Studies on the Mechanism of Thermal Decomposition of Unsaturated Polyester Resins with Reduced Flammability

The thermal decomposition of halogenated unsaturated polyester resins (UPRs), flame retarded by zinc hydroxystannate and cross-linked with styrene, has been investigated by thermogravimetry (TG) in both dynamic and isothermal mode, and by TG coupled on-line with Fourier transform infra red spectroscopy (TG-FTIR) or mass spectroscopy (TG-MS). The data from TG analysis show that degradation proceeds in two steps for unmodified as well as modified UPR, but the char residue at 500°C was considerably higher for flame-retarded resins. Analysis of the volatile decomposition products by FT-IR and MS spectrocopy revealed that during controlled heating, the emission of phthalic anhydride takes place at ca. 330°C, followed by styrene at ca. 380°C., and a complex mixture of other aromatic compounds are evolved. Further studies by TG-FTIR revealed that evolution of CO₂ is considerably lowered by zinc hydroxystannate, andat a later stage of decomposition the evolution of H₂O and HBr (traces) can be identified. On the basis of the results obtained a mechanism of decomposition was proposed that takes into account the reaction of tin compounds with the products of ester linkage rupture processes.