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Meeting FR Goals Using Polymer Additive Systems Edward D. Weil. INTRODUCTION Based on experience and theory, it should be possible to find effective low-level additives to further reduce ignitability; heat release rate; and smoke of even the best flame-retardant low- smoke plastics, composites, adhesives, and fabrics. In some cases, there are empirical guide- lines; in other cases theory points out promising directions. Even where mechanistic knowledge is sparse, reasonable working hypotheses can point the way to effective additives and, if pursued, also help firm up scientific understarlding. The key material properties suggested in mathematical fire models can also provide ideas for additives. The use of additives should not be viewed as competing with synthesis of new polymeric materials but as a way of complementing, enhancing, and, in the best cases, synergizing the flame-retardant performance of advanced polymers. 7 TRENDS IN FLAME RETARDANT COMPOUNDING: BUILDING A SYSTEM OF INTERACTIVE INGREDIENTS As shown by research in academic laboratories, and by industrial research as disclosed in the recent patent literature, there is a trend toward more multicomponent flame-retardant systems. A system can be built up using a "tool kits comprising ignition retarders, melt-rheology control agents, mass-transfer retardants, heat-transfer retardants, heat sinks, char initiators, 4; ~, ~" charting catalysts, char-strengthening agents, char-oxidation preventatives, and noncarbon barrier-forming agents. Just as modern rubber and vinyl formulations typically have a substantial number of cooperating ingredients, in the same way we can expect that the most advanced flame-retardant systems will have several additives with, in many cases, positive interactions between them. In the final optimization stage, computerized statistical methods are available to f~ne-tune such multicomponent systems, either by traditional experimental design and optimization algorithms (Cornell, 1990) or by more recently developed "neural network" methodology (Gill and Shutt, 19921. SOME PERSPECTIVES ON THE PROBLEM Most of the scientific and patent literature in the flame retardancy field relates to enabling flammable commodity polymers to pass fairly lenient small-scale tests such as UL-94. Most of *Polytechnic University, Brooklyn, New York. 129
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130 Improved Fire- arut Smoke-Resistant Materials the commodity thermoplastics form little char and much volatile fuel. On the other hand, the high-performance engineering thermoplastics, exemplified by polysulfones, polyetherimides, and polyetherkelones, have a high char yield and inherently low heat release. There is much less background on making these excellent fire-resistant polymers even more flame retardant, because the motivation has not been very great. Programs such as that which the Federal Aviation Administration is developing will "push the limits" in new and relatively unexplored territory. The general strategy for making already good high-char-yielding polymers perform even better with respect to aircraft cabin escape time is to increase their time of ignition, to delay as well as tower the peak rate of heat release, and to retard as well as lower the smoke release. All of these improvements take into account the goal of increased escape time. SPECIFIC TACTICS Retardation of Ignition by Reflecting Radiant Heat In venous mathematical models for the rate of generation of ignitable gases, re-radiated heat is subtracted from the externally applied heat (DiBlasi et al., 1991~. It follows from other models that increasing the re-radiated heat should retard ignition (Dimitr~ou et al., 19X9; Delichatsios et al., 19911. Empirical work done in Russia on this aspect with decorative plastics showed that spectral reflecting capacity of the materials was important in controlling the ignition time under radiant heating conditions (Shabalin and Mozolevskaya, 19891. Factory Mutual work has also shown a large difference in ignition time between dark-coated and uncoated polymethy~metacrylate ~· - ~· . ~ · ~m ~ (PMMA) (rlewarson, levy. Absorplance of radiant heat by ditterent varieties of wood, In relationship to the spectral disixibucion of the ignition source, has been shown to have a major effect on the time of piloted ignition of wood (Wesson et al., 19711. The opportunity to retard ignition by using a reflective surface seems perhaps to have been overlooked in plastics flame-retardant technology, although protective clothing and fire fighters' gear with heat reflective properties have been successful in the field. On the empirical side, it is common knowledge that light colors reflect heat better than dark colors, and the choice of color for clothing, vehicles, and houses often makes use of this factor. It is probably less commonly a matter of experience that the effect can be dramatic with very high irradiances. Photographs taken after an atomic bomb blast show this. In one instance, the pattern of a kimono was thermally etched onto the back of a Hiroshima survivor. In another instance, a shirt exposed to the same heat source shows the dark stripes carbonized whereas the white stripes are not even scorched. Designers of commercial aircraft cabin interiors usually incline toward light shades but the colors are decided on aesthetic considerations. The spectral behavior that matters the most is not visible light absorption but infrared. Perhaps this factor should be taken into account or put on a sound scientific basis, or even made the subject of regulation. Looking again at ignition theory, we find a number of models, ranging from theoretically derived to empirical. Without presuming to get into a discussion of their merits, we can note the
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Edward D. Well 131 input variables that occur in these ignition models. Empirical studies show that ignition is more complicated than it seems at first examination. The evolution of a fuel which in admixture with air gives an ignitable composition is a first requisite; but, in the absence of a pilot flame, a combination of conduction and infrared absorption by both the vapor and the surface is decisive (DiBlasi et al., 1991~. Catalytic wall effects can influence ignition either positively or negatively (VIachos et al., 1994~. It has been known for a long time that piloted ignition time is a function of incident irradiation, the spectral distribution of the ignition source, and the optical properties of the material (absorptance and re-radiation). A mathematical mode} derived at the University of Oklahoma (Haliman et al., 1972, 1974) involving terms for reflectivity and absorptivity provides a surprisingly good correlation to observed ignition time, considering that the data are taken from the literature and also that one of the terms in the equation is surface ignition temperature, the measurement of which is fraught with difficulties. Some appreciation of these difficulties can be gained by noting the exchange of views between two groups of researchers trying to measure the same ignition temperatures (Thomson and Drysdale, 1987; Drysdale and Thomson, 1988; Kashiwagi, 1988~. Tt is quite possible that reflective pigments such as titanium dioxide (TiO2) or, even better, the pearlescent pigments comprising TiO2 deposited on mica, and various infrared reflective clays, may be useful and practical for ignition retardation. This topic has been scarcely studied. Obviously, the reflective pigments would be rapidly defeated by surface charring. Raising Ignition Temperature and Effective Heat Capacity by Providing Heat Sinks There is scattered evidence that the heat sink effect is an important mode of action for additives such as melamine in lower-temperature polymers such as urethane foams (Ball and Appleyard, 1989~. The heat capacity effects alone were used to explain the relative effectiveness of halogen compounds in the flame Worsen, 1974, 1975~. We obtain essentially the same flame-retardant efficacy in an ABS (acrylonitrile- butadiene-styrene) char-former blend from the very stable and nonreactive tnpheny~phosphine oxide as from enpheny} phosphate, and that suggests that both the phosphine oxide and the phosphate may be at least in part working as a heat sink. This heat sink approach could be extended to high-performance polymers by rational selection of additives, as well as by providing means for the polymer itself to decompose endothermically in the ignition flame- spread temperature range. The heat sink approach need not be inefficient if used in a sophisticated way with a good match of the temperature of the endotherm of the additive to the initial polymer decomposition temperature. One believable German patent reference actually gives data showing an improvement from UL-94 V! to V0 resulting from the addition of melamine at ~ percent in a complex flame-retarded styrenic system (Feldmann et al., 1989~. In our laboratory at Polytechnic University, we have repeatedly obtained useful incremental flame-retardant effects (V! improvement to V0) in styrenics and polyamides from addition of melamine in the 5 percent range, likewise with 5 percent tripheny~phosphine oxide, .
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132 Improved Fire- arm Smoke-Resistara Materials in styrenics or polyamides, generally along with other flame retardants working by other mechanisms. The heat sink can be provided in the polymer phase (retarding ignition) or in the preheat zone of Me flame (weakening We flamed. In this way, the effective auto-ignition temperature of a plastic can be raised. An increase in effective heat capacity can be simulated by endothermic components. There is some reason to believe that endothermically decomposable gases, such as ammonia, even though they can burn, can provide endotherms in the preheat (pre-oxidative) flame zone and thus serve as gas-phase fire suppressants. Additives that evolve ammonia seem to be effective as flame retardants (Little, 1964; Pitts, 1971; Costa et al., 1991; Cullis et al., 1991; Garo et al., 19921. The endothermic contribution of volatilized flame retardants in the preheat and combustion zones of the flame have been given very little notice in the flame-retardancy literature. This neglect is becoming increasingly untenable in the face of some remarkable quantitative calculations, in good agreement with experiment, estimating the efficacy of many flame extinguishants on the basis of mainly endothermic decomposition (Sheinson et al., 1989) or entirely endothermic decomposition (Ewing et al., 198S, 1989, 19941. In a recent development of a computable model for the oxygen index, one investigator found that the model conformed to experiment only if a term was included for endothermic pyrolysis of the vaporized fuel (Bucsi and Rychly, 1992), and the computed oxygen index was quite sensitive to this quantity. Heat loss not only by endothermicity and heat capacity but also by post-ignition re- radiation is another factor that may be exploited, according to the views of a Russian research group (Khalturinsky and Berlin, 1990~. This approach seems to have had very little attention. The Russian workers believe that the reflection or re-radiation of heat may explain the unusual flame-retardant action they found with antimony oxide in a nonhalogen epoxy system. Taking Advantage of the DamkohIer Number Effect: Removing the Fue! from the Combustion Zone before most has Burned The Damkohier number is a generalized dimensionless variable that can express the ratio of the average retention time of fuel in the combustion zone divided by the average time to undergo combustion. A certain critical Damkohier number must be reached for ignition to occur and another critical Damkohier number must be reached for propagation (Williams, 1974; Sohrab and Williams, 1981~. Extinction will occur when the system drops below a critical Damkohier number. Viewed in terms of the DamkohIer number, combustion can be prevented by increasing the denominator (i.e., retarding the rate of the combustion reaction) or by decreasing the numerator (i.e., reducing the retention time of the fuel in the combustion zone). Doing both at the same time gives an even better result. While inhibiting the rate of the combustion reactions is a familiar strategy, propelling the fuel out of the reaction zone is not so familiar, except incidental to some other function of an additive. We (Zhu et al., 1995) found, much to our surprise, that, in a research project on
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Edward D. Well 133 flame-retarding ethylene-vinyl acetate copolymer, adding a metal nitrate paradoxically gave us a flame-retardant effect. After eliminating other possibilities, we were left with the likely explanation that the sudden surge of carbon dioxide and nitrogen oxides, just before polymer ignition, was perhaps pushing whatever fuel gas was evolved out of the reaction zone before it could be combusted, while also slowing down the combustion rate by dilution of the fuel. With additives such as melamine, such enhancement of gas evolution would take place concurrently with an endothermic effect. Wig our nitrate additives, gas evolution is concurrent with an endothermic reaction that generates these gases yet the flame-retardant effect is seen. This curious effect of nitrates probably illustrates flame retardancy achieved by decreasing the Damkohler number. Shifting Degradation Pathway Away from Volatile Fuel Release to Cross-Linking and/or Generation of Vapors of Low Fuel Value The more familiar approach of shifting degradation pathway is the likely basis for the highly effective phosphorus-based flame-retardant systems in cellulose; the degradation is shifted from volatile levoglucosan to formation of cross-linked char (Barker and Hendrix, 1979; LeVan, 1984; Lewin, 1984~. Enhanced water release is undoubtedly also an important factor. A quantitative study of the thermal effects of flame-retarding cellulose by alkali showed that the reduction of the fuel content of the volatiles counteracted the enhanced rate of degradation and enhanced volatile generation (Chen et al., 1991~. Cross-linking, char enhancement, and water release are probably the main modes of action of phosphorus flame retardants in certain other systems, for example, in rigid urethane foams. However, phosphorus flame retardants have several distinct other modes of action, including endothermic effects, radical scavenging effects, and coating effects (Well, 1992b). Recent work at the National Institute of Standards and Technology (NIST) provided quantum chemical support to the concept that cross-linking should help flame retardancy (Nyden et al., 1992~. However, this idea has for decades been in the "tool kit" of the empirical flame- retardant system designer. It has been well known that the initial formation of thermally stable cross-links begins the process of char formation and retards the formation of volatile fuels (Hindersinn and Witschard, 1978; Brauman, 1979a). For instance, the dechloranes together with certain metal oxides have been known as high-temperature vulcanizing agents for polyolefin elastomers; when they are used as flame retardants, the result is greatly increased char yield. This concept has been shown to work with a fairly good range of thermoplastics where there is a functional group that can be exploited for purposes of cross-lin~ng. For example, acid catalysts flame retard PMMA, apparently by causing anhydride links to form, and metal compounds in some cases cause salt cross-links to form (Gruntfest and Young, 1962; Wilkie et al., 1989~. Various other polymer-specif~c catalytic systems have been found. One example found at Polytechnic University is the action of zinc chloride (ZnCl2) as a catalyst for trimerization of the nitrite group in styrene-acrylonitrile copolymers, with resultant char enhancement (Pearce and Kwei, 1994~.
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134 Improved Fire- arm Smoke-Resistant Materials The sulfonate salt flame retardants, which work at very low levels specifically in polycarbonates, have been shown to be catalysts for the formation of an intumescent char (Ballistren et al., 1988), although increased carbon dioxide (CO2) evolution may also have a DamkohIer number effect. Another polymer-specif~c additive that is almost surely catalytic is platinum used at low levels in peroxide-cured silica-fi~led silicone rubbers (MacLaury, 1979~. The details of the catalytic effect of platinum are not clear, although a French study suggests that it somehow inhibits depolymerization of the siloxane structure (Lagarde and Lahaye, 1977~. However, this is one of the most efficient flame-retardant additives known, having a detectable effect at parts- per-million levels; and it affords reason for hoping that other such highly efficient catalyst systems may be possible. Somewhat related to these ideas is the concept of scavenging radicals in the decomposing polymer to perhaps retard decomposition to volatile fuel. At the typical temperatures of burning polymers, above 300 °C at least, there are few effective radical scavengers. The classical antioxidants do not work at these temperatures. One of the many proposed mechanisms for red phosphorus, an effective flame retardant, is radical scavenging in the decomposing polymer phase; the rather weak evidence is that red phosphorus retards pyrolytic degradation of polyethylene (Peters, 1979~. There are several other modes of action of red phosphorus and it is difficult to assess the relative contribution of each. In principle, the radical scavenging approach should work best in those polymers that fragmentize under radical initiation, but the same additives might antagonize flame retardancy in polymers that cross-link with radical initiation. Some marginal flame-retardant effects of antioxidants have been observed (Weil, 19871. Rapid Formation of Barriers to Heat and Mass Transfer Any char formation should be beneficial, at the very least by representing material that escapes burning, but the flame-retardant effect of char is obviously greater the better the char layer selves as a barrier to heat and mass transfer and the more quickly it is formed. Intumescent, that is, foamed and expanded, chars are clearly desirable. The relationship of char yield (as measured by thermogravimetric analysis (TGA) under nitrogen) to oxygen index has been well demonstrated for charrable polymers (Van Krevelen, 1975, 1990), but there are many complicating factors in the relationship of charring when flame retardancy is measured by other means, or with polymers that do not char well. One important factor in the relationship of char to flammability is the rate of char formation. This factor has only recently been noted in flame-retardancy studies (Gnedin et al., 1993~. In a recent study done in our laboratory using combinations of two phosphorus-containing additives, one being melamine phosphate and the other being a phosphonate ester, we found a useful synergism; interestingly, the flame retardancy was found to correlate not to the quantity of char but to the rate of char formation (Zhu et al., 1995~. As have earlier workers at American Cyanamid and B.F. Goodrich (Granzow, 1978; Savides et al., 1979; Hall et al., 1985), we at Polytechnic University have found venous synergistic combinations of phosphorus compounds by patient trial and error, and we now think
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Edward D. Well 135 that the basis of this useful phenomenon may be involved with rate of char as well as with amount of char. Another important aspect of char is the nature of its surface. in a recent Russian study, a phosphorus additive was shown to produce a carbonized layer with a smooth metric reflection in a filled epoxy resin while, without the phosphorus, the surface was visibly heterogeneous (Kodolov et al., 1993~. The principal flame-retardant effect of the phosphoms additive expressed itself as a pronounced ignition delay. Phosphoric or polyphosphoric acid was found to be present in or on the surface of the char from a phosphoms-flame-retarded polymer. This has been proposed to have a protective effect (Brauman, 1977a, by. We have confirmed the formation of a phosphoric acid layer in the UL-94 ignition of a thermoplastic polyester and we found direct evidence for its flame-retardant effect. This is probably an important part of the flame-retardant action of phosphoms additives. Further Consideration of Charring Catalysis Dehydrogenation and oxidative dehydrogenation catalysts are a generic class of additives that a priori should accelerate char formation under oxidative conditions with water as the byproduct. The char would not only have value as a barrier to heat and mass transfer, but its carbon would represent material left unburnt and obviously not contributing to heat release. The heat of combustion of a reaction consuming only the hydrogen of an aliphatic hydrocarbon polymer molecule is only about one-third the heat of combustion of both the hydrogen and the carbon (Well et al., 1990~. The heat of combustion discrepancy gets even greater for high- performance polymers typically having a high carbon/hydrogen ratio. The literature shows a few possible cases where the catalytic dehydrogenation approach may have actually worked well. For example, polypropylene has been flame retarded with 2 percent chromium introduced as chromyl groups (Chien and Kiang, 19801. Zinc acetylacetonate at ~ percent was shown to make polypropylene self-extinguishing by the D-635 test and increased char formation was exhibited by this system (Cullis and Hirschler, 19841. Recently, a Japanese company disclosed data in a patent that showed 0.02-0.05 percent of dehydrogenatively active metal-oxide combinations (Cr2O3-ZnO, Fe2O3-ZnO, or Al2O3-MnO) synergized alumina t~ihydrate (ATM) additives in polyolefins and prevented drip; they proposed a dehydrogenation mechanism (Shinyoji et al., 1987~. Another Japanese company disclosed the synergizing of the flame-retardant effect of magnesium hydroxide in ethylene-viny} acetate copolymer by small amounts of nickel, iron, manganese, or copper compounds, all plausible dehydrogenation catalysts (Kanemitsuya, 19911. At Polytechnic, we found that iron oxides and various other iron compounds are effective components of flame-retardant systems in certain polyamides (Weil et al., 1991~. Char morphology was found to be affected by these iron additives. The char appeared shiny, continuous, and free of cracks. While the mechanism of action has not been definitely determined, we note that iron is associated with dehydrogenative catalysis, but also that pyrolysis of certain polymers containing iron compounds have been shown to lead to strong glassy carbons (Kammereck et al., 19741. The mechanism of our iron effect is still being studied. ~,
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136 Improved Fire- aru] Smoke-Resistara Materials Another stoichiometric equivalent to dehydrogenation is oxidation to hydroxy, hydroperoxy, or keto functionality followed by dehydration thereof. This concept of oxidation as a precursor to charring has been discussed by the NIST group (Nyden et al., 1992~. Some experimental evidence is available for the fortuitous occurrence of this sequence of steps: Delobe} and colleagues recently presented evidence for the char-forming reaction of polypropylene oxidation products and ammonium polyphosphate in a system where part of the char is formed from pentaerythrito! (Delobe} et al., 19891. They have evidence for the presence of a phosphoric or polyphosphoric acid coating and possibly for phosphorylated carbonaceous structures (Delobel et al., 1989, 1991~. It would appear timely to explore such flame-retardant applications for dehydrogenation or oxidative dehydrogenation catalysis, since recent progress has been made in this type of catalysis in the petrochemical field. It should be now that this catalytic chemistry does not require particularly toxic metals or particularly expensive metals; some of the catalysts reported in the petrochemical field are based on zirconium, aluminum, zinc, molybdenum, and iron. Polymer additives based on this type of catalysis seem feasible. Of course, we cannot be sure that the same catalysts that promote dehydrogenation or oxidative dehydrogenation of small petrochemical molecules will work for polymers in inducing char formation at fire exposure temperatures. In our own recent work at Polytechnic, we have pursued this working hypothesis and have found some synergistic results of known dehydrogenation or oxidative dehydrogenation catalysts with other flame retardants, but have not found any manifestation of this effect strong enough to provide us with a "stand-alone" flame retardant. However, we believe that the advanced thermoplastics should actually respond better to this type of catalytic additive (less heat of combustion to counteract the effect, and inherently higher char yield to assist the effect). A substantial body of knowledge of carbonization catalysis is available in other fields of technology, such as conversion of coal to coke, production of carbon fibers and electrodes from pitch, and production of carbon-carbon composites. For example, in making chemically activated carbons from wood, it is typical to use zinc chloride or phosphoric acid. In coal liquefaction by pyrolysis, divalent metal cations increase the char/liquid ratio (Whitehurst en al. ,- 1980; Serio et al., 1993~. This literature is too extensive to review here. Fuel technology journals often discuss ca~ycic factors in carbonization chemistry, and this discussion may suggest leads to char- inducing polymer additives. It may be useful to consider the pathways for the conversion of the carbon content of the polymer, first to cross-linked polymer, then to amorphous carbon-rich material, then to partially organized "turbostratic" or "mesophase" carbon structures, and finally to graphitic structures. A laser Raman study at GE showed that the end product was much the same for several polymers but that the intervening steps were different (Factor, 19901. Much of the literature on carbonization and graphitization deals with the process at temperatures above the likely temperatures of char and at time scales beyond that which we are concerned with in flame-retarding polymers. Nevertheless, there are clues that may suggest useful polymer additives. Boron appears to be a uniquely effective graphitization catalyst in both the turbostratic phase and the graphitization phase (Oya et al., 19791. A wide variety of metals has been shown to have catalytic activity in graphitization (Oya and Marsh, 19821. It was found that the dehydrogenation and carbonization of coal tar pitch can be catalyzed below 700 °C by
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Edward D. Well 137 finely divided molybdenum particles dshihara et al., 1993). A study in a Russian petrochemical laboratory (Galimov and Rakhimov, 1990) shows that phosphorus acids can catalyze the condensation of polycondensed aromatic hydrocarbons to form coke. Besides the carbon structure on the molecular level, the larger-scale morphology is clearly important. The ideal morphology is that of a closed cell foam, as shown by microphotography (Bertelli et al., 1989). Phosphorus additives appeared helpful in this study. We found that iron compounds improved the macroscopic structure of our char from nylon-4,6 with polyphenylene oxide (Well et al., 1991). In one recent study on polyolefins with an intumescent flame-retardant additive system, the further addition of SnO2 (which reacts with the char) was found to cause the char to have an undesirable porous flaky structure, whereas TiO2, acting perhaps as a binder, was helpful in producing a coherent foam of good insulating character (Scharf et al., 1990). Special Problems of Catalyst Design and Possible Solutions The concept of char-promoting catalysts poses a dilemma when one attempts to apply it to polymers. On the one hand, if the catalyst is heterogeneous, polymers are not likely to be able to penetrate more than superficially, and product molecules may not be able to desorb. So a good dehydrogenation catalyst may only produce a local surface char of little protective value and may instantly become fouled. On the other hand, while homogeneous catalysts might not be expected to have this defect, especially if they can be dissolved in the polymer, such catalysts are fewer, they usually involve expensive ligands, and they are for the most part limited in their thermal stability. A proof of concept is the use of Wilkinson's Catalyst as a flame retardant for poly~methy! methacrylate), working probably by cross-linking (Sirdesai and Wilkie, 19891. This catalyst is admittedly too expensive and not active enough for real use, but such work stimulates the search for further catalysts and mechanisms of action. A possible escape from this dilemma is the use of nanoscale catalyst particles, having large surface area, little need for penetration of reactants into their interior, and possibly good thermal stability. A nice prototype for this concept is the recent finding of the effective cross- linking (probable smoke-suppressant) effect in polyvinylchloride of colloidally dispersed copper (leng et al., 19941. Introduction of Polymeric Additives to Provide Char Although most advanced thermoplastics such as polyetherimides and polysulfones are good char farmers in their own right, they could benefit by further increase in char yield and they may likely need improvement in rate of charring. The addition of a good char-forming polymer to a less charrable polymer has been recognized as a valid approach to building a flame-retardant polymer system. One successful prototype is Noryl@, a blend of polyphenylene oxide (PPO) which is a good char former with high impact polystyrene, a poor char former. GE researchers have shown that the char forming
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138 Improved Fire- aM Smoke-Resistant Materials character of the PPO is strongly contributory to the overall flame-retardant effect; it is only necessary to add a moderate amount of an aryl phosphate to overcome the flammability of the pyrolysis products of the high-impact polystyrene component of the blend (Carnahan et al., 19791. Noryl. contains PPO as a major component. We find that even small additive quantities of PPO, down in the 5 percent range, are quite useful when we are building a flame-retardant system for a poorly charrable polymer such as a nylon or a styrenic (Weir et al., 1991; Well, 1992b). The rate of char formation seems to have been less studied than the char yield. A recent study at Polytechnic University suggested that rate of char was the key variable in the flame- retardant effect of a series of related char-forming polyphenylene oxides (Zhu et al., 1995). We also found some statistical evidence for different flame-retardant contributions of fast char farmers and slow char farmers in a study of flame retardancy of styrenics (Weil, 1992b). We think that it will be found productive to add fast char farmers even to those engineering thermoplastics that provide, by themselves, a good char yield. We find fast char farmers to include certain novolacs. Some other plausible examples are the special novolac and "cardo" molecules described by Polytechnic University researchers (tin and Pearce, 1979, 198 1a, b; Lin et al., 1981; Lo, 1981; Pearce, 1984). Introduction of Reactive Cross-Linking Additives Even if the matrix polymer itself can char, the rate of polymer chain-polymer chain reactions leading to cross-linking may be relatively slow (Arrhenius pre-exponential term may be small) compared to the rate of reaction of some smaller molecule capable of causing cross- linking. Proof of concept was done years ago at SRI (Brauman, 1979b) using xylylene dichloride plus latent Friedel-Crafts catalyst in a styrenic polymer (a poorly charrable polymer that was thus induced to char). Later industrial researchers used less toxic bifunctional benzylating agents (Clubley et al., 1981). This approach may be somewhat polymer-specific. However, most or all of the high- performance engineering thermoplastics have aromatic groups in their backbone, and in many instances they have rather electron-rich aromatic groups bearing oxygen or nitrogen sub stituents. Thus, polyfunctional allylating agents or other types of polyfunctional electrophilic reagents should be able to cross-link these chains. Finding suitable additives of this type is a challenge, because they must not be reactive at processing temperatures but must enter into reaction quickly under fire-exposure temperatures. The delayed action could come from either the reagent or the catalyst. Some interesting hints and clues may be found in the literature pointing to possible polyfunctional allylating agents. For example, it was shown that in high-impact polystyrene with an antimony/halogen flame retardant, polybutadiene acted as an anti-dnp agent (Wagner and loesten, 19761. It is tempting to consider that polybutadiene may have been allylating the styrenic component. By itself, under these same conditions, polybutadiene underwent reactions suggesting cationic mechanisms.
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Edward D. Well 139 hnprov~ng the Barrier Quality by Introducing Heteroelements The N char" barrier to heat and mass transfer needs to be coherent, adherent, fast forming, and preferably somewhat intumesced with closed cells, but the barrier material need not be entirely or even primarily carbonaceous. The efficacy of a noncombustible noncarbon barrier layer was shown some years ago (Ellard, 19731. In this study of intumescent coatings, an advantageous protective effect was obtained when a refractory "char" component remained even if much of the carbon content from the polymer matrix was burned away. Glassy antimonate or phosphate barrier materials were shown to be effective. TiO2 and mica were also effective. These refractory barriers also had high infrared reflectance. Although this study was directed to coatings, it should be applicable to plastics. GE has done extensive work in including siloxane units in polymers with resultant formation of improved fire-barrier material. An excellent basic study by Kambour shows that this is a subtle effect with a distinct maximum of flame-retardant performance versus composition. In one study, the peak of flame retardancy coincided with a peals of char strength (Kambour et al., 19811. It is significant that some recent research at NIST showed that in polycarbonate modified by silicone, while the rate of heat release was greatly reduced, the piloted ignition time was shortened and the flame-spread rate was increased. Apparently, the intumesced char did not form fast enough (Kashiwagi et al., 19931. Some flame-retardant systems with rather low levels of silicone structures as additives have been described by GE authors (Frye, 1984; Schroll and MacLaury, 1984; MacLaury, 1990~. A recent example, which showed a very impressive reduction of rate of heat release by levels as low as 1 percent of a silicone additive in polystyrene, was presented by Dow Corning researchers (Page et al., 19931. Physical Reinforcement of the Barrier As char (or any other barrier material) forms, it can also build up stresses from shrinkage or stretching and from gases attempting to break out, and it can become cracked or porous. It has been shown (Gibov et al., 1990) that the underlying molten polymer and pyrolysates from the polymer can even come to the surface by capillarity through char. It has also been shown that some additives can aggravate this situation by causing perforations in the char; other additives can give more coherent and smooth char (Scharf, 19921. To prevent this failure of the barrier, an approach known in the art of fire-retardant coatings is to use "bridging" additives (Anderson et al., 19851. We have found that in thermoplastics, small amounts of high-aspect materials such as mica or woliastonite can help, even in the range of a few percent. We have also seen evidence of synergism between certain high-aspect mineral additives, which we attribute to particularly effective bridging of the char. We have also noted a small amount of evidence that coupling agents may aid these mineral additives in binding chars (Well, 1987), although some of the cited effects may have been from improved dispersion. There are even some indications that flame retardancy may peak
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140 at some low loading level Improved Fire- aM Smoke-Resistant Materials of coupling agent, so that the effect may require very little coupling agent and thus be economical. There is one Russian report that an elastomer containing an organophilicized clay gave the best flame retardancy and the optimum tensile strength at the same critical loading of surface-modifying agent (Kireenkova and Zuev, 19681. · . . Preventing Oxidative Destruction and Smoldering of the Barrier During or after the formation of a char barrier, it can also burn away. The protection of char from smoldering is known to be possible using phosphorus or boron compounds, but this process has not been studied very systematically, nor under high thermal input conditions in regard to flame-retardant plastics. There is a considerable body of knowledge on preventing carbon-carbon composites from burning away at high temperatures; small amounts of boron (as boron oxide) or phosphorus (in the +5 oxidation stated can help. As little as 0. ~ percent of a phosphorus containing adsorbate on graphite can reduce the rate of oxidation by 75 Dercent. so the effect is a powerful one (McKee et al., 19841. A A- r If this effect is to be made use of, optimum boron and phosphorus additives should be sought. Boron and phosphorus compounds cover a wide range in volatility, and some chemical ingenuity may be needed to reduce the volatility losses. In our laboratory, we have studied phosphorus compounds from the standpoint of, on the one hand, release to the vapor phase, and on the other hand, retention in and on the char. We consider that especially interesting phosphorus compounds from the standpoint of nonvolatility are the thermally stable phospham, phosphorus nitrides, and oxynitrides (Wei! et al., 1993a, by. Another approach to preventing the burning away of the char is to produce a ceramic or glassy protective layer. The protection of char is perhaps the most logical use of a low-melting ceramic- or glass-forming component. This approach has been studied in connection with protection of carbon fibers and carbon-carbon composites. Boron oxide from ammonium borate and boric acid appears protective up to 1000 °C and appears to not only form a coating but actually block the active sites on the carbon where oxygen attack occurs bY several mechanisms (Iones and Thrower, 1988, 19911. A wide range of berates, ~, , , _ .~. . ~ .~. . ~ .~ . ,. ~ ~ . ~ ~ ,. . . silicates, noroslllcates, and other ceramic coatings nas Been reviewed, as well as tne principles governing their performance in protecting carbon at various temperature ranges (McKee, 1986; Strife and Sheehan, 1988; Sheehan, 1990~. In principle, a very-Iow-melting glass could even offer protection at a pre-char stage. This approach has a long history. British patent 551 (1735!) discloses berates, alum, and other low-melting salts for protecting canvas against fire. The use of low-melting glass-forming salts has been well demonstrated by work done at B.F. Goodrich with glass-forming compositions such as nickel sulfates and ammonium pentaborate (Myers and Licursi, 1985; Myers et al., 1985; Kroenke, 19861. These low-melting glass formulations provided respectably good flame retardancy to a wide variety of plastics. However, these are water-soluble salts. It happens that the same factors that cause a glass to be low melting usually tend to cause it to be water-soluble. A number of intriguing proprietary fire protective coatings claimed to work at very high temperatures are probably something of this sort-alkali silicates or berates which, after an organic matrix has burnt away, leave a good hire-protective barrier. A number of such
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Edward D. Well 141 compositions, using fusible inorganic materials added to organic matrices, are described in the patent literature. One recent British patent application (Crompton, 1988) manages to name frits, basalt, sodium silicate, ceramic fibers, mullite, etc., for use in phenolics, polyester resins, ethylene vinyl acetate, and a variety of other thermoplastics. The new idea claimed is to use two different fries with different melting ranges. A recent German patent application refers explicitly to reduced-heat-release and low- smoke aircraft cabin materials and attempts to cover oxides and oxygen acid salts of groups IlI-V of the periodic table as additives for all possible aromatic thermoplastics. The examples seem limited to antimony oxide and zinc borate, but the claims are remarkably broad (Buchert et al., 1990~. Corning has devised a family of phosphate glasses whose melting range overlaps that of the higher-temperature engineering thermoplastics. They are marketing these glass-polymer blends for high-performance, dimensionally very stable. hiah-modulus molding compositions (Quinn and Beall, 19921. . , , ~ _O ICT has recently introduced CEPREE, a fairly low melting inorganic additive which, after its organic matrix has largely or completely burned out, begins to melt at 350 °C and form a glassy fire barrier that eventually devitrifies and forms a ceramic fire barrier (Smigie, 1992~. Although these glass- or ceramic-forming compositions can afford a high degree of fire protection under high heat load, they would have some inherent problems from the standpoint of aircraft uses. They have rather high densities and perhaps greater rigidity than desired. impact strength may be compromised. Also, they may not work quickly enough. Nevertheless, it would seem that these glass-polymer compositions should be added to the fire-retardant compounders' Tool kit" anyway. To overcome the density problem and stiffness, the commercial glass-polymer blends can be considered as a masterbatch and let down to some much lower glass concentration, where the glass will not have to carry the full burden of being the flame-retardant component. Such an additive may very well cooperate with the other flame-retardant components and, in the best cases, may synergize. At the very least, when the formulation has carbonized, the glass should remain to protect the char. A further approach to low-melting glasses are materials such as amorphous phosphorus oxynitnde (Well et al., 1993a, by, which have a fairly low density compared to glasses containing metals. They can be water resistant (Bunker et al., 19871. These have been only · ~ ~ ~ ~ , ~ , ~ ~ 1 _ _ _ ~ ~ 1 ~ 1 1 1 ~ ~ cursorily explored as flame retardants and are interesting because ot their potentially tow COSt and the possibility that they offer several modes of flame-retardant action. Other clues have been found to low-melting glasses that might be useful in flame retardancy. It was recently shown that zinc borate appears to help the wintering of alumina, giving a better fire barrier than the alumina alone provides (Shen, 1987~. Low-melting glasses based on modified zinc berates have been described as low-melting fluxes for porcelain glazes (Jackson, 1989), but seem not to have been explored in flame retardancy. Smoke Considerations Through use of polymers that are inherently good char farmers along with additives to further improve the flame retardancy by methods other than flame-zone inhibition, smoke should
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142 Improved Fire- and Smo1~-Resistant Materials be a minimal problem. Combustion toxicity likewise is not expected to be a major problem for the same reason. Carbon monoxide, formed by the incomplete combustion of any organic material, appears to be He main killer, and its yield is dominantly controlled by fuel/air ratio and temperature (Hirschier et al., 19931. Reducing the amount of material burnt seems to be the key to low smoke and toxic gas formation. A particular point of view we think should be kept in mind is that visible smoke, as measured by a photocell, may not represent the true hazard in regard to aircraft cabin safely. Some vapors are perfectly transparent to the photocell, a typical example being acrolein (formed from some organics), but they may prevent vision by being extreme eye irritants and thus inhibit exit from an aircraft cabin. Fonnaalat~ng a Flame Retardancy and Smoke Suppression "Package" The large number of different modes of action for flame retardants (and smoke suppressants) has the practical importance that these modes can be combined, frequently with additive results, and in quite a few cases with synergistic results (Well, 1975, 1992a). Building up a flame retardant/Iow smoke formulation by using combinations of the available "tools" is facilitated by modern software for experiment design and formula development. A combination of theory, working hypotheses, and statistically planned expenmentacion seems to us to be the most cost-effective way to meet any advanced performance goal. The Structural Aspect of Flame Retar~ncy Layer Effects Aircraft manufacturers are accustomed to using multilayered panels. It may be sufficient to have one layer that is extraordinarily fire-resistant, provided that the other layers do not defeat this effect. By the strategy of using a "super-f.r. layer," the desired mechanical, aesthetic, and processing properties of the overall structure need' not be unduly sacrificed. Any compositions that the proposed research program develops obviously need to be tested in layered structures with other likely materials. SUMMARY As an alternative or as a supplement to finding totally new advanced polymers, it seems to us that an effective strategy for attaining very high performance goals is to try to enhance the performance of the best plastics, or even the "next best" plastics, through use of relatively low levels of functional additives. The use of multiple additives with differing but cooperating modes of activity in optimized combinations found through experimental design seems likely to be productive. Additive types are suggested based on reflectance, endothermicity in the condensed phase, endothermicity in the gas phase, inert gas emission, inhibition of decomposition reactions, char
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Edward D. Well 143 quantity enhancement, rate of char enhancement, char strengthening, catalysis of pre-char chemistry, cross-linking, formation of solid noncarbon barriers, enhancement of the strength of carbonaceous and noncarbon barriers, coating of char, and inhibition of the oxidation of carbonaceous barriers. There is some precedent for each of these actions, and optimized combinations of these actions should be possible. REFERENCES Anderson, C.E., I. Dziuk, W.A. Mallow, and I. Buckmaster. 1985. Intumescent reaction mechanisms. Journal of Fire Sciences 3: 161-194. Ballistren, A., G. Montaudo, E. Scampornno, C. Puglisi, D. Vitalini, and S. Cucinella. 1988. Tntumescent flame retardants for polymers. IV. The polycarbonate-aromatic sulfonates system. Journal of Polymer Science, Part A: Polymer Chemistry 26:2113-2127. Barker, R.H., and I.E. Hendrix. 1979. Flame retardance of cotton. Pp. I-65 in Flame Retardancy of Polymeric Materials, Vol. 5. W.C. Kuryla and A.~. Papa, eds. New York: Marce! Dekker, Inc. Batt, A.M., and P. Appleyard. 1989. The mechanism and performance of combustion modified flexible foams in small scale fire tests. Journal of Fire Sciences 7:338-363. Bertelli, G., G. Camino, E. Marchetti, L. Costa, and R. Locatelli. 1989. Structural studies on chars from fire retardant intumescent systems. Die Angewandte Malcromolekulare Chemie 169:137-142. Brau man, S. 1977a. Phosphorus flame retardance in polymers. I. General mode of action. Journal of Fire Retardant Chemistry 4:~-37. Brauman, S. 1977b. Phosphorus flame retardance in polymers. IT. Retardant-polymer substrate interactions. Journal of Fire Retardant Chemistry 4:38. Brauman, S. 1979a. Char-forming synthetic polymers. I. Combustion evaluation. Journal of Fire Retardant Chemistry 6:249-264. Brauman, S. 1979b. Fnedel-Crafts reagents as charring agents in impact polystyrene. Journal Of Polymer Science, Polymer Chemistry Edition 17:1129-1144. Buchert, H., G. Heinz, P. Itteman, M. Kopietz, J. Koch, W. Eberle, and H. Zeiner (to BASF). 1990. Temperature-resistant thermoplastic molding compounds. German Patent Application 3829712 (March 15, 1990~. Bucsi, A., and J. Rychly. 1992. A theoretical! approach to understanding the connection between ignitability and flammability parameters of organic polymers. Polymer Degradation and Stability 38:33-40. Bunker, B.C., G.W. Arnold, M. Rajaram, and D.E. Day. 1987. Corrosion of phosphorus oxynitride glasses in water and humid air. Journal of the American Ceramic Society 70(6):425-430. Carnahan, J., W. Haaf, G. Nelson, G. Lee, V. Abolins, and P. Shank. 1979. Paper presented at the 4th International Conference on Flammability and Safety, San Francisco, January. Chen, V., A. Frendi, S.S. Tewari, and M. Sibulkin. 1991. Effects of fire retardant addition on the combustion properties of a charting fuel. Pp. 527-536 in Fire Safety
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