The Preparation And Heat-resistant Property’s Research Of Polyimide/Sericite Nano Composite Insulating Varnish

Polyimide (PI) is a molecular structure containing imide base of aromatic heterocyclic polymer compounds.With excellent high and low temperature resistance, the range of application:-269℃～400℃; Excellent physical and mechanical properties which is just lower than the carbon fiber; Good electrical insulating performance and keeping stable within the scope of the wide frequency, high temperature field; Excellent chemical stability:Irradiation resistance, resistance to a variety of organic solvents, etc. The high temperature resistance and insulation varnish made from PI had used as insulating treatment in nuke industry and related with aerospace field; special fibre had used as high temperature medium, high temperature filter material and bulletproof fireproof fabric; Ultra high temperature engineering plastics used as aerospace and rocket parts; Plate and rod used as construction material such as since the lubrication, sealing and insulation. But the PI enameled wire insulation varnish the highest heat resistance level is 350℃ in foreign, and 240℃ in our country. Now, to further improve the heat performance of PI is becoming the pressing problem.Sericite and montmorillonite (MMT) as cheap and available layered silicate material, were widely used in rubber, plastic, coating, papermaking, etc field because of the the excellent mechanical properties, insulation and resistance to high temperature performance, In order to further improve the performance of sericite and montmorillonit and increase the compatibility between the polymer and them, in this text we proposed that prepared the sericite and silicon dioxide hybrid system (Sericite-SiO2), and montmorillonit and silicon dioxide hybrid system (MMT-SiO2) respectively by the Sol-Gel method. Specifically, during the hydrolysis and condensation reactions of tetraethoxysilane (TEOS), the silicon dioxide interact with layered silicate in acid circumstance (PH is about 2) to achieve the modified sericite or MMT by silicon dioxide. SEM and XRD results indicated that the SiO2 had well formed on the surface of sericite to form the compact inorganic layer. For MMT, the SiO2 had well formed on the surface of it and exfoliated the MMT slices. Next, thePAA/Sericite-SiO2, PAA/MMT-SiO2 composite material were obtained by the physical blending of PAA and the hybrid of Sericite-SiO2, MMT-SiO2. Then, the PI/Sericite-SiO2, PI/MMT-SiO2 composite was prepared by the thermal imidization of PAA/Sericite-SiO2 and PAA/MMT/SiO2 composite. SEM and XRD results indicated that the moderate pure inorganic hybrid system dispersed evenly in the PI matrix to form the compact inorganic layer network. Thermogravimetric analysis (TGA) and electrical test results show that because of the Sericite-SiO2, MMT-SiO2 were formed in the course of the PI substrate good dense inorganic network structure. The temperature of initial decomposition of polyimide composite materials (PI-5wt%) filled by Sericite-SiO2 hybrid system was improved 40℃ and 17℃ compared with pure PI (PI-0) and polyimide film by adding the same amount of pure sericite (PI-raw). The electrical resistivity of PI-5wt% was 70 times of PI-0 and 2 times of PI-raw. The breakdown voltage had been similarly improved. The temperature of initial decomposition of polyimide composite materials (PI-5wt%) filled by MMT-SiO2 hybrid system was improved 28℃ and 18℃ compared with pure PI (PI-0) and polyimide film by adding the same amount of pure MMT (PI-raw). The electrical resistivity of PI-5wt% was 30 times of PI-0 and 1.6 times of PI-raw. The breakdown voltage had been similarly improved. So the layered silicate sericite and MMT had synergistically improved the thermal stability, electrical resistivity and intensity of electric shock of PI film.In order to further improve the dispersion of sericite in polymer matrix, in text we used the chitosan which was widely used in bionic materials to improve the orientation texture of sericite. The chitosan/sericite composite film was achieved by blending platelike sericite and common chitosan. The Scanning electron microscope result indicated that the composite film had typical "brick-mortar" orientation microstructure. With the incorporation of sericite, the mechanical properties (tensile strength and elongation at break) had been improved apparently and preserve the good flexibility on account of the unique structure. Meanwhile, the composite film had the excellent transparency and insulation.