| 摘要 |
A substantially normally crystalline polyolefine resin having a melt index of at least 1,0, when determined as described in ASTM Test Method D-1236-52T, is melt-spun to form continuous filaments which are stretched so that their length is increased by at least 200% at a temperature of at least 100 DEG C. and not higher than 5 DEG C. below the melting point of the resin. The polyolefine may initially have had a melt index in the range 0,001 to below 1,0, and may have had its melt index raised to fall within the range 1,0 to 200 by heating it under controlled conditions of time and temperature such that it undergoes pyrolysis to a degree sufficient to raise the melt index by the required amount without extensive molecular degradation of the polymer. The depolymerization of the polymer is effected as far as possible in such a manner that only the higher molecular weight components of the polymer are depolymerized, the range of molecular weights of the components of the polymer thus being narrowed. The preferred ranges of melt index are 3,0-15, 2,0-50 and 5-20 for polyethylene, polypropylene and a mixture of 80 mole per cent 3-methyl-1-butene and 20 mole per cent 4-methyl-1-pentene respectively. The pyrolysis step may be combined with the spinning operation, cooling and remelting of the pyrolyzed polymer thus being avoided. Pyrolysis temperatures and corresponding times may be within the range 325-450 DEG C. and 600-10 seconds. The polyolefines include those prepared from branched and unbranched aliphatic a, b-unsaturated olefine monomers containing 2-20, and preferably 2-12, carbon atoms. In the case of branched monomers, the branch chain must be positioned at least 1 and preferably 2 carbon atoms removed from the nearest ethylenically unsaturated carbon atom. In an example, a polyethylene resin having a melt index of 0,18 is pyrolyzed in a mill at 397 DEG C. to increase its melt index to 9,5 and is melt-spun at 200 DEG C. to form filaments, which are solidified by a concurrent flow of air at room temperature and then wound on to a bobbin. The yarn is then passed through a tube supplied with steam at 2 atmospheres absolute pressure corresponding with a temperature of 121 DEG C., and stretched so that its length is increased by 890%. Specification 823,309 is referred to.
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