BACKGROUND TO THE INVENTION

Field of the invention The present invention relates to a layered composite structure, and a method for manufacturing the same. The present invention also relates to a two-layered coating for a cellular core.

Related art

WO 2012/056202 discloses composite load bearing panels, resin-impregnated fibre mats and methods for their manufacture. Figs. 1 and 2 schematically illustrate a composite load bearing panel according to WO 2012/056202. Fig. 1 shows the separated components of the panel 1 during manufacture, and the mould surfaces 5a, 5b used to press them together. Fig. 2 shows the assembled panel 1. The panel 1 has a sandwich construction with a paper or card cellular core 2 and upper and lower skin layers 3, 4. Each skin layer in the assembled panel 1 comprises natural fibre and cured bio-resin.

These are each formed using a natural fibre mat with bio-resin applied. Each skin layer 3, 4 has a first face 3a, 4a and a second face 3b, 4b, the first face 3a, 4a having applied a first bio-resin, the second face 3b, 4b having applied a second bio-resin. The second face 3b, 4b for each skin layer 3, 4 is in contact with and bonded to the core 2. The second bio-resin differs from the first bio-resin in terms of composition, quantity, curing temperature, curing speed, viscosity and/or water content.

It is also known in the art to produce sandwich panels using flax-fibre composites, as in Khalfallah et al "Flax/Acrodur ^® sandwich panel: an innovative ecomaterial for automotive applications", (JEC Composites Magazine / No. 89, May 2014). The sandwich panels have three layers of flax fibre reinforcement impregnated with Acrodur ^® thermoset polyester acrylic resin, the three layers being stacked in a direction suitable for the envisaged applied stresses. Cardboard honeycombs are placed between the skins. In the production of the flax fibres, the bundles forming the flax stems are decorticated and separated, then the remaining shives and woody core of the stem are removed by scutching. The scutched flax is then hackled to obtain long flax fibres. These bundles are then stretched and maintained in a unidirection direction and sprayed with a water mist to promote cohesion of the fibres. The fibres are then dried in an oven and are subsequently impregnated with the Acrodur ^® resin.

SUMMARY OF THE INVENTION

The present inventors have realised that in the manufacturing process described in WO 2012/056202, the fibres in the skin layers are partially aligned during the carding part of the process to form the natural fibre mat, but they are not fully extended (i.e. straight). As a consequence, when the panel is put under a load transverse to its major surface, many of the fibres do not contribute to resisting deflection of the panel. Furthermore, when the fibre mat is needle punched to consolidate the layers, the needles can cause a substantial number of fibres to be displaced in the direction perpendicular to the plane of the mat. These fibres similarly cannot contribute substantially to the stiffness of the panel.

The production process in the article by Khalfallah et al. involves laying all the fibres in one direction. This is complex and expensive. During this process, if the fibres are only partially aligned, their strength cannot be fully harnessed because the resultant board will deflect until the fibres are fully aligned and the load fully transferred. Furthermore, crimping can occur during the scutching process, also contributing to the incomplete alignment or stretching of the fibres.

Many natural fibres have high specific strength and modulus, when straight. To make use of these properties in composite load-bearing applications, natural fibres should be introduced in a composite material in a suitable orientation. It is possible to conceive of strategies to reinforce non-woven natural fibre mats using specially prepared low twist yarns or unidirectional tapes of reinforcing fibres. The high production cost, and, as in the Khalfallah et al article discussed above, potential unreliability of incorporating aligned fibres in a non-woven material tends to be a serious disadvantage and so such approaches are not used in industrial scale production of composite parts for use in the automotive or construction industries, for example.

The present invention has been devised in order to address at least one of the above problems. Preferably, the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems.

In a first preferred aspect, the present invention provides a layered composite structure having a cellular layer and at least one skin layer bonded to the cellular layer, the skin layer having first and second sublayers, the first sublayer comprising a fibrous material component comprising a network of fibres and a resin-based binder component, and the second sublayer comprising a veneer.

Preferably, the cellular layer is a core layer. In this case, the layered composite structure preferably has a further skin layer, bonded on the opposite side of the cellular layer. The layered composite structure may therefore form a sandwich structure. The further skin layer may include similar first and second sublayers, but this is not necessarily essential. Preferably, the further skin layer includes a veneer. Preferably the further skin layer also includes a resin-based binder component. The veneer is preferably a wood veneer. Wood veneer is well known. Historically, it has been used for decorative purposes, to provide an attractive surface when bonded to a structural support (e.g. plain wood or engineered wood such as plywood, chipboard or MDF). Preferably, the wood veneer substantially preserves the wood structure, i.e. the wood grain, of the tree from which the wood veneer is produced. The wood veneer is thus preferably a planar or curved section of a tree.

The wood veneer may be obtained by "peeling" the layer from the trunk of a tree, or by slicing large blocks of wood known as flitches. There may be other ways of obtaining the wood veneer. As indicated above, it is preferable that the wood veneer is taken in a manner which preserves the grain of the wood from which the layer is taken. Preferably, the grain of the wood in the wood veneer runs in a direction which is substantially uniform across the wood veneer. Most preferably, the grain of the wood in the wood veneer runs in a direction which is substantially parallel to the plane of the wood veneer. In solid wood, the cellulose fibres are substantially aligned with each other, in a direction dictated by the vertical growth direction of the tree from which the wood is taken. For this reason, the fibres in solid wood are substantially straight and substantially parallel. As a result, when the layered composite structure is loaded in a direction substantially perpendicular to the direction of the grain, the fibres within the wood veneer are able to take up the load without relative movement of the fibres. As a result, the stiffness of the layered composite structure is substantially improved. Alternatively, the veneer may be a bamboo veneer or a leaf veneer such as banana leaf veneer. Preferably, the veneer (second sublayer) is thinner than the first sublayer in the thickness direction of the layered composite structure. It is also preferable that the veneer has uniform thickness, at least over 80% of its area, more preferably over all of its area. It is preferable that the veneer has a thickness not greater than 3mm. More preferably the veneer has a thickness of up to 2mm, or more preferably a thickness of up to 1mm thick or less than 1mm. Still more preferably, the veneer may have a thickness between 0.5 and 0.7mm. The veneer may have a thickness down to any suitable lower limit, depending on the process used to cut the veneer from the original wood. For example, the veneer may have a thickness of not less than 0.2mm. It is possible, alternatively, for the second sublayer to be thicker than the first sublayer, for example, where the first sublayer acts only as a carrier for the resin to bond the veneer to the core.

The resin-based binder component of the skin layer may be a thermoplastic resin.

Suitable materials include polyethylene, polypropylene, polylactic acid or poly vinyl chloride. Alternatively, the resin-based binder component may be a thermosetting resin such as acrylic, epoxy, phenolic, urea-formaldehyde, melamine, unsaturated polyester, or polyurethane. It is preferable that renewable thermosetting resins are used, such as epoxydized plant oil resins, cashew oil resins or furan-based resins, for environmental reasons. Most preferably, a furan resin comprising a polyfurfuryl alcohol polymer and an acid catalyst is used.

It is preferable that the resin-based binder component also acts to bond the first and second sublayers. This facilitates a simple manufacturing process.

The fibrous material component of the skin layer preferably comprises carded fibres. Suitable fibres may be cross-lapped. Carded and cross-lapped fibres may then be needle punched in order to form a fibre mat.

The fibrous material component of the skin layer preferably comprises natural fibres. Natural fibres have in their elementary form high specific strength and stiffness like mineral fibres. The natural fibres of the fibrous material component are preferably at least partially aligned, to provide additional stiffness for the composite. Examples of natural fibres which may be used are flax, hemp, jute, banana leaf, sisal leaf, pineapple leaf, cotton, coir, oil palm, bamboo, wood pulp or E-glass or basalt. Preferably flax or hemp are used as the natural fibre. In alternative embodiments, the fibrous material component of the skin layer may include mineral fibres. Glass fibres such as e-glass fibres may be used. Naturally-sourced mineral fibres such as basalt fibres may be used.

The cellular layer may have an open cell format. For example, the cellular layer preferably has a honeycomb structure. More preferably, the cellular layer has a honeycomb structure comprising a plurality of substantially parallel hollow channels. The cellular layer is preferably made of a cellulosic material such as paper or card. However, synthetic materials can be used, including polymeric materials such as aromatic polyamides (aramid or meta-aramid). A honeycomb structure is provided with several air spaces, which give the structure a low density, while maintaining mechanical strength. The structure is particularly resilient to compression in a direction substantially parallel to the substantially parallel hollow channels, and also particularly resilient against flexure in directions which attempt to bend the channels. Provision of a cellular layer made from paper or card can provide a suitably low density and is advantageous in terms of its use of natural materials.

Alternatively, the cellular layer may comprise a foam. Suitable foams can be open cell or closed cell. A suitable closed-cell foam is expanded polypropylene. The first sublayer may be in contact with the cellular layer. This is advantageous because the cellular layer may bond well with the first sublayer in view of the resin component able to form a bonding bridge between the first sublayer and the cellular layer.

The second sublayer (comprising veneer) may extend substantially across the full length of the first sublayer. Additionally or alternatively the second sublayer may extend substantially across the full width of the first sublayer. This may provide a particularly stiff structure. However, it is possible for the second sublayer to be present only in part of the skin layer. When the veneer only partially covers the composite structure, it is preferable that the outer surface of the veneer lies substantially flush with the outer surface of the first sublayer which is not covered by the veneer. This allows the peripheral regions of the composite structure to be formed without veneer. This is advantageous in particular where these peripheral regions are formed with a sealed edge (i.e. the cellular layer is not exposed at the edge) or a profiled shape. Such

arrangements may be desirable in order to protect the cellular core and/or to provide a desired finish to the structure. With this arrangement, it is particularly preferred that the opposing side of the composite structure includes a corresponding skin layer. This preferably has a corresponding format of first and second sublayers, the second sublayer being a veneer.

A particularly preferred arrangement of the composite laminate is an arrangement of a cellular core and opposing skin layers, each skin layer comprising a first sublayer and a second sublayer which is a veneer, wherein the veneer extends only part way across the full width and/or only part way across the full length of the first sublayer, and a surface of the first sublayer not covered with the veneer lies substantially flush with a surface of the veneer. This arrangement allows sealed edges to be formed around the periphery of the composite. Furthermore, the arrangement of veneers on opposing sides of the cellular core provides additional impact resistance and bending stiffness.

In a second preferred aspect, the present invention provides a method of manufacturing the layered composite structure of the first aspect of the invention, including the steps of:

(a) arranging a cellular layer, a fibre mat with resin and a veneer into a stack;

(b) pressing and heating said stack between opposing pressing surfaces, in order to solidify the resin and form said skin layer bonded to said cellular layer.

The fibre mat may be at least partially impregnated with resin. This may be done before the arrangement of the cellular layer, fibre mat and veneer into the stack.

It is preferable that the veneer is pressed into the fibre mat during the pressing and heating step. Where the veneer does not cover the entire fibre mat, the result of this is that the outer surface of the veneer lies flush with the exposed surface of the first sublayer (formed from the fibre mat and resin) to the extent that the forming tool (the relevant pressing surface) has a continuous surface. In order to achieve this, it is possible that, during the step of arranging the components into a stack, the veneer is held at one (or both) of the opposing pressing surfaces. This allows the method of WO 2012/056202 to be adapted without expensive changes to the equipment. This is made possible because the fibres in the veneer are typically aligned and straight, and so there is no need for additional steps to ensure alignment of the fibres in the veneer.

The forming tool (the opposed pressing surfaces) may have different shapes depending on the overall shape of the layered composite structure required. Furthermore one or both of the opposed pressing surfaces may move during the pressing and heating step.

In a third preferred aspect, the present invention provides a layered composite structure having first and second sublayers, the first sublayer comprising a fibrous material component comprising a network of fibres and resin-based binder component, and the second sublayer comprising a veneer.

Preferably, the veneer is bonded to the first sublayer due to the resin component, which additionally acts to bind the fibres of the first sublayer together. Further preferred features of the veneer are as set out with respect to other aspects of the invention.

Preferably, the fibrous material component comprises natural fibres. It is of particular interest in the third aspect of the invention that the resin is a bio-resin as disclosed as a preferred feature with respect to the first and second aspects. In this way, the layered composite structure of the third aspect can be formed of only natural components, which is advantageous for ecological reasons.

In particular where the veneer is a wood veneer, it may be used for a decorative purpose, at least in part. In that case, it can be advantageous to use a wood veneer in which the grain direction is not parallel to the surface of the wood veneer. Some applications use burr veneer, for example, in which the wood grain is highly distorted and prized for its decorative uses. At present, when decorative veneers are used, for example, in luxury cars, they are typically applied to aluminium (or other metallic) substrates. The veneer and the metal have substantially different coefficients of thermal expansion. Therefore, fluctuations in temperature cause differential thermal expansion, which can lead to cracking or detachment of the veneer. In the present invention, it is possible for the first and second sublayers to have similar coefficients of thermal expansion.

In a fourth preferred aspect, the present invention provides a method of manufacturing the layered composite structure of the third aspect of the invention, including the steps of:

(a) arranging a fibre mat with resin and a veneer into a stack;

(b) pressing and heating said stack between opposing pressing surfaces, in order to solidify the resin and bond the veneer to the fibre mat, thereby forming the first and second sublayers.

It is additionally considered that the use of veneers in any composite material comprising thermosetting furan-based resins is new. Accordingly, such use and composite materials comprising veneer and thermosetting furan-based resin constitute respectively fifth and sixth independent aspects of the invention.

In a seventh aspect, the present invention provides a method of manufacturing a layered composite structure having a cellular layer and at least one skin layer bonded to the cellular layer, the skin layer having first and second sublayers, the first sublayer comprising a fibrous material component comprising a network of fibres and resin-based binder component, and the second sublayer comprising a veneer, the method including the steps:

(a) providing a cellular layer, a fibre mat and a veneer;

(b) impregnating the fibre mat with uncured resin; and then

(c) arranging the cellular layer, uncured resin-impregnated fibre mat and veneer into a stack; and then

(d) pressing and heating said stack between opposing pressing surfaces, in order to cure the resin and compress the fibre mat to form the first sublayer, thereby forming a bond between the cellular layer and the first sublayer via the cured resin and forming a bond between the first sublayer and the veneer via the cured resin in the same pressing and heating step. This method is advantageous as the fibre mat is bonded to the cellular core and veneer in a single curing process. This "one-shot" process, provides reduced manufacturing times, as the all the layers are bonded in a single pressing and heating step, rather than repeated steps of heating, curing and cooling to bond each of the layers separately.

The first, second, third, fourth, fifth sixth and/or seventh aspects of the invention may be combined with each other, in any combination. Additionally or alternatively, to the extent that they are compatible, they may include any combination of the above mentioned optional features of any aspect of the invention.

Further optional features of the invention are set out below.

Preferably, a first resin is provided at the first face (A face) of the natural fibre mat and a second resin is provided at the second face (B face) of the natural fibre mat. The second resin may differ from the first resin in terms of composition, quantity, curing temperature, curing speed, viscosity and/or water content. This is advantageous since the two faces of the mat typically must perform different tasks, as discussed in WO 2012/056202. Preferably, the second resin differs from the first resin at least in that the second resin cures faster and/or at a lower temperature than the first resin. Preferably, for each natural fibre mat, the second face (B face) of the impregnated natural fibre mat is in contact with the core. In the method of manufacture of the layered composite structure, there is preferably formed a wet sandwich construction which is pressed and heated so as to compress (at least in part) the impregnated natural fibre mat and cure the first and second resins.

The application of the resin to the respective faces of the natural fibre mat may be achieved through different processes. For example one face may be contact coated, e.g. using a roller coating process. The other side may be sprayed or applied with a blade coater. Alternative coating processes include sprinkling, painting, dipping, and full bath impregnation. The purpose of using different application processes relates to the different resins having different physical properties (e.g. different viscosity) and so the different resins are most efficiently applied to the mat using different application processes. Furthermore, the different application processes allow different quantities of resin to be applied to each face.

The characteristics of the final product can be varied considerably according to need by varying the distribution of the resin through the natural fibre mat, its thickness and the pressure applied. The process of impregnation of the resin into the natural fibre mat can be in the form of a continuous process. In that case, the width of the mat is limited only by the width of the coating machinery. Furthermore, the manufacture of the board itself (including the pressing and curing step) can be part of the continuous process. The resulting sheet of board may then be guillotined to size as it emerges from the production machinery.

Alternatively, the resin-impregnated natural fibre mat can be used as a prepreg, the prepreg being cut to length as desired, applied to the core and then pressed and cured in a static press.

The thickness of the cellular layer is preferably at least 5mm or at least 10mm. The thickness of the other each skin layer (the skin layer being the resin impregnated natural fibre mat after the curing step) is preferably at least 5 times smaller than the thickness of the cellular layer.

For each skin layer, the natural fibres are preferably arranged substantially randomly but substantially parallel to each face of the skin layer. The natural fibres may have an average length of at least 10mm, more preferably at least 20mm, at least 30mm, at least 40mm, at least 50mm, at least 60mm or at least 70mm. The fibres may be processed (e.g. cut) to have a maximum length of up to 150mm, for example. The fibres may have a length which is longer (preferably substantially longer, e.g. at least 10 times longer) than the thickness of the first sublayer. Preferably, the natural fibres are plant-derived fibres. Preferably, the plant-derived fibres are one or more selected from the following: hemp, jute, flax, ramie, kenaf, rattan, soya bean fibre, okra fibre, cotton, vine fibre, peat fibre, kapok fibre, sisal fibre, banana fibre or other similar types of bast fibre material. Such fibres are considered to be annually renewable, in that they are based on a crop which can be grown, harvested and renewed annually.

Preferably, the natural fibre mat is a non-woven mat formed by needle punching.

Alternatively, air laying may be used. Non-woven mats are preferred. However, woven mats may be used. The area density of the natural fibre mat may be in the range 300- 3000 grams per square metre (gsm).

Preferably, the natural fibre mat is dried before use in the process. A natural fibre mat with moisture content of less than 5wt% is suitable, e.g. about 3wt%.

The curing process for the board includes heating. The heating step typically heats the structure to a temperature of at least 130°C. Preferably, in this step, the structure is heated to a temperature of 250°C or lower, more preferably 200°C or lower. A typical temperature for this heating step is 150-190°C.

The curing process for the panel also includes pressing. By the combination of heating and pressing, at least part of the impregnated natural fibre mat is typically permanently compressed so that the density of the skin layer after heating and pressing is at least three times (preferably at least four times or at least five times) the density of the impregnated natural fibre mat before heating and pressing. This may apply particularly to the A face of the natural fibre mat, but for the reasons explained in more detail below may not apply to the B face of the natural fibre mat.

The bio-resin may be derived from sugar cane. One example of a "bio-resin" is a furan resin, although other bio-resins are available. Preferably, the bio-resin comprises furfural (furan-2-carbaldehyde) or a derivative of furfural such as furfuryl alcohol. Furan resins are typically made by self polymerisation of furfuryl alcohol and/or furfural. Alternatively, suitable resins can be made by copolymerisation of furfuryl alcohol and/or furfural with another resin (e.g. a thermosetting resin) or its monomers. Examples of the latter are furfuryl alcohol - urea formaldehyde and furfuryl alcohol - phenolic blends. In a preferred embodiment, the resin may therefore be a polyfurfuryl alcohol, a liquid polymer which self-crosslinks in the presence of an acid catalyst. For example, a furan resin may be produced in which furfural replaces formaldehyde in a conventional production of a phenolic resin. The furan resin cross links (cures) in the presence of a strong acid catalyst via condensation reactions. Furfural is an aromatic aldehyde, and is derived from pentose (C5) sugars, and is obtainable from a variety of agricultural byproducts. It is typically synthesized by the acid hydrolysis and steam distillation of agricultural byproducts such as corn cobs, rice hulls, oat hulls and sugar cane bagasse. Further details relating to furan resins whose use is contemplated in the present invention is set out in "Handbook of Thermoset Plastics", edited by Sidney H. Goodman, Edition 2, Published by William Andrew, 1998, ISBN 0815514212,

9780815514213, Chapter 3: Amino and Furan Resins, by Christopher C. Ibeh, the content of which is incorporated herein by reference in its entirety. Furan resins are of particular interest because they are derived from natural, renewable sources, they bond well to natural fibres and they have good flame-retardancy properties.

The bio-resin preferably includes an acid catalyst. The catalyst promotes curing via condensation reactions, releasing water vapour. The bio-resin may further include a blocker component. The function of the blocker component is to affect the curing behaviour of the bio-resin. Thus, the first and second bio-resins may differ in

composition, based on the content and/or type of catalyst and/or the content and/or type of blocker component. This allows the second bio-resin to cure faster and/or at a lower temperature than the first bio-resin component.

The present inventors have previously realised in particular that if the bio-resins applied to the first and second layers are identical, then during the curing step in which the wet sandwich is heated and pressed, the bio-resin at the first face of the natural fibre mat will cure first, in view of the temperature gradient across the wet sandwich. However, the cured bio-resin can form a thermal barrier, thus reducing the flow of heat to the second face of the mat for curing of the bio-resin at that location. This results either in incomplete curing of the bio-resin at the second face or in an unacceptably long curing time for the panel. Without wishing to be bound by theory, the present inventors therefore consider that promoting curing of the bio-resin at the second face at a lower temperature and/or at a faster rate than the bio-resin at the first face can lead to significantly improved properties of the cured panel.

References in this disclosure to the second bio-resin cures faster and/or at a lower temperature than the first bio-resin relate to analysis of the respective bio-resins under identical conditions when subjected to heating up to their respective curing temperatures at the same rate of heating. It will be understood that the behaviour of the first and second bio-resins in situ in the composite depends heavily on the thermal gradient across the composite. However, in situ during the process of the present invention, it is preferred that the second bio-resin is cured before the first bio-resin is completely cured. Moisture management may also be important during the curing process. Curing of the bio-resin typically takes place via condensation reactions and therefore releases water vapour. Since the core of the structure may be formed of paper or card, release of large quantities of water vapour into the core can cause unacceptable reductions in strength of the core. Furthermore, it is considered that if the bio-resin at the first face of the natural fibre mat is allowed to cure before the bio-resin at the second face, then moisture generated during curing of the second face may become trapped in the core, leading to weakening of the core. Again, the inventors make these observations without wishing to be bound by theory.

During the curing step, preferably steam release means are provided. The curing step typically takes place in a heated moulding press. Known moulding presses are known for moulding polyurethane-containing products. Such moulding presses are typically operated in a carefully sealed condition, in view of the health and safety issues surrounding the curing of polyurethane. However, for the panels of the present invention, it is preferred instead that a steam vent is provided. The steam vent may take the form of an additional process step, in which the mould press is opened (at least partially) during the curing step in order to release steam that has been generated during the heating and curing of the panel. In that case, the mould press is typically then closed again to complete the curing of the panel. The steam vent may take place at 1 minute or less from the closure of the moulding press, more preferably at 45 seconds or less from the closure of the moulding press. Additionally or alternatively, the steam vent may take the form of a gap provided in the mould press throughout the curing step, the gap being placed so as to provide a suitable exit route for steam generated during the heating and curing of the panel. The steam vent can also be achieved by the use of holes (e.g.

drilled holes) in the tool face, providing a steam exit route out of the tool. It is possible to consider the cycle time of the manufacture of a panel according to the present invention. The cycle time is considered to be the time between corresponding steps in the manufacture of a first panel and a subsequent panel in the same moulding apparatus. Preferably, the cycle time here is 150 seconds or less. More preferably, the cycle time is 120 seconds or less, e.g. about 100 seconds. For large panels, however, the cycle times may be longer, e.g. up to 240 seconds.

Preferably, the viscosity of the first bio-resin at room temperature is lower than that of the second bio-resin under the same conditions. This allows the first bio-resin to penetrate more deeply into the natural fibre mat. The second bio-resin in contrast penetrates less deeply and so is available for providing a secure bond between the core and the skin layer. In order to provide this difference in viscosity, the first and second bio-resins, at the time of application to the natural fibre mat, may differ in water content. It is preferred that the first bio-resin has a higher water content than the second bio-resin. In view of this higher water content, the partially impregnated natural fibre mat may be subjected to a drying step after application of the first bio-resin and before the application of the second bio-resin. The water content of the second bio-resin is preferably 10wt% or less. The water content of the first bio-resin (after an optional drying step of the impregnated mat) is preferably 10wt% or less. However, the water content of the first bio-resin at the time of impregnation into the first face of the natural fibre mat may be higher than 0wt%, e.g. 15wt% or higher, or 20wt% or higher.

The first bio-resin preferably also includes a mould-release agent. This is preferred in order to assist in release of the cured panel from a corresponding mould. Preferably, the second bio-resin does not include a mould-release agent, since it is not needed at the interface between the paper core and the skin layer. Inclusion of a mould-release agent in the second bio-resin would likely have the effect of deleteriously reducing the strength of the panel.

The first bio-resin preferably also includes a filler component. The filler component can provide greater structural integrity to the free faces of the panel after curing. The filler is preferably a particulate filler. The average particle size of the filler may be less than 100pm, e.g. about 30pm. The first bio-resin may include up to about 30wt% filler, more preferably up to about 20wt% filler.

Preferably, different amounts by weight of bio-resin are applied to the first and second faces of the natural fibre mat. Typically, more bio-resin is applied to the first face than to the second face. For example, 1.5-3 times by weight the amount of bio-resin may be applied to the first face than to the second face. Typically, the bio-resin may be applied to the second face in an amount of 100-200gnr ² . Typically, the bio-resin may be applied to the first face in an amount of 200-500gm ^"2 . In some embodiments, it is preferred for the fibre mats to be fully impregnated with the first resin prior to the drying step.

The basis weight of the natural fibre mat is preferably in the range 400-800gm ^-2 .

In the manufacturing process, a pressure difference may be applied across the

impregnated natural fibre mats, so that there is a higher pressure in the core region compared with the A-face of the natural fibre mats. This may be achieved by applying a vacuum in the mould to pull the A-face of the natural fibre mats into contact with the mould. Additionally or alternatively pressurised gas (e.g. compressed air) may be injected into the region between the B-faces of the natural fibre mats. This pressure difference assists in forcing the natural fibre mats into contact with the mould. Where the mould is heated, this therefore assists in heating the bio-resin and the expandable component.

Where a separate core of the construction is provided (e.g. a paper or card honeycomb cellular core), then providing the expandable component at the B-face of the natural fibre mat has additional advantages. Typical cores are relatively fragile without the protection provided by the skins of the composite panel. Including the expandable component at the B-face can increase the contact area between the B-face and the core. This improves the adhesion between the B-face and the core and improves the stiffness of the composite panel. As mentioned above, in certain embodiments it is possible for the fibrous material component to be non-plant-derived, such as glass fibres or mineral fibres such as basalt. Additionally, it is possible for the resin component to be synthetic, such a polyurethane. Still further, it is possible for the cellular material layer (if present) to be a polymeric foam material. Further optional features of the invention are set out below. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Fig. 1 shows the manufacturing process used to produce a composite load bearing panel, as described in WO 2012/056202, described above. Fig. 2 shows the fully assembled panel of WO 2012/056202, described above.

Fig. 3a shows a cross section of a layered composite structure according to a first embodiment of the present invention.

Fig. 3b shows the manufacturing process used to produce a layered composite structure according to a first embodiment of the present invention.

Fig. 4a shows a cross section of a layered composite structure according to a second embodiment of the present invention.

Fig. 4b shows the manufacturing process used to produce a layered composite structure according to a second embodiment of the present invention. Fig. 5a shows a cross section of a layered composite structure according to a third embodiment of the present invention.

Fig. 5b shows the manufacturing process used to produce a layered composite structure according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS. AND FURTHER OPTIONAL FEATURES OF THE INVENTION

Fig. 3a and Fig. 3b show, respectively, a schematic cross sectional view of the final assembled version of a layered composite structure 20, and a representation of the manufacturing process, according to a first embodiment of the present invention.

Layered composite structure 20 has cellular layer (core) 22, skin sublayer 26, and wood veneer sublayer 30. The cellular core 22 has an upper surface 24a and a lower surface 24b. Similarly, skin sublayer 26 has an upper surface 26a and a lower surface 26b and top veneer sublayer 30 has an upper surface 30a and a lower surface 30b. The skin sublayer 26 consists of a fibrous material and a resin-based binder component (not shown). Together, the wood veneer sublayer 30 and the skin sublayer 26 make up the skin layer. When assembled, as in Fig. 3a, the lower surface 30b of the wood veneer sublayer 30 is in contact with and bonded to the upper surface 26a of the skin sublayer 26, and the lower surface 26b of the skin sublayer 26 is in contact with and bonded to the upper surface 24a of the cellular core 22.

In order to manufacture a layered composite structure 20 according to the first embodiment of the present invention, a stack is formed, of the wood veneer 30, a resin- impregnated fibre mat 26 and the cellular core 22. During manufacture, the resin component of the skin layer is wet, and there is wet resin exposed on the upper surface 26a and lower surface 26b of the skin sublayer 26. Once the stack is assembled, top pressing surface 34a and bottom pressing surface 34b of a forming tool move towards each other (indicated by the large solid arrows), pressing the components of the stack together, and simultaneously, the stack is heated. The heating cures the resin which bonds the components together. In the present embodiment, as is clear from Figs. 3a and 3b, the top wood veneer 30 covers the whole of the skin layer.

Fig. 4a and Fig. 4b show, respectively, a cross sectional view of the final assembled version of a layered composite structure 40 and a schematic view of the manufacturing process for it, according to a second embodiment of the present invention. This is similar to the first embodiment, except that here there is explicitly provided a second skin layer in order to sandwich the core.

Layered composite structure 40 is composed of five main layers. These are a bottom wood veneer sublayer 52, a bottom skin sublayer 48, a cellular core 44, a top skin sublayer 46, and a top wood veneer sublayer 50. The cellular core 44 has an upper surface 44a and a lower surface 44b. Similarly, the top skin sublayer 46 has an upper surface 46a and a lower surface 46b; the bottom skin sublayer 48 has an upper surface 48a and a lower surface 48b; the top wood veneer sublayer 50 has an upper surface 50a and a lower surface 50b and the bottom wood veneer sublayer 52 has an upper surface 52a and a lower surface 52b. The top and bottom skin layers each consist of a fibrous material and a resin-based binder component (not shown). Together, the top wood veneer sublayer 50 and the top skin sublayer 46 make up one skin layer, and the bottom wood veneer sublayer 52 and the bottom skin sublayer 48 make up another skin layer. When assembled, as in Fig. 4a, the lower surface 50b of the top wood veneer sublayer 50 is in contact with the upper surface 46a of the top skin sublayer 46, and the lower surface 46b of the top skin sublayer is in contact with the upper surface of the cellular core 44a. Also, the upper surface 52a of the bottom wood veneer sublayer 52 is in contact with the lower surface 48b of the bottom skin sublayer 48b and the upper surface 48a of the bottom skin sublayer 48 is in contact with the lower surface 44b of the cellular core 44.

In order to manufacture a layered composite structure according to the second embodiment of the present invention, a stack is formed, of the top wood veneer sublayer 50, a first a resin-impregnated fibre mat, the cellular core 44, a second resin-impregnated fibre mat and the bottom wood veneer sublayer 48. During manufacture, the resin component of the top and bottom skin layers 46, 48 is wet, and there is wet resin exposed on the upper surface 46a, 48a and lower surface 46b, 48b of the top and bottom skin layers 46, 48. Once the stack is assembled, top pressing surface 54a and bottom pressing surface 54b of the forming tool move towards each other (indicated by the large solid arrows), pressing the components of the stack together and simultaneously, the stack is heated. The heating cures the resin in the top skin layer and the bottom skin layer, which bonds the components together. In the present embodiment, as is clear from Figs. 4a and 4b, the top wood veneer sublayer 50 covers the whole of the top skin layer and the bottom wood veneer 52 covers the whole of the bottom skin layer.

Fig. 5a and Fig. 5b show, respectively, a cross sectional view of the final assembled version of a layered composite structure 60 and a schematic view of the manufacturing process for it, according to a third embodiment of the present invention. In this embodiment, unlike the two previously described, the top veneer sublayer 70 does not cover the whole of the top skin sublayer 66.

Layered composite structure 60 is composed of a cellular core 64, a top skin sublayer 66, and a top wood veneer sublayer 70. The cellular core 64 has an upper surface 64a and a lower surface 64b. Similarly, the top skin sublayer 66 has an upper surface 66a and a lower surface 66b and the top veneer sublayer 70 has an upper surface 70a and a lower surface 70b. The top skin sublayer 66 consists of a fibrous material and a resin-based binder component (not shown). Together, the top wood veneer sublayer 70 and the top skin sublayer 66 make up the skin layer. When assembled, as in Fig. 5a, the lower surface 70b of the top wood veneer sublayer 70 is in contact with the upper surface 66a of the top skin sublayer 66, and the lower surface 66b of the top skin sublayer 66 is in contact with the upper surface 64a of the cellular core 64.

In order to manufacture a layered composite structure 60 according to the third embodiment of the present invention, a stack is formed, of the top wood veneer sublayer 70, a natural fibre mat impregnated with resin and the cellular core 64. During

manufacture, the resin component is wet, and there is wet resin exposed on the upper surface and lower surface of the resin-impregnated natural fibre mat. Once the stack is assembled, top pressing surface 74a and bottom pressing surface 74b of the forming tool move towards each other (indicated by the large solid arrows), pressing the components of the stack together. In this embodiment, because the length or width of the top wood veneer 70 is smaller than the top skin layer 66, during manufacture, the veneer 70 is pressed into the natural fibre mat. As a result, the upper surfaces of the skin sublayer 66 and the wood veneer sublayer are flush with each other. The stack is heated, as with other embodiments, which cures the resin in the top skin layer 66, fixing its shape, and bonding the layers together - resulting in a layered composite structure as in Fig. 5a. The upper surface 70a of the top wood veneer sublayer 70 and part of the upper surface 66a of the top skin sublayer 66 are therefore flush in this embodiment, when it is removed from the pressing surfaces 74a, 74b.

In an embodiment of the invention, a layered composite structure having a cellular layer and skin layers bonded to opposing sides of the cellular layer is provided. Each skin layer has first and second sublayers, the first sublayer comprising a fibrous material component comprising a network of fibres and resin-based binder component, and the second sublayer is a veneer, wherein the veneer extends only part way across the full width and/or only part way across the full length of the first sublayer, and a surface of the first sublayer not covered with the veneer lies substantially flush with a surface of the veneer. In this embodiment, the veneers are formed on opposing upper and lower surfaces of the cellular layer. In this arrangement the composite structure has increased impact resistance on the upper surface provided by the veneer on the upper surface, compared with corresponding composite structures formed without a veneer layer.

Additionally on the opposing lower surface the composite structure also has increased bending stiffness provided by the opposing veneer. Furthermore, the presence of both opposing veneers when forming the composite structure provides increased resistance to warping of the composite structure.

In order to illustrate the basic mechanical properties of various natural fibres, Table 1 is provided.

Table 1

Fibre Density Tensile Specific tensile Tensile Specific teasile Failure

(gem ^'"3 ) modulus (GPa) modulus (GPa/gcm ^-3 ) strength (MPa) strength (MPa gcm ^--* ) strain (%■)

Bast

Flax 1 .45-1.55 28-100 19-65 343-1035 237-668 2.7-3.2

Hemp 1.45-1.55 32-60 22-39 310-900 214-581 1.3-2.1

Jute 1.35-1.45 25-55 19-38 393-773 291-533 1.4-3.1

Leaf

Sisal 1.40-1.45 9-28 6-19 347-700 248-483 2.0-2.9

Pineapple 1.44-1.56 6-42 4-27 170-727 118-466 0.8-1.6

Banana 1.30-1.35 8-32 6-24 503-790 387-585 3.0-10.0

Seed

Cotton 1.50-1.60 5-13 3-8 287-597 191-373 6.0-8.0

Coir 1.10-1.20 4-6 3-5 131-175 1 19-146 15.0-30.0

OiJ palm 0.70-1.55 3-4 2-4 248 160-354 25.0

Other

Bamboo 0.60- i .10 1 1-30 18-27 140-230 210-233 1.3

Wood pulp" 1.30-1.50 40 26-31 1000 667-769 4.4

E-glass 2.55 78.5 31 1 56 767 2.5

This shows that wood fibres have a similar high strength high modulus to other natural fibres such as flax and hemp.

Examples The technical advantages provided by the present invention are further illustrated in the following examples.

EXAMPLE 1

In the case of a 1000 mm x 333 mm x 20 mm panel, formed from two sheets of 1000 gsm bio-resin-impregnated natural fibre mats comprising 600 gsm Flax mat and 400 gsm resin, either side of a paper honeycomb core (Axxor paper core, 8mm cell size, 150 gsm paper) with a 1080 gsm core weight, integration of a single strip of birch veneer 800 mm x 150 mm x 0.5 mm (wood fibres aligned parallel to the surface of the veneer) into the underside gives a 40% reduction in deflection during a bending test after a 44 hour normal thermal cycle (11 hours at 40° C followed by a transition and 11 hours at 90° C) with a constant 75 kg load.

EXAMPLE 2

In a second, more basic experiment, three pieces (A, B, C) of FibriCard, 40 mm wide were subjected to 3 point flexure tests. The FibriCard consists of:

- Fibricard: 600 g/m ² Flax mats, from EcoTechnilin, with BioRez 120614 (including M6-silica filler at 250 g/m ² ) on one surface and BioRez 080101 (at 150 g/m ² ) on the other. - 10 mm thick paper honeycomb from Axxor.

- veneers: 0.7mm oak veneers at widths of 7mm and 20mm

Piece A had no veneer, Piece B had 7 mm wide veneer strips along its top and bottom, and Piece C had 20 mm wide veneer strips along its top and bottom. The results of the experiments are summarised in the tables below:

EXAMPLE 3

Three further tests were carried out on 1000 gsm FibriPreg comprising a 600 gsm flax mat and 4000 gsm resin, with an Axxor paper core, 8mm cell cize, 150 gsm paper. Core weight 1250 gsm.

Static deflection test on 23 mm thick board supported only on outside edges:

- Standard FibriCard board - 15 mm deflection - 21 kg load applied on a 50 mm disc. - Same with birch veneered both sides - 15 mm deflection - 53 kg load applied on a 50 mm disc.

Climatic deflection test with 75 kg constant loading: 4 cycles (each of 1.5 hours at -40° C then 4 hours at +90° C with 95% humidity, with 21 ° C conditioning steps in between)

- Board without veneer - Fail on front board (over 30 mm deflection)

- Board with veneer - Pass, maximum platen deflection at front 20 mm at rear 12 mm (maximum allowed 30 mm)

Local 25 kg loading test

- Tested without veneer - Fail on front board 14 mm deflection (max 10mm)

- Tested with veneer - Pass on front board 6.4 mm deflection (max 10mm).

EXAMPLE 4

3 point bend test on 1090mm x 333mm x 20mm panel using 25kg load applied via a 75mm disc. Test were conducted with and without 2 x 40g veneer birch panels, 0.5mm thick, 800m x 150mm.

In a further test, the panels in the first row of data withstood the impact of a 6kg block dropped from a height of 1 m without cracking when the veneer was added to both sides. The panels cracked when a sample without the veneers was tested in an identical manner. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.