The Synthesis And Performance Study Of Heat-resistant Structural Adhesive Based On Polyimide

Abstract:Polyimide is one of the heterocyclic polymer materials used as heat-resistant structural adhesives in the early times.It possessed excellent heat, aging, chemical and atomic radiation restant property, good mechnical and electrical insulating property, as well as low thermal expansion coefficient, and has become one of the chief heat-resistant structural adhesives. However, the electronic polarization and crystallinity of traditional PI lead to strong interaction and tight accumulation of molecular chains, which made PIs infusible and insoluble, as well as hard and brittle, limiting the application of high cost PI. In our study, flexible groups was introduced into PI moleculars using ternary polymerization so as to prepare a relatively lower cost high-temperature PI adhesive with good bonding performance.A series of polyamides (PAAs) and polyimides with different content of BAPP were synthesized using two-step method, with4,4’-Diaminodiphenylmethane (MDA) and2,2’-Bis [4-(4-aminophenoxy) phenyl] propane (BAPP) as the diamines, and3,3’,4,4’-Benzophenonetetracarboxylic acid dianhydride (BTDA) as the dianhydride, and were characterized by means of FT-IR,1H-NMR, TGA, DSC and DTA, and so on. Results showed that the monomers had been transformed into PAA completely. Weight loss rate during the imidization process from PAA to PI were about13%. Besides, it was indicated that imidization process mainly occurred between the temperature range of150℃and280℃. It turns out that the crystallinity declined with the increase of the BAPP content.PI films and bonded joints were produced through theromal imidization. The PI films were characterized by means of FT-IR, TGA, DSC, DMA and XRD, and so on. Results showed that these PIs had been fully imidized and possessed excellent thermal stability, whose initial decomposition temperature and glass transition temperature (Tg) was approximately500℃and300℃, respectively. All the three copolymides showed high loss factor around Tg, and it raised with the increase of BAPP content.Single lap shear tensile test was used to analysis the adhesive performance of PI adhesives, and the influence of monomer type, molar ratio of two diamines, reaction medium, solid content, surface roughness of adherends and imidization process on it was studied. Results demonstrated that PI adhesive with the formula of MDA:BAPP:BTDA=1:2:3and solid content of25wt%had better adhesive performance under room temperature. Besides, higher bonding strength could be obtained when the adherends were burnished with800#sandpapers and the bonding joint was cured under the temperature of150℃,250℃and350℃each for one hour. The PI adhesive showed good bonding strength under high temperature, lap shear strength under the temperature of150℃,250℃and350℃were4.15MPa,2.45MPa and1.91MPa, respectively, which can meet the requirement of high-temperature adhesion in fields such as automobile brake industry.