Process of making a compacted polyolefin article

Details Process of making a compacted polyolefin article Process of making a compacted polyolefin article

1999-04-02

2001-11-06

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

Shaping against forming surface

Reexamination Certificate

active

06312638

ABSTRACT:

This invention relates to processes for the production of polymer sheet materials from oriented olefin polymer fibres and to the products of such processes.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 5,135,804 describes a 1-step compaction process for making a film, such as for a sail, from polyethylene fibres. The examples use a range of pressures, of from 4.6 MPa to 46 MPa. The process is exclusively for application to gel spun fibres having a weight average molecular weight of at least about 500,000, preferably at least about a million and more preferably between about 2 million and about six million.
It would be desirable to use a compaction process employing a low pressure, for example in an autoclave, for melt spun fibres but none is available. There are compaction processes for melt spun fibres, but they are 2-step processes, employing high compaction pressures.
DESCRIPTION OF THE INVENTION
There is a substantial body of literature about such processes, for example the following articles: “The hot compaction of high modulus melt-spun polyethylene fibres”, Hine et al, Jnl. Materials Science 28 (1993), 316-324; “Morphology of compacted polyethylene fibres”, R. H. Olley et al, Jnl. Materials Science 28 (1993), 1107-1112; “Compaction of high-modulus melt-spun polyethylene fibres at temperatures above and below optimum”, M. A. Kabeel et al, Jnl. Materials Science 29 (1994), 4694-4699; “Differential melting in compacted high-modulus melt-spun polyethylene fibres”, M. A. Kabeel et al, Jnl. Materials Science 30 (1995), 601-606; “The hot compaction of polyethylene terephthalate”, J. Rasburn et al, Jnl. Materials Science 30 (1995), 615-622; “The hot compaction of polypropylene fibres”, M. I. Abo El-Maaty et al, Jnl. Materials Science 31 (1996), 1157-1163.
GB 2253420B describes a 2-step compaction process whereby an assembly of fibres of an oriented polymer may be hot compacted to form a sheet having good mechanical properties. The process involves an initial processing step in which the fibres are brought to and held at the compaction temperature whilst subject to a pressure sufficient to maintain the fibres in contact, the contact pressure, and thereafter compacted at a higher pressure for a few seconds, the compaction pressure. In the process a proportion of the fibre surfaces—most preferably from 5 to 10% by weight—melts and subsequently recrystallises on cooling. This recrystallised phase binds the fibres together. The contact pressure is stated to be preferably in the range 0.5 to 2.0 MPa, and the compaction pressure 40 to 50 MPa.
We have now found that, surprisingly to us and quite unexpectedly in the light of the earlier work described above, melt spun polyolefin fibres can be compacted into monolithic articles by a process which does not require a high compaction pressure.
In accordance with a first aspect of the present invention there is provided a process for the production of a monolithic article in which process an assembly of melt formed fibres of an oriented polyolefin polymer is subjected to a compaction step wherein the assembly of fibres is maintained in intimate contact at an elevated temperature sufficient to melt a proportion of the polymer, and is subjected to a compaction pressure not exceeding 10 MPa.
In certain embodiments the process of the invention may employ a uniform pressure throughout.
In certain embodiments the process of the invention may involve a change in the pressure applied, but such that the maximum pressure applied is 10 MPa. In such embodiments a lower, contact, pressure may initially be applied, sufficient to maintain the fibres in contact, followed by the higher pressure, referred to as the compaction pressure.
Preferably the monolithic article is an article which is shape stable under its own weight, for example a plaque.
The term “melt formed fibres” is used herein in a broad sense to denote strands of polyolefin polymer, formed by any process in which the strands are formed via molten polymer. The melt formed fibres may be non-woven melt spun fibres laid in a web, or may be melt spun fibres comprised within yarns, or may be constituted by bands or fibrillated tapes, for example formed by slitting melt formed films. If comprised within yarns, or constituted by bands or fibrillated tapes, those yarns, bands or fibrillated tapes may be laid together or they may be formed into a fabric, for example by weaving or knitting.
Preferably the fibres used in the present invention have a weight average molecular weight in the range 10,000 to 400,000, preferably 50,000 to 200,000.
The polyolefin polymer can be selected from polyethylene, polypropylene or polybutylene, or copolymers comprising at least one of those olefin polymers. The polyolefin polymer used in the process of the present invention may suitably be a polypropylene homopolymer or a copolymer containing a major proportion of polypropylene. Advantageously it may be a polyethylene homopolymer or a copolymer containing a major proportion of polyethylene.
A polyethylene copolymer comprising a major proportion of polyethylene as defined herein is one comprising more than 50% by weight of polyethylene. Preferably, it comprises more than 70% by weight of polyethylene, most preferably, more than 85% by weight of polyethylene.
A polyethylene polymer as defined herein may be unsubstituted, or substituted, for example by halogen atoms, preferably fluorine or chlorine atoms. Unsubstituted polyethylene polymers are however preferred.
A polyethylene copolymer comprising a major proportion of polyethylene may have one or more different copolymers, following copolymerisation of ethylene with, for example, one or more of propylene, butylene, butadiene, vinyl chloride, styrene or tetrafluoroethylene. Such a polyethylene copolymer may be a random copolymer, or a block or graft copolymer. A preferred polyethylene copolymer is a propylene-ethylene copolymer, having a major proportion of polyethylene and a minor proportion of polypropylene.
A polypropylene copolymer comprising a major proportion of polypropylene as defined herein is one comprising more than 50% by weight of polypropylene. Preferably, it comprises more than 70% by weight of polypropylene, most preferably, more than 85% by weight of polypropylene.
A polypropylene polymer as defined herein may be unsubstituted, or substituted, for example by halogen atoms, preferably fluorine or chlorine atoms. Unsubstituted polypropylene polymers are however preferred.
A polypropylene copolymer comprising a major proportion of polypropylene may have one or more different copolymers, following copolymerisation of propylene with, for example, one or more of ethylene, butylene, butadiene, vinyl chloride, styrene or tetrafluoroethylene. Such a polypropylene copolymer may be a random copolymer, or a block or graft copolymer. A preferred polypropylene copolymer is a propylene-ethylene copolymer, having a major proportion of polypropylene and a minor proportion of polyethylene.
An inorganic filler material may be present. An inorganic filler material, when present, may be present in an amount up to 60 vol % of the article, preferably 20 to 50 vol %. An inorganic filler material may, for example, be selected from silica, talc, mica, graphite, metal oxides, carbonates and hydroxides and apatite, for example hydroxyapatite, a biocompatible calcium phosphate ceramic.
In accordance with the present invention there is provided a polyolefin polymer monolith prepared in accordance with the process of the invention, as defined above.
In relation to the compaction the description in GB 2253420B is still broadly applicable to the modified process of the present invention, for example in relation to treatment times, temperatures, proportion of material which is to melt, the assembly of the fibres and molecular weights and the description of GB 2253420B may be regarded as incorporated into the present specification by reference, insofar as it applies to the production of polyolefin articles. However the pressure conditions are different, as described below.
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