Preparation of a thiophenol/formaldehyde resin; its structure and the mechanism of its pyrolysis

Abstract

A thiophenol/formaldehyde resin (TPFR) was synthesized under basic (NaOH) conditions, and its structure was thermally investigated by isothermal pyrolysis, under fluidizedbed conditions, at various temperatures. The pyro-products, volatilized during pyrolysis, were trapped and most of them were identified by GC and GC/MS to be sulfur-containing compounds, i.e. thiophenol, methylthiobenzene, benzothiophenes, diphenyl sulfide, diphenyl disulfide, thioxanthene, thianthrene and p-mercaptobenzyl phenyl sulfide (MBPS). The relative distribution of the condensed pyro-products was temperature dependent. The kinetic profiles of the pyrolysis were evaluated in relation to the spatial structure. The thermal behavior of a structurally similar monomer, benzyl phenyl sulfide (BPS), was studied under the same pyrolytic conditions. The similarities and the differences between the thermal behaviors of BPS and TPFR are discussed and decomposition mechanisms are proposed. The main conclusions of this work are: (i) TPFR is a highly branched polymer built from BPS units; (ii) cleavage of the benzylic CS bond, which is the weakest bond in the resin, is the key step in the formation of most of the pyro-products; (iii) the structure of TPFR indicates that the polymerizations mechanism of thiophenol and formaldehyde differs considerably from that of phenol/formaldehyde resins. The proposed mechanisms for polymerization and for thermal decomposition are discussed in relation to our previous studies on phenol/formaldehyde resins.