Patent GB-990337-A (GB)
Title: Improvements in or relating to the manufacture of stretched films from polyethers
IPC: B29C55/06
Filed: 1963-02-19
Abstract: 990,337. Treating polyether films. ONDERZOEKINGSINSTITUUT RESEARCH N.V. Feb. 19, 1963 [Feb. 22, 1962], No. 6611/63. Drawings to Specification. Heading B5B. [Also in Division H1] Films which have, for example, been made by casting on to a moving casting surface a solution in a volatile organic solvent of a polyether having the general formula in which n is an integer having a value of at least 100, preferably between 200 and 300, and R and R 1  are alkyl groups containing not more " than two carbon atoms, and by thereafter evaporating off the volatile solvent and stripping the residual film from the casting surface, are stretched longitudinally at a stretch ratio of 1.5 to 4.75 between feed and draw rolls while being passed over and in sliding contact with a solid curved surface of a heat-conducting body, such as the circumferential surface of a fixed metal rod or bar which is at right angles to the path of travel of the film, and which is maintained at 190‹ to 210‹ C., preferably 203‹ to 207‹ C. The rod may have a diameter of at least 2 cm., the angle of deflection of the path of travel of the film may be at least 90 degrees of arc and the film may be stretched at a stretch ratio of at least 2.5 in about one minute. In the example, a polyether prepared by polymerization of 2,6-dimethyl phenol and having a molecular weight of 28,000, a glass transition temperature of 220‹ C., a dielectric constant of 2.5, and a loss angle of 10 x 10<SP>-4</SP> at 50 c.p.s. is dissolved in a 3: 2 v/v mixture of methylene chloride and chloroform to form a 10% solution and a film of the solution is cast on to a travelling endless belt. The solvents are evaporated off and the residual film is stripped from the belt, caused to travel in a sinuous path over three rotating feed rolls and thereafter is drawn by means of three draw rolls over and in sliding contact with a fixed metal rod having a diameter of 5 cm. and positioned at right angles to the path of travel of the film. The rod is maintained at 203‹ C. by means of an internal electric heater, the angle included between the portions of the travelling web of film approaching and leaving the rod respectively is 90 degrees and the ratio at which the film is stretched is 2.5. The electric properties of films made according to the invention render them suitable for use as insulating strips in the manufacture of electric cables. They may be helically wound around a conductor in a known manner and after this layer has been sheathed, the cable is filled with an insulating liquid which may be one of the usual types. It is, however, preferred to use insulating liquids in which the ratio between the dielectric constant of the solid insulating medium and that of the liquid lies between 0.9 and 1.1. Cable liquids which may be employed with the polyether films of the invention are silicon coil 200/3 (# = 2.52) and high viscosity cable oils. In order to ensure that the cables remain completely filled with cable liquid while they are in use, the surface of the strips of film may be ruffled on one side by means of heated profile rolls which emboss a point or line pattern on the film surface, preferably in the form of line-shaped goffering transverse to the length of the strips.