Polymers, burning thermal degradation

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. 

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). 

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)... 

In the last two decades, RH and the products of its thermal degradation are often used as fillers in paper, paint [5], polymers [42, 43], polymeric composites [44 7], mbber [48-50], cement [51, 52], adhesives, and fertilizers [5]. By the addition of fillers, the mechanical, thermal, chemical, and other properties of the material are improved. Controlled burning of RH in air or inert atmosphere yields two grades of fillers, namely white rice husk ash (WRHA) and black rice husk ash (BRHA). Both these RHA have been used as fillers in polyethylene [53-55], polypropylene [56-61], polystyrene [62, 63], etc. [64, 65]. Tremendous opportunities exist in more exhaustive research on the RHA-polymer composites, which can lead to the futuristic organic-inorganic hybrid materials with specific properties. In this respect, the aim of the present work is to describe the possibility for utilization of RRHs and the products of its thermal degradation as fillers in different polymer plastic composites. 

Zhu and co-workers [84] have studied the thermal degradation of a new flame-retardant phosphate methacrylate polymer. Degradation was monitored by in situ Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman measurements. The carbon structure of the final char after burning was examined. 

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 ... 

As is shown in the burning cycle, the burning of a polymer occurs by the combustion of the volatile products emitted during the oxidative thermal degradation of the polymer. If the degradation mechanism could be altered to produce more char and less volatiles the flame retardancy of the formulation should be decreased. Many flame retardants act in this way, promoting low energy, solid-state reactions which lead to the carbonization of the polymer and a carbonaceous char on the surface. 

The strategy for inducing intumescent behaviour in plastics depends first of all on the abihty of the polymer to leave a substantial amount of residue by thermal degradation ( char ) which is stable at the temperature of burning. In this case the polymer itself can become the char source. 

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