Process for the production of polypropylene monofilaments

摘要

Polypropylene monofilaments are made by melting a normally solid high molecular weight polypropylene containing a major amount of the isotactic form, and extruding the melt from a spinneret downwardly through a gaseous medium, especially air, into a bath of cooling liquid, the distance between the spinneret and the surface of the liquid being at least 8 inches. Polypropylene containing at least three times as much by weight of isotactic polymer as atactic polymer is preferred. The spinning temperature may be up to 300 DEG C. but is generally 215 DEG -250 DEG C. Monofilaments having diameters in the unstretched condition of between 0,010 and 0,060 inches may be made using a spinneret-liquid surface distance of from 20-100 inches. The cooling liquid is generally water but other non-solvents for the polymer may be used, e.g. glycerol, and the temperature of the cooling liquid may be from room temperature to 60 DEG C. or even 70 DEG C. Desirably, the extruded filaments are passed from the spinneret to the cooling liquid through a tube or several tubes in series so as to shield the filaments from draughts. After passage through the cooling liquid the filaments may be stretched to at least 3,5 times their original length, suitably not more than 12 times and desirably from 4-9 times at temperatures above 45 DEG C., preferably from 90 DEG -160 DEG C., in a steam atmosphere, or in hot air, hot organic vapours as for example alcohols, e.g. n-butanol, or by passing over a heated metal plate or snubbing pin. Stretching may be carried out continuously with the extrusion and quenching steps or as a separate operation after the filaments have been collected. The stretched monofilaments may be heated to an elevated temperature, suitably at least 80 DEG C., preferably 100 DEG -160 DEG C., while prevented from contracting by more than 20% of their stretched length. Monofilaments intended for brushes are preferably heat-treated while prevented from contracting, e.g. by immersion in boiling water for 1-90 minutes while held at constant length, as this improves their recovery properties, but monofilaments for weaving are more suitably heat-treated continuously with the stretching stage by passage around relaxing rolls having a peripheral speed not more than 20% slower than the fast stretching rolls, a chamber heated by steam, air, or other hot inert vapour being interposed between the fast stretching rolls and the relaxing rolls. Apparatus as shown comprises a screw extruder 1 terminating in a spinneret 2. Metal tubes 4 and 5 shield the filaments 14 from draughts. The surface of the water bath 3 is 40 inches below the spinneret. Tube 4 is 3 inches diameter and 15 inches long and protrudes 4 inches into tube 5 which is 12 inches diameter. A comb guide 6 prevents the filaments from sticking together. 7 is a set of slow rolls separated by steam chamber 8 from a set of fast rolls 9, and 10 is a 30-inch diameter reel for winding up the stretched filaments. Monofilaments so made show a reduced ovality compared with conventional monofilaments.