GB694870A - Process for increased clarity and transparency of polyethylene products - Google Patents

Abstract

The clarity of solid polyethylene extruded film is improved by heating the film after continuous extrusion at, e.g., 240-500 DEG F., for a minimum clearing time while the main central portion of the film is out of contact with solid <PICT:0694870/IV (a)/1> <PICT:0694870/IV (a)/2> <PICT:0694870/IV (a)/3> objects. The film is then quenched to below 180 DEG F. in a liquid bath. The minimum clearing time, after which there is no readily visible improvement in clarity of the film, may be determined from the expression <FORM:0694870/IV (a)/1> where t = clearing time in seconds, h = sheet thickness in mils., and T = temperature in DEG F. An extrusion apparatus described (Fig. 1) includes a unit in which the hot film is extruded from a die D, gripped at the edges by rollers 2 and stretched laterally in a heating zone held at 240-500 DEG F. by heaters 3, 6a. The film is quenched in a bath 4, dried in a hot air oven 18 at less than 240 DEG F. and rolled at 17. The quenching unit (Figs. 2, 4) has festooning rolls 10, 11, 12, the intermediate roll 11 being journalled in blocks 13 to slide vertically, while a notch and sliding bolt 16 holds the roller 11 in or out of position, or alternatively, the tension is maintained by the weight of roller 11.