Char yields, polymer flammability

It is known that increased char yield is usually associated with improved flammability behavior ( 1). This can be understood if one considers that the volatile flammable products can only diffuse with difficulty through the char, and that the thermal conductivity of a porous char layer is relatively poor (2). The structure of the polymer can contribute to the amount of char formed based on the character of the functional groups present and the nature of the backbone (2,3). Ritchie ( ) found that for a series of unsaturated polyesters and their copolymers, the temperatures at which carbon dioxide is eliminated was in the range of 280 to 345°C depending on the structure of the polyester. Aliphatic polyesters and their copolymers have less thermal... 

Char yield is increasingly regarded as an important measure of fire performance. The formation of large quantities of char (from organic resins, invariably carbonaceous) has several advantages. First, conversion of resin to inert char rather than volatile flammable products reduces the fuel available to feed the fire. Furthermore, a carbonaceous char will be selfextinguishing. In addition, a surface char seals the polymer surface, preventing the escape of volatile materials. 

Thermal relationships between flammability and structure/composition of these polymers was explored. It is found that BPC Il-polyarylate is an extremely fire-resistant thermoplastic that can be used as an efficient flame-retardant agent to be blended with the other polymers. Chakon Il-polyarylate is of interest as a UV/visibk-sensitive polymer with a relatively low HRR and a high char yield. Pyrolysis combustion flow calorimetry (PCFC) results show that the total heat of combustion of the copolymers or blends changes linearly with the composition, but the change of maximum HRR and char yield depends greatly on the chemical structure of the components. 

Polypropylene, like polyethylene (PE) and polystyrene (PS) when ignited bums rapidly, completely and leaves little or no char. Reducing the flammability of inherently non-charring polymers through char enhancement presents a particular challenge. In the process of systematically evaluating additives expected to enhance char formation, we found that silica gel when combined with potassium carbonate not only increased char yields but also reduced the... 

The effect that silica gel/K2C03 has on the flammability of PS is shown in Table 5.5. The results are similar, but reduced in magnitude, to those for PP. Use of silica gel /K2CO3 additives causes the otherwise non-char-forming PS to produce a char yield of 6% (16% residue yield). The additives reduce the rate of heat release by 31%, reduce the total heat release by 11%, and similar to PP, had little effect on the CO yield and soot. The cone calorimeter results for PMMA are also shown in Table 5.5. PMMA is an inherently non-char-forming thermoplastic polymer however, using less than half the usual amount of additives, the reduction in flammability was comparable to PP and PS. Even at this low level... 

PVA is one of the few linear non-halogenated aliphatic polymers that has a significant char yield (3-5%) when burned. The flammability properties of PVA, the structure of the char and the processes by which char forms, have been studied previously. The data for the performance of the additives individually, combined, and at different concentrations are shown in Table 5.5. Potassium carbonate used alone with PVA (mass fraction of 10%) reduces the peak RHR by 47%, while silica gel used alone with PVA (mass fraction of 10%) reduces the peak RHR by 57%. 

Cellulose, like PVA, gives a measurable char yield when combusted (3-4%) and in view of the promising results seen for PVA, and since cellulose is a commercially important polymer, its flammability properties were examined in the presence of silica gel/ K2CO3 additive. Cellulose, in the presence of the additives, like PVA showed a significant increase in the amount of carbonaceous char, 32% (39% residue yield). The peak heat release rate was reduced by 52%, and the total heat release was reduced by 66%. Like PVA, but in contrast to the results for PP, PS, and PMMA, the heat of combustion was reduced (by 53%). The CO yield was increased by -50%, primarily from incomplete oxidation at the end of the combustion, and the soot was ecreased by 26%. 

Table 1 ABS/PVC/FeOOH blends, char yields and related flammability data (First published in Polymer, 35(2), 344 (1994))... 

It is of note that the respective amounts of char and flammable volatiles produced by the thermal decomposition of the composite are highly dependent on the chemical nature of the organic phases, i.e., the polymer matrix and synthetic fibres, if present (Levchnick and Wilkie, 2000 Mouritz, 2007). As regards the main thermosetting polymers used in construction (i.e., polyesters, vinylesters and epoxies), pyrolysis yields a large amount of volatiles but retains a small amount of char (10-20% of the initial mass). FRP composites based on these thermoset matrices are thus highly flammable materials. 

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