Process for drawing gel-spun polyethylene yarns
View Patent ↗Gel-spun multi-filament polyethylene yarns possessing a high degree of molecular and crystalline order, and to the drawing methods by which they are produced. The drawn yarns are useful in impact absorption and ballistic resistance for body armor, helmets, breast plates, helicopter seats, spall shields, and other applications; composite sports equipment such as kayaks, canoes, bicycles and boats; and in fishing line, sails, ropes, sutures and fabrics.
1. A process for drawing a gel-spun multi-filament yarn comprising:
a) forming a gel-spun polyethylene multi-filament feed yarn comprising a polyethylene having an intrinsic viscosity in decalin at 135° C. of from about 5 dl/g to 35 dl/g, fewer than about two methyl groups per thousand carbon atoms, and less than about 2 wt. % of other constituents;
b) passing said feed yarn at a speed of V 1 meters/minute into a forced convection air oven having a yarn path length of L meters, wherein one or more zones are present along the yarn path having zone temperatures from about 130° C. to 160° C.;
c) passing said feed yarn continuously through said oven and out of said oven at an exit speed of V 2 meters/minute wherein the following equations are satisfied:
0.25≦ L/V 1 ≦20, min
3≦ V 2 / V 1 ≦20
1.7≦( V 2 − V 1 )/ L≦ 60, min −1
0.20≦2 L/ ( V 1 + V 2 )≦0.24 min.
2. The process of claim 1 wherein the yarn is drawn at constant tension throughout the oven neglecting the effect of air drag.
3. The process of claim 1 wherein the yarn path in said oven is a straight line from inlet to outlet.
4. The process of claim 1 wherein the feed yarn comprises a polyethylene having an intrinsic viscosity in decalin at 135° C. of from about 8 dl/g to 30 dl/g, fewer than about one methyl groups per thousand carbon atoms, and less than about 1 wt. % of other constituents, said feed yarn having a tenacity of from about 2 to 76 g/d as measured by ASTM D2256-97.
5. The process of claim 4 wherein the feed yarn has a tenacity of from about 5 to 66 g/d.
6. The process of claim 4 wherein the feed yarn comprises a polyethylene having an intrinsic viscosity in decalin at 135° C. of from about 10 dl/g to 20 dl/g, fewer than 0.5 methyl groups per thousand carbon atoms, said feed yarn having a tenacity of from about 7 to 51 g/d.