Apparatus for producing polypropylene sheet

Apparatus for producing an abrasion resistant sheet from linearly crystalizable polypropylene material, said apparatus comprising:

• (a) extrusion means for extruding a sheet of said material at a first temperature at which said material is rendered flowable;
• (b) casting means including plural driven, heat transfer casting rolls for conveying said sheet in a forward direction and for forming said sheet, said casting means including means for maintaining said casting rolls at a temperature below said first temperature whereby said sheet is cooled to a second temperature below said first temperature;
• (c) cooling means including a cooling bath for cooling said sheet to a third temperature below said second temperature;
• (d) heated compression means including heated and driven compression rolls for maintaining said sheet at a fourth temperature, for compressing said sheet, and for causing flow of said material in said sheet in the forward direction so as to reduce the cross-sectional thickness of said sheet;
• (e) tensioning means for placing said sheet under tension in the forward direction in conjunction with said compression rolls so as to cause orientation of the macromolecular chains in said material of said sheet as said sheet exits said compression rolls, said tensioning means including
• (1) one or more chill rolls that are cooled so as to cool said sheet to a fifth temperature below said fourth temperature;
• (2) one or more surface treatment rolls that are heated for reheating a surface of said sheet to a sixth temperature above said fifth temperature and between about 350 degrees F. and about 525 degrees F. for a time period sufficient to cause surface fusion to a depth between about 0.1 mil and about 3 mils but for a time period insufficient to cause fusion in the entire thickness of said sheet;
• and
• (3) one or more stress relieving rolls heated to a seventh temperature below said sixth temperature so as to cause said sheet to reach a uniform temperature throughout its cross section for effecting stabilization of said sheet;
• whereby the rolls of said tensioning means not only place said sheet under tension in conjunction with said compression rolls for effecting said orientation of the macromolecular chains, but also simultaneously effect said cooling of said sheet, said surface fusion of said sheet, and said stabilization of said sheet.