| 摘要 |
<p>Polypropylene of which a major proportion is isotactic is melted, preferably while being compressed, and the molten polymer is extruded into filaments which are solidified in a controlled temperature zone at least 25 DEG C. below their melting point and wound up at a speed sufficient to impart to them a birefringence of at least 20 X 10-3 and at most 50 X 10-3. The filaments may be subjected to a separate drawing step at a draw ratio of less than 3 : 1 to impart a birefringence of at least 30 X 10-3 and at most 50 X 10-3, the separate drawing step being carried out at speeds of the order of 100 metres per minute. Preferably, however, filaments having a birefringence of at least 30 X 10-3 and at most 50 X 10-3 are made by cooling the filaments in a controlled temperature zone between 20 DEG and at least 30 DEG C. below the melting point of the polypropylene and winding the filaments at a speed of at least 900 metres per minute. The controlled temperature zone may comprise a fluid, e.g. water or air, heated to the specified temperature. The distance between the wind-up device and the solidification zone is dependent on the nature of the cooling fluid and the rate of wind-up and may be, e.g., at least 125 cms. for at least 900 metres per minute when the fluid is air and at least 25 cms. for at least 500 metres per minute when the fluid is water. Specification 818,100 is referred to.</p> |
