Research Of High Performance Poltimide And Epoxy Resin Adhesive System

Aromatic polyimide(PI), being one of the high-performance polymers, exhibits good thermal stability, mechanical strength, chemical solvent resistance, dimensional stability, and excellent radiation resistance to creep and wear. Due to their unique and specific properties, Pis have been widely used as films, composite matrix, coatings, membranes, and high-temperature structural adhesives. The high glass transition temperatures (Tg) of these polyimides are attributed to their stiff polymer backbones which contain aromatic rings. PIs can be applied in adhesive joints in different forms. Some of the common methods used for this purpose are PI films, glass fiber cloth impregnated with PAA solution. Polyimide adhesive system can be able to use for a long time between 260-350?. However, most polyimides are often insoluble and intractable in their fully imidized form and therefore, show poor processability. When it used as adhesive independently, limits its application because of poor adhesion, high curing temperature, large molding shrinkage restrict their application. Compared to polyimide, epoxy resin(EP) possess excellent adhesion properties, lower molding temperature, but the relative lower thermal stability. Owing to its excellent performance, EP plays a significant role in the adhesive area. Therefore, great attentions have been drawn for the combining of the advantages of PI and EP materials.The methods of polyimide modified epoxy resin have three aspects as reported:(1) Physical blending of unreactive linear polyimides, Hay et al studied the molding of linear unreactive polyimide for toughening of the epoxy-resin-diaminodiphenyl-sulfone system. The fracture toughness was increased with no loss of modulus, but there was a little influence on the thermal capability across the range of modifying polyimides. (2) Bringing the imide ring into the epoxy resin skeleton, Tao developed a novel imide ring and siloxane-containing cycloaliphatic epoxy compound BISE by a two-step procedure, which was then thermally cured with alicyclic anhydrides. They found that fully cured BISE epoxy resins had good thermal stability with thermal decomposition temperature. They also found that BISE epoxy resins exhibited good mechanical properties both flexural strengths and tensile strengths.(3)Using polyimide as curing agent to cure epoxy resin, T. Agag made another approach to prepare PI-EP composites with high thermal stability and good mechanical properties by using soluble reactive polyimide containing hydroxyl functionalities as hardener. The presence of polyimide is capable of exhibiting flexibility characteristics and noticeable thermal stability as well as being curing agents on their own. Soo-Jin Parkobtaineda sulfone-containing epoxy resin cured with phthalic anhydrideand tetrahydrophthalic anhydride. They also found that the tetrahydrophthalic anhydride system showed better thermal properties, including glass transition temperature, initial decomposition temperature and activation energy for decomposition than those of the phthalic anhydride system. All the previous works above make us believe that by choosing appropriate method, it is possible to synthesize a PI-EP composite that can finely combine the advantages of the two materials.In this study, we made another approach to prepare PI/EP composites with high thermal stability and good adhesion properties. Fluorine polyimide with design molecular weight of about 2000 was used as PI moieties for its good solubility and was end capped with KH550. EP containing siloxane (SEP)was employed according to Zhang's research. With the help of hydrolytic polycondensation of siloxane, a highly cross-linked polymer can be derived. The condensation process is shown in Scheme 1.The synthesized polyimide, silcane ended polyimide, and SEP are characterized by Fourier transform infrared (FTIR) spectroscopy and'HNMR.Microstructure of the crosslinking system was investigated with scanning electron microscopy (SEM). Effects of material composition on gas barrier, thermal stability, mechanical strength, and optical clarity of epoxy resin, and a series of materials are also studied by gel permeation chromatography(GPC), thermogravimetric analysis (TGA), differential scanning calorimetry(DSC), respectively. The chemical composition and bond properties of the system are discussed herein, and more details of the novel two-step synthetic route ofsilcane ended polyimide will be described in a subsequent paper.