GB1287527A - Polyolefine films - Google Patents

Abstract

1287527 Polyethylene films IMPERIAL CHEMICAL INDUSTRIES Ltd 20 April 1971 [21 April 1970] 18992/70 Heading B5B A process of producing a film or tape of high density polyethylene or a copolymer thereof having a shrinkage not exceeding 10% in either the longitudinal or transverse directions when maintained for five minutes at a temperature of 120‹ C., without heat setting, comprises heating a film or tape of high density polyethylene or a copolymer thereof having a density greater than 0À935 gm./cm.<SP>3</SP>, after extrusion, and optionally, cooling to a temperature within the range of from 18‹ to 3‹ C. below the crystalline melting point of the polymer or copolymer and biaxially stretching the film or tape at draw ratios of at least 6À5 : 1 in mutually perpendicular directions in the plane of the film or tape at that temperature, the maximum temperature to which the film is heated being the stretching temperature. The film or tape may be stretched simultaneously in each direction, and a balanced film or tape may be made by employing substantially equal draw ratios in the two directions. Preferred biaxial draw ratios are from 6À5 : 1 to 9 : 1. Preferred stretching temperatures are from 128‹ to 134‹C. Polyethylene or polyethylene copoylmer film so made may have a shrinkage in the longitudinal and transverse directions not exceeding 5%, and a tensile strength at break in the longitudinal and transverse directions exceeding 20,000 p.s.i. Such film may have a twist retention of about half a twist after twisting 1À5 times and being allowed to relax for about 24 hours. A preferred polyethylene or copolymer film has a shrinkage in the longitudinal and transverse directions of 2 to 4% when heated for five minutes at 120‹ C., a tensile strength at break in excess of 25,000 p.s.i., and an elastic modulus (1% secant) greater than 3 x 10<SP>5</SP> p.s.i. Films so made may be utilized as packaging material, and are especially useful for twist wrapping.