WO1997043121A1

WO1997043121A1 - Thermoformable composite panel and method for making same

Info

Publication number: WO1997043121A1
Application number: PCT/EP1997/002430
Authority: WO
Inventors: Adriano Odino; Marco Colatarci; Didier Delimoy; Claude Dehennau
Original assignee: Solvay (Société Anonyme)
Priority date: 1996-05-16
Filing date: 1997-05-03
Publication date: 1997-11-20
Also published as: JP2000510062A; CN1225604A; AU2897197A; EP0898511A1

Abstract

A composite panel with at least three layers containing a thermoplastic material, including an inner layer (A) containing cellulose particles dispersed therein, and two side layers (B) arranged on either side of said inner layer, reinforced by reinforcing fibre fabrics and substantially free of cellulose particles, is disclosed. A method for thermoforming said composite panel, wherein said layers are simultaneously assembled and thermoformed without prior assembly of layers (A) and (B), is also disclosed.

Description

Thermoformable composite panel and process for its manufacture

The present invention relates to a composite panel based on thermoplastic material as well as a thermoforming method for manufacturing it.

In many industrial applications, such as for example the interior trim of land, sea or air vehicles, it is desirable to have composite panels which are light and inexpensive while having good mechanical properties, in particular as regards their impact and flexural strength. Preferably, these mechanical properties should not be excessively affected by temperatures of the order of 60 to 100 ° C., which are commonly reached near propulsion or heating devices, or even when a non-ventilated vehicle is exposed to the sun

It is also important to be able to easily give these panels various shapes adapted to their destination, for example so that some of their parts can serve as armrests, storage cavities, recesses for handles, etc. These panels must therefore be capable of undergoing thermoforming, even deep, without tearing.

There are commercially available plates of thermoplastic material loaded with wood particles, which are widely used in the automotive industry. Although these plates can in certain cases pose problems if they are subjected to an extremely deep thermoforming, it would be advantageous, given their low cost and their low density, to be able to use them as starting material for manufacturing composite panels presenting the desired properties.

To improve the mechanical properties of these plates, one can in particular consider reinforcing them with metal bars, ribs, etc., but these solutions are complex and prevent thermoforming of the panels comprising such reinforced plates.

Furthermore, good mechanical properties can be obtained by using plates of thermoplastic material comprising reinforcing fibers dispersed within them. However, these fibers generally increase the risk of tearing in the case of deep thermoforming, and these plates have a high density.

As an example, document DE 204831 1 describes a composite panel based on a thermoplastic material, comprising one or more felts (mats) or fabrics of reinforcing fibers, preferably textiles, as well as wood fibers uniformly dispersed throughout its thickness. This last characteristic is disadvantageous: in particular, it reduces the mechanical strength of the panel and increases the risk of tearing during its thermoforming. Without this explanation being limiting, it is suspected that the proximity, and a fortiori the contact, of the wood fibers with the reinforcing fibers could moreover harm the anchoring of the latter within the thermoplastic material, thus increasing the risk of tearing during deep thermoforming. In the document DE 2048311, thermoforming is also absolutely not mentioned. In addition, a single thermoplastic material is used for the entire panel, which notably excludes the use of an optimized material, for example less expensive, in the central area. Finally, the manufacturing process used in this document does not guarantee the uniformity of the thickness of each of the layers of the panel obtained. The present invention therefore aims to provide a lightweight composite panel, simple to manufacture, inexpensive, having good resistance to bending and impact, and capable of withstanding without tearing a thermoforming, even deep. It is further desirable that this panel can be manufactured in a simple manner from a plate of thermoplastic material reinforced with cellulosic particles, such as a WOOD-STOCK® plate.

More specifically, the invention relates to a composite panel comprising at least three layers based on thermoplastic material, among which an internal layer (A) containing cellulose particles dispersed therein, as well as two lateral layers (B), arranged on either side of said internal layer, reinforced with fabrics of reinforcing fibers and substantially free of cellulosic particles.

The thermoplastic material constituting each of the layers (A) and (B) essentially comprises one or more thermoplastic polymers, such as for example polyolefins, polyamides, fluorinated polymers or vinyl polymers. Very good results have been obtained with polyolefins such as homo- and copolymers of α-olefins, and in particular propylene. The copolymers optionally used advantageously comprise at least 70% by mass of propylene. Excellent results have been obtained with polypropylene (PP) homopolymer, that is to say comprising at least 99% by mass of propylene. Besides this or these polymers, the thermoplastic material may also comprise one or more usual additives such as stabilizers, lubricants, antioxidants, pigments, antistatic agents, compatibilizers, coupling agents, etc. The amounts of such additives can be any; they are generally moderate. Preferably, no additive is present in amounts exceeding 10% relative to the mass of the thermoplastic material.

The thermoplastics constituting the internal layer (A) and each of the two lateral layers (B) can be identical or different. Advantageously, a different thermoplastic material is used for the inner layer (A), in particular having less homogeneous or lower mechanical properties than the one or more thermoplastic materials constituting the side layers (B). It may in particular be a foam or, more advantageously, a less expensive recycled thermoplastic material. In the case where the thermoplastic materials constituting two adjacent layers are of different natures, it is desirable that the polymers included in these thermoplastic materials have sufficient mutual compatibility, so that it is not necessary to interpose an adhesive layer between these two layers. It can advantageously be different polyolefins. According to an advantageous variant, the thermoplastic material constituting each of the layers (A) and (B) comprises at least 50% by mass of one or more polyolefins (relative to the total mass of the polymers of the layer in question). Preferably, one or more propylene polymers are used as polyolefin (s). In a particularly preferred manner, the thermoplastic material constituting each of the layers (A) and (B) comprises at least 70% by mass of propylene, which may as well be contained in a homopolymer, in a copolymer or in a mixture of several homo - And / or copolymers.

Any type of cellulosic particle can be used in the internal layer (A), in particular sawdust, wood flour, wood fibers, particles of paper or cardboard, or vegetable fibers such as fibers. flax, cotton or bamboo, straw waste, and mixtures thereof. These particles preferably have average dimensions of about 0.1 to 3 mm. It is desirable that their water content does not exceed 15% by mass. In order to improve the adhesion of the cellulosic particles to the thermoplastic material constituting the internal layer (A), it may be useful to make them compatible, by example by addition of a small amount of compatibilizing agents such as unsaturated organosilanes (vinyltriethoxysilane, gamrnamethacryloxypropyltrimethoxysilane, etc.), as well as possibly one or peroxides. The effect of such compatibilizers can be further increased by the joint use of small amounts of suitable crosslinking agents, for example poly- tri-, tetra- or penta-acrylates. Another compatibilization method consists in using a thermoplastic material comprising one or more polymers modified so as to exhibit an increased affinity with respect to cellulosic particles, such as a polyolefin grafted with maleic anhydride.

The concentration of the cellulosic particles within the internal layer (A) is generally at least 30 parts by mass (relative to 100 parts by mass of thermoplastic material), preferably at least 70 parts. Furthermore, this concentration is generally at most 250 parts, and preferably at most 150 parts.

For example, very good results have been obtained using as internal layer (A) PP plates loaded with wood particles sold under the brand WOOD-STOCK® by the company GO R. Applicazioni Speciali. Each of the two lateral layers (B) comprises one or more fabrics of reinforcing fibers, which can be of any known type. It is preferred to use inorganic fibers, for example carbon, glass or metal fibers. Very good results have been obtained with glass fiber fabrics. These fibers have a long length, generally several decimeters; their length often corresponds at least to the length or width of the panel. These fibers are advantageously compatible, or made compatible, with the thermoplastic material; to this end, they can in particular be provided with an appropriate size, for example based on silanes. Several different types of fibers can be used together. It is preferable that the fiber fabrics used have a structure favoring their anchoring within the thermoplastic material. One can in particular, for this purpose, use open mesh fabrics, so that the thermoplastic material can penetrate there. The side layers (B) can also be made from fabrics of reinforcing fibers and fibers of thermally plastics material. Further details on the fabrication of layers (A) and (B) are provided below, in relation to the process. Whatever the mode of manufacture of the lateral layers (B), their constituent thermoplastic material is preferably homogeneous; in particular, it does not have a fibrous structure, even in the case of layers (B) made from fabrics co-mixed with reinforcing fibers and thermoplastic fibers.

The use of reinforcement fiber fabrics leads to higher mechanical performance than other types of fiber-based reinforcement such as uniformly dispersed short fibers or felts (mats). In addition, it greatly reduces the risk of tearing the panel in the event of deep thermoforming. This result is surprising insofar as the deformability of an article based on thermoplastic material reinforced with a fiber fabric is a priori considered to be lower than if this article was reinforced with short fibers, for example.

The concentration of reinforcing fibers within each of the side layers (B) is generally of the order of 10 to 70% (relative to the total weight of each of the layers (B)). When this concentration is expressed relative to the mass of the entire panel, we arrive at significantly lower values than in panels uniformly loaded with fibers, for comparable mechanical properties, which leads to a specific mass and a lower material costs.

As explained previously, the two lateral layers (B) are substantially free of cellulosic particles, that is to say that they contain less than 5% by mass thereof. Ideally, these layers are completely free of cellulosic particles. In addition to the constituents mentioned above, the layers (A) and / or (B) can optionally contain one or more conventional inorganic fillers such as calcium carbonate, talc, etc.

The thicknesses of the layers (A) and (B) can be freely chosen according to the requirements imposed on the composite panel. The thickness of the inner layer (A) is generally at least 1 mm. It is generally at most 4 mm. Furthermore, the thickness of each of the lateral layers (B) is generally at least 0.1 mm. It is also generally at most 0.5 mm.

According to an advantageous variant, the ratio between the thickness of the central layer (A) and that of each of the lateral layers (B) is between 2 and 40. In addition to the internal layer (A) and the two lateral layers (B), the composite panel of the invention may possibly comprise one or more other layers of any material, provided that their presence does not disturb the thermoforming of the panel. These additional layers are however preferably also based on thermoplastic material. It may in particular be a thin decorative layer of thermoplastic material applied to at least part of the outer surface of one of the side layers (B), or of each of the two side layers (B). Such decorative layers can for example be made from PVC or polyolefins, optionally in the form of a foam layer with a closed surface, and can optionally be grained or textured. If necessary, a layer of adhesive can be interposed between a side layer (B) and any adjacent surface decorative layer.

The lateral layers (B) are not necessarily immediately adjacent to the internal layer (A). Furthermore, one or more other layers may optionally be arranged on the side of a side layer (B) opposite the side where the inner layer (A) is located, that is to say closer to one of the two exterior surfaces of the panel. Thus, for example, composite panels having the following structures could meet the definition of the invention B / A / B, D / B / A / B / D, D / B / A / B, B / D / A / B, B / D / A / D / B, D / B / D / A / B (D representing any layer, or even several layers whatever). For reasons of mechanical strength, it is however desirable that the side layers (B), and in particular the reinforcing fiber fabrics which they contain, are not too far from the exterior surfaces of the panel. It is preferred that the distance (measured perpendicular to the thickness of the panel) separating the center of the thickness of each side layer (B) from the nearest outer surface of the panel does not exceed the distance separating the centers of the thickness of layers (A) and (B).

The composite panels of the invention can in particular be used in the interior trim of vehicles, for example for manufacturing dashboards, rear panels, door trim, bodywork elements, etc.

The panels can be manufactured by any known process, continuous or discontinuous, in particular by rolling or hot pressing of the different layers In the case where the panels are produced from thermoplastic sheets reinforced with cellulosic particles such as sheets WOOD-STOCK, the manufacture of the composite panels of the invention can advantageously be carried out in line with the manufacture of said plates, which avoids having to heat them. Another particularly advantageous method of manufacturing the panels is described below. Because of the advantages offered by the panels of the invention when thermoformed, the invention also relates to a panel as described above, thermoformed. The term “thermoformed panel” is intended to denote a panel which has been shaped so that at least one of its parts undergoes a deformation which, measured perpendicular to the mean plane of the panel, is at least 2 times its thickness, and in particular at least 10 times its thickness. Another aspect of the present invention relates to a particular method allowing the manufacture of a composite panel as described above. According to the previously known methods, a first manufacturing step makes it possible to obtain a flat composite panel, for example by rolling or hot pressing of different layers. This panel can then be thermoformed in a second step. Thermoforming is therefore applied to a monolithic composite panel, sometimes thick, which generally leads to tearing in the event of deep thermoforming. In general, thermoforming consists in placing the panel between two half-molds each having the shape of the article which it is desired to obtain, and applying by means of these a high pressure to the panel, after having heated it. . The two half-molds are generally qualified as male and female half-molds respectively, according to their shape.

The present invention also aims to provide a simple method of manufacturing composite panels as defined above, which makes it possible to assemble their different layers and to thermoform them, even deeply, simultaneously, without causing tearing.

More specifically, another object of the present invention relates to a process for thermoforming a composite panel comprising at least three layers based on thermoplastic material, among which an internal layer (A) containing cellulosic particles dispersed therein, as well as two lateral layers (B), arranged on either side of said internal layer, reinforced with fabrics of reinforcing fibers and substantially free of cellulosic particles, according to which these different layers are assembled and thermoformed simultaneously, without prior assembly layers (A) and (B). This process combines assembly and thermoforming, which leads to saving time and energy, since only one heating is enough to assemble the different layers and simultaneously thermoform the composite panel thus obtained.

The fact that the assembly and the thermoforming of the different layers are simultaneous means that these two operations take place at the same time - and therefore necessarily in the same mold - for each of the zones of the panel, considered however in isolation from each other. This does not therefore exclude that the layers are assembled and thermoformed in certain areas of the panel before others, which is particularly the case when using a mold comprising one or more retractable parts, initially protruding, making it possible to assemble and simultaneously thermoform the layers in one or more strongly deformed zones of the panel before proceeding to their assembly and simultaneous thermoforming in the other zones of the panel.

Other advantages of this process are linked to the properties of the composite panels which it makes it possible to obtain. Thus, surprisingly, it has been found that this method leads to panels having a significantly better surface finish than the known methods. In addition, it has been found that carrying out the implementation without prior joining of the layers (A) and (B) considerably reduces the risks of tearing during thermoforming, in particular in the case of deep thermoforming. By deep thermoforming is meant a thermoforming such that at least part of the panel undergoes during thermoforming a deformation which, measured perpendicular to the mean plane of the panel, is at least 5 times its thickness, and in particular at least 15 times its thickness. The method is especially advantageous when the slope of at least one zone of the edge of the deformed part forms an angle of at least 30 ° relative to the mean plane of the panel.

Generally, before proceeding to thermoforming, the various layers are placed in a metal frame, in such a way, however, that their relative sliding is possible. To this end, it is possible, for example, to use a frame comprising two similar parts which are placed on either side of the different layers stacked on top of each other, so as to pinch and stretch them while subjecting them to a definite pressure on their periphery. This pressure must be high enough so that the different layers are taut and substantially flat, but low enough to allow displacement of the layers or of some of their parts if necessary, to avoid any tearing. One can in particular use a frame provided springs or cylinders arranged at regular intervals on its periphery, so as to exert a determined pressure on the periphery of the stack of layers. It is desirable that the different layers have dimensions greater than those of this frame, and are arranged therein so as to extend beyond it, so that they always remain pinched by the frame even after a slight lateral movement, if any. It is moreover obvious that the frame used must have dimensions greater than those of the mold, so as not to prevent its closing.

For thermoforming, the different layers can be preheated together, after having been superimposed, or else separately, before being superimposed.

According to a first alternative embodiment, the stacking of the different layers, secured by a frame as described above or by any equivalent device, is then placed in an oven, for example infrared, in which it is heated beyond of the processing temperature of the thermoplastic which has the highest processing temperature. For a semi-crystalline thermoplastic material (polyolefins, etc.), this temperature is slightly higher than its melting temperature (Tf); for an amorphous thermoplastic material (PVC, etc.), it is generally of the order of Tg + 100 ° C., T „designating its glass transition temperature. For example, when each of the layers (A) and (B) is based on PP (Tf ≈

160 ° C), the stack is heated to a temperature of about 190 ° C (± 20 ° C).

Although the different layers are not assembled before their thermoforming, they are stacked on top of each other, and their simple contact is generally sufficient to ensure the heating of the internal layer through the external layers The choice of infrared radiation the appropriate wavelength also makes it possible to influence its penetration into the layers of thermoplastic material. It can also be useful to confer on the different layers different absorption powers by means of pigments and / or of suitable charges. Thus, for example, very good results have been obtained using non-pigmented side layers and an inner layer (A) of dark color.

You can also preheat the inner layer (A) separately, then add the other non-preheated layers: since these are generally thinner, their simple contact with the inner layer will generally bring them to the desired temperature.

Then, the preheated stack is quickly introduced into a mold thermoforming, where the layers are simultaneously assembled and thermoformed by applying high pressure to them. Generally, the mold is cooled, and the panel is allowed to cool therein under pressure to a determined temperature, for example up to 50 ° C., before extracting it. The duration of this pressing depends in particular on the thickness of the panel.

According to another variant, the heating and the thermoforming can both take place in the mold. Thus, for example, the stack of different layers can be placed directly, without preheating, between two half-molds fixed to the two jaws of a heating press. These jaws, and therefore the mold, are first partially closed, so that the two half-molds come into contact with the stack. The mold is not completely closed until the stack has reached the temperature necessary for thermoforming. It goes without saying that the residence time in the heating press will require in this case a much longer duration than in the previous case, generally of the order of several minutes, in order to allow each of the layers to reach the necessary temperature. thermoforming. Although after thermoforming, the panel can possibly be cooled in the same press, this particular variant would require the press to alternate heating and cooling, which would increase the duration of the process. It is therefore preferable, when this variant is chosen, to use a heating press and a cooled press. In this case, once the thermoforming proper is completed, the mold is detached from the jaws of the heating press, without opening it, and it is installed in the cooled press, where the panel it contains can cool under pressure. As explained above, the layers (A) and (B) are not joined before their thermoforming, neither directly nor indirectly, but are simply stacked on top of each other. Regarding any additional layers, they can either be pre-assembled with one of the layers (A) and (B), or else be previously joined to any other layer. In some cases, for example, we may have side layers (B) already coated with a thin decorative layer (D): in this case, we can assemble and thermoform in one step the 3 "layers" (D + B), A and (B + D).

According to an advantageous variant, the method is carried out in a thermoforming mold comprising a male half-mold and a female half-mold, and a vacuum is created in the female half-mold during at least part of the thermoforming; preferably, the pressure is reduced to a value of 0.05 to 0.02 MPa. According to this variant, the stack of layers is deposited on the female half-mold, in contact with its entire periphery, which defines a closed volume as far as the shape of the female half-mold lends itself to it. We then put this volume in depression. The vacuum thus applied generally does not in itself cause the application of the stack of layers against the entire surface of the female half-mold. However, it makes it possible to avoid that only the mold exerts pressure on the stack of layers, and to make this pressure more homogeneous; in fact, the pressure exerted by the mold is generally very localized, in particular at the edges of the deformed zones, which increases the risks of tearing in these places. The vacuum can in particular be obtained by providing in the female half-mold one or more very small diameter openings which are connected to an external vacuum pump. This variant reduces the risk of tearing the panel and makes it possible to obtain a surface quality of surprising quality. According to a simple variant, both the internal layer (A) and the lateral layers (B) are manufactured separately, in steps prior to thermoforming, and are each in the form of homogeneous sheets of thermoplastic material comprising different reinforcements or fillers .

The internal layer (A) can be produced in a manner known per se, for example by the extrusion of a thermoplastic material with which the desired quantity of cellulose particles has been mixed, by means of an extruder provided with a flat die, possibly followed by a grille.

The side layers (B) can also be produced by any suitable technique, for example by impregnating a fabric of fibers with a molten thermoplastic material, or by laminating a fabric of fibers between two sheets of heated thermoplastic material. . The lateral layers (B) can also be produced from one or more fabrics of reinforcing fibers and fibers of thermoplastic material ("knitted fabrics"), such as for example the composite polypropylene-glass fibers TWINTEX® from VETROTEX. In fact, by bringing such fabrics mixed together at a temperature higher than the melting temperature of the fibers of thermoplastic material which they contain, by pressing them then by cooling them (in short, by "consolidating" these fabrics), one also obtains homogeneous sheets of thermoplastic material reinforced with reinforcing fiber fabrics. In plates of this type obtained from mixed fabrics, the anchoring of the reinforcing fibers within the thermoplastic material is particularly good. According to another advantageous variant, the lateral layers (B) are not used in the form of such homogeneous plates, but are directly used in the form of fabrics made up of reinforcing fibers and fibers of thermoplastic material, such as described above, without prior step of consolidation of these fabrics into homogeneous plates (that is to say plates of which the constituent thermoplastic material is homogeneous). The use of such mixed fabrics leads to additional advantages: in particular, it makes it possible to carry out even deeper thermoformings without the risk of tearing. In addition, the panels thus obtained are more homogeneous.

In preferred combed fabrics, the proportion of reinforcing fibers is 30 to 70% relative to the total weight of these fabrics. Examples

The following examples illustrate, without limitation, the operation and the advantages of the panels of the invention and their manufacturing process. Example 3 is in accordance with the invention, and Examples 1R, 2R and 4R are given for comparison. Examples 1R to 4R - Properties of different composite panels

We made 4 different composite panels, by a thermoforming process in two stages: the heating and the thermoforming proper in a first press, heated, and a cooling under pressure in a second, cooled press, in which we transfer the mold containing the panel after thermoforming. The operating conditions were as follows:

The different panels measured 250 x 350 mm, and had the following structure: (1R) WOOD-STOCK plate (marketed by G. O R. Applicazioni Spécial.), made of PP loaded with 50% by mass of wood particles, 2.5 mm thick.

(2R) panel obtained by hot pressing of a stack of 4 layers of balanced fabrics (that is to say comprising the same number of fibers per unit of length in each of the two main directions) of fibers streaked with glass and of PP. These fabrics have a surface mass of 600 g / m 2 and a mass content of glass fibers of 60%.

(3) panel according to the invention, consisting of a central core which is a WOOD-STOCK plate 1.6 mm thick (content by weight of wood fibers

: 50%), trapped between two lateral layers with a thickness of 0.35 mm, made from knitted fabrics as described above.

(4R) AZDEL® PM 10400 plate, consisting of a polypropylene plate reinforced with an isotropic felt (mat) of long glass fibers.

The composite panels thus produced had the following properties. The impact resistance was evaluated by means of a non-notched Charpy test (standard

ISO 179 (1993)).

It is clearly noted that the panels according to the invention have a low specific mass (D), good impact resistance, as well as a high flexural modulus (in particular when expressed by unit of mass - cf. reports F20 / D and F] θf / R), ^and relatively independent of temperature (see the ratio F100 / ^{F20), despite} a relatively low content of glass fibers (23%). Examples 5 - Thermoforming

We install successively in a frame

- a fabric of fibers streaked with glass and polypropylene as described above,

- a 2 mm thick WOOD-STOCK plate, and - another layer of the same fabric of ribbed fibers

The stack of the three layers is heated by infrared radiation to a temperature of 190 ° C., and introduced into a thermoforming mold conditioned at a temperature of 30 ° C. ± 10 ° C., the male part of which is fixed to the upper plate. of a press, has a frustoconical protrusion 22 cm in diameter (at the base) and 55 mm high, with a draft angle of 5 °, and the female part of which, fixed to the lower plate of the press, comprises a correspondingly shaped cavity. Pressing is carried out under a pressure of 1.6 MPa. At the opening of the mold, we notice that the internal layer (WOOD-STOCK plate) is not torn and adheres to the 2 lateral layers; the panel obtained does not exhibit any abnormal deformation (other than that targeted). Comparative examples 6R and 7R - Thermoforming

Example 5 is repeated using exclusively a WOOD-STOCK plate, without any side layer (with preheating at 175 ° C instead of 190 ° C). The panel obtained after thermoforming is torn. Example 5 is repeated using exclusively an Azdel plate identical to that used in Example 4R, without any side layer (with preheating to 190 ° C). The panel obtained is also torn.

Claims

R E V E N D I C A T I O N S

1 - Composite panel comprising at least three layers based on thermoplastic material, among which an internal layer (A) containing cellulosic particles dispersed therein, as well as two lateral layers (B), arranged on either side of said inner layer, reinforced with reinforcing fiber fabrics and substantially free of cellulosic particles

2 - Panel according to claim 1, wherein the thermoplastic material constituting each of the layers (A) and (B) comprises at least 50% by mass of one or more polyolefins

3 - Panel according to one of the preceding claims, wherein the thermoplastic material constituting each of the layers (A) and (B) comprises at least 70% by mass of propylene

4 - Panel according to one of the preceding claims, wherein the reinforcing fiber fabrics are glass fiber fabrics

5 - Panel according to one of the preceding claims, in which the thermoplastic material constituting the lateral layers (B) is homogeneous

6 - Panel according to one of the preceding claims, thermoformed

7 - Process for thermoforming a composite panel comprising at least three layers based on thermoplastic material, including an internal layer (A) containing cellulosic particles dispersed therein, as well as two side layers (B), arranged on the side and on the other side of said internal layer, reinforced with fabrics of reinforcing fibers and substantially free of cellulosic particles, according to which these different layers are assembled and thermoformed simultaneously, without prior assembly of layers (A) and (B)

8 - Method according to the preceding claim, wherein at least part of the panel undergoes during thermoforming a deformation which, measured perpendicular to the mean plane of the panel, is at least 5 times its thickness. 9 - Method according to one of claims 7 or 8, wherein the method is carried out in a thermoforming mold comprising a male half-mold and a female half-mold, and a depression is created in the female half-mold during minus part of thermoforming.

10 - Method according to one of claims 7 to 9, wherein the side layers (B) are directly used in the form of fabrics mixed with reinforcing fibers and fibers of thermoplastic material, without prior consolidation step of these homogeneous plate fabrics.