Studies On The Modifications Of Bisphenol A-based Phthalonitrile Resins And Their Corresponding Composites

Bisphenol A-based phthalonitrile resin,as a typical phthalonitrile resin,exhibit excellent mechanical,thermal,temperature-resistant and other physical and chemical properties,and they have potential application prospects in aerospace,ship warships,electronic materials,automotive industry,adhesives and other fields.However,the special molecular structure also results in high melting point,difficult dissolution,high processing temperature,high curing temperature and long curing time,which limits its practical application.To design the molecular structure of the modified Bisphenol A-based phthalonitrile resins,it is a challenge to balance the processing properties of the resin and the final properties of the composites.This study expects that the outstanding curing performance of amino compound modified resin systems and the good processing performance of hydroxyl compound modified systems could be integrated by molecular structure design.In this paper,amino compound 4,4-diaminodiphenyl sulfone(DDS)was selected to catalyze the formation of rigid resin prepolymer(PNP)from bisphenol A nitrile-based resin monomer(BAPH).Then different hydroxyl compounds were copolymerized with PNP to from PNP/hydroxyl compound copolymers.The effects of different hydroxyl compounds on the curing behavior,reaction mechanism,product structure,processing properties and final properties of copolymer resins were investigated by DSC,reaction kinetics,FTIR,UV-vis,SEM,DAR,ST,MT,SEM and TGA.The relationship between copolymerization structure and properties was established.The similarities and differences of copolymerization were summarized.The method of carbon fiber or multi-component and multi-size fillers were used to reinforce the obtained copolymers.The properties of copolymer composites at room temperature,high temperature,and after high temperature aging were compared.The research can help further enrich phthalonitrile-based copolymerization systems,recognize mechanism and behavior of phthalonitrile copolymerization reaction,and expand their application fields.The chain-extension copolymerization reaction of BPA monomer with PNP is a repetitive heteropoly reaction.When the conversion is less than 0.4,the reaction belongs to irregular diffusion reaction,corresponding to the step-by-step reaction of hydroxyl group on BPA and nitrile group on PNP to form indole ring.When the conversion is more than 0.4,the reaction belongs to self-catalytic reaction,corresponding to the formation of phthalocyanine rings by the spontaneous assembly of indole rings.As linear phenolic resin(PEP),which can directly provide hydroxyl groups,is constituted by a series of BPA segments through-CH2-parallel link,its chain-extension copolymerization reaction with PNP is similar to that of BPA.In essence,it is a reaction of the hydroxyl group on PEP and nitrile group on PNP.The copolymerization of benzoxazine(BZ)resin with PNP is a multi-step complex reaction.The first reaction is that oxazine ring-opening self-polymerization produces hydroxyl groups.It is a one-step autocatalytic reaction and its activation energy increases with the conversion.The second reaction is the copolymerization of hydroxyl groups on BZ after ring-opening with nitrile groups on PNP.It is a multi-step complex reaction,and the conversion decreases with the increase of the conversion.The first and the second reaction form a competitive relationship.Compared with those of PNP,the curing properties and processabilities of PNP/BPA,PNP/PEP and PNP/BZ copolymers with segregated structures were improved obviously,i.e.the curing temperature was decreased,the complex viscosity was decreased,the gelation time was shortened,the solubility in low boiling point solvent was improved,and the wet prepreg process was satisfied.Compared with those PNP/GF composites,the copolymer resin/GF composites obtained under relatively mild preparation conditions have better mechanical properties,interfacial properties and heat resistance.By analogy,the above-mentioned hydroxyl compounds were further upgraded,and the results confirmed that the designed PNP/hydroxyl compound copolymers can exhibit both good processing properties and excellent final performance.Among them,the key factor affecting the curing behavior of copolymer resin is the source way of hydroxyl compound,that is,whether the compound directly or indirectly provides hydroxyl.The key factor affecting the rheological properties of copolymer is the type of hydroxyl compounds available,that is,whether the compound is monomer or resin.Mechanical properties of PNP/hydroxyl compound/CF composites at room temperature are determined by the copolymer structure of resins,PNP/BZCN/CF exhibited the best comprehensive mechanical properties since PNP/BZCN copolymer has a "network connected network" structure.In a word,compared with those of PNP/CF composites,the processing conditions of PNP/hydroxyl compound/CF composites are milder,but their comprehensive properties at room temperature,at 300 ℃ and after aging at high temperature are better.Those composites have obvious high-performance characteristics and the service temperature may reach to 350 ℃.However,after aging at 400 ℃ for 50 h,the mass retention rate was only 66%,due to the more serious decomposition of resins.To further improve the performance of PNP/hydroxyl compound copolymer composites,the multi-element and multi-size reinforcing scheme was used and silicon powder(MS)was in situ introduced into PNP/BP copolymer.Compared with PNP/BP/GF composites,PNP/BP/MS/GF composites have better mechanical properties and high performance material characteristics.This is mainly attributed to additional reinforcement provide by MS,resulting in the reduction of free volume and matrix enrichment area within the composite.After thermal aging at 400 ℃ for 1 h and 12 h,the mechanical properties of PNP/BP/MS/GF composites were maintained at 90% and 70% respectively.The flexural modulus and strength of all PNP/BP/MS/GF composites are still higher than 10 GPa and 300 MPa even after thermal aging at 400 ℃ for 24 h,which can provide sufficient mechanical properties for most applications.Scanning electron microscopy results show that with the aging process,the resin on the surface of the fiber becomes thinner,cracked or even disappeared,and the interfacial composite structure between the fiber and the fiber is destroyed until it disintegrates.The high performances of the modified Bisphenol A-based phthalonitrile composites were further realized by multi-element and multi-size reinforcement,which can be used for a short time at 400 ℃.It may have certain inspiration and reference significance for the future design of other high performance thermosetting resin composites.