Polyimide High Temperature Resistant Polymer Material

Typical high temperature resistant polymer materials include polyimide, trapezoidal polymer, polyphenylene sulfide resin, etc.

Polyimides are generally obtained by polycondensation of binary anhydrides and diamines. According to the structure, polyimides can be divided into aliphatic polyimides and aromatic polyimides. Aliphatic polyimide is not resistant to high temperature and has poor practicability. Aromatic polyimides can be divided into several types. According to the type of chain extension and crosslinking reaction, aromatic polyimides can also be divided into condensation type and addition type.

(1) Polycondensated polyimide

It is prepared from aromatic diamine and aromatic dianhydride. The preparation process consists of two steps: polycondensation and amidation:

① The formation of high molecular weight linear polyamic acid prepolymer can be completed instantaneously at room temperature. Polar solvents are used.

② Under the conditions of heating and pressure, water or alkyl alcohol is removed from the polyamide acid solution, and polyimide is formed by amidation or curing.

Due to the high reaction temperature in the second step, small molecules are released, which is easy to produce pores, resulting in the decline of product quality. There are two improved synthesis routes:

① Polyimide, the precursor of polyimide, was synthesized first and then isomerized to polyimide. No small molecules were released during the isomerization reaction.

② Tetra acid and diamine first form nylon salt. Then it is heated and dehydrated to produce polyimide Avoid the use of polar solvents with high boiling points in the reaction, and some varieties can also be granulated by reactive extrusion in an extraction extruder.

Pyromellitic anhydride diphenyl imide is the first commercialized polyimide, but it is insoluble in organic solvents and has poor processing performance, so its use is limited The development of soluble and fusible polyimides has attracted more and more attention. At present, the main ways to improve polyimide are:

① Chemical bonds with large flexibility and high thermal stability are introduced into the main chain.

② Substitute groups with asymmetric structure and high thermal stability, such as m-phenylene or o-phenylene, are introduced into the main chain, or polyimide monomers with non planar twisted structure are used.

③ Introduce large side groups, such as phenyl or fluorenyl.

④ For example, two different binary anhydrides or two different diamines are used for CO condensation polymerization.

(2) Additive polyimide

Polymaleimide, norbornenyl terminated polyimide resin and other polyimide amine resins must be cured by addition polymerization, which is called addition polyimide. Addition polyimides are usually prepared by homopolymerization or copolymerization of low molecular weight polymers with cyclic structure with unsaturated groups at the ends through unsaturated end groups.

There are two kinds of crosslinking reactions of polyimides with addition polymers: end group reaction and bismaleimide reaction. Among them, end group reactions can be further divided into two types: end group reaction a (dicarboxylic acid end group) and end group reaction B (acetylene end group).

① End group reaction A.

The most important resin obtained from the crosslinking of dicarboxylic acid end group is the monomer in situ polymerization polyimide resin (PMR) developed by NASA Lewis Research Center. PMR polyimide resin is the solution of dialkyl ester of aromatic tetra acid, aromatic binary amine and monoalkyl ester (NE) of 5-norbornene-2,3-dicarboxylic acid in an alkyl alcohol. This resin can be directly used to impregnate fibers. At present, there are two kinds of PMR polyimide resins, PMR-15 (first generation) and pmr-ll (second generation).

② End group reaction B.

End group reaction B means that two polyimide monomers with acetylene end groups are connected together and then connected with other polyimide monomers to form acetylene end group polyimide.

③ Polybismaleimide (BMI).

BMI is a branch of additive polyimide, which is obtained by condensation polymerization of maleic anhydride and aromatic diamine. The operating temperature is 150 ℃～ 250 ℃.

The crosslink density of pure BMI is too high, and the brittleness of BMI needs to be improved by modification There are usually polyamino modified bismaleimide, epoxy modified bismaleimide and other modified systems.

Polyamino modified bismaleimide (pabmi) has the same viscoelasticity as thermosetting resin and can be processed by general methods; When curing, there is no small molecule volatilization, and the composite material produced is free of free air L; Easy to mix with various fillers; The price is relatively cheap; Stable performance. Bismaleimide modified by epoxy resin has high cohesion.