WO2018019789A1

WO2018019789A1 - Laminated composite material and method for manufacturing laminated composite material

Info

Publication number: WO2018019789A1
Application number: PCT/EP2017/068685
Authority: WO
Inventors: Timothy Sweatman; Karim Behlouli
Original assignee: Eco-Technilin Sas
Current assignee: Eco-Technilin Sas
Priority date: 2016-07-25
Filing date: 2017-07-24
Publication date: 2018-02-01
Anticipated expiration: 2019-01-25
Also published as: GB201612854D0

Abstract

A laminated composite material has a core, skin layers and adhesive mat layers provided between the core and the respective skin layers at each side of the core. Each adhesive mat layer comprises a network of natural fibres at least partially impregnated with a resin- based binder component, the resin-based binder component bonding to the core and to the skin layers. The skin layers comprise paper. The skin layer has sub-layers, at least one of these sub-layers being a paper sub-layer. The skin layer includes at least one moisture-resistant sub-layer. The skin layer has a moisture resistance which is expressed in terms of Water Vapour Transmission Rate (WVTR) which is not more than 24 g/m2 x d at 23°C, 85% r.h. (grams of water per m2 permeating over a 24 hour period).

Description

LAMINATED COMPOSITE MATERIAL AND METHOD FOR MANUFACTURING LAMINATED COMPOSITE MATERIAL

BACKGROUND TO THE INVENTION

Field of the invention

The present invention relates to a laminated composite material and methods for manufacturing a laminated composite material. Such materials are of interest for load bearing applications.

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. Composite materials are known having cellular core layers with skin layers bonded at opposing faces, for stiffness. Cellular core layers formed of aluminium foil are known. Skin layers may include inorganic fibre reinforced plastics, such as glass fibre reinforced plastics or carbon fibre reinforced plastics. The skin layers are typically bonded to the cellular core layers using a separate adhesive layer. Such composite materials are known, for example, for aerospace applications. Suitable known adhesive layers for use in these composite materials include epoxy or BMI bis-maleimide (produced by the condensation reaction of a diamine with maleic anhydride) adhesives. Suitable adhesives are known, for example, from Hexcel with their Redux products

(http://www.hexcel.com/Resources/SelectorGuides/Redux SelectorGuide.pdf). Some suitable adhesives have an expanding, foaming action to link with the walls of a honeycomb core of the composite. Alternative adhesive layers are known from Gurit (http://www.qurit.com/adhesive-films.asDx).

Recent work by the present inventors is disclosed in WO 2016/06301 1 , in which a laminated composite material has a low density core and skin layers. The skin layers are formed by needlepunching natural fibres to form a mat. The skin layers are bonded to the low density core using a resin such as furan resin.

SUMMARY OF THE INVENTION

The present inventors have realised that still further developments of materials based primarily on sustainably-sourced natural materials can provide unexpectedly good performance for laminated composite materials. This is of importance for developing environmentally friendly composite materials, which are needed in order to replace known materials which can be environmentally damaging, and preferably improve upon the load bearing performance of known materials in various condition.

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 laminated composite material having:

a core;

skin layers provided at the outer facing surfaces of the laminated composite material;

adhesive mat layers provided between the core and the respective skin layers at each side of the core, wherein each adhesive mat layer comprises a network of natural fibres at least partially impregnated with a resin-based binder component, the resin-based binder component bonding to the core and to the skin layers, wherein the skin layers comprise paper.

In a second preferred aspect, the present invention provides a method for manufacturing a laminated composite material, including the steps:

providing a core;

providing skin layers comprising paper;

providing adhesive mat layers, wherein each adhesive mat layer comprises a network of natural fibres at least partially impregnated with a resin-based binder component;

arranging the core, the skin layers and the adhesive mat layers into a stack so that the adhesive mat layers are interposed respectively between the core and the skin layers at opposite sides of the core; and

pressing and heating the stack in order to bond the skin layers to the core via the resin-based binder component. 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.

Preferably, each adhesive mat layer has a greater thickness than each skin layer.

Similarly, preferably the core has a greater thickness than the adhesive mat layer. In preferred embodiments, the core has a thickness which is greater than the combined thickness of the adhesive mat layers and the skin layers. The core may for example have a thickness in the range 3-60mm, more preferably 3-30mm.

Preferably, therefore, the adhesive mat layer acts in part as an adhesive sheet, to bond the skin layer to the core layer. The present inventors have found that this is a surprisingly convenient way to bond paper-based skin layers with the core. 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 (PFA) polymer and an acid catalyst is used. Preferably, the acid catalyst is paratoluene sulphonic acid.

It is preferable that the resin-based binder component acts to bond the skin layer and the adhesive mat layer. However, it is possible (and preferred) for the resin-based binder component to be provided only in the adhesive mat layer. The adhesive mat layer can be pre-impregnated with the resin-based binder in a convenient manner, in the components which are subsequently brought together to form the composite. This also allows the adhesive mat layer to be bonded to the core using the resin-based binder component originating in the adhesive mat layer.

The natural fibres of the adhesive mat layer preferably comprise 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.

Natural fibres have in their elementary form high specific strength and stiffness like mineral fibres. The natural fibres 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 or wood pulp. Preferably flax or hemp are used as the natural fibre.

The core may be cellular. The core may have an open cell format. For example, the core preferably has a honeycomb structure. More preferably, the core has a honeycomb structure comprising a plurality of substantially parallel hollow channels. The core may be 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 core can provide a suitably low density and is advantageous in terms of its use of natural materials. Alternatively, the core may comprise a foam. Suitable foams can be open cell or closed cell. A suitable closed-cell foam is expanded polypropylene. The resin component is preferably capable of forming a bonding bridge between the core and the skin layers.

The fibre mat, to be used in the adhesive mat layer is at least partially impregnated with resin. This impregnation is preferably done before the arrangement of the adhesive mat layer, the core and the skin layers into a stack suitable for heating and pressing to form the laminated composite.

In the method of the second aspect, the heating and pressing may be carried out by opposed pressing surfaces of a forming tool. The forming tool may have different shapes depending on the overall shape of the composite required. Furthermore one or both of the opposed pressing surfaces may move during the pressing and heating step.

Optionally, a first resin is provided at a first face (A face) of the natural fibre mat and a second resin is provided at a 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 for contact with the core (low density layer). In the method of manufacture of the present invention, 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 composite itself (including the pressing and curing step) can be part of the continuous process. The resulting sheet of composite material 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 pre- preg, the pre-preg being cut to length as desired, applied to the core and then pressed and cured in a static press. Alternatively, in a preferred approach, the adhesive mat layer is impregnated with a single resin. This is preferably done by immersion of the natural fibre mat into the resin. Subsequently, excess resin may be removed from the mat. This may be done, for example, by squeezing. Squeezing between rollers is suitable. The mat may

subsequently be subjected to a drying process, for example in an oven. This is useful in order to reduce the water content of the impregnated mat.

The thickness of the core is preferably at least 3mm or at least 5mm. The thickness of each skin layer is preferably at most 200 μηη.

The thickness of the adhesive mat layer (in the final product) is preferably at least 5 times smaller than the thickness of the core.

For the adhesive mat 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 and/or third core 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 100- 3000 grams per square metre (gsm). However, it has been found that the present invention is particularly well suited to the manufacture of low area density laminated composite materials. With this in mind, preferably the area density of the natural fibre mat is not more than 600 gsm, preferably not more than 500 gsm. For example, an area density of about 300 gsm or 450 gsm may be suitable.

The impregnation of the natural fibre mat with resin may be with at least 100 gsm resin, in order to form the adhesive mat layer. More preferably, the adhesive mat layer may include at least 200 gsm resin. The adhesive mat layer may include up to about 500 gsm resin, for example.

Preferably, the area density of the core is in the range 200 to 500 gsm. This is typical, for example, for a 20mm thick core. Preferably, the combined area density of the adhesive mat layer and skin layer on one side of the core in the finished laminated composite is not more than 900 gsm.

Considering the mechanical performance of the composite material, this is a remarkably low area density, contributing to the attractiveness of the material for various applications The combined area density of the adhesive mat layer and skin layer may, for example, be 400 gsm or higher, for practical purposes. It has been found that the performance of finished laminated composites according to embodiments of the invention with combined area density of the adhesive mat layer and skin layer of 700-800 gsm is particularly suitable.

Preferably, the area density of the skin layer is at most 300 gsm. More preferably the area density of the skin layer is at most 250 gsm. The area density of the skin layer may for example be at least 50 gsm, more preferably at least 100 gsm.

As set out above, the skin layer comprises paper. Preferably, the skin layer has sublayers. Preferably, at least one of these sub-layers is a paper sub-layer. The paper sub- layer may be kraft paper. Kraft paper is known for its strength. It is formed using the kraft process in which most of the lignin from the original wood is removed.

Preferably, the skin layer includes at least one moisture-resistant sub-layer. The moisture-resistant sub-layer may be provided at an external face of the laminated composite. Alternatively, it may be provided sandwiched between other sub-layers of the skin layer. The moisture-resistant sub-layer may be formed of a polymeric material. For example, a polyethylene terephthalate (PET) layer may be used. Additionally or alternatively a polyester (PE) layer may be used. This is of particular interest for the outermost-facing sub-layer of the skin layer, because the use of such a sub-layer helps to withstand the temperature during moulding of the laminated composite.

The skin layer has a moisture resistance which is expressed in terms of Water Vapour Transmission Rate (WVTR). This is measurable using test standard DIN 53122. WVTR values may be measured in g/m² per day a given relative humidity and temperature. A gravimetric method or an electrolysis method can be used. It is recognized in the art that a complete moisture barrier uses a standard of an aluminium foil of 8 μηη thickness. Compared with this standard, the skin layer used in the preferred embodiments of the present invention allow some moisture to pass through. Preferably, the area density of the moisture-resistant sub-layer is at least 25 gsm, more preferably at least 35 gsm. The area density of the moisture-resistant sub-layer may be up to 60 gsm, more preferably up to 50 gsm.

Preferably, the WVTR for the skin layer is not more than 24 g/m² x d at 23°C, 85% r.h. (grams of water per m² permeating over a 24 hour period). More preferably, the WVTR for the skin layer is not more than 20 g/m² x d at 23°C, 85% r.h. Still more preferably, the WVTR for the skin layer is not more than 16 g/m² x d at 23°C, 85% r.h.

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 for the panel also includes pressing. By the combination of heating and pressing, at least part of the adhesive mat layer is typically permanently compressed so that the density of the adhesive mat layer after heating and pressing is at least three times (preferably at least four times or at least five times) the density of the adhesive mat layer before heating and pressing.

Where the resin is a bio-resin, 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 by-products. It is typically synthesized by the acid hydrolysis and steam distillation of agricultural by-products 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. 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.

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 materials 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 material. In that case, the mould press is typically then closed again to complete the curing of the material. 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 material. 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 composite material according to the present invention. The cycle time is considered to be the time between corresponding steps in the manufacture of a first composite material and a subsequent composite material 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.

In previously-known approaches, it has been found to be necessary to include a mould release agent in the process. A mould release agent would typically be applied to the faces of the mould tool that make contact with the outer surfaces of the stack to be pressed to form the laminated composite. This was found to be necessary in order to avoid the laminated composite sticking to the mould, and causing difficulties with extraction of the laminated composite from the mould. Furthermore, the failure to use a mould release agent would typically require the mould to be cleaned between heating and pressing cycles.

In the preferred embodiments of the present invention, it is found that the use of a paper- based skin layer means that the method can be carried out without a mould release agent. This is because the paper has good mould release properties, such that sticking of the laminated composite to the mould is not a problem and cleaning of the mould between heating and pressing cycles is not necessary. Thus, preferably, in the method, there includes the manufacture of a first laminated composite panel according to the second aspect followed immediately by the manufacture of a second laminated composite panel according to the second aspect without any intervening step of applying a mould release agent to the mould or to the stack. Similarly, preferably, in the method, there includes the manufacture of a first laminated composite panel according to the second aspect followed immediately by the manufacture of a second laminated composite panel according to the second aspect without any intervening step of cleaning the mould or to the stack.

In a further development, the present inventors have realised that in some embodiments, it may not be necessary to include a separate core. Accordingly, in a third preferred aspect, the present invention provides a laminated composite material having:

an adhesive mat layer provided between the respective skin layers, wherein the adhesive mat layer comprises a network of natural fibres at least partially impregnated with a resin-based binder component, the resin-based binder component bonding to the skin layers,

wherein the skin layers comprise paper.

In a fourth preferred aspect, the present invention provides a method for manufacturing a laminated composite material, including the steps:

providing skin layers comprising paper;

providing an adhesive mat layer, wherein the adhesive mat layer comprises a network of natural fibres at least partially impregnated with a resin-based binder component;

arranging the skin layers and the adhesive mat layer into a stack so that the adhesive mat layers are interposed between the skin layers; and

pressing and heating the stack in order to bond the skin layers to the adhesive mat layer via the resin-based binder component.

The optional features set out with respect to the first and second aspects of the invention also apply to the third and fourth aspects of the invention, unless the context demands otherwise.

The resultant composite material has surprisingly good mechanical properties, and is considered to be of particular use in furniture applications, for example as a panel in a flat packed furniture kit. This composite material preferably has a thickness of less than 10mm, for example not more than 6mm, or about 4mm. The adhesive mat layer preferably has an area density of 1000-1500 gsm, for example about 1200 gsm.

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. 3 shows a schematic cross section of a laminated composite material according to an embodiment of the present invention. Fig. 4 shows the manufacturing process used to produce the laminated composite material of Fig. 3.

Fig. 5 shows a cross section of a laminated composite material according to another embodiment of the present invention.

Fig. 6 shows the manufacturing process used to produce a laminated composite material according to another embodiment of the present invention.

Fig. 7 shows a schematic cross section of a laminated composite according to an embodiment of the present invention and an enlarged view showing the compression of the natural fibre layer where the honeycomb core walls meet the natural fibre layer. Fig. 8 shows a graph of the temperature variation applied to composite panels during testing. The conditions of humidity were 95%.

Fig. 9 shows an arrangement for mechanical testing of a composite laminated panel. Figs. 10-12 show load-deflection graphs for various composite laminated panels.

Fig. 13 shows the deflection of various composite laminated panels at ambient temperature or at 80°C with 50kg load.

Fig. 14 shows the deflection of various composite laminated panels at ambient temperature or at 80°C with 100kg load.

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

The embodiments of the invention described below use skin layers comprising paper. In preferred embodiments, the skin layers are moisture resistant, but this is not necessarily essential in all embodiments.

Fig. 3 and Fig. 4 show, respectively, a cross sectional view of the finished laminated composite material 40 after manufacture, and a schematic view of the manufacturing process for it, according to an embodiment of the present invention. In Fig. 3, laminated composite material 40 is composed of five main layers. These are bottom skin layer 52, bottom adhesive mat layer 48, cellular core 44, top adhesive mat layer 46, and top skin layer 50. Note that these layers are not shown to scale.

The cellular core 44 has upper surface 44a and lower surface 44b. Similarly, the top adhesive mat layer 46 has upper surface 46a and lower surface 46b. The bottom adhesive mat layer 48 has upper surface 48a and lower surface 48b. The top skin layer 50 has upper surface 50a and lower surface 50b. The bottom skin layer 52 has upper surface 52a and lower surface 52b.

The top and bottom adhesive mat layer 46, 48 each consist of a fibrous material and a resin-based binder component (not shown). The top and bottom skin layers 50, 52 consist of water-resistant paper.

When assembled, as in Fig. 3, the lower surface 50b of the top skin layer 50 is in contact with the upper surface 46a of the top adhesive mat layer 46, and the lower surface 46b of the top adhesive mat layer 46 is in contact with the upper surface of the cellular core 44a. Also, the upper surface 52a of the bottom skin layer 52 is in contact with the lower surface 48b of the bottom adhesive mat layer 48b and the upper surface 48a of the bottom adhesive mat layer 48 is in contact with the lower surface 44b of the cellular core 44.

In order to manufacture a laminated composite material according to the preferred embodiment of the present invention, a stack is formed, of the top skin layer 50, a first resin-impregnated fibre mat 46, the cellular core 44, a second resin-impregnated fibre mat 48 and the bottom skin layer 52.

During manufacture, the resin component of the top and bottom adhesive mat 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 adhesive mat 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 adhesive mat layer and the bottom adhesive mat layer, which bonds the components together. Known composite materials using a foaming-type adhesive provide useful but limited adhesion to the honeycomb walls of the low density cellular core. In the preferred embodiments of the present invention, by using a relatively lofty (i.e. low bulk density) natural fibre (preferably non-woven) mat, it is possible not only to lock in the honeycomb cells but also to add to the reinforcement of the composite panel using the strength of the natural fibres. In embodiments where the natural fibre mat is non-woven, the natural fibres tend to have free ends sticking out from the mat surface. These are pushed inside the cells of the low density core during moulding, assisting in the provision of this locking effect.

In effect, the preferred embodiments of the present invention use nature fibre mats impregnated with resin as an adhesive system to bond skin layers to a low density cellular core. This avoids the use of a separate adhesive film typically used in the construction of low density composite panels.

Figs. 5 and 6 show, respectively, a cross sectional view of the final assembled version of a laminated composite material 60 and a schematic view of the manufacturing process for it, according to another embodiment of the present invention. In this embodiment, unlike the embodiment previously described, there is no cellular core layer.

Laminated composite material 60 is composed of a natural fibre and resin adhesive mat layer 64, a top skin layer 66 formed of paper and a bottom skin layer 68 also formed of paper.

The adhesive mat layer 64 has an upper surface 64a and a lower surface 64b. Similarly, the top skin layer 66 has an upper surface 66a and a lower surface 66b. When assembled, as in Fig. 5, the lower surface 66b of the top skin layer 66 is in contact with the upper surface 64a of the adhesive mat layer 64. The lower surface 64b of the adhesive mat layer 64 is in contact with the upper surface 68a of the bottom skin layer 68.

In order to manufacture a layered composite structure 60 according to this embodiment of the present invention, a stack is formed, of the top skin layer 66, an adhesive mat layer 64 impregnated with resin and the bottom skin layer 68. 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, the top pressing surface and bottom pressing surface of the forming tool move towards each other (indicated by the large solid arrows), pressing the components of the stack together. The stack is heated, as with other embodiments, which cures the resin in the adhesive mat layer 64, fixing its shape, and bonding the layers together - resulting in a laminated composite material as in Fig. 5.

Fig. 7 shows a schematic cross section of a laminated composite according to an embodiment of the present invention. Also shown is an enlarged view. This shows the skin layer 104 laminated with the natural fibre and resin layer 102. During moulding, the structure is compressed. The honeycomb core walls 106, do not permanently deform. Instead, the walls 106 are pressed into the natural fibre mat. The result of this is as shown in Fig. 7, in which the honeycomb walls 106 are bonded securely to the resin- impregnated natural fibre mats due to the increase in bonding length provided by the pinching of the natural fibre mat between the honeycomb wall and the skin layer.

Typically, laminated composites using liquid resin systems take around 30 minutes to cure. In contrast, the preferred embodiments of the present invention use a resin which can be cured in about 40 s by itself and when in a sandwich construction in than 3 minutes. Known liquid resins are phenolic systems which are known for use in multilayer laminates. These for example may be applied at around 40°C then have to be held at 60°C during the gel phase. They are then de-moulded and finally post cured. This is complex and time-consuming process, especially when compared with the straight- forward curing processes used in the preferred embodiments of the present invention.

The use of low cost materials if of interest to form useful laminated composite materials as efficiently as possible. The environmental sustainability of the materials used in the preferred embodiments is of great interest. The bio-resin and the natural fibres are clearly understood as having low environmental impact. Similarly, paper has low environmental impact, often coming from recycled sources. In order to form the resin-impregnated natural fibre mat, a non-woven natural fibre mat is fully immersed in aqueous furan-based bioresin and is then passed through rollers to remove excess liquid before optionally being passed through an oven to remove excess water. This drying stage is necessary in particular if the cellular core material is to be a paper or card core. This drying step can be omitted if the cellular core is foam, plastic or aramid.

The result of this process is a mat pre-impregnated with bioresin in which every fibre is covered with resin. Optionally the impregnated mat may be passed through a roller-coater so that additional concentrated bioresin may be applied to one or both sides of the mat, as explained above in relation to A and B faces of the impregnated mat. The mat is then either cut into panels or supplied in rolls.

Optionally the impregnated mat may be cured ore part-cured in an oven in uncompressed state before the additional concentrated bioresin is applied in the roller- coater. This serves to rigidify the core in a less dense form before the mineral skins are applied, giving improved structural and acoustic performance for some applications. In this case the mat is supplied in panels.

In order to form the composite material, a five layer structure can be formed in which two layers of resin-impregnated natural fibre mats and two paper-based layers are disposed either side of a closed cell or honeycomb core. The two layers of impregnated natural fibre mat are placed between the cellular core and the paper-based skin layers. This laminate is placed into a heated press. On heating and compression, the resin on the inside forms the bond to the core and the resin on the outside forms the bond to the paper-based sink layers.

The cycle time of this one-shot process is typically between 40 seconds and three minutes, considerably shorter than other resin systems and composites which require more layers and several curing phases. EXAMPLES

Examples of the invention used adhesive mat layers formed from full-bath-impregnated needle-punched flax mats. The skin layers were waterproof Kraft paper outer layers (sourced from Mondi http://www.mondigroup.com/products/desktopdefault.aspx/tabid- 1941/).

The skin layers provide a water resistant barrier which ensure that the laminated composite material retains high mechanical performance even under high-humidity conditions. Additionally, and unexpectedly, the skin layers provide excellent mechanical performance. Non woven mats with an area density of 300 gsm or 450 gsm were used. These were formed from flax or a combination of other natural fibres and cotton shoddy (recycled cotton textiles). For the skin layers, a variety of different Kraft papers with different weights were used:

(a) regular (non-water-resistant) paper from 100 to 250 gsm

(b) a paper which comprised a polyethylene layer sandwiched between 2 regular paper sheets from 125 to 250 gsm

(c) the same 250 gsm paper as (b) with an additional layer of polyester on the outside (to add strength and to help withstand the 180 degree temperature during moulding).

The evaluation process involved forming sealed 600 x 300 mm boards (or 330 x 740 mm boards) and then putting them on a test rig to test deflection. The boards were "sealed" in the sense that the top and bottom skin layers were adhered directly to each other around the periphery of the board.

A typical board manufactured in this way was a perfectly sealed board formed from a 450 gsm flax mat (impregnated with 300 gsm resin) and 250 gsm paper/polyethylene/paper skin. The core was formed from cardboard honeycomb of thickness about 24mm. This board could withstand a 100 kg load applied through a 150mm diameter disc and only gave a 16mm deflection. This significantly outperforms a typical requirement for automotive load floor component to have a deflection under such test conditions of less than 20mm. The effect of climatic changes was assessed by subjecting the panel to a series of variations of temperature as shown in Fig. 8, at 95% humidity. For each panel, 5 cycles (1 day per cycle) were carried out with a load of 75kg placed on top of the panel, spread across the panel. To pass the test, the panel has to resist 5 climatic cycles with the load on top with a deflection of less than 15mm. During the test, the panel is supported by 15mm on both edges across the width.

The arrangement of the composite panel during mechanical testing is shown in Fig. 9A. Here, the panel has width 740mm, width 330mm, and the panel is supported only at the outer 15mm at each side and the panel has thickness 25mm. The load is supported at the centre of the panel or distributed across the panel, as described. For load floor testing, the arrangement of the composite panel is shown in Fig. 9B. Here, the panel has width 740mm, width 330mm, and the panel is supported only at the outer 30mm at each side for a depth of 64mm. The panel has thickness 25mm. The load is supported at the centre of the side of the panel away from the supports.

For comparative purposes, a composite panel was prepared without any paper skin layers. Effectively, therefore, the adhesive mat layer provides the outer skin of the composite panel. The performance of this panel is reported below as Comparative Example 1 .

Comparative Example 1 - cardboard honeycomb and adhesive mat layers only

Table 1A

Example 2 - cardboard honeycomb, adhesive mat layers, water resistant paper skin layers

Table 1 B

Weight (g) 667

Dimensions

740 x 330 x 25

(mm)

100Kg Bend Max. Deflection

Test 12.7 (mm) Load Removed

0.7

Pass + 50g weight

Climatic Test

increase

The effect of area density of the adhesive mat layer and the effect of the paper skin layers was assessed. The adhesive mat layers used are reported in Table 2.

Table 2

Fibres in % Volatile of Area weight

Identifier Mat weight

adhesive mat mat (g/m²)

1.1 Flax 300 GSM 10.2% 475

1.2 Flax 300 GSM 8.7% 479

1.3 Flax 300 GSM 9.4% 486

2.1 Flax 450 GSM 9.7% 763

2.2 Flax 450 GSM 1 1 .3% 803

60%Acryl/ 40%

3.1 325 GSM 1 1 .8% 632

Jute

60% Acryl / 40%

3.2 325 GSM 1 1 .5% 661

Jute

4.2 Flax 600 GSM 10.2% 1202

The paper skin layers are reported in Table 3. Table 3 - Paper characteristics:

Paper name Paper construction

150 Mondi 150 GSM Kraft paper and PE / PET film outside

100 GSM Kraft paper + 15 GSM PE film+ 40 GSM Kraft

155 Mondi paper

275 Kraft 275 GSM Kraft paper

In all samples, the core was a cardboard honeycomb of thickness 24mm. In order to provide a suitable number of data points, small samples were tested in 3 point bending. The samples were of dimensions 25mm thick, 200mm long and 10mm deep.

Figs. 10-12 report force-deflection data for samples tested at ambient temperature.

As can be seen, the use of kraft paper alone as the skin layer provides a substantial benefit in terms of stiffness of the composite panel at ambient conditions.

The composite panels had the area density reported in Table 4.

Table 4

Adhesive mat layer and skin layer Panel weight (g) Panel area density (g/m²)

1.1 with 150 Mondi Paper 565 2 158

1.1 with 155 Mondi Paper 622 2 376

1.1 with 275 Kraft Paper 615 2 349

2.1 with 150 Mondi 734 2 804

2.1 with 155 Mondi 720 2 750

2.1 with 275 Kraft 791 3 021

2.2 with 150 Mondi Paper 742 2 834

2.2 with 155 Mondi Paper 731 2 792

2.2 with 275 Mondi Paper 795 2 997

3.1 with 150 Mondi Paper 650 2 483

3.1 with 155 Mondi Paper 623 2 380

3.1 with 275 Kraft Paper 684 2 613

3.2 with 150 Mondi 658 2 513

3.2 with 155 Mondi 635 2 426

3.2 with 275 Kraft 739 2 823

4.2 only - no paper skin layer 845 3 228

4.2 with 150 Mondi 910 3 476

4.2 with 155 Mondi 961 3 671 Adhesive mat layer and skin layer Panel weight (g) Panel area density (g/m²)

4.2 with 275 Kraft 1028 3 927

Flax with furan resin coated 812 3 062

Further testing was undertaken, using sample panels of thickness 20mm, length 780mm and depth 340mm. Samples were initially tested with a 50kg load at 80°C using a corresponding testing arrangement to that shown in Fig. 9A. Selected samples were then tested with a 100kg load at 80°C using a corresponding testing arrangement to that shown in Fig. 9A.

The performance of the panels at 50kg loading is reported in Fig. 13. The performance of the panels at 100kg loading is reported in Fig. 14.

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.

Claims

1. A laminated composite material having:

a core;

2. A laminated composite material according to claim 1 , wherein the resin-based binder component comprises a polyfurfuryl alcohol (PFA) polymer.

3. A laminated composite material according to claim 1 or claim 2, wherein the core is cellular.

4. A laminated composite material according to any one of claims 1 to 3, wherein the core is formed of paper or card.

5. A laminated composite material according to any one of claims 1 to 4, wherein the area density of the skin layer is at most 300 gsm.

6. A laminated composite material according to any one of claims 1 to 5, wherein the skin layer has sub-layers, at least one of these sub-layers being a paper sub-layer.

7. A laminated composite material according to claim 6, wherein the skin layer includes at least one moisture-resistant sub-layer.

8. A laminated composite material according to any one of claims 1 to 7, wherein the skin layer has a moisture resistance which is expressed in terms of Water Vapour Transmission Rate (WVTR) which is not more than 24 g/m² x d at 23°C, 85% r.h. (grams of water per m² permeating over a 24 hour period).

9. A laminated composite material having: skin layers provided at the outer facing surfaces of the laminated composite material;

wherein the skin layers comprise paper.

10. A flat packed furniture kit comprising a panel formed of a laminated composite material according to claim 9.

1 1. A method for manufacturing a laminated composite material, including the steps: providing a core;

providing skin layers comprising paper;

pressing and heating the stack in order to bond the skin layers to the core via the resin-based binder component.

12. A method according to claim 1 1 , wherein the adhesive mat layers are

impregnated with the resin-based binder by immersion of the mat into the resin.

13. A method according to claim 12, wherein excess resin is removed from the mat after impregnation.

14. A method according to any one of claims 1 1 to 13, wherein the method is carried out to manufacture two laminated composite material panels, without any intervening step of applying a mould release agent to the mould or to the stack, and without any intervening step of cleaning the mould or to the stack, between the manufacture of the two laminated composite material panels.

15. A method for manufacturing a laminated composite material, including the steps: providing skin layers comprising paper;