MANUFACTURE OF ELONGATE MEMBERS

Abstract: A melt article, e.g. a fibre or film, or a fusible acrylonitrile polymer can be cross-linked to render it infusible. The thusly cross-linked article can be oxidised and subsequently carbonised or graphitised in similar manner to articles made by wet- or dry-extrusion of infusible acrylonitrile polymers.

Description: MANUFACTURE OF ELONGATE MEMBERS Field of the invention This invention relates to the manufacture of elongate members based on acrylonitrile polymers and to their oxidation, carbonisation and graphitisation. Background art Acrylonitrile polymers are well-known and are described for example in articles in Encyclopaedia of Polymer Science and Engineering, Volume 1 ( iley-Interscience, 1985) entitled "Acrylonitrile Polymers", at page 426, and "Acrylic Fibers", at page 334. Acrylic polymers are defined as polymers which contain at least 85% by weight acrylonitrile monomer units. Modacrylic polymers are defined as polymers which contain 40 to 85% by weight acrylonitrile monomer units. Many acrylonitrile polymers, including a large proportion of acrylic polymers, are infusible and decompose without melting when they are heated. Processes are known by which such polymers can be processed into elongate members, for example fibres, using wet- or dry-extrusion techniques. These involve the extrusion of a solution of the polymer in a suitable solvent into a coagulating bath or an evaporative atmosphere, respectively. Processes are also known whereby elongate members can be manufactured by extrusion of a plasticised acrylonitrile polymer under dry-spinning conditions, followed by removal of the plasticiser. Fusible acrylonitrile polymers are also known and are described for example in US-A-5 , 106 , 925 and US-A-5 , 286 , 828 (both Curatolo et al . ) , the contents of which are incorporated herein by way of reference. Such fusible acrylonitrile polymers can be melt-extruded to form an elongate member at temperatures below the temperature at which they decompose. In general, such polymers may alternatively be extruded by wet- and dry-spinning techniques. Carbon fibres are well-known and are described for example in an article in Encyclopaedia of Polymer Science and Engineering, Volume 2 (Wiley-Interscience, 1985) entitled "Carbon Fibers", at page 640. Carbon fibres are made by controlled pyrolysis of an organic precursor, for example an acrylonitrile polymer, in fibrous form. Pyrolysis of acrylonitrile polymers typically involves a number of pyrolysis steps. The first step, which may be called the stabilisation step or the oxidation, pre-oxidation or thermosetting step, is carried out at relatively low temperatures, typically in the range 200-450°C, usually in air. The oxidised fibre may then be converted to carbon fibre (carbonised) by heating in an inert atmosphere at 1000-2000 °C, or it may be graphitised by heating in an inert atmosphere at temperatures in excess of 2500 °C. Melt-extrusion techniques have certain advantages over dry- and wet-extrusion techniques for the manufacture of elongate members, notably in the relative simplicity of the apparatus required. Furthermore, it is known that melt-extrusion often lends itself more readily to the production of extruded members of modified cross-section by extrusion though a die of modified cross -section than do wet- and dry-extrusion techniques. However, it will readily be appreciated that melt-extruded fusible acrylonitrile polymers cannot readily be pyrolysed to form oxidised, carbonised or graphitised articles . Such polymers melt below the temperature at which they can be oxidised. Disclosure of the invention The present invention provides a method for manufacturing an elongate member comprising the steps of (1) melt -extruding a fusible acrylonitrile polymer to produce an elongate member, and as characterising step (2) cross -linking said polymer in said elongate member, thereby rendering it infusible. The elongate member may take the form of a film or preferably a fibre. Suitable fusible acrylonitrile polymers for use in the invention and methods for their manufacture and melt-extrusion are described for example in US-A-5 , 106 , 925 and US-A-5, 286, 828. The fusible acrylonitrile polymer generally contains at least 50 percent by weight acrylonitrile monomer units. The cross -linking step (2) may be effected in a variety of ways. For example, the extruded elongate member may be exposed to ionising radiation so as to induce cross-linking. Alternatively, the elongate member may be treated with a chemical reagent which serves to induce cross-linking in acrylonitrile polymers, for example ammonia, hydrazine or a primary or secondary amine . Further alternatively, the fusible acrylonitrile polymer may comprise monomer units which possess a cross- linkable group other than the nitrile group, for example olefinic or hydroxyl groups. Cross -linking may in such cases be induced in the melt -extruded elongate member by subjecting it to conditions which cause reaction of the cross -linkable group, for example in the case of an olefinic group exposure to UV radiation or in the case of a hydroxyl group etherification or reaction with a bifunctional reagent. Care should be taken to avoid excessive cross -linking before and during the melt-extrusion step, because this may adversely affect the physical properties of the melt-extruded member. The cross -linkable group should accordingly possess sufficient thermal stability to resist excessive cross -linking during melt -extrusion. Further alternatively, the acrylonitrile polymer may comprise latent cross- linkable groups. For example, it may comprise t-butyl vinyl ether monomer units serving to provide latent cross-linkable groups, whereby the ether groups can be hydrolysed after melt-extrusion to yield hydroxymethylene groups as cross -linkable groups. The cross-linked infusible polymer is preferably one which does not show any tendency to melt below about 300 * C. This permits the cross-linked polymer to be oxidised and subsequently pyrolysed using techniques generally known for acrylonitrile elongate members, including fibres. The invention further provides a method which further includes the step of (3) pyrolysing the cross- linked elongate member under oxidising conditions, thereby producing an oxidised acrylonitrile elongate member. The invention yet further provides a method which in addition to step (3) further includes the step of (4) pyrolysing said oxidised elongate member under anaerobic conditions, thereby producing a carbonised or a graphitised elongate member. Cross -linked elongate members produced by the method of the invention in the form of fibres are suited to the manufacture of carbon fibres, for example for use in fire-retardant materials and in battery separators.

Claims: CLAIMS 1. A method for manufacturing an elongate member comprising the steps of (1) melt-extruding a fusible acrylonitrile polymer to produce an elongate member, and as characterising step (2) cross-linking said polymer in said elongate member, thereby rendering it infusible. 2. The method according to claim 1, further including the step (3) of pyrolysing said elongate member after cross- linking said polymer, said pyrolysing being carried out under oxidising conditions, thereby producing an oxidised elongate member. 3. The method according to claim 2, further including the step of (4) pyrolysing said oxidised elongate member under anaerobic conditions, thereby producing a carbonised or graphitised elongate member. 4. The method according to any of claims 1 to 3 , characterised in that the elongate member is a fibre. 5. The use of an elongate member produced by a method as claimed in any of claims 1 for the manufacture of carbon fibres.

Inventors: SERVICE DAVID MAC, SERVICE, DAVID, MAC
Applicants: COURTAULDS FIBRES HOLDINGS LTD, SERVICE DAVID MAC, COURTAULDS FIBRES (HOLDINGS) LIMITED, SERVICE, DAVID, MAC
Application year: 1998
Main classification: D01F9
IPC codes: C08J5, D01F6, D01F9, D01F11, D06M10, D06M11, D06M13, D06M101