Flame retardancy generally depends on the amount of halogenated compounds in the resin (mainly the content of chlorine and bromine). Chlorine is added to the resin in two ways: one is in a mixed state, such as chlorinated paraffin: the other is in a combined state, that is, in a stable bonding state. It is more effective to use chlorinated aromatic dibasic acids (such as tetrachlorophthalic anhydride, hexachlorobridge methylenephthalic anhydride) as saturated acids to participate in the reaction. Tetrabromophthalic anhydride, dibromo neopentyl glycol and other bromine can add fluorine element to the chlorine-containing compound, which can significantly improve the thermal stability of the resin. For example, difluorotetrachloromethylene phthalic acid, maleic anhydride, and ethylene glycol polymer lose weight of 6.7% after 300 days at 200°C: under the same conditions, the product obtained from hexachloromethylene phthalic anhydride Weightlessness 14.4%: Ordinary phthalic anhydride product weightlessness 25.6%.
If the polarity of the polyester molecule decreases and the hydrocarbon content increases, the electrical insulation properties of the resin increase, but the mechanical properties decrease. The decrease in polar groups affects the strength of the resin.
The unreacted hydroxyl and carboxyl groups in the resin will reduce the electrical insulation. It is necessary to minimize the unreacted hydroxyl and carboxyl groups in the polyester. To this end, the methyl ester of the dibasic acid can be used to make the resin through the transesterification method. At this time, the residual carboxyl group and methyl group are combined, and the acid value can be reduced to a very low value.
In addition, it is best not to use glycols containing oxygen bridges, because the ether bond also increases the polarity of the resin.