Preparation And Corona-Resistance Properties Of Thermosetting Polyimide/Inorganic Oxide Nano-Composite

Polymer/inorganic nano-composites have attracted extensive research interests and become an important research topic in materials science due to their unique properties superior to single material and conventional composites. Many properties of nano-composites are greatly enhanced by the inorganic particles well dispersed in polymer matrix in nanometer scale and therefore nano-composites always have the advantages of inorganic and polymer materials. Polyimide (PI), one of the most important super-engineering plastics, exhibits excellent physical, chemical and electrical properties at a large temperature range and its products are widely used. But few reports can be found on its nano-doping modification in application to insulating adhesives and varnish materials.In this paper, bismaleimide (BMI) resin was used as basic resin and the polymer of high polymerization degree (n=15) as organic precursor to improve toughness and solubility of materials. The materials as prepared can be dissolved in dimethylacetamide (DMAc), dimethylformamide (DMF), 1-methyl-2-pyrrolidinone (NMP) and other aprotic polar solvent. Cross-linked network forming by double bonds in thermosetting PI can greatly enhance breakdown strength and corona-resistant time of the materials and improve compatibility between inorganic and organic phases with better dispersed inorganic nanoparticles in the network.The present study aimed at developing thermosetting PI hybrid materials doped with alumina nano-particles. Alumina nano-particles were prepared via sol-gel method using isopropanol aluminum as precursor. Alumina/PI hybrid materials were prepared using anti-feed method. The experimental results revealed that heat resistant and corona resistant properties increase linearly with the amount of alumina. The corona resistance time for the 25μm thick sample with 16wt% alumina on the steel plate was 62h, implying that the hybrid materials possessed better corona resistant properties than Kapton100CR because of good thermal conductivity of steel plate. Compared with 23wt% alumina content of Kapton100CR, the doping dosage of alumina in the hybrid materials was only 16wt%. The breakdown strength of the hybrid material was 196kV/mm, 280% times that of national standards. Additionally, the corona resistant properties were related to thickness of the material. The thicker the material, the longer the corona-resistace time.Alumina/EP-PI was also prepared in the paper. With the increase of inorganic doping, heat resistance and corona resistance were enhanced, dielectric constant and dielectric loss increased. When the content of inorganic doping was 16wt% and the thickness of sample was 25μm and the breakdown strength was 60kV/mm, the corona resistant time was 49h, closed to that of Kapton100CR. The breakdown strength was 180kV/mm, 250% times that of national standards. The results of shear strength and rapid tensile strength showed that the strength of alumina/EP-PI was much higher than that of alumina/PI thanks to the addition of epoxy resin in matrix. The hydroxyl groups in epoxy resin contributed to the adhesive strength.Fourier transform infrared spectroscopy (FTIR) was used to analyze chemical structure of composites. The aluminum content was analyzed by point scanning and face scanning of energy dispersive spectrdmeter (EDS). The influences of coupling agent (vinyl (6-methoxy- ethoxy) silane (A-172)) on the dispersibility of inorganic particles and aggregation structure of composites were evaluated by scanning eletronic microscope (SEM) and atomic force microscopy (AFM). The results showed that dispersibility of inorganic particles in organic matrix was promoted by coupling agent which could weaken the interactions of inorganic active group and decrease conglomeration of secondary particles.The experimental results confirmed that performances of nano-composites were different from the traditional materials. The active groups in alumina can form chemical bonds with active crosslink groups in the organic matrix. Such interaction promoted the formation of interpenetrated network (IPN) structure between two phases. The interpenetration and encapsulation of two phases were enhanced with the increasing content of alumina. The interfacial structure played an important role in the unique performances of nano-composites which also depended on the surface effect, the size effect and quantum effects.