Patent GB-1425747-A (GB)
Title: Extrusion process
IPC: B29C48/08
Filed: 1973-04-05
Abstract: 1425747 Extruding crystallized polymers IMPERIAL CHEMICAL INDUSTRIES Ltd 19 March 1974 [5 April 1973] 16291/73 Headings B5A and B5B A crystallizable synthetic polymer is extruded in a crystallized oriented state, e.g. as a film, by delivering separate streams of the molten polymer to a confluence, upstream of the extrusion outlet passage and at which the polymer is subjected to a velocity gradient in the direction of extrusion which aligns the polymer molecules, under temperature and pressure conditions inducing the crystallization. The extrudate modulus is comparable with that obtained by solid phase stretching, but with a lower tendency to fibrillate. The extrudate may comprise flat films extruded on to a casting drum, or inflated tubular films, or a coating extruded on to a textile, paper, or another polymeric film, and comprising high or low density polyethylene, polypropylene, propylene or ethylene copolymers, polyamides, and specified polyesters. The extrudate may also comprise a foamed film. The extrusion temperature is above the crystallite melting point, and the crystallization is effected by the tensile stress produced by the velocity gradient, and which may be assisted by a nucleating agent or the use of a previously crystallized polymer. The separate streams may be delivered from separate sources, or by division from a single stream as in the embodiments. The example relates to the extrusion of a tape of high density polyethylene, by the die of the Fig. 2 embodiment. In the Fig. 1 embodiment (not shown) the confluence is effected upstream of a linear slot outlet, from a pair of streams divided from a single stream around a preferably cooled torpedo, and provides a crystallized core aligned in the extrusion direction and sandwiched between outer molten unaligned polymer layers. In the Fig.- 2 modification (not shown) the torpedo is replaced by a pair of members spaced apart to provide a third slower moving stream, intermediate a pair of faster moving streams, and which third stream is accelerated at the confluence by the faster moving streams to effect the alignment. In the Fig. 3 modification of the Fig. 2 embodiment, a further pair of spaced apart members 12, 13 is provided so that parts of the faster outer moving streams from passages 7, 8 effect a confluence, via passages, 18, 19, with the slower moving stream from passage 11 and flows through passage 15, and the other parts of the streams from passages 7, 8 effect a further aligning confluence, via passages 16, 17, with the combined stream from passage 15. In the Fig. 6 modification of the Fig. 2 embodiment, parts of the unaligned and uncrystallized outer layers of molten material are removed for recirculation by die knife members 32 which are transversely adjustable. In the Fig. 4 embodiment, a radially directed tubular film outlet 21, 22, has each of a pair of streams delivered to it in opposite directions from a passage 26, by annular passages 27, 28, producing a radial velocity gradient and alignment, together with a circumferential alignment arising from radially outward flow through the outlet 20, 21. Passage 26 communicases directly with passage 27 defined around a tube 24, and with passage 28 via a passage through, and slots 25 at opposite ends of, the tube 24. In the Fig. 5 embodiment (not shown), a similar radially directed tubular film outlet is applied via inner and outer annular streams delivered in the same direction to the confluence, with the inner stream preferably being slower flowing. In the Fig. 7 embodiment, biaxially aligned tubular extrudate issues from an outlet defined between oppositely rotatable die members 37, 38, and upstream of which the confluence is effected from a pair of outer, and an inner stream, provided by annular members 39.