Polymer films

Claim 1

A stretched polyolefin film suitable for use as a packaging material, having a substantially uniform cellular structure produced with the use of a blowing agent and consisting essentially of a crystalline linear high density polyethylene having a broad molecular weight distribution and a low Melt Flow Index, which film has the following limitations: (a) it comprises no more than 14 percent by volume of the film of closed cells; (b) it has an apparent density which is 90 to 20 percent of the density of the starting polyethylene composition: (c) it has a cell factor equal to or less than +0.65, which factor may be zero or negative, determined according to the formula F=T-2.0/Z wherein F is the cell factor, T is the thickness of the film in mils and Z is the percentage by volume of the film of closed cells, determined according to ultimate density calculations; (d) it contains 0 to 25 percent by weight based on the polyethylene of at least one additive selected from the group consisting of fillers and pigments; (e) it has a thickness of up to 11 mils; (f) it comprises cells with dimensions such that the average cell volume is not greater than 10 -4 cc; (g) it has been stretched in the plane of the film by an amount corresponding to a stretch ratio of at least 1.1 to 1 in the machine in direction or the transverse direction or both; (h) it contains at least 50,000 cells/cc of film; (i) it contains residues from the decomposition of about 0.1-1 percent by weight of a chemical blowing agent; (j) it has a water vapour permeability of less than 500 gm/m 2 .24 hrs; (k) it has an air permeability of les than 500 ml/m 2 .sec; and (l) the majority of cells that are open do not communicate with both major surfaces of the film.