Method of biaxially stretching thermoplastic material

• Main Authors: HOFER PETER H, HIGGINS WILLIAM T
• Format: Patent
• Language: English
• Published: 29.10.1963
• Subjects: AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING; PERFORMING OPERATIONS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDEDFOR; SHAPING OR JOINING OF PLASTICS; TRANSPORTING; WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE, IN GENERAL

Tubular films are continuously produced by extruding an organic polymeric thermoplastic material through an annular extrusion orifice at a temperature above the first order phase transition temperature of the material to form seamless tubing, withdrawing the tubing from the orifice at a rate greater than the rate of extrusion, cooling the tubing to a temperature between the second order phase transition temperature of the tubing material and a temperature 20 DEG C. higher than the latter, re-heating the cooled tubing to a temperature more than 20 DEG C. above the second order phase transition temperature and within a temperature range at which the tubing can be molecularly oriented by a stretching operation, introducing and maintaining a gas within the interior of the tubing under a pressure sufficient to stretch the reheated tubing to a diameter greater than its extrusion diameter, and cooling the resultant tubular film. The process is particularly applicable to the production of film from rigid styrene polymer and rigid vinyl polymers, including polymers of para-chloro-styrene, alphamethyl styrene, para-ethylstyrene and the vinyl naphthalenes, copolymers of styrene with acrylic acid esters such as butyl methacrylate, hexyl methacrylate and 2-ethyl hexyl acrylate, or with nitriles such as acrylonitrile, polyvinyl chloride and vinyl chloride-vinyl acetate copolymers. The process is also applicable, however, to the production of films of cellulose acetate, polyethylene and polypropylene. In an example, a rigid polystyrene having a tensile modulus of 400,000 p.s.i., a molecular weight of 52,000, and a second order phase transition temperature of 82 DEG C. is extruded at 183 DEG C. to form tubing having a diameter of 2 inches and a wall thickness of 20 mils. The tubing is withdrawn vertically upwards from the extrusion die first through a zone in which it is cooled to 100 DEG C. by means of air directed on to its exterior from a cooling ring encircling the tubing, and then through a zone in which it is re-heated to 133 DEG C. by means of 8 infra-red lamps arranged in circular fashion around the extruded tubing. From the heating zone the tubing passes to two co-operating pinch rolls which withdraw the tubing from the die at ten times the extrusion rate. The re-heated tubing is inflated by means of air to about eight times the extrusion diameter. Specification 922,083 is referred to.