Effects of hydrotalcites and tris (1-chloro-2-propyl) phosphate on thermal stability, cellular structure and fire resistance of isocyanate-based polyimide foams

Isocyanate-based polyimide foams (PIFs) with different dosages of liquid tri (1-chloro-2-propyl) phosphate (TCPP) and micro-sized hydrotalcites (LDHs) particles alone, as well as different mixing ratios of TCPP and LDHs, were prepared via a one-step process in this work. Limiting oxygen index (LOI) and cone calorimeter test (CCT) results indicated that TCPP exhibited more pronounced flame retardant efficiency than LDHs for isocyanate-based PIFs. However, scanning electron microscopy images and digital macrostructural images results showed that, in contrast to LDHs, when the dosage of TCPP exceeded 10% it caused a clear cracking effects on the macro-cellular structure and opening cell effects on the micro-cellular structure for isocyanate-based PIFs. Because the dramatically volatilization of TCPP during the postcuring process caused obvious cellular contraction in the isocyanate-based PIFs. Meanwhile, the use of TCPP also obviously decreased the thermal stability of isocyanate-based PIFs unlike LDHs. However, when these two flame retardants were used in combination, they could effectively enhance the fire resistance and ensure macro- and micro-cellular structures of the isocyanate-based PIFs, unlike standalone use of TCPP and LDHs. When 10% TCPP was simultaneously used with 10% LDHs, the macro- and micro-cellular structures of the resultant foams were clearly improved compared with foams prepared using TCPP only. These results were believed to be attributable to LDHs dispersion in the foams, which enhanced the strength of cellular windows and skeletons, then restrained the cell contraction. Compared with foams without flame retardants, the fire resistance of the isocyanate-based PIFs prepared with 10% TCPP and 10% LDHs was obviously enhanced; specially, the LOI was enhanced by 29.4%, and the peak of heat release rate (PHRR) decreased by 36.1%. Thus, the use of liquid and solid flame retardants in combination may effectively yield isocyanate-based PIFs with high quality cellular structures and excellent fire resistance.