Method of making polyurethane thread and thread produced by such method

Claim 1

IN A METHOD OF MAKING A POLYURETHANE THREAD WHEREIN A STREAM OF A PREPOLYMER, FORMED BY HEATING, AT A TEMPERATURE OF 90*-100*C. FOR A PERIOD OF FROM 90 TO 120 MINUTES, A MIXTURE OF A POLYESTER DERIVED FROM A GLYCOL AND A DICARBOXYLIC ACID, SAID POLYESTER HAVING A MOLECULAR WEIGHT OF FROM 500 TO 5,000, AND AN ACID VALUE LESS THAN 6, WITH FROM 1.8 TO 2.1 MOLES OF AROMATIC DIISOCYANATE, UNTIL THE REACTION IS SUBSTANTIALLY COMPLETE, AS EVIDENCED BY THE FACT THAT A SUBSTANTIALLY CONSTANT VISCOSITY IS ACHIEVED, TO FORM A SOLUBLE, UNCURED, LIQUID MATERIAL WHICH IS AN ESSENTIALLY LINEAR POLYURETHANE PREPOLYMER HAVING TERMINAL ISOCYANATE GROUPS, IS EXTRUDED AND SAID EXTRUDED STREAM IS PASSED INTO A SETTING BATH COMPRISING AN ALIPHATIC DIPRIMARY DIAMINE, WHEREBY THE SURFACE OF THE EXTRUDED STREAM IS CURED TO A SOLID STATE BY THE ACTION OF SAID DIAMINE, AND THE CURE OF THE EXTRUDED STREAM IS THEREAFTER COMPLETED BY THE ACTION OF WATER, TO FORM A CURED POLYURETHANE THREAD, THE IMPROVEMENT COMPRISING THE STEP OF FURTHER HEATING THE SAID LINEAR POLYURETHANE PREPOLYMER, SUBSEQUENT TO THE ACHIEVEMENT OF SAID SUBSTANTIALLY CONSTANT VISCOSITY BUT PRIOR TO THE SAID EXTRUSION STEP, AT A TEMPERATURE OF 90* TO 100*C. FOR A PERIOD OF FROM 48 TO 198 HOURS, TO PROVIDE A MODIFIED, LIQUID, UNCURED, ESSENTIALLY LINEAR, SOLUBLE POLYURETHANE PREPOLYMER HAVING TERMINAL ISOCYANATE GROUPS, WHEREBY THE THREAD FINALLY PRODUCED FROM SUCH MODIFIED PREPOLYMER BY THE SAID METHOD IS CHARACTERIZED BY A PERMANENT SET OF FROM 2 TO 10%.