ISOTHERMAL FLUIDIZED-BED STUDIES ON THE KINETICS AND PYRE-PRODUCTS OF LINEAR AND BRANCHED POLY(P-PHENYLENE SULFIDE) AND PROPOSED MECHANISMS

The isothermal kinetic behaviour under fluidized-bed conditions of linear poly(p-phenylene sulfide) (LPPS) and the branched polymer (BPPS), formed by curing at 340 degrees C, is presented. The FID monitoring of the products volatilized at 575, 525 and 475 degrees C shows different profiles for LPPS and BPPS and supports the concept that the thermal stability of the poly(phenylene sulfide) (PPS) increases with an increase in the degree of branching. This is also supported by differential scanning calorimetry (DSC) and thermogravimetry results which indicate that the weight loss on thermal degradation starts at 430 degrees C. The pyro-products volatilized during pyrolysis were trapped, and GC and GC/MS analyses for both LPPS and BPPS showed two families of compounds: (i) fragments of Ph-S unit structure; (ii) benzothiophenic structures (mostly dibenzothiophenes). The ratios of products as well as kinetic parameters lead us to conclude that: (A) the activation energy required for the formation of type (ii) products is higher than for type (i); (B) an increased degree of branching facilitates the formation of dibenzothiophenes; (C) the activation energy required to form dibenzothiophenes of higher molecular weight pyre-products, e.g. 1,4-S-phenyl-S'-2-dibenzothiophene benzenedithiol, is lower than that needed to form smaller molecules, e.g. dibenzothiophene. Mechanistically, it is proposed that ''low temperature'' pyrolysis is controlled by cleavage of the phenyl-sulfur bond via free radicals which restructure to form BPPS with small weight loss in the chain transfer processes. At higher temperatures the free radicals may abstract hybrogen atoms from the polymer structure to form dibenzothiophenes and volatile pyre-products. All changes are monitored quantitatively, and atomic ratio (H/C, S/C) analyses of the residue are discussed.