WO2017046577A1

WO2017046577A1 - Composite material

Info

Publication number: WO2017046577A1
Application number: PCT/GB2016/052821
Authority: WO
Inventors: Roger Ford
Original assignee: Integrated Materials Technology Ltd
Current assignee: Integrated Materials Technology Ltd
Priority date: 2015-09-14
Filing date: 2016-09-13
Publication date: 2017-03-23
Anticipated expiration: 2018-03-14
Also published as: GB201516245D0

Abstract

A method for preparing a composite material layer (400, 500, 600), said composite material layer (400, 500, 600, 700, 800) comprising discrete fibre domains (402, 502, 602, 702) comprising discontinuous fibres, wherein a said discontinuous fibre does not extend beyond a said fibre domain, the method comprising: providing a continuous pre-preg layer comprising reinforcing fibres dispersed in a matrix, wherein said reinforcing fibres are generally aligned in a fibre alignment direction; making a plurality of first disconnected cuts through an entire thickness of said pre-preg layer, wherein said first disconnected cuts are separated in said fibre alignment direction, and wherein said first disconnected cuts have a component perpendicular to said fibre alignment direction; and making one or more of second disconnected cuts through an entire thickness of said pre-preg layer, wherein a said second disconnected cut is made along a line which is generally parallel to said fibre alignment direction, and wherein a said line connects corresponding, respective end points of said first disconnected cuts.

Description

Composite material

FIELD OF THE INVENTION

This invention relates to composite materials comprising a structure of discrete fibre domains comprising discontinuous fibres, wherein a said discontinuous fibre does not extend beyond a fibre domain, and to apparatus and methods for preparing such composite materials.

BACKGROUND TO THE INVENTION

It is common practice to reinforce solid materials with fibres of high strength and stiffness. If high performance is required, highly aligned continuous fibres are used that maximise their mechanical properties in the direction of fibre alignment. However there are problems with such materials which arise from the continuity of the reinforcing fibres. When such materials are formed the continuity of the fibre reinforcement can prevent the necessary flow of the material within the mould, particularly in the directions of the fibre axes involved. Furthermore, severe fibre disturbance and even breakage can occur during forming because of the inextensible nature of the reinforcing fibres. These problems may potentially be overcome by converting the continuous fibres to a discontinuous form. However in practice, obtaining the desired improvement in deformation behaviour whilst maintaining the required level of mechanical performance has proved to be problematic.

Several different approaches have been used to produce discontinuous aligned fibre products, as described in, e.g. GB 1389539 A; US 4,552,805 A; US 4,759,985 A; US 2010/0233423 A1 ; JPH 021 15236 A; JP 60224530 A; JP 2009-286817 A; US 2010/0028593 A1 ; US 6,454,893 B1 ; EP 0 768 942 B1 ; US 6,027,786 A; US 5910361 A; US 7387828 B2; US 5487941 A; Edwards, H., Evans, N.P., "A method for the production of high quality aligned short fibre mats and their composites", Advances in Composite Materials vol. 1 , 26 - 29 Aug 1980, Paris, France, Oxford: Pargamon Press, 1620 - 1635; Chang, I.Y. and J. F. Pratte, "LDF Thermoplastic Composites Technology", J. Thermoplastic Comp. Mat., 4, 227 - 252 (1991).

However, none of these approaches has delivered a commercially viable structural composite material. Those based on chopped fibres were limited to short fibre lengths, >5mm, leading to unacceptable mechanical properties, in particular impact resistance. Those based on stretch- broken fibres suffered from excessive deformation yield stresses which made forming difficult. The approach making use of fugitive yarns proved too cumbersome and degraded the fibre alignment during the fabric production and scouring stages, whilst the micro-perforation process, although achieving a major reduction in peak yield stress, failed to deliver the desired improvement in forming quality. The introduction of slits into unidirectional pre-preg or impregnated fabric has been used to produce materials that are used in the preparation of composite moulding tools, but their mechanical performance is not suitable for structural applications.

There is therefore a need for further improvements of such composite materials, and of methods for preparing them.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is therefore provided a method for preparing a composite material layer, said composite material layer comprising discrete fibre domains comprising discontinuous fibres, wherein a said discontinuous fibre does not extend beyond a said fibre domain, the method comprising: providing a continuous pre-preg layer comprising reinforcing fibres dispersed in a matrix, wherein said reinforcing fibres are generally aligned in a fibre alignment direction; making a plurality of first disconnected cuts through an entire thickness of said pre-preg layer, wherein said first disconnected cuts are separated in said fibre alignment direction, and wherein said first disconnected cuts have a component perpendicular to said fibre alignment direction; and making one or more of second disconnected cuts through an entire thickness of said pre-preg layer, wherein a said second disconnected cut is made along a line which is generally parallel to said fibre alignment direction, and wherein a said line connects corresponding, respective end points of said first disconnected cuts.

The inventors have realised that composite materials which comprise discontinuous, aligned fibres in a matrix, whereby a fibre does not extend beyond a fibre domain, as described herein have a wider, specifiable envelope of mechanical properties than existing structural composites, and can therefore be provided in a wider range of specifiable product forms. The inventors have found that it is crucial to the mechanical properties, in particular the ductile response, of composite materials in a continuous form to prepare elements with discrete fibre domains which are free of fibre bridging, i.e. fibres which span from one fibre domain to another. This has been found to be particularly important in order to decrease the peak yield stress occurring on deformation of the composite material. A composite material layer comprising discrete fibre domains comprising discontinuous fibres allows for providing materials with a degree of ductile response which facilitates the detection of local damage and thereby improves safety in use of the composite material layer, reduces safety margins and the frequency of maintenance schedules.

The inventors have realised that eliminating fibre bridging is important because it hinders fibre domain movement during the forming process. Fibre bridging occurs across the contiguous edges of adjacent fibre domains because of fibre misalignment in commercial pre-pregs. Fibre bridging does not occur between the in-plane faces of fibre domains because of the inter-ply matrix layer formed between plies during laminate consolidation.

Fibre bridging is avoided by preparing the pattern of first and second cuts into the pre-preg layer as described herein.

The discrete fibre domains of the composite material layer may be defined by the first and second disconnected cuts, whereby an intersection or link between a cut of the first disconnected cuts and a line along which a second disconnected cut is made may define a corner of a fibre domain. A distance or average distance between one of the first cuts and a neighbouring, second one of the first cuts may thereby define a domain length of a fibre domain. A distance or average distance between a line along which a second disconnected cut is prepared and a second, neighbouring line may define a domain width of a fibre domain. In some embodiments, the line along which the one or more second cuts is/are made may be at a certain distance from an edge of the pre-preg layer. In these embodiments, the width of a fibre domain is determined by the distance between the edge of the pre-preg layer and the line along which the one or more second cuts is/are made.

It will be appreciated that a single first cut and a single second cut may be made into the pre- preg layer. Two adjacent corners may then be used to define the fibre domain together with the first and second cuts. This process may then be repeated and the resulting pre-preg material may subsequently be assembled.

The first disconnected cuts may have a finite length L and may be made into the pre-preg layer normal to or at an angle Θ to the fibre alignment direction. A domain width of a fibre domain is, in this case, w = L x sine. As outlined above, the distance (or average distance in case the first cuts are not made normal to the fibre alignment direction) along the fibre alignment direction between successive cuts of the first cuts defines the fibre domain length.

Methods of preparing such composite materials as described herein allow for preparing such continuous composite material layers which do not contain fibres which span from one fibre domain to a neighbouring fibre domain. In the above method, fibre bridging may be prevented by making the first and second cuts into the pre-preg layer. By making, in particular, second cuts into the pre-preg layer along lines which are generally parallel to the fibre alignment direction, the probability of fibres spanning from one fibre domain to a neighbouring fibre domain is minimised.

The second cuts may consist of a line of disconnected cuts having a straight, elongated shape. Preferably, the gaps between neighbouring cuts of the second disconnected cuts are as small as required (for example a few tens of micrometres, or a few micrometres, or less), to prevent fibres from spanning from one fibre domain via such a gap into another, neighbouring fibre domain. The second disconnected cuts may define perforations, holes, or other shapes.

Connections between the corresponding, respective end points of the cuts (i.e. the lines along which the second disconnected cuts are made) may be aligned substantially parallel to the fibre alignment direction. The probability of fibre bridging between neighbouring fibre domains may thereby be reduced significantly. It is to be noted that the majority of fibres are parallel to the general fibre alignment direction. However, a small fraction of fibres may form a small angle of, for example, 5 degrees or less with the fibre alignment direction. By preparing the second disconnected cuts parallel to the general fibre alignment direction, the probability of a fibre spanning from a fibre domain via a gap between two of the second disconnected cuts to a neighbouring fibre domain may be minimised or negligible.

It will be understood that the angle between a fibre and the general fibre alignment direction (i.e. the fibre misalignment) may follow a certain distribution, for example a Gaussian distribution. Moreover the angle of maximum fibre misalignment may vary between different pre-preg materials. The embodiments of the methods described herein allow for preventing fibre bridging in such cases.

Generally, uni-directional pre-preg layers include a significant percentage of misaligned fibres normally accepted to fall within a range of +/- 5% of the general fibre alignment direction. Such fibre misalignment may facilitate the coupling of the contiguous edges of adjacent fibre domains and may activate an element of intra-ply shear within the fibre domains during axial deformation.

In a preferred embodiment of the method, a spacing p between a said second disconnected cut and a neighbouring said second disconnected cut on a same said line is equal to or less than x / tan Φ, where x is a width of a said second disconnected cut and Φ is a threshold angle between a said fibre of said pre-preg layer and said fibre alignment direction. In some embodiments, the threshold angle may be the maximum angle of misalignment of fibres in the pre-preg layer. In some embodiments, the threshold angle Φ may be, for example, 5 degrees, 4.5 degrees, 4 degrees, or less than 4 degrees. This may ensure that all fibres which previously extended beyond a single fibre domain are cut off by making the second disconnected cuts into the pre- preg layer.

By making the second disconnected cuts into the pre-preg layer along one or more lines as outlined above, fibre bridging is completely prevented or prevented to an acceptable threshold.

In a preferred embodiment of the method, a said second disconnected cut is made such that there is substantially no fibre bridging between adjacent fibre domains. As the said second cut connects, in this embodiment, with the corresponding, respective ends of the first cuts, a complete severing of all the fibres which bridge the contiguous, in-plane edges of adjacent fibre domains may be ensured.

In a related aspect of the invention, there is provided a method for preparing a composite material layer, the composite material layer comprising discrete fibre domains comprising discontinuous fibres, wherein a said discontinuous fibre does not extend beyond a said fibre domain, the method comprising: providing a continuous pre-preg layer comprising reinforcing fibres dispersed in a matrix, wherein the reinforcing fibres are generally aligned in a fibre alignment direction; making a plurality of first disconnected cuts through an entire thickness of the pre-preg layer, wherein the first disconnected cuts are separated in the fibre alignment direction, and wherein the first disconnected cuts have a component perpendicular to the fibre alignment direction; and making one or more of second cuts through an entire thickness of the pre-preg layer, wherein a said second cut is made substantially continuously along a line which is generally parallel to the fibre alignment direction, and wherein a said line connects corresponding, respective end points of the first disconnected cuts. In a preferred embodiment of the method, making the first disconnected cuts comprises making a first row of the first cuts, and making a second row of the first cuts, wherein a said cut of the first row and a said cut of the second row are disconnected and offset in the fibre alignment direction, and wherein a said line connects the corresponding, respective end points of the first cuts of the first row and simultaneously connects the corresponding, respective end points of the first cuts of the second row. As a line along which the one or more second disconnected cuts are made may connect corresponding, respective end points of cuts of different, neighbouring rows simultaneously, the manufacturing process may be simplified and/or accelerated.

In a further preferred embodiment, the method further comprises applying a coating to a surface of the pre-preg layer prior to making the one or more second disconnected cuts, wherein the coating is compatible with the matrix.

The coating (which may be, for example, a polymer coating) is considered compatible with the matrix if the coating fuses with the matrix to form a single phase.

By applying a coating to the pre-preg layer prior to making the one or more second disconnected cuts into the pre-preg layer, the coating may wholly or partly penetrate the pre-preg layer to hold the composite layer together during and after preparing the one or more second disconnected cuts. This may be particularly important if a cut of the second disconnected cuts connects corresponding, respective end points of multiple cuts of the first disconnected cuts while spanning over an entire length of the pre-preg layer. Without a coating, the pre-preg layer may in a case when a cut of the second cuts spans over an entire length of the pre-preg layer fall apart into two or more pieces or strips, which may in some instances be undesired.

Apart from ensuring cohesion between axially adjacent fibre domains, this coating may be chosen to improve the mechanical properties of the resulting composite material layer. It will be appreciated that the coating may thereby be chosen dependent on the subsequent use of the composite material layer.

In a further preferred embodiment of the method, the plurality of first disconnected cuts and the one or more second cuts are made substantially simultaneously, for example prior to collecting the processed material on a single reel. This embodiment may be particularly useful for laminate sheet production as a wider sheet material may be employed. A coating, for example a polymer coating, may in this embodiment be applied to the pre-preg layer after making the first and second cuts into the pre-preg layer, which may improve the mechanical properties of the resulting composite material layer, as outlined above.

It will be appreciated that, in some embodiments, the order in which the first and second cuts are made into the pre-preg layer may be changed. In some embodiments, the first disconnected cuts are made into the pre-preg layer prior to making the one or more second cuts. Alternatively, the one or more second cuts may be made prior to the plurality of first disconnected cuts. In the latter case, the method may further comprising applying a coating to a surface of the pre-preg layer prior to making the plurality of first disconnected cuts. In this embodiment, the coating may not fuse with the matrix such that the coating encapsulates the matrix.

In a preferred embodiment, the coating comprises a polymer coating. A polymer coating may be prepared, for example from a solution and may be deposited by comparatively inexpensive deposition techniques. Coating means include, but are not limited to, dipping techniques, knife coating, die extrusion, roller systems, powder coating and others. A specific coating may be preferred depending on the pre-preg type and/or coating to be applied.

The coating may be applied to one or both surfaces of the pre-preg layer. Coating the pre-preg layer on both surfaces may be preferable to increase strength of cohesion between adjacent fibre domains, in particular since the coating may in this way fuse with a larger volume of the matrix to form a single phase compared to applying the coating on one side of the pre-preg layer only. Whether it is preferable to apply the coating to a single side of the pre-preg layer or to both sides of the pre-preg layer may depend on the use to which the resulting composite material layer may be applied.

It will be understood that in embodiments, in which the one or more second cuts are continuously made along one or more lines which are generally parallel to the fibre alignment direction, the narrow tapes or strips comprising a sequence of individual discrete fibre domains may be obtained, which, in some embodiments, are linked via the coating. The tapes or strips may be collected on a set of reels by means known in the art.

The pre-preg layer may, in some embodiments, be covered on one side by a backing paper. This may be particularly useful in embodiments in which the matrix comprises a thermoset matrix, as the backing paper provides for an enhanced cohesion between neighbouring fibre domains. It will be appreciated that the composite materials described herein may have different shapes. The shape and size of a composite material strip or layer prepared according to embodiments of the methods described herein may be controlled by the shape, and/or length, and/or distribution of cuts.

In order to achieve complete conversion of the pre-preg material to a single shape of discrete fibre domains, the pattern of cuts defining the shape may have two-dimensional translational symmetry. The shapes of the discrete fibre domains may have rotational symmetry C_n where _n = 1 ,2,3,4,6; leading to oblique, rectangular, rhombic, square or hexagonal discrete fibre domains. In some embodiments, however, in the absence of this restriction methods described herein may provide for making materials which may comprise more than one size or shape of discrete fibre domains, which may be distributed transversely or longitudinally with respect to the fibre alignment direction and have a regular, or irregular distribution depending on the use to which the material will be applied. In this case the choice of design for the pattern of cuts may depend on the shape into which the material will be formed.

Preferably, a hexagonal symmetry may be employed as the composite material layer may then have shearing properties which may be the same in all directions (in the plane).

It may further be preferable to choose a certain shape of the fibre domains dependent on the subsequent use and/or shape of the composite material layer. It may further be preferable to choose the shape of the fibre domains such that the majority of fibre domains may have a first shape, and a minority of the fibre domains may have another shape, in order to ensure that the majority of the pre-preg layer material is used when preparing the composite material layer, i.e. to not waste some of the pre-preg layer material.

It will be understood that the shape and size of the domains may be adjusted depending on the specific subsequent use of the composite material.

In this example, a fibre domain has a width of approximately 4 - 60 mm and a length of approximately 50 - 150 mm. However, as outlined above, these parameters may be adjusted according to specific application of the composite material layer. The cuts may be made into the pre-preg layer such that the resulting fibre domains may have a rectangular, trapezoidal, rhomboidal or other shape.

In a preferred embodiment of the method, the first disconnected cuts are substantially parallel. However, depending on the required shape of the composite material strip, the cuts may not be parallel to each other, or only some of the cuts may be parallel to each other. Preparing cuts which are substantially parallel may simplify the manufacturing process.

In a further preferred embodiment of the method, an angle between a said first disconnected cut and a said line along which the one or more second cuts are made is substantially 90 degrees. A composite material layer may be prepared whereby the shape of a fibre domain is substantially rectangular or trapezoidal.

Alternatively, the shape of a domain may be rhomboidal. Therefore, in a preferred embodiment of the method, an angle between one or more of the first cuts and a said line is less than 90 degrees. It will be appreciated that a combination of rectangular-shaped fibre domains, rhombus-shaped, trapezoidal-shaped, or otherwise shaped fibre domains may be prepared in a composite material strip, as required.

It may be preferable to prepare fibre domains with different lateral dimensions. "Lateral" refers here to a plane of the pre-preg layer.

Therefore, in a preferred embodiment of the method, two or more of the fibre domains each have different domain lengths and/or different domain widths and/or different domain shapes. This may be preferable depending on, for example, a specific shape of an object the composite material layer may be applied to. This may be achieved by varying the length of some of the cuts, or, equivalently, by varying the distance between some of the lines along which the second cuts are made.

It will be understood that the order in which the cuts are prepared may vary. In one example, a first set of (first or second) disconnected cuts is prepared across the width of the pre-preg layer, one cut for each row of cuts. A second set of cuts may then be prepared across the width of the pre-preg at a different position, one further cut for each row or a separate row. Alternatively, cuts may be prepared for one entire row before preparing cuts for one or more other rows. It will be understood that alternative sequences for preparing cuts in the pre-preg layer may be used. In a further preferred embodiment, the coating comprises a polymer coating. The polymer coating may be compatible with the matrix, i.e. it may fuse with the matrix to form a single phase. When the coating is applied to the pre-preg layer, the coating may form a bonding with the surface molecules of the pre-preg layer. Coating techniques include, but are not limited to, dipping techniques, knife coating technologies, slot die technologies, roller systems, powder scattering, hot-melt technologies, and others. It will be appreciated that a specific coating technique may be preferred depending on the type of pre-preg and/or coating to be applied to the pre-preg.

In a further preferred embodiment of the method, the matrix comprises a thermoplastic matrix or a thermoset matrix. The thermoplastic matrix may comprise a thermoplastic polymer, such as, for example, a polyamide (PA), polyether ether ketone (PEEK), poly ether ketone ketone (PEKK), polyaryletherketone (PAEK), polyethersulfone (PES), polyphenylene sulphide (PPS), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyvinyl chloride (PVC), polystyrene (PS), ethylene vinyl acetate (EVA), poly(methyl methacrylate) (PMMA), acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), polybenzimidazole (PBI), polycarbonate (PC), polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), and others. The thermoset matrix may comprise, for example, epoxy, vinyl ester, phenolic resins, or other materials.

The reinforcing fibres may be, but are not limited to, glass fibres, carbon fibres, cellulose fibres, high strength polymeric fibres such as aramid or ultra-high molecular weight (UHMW) polyethylene and cellulose fibres, and other fibres. In some embodiments, the composite layer may be comprised of a matrix with a combination of fibre types.

In a preferred embodiment of the method, the first disconnected cuts are substantially perpendicular to the alignment direction of the reinforcing fibres. Preferably, corresponding, respective end points of the first disconnected cuts are aligned in the fibre alignment direction such that the second cuts are prepared in the fibre alignment direction to thereby minimise the probability of fibre bridging between adjacent fibre domains. It will be appreciated that fibre bridging is prevented where the second cuts are prepared continuously along the entire length of the pre-preg layer. It may be preferable to prepare a composite material layer as outlined above whereby the material has a plurality of plies. Therefore, in a preferred embodiment of the method, the continuous pre-preg layer comprises one or more plies each comprising reinforcing fibres dispersed in a matrix. This may advantageously allow for a rapid way of producing a composite material layer as described herein, which is more balanced with regard to its mechanical properties.

Preferably, a first said fibre alignment direction of fibres in a first said ply has a component which is perpendicular to a second said fibre alignment direction of fibres in a second said ply, in particular wherein the first and second fibre alignment directions of the first and second plies, respectively, are perpendicular to each other. This may advantageously allow for a rapid way of producing a composite material layer as described herein, which is even more balanced with regard to its mechanical properties.

In embodiments in which a plurality of plies are used, it may be advantageous to make the second cuts continuously into the pre-preg layer in order to avoid fibre bridging in all plies, in particular as the general fibre alignment directions in the different plies may not be parallel to each other. If in these embodiments the second cuts were made discontinuously into the pre- preg layer, the second cuts may allow for avoiding fibre bridging in one or more plies, but not necessarily all plies if the general fibre alignment directions are not parallel to each other in all plies.

The plies may have a hybridised composition involving more than one fibre type. The combination of types of fibres and matrices for each of the plies, and the combination of respective plies may be determined by the specific requirements of the composite material.

In embodiments, a single ply has a thickness of about 0.1 - 0.3 mm. But plies with other thicknesses may be provided depending on the subsequent use of the composite material layer.

In a further preferred embodiment of the method, the composite material layer comprises one or more fibre types which are hybridised. If a combination of fibres with different mechanical properties is used (for example, relatively softer and stronger fibres and/or more or less extensible fibres), the property (or properties) of one of the fibre types may override that of the others such that a single (or a few) fibre types may dominate the mechanical properties of the composite material layer when the fibre are provided in a continuous form (i.e. bridge from one fibre domain to one or more other fibre domains). For example, in a composite material layer with continuous fibres, the fibre type with the lowest extension capability dominates the extension capability of the whole composite material layer. However, as embodiments described herein allow for obtaining a composite material layer with discontinuous fibres, i.e. fibres which do not bridge from one fibre domain to another fibre domain, the composite material layer exhibits a blend of properties derived from each of the fibre types used. Embodiments described herein therefore allow for obtaining a larger variety of composite material layer exhibiting different, tuneable mechanical properties.

Preferably, the cuts may be prepared in the pre-preg layer using laser light. Although this may be the preferred technique due to being a non-contact technique, its reliability and/or the short time in which cuts may be prepared in the pre-preg layer, other techniques may be exploited, and the preferred technique may depend on the type of pre-preg to be cut. Therefore, in a preferred embodiment of the method, the first and/or second cuts are made using rotary die cutting, flat die cutting, shear cutting, crush slitting, mechanical fracture by controlled flexing or impact or, preferably, laser-cutting.

Composite materials layers comprising discrete fibre domains prepared with any of the above methods may in some embodiments subsequently be assembled as required before being heated and/or consolidated to produce semi-finished materials of various forms, such as, for example, a sheet, profile, rod or tube.

In a related aspect of the invention, there is provided a composite material layer comprising a plurality of discrete fibre domains comprising discontinuous, reinforcing fibres dispersed in a matrix, wherein a said discontinuous, reinforcing fibre does not extend beyond a said fibre domain, wherein a plurality of first disconnected cuts and one or more second disconnected or continuous cuts in said composite material layer define said fibre domains, wherein said first cuts are separated in a fibre alignment direction of said composite material layer, wherein said first cuts have a component perpendicular to said fibre alignment direction, wherein a said second cut is provided along a line which is substantially parallel to said fibre alignment direction, and wherein a said line connects corresponding, respective end points of said first cuts.

As outlined above, discrete fibre domains of the composite material layer may be defined by the first and second cuts, whereby intersections or links between the first cuts and a said line along which a said second cut is made may define corners of the fibre domains. In embodiments in which one second cut is made, a fibre domain may be defined by the first cuts, the second cut as well as the edge of the composite material layer. A distance or average distance between a cut of the first cuts and a neighbouring cut of the first cuts may thereby define a domain length of a fibre domain. A distance or average distance between a line along which a second cut is prepared and a second, neighbouring line may define a domain width of a fibre domain. In embodiments in which a single second cut is made, the domain width may be defined by a distance or average distance between the second cut and an edge of the composite material layer.

Such a composite material layer may be free from fibre bridging between adjacent fibre domains, resulting in advantageous properties of the composite material layer, as outlined above.

In a related aspect of the invention, there is provided a composite material layer comprising: a plurality of discrete fibre domains comprising discontinuous, reinforcing fibres dispersed in a matrix, in particular a thermoplastic or thermoset matrix, wherein a said discontinuous fibre does not extend beyond a said fibre domain; and a coating, in particular a polymer coating, on at least a first surface of the fibre domains configured to fuse with said matrix to connect the fibre domains to each other to form the composite material layer. As outlined above, the coating may thereby improve the mechanical properties of the pre-preg layer, e.g. by strengthening the structure.

In a further related aspect of the invention, there is provided a composite material layer comprising: a plurality of discrete fibre domains comprising discontinuous, reinforcing fibres dispersed in a matrix, in particular a thermoplastic or thermoset matrix, wherein a said discontinuous fibre does not extend beyond a said fibre domain; and a coating, in particular a polymer coating, which encapsulates said matrix. The coating may hereby not fuse with the matrix to form a single phase such that the coating encapsulates the matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described by way of example only, with reference to the accompanying Figures, wherein like numerals refer to like parts throughout, and in which: Figure 1 shows a schematic top-view of a composite material layer according to a first embodiment of the present invention;

Figure 2 shows a schematic top-view of a composite material layer according to a second embodiment of the present invention;

Figure 3 shows a schematic top-view of a composite material layer according to a third embodiment of the present invention;

Figure 4 shows a schematic cross-sectional side view of a composite material layer according to a fourth embodiment of the present invention;

Figure 5 shows a schematic top-view of a composite material layer according to a fifth embodiment of the present invention;

Figures 6a - c show schematic top-views of composite material layers according to a sixth embodiment of the present invention;

Figures 7a - c show distribution of fibres along the fibre alignment direction in a pre-preg layer and the relation between the spacing, p, between adjacent disconnected cuts in the line of cuts comprising a second cut and the width of such cuts according to embodiments of the present invention;

Figure 8 shows a schematic cross-sectional side-view of a composite material layer according to embodiments of the present invention;

Figures 9a and 9b show a flowchart of a method and a schematic of an apparatus, respectively, for preparing a composite material layer according to a first preferred embodiment of the present invention;

Figures 10a, 10b and 10c show a flowchart of a method for preparing a composite material layer, a schematic cross-sectional side view of the resulting composite material layer and an apparatus for preparing a composite material layer according to a second preferred embodiment of the present invention; Figures 11 a and 1 1 b show a flowchart of a method and an apparatus, respectively, for preparing a composite material layer according to a third preferred embodiment of the present invention; and

Figure 12 shows a schematic cross-sectional side view of a composite material layer prepared according to the third preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Overview

Broadly speaking embodiments of the present invention aim to provide aligned discontinuous fibre materials which offer the needed balance of mechanical and deformation properties required for commercially acceptable structural applications.

The inventors have realised, that if the formability of composite materials, comprised of fibre domains created when a pattern of first transverse cuts [T cuts] is introduced into unidirectional [UD] material, is to be improved, a method must be provided for removing the fibre bridging that exists between adjacent fibre domains, As those skilled in the art will be aware, there is a significant degree of fibre misalignment present in commercial UD materials which couples the contiguous axial edges of adjacent fibre domains. During deformation this bridging activates intra-ply shear within adjacent fibre domains and thus augments the deformation yield stress and hinders fibre domain movement during the forming process. Similar bridging does not occur between the in-plane faces of stacked fibre domains because of the inter-ply matrix layer formed between stacked plies during laminate consolidation. The necessary decoupling of adjacent fibre domains can be provided if second cuts parallel to the fibre alignment axis [P cuts] are introduced into the material, which link up with ends of the existing transverse T cuts, thereby creating discrete fibre domains comprised of fibres that do not extend beyond their own fibre domain.

In broad terms we therefore describe a method comprising: providing a continuous pre-preg layer comprising reinforcing fibres dispersed in a matrix, where the reinforcing fibres are generally aligned in a fibre alignment direction; and making a plurality of sets of disconnected T cuts through an entire thickness of the pre-preg layer. Successive sets of disconnected T cuts are separated in said fibre alignment direction, and the disconnected T cuts have a component perpendicular to the fibre alignment direction. The method preferably also involves making one or more continuous P cuts through an entire thickness of said pre-preg layer. A continuous P cut is made along a line which is generally parallel to the fibre alignment direction, and a continuous P cut connects corresponding, respective end points of the sets of disconnected T cuts.

Methods of preparing such composite materials as described herein allow for preparing such continuous composite material layers which do not contain fibres which span from one fibre domain to a neighbouring fibre domain. In the above method, by making, in particular, P cuts into the pre-preg layer along lines which are generally parallel to the fibre alignment direction, the probability of fibres spanning from one fibre domain to a neighbouring fibre domain is removed.

The discrete fibre domains of the composite material layer may then be defined by the sets of disconnected T cuts and continuous P cuts, whereby an intersection or link between a T cut and a P cut defines a corner of a fibre domain. A distance or average distance between one set of T cuts and a neighbouring, set of T cuts thereby defines a domain length of a fibre domain. A distance between a continuous P cut and a neighbouring continuous P cut thereby defines a domain width of a fibre domain. In some embodiments, the line along which the one or more continuous P cuts is/are made may be at a certain distance from an edge of the pre-preg layer. In these embodiments, the width of a fibre domain is determined by the distance between the edge of the pre-preg layer and the line along which the one or more P cuts is/are made.

In another embodiment the T cuts, may not be made normal to the fibre alignment axis, but at an angle Θ to the fibre alignment direction. In this case the width of the fibre domain, w, is given by w = L x sine, and the average distance between neighbouring sets of T cuts defines the fibre domain length.

In another embodiment of the method, the disconnected T cuts may be substantially parallel. However, depending on the required shape of the discrete fibre domains, the cuts may not be parallel to each other, or only some of the cuts may be parallel to each other.

Because the origin of the fibre bridging is associated with misalignment of the continuous fibres in the pre-preg material, it will be appreciated by those skilled in the art that the extent of such misalignment is dependent on the care with which the pre-preg is prepared. In the case of commercial pre-preg it is generally accepted that that the majority of misaligned fibres fall within a generally normal distribution between +/- 5% of the fibre alignment direction. In these circumstances it is possible to sever these misaligned fibres, without requiring the P cuts to be continuous, if the P cuts comprise a line of linear cuts, spaced apart by a length p, where p = x/tanO, and where x is the width of the P cut and Φ, in some embodiments, the maximum angle of misalignment exhibited by the pre-preg fibres. Then if the spacing is equal or less than x tanO, fibre bridging from one fibre domain via the material existing between adjacent disconnected P cuts can be effectively prevented. These effectively continuous P cuts may take the form of slots, perforations or holes of varied shape.

It will also be understood that the order in which the cuts are prepared may vary. T cuts may precede P cuts and visa-versa depending on the application for which the material is intended. If narrow tape material is required for winding operations, P cuts may precede or follow T cuts whilst P and T cuts may be introduced simultaneously if wide width material is required for sheet or panel applications.

It will be appreciated that the P and T cuts will be the origin of voids in the pre-preg layer, which should be minimised and generally not exceed 0.5% of the pre-preg volume if laminate or shaped part quality is to be retained. Calculations of the potential void levels created by the required sets of T and P cuts are laid out in Tables 1 and 2 below. These calculations are based on laser cuts having a specific width in these examples of 0.01 mm, 0.03mm, 0.05mm, 0.07mm and 0.09mm. In these examples, the fibre domains have a length which varies from 10mm to 160mm. It will be understood that these values are only examples.

Table 1 : Calculated % voids associated with sets of T cuts:

Table 2: Calculated % voids associated with sets of P cuts comprising lines of linear cuts which effectively remove fibre bridging between adjacent fibre domains for the case where misalignment within the fibres of the composite pre-preg layer does not exceed 5 degrees.

Accordingly, the total level of voids created in embodiments of the method in which the P cuts consisting of lines of linear cuts which effectively remove fibre bridging between adjacent fibre domains for the case where misalignment within the fibres of the composite pre-preg layer does not exceed 5 degrees are laid out below in Table 3.

Table 3: calculated % voids associated with embodiments of the method in which the second cuts are made discontinuously along a line into the pre-preg layer.

ο,οι 0.03 3.05 3.8^" 8,88

4

1© a t? 0.35» 8.5/ 8 0.885 1,848

20 0.067 0>S80 a 0,4?S 8,538

80 0.800 0.038 0.133 0,1 3 0.283

:50 0,008 0.107 8. 158 0,282 a

>s<-;-> 10 0,108 ø,0¾ 8,:838 0.783 8.073

2 0,0§8 0.173 0,4 3 8,523

0 0.033 IW 0.183 0,238 0.283

80 .02 0.003 0, 8 3. 131 0.185

180 0.014 0,044 8,088 0,10? 8,1 £8

1§

10 0,104 m 8,518 0.731 8,t3~

20 0,0§4 0.182 8,£38 0.081 8.42?

0 0,820 0,031 0,144 0,23 0,282

80 0.05? 0.03Ϊ 0.082 0,118 0.180

180 0. 10 0.032 8,888 0,078 8,883

32

10 , 2 *** 8.518 0,718 8,018

go 0.052 0. 1 8 0.280 0,33 0.483

0 0.02? 0_; i 0.185 0,131 0.243

30 0.018 0.044 8,078 0.184 8,181

18S 0.033 0,083 0,041 0.080 8.374

The total level of voids created using embodiments of the method in which the P cuts are genuinely continuous is that laid out in Table 1 , because here the set of P cuts separates the pre-preg layer into a set of individual discrete fibre domains linked through an applied coating.

It will be seen by inspection that total levels of voids created using embodiments either genuinely continuous or effectively continuous P cuts present no problem with regard to the level of voids required to ensure the quality of laminate and shaped parts, providing that the cut widths, or hole sizes in the case of effectively continuous P cuts are appropriately chosen for the required size of discrete fibre domain.

The method of severing the fibres within the pre-preg layer may include, but is not limited to, rotary or flat die cutting, ultrasonic assisted blade cutting, crush slitting or a highly focused beam of high energy radiation. In the case of severing fibres within a pre-preg layer where the width of the pre-preg layer is that of a single fibre domain, mechanical means including, but not limited to controlled flexing or impact may be used to fracture the fibres.

Preferably, the cuts may be prepared in the pre-preg layer using pulsed laser radiation, a non- contact technique that offers the long term reliability desirable for a continuous manufacturing process, but other techniques may be employed, the preference depending on the type of pre- preg to be processed.

The reinforcing fibres of the composite layer may be, but are not limited to, glass fibres, carbon fibres, cellulose fibres, high strength polymeric fibres such as aramid or ultra-high molecular weight (UHMW) polyethylene and cellulose fibres, and other fibres. The composite layer may also have a hybridised composition involving more than one fibre type. The combination of types of fibres and matrices for each of the plies, and the combination of respective plies may be determined by the specific manner in which the composite material will be used.

The matrix may comprise a thermoplastic matrix or a thermoset matrix. In particular the thermoplastic matrix may comprise any of the polymers listed in the Summary of the Invention.

It may be preferable to prepare a composite material layer as outlined above whereby the material comprises a plurality of plies. In a preferred embodiment of the method, the composite material layer may comprises two or more plies, each comprising reinforcing fibres dispersed in a matrix with the sequence of the fibre alignment directions in the stacked plies being determined by the mechanical performance required of the resulting material. In such a case the embodiment of the method will be restricted to the use of genuinely continuous P cuts with the T cuts.

It is customary for a single ply to have a thickness of about 0.1 - 0.3 mm. But plies with other thicknesses may be provided depending on the subsequent use of the composite material layer. In a further aspect of the present invention a coating is applied to a surface of the pre-preg layer wherein the coating is compatible with the matrix. The coating, for example a polymer, is considered compatible with the matrix if the coating can fuse with the matrix to form a continuous phase.

The coating may be applied to one or both surfaces of the pre-preg layer. Coating the composite material layer on both surfaces may be preferable to increase the cohesion between adjacent discrete fibre domains. Apart from ensuring cohesion between axially adjacent discrete fibre domains, it will be appreciated that the coating may also be chosen to modify the properties of the resulting composite material layer. The degree to which the fibre volume fraction of the composite layer will be reduced by coating will depend on the coating thickness. For materials employed in structural applications it is desirable that the fibre volume fraction should not fall below 0.55. The maximum permissible thickness of the coating will therefore depend on the fibre volume fraction of the uncoated composite material layer. Accordingly, it will be appreciated that the choice of coating may depend on the subsequent use of the composite material layer.

The coating may be applied to the pre-preg layer before or after the introduction of the sets of T and P cuts depending on the chosen embodiment of the method.

In embodiments in which the pre-preg layer is coated, coating means may include, but are not limited to, dipping techniques, knife coating, die extrusion, roller systems, film lamination, powder coating and others. It will be appreciated that a specific coating technique may be preferred depending on the pre-preg type and/or the coating to be applied. Preferably, the coating is compatible with the composite matrix, effectively fusing with the matrix to form a continuous matrix phase.

It will be appreciated that the composite materials described herein may comprise different shapes of discrete fibre domains which will be defined by the shape, and/or length, and/or distribution of the sets of T and P cuts. There may be reasons for adjusting the size and shape of discrete fibre domains to achieve the desired balance of the mechanical and deformation properties or situations where maximising material usage is important.

In order to achieve complete conversion of the pre-preg material into discrete fibre domains, the pattern of cuts defining the shape of the discrete fibre domains must have two-dimensional translational symmetry. In this case the shapes of the discrete fibre domains may be restricted to those having rotational symmetry C_n where _n = 1 ,2,3,4,6; giving oblique, rectangular, rhombic, square or hexagonal discrete fibre domains.

In other embodiments, however, where material utilisation is not a primary consideration the methods described herein may provide for making materials which may comprise more than one size or shape of discrete fibre domains, which may be distributed transversely or longitudinally with respect to the fibre alignment direction and have a regular, or irregular distribution depending on the use to which the material will be applied. In this case the choice of design for the pattern of cuts may depend on the shape into which the material will be formed.

In a first preferred embodiment of the method, the sets of discontinuous T* cuts and the set of effectively continuous P cuts, which connect with the ends of the said T cuts, are simultaneously introduced into a wide tape of UD pre-preg material by means well known in the art, prior to collecting the processed material on a single reel, by means well known in the art. This embodiment is particularly useful for laminate sheet production

In a second preferred embodiment of the method, the sets of discontinuous T cuts are introduced into a supply of UD pre-preg material by means well known in the art, after which the pre-preg material is coated with a compatible polymer by means well known in the art. This coating may be applied to a single or both sides of the pre-preg, depending on the use to which the resulting material will be applied. Thereafter a set of continuous P cuts, which connect with the ends of the sets of T' cuts, are made in the coated pre-preg, using means well known in the art, before collecting a set of narrow tapes, comprised of axially linked individual discrete fibre domains on a set of reels by means well known in the art. These narrow tapes comprising a sequence of individual discrete fibre domains linked by the coating are particularly suitable for tape placement and winding operations.

In a third preferred embodiment of the method a supply of UD pre-preg material is slit into a set of narrow tapes of the required fibre domain width by means well known in the art, after which an individual narrow tape is coated with a compatible polymer, which has a melt temperature somewhat lower than that of the pre-preg matrix, using means well known in the art. By this means the pre-preg tape is given a coated sleeve without fusion of the matrix and coating taking place his narrow coated tape is then subjected to periodic mechanical stress, which fractures the fibres, before the tape is collected on a reel by means well known in the art. As described above there is provided a composite material layer comprising a plurality of discrete fibre domains comprising discontinuous, reinforcing fibres dispersed in a matrix, wherein a said discontinuous, reinforcing fibre does not extend beyond a said fibre domain, wherein a plurality of disconnected T cuts and one or more continuous or effectively continuous P cuts in said composite material layer define said discrete fibre domains, wherein said T cuts are separated in a fibre alignment direction of said composite material layer, wherein said T cuts have a component perpendicular to said fibre alignment direction, wherein a said P cut is provided along a line which is substantially parallel to said fibre alignment direction, and wherein a said line connects corresponding, respective end points of said T cuts.

As outlined above, discrete fibre domains of the composite material layer may be defined by the T and P cuts, whereby intersections or links between the T cuts and a said line along which a said P cut is made may define corners of the fibre domains. In embodiments in which one P cut is made, a fibre domain may be defined by the T cuts, and the P cut as well as the edge of the composite material layer. A distance or average distance between a set of T cuts and a neighbouring set of T cuts may thereby define a domain length of a fibre domain. A distance or average distance between a line along which a P cut is prepared and its neighbouring line may define a domain width of a fibre domain. In embodiments in which a single P cut is made, the domain width may be defined by a distance or average distance between the P cut and an edge of the composite material layer. Such a composite material layer may be free from fibre bridging between adjacent fibre domains, resulting in advantageous properties of the composite material layer, as outlined above.

Composite materials layers comprising discrete fibre domains prepared with any of the above methods may in some embodiments subsequently be assembled as required before being heated and consolidated to produce materials of various forms, such as, for example, a sheet, profile, rod or tube.

Detailed example

In one preferred embodiment of the method, a supply of 150mm width PEKK/AS4 carbon fibre pre-preg, 61 % fibre by volume, was fed under controlled tension through a roller system consisting of rotating datum guides and a pair of parallel un-driven rollers, which define the focal plane of a high speed Cambridge Technology galvanometer scanner. A SPI SP -200C 200W laser, wavelength 1075 nm, beam diameter 5+/-0.7mm and M² 1.1 , was then focussed via an 80mm f-theta lens to deliver a minimum focal spot of 0.032mm at the work piece to introduce the sets of T cuts into the pre-preg. The resulting processed pre-preg was then drawn through a 150mm width heated extrusion die, supplied with PEEK resin by a co-rotating twin screw extruder, which could coat the pre-preg on one or both sides. After cooling the coated tape to 30°C in a temperature controlled water bath, the set of continuous P cuts was introduced into the coated tape by a multiple shear slitting unit. The resulting slit tapes, consisting of a linked sequence of individual discrete fibre domains, were then collected on a set of driven reels with a minimum core size of 100mm. Integration of the laser power, the galvanometer scanner movement, the polymer extrusion rate and the line speed was provided by a CNC system.

Referring to Figure 1 , this shows a schematic top-view of a processed composite material layer [100] comprising a sequence of rectangular discrete fibre domains [102], where A is the fibre alignment direction, L is the length and W the width of an individual domain.

Figure 2 shows a schematic top-view of a processed composite material layer [200] comprising a sequence of rectangular discrete fibre domains of different domain lengths [201 & 204], where A is the fibre alignment direction.

Figure 3 shows a schematic top-view of a processed composite material layer [300] comprising a sequence of rhombic discrete fibre domains [302], where A is the fibre alignment direction.

Figure 4 shows a schematic longitudinal cross-sectional view of stacked, processed composite material layers [400] comprising discrete fibre domains [402] where A is the fibre alignment direction.

Figure 5 shows a schematic top-view of a processed composite material layer [500] comprising domains [502, 504] with different shapes. In this example, some domains have a rectangular shape [504], while some domains have a rhombic (trapezoidal) shape [502]. It will be appreciated that many other shapes and/or combination of shapes may be desirable, depending on the specific application in which the composite material layer is used.

Figures 6a to 6c show schematic top-views [600] of processed composite material layers illustrating the T cuts 602 and P cuts 604 used to create discrete fibre domains, where A is the fibre alignment direction. Whilst T cuts 602 are always continuous, P cuts may in some cases be discontinuous where the spacing between individual component cuts of the P cut is sufficiently small to ensure that no fibre bridging between adjacent fibre domains occurs.

Figures 7a shows an example of the general distribution of misaligned fibres about the fibre alignment direction in a commercial pre-preg layer, where a is the angle of fibre misalignment.

Figure 7b illustrates the spacing required between component cuts of effectively continuous P cuts [702] where x is the width of the component cuts and p is the desired spacing.

Figure 7c illustrates the relationship between the required spacing between component cuts of effectively continuous P cuts and the width of the component cuts for the case where misalignment within the fibres of the composite pre-preg layer does not exceed 5 degrees.

Figure 8 shows a composite material layer [800] which comprises a first ply [802] and a second ply [804]. "A" indicates the general fibre alignment direction in each of the plies [802 & 804]. In this example, the fibre alignment directions of the plies [802 & 804] are perpendicular to each other. As outlined above, the composite material layer which may be prepared this way allows for a more rapid manufacture for producing a material with more balanced mechanical properties.

Figure 9a shows a flowchart according to the first preferred embodiment of the method of the present invention. The pre-preg layer is unwounded at step [902]. T and P cuts are inserted into the pre-preg layer at step [904]. In this example, the pre-preg is then rewound at step [906].

Figure 9b is a schematic diagram of the apparatus [910] required for processing a composite material layer according to the first preferred embodiment of the method of the present invention where a winding unit [912] is loaded with a package of the composite material layer [914] that is fed through a roller system [916] which positions it precisely under a galvo-scanner [920] which delivers appropriately pulsed radiation from a laser [918] to sever the fibres of the composite layer before collecting a sheet of processed material [922] on a further winding unit [912]. B is the machine direction.

Figure 10a shows a flowchart according to the second preferred embodiment of the method of the present invention. The pre-preg layer is unwounded at step [1002]. T cuts are then inserted into the pre-preg layer using a laser at step [1004]. The material is then coated with a coating at step [1006]. P cuts are then inserted by shear slitting at step [1008], before the material is rewound at step [1010].

Figure 10b is a schematic drawing of the cross section of an individual fibre domain [1012] according to the second preferred embodiment of the method of the present invention showing the composite layer [1016] and its coating [1014].

Figure 10c is a schematic diagram of the apparatus [1020] required for processing a composite material layer according to the second preferred embodiment of the method of the present invention. A winding unit [1022] is loaded with a package of the composite material layer which is fed through a roller system [1024] which positions it under a galvo-scanner [1228] which delivers appropriately pulsed radiation from a laser [1226] to sever the fibres of the composite layer. The material then passes through a coating unit [1030] which is fed by an extruder [1032]. The coated material then passes through a temperature controlled water bath [1034] and a slitting unit [1036] before being collected as sequences of individual discrete fibre domains [1038] on a further winding unit [1022]. B is the machine direction.

Figure 1 1 a shows a flowchart according to the third preferred embodiment of the method of the present invention. The pre-preg layer is unwound at step [1 102]. P cuts are then inserted into the pre-preg layer, in this example, by shear slitting at step [1 104]. The material is then coated with a coating at step [1 106]. T cuts are then inserted, in this example, into the layer using a mechanical fracturing process at step [1 108]. The layer is then rewound at step [1 1 10].

Figure 1 1 b is a schematic diagram of the apparatus [1 120] required for processing a composite material layer according to the third preferred embodiment of the method of the present invention. A winding unit [1 122] is loaded with a package of the composite material layer [1 124] which passes through a slitting unit [1 126]. The slit material is then fed through a coating unit [1 128] which is fed by an extruder [1 130]. The coated material is then passed through a temperature controlled water bath before entering mechanical fracturing units [1 134 & 1 138] and being collected as sequences of individual discrete fibre domains on a further winding unit [1 122]. B is the machine direction.

Figure 12 is a schematic drawing of the cross section of an individual fibre domain [1200] according to the third preferred embodiment of the method of the present invention showing the composite material layer [1204] and the encapsulating coating [1202]. Variants

One variant of the above described technique involves coating the pre-preg and impacting the coated pre-preg to prepare the composite material layer with discrete fibre domains.

Thus in a related aspect of the invention, there is provided a method of preparing discrete fibre domains comprising discontinuous fibres in a composite material strip (or layer), the method comprising: providing a continuous fibre reinforcement tow; impregnating said continuous fibre reinforcement tow with a matrix resin; and subjecting said impregnated continuous fibre reinforcement tow to an impact force which is controlled to fracture said fibres to obtain discrete fibre domains while preserving said matrix such that said discrete fibre domains are connected to each other via said matrix, wherein a said discontinuous fibre does not extend beyond a said fibre domain.

Fibre bridging may be prevented in this method due to the fracturing of the fibres in areas which define boarders of a fibre domain.

The continuous fibre reinforcement tow may be of a specific Tex which may be chosen dependent on the specific application of the composite material.

Various techniques for impregnating the continuous fibre reinforcement tow may be exploited, and it will be appreciated that a certain technique may be preferred over other techniques depending on the materials used.

In some examples, the continuous fibre reinforcement tow or pre-preg layer may be coated with a coating, for example a polymer coating. In some examples, the coating may not fuse with the matrix (or matrix resin) to form a single phase.

Precursor elements obtained by impregnating the continuous fibre reinforcement tow may be subjected to a period impact while the tow is moved under an impact unit. Alternatively, the impactor may be moved over the impregnated elements to obtain the desired fibre domains at specific locations of the impregnated elements. The fibres are thereby fractured whilst leaving the surrounding matrix intact, such that the matrix bonds the fibre domains to each other. It will be appreciated that the force of impacting the impregnated continuous fibre reinforcement tow to fracture fibres without fracturing the matrix should be adjusted depending on the specific material combination of fibres and matrix.

In a preferred embodiment, the method further comprises shaping the impregnated continuous fibre reinforcement tow prior to said fracturing, wherein the shaping comprises: feeding the impregnated continuous fibre reinforcement tow through a heated die having a cross-section, in particular a rectangular cross-section, which the impregnated continuous fibre reinforcement tow is to adapt; and cooling and straightening the impregnated continuous fibre reinforcement tow. Shaping the impregnated continuous fibre reinforcement tow may be preferably performed prior to the fracturing as the shaping may be easier on a pristine impregnated continuous fibre reinforcement tow.

Adequate residence time may be provided in an impregnation unit to ensure complete impregnation before the impregnated tow passes through the heated die and is then cooled and straightened.

In a preferred embodiment of the method, the cooling comprises controlling a temperature in a water bath for cooling the impregnated continuous fibre reinforcement tow. This may be preferable as the cooling rate may be precisely controlled using the water bath.

The fibre domain size, and/or shape, and/or pattern prepared with the "impacting" method may be equivalent to those patterns which may be prepared using the "cutting" method outlined above.

Composite materials strips comprising discrete fibre domains prepared with any of the above methods may then be assembled as required before being heated and consolidated to produce semi-finished materials of various forms, such as, for example, a pre-preg, rod or tube.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art and lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS:

1. A method for preparing a composite material layer, said composite material layer comprising discrete fibre domains comprising discontinuous fibres, wherein a said discontinuous fibre does not extend beyond a said fibre domain, the method comprising:

providing a continuous pre-preg layer comprising reinforcing fibres dispersed in a matrix, wherein said reinforcing fibres are generally aligned in a fibre alignment direction;

making a plurality of first disconnected cuts through an entire thickness of said pre-preg layer, wherein said first disconnected cuts are separated in said fibre alignment direction, and wherein said first disconnected cuts have a component perpendicular to said fibre alignment direction; and

making one or more of second disconnected cuts through an entire thickness of said pre- preg layer, wherein a said second disconnected cut is made along a line which is generally parallel to said fibre alignment direction, and wherein a said line connects corresponding, respective end points of said first disconnected cuts.

2. A method as claimed in claim 1 , wherein a spacing p between a said second disconnected cut and a neighbouring said second disconnected cut on a same said line is equal to or less than x / tan Φ, where x is a width of a said second disconnected cut and Φ is a threshold angle between a said fibre of said pre-preg layer and said fibre alignment direction.

3. A method as claimed in any preceding claim, wherein a said second disconnected cut is made such that there is substantially no fibre bridging between adjacent fibre domains.

4. A method for preparing a composite material layer, said composite material layer comprising discrete fibre domains comprising discontinuous fibres, wherein a said discontinuous fibre does not extend beyond a said fibre domain, the method comprising:

making a plurality of first disconnected cuts through an entire thickness of said pre-preg layer, wherein said first disconnected cuts are separated in said fibre alignment direction, and wherein said first disconnected cuts have a component perpendicular to said fibre alignment direction; and making one or more of second cuts through an entire thickness of said pre-preg layer, wherein a said second cut is made substantially continuously along a line which is generally parallel to said fibre alignment direction, and wherein a said line connects corresponding, respective end points of said first disconnected cuts.

5. A method as claimed in any preceding claim, wherein making said first disconnected cuts comprises making a first row of said first cuts, and making a second row of said first cuts, wherein a said cut of said first row and a said cut of said second row are disconnected and offset in said fibre alignment direction, and wherein a said line connects said corresponding, respective end points of said first cuts of said first row and simultaneously connects said corresponding, respective end points of said first cuts of said second row.

6. A method as claimed in any preceding claim, further comprising applying a coating to a surface of said pre-preg layer prior to making said one or more second cuts, wherein said coating is compatible with said matrix.

7. A method as claimed in any one of claims 1 to 5, wherein said plurality of first disconnected cuts and said one or more second cuts are made substantially simultaneously.

8. A method as claimed in claim 7, further comprising applying a coating to a surface of said pre-preg layer prior to making said first and second cuts, wherein said coating is compatible with said matrix.

9. A method as claimed in any one of claims 1 to 5, wherein said one or more second cuts are made prior to said plurality of first disconnected cuts.

10. A method as claimed in claim 9, further comprising applying a coating to a surface of said pre-preg layer prior to making said plurality of first disconnected cuts.

1 1 . A method as claimed in any one of claims 6, 8 or 10, wherein said coating comprises a polymer coating.

12. A method as claimed in any one of claims 6, 8, 10 or 1 1 , wherein said pre-preg layer is coated with said coating on both surfaces of said pre-preg layer.

13. A method as claimed in any preceding claim, wherein said first disconnected cuts are substantially parallel.

14. A method as claimed in any preceding claim, wherein an angle between a said first disconnected cut and a said line is substantially 90 degrees.

15. A method as claimed in any preceding claim, wherein an angle between one or more of said first cuts and a said line is less than 90 degrees.

16. A method as claimed in any preceding claim, wherein two or more of said fibre domains each have different domain lengths and/or different domain widths and/or different domain shapes.

17. A method as claimed in any preceding claim, wherein said matrix comprises a thermoplastic matrix or a thermoset matrix.

18. A method as claimed in any preceding claim, wherein said first disconnected cuts are made substantially perpendicular to said fibre alignment direction.

19. A method as claimed in any preceding claim, wherein said continuous pre-preg layer comprises one or more plies each comprising reinforcing fibres dispersed in a said matrix.

20. A method as claimed in claim 19, wherein a first said fibre alignment direction of fibres in a first said ply has a component which is perpendicular to a second said fibre alignment direction of fibres in a second said ply, in particular wherein the first and second fibre alignment directions of the first and second plies, respectively, are perpendicular to each other.

21 . A method as claimed in any preceding claim, wherein said composite material layer comprises one or more fibre types which are hybridised.

22. A method as claimed in any preceding claim, wherein said first and/or second cuts are made using rotary die cutting, flat die cutting, shear cutting, crush slitting, mechanical fracture by controlled flexing or impact or, preferably, laser-cutting.

23. A composite material layer prepared using the method of any one of the preceding claims.

24. A composite material layer comprising a plurality of discrete fibre domains comprising discontinuous, reinforcing fibres dispersed in a matrix, wherein a said discontinuous, reinforcing fibre does not extend beyond a said fibre domain, wherein a plurality of first disconnected cuts and one or more second disconnected or continuous cuts in said composite material layer define said fibre domains, wherein said first cuts are separated in a fibre alignment direction of said composite material layer, wherein said first cuts have a component perpendicular to said fibre alignment direction, wherein a said second cut is provided along a line which is substantially parallel to said fibre alignment direction, and wherein a said line connects corresponding, respective end points of said first cuts.

25. A composite material layer comprising:

a plurality of discrete fibre domains comprising discontinuous, reinforcing fibres dispersed in a matrix, in particular a thermoplastic or thermoset matrix, wherein a said discontinuous fibre does not extend beyond a said fibre domain; and

a coating, in particular a polymer coating, on at least a first surface of said fibre domains configured to fuse with said matrix to connect said fibre domains to each other to form said composite material layer.

26. A composite material layer comprising:

a coating, in particular a polymer coating, which encapsulates said matrix.

27. A composite material layer substantially as described and/or illustrated herein in the description and drawings.