SynopsisThe investigation of the thermal degradation of the char-forming phenol-formaldehyde resins is conducted to provide information for the systematic design of high temperature flame-resistant phenolics. Three different processes of curing are used (1) Formaldehyde or s-trioxane is reacted with m-substituted phenol-formaldehyde oligomers under acidic conditions to give the methylene bridged-novolac resins. (2) Phenol and rn-substituted phenols are reacted with CH20 under basic conditions and then heated to give the methylene bridged resol resins. (3) p-Terephthaloyl chloride and m-and p-substituted novolac oligomers are reacted to give cured resins with ester linkages. The evaluation of the effect of various substituents indicates that the oxygen index (01) increases from about 33 for unsubstituted phenolics to about 75 for meta-halogen substituted phenolics. The evaluation of the effect of various crosslinking agents shows that the 01 for CHzO-cured phenolics is 75 as compared to 50 for the trioxane cured phenolics and to 40 for the terephthaloyl chloride cured phenolics. A set of phenolic copolymers with different weight percentage content of halogen substituted phenols are synthesized as novolacs and resols. The results surprisingly indicate no increase of 01 for the cured novolac copolymers, whereas the increase is observed for the cured resol copolymers. The activation energy for the thermooxidative degradation of the cured novolacs is about 12-15 kJ/mol lower as compared to tpt of the cured resols.
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