Abstract: An aqueous slurry comprising fibrous particles of a heat-resistant aromatic polymer and short fibers made of said polymer or insulating inorganic short fibers, or a mixture thereof, is made into a wet web and, after adjusting the water content to 50 to 95 weight %, said wet web is laid up in a desired number of layers, and then the laminate is dewatered and dried for integration under pressure at a temperature not exceeding the softening point of said aromatic polymer, thereby obtaining an oil-impregnatable insulating board with high strength and capable of uniform impregnation of oil therein.

Abstract: A heat resistant thin paper excellent in electrical insulating property can be obtained by subjecting pulp-like particles of aromatic polyamide or aromatic polyamide-imide, which have a low freeness and a low ion content to wet paper-making process, drying and then making the resulting web dense under a pressure.

Abstract: A rock wool composite material obtained by mixing a fine rock wool having a diameter of 1 to 10.mu. and a length of 70 to 500.mu. and containing substantially no shots with a pulp-like substance of a heat-resistant aromatic polymer having a freeness value of 150 seconds or more and less than 500 seconds, is excellent in heat resistance, incombustibility, heat insulation and electrical insulation and is advantageously used as a substitute for an asbestos sheet or board.

Abstract: A laminated composite comprising a web of heat-resistant base material having a sufficient structural strength and a web of tiny mica flakes laid on said web of base material by multilayer-paper making. This composite can be manufactured with good operational efficiency, is improved in mica retention and is excellent in varnish-impregnability, heat-resistance, mechanical strengths and electric insulation.

Abstract: An electrical insulating material composed of a mica flake layer and a heat resistant base material layer is prepared by superposing the mica flake sheet containing at most 9% by weight of pulp-like particles of a heat resistant polymer based thereon on the heat resistant base material sheet, both of which sheets being dried, and heating them under pressure. The electrical insulating material has excellent in structural strength, varnish-impregnability, and is resist to pealing off of the mica flakes.