High Performance Intrinsic Flame Retardant Bismaleimide Resins Based On Disulfide Bond And Their Reprocessibility

High performance thermosetting resins(HPTRs)play a pivotal role in many cutting-edge fields such as new energy,printed circuit board,electronic packaging,aerospace and so on owing to their excellent integrated performances.Requirements for heat resistance,flame retardancy and recyclability of HPTRs have continuously increased with the dramatic growth of modern science and technology.Phosphorus-containing flame retardants are proved to be a type of effective halogen-free flame retardants,however,they often deteriorate the heat resistance of heat-resistant resins,and their negative impacts on the environment cannot be neglected.Therefore,developing halogen-free and non-phosphorus flame-retarding polymers and evaluating their reprocessibility have attracted attention from researchers in recent years.Bismaleimide(BMI)resins are typical representatives of HPTRs,however they also have inherent defects such as inflammability and poor recyclability as almost all thermosetting resins.This thesis focuses on these two intrinsic problems of HPTRs by taking BMI resins as an example.Firstly,a novel disulfide-containing aryl allyl ether compound(DS)was designed and synthesized,then BDS resins were prepared by the copolymerization of DS and N,N'-4,4,-bismaleimidodiphenylmethane(BDM).The relationships between the structure and the performances of BDS resins were systematically investigated and compared with the classic BDM/2,2'-diallylbisphenol A(DB)resin(BDB-0.86),which is considered as one of the modified bismaleimide resins with the best integrated performances.Results show that the mole ratio of DS and DBM has significant effects on the performances of BDS resins.Glass transition temperature(Tg)and flexural modulus decreases as the mole ratio increases,while the impact strength increases and the flexural strength initially rises then drops.Among them,BDS-0.86 shows the best integrated performances.BDS-0.86 resin exhibits outstanding flame retardancy,the limiting oxygen index reaches 32.6%.The heat releasing capacity,peak heat release rate and total heat release of BDS-0.86 are only 33.2%,33.2%and 56.9%of the corresponding values of BDB-0.86,respectively.In addition,the flexural strength of BDS-0.86 reaches 128.2�8.6 MPa,which is comparable with that of BDB-0.86.Meanwhile,Tg of BDS-0.86 reaches up to 336?,which is not only 27? higher than that of BDB-0.86,but also the highest value among halogen-free and non-phosphorus flame-retarding BMI resins.The origin of superior integrated performances of BDS-0.86 was intensively discussed from molecular structures and curing mechanism of resins.Secondly,BDS-0.86,which possesses the best integrated performances,was chosen to investigate its reprocessibility.The reprocessing conditions were determined by thermogravimetric and dynamic mechanical analyses,then the resin was grinded into powders and remolded at 320? for 2 h with a pressure of 40 MPa.Fourier transform infrared(FT-IR)spectra and tensile tests were uesd to investigate the changes in chemical structure and mechanical properties of resins before and after remolding.Results show that there is no significant differences in FT-IR spectra of BDS-0.86 resins before and after remolding,while the tensile strength,tensile modulus and elongation at break of remolded samples are 43.1%,53.1%and 75.0%of that of pristine BDS-0.86 resin,respectively.These values are at low level compared with reprocessible thermosetting resins based on disulfide bond reported in literatures,this is attributed to the narrow gap between Tg and the temperature that decomposition occurs.Interestingly,the tensile properties of remolded BDS-0.86 are taking the leading positions in reprocessible thermosetting resins based on disulfide bond reported so far.