Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Patent
1986-05-20
1989-07-25
Thurlow, Jeffery
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
2642092, 264216, 264310, 26433117, 4253823, 528502, B29C 4124
Patent
active
048511730
DESCRIPTION:
BRIEF SUMMARY
This invention relates to oriented polymer films; more particularly, this invention relates to processes for the continuous preparation of oriented films of polymer materials; to apparatus which may be used in such processes; to oriented films of polymer materials so prepared and to certain oriented films as new products; and to composite materials comprising the oriented films.
It is known that a dilute solution of linear polyethylene of very high molecular weight (for example, M.sub.w about 1.5.times.10.sup.6) in xylene at a temperature up to about 125.degree. C. will form a fibre when seeded from a surface in contact with the solution when this solution is subjected to Couette flow. (A. Zwijnenburg and A. J. Pennings, Colloid & Polymer Sci: 254, p. 868 (1976).) The fibre can be continuously removed as a thread line and reeled. Such fibres are found to have very high ultimate tensile strengths and Young's moduli (typically 1 GPa and 100 GPa, respectively).
More recently, it has been shown that by allowing such a fibre to seed from a rotating conical surface it may grow in a helical coil around the surface and will self-coalesce to form a ring of linear polyethylene film from the polyethylene solution. (A. Coombes and A. Keller, J. Polym. Sci. Polym. Phys. Ed., 17 p. 1637 (1979).) Furthermore, it has also been shown that the incorporation of polypropylene into the solution can similarly lead to films of a polymer material which is a varying blend of linear polyethylene and polypropylene. (A. Coombes, C. G. Cannon and A. Keller, J. Polym. Sci. Polym. Phys. Ed., 17 p. 1957 (1979).)
This invention seeks to provide a process for the continuous preparation for oriented films of polymer materials which is more suited to commercial production.
According, therefore, to one aspect of this invention, there is provided a process for the continuous preparation of an oriented film of a polymer material, which process comprises:
(i) shearing at least one film of a solution and/or gel of the polymer material between at least one set of two surfaces, at least one of which surfaces is endless and in relative motion; and
(ii) continuously removing the or each oriented film of polymer material from one of the surfaces on which it is formed.
By "film of solution and/or gel" is meant herein a thin layer, typically of a thickness not greater than 3 mm.
The process of the present invention is applicable to any thermoplastic organic polymer material, preferably a crystallisable such polymer material, provided only that the polymer material comprises at least a fraction of sufficiently high molecular weight and that this latter material can form a solution and/or gel. Examples include linear vinyl hydrocarbon polymers, polyethylene oxide, polyacetals such as polyoxymethylene and polyacetaldehyde, aliphatic polyamides, polyesters such as polyethylene terephthalate and fluorinated polymers such as polyvinylidene difluoride.
Preferably, the polymer material comprises a polyolefin, for example polyethylene, polypropylene, a polyethylene-polypropylene copolymer or a polyolefin blend comprising at least one thereof, especially linear polyethylene.
It is desirable that the polymer material has a weight average molecular weight (M.sub.w) greater than about 500,000, desirably greater than about 600,000. Below this value there is not sufficient of the polymer material of sufficiently high molecular entanglement and/or relaxation time to enable highly oriented films, in which the polymer material is believed to exist as extended chain fibrils, to be formed by the process of the present invention with commercial viability, if at all. Preferably, M.sub.w is greater than about 750,000, desirably greater than about 850,000, especially greater than about 1,000,000.
The film of solution and/or gel of the polymer material is preferably less than 3 mm thick, especially less than 2 mm thick. It is particularly preferred that the film is 1 mm thick or less; for example, no greater than 0.5 mm.
It is highly desirable, and usually necessary, to maintai
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Mackley and Sapsford, Techniques of Preparing High Strength, High Stiffness Polyethylene Fibers by Solution Processing.
Mackley Malcolm R.
Sapsford Gary S.
National Research Development Corporation
Tentoni Leo B.
Thurlow Jeffery
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