| 摘要 |
Polypropylene films are made by casting on to a heated solid casting surface a layer of a solution of a solid crystallisable linear high polymer of propylene in a solvent which can be volatilized by heating without decomposition, stripping from the casting surface the film thereby formed and imparting molecular orientation to the film by stretching it at an elevated temperature in two directions at right angles to one another. The preferred solvent is kerosine, especially kerosine boiling between 170 DEG C. and 250 DEG C. The polypropylene preferably has an intrinsic viscosity, as measured in tetrahydronaphthalene at 135 DEG C., of 0,5 to 7. The film may be stretched at 80 DEG to 150 DEG C. with a stretching ratio of 1:3 to 1:12 and subsequently stabilized by maintaining it at 130 DEG -150 DEG C. for 2-15 seconds while under tension. In an example, a 30% by weight solution of polypropylene (of intrinsic viscosity 4,3) in a kerosine fraction boiling at 200 DEG to 210 DEG C. is cast into a film on a rotating casting drum the surface of which is heated by means of an internal jacket through which a liquid at 175 DEG C. is circulated to evaporate the solvent. A countercurrent of air at 175 DEG C. is passed through an annular evaporating chamber surrounding most of the circumference of the drum. After a residence time of 25 minutes on the drum the film is stripped from the drum, quenched in water and wound up. The film is then stretched in a bath of ethylene glycol at 135 DEG C. first longitudinally with a stretch ratio of 1:4 and then transversely with a stretch ratio of 1:3. The film is then heated at 130 DEG -140 DEG C. for 3 seconds while held stretched. The polypropylene solution contains 0,15% by weight of diphenylethylphosphite as antioxidant. The process permits the production of polypropylene films having a molecular weight higher than that of films produced by melt extrusion. |
