FIELD OF THE INVENTION

The present invention relates to fibre composite materials, and to a method for manufacturing them.

Composite materials have been in use, in particular for aeronautic and space applications, for the past 40 years mostly thanks to their high specific mechanical properties. In recent years, composites made from natural fibres have received increasing attention in light of the growing environmental awareness. Due to their low cost, low

environmental impact, and relatively high specific mechanical properties, natural fibres are emerging as a new alternative to glass fibres as a reinforcement in composites.

So far, several types of natural fibres have been extracted and characterized, such as flax, hemp, jute, ramie, kenaf, sisal, henequen, bamboo, silk, or cotton. Currently, flax and hemp fibres appear to be amongst the most promising, due to their combination of high specific mechanical properties, availability, low energy-use in production, low amount of plant care under growth, little need for irrigation, and zero carbon footprint when considering the entire fibre life cycle. In addition to outperforming glass fibres regarding their specific stiffness, flax fibres have been reported to have outstanding damping properties, and the energy required to produce 1 kg of flax fibre mat is only 1/6 of the glass fibre-, and 1/13 of the carbon fibre equivalent. When a natural fibre-based material is gained completely from renewable resources it is typically known as a biopolymer. An alternative approach for designing natural fibre-based composites is the use of thermoplastic polymers, with natural fibres used as reinforcement of this polymer material. Thermoplastic polymers such as PET or Polypropylene are well suited for reuse by means of recycling steps: In combination with natural fibres such as hemp, the mechanical properties remained relatively constant from one recycling step to the other. Biopolymers based on natural fibres have the advantage of being easier to machi ne, since the fibres are less abrasive than most synthetic fibres. The present invention is dedicated to materials based at least in part on natural fibre-based materials.

According to one aspect, the present invention relates to a method for manufacturing a composite material, comprising:

stacking layers of foam or wood, with layers of natural fibre-based composite;

bonding said layers;

obtaining a plurality of structured parts by cutting the obtained stack perpendicular to the layer planes, whereas the thickness of said parts is between 1 mm and several dm;

optionally, shaping said structured parts by milling, sanding and/or machining.

This manufacturing process can be carried out in a continuous or batch process.

According to another aspect, the present invention relates to a sheet of composite material, comprising:

an upper face and a lower face;

a stack of layers of foam or wood adhesively bonded with layers of natural fibre-based composite, wherein said layers extend in a direction substantially perpendicular to at least one of said faces. The height of the stack might be higher than the width of said layers between said upper face and said lower face.

This structural sheet can be used as a core for various objects, such as ski cores, snowboards cores, blades for wind turbines, aeronautic parts, roof or ceiling parts for vehicles etc. This core combines the low density of a cellular solid (foam, wood etc) with the high specific-, anisotropic-, and tailorable properties of fibre reinforced polymers (FRPs). Key advantages of this material include:

• Significantly increased compression and shear moduli in one

direction (3-100 times higher) vs. foam only;

• Significantly increased compression and shear strength in one

direction (2-20 times higher);

• Significantly increased core-face adhesion (+50%-150%) vs. standard foam due to optimum stress transfer from the faces into the core; · Little additional weight and cost, since (i) little material is added to the foam, and (ii) possibility of continuous process;

• Improved adhesion between the natural fibres and the resin,

compared to the best possible adhesion between glass or carbon fibres and the resin. The invention will be better understood with the description of preferred embodiments illustrated by the figures in which:

Figure 1 a illustrates a stack of foams and natural fiber reinforced polymers (FRP) using non-crimp fabrics made from UD fiber layers oriented at +/-45 ⁰ as the FRP layer Figure 1 b illustrates a stack of foams and natural FRPs using balanced woven fabrics as the FRP layer

Figure 2a illustrates a bonded Stack of foams- and natural fiber reinforced polymers (FRP) layers

Figure 3a illustrates examples of cutting lines Figure 3b illustrates an example of resulting sheets

Figure 4a illustrates an example of machined profile, here a ski core

Detailed description of the invention We will now describe an example of manufacturing process according to the invention. In this example, the process comprises four main steps I to IV.

The Process l/l V: Stacking of Layers

During the first step, as illustrated on Figures 1A and 1 B, layers 10 of a structural solid (preferentially a foam plate or wood plate such as balsa or other lightweight wood) are stacked with layers 20; 200, 201 of polymer reinforced with natural fibers, whereas:

• The foam or wood layer 10 can be of variable thickness t, preferably in the range 0.5 cm < t < 10 cm · The foam or wood layers 10 may be made from any commonly

known polymer foam, such as PVC, PP, PET or other thermoplastic polyester, PU, PS, SAN, or any natural cellular solid such as wood or cork etc. In a preferred embodiment, the solid is made from a recyclable material, such as PET or PP. In a preferred embodiment, the solid is made of a thermoplastic material that can be easily bonded with an adjacent layer of the same material, by applying heat and pressure.

• Different types of wood or foam layers 10 may be used within a

same stack 1 , for example a mix between foam layers, wood layers, or several layers of wood/foam with different densities. • The number of natural fibre layers 20; 200, 201 between each layer of foam or wood may vary; preferably, one layer is used as in Figure 1 B, but two or more layers could be used, as in Figure 1 A.

• The natural fibre layers 20; 200, 201 may be made of a fabric, such as a stitched or weaved fabric, a non-crimp fabric, a multi-actial non crimp fabric etc.

• The fibres constituting the natural fibre layers are preferably natural fibres (kenaf, bamboo, flax, hemp, jute, ramie, sisal, henequen, silk, wool, or cotton etc), but may additionally also include any polymer fibre.

According to various embodiments of the invention, at least one of the natural fibre layers 20; 200, 201 comprises:

(i) a 45°balanced woven fabric, i.e. a fabric with natural fibers oriented at +45° relative to the length direction of said layers (0°is parallel, 90° perpendicular to the length direction of the foam or wood plate 10). Different orientations could be used in successive different layers;

(ii) a ±45°balanced woven fabric, i.e. a fabric with natural fibers oriented at +45° and other natural fibers oriented at -45° relative to the length direction of said layers;

(iis) a fabric with natural fibers oriented at +45°, other natural fibers oriented at -45° and still other natural fibres oriented at 90° relative to the length direction of said layers.

The use of fabrics 20 comprising one single layer, without any sublayers, reduces the overall weigth of the product.

As will be described, at least some of the natural fibre layers 20; 200, 201 can comprise a fabric in which thermoplastic fibres are embedded and mixed with the natural fibres. In one embodiment, the constituting yarns may be comingled yarns that contain both natural- and polymer fibres.

The Process ll/IV: Bonding

As illustrated on Figure 2A, the different layers 10, 20, 200, 201 are adhesively bonded to each other, so as to produce a plate 1 comprising a stack 1 of layers 10, 20 substantially parallel to the upper side and/or to the lower side of the plate.

Different methods could be used for bonding the natural fibre layers with the foam or wood layersJf the natural fibre layers 20; 200, 201 already include a thermoplastic material, for example as additional yarns in addition to the natural fibre yarns, or as comingled yarns, bonding may be achieved by heating and pressing the stack 1 so as to melt the

thermoplastic material and weld it with the foam or wood 10.

Alternatively, the natural fibre layers 20; 200, 201 may be impregnated before or during stacking with a thermoplastic powder that will be welded with the foam or wood layers 10. In a preferred embodiment, the material for the thermoplastic fibres/yarns respectively for thermoplastic powder is the same than the material for the foam; this improves the quality of the bonding, and makes recycling of the end-product easier. This material could be PET, polypropylene, PA, PE, PVC, TPU, PS, EVA, SAN, PLA or thermoplastic polyester such as PET for example

In an alternative embodiment, the natural fibre fabric layers 20; 200, 201 may be impregnated during the stacking process, or may have previously entirely or partly been impregnated with an adhesive material, such as a thermoset resin, for example an epoxy resin. Alternatively, a thermoplastic or adhesive film may be placed between one layer 20; 200, 201 of natural fibre-based composite and one layer 10 of foam or wood, and used for bonding those layers together. The use of an additional adhesive has the advantage of improving the heat resistance, for example if the product is further processed using additional steps involving heat; however, it makes recycling much more difficult. The Process 11 l/l V: Cutting

As illustrated on Figure 3A, the obtained composite material 1 is cut perpendicular to the plate and to the layer planes. This results in sheets 100 shown in Figure 3B having upper faces 103 and lower faces 102 corresponding to the cut planes, i.e. perpendicular to the layer planes. The cut might be straight, as illustrated on Figures 3A and 3B, or curved (not illustrated), resulting in sheets having a lower and/or an upper face 102, 103 which are not flat.

• The thickness of the sheets 100 (i.e., the distance between adjacent cut planes) may vary between 1 mm and several dm

• The cutting lines may or may not be parallel to each other, thus

producing sheets 100 with lower and upper faces which are parallel or not parallel to one another. Different sheets may have different thicknesses and/or different vertical sections. · In the case where a thermoset resin is used in the natural FRP layer, a post-curing step may be added to increase the glass transition temperature (T _g ) of the polymer or resin

The Process IV/IV: Milling/Sanding

As illustrated on Figure 4A, the resulting sheet 100 can be used as a structural core material, which may be milled/sanded/machined:

• To change its shape in the plane (e.g. to obtain ski- or snowboard shape)

• To change its thickness along the sheet (in any direction, e.g.

according to ski thickness) · Preferably, computer numerically controlled (CNC) machines are used for milling Applications

The structural core material disclosed in this invention may potentially be used in a wide variety of applications, such as:

• Cores for skis

• Cores for snowboards

• Structural core materials in the wind energy industry (blades), marine, transportation (sandwich structures for floors), aerospace, panels in civil engineering or building etc.

The product may be made in batch; however, an economically most efficient option consists of a continuous production line combining extrusion-foaming layers of foam in-between impregnated FRP layers, followed by consolidating and cutting to right shape