Method for producing heat resistant synthetic resin tubes

#The present invention relates to a novel method for producing heat resistant synthetic resin tubes having a thickness of not more than 0.3 mm.

#Heretofore, a large number of insulating synthetic resin tubes have been used for electric wiring in apparatus. Recently, the apparatus have become small in size, so that the temperature in the apparatus has risen raised and for as the synthetic resin tubes used, fineness and further excellent heat deformation resistance and heat deterioration resistance has been required. However, the heat resistant resins are generally difficult to soften by heating and therefore it is difficult or impossible to fabricate said resins into tubes by extrusion as in the case of ordinary resins. Accordingly, the typical heat resistant synthetic insulating tubes that satisfy the above described requirements have been produced by slitting polyimide film with adhesive into a tape and winding said tape spirally.

#However, the heat resistance of the above-described insulating tube produced from the heat resistant resin film, depends upon the heat resistance of the adhesive used and even at the temperature which the film itself can fully endure, the adhesive deteriorates or is softened and the tube is broken or the properties are lost. Furthermore, when such a spirally wound tube is produced from the film, a part of the width of the film is overlapped, so that the electric insulating property is determined by the portion where the thickness of the tube is smallest. Further the mechanical properties of said tube, such as tensile strength, elongation etc., depend upon this adhesive and are unsatisfactory. Furthermore, in this case, various steps, for example a step for producing a film from a starting resin, a step for applying an adhesive on the film, a step for slitting the thus treated film into a tape, a step for winding the tape on a mandrel and a step for curing the adhesive by a heat treatment, are necessary, resulting in high production cost for the tube.

#The following examples are given for the purpose of illustration of this invention and are not intended as limitations thereof.

#A silicone resin (Trademark: Silicone KS 701, made by Shinetsu Kagaku Co.) was applied once on a copper wire having a diameter of 0.5 mm, passed through a baking furnace having a length of 4 m and kept at a temperature of 400.degree. C, at a velocity of 6 m/min, to form a thin silicone resin layer on the wire, and successively a polyimide precursor varnish (Trademark: ML Varnish, made by Du Pont Co., main component: polyamide-acid) was applied and cured 12 times on the above treated wire, while the wire was passed through the furnace at the same velocity, to obtain a resin coated wire, 49 .mu. in the thickness of the coated layer, which was taken up on a bobbin.

#The resin coated wire was taken out from the bobbin, and wound in 10 turns around a rubber coated roll having a diameter of 50 cm and further in 10 turns around another rubber coated roll having a diameter of 50 cm which is arranged at a distance of about 2 m from the first roll and finally wound aroound a take-up bobbin. Then, the rotating rates of the two rolls were adjusted so as to draw the wire to about 121% of its original length between the two rolls. The wire was then taken up on a take-up bobbin while being drawn continuously. Then, the resin coated wire was taken out from the take-up bobbin, and only the tubular resin layer was cut at about 1 m intervals. The wire was subsequently passed twice through a furnace kept at a temperature of 300.degree. C to cause the resin layer to shrink sufficiently, and the wire was taken up on another bobbin. Then, the wire was cut-off at the portions where the copper wire was exposed, and the copper wires were pulled out to obtain polyimide resin tubes.