BACKGROUND TO THE INVENTION

Field of the invention

The present invention relates to a process for the manufacture of a laminated composite, and to the resultant laminated composite. Related art

In WO 2012/056202, a process for producing paper honeycomb cored composite panels in a single heating and moulding process is described, using a natural fibre mat pre- impregnated with bio-resin. This process is shown schematically in Fig. 1 , with the resulting structure shown in Fig. 3.

A sheet of honeycomb material 1 , which has a series of hollow cells 1 b defined and separated by partition walls 1a, is placed between two skins 2a, 2b of natural fibre mat pre-impregnated with bio-resin. The partition walls are aligned with a principal axis direction of the cells. The honeycomb material becomes the core of the resultant laminated composite, having a relatively low density. The stack of the honeycomb core and the skins is then pressed and heated by pressing surfaces 3 and 4. Upper pressing surface 3 has a recessed portion 3a. Due to the shapes of the upper pressing surface 3 and lower pressing surface 4, the edges of the stack can be closed by pinching the skins 2a, 2b together to give a sealed edge. This forms, in one heating and moulding step, the composite 10 shown in Fig. 3.

Other known processes for the manufacture of laminated composites generally involve additional processes and tools. For example, additional tools may be used to pre form the top and bottom skins for the panel with the sandwich subsequently being formed in a third tool using a pre-cut paper honeycomb blank. These additional stages require additional tooling cost, cycle time and manpower.

There are several different types of paper honeycomb core, but for structural applications in load-bearing panels, such as those for interest for automotive applications, there are essentially two types: pre-laminated cores, and expandable cores. Pre-laminated cores have a corrugated -type sinusoidal wave layer of paper held in form by adjacent flat pieces of paper. Layers of these corrugated and flat papers are glued on top of each other to form a block. The block is then sliced in a direction perpendicular to the direction of the layers, to form panels of a desired thickness. The cells are defined by the flat and sinusoidal papers. An example is shown in Fig. 4. The advantage of such a pre- laminated core is rigidity, both parallel and perpendicular to the principal axis direction of the channels. The disadvantages of such a pre-laminated core include weight and cost (both per panel, and transport costs, in view of the volume of the panel). Expandable cores are generally cheaper to manufacture and to transport. They can be shipped in a compressed form, with little volume in the cells, and then expanded on site and as required into flat panels. An example of an expandable core is shown in Fig. 5, partially expanded. The equipment required to expand the cores is neither expensive nor complicated, as it simply expands the core by applying a lateral force. The paper is then moistened and dried, in order that it keeps its shape. The disadvantage of expandable cores, when compared to the pre-laminated cores, is that they have lower rigidity. This lower rigidity is particularly problematic when a force is applied laterally, i.e. in a direction perpendicular to the principle axis of the cells, which acts to collapse the honeycomb back into its unexpanded form.

SUMMARY OF THE INVENTION

When expandable cores are used in the process described above and illustrated in Figs. 1 and 2, the present inventors have found that the lack of lateral rigidity can cause areas where one or both of the skin layers are not sufficiently compressed and therefore the integrity and surface definition of the resultant panel can be compromised. This can be especially evident at the edges of the panel where the honeycomb core does not offer much resistant force during lateral compression. 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.

Accordingly, in a first preferred aspect, the present invention provides a process for manufacturing a laminated composite of a cellular material and at least one skin layer, the cellular material having a plurality of hollow cells defined and separated by partition walls aligned with a principal axis direction of the cells, the process including the steps:

(a) at least partially crushing the partition walls, by compression, in a peripheral region of the cellular material, in a direction substantially parallel to the principal axis direction; and then

(b) bringing the cellular material treated in step (a) into contact with the at least one skin layer; and then

(c) bonding the at least one skin layer and the cellular material to form the laminated composite.

In a second preferred aspect, the present invention provides a laminated composite produced by or obtainable by a process according to the first aspect.

The inventors have surprisingly found that subjecting the cellular material to crushing in this way provides the significant technical advantage of improving the mechanical properties of the cellular material in a direction perpendicular to the principal axis. This has relevance in terms of the performance of the cellular material during the process for manufacturing the laminated composite and can ensure good edge definition of the laminated composite.

The first and/or second aspect of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features.

It is intended that step (a), in which the partition walls are crushed, takes place before the cellular material is brought into contact with the skin layer in step (b). Thus, it is intended that step (a) takes place when the cellular material is not in contact with the skin layer.

In the process of the invention, the cellular material may have a thickness, immediately before crushing which is referred to here as the starting thickness. In order to crush the partition walls of the cellular material, the cellular material may be compressed by a compressive strain corresponding to 90% to 10% of the starting thickness of the cellular material. Preferably, the compressive strain corresponds to down to 20% of the starting thickness of the cellular material. Preferably the compressive strain corresponds to from 80% and below of the starting thickness of the cellular material. This definition of the compressive strain ignores possible elastic recovery of the partition walls. However, it is intended that during the crushing step the partition walls suffer permanent deformation, i.e. plastic deformation.

Preferably, the extent of the compression of the cellular material is substantially uniform across the whole peripheral region subjected to crushing. It is suitable, for example, for the cellular material to be at least partially crushed for at least 50% of the perimeter of the cellular material. Thus, for a panel of cellular material of a particular shape, at least 50% of the perimeter of that shape is subjected to crushing. In some embodiments, the peripheral region may extend fully around the perimeter of the cellular material. This allows the effect of the crushing to assist in maintaining a desired shape of the cellular material during the process.

Preferably, before the crushing step, the cellular material is subjected to a lateral expansion, to open the cells. It is intended that the lateral expansion is in a direction perpendicular to the principal axis. Thus, it is preferred that the cellular material is of the expandable type discussed above. This material tends to be available at relatively low cost, in view of low production costs and low transport costs. The crushing step helps to maintain the cellular material in the desired expanded form during the process of manufacture of the laminated composite. This allows the cellular material to maintain the desired form during the subsequent steps (b) and (c) of bringing the cellular material into contact with the skin layer and carrying out the bonding step.

Preferably, the peripheral region of the cellular material, in which the partition walls are crushed, has a width of at least one cell, measured in a direction perpendicular to the principal axis. Where the cells have different widths in different directions perpendicular to the principal axis, preferably the width used is the maximum width. More preferably the width is at least two cells, or at least three cells, or at least four cells, measured in a direction perpendicular to the principal axis. Preferably, the peripheral region of the cellular material, in which the partition walls are crushed, has a substantially uniform width, measured in a direction perpendicular to the principal axis towards the centre of the cellular material.

Preferably, there remains a region of the cellular material which is not subjected to crushing. Typically, this region is in a central part of the cellular material, where the stiffness and strength of the composite is best served by having non-crushed partition walls in the cellular material. It is also possible for there to remain a non-crushed region at or near the periphery of the cellular material, for example closer to the periphery than the crushed region. The crushing may be carried out by applying pressure to the cellular material with a tool having substantially the same shape as the peripheral region. In this way, it can easily be ensured that the same degree of compression is applied to the partition walls in the peripheral region. Furthermore, this allows the crushing step to be carried out quickly and accurately.

After the crushing step, preferably the resulting cellular material has a step-shaped height profile from the peripheral region to a non-crushed region. This may be formed only at one face of the cellular material. Alternatively, the step-shaped height profile may be formed at both faces of the cellular material.

Preferably, the cellular material, once expanded (if required), has a honeycomb structure.

Preferably, the cellular material is made of a cellulosic material such as paper or card. Thus, the partition walls are preferably formed from paper or card. Reference in this disclosure to "paper honeycomb" is intended to cover paper and/or card honeycomb.

Preferably, the at least one skin layer is a fibrous material component comprising a network of fibres and a resin-based binder component. For example there may be used a fibre mat such as a natural fibre mat. The resin-based binder component may be a bio- resin.

Preferably, the at least one skin layer includes a top skin layer and a bottom skin layer, together sandwiching the cellular material as a core layer. In the process of the invention it is preferred that the core and the at least one skin layer are bonded by pressing and heating. This has the effect of pressing the partition walls and the skin layer together and the effect of curing the resin-based binder component to densify the skin layer and bond with the partition walls. The pressing and heating step may provide a profiled shape to the composite. In particular, a profiled shape may be provided at the periphery of the composite. For example, the periphery of the composite may have a tapered shape. The taper may be substantially linear or curved. Preferably, where top and bottom skin layers are provided, these layers are brought together at the periphery of the composite, in order to provide a continuous outer surface for the composite. The present inventors have found that at this tapered part, the crushing of the partition walls in the corresponding region of the cellular material allows the cellular material to provide sufficient resistance to lateral displacement that the resulting composite has good edge definition and adequate bonding of the skins to the cellular material 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.

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 following discussion relates to natural fibre mats but can be adapted for other fibres as appropriate. In order to form a resin-impregnated natural fibre mat, 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 cellular material. In the method of manufacture of the laminated composite, 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 material, before crushing, 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 material.

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 skin layer.

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 composite 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 preferably 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,

97808 5514213, 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 comprising a laminated composite 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 10wt%, 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 ΙΟΌμΐΎΐ, e.g. about 30μιτι. 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. 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. 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 a schematic exploded cross sectional view of a known process for forming a laminated composite panel using a cellular core and resin-impregnated natural fibre mat skins, using a heated press.

Fig. 2 shows a schematic cross sectional view of a composite panel being formed in the process depicted in Fig. 1.

Fig. 3 shows a schematic cross sectional view of the product obtained by the process of Fig. 1.

Fig. 4 shows a perspective view of a cellular core formed by arranging corrugated and flat card.

Fig. 5 shows a perspective view of an expandable cellular core, for use in preferred embodiments of the invention.

Fig. 6 shows a schematic view of an expandable cellular core, used to illustrate a technical concept behind the present invention.

Fig, 7 shows a process of partially crushing a peripheral region of an expandable core, according to an embodiment of the invention.

Fig. 8 shows a schematic cross sectional view of a partially crushed core for use in an embodiment of the invention.

Fig. 9 shows a schematic cross sectional view of a composite panel formed in accordance with an embodiment of the invention.

Fig, 10 shows a schematic cross section view of the panel of Fig. 9 with a carpet fitted. Fig, 11 shows an illustration of a test protocol for measuring the resistance of the core to lateral compression.

Figs, 12, 13 and 14 show schematic perspective magnified views of crushed peripheral regions of core, with the adjacent non-crushed regions.

Fig. 15 shows a schematic plan view of an entire core, for use in a composite panel of corresponding shape, with a peripheral crushed region. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER OPTIONAL FEATURES OF THE INVENTION In the field of automotive technology, for example, it can be extremely advantageous if parts and assemblies can be formed using relatively few processes and stages. The present inventors have previously disclosed the use of a natural fibre (flax) mat pre- impregnated with bio-resin, for use in manufacturing laminated composite panels. The inventors have pioneered the forming and trimming of composite panels which use paper-honeycomb-cores, in a single process. Two sheets of the bio-resin impregnated natural fibre mat are positioned either side of a sheet of expanded paper honeycomb and then placed into a press with heated male and female tools, as shown in Fig, 1 , described earlier. To add overall rigidity and to prevent the core from excess moisture, the tools can close the edges of the composite sandwich by pinching the skins together to give a sealed edge, as shown in Fig, 3, described earlier. This forms and trims a paper-honeycomb- cored composite in a one-shot process using a single pair of tools. Known composite panel forming processes generally involve additional processes and tools. For example, additional tools may be used to pre-form the top and bottom skins of the panel, with the sandwich subsequently being formed in a third tool using a pre-cut paper honeycomb blank. Such additional stages require additional tooling cost, cycle time and manpower.

There are different types of paper honeycomb core, but for structural applications in load bearing panels, there are essentially two types: pre-laminated cores (such as those offered by Swap (www.swap-sachsen.de/en/products/#honeycomb-board-unlaminate d) and expandable cores (such as those offered by Axxor

www.axxor.eu/paginas/whypaperhonevcomb).

Fig. 4 shows a perspective view of a cellular core formed by arranging corrugated and flat card, i.e. of pre-laminated type. Fig. 5 shows a perspective view of an expandable cellular core, for use in preferred embodiments of the invention. The pre-laminated cores have a corrugated layer of paper held in form by one or two flat pieces of paper. Multiple layers of these corrugated-flat combination layers are glued on top of each other to form a block. The whole assembly is then rotated through 90° and sliced into panels of the desired thickness.

The advantage of the pre-laminated core is rigidity in the direction perpendicular to the cut surface (parallel to the principal axes of the cells) and in directions parallel to the cut surface (perpendicular to the principal axes of the cells). The disadvantage is weight and cost (per panel and transport).

Expandable cores, such as shown in Fig. 5, are generally cheaper to buy and to transport, being shipped in a compressed form (with little or no volume within the cells) and then expanded by customers onsite into flat panels. Typically, they also have a lower density when expanded than the pre-laminated cores. The equipment needed to expand the cores is not expensive or complex. The core is simply expanded by applying a lateral force. The core is moistened and then dried, thus retaining the shape.

Fig. 6 shows a schematic view of a cellular core 20 and having upper 22 and lower 24 faces.

The disadvantage of expandable cores when compared to the pre-laminated, corrugated cores, is reduced rigidity, particularly in response to a force applied perpendicular to the principal axes of the cell - a force which is essentially trying to collapse the honeycomb into a non-expanded form. We refer to the direction of such a force as "lateral" in this disclosure. This corresponds to the direction of the narrow horizontal arrows L in Fig. 6. The direction parallel to the principal axes of the cells is referred to here as

"perpendicular" in this disclosure. This corresponds to the direction of the large vertical arrows P in Fig. 6. The present inventors have found that when the cheaper, lighter expandable cores are used in a one-shot process as described above, this lack of lateral rigidity can cause areas where the skin layers are not sufficiently compressed, and therefore the integrity and surface definition of the resultant panel can be compromised. This can be evident in particular at the edges of the panel where the honeycomb core does not offer a suitable reaction force to allow compression of the skin between the mould and the core, as shown by the location and direction of the arrows D in Fig. 3. Fig. 3 shows the panel being formed in the mold shown open in Fig. 1. In regions of the panel where the core can provide a suitable reaction force to the pressure of the mould, the skin layers are compressed and suitably cured. However, in the location and direction of arrows D in Fig. 3, the core has inadequate strength, and so the skin is not suitably compressed, may not adequately cure as quickly as for the remainder of the panel, and may have unsuitable edge definition for commercial purposes.

The present inventors, on finding and investigating this problem, found that, surprisingly, the lateral rigidity of the expandable paper honeycomb core can be greatly enhanced by pre-compressing the core where required. This can be done using a simple tool that compresses the core around the peripheral region of the core in a direction parallel to the principal axis of the cells of the core.

A process according to an embodiment of the present invention is illustrated in Fig. 7, this being a modification of the view shown in Fig. 6, and so similar features are not described again. A peripheral region 30 of the core is subjected to partial crushing, by compressive force C, applied selectively to peripheral region 30, at one face only of the core. The result of this is that the core is crushed only in this peripheral region, leading to there being a step 32 between the crushed region and the remainder of the face 22 of the core. This is shown in Fig. 8.

A suitable form for the final composite panel is shown in Fig. 9, having the profiled edge region similar to that in Fig, 2 and the step formed by the partial crushing of the peripheral region.

As shown in Fig. 10, the step can be used to define a recess for mounting a carpet 34. Such an approach allows the carpet to be wrapped around the edge of the composite panel for cosmetic reasons, particularly when used for load bearing panels for

automotive applications. Such an approach allows there to be no ridge or step where the carpet ends. This is cosmetically desirable and reduces the tendency of the carpet to unpeel during use.

The partial crushing of the expanded core at the peripheral region is found to increase the resistance of the core to lateral deformation during the moulding of the composite panel. This assists in forming the required edge definition of the panel and provides superior appearance and mechanical performance at the peripheral region of the panel. To quantify the increase in resistance of the core to lateral deformation, a 1 metre length of expandable Axxor-type paper honeycomb core with 20mm thickness was provided. A peripheral region A of the upper face was selected and was partially crushed by 3mm, as illustrated in Fig. 11 , the remainder of the core being region B. Thus, the 20mm thickness was reduced to 7mm in this peripheral region A.

The response of the core to compression force in directions L to the edge of the core. The force necessary to compress the core per unit length in directions L was measured. The un-compressed region B showed a compression rate of - 6.9 Newtons / mm. The compressed region A showed a compression rate of - 23.4 Newtons / mm. This demonstrates the substantial advantage available by a relatively small partial crushing of the core. Figs. 12, 13 and 14 show schematic perspective magnified views of crushed peripheral regions of core, with the adjacent non-crushed regions. Fig. 15 shows a plan view of an entire core, for use in a composite panel of corresponding shape, with a peripheral crushed region. As in Fig. 15, it is advantageous for the peripheral region to extend around most of the periphery of the panel, but not necessarily all, whilst still achieving a substantial improvement to resistance to lateral deformation during the forming process.

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.