Study On Toughening Of Benzoxazine Resins And Their Carbon Fiber Composites

Benzoxazine(BOZ),a novel class of thermosetting resins,display many excellent properties,such as excellent mechanical properties,near zero volumetric shrinkage,low water absorption and high char yield on decomposition.BOZ resins have been widly used in aerospace,electronic packaging materials,rail transportation and other fields.However,the shortcoming of BOZ is an inherent brittleness as is common for thermosets,which limits their application.The purpose of this paper is to improve the toughness of BOZ resin without sacrificing its strength and thermal properties.This paper's researching idea is to synthesize a new type of hyperbranched epoxy resin as a toughening agent and blend it with BOZ to balance the toughness,strength,modulus and thermal properties of the resin.Secondly,PPBES/HBPEE/PA modified carbon fiber(CF)/BOZ composite was prepared by bulk resin modification and Interlaminar toughening.Finally,organic-inorganic hybrid toughening agent GO-HE was prepared by combining organic HBPEE with inorganic graphene oxide.The GO-HE was used to toughening of BOZ resin and CF/BOZ composite,respectively.The main work of this paper is as follows:Firstly,novel hyperbranched polyether epoxies(HBEs)containing benzene,biphenyl and bisfluorene(named HBE-1,HBE-2,HBE-3)were synthesized,respectively.Then,these highly effective HBEs were incorporated into polybenzoxazine(PBOZ)network at different concentration by solution blending method.Effects of backbone and loading amounts on the curing behavior,mechanical and thermal properties of BOZ/HBE hybrid have been investigated.The general trend shows that the impact strength,flexural strength and storage modulus all first increase and then decrease with the three kinds of filler content.Among the backbone structure study,HBE-3 which has the highest proportion of benzene rings can simultaneously improve impact strength,flexural strength and storage modulus.In contrast,HBE-1 and HBE-2,which have less benzene rings and smaller intramolecular cavities,perform worse in mechanical and thermal properties.Both backbone structure and loading of HBEs play important roles in determining the crosslink density and structures of non-phase separated network.Secondly,in order to investigate the effect of molecular weight and loading amounts of hyperbranched epoxy resin on the properties of blend system,HBPEE containing bisfluorene was used as a toughener for bis-benzoxazine resin(MDA-BOZ).Effects of loadings and molecular weight of HBPEE on curing behavior,rheology properties,static mechanical,dynamic mechanical and thermal properties of the hybrid systems have been studied.Results show hybrids containing HBPEE-2(Mn=3900)modifier possesses optimal comprehensive properties.Synchronous improvement of impact strength,flexural strength and storage modulus was found in hybrids containing 5%HBPEE-2 and 5%HBPEE-3,where HBPEE served as a non-phase-separation modifier confirmed by SEM and DMA.Notably,there was a 2.8-fold improvement in impact strength for hybrids containing 10%HBPEE-2,compared to the same properties of samples without HBPEE.This was achieved without any compromise of flexural modulus,flexural strength,E',thermal stability and glass transition temperature(Tg).The results demonstrate that multiple factors,such as enhanced crosslinking density(ve),rigidity of the toughener,reduced hydrogen bond network and higher fractional free volume,contributed to such improvement.In order to expand the application of BOZ resin in the field of carbon fiber composites,the toughening of CF/BOZ composites were studied in this paper.PPBES,HBPEE and PA-6 particles were incorporated into the interlayer of CF/BOZ composites.The performance of CF/BOZ was also investigated.The results have shown that the GIC,flexural strength,flexural moduli and ILSS of PPBES/HBPEE/PA-6 modified composites increased by 177%,9%,17%,19%compared to that of unmodified composites.At the same time,the Tg of modified system has not been compromised.The reason is that PA particles help to prevent the crack growth in the interlayer due to the filtration of the fiber bundle.Secondly,PPBES and HBPEE,which played a reinforcing and toughening role,were uniformly distributed in the whole composites and the strength was improved simultaneously.The fracture surface of composites was observed by SEM,and the toughening mechanism was discussed.The toughening mechanism mainly includes PA particle pull-out,crack tip blunting caused by HBPEE intramolecular cavities,ductile tearing of PPBES continuous phase and plastic deformation of PBOZ inversion phase.In conclusion,the incorporation of PPBES,HBPEE and PA-6 particles has greatly improved the toughness of CF/BOZ composites without reducing the strength,heat resistance and hygrothermal properties of the composites.Hyperbranched polyether epoxy(HBPEE)grafted graphene oxide(GO-HE)was designed to improve the dispersion/exfoliation and interfacial interaction between graphene oxide and benzoxazine(BOZ).The structures and morphologies of graphene oxide(GO)and GO-HE sheets were characterized by FT-IR,XRD,XPS,Raman spectroscopy,TEM and AFM.BOZ composites containing GO and GO-HE were prepared with different loadings and systemically investigated.GO and GO-HE produced 88%and 139%improvements,respectively,in impact strength at 0.05wt%loading.The three-point bending test indicated that the BOZ/GO-HE composites exhibited higher flexural strength and modulus than the neat BOZ or BOZ/GO composites.For BOZ composites containing 0.05wt%GO-HE,the flexural strength increased by 14.4%,and the flexural modulus increased by 10.0%compared to that of neat BOZ.Furthermore,the Tg and thermal stability of BOZ/GO-HE composites were significantly improved compared to those of neat BOZ and BOZ/GO composites.These improvements can be ascribed to better dispersion of GO-HE and a stronger interfacial interaction between GO-HE and the matrix,according to TEM and SEM analyses.The T700 carbon fiber was combined with pure BOZ,BOZ/GO and BOZ/GO-HE by the method of moulding molding,respectively.The CF/BOZ,CF/BOZ/GO and CF/BOZ/GO-HE composites were obtained.The results showed that the flexural strength and ILSS increased firstly and then decreased with the increase of filler,and the toughening effect of GO-HE was better than that of GO.When the GO-HE loading is 0.05wt%,the properties of the composites are improved by 160%,the flexural strength is increased by 26%,and the flexural modulus is increased by 19%.The toughening mechanism of GO-HE was discussed,mainly including crack pining,crack deflection and plastic deformation of matrix resin around the GO-HE sheets.The reason why the toughening effect of GO-HE is better than GO is that the dispersion and interfacial strength of GO-HE in BOZ resin are better than that of GO.