Some Recent Aspects of Polymer Flammability
There are various stages in the pyrolytic degradation and combustion of volatile products leading to the flammability behavior of polymers. Mechanisms to decrease flammability involve modifying condensed phase or volatile phase reactions. The reported studies relate our recent progress in the understanding and controll of these mechanisms.
Some general known relationship between flammability and polymer structure are reviewed. Our studies on relating polymer end groups and molecular weights to flammability indicated that the known thermal degradation mechanisms for nylon 6 and polyethylene terephthalate (PET) are, in part, related to their flammability behavior.
The question of the flammability behavior of similar flame retarding structures when used as additives or as comonomers in PET is discussed. For the case of structures related to tetrabromo-bisphenol-A, there was little difference, but for those containing triphenylphosphine oxide related structures a switch from volatile phase to condensed phase mechanisms was possible.
Efforts in regard to understanding char formation mechanisms and improvements in the amount of char have been studied for systems such as polystyrene, cardopolymers, phenol-formaldehyde resins, aromatic polyamides and styrylpyridine based polymers. Increased char formation occurred when thermally stable thermally induced crosslinking structures occurred and/or thermally stable aromatic rings wre produced.
Crosslinking could be induced in polystyrene structures containing vinylbenzyl chloride as a comonomer which enhanced char formation. For the cardopolumers, the phenolphthalein based polymers were shown to have increased char formation because of the rearrangement of the lactide group to a thermally stable ester crosslink. Although other cardopolymers showed improvements in flammability, the mechanisms were not explored. Vapor phase and condensed phase mechanisms were applicable for subtituted phenolic resins but mechanisms for the latter were not elucidated.
Specific halogen substituted aromatic polyamides have given substantial increases in char formation due to the formation, in part, of thermally stable benzoxazole units. Another system in which ring formation accounts for increased char formation were those which contained styrylpyridine units. In this case a Diels Alder addition reaction could account for these results.
KeywordsFlame Retardant Oxygen Index Char Yield Char Formation Aromatic Polyamide
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