Synthesis And Modification Of Phthalonitrile Resins For Adhesive Materials

Synthetic resin adhesives are increasingly widely used in aviation,aerospace,shipbuilding,and other fields.The design concepts have been more stringent to the adhesive to obtain high-temperature resistance,easy processing,long life,high toughness,and low dielectric performance.Phthalonitrile(PN)is a type of high-performance thermosetting resin,which was developed by the U.S.Naval Laboratory in the 1970s.These resins not only possess excellent heat resistance,mechanical properties,and flame retardant properties but also own an excellent adhesion due to the polar cyano groups in the molecule,which make them have potential in structural bonding applications.In this paper,many new types of PN resins were designed from the perspective of molecular design,owning a low melting point,autocatalysis,and imide ring structure,which reach the special requirements of processability and high-temperature resistance for the high-temperature resistant resin adhesives at the same time.Many strategies have been performed to satisfy the construction technology and bonding requirements of high-temperature resistant adhesives,and decrease the sacrifice of the high-temperature resistance and mechanical properties of the resin to be least by matching and modifying the different resins.Aiming at the problems of high melting temperature and poor processibility of the traditional PN resins,we synthesized four kinds of low-melting PN resin monomers via nucleophilic substitution reaction.Compared with the biphenyl PN,the melting point of the resin monomers prepared in this paper was significantly lower due to the reduced rotational resistance in the molecular chain.The PN resin prepolymers were prepared with p-phenylenediamine as a curing accelerator,and the influence of resin structure on processing,thermal,thermomechanical,and adhesive properties was investigated.The results showed that the PN prepolymers had a wide processing temperature window(47-99℃),the glass transition temperature(T_g)of the cured products was 255-311℃,and the concentration of cyano functional groups and crosslinking density played a leading role in increasing the T_g.Due to the introduction of flexible groups,the thermal stability of the cured resins under a nitrogen atmosphere was slightly reduced,while the lap shear strength(LSS)at room temperature and the high temperature was significantly improved.Among them,the LSS of the bisphenol P-type PN(BPPN)at room temperature and 300℃were 22.7 MPa and 12.8MPa,respectively.Therefore,BPPN possessed a better comprehensive performance.Two autocatalytic diamine-functional PN resin monomers,namely PN-m and PN-p,were synthesized through nitration,reduction,and nucleophilic substitution using bisphenol A as raw material,aiming to solve the problem that the cured PN resins were not dense caused by the volatility of the small molecule external curing accelerator.The effect of the curing process on the thermal properties,T_g,and bonding properties of diamine-functional PN was studied.Then the diamine-functional PN resins were blended with BPPN,and the effects of the type and amount of diamine-functional PN on the overall performance of the autocatalytic PN/BPPN blends were systematically studied.The results showed that diamine-functional PN monomers had a self-polymerized behavior without any external accelerator,and an appropriate increase in the heat treatment temperature could improve their thermal stability and T_gbut had little effect on the bonding performance.Because of the difference in the substitution positions of cyano groups,the reactivity of PN-m/BPPN was higher than PN-p/BPPN under an equal concentration of diamine-functional PN.With the increase of the diamine-functional PN,the curing speed of the resin systems was accelerated,and the T_gof the cured product gradually increased and then tended to be stable,while the thermal decomposition temperature and adhesive properties first increased and then decreased.The PN-p/BPPN resin system(MPN)with a PN-p content of 10 wt%could be used as a resin matrix for high-temperature structural bonding.To further improve the temperature resistance properties of PN resin,four kinds of imide-containing PN monomers were synthesized.The effects of the symmetry of the molecular structure and the introduction of methyl side groups on the comprehensive properties of the PN resins were investigated in detail.The results indicated that when the nitrogen atom on the imide ring and the ether bond was arranged in meta positions on the same benzene ring,the resin had a lower melting temperature and higher bonding performance.When the methyl pendant groups were introduced into the resin molecule,their solubility,T_g,and carbon residue rate at high temperature could be further improved.When the imide-containing PN resins were used for bonding materials,they did not contain organic solvents and no small molecules were released during the curing process.The results showed that the LSS was 15.4-24.4 MPa at 25℃and 6.5-10.8 MPa at 300℃,respectively,and could be used for large-area structural bonding.At the same time,the problem of poor processibility for traditional polyimide adhesives was solved by this method.The copolymerized polyarylether nitrile (PFPEN) and divinylsiloxane-dibenzocyclobutene(DVS-BCB)were used to modify MPN,respectively,to further improve the overall performance of the MPN resin,and the corresponding modification mechanisms were analyzed.The entanglement of the macromolecular chain of PFPEN could hinder the cyano polymerization reaction,but it has not changed the curing mechanism of the MPN resin.With the increase of PFPEN content,the fracture morphology of the resin system was gradually transformed from brittle rupture to ductile rupture.The mechanical properties were significantly improved,while the modified resins still maintained a good heat resistance and thermomechanical properties.DVS-BCB had a stimulating effect on the polymerization of MPN,and the processing technology was significantly improved as well.The addition of a small amount of DVS-BCB could destroy the regularity of the cross-linked structure,as a result,the toughness of the blend system was slightly improved,however,the thermal stability and thermomechanical properties of the modified resins were better than those of the pure MPN.Finally,a modified PN resin film adhesive was prepared by several kinds of PN resins prepared in this paper and the modifiers mentioned above.The film adhesive had an excellent thermal aging resistance and damp heat resistance,which could be used for structural bonding of various materials in the field of high-temperature resistance.