Polystyrene Flammability char formation

The principles needed to design a polymer of low flammability are reasonably well understood and have been systematized by Van Krevelen (5). A number of methods have been found for modifying the structure of an inherently flammable polymer to make it respond better to conventional flame retardant systems. For example, extensive work by Pearce et al. at Polytechnic (38, 39) has demonstrated that incorporation of certain ring systems such as phthalide or fluorenone structures into a polymer can greatly increase char and thus flame resistance. Pearce, et al. also showed that increased char formation from polystyrene could be achieved by the introduction of chloromethyl groups on the aromatic rings, along with the addition of antimony oxide or zinc oxide to provide a latent Friedel-Crafts catalyst. 

In some types of nanocomposites the nature and aspect ratios of the assembled silicate structures may affect their ability to resume a reasonably coherent structure during char formation. Such structures would be less effective in reducing movement of volatiles and new fuel and so have a reduced flame retardaney eapability. This latter phenomenon was found in some polystyrene nanocomposites that utilised fluorohectorite. PS composites using MMT did show a decrease in flammability. 

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... 

Crosslinking could be induced in polystyrene structures containing vinylbenzyl chloride as a comonomer which enhanced char formation. For the cardopol3nners, 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 showec improvements in flammability, the mechanisms were not explored. Vapor phase and condensed phase mechanisms were applicable for substituted phenolic resins but mechanisms for the latter were not elucidated. 

Morgan, A.B. Harris, R.H. Kashiwagi, T. Chyall, L.J. Gilman, J.W. Flammability of polystyrene layered silicate (clay) nanocomposites carbonaceous char formation. Fire Mater. 2002, 26(6), 247. 

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