Web-stretching process

Claim 1

In a process for the production of a biaxially oriented plastic web by stretching an orientable heated web of a thermoplastic resin composition extending in longitudinal and traverse directions and having longitudinally extending edges, simultaneously in the traverse and longitudinal directions, by gripping both edges of the orientable web at a plurality of longitudinally opposed locations adjacent to the longitudinal edges thereof with a plurality of opposed web gripping means, each of which is mounted on a separate carriage means, which carriage means are constrained to follow opposed rail means, which rail means are arranged in mirror-image paths and define at least a parallel path section, a first-bend path section, a diverging path section, a second-bend path section and a second parallel path section, which carriage means on each rail means are joined by a continuous chain of articulated links, which chain is driven so as to propel said carriage means along said rail means, whereby said gripping means advance said web along said rail means, which links engage guide means, coextensive with the rail means, at a point upstream of the parallel path section, in a manner which collapses the links and thereby moves the carriages in relatively close proximity to each other upstream of the point at which the web-gripping means mounted thereon grip the edges of the web, which guide means extend along the first-bend path section and along the diverging path section in a manner which unfolds the links and spreads the carriages apart both longitudinally and traversely, thereby biaxially drawing the web, and which guide means extend to the second-bend path section, the improvement wherein the oriented web is a continuous sheet formed from a continuous sheet of the orientable web; wherein the plastic resin composition comprises a mixture of (a) 100 parts by weight of crystalline polyolefin selected from propylene polymers, high density ethylene polymers and mixtures thereof, (b) 30 to 50 parts by weight of styrene polymer, and (c) 0 to 100 parts, per hundred parts (a), of one or both of dispersed pigments and fillers; wherein each of the gripping means has a gripping area with a midpart so disposed with respect to the carriage upon which the gripping means is mounted that the gripping area travels the same distance as that travelled by the carriage in the first-bend and second-bend path sections, thereby minimizing scalloping of the sheet adjacent the longitudinal edge portions thereof; wherein the sheet is heated to 115° C. to 130° C. when component (a) is an ethylene polymer and to 130° C. to 160° C. when component (a) is a propylene polymer; and wherein the sheet is biaxially drawn as the sheet advances over the diverging path section so as to have a final stretch ratio in both the traverse and longitudinal directions in the range of 3:1 to 10:1 in the second parallel path section compared to the first parallel path section.