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Fire Properties of Future Material Candidates Charles A. Wilkie* BACKGROUND If we wish to look at future materials that may provide a safer fire environment, it is useful to first consider the past. Currendy we attempt to produce fire-safe materials by the additive route and by He synthesis of materials that one hopes may be inherently more fire safe. The additive route means that something is added to the polymer, either in a physical or a chemical fashion, to make that material more resistant to burning. The commonly useful additives include alumina trihydrate and magnesium hydroxide, halogens, phosphorus, antimony, and synergistic combinations of these, sometimes including nitrogen or other elements. Additives may function in the vapor phase, as radical scavengers to quench the flame, or in the condensed phase. Condensed-phase retardants either change the mode of degradation of the polymer so that the production of volatile species is reduced or promote the formation of a thermally insulating char layer on the surface of the polymer; frequently the mode of degradation is changed to one that produces char. This char layer insulates the feedback of energy from the fire to the polymer and tends to prevent volatilization of material from the polymer. Condensed-phase retardants offer, in my opinion, the best chance to retard the burning of polymers because they provide a means to prevent, rather than quench, a fire. Currently, vapor-phase additives are more commonly used, but ~ believe that this will change in the future. Alumina trihydrate (ATM) and magnesium hydroxide decompose endothermically and remove heat so that all of the applied heat is not directed at the polymer. They also release water, and some of the energy is taken on by the water. These additives are effective, but very large loadings are required, and this has a deleterious effect on the physical properties of the polymer. Halogens, mainly bromine compounds but to some extent also chlorine compounds, form HX in the vapor phase. This will interact with the hydrogen and hyciroxy radicals that make up the flame and remove Hem so that the flame is quenched. Again, these materials are effective but present a problem because a great amount of HX is released. Many compounds of phosphorus, including elemental phosphorus and phosphates, have been used and are currently being used as flame retardants. The majority of additives that have been used as flame retardants for polymers in the past have relied on action in the gas phase. These materials do quench fires, but they offer disadvantages due to toxicity and loading. A very significant problem is the lack of portability of additives from one polymer to another. Because most flame retardants have been devised based upon a trial-and-error approach, there is tilde knowledge about how the additive interacts with the polymer. This means that no - *Department of Chemistry, Marquette University, Milwaukee, Wisconsin. ~5
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116 Improved Fire- and Smoke-Resistant Materials one can predict if an additive that is effective for one polymer will be effective for another polymer. A PREDICTION OF THE FUTURE OF FLAME RETARDANCY The route to the future is clear. Both the synthesis of new polymers that incorporate flame retardant elements and the use of condensed-phase additives will continue. We must continue to perfect the additives and the kind of chemistry that these additives perform and we must develop new additives that will perform new chemistry. The role of heat release rate in the evaluation of a potential flame-retardant system must be considered. It is my contention in this paper that the continued use of the trial-and-error method of additive selection that has been followed for years will not prove especially fruitful. The role of flame-retardant elements is, for the most part, well known; some of the modes of action are delineated above. As in almost any somewhat-mature area, only incremental improvements are to be expected if one follows the path well trodden. Major advances (and major disappointments) may occur from new approaches. In this naDer ~ suggest new routes that may be explored in order to develop the flame .' , . . ~ . ~ ~ . . .. . . ~ . . . .. .. retardant systems of the future. In essence, I suggest that we must forget about the so-cai~ect flame-retardant elements and, instead, concentrate on chemical reactions that will change the degradation pathway of the polymer so that a cross-linked char is produced on the surface of the polymer rather than a significant fraction of volatiles. Most thermoplastics degrade to produce little, if any, char and high yields of volatiles. Some of the newer, high-performance engineering thermoplastics, such as polysulfones or polyetherketones, give high char yields with low heat release rates. One model for the future is to produce thermoplastic systems that Will char in the same way as these engineering thermoplastics. Continued development in the high-performance engineering thermoplastics to make them even more thermally resistant is also required. Char formation to give an adherent, thermally insulating layer on the surface of the polymer prevents the feedback of thermal energy to the underlying polymer and effectively prevents its burning. Char formation offers the opportunity of portability from one polymer to another, and this offers great possibilities for the design of new flame retardants. ~ believe that we must begin to design flame retardants to perform a specific task upon interaction with the polymer and that a systematic investigation of reactions between polymers and additives will enable this design. A schematic diagram showing how char formation may proceed and its effect is shown in Figure I. In this diagram, at the top, a thermoplastic is shown. If one adds energy, that is, increases the temperature, bonds may begin to break. Bond cleavage will give rise to small molecules that can escape from the polymer and enter the flame zone. If one finds the reagents that are necessary to cross-link the polymer strands, then, when the same amount of thermal energy is added to the polymer, bonds will still break; however, the strands will still be attached to the polymer and no volatiles can escape. Significantly greater temperatures are required to break the bond that will liberate the sneak molecules and give rise to a flame. Recent work from
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Charles A. Wilhe l , crosslinking thermal cleavage thermal cleavage S=C cleavage FIGURE 1 Schematic representation of the formation and effect of char. the National Institute of Standards and Technology (NIST) has provided theoretical support for the concept of cross-linking to provide flame retardancy (Nyden et al., 19921. It must be considered that an organic polymer will always eventually burn. We can attempt to increase the ignition temperature and to reduce the evolution of toxic gases and smoke from the material, but it will eventually burn. If we wish to have materials that will not burn, then we are forced to consider inorganic materials. Phospham is an iminophosphazene polymer of unusually high thermal stability, and this material, either by itself or in combination with other polymers, offers some potential (Wei} and Patel, 1994~. Other inorganic polymers also offer promise, but this topic, as well as the synthesis of new inherently flame-retarded polymers, is outside the scope of this paper. This paper is limited strictly to an examination of some of the types of reactions that might be performed on polymers with the goal of char formation. Some of these reactions will be limited to specific polymers, others will be more broadly applicable. A problem currently facing the designer of flame retardants is the lack of applicability of an additive from one polymer to another. This is a most unfortunate situation. it is imperative that we have a detailed understanding of the interaction of an additive with a polymer so that one may predict the applicability of an additive to another polymer. Our goal should be to develop somewhat complete mechanistic information about the course of the chemical reactions that ensue between the polymer and the additive; only in this way can one learn if a particular additive will be useful with another polymer. Temperature Requirements For any flame-retardant reaction that one may consider, it is important to allow no reaction under normal conditions of temperature. The normal temperatures to which a polymer
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118 Improved Fire- arm Smoke-Resistant Materials is exposed go up to the processing temperature of that polymer. Thus, there must be no reaction until the temperature is above the processing temperature with some margin of safety. A realistic measure of the margin of safety is about 50 °C; practically, this means that the temperature at which the reactions to be considered commence must be at least 300 °C. The higher the temperature, the greater the possibility for degradation reactions of the polymer, so the onset temperature for the reaction must not be unduly high. It is probably useful to consider an onset temperature of 300 °C to 350 °C. Types of Reactions For the purposes of this paper, ~ have arbitrarily divided reactions into three classifications: oxidation-reduction reactions; acid-base reactions; and organic reactions. This is a very arbitrary distinction, but it is a useful way to keep track of reaction types. Oxidation-Reduction Reactions The ultimate products that are obtained when an organic polymer is burned are carbon dioxide and water. The possible oxidation states that are available to carbon range from -4 to +4; of course, in carbon dioxide, the oxidation state of +4 is attained. In a typical polymer, the oxidation state of carbon will range between slightly negative numbers to values near zero. This means that one can carry out an oxidation to increase the oxidation level by only a little, perhaps up to the level of zero, which describes graphite. There is a wide variety of reagents that are regularly used in either inorganic or analytical chemistry as oxidants, including permanganate, Bichromate, nitrate, element chlorine or bromine, and others. In addition, there are many much-milder oxidizing agents that may prove useful. These include both the transition elements, which have two possible oxidation states (e.g., iron), and those nontransition elements that have more than one accessible oxidation state (e.g., tin). Oxidation may be most useful for those polymers for which the average oxidation state of carbon is relatively low. This may mean that oxidation would be most applicable to hydrocarbon polymers and not those that contain functional groups. Since very little, if any, of this chemistry has been performed, it is difficult to make general statements. The choice of oxidizing agents is not an easy one, especially considering the severe temperature requirements. A strong oxidizing agent will likely effect reaction at lower temperatures so the probable initial choice is a somewhat weak agent whose strength may increase with temperature. The Nernst equation tells us that, in aqueous solution, the potential for any reaction will increase with temperature. Oxidation reactions will not be performed in aqueous solution, but it is reasonable to assume that the oxidation potential will increase with temperature. Thus, oxidizing agents such as Sn4+ and Cu2+ may be a good starting point. The exact choice of materials to be studied is probably somewhat unimportant. The idea is to determine if the use of oxidizing agents is a valid approach to flame retardancy. Thus, nitrate, and even permanganate, should be examined. At the recent American Chemical Society meeting, Zaikov's presentation focused on oxidizing agents (Zaikov, 1994~. The message that he imparted to the audience was that "oxidation is your
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Charles A. Wilkie 119 friend." These preliminary results are all that is available, but they do suggest that this may be a fruitful area for investigation. The expectation is that an effective oxidizing additive will increase the char yield in a thermogravimetric analysis. The first step in this area would be to compare various classes of polymers with a selection of oxidants and determine by thermogravimetric analysis (TGA) the weight-Ioss behavior. Note that even though it is expected that oxidizing conditions will be most effective for hydrocarbon-type polymers, it is not recommended that only these be examined. It is important to have a comparison between polymer types to understand how and why the reactions occur and learn how to apply them. Even better than this would be to perform a TGA-Fourier Transform Infrared (FTIR) experiment and compare the starting polymer with the polymer-additive combination. The advantage of this approach is that it permits not only the determination of the fraction of weight that is lost but also an identification of the gases that give rise to that weight loss (Mittleman et al., 1994~. It is also important to perform cone calorimetry to determine the rate of heat release and associated information. If oxidation is useful for polymers in which the average oxidation state is fairly low, then reduction may be most useful for those polymers in which the oxidation state is fairly high, such as functionalized polymers, especially those that contain carbonyls. The collection of reducing agents is not as large as that of oxidizing agents. In an inorganic or analytical laboratory the common reducing agents are metals, especially the alkali metals. These clearly cannot be used for polymers, and some other material must be sought. In principle any metal can behave as a reductant; thus, the incorporation of metals into polymers may effect reduction. Metals that should be considered include zinc, manganese, aluminum, iron, and chromium. These metals include the strongest reducing agents that have reasonable water stability. One of the more potent, yet usable, inorganic reducing agents is Cr2+. The chemistry of chromium (II) is controlled by the fact that compounds containing this ion exist as multiple metal-metal bond species (Cotton, 1978~. The ligands surrounding the metals control the bond length and probably the reactivity. This species may easily be oxidized to the Cr3+ state. The oxidation of Cr2+ is so facile that in some compounds air is able to effect the oxidation. The choice of ligands that surround the low-valent chromium dictates its ease of oxidation and, thus, its ability as a reducing agent. The logical starting point for the investigation of reducing agents is to blend Cr2+ in one of its many forms with a variety of polymers that contain carbonyl groups or similar fi~nccionalities which will lead to a relatively high oxidation, as well as with some hydrocarbon polymers, to check on the question of the importance of oxidation number. It must be recognized that Cr2+ is a somewhat exotic reducing agent. I am not proposing this as a flame retardant but rather as a starting point to learn about the effect of reducing agents on the thermal degradation of polymers. In addition, these same polymers should be combined with some of the metals mentioned above to determine the relative effectiveness of the two different classes of reductants. Another oxidation-reduction reaction is oxidative insertion into some bond. This reaction may not be immediately recognized as an oxidation. The normal material that will participate in an oxidative insertion is some transition metal compound that does not fulfill the I8-electron rule. An example of this is Wilkinson's catalyst, ClRh(PPh3)3, which is best known as a homogenous hydrogenation catalyst. In work that has been performed in my laboratories at Marquette University, we have shown that this material will insert into a C-O bond of
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120 Improved Fire- aru] Smoke-Resistant Materials poly~methy! methacrylate3 (PMMA) (Sirdesai and Wilkie, 1989a, by. The cost of this rhodium compound is such that it could never be used as a flame retardant, but it is important to examine the potential of such reactions and to use them to identify other materials that may be useful. Since this rhodium compound functions as a hydrogenation catalyst, other hydrogenation catalysts should be investigated, not only with PMMA but also with other polymers, to determine the breadth of this reaction (ColIman, 19681. Acid-Base Chemistry There are two different types of acids and bases, Bronsted and Lewis. A Bronsted acid is defined in terms of its ability to donate a proton while a base is a proton acceptor. Thus? Bronsted chemistry will only be important in those cases where the polymer contains acidic or basic functionalities. This severely limits our ability to affect a polymer based upon this chemistry. For those special cases where the polymer is acidic, for example, poly~methacrylic acid), the presence of some basic additive that will react only at elevated temperatures may cause cross-linking through ionomer formation. Again there is the problem of temperature and acid- base strength. For a polymer like poly~methacrylic acid), the basic additive must be one that will not react at room temperature but will react at elevated temperatures. This means it must be a very weak base at room temperature, and, because cross-linking is the goal, it must be difunctional. ~ am not convinced that this area can lead to much useful chemistry. The Lewis description of acids and bases is more useful for our consideration. A Lewis acid is defined as an electron pair acceptor, and a Lewis base is an electron pair donor. Any polymer that contains either oxygen or nitrogen will have Lewis basicity, and it may be possible to react this with a Lewis base. The reaction of choice would utilize a bifunctional Lewis base so that it can bridge two groups and effect cross-linking. Unfortunately the dative bond between the acid and base is not very strong and will probably be cleaved easily at reaction temperature. It is possible to design the Lewis base such that it may undergo further reaction with the polymer so that the bond is converted into a strong covalent bond. An example from organome~lic chemistry is shown in Figure 2. CH H ~ 3 ~ 2 (CH3~2Ga N(CH3~2 FIGURE 2 Example of organometallic chemistry. (CH3~2G0- N(CH3~2 4 (CHOSEN-Ga(CH3~2 In order to carry out this chemistry it is necessary that there be a group on the polymer that may thermally combine with some group on the Lewis base. This will require very careful design but does offer some potential for polymers. As an example of this chemistry, I cite the
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Charles A. Wilkie 121 work of McNeill on blends of zinc bromide and poly~methy} methacrylate3 (McNeil! and McGuiness, 1984a, by. The Zn2+ ion aces as a Lewis acid, coordinating to the carbony} oxygens. A thermal reaction then leads to the evolution of methyl bromide with the formation of a zinc salt of poly~methacrylic acid). It is obvious that this type of chemistry will only be effective for those polymers that contain Lewis basic sites; there should be no effect on hydrocarbon polymers. The logical starting point is to carry out reactions involving venous Lewis acids zinc and tin compounds come immediately to mind to see what the effect of these will be on the degradation of the polymer. In my laboratory, we have investigated reactions of PMMA with several different tin compounds that may have some acidic character at elevated temperatures (Chandrasiri and Wilkie, 1994a, by. The results of these investigations indicate an increase in char formation with the formation of tin salts of the polymer. The ligands on the tin give volatile materials that can easily burn. It will be necessary to carefully design the additive that is used so that the fragments that are lost from the acid will not contribute to the flammability of the polymer-additive blend. This may be accomplished by using a reactive rather than an additive and actually having the Lewis acid be a part of the polymer backbone. These investigations must be extended to other polymers in order to explore the generality of these reaction systems. Organic Reactions There is a wide variety of organic reactions that may be considered. These may be classified as addition, substitution, and elimination reactions. An addition reaction means that something is added to the substrate, usually addition across a multiple bond. In the majority of polymers, double bonds are absent, so addition reactions may prove useful only in limited cases. These limit cases include polybuladiene, copolymers containing butadiene, and related materials. Here it may be possible to perform some addition reaction across the double bond. The ideal material to add is some difunctional compound that can effect cross-linking of the polymer. The first possibility that comes to mind is some ~x,2' species that can form a diradical under thermal conditions and may add across the double bonds in two different polymer strands and effect cross-linking. The Diels-Alder reaction may also be utilized for materials that contain double bonds. The polymer strands can function as the dienophile and some additive as the diene. The normal reactive dienophile contains functional groups that activate it for this reaction. In this case, since we do not wane reaction to commence until elevated temperatures are reached, it may be an advantage that the polymer is not activated. The diene must be chosen carefully so that it will be thermally stable, yet not volatile, at flame temperatures. The choice of diene will be critical to the success of the reaction. There appears to be no work performed on Diels-Alder chemistry on polymers; this is not surprising because the double bond in a butadiene unit is an Inactivated, and, therefore, an attractive dienophile. Thermal activation is a distinct possibility, and it is recommended that attempts be made to mode! this reaction with simple dienes to determine if the reaction has potential for achieving cross-linking of polymers.
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122 Improved Fire- arm Smoke-Resistant Materials Substitution reactions should prove more useful than addition reactions. In a substitution reaction, one replaces one atom or group with another. Friedel-Crafts substitution reactions may prove useful for aromatic systems (Brauman, 1979; Rabek and Lucki, 1988~. If one can identify a suitable catalyst that will function only at high temperatures, then a difunctional additive can easily be identified to effect this reaction. The usual catalysts for Friedel-Crafts chemistry are strong Lewis and Bronx acids. Strong Lewis acids are not suitable because they are hydrolytically unstable. A Bronx acid, on the other hand, should increase in strength as temperature increases. This implies that a Bronsted acid, which is weals at room temperature, may become strong enough at elevated temperatures to function as an effective catalyst for the reaction. The alkylating or acylating reagent that is used must be at least difunctional; the usual reagent is a halide. A halide would be unsuitable here because it would lead to the evolution of HCI. Alcohols have also been used as alkylating agents for Friedel-Crafts chemistry, and these offer the great advantage that water is the byproduct. Not only does this remove the toxicity problem, but the presence of water will dilute the flame, and some heat will be wasted in heating up the water rather than the polymer. This type of chemistry appears to offer a real advantage for polymers that contain aromatic rings, such as polystyrene and copolymers containing styrene. The problems then that are presented in making Friedel-Crafts chemistry a viable means for cross-linkina of benzene rings in stvrenic and related polymers are two: catalyst and _ _ ~ ~ ~ of .. . .. , . .. . . . .. ~ .. em. . . . . . . . . . ~ ~ . . . . alkylating/acylating agent identification. The catalyst must be completely ~nenect~ve at modest temperatures but become effective when the polymer is subjected to a thermal stress. The catalyst must "turn on" at about 300 °C and promote the reaction. The akylating or acylating agents must be difunctional, nonvolatile at fire temperatures, and thermally stable. The best alkylating agents will probably be based upon aromatic chemistry so that the stronger aromatic bonds will be available for thermal resistance. Substitution reactions for nonaromatic substrates will no doubt involve the functional groups on the polymer in some way, and these reactions will be dealt with later. Elimination reactions invoke the loss of some species with the formation of a multiple bond. This reaction is well known in polymer degradation because it is the pathway by which polymers, such as polyvinyl chloride (PVC) or polyvinyl acetate, degrade. In this arena, this reaction is known as chain-stripping. The degradation of PVC is shown in Figure 3. One way in which this reaction may prove useful would be in combination with an addition reaction. If a polymer, such as PVC could be activated to undergo chain-stripping and if there were an additive present that could add to that double bond and effect cross-linking, this would be a desirable result. This is rather exotic chemistry because a minimum of two additives are required: one to facilitate the chain- stripping reaction and one for the addition. This is not likely to be an area where immediate progress can be made. Functional Group Reactions The great majority of polymers contain functional groups. These functional groups include esters, acids, nitrites, ketones, and halides. The literature of organic chemistry is replete with examples of reactions that may be carried out on these various functional groups. In small- molecule chemistry these reactions are normally designed to convert one material into another.
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Charies A. Wrinkle FIGURE 3 Degradation of polyvinyl chloride. 123 -CH2- CHCI CH~CHCI~ heat ~ CH CH CH2-CHCI~ In this arena, we would be looking for processes that could effect the cross-lin~ng of the polymers. This is a straightforward extension of the types of organic reactions that have been carried out for many years, and a collaborative effort between one who is versed in flame retardancy and one who is versed in organic reactions should lead to some interesting results. As an example of this type of chemistry, ~ will cite the fact that a hydrogen that is adjacent to a carbony} group is more acidic than other hydrogens. Use may be made of this to effect an allylation at a site adjacent to the carbonyI. if one uses some difunctional alkylating agent, it is possible to effect cross-linking of the polymer chains. S;ystematic Studies of Additives aru! Polymers ~ think that it is important that one think not only about materials that may be used as flame retardants but also broadly consider venous classes of compounds and reaction types. If we can learn how a particular class of reagents interacts with a variety of polymers, we can then use this information to begin to design an additive (or reactive) flame retardant that will participate in the desired reaction. In this regard ~ mention the great number of investigations that have been carried out in McNeill's laboratory and in my laboratory on the effect of additives on the thermal degradation of PMMA (Camino et al., 1978; lamieson and McNeill, 1978; Wilkie et al., 1981, 1991a, b; McNeill and McGuiness, 1984a, b; Brown et al., 1986; McNeill and Liggat, 1990, 1992; Beer et al., 1992; Chandrasiri et al., 1994; Chandrasin and Wilkie, 1994a, b). The combined knowledge that results from the sum of these investigations should be enough to lead one to begin the process of designing additives that will be useful for flame retardance of PMMA. It is only when we understand how an additive affects the chemistry of degradation that we can apply that additive to other polymers. It is imperative that systematic, mechanistic studies on the effects of additives on the thermal degradation of polymers begin soon.
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124 Surface Treatment of Polymers Improved Fire- arm Smoke-Resistant Materials Of all of the current schemes used to achieve flame retardancy, ~ believe that only one, intumescence, will stand the test of time and continue to be used into the next century. Tntumescent flame-retardant systems are those that thermally form a voluminous foamed char layer that protects the underlying polymer (Camino et al., 197S, 1984a, b, c, 6, 1985a, b, 1988, 1989, 1990a, b, 1991; Camino and Costa, 1986, 1988; Impallomeni et al., 1986; Bertelli et al., 1989a, b, c; Marchal et al., 19941. An intumescent system consists of an inorganic acid or a compound that thermally generates an acid and a polyhydric compound in addition to the polymer to be protected. Ammonium polyphosphate and pentaerythrito! have been used with a polymer such as polypropylene. In the condensed phase these will form a multicellular char layer on the surface of the polymer. The intumescence approach has great appeal because it appears to offer applicability to a variety of polymeric systems. The mechanism of intumescence is now somewhat understood for the ammonium polyphosphate-pentaerythritol-polypropylene system. There has not been much application to other polymeric sYstems. and this approach must be extended to other PolYmers. It is also or-- ~-~ ~ ~-~ ----- --err- - r - ~ - -- necessary to determine the best choices for the acid former and the char former. The question of potential toxicity of these additives must also be addressed. An alternative process to form this char layer is to graft some char-forming monomer onto the polymer. If one is able to graft a monomer onto the surface of the polymer that will thermally decompose before the polymer does, then this can form an adherent char layer on the surface that will effectively insulate the polymer from the flame and prevent degradation. In my laboratory we have had success using methacrylic acid grafted onto acrylonitrile-butadiene- styrene terpolymer, ABS. When the methacrylic acid is converted to its sodium salt by treatment with sodium hv~roxide. this offers extensive Drotection to the underlvina ABS (Suzuki and , ~ ~ ~ ~ ~ ~~ · ~ ~ ~ Or ·. , ~ i' . i' ~ ~ · ~ 1 ~ · 11 Wllkle, 1993). ot course, It may not be necessary that the cnar-tormlng layer be cnemlca~ly attached to the polymer. It may be sufficient that there merely be a layer of char former at the surface of the polymer. The importance of chemical attachment versus physical attachment of this layer to the polymer remains to be seen. One of the significant advantages of this approach is that it should be applicable to a wide variety of polymers. If one can apply a surface layer of poly~sodium methacrylate) or some other good char former to a polymeric substrate, either by physical or chemical means, and if this degrades to char before or at the same time as the substrate degrades, one has an effective flame-retardant system. Further developments in this area require the identification of suitable char farmers whose degradation temperatures are known, along with suitable methods to apply this to the appropriate substrate. A variety of char farmers is required because one will need to "tune" the degradation temperature of the char former to that of the substrate. These systems must be optimized to determine the match that is required between degradation temperatures of substrate and coating.
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Charles A. Willie E:xper~merual Protocol 125 Many reactions have been suggest in this paper, and evaluation and determination of how to spend limited resources to maximize the return have a critical role. ~ believe that modeling studies on small molecules have a place in this study. Modeling studies are not the first step, however. One must first demonstrate that the indicated reaction will work on the polymeric substrates. For instance, for the redox reactions, the first step should be to carry out a reaction, probably by thermogravimetric analysis, between a blend of the oxidizing agent and several polymers. If an increased char yield for the polymer is obtained, this result would then permit mode} compound studies. The mode} must accurately represent the structural features of the polymer and it must be sufficiently nonvolatile to be available at the temperature of the reaction. There will certainly be differences in behavior between the mode} and the polymer. The mode} study should be used only to establish some of the steps in the reaction pathway. The actual course of the reaction between the polymer and the additive can then be investigated by the use of TGA-FTIR and related techniques. CONCLUSION The premise of this paper is that char formation is desirable to achieve flame retardancy of polymers and that processes that are well understood chemically, and, therefore, extendable to other polymers, are desirable. Reactions that effect the formation of char and have wide applicability to a variety of polymers will be most advantageous. ~ have presented many different chemical reactions that may lead to the formation of char. It is my hope that the flame-retardant community will have the opportunity to try out many of these reactions and that some may prove to be fruitful. REFERENCES Beer, R.S., C.A. Wilkie, and M.~. Mittleman. 1992. The interaction of poly~methy! methacrylate3 and chromium chloride: Transfer of methyl groups from the ester to the main chain. Journal of Applied Polymer Science 46:1095-!102. Bertelli, G., G. Camino, E. Marchetti, L. Costa, and R. Locatelli. 1989a. Structural studies on chars from fire retardant intumescent systems. Die Angewandte Makromolekulare Chemie 169:137-142. Bertelli, G., E. Marchetti, G. Camino, L. Costa, and R. Locatelli. 1989b. Intumescent fire retardant systems: Effect of fitters on char structure. Die Angewandte Makromolekulare Chemie 172:153-163. Bertelli, G., G. Camino, E. Marchetti, L. Costa, E. Casorati, and R. Locatelli. 1989c. Parameters affecting fire retardant effectiveness in intumescent systems. Polymer Degradation and Stability 25:277-292. Brauman, S.K. 1979. Friedel-Crafts reagents as charring agents in impact polystyrene. Journal of Polymer Science: Polymer Chemistry 17: ~ 129-! 144.
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Representative terms from entire chapter: