Carbon Fibre Tow

Abstract

The degree of alignment of carbon fibres in a continuous carbon fibre tow is improved by applying a resin solution to the tow at a point where the tow is still travelling under the same tension and in the same direction as that in which it travelled through the furnace. Application of the resin, e.g., polyvinyl alcohol in a volatile organic solvent, may be effected by means of a porous, e.g., felt, spreader without substantially altering the direction of travel of the tow.

Description

This invention relates to the production of carbon fibre tows. As is well known, the standard procedure starts with a polyacrylonitrile fibre tow, and comprises the following steps:

1. An optional oxidation at a temperature in the range 200.degree.C to 275.degree.C.

2. Carbonisation at a temperature of the order of 1,000.degree.C to 1,500.degree.C.

3. An optional graphitisation at a temperature in the range 2,250.degree.C to 275.degree.C.

4. An optional surface oxidation treatment. The present invention relates to a continuous process for the production of carbon fibres on a relatively large scale. It will be apparent that it is desirable to produce the carbon fibres in long lengths, and indeed, for the manufacture of artefacts from the carbon fibres, the longer the length the better. It is therefore desirable to use as a starting material long lengths of PAN fibre. Similarly, for the sake of cheapness, it is desirable to use a commercially available form of PAN fibre rather than to use a special fibre which, it will be understood, would inevitably be more expensive. Finally, our tests have shown that taking into account the inevitable shrinkage in diameter of the carbon fibres as compared with the precursor material, it is desirable to make use of a precursor material of about 1.5-denier.

In order to use carbon fibres, the present practice is normally to use them as a reinforcement in a matrix, and for many purposes it is extremely desirable that the carbon fibres should be available in the form of a tape or band some 4 in. wide, since this form enables the carbon fibres to be surface-treated if necessary and then to be preimpregnated with the matrix material. It is relatively simple thereafter to split the wide tape up into narrower tapes should this be desirable.

The normal manufacture of PAN fibres for conventional textile purposes results in two possible forms of fibre, namely straight fibre and crimped fibre which is produced by forcing the tow through a heated stuffing box. In practice we have found that straight filament fibre has a false twist which encourages fibre entanglement so that it is extremely difficult to spread the initially circular tow into an even tape of fibres. On the other hand, with crimped fibre, due to the final crimping stage which is carried out in the form of a wide band, the fibres naturally fall into the necessary tape form and therefore we prefer to use crimped fibres in the process of forming carbon fibres. However, it is necessary to remove the crimp before the fibres are oxidised, and also in practice we find that the crimped fibres are normally provided as 3-denier or thicker fibres and so must be reduced in thickness before they form the most desirable carbon fibres.

In the process described in British Pat. Specification No. 1110791, it is taught that at least a part of the operation is performed with the tow under tension in order to increase the ultimate tensile strength and Youngs modulus of the product. Tension is generally applied by means of a roller assembly.

When the emerging carbon fibres reach the outlet rollers, two factors operate to spoil the alignment of individual fibres within the tow. Firstly, in those cases where the tow is wrapped around part of the periphery of a roller, the tow has thickness so that some fibres pass around the roller at a greater radius than others. Secondly, it is not practicable to apply much tension to the fibres for winding up for storage, so that on the outlet side of the tensioning rollers, tension in the tow is relaxed.

It is an object of the present invention to improve the degree of alignment of the carbon fibres in the tow as stored.

The present invention provides a method of producing a carbon fibre tow which method comprises continuously drawing a carbonisable fibre tow linearly under tension through a carbonising furnace and applying a resin solution to the carbon fibre tow, at a point where the tow is still under tension and is still travelling in the direction in which it travelled through the furnace, so as to retain the alignment of the carbon fibres in the tow substantially as it emerged from the furnace.

The carbonisable fibre tow may be a polyacrylonitrile fibre tow either before or after the optional preliminary oxidation treatment. As the oxidation treatment is rather slow, it may be easier to wind up the oxidised tow, rather than to transfer it direct to the carbonising furnace. The oxidised fibres retain some elasticity, and the oxidised tow can be wound up without excessive loss of alignment of the individual fibres.

When a graphitisation and/or surface oxidation treatment is carried out after carbonisation, the tow is drawn successively without any alteration in direction through the various treatment zones prior to the resin application.

The resin solution may be applied to the carbon fibre tow by any convenient method which does not involve loss of alignment of the individual fibres. Spraying of the resin solution and application by means of a porous, e.g., felt, spreader are suitable methods, the latter being preferred. Passage of the tow through a trough of the resin solution is not a suitable method if it involves loss of alignment of the individual fibres on passage round a roller.

The solvent of the resin solution is preferably rapidly volatile. Conventional organic solvents appropriate to the resin chosen, e.g., acetone or ether, are generally suitable.

The nature of the resin is not critical to this invention. In general, the resin will be chosen with regard for the matrix which the carbon fibre tow is to reinforce. Polyvinyl alcohol, being water-soluble, is suitable for fibre intended for a further stage of wet surface treatment or for metal composites. Epoxy resins, polyvinyl acetate systems, polyester and polyamide resins are suitable for other applications of the fibres. Further possible resins will be readily apparent to the reader.

The purpose of the resin application is to prevent or reduce accidental breakage and loss of the fibres, which are delicate and slippery, during normal handling. A relatively small amount of resin, e.g. from 1 to 5 percent, preferably about 2.5 percent by weight on the weight of the carbon fibre, is sufficient for this purpose. The resin coating also greatly improves the grip between the fibres and the rollers.

The concentration of the resin solution is not critical. As the amount of resin used is small, it may be convenient to use a rather dilute solution, e.g. from 0.1 percent to 5 percent by weight, of the resin.

The present invention also provides apparatus for producing a carbon fibre tow, which apparatus comprises a carbonising furnace, means for continuously drawing a carbonisable fibre tow linearly therethrough under tension, and means for applying a liquid to the tow, without substantially altering its direction of travel, at a point between the carbonising furnace and the drawing means. The tension is not critical; we find it convenient to use a tension of from 2 to 16 Kg, e.g., 8 Kg, for a 160,000 3-denier filament tow.

A specific embodiment of the invention will now be described with reference to the accompanying drawing, which is a diagrammatic representation of apparatus according to the invention.

The apparatus comprises a carbonisation furnace 10, a graphitisation furnace 11, and a gaseous surface treatment zone 12 in succession, separated from each other and from the atmosphere by gas locks 13. Adjacent the final gas lock is a felt spreader 14, supplied with a 2 percent solution of polyvinyl alcohol in acetone from a reservoir 15. An extraction hood 17 is positioned next to the felt spreader 14.

In operation a tow of previously oxidised polyacrylonitrile fibres 18 is drawn by means of a pair of driven rollers 22 and 23 from a supply (not shown) through the nip between rollers 19 and 20, which are braked to provide the desired degree of tension in the tow between rollers 19, 20 and rollers 22, 23. The tow passes successively through furnaces 10 and 11 and treatment zone 12; past the felt spreader 14 where it picks up approximately its own weight of the resin solution; past the extraction hood 17, where the acetone is evaporated off and the polyvinyl alcohol sets and causes the individual carbon fibres in the tow to adhere lightly together; around the freely rotating roller 21 which is fitted with a revolution counter (not shown) to measure the quantity of fibre passing; through the nip between driven rollers 22 and 23, where the tension on the tow is relaxed; and finally on to a drum 25, where it is rolled up interleaved with paper from a supply 26.

Although the felt spreader 14 is illustrated as being above the tow 18, and the extraction hood 17 below it, it may be found more convenient to apply the resin solution to the tow 18 from below and to extract volatile solvent from above.

The carbon fibres are slippery and difficult to handle, and it is not easy to impart a controlled tension to the tow by means of a pair of conventional driven pinch rollers, even when they are surfaced with a non-slip material. In the roller combination, 22, 23, which is used in the embodiment of this invention, the roller 22 swings on a pivot 24. Also the tow, in addition to passing through the nip between the two rollers 22 and 23, passes round a substantial portion of the circumference of each. Thus the tension in the tow increases the pressure (and hence grip) between the rollers.

Claims

1. A method of producing a carbon fiber tow which comprises continuously drawing a carbonizable fiber tow unidirectionally and substantially linearly under tension through a carbonizing furnace to carbonize said fiber, drawing the carbon fiber tow from said furnace so that the fiber travels in substantially the same linear direction in which it travelled through the furnace, applying a resin solution to the tow and allowing said resin to substantially set while the tow is still under tension and travelling in substantially the said linear direction so as to preserve the alignment of the carbon fibers in the tow substantially as it emerges

2. A method as claimed in claim 1, wherein the carbonizable fiber tow is a

3. A method according to claim 2, wherein the carbonizable polycrylonitrile fiber tow is subjected to a preliminary oxidation treatment before

4. A method according to claim 2, wherein the carbonizable polyacrylonitrile tow is subjected to an oxidation treatment after

5. A method as claimed in claim 1, wherein the carbon fiber tow is subjected to a graphitization treatment by being drawn without any alteration in direction through the treatment zone or zones prior to the

6. A method according to claim 5, wherein the carbon fiber tow is also subjected to a surface oxidation treatment by being drawn without any alteration in direction through the treatment zones prior to the resin

7. A method as claimed in claim 1, wherein the resin solution is applied to

8. A method as claimed in claim 1, wherein the resin is applied in solution

9. A method as claimed in claim 11, wherein the resin is polyvinyl alcohol,

10. A method as claimed in claim 11, wherein the amount of resin applied is

11. A method as claimed in claim 11, wherein the resin is applied in the form of a solution containing from 0.1 to 5 percent by weight of the

12. A method according to claim 1 wherein the carbon fiber tow is subjected to a surface oxidation treatment by being drawn without any alteration in direction through the treatment zone or zones prior to the resin application.