WO2000075410A1

WO2000075410A1 - Textile structures based upon multifilament fibres and method for producing same

Info

Publication number: WO2000075410A1
Application number: PCT/GB2000/002155
Authority: WO
Inventors: Eric Taylor; Norman Ridge
Original assignee: Carr Reinforcements Limited
Priority date: 1999-06-05
Filing date: 2000-06-05
Publication date: 2000-12-14
Also published as: GB2364328A8; GB0127038D0; GB9913119D0; GB2364328A; AU5233900A

Abstract

A method for making a fabric comprising making an open fabric out of tows of fibre, and spreading the fibre tows to produce a fabric having a closed structure.

Description

TEXTILE STRUCTURES BASED UPON MULTIFILAMENT FIBRES AND METHOD FOR PRODUCING SAME

This invention relates to the field of textile structures intended primarily for the production of composite materials by impregnation with resins It more particularly relates to a warp and weft fabric produced from multifilament high performance tows, with a relatively high weight per unit length, for a relatively low weight per unit area, and to a method for producing such structures It is often required that the textile structures used to manufacture composite materials are tightly woven, so as to retain a regular geometry and an appropriate handling capacity, whilst at the same time allowing sufficient penetration of the resin during manufacture of the composite As a result, satisfactory mechanical properties are obtained in the final composite Thus it is considered necessary to use fine tows to manufacture such tightly woven structures

In order to produce a fabric with the desired weight per unit area for the structure, a fibre assembly or tow giving good cover factor is chosen In other words, a tow which does not exhibit large porosities in the weave and which leads to a high fibre to volume ratio (FVR), defined as follows

FVR = Weight of fabric/density of the fibre material Unit width x unit length x thickness

The fibre volume ratio can be calculated for any point in a fabric If the FVR is low the resultant fabric is unsatisfactory as a reinforcement for composite structures due to the high porosity present in the fabric Therefore the lower the weight per unit area of the woven fabric required the lower the linear density of the tow used

However, fine tows are relatively expensive and this is particularly true for carbon fibres currently available on the market For example the price of Ik (1000 filaments) carbon fibre tows is about four times that of 3k tows, six to eight times that of 6k carbon tows, nine to eleven times of 12k carbon tows and 12 to 14 times that of 80k tows

Hence it is conventionally advantageous to use coarser tows, where the price decreases as the coarseness increases to produce a fabric of a given weight However, if fine tows are replaced by coarser tows, whilst keeping the same weight per unit area, for example, four 3k tows by one 12k tow, the resultant holes in the fabric are larger than they are for higher weights per unit area The FVR is low, the fabric porosity high, and the fabric is unsuitable as a composite reinforcement material Coarser tows are therefore unsuited for use in textile structures where the weight per unit area is relatively low when conventional weaving methods are used In essence the structures formed are too open, and additionally they cannot be easily handled as they leave the weaving loom As a result the use of coarser tows is currently limited to fabrics with relatively high weights per unit area

Figure 1 illustrates the difference between a fabric woven with a coarse tow (IA) and a fine tow (IB) for a fabric of the same weight per unit area This clearly shows the open structure obtained with the coarser tows compared to the "closed" structure obtained from the fine tows

An analysis of the balanced carbon fabrics available on the market, i e where the weight of the warp is identical to the weight of the weft, and having a uniform surface with no porosity, leads to a relationship between the tow used and the weight per unit area of the fabric Table one illustrates the relationship between the fibre type used and the weight per unit area of the fabric

TABLE 1 Standard fabrics, relationship between tow tex value, weight and construction of a woven carbon fibre fabric

Construction Weight G/m2 Tow filaments Ends/cm x Picks/cm

120 Ik 9 5 x 9 5/cm

200 3k 5 0 x 5 0/cm

375 6k 4 69 x 4 69/cm

660 12k 4 2 x 4 2/cm

This relationship can be summarised as follows

Ik tows are used for fabrics whose weight per unit area is generally between 90 and 210 g/m² For 3k tows the weight per unit area of fabric is generally between 160 and 400 g/m², for 6k tows it is from 260 to 600 g/m², and for 12k tows it is between 465 and 800 g/r i It must be remembered that fabrics with lower weights than these figures can be produced from different tows, But the resultant fabric can have a high porosity, which is not compatible with the objective of good cover factor

The present invention provides a method for making fabrics, and fabrics so made, from fibre tows, including carbon fibre tows, whose linear density is relatively high with respect to the weight per unit area of the fabric having a porosity or FVR compatible with its use in the manufacture of a resinated composite material having satisfactory mechanical properties

According to one aspect of the present invention there is provided a method for making a fabric having a substantially closed structure with a given weight per unit area and construction using tows of fibres which are coarse relative to said given weight per unit area and construction, said method comprising making an open fabric out of said coarse tows of fibres and spreading the tows to produce a fabric having said substantially closed structure, characterised by producing the open structure fabric using coarse tows which are sized, desizmg the fabric and spreading the tows

Use of sized tows facilitates handling of the tows and production of the fabrics from the tows, e g by weaving

Conventionally the size is selected for compatabiiity with the resin system with which the fabric is subseguently used in the production of composites By desizmg the fabric, the ability to spread the tows is considerably enhanced so that relatively inexpensive, coarse tows may be employed to produce the fabric since the enhanced spreading allows a closed structure to be secured despite the coarseness of the tows employed

Thus, for example, the resulting fabric may comprise a plurality of multi-filament ribbons each derived from a generally round tow (i e generally circular or elliptical) forming a closed structure, the fabric having a filament count/weight per unit area ratio which is greater than that associated with conventionally produced fabrics For instance, the filament count/weight per unit area ratio may be at least 40 g m² more preferably at least 50 g ¹ m², e g in excess of 66 g ¹ m² Although the coarse tow prior to production of the fabric will be generally round, we do not exclude the possibility that it may be of a flat configuration, e g tape or nbbon- ke

It has been found that, by removing the size from the tows, a highly drapeable fabric is achievable largely independently of the fabrication technigue (e g weave style), fibre density and thickness This is an important consideration when producing composites of complex configuration The drapeability of the fabric may be retained to a large extent even where the fabric is re-sized since the size level needed e g for compatabiiity with the impregnating resin during subsquent use of the fabric in composite production may be substantially less than the size level employed for tow handling purposes during weaving

The tows may be incorporated into the fabric with zero or substantially zero twist/m However, the method of the invention may be implemented using twisted tows having for example at least 5 turns/m or more, and even in excess of 10 turns/m The use of twisted tows is made possible by the desizmg step since the desizmg step greatly facilitates spreading of the tows even when twist is present

By producing a fabric from a coarse tow that is then spread, a thinner fabric can be achieved As a conseguence, by using a thinner fabric obtained in this way, a high FVR is achievable which confers good mechanical properties when used in a composite By virtue of the present invention, more plies of fabric can be used in a given volume compared with a thick heavy fabric

The size may be water soluble, e g glycerine and water-soluble epoxy and polyester-based sizes

Usually the size is present on the tow prior to fabric production in an amount of at least 0 2%, e g from 0 2% to 10%, by weight Normally the size is present on the tow prior to fabric production in an amount of at least 0 5%, more preferably at least 0 7% and typically at least 1 0% by weight (i e by weight with reference to the unsized tow)

The tows may be used to produce a multiple layer fabric, the tows used being coarse relative to said given weight per unit area and construction of each layer, in the case of a multi-layer fabric, the ratio of filament count to weight per unit area in each layer of the fabric is preferably not less than 50 g ¹ n , more preferably not less than 60 g^"1 m² and most preferably in excess of 66 g ¹ m², e g at least 75 g ¹ m²

Heating of the fabric may be effected during the de-sizing process

A water soluble size can be removed either by washing, by agitation in hot water or by the application of heat

The fibre size can, however, be any other conventional size such as a substantially non-soluble epoxy, polyester, acrylic or thermoplastic or thermoplastic containing size, and, after weaving, the epoxy or other size can, for example in the case of carbon fibre fabrics, be removed by burning off using an oven or a furnace

Alternatively, a substantially water-insoluble epoxy or other size can be removed using an appropriate solvent

By removing sufficient size to render the filaments mobile, spreading of the tows may occur during the de-sizing (e g washing) process Surprisingly, it has been found that a washing process is particularly effective in spreading the tows even to the extent that a major part, if not substantially all, of tow spreading occurs during desizmg Tow spreading is especially effective during desizmg where the size employed is water soluble glycerine or epoxy size

Desizmg is advantageously carried out with the fabric immersed in a body of liquid, e g water, which is preferably heated The fabric or body of liquid may be agitated during the desizmg step so as to aid tow spreading Such agitation may for example be provided by conducting the desizmg step under boiling conditions so that the bubbling accompanying boiling creates agitation of the fabric

Typically, desizmg reduces the size level or content of the tows to the extent necessary to facilitate spreading of the tows Thus, reduction in size level may but need not necessarily be of the order of at least 70%, e g at least 80% In the case of a glycerine size for instance, desizmg is carried out so as to remove substantially all of the size In some implementations of the invention, especially where the size used on the tow prior to weaving is suitable, the desizmg step is carried out so as to remove sufficient size to facilitate spreading of the tow during and/or after desizmg while retaining sufficient size in the structure so that re-sizing for e g chemical compatabiiity with resin is obviated In practice, the amount of resin-compatible size required is usually somewhat less, e g at least 50% less, than that required for the purpose of facilitating tow handling during weaving

After removal of the size the fabric can be dried, for example by the use of drying cylinders or ovens or infra-red heaters

The fibres in the fibre tows of the fabric can be spread either simultaneously with the drying operation and/or separately using apparatus such as spreader bars, banana rollers, air knives or combs, rollers, cylinders, ultrasonic spreaders, air bonding jets, smooth, grooved or crowned rollers, drums, water jets or any appropriate combination of these types of apparatus

After the fabric has been spread, it may be re-sized, for example by coating with a thermoplastic or thermosettmg resin compatible size which may be chemically different from or chemically the same or similar to the size present on the tows prior to production of the fabric Alternatively, as indicated above, where the size employed is suitable, the desizmg step may be carried out so as to leave a residual size level in the structure so that re-sizing is obviated Where re-sizing is undertaken, the amount of size used is typically less than the amount used on the tow prior to fabric production, e g at least about 50% less by weight

Where the fabric after desizmg is subsequently re-sized or retains sufficient size to obviate re-sizing, it will be appreciated that its drape properties and hence its ability to follow contours may be significantly enhanced compared with those of the fabric prior to desizmg One feature of the invention therefore resides in using tows which are sized in preparation for the fabric production operation, undergo desizmg and spreading thereby producing fabric with good drape properties and retaining improved drape properties through the use of reduced size levels (through re-sizing or by partially desizmg the fabric) when preparing the fabric for use with a resin system Carbon fibre tows in particular as obtained from manufacturers are usually already sized with a size at a level suitable for handling during weaving and using a size compatible with the resin system to be contacted with the fabric The process of the present invention provides considerable scope for enhancing the drape properties of the fabric produced through tow spreading and control of size level in the fabric end product

A further feature of the invention resides in producing a fabric by weaving carbon fibre tows which are sized with a water-soluble size such as glycerine Although it is known to produce carbon fibre tows which are sized with glycerine, such tows are conventionally used to produce chopped fibres and not for weaving

The invention can be applied to a wide range of types of fibres, for example, carbon, aramid, glass, ultra-high molecular weight polyethylene, polyester, polyamide and metal coated fibres, e g metal coated aramid fibres and nickel coated carbon fibres The types of carbon fibres can be high strength, intermediate modulus, high-modulus or ultra-high-modulus

The carbon fibre tows can contain a range of numbers of fibres e g IK, 3K, 6K, 12K, 24K, 48K and 50K etc Tow types are sometimes designated by tex count, i e 66 tex - 1 k, 200 tex (3k), 400 tex (6k), 800 tex (12k)

The invention will now be more particularly described with reference to the accompanying drawings in which

Figures IA and IB show respectively a fabric woven with a coarse yarn and a fine yarn,

Figures 2A and 2B show respectively a sized woven fabric using coarse yarns and a de-sized woven fabric using coarse yarns

Figures 3, 4 and 5 illustrate diagrammatically a first method of making a fabric according to the present invention,

Figures 6 and 7 show a second method of processing a fabric according to the present invention,

Figure 8 shows diagrammatically a third method of processing a fabric according to the present invention,

Figure 9 shows a fourth method of processing a fabric according to the present invention, and

Figures 10 and 11 show another method of processing a fabric according to the present invention

Referring to Figure 3, a 12k carbon fibre tow having a nominal 5% by weight of glycerine size on it (Grafil 34-700-12k-5%-catalogue reference of Grafil Inc) is woven on a loom (18) into a fabric (12) with a construction, i e ends and picks/cm, calculated to give the desired weight in the fabric for the intended end use, for example 200 g/m² = 1 25 ends/cm x 1 25 picks/cm of 12k fibre tow The resulting fabric produced is very open in its structure and is at this stage totally unsuitable as a fabric for composite manufacture Referring to Figure 4, the fabric is then transferred from the loom (18) and wound onto a mandrel (20) and placed in a "jig" (32) It is then threaded around the various rollers in the jig and attached to a second mandrel (22) A jig tank (28) is then filled with water which is then heated to the required temperature by means of steam pipes (30) in the bottom of the jig

The fabric (12) is then run back and forth from one mandrel to the other through the boiling water In the method described in this invention, the water is kept at boiling point by means of the steam pipes (30) located in the bottom of the jig (32) The effect of this is to not only heat the water up but also to agitate it by means of the escaping steam As a result the glycerine size is washed off the carbon fibres, thus allowing them to become very mobile within the fabric Due to the water being violently agitated, the desized carbon fibres begin to spread out and fill the spaces in the fabric The number of cycles required in the jig varies according to the weight of the fabric woven, which determines the ease with which the glycerine size can be removed by the washing process It will be appreciated that, to facilitate spreading of the tows, other means may be provided to effect agitation of the fabric in addition to or instead of steam bubbles Where glycerine is used as the size, substantially all of the size will usually be removed during the de-sizing step

Referring to Figure 5, after a predetermined number of cycles in the jig the desized carbon fibre fabric is wound onto one of the mandrels and removed from the jig and transferred to heated drying cylinders (36) The fabric (34) is then threaded around the drying cylinders and attached to a take up fabric roller (38) The drying cylinders are heated to such a temperature that, when the fabric is run around the cylinders the water contained in the fabric is driven off and the fabric dried completely At the same time the tensioning of the fabric as it passes around the relatively large diameter of the drying cylinders causes the desized carbon fibres further to spread out As a consequence any spaces that had been left in the woven fabric from the washing operation are filled by the desized carbon fibres Ideally, at the end of the process, all spaces in the fabric should be evenly and substantially completely filled in

Figures 2A and 2B illustrate a carbon fibre fabric before and after processing by the described method The end result of this process is a very flexible fabric with a good cosmetic appearance and an even spacing of the fibres It must be remembered that the fabric described above is for illustrative purposes only The process is applicable to any carbon fibre and indeed any synthetic fibre, which is capable of being sized with a water-soluble size and having it subsequently, removed by means of washing in either hot or cold water

Further examples of fabrics produced by the process as described above will now be given to illustrate the potential of the process

Table 1 lists commonly available standard carbon fibre fabrics produced by numerous weavers Table 2 lists the standard fabrics and their replacements by fabrics woven from heavier tows than would be possible by conventional weaving methods, but which is made possible by the present invention

TABLE 2 Standard woven fabrics and their replacements by fabrics produced by desizmg

(This is not to be taken as a definitive list of exact replacements for a given currently available commercial fabric Other alternative constructions may be feasible, using a range of fibres or even a combination of different fibre types )

The process as described is not limited to what is termed a "balanced" fabric, in other words those for which the weight of the warp fibres is identical to the weight of the weft fibres It can also be used for fabrics which are described as uni-directional fabrics, where the ratio by weight of the warp fibres and the weft fibres (or that of the weft fibres and the warp fιbres)ιs say, greater than 80 20 Example number 4 describes a uni-directional fabric which may be produced by the process as described in this patent

Example 1

The two fabrics are balanced fabrics woven from carbon fibres Soficar FT300B-3000-40B (standard fabric) and Grafil Inc 34-700 12000 5%-G (product references of the respective manufacturers) Both fabrics have a weight per unit area of 200g/m² The Grafil product may be obtained as a sized product from the manufacturer and the particular product specified has a glycerine size level of 5% by weight relative to the unsized tow This product is normally supplied for use in producing chopped fibres and is not normally used for weaving

Standard fabric Desized fabric construction construction

Weight Ends/cm Picks/cm Ends/cm Picks/cm

200g/m² 5 0 5 0 1 25 1 25

The reference here to desized will be understood to refer to desizmg effected in accordance with the method of the invention

Example 2

Two fabrics, being balanced fabrics each having a weight per unit area of 220g/m², when woven from aramid fibres comprising respectively Kevlar 49, T968, 1580 dtex (standard fabric) and Kevlar 49 3160 dtex (product references of Du Pont de Nemours) can be expected to produce the following results after the latter product, initially sized with a water soluble size, is subjected to desizmg following weaving

Standard fabric Desized fabric construction construction

Weight Ends/cm Picks/cm Ends/cm Picks/cm

200g/m² 6 7 6 7 3 48 3 48

In this Example, Kevlar product for the desized product has a filament count which is twice that of the product used for the standard fabric

Example 3

Where two fabrics, being balanced fabrics, are produced from respective glass fibres, namely glass roving 2002, 600 tex, (standard fabric) and glass roving 2002, 2400 tex (product references of PPG), the following results can be expected after the latter product has been subjected to desizmg Standard fabric Desized fabric construction construction

Weight Ends/cm Picks/cm Ends/cm Picks/cm

600g/m² 5 0 5 0 1 25 1 25

In this Example, the desized fabric glass fibre product has a filament count which 4 times greater than that of the standard fabric glass fibre product

Example 4

The two fabrics in this example are uni-directional fabrics produced from carbon fibres, with a nominal weight per unit area of 400g/m² The weave used is plain weave and the warp to weft ratio is 85 15

Standard fabric

Warp fibre, 34-700-12000-1 2% (catalogue ref of Grafil Inc)

Weft fibre, FT300B-3000-40B (catalogue ref of Soficar SA)

Desized fabric

Warp fibre, 34-700-48000 5%GS (catalogue ref of Grafil Inc)

Weft fibre, FT300B-3000-40B (catalogue ref of Soficar SA)

Standard fabric Desized fabric construction construction

Ends/cm Picks/cm Ends/cm Picks/cm

4 2 3 0 1 14 3 0

Example 5

The two fabrics in this example are hybrid fabrics, consisting of a combination of carbon and aramid fibres

Standard fabric

Warp fibre FT300B-3000-40B (catalogue ref of Soficar SA)

Kevlar 49, 1580 dtex, t968 (catalogue ref of Du Pont de Nemours)

Arrangement Warp - 1 carbon fibre to 1 Kevlar fibre

Weft fibre as Warp fibre above and with the same fibre arrangement Desized fabric

Warp fibre FT300B-3000-B (catalogue ref of Soficar SA) Kevlar 49, 2400 dtex (catalogue ref of Du Pont de Nemours) Arrangement, 1 carbon fibre to 1 Kevlar fibre

Weft fibre, as warp fibre above and with the same fibre arrangement Standard fabric Desized fabric construction construction

Weight Ends/cm Picks/cm End/cm Picks/cm 220g/m² 6 42 6 3 5 09 5 09

Although the present invention is particularly applicable to the production of carbon fibre-based fabrics, fibres other than carbon fibres can be used Moreover the carbon fibres described are known in the carbon fibre industry as high strength, intermediate modulus fibres But the process as described in this invention can be used with any fibre type or fibre tenacity, or combination thereof, that is capable of being supplied with a water soluble or other size and having the size removed by washing in water or otherwise and dried at the relevant temperature to drive the water off completely Fibres used may also be with or without twist

Table 3 lists various fibres that may be used in conjunction with this invention It must not be taken as an exhaustive list of all the fibres capable of being processed by the present invention, merely as an illustration

TABLE 3 Examples of fibres that can be used in the invention as described in this patent

CARBON FIBRES

High strength intermediate modulus e g Grafil 34-700-3k, Grafil 34-700-12k, 34-600-48k amd Toray

T300-3000-40B

High strength e g Toray T800 types or equivalent Intermediate modulus

High modulus e g Toray M46, M55 Mitsubishi Rayon K13710 etc Ultra high modulus

Fibre tex counts Ik, 3k, 6k, 12k, 24k, 48k, 50k, 80k, and above

OTHER FIBRE TYPES

Ceramic Aramid

Glass

Ultra high molecular weight polyethylene (UHMWP) e g Dyneema, Spectra, Vectran Fibres

Polyester, Polyamide

Thermoplastic fibres

The current invention has described the process applied solely to a woven fabric produced by conventional weaving technology The process is not limited to woven fabrics, but can also be used with textile structures or fabrics produced by other textile technologies some of which are listed below

Woven shapes

Warp and weft knitted fabrics

Braided fabrics

Non-woven uni-directional fabrics High modulus woven fabrics Multi-layer or three dimensional fabrics Tnaxial woven fabrics

For example, the fabric may be a stitched multi-axial fabric in which two or more layers are stitched together in known manner to give a multi-layer fabric and may be produced in a wide range of weights and styles, e g biaxial, tπaxial, guadnaxial combinations Thus, by knitting an open weave fabric structure using a large sized tow, the size can be washed off or otherwise removed to produce a highly drapable, lightweight fabric

Three dimensional fabrics comprise multiple layers of fabric woven together to form an mtergrated fibre network with fibres in the Z direction The fabric layers may be held together by weaving of selected tows through the thickness of the fabric, which tows can be taken from any of the layers Various 3-D weave architectures are possible, including angle interlock, 3X, 3X with warp stuffer yarns and orthogonal 3-D, as described in the textbook "3-D textile reinforcements in composite materials" published in 1999 by Woodhead Publishing Limited

A high modulus fabric is one in which a fine secondary fibre is woven in with the mam reinforcing tow in such a way that the fine secondary fibre takes up all of the crimp leaving the mam tow substantially perfectly flat (see the above mentioned textbook)

The fabrics may be produced by batch production methods, or by continuous processes

Figure 6 illustrates a continuous process to produce a desized carbon fibre fabric which is similar to that shown in Figures 4 and 5

Figure 7 illustrates the continuous desizmg process with alternative means of spreading The desized fabric (12) is passed over spreading rollers (40) and then through an air comb (42) to further spread the carbon fibres if reguired Thence the fabric can pass through a further drying oven (44) onto a take-up roller (46)

So far only the de-sizing and spreading of a carbon fibre fabric has been described, but additionally other processes may be added on to the process such as "re-sizing" the fabric The purpose behind re-sizing the fabric is that it promotes adhesion to different matrix systems i e polyester, epoxy, and thermoplastic resin systems In fact the process may be very advantageous because at present most carbon fibre is supplied mainly with an epoxy size system This size is mainly compatible with epoxy resms and not others such as polyester or thermoplastics resins For compatabiiity with other resin systems, carbon fibres have to be supplied with special sizes Such fibres may not be economical or the fibres may not be even available commercially Using the technology as described herein, fabrics with different size systems could be produced economically and in small guantities

Figure 8 illustrates the fabric desizmg process with the additional sizing stage and shows essentially a combination of the features shown and described above with reference to Figures 6 and 7 Similar or identical components in Figure 8 have been given the same reference numbers as those shown in Figures 6 and 7 However, in addition to the components shown in Figures 6 and 7, the apparatus shown in Figure 8 also includes a drying oven (56) for use if reguired between the continuous desizmg hot water bath (28) and an air comb (58) between the spreader bars or cylinders (36) and the size bath (50) The size in the bath (50) can be applied to the fabric by means of a lick roller (60)

Glycerine has been particularly referred to above because it is a commercially available water soluble size commonly used by carbon fibre producers for the manufacture of chopped fibres and wet laid non-woven fabrics Therefore such fibres are readily available and at commercially advantageous prices because they do not have to be manufactured specially This does not mean that any other water-soluble size cannot be used In fact any size that is capable of being removed by water or otherwise can potentially be used, such as polyvinyl pyrro dine (PVP), or a low molecular weight uncured epoxy resin, or polyester resin It must preferably be capable of being easily applied to the carbon fibres during manufacture and able to act as a processing aid during the weaving process

Alternative means of de-sizing will now be described

Solvent de-sizing

The use of a solvent may be preferred where there are problems associated with the use of water and the possibility that it may not be all removed during the drying stage It is well known that water has a deleterious effect on the mechanical properties of a carbon fibre composite or, indeed, a composite produced from any other type of fibre Therefore use of a size that can be easily dissolved in a solvent may be preferable in preference to the use of water

A fabric may be woven from large tow fibres sized with either standard size systems or sizes that are easily removed by a solvent The fabric is then immersed in a solvent and processed by a method such as described above or by other means such as continuous de-sizing The carbon fibres could be spread during the de-sizing process or during the drying stage in a manner similar to that described above

Heat desizing

An alternative method to both the water and solvent de-sizing technigues is to burn the size off the fabric Two methodswill be described

Method A

This method is well known in the ceramic matrix composites industry in which a brittle ceramic fibre is sized with a 'binder' which allows the brittle ceramic fibre to be processed i e woven After weaving, the size (or binder) is removed by burning the size off at an appropriate temperature Typically such sizes are based upon acrylic polymers, though other binder types may be used Such a binder needs to be capable of being removed at comparatively low temperatures and leave no unwanted residues or contamination on the surface of the carbon fibre Such a method could be used to de-size a carbon fibre fabric prior to the spreading of the carbon fibre in the fabric The use of such a means would limit it to those fibres that are capable of withstanding the temperature at which the size needs to be removed

Method B

The second means by which a 'desized' carbon fibre fabric could be produced would be to weave a fabric from yarn sized conventionally, e g epoxy resin sized Then by means of heat, burn off the size at the reguired temperature, and for the necessary period of time to produce a desized fabric, after which the carbon fibres could be spread by the various methods as described in the present invention

Two means by which heat desizmg could be achieved are described below although again it must be accepted that there may be other methods by which a size could be burnt off a fabric

Batch production Table 4 lists the stages in a potential process by means of using of an oven Such an oven could be one as used by glassfibre weavers, which is known in the industry as a batch heat cleaning oven The oven will have both a temperature and time controller to ensure that the fabric is in the oven for the desired length of time at the correct temperature, to ensure complete removal of the size without damaging the carbon fibre fabric

TABLE 4 Stages in a batch heat de-sizing process

1 Weave "sized" fabric

2 Wind onto perforated steel mandrel

3 Place fabric into oven,

4 Heat to reguired temperature for the necessary time to remove the size

5 Switch off oven and allow to cool

6 Remove fabric from oven and the spread the fibres

Continuous Production

Figure 9 illustrates a production method using a furnace, and having a re-sizing stage In Figure 9 there is shown a continuous desiz g apparatus for carbon fibre fabric by heat treatment and subseguent adhesion promoter application Fabric from the loom (18) passes from a take-off roller (62) over tension rollers (64) and through a furnace or oven (66) which is heated by any appropriate means around a roller (68)

The desized fabric (12) then passes in sequence over spreader bars or cylinders or any appropriate apparatus (70) or air combs (72) to spread the fibres, a size or adhesion promoter applicator (84), and onto a take-up roller (76)

Weaving of un-sized carbon fibres

Whilst this is extremely difficult and has not proved to be a commercially viable method due to the very fragile nature of the carbon fibre, which are easily broken during the weaving process, it has been achieved by several weavers It is equally possible, even if difficult, to weave fabrics from tows that are larger for the weight of fabric than is normally used as described in this invention These large tows are then spread to fill the space in order to produce an acceptable carbon fabric for composites manufacture

Thus, in a further aspect of the present invention there is provided a method for making a fabric having a substantially closed structure with a given weight per unit area and construction using tows of fibres which are coarse relative to said given weight per unit area and construction, said method comprising making an open fabric using coarse unsized tows of carbon fibres and spreading the tows to produce a fabric having said substantially closed structure Such fabrics can have their large tows spread by the various methods described in this invention i e spreader bars, air comb, etc The fabrics then can also be sized to give compatibility with the various resins in which they will be incorporated

Another route to producing, e g weaving, unsized carbon fibre fabric from unsized carbon fibres would be to wrap the fibres with a protective yarn Once the carbon fibres have ben wrapped with the protective yarns, they are then processed, e g by weaving, as normal to produce the fabric The protective yarns are then removed in a further process, as by dissolving them Such a method could incorporate any of the desizmg processes as previously described For example-the carbon fibre could be wrapped with a water-soluble yarn and this yarn subsequently removed by washing in water Alternatively a yarn that was easily removed by heat could be used, such as an acrylic fibre Once the fabric has been woven, the acrylic fibre is then removed by burning it off, either in an oven or in a furnace The carbon fibre tows are then spread by using any of the methods described above

Additional processes can be performed continuously with the production of the fabrics as described above, such, for example, as pre-preggmg (solvent or hot melt impregnation of a fabric with a resin) or thermoplastic film stacking or double band sheet production Such continuous processing will minimise the handling of what can, in the case of carbon fibres, be a very fragile and delicate fabric A fabric made in accordance with the present invention may be used in other composite manufacturing techniques such as wet lay up resin transfer moulding and its variants such as

RIFT (resin in fusion under a flexible tool

SCRIMP (Seeman Composite Resin Infusion Moulding Process)

SPRINT (SP Systems Limited, Resin Infusion Technology)

Rotational moulding (thermoset and thermoplastic composites)

Resin Film Infusion

Figures 10 and 11 show the thermoplastic and thermoset impregnation processes respectively Figure 10 illustrates diagrammatically an apparatus for the thermoplastic or thermoset impregnation of desized and spread fabric using a film of polymer The fabric (12) passes through the nip of a pair of rollers (78, 80) together with a thermoplastic or thermosettmg film (84) on each side of the un-sized or re-sized fabric The sandwich of the fabric (12) and film (84) passes through a heating zone (86) and a cooling zone (88) by means of two continuous drive belts (90, 92) The thermoplastic or thermoset sheet (12a) of fabric and film is then reeled on up a take-up roller (90), or cut into flat sheets by means of a flying saw and stacked

Figure 11 shows an apparatus for the solvent impregnation with a thermosettmg resin of a de-sized or re-sized and spread fabric

The desized or re-sized fabric (12) passes over a roller (93) in a dip-bath of resin in solvent (94) and thence into a heated tower (96) The resin-impregnated fabric is then provided with a release film (98) and passes on to a take-up roller (100) through the nip of a pair of rollers (102, 104)

A fabric woven from conventionally sized carbon fibre, e g using an epoxy size, can be produced as follows

The epoxy-sized carbon fibre fabric is woven in the normal way and the size is removed by heat by means of any of the methods described above

The fabric is then either removed from the heat and fibres spread by any appropriate apparatus, or they are spread after the desized fabric emerges from the continuous desizmg furnace

An adhesion promoter may be applied to the de-sized and spread fabric again by any conventional apparatus

It should be noted that the method of impregnation is not limited to those described Other techniques may be used for either matrix system such as hot melt coating, film infusion, or powder coating

Again it must not be taken that the method of drying the fabric as described in this invention is the only means to achieve that result There are many other means that can give the desired end result Other techniques such as ovens, infra-red heaters, microwave ovens, etc, may be used

Spreading of the fibres within the fabric can be achieved by a variety of means, many of which are well known to practitioners in the textile industry and are commercially available pieces of equipment from various suppliers Some of these alternative spreading techniques are now described below, although it should not be taken as a comprehensive list

Spreader bars

Banana rollers

Air knives or combs

Rollers

Cylinders

Ultra-sonic spreaders

Air banding jets

Smooth, grooved or crowned rollers

Drums

Water jets

Vibratory techniques

It must be remembered that the spreading process is not limited to any one technique, but can be undertaken by using a combination of any of the above methods

It is also envisaged that the process as described in this invention may not always be undertaken "off loom" The possibility exists for it to be incorporated as an additional stage during weaving Thus the fabric could be desized and spread immediately after weaving

Typically applications of fabrics made in accordance with the invention include production of automotive and motor cycle parts and components, boats, racing car structures and components, sports equipment such as skis, fishing rods and kayaks, tooling components, etc

Claims

1 A method for making a fabric having a substantially closed structure with a given weight per unit area and construction using tows of fibres which are coarse relative to said given weight per unit area and construction, said method comprising making an open fabric out of said coarse tows of fibres and spreading the tows to produce a fabric having said substantially closed structure, characterised by producing the open structure fabric using coarse tows which are sized, desiz g the fabric and spreading the tows

2 A method as claimed in Claim 1 in which spreading of the fibres is effected during desizmg

3 A method as claimed in Claim 1 or 2 in which spreading of the fibres is effected after desizmg

4 A method as claimed in Claim 3 in which spreading of the fibres is effected both during and after desizmg

5 A method as claimed in Claim 4 in which spreading of the fibres is effected primarily during desizmg

6 A method as claimed in any one of Claims 1 to 5 in which the size is water soluble

7 A method as claimed in Claim 6 in which the size is glycerine or a water-soluble epoxy or polyester size

8 A method as claimed in any one of the preceding claims in which the size is removed by washing, optionally while the fabric is immersed in a body of liquid

9 A method as claimed in any one of the preceding claims in which the fabric is agitated to remove the size, optionally by washing the fabric and generating bubbles within the washing medium

10 A method as claimed in any one of the preceding claims in which the fabric is heated to remove the size

11 A method as claimed in any one of the preceding claims in which, following removal of the size, the fabric is dried, for example, by use of drying cylinders, or ovens or infra-red heaters, optionally with tensionmg of the fabric

12 A method as claimed in Claim 11 in which the fibres are spread simultaneously with the drying operation, optionally with tensionmg of the fabric

13 A method as claimed in Claim 11 or 12 in which the fibres are spread after the drying operation

14 A method according to any one of the preceding claims in which the fibres are spread using apparatus such as spreader bars, banana rollers, air knives or combs, rollers, cylinders, ultra sonic spreaders, air banding jets, smooth grooved or crowned rollers, water jets or drums or any combination of the aforesaid types of apparatus

15 A method as claimed in any one of the preceding claims in which the size is removed using an appropriate solvent

16 A method as claimed in any one of Claims 1 to 6 in which the size is a non-water soluble size such as an epoxy size and desizmg is effected by burning off, for example using an oven or furnace

17 A method as claimed in any one of the preceding claims in which following spreading of the tows the fabric is re-sized

18 A method as claimed in Claim 17 in which the size used for re-sizing of the fabric differs chemically from that used for sizing the tows prior to production of the fabric

19 A method as claimed in Claim 17 or 18 in which the size used to size the tows prior to production of the fabric is a water-soluble size and that used to re-size the fabric is less water-soluble or water-insoluble

20 A method as claimed in any one of Claims 17 to 19 in which the amount of size used, on a weight percent basis, for resizing is less, e g about at least 50% less, than the size level present on the tows prior to fabric production

21 A method as claimed in any one of Claims 1 to 16 in which the desizmg step is carried out in such a way as to retain a residual amount of size in the fabric and in which the fabric is subsequently contacted with a resin or resin system which is compatible with the size

22 A method as claimed in Claim 21 in which the resin or resin system is contacted with the fabric without re-sizing the same

23 A method as claimed in any one of the preceding claims comprising contacting the fabric with a resin system

24 A method as claimed in Claim 23 in which the fabric is impregnated with a thermoplastic or thermosettmg resin

25 A method for use in the production of composites comprising making a fabric having a substantially closed structure with a given weight per unit area and construction using tows of fibres which are coarse relative to said given weight per unit area and construction, said closed structure fabric being produced by making an open fabric using coarse tows which are sized with a water-soluble size, washing the open structure fabric to remove size therefrom while spreading the tows, and drying the washed fabric, optionally causing further spreading of the tows during and/or after drying, whereby the tows are converted to ribbon-like form to produce the closed structure fabric, re-sizing the dried closed structure fabric using a different size, and contacting the re-sized fabric with a resin or resin system, said different size being one which is compatible with the resin or resin system

26 A method as claimed in any one of the preceding claims in which the tows prior to fabric production are untwisted

27 A method as claimed in any one of the preceding claims in which the tows prior to fabric production are twisted, optionally with at least 5 turns/m

28 A method as claimed in any one of the preceding claims in which the fibres are carbon fibres

29 A method as claimed in any one of Claims 1 to 22 in which the fibres are selected from aramid fibres, glass fibres, polyethylene fibres (preferably ultra-high molecular weight polyethylene fibres), polyester fibres and polyamide fibres

30 A method as claimed in any one of the preceding claims in which the fibres are metal coated fibres, e g nickel coated carbon fibres Not fiirnisched at time of publication

using unsized coarse tows wrapped with a protective yarn, which protective yarns are subsequently removed by dissolving the same or by heating or burning off

43 A method for making a fabric having a substantially closed structure with a given weight per unit area and construction using tows of fibres which are coarse relative to said given weight per unit area and construction, said method comprising making an open fabric using coarse unsized tows of carbon fibres and spreading the tows to produce a fabric having said substantially closed structure

44 A method for making a fabric by weaving using carbon fibre tows which are sized with a water-soluble size, optionally glycerine or a water-soluble epoxy or polyester size

45 A fabric obtainable by the method as claimed in any one of the preceding claims

46 A fabric comprising at least one multifilament tow accommodated in the fabric in substantially nbbon-like form and having a filament count/weight per unit area ratio in excess of 66 g m²

47 A fabric as claimed in Claim 46 in which the multifilament tows are accommodated within the fabric with a zero or substantially zero

48 A fabric as claimed in Claim 46 in which the multifilament tows are twisted, optionally with at least 5 turns/m