Polypropylene sheet for projection screens

摘要

Sheet is produced by pouring or extruding a polypropylene composition of thickness 0.5 to 2 mm. on a heat transfer surface for sufficient time for the sheet to solidify, the surface being maintained between 85 degrees and 170 degrees C., and then maintaining the sheet surfaces at 85 degrees to 160 degrees C. until three quarters at least of the sheet has become crystalline. The composition should have a flow index (A.S.T.M. D 1238) of at least 90, and the sheet should contain large spherulites in its central core of 15 - 45 micron size, with smaller spherulites in the outer layers. Additives may be included. The screen is of an improved transparent diffracting type which is robust and reduces scintillation.