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Abstract

A method of preparing a high strength sheet material comprising (a) forming a laminate comprising at least two layers of a thermoplastic polymer material, each layer having a fibrillar grain structure providing a predominant direction of splittability in each said layer, and wherein the layers are bonded to one another with the said predominant directions of splittability generally transverse to each other, (b) biaxially orienting the molecules of said layers by stretching the layers in substantially uniaxial steps, at least the transverse stretching step thereof being effected by applying pressure to the surface of the laminate along lines extending substantially in the longitudinal direction of the laminate to impart thereto a transversely waved or sinuous configuration, and (c) subjecting the biaxially oriented laminate while in the sinuous condition to a heat treatment while allowing at least 7% shrinkage of the laminate to take place in at least its transverse direction. The sinuously configured laminate is preferably brought into contact with a heated body having a surface that is flat in a transverse direction so that the crests on the sides of the sinuous configuration adjacent said body are heated more rapidly than the remainder of the laminate.

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First Claim