Unsaturated polyesters containing varying amounts of HET acid (1,4,5,6,7,7-hexachloro-5norbornene-2,3-dicarboxylic acid) were synthesized and cured with styrene to give flame-retarding unsaturated polyester resins. The thermal properties were investigated by DTA (differential thermal analysis), TVA (thermal volatilization analysis), SATVA (subambient thermal volatilization analysis), and weight loss measurements. The studies revealed that the reverse Diels-Alder reaction of the HET acid units present in the polyester backbone proceeds in the temperature range 300-350°C and yields hexachlorocyclopentadiene. The influence of the HET acid content on the mass distribution of the products of pyrolysis (450°C) was analyzed b means of a gas chromatograph coupled with a mass spectrometer. With increasing HET acid content, the total amount of aromatics evolved from the cured resins is decreased. Concurrently, the relative amount of styrene produced is decreased while the relative amounts of toluene and ethyl benzene are increased. This finding strongly suggests that the hexachlorocyclopentadiene interferes as a chain transfer agent with the unzipping reaction of the polystyrene chains. Further support for this interpretation is derived from the enhanced char formation with increasing HET acid content, as observed by the weight loss measurements.
Hexachlorocyclopentadiene
Maleic acid HET acid
It is well known that the double bond of HET acid does not participate in the