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Characterization of Flame Retarded Polymer Combustion Chars by Solid-State 13C and 29Si NMR and EPR.


pdf icon Characterization of Flame Retarded Polymer Combustion Chars by Solid-State 13C and 29Si NMR and EPR. (435 K)
Gilman, J. W.; Lomakin, S. M.; Kashiwagi, T.; VanderHart, D. L.; Nagy, V.

Polymer Preprints, Vol. 38, No. 1, 802-803, April 1997 AND Fire and Materials, Vol. 22, No. 2, 61-67, March/April 1998,

Keywords:

char; nuclear magnetic resonance; electron paramagnetic resonance; radicals; char formation; polymers; flammability

Abstract:

Additives that increase the amount of charcoal-like residue of carbonaceous char that forms during polymer combustion are very effective fire retardants. However, very little is understood about the detailed structure of char or how it forms. Our research efforts focus on reducing polymer flammability by promoting char formation. Char formation reduces the amount of small, volatile polymer pyrolysis fragments, or fuel, available for burning in the gas phase; this in turn reduces the amount of heat released and fed back to the polymer surface. The char also insulates the underlying polymer, due to its low thermal conductivity, and reradiates externally impinging energy away from the polymer. The char must also function as a mass transport barrier, by physically delaying the volatilization of decomposition products and or trapping decomposition products through chemical reaction. The physical structure of the char is important in this role. Foamy char structure appears to be more fire resistant than brittle, thin char. This char enhancing approach is most successful when the polymer chars rapidly and early in the burning process. To be useful, the charring process must occur at a temperature above the polymer processing temperature but below the temperature where rapid gassification of the polymer to combustible fuel occurs. We report here the 29Si, 13C NMR and EPR analysis of polymer chars. We found that the ratio of non-protonated carbon to protonated carbon is ~ 1.5 + 01.1. It appears that the additives do not change the type if char formed, but they do change the rate of char formation relative to the rate of fuel generation, since the char yield is higher and the flammability (due to a lower fuel generation rate) is reduced.