Preparation And Modification Of Isocyanate-based Polyimide Foams

Polyimide(PI)foam is one of the best thermal stability material in polymer foams with excellent flame retardancy as well as fine mechanical properties,which is widely used in aerospace and electronics industries.However,the production process of PI foam is difficult to control and costly,which greatly limits the wide application of PI foams.Isocyanate-based PI foams are increasingly concerned by researchers due to the simple preparation process,affordable equipment requirements and low manufacturing cost in recent years.Firstly,a series of PI foams with different ratio of the amounts of functional groups of isocyanate/anhydride were obtained with dianhydride and isocyanate as the raw materials in this work,the cell structure,apparent density,mechanical properties,thermal and flame retardant properties of the resulting foams were characterized.The results showed that PI foams prepared in this study exhibited high open-cell ratio,the expansion rate and specific compression strength increased initially and then decreased with the increase of isocyanate index.Correspondingly,both the thermal stability and flame retardancy reached optimum when the isocyanate index was 1.2.The contents of two catalysts,dibutyltin dilaurate(DBTDL)and triethanolamine(TEOA)were changed,respectively.The catalytic effect of two catalysts in the synthesis of PI foams was studied.The results showed that when the DBTDL content was 1.85% and the TEOA content was 1.48%,PI foam exhibited fine mechanical properties as well as excellent flame retardancy.DBTDL catalyst was sensitive to the gel reaction during the preparation of the prepolymer,which can effectively enhance the chain growth,so the flame retardant performance of PI foams significantly improved with the increase of DBTDL.Meanwhile,the TEOA catalyst had a great influence on the foaming process,not only accelerated the foaming reaction but also as reactant played a role in crosslinking,resulting in dense cell morphology,so the mechanical performance of PI foams greatly enhanced with the increase of TEOA.Secondly,a kind of liquid physical foaming agent,HCFC-141 b,was introduced and used in combination with water to prepare PI foams,which was intended to reduce the viscosity of the prepolymer while reducing side reactions of the system.It had been found that an increase in 141 b effectively reduced the viscosity of the prepolymer,so that the foaming reaction can be carried out more easily.With the increase of 141 b content,the open-cell content of PI foams showed a downward trend,the compressive strength increased initially and then decreased,and reached the maximum value when the 141 b content was 10%.The TG-FTIR combined data showed that the addition of 141 b effectively reduced side reactions of the system.However,the incorporation of excess 141 b caused the foam to collapse,which in turn affected the performance of the foam.When the content of 141 b was 10%,the overall performance of PI foam was optimized.Afterwards,graphene oxide(GO)was used as chain growth points to prepare GO/PI composite foams by in situ polymerization,then the surface modification of GO by dopamine(Do@GO)improved the dispersion of GO in the foam matrix.The results showed that the addition of GO increased the degree of imidization of PI foams,the mechanical properties and thermal stability of PI foams were also improved.GO exhibited fine dispersion in solvent by forming ?-? conjugate with dopamine,further improving the thermal stability and flame retardancy of PI foams,high-performance GO/PI composite foams were obtained.Finally,red phosphorus(RP)was grafted to GO and then introduced into PI foams to prepare flame-retardant foams.The results showed that the addition of RP reduced the mean cell diameter of PI foams,which not only improved the mechanical properties,but also enhanced the flame retardancy of PI foams,the limiting oxygen index(LOI)was improved by 4.3% and the peak heat release rate(PHRR)was reduced by 39%.The addition of RP increased the heterogeneous nucleation points of PI foams during the foaming process,which acted as condensed phase flame retardant in the polymer matrix during combustion,so the mechanical properties and flame retardancy of PI foams were significantly improved.