Stretching of Polypivalolactone Yarns

1,184,726. Stretching polypivalolactone yarns. SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ N.V. 5 Aug., 1968 [7 Aug., 1967], No. 36078/67. Heading B5B. [Also in Division D1] A polypivalolactone yarn has a tenacity of at least 5 grams per denier and an elongation of not more than 70%, especially not more than 20%. The preferred starting material is a polypivalolactone having a minimum intrinsic viscosity of 0À75, preferably between 2 and 4, when measured in trifluoroacetic acid at 25‹ C. The term polypivalolactone is defined as a linear polyester consisting essentially of recurring ester structural units of the formula: -CH 2 -C(CH 3 ) 2 -C(O)O-. Preferably, the sole polymeric fibre forming constituent in the polypivalolactone is a pivalolactone homopolymer. The polypivalolactone, however, may also consist of copolymers or polymer blends containing at least 75 mole per cent of the ester structural units derived from pivalolactone. Suitable copolymeric components are those derived from #-propiolactone, α-ethyl-α-methyl-#-propiolactone and α,α-diethyl-#-propiolactone. Blends of polypivalolactones with up to 25 mole per cent of other polymers, in particular polyamides, such as nylon 66 and nylon 12, are also suitable in the preparation of the polypivalolactone yarn. Conventional additives, e.g. dyes, pigments, dye-assistants, and stabilizers may be present. The starting polypivalolactone yarn may be obtained by drawing off polypivalolactone filaments, melt-extruded through a die-plate, in a zone situated immediately next to the die-plate and heated to a temperature between 175‹ and 305‹ C., to a diameter between ¢ and 1/40th of the diameter at which the filament was extruded and thereafter quickly cooling the filaments to below the recrystallization temperature. Thus the yarn may be conducted through an annealing tube 10 to 20 cms. long and heated to 270‹ to 290‹ C. and cooled in room temperature air, or conducted through the annealing tube as before but subsequently shock-cooled in a water bath located beneath the annealing tube. Suitably, the yarn is stretched in a first stage at between 20‹ and 220‹ C. at such a stretch ratio as to leave 10 to 200% elongation in the yarn, followed by a heat relaxation at a temperature between 110‹ and 220‹ C. without applying stretch and, in a third stage, restretching the yarn at a temperature between 100‹ and 220‹ C. at such a stretch ratio as to leave 5 to 70% elongation in the yarn, the yarn having a tenacity of at least