Effect Of Graphite And Its Derivatives On Flame Resistance Of Isocyanate-based Polyimide Foam

Isocyanate-based polyimide foams (IBPIFs) were rapidly prepared via a simple free-foaming process. Compared to powder-foaming process, free-foaming process has many advantages including short process, low requirements and extensive raw materials for process.However, there's by-product of ureido in IBPIF with regular cell structures. It causes a great decrease in flame resistance and safety of IBPIF. In this paper, natural graphite (NG),expandable graphite (EG) and graphene (GNS) were added into IBPIFs, respectively. Then the effect of graphite and its derivatives on flame resistance of IBPIF was investigated.NG/IBPIFs with regular cell structures were prepared. Their thermal stability, flame resistance and smoke release behavior were evaluated. The results show natural graphite can improve the flame resistance of IBPIFs. When the amount of natural graphite added was 20%,the limiting oxygen index (LOI) of NG/IBPIFs can reach 22.3%. Additionally, it can effectively postpone ignition time of IBPIFs, thus delay the time of occurrence of fire.It is owing to the good thermal conductivity of NG which can accelerate heat diffusion.However,it has little effect on combustion behavior and smoke release behavior.EG/IBPIFs with regular cell structures were synthesized. The thermal stability, flame resistance and smoke release behavior of EG/IBPIFs were characterized. The results indicated that the expandable graphite added can effectively improve the flame resistance of IBPIFs.This is because non-flammable gases was provided by igniting EG. A worm-like, dense and expanded carbon layer formed to shield the flame and combustible gas barrier. When the amount of the expandable graphite was 20%, the LOI of EG/IBPIFs can be increased to 32.4%. The flame retardant level can reach flame retardant grade. Meanwhile, combustion behavior and smoke release behavior of IBPIF also can be greatly improved, thus the safety in use is enhanced.Isocyanate was grafted on the graphene oxide (GO) sheets, at first a reaction between the graphene oxygen containing groups and the isocyanate group took place then the foaming black material which contains iGO (isocyanate-modified graphene oxide) was prepared.Eventually, GNS/IBPIFs with regular cell structures were obtained. The thermal stability and flame resistance of GNS/IBPIFs were measured. The results show that graphene oxide can improve the flame retardant of IBPIF. This is due to the formation of a layered lattice structure and promotion of a dense carbon layer who can play a physical barrier effect when GNS is ignite. With the increasing mount of GO added, the LOI of GNS/IBPIFs have a small increase.The reason is that GO has an agglomeration and can't dispersed unevenly. When the amount of GO added was 4%, the LOI can be increased to 22.2%. Meanwhile, GO as additive almost has no effect on the thermal stability.