Patent ID: 7736561

Claim:
A process of producing a high strength polyethylene multi-filament yarn comprising the steps of: a) forming a slurry of an ultrahigh molecular weight polyethylene (UHMWPE) having an initial intrinsic viscosity (IV 0 ) at least about 10 dl/g and an antioxidant in a solvent for said UHMWPE having an atmospheric pressure boiling point at least about 180° C. at a temperature that is below the temperature at which the UHMWPE will dissolve in said solvent, the weight ratio of said UHMWPE to said solvent being from about 5:95 to about 95:5 and the weight ratio of said antioxidant to said solvent being from about 10 parts per million to about 1000 parts per million; b) feeding said slurry to an intermeshing co-rotating twin screw extruder wherein the screws in said twin screw extruder are comprised of multiple segments having a root diameter of D r millimeters, an outside diameter of D o millimeters, the ratio D r /D o is R, the rotational speed of the screws is ω revolutions per minute and following relationships are satisfied 0.84 ≥ R ≥ 0.55 ω ⁢ ⁢ D 0 ⁡ ( ( 1 + R ) ( 1 - R ) ) ≥ 70 , 000 , millimeters ⁢ / ⁢ ⁢ min c) forming a liquid mixture in said extruder of said UHMWPE and said antioxidant, in said solvent at a temperature from about 140° C. to about 320° C., the intrinsic viscosity of the UHMWPE in said mixture being reduced to a value of from about 0.2 IV 0 to about 0.7 IV 0 in passing through said extruder; d) passing said liquid mixture through a heated vessel at a temperature at least about 140° C., said vessel having a volume such that the average residence time in said vessel is from about 2 to about 120 minutes, whereby a solution of the UHMWPE is formed; e) passing the thus-formed solution through a spinneret to form solution filaments; f) passing said solution filaments from said spinneret through a short gaseous space into a liquid quench bath at a temperature less than about 35° C. wherein said solution filaments are rapidly cooled to form gel filaments; g) removing the solvent from the gel filaments to form solid filaments; and h) stretching at least one of the solution filaments, the gel filaments and the solid filaments in one or more stages to a combined stretch ratio of at least about 10:1, wherein a stretch of at least about 2:1 is applied to the solid filaments to form a high strength multi-filament UHMWPE yarn, said UHMWPE yarn having a tenacity at least about 40 g/d, and having an intrinsic viscosity from 0.2 IV 0 to 0.65 IV 0 .