GB938766A - Transparent polyethylene film - Google Patents

Abstract

A polyethylene film up to 50 mils. thick is caused to imbibe a liquid having a swelling action on polyethylene at the imbibition temperature, immersed in a solution of an organic compound capable of generating free radicals at the melting point of the polyethylene, cured at from 10 DEG C. below to 100 DEG C. above its melting point in the absence of oxygen, stretched at 10 DEG C. below the melting point or higher, the temperature used and the duration of heating being such that the polyethylene does not suffer substantial thermal degradation, especially at a temperature within \sB10 DEG C. of the melting point, and cooled to form a transparent, shrinkable film. Specified swelling agents include benzene, cyclohexane and carbon tetrachloride, and organic peroxides, especially benzoyl peroxide, and dicumyl peroxide, and t-butyl peracetate and t-butyl perbenzoate may be used as free-radical generating compounds, suitably in an amount of 0,2-10% based on the weight of the polyethylene. A film may be simultaneously swollen and caused to take up the free radical-generating compound by immersion in a solution of the latter in the swelling agent at a temperature at which the polyethylene is swollen thereby. Alternatively the film may be first caused to imbibe the swelling agent at a temperature of 40 DEG C. or above (e.g. polyethylene of density 0,92 may be caused to imbibe benzene at 50 DEG -70 DEG C.), and then immersed in a solution of the free-radical generating compound at about room temperature. Prior to c curing the swelling agent may if desired be removed by evaporation, and stretching of both tubular and flat films may be effected by blowing. A flat film may be clamped between two horizontal plates, the top plate having a cut-out circular portion in its centre, and the bottom plate a corresponding recess. The whole is immersed in a liquid held at a suitable temperature, in a closed pressure system, and air introduced into the recess to stretch the film. Shrinkage of the stretched film may be effected by heating it to a temperature not lower than 25 DEG C. below the melting point of the polyethylene. Specification 471,590 and U.S.A. Specification 2,816,883 are referred to.