Various types of polyimide and how to use it

The polycondensation type aromatic polyimide is prepared by the reaction of aromatic diamine and aromatic dianhydride, aromatic tetracarboxylic acid or aromatic tetracarboxylic acid dialkyl ester. Since the synthesis of polycondensation polyimide is carried out in high-boiling aprotic polar solvents such as dimethylformamide, N-methylpyrrolidone, etc., polyimide composites are usually formed by prepreg process, these high-boiling aprotic polar solvents are difficult to volatilize cleanly during the preparation of prepregs, and volatiles are also released during the cyclization (imidization) of polyamic acid, which is easy to be used in composite products. Pores are generated, and it is difficult to obtain high-quality, void-free composites. Therefore, polycondensation polyimide has been less used as the matrix resin of composite materials, mainly used to manufacture polyimide films and coatings.

polyaddition

Since polycondensation type polyimides have the above-mentioned disadvantages, in order to overcome these disadvantages, addition polymerization type polyimides have been successively developed. The widely used ones are mainly polybismaleimide and norbornene-terminated polyimide. Usually, these resins are low molecular weight polyimides with unsaturated groups at the ends, which are polymerized through unsaturated end groups when used.

(1) Polybismaleimide

Polybismaleimide is formed by polycondensation of maleic anhydride and aromatic diamine. Compared with polyimide, its performance is not bad, but the synthesis process is simple, the post-processing is easy, and the cost is low, and it can be easily made into various composite materials. But the cured product is brittle.

(2) Norbornene-terminated polyimide resin

The most important of them is a type of PMR (for insitu polymerization of monomer reactants, in-situ polymerization of monomer reactants) type polyimide resin developed by NASA Lewis Research Center. PMR type polyimide resins are made by dissolving monomers such as dialkyl esters of aromatic tetracarboxylic acids, aromatic diamines, and monoalkyl esters of 5-norbornene-2,3-dicarboxylic acid in one In an alkyl alcohol such as methanol or ethanol, the solution can be used directly to impregnate fibers.