Study On The Preparation And Properties Of Phthalonitrile Resin And Its Composites

With the increasing demand for high temperature resistant materials for aerospace,the development of high temperature resistant resin matrix composites has become the focus of attention.The properties of resin matrix composites mainly depend on the properties of the matrix resin,so it is urgent to develop a resin matrix with high temperature resistance and excellent mechanical properties.Phthalonitrile resin is a high-performance thermosetting resin with good flame retardancy,high temperature resistance and good mechanical properties.However,these resins also have some disadvantages,such as low toughness,long curing time,and poor processing properties,which limit the wide application of phthalonitrile resins.Therefore,this paper focuses on the synthesis of mono/dihydroxy substituted phthalonitrile resin,analysis of curing kinetics and PEK-C toughening and modification of mono/dihydroxy substituted phthalonitrile resin..In this paper,resorcinol and 4-nitrophthalonitrile are used as reactants,dimethyl sulfoxide（DMSO）and dimethylformamide（DMF）are used as solvents,and anhydrous potassium carbonate（K₂CO₃）is used as catalyst.A series of phthalonitrile resin monomers with different material ratios and different synthesis conditions were prepared and analyzed by infrared spectroscopy（FT-IR）,differential scanning calorimetry（DSC）and thermogravimetric analysis（TGA）.The structure and curing characteristics of phthalonitrile resin were studied,and the optimal process conditions and properties of phthalonitrile resin were determined:the reaction temperature was 80℃,the reaction time was 16 h,resorcinol,4-nitro The molar ratio of phthalonitrile and anhydrous K₂CO₃ was 1:2:3,and the solvents were DMSO and DMF.The results of thermogravimetric test are:T_5%in Air atmosphere is 541℃,T_5%in N₂ atmosphere and carbon residue rate at 800℃ are 551℃ and 75%,respectivelyThe non-isothermal DSC test method was used to study the curing behavior,and the curing process of phthalonitrile resin was determined:200℃/1 h+250℃/1 h+300℃/1 h+350℃/4h;The average apparent activation energy Ea of the cured system obtained by the Kissinger and Ozawa method was 85.06 k J·mol,and the reaction order n was 0.951;the relationship between the activation energy and curing degree of the phthalonitrile resin system was established by the equal conversion rate integral method,combined with infrared spectroscopy analysis to further explain the curing reaction mechanism of phthalonitrile resin.In order to improve the toughness of phthalonitrile resin,this paper adopts PEK-C to toughen phthalonitrile resin.The study found that:with the increase of PEK-C content,T_5%decreased,and the carbon residue decreased first and then stabilized;with the increase of heat treatment time,the mass loss increased and the loss rate became slower and slower,and the resin quality was retained after 25 h.When the PEK-C content reaches 6 wt%,the comprehensive mechanical properties of PEK-C/DDS/BPN composites are the best,with tensile strength,compressive strength and bending strength of 46.28 Mpa,256.46 MPa,101.00MPa respectively,the elongation at break increased from 0.57%to 1.26%,an increase of 121.05%.The heat resistance performance decreases,when the PEK-C content reaches 8 wt%,the T_5%（Air atmosphere）decreases by 22.15%from 465℃ to 362℃,and the T_5%（N₂ atmosphere）decreases from 469℃ to 394℃,decreased by 15.99%,and the carbon residue rate first decreased with the increase of PEK-C content,and when the PEK-C content increased to 4 wt%,the carbon residue rate became stable.Using microscopic analysis methods such as SEM,the mechanism of PEK-C toughening phthalonitrile resin was studied.It was found that the toughening mechanism was that PEK-C and BPN resin cooperated to form a network structure.PEK-C formed a network structure through bridging and shearing The phthalonitrile resin is toughened and strengthened by yielding,and the micro-cracks are deflected by filling the defects of the BPN resin,and the cracks change from unidirectional to complex and multi-directional,thus realizing the transformation of the brittle fracture of the BPN resin system into a certain degree of toughness fracture.