Fibre reinforced polymer composite sheet with moisture barrier surface layer, multilayer composite material with said sheet, composite product with said material and manufacturing method of said sheet

Abstract

The invention concerns a fibre reinforced polymer composite sheet (20), comprising impregnated fibres in a matrix (14), said matrix (14) comprising a polymeric material as main component, characterised in that it further comprises flake-like elements (12) which are fluid-impermeable, in at least one of the two surface layers, forming a moisture barrier surface layer (22). The invention concerns also a multilayer composite material with said fibre reinforced polymer composite sheet (20), a composite product with said multilayer composite material and the manufacturing method of said fibre reinforced polymer composite sheet (20). This moisture barrier surface layer (22) significantly reduces moisture uptake in said fibre reinforced polymer composite sheet (20), enabling protection under harsh humidity and temperature conditions.

Description

Fibre reinforced polymer composite sheet with moisture barrier surface layer, multilayer composite material with said sheet, composite product with said material and manufacturing method of said sheetTechnical domain

[0001] The present invention concerns the field of composite product comprising reinforcement structure via fibre reinforced polymer composite sheet(s) contained as one layer or as multiple layers in the composite product. The present invention notably relates to, but is not limited to, composite products with natural fibres.

[0002] The present invention notably relates to, but is not limited to, thinwalled composite products. In the present text, a thin-walled composite product is a composite product having the shape of a sheet or a plate or the shape of a three-dimensional shell. Other types of composite products are also concerned with the present invention, in particular but not limited to, sandwich structures, laminates or other thick arrangements.

[0003] Such composite products are, for example, a car body part, a sports article, a piece of furniture, a boat hull, or a part of an aircraft, which may have different geometric arrangements, such as a panel, a sheet, a tube or any other hollow three-dimensional geometry, more generally, such a composite product, including a thin-walled composite product, can have a variety of geometries, including a flat plate or sheet, a non-flat plate or sheet, and in particular a plate or sheet with a convex surface and a concave surface, or a corrugated plate or sheet, a three-dimensional hollow shape, and in particular a hollow tube with a circular cross-section, a polygonal cross-section or any other shape, and in particular any thin-walled three-dimensional shell.

[0004] There are various arrangements of reinforced composite products, for example, a composite product having a matrix made of plastics material of a polymer or a resin and a reinforcement which may be in the form of a preform with yarns.

[0005] In case of a thin-walled reinforced composite product, this has a thin wall, i.e. it is generally initially in the form of a sheet, plate or panel, oneof the dimensions of which is much smaller (at least 10 times smaller) than the other two dimensions of the product.

[0006] Such fibre reinforced polymer composite sheet(s) which is(are) provided to manufacture the composite product consist(s) usually as what is known as "prepreg", which is a term for "pre-impregnated" composite fibres. These fibres or yarns may either take the form of a weave, multi-axial fabrics or are unidirectional. As a result of the impregnation step, they already contain an amount of the matrix material used to bond them together and to other components during manufacture.The main idea behind a prepreg material is the use of anisotropic mechanical properties along the fibres, while the polymer matrix provides filling properties, keeping the fibres in a single system.In the impregnation step, this matrix can be a partially cured matrix comprising a polymeric material, which is a thermosetting polymer, as main component, so that the fibres are impregnated in said matrix. Those possibles thermosetting polymers are often epoxy or phenolic resin. The partially cured state of the matrix during and after impregnation allow easy handling.This matrix can alternatively contain a thermoplastic polymer as polymeric material, which is fully polymerized and solid after the impregnation step. In some other cases, the matrix contains as polymeric material a thermoplastic material mixed with liquid rubbers or resins.

[0007] For some applications, composite products are such as described in EP2648890, EP3999326 and EP4058623. In such applications, these are thinwalled composite product with one ribbed face which shape has to be kept unchanged, namely the ribs should not be flattened nor sink too much into the thin-walled composite to keep good mechanical properties provided by the ribs to the product. In such thin-walled composite products, ribs form together a grid with a defined shape and high adhesion to the underlaying composite layer.

[0008] In some cases, those thin-walled composite products have relief elements on one side, with those relief, notably protruding elements, forming stiffeners which volume has to be kept unchanged or relatively not too much lowered during the manufacturing process of the composite product and during the lifetime of the product.

[0009] Also, more generally, depending on the application of these composite products, there could be degradation of the mechanical properties, also with possible visual modification of these products, due to the presence of moisture in the environment where these products are used.This problem is particularly relevant for composite products using natural fibres as reinforcement.

[0010] It is known that moisture sorption is responsible for a significant swelling of natural fibres, which can severely impact composite product made with these natural fibres both on mechanical properties and aesthetics.Among other things, this fibre swelling reduces the mechanical properties of the fibres, contributes to delamination between the fibres and the matrix, which strongly degrades mechanical resistance of a fibre reinforced polymer composite sheet or any composite product comprising a fibre reinforced polymer composite.Natural fibres embedded in a polymer matrix are indeed particularly subject to this issue, due to the moisture diffusivity of the polymer matrix and of the fibres network itself.

[0011] In some cases, natural fibre reinforcement is used as both a composite reinforcement and visual solution, notably in the automotive sector. This situation requires that the protective layer be clear and transparent, while effectively also preventing the moisture sorption under harsh humidity and temperature conditions.Related art

[0012] The applicant is not aware of a fibre treatment that could significantly reduce moisture uptake, nor significantly reduce swelling caused by moisture uptake in a fibre reinforced polymer composite sheet. There is research literature on this topic, with some studies presenting solutions reducing slightly water absorption, but not sufficiently.

[0013] The applicant tested protective layers currently used in automotive and aerospace industry, including polyurethanes (PU), polyesters, acrylics and cross-linking thermoplastics, with various thicknesses. These tests were combined with some other presumed moisture prevention provisions, including :- increasing the resin content forming the polymeric material of the matrix, inthe laminate which is formed by stacked fibre reinforced polymer composite sheets,- Inclusion of nonwoven glass fibre fleece or glass fibre weave or nonwoven polyester fleece as an additional surface layers, to create a thick and possibly transparent layer on top of the laminate after impregnation by the matrix, and- fibre treatments: desizing, hornification, mercerization, alkalization, citrate, bleaching, bleaching and dyeing.

[0014] Despite measurable differences in fibre swelling and surface topology after wet ageing, none of these approaches, even in combination, came close to solving the problem. The applicant's assumption is based on too high moisture sorption, at least at the surface layer, which in all cases reaches the fibres, thereby causing significant damage in the fibre reinforced polymer composite sheet or in any multilayer composite material or product comprising a fibre reinforced polymer composite.

[0015] It is therefore not known any protective layer including common automotive-grade coat systems, clear or unclear, which provides adequate protection for natural fibre composites under conditions required by standards, namely aging tests under harsh humidity and temperature conditions, at any reasonable thickness.Short disclosure of the invention

[0016] An aim of the present invention is the provision of a fibre reinforced polymer composite sheet that can be used as subcomponent in a composite product, and which overcomes the shortcomings and limitations of the state of the art in relation with moisture absorption.

[0017] Another aim of the invention concerns the supplying of a multilayer composite material as intermediate product, with such a fibre reinforced polymer composite sheet, in order to prevent or at least limit moisture absorption.

[0018] Also, the present invention relates to composite products obtained with such fibre reinforced polymer composite sheet or with such multilayer composite material, including thin-walled composite products.

[0019] Hence, the objective of the present invention relies on providing a solution preventing moisture from ever, or quite ever, reaching the fibres altogether, by sealing the composite behind a strong moisture barrier.

[0020] According to the invention, these aims are attained by the object of the attached claims, and especially by a fibre reinforced polymer composite sheet, comprising impregnated fibres in a matrix, said matrix comprising a polymeric material as main component, characterised in that it further comprises flake-like elements which are fluid-impermeable, in at least one of the two surface layers, forming a moisture barrier surface layer.

[0021] With respect to what is known in the art, the invention provides the advantage that the incorporation of flake-like elements which are fluid- impermeable allows to form a true and efficient moisture barrier surface layer.The flake-like elements being fluid-impermeable, means that each flake-like element is both liquid impermeable and gas impermeable, therefore water impermeable both for liquid water or gaseous water (steam).This barrier surface layer resulting from the presence of flake-like elements within the fibres and matrix system (such as a prepreg), can be obtained easily without degradation of the mechanical properties and aesthetics of the final product.

[0022] Flake-like elements have a flat or almost flat geometry, with more or less important planar extension. One understands the orientation of those flake-like elements within the moisture barrier surface layer of fibre reinforced polymer composite sheet modifies the efficiency of moisture sorption prevention.Preferably, the flake-like elements are arranged in a close to parallel relative orientation between them and with respect to the sheet surface.In possible embodiments, on average said flake-like elements are arranged in the barrier surface layer in a relative orientation with respect to the surface of the fibre reinforced polymer composite sheet which is less than 45°, or less than 30°, or less than 20°, or less than 15°. "On average" means either mass average, or number average, or surface average of said flake-like elements. In possible embodiments, at least 80% of said flake-like elements are arranged in the barrier surface layer in a relative orientation with respect to the surface of the fibre reinforced polymer composite sheet which is less than 45°, or less than 30°, or less than 20°, or less than 15°.This orientation preference can be fulfilled using another parameter called the orientation factor. This so-called "orientation factor" is denoted S' in tortuosity models to account for the average orientation ("Modeling the Barrier Properties of Polymer-Layered Silicate Nanocomposites" Bharadwaj, 2001; Macromolecules 2001, 34, 9189-9192).Even at S' = 0 (true random), flake-like elements as fillers already provide a much better barrier than granular fillers. S' is one of several more parameters in tortuosity models (e.g. aspect ratio, filler content, number of layers in the stack).When using said orientation factor parameter, depending on embodiments, said flake-like elements are can be arranged in the barrier surface layer with an orientation factor S' between 0.5 and 1, or with an orientation factor S' between 0.6 and 1 or with an orientation factor S' between 0.8 and 1.

[0023] In possible embodiments of the fibre reinforced polymer composite sheet, said flake-like elements have a ratio between the average diameter L and the average thickness W, which is higher than 10 (L / W > 10). Such provision allows to provide flake-like elements having a generally flattened geometry.

[0024] In some cases, the appearance of the prepreg or fibre reinforced polymer composite sheet requires transparency, so that the fibres embedded in the matrix are visible. In those cases, said matrix and said flake-like elements are transparent to visible spectrum, which includes said polymeric material being also transparent to visible spectrum. In the present text, such visible spectrum is defined for light wavelengths between 380 nanometers (nm) and 700 nanometers (nm).

[0025] In some embodiments of the fibre reinforced polymer composite sheet, said barrier surface layer has a thickness comprised between 30 pm to 500 pm, or between 50 pm and 300 pm.

[0026] In some embodiments of the fibre reinforced polymer composite sheet, the flake-like elements content in the fibre reinforced polymer composite sheet is at least 5 g / m2, or between 5 and 200 g / m2, or between 5 and 100 g / m2. Such flake-like elements content is determined as the surface mass, i.e. grams of flake-like elements per unit area in square meter of the fibre reinforced polymer composite sheet. In some embodiments of the fibre reinforced polymer composite sheet, the flake-like elements content in thefibre reinforced polymer composite sheet is at least 0.01 g / m2 / nm or between 0.01 g / m2 / nm and 0.1 g / m2 / nm. In this case, the flake-like elements content is determined with the ratio between the flakes areal weight (FAW in g / m2) and t (average flakes thickness in nanometers), namely FAW / t.

[0027] In some embodiments of the fibre reinforced polymer composite sheet, said fibres form a nonwoven fabric or a woven fabric.

[0028] In a first possible variant of the fibre reinforced polymer composite sheet, the matrix is partially cured and said polymeric material comprises a thermosetting polymer, such as epoxy resin, phenolic resin, polyester resin, polyfurfuryl alcohol, polyurethane resin, cyanate ester resin or a combination thereof.

[0029] In a second possible variant of the fibre reinforced polymer composite sheet, said polymeric material comprises a thermoplastic polymer. In that variant, the matrix is fully cured and molten at the time of forming the moisture barrier surface layer, the curing step being performed after formation of the whole stack forming the final thin-walled composite product.

[0030] The present invention also concerns a multilayer composite material comprising a fibre reinforced polymer composite sheet as defined in the present text, and a fibre carrier such as a glass fleece or fabric, a polyamide (PA) fleece or fabric, or a polyester fleece or fabric, said fibre carrier being located within said moisture barrier surface layer or adjacent said moisture barrier surface layer.

[0031] According to the invention, more generally, the fibre reinforced polymer composite sheet as defined in the present text can form the element placed at the surface of a stacked of several planar elements assembled together to constitute a sandwich like material. In that situation, the sandwich like material will be placed in any assembly or composite part at the surface thereof, with the moisture barrier surface layer of the fibre reinforced polymer composite sheet forming the external layer enabling moisture protection of the sandwich like material.

[0032] Therefore, the present invention also concerns a multilayer composite material comprising a fibre reinforced polymer composite sheet as defined in the present text, and on the face of the fibre reinforced polymer composite sheet which do not form a barrier surface layer, a stack of several layers, wherein each layer comprising impregnated fibres in a partially cured matrix comprising a polymeric material as main component.

[0033] In another possible implementation of the invention, it is proposed a multilayer composite material comprising a stack of a plurality of layers of fibrous materials, wherein each layer comprises impregnated fibres in a partially cured matrix comprising a polymeric material as main component, and on each side of the stack a fibre reinforced polymer composite sheet as defined in the present text, the side of the fibre reinforced polymer composite sheet forming a moisture barrier surface layer on the exterior of the multilayer composite material.

[0034] In any of such situations, the multilayer composite material can be a stack of traditional prepregs with one (or two) specific prepreg(s) formed by the fibre reinforced polymer composite sheet as defined in the present text, as a top layer (or two top layers on opposite sides) that will be exposed to wet ageing.

[0035] The present invention is also directed to a composite product comprising a multilayer composite material as defined in any of the previous paragraphs, where the matrix comprises a thermosetting polymer which is cured.

[0036] The invention also concerns a composite product which is a thinwalled reinforced composite product.

[0037] According to the invention, it is also provided a manufacturing method for producing a fibre reinforced polymer composite sheet as previously presented.Such a manufacturing method for producing a fibre reinforced polymer composite sheet, comprises the following steps: i) providing flake-like elements which are fluid-impermeable, ii) providing liquid matrix comprising a polymeric material as main component, iii) forming a film with at least said flake-like elements and said matrix,iv) providing a reinforcement fibre fabric, v) superimposing said film with said reinforcement fibre fabric, forming thereby a coupled assembly of layers, and vi) compressing the coupled assembly of layers to form said fibre reinforced polymer composite sheet, whereby the polymeric material impregnates the reinforcement fibre fabric, the flake-like elements mixed in polymeric material creates a moisture barrier surface layer in said fibre reinforced polymer composite sheet.Depending on the application of the fibre reinforced polymer composite sheet the liquid matrix could be uncured thermoset or molten thermoplastic.

[0038] This fibre reinforced polymer composite sheet is a moisture barrier improved fibre reinforced polymer composite sheet, namely a moisture barrier prepreg. This will later be cured during the moulding of parts or products incorporating this fibre-reinforced polymer composite sheet.

[0039] During step vi), the compression can be heat compression or cold compression, the impregnation by the polymeric material of the reinforcement fibre fabric having to be guaranteed. In parallel, during the compression step vi), the flake-like elements do not penetrate nor impregnate the reinforcement fibre fabric but are concentrated and come closer to each other due to the departure of an important part of the polymeric material from the film by impregnation of the reinforcement fibre fabric.This highly concentrated stack of flake-like elements is used to create a moisture-blocking top layer. This is described in more detail below.

[0040] In a first possible variant of the manufacturing method for producing a fibre reinforced polymer composite sheet, and said polymeric material comprises a thermosetting polymer, such as epoxy resin, phenolic resin, polyester resin, polyfurfuryl alcohol, poly-urethane resin, cyanate ester resin or a combination thereof. This liquid matrix is uncured at the moment of mixing with flake-like elements. It will be partially cured after mixing or after impregnation of the reinforcement fibre fabric.

[0041] In a second possible variant of the manufacturing method for producing a fibre reinforced polymer composite sheet, said polymeric material comprises a thermoplastic polymer.

[0042] During step iii), in a first variant, the flake-like elements are mixed with the matrix to form a mixture, and the mixture is drawn or stretched to form the film. In a second variant, the matrix is drawn to form a matrix film, and the flake-like elements are scattered or dispersed on the matrix film to form the film.

[0043] In some embodiments, in step ii) the liquid matrix is carried by a fibre carrier and wherein in step iii) the film (is formed with said fibre carrier, said matrix and said flake-like elements.

[0044] In relation with the previously defined fibre reinforced polymer composite sheet and manufacturing method according to the present invention, the following provisions may be implemented, alone or in combination:- said flake-like elements are made from anyone of the following materials or a combination thereof : glass, metal oxide-coated glass, metal oxide-coated polymer, metal coated-polymer, metals, polymers and ceramics such as mica, malachite;- said flake-like elements have an average width comprised between 5 pm and 2000 pm, or between 50 pm and 500 pm, or between 100 pm and 200 pm, and an average thickness comprised between 0.1 pm and 5.0 pm, or between 0.1 pm and 1.0 pm;- said fibres are made from anyone of the following materials: vegetal natural fibres, animal fibres, glass fibres, carbon fibres and aramid fibres and a combination thereof. The vegetal natural fibres can be made of any of the following fibres or a combination thereof : flax, hemp, sisal, jute, abaca, kenaf, nettle, ramie, henequen, pineapple, banana, and palm.Short description of the drawings

[0045] Exemplar embodiments of the invention are disclosed in the description and illustrated by the drawings in which:Figure 1 schematically illustrates the principle of formation of the fibre reinforced polymer composite sheet according to the present invention,Figure 2 shows the resulting reinforced polymer composite sheet according to the present invention, with the moisture barrier surface layer,Figure 3 is a representation of the mixture of fluid-impermeable flakelike elements with a liquid matrix,Figure 4 shows an example of the various steps of a manufacturing method that can be carried out, in order to produce the reinforced polymer composite sheet according to the present invention,Figure 5 illustrates in principle the moisture barrier performance resulting from the use of fluid-impermeable flake-like elements,Figure 6 shows an enlarged portion VI of Figure 2, with a portion of the moisture barrier surface layer and of adjacent impregnated reinforcement fibre fabric,Figure 7 illustrates the characterization of actual improvement in the prevention of moisture absorption in a fibre reinforced polymer composite sheet according to the present invention,Figure 8 illustrates a possible variant for the formation of the fibre reinforced polymer composite sheet according to the present invention, with the presence of a fibre carrier, forming thereby a multilayer composite material,Figure 9 shows the resulting reinforced polymer composite sheet according to the variant of figure 8, with the moisture barrier surface layer, andFigure 10 shows a portion X of Figure 9, with a portion of the moisture barrier surface layer comprising a fibre carrier, a portion of adjacent impregnated reinforcement fibre fabric, and of a portion of a mould, at large scale.Examples of embodiments of the present invention

[0046] A film 10 formed by the combination of flake-like elements 12 embedded in a matrix 14 is shown on the underside of Figure 1 . A reinforcement fibre fabric 16 is shown on the upper side of Figure 1 : this is for example a weave of fibres, including natural fibres. Such a reinforcement fibre fabric 16 can consist of any assembly of fibres or yarns which are classically impregnated with a matrix to form a prepreg material.

[0047] In order to form a fibre reinforced polymer composite sheet 20 according to the invention, the film 10 is superposed to the reinforcement fibre fabric 16. Then the matrix 14 of the film is forced to impregnate the reinforcement fibre fabric 16 as shown by the arrow in the simplified representation of figure 2, forming thereby an impregnated reinforcement fibre fabric 16'. Due to the natural filtering effect of the reinforcement fibre fabric 16, and since an important portion of the matrix 14 of the film 10 penetrates the reinforcement fibre fabric 16, the flake-like elements 12 move closer together during said impregnation and concentrate to form a barrier layer at the surface of the stack (on the bottom side on figure 2). This barrier layer is a moisture barrier surface layer 22. Figure 6 illustrates at a larger scale the portion VI of figure 2, which gives a representation of the content, close relationship and attachment between the moisture barrier surface layer 22 and the impregnated reinforcement fibre fabric 16', which both contain the the matrix 14.

[0048] This moisture barrier surface layer 22 is constituted by the same components as the film 10, namely the matrix 14 and the flake-like elements 12, with a higher concentration of the flake-like elements 12 with respect to the original film 10. This high concentration of the flake-like elements 12 contributes to an orientation of the flake-like elements 12 that is conducive to preventing moisture penetration, in fact, the flake-like elements 12 tend to be positioned in a sort of stack, more or less parallel to each other as can be seen in figure 5.

[0049] Typically, the flake-like elements content in the fibre reinforced polymer composite sheet 20, considered as the as the surface mass, i.e. mass of flake-like elements per unit area, is at least 5 g / m2, or between 5 and 200 g / m2, or between 5 and 100 g / m2. In some other cases, notably with flake-like elements 12 sufficiently thick, the flake-like elements content in the fibrereinforced polymer composite sheet 20 is at least 10 g / m2, or between 10 and 200 g / m2, or between 10 and 100 g / m2.

[0050] Considering the average thickness t (nm) of the flake-like elements 12, the ratio between the flakes areal weight (FAW in g / m2) and t (average flakes thickness) is typically above FAW / t = 0.01 g / m2 / nm, which means in SI units having a flake-like elements content of at least 107g / m3. This parameter can be in the range FAW / t = 0.01 - 0.10 g / m2 / nm, which means in SI units having a flake-like elements content between 107g / m3and 108g / m3in the fibre reinforced polymer composite sheet 20.

[0051] The flake-like elements 12 can be glass flake-like elements 12. The flake-like elements 12 can also be made from anyone of the following materials or a combination thereof : glass, metal oxide-coated glass, metal oxide-coated polymer, metal coated-polymer, metals, polymers and ceramics such as mica, malachite.The flake-like elements 12 can have an average width comprised between 5 pm and 2000 pm, or between 50 pm and 500 pm, or between 100 pm and 200 pm, and an average thickness comprised between 0.1 pm and 5.0 pm, or between 0.1 pm and 1.0 pm.

[0052] In the fibre reinforced polymer composite sheet 20, this moisture barrier surface layer 22 is attached to the impregnated reinforcement fibre fabric 16'. In other words, this moisture barrier surface layer 22 is integral with the impregnated reinforcement fibre fabric 16'. This means the barrier surface layer 22 has become an integral part of the impregnated reinforcement fibre fabric 16'. This moisture barrier surface layer 22 has a reduced thickness with respect to the film 10, whereas the thickness of the impregnated reinforcement fibre fabric 16' is close to the thickness of the reinforcement fibre fabric 16. Typically, the barrier surface layer 22 has a thickness comprised between 30 pm to 500 pm, or between 50 pm and 300 pm.

[0053] This constitutes a fibre reinforced polymer composite sheet 20 with one side formed by the moisture barrier surface layer 22 and the other side formed by the impregnated reinforcement fibre fabric 16'. This result is therefore obtained via a one-sided, through-thickness pre-impregnation method of a reinforcement fibre fabric 16.

[0054] To obtain the film 10, some flake-like elements 12 are mixed with a liquid matrix 14, which results in a mixture 15 as shown in figure 3.In a possible embodiment, this matrix 14 is an uncured thermoset matrix comprises usual components of a liquid resin system components (resin, hardener, activator, other components such as additives). All sorts of resin can be used, such as an epoxy resin, but other resins can be used, such as phenolic resin, polyester resin, polyurethane resin, polyfurfuryl alcohol or cyanate ester resin. A combination of any of these indicated components can also be used as polymeric material forming the main component of the matrix 14.In another possible embodiment, this matrix 14 is a molten thermoplastic material or comprises a polymeric material constituted of or containing a thermoplastic polymer or a mixture of thermoplastic polymers.

[0055] In the next step, this mixture 15 is treated to form the film 10 that can be obtained from a drawing step, using a roller 101 as shown on the left side of figure 4. Instead of mixing before drawing a film, the flake-like elements 12 could be scattered onto a film made from a drawn liquid matrix 14 before coupling with the reinforcement fibre fabric 16.

[0056] Then, as previously explained in relation to figure 1, the film 10 is coupled with a dry fabric reinforcement consisting of the reinforcement fibre fabric 16. This is visible in the middle of the figure 4 where the film 10 and the reinforcement fibre fabric 16 are superposed. After formation of the stack between the reinforcement fibre fabric 16 and the film 10, there are several possible ways to obtain the fibre reinforced polymer composite sheet 20.

[0057] On the right side of figure 4, this fibre reinforced polymer composite sheet 20 results from a calendering operation where the stack formed by the superimposed reinforcement fibre fabric 16 and film 10, is passed between rollers 102, 103 heated under pressure. At the outlet of these rollers (on the far right of figure 4), the fibre reinforced polymer composite sheet 20 is formed with a section similar to the representation of figure 2 previously discussed. Therefore, as explained above, during calendering, the coupled film 10 and reinforcement fibre fabric 16 are compressed, thereby forcing the matrix 14 (resin system) to impregnate the reinforcement fibre fabric 16, leaving as much as possible flake-like elements 12 accumulated on its surface le in the thus formed moisture barrier surface layer 22.

[0058] Instead of calendaring of the stacked film 10 and reinforcement fibre fabric 16, their assembling can be obtained by pressing them in any other way. For instance, the assembling of the film 10 and reinforcement fibre fabric 16 can be obtained by pressing them in a press, which is a heated press or a not heated press. This pressing operation can be implemented in a continuous way like with a double belt press.

[0059] The structure of the moisture barrier surface layer 22 is illustrated in a more realistic way in Figure 5. The flake-like elements 12 are close to each other and are preferably arranged in a close to parallel relative orientation between them and with respect to the sheet surface 16a.

[0060] Considering the moisture barrier surface layer 22, the presence of flake-like elements 12 which are impermeable as fillers in the polymeric matrix 14 gives rises to a permeation path which is much longer as it would be with granular filler. The use of flake-like elements creates a tortuosity parameter that describes the ratio between a (water) molecules' average diffusion path length, and the polymer film's thickness. The diffusion path 22a is shown in figure 5: the diffusion path length being far larger than the thickness of the moisture barrier surface layer 22, this reduces the permeability of the moisture barrier surface layer 22. The tortuosity is directly related to the total surface area of flake-like fillers, which itself is tied to their average aspect ratio, all other things being equal (e.g. filler mass fraction). It is also linked to the flake's relative orientation (random vs. parallel).

[0061] This orientation can be characterized using the "orientation factor" which is denoted S' in tortuosity models to account for the average orientation ("Modeling the Barrier Properties of Polymer-Layered Silicate Nanocomposites" Bharadwaj, 2001; Macromolecules 2001, 34, 9189-9192). In a situation of perfect parallelism (av. 0°) as orientation of the flake-like elements with respect to the surface of the polymeric film, the orientation factor S' = 1.In a situation of perfect perpendicularity (av. 90°) as orientation of the flakelike elements with respect to the surface of the polymeric film, the orientation factor S' = -0.5.In a situation of true random orientation (av. 54.74°) of the flake-like elements with respect to the surface of the polymeric film, the orientation factor S' = 0.In a situation of average orientation at 30°of the flake-like elements with respect to the surface of the polymeric film, the orientation factor S' = 0.625.

[0062] In the present situation, in order to obtain moisture prevention, the flake-like elements can be arranged in the barrier surface layer 22 with an orientation factor S' between 0.5 and 1, or with an orientation factor S' between 0.6 and 1 or with an orientation factor S' between 0.8 and 1.As described in “A review on predictive tortuosity models for composite films", J. Coat. Technol. Res., 19 (3) 699-716, 2022, using the angle correspondence with calculated with Eq. (38) on page 708, namely: S’ =1 / 2( 3 cos2e -1), where 0 is the angle between the penetrant flow direction and normal to the flake layer or flake main plane.

[0063] Also, the applicant made some tests with flake-like elements 12 being glass flakes, which results are illustrated in figure 7.These glass flakes were Glassflake® GF100M epoxy-silane treated, having a 1.0-1.3 pm thickness, 120 pm average width. These glass flakes were deposited onto epoxy films.Flax-epoxy prepregs were produced via a standard film stacking process, by coupling the glass flake-covered epoxy films with flax fabrics, and impregnating using a hot press, forming thereby a fibre reinforced polymer composite sheet.These fibre reinforced polymer composite sheets were cured in a standard autoclave process, with the glass flake side on the surface. The sheets formed plates which were then painted with a standard automotive polyurethane (PU) clear coat system, and placed in an accelerated hygrothermal aging chamber following automotive qualification standards.A very clear and compelling improvement of the surface aspect after hygrothermal aging was shown with increasing glass flakes content.

[0064] The improvement in moisture absorption of the composite sheets is visible in figure 7 with the measurement of surface waviness ("Surface wavescan RMS") for a composite sheet without glass flakes (on the left side of figure 7 as "unaged reference"), for the same composite sheet without glass flakes after hygrothermal aging (on the middle of figure 7 as "Aged -no flakes") and for a composite sheet with glass flakes after hygrothermal aging aging (on the right side of figure 7 as "Aged -with flakes").

[0065] As visible in figure 7, thanks to the moisture barrier surface layer 22, the fibre reinforced polymer composite sheet 20 according to the invention resists hygrothermal aging with a strictly limited rise in the surface waviness in comparison with a standard prepreg formed by impregnated fibres in a matrix exempt of flake-like elements.

[0066] Such fibre reinforced polymer composite sheet 20 according to the invention can be used to form a multilayer composite material.

[0067] In a possible embodiment of such multilayer composite material comprising a stack of a plurality of layers of fibrous materials, wherein each layer comprises impregnated fibres in a partially cured matrix 14 comprising a polymeric material as main component, there is one fibre reinforced polymer composite sheet 20 according to the invention on one side of the stack only. Such side is therefore equipped with a moisture barrier surface layer 22. Such side forms the exterior of the multilayer composite material. In such arrangement, there is only one reinforced polymer composite sheet 20.

[0068] In another possible embodiment of such a multilayer composite material comprising a stack of a plurality of layers of fibrous materials, wherein each layer comprises impregnated fibres in a partially cured matrix 14 comprising a polymeric material as main component, there is one fibre reinforced polymer composite sheet 20 according to the invention on each side of the stack. In such arrangement, there are two reinforced polymer composite sheets 20, one on each side of the stack of a plurality of layers of fibrous materials, both sides of the multilayer composite material are therefore equipped with a moisture barrier surface layer 22. Such sides form the exterior of the multilayer composite material.

[0069] One understands that, in order to avoid moisture penetration in the composite part, the fibre reinforced polymer composite sheet 20 with a moisture barrier surface layer 22 can be used either on one face or on both faces of the stack of layers of the multilayer composite material forming the composite part.

[0070] In a possible variant which is applicable for all embodiments, the fibre reinforced polymer composite sheet 20 further contains a fibre carrier 18 such as a glass fleece or fabric, a polyamide (PA) fleece or fabric, or a polyester fleece or fabric, said fibre carrier being located within said moisturebarrier surface layer 22 or adjacent said moisture barrier surface layer 22. Such fibre carrier 18 can be a fleece, or any convenient nonwoven carrier, or a light woven fabric which can be open enough not to contribute filtering the flakelike elements 12.

[0071] This variant for the fibre reinforced polymer composite sheet 20 is presented in relation to figures 8 and 9.

[0072] The film 10 shown on the underside of Figure 8, contains the combination of flake-like elements 12 embedded in a matrix 14 and contains further a fibre carrier 18. This fibre carrier 18 serves as support to the mixture of matrix 14 and flake-like elements 12. This fibre carrier 18 is impregnated with the matrix 14 and the flake-like elements 12 can be present either within the whole volume of the matrix, on both sides of the fibre carrier 18 (as visible in figure 8), or within the matrix but only on one side of the fibre carrier 18. In this variant, thanks to the presence of the fibre carrier 18, the film 10 containing the fibre carrier 18 can be prepared in advance and form a self-supporting element, sufficiently resistant to be handled. This could be obtained with a standalone resin film with fibre carrier existing as commercial product, on which flake-like elements 12 are scattered. On the upper side of Figure 8, a reinforcement fibre fabric 16 similar to that of figure 1 is shown, in a state already impregnated with the matrix 14. This could be a nonimpregnated reinforcement fibre fabric 16.

[0073] In order to form a fibre reinforced polymer composite sheet 20 according to the invention, the film 10 containing the fibre carrier 18 is superposed to the reinforcement fibre fabric 16. Preferably, they are attached together, for instance through a calendaring operation. This calendaring operation is in most cases effective to lead to an at least partial filtration of the flake-like elements 12 with flowing of the matrix 14 from the film 10 into the reinforcement fibre fabric 16. In another later step, shown in figure 9, the reinforced polymer composite sheet 20 is cut with the required shape and is placed in the lower part of a mould 30, with the film 10 as external lowest layer. In this situation, the reinforced polymer composite sheet 20 comprises the Impregnated reinforcement fibre fabric 16' and the film 10 with the fibre carrier 18. It is to be understood that (an)other layer(s) (not shown) can be present in the mould, on top of the reinforced polymer composite sheet 20 visible on Figure 9. This or these layer(s) can be prepreg layer(s), each prepreg layer comprising impregnated fibres in a partially cured matrix comprising apolymeric material as main component, or other types of layers such as dry reinforcement fabrics.

[0074] During the moulding step, the matrix 14 of the film is forced to migrate upwards and to impregnate the reinforcement fibre fabric 16 and any other upper layer(s) as shown by the arrow in the simplified representation of figure 9. Due to this migration and natural filtering effect of the reinforcement fibre fabric 16, the flake-like elements 12 move closer to each other during said impregnation and concentrate to form a moisture barrier 22 layer at the lowest surface of the stack (on the bottom side on figure 9). Depending on the nature of the fibre carrier 18, the latter can also act as a filter for the flake-like elements 12.

[0075] In another possible embodiment (not shown), the fibre reinforced polymer composite sheet 20 of figure 2 is stacked on several prepreg layers, each prepreg layer comprising impregnated fibres in a partially cured matrix comprising a polymeric material as main component. In this case, the fibre reinforced polymer composite sheet 20 is kept as element placed on the outer place of the stack, so that the moisture barrier surface layer 22 of the sheet 20 forms also the surface layer of the stack.

[0076] After curing of this multilayer composite material, a composite product is obtained.

[0077] As possible non-limited application of the composite product according to the present invention, such composite product can be a portion of an article of the following list : an automobile body part, in particular a door, a roof, a hood, a fender, a spoiler, a front or rear bumper, an aerodynamic kit; an automobile interior part, in particular a door cover, a dashboard, a centre console, a pillar trim, a trunk trim, a roof; a sport article, in particular a canoe, a kayak, a boat hull, a rod saddle, a bicycle saddle, a bicycle frame, a bicycle handlebar, a baseball bat, a paddle, a ski or walking stick; an element of furniture or a part of the interior of an airplane, in particular a side panel, a ceiling panel, a luggage compartment; and an aerodynamic part of a light airplane, in particular an engine cowling, a wheel cowling; and an aerodynamic fairing of a mobile machine.Reference signs used in the figuresFilmFlake-like elementsMatrixMixtureReinforcement fibre fabric ' Impregnated reinforcement fibre fabric a Surface of the Impregnated Reinforcement fibre fabricFibre carrier (fleece or fabric carrier)Fibre reinforced polymer composite sheetMoisture barrier surface layer a Diffusion pathMould (lower part) 1 Roller 2 Roller 3 Roller

Claims

Claims1. Fibre reinforced polymer composite sheet (20), comprising impregnated fibres in a matrix (14), said matrix (14) comprising a polymeric material as main component, characterised in that it further comprises flake-like elements (12) which are fluid-impermeable, in at least one of the two surface layers, forming a moisture barrier surface layer (22).

2. Fibre reinforced polymer composite sheet (20) according to claim 1, wherein said matrix (14) and said flake-like elements (12) are transparent to visible spectrum.

3. Fibre reinforced polymer composite sheet (20) according to claim 1 or 2, wherein said flake-like elements (12) are made from anyone of the following materials or a combination thereof: glass, metal oxide-coated glass, metal oxide-coated polymer, metal coated-polymer, metals, polymers and ceramics such as mica, malachite.

4. Fibre reinforced polymer composite sheet (20) according to any of claims 1 to 3, wherein at least 80% of said flake-like elements (12) are arranged in the barrier surface layer (22) in a relative orientation with respect to the sheet (20) surface which is less than 45°.

5. Fibre reinforced polymer composite sheet (20) according to any of claims 1 to 4, wherein said flake-like elements (12) are arranged in the barrier surface layer (22) with an orientation factor S' between 0.5 and 1, or with an orientation factor S' between 0.6 and 1 or with an orientation factor S' between 0.8 and 16. Fibre reinforced polymer composite sheet (20) according to any of claims 1 to 5, wherein said flake-like elements (12) have a ratio between the average diameter L and the average thickness W, which is higher than 10 (L / W > 10).

7. Fibre reinforced polymer composite sheet (20) according to any of claims 1 to 6, wherein said flake-like elements (12) have an average width comprised between 5 pm and 2000 pm, or between 50 pm and 500 pm, or between 100 pm and 200 pm, and an average thickness comprised between 0.1 pm and 5.0 pm, or between 0.1 pm and 1.0 pm.

8. Fibre reinforced polymer composite sheet (20) according to any of claims 1 to 7, wherein said barrier surface layer (22) has a thickness comprised between 30 pm to 500 pm, or between 50 pm and 300 pm.

9. Fibre reinforced polymer composite sheet (20) according to any of claims 1 to 8, wherein the flake-like elements (12) content is at least 5 g / m2, or between 5 and 200 g / m2, or between 5 and 100 g / m2.

10. Fibre reinforced polymer composite sheet (20) according to any of claims 1 to 9, wherein the flake-like elements (12) content is at least 0.01 g / m2 / nm or between 0.01 g / m2 / nm and 0.1 g / m2 / nm.

11. Fibre reinforced polymer composite sheet (20) according to any of claims 1 to 10, wherein said fibres form a nonwoven fabric or a woven fabric.

12. Fibre reinforced polymer composite sheet (20) according to any of claims 1 to 11, wherein said fibres are made from anyone of the following materials: vegetal natural fibres, animal fibres, glass fibres, carbon fibres and aramid fibres.

13. Fibre reinforced polymer composite sheet (20) according to any of claims 1 to 12, where the matrix (14) is partially cured and wherein said polymeric material comprises a thermosetting polymer, such as epoxy resin, phenolic resin, polyester resin, polyfurfuryl alcohol, poly-urethane resin, cyanate ester resin or a combination thereof.

14. Fibre reinforced polymer composite sheet (20) according to any of claims 1 to 12, wherein said polymeric material comprises a thermoplastic polymer.

15. Multilayer composite material comprising a fibre reinforced polymer composite sheet (20) as in any of the preceding claims 1 to 14, and a fibre carrier such as a glass fleece or fabric, a polyamide (PA) fleece or fabric, or a polyester fleece or fabric, said fibre carrier being located within said moisture barrier surface layer (22) or adjacent said moisture barrier surface layer (22).

16. Multilayer composite material comprising a fibre reinforced polymer composite sheet (20) as in any of the preceding claims 1 to 14, and on the face of the fibre reinforced polymer composite sheet (20) which do not form a moisture barrier surface layer (22), a stack of several layers, wherein each layer comprises impregnated fibres in a partially cured matrix (14) comprising a polymeric material as main component.

17. Multilayer composite material comprising a stack of a plurality of layers of fibrous materials, wherein each layer comprises impregnated fibres in a partially cured matrix (14) comprising a polymeric material as main component, and on each side of the stack a fibre reinforced polymer composite sheet (20) as in any of the preceding claims 1 to 14, the side of the fibre reinforced polymer composite sheet (20) forming a moisture barrier surface layer (22) on the exterior of the multilayer composite material.

18. Composite product comprising a multilayer composite material as in any of claims 15 to 17, where the matrix (14) comprises a thermosetting polymer which is cured.

19. Composite product according to the preceding claim forming a portion of an article of the following list : an automobile body part, in particular a door, a roof, a hood, a fender, a spoiler, a front or rear bumper, anaerodynamic kit; an automobile interior part, in particular a door cover, a dashboard, a centre console, a pillar trim, a trunk trim, a roof; a sport article, in particular a canoe, a kayak, a light boat hull, a rod saddle, a bicycle saddle, a bicycle frame, a bicycle handlebar, a baseball bat, a paddle, a ski or walking stick; an element of furniture or a part of the interior of an airplane, in particular a side panel, a ceiling panel, a luggage compartment; and an aerodynamic part of a light airplane, in particular an engine cowling, a wheel cowling; and an aerodynamic fairing of a mobile machine.

20. Composite product according to claim 18 or 19, wherein said composite product is a thin-walled reinforced composite product.

21. Manufacturing method for producing a fibre reinforced polymer composite sheet (20), comprising the following steps: i) providing flake-like elements (12) which are fluid-impermeable, ii) providing liquid matrix (14) comprising a polymeric material as main component, iii) forming a film (10) with at least said flake-like elements (12) and said matrix (14), iv) providing a reinforcement fibre fabric (16), v) superimposing said film (10) with said reinforcement fibre fabric (16), forming thereby a coupled assembly of layers, and vi) compressing the coupled assembly of layers to form said fibre reinforced polymer composite sheet (20), whereby the polymeric material impregnates the reinforcement fibre fabric, the flake-like elements (12) mixed in polymeric material creates a moisture barrier surface layer (22) in said fibre reinforced polymer composite sheet (20).

22. Manufacturing method of claim 21, wherein in step iii) the flake-like elements (12) are mixed with the matrix (14), forming a mixture (15), and said mixture is drawn to form said film (10).

23. Manufacturing method of claim 21, wherein in step iii) the matrix (14) is drawn to form a matrix film, and the flake-like elements (12) are scattered on said matrix (14) to form said film (10).

24. Manufacturing method of claim 21, wherein in step ii) the liquid matrix (14) is carried by a fibre carrier (18) and wherein in step iii) the film (10) is formed with said fibre carrier (18), said matrix (14) and said flake-like elements (12).

25. Manufacturing method of any of claims 21 to 24, wherein said matrix (14) and said flake-like elements (12) are transparent to visible spectrum.

26. Manufacturing method of any of claims 21 to 25, wherein said flakelike elements (12) are made from anyone of the following materials or a combination thereof: glass, metal oxide-coated glass, metal oxide-coated polymer, metal coated-polymer, metals, polymers and ceramics such as mica, malachite.

27. Manufacturing method of any of claims 21 to 26, wherein said flakelike elements (12) have a ratio between the average diameter L and the average thickness W, which is higher than 10 (L / W > 10).

28. Manufacturing method of any of claims 21 to 27, wherein said flakelike elements (12) have an average width comprised between 5 pm and 2000 pm, or between 50 pm and 500 pm, or between 100 pm and 200 pm, and an average thickness comprised between 0.1 pm and 5.0 pm, or between 0.1 pm and 1.0 pm.

29. Manufacturing method of any of claims 21 to 28, wherein said fibres are made from anyone of the following materials: vegetal natural fibres, animal fibres, glass fibres, carbon fibres and aramid fibres.

30. Manufacturing method of any of claims 21 to 29, wherein said polymeric material comprises a thermosetting polymer, such as epoxy resin, phenolic resin, polyester resin, polyfurfuryl alcohol, poly-urethane resin, cyanate ester resin or a combination thereof.

31. Manufacturing method of any of claims 21 to 29, wherein said polymeric material comprises a thermoplastic polymer.

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