IMPROVEMENTS IN OR RELATED TO FIBRES

IMPROVEMENTS IN OR RELATED TO FIBRES

The present invention relates to fibres of the type used in engineering structures, the so-called "engineering fibres", as distinct from optical fibres and plant or animal fibres. The term "fibre" as used herein includes filament, whisker and the like, as well as fibres comprising a plurality of filaments or whiskers which have been spun together to form a thread or yarn.

Engineering fibres, their manufacture and their uses are well known. Among known inorganic fibres are glass fibres, carbon fibres such as graphite fibres, metal whiskers such as tungsten whiskers, ceramic whiskers such as silicon carbide whiskers, and boron fibres. Among known organic fibres are those formed from lyotropic liquid crystalline main-chain polymers such as poly-para-phenylene terephthalamide, and from thermotropic liquid crystalline main-chain polymers such as poly (chloro-p-phenylene terephthalate-co-naphthalene -2,6-di-carboxylate) . Such organic fibres and their production and properties, are described in detail in a, monograph dated 1984 by M.G. Dobb and J.E. Mclntyre entitled "Properties and Applications of liquid crystalline main-chain polymers" which is available from the Department of Textile Industries of the University of Leeds.

Generally, engineering fibres possess very high tensile moduli and., tensile strengths. The Young's moduli, of such fibres are generally in excess of those of amorphous glass,i.e. greater than about 35GPa. However, their other physical properties such as impact resist¬ ance and toughness can vary considerably. A good example of such differing secondary properties is carbon fibres and KEVLAR fibres. Whilst the former possess a very high compressive strength but a relatively high brittleness, the latter possess a much lower compressive strength but a very high impact resistanc and toughness. In applications where no one type of fibre fully meets all of the required fibre properties, it is known to use combinations of fibres of two or more different materials either woven together usually at an angle to each other, or else embedded together in a common matrix, e.g. an epoxy resin. In a limited number of uses.

such as aeronautics ana astronautics , such admixtures of fibres are not satisfactory.

The present invention therefore seeks to provide fibres and fibre-reinforced materials which are superior in certain respects to known fibres and their ^•known combinations, for example in tolerance to impact.

In accordance with the present invention there is provided an engineering fibre comprising a core of a first material and an outer layer of a second material wherein both the first and the second materials are in the form of fibres. These multi-component fibres will be referred to hereinafter as "hybrid fibres". Preferably the outer layer covers substantially the entire surface of the core. Optionally, but not essentially, the first and second materials have coefficients of thermal expansion which are substantially equal.

In order to ensure good adhesion between the core layer and outer layer the surface of the core can be modified or else at least partial chemical bonding can be arranged to take place therebetween. The surface modification of ^' the core layer can take the form of surface etching or the provision of an intermediate layer such as that described in EP-A-175484.

The hybrid fibres of the present invention can be coated either partially or completely with one or more layers of a third or subsequent material, such further materials themselves optionally being in the form of fibres.

The hybrid fibres can be spun or woven together in the form of a thread, rope, cloth or tape. The hybrid fibres and threads, rope, cloth or tape made therefrom can be embedded in a matrix material to form a composite structure.

The main advantages of the present hybrid fibres over existing single component fibres are realised when the impact resistance and the toughness of the second material is higher than that of the first material. Generally this is achieved when the first material is an inorganic material and the second material is an organic material.

SUBSTITUTE SHEE In its most preferred embodiment, the first material is carbon and the second material is either a lyotropic or thermotropic liquid crystalline main-chain polymer, especially poly-para-phenylene terephthalamide.

The present invention also provides a method of making a hybrid fibre which method comprises providing the first material in the form of a fibre, at least partially coating the fibre of the first material with the second material, and subjecting the at least partially coated fibre to processing conditions such that the second material at least partially coating the fibre is transformed into a fibre.

Generally, the core is made as a fibre by any of the conventional processes known to produce such fibres, and then the outer layer is made as a fibre around the core by any of the conventional processes known to produce such fibres but modified, if necessary, to allow for the presence of the core.

In the preferred embodiment a hybrid fibre is produced by first producing a carbon fibre by any of the normal commercial processes and then drawing this manufactured carbon fibre through a solution of the liquid crystal or nematic form of poly-para-phenylene terephthalamide so as to coat the fibre completely, and then processing the material of the second layer to transform that material into a fibre.

TITUTESHEET In certain circumstances it may be desired to produce a coating which does not cover the entire surface of the core fibre being coated.

Another example of a hybrid fibre comprises a core of boron with next a layer of glass (optionally in the form of a fibre) and then a layer of a thermotropic liquid crystalline main-chain polymer such as poly

(chloro-p-phenylene terephthalate-co-rraphthalene-2,6- dicarboxylate) (70/30 molar) .

In order to produce a composite structure using a hybrid fibre in which the outmost layer is a thermotropic liquid-crystalline main-chain polymer, the fibres can be laid up singly or as a spun thread in or on a mould, either as individual strands orientated to suit the particular needs of the end product or else in the form of a woven cloth or tape, and the whole composite or mould heated under pressure or vacuum as may be desired, so that the outermost layer of the polymer is re-melted to allow this layer to act as the adhesive or matrix for the whole component.

The laying up of such component fibres or hybrid fibres in any desired shape or mould may be facilitated by pre-heating or by using a heated roller or guide to press the component fibres or hybrid fibres, whether individually or in groups, into intimate contact with previously laid component fibres or hybrid fibres. The result of such laying up with heated hybrid fibres is that fusion of the outer layers of successive fibres is achieved in situ in the mould. For other types of hybrid fibres standard composite structure-forming methods can be used. Hybrid fibres may also comprise more than one thermotropic polymer or other suitable material which during the composite's manufacture or re-melting at another stage polymerise further or otherwise react with each other or with any other material introduced at that stage, earlier, or later, to achieve for the finished product even more advantageous properties through intimate contact while constrained as extended chain polymers by either the relatively small diameter of the original fibre or by the close proximity of other such fibres within the overall matrix of a composite material.

The combinations of possible materials are many and can be selected according to the end use and the environment of its use. For example, because aramids are degraded by high ultra-violet light levels, hybrid fibres having aramids as their outermost layer can be coated with a further layer of an opaque glass, optionally also in the form of a fibre. The present invention includes hybrid fibres having a core of an aramid and an outer layer of glass in fibre form.

Glass can also be used as an anealing agent where the heat of the molten glass (through which perhaps a component fibre or hybrid fibre is drawn) provides the heat for post extrusion anealing of that component or entire hybrid fibre in order to improve the properties of the finished hybrid fibre. The thickness of molten glass can be varied to vary the amount of heat input to such an anealing process.

Like ordinary fibres, hybrid fibres will allow the production of composite components in narrow or wide sheets or blocks, as standard shapes, equivalent to the stock range normally available through other base material producers such as steel or wood. The particular hybrid fibre combination can be selected to suit the particular need.

The manufacture of hybrid fibres may present difficulties where simple extrusion or drawing through a small capillary or spinerette is., not enough to impart the best properties to any particular layer of the hybrid fibre. In such a case relative rotation of one or more layers of the hybrid fibre may be of assistance.

Rotation of the core fibre or the hybrid fibre as a whole can be achieved by rotating the entire feed or draw mechanism in proportion to the throughput. It may, however, be easier to rotate the extrusion die, capillary or spinerette alone. Then again it may only need the feed material for the coating layer to be rotated, with the core fibre or hybrid fibre being pulled therethrough.

Other layers of such a hybrid fibre may be deposited by condensing the appropriate pre-cursor material or materials from a vapour or powder, either separately or together, to produce the desired layer or layers, one layer perhaps acting as an adhesive for another.

Some liquid-crystalline main-chain polymers are normally given their best characteristics by being extended or pulled and/or twisted during their production. This may be achieved by passing a component fibre or hybrid fibre through one or more drag tubes which contain a suitable friction-generating material such as smooth sand, glass microspheres, or a liquid of a suitable viscosity. Combinations of such materials may be "used.

In an alternative construction the drag tube may be provided with channelling jets for a suitable fluid such as air, the orientation of the jets being such as to create drag on the material passing through the tube.

The drag tube may be pressurised, rotated, vibrated, ventilated, heated or cooled as may be desired. The friction produced by such a tube can be arranged to be uniform or to vary along its length as is known by those skilled in the art.

Instead of utilising a separate drag tube, it may be possible for a particular apparatus to arrange for the capillary, die or spinerette from which the component fibre or hybrid fibre is emerging from the pre-cursor material to be of such a length and shape as to produce the required drag characteristics upon the fibre being formed.

It may not be possible in certain cases to produce extended lengths of fibre on a continuous basis because, for example,- . the core fibres may only be available in short lengths, such as in the case of carbon fibre. In that event, provision is made to inject fresh core fibres or hybrid fibres into the process as necessary. Such injection may be through attachment to a projectile which is fired through the material being processed. Alternatively a simple air jet could be used to create a small hole or tunnel through which fresh fibres might be passed to allow longer lengths of hybrid fibre to be produced.

Continuous production may also be facilitated by arranging for the feed of the core fibre or hybrid fibre to be from coils in such a way that the fibre is drawn out of the top of an open container, in the manner of the wire following a wire-guided missile or as from the top of a coil of rope. The other end of the fibre or hybrid fibre would then be suitably attached or otherwise bonded to the beginning of another container full of fibre which would itself be in a correct position to allow the fibre contained therein to be also drawn off, and so on.

By arranging for the outermost layer of material of a hybrid fibre to be a thermotropic polymer, thermotropic liquid crystalline main chain polymer, or thermoplastic polymer such as PEEK (poly-ether-ether-ketone) , it becomes possible during the production process to re-melt this layer so as to act as the adhesive or matrix for a bundle of such fibres within a composite structure thereby producing a composite engineering structure.