Polymers, burning degradation

Reich, L. and Stivala, S. S. Elements of Polymer Degradation. McGraw-Hill, New York 1971 Tkac, A. Radical processes in polymer burning and its retardation. I. ESR methods for studying the thermal decomposition of polymers in the preflame and flame zones. J. Polym. Sci. Polym. Chem. Ed., 19, 1475 (1981)... 

As shown in the burning cycle, a polymer burns via the vapour phase combustion of the volatile products produced during its thermal oxidative degradation. The vapour phase combustion is a free radical process which can be simplified and expressed as follows, where RCH3 is representative of the hydrocarbon undergoing combustion ... 

Resistance to burning depends on many factors. It is, however, to be noted that those polymers that only burn in air enriched with oxygen tend to have high carbon hydrogen ratios and/or may also emit materials during degradation, such as hydrogen chloride, that are inherent flame-retardants. 

It should not be thought, however, that perfluorocarbons are completely inert toward combustion. Even the very inert perfluorocarbon polymer polytetrafluoroethylene [PTFE, Du Pont s Teflon F(CF2CF2)nF] is thermodynamically unstable in oxygen with respect to CO2 and CF4 (Exercise 12.6) and can burn in a 95% 02/5% N2 mixture at 0.1 MPa, although combustion is hard to initiate because of the nonvolatility of PTFE and the resistance of the thermal degradation products to oxidation. Conflagrations involving more reactive, volatile fluorocarbons such as perfluoro-toluene have been reported.15... 

Crosslinking has no specific direct effect on thermal degradation crosslinks can be either weak points (e.g., tertiary carbons in polyester or anhydride-cured epoxies) or thermostable structural units (e.g., trisubstituted aromatic rings in phenolics, certain epoxies, or certain thermostable polymers). Indirect effects can be observed essentially above Tg crosslinking reduces free volume and thus decreases 02 diffusivity. It also prevents melting, which can be favorable in burning contexts. 

T.R. Hull, D. Price, Y. Liu, C.L. Wills, and J. Brady, An investigation into the decomposition and burning behavior of ethylene-vinyl acetate copolymer nanocomposite materials. Polym. Degrad. Stab., 82, 365-371 (2003). 

Price, D. Liu, Y. Milnes, GJ. et al. Burning behaviour of foam/cotton fabric combinations in the cone calorimeter. Polym. Degrad. Stab. 2002, 77, 213-220. 

To prevent degradation and burning, small quantities of chemicals are added to the polymers. Although the ratio of these chemicals to polymers is small, their effect is decisive for the endurance and life of the polymers. 1968 s consumption of stabilizers and fire retardants is expected to pass the 350-million pounds mark for plastics and rubbers, as 1967 figures indicate (see Table I). 

In this way one mole of water reacts with two moles of isocyanate. For a urethane prepolymer with an equivalent weight of about 300, it only takes 9 grams of water to effectively consume the prepolymer. This can lead not only to degraded physical properties, but it also forms gas bubbles in the proplnt which lead to voids and higher than predicted burning rates. This sensitivity to water content also exists with hydroxy-terminated polybutadiene polymers cured with... 

Wood burns because the cell wall polymers undergo hydrolysis, oxidation, dehydration, and pyrolysis reactions with increasing temperature to give off volatile, flammable gases. The lignin component contributes more to char formation than do the cellulose components, and the charred layer helps insulate the wood from further thermal degradation see Chapter 13). 

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