Patent GB-1217363-A (GB)
Title: Tough heat-resistant polyimide film, process of making and tapes made therefrom
IPC: H01B3/30
Filed: 1968-01-23
Abstract: 1,217,363. Polyimide films; laminates. MINNESOTA MINING & MFG. CO. 23 Jan., 1968 [24 Jan., 1967], No. 3586/68. Headings B5B and B5N. [Also in Divisions B2 and H1]  In processes of producing polyimide films by reacting a tetracarboxylic acid dianhydride with an organic diamine in a solvent to form a solution of polyamide acid, said polyamide acid in solution thereafter being applied to a temporary support and chemically dehydrated with an anhydride or by means of heat in the presence of a tertiary amine catalyst to form a solvent-swollen polyimide film, the solvent being thereafter eliminated from the film, the characteristic feature that the solventswollen film is allowed to shrink in at least one direction in the plane of the film at least 10% but less than it would shrink if unrestrained. The film thus produced has an ultimate elongation and tensile strength in said one direction of up to 50% more than that of a film of identical composition and manufacture but which has been prevented from shrinking in said one direction. The solvent-swollen polyimide film may be allowed to shrink at least 10% but less than 50% in mutually perpendicular directions in the plane of the film. In either case the solvent-free polyimide film obtained may be heated and oriented in said one direction to an extent such that the ultimate dimension in said one direction is at least 10% less than if the solvent-swollen film had been totally restrained from shrinkage in said one direction. The chemical dehydrating agent and tertiary amine catalyst may be present in the polyamide acid solution before it is coated on the temporary support. The polyimide film may be produced continuously by extruding the solution of polyamide-acid, dehydrating agent and catalyst continuously on to a cesting roll heated to 55-75‹ C., maintaining the extruded solution in contact with the casting roll until liquid exudes from the film formed on the roll, thereby causing the film to separate from the roll, removing the film from the roll and passing it over a hot can stack, gradually increasing the temperature to at least 95‹ C., whereby the liquid content of the film may be reduced to about 15%, winding the resultant film on a winder roll which is driven at a slower peripheral speed than the casting roll, holding the dimensions of the film fixed, and heating it to at least about 315‹ C. to remove the remainder of the solvent. The film may thereafter be part cured for about 1 to 5 minutes at about 400‹ C. to further improve it tensile strength. An electrical insulating tape suitable for installation where it is subjected to high temperatures for long periods of time, and which can be wrapped around electrical conductors, caused to adhere in place and thereafter bent severely without cracking or breaking comprises the polyimide film made as described above, an adhesive being firmly bonded to one face thereof. The adhesive may be a normally tacky and pressure-sensitive polysiloxane or a thermoplastic copolymer of tetrafluoroethylene and hexafluoropropylene. Such tape may subsequently be laminated on its adhesive side to a PTFE or coated creped paper liner. The fluoropolymer adhesive may be applied as a film which is not laminated to the tape following treatment with a polyalkylene-imine or -polyamine or a cation bearing quaternary ammonium chloride.