GB773718A - Processes for treating films made from solid polymers of ethylene - Google Patents

Abstract

An unoriented polyethylene film is rolled out in one direction in at least three successive stages, to effect successive reductions in the thickness of the film and orientation thereof, the film being maintained during rolling at a temperature between room temperature and the upper limit of the crystalline melting temperature range of the film. The lower limit of this range is defined as the lowest temperature at which the normal crystalline state of the film, e.g. its state at room temperature, begins to disappear at an appreciably rapid rate as evidenced by X-ray examination, infra-red techniques or density or heat-capacity measurements. The upper limit of the range is defined as the lowest temperature at which the crystalline structure of the film is no longer evident. The temperature of the film during rolling is directly determined by the temperature of the rolls, the film being at room temperature as it enters the rolls. In the example, polyethylene at 265 DEG C. is extruded downwards through air into a water quench bath at 60 DEG C. to form a film 6 mils. thick. The film is withdrawn from the quench bath over an unwind mandrel by means of a pair of tension rolls and passed through a drying tower. The film is withdrawn from the tower and conveyed, under tension maintained by another pair of tension rolls and a mandrel, into the nip of the first of five pairs of calender rolls, the nip being set at a thickness less than that of the extruded film. The rolling operation is carried out using uncrowned, polished, stainless steel rolls 10 inch in diameter maintained at 65 DEG C. and rotating at 3 r.p.m. A small amount of silicone mould release oil is added as a slip agent. The film is passed in succession through the pairs of calender rolls, the nip between the faces of each successive set of rolls being successively reduced by an amount such that the distance between the roll faces is in every case less than the thickness of the entering film. After passing through the last of the calender rolls, the film is passed over a set of tension rolls to a wind-up mandrel, and is wound up as a roll. The thickness of the rolled film is 1.3 mils. The polyethylene film material may contain up to 25 per cent of substances copolymerizable with polyethylene such as styrene, isobutylene, vinyl acetate, vinyl chloride and other ethylenically unsaturated compounds. The treated films have improved gloss and clarity, decreased elongation and increased longitudinal and transverse tensile strength, the mechanical properties of the film being substantially the same in both directions. Methods of determining the crystalline melting temperature range are described and literature references are given.