Bi-DOPO Structure Flame Retardants with or without Reactive Group: Their Effects on Thermal Stability and Flammability of Unsaturated Polyester

A novel reactive phosphorus-containing flame retardant 1,4-phenylene-bis((6-oxido-6H-dibenz[c,e][1,2]-oxaphosphorinyl) methylene) diacrylate (TDCAA-DOPO), with two symmetrical 9,10-dihydro-9-oxa-10-phosphaphena-nthrene-10-oxide structure (bi-DOPO), was synthesized and combined with unsaturated polyester resin (UPR) via cross-linking to prepare flame-retardant UPR (FR-UPR). To make clear both the effects of the bi-DOPO structure and the cross-linking reaction on the thermal stability and flame retardancy of UPR, an additive flame retardant that also owned bi-DOPO structure but without a vinyl bond, 1,4-phenylene-bis((6-oxido-6H-dibenz[c,e][1,2]-oxaphosphorinyl) carbinol) (TDCA-DOPO), was used as a contrast. The results showed that the incorporation of TDCAA-DOPO or TDCA-DOPO into UPR can reduce its peak heat release rate (PHRR) and total heat release (THR), as well as improve the limiting oxygen index (LOI) value. As compared to TDCA-DOPO, the reactive TDCAA-DOPO endowed FR-UPR with enhanced thermal stability, residual char, and glass transition temperature (Tg), which were even higher than those of pure UPR. Besides, their flame-retardant mechanism for UPR had been well investigated.