [34]

of the material as a whole depend on the properties of the matrix. The role of the carbon frameworks is not so much to strengthen the matrix as much as to redistribute the applied load since the CFW, with a higher elastic modulus and ultimate tensile strength than the polymer matrix, takes on the main share of the load [29]. The influence of CFW on the process of thermolysis is confirmed by the results of an integrated thermal analysis of FRICT composed of melamine, pentaerythritol, ammonium polyphosphate and a vinyl acetate copolymer binder. FPCFC samples modified with an endometallofullerene solution (from iron) Fe@C60 in dimethylformamide (DMF) (in amounts of 0.01; 0.03; 0.05%) and with carbon black, enriched with fullerenes in quantities of 0.9 and 1.5%, were analyzed in comparison to the control sample (Figs. 4.8, 4.9 and 4.10). The research included thermogravimetric (TGA), differential thermogravimetric (DTGA) and differential thermal (DTA) analyses. The thermal analysis was conducted in an oxidizing atmosphere. The sample weights were 50 mg (weighing error ±0.1 mg). The temperature was measured with a platinum and platinum-rhodium thermocouple (PP-1) with an error of ±2 °C in a temperature range from 20 to 800 °C. The heating rate was 10 °C/min. The comparison substance was Al2 O3 [35, 38]. The endothermic effects observed on all curves of the DTA (Figs. 4.8, 4.9 and 4.10) at the initial stage of thermolysis of the intumescent composition, in a temperature range of 170–270 °C, are linked with the thermolysis of the film former and the rearrangement and dehydration of pentaerythritol. The exothermic peak in a temperature range from 270 to 300 °C is related to the synthesis of the polymer-oligomeric framework of the charred layer. The endoeffect in the range of 300–450 °C is linked with the destruction of the APP and the formation of a spatial structure of the charred layer. At this same temperature range, half of the mass of the sample is lost. This is the evidence of the intense gas evolution which occurs during the swelling of the wireframe resin. Carbonization is represented by an endothermic peak in a temperature

4 Intumescent Nanocoatings for Fire Safety

125

Fig. 4.7 Surface relief of the charred layer of a flame-retardant coating obtained by AFM: a, c, g surface relief of a sample containing C60 fullerenes; e 3-D image of the surface relief of the sample containing C60 fullerenes; b, d, h surface relief of the control sample; f 3-D image of the surface relief of the control sample [34]

range of 400–550 °C. The observable exothermic effects at high temperatures (over 480 °C) can be due to the formation of condensed structures and oxidation processes. The presence of CFW in the composition changes the structure of the endoeffects in the range of 170–300 °C, shifting the individual peaks that are characteristic for the start of destruction to higher temperatures. The exoeffects known for ending the process are, conversely, moved to lower temperatures.

126

4 Intumescent Nanocoatings for Fire Safety

Fig. 4.8 Results of thermal analysis of a control sample of FPCFC; b FPCFC containing 0.9% carbon black enriched with fullerenes [38]

Fig. 4.9 Results of thermal analysis of a FPCFC containing 1.5% carbon black enriched with fullerene; b FPCFC containing 0.01% (mass) solution Fe@C60 in DMF [38]

In samples containing carbon black, additional exothermic peaks show up in a temperature range of 620–680 °C which were not observed in the control sample and samples containing Fe@C60 . In the presence of CFW, decomposition is more intense. At the same time, however, as suggested by the curve types of the TGA (Figs. 4.8, 4.9 and 4.10), nanoadditives increase the induction period of decomposition, and the heat resistance of all samples and the char residue increases by 4–6% in comparison to the control sample. Next, the flame-retardant characteristics of samples modified with the examined CFW (Table 4.1) were evaluated in comparison to compositions containing 10% hallow corundum microspheres [38]. The swelling coefficient and the time to reach a temperature of 500 °C on the unheated side of the protected char-forming composition of the sample were chosen as the comparative parameters. During the testing process,

4 Intumescent Nanocoatings for Fire Safety

127

Fig. 4.10 Results of thermal analysis of FPCFC containing: a 0.03% (mass); b 0.05% (mass) solution Fe@C60 in DMF [38]

Table 4.1 Results of thermo-physical tests in a muffle furnace [38] No.

Modifying additive

Amount (%)

Thickness of FRICT (mm)

Thickness of charred layer (mm)

Kexp

Time to reach 500 °C (min)

1

–

–

0.3

10.1

33.7

10

2

C60 –C70 carbon black

0.9

0.3

12.9

43

28

3

C60 –C70 carbon black

1.5

0.3

12.2

40.6

33

4

Fe@C60

0.01

0.3

12.4

41.3

37

5

MC-Al2 O3

10

0.3

13.4

44.7

35

6

FePc

10

0.3

13.6

45.3

75

all modified samples demonstrated a significant increase in performance characteristics. It should also be noted that the most significant increase in flame-retardant effectiveness was observed when introducing into the flame-retardant composition additives developed by the authors using complex compounds of tetraazotetrabenzoporphyrins with transition metals that are precursors in the synthesis of carbon frameworks [38]. The mechanism of their effect on the thermolysis of FPCFC needs to be studied further. However, it is worth noting that introducing these compounds into the composition improves the microstructure of the charred layer (Fig. 4.11). By using them, a number of intumescent materials with an improved flame-retardant effectiveness have been developed and put into production. For further comparison of the flame-retardant effectiveness of intumescent material containing CFW, thermo-physical tests were conducted at the test lab of FNPP Gefest, Ltd. in a facility for thermo-physical testing at temperature conditions matching that of a “standard fire”. The flame-retardant effectiveness of a coating

128

4 Intumescent Nanocoatings for Fire Safety

Fig. 4.11 Images of the microstructure of the charred layer captured using scanning electron microscopy: a control sample (×700); b control sample (×1600); c sample modified with copper phthalocyanine (×500); d sample modified with copper phthalocyanine additive (×1400) [38]

containing 0.9% carbon black with C60 –C70 grew, practically doubling in comparison to the control sample (Figs. 4.12 and 4.13). In order to establish the flame-retardant effectiveness of an intumescent material containing CFW, standardized [75, 76] fire tests, most closely resembling the conditions of a real fire, were conducted at the accredited fire-safety testing center Pozhpolitest, ANO, certified by “Elektrocert”. The results of the standardized tests also confirmed the hypothesis of the increased flame-retardant effectiveness of materials containing nanobodies. The flame-retardant effectiveness of the tested samples containing 0.9% CFW increased by 10 min in comparison to the control sample [4]. Thus, carbon nanoproducts are of great interest when creating flame-retardant materials with improved properties. However, it is well known that pure nanotechnology products (such as fullerenes and CNT) are costly [77]. The high price of CFW (single-wall nanotubes cost tens or even hundreds of dollars per gram) dictates the choice of nanomaterials which, along with their inherent high-quality indicators, are available to industrial producers of nanocomposites, both from the side of production volume as well as in terms of the commercial potential to sell the product [78].

4.1 Influence of Graphene Structures on the Properties …

129

Fig. 4.12 Results of thermo-physical lab tests of intumescent coatings of: a control sample; b sample containing 0.9% carbon black with CFW [4]

Fig. 4.13 Results of thermo-physical lab tests of a sample of intumescent material containing, 0.9% carbon black with CFW [4]

4.1 Influence of Graphene Structures on the Properties of Intumescent Compositions The production of fullerenes and nanotubes is currently costly, and using them in flame-retardant compositions causes the cost of the material to increase manifold. Therefore, an advisable replacement for CFW-containing carbon black is the more affordable carbon nanoproducts from graphene and intercalated or thermally

130

4 Intumescent Nanocoatings for Fire Safety

expanded graphites. The cost of these products is significantly lower [79], while the impact on the characteristics of the char, as explained below, is practically the same. Intercalated graphites (IG) of various types (various degrees of intercalation) are widely used in flame-retardant materials as an independent intumescent component [30, 80–83]. They can be obtained by exposing crystalline graphite to sulfuric or nitric acid in the presence of an additional oxidizing agent, for example, hydrogen peroxide. Similar graphites are also obtained through an electrochemical method. Intercalated graphite is relatively inexpensive and available in a range of different expansion temperatures. It retains the heat-resistant properties of solid graphite and expands enough to be used in intumescent FPCFC. One disadvantage of the expanded graphite is its amorphous form and lightness, which can cause the intumescent layer to “deflate” from the surface of the protected material due to the turbulent gas flows produced in a fire [84]. As a rule, graphites which start to swell at a relatively low temperature range of around 160–180 °C are used in intumescent compositions. IG is added to the coating at 6–30% by weight. The more IG t in the composition, the more difficult it is to ensure sufficient strength properties for the charred layer. Some well-known compounds [81] include only expanding graphite and some type of binding agent. However, these compounds are only effective in specific limited amounts, since the expanding graphite crumbles from the protected surface after the binding agent has burned up, unless certain blocking elements prevent this. In other known compounds [82], the IG is kept from collapsing by a classic intumescent composition—a resin binding the thermally expanded graphite (TEG) is formed during thermolysis. However, in these materials the charred layer, classically characteristic for APP-MA-PE systems as a foam-cell carbonized structure, does not form. The IG imposes its own “fibrous” structure on the intumescent layer (Fig. 4.14) [6]. In this case, such materials are sufficiently effective [81]. Another method for using intercalated graphite was discovered. It can be used as an additive to traditional recipe compositions in small quantities as a component which increases the performance characteristics of the charred layer. In terms of the nature of the structure of intercalated graphite and the similarities of the surface electron structure of

Fig. 4.14 Graphene and its positioning in a graphite structure [50]

4.1 Influence of Graphene Structures on the Properties …

131

its graphene layers (Fig. 4.14) with that of carbon nanobodies, a similarity in the catalytic influence of these comparable objects is proposed. This is shown using an example of comparing the influence of CFW and TEG on the indicators of the intumescent material. IG is not used so that its own intumescent contribution, missing from nanobodies, is excluded. Carbon structures (Tables 4.2 and 4.3) were introduced to the ingredients of certified water-dispersive flame-retardant paint for metalwork. Intercalated graphite of the make ADT 351, thermally expanded in a muffle furnace at a temperature of 600 °C, was used for the thermally expanded graphite (TEG). The results of the comparative tests of intumescent CFW compositions are shown in Tables 4.4 and 4.5 and in Fig. 4.15. When the TEG in the composition equaled 0.7%, the intumescent indicators were at a maximum. Next, compositions were tested with just the amount of CFW needed to achieve a comparative result. The lack of an adhesive or mixed nature of destruction (Table 4.3) during separation of the coating is an evidence that the adhesive binding strength of the coating Table 4.2 Characteristics for the mixed carbon black of C60–70 fullerenes. Reproduced with permission from [34]. Copyright Springer Nature 2020

Table 4.3 Characteristics for CNT of the manufacturer “Nano Tech Centre”

Diameter of particles (nm)

0.72–0.75

Density (g/cm3 )

1.65

Outer diameter (nm)

8–15

Inner diameter (nm)

4–8

Length (μm)

2 and over

Thermal stability (°C)

600

Active surface (m2 /h)

300–320

Impurity content (%)

~5

Density (g/cm3 )

0.03–0.05

Table 4.4 Results of research on the characteristics of the original coating: normal separation adhesion and Buchholz microhardness. Reproduced with permission from [34]. Copyright Springer Nature 2020 No.

Type of CFW

Amount (%)

Adhesion (MPa); nature of separation

Value of microhardness

1

–

0

1.5 adhesive

73

2

C60–70

0.7

2.5 cohesive

81

3

CNT

0.7

3.0 cohesive

86

4

TEG

0.7

1.8 mixed

80

132

4 Intumescent Nanocoatings for Fire Safety

Table 4.5 The proportion of the charred layer remaining on the substrate surface after the mechanical effects of a flashback. Reproduced with permission from [34]. Copyright Springer Nature 2020 No.

Type of CFW

Amount (%)

Proportion of the charred layer remaining (%)

1

–

0

45

2

C60–70

0.7

87

3

CNT

0.7

93

4

TEG

0.7

64

Fig. 4.15 Change of the swelling coefficient from the amount of TEG and CNT in the intumescent composition. Reproduced with permission from [90]. Copyright Springer Nature 2010

with the substrate is greater than the specific force of the separation. This kind of phenomenon is seen in cases with samples containing CNT and C60 . When introducing TEG, the adhesion also increases, although to a lesser degree. All of the data obtained illustrates an increase in the performance characteristics of intumescent coatings in the presence of CFW, including TEG. It is clear that small (up to 3%) additives of IG [82] work in the same way. The lower effectiveness of thermally expanded graphite can be explained by the lack of steric regularity of the monolayers in contrast with the highly organized nanobodies. In order to clarify the type of influence of the CFW, a microscale calorimetric analysis of flame-retardant compositions modified with commercially available carbon nanoadditives in comparison with a sample modified with nanotubes was additionally conducted. The following types of CFW were added into the composition of certified water-dispersive flame-retardant paint of intumescent-type OSM-1 Gefest, present on the market, in the amount of 0.9%: single-wall carbon nanotubes (NanoTechCenter, Ltd., Tambov), thermally expanded at 600 °C, and crushed intercalated graphite of

4.1 Influence of Graphene Structures on the Properties …

133

the make METOPAC EG 803-95(99), graphene structures of the trademark SHS-Gr (Faktoriya-LS, Ltd., Saint Petersburg), where the carbonization of SHS-Gr_L starch and SHS-Gr_L lignin is carried out. SHS-Gr structures are similar in structure to the multiwall graphene obtained by pilling graphite using common techniques [85]. The characteristics of the graphene samples are given in Table 4.6. Data from the scanning and transmission electron microscopy of the make SHS-Gr_S are shown in Fig. 4.16. FPCFC samples were studied using microscale combustion calorimetry on the device, model “MSS-2” of the manufacturer Govmark, Farmingdale, NY, USA, according to ASTM D7309 (method A) (“standard test method for determining flammability characteristics of plastics and other solid materials using microscale combustion calorimetry”). The FPCFC sample (~5 mg) was loaded into the device and was then heated to a given temperature at a linear heating rate (20 °C/min) in a stream of nitrogen. Figure 4.17 shows the comparative results of the microscale calorimetric testing of samples of flame-retardant compositions in the form of the graphic dependencies of the heat release power during the oxidation of volatile products from the temperature in the combustion chamber. The red curve corresponds to the initial compound, while the black represents the compound modified by a certain nanoadditive. The results of the microscale combustion calorimetry show that for all samples containing CFW, the nature of the pyrolysis changes at the initial stage. At the same time, an increase in the curve maximum in a decomposition temperature range of Table 4.6 Characteristics for graphene structures [35]

Parameters

SHS-Gr_L

SHS-Gr_S

Number of graphene walls

2–5

2–3

Impurities (%)