1–110
Introduction
Solid polymeric materials undergo both physical and chemical changes when heat is applied; this will usually result in undesirable changes to the properties of the material. A clear distinction needs to be made between thermal decomposition and thermal degradation. The American Society for Testing and Materials’ (ASTM) def-initions should provide helpful guidelines. Thermal de-composition is “a process of extensive chemical species change caud by heat.”1Thermal degradation is “a process whereby the action of heat or elevated tempera-ture on a material, product, or asmbly caus a loss of physical, mechanical, or electrical properties.”1In terms of fire, the important change is thermal decomposition,whereby the chemical decomposition of a solid material generates gaous fuel vapors, which can burn above the solid material. In order for the process to be lf-sustain-ing, it is necessary for the burning gas to feed back suf-ficient heat to the material to continue the production of gaous fuel vapors or volatiles. As such, the process can be a continuous feedback loop if the material continues burning. In that ca, heat transferred to the polymer caus the generation of flammable volatiles; the volatiles react with the oxygen in the air above the poly-mer to generate heat, and a part of this heat is transferred back to the polymer to continue the process. (See Figure 1-7.1.) This ch
apter is concerned with chemical and phys-ical aspects of thermal decomposition of polymers. The chemical process are responsible for the generation of flammable volatiles while physical changes, such as melt-ing and charring, can markedly alter the decomposition and burning characteristics of a material.
The gasification of polymers is generally much more complicated than that of flammable liquids. For most flammable liquids, the gasification process is simply evap-oration. The liquid evaporates at a rate required to main-tain the equilibrium vapor pressure above the liquid. In the ca of polymeric materials, the original material itlf is esntially involatile, and the quite large molecules must be broken down into smaller molecules that can va-porize. In most cas, a solid polymer breaks down into a variety of smaller molecular fragments made up of a num-ber of different chemical species. Hence, each of the frag-ments has a different equilibrium vapor pressure. The lighter of the molecular fragments will vaporize immedi-ately upon their creation while other heavier molecules will remain in the condend pha (solid or liquid) for some time. While remaining in the condend pha, the heavier molecules may undergo further decomposition to lighter fragments which are more easily vaporized. Some polymers break down completely so that virtually no solid residue remains. More often, however, not all the original fuel becomes fuel vapors since solid residues are l
eft be-hind. The residues can be carbonaceous (char), inor-ganic (originating from heteroatoms contained in the original polymer, either within the structure or as a result of additive incorporations), or a combination of both.Charring materials, such as wood, leave large fractions of the original carbon content as carbonaceous residue, often as a porous char. When thermal decomposition of deeper layers of such a material continues, the volatiles produced must pass through the char above them to reach the sur-face. During this travel, the hot char may cau condary reactions to occur in the volatiles. Carbonaceous chars can be intumescent layers, when appropriately formed, which slow down further thermal decomposition considerably.Inorganic residues, on the other hand, can form glassy lay-
Dr. Craig L. Beyler is the technical director of Hughes Associates,Fire Science and Engineering. He was the founding editor of the Journal of Fire Protection Engineering and rves on a wide range of committees in the fire rearch community.
Dr. Marcelo M. Hirschler is an independent consultant on fire safety with GBH International. He has over two decades of experience re-arching fire and polymers and has managed a plastics industry fire testing and rearch laboratory for ven years. He now rves on a variety of committees addressing the development of fire standards and codes, has published extensively, and is an associ
ate editor of the journal Fire and Materials.
SECTION ONE
CHAPTER 7
coronerThermal Decomposition
of Polymers
Craig L.Beyler and Marcelo M.Hirschler
ers that may then become impenetrable to volatiles and protect the underlying layers from any further thermal breakdown. Unless such inorganic barriers form, purely carbonaceous chars can always be burned by surface oxi-dation at higher temperatures.
As this brief description of the thermal decomposi-tion process indicates, the chemical process are varied and complex. The rate, mechanism, and product compo-sition of the thermal decomposition process depend both on the physical properties of the original material and on its chemical composition.
Polymeric Materials
Polymeric materials can be classified in a variety of ways.2First, polymers are often classified, bad on their origin, into natural and synthetic (and sometimes includ-ing a third category of minatural or synthetic modifica-tions of natural polymers). However, more uful is a classification bad on physical properties, in particular the elastic modulus and the degree of elongation. Follow-ing this criterion, polymers can be classified into elas-tomers, plastics, and fibers. Elastomers (or rubbers) are characterized by a long-range extensibility that is almost completely reversible at room temperature. Plastics have only partially reversible deformability, while fibers have very high tensile strength but low extensibility. Plastics can be further subdivided into thermoplastics (who de-formation at elevated temperatures is reversible) and thermots (which undergo irreversible changes when heated). Elastomers have elastic moduli between 105and 106N/m2, while plastics have moduli between 107and 108N/m2, and fibers have moduli between 109and 1010 N/m2. In terms of the elongation, elastomers can be stretched roughly up to 500 to 1000 percent, plastics be-tween 100 to 200 percent, and fibers only 10 to 30 percent before fracture of the material is complete.
Polymers can also be classified in terms of their chemical composition; this gives a very important in
dica-tion as to their reactivity, including their mechanism of thermal decomposition and their fire performance.
The main carbonaceous polymers with no het-eroatoms are polyolefins, polydienes, and aromatic hydro-carbon polymers (typically styrenics). The main poly-olefins are thermoplastics: polyethylene [repeating unit: >(CH2>CH2)>] and polypropylene {repeating unit: >[CH(CH3)>CH2]>}, which are two of the three most widely ud synthetic polymers. Polydienes are generally elastomeric and contain one double bond per repeating unit. Other than polyisoprene (which can be synthetic or natural, e.g., natural rubber) and polybutadiene (ud mostly as substitutes for rubber), most other polydienes are ud as copolymers or blends with other materials [e.g., in ABS (acrylonitrile butadiene styrene terpolymers), SBR (styrene butadiene rubbers), MBS (methyl methacry-late butadiene styrene terpolymers), and EPDM (ethylene propylene diene rubbers)]. They are primarily ud for their high abrasion resistance and high impact strength. The most important aromatic hydrocarbon polymers are bad on polystyrene {repeating unit: >[CH(phenyl)> CH2]>}. It is extensively ud as a foam and as a plastic for injection-molded articles. A number of styrenic copoly-mers also have tremendous usage, e.g., principally, ABS, styrene acrylonitrile polymers (SAN), and MBS.
The most important oxygen-containing polymers are cellulosics, polyacrylics, and polyesters. Polyacrylics are the only major oxygen-containing polymers with carbon–carbon chains. The most important oxygen-containing natural materials are cellulosics, mostly wood and pa-per products. Different grades of wood contain 20 to 50percent cellulo. The most widely ud polyacrylic is poly(methyl methacrylate) (PMMA) {repeating unit: >[CH2>C(CH3)>CO–OCH3]>}. PMMA is valued for its high light transmittance, dyeability, and transparency. The most important polyesters are manufactured from glycols, for example, polyethylene terephthalate (PET) or poly-butylene terephthalate (PBT), or from biphenol A(poly-carbonate). They are ud as engineering thermoplastics, as fibers, for injection-molded articles, and unbreakable replacements for glass. Other oxygenated polymers in-clude phenolic resins (produced by the condensation of phenols and aldehydes, which are often ud as polymeric additives), polyethers [such as polyphenylene oxide (PPO), a very thermally stable engineering polymer], and polyacetals (such as polyformaldehyde, ud for its in-ten hardness and resistance to solvents).
Nitrogen-containing materials include nylons, poly-urethanes, polyamides, and polyacrylonitrile. Nylons, having repeating units containing the characteristic group >CO>NH>, are made into fibers and also into a number of injection-molded articles. Nylons are synthetic aliphatic polyamides. There
are also natural polyamides (e.g., wool, silk, and leather) and synthetic aromatic polyamides (of exceptionally high thermal stability and ud for protec-tive clothing). Polyurethanes (PU), with repeating units containing the characteristic group >NH>COO>, are normally manufactured from the condensation of polyiso-cyanates and polyols. Their principal area of application is as foams (flexible and rigid), or as thermal insulation.
Thermal Decomposition of Polymers1–111
Figure 1-7.1.Energy feedback loop required for sus-
tained burning.
Other polyurethanes are made into thermoplastic elas-tomers, which are chemically very inert. Both the types of polymers have carbon–nitrogen chains, but nitrogen can also be contained in materials with carbon–carbon chains, the main example being polyacrylonitrile [repeat-ing unit: >(CH2>CH>CN>)]. It is ud mostly to make into fibers and as a constituent of engineering copolymers (e.g., SAN, ABS).
Chlorine-containing polymers are exemplified by poly(vinyl chloride) [PVC, repeating unit: >(CH2>CHCl)>]. It is the most widely ud synthetic polymer, together with polyethylene and polypropylene. It is unique in that it is ud both as a rigid material (unplasticized) and as a flexi-ble material (plasticized). Flexibility is achieved by adding plasticizers or flexibilizers. Through the additional chlori-nation of PVC, another member of the family of vinyl ma-terials is made: chlorinated poly(vinyl chloride) (CPVC) with very different physical and fire properties from PVC. Two other chlorinated materials are of commercial interest: (1)polychloroprene (a polydiene, ud for oil-resistant wire and cable materials and resilient foams) and (2)poly(vinylidene chloride) [PVDC, with a repeating unit:
never say die>(CH2>CCl2)>ud for making films and fibers]. All the polymers have carbon–carbon chains.
Fluorine-containing polymers are characterized by high thermal and chemical inertness and low coefficient of friction. The most important material in the family is polytetrafluoroethylene (PTFE); others are poly(vinyli-dene fluoride) (PVDF), poly(vinyl fluoride) (PVF), and fluorinated ethylene polymers (FEP).
Physical Process
The various physical process that occur during ther-mal decomposition can depend on the nature of the mate-rial. For example, as thermotting polymeric materials are infusible and insoluble once they have been formed, simple pha changes upon heating are not possible. Ther-moplastics, on the other hand, can be softened by heating without irreversible changes to the material, provided heating does not exceed the minimum thermal decompo-sition temperature. This provides a major advantage for thermoplastic materials in the ea of molding or thermo forming of products.
The physical behavior of thermoplastics in heating is dependent on the degree of order in molecular packing, i.e., the degree of crystallinity. For crystalline materials, there exists a well-defined melting temperature. Materi-als that do not posss this ordered internal packing are amorphous. An example of an amorphous material is window glass. While it appears to be a solid, it is in fact a fluid t
hat over long periods of time (centuries) will flow noticeably. Despite this, at low temperatures amorphous materials do have structural properties of normal solids. At a temperature known as the glass transition tempera-ture in polymers, the material starts a transition toward a soft and rubbery state. For example, when using a rubber band, one would hope to u the material above its glass transition temperature. However, for materials requiring rigidity and compressive strength, the glass transition temperature is an upper limit for practical u. In theo-retical terms, this “deformability” of a polymer can be ex-presd as the ratio of the deformation (strain) resulting from a constant stress applied. Figure 1-7.2 shows an ide-alized view of the effect on the deformability of thermo-plastics of increasing the temperature: a two-step increa. In practice, it can be stated that the glass transi-tion temperature is the upper limit for u of a plastic material (as defined above, bad on its elastic modulus and elongation) and the lower limit for u of an elas-tomeric material. Furthermore, many materials may not achieve a viscous state since they begin undergoing ther-mal decomposition before the polymer melts. Some typi-cal glass transition temperatures are given in Table 1-7.1. As this type of physical transformation is less well de-fined than a pha transformation, it is known as a c-ond order transition. Typically, materials are only partially crystalline, and, hence, the melting temperature is less well defined, usually extending over a range of 10°C or more.
Neither thermotting nor cellulosic materials have a fluid state. Due to their structure, it is not possible for the original material to change state at temperatures be-low that at which thermal decomposition occurs. Hence, there are no notable physical transformations in the ma-terial before decomposition. In cellulosic materials, there is an important mi-physical change that always occurs on heating: desorption of the adsorbed water. As the wa-ter is both physically and chemically adsorbed, the tem-perature and rate of desorption will vary with the material. The activation energy for physical desorption of water is 30 to 40 kJ/mol, and it starts occurring at tem-peratures somewhat lower than the boiling point of wa-ter (100°C).
Many materials (whether cellulosic, thermotting, or thermoplastic) produce carbonaceous chars on thermal decomposition. The physical structure of the chars will strongly affect the continued thermal decomposition
1–112Fundamentals
Figure 1-7.2.Idealized view of effect on deformability
of thermoplastics with increasing temperature.
process. Very often the physical characteristics of the char will dictate the rate of thermal decomposition of the remainder of the polymer. Among the most important characteristics of char are density, continuity, coherence, adherence, oxidation-resistance, thermal insulation prop-erties, and permeability.3Low-density–high-porosity chars tend to be good thermal insulators; they can significantly inhibit the flow of heat from the gaous combustion zone back to the condend pha behind it, and thus slow down the thermal decomposition process. This is one of the better means of
decreasing the flammability of a poly-mer (through additive or reactive flame retardants).1,3,4 As the char layer thickens, the heat flux to the virgin ma-terial decreas, and the decomposition rate is reduced. The char itlf can undergo glowing combustion when it is expod to air. However, it is unlikely that both glow-ing combustion of the char and significant gas-pha combustion can occur simultaneously in the same zone above the surface, since the flow of volatiles through the char will tend to exclude air from direct contact with the char. Therefore, in general, solid-pha char combustion tends to occur after volatilization has largely ended.
Chemical Process
The thermal decomposition of polymers may pro-ceed by oxidative process or simply by the action of heat. In many polymers, the thermal decomposition process are accelerated by oxidants (such as air or oxy-gen). In that ca, the minimum decomposition tempera-tures are lower in the prence of an oxidant. This significantly complicates the problem of predicting ther-mal decomposition rates, as the prediction of the concen-tration of oxygen at the polymer surface during thermal decomposition or combustion is quite difficult. Despite its importance to fire, there have been many fewer studies of thermal decomposition process in oxygen or air than in inert atmospheres.
Thermal Decomposition of Polymers1–113
Polymer
Acetal
Acrylonitrile-butadiene-styrene Cellulo
Ethylene-vinyl acetate Fluorinated ethylene propylene High-density polyethylene
Low-density polyethylene
Natural rubber
Nylon 11
Nylon 6
Nylon 6-10
Nylon 6-6
Polyacrylonitrile
Poly(butene 1)
Polybutylene
Poly(butylene terephthalate) Polycarbonate Polychlorotrifluoroethylene
Poly(ether ether ketone)
Poly(ether imide)
恳切
Poly(ethylene terephthalate)
Poly(hexene 1)
Poly(methylbutene 1) Polymethylene
Poly(methyl methacrylate) Polyoxymethylene
Poly(pentene 1)
Poly(3-phenylbutene 1)
Poly(phenylene oxide)/polystyrene Poly(phenylene sulphide) Polypropylene
Polystyrene
Polysulphone Polytetrafluoroethylene
Poly(vinyl chloride)
Poly(vinylidene chloride)
Poly(vinylidene fluoride)
Poly(p-xylene)
Styrene-acrylonitrile % Crystalline
high
low
high
high
high
95
60
low
high
low最新国际音标
high
low
high
high
high
100
low
75–80
low
high
65
low
low
100
5–15
high
high
low
Glass T ransition
T emperature (°C)
91–110
–125
–25
75
50
57
140
124–142
126
40
145–150
45
143
217
70
anna aj50
–85
100–135
88–93国庆节英语简介50字
–20
>80
190
125
80–85
–18
–30– –20
fresh是什么意思
100–120
Crystalline Melting
Temperature (°C)
175–181
110–125
decompos
65–110
275
130–135
109–125
30
185–195
215–220
215
250–260
317
124–142
126
232–267
215–230
220
334
265
55
300
136
90–105
new city175–180
130
360
成熟英语110–135
277–282
170
230
190
327
75–105 (212)
210
160–170
>400
120
Table 1-7.1Glass Transition and Crystalline Melting Temperatures
It is worthwhile highlighting, however, that some very detailed measurements of oxygen concentrations and of the effects of oxidants have been made by Stuetz et al. in the 1970s5and more recently by Kashiwagi et al.,6–10Brau-man,11and Gijsman et al.12Stuetz found that oxygen can penetrate down to at least 10 mm below the surface of polypropylene. Moreover, for both polyethylene and polypropylene, this access to oxygen is very important in determining thermal decomposition rates and mecha-nisms. Another study of oxygen concentration inside polymers durin
g thermal decomposition, by Brauman,11 suggests that the thermal decomposition of polypropylene is affected by the prence of oxygen (a fact confirmed more recently by Gijsman et al.12) while poly(methyl methacrylate) thermal decomposition is not. Kashiwagi found that a number of properties affect the thermal and oxidative decomposition of thermoplastics, particularly molecular weight, prior thermal damage, weak linkages, and primary radicals. Of particular interest is the fact that the effect of oxygen (or air) on thermal decomposition de-pends on the mechanism of polymerization: free-radical polymerization leads to a neutralization of the effect of oxygen. A study on poly(vinylidene fluoride) indicated that the effect of oxygen can lead to changes in both reac-tion rate and kinetic order of reaction.13
agony是什么意思Kashiwagi’s work in particular has resulted in the de-velopment of models for the kinetics of general random-chain scission thermal decomposition,14as well as for the thermal decomposition of cellulosics15and thermo-plastics.16
There are a number of general class of chemical mechanisms important in the thermal decomposition of polymers: (1)random-chain scission, in which chain scis-sions occur at apparently random locations in the poly-mer chain; (2)end-chain scission, in which individual
monomer units are successively removed at the chain end; (3)chain-stripping, in which atoms or groups not part of the polymer chain (or backbone) are cleaved; and (4)cross-linking, in which bonds are created between polymer chains. The are discusd in some detail un-der General Chemical Mechanisms, later in this chapter. It is sufficient here to note that thermal decomposition of a polymer generally involves more than one of the class of reactions. Nonetheless, the general class provide a conceptual framework uful for understand-ing and classifying polymer decomposition behavior.
Interaction of Chemical
and Physical Process
The nature of the volatile products of thermal decom-position is dictated by the chemical and physical properties of both the polymer and the products of decomposition. The size of the molecular fragments must be small enough to be volatile at the decomposition temperature. This effec-tively ts an upper limit on the molecular weight of the volatiles. If larger chain fragments are created, they will re-main in the condend pha and will be further decom-pod to smaller fragments, which can vaporize.
Figure 1-7.3 shows examples of the range of chemical or physical changes that can occur when a solid polymer is volatilized. The changes range from simple pha trans-formations (solid going to liquid and then to gas, at the top of the figure), to complex combinations of chemical and physical changes (in the lower part of the figure). Wa-ter and many other liquids forming crystalline solids on freezing (e.g., most flammable liquids) undergo straight-forward physical pha changes. Sublimation, i.e., the di-rect pha change from a solid to a gas, without going through the liquid pha, will happen with materials such as carbon dioxide (e.g., CO2, dry gas) or methenamine at normal temperatures and pressures. Methenamine is of interest in fires becau methenamine pills are the igni-tion source in a standard test for carpets, ASTM D2859,17 ud in mandatory national regulations.18,19Thermoplas-tics can melt without chemical reaction to form a viscous state (polymer melt), but they often decompo thermally before melting. This polymer melt can then decompo into smaller liquid or gaous fragments. The liquid frag-ments will then decompo further until they, too, are suf-ficiently volatile to vaporize. Some polymers, especially thermots or cellulosics, have even more complex de-composition mechanisms. Polyurethanes (particularly flexible foams) can decompo by three different mecha-nisms. One of them involves the formation of gaous isocyanates, which can then repolymerize in the gas pha and conden as a “yellow smoke.” The iso-
1–114Fundamentals
Figure 1-7.3.Physical and chemical changes during
thermal decomposition.
