Effect of chlorination and bromination on the dielectric characteristics of bisphenol‐A polyesters

The variation of dielectric behaviour with temperature of three crosslinked polyester resins based on the condensates of chloro-methyl acetate and bisphenol-A, 2,2-di-(4-hydroxy 3,5-dichloro phenyl) propane and 2,2-di-(4-hydroxy 3,5-dibromo phenyl) propane were studied in the microwave frequency region at wave length 3.21 cm. The dielectric constants of these polyesters are 2.93, 2.91 and 2.77 respectively and do not change with temperature in the 30–85°C region. The decrease in dielectric constant after incorporating 2,2-di-(4-hydroxy 3,5-dichloro phenyl) propane and 2,2-di-(4-hydroxy 3,5-dibromo phenyl) propane in the polyester chain is attributed to the decrease in the number of ester and ether groups in the polyester chain per unit weight of the cured polyesters. It is found that the loss factor for bisphenol-A based polyester resin is maximum at 55°C and the relaxation time of this polyester is 1–7 · 10-11 sec at 55°C. It can be assigned to the free motion of ether linkages due to the absence of halide groups at 3,5 position of bisphenol-A incorporated in the polyester chain. This can also be attributed to the absence of secondary forces between the chains due to the presence of bulky p-phenylene groups in the polyester chain and crosslinking by bulky molecules like styrene. In case of chlorinated and brominated polyesters, the motion about ether and ester linkages is difficult. The chlorinated bisphenol-A polyester has less variation in loss factor as compared to that of bromo polyester. It may be due to the homo polystyrene or large polystyrene segments present in bromo polyester. These results are also supported by the studies on hydrolytic stability of their base resins and by the heat distortion point of the crosslinked polyesters.