From Fig. 2, it can be seen that PURF mainly had three weight loss range at 100–200 °C, 200–400 °C and 600–800 °C. The weight loss peak in 100–200 °C was caused by the loss of absorbed water in the sample. In range of 200–400 °C, the PURF curve showed a double peak because the decomposition of PURF involved several processes [14], which led to the generation of either cross-linked structures or gas. Specifically, the first peak at 280 °C could be ascribed to the loss of PAPI segments from the decomposition of urethane, while the second peak at 320 °C could be assigned to the gasification of PAPI segments from the decomposition of urea bonding. The slow weight loss of PURF at 400–600 °C was related to the degradation of soft segments. At 600 °C or above, the slow weight loss of PURF was supposed to the loss of fragment from unstable char layer.
In comparison with PURF, PEPA modified PURFs showed slight weight loss at 100–200 °C, and the weight loss increased with the PEPA content at 250–380 °C. It can be proposed that the weight loss here was caused by two reasons, one was the similar weight loss as pure PURF, and the other was caused by the decomposition of PURFs catalyzed by phosphorus acid that was generated by the degradation of PEPA. At 380–450 °C, the weight loss could be attributed to the dehydrogenation and dehydration of secondary polymers generated from soft segments. Finally, the weight loss became very slight at above 600 °C.