GB1253272A - Polyolefin films and aeronautical balloons produced therefrom - Google Patents

Abstract

1,253,272. Polyolefin films for aeronautical balloons. UNION CARBIDE CORP. 17 Jan., 1969 [18 Jan., 1968], No. 2974/69. Heading B5B. [Also in Division B7] Polyolefin films suitable for forming aeronautical balloons have been biaxially oriented in the machine direction and in the transverse direction in each case by stretching them at a stretch ratio of at least 3 : 1 and have been obtained from a polyolefin having an intrinsic viscosity in decalin at 130‹ C. of at least 1À2, the polyolefin being either a homopolymer of ethylene or a copolymer of at least 50% by weight of ethylene with another monomer (such as vinyl acetate, methyl acrylate, acrylic acid, ethyl acrylate, neohexene, methacrylic acid or propylene), the films having a "cold brittleness temperature" of -99‹ C. or lower, as determined in accordance with the method designated M11-P-4640A (U.S.A.F.). The film may be formed from an ethylene homopolymer having a density of up to 0À925. The intrinsic viscosity of the polymer is measured according to A.S.T.M. procedure D-1601-58T. The polyolefins may be formed into tubular films and may be biaxially oriented by inflation while in this form, or the films may be in sheet form and biaxially oriented by known tentering techniques. Preferably the biaxially oriented films are produced by the known "doublebubble" process. In this process a primary tube is first formed by melt extrusion from a die, inflated by admission of air, stretched at a temperature above its melting point and collapsed. The "blown tube" thus formed is then reinflated to form an isolated bubble therein and advanced through a heating zone to raise the film to its draw temperature. In the draw zone the film is radially expanded in both the transverse and machine directions at a temperature below the melting point of the film such that orientation occurs in both directions. The preferred draw temperature is 15‹ to 50‹ F. below the crystalline melting point of the film polymer. The biaxially oriented film is preferably annealed to eliminate the tendency of the film to pucker when being heat-sealed. In an example, a film 1À0 mil. thick formed from a polyethylene having a density of 0À9175 and an intrinsic viscosity (in decalin at 130‹ C.) of 1À3 is biaxially oriented at a stretch ratio of 3À4 : 1. When tested it is found to have a cold brittleness temperature below - 150‹ C. Aeronautical balloons are made from films produced according to the invention by cutting the films into a number of pieces or gores of predetermined shape and sealing the pieces to each other, usually by heat-sealing, along their edges.