The goal of the current fire-resistant materials research at the Federal Aviation Administration (FAA) is to provide an "order-of-magnitude" improvement in fire resistance compared with current materials. While the goal of an order-of-magnitude improvement is difficult to define considering the multitude of performance metrics and fire scenarios, the ultimate FAA goal to "eliminate fire as a cause of fatalities" (FAA, 1993:1) requires substantial improvements in materials. To achieve such an ambitious goal will require a more fundamental understanding of polymer burning processes and the effects of materials composition, structure, and properties on flammability.
This chapter describes the polymer combustion process and the factors influencing polymer flammability; identifies promising areas for materials research and development; describes factors that will affect selection and application of new materials in a manufacturing environment; and describes modeling techniques that could aid in the understanding of polymer combustion processes and molecular design.
Polymer combustion is a complex process consisting of a sequence of events including thermal degradation, char formation, transport of degradation products, ignition, and fire growth. An understanding of the factors that influence each event is critical in the development and characterization of new fire-resistant materials.
The amount of energy absorbed by a polymeric material exposed to an external heat source depends on the level and the spectral characteristics of the radiant flux, the in-depth absorption characteristics of the material, and its surface reflectance with respect to the emission spectrum of the incident radiation (Hallman et al., 1978; Kashiwagi, 1981). If the effective absorption coefficient of the material with respect to the thermal radiation is small, a large amount of the material beneath the surface is heated, which slows the rate at which the material approaches its degradation temperature range. However, if the effective absorption coefficient is large, most of the radiation is absorbed close to the surface, and a thin layer of the material is rapidly heated to its degradation temperature range.
When temperatures near the material surface become high, thermal degradation reactions occur and small gaseous degradation products are evolved. The majority of the evolved products are combustible, and their chemical composition depends on the chemical structure of the polymer combined with the degradation conditions. Although the global thermal degradation mechanisms of many different polymers are understood, detailed degradation reaction mechanisms are still not well understood, and degradation mechanisms are especially complex in the case of combinations of materials.
The thermal degradation mechanisms of thermally stable engineering plastics are difficult to understand in detail. Generally, these materials form crosslinks and condensed ring systems that lead to char during degradation. Although only a limited number of analytical methods are available to study changes in chemical structure of the polymer residues because charred or crosslinked samples do not dissolve in solvents, these have not been extensively used and much remains to be done.
Generally, polybutadiene polymers, polyacrylonitrile, poly (vinyl chloride) and many aromatic and heterocyclic backbone polymers can form char under the appropriate conditions. Common to the pyrolysis of all these polymers is the formation of conjugated multiple bonds, transition from a linear to a crosslinked structure, possible formation of ring structures, and an increase of the aromaticity of the polymer residue. General features of the pyrolysis and char of polymers containing aromatic carbon or heterocyclic links in the main chain of the polymer structure have been derived (van Krevelen, 1975; Aseeva and Zaikov, 1985). These features include:
thermal stability and char yield increase with the relative number of aromatic groups in the main chain per monomer unit of the polymer chain, and
thermal stability of heterocyclic polymers increases with the aromatic content of the heterocycles.
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft 4 Development of Candidate Materials for Future Interiors The goal of the current fire-resistant materials research at the Federal Aviation Administration (FAA) is to provide an "order-of-magnitude" improvement in fire resistance compared with current materials. While the goal of an order-of-magnitude improvement is difficult to define considering the multitude of performance metrics and fire scenarios, the ultimate FAA goal to "eliminate fire as a cause of fatalities" (FAA, 1993:1) requires substantial improvements in materials. To achieve such an ambitious goal will require a more fundamental understanding of polymer burning processes and the effects of materials composition, structure, and properties on flammability. This chapter describes the polymer combustion process and the factors influencing polymer flammability; identifies promising areas for materials research and development; describes factors that will affect selection and application of new materials in a manufacturing environment; and describes modeling techniques that could aid in the understanding of polymer combustion processes and molecular design. COMBUSTION OF POLYMERS Polymer combustion is a complex process consisting of a sequence of events including thermal degradation, char formation, transport of degradation products, ignition, and fire growth. An understanding of the factors that influence each event is critical in the development and characterization of new fire-resistant materials. The amount of energy absorbed by a polymeric material exposed to an external heat source depends on the level and the spectral characteristics of the radiant flux, the in-depth absorption characteristics of the material, and its surface reflectance with respect to the emission spectrum of the incident radiation (Hallman et al., 1978; Kashiwagi, 1981). If the effective absorption coefficient of the material with respect to the thermal radiation is small, a large amount of the material beneath the surface is heated, which slows the rate at which the material approaches its degradation temperature range. However, if the effective absorption coefficient is large, most of the radiation is absorbed close to the surface, and a thin layer of the material is rapidly heated to its degradation temperature range. Thermal Degradation When temperatures near the material surface become high, thermal degradation reactions occur and small gaseous degradation products are evolved. The majority of the evolved products are combustible, and their chemical composition depends on the chemical structure of the polymer combined with the degradation conditions. Although the global thermal degradation mechanisms of many different polymers are understood, detailed degradation reaction mechanisms are still not well understood, and degradation mechanisms are especially complex in the case of combinations of materials. The thermal degradation mechanisms of thermally stable engineering plastics are difficult to understand in detail. Generally, these materials form crosslinks and condensed ring systems that lead to char during degradation. Although only a limited number of analytical methods are available to study changes in chemical structure of the polymer residues because charred or crosslinked samples do not dissolve in solvents, these have not been extensively used and much remains to be done. Char Formation Generally, polybutadiene polymers, polyacrylonitrile, poly (vinyl chloride) and many aromatic and heterocyclic backbone polymers can form char under the appropriate conditions. Common to the pyrolysis of all these polymers is the formation of conjugated multiple bonds, transition from a linear to a crosslinked structure, possible formation of ring structures, and an increase of the aromaticity of the polymer residue. General features of the pyrolysis and char of polymers containing aromatic carbon or heterocyclic links in the main chain of the polymer structure have been derived (van Krevelen, 1975; Aseeva and Zaikov, 1985). These features include: thermal stability and char yield increase with the relative number of aromatic groups in the main chain per monomer unit of the polymer chain, and thermal stability of heterocyclic polymers increases with the aromatic content of the heterocycles.
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft Since char is mainly composed of carbon and hydrogen, decreased amounts of carbon and hydrogen are released from char-forming materials to the gas phase as combustible gaseous products. Furthermore, since thermal conductivity of char is generally much lower than that of a polymer, a char layer acts as an excellent thermal insulation layer to protect virgin polymer underneath and as barrier to the passage of these gases. However, the detailed chemical reaction steps to form char as well as the chemical structure of char are not well understood. Studies are urgently needed to understand these reactions so as to better enhance the char formation rate and its amount. Transport of Degradation Products As a thermal wave penetrates into the interior of a polymer, a highly complex generation and transport of degradation products occurs from the interior of the polymer outward through a strong viscosity gradient. The viscosity gradient has a significant influence on transport behavior. It appears that the transport process of the sub-surface degradation products supplies a majority of degradation products to the sample surface. For char-forming materials, sub-surface degradation products, which are dominant in this case, are transported through the many cracks that form in a hard and porous char. However, if an intumescent char is formed instead of hard, porous char, the transport of the sub-surface degradation products becomes more complex. The effectiveness of the thermal insulation of a char appears to depend on its physical structure. An intumescent, foamy char tends to have better insulation characteristics than a hard, brittle, dense char. An important macroscopic transport process for thermo-plastic polymers is melting and dripping during burning. Since an aircraft interior consists essentially of a floor, two walls, and a ceiling, if thermoplastic materials on the walls and the ceiling are heated to well above their glass transition temperatures or melt temperatures (for crystalline polymers) by the thermal radiation coming through an opening such as a door, there could be a significant amount of melting and dripping of interior materials. Although such behavior might be helpful in certain fire scenarios, here it appears to increase the hazard; melting and dripping from wall and ceiling panels may interfere with the evacuation of passengers and crew, as well as enhancing fire growth. Ignition Ignition occurs after sufficient amounts of combustible degradation products reach the gas phase. Generally, non-charring polymeric materials (e.g., polyolefins) degrade below 400°C which is too low to produce nonpiloted ignition (Kashiwagi, 1981). However, char-forming materials (e.g., phenolics) could reach a surface temperature high enough (due to low thermal conductivity of the char) to allow nonpiloted ignition. However, the limiting requirement to cause piloted ignition is a sufficient supply of gaseous, combustible degradation products. Therefore, piloted ignition tends to occur much earlier than nonpiloted ignition (Kashiwagi, 1981). Fire Growth After ignition, the growth of fire is determined by the flame-spread characteristics of materials. The total heat release rate of a fire is determined by the integral of the burning surface area times the local burning rate per unit surface area. Local heat release rates are the result of a complex coupling between condensed and gas-phase phenomena. Flame spread depends on continued generation and transport of degradation products to the flame. The generation rate of combustible degradation products is determined by the heat-and mass-transport processes and also by the chemical degradation reactions. DEVELOPMENT OF MATERIALS The relationship between materials chemical structure, composition, and fire performance is presently understood on a general, empirical basis (Weil, 1995; Wilkie, 1995). Previous sections of this report described the burning process as it applies to aircraft interiors and identified important properties and analysis methods to understand combustion and to evaluate materials performance. Based on this understanding of the critical issues and requirements and the structure performance relationships, approaches for significantly improving fire resistance by either affecting the pyrolysis rate or degradation product composition, or by changing the gas-phase reaction rates can be identified. This section outlines potential advances in fire-safe materials, including improvements in organic materials, organic/inorganic materials, inorganic preceramic polymers, innovative material systems, and advanced additive approaches. In addition to the advances described in this report, future prospects for improved fire-resistant materials are described in the proceedings of the conference that the committee hosted (NRC, 1995). Reviews are included that describe research and trends in fire resistant polymers (Wilkie, 1995), additive concepts (Weil, 1995), and inorganic and organometallic polymers (Zeldin, 1995). Approach Research to develop a fundamental understanding of chemical structure is most likely to contribute to the development of more-fire-resistant products and reduced flammability
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft characteristics of aircraft interior materials (Sorathia and Beck, 1995). A number of fundamental properties of polymeric materials are important in determining behavior under various fire conditions (Pearce, 1986; Troitzsch, 1990; FAA, 1993; Sorathia and Beck, 1993; Tewarson, 1993; Wilkie, 1993; Lyon, 1994; Nelson, 1995). Among the important physical properties are the morphology (especially the degree of crystallinity), crystalline melting temperature (Tm), glass transition temperature (Tg), thermal conductivity, density, and thermal capacity. Chemical factors include the heat of combustion, the heat of gasification, the minimum surface temperature at which a self-sustaining flame can be established, the products of decomposition, the tendency of the material to form char, and the heat release rate. As described earlier in this chapter, materials that form a char can produce a barrier at the early stage of burning which can insulate the interior material from further decomposition. To be effective, an adherent thermally insulating layer of char must form on the surface of the polymer to retard the feedback of thermal energy to the underlying material and effectively retard its burning. Char formation offers the opportunity of generalizing fire resistance among a variety of polymeric materials. An important goal should be to develop as much mechanistic information as possible about chemical reactions and physical processes that produce effective char. Also, the effect of char formation on toxicity needs to be characterized. There are three approaches to developing improved fire-resistant polymers. First, engineering polymers, including thermoplastics such as polycarbonate and polyamides (nylon 6, 6 and nylon 6) and thermosets such as unsaturated or vinyl polyester and phenolic could be improved with additives, coatings, or intumescent agents. This could be accomplished in the near term and may be the lowest cost approach, but it is not clear that the significant improvements sought could be attained. Second, further understanding of the degradation of current high-temperature specialty polymers could lead to new approaches for enhancing the yield and mechanical integrity of char, which usually results in improved fire resistance. Current specialty polymers include thermoplastics such as polytetrafluoroethylene copolymers, polyphenylene sulfide (e.g., Ryton®), polyarylene ether ketones (e.g., polyetheretherketone and polyetherketoneketone), related systems such as polyarylene ether sulfone (e.g., Radel®), liquid-crystalline polyesters, (e.g., Vectra ®), and the polyether imides (e.g., Ultem®). Thermosets such as cyanate ester, bismaleimide, certain polyimides, and polybenzimidazole are also classified as specialty polymers. The modification of current commercial specialty polymers could provide the best performance in the near term (within 10 years). Finally, new high-performance thermally stable materials, possibly composed of organic/inorganic systems, copolymers, including novel copolymers containing silicon or phosphorous (Nelson, 1995), polymer blends and alloys, and glasses, ceramics, and laminates could be developed to provide additional improvements in fire resistance. This approach has the potential to provide the greatest improvement in fire performance, but is considered a long-term (10 years) development. As described in Chapter 2, there is a wide range of material systems used to produce aircraft interior components, including fiber-reinforced polymeric composites, adhesives, decorative layers, thermoplastic molding compounds and formable sheets, textiles, rigid and flexible urethane foams, fibrous mats, acrylic or polycarbonate window transparencies, and elastomers. While this study focuses on the long-term challenge of developing better fire-resistant materials, the ultimate challenge is not just to develop improved polymers, but to also develop a range of products to produce components that are applicable in interiors applications. It will be important to identify alternate markets for new fire-resistant materials before committing to a development program. The small size of the potential market for materials solely for use in aircraft interior components often does not justify the expense to the suppliers of development and qualification. When asked by the committee to identify drivers for (and barriers against) the development and application of improved fire-resistant materials, industry representatives indicated that "the high cost of qualification and certification of a new material for aircraft applications makes embarking on a material development and implementation program risky for both the materials supplier and the aircraft manufacturer, and without alternative uses for new developments to increase utilization, justifying development of new materials for the limited market will be difficult" (NRC, 1995:238). Thermosets The performance, processibility, and light weight of honeycomb sandwich structures produced from polymeric composites that use thermosetting resin matrices have made them the material of choice in most current cabin and cargo liner applications. Prior to the development of the 1980s flammability regulations, the predominant matrix materials were epoxy-based. However, the fire resistance of conventional epoxy networks was not adequate for conformance with interior heat release regulations. To meet the 1990 heat release requirements (65/65) described in Chapter 2, materials suppliers and manufacturers developed phenolic matrix systems with increased char yield relative to previous phenolic systems and the incumbent epoxy systems that provided flammability and heat release characteristics in compliance with the regulations. Phenolic systems, however, exhibit inferior mechanical properties, some difficulties in processing due to the evolution of volatiles such as formaldehyde, and production concerns centered
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft around the health and safety of workers handling the uncured resin. Alternative cure systems (perhaps epoxy novolacs) could be considered. Future developments in thermosetting organic-matrix composites will focus on improved methods of production, increased strength and toughness, and improved flammability characteristics (primarily through structural variations that increase char yield). Multifunctional cyanate ester systems based on phenol-formaldehyde backbones are currently available and are finding increased usage, especially in electronic components (due to their low dielectric loss). They cure without generation of volatiles and exhibit exceptional thermal stability and high char yield (Das et al., 1990). Related systems have also been toughened to improve fracture strength (Srinivasan et al., 1994). Like cyanate ester systems, high-performance thermosetting polymers, including bismaleimides and polyimides, and polybenzimidazoles promise very good fire resistance due to their thermal stability and high char yields. However, high processing temperatures, long cycle times, and high monomer costs detract from their commercial potential and their utility. Also, for thermosetting systems, high-temperature stability is gained at the expense of resin toughness. The mechanical properties and durability reflect the brittle nature of many of these systems. Research continues on approaches to toughening high-temperature thermosets through the addition of a discrete second phase, or cure sites that allow chain extension, or development of an interpenetrating network containing high-temperature thermoplastics. Thermoplastics Some high-performance thermoplastics, such as the poly (arylene ether sulfones), polyetherimide, polyphenylene sulfide, and the poly (arylene ether ketones) (e.g., polyetheretherketone, polyetherketoneketone), provide attractive fire resistance due to their high char yields and low heat release rates. These materials have found use on current commercial aircraft, particularly in injection molding and thermoformed sheet product forms. The use of thermoplastic matrix composites has the potential to provide outstanding fire resistance with greater durability than thermosets, without a weight penalty (Diehl, 1993). There are potential processing advantages, including improved surface finish, shorter process cycles, and the ability to form colored and textured surfaces without additional steps. However, the application of high-performance thermoplastic composites has been limited primarily by their high cost, but also by their high melt viscosity, and high processing temperatures and pressures. Current high-performance thermoplastics exhibit outstanding fire resistance without additives (Lyon, 1994). Further improvements may be possible through the use of fire-retarding additives or highly fire-resistant thin films or coatings. Innovative additive and coating approaches are described later in this chapter. Organic/Inorganic Polymers Organic/inorganic polymers, wherein the inorganic portion contains elements such as silicon, phosphorus, and sulfur, could develop into useful fire-resistant materials. The systems that are important here include those that contain phosphorus, silicon, nitrogen, and perhaps metal containing polymers. The use of silicon has already been demonstrated to some extent. It is known that if one synthesizes polycarbonate siloxane block or segmented copolymers, or polyimide siloxane block or graft copolymers, and exposes them to high temperatures, the resulting materials show significant char that is thought to be a residual organosilicate (Noshay and McGrath, 1977). Another suggested mechanism is that certain of the segmented siloxane copolymers can undergo intumescence, allowing for a volume expansion near the fire source. The relatively stable residue of intumescent char can greatly retard the further entry of the flame to the bulk. Organophosphorus chemistry is another viable route to nonhalogen containing polymeric materials with high char yield and inherent high flame resistance. There are many examples of the phosphorus-containing systems that have produced desirable effects (Deshpande et al., 1970; Zeldin et al., 1981). Phosphine-oxide containing thermoplastic polymers have been demonstrated to be also outstanding candidates for hydrolytically stable resins with char yield (Smith et al., 1991). This approach has been extended recently to polycarbonates and polyamides (Knauss, 1994; Wan, 1994; Nelson, 1995). Mechanistic studies are needed to determine the role of in-chain inorganic systems such as phosphorus or silicon to allow for full realization of the potential of this approach. Organic/inorganic hybrids including ketone-phosphorus combinations (Calvert, 1994), as well as copolymers containing phosphorus and sulfur and polyimides containing nitrogen and phosphorus, also show promise. These materials can be synthesized from a variety of different metal alkoxides and organic polymers to create a very fine morphological structure wherein both inorganic oxides and inorganic polymers can coexist. These materials have been referred to as ceramers (Wilkes et al., 1990), ormosils or ormocers (Schmidt, 1988), and polycerams (Boulton et al., 1990). Most of the work to date has been with silicon alkoxides in combination with functionalized short-chain oligomers. Oxides of titanium and zirconium have also been investigated. Further work in this area could lead to important new fire-safe materials. At this point, the high modulus materials have been generated mostly at the expense of developing extreme brittleness (Coltrain
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft et al., 1993). However, there have been isolated reports of siloxane-modified silicates (Spinu and McGrath, 1994) that exhibit more attractive mechanical properties. Novel blends of phosphate glass with engineering polymer that have similar melt characteristics have been produced (Quinn and Beall, 1992). These materials are intended to have the capability to be processed using standard thermoplastic processes. Work is needed to evaluate the performance and processibility of these materials. Inorganic Materials Highly inorganic materials, including inorganic and organometallic polymers offer substantially improved thermal stability and fire resistance compared with more conventional systems. Examples of inorganic and preceramic polymers include polysiloxanes, polysilazanes, polycarbosilanes, polysilanes, and polyphosphazenes (Zeldin, 1995). These polymers have the potential for extremely fire-resistant behavior, however the high cost and limited processibility have restricted their use to specialty areas such as electronic parts. While the potential for long-term benefit is great, it is not clear how soon they could be developed for practical applications in aircraft interiors. Application of inorganic or organometallic polymers may depend on advanced concepts such as thin films or blends as described in the following section. New and Modified Materials Since cost is such a critical issue in materials applications on commercial transports, technologies need to be developed to make advanced concepts for enhanced fire performance, such as thin films, blends, coatings, intelligent materials and nanostructures, more practical. One promising physical approach is the use of a protective coating over a polymer substrate. The coating could be a highly thermally stable polymer layer (e.g., inorganic polymers) or one that generates an intumescent char layer when exposed to external heat. The thickness of the coating and its adhesion to the base material would determine how long it protects the bulk polymer substrate. A similar concept is to include a heat-sink additive such as aluminum trihydrate using imbedded microcapsules that are micron size (Khalturinskii and Berlin, 1990). Water or flame retardants can be included inside of the capsules. When a polymer having a large number of such capsules is heated to elevated temperatures, the capsules burst and release their ingredients into the flame. The sudden release of water or flame retardants tends to blow off the flame or extinguish it. Since the size of the capsules is so small, their effect on the physical properties of the original polymer is claimed to be negligible. ''Intelligent'' materials approaches may provide a means to selectively change the properties of a material to provide improved fire performance. For example, a crosslinking reaction, initiated at a predetermined, elevated temperature could greatly improve thermal stability and char formation. One such material could be polyimides with a phenylethynyl cure site that does not exothermically react until about 350 or 400°C (Meyer et al., 1994). Another promising new approach is the use of nanostructured materials. Generally, materials with grains or particles 1–100 nanometers across, or with layers or filaments of similar thickness, are considered nanostructured materials. These materials can lead to dramatically improved or altered mechanical, optical, and magnetic properties. The properties of nanostructured materials are determined by a complex interplay among the building blocks and the interfaces between them. While the mechanism of these interactions are not clearly understood, it has been reported that when molecules are intercalated in silicates, their thermal and oxidative stability increases dramatically (Dagani, 1992). The molecules' confinement inside the ceramic lattice apparently protects them from engaging in degradative behavior. It is not clear whether the flammability properties of polymers blended with such nanostructured materials are significantly affected, and feasibility studies are urgently needed. Since this approach does not depend on the polymer base, there might be significant potential. Flame-Retardant Additives Flame-retardant (FR) additives have been extensively used to decrease polymer flammability. This is the most common approach since it does not usually affect the in-plant commercial polymer manufacturing procedure. The additives are usually mixed with the manufactured polymer, blended, and then produced as a flammability retarded product (Pearce, 1986). Various mechanisms for reducing polymer flammability by this route have been reviewed (Pearce, 1986). These mechanisms included volatile-phase active FRs that inhibit the combustion process, condensed-phase active FRs that lead to char or intumescence, and FRs that endothermically lose volatile components (e.g., H2O). FR additives may function by one or more of these mechanisms in a particular polymer system. To date, most FR additive systems have incorporated either chlorine or bromine, phosphorous, antimony or boron-related compounds, or inorganic hydrates such as alumina trihydrate. Relatively large amounts of FR are required, and there can be large changes in the balance of properties dependent on whether the FR behaves as a plasticizer or a filler. The search for newer FR systems concentrates on using smaller amounts of FR to allow retention of overall properties of the original
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft polymer; replacement of certain vapor-phase modifying systems such as halogen compounds, since halogen compounds can produce eye and lung irritation during a fire and also cause corrosion problems and post-exposure toxic effects; and decreasing formation of volatile components by increasing condensed-phase reactions leading to enhancement of char and intumescence (Whang and Pearce, 1990). An ideal FR additive/blend is one that helps to form crosslinks and to enhance cyclization in an originally linear polymer (easier processing) at temperatures well above processing temperature, thus forming char. The mechanisms through which char layers improve fire resistance were described earlier in this chapter. The effectiveness of the thermal insulation of char appears to depend on its physical structure. An intumescent, foamy char consisting of numerous small bubbles tends to have better insulation characteristics than a hard, brittle, dense char. However, char-enhancing FR tend to depend on the polymer structure, and their effectiveness is not as nearly universal as halogenated FR. Since currently used polymers in the aircraft interior already form a certain amount of char, the enhancement of char amount and the formation of the desirable foamy char in these polymers might not be as difficult as in the case for polyolefin polymers which do not generally form char. However, the level of understanding of thermal degradation mechanisms of engineering plastics is much less than that for polyolefins. This lack of understanding is caused in part by the lack of many analytical tools to characterize the chemical structure of polymer residues that do not normally dissolve in solvents. However, there are techniques such as Fourier-transform infrared spectroscopy and solid-state nuclear magnetic resonance that could be applied to these systems and coupled with microscopy studies. These approaches and other new FR approaches based on reflectance, endothermicity in the condensed phase, endothermicity in the gas phase, inert gas emission, inhibition of decomposition reactions, char quantity enhancement, rate of char enhancement, char strengthening, catalysis of pre-char chemistry, crosslinking, 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 hold the possibility for significantly improved fire resistance (Weil, 1995). Hybrid approaches or synergistic approaches that use multiple additives with differing but cooperating modes of activity in optimized combinations should also be explored (Weil, 1995). Toxicant Suppressants A toxicant suppressant is a chemical that, when added to a combustible material, significantly reduces or prevents one or more toxic gases from being generated during thermal decomposition. The resultant gas effluent should be less toxic than that from the untreated material (i.e., the toxic gas whose concentration is being lowered should not be converted to an equally or more toxic product). Toxicant suppressants are a relatively new concept arising from work demonstrating that the addition of relatively small amounts (0.1–1 percent by weight) of copper compounds to flexible polyurethane (FPU) foam significantly reduced the generation of hydrogen cyanide (HCN), as well as the overall toxicity of the combustion products, when the foam was thermally decomposed (Levin et al., 1988, 1990, 1992). The copper compounds could be added to the foam during formulation without changing the flammability characteristics or physical properties of the foam. Small-scale as well as full-scale room fire tests showed that the toxicant suppressant reduced HCN levels. The use of melamine-treated FPU is becoming more common because melamine is nonhalogen, inexpensive, and has little effect on processing (Weil and Choudhary, 1995; Weil and Zhu, 1995). It is one of two FPU foams currently allowed in Great Britain. Small-scale tests indicated that a melamine-treated FPU generated at least six times more HCN than an equal amount of a non-melamine-treated foam (Braun et al., 1990, 1991; Levin et al., 1992). The presence of Cu2O reduced the HCN from the melamine-treated foam by 90 percent. In other experiments, a wool fabric treated with copper generated 50 percent less HCN than the untreated fabric (B.C. Levin, unpublished data, 1995). Smaller-scale work also showed that the concentrations of HCN generated from the thermal decomposition of thin films of polyurethane at 300 and 400°C decreased when flowed through copper metal films (Jellinek and Takada, 1977; Jellinek et al., 1978). These experiments indicated that the copper is probably acting as an oxidative catalyst which would decompose gaseous HCN into N2, CO2, H2O, and small amounts of nitrogen oxides. Further research is needed to validate this molecular mechanism. The FPU and wool results suggest a more universal effect, namely that treating nitrogen-containing materials with copper compounds could reduce the HCN generated when those materials are exposed to fire conditions. Taking these results one step further, one could develop other toxicant suppressants which when added to materials and products would now prevent or significantly reduce the toxic effluents that are generated when they are exposed to a fire environment. MANUFACTURING In addition to the physical and mechanical properties described in Chapter 2 and fire-resistance and toxicity properties described in Chapter 3, improved materials must exhibit processibility and durability necessary for aircraft application and service. Since interior materials do not contribute to the flight
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft performance of the aircraft, component cost, especially life-cycle costs, are critical to their application. New materials must be compatible with manufacturing processes and health and safety considerations (from its initial manufacturing to its final disposal), be available in a variety of product forms, and be cost competitive to be considered for commercial aircraft applications. Manufacturing Processes While longer-term developments may have more processing flexibility, new materials developed for aircraft interior applications in the near term need to be compatible with the current methods and equipment. A large investment has been made in manufacturing infrastructure, including automated cutters, presses and autoclaves, and machining equipment. For the most part, the existing methods and equipment offer significant flexibility, allowing a range of materials and constructions to be processed. For near-term applications, developers of new materials must recognize and consider the limitations imposed by existing capabilities. By far the most prevalent construction found in current aircraft interiors is composite face skins bonded to nonmetallic (usually aramid-reinforced) honeycomb core (Berg, 1995). The extreme light weight of this type of construction will be hard to match with alternate constructions. Hence, thin-skinned honeycomb panels will likely remain the construction of choice for stowage bins and cabin and cargo liner applications. Manufacturing processes of the future will emphasize cost reduction. Cost reductions in the autoclave or press molding processes currently used in the aircraft industry will likely result from decreasing the high labor intensity of manufacturing steps, including ply preparation (kitting), orientation (layup), molding, surfacing, bonding of decorative surfaces, and part trim. Automation of cutting and ply location processes have shown promise, and improvements could further reduce the cost of these operations. For molding and decorating operations, reduction in the number of process steps and decreasing cycle times hold the most promise. For example, combining molding and application of decorative layers would not only eliminate a bonding cycle, but would preclude the need for highly labor-intensive sanding and surfacing operations currently used to produce a smooth surface. Low-temperature-curing polymers, quick-cure systems, or alternate heat sources (e.g., electromagnetic) could reduce costs by increasing throughput, decreasing energy consumption, simplifying in-process repair, or eliminating the need for part staging to optimize equipment use. Finally, net or near-net shape molding processes could reduce material scrap and eliminate or reduce machining and trim operations. Unreinforced thermoplastics, including polyetherimide (Ultem), polyetherketoneketone (Declar), and polyarylene ether sulfone (Radel), already find wide use in nonstructural components of current aircraft interiors due to their fire resistance and the advantages of thermoplastic processes, especially surface finish and process cycle time. The most common processes are sheet thermoforming and injection molding. Developing continuous-fiber reinforced thermoplastic composites to take advantage of thermoplastic processing advantages could provide significant savings in manufacturing costs in future aircraft (Guard and Peterson, 1993; Guard, 1994). Thermoplastic composites could offer the following advantages in future production (Diehi, 1993): cheaper tooling, especially for the short production runs typical of commercial aircraft; more versatile production methods; short production cycles; elimination of hand finishing; more durable parts without weight penalty; integral color, pattern, and texture; potential for recycling; and better specific fire behavior without loss of durability or appearance. In general, thermoplastic composites have failed to realize their potential. Most of the manufacturing processing difficulties associated with thermoplastic composites, especially in the production of honeycomb sandwich constructions, result from the high temperatures and pressure required to consolidate these high-viscosity materials. Development of manufacturing processes and compatible materials for co-bonding, secondary bonding, and decorative processes are required before the full potential of thermoplastics can be realized. Environmental and Health Considerations The environmental impacts of new fire-resistant or firesafe materials need to be characterized. The historical approach has been to examine the effects of any new process or product on the environment and install mechanisms to mitigate pollution or to clean up the already polluted environment. More recently, environmental issues have been addressed during the design and development phases. There are myriad government and industry programs that emphasize the "design for the environment" approach (OTA, 1992). There are two aspects to consider in the examination of new chemical compounds and products: the effect of the new product on the environment, including adverse effects of the product' s manufacture, byproducts, waste products, or disposal on the environment (air, land, or waters); and the effect of the product's manufacture, byproducts, and disposal on the health and well-being of the humans
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft who are working with, using, or living in the vicinity of the plant making the product or its byproducts or disposal. Environmental and health and safety issues that influence materials and processes for aircraft interiors include the restrictions on the release of volatile organic compounds, the release of solid and liquid wastes, and the use of toxic or irritant chemicals. Limitations of the release of volatile organic compounds affects materials and processes, including paints and finishes, cleaning and surface preparation, and adhesive bonding. The need to reduce hazardous solid and liquid wastes has significant influence on the disposal of carbon-fiber composite materials,1 heavy metal compounds such as chromium and cadmium that are used in corrosion prevention finishes,2 and solutions from manufacturing processes such as cleaning and surface preparation. The complete evaluation of the toxicity or irritancy of a compound can take as long as five years and is quite costly. The compound may present a concern if: the substance may cause carcinogenic effects based ontests or similarity to known carcinogens; the substance may cause acutely toxic effects based ontests or similarity to known toxicants; the substance may cause serious chronic effects, serious acute effects, or developmentally toxic effects under reasonably anticipated conditions; the substance may cause significant adverse environmental effects under reasonably anticipated exposures; or concern exists about the health or environmental effects of one or more impurities or byproducts of the substance. All of the health and safety issues have to be weighted in relation to the potential exposure of individuals (i.e., risk assessment). For example, a new material could be toxic and still be acceptable if exposure can be minimized by taking the appropriate actions during its manufacture and use. The toxicity has to be assessed, however, to determine the extent to which such actions are necessary. THEORETICAL MODELING OF POLYMER COMBUSTION Theoretical modeling can be used to predict the influence of material structure on fire performance. A broad range of models of combustion processes have been developed on scales from molecular to intermediate-and full-scale compartment models described in Chapter 3 and Appendix C. The theoretical models discussed in the following sections describe many different aspects of polymer combustion outlined earlier in this chapter. Together, these models could aid in the understanding of polymer combustion processes and in the molecular design of more fire-resistant materials. Heat Transfer Models One-dimensional heat transfer models including representation of in-depth absorption in the material are currently available. However, the heat transfer process from the hot surface to the interior of a honeycomb aircraft interior component is very complex (heat conduction occurs through the polymer resin layer, but convective plus radiative heat transfer occur in the honeycomb cells), and an accurate calculation methodology should be developed. Thermal Degradation Models The details of the thermal degradation of relatively simple polymers such as polyethylene, polystyrene, and poly (methylmethacrylate) have been studied using molecular dynamic calculations (Nyden and Noid, 1991; Nyden et al., 1992), the Monte Carlo method (Guaita and Chiantoe, 1985; Guaita et al., 1985), and kinetic calculations (Boyd, 1970; Kashiwagi et al., 1989). Molecular dynamics is a computational technique that has been used to simulate the reaction kinetics of small gas-phase molecules, as well as to study conformational motions in synthetic and natural polymers. More recently, this technique has been applied to investigate the thermal degradation of vinyl polymers and to the design of flame resistant polymers (Nyden and Noid, 1991; Nyden et al., 1992). Although molecular dynamic calculations require a large, high-speed supercomputer, the results simulate the actual dynamic behavior of polymer chains. Molecular dynamics can provide a realistic description of the thermal degradation of polymers and should, therefore, aid in the development of fire-retardant treatments for these materials. Future work will extend these techniques to the combustion of more complex and thermally stable systems such as those used for aircraft interiors. 1 Carbon-fiber composite materials are disposed of as a hazardous solid waste to avoid incineration. Airborne carbon fibers are considered a threat to electrical systems. 2 Chromium compounds are used in primers in most painted components as well as on metallic support structures, seat tracks, and lavatories. Chromium and cadmium compounds are used in fastener treatments. While the total amount of material may be small, restrictions put on these materials could have a significant influence on future materials selection and development programs.
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft The Monte Carlo method uses a probability analysis on each set of polymer fractions which consist of a different number of monomeric units. The total set of fractions is equal to the molecular-size distribution of the polymer sample. The change in population in each set is calculated at each time step. Small predetermined macromolecular units, of a predetermined size, that result from the analysis are considered to be evolvable and are counted as lost weight. Kinetic methods calculate changes in the concentration of polymer molecules and also in the concentration of polymer radicals for each degree of polymerization from the monomer unit to the longest chain. The rate of change in the concentration of each species is expressed by an ordinary differential equation. Numerous differential equations (up to 10,000) are coupled, generally requiring numerical calculations to provide solutions except for specific cases where analytical solutions were derived from approximations. These approximations are based on knowledge of the degradation mechanisms and kinetic expressions for rates of chain initiation reaction, depolymerization, termination reactions, and any other relevant reactions (Boyd, 1970). All these calculations are based on an imposed, spatially uniform temperature and assume no transport processes are involved. There are no theoretical degradation models available that describe the formation of char. Transport Models The complex transport processes of degradation products through materials described earlier in this chapter, have been ignored in existing models. Instead, it has been assumed that the degradation products generated in the sub-surface region are instantaneously transported to the sample surface. More progress is needed in modeling complex transport processes. At present, there are no theoretical models to describe polymer melting and dripping during burning. One needs to understand when melting happens, how fast polymer melt flows, when dripping occurs, how fast dripping occurs, and how much drips. Such modeling is urgently needed to estimate the importance of melt flow and dripping in fire growth. For the overall modeling of the gasification process, current models generally consist of a time-dependent heat conduction equation with a one-step global degradation reaction. The degradation reaction is approximated at the surface or in the bulk of the polymer. Although mass transport of degradation products from the inside of the sample to the sample surface has been included for wood and bubble growth of coal, these processes are hardly included or even considered for the gasification of polymers except in works of Wichman (1986). Mass transport models for polymers should include nucleation and growth of bubbles and bubble transport processes created by melt viscosity gradient, surface tension gradients, and gravity. Inclusion of such processes, with the addition of char-forming reactions, is also needed for an intumescent char model. Ignition Models Generalized ignition processes are reasonably well understood, and two different types of theoretical models are available. Simplified models, the so-called "thermal models," are based on the assumption that the heat-up time of the material is the rate-controlling step and that the chemical reaction time is much shorter. In such a model, an ignition delay time is expressed as the time when the surface temperature of the material reaches its pyrolysis temperature which is taken to be a fixed value. Ignition delay time is correlated inversely with the square of the thermal radiant flux, providing that degradation occurs at a nearly fixed temperature.3 For a char-forming material, ignition delay time could still be correlated inversely with the nth power of the thermal radiant flux by selecting an appropriate, inferred degradation temperature. Generally, these correlations are derived using experimentally measured results, and the validity of the correlations is limited to the experimental conditions. More-detailed theoretical ignition models solving energy, species equations in the gas phase, and an energy equation in the condensed phase have been developed for the auto-ignition mode (no pilot ignition) in a one-dimensional configuration (Kashiwagi, 1974), at a stagnation point (Amos and Fernandez-Pello, 1988), or for piloted ignition in a boundary layer (Tzeng et al., 1990). However, it is still extremely difficult to quantitatively predict ignition delay time at a specified external radiant flux starting with only information on the chemical structure of a polymeric material. The more complicated models require more-detailed information on thermal and physical properties, especially regarding chemical reactions and their rates. Therefore, it would be unrealistic to attempt to quantitatively predict the ignition characteristics of materials currently used in an aircraft because information on detailed chemical composition of the materials is generally not available. However, it is feasible and desirable to use detailed models as a guideline to improve ignition characteristics during the development of a fire-resistant polymer of known composition. Fire-Growth Models After ignition, the growth of fire is determined by the flame-spread characteristics of materials. The most studied case is for well-defined ignition at the bottom of a flat 3 This condition might be attained for non-char-forming polymeric materials having a high activation energy for a global one-step degradation reaction such as certain poly (methylmethacrylate) polymers.
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft vertical wall and flame spread upward. A large amount of experimental data and theoretical models are currently available that correlate flame-spread characteristics with materials flammability properties (Hasemi, 1986; Saito et al., 1989; Cleary and Quintiere, 1991). The calculated results show a reasonable agreement with the experimental data (Cleary and Quintiere, 1991). However, the effects of melting or dripping on upward flame spread might be important if thermoplastics are used. Another important flame-spread configuration for an airplane is flame spread under the ceiling. The limited studies that are available (Agrawal and Atreya, 1992; Atreya and Mekki, 1992) show that unusual flame instability might occur in this configuration. It is extremely difficult to predict the upward flame-spread process if only the chemical structure of the material and its dimensions are provided. Even more-detailed flame-spread models can be developed, including detailed chemical reactions, but their predictive capability would be still semi-quantitative. Flame-spread and burning characteristics of upholstered furniture have been extensively studied. A theoretical model of fire growth on an upholstered chair, based on detailed radiative shape-factor calculation between the burning area and unburned region with empirical correlation for flame spread, has been reported (Dietenberger, 1992). However, the validity of the model and its accuracy have not yet been well established. These data and models might be of use for aircraft seats to describe how these seats bum in the event of a fire accident. It is important to be able to predict the occurrence of flashover based on calculated local heat release rates. Given the heat feedback rate from a flame to the material surface, the generation rate of combustible degradation products is determined from the processes of heat-and mass-transport rates processes and also from degradation chemical reaction kinetics. Given the rate of supply of the combustible degradation products and their chemical composition, characteristics of the flame such as flame height and heat release rate are determined. Thus, the direct coupling between the energy feedback rate and the rate of supply of the combustible degradation products determines local heat release rate per unit surface area. Experimental results show that a flame tends to become more optically opaque with increases in flame size, and radiative feedback dominates over convective feedback; also radiative feedback is enhanced for a flame generated by the degradation products having higher aromatic content. At present, it is extremely difficult to calculate radiative transfer in a turbulent flame. There are still uncertainties in how to accurately model turbulence, formation and destruction of soot particulates (the radiation source), and how to calculate radiative heat transfer efficiently. Since fire in an aircraft cabin is affected by interaction with its surroundings, such as air entrainment through openings or by the interaction of hot ceiling and walls with burning items, compartment fire models described in Chapter 3 and Appendix C are an important element in understanding an aircraft fire.