Thermal degradation and fire behavior of thermoset resins modified with phosphorus containing styrene

In order to improve the fire behavior of unsaturated polyester resins (UPR) a phosphorus containing monomer, dimethyl-vinylbenzyl phosphonate (or S1), was used to replace a part of the reactive diluent (styrene). UPR with various phosphorus contents were prepared. Thermal degradation was studied using thermogravimetric analysis (TGA) and pyrolysis combustion flow microcalorimeter (PCFC), while fire behavior was investigated by cone calorimeter. The phosphorus yield in the residue was determined using energy-dispersive X-ray (EDX).TGA pointed out a decrease in thermal stability with increasing S1 content but also an important increase in char formation. Cone calorimetry highlighted pHRR decrease, and from 50% of styrene substitution a barrier effect was evidenced. Residue analysis enabled to determine the partition of phosphorus between vapor and condensed phase. It was shown that this partition depends on the degree of styrene substitution. At high S1 content, interactions between phosphonate groups enable a very efficient condensed phase action by creation of a charred protective barrier. This strategy was also successfully transposed to a vinyl ester resin (VER).