Abstract: A heat-resistant prepreg uses at a substrate a heat-resistant material made of heat-resistant organic fibers, inorganic fibers, etc. and as a matrix a heat-resistant mixture containing at least 10% by weight of a borosiloxane resin composition. This heat-resistant prepreg is obtained by impregnating the substrate at a temperature in the range of 200.degree. to 300.degree. C. to such an extent that at least 20% of the amount of phenyl groups bonded to silicon atoms in the aforementioned borosiloxane resin composition prior to the impregnating and drying steps will remain after the heat treatment.

Abstract: A polyborosiloxane composition to be used for the formation of an insulating layer on an electric conductor comprises (A) a polyborosiloxane produced by the reaction of boric acid, a boric acid derivative, or a halogenated boron with a silane compound and a silicone oil, (B) a silicone resin or a synthetic resin having a nitrogen-containing heterocyclic ring in the molecule, (C) an inorganic filler, and (D) a common solvent for (A) and (B). An insulated electric wire excellent in thermal resistance is obtained by applying the composition to an electric conductor and baking the applied layer of the composition.

Abstract: Insulating varnishes capable of forming a film possessing excellent electrical and mechanical properties can be obtained by reacting a reaction product of (1) a reactant of a dicarboxylic acid or a tricarboxylic acid or a derivative thereof comprising an aromatic dicarboxylic acid or a derivative thereof in an amount of at least 95 equivalent % [the term "equivalent" as used herein means (number of moles) x (number of reactive functional groups in one molecule)] of the total of the (1), (2) a primary diol and (3) an aliphatic polyhydric alcohol having at least 3 hydroxyl groups with (4) an aromatic teracarboxylic acid or derivative thereof, and subsequently dispersing the reaction product into an aqueous solution containing ammonia or an organic amine.