Biaxially-oriented metallocene-based polypropylene films

Abstract

A process for the production of biaxially-oriented polypropylene film involving the provision of a polypropylene polymer produced by the polymerization of propylene in the presence of a metallocene catalyst characterized by a bridged racemic bis(indenyl) ligand substituted at the proximal position. The polypropylene contains 0.5 to 2% 2,1 insertions and has an isotacticity of at least 96% meso pentads and at least 99% meso diads. The film is formed by stressing the polymer produced from a slot die in the machine direction at a stretch ratio of about 5 or 6 and in the transverse direction at a stretch ratio of about 8 or 9 to produce a biaxially-oriented film having a non-uniform melt temperature of a peak value of less than 160.degree. C. The polypropylene polymer has meltflow index of less than 5 grams per 10 minutes and has an average molecular weight within the range of 100,000-400,000 grams per mole. The film, when configured having a thickness of 18 microns, is characterized by a machine direction secant modulus of at least 1400 MPa, a transverse direction secant modulus of at least 2200 MPa, a kinetic film-to-film co-efficient of friction of no more than 0.5, and permeabilities to water and oxygen of no more than 2.6 g/m.sup.2 /d and 2400 c.sup.3/ m.sup.2 /d, respectively. The film is further characterized by shrinkage factors of at least 8% in a transverse dimension and at least 16% in a transverse direction when heated to a temperature of at least 140.degree. C.

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