Thermal Stability and Flammability of Phosphorus-Containing Polyisocyanurate Foams

Introduction. The high flammability of polymeric thermal insulation materials (foams) significantly limits their wider application in the construction industry. To reduce the flammability of these materials, flame retardants based on various phosphorus- and nitrogen-containing compounds are used. Of particular interest for reducing the flammability of polyisocyanurate foams (PIR) is a mixture of melamine and ammonium polyphosphate (APP). The aim of this study was to determine the effect of the content and ratio of melamine and APP on the flammability and main patterns of thermal and thermooxidative degradation of PIR. Research objectives. Production of PIR with varying content and ratio of melamine and APP; determination of thermal stability and flammability of phosphorus-containing foams; analysis of the obtained results on flammability assessment, thermal and thermooxidative degradation of phosphorus-containing PIR. PIR with varying content and ratio of melamine and APP were used as research objects. The thermal stability of phosphorus-containing PIR was studied using a DuPont-9900 multimodal thermoanalytical complex and TGA-951 thermobalance. The flammability of foams was determined by the limiting oxygen index method according to GOST 21793-76. Results. It was shown that the formation of char residue during pyrolysis of phosphorus-containing PIR depends on the content of the studied flame retardants and the ratio of melamine to APP. During thermal degradation of PIR at a melamine:APP ratio of 62:38, the char yield exceeds by 1.27–1.63 times the char output at a ratio of these compounds equal to 50:50. It was revealed that the observed effect is the main factor determining the higher flame-retardant efficiency of melamine-APP mixtures at a ratio of 62:38. Conclusions. It was established that moderately flammable polyisocyanurate foams (oxygen index exceeding 27%) can be obtained at a concentration of the studied phosphorus-nitrogen flame retardants above 8–15 wt.%, depending on the melamine:APP ratio. A linear dependence of the oxygen index (OI) of foams on the char residue value during thermal degradation of phosphorus-containing PIR was observed. The mechanism of flammability reduction in the developed phosphorus-containing polyisocyanurates is based on enhanced carbonization of the polymer matrix and foaming of the char layer due to melamine decomposition.