Process for the productions of multi-cellular stretched articles

Claim 1

A process for the production of a multi-cellular stretched article, which process comprises the steps of forming a composition comprising mixing a melted blend of a stretchable thermoplastic organic polymer matrix and an additive which reduces the density of the resulting article and is non-molten or non-softened in the normal orientation temperature range of the matrix polymer, at a temperature at which the matrix polymer and additive are molten, forming an article from said composition, and then stretching the article at a temperature which is within the normal orientation temperature range for the matrix polymer so that micro-voids are formed in the matrix polymer, said additive constituting from about 2 to 50% by weight of the total weight of the blend, being at least partially incompatible with the matrix polymer in the concentration employed when and at the temperature at which said article is subsequently stretched, being selected from the group consisting of organic polymeric, organic acids, and those metal salts of organic acids other than metal resinates and rosin esters, except when the matrix polymer is a crystallizable polymer other than an olefin, in which case said additive is selected from the group consisting of organic acids and their metal salts other than metal resinates and rosin esters, the temperature at which said article is stretched being below the glass-transition temperature of said additive when said additive is a polymer, and below the melting or softening point of the additive when said additive is an organic acid, organic derivative or a modified or unmodified natural product.