EP3938162A1

EP3938162A1 - Process for manufacturing a preimpregnated fibrous reinforcement from a thermoplastic nonwoven and a natural plant fiber reinforcement, and preimpregnated fibrous reinforcement obtained

Info

Publication number: EP3938162A1
Application number: EP20709228.9A
Authority: EP
Inventors: Thibault ROUMIER; Floran PIERRE; Karim Behlouli
Original assignee: Lineo
Current assignee: Lineo
Priority date: 2019-03-12
Filing date: 2020-03-12
Publication date: 2022-01-19
Also published as: FR3093669B1; WO2020182959A1; FR3093669A1

Abstract

Process for manufacturing a preimpregnated fibrous reinforcement (1) composed of a reinforcement made of natural plant fibers (2) impregnated with a thermoplastic polymer (3), comprising the steps: placing a first strip (4) of reinforcement made of natural plant fibers (2) and a second strip (5) of thermoplastic nonwoven (3) with a melting temperature Tm in contact with one another; said strips (4,5) are conveyed to first a heating zone (9) maintained at a temperature T1 below Tm; said strips (4,5) are conveyed to a second heating zone (11) maintained at a temperature T2 above Tm; said complex (13) is pressed and is conveyed to a cooling zone (15) in order to obtain said reinforcement.

Description

METHOD OF MANUFACTURING A PRE-IMPREGNATED FIBROUS REINFORCEMENT FROM A THERMOPLASTIC NON-WOVEN AND A FIBER REINFORCEMENT

NATURAL PLANTS, AND PRE-IMPREGN FIBROUS REINFORCEMENT OBTAINED

The present invention relates to a method of manufacturing a composite material.

More particularly, the present invention relates to a process for manufacturing a fibrous material prepreg with thermoplastic polymer, designated by the term prepreg fibrous reinforcement.

The invention also relates to a prepreg fiber reinforcement capable of being obtained by carrying out the process of the present invention.

This prepreg reinforcing material obtained by implementing the process of the present invention consists, on the one hand, of a thermoplastic nonwoven, preferably of synthetic polymer of polypropylene (PP) type or of polylactic acid ( PLA), or any type of thermoplastic polymer, and, on the other hand, by a reinforcing web of natural vegetable fibers, short or long, for example in linen.

In the state of the art, the manufacture of a thermosetting prepreg material obtained by hot calendering, between two rollers, of unidirectional reinforcements made of natural fibers, or even by integral impregnation of a layer of fibers in a thermosetting resin bath, followed by drying.

Thus, it was imagined, to impregnate unidirectional reinforcements in natural fibers to implement the technique of “film stacking”, consisting of heating and simultaneous compression of alternating layers, namely at least one layer of a polymer film. and at least one layer of dry fibers.

For example, this operation can be carried out by hot calendering of a thermoplastic polymer film, between two cylinders or rolls heated and driven in rotation.

This process has the advantage of being easy to implement, in particular as regards hot calendering. In addition, the films thermoplastic polymers being particularly widespread on the market, this technique is particularly economical.

Thus, the film stacking technique has been particularly widespread for many years.

However, this technique has the disadvantage of being quite restrictive for the impregnation of reinforcements made of natural fibers which may contain residual moisture.

Indeed, this residual moisture not evacuated during the compression by calendering, for example, is responsible for the formation of bubbles or microbubbles, in the final material, resulting in the creation of porosity, the bubbles being responsible for aesthetic problems on the final material, which then does not have a smooth and homogeneous surface, but also, and above all, has degraded mechanical characteristics.

It should also be noted that the melt flow index (or MFI for “Melt Flow Index”) of thermoplastic films is generally considered to be particularly low, with a maximum of the order of 8 g / 10 min, knowing that the melt index. fluidity is defined as the mass of preheated polymer, in grams, flowing in 10 minutes through a capillary of specific diameter and length, under the pressure of a standard weight piston. Such a low melt index implies that the resin, after melting the polymer, will not permeate the natural fiber backing optimally.

In other words, the lower the melt index of a polymer, the less fluid the resin will be and, consequently, the less well the reinforcement will impregnate.

In an attempt to overcome, at least in part, the drawbacks of film stacking, the method of sprinkling a thermoplastic polymer powder on the reinforcement of natural fibers, combined with hot calendering, is increasingly widespread.

In fact, such a technique allows, compared to film stacking, an improvement in the impregnation of the reinforcement, due in particular to the melt index of the powder, of the order of 50 g / 10 min, which is substantially greater than the melt index of the thermoplastic film. Residual moisture from natural fibers is also more easily removed during compression. However, moisture wicking can be further improved to achieve optimum final material quality.

It should also be noted that the sprinkling method remains a relatively expensive technology, as it requires significant investments, in particular in the purchase of a sprinkling and / or calendering module, for companies not having previously equipped them.

In the state of the art, also known from the patent document published under the number WO 2017/134356, a method for producing three-dimensional preforms of composite material, by forming initial preforms, with webs.

More particularly, in the field of manufacturing parts made of composite material, it is known to manufacture a three-dimensional preform, by first producing an initial preform of several layers, or folds, of fibers which are superimposed, followed by a step of forming the initial preform in a press, generally hot, in order to obtain, finally, a three-dimensional preform of the desired shape. This will then be used for the manufacture of a composite part.

To avoid the pleating of the layers of fibers during the production of complex shapes, a method is proposed here for producing a three-dimensional preform in which the initial preform, comprising several superimposed plies and each consisting of unidirectional continuous fibers and a first polymer, is then, during forming, sandwiched between two external webs, arranged at the interfaces between said initial preform and the male tool and between this preform and the female tool.

The outer webs are made of a second polymer, different from the first, thermoplastic or thermosetting, preferably polyamide, and remain solid while deforming during forming, this step being carried out at a forming temperature below the melting temperature. of said second polymer, while the first polymer constituting the plies of the initial preform is liquid or viscous at the forming temperature.

However, if such a technology seems to have a certain efficiency in avoiding the pleating of the layers of fibers during the production of complex shapes, the aim here is to manufacture a complex three-dimensional preform, requiring the implementation of different stages in a long process to implement, while the problem of the evacuation of residual moisture in the manufacture of pre-reinforcements. impregnated is not resolved.

Indeed, in this document of the state of the art, if it is mentioned the production of so-called dry preforms, comprising a certain proportion of binder allowing a subsequent impregnation operation to form a composite part, or the production of an initial preform from fibers pre-impregnated with a first thermoplastic or thermosetting polymer, no indication is given on the production of such fibers exhibiting optimum quality, both in terms of mechanical and aesthetic characteristics.

Also known from French patent application FR 2 939 069 is a process for preparing a tape of reinforcing threads or filaments, having a given width that is substantially constant over its entire length, associated, on each of the faces of said tape. , to a polymeric binder.

In this process, the first step is to adjust the width of the tape to the desired width, by means of calibration means, before associating the tape, on each of its faces, with a polymeric binder making it possible to ensure homogeneous cohesion of the tape, so that the total mass of the binder does not exceed 25% of the total mass of the tape obtained.

Said binder affixed to both sides of the reinforcing tape is in the form of a powder of one or more thermoplastic and / or thermosetting polymers or of a nonwoven or of a fabric made of one or more thermoplastic polymers and / or thermosets.

In this document, the tapes are made of carbon fibers or other mineral material, ceramic, glass, silica, basalt or aramid, and the problem of the evacuation of moisture from a reinforcing veil of natural plant fibers is not not resolved.

In addition, a proportion of binder, less than 25% of the total mass of the tape obtained, serves to consolidate the web of carbon fibers, and does not allow not a good impregnation of said fibers. The carbon web is thus made easy to handle, but requires additional impregnation to obtain a ready-to-use composite material.

In the international application WO 2007/1 10660, a process is described for manufacturing composite products, in which a liquid phenolic or polyester resin containing at least one catalyst is mixed with vegetable fibers dried at 100 ° C for 1 h in an oven. , before adding the resin mixed with a catalyst to the fibers, or at 60 ° C for 3 hours in an oven, after the fibers have been soaked in the resin mixed with a catalyst.

The resulting mixture is then compacted at high temperature to obtain a composite product.

The removal of residual moisture contained in the plant fibers here is likely not to be optimal, especially in the case where the fibers are soaked in the phenolic resin before the drying step.

In addition, regardless of when the drying takes place, before or after adding the resin / catalyst mixture to the fibers, this step requires the use of an oven and this step is time consuming; in fact, at least 1 hour and up to 3 hours are necessary to remove residual moisture.

Another drawback of this process is that the impregnation of the plant fibers with the phenolic resin is likely not to be homogeneous at all points of the composite product finally obtained, so that the mechanical characteristics of the product will not be the same throughout. surface of it.

Thus, the invention offers the possibility of alleviating, at least in part, the various drawbacks of the state of the art by proposing a process for manufacturing pre-impregnated natural vegetable fibers exhibiting optimum impregnation of the reinforcement as well as mechanical characteristics. and optimal aesthetics for the subsequent manufacture of a high quality composite material, which process has allowed residual moisture to be evacuated from natural plant fibers.

In particular, the final product obtained by implementing the process exhibits particularly advantageous stress / stiffness values as well as an absence of very low porosity or porosity. To this end, the present invention relates to a method of manufacturing a prepreg fiber reinforcement composed of a reinforcement of natural plant fibers impregnated with a thermoplastic polymer, said prepreg fiber reinforcement being intended to be used in the manufacture of a part made of composite material, said method being characterized in that it comprises, at least, the following steps:

- Using a first reinforcing strip of natural plant fibers and at least one second strip of thermoplastic nonwoven, the melting point of which is denoted TF;

- Said first reinforcing strip of natural plant fibers and said second strip of thermoplastic nonwoven are placed in contact with each other;

- Said first strip and said second strip are conveyed to the level of a first heating zone within which a temperature T1 lower than the melting temperature TF is applied, for the evacuation of the residual humidity of the first strip of reinforcement in natural fibers;

- Said first strip and said second strip are then conveyed, in contact with one another, at a second heating zone at which a temperature T2 is applied higher than the melting temperature TF, for the melting said second strip of thermoplastic nonwoven and obtaining a complex consisting of natural plant fibers and molten thermoplastic nonwoven;

- said complex is pressed;

- said complex is conveyed to the level of a cooling zone to finally obtain said prepreg fiber reinforcement;

- optionally, said prepreg fiber reinforcement is wound in the form of a roll.

In a first variant of the process of the invention, a single strip of thermoplastic nonwoven is used.

In another variant of this process, a second strip and a third strip of thermoplastic nonwoven are used, one coming on a first, upper face of said first reinforcing fiber strip. natural plant fibers and the other coming on a second, lower face of said first reinforcing strip of natural plant fibers.

Most preferably, a temperature T1 is applied within said first heating zone of between 60 and 250 ° C.

With regard to said second heating zone, a temperature T2 of between 100 and 280 ° C. is applied to it, advantageously.

Interestingly, a pressure is applied during the pressing step of between 10 and 150 N / cm ² , preferably equal to 80 N / cm ² .

As regards said first reinforcing strip of natural vegetable fibers, long or short, preferably flax fibers or jute fibers or hemp fibers or nettle fibers or kenaf fibers, or a mixture of these.

According to another feature of the invention, a second strip of thermoplastic nonwoven fabric is used, made from polypropylene and its derivatives, or polylactic acid (PLA) and its derivatives, or polyamide and its derivatives, or high or low density polyethylene and its derivatives, or polyester and its derivatives, or a mixture of several of these thermoplastic compounds.

According to a very preferred exemplary embodiment, a first reinforcing strip of natural plant fibers comprising unidirectional long flax fibers and a second strip of thermoplastic nonwoven made from polypropylene is used.

The present invention also relates to a prepreg fiber reinforcement capable of being obtained by implementing the aforementioned manufacturing process, said reinforcement being characterized in that it is composed of a first reinforcing strip of natural plant fibers. and at least a second strip of thermoplastic nonwoven.

Most preferably, the prepreg fiber reinforcement according to the invention consists only of a first reinforcing strip of natural plant fibers and of a second strip of thermoplastic nonwoven.

Advantageously, said natural plant fibers of said reinforcing strip of the prepreg fiber reinforcement are flax fibers, while the thermoplastic material of said thermoplastic nonwoven web is polypropylene.

The prepreg fiber reinforcement of the invention preferably comprises a proportion of thermoplastic nonwoven greater than or equal to 30% and less than or equal to 80% by mass relative to the total mass of the reinforcement, while the volume ratio Vf% of natural plant fibers in this reinforcement is between 15 and 70% by volume.

Even more preferably, the proportion of thermoplastic nonwoven in this reinforcement is between 35 and 40% by mass, relative to the total mass of the reinforcement, while the volume ratio Vf% of natural plant fibers is between 45 and 55 % of the volume of the reinforcement.

The invention also relates to a part made of composite material comprising at least one such prepreg fiber reinforcement and a core made of honeycomb, cardboard, foam or thermoplastic polymer.

The method which is the subject of the present invention has many advantages, and in particular that of providing a material of the reinforcement type made from natural plant fibers, short or long, in particular from flax fibers, homogeneously and optimally pre-impregnated with a polymer. thermoplastic, of a higher quality than the quality of prepreg substrate currently obtained by the implementation of known processes.

This improvement in quality results in particular from better evacuation of residual moisture from natural plant fibers at the time of heating, by the use of a layer of thermoplastic nonwoven associated with the layer of natural fibers, resulting in a optimum and homogeneous impregnation of these fibers by the polymer in the form of a strip of thermoplastic nonwoven material.

In addition, the prepreg fiber reinforcement according to the invention has optimal mechanical qualities such that it can constitute a composite material directly ready for use, or else be combined with another element to form a part made of composite material. .

Other characteristics and advantages of the invention will emerge from the detailed description which will follow of the non-limiting embodiments of the invention, with reference to the single appended figure described in more detail below:

- Figure 1 shows, schematically, and in a side view, the different steps of a particular and preferred embodiment of the manufacturing process of a prepreg fiber reinforcement of the invention.

With reference to the appended FIG. 1, the subject of the present invention is in particular a process for manufacturing a prepreg fiber reinforcement 1.

According to the invention, said prepreg fiber reinforcement 1, obtained by implementing the present process, is composed at least, on the one hand, of a reinforcement of natural plant fibers 2 and, on the other hand, of 'a thermoplastic polymer 3, of which said reinforcement of natural plant fibers 2 is intended to be impregnated during the process of the invention.

For this purpose, in a first step of the present process for manufacturing a prepreg fiber reinforcement 1, a first strip 4 of such a reinforcement made of natural plant fibers 2 and at least a second strip 5 of a material are used. thermoplastic nonwoven 3, the latter having a defined melting point TF.

As a preliminary remark, a particularly preferred embodiment of the method of the invention will be described below, with reference to FIG. 1, in which only a second strip 5 of a thermoplastic nonwoven material 3 is used, coupled to the reinforcement band 2 in natural vegetable fibers.

This being the case, it is also conceivable, in another variant embodiment, to use two strips 5 of a thermoplastic nonwoven material 3, advantageously of the same nature, but which can also be made from different thermoplastic nonwoven materials. , said two strips of thermoplastic nonwoven 3 then sandwiching the reinforcing strip of natural plant fibers 2. The latter then constitutes the core while the strips of thermoplastic nonwoven on one side and on the other of this core of natural vegetable fibers constitute the skins. It should also be noted that, within the meaning of the present invention, by thermoplastic nonwoven material is meant a material in the form of a web, a sheet or a mat of thermoplastic fibers arranged randomly and held together. different ways.

The thermoplastic nonwoven material 3 used in the present invention therefore has a certain porosity, in particular compared to the thermoplastic films or fabrics, therefore non-porous, used in the processes for manufacturing composite materials pre-impregnated with l state of the art.

Preferably, this thermoplastic nonwoven material 3 is made from polypropylene (PP) and its derivatives, or polylactic acid (PLA) and its derivatives, or polyamide (nylon) and its derivatives, or polyethylene high or low density and its derivatives, or polyester and its derivatives or even a mixture of these thermoplastic compounds or any other thermoplastic polymer.

Even more preferably, the thermoplastic nonwoven material 3 is made from polypropylene.

As regards now the reinforcement 2 made of natural vegetable fibers, in a particularly advantageous embodiment, the latter is based on natural vegetable fibers, short or long, of flax, or of jute fibers, or of hemp, or nettle fibers, or kenaf fibers, or any other type of natural plant fibers potentially usable in the composite field, or else based on a mixture of these aforementioned natural plant fibers.

Most preferably, in the invention, use is made of a reinforcement made of natural vegetable flax fibers 2, the fibers of which are advantageously long, having a length of for example between 1 and 60 cm, preferably between 10 and 40 cm and, more more preferably still substantially equal to or equal to 30 cm and 100% oriented, in other words unidirectional natural flax fibers.

In a second step of the method which is the subject of the present invention, said first reinforcing strip 4 of natural plant fibers 2 is placed and said second strip 5 of thermoplastic nonwoven 3 in contact with one another.

In an exemplary embodiment of said method, the first 4 and second 5 bands are brought into contact by a device 6, illustrated in FIG. 1, and allowing automated implementation of the method of the invention.

Such a device 6 preferably comprises, among other things, at least, means 7 for unwinding a first reinforcing roll of natural plant fibers 2 as well as means 8 for unwinding a second roll of thermoplastic nonwoven 3 for the formation, respectively, of said first 4 and second 5 bands, the latter gradually approaching until they come into contact with one another.

In the alternative embodiment of the method, not shown, in which a first reinforcing strip 4 made of natural plant fibers 2, a second strip 5, and a third strip, of a thermoplastic nonwoven material 3, said second and said third strip sandwiching said first strip 4, the device 6 for implementing the method of the invention, further comprises means for unwinding this third strip of thermoplastic nonwoven.

In this hypothesis, the two rolls of nonwoven are positioned on either side of the reinforcing roll 2, at the inlet of the device 6, so that the second strip of thermoplastic nonwoven 3 comes into contact on a first face. upper part of said first reinforcing strip 4 2 and that the third strip comes into contact on its opposite lower face.

Coming back to the variant embodiment of the method of the invention in which only a first reinforcing strip 4 of natural plant fibers 2 and a second strip 5 of thermoplastic nonwoven are used, the gradual bringing together of the two strips 4 and 5 is carried out, most preferably, during a step in which the first reinforcing strip 4 of natural plant fibers 2 and the second strip 5 of thermoplastic nonwoven 3 are conveyed to the level of a first heating zone 9 included in said device 6. Within this first heating zone 9, a temperature below the melting point TF of the thermoplastic nonwoven material 3 is advantageously applied. Such a temperature is reached through suitable heating means 10 shown in FIG. attached, by heating means located in the upper part and in the lower part of said device 6.

When the two bands 4 and 5 pass within this first heating zone 9, while approaching to come into contact with one another, it allows, in a particularly advantageous manner, a drying of the natural plant fibers and a evacuation of moisture that may still be present at the level of the reinforcing material made of natural plant fibers 2 of the first strip 4, through the porous structure of the thermoplastic nonwoven material 3 of which the second strip 5 is made.

Most preferably, the drying of the natural plant fibers of the first strip 4, and therefore the evacuation of the residual moisture from these, is carried out for a period of between 10 seconds and 5 min, the drying time being dependent, in particular, parameters and in particular the proportion of natural plant fibers 2 and of thermoplastic polymer.

In other words, the passage time of the two bands 4 and 5 within the first heating zone 9 is between 10 seconds and 5 min.

This drying is, therefore, particularly rapid, while allowing the production of a final product with very good mechanical performance.

Note, moreover, that in the attached figure, the first strip 4 of the reinforcement of natural plant fibers 2 is shown as being positioned above the second strip 5 of thermoplastic nonwoven material 3 for moisture evacuation. residual within said first strip 4, during the first steps of the method of the invention, this positioning not being, however, limiting of the method which is the subject of the invention.

Indeed, it is quite possible for the positioning to be the reverse, that is to say that the second strip 5 of thermoplastic nonwoven material 3 is located above said first strip 4, and that the evacuation of residual moisture from the reinforcement of natural plant fibers 2 is just as optimal.

Note also, still with reference to the schematic illustration of the attached figure, that the two bands 4, 5 are shown as not being in contact with each other at the inlet of the first heating zone 9, and coming closer to one another as said two bands 4, 5 advance within this zone 9, between the upper heating means and the lower heating means.

It is however conceivable that said two bands 4, 5 are already in contact with each other at the entrance of said first heating zone 9 and that they are routed all along this zone 9 in contact with the one on the other, without however altering the evacuation of residual moisture present within the first strip 4 of the reinforcement of natural plant fibers 2, due to the porosity of the thermoplastic nonwoven material 3 of the strip 5.

In this first heating zone 9, a temperature T1 of between 60 and 250 ° C. is preferably applied, this being chosen according to the nature of the thermoplastic nonwoven material 3 of which the second strip 5 is made, taking into account the fact that this temperature T1 applied must imperatively be lower than the melting temperature TF of said material 3.

Thus, in the particularly preferred embodiment in which the thermoplastic nonwoven material 3 consists of polypropylene, the melting point of which TF is generally considered to be of the order of 170 ° C depending on its tacticity, in d ' other terms depending on the degree and form of regularity of the distribution of the substituent groups with respect to the main aliphatic chain, a temperature T1 will preferably be applied, in the first heating zone 9, of less than 160 ° C.

Even more preferably, this temperature T1 applied within said first heating zone 9 is as high as possible, while avoiding both the melting of the thermoplastic nonwoven material 3 of the strip 5 and the thermal degradation of the natural plant fibers 2. of the strip 4, in order to evacuate the humidity contained in said fibers 2 as quickly as possible.

Note that, as the passage time of the strips 4, 5 of plant fibers 2 and of thermoplastic non-woven material 3 in the first heating zone 9 is relatively short, the temperature applied at the level of the heating means can be between 150 and 200 ° C. Indeed, even if the melting temperature of polypropylene is of the order of 170 ° C, the polypropylene will not be present long enough in said first heating zone 9 to reach this temperature and begin to melt.

However, the effective elimination of this moisture obtained by the implementation of the method of the invention makes it possible to avoid the formation of air bubbles or microbubbles due to the vaporization of the humidity in the resin during the curing. of the prepreg. By means of the process of the invention and the drying of the plant fibers, the creation of a porosity in the final product, namely the prepreg fiber reinforcement 1, which then has optimum quality, in particular in terms of mechanical characteristics.

Indeed, the presence of porosity at one point of a material is necessarily synonymous with degraded mechanical characteristics at this point, compared to the mechanical characteristics of other areas of the material not exhibiting this porosity. The process of the invention makes it possible to avoid this and to ensure that a prepreg fiber reinforcement 1 is obtained which has mechanical and aesthetic characteristics that are homogeneous and optimal over its entire surface.

Following this first heating step, the main function of which is the removal of residual moisture from the natural plant fibers 2, said first strip 4 and said second strip 5 are then conveyed, in contact with each other, at the level a second heating zone 11, which also comprises said device 6.

This second heating zone 1 1 is preferably located immediately downstream of the first heating zone 9, taking into account the direction of routing of the bands 4, 5, shown by the arrow in the accompanying figure. At this second heating zone 11, a temperature T2 is applied which is, this time, higher than the melting point TF of the thermoplastic nonwoven material 3.

Advantageously, as illustrated in the accompanying figure, this temperature T2 is applied through the action of suitable heating means 12 shown in the accompanying figure, by heating means in the upper part and in the lower part of said device 6.

In this second heating zone 1 1, a temperature T2 of between 100 and 280 ° C is preferably applied, the latter being, like the temperature T1, chosen according to the thermoplastic nonwoven material 3 of which it is made. the second strip 5.

The temperature T2 must, this time, imperatively be higher than the melting temperature TF of said material 3.

Returning to the example in which the thermoplastic nonwoven material 3 consists of polypropylene, the melting point of which TF is approximately 170 ° C, a temperature T2 will preferably be applied in the second heating zone 1 1 greater than 180 ° C and up to 220 ° C, and more preferably still, a temperature T2 of the order of 200 ° C.

The application, within this second heating zone 1 1, of a temperature T2 higher than the melting temperature TF, allows, once the evacuation of moisture in the first heating zone 9, to cause the fusion of said second strip 5 of thermoplastic nonwoven 3 and the homogeneous diffusion of this molten thermoplastic nonwoven within the natural plant fibers of the reinforcement 2.

A complex 13 is then obtained consisting of natural plant fibers and molten thermoplastic nonwoven.

Particularly advantageously, in the complex 13, and therefore in the final prepreg fiber reinforcement 1, the proportion of thermoplastic nonwoven is greater than or equal to 30% and less than or equal to 80% by mass relative to the mass total of complex 13, the proportion of natural plant fibers being between 20 and 70% by mass relative to the total mass of the complex, while the volume ratio of natural fibers plant (noted Vf%) in this complex 13 is between 15 and 70% by volume.

Even more advantageously, the proportion of thermoplastic nonwoven in this complex 13 is between 35 and 40% by mass, relative to the total mass of the complex 13, while the rate by volume of natural vegetable fibers Vf% is between 45 and 55% of the volume of complex 13.

Such proportions and volume rates make it possible to completely encompass the natural plant fibers 2 of the first strip 4 of thermoplastic nonwoven 3 in the complex 13, in order to have, at the level of the final prepreg 1 fibrous reinforcement, optimum transmission of mechanical forces between the natural plant fibers and the thermoplastic polymer binder matrix.

The complex 13 thus obtained is then conveyed to a calendering device 14, which comprises the device 6 for implementing the method of the invention, for carrying out the pressing step.

This calendering device 14 is advantageously located immediately downstream of the second heating zone 12, taking into account the direction of conveyance of the strips 4, 5, then of the complex 13, inside said device 6.

The calendering device 14 is preferably in the form of a first pressure roller 14a, operating from above and a second pressure roller 14b operating from below, the strip 13 consisting of natural fibers and non-woven fabric. molten thermoplastic passing between said two pressure rollers 14a, 14b, to guarantee a homogenization of the diffusion of the molten thermoplastic nonwoven throughout the thickness of the natural plant fibers 2.

Preferably, but not limitingly, a pressure is applied, during this pressing step, preferably by means of said calendering device 14, of between 10 and 150 N / cm ² , preferably equal to 80 N / cm ² .

In another exemplary embodiment of the method of manufacturing a prepreg fiber reinforcement 1 according to the invention, not shown in the figures, it is conceivable that the step during which the temperature T2 is applied above the melting temperature TF of the thermoplastic 3, to obtain the complex 13, and the step during which the said complex 13 is pressed, are carried out simultaneously.

In such an exemplary embodiment, not shown in the accompanying figure, the calendering device 14 provides, in addition to the pressing function, a heating function up to a temperature T2 above the melting temperature TF of the thermoplastic 3.

This being the case, whatever the embodiment chosen, once the step of pressing the complex 13 consisting of natural plant fibers and molten thermoplastic nonwoven has been carried out, this complex 13 is entrained in a cooling zone 15. that the device 6 advantageously comprises.

This cooling zone 15 is preferably located immediately downstream of the calendering device 14, taking into account the direction of travel of the strips 4, 5 and then of the complex 13, inside said device 6.

At the level of this cooling zone 15, an ambient temperature, of the order of 20 ° C, or a lower temperature, between 5 and 15 ° C, is preferably applied through suitable means, for example in the form refrigeration means 16 in the upper part and in the lower part of the device 6.

The presence of this cooling zone 15 makes it possible to cool the complex 13 for a solidification of the latter, more particularly through a solidification of the thermoplastic 3 distributed homogeneously within the natural plant fibers 2.

Thus, at the outlet of this cooling zone 15, the desired end product is obtained, namely the prepreg fiber reinforcement 1.

Optionally, this prepreg fiber reinforcement 1 is then wound in the form of a roll 17 through suitable winding means, for example in the form of a winder, in order to be able to easily store the final product 1 obtained, in waiting for the use of this prepreg fiber reinforcement 1 to manufacture, in particular, parts of composite material in various fields such as automotive, aeronautics, mechanics, etc.

Note that, throughout the manufacturing process of a prepreg fiber reinforcement 1 according to the invention, by means of the device 6, the routing of the bands 4 and 5, then of the complex 13 and finally of the said pre-impregnated fiber reinforcement. impregnated 1 is carried out by means of suitable drive means coupled, preferably, to motor means.

Advantageously, for the automated implementation of the method of the invention, a device 6 consisting of a double band press is used, allowing, throughout the path of the bands 4 and 5 within this device 6, 'application of a light compression between these two bands, favoring a pre-assembly thereof, in addition to the calendering device 14 at which the pressure is really applied for the shaping of the product.

The prepreg reinforcement 1 obtained by the implementation of the process according to the present invention can then, in a particularly advantageous manner, enter into the composition of three-dimensional preforms having optimal qualities, by a hot forming operation, also called a dyeing operation. stamping, before forming, by still subsequent operations, molding and / or draping, a part of composite material.

The method of the present invention makes it possible to obtain a prepreg reinforcement 1, formed of a thermoplastic nonwoven 3, calendered on a reinforcing linen web 2, having optimized mechanical and aesthetic characteristics, and which can be used in the production. manufacture of parts in composite material, in particular for the field of mechanics or transport such as the automobile, aeronautics, space or even the railway field.

Thus, the present invention also relates to a prepreg fiber reinforcement 1 composed of a first reinforcing strip 4 of natural plant fibers 2 and at least one second strip 5 of thermoplastic nonwoven 3, said reinforcement 1 being capable of being obtained by implementing the process of the present invention.

In a particularly advantageous manner, the prepreg fiber reinforcement 1 according to the invention comprises a proportion of nonwoven thermoplastic greater than or equal to 30% and less than or equal to 80% by mass relative to the total mass of reinforcement 1, preferably between 30 and 60% by mass. The proportion of natural plant fibers is between 20 and 70% by mass relative to the total mass of said reinforcement 1, preferably between 40 and 70% by mass, while the volume content of natural plant fibers (noted Vf%) in this reinforcement 1 is between 15 and 70% by volume, preferably 35 and 60% by volume.

Even more advantageously, the proportion of thermoplastic nonwoven in this prepreg fiber reinforcement 1 is between 35 and 40% by mass, relative to the total mass of the reinforcement 1, while the volume rate of natural vegetable fibers Vf% is between 45 and 55% of the volume of reinforcement 1.

The proportions of thermoplastic nonwoven and the volume ratio of natural plant fibers will be adapted according to the envisaged applications, the density of the potential thermoplastic polymer which may vary between 0.9 and 1.3 optionally, and the nature of the fibers.

This prepreg fiber reinforcement 1 has, due to its composition containing at least one strip 5 of thermoplastic nonwoven 3, optimum and homogeneous mechanical characteristics over its entire surface, the nonwoven strip in question having impregnated, with homogeneously, the whole of the first reinforcing strip 4 of natural vegetable fibers 2.

More particularly, said prepreg fiber reinforcement 1 consists only of a first reinforcing strip 4 of natural plant fibers 2 and of a second strip 5 of thermoplastic nonwoven 3.

Most preferably, the natural plant fibers of said reinforcing strip at 2 are flax fibers, while the thermoplastic material of said strip 5 of thermoplastic nonwoven 3 is polypropylene.

In this particular embodiment, advantageously, the volume ratio of flax fibers Vf% is of the order of, or equal to, 50% by volume while the proportion of thermoplastic nonwoven polymer 3 is order of, or equal to, 37% by mass relative to the total mass of reinforcement 1.

The present invention also relates to a part made of composite material comprising at least one such prepreg fiber reinforcement 1. In particular, the part made of composite material obtained by using the prepreg reinforcement 1 according to the invention may consist of a panel with a core of honeycomb, cardboard, foam or thermoplastic polymer, which would thus be , advantageously, free from formaldehyde.

It should be noted that the technical characteristics which have been specified above in connection with the detailed description of the process for manufacturing a prepreg fiber reinforcement 1 of the invention, in particular the nature and properties of the natural plant fibers and the nature of the thermoplastic nonwoven, the proportions of the constituents, etc., are applicable to the prepreg fiber reinforcement product 1 and vice versa.

Example: definition of the mechanical performance of prepreg fiber reinforcement according to the invention

The process parameters and the characteristics of the starting products (volume ratio of natural vegetable fibers Vf%, natural vegetable flax fibers - "flaxtape", thermoplastic polypropylene non-woven material - "spun PP"), heating temperature, heating time heater, etc., are shown below.

Note that the heating temperature, in the first heating zone where the plant fibers are dried, is indicated as being at 200 ° C, taking into account the fact that the heating time is relatively short, the thermoplastic nonwoven strip polypropylene does not have time to reach such a temperature of 200 ° C and remains at a temperature below its melting point.

Elementary and mechanical characterizations:

Elementary characterizations:

The density is obtained by calculating the ratio between the mass and the volume of the product. Mf%, Vf%: Use of the law of mixtures with the mass of the thermoplastic nonwoven as a reference, weighing of the final composite, and deduction of the density of fibers after processing (evaporation of residual moisture from the fibers).

For the porosity: Determination of Vm% and Vf% as a function of the mass of the composite and of resin, then determination of the porosity with% Composite volume = Vf% + Vm% + Vp% (porosity).

Mechanical characterizations:

The standard used is ISO 527-5 (UD glass composite tensile strength).

For the determination of the stiffness moduli, unidirectional biocomposites not having linear behavior during their entire deformation on a tensile test, two calculation ranges were selected in order to determine two different stiffness moduli depending on the deformations of the sample. . A first range is located between 0.01% and 0.15% of strain called “Rigidity_Petite strain” and another range between 0.4% and 0.7% called “Rigidity_Great strain”.

With regard to specific stiffness, this refers to the stiffness / density ratio of the sample.

These mechanical values are not exhaustive, and are examples with a composite obtained under certain conditions, optimized for these fiber ratios.

These tests demonstrate that the prepreg fiber reinforcements obtained by means of the process of the invention exhibit optimum mechanical characteristics.

Claims

1. A method of manufacturing a prepreg fiber reinforcement (1) composed of a reinforcement of natural plant fibers (2) impregnated with a thermoplastic polymer (3), said prepreg fiber reinforcement (1) being intended for enter into the manufacture of a part in composite material, said method being characterized in that it comprises, at least, the following steps:

- A first reinforcing strip (4) of natural plant fibers (2) and at least one second strip (5) of thermoplastic nonwoven (3), the melting point of which is denoted TF is used;

- Said first reinforcing strip (4) of natural plant fibers (2) and said second strip (5) of thermoplastic nonwoven (3) are placed in contact with each other;

- said first strip (4) and said second strip are conveyed

(5) at the level of a first heating zone (9) within which a temperature T1 lower than the melting temperature TF is applied, for the evacuation of residual moisture from the first reinforcing strip (4) of natural vegetable fibers (2);

- Then feeds said first strip (4) and said second strip (5), in contact with each other, at a second heating zone (1 1) at which a temperature T2 is applied greater than the melting point TF, for melting said second strip (5) of thermoplastic nonwoven (3) and obtaining a complex (13) made up of natural fibers and molten thermoplastic nonwoven;

- said complex (13) is pressed;

- said complex is conveyed to a cooling zone (15) to finally obtain said prepreg fiber reinforcement (1);

- Optionally, said prepreg fiber reinforcement (1) is wound in the form of a roll (17).

2. A method of manufacturing a prepreg fiber reinforcement (1) according to the preceding claim, characterized in that a single strip (5) of thermoplastic nonwoven (3) is used.

3. A method of manufacturing a prepreg fiber reinforcement (1) according to claim 1, characterized in that one uses a second strip (5) of thermoplastic nonwoven (3) and a third strip of non- thermoplastic woven fabric (3), one coming on a first upper face of said first reinforcing strip (4) made of natural plant fibers (2) and the other coming on a second lower face of said first reinforcing strip (4) in natural vegetable fibers (2).

4. A method of manufacturing a prepreg fiber reinforcement (1) according to any one of the preceding claims, characterized in that a temperature T1 is applied within said first heating zone (9) of between 60 and 250 ° C.

5. A method of manufacturing a prepreg fiber reinforcement (1) according to any one of the preceding claims, characterized in that a temperature T2 is applied within said second heating zone (1 1) of between 100 and 280 ° C.

6. A method of manufacturing a prepreg fiber reinforcement (1) according to any one of the preceding claims, characterized in that a pressure is applied during the pressing step of between 10 and 150 N / cm ² , preferably equal to 80 N / cm ² .

7. A method of manufacturing a prepreg fiber reinforcement (1) according to any one of the preceding claims, characterized in that one uses a first reinforcing strip (4) of natural plant fibers (2), long or short, comprising flax fibers or jute fibers or hemp fibers or nettle fibers or kenaf fibers, or a mixture thereof.

8. A method of manufacturing a prepreg fiber reinforcement (1) according to any one of the preceding claims, characterized in that one uses a second strip (5) of thermoplastic nonwoven (3) made from polypropylene and its derivatives, or polylactic acid (PLA) and its derivatives, or polyamide and its derivatives, or high or low density polyethylene and its derivatives, or polyester and its derivatives or a mixture of several of these thermoplastic compounds.

9. A method of manufacturing a prepreg fiber reinforcement (1) according to any one of the preceding claims, characterized in that one uses a first reinforcement strip (4) of natural plant fibers (2) comprising unidirectional long flax fibers and a second strip (5) of thermoplastic nonwoven (3) made from polypropylene.

10. Pre-impregnated fibrous reinforcement (1) obtainable by the implementation of the manufacturing process according to any one of the preceding claims, said reinforcement being characterized in that it is composed of a first strip ( 4) reinforcement of natural plant fibers (2) and at least a second strip (5) of thermoplastic nonwoven (3).

11. Pre-impregnated fibrous reinforcement (1) according to the preceding claim characterized in that it consists only of a first strip (4) of reinforcement of natural plant fibers (2) and by a second strip (5) of non- thermoplastic woven (3).

12. Pre-impregnated fibrous reinforcement (1) according to any one of claims 10 or 1 1 characterized in that said natural plant fibers of said reinforcing strip in (2) are flax fibers, while the thermoplastic material of said web (5) of thermoplastic nonwoven (3) is polypropylene.

13. Pre-impregnated fibrous reinforcement (1) according to any one of claims 10 to 12 characterized in that it comprises a proportion of thermoplastic nonwoven (3) greater than or equal to 30% and less than or equal to 80% by mass relative to the total mass of the reinforcement (1), while the volume rate (Vf%) of natural plant fibers (2) in this reinforcement (1) is between 15 and 70% by volume.

14. Pre-impregnated fibrous reinforcement (1) according to claim 13 characterized in that the proportion of thermoplastic nonwoven (3) in this reinforcement (1) is between 35 and 40% by mass, relative to to the total mass of the reinforcement (1), while the volume rate (Vf%) of natural plant fibers (2) is between 45 and 55% of the volume of the reinforcement (1).

15. Composite material part characterized in that it comprises at least one prepreg fiber reinforcement (1) according to any one of claims 10 to 14 and a core of honeycomb, cardboard, foam or thermoplastic polymer.