Easy route to control density of polyimide foams and impact on mechanical and combustion behaviors

Polyimide foams (PIFs) with different density are rapidly formed basing on water foaming technology of multi-anhydride and multi-isocyanate system in airtight mold. Density of PIFs is controlled by easily route that regulate the ratio of main components total mass and mold cavity volume. Polyimide proportion of PIFs of different series is adjusted by refilled multi-anhydride method. Analyses curves show that density exerts minimal effect on molecular structure of PIFs in the same series. In all series, cellular shapes gradually change from polyhedral to spherical and cellular skeletons become thicker and thicker with the increasing of PIFs density, and the increase in density leads to an obviously enhancement in mechanical properties, including a nearly 300% increase in compression strength, and compression modulus shows an approximately linear growth trend with an increase rate of 6–8 Pa/g. Moreover, the Fourier transform infrared (FT-IR) spectra and thermal gravimetric analysis (TGA) curves show that the density of foams makes slight effect on the molecular structure of the matrix resin in the same series. In all series, flame retardance is also improved with increase in foam density because of the thicker char layer, especially inflected by obviously decrease in peak of heat release rate (PHRR) curves and peak of smoke production rate (PSPR), but increase in limiting oxygen index (LOI) value is only 1–4%. Meanwhile, the effect of density on combustion behavior decrease with the increase in polyimide proportion.