WO2014009381A1

WO2014009381A1 - Novel light composite materials, methods for manufacturing same, and uses thereof

Info

Publication number: WO2014009381A1
Application number: PCT/EP2013/064509
Authority: WO
Inventors: Pascal Trouillot; Christian FREYDIER; Nicolas Rumeau
Original assignee: Roxel France
Priority date: 2012-07-13
Filing date: 2013-07-09
Publication date: 2014-01-16
Also published as: FR2993202B1; FR2993202A1

Abstract

The present invention relates to a light composite material consisting of a substrate (15) made of natural fibers and having the structure of felt, wherein: the felt is needle-punched on both surfaces thereof by providing complementary polyethylene fibers; a heat-curable matrix (16) consisting of an aqueous-base resin and an expansion agent is dispersed in the matrix, the expansion of which is initiated by heating same to a given temperature; the weight proportion of the expansion agent in the matrix (16) is between 10% and 15%; and said heat-curable matrix (16) is integrated into the substrate by impregnation. The weight proportions of the substrate (15) and the heat-curable matrix (16) are defined so as to obtain an impregnated substrate having the following weight proportions after a dehumidifying step (12): 10% to 20% of substrate (15) that consists of natural fibers, and 80% to 90% of heat-curable matrix (16).

Description

New lightweight composite materials, their manufacturing processes and their uses

FIELD OF THE INVENTION

The present invention relates to the field of composite materials based on fibrous materials and resins. It relates more particularly to the field of composite materials made from natural fibrous materials and aqueous resins.

BACKGROUND OF THE INVENTION - PRIOR ART Nowadays, composite materials are commonly used for more or less specialized applications. These composite materials are generally manufactured, in known manner, from an absorbent material, woven or non-woven, organic or not, impregnated with a thermosetting resin. The application EP 0 041 054 describes in particular the formation of such materials. Certain composite materials are in particular made of glass fibers, mineral fibers, cellulose or polyester fibers, impregnated with thermosetting resins based on urea-formaldehyde, phenol-formaldehyde, resorcinol-formaldehyde or melamine-formaldehyde combinations.

In general, the preparation of such materials comprises impregnating the absorbent material fibers with a resin solution, for example a phenol-formaldehyde resin, in which microspheres consisting of a polymer material, of the vinylidene chloride type, are dispersed. acrylonitrile, containing an expander agent of the isobutane type, for example.

The European patent application EP 0 102 335 describes in particular, in its example, a process for producing a composite material using cellulose fibers, the process comprising mixing microspheres with a suspension of cellulose fibers. After dehumidification, the fibrous network is calendered and heated to 120 ° C to initiate the expansion. The material thus expanded is impregnated with a solution aqueous phenolic resin then dried in the microwave and the resin is crosslinked.

The method described in this application has the advantage of allowing a realization of the composite material in two steps which may be, to a certain extent, spaced apart in time. On the other hand it requires two baking phases, the first to react the microspheres acting as an expansion agent and initiate the expansion of the material, the second to crosslink the resin and impart rigidity to the material. As a result, the implementation of such a method has a certain complexity.

The patent application FR 2 956 664 describes the implementation of composite materials in a process comprising four steps, including a step of impregnating by calendering a fiber reinforcement with a resin-blowing agent mixture, followed by a step of volumic expansion under hot pressure. Although having the advantage of requiring only one baking step (crosslinking), this type of process proves unsuitable for the production of lightened composite material having a high mechanical and thermal resistance, in particular because of difficulties in control and reliability of the impregnation rate of the resin-blowing agent mixture within the fibrous reinforcement.

Also known from patent application WO201 1 / 101,343, filed by the Applicant, a composite material consisting of a significant proportion of fibrous reinforcement and the principle of a four-step process for the implementation of such materials. If certain performances of this material, in particular the mechanical and thermal strength, are advantageous, its high density does not allow to consider an industrial application in the field of transport, and in particular in the aeronautical field. In addition, the proposed method is relatively complex and difficult to make reliable. As previously described, the impregnation phase carried out according to a calendering process may in practice prove to be difficult to control, leading to concentration gradients that may be critical with respect to the resistance of the material in mechanical environments. and particular thermal. In addition, the process as described uses a dehumidification phase complex since requiring a sequence of thermal cycles in climatic chamber. Finally, the elimination phase of the volatile elements at the end of the compression cycle also complicates the process, in particular when significant industrial rates are sought.

PRESENTATION OF THE INVENTION

In the present invention, a lightened material having both a contained density and an excellent fire resistance is implemented by means of a simple and reliable alternative method.

The lightened material has a large proportion of thermosetting resin, which generates the formation by pyrolysis of a high carbon film, generally called carbonaceous tank by the skilled person, to significantly limit the heat propagation at through the material. Moreover, the fibrous substrate of the material ensures satisfactory mechanical strength of the material, which makes it possible to avoid any breakage or collapse of the structure on which the material is mounted (door, partition, etc.). The definition of the composite material associated with the implementation of a suitable manufacturing process makes it possible to have an industrializable product that is reasonably priced and whose performance is highly competitive for the applications envisaged for the material.

Another advantage described by the present invention consists in the implementation of steps consisting of the addition of reinforcing skins to the surface of the composite material advantageously for mechanically reinforcing the composite material.

For this purpose, the subject of the invention is a lightened composite material consisting of:

- A natural fiber substrate having the structure of a felt, the felt being needled on these two surfaces by providing a complementary fiber polyethylene;

a thermosetting matrix, integrated into the substrate by impregnation, formed of an aqueous base resin and an expansion agent dispersed in the matrix whose expansion is initiated by bringing it to a given temperature; the mass proportion of expansion agent in the matrix being between 10% and 15%, and in that the mass proportions of substrate and thermosetting matrix are defined so as to obtain a final composite material having the following mass proportions:

between 10% and 20% of natural fiber substrate, between 80% and 90% of thermosetting matrix;

The invention also relates to a method for manufacturing the composite material described above and comprising the following steps:

a first step of impregnating the substrate with the thermosetting matrix, the impregnation being carried out by capillarity by at least one face of the substrate,

a second stage of dehumidification of the impregnated substrate, the dehumidification being carried out by steaming and forced ventilation, the steaming being carried out at a controlled temperature of between 27 ° C. and 33 ° C. for a duration of between 25 and 30 hours,

a third expansion step, capable of activating the blowing agent and of crosslinking the resin by raising the temperature of the impregnated substrate; the substrate being under pressure stress. Because of its intrinsic characteristics, and in particular the combination of a contained density and the very competitive performances of mechanical and thermal behavior, the material according to the invention can advantageously be used to produce thermal insulation and protection elements. particularly in the field of aeronautics and, more generally, transport, where the concern to make gains on the onboard mass is a constant concern. The material according to the invention can thus advantageously be used to produce:

- inspection hatches mounted on the structure of airplanes or helicopters and for inspecting the interior of certain parts of this structure,

- interior bulkheads intended to accommodate the interior of vehicles, in particular aircraft (cabin and cockpit), riders (floor and ceiling fasteners) intended to hold said vehicle in position bulkheads or furniture items intended to equip the same vehicles,

thermal protective coatings for hoods, in particular hoods allowing access to mechanical shafts on aircraft or helicopter engines,

thermal protection elements of engine nacelles,

- protective packaging and transport of objects sensitive to thermal attack.

- core materials for sound, acoustic, thermal and mechanical insulation.

DESCRIPTION OF THE FIGURES

The characteristics and advantages of the invention will be better appreciated thanks to the following description, which is based on the appended figures which represent:

the figurel, the flowchart of principle of the sequence of steps of the method of manufacturing the composite material according to the invention;

FIG. 2, an illustration of a preferred embodiment of the impregnation stage of the process according to the invention;

- Figure 3, a timing diagram of the stoving implemented in the context of a preferred embodiment of the dehumidification step of the method according to the invention;

FIGS. 4 and 5 are illustrations of the implementation of the expansion step of the method according to the invention,

FIG. 6, an illustration of a preferred embodiment of a complementary step of draping the material by a prepreg composite material,

FIG. 7, a statistical measurement result of the Weibull module made on a sample of short basalt fibers selected for producing the composite material.

For the sake of clarity, the same elements will bear the same references in the different figures. DETAILED DESCRIPTION

According to a first object, the present invention therefore relates to a new type of lightened composite material comprising a support, a substrate, formed of fibers of natural origin, impregnated with a mixture of one or more thermosetting resins and a expansion agent.

The fibers of natural origin are mineral or vegetable fibers in the form of felt, basalt, flax, hemp, corn, sunflower, or bamboo fibers in particular. The natural fibers that can be used according to the invention can come from up to 30% of recyclable materials.

Felt means a nonwoven manufactured sheet consisting of webs or webs of fibers oriented in a particular direction or at random. Said felt can be in any form giving it sufficient porosity to be impregnated.

The use of natural fibers in the form of a felt advantageously allows a voluminal expansion of the fibers (i.e. in the three dimensions). The fibers can thus be distributed uniformly in the thickness of the substrate thus ensuring homogeneity of the final product. This volume expansion further contributes to the mechanical strength as well as the acoustic, phonic and thermal performance of the composite material formed. The use of a felt also makes it possible to achieve only one cooking phase, unlike the patent application EP 0102335, thus limiting the energy consumption.

It should be noted that fibers of natural origin used for the manufacture of felts are generally obtained by grinding, which makes their moisture content completely random. This water content, very variable, makes their specific use compared to the fibers usually used to make composite materials, such as glass fibers. Indeed, the presence of water strongly impacts the reproducibility of the final product if the manufacturing processes used do not take into account the variability of the moisture of the fibers. The use of fibers of natural origin therefore requires the evacuation of water vapor, which is not the case for the glass or cellulose fibers usually used in composite materials. Thus, the methods described in the EP 0 041 054 and EP 0 1 02 335 can not be applied as such to natural fibers.

As has been said previously, the fibers of the substrate, used to produce the composite material according to the invention, are fibers of natural origin, among which mention may notably be made of basalt, flax, hemp, sunflower, bamboo or corn. However, in the case where the desired final product is characterized by excellent heat resistance as well as resistance to strong mechanical stresses, the fibers used are preferably basalt fibers. In fact, fibers of plant origin whose degradation under the action of a heat flux occurs at a lower temperature are not adapted to the constitution of a final material intended to be subjected to constrictive thermal environments.

According to a preferred embodiment of a material according to the invention adapted to constrictive thermal environments, short fibers, also known by the Anglo-Saxon name chopped fibers, of basalt of Ukrainian origin are used, or in the absence of Russian origin which are characterized in particular by a relatively high olivine content, typically greater than 1 5% (mass percentage). This olivine level advantageously makes it possible to obtain a good regularity of the diameter of the fibers obtained by spinning and confers on these fibers advantageous mechanical strength and thermal performance.

Thus, the short basalt fiber used in the present invention preferably has the characteristics referenced in Table 1 below.

Characteristics Values Units

Average diameter 13 to 17 μηι

Olivine content 15 - 25%

Melting temperature> 1000 ° C

Breaking strain 1350 to 1700 MPa

60 to 80 GPa elastic module

Elongation at break 2.5 to 4 o // o

Weibull module 3.9 to 5.2 m

density 2.7 g / cm ³ Table 1: characteristics of short basalt fibers

The selection of natural fibers determines the performance of the final composite material. A first important characteristic relates to the average diameter of the fibers, between 1 3 and 1 7 μηι. These so-called short fibers make it possible to have a felt which is destructured, or in other words does not have a preferential alignment, unlike a felt made by weaving longer fibers. The implementation of such a felt advantageously makes it possible to ensure, even for high felt thickness, to maintain a fast and effective impregnation of the resin in the heart of the felt. It becomes possible to implement felts of significant thickness while preserving limited impregnation times of the felt by the resin.

A second key characteristic is the resistance of the fibers to fracture; this characteristic is quantified by means of the Weibull modulus, a parameter well known to those skilled in the art, measured by uni-axial tensile tests on unit fibers.

The Weibull module reports on the distribution of defects and the sensitivity of the material to these defects. It is determined empirically statistically on a unit fiber sample. The classical determination is made from the following function called "two parameters":

P _R = 1 - Jv exp (- (σ / σοΓ) dV, where PR represents the rupture probability, σ the breaking stress, σ ₀ a normalization stress, V the tested volume and m the Weibull modulus.

In the case of a constant tested volume, the Weibull modulus can be determined by the following formula:

Ln Ln (1 / (1 - P _R )) = m Ln (σ) + K, in which K = - m Ln (σ ₀ ) + Ln V. FIG. 7 represents an experimental result of measurement of the Weibull modulus carried out on a sample of short basalt fibers used for the implementation of the composite material according to the invention.

The selection of short fibers comprising a Weibull modulus of between 3.9 and 5.2 m ensures very competitive performance, particularly in terms of mechanical strength, as described below in Table 3.

The felts used are preferably needled on both sides, according to techniques known elsewhere, to ensure the integrity of the felt during the production cycle of the expanded composite material.

The needling is preferably carried out by the addition of a complementary polyethylene fiber.

In a preferred embodiment of the invention, the felts will have a thickness of between 4 and 6 mm, the variation in thickness constituting a lever for setting the density level of the expanded composite material.

For reasons of control of the felt impregnation process

(Optimized wetting of the fibers in the three directions of space) by the resin, it will be sought to avoid any pretreatment of the fiber, of the sizing type, for example a silane. However, the lightened composite material produced by means of the impregnation method according to the invention, described below, advantageously allows to tolerate a sizing rate between

0.1 and 0.5%. The resins used to make the composite material are preferably water-based resins such as phenolic resins and especially phenol-formaldehyde resins. These resins are, advantageously, particularly powerful to ensure the final composite material excellent heat resistance. Indeed, their exposure to heat produced on the surface, by a very endothermic chemical transformation phenomenon, a protective carbon layer that hinders the progression of this heat. In addition, while burning, these resins do not produce toxic fumes, which allows their use for interior equipment cabins intended to equip passenger vehicles. Alternatively, the resins used may be natural water-based resins derived from the bio-source, such as wood resins or phenolic resins of vegetable origin (grape or wood tannins).

The viscosity of the resin used is adapted to the density of the felt used to make the material, so as to impregnate the felt thickness, homogeneously. The use of a resin whose viscosity is not suitable, will lead to the constitution of a final product inhomogeneous or having a ratio between the mass percentage of resin and the mass percentage of fibers away from the values conferring on the final product the mechanical, thermal and acoustic characteristics.

Thus, it may be noted, by way of indication, that a resin with a viscosity of 300 mPa.s allows the homogeneous impregnation of a basalt felt with a density of 780 g / m 2. It should be noted that, if necessary, it is possible to add a solvent to adjust the viscosity level.

The blowing agent may be chosen from the expander usually used for this type of use. In a preferred embodiment of the invention, the blowing agent is preferably composed of Expancel® type microspheres marketed by Akzo Nobel. The blowing agent may also be of the Isobutane type or may also be chosen from natural or chemical yeasts.

According to the invention, the final composite material comprises by weight between 10% and 20% of fibers of natural origin and between 80% and 90% of thermosetting matrix consisting of thermosetting resin and the blowing agent.

The expander is between 5% and 25% by weight of the final composite material.

In the initial aqueous mixture, used for the impregnation of the substrate, the thermosetting matrix consists of 85% resin, 10% blowing agent and 5% solvent, which in this case will be water. Thus, by respecting such proportions, and in particular by maintaining a mass proportion of fibrous reinforcement of less than 20%, using the manufacturing method described in the following text, a composite material with very advantageous performance is produced. In the first place, the composite material provides both very competitive mechanical and thermal performance while having a reduced density. These performances make it a particularly suitable material for applications in the field of transport (aircraft cabin layout, thermal boxes and bulkheads, aircraft inspection hatches, etc.).

The composite material also has the following characteristics:

- good fiber homogeneity which allows a consistent expansion in the 3 axes. This homogeneity being defined by a distribution (random) of fiber concentration almost constant in the three dimensions of the material;

- A variable density and adjustable, by manufacture, between 100 kg / m3 and 500 Kg / m3. The adaptation of the density to the intended application thus makes it possible to make gains on the on-board masses;

a variable and adjustable thickness, by manufacture, between 6 mm and 30 mm;

- mechanical characteristics in accordance with the requirements of the aeronautical and transport sectors, such that the material can be considered as a structural element, in particular in certain interior fittings applications for passenger vehicle cabs, the characteristics of strength and appearance of the vehicle, material obtained being for example suitable to make its use attractive for the realization of civil aircraft cabin furniture;

- thermal and acoustic characteristics that make it suitable for use as a thermal and acoustic insulation material for any type of application requiring excellent thermal attenuation capabilities (including fire protection applications) and / or acoustic. The composite materials according to the invention make it possible in particular to produce structures meeting the fire / smoke standards in force in the various fields of activity, such as in particular air transport, in particular the ISO 2685 (Edition 1998) or the FAA AC 20 standard. -135, or even fireproof structures within the meaning of this latter standard, their fire resistance being a function of the thickness and density of the material used.

- an ease of elimination related to the use of natural fibers, in particular basalt fibers, which do not constitute, by intrinsic physical characteristics (minimum diameter of the spun fiber, almost impossible cracking along the length of the fiber ), a health hazard for the user of the material (fiber diameter greater than the limit of "breathability") and subsequently for those responsible for the dismantling operations, at the end of life, of articles made from composite material according to the invention.

The composite material according to the invention also does not alter the transmission of radio waves and can thus be used in protection applications for transmitting equipment, reception, in radar equipment for example.

In particular embodiments, the composite material according to the invention may comprise ingredients additional to those described above, with the aim of conferring on it additional properties such as the resistance to chemical contaminating agents or even an increase or a decrease in its electrical resistivity, or even an aesthetic character. These ingredients may in particular be added to the aqueous mixture of resin and blowing agent, in particular when it comes, for example, to anti-bacterial agents (of the lauryl-dimethyl-benzyl ammonium chloride type) or of organometallic pigments (dyes) dispersed in aqueous phase.

The composite material according to the invention may also comprise other materials and additives generally used for the envisaged applications: release agents, and flame retardants for fibers of vegetable origin in particular.

The composite material according to the invention, as described in the preceding text, is produced by implementing the original method described in the rest of the text and summarized by the flowchart of FIG. The manufacturing process implemented comprises the following steps: a first step 1 1 of impregnating the substrate 15, constituted by the fiber felt, by the thermosetting matrix 16,

a second dehumidification step 12 of the impregnated substrate 23,

a third step 13 for expanding the impregnated and dehumidified substrate, capable of activating the blowing agent and then of crosslinking,

Advantageously, these three steps are preceded by a preliminary step of preparation of the thermosetting matrix 16, carried out in a controlled thermal environment in order to control the rheology of the thermosetting matrix 16.

Finally, these manufacturing steps are advantageously followed by a final step 14 of stabilizing the material manufactured.

As illustrated in FIG. 2, the impregnation step 1 1 of the fibers constituting the substrate 15 is preferably carried out according to the application of the principle of capillarity, the resin and blowing agent mixture constituting the thermosetting matrix 16 deposited. at the surface diffusing within the substrate 15, as represented by the arrows 21. The mass content of resin and blowing agent 16 in the felt 15, or impregnation rate, is in this case a function of the flow rate of the resin / blowing agent mixture, as well as the running speed of the felt. Ideally, the impregnation is carried out by applying a displacement speed 22 of the substrate 15, or fibrous reinforcement, slaved to the diffusion capacity of the resin as well as the permeability level of the fibrous reinforcement. In order to ensure a perfect concentration isotropy throughout the volume of the felt, it will optionally be impregnated by capillarity on both sides of the felt, this operation being in the case of the present invention carried out continuously during the movement of the felt. fibrous reinforcement.

Alternatively, the impregnation of the substrate can also be performed within a multi-cast mold that allows the production of monolithic parts of complex and functionalized geometries (device integration or measurement sensors for example), thus limiting the mechanical assembly operations in the fields of application concerned. At the end of the impregnation stage, an impregnated substrate 23 of thermosetting matrix is obtained, which has substantially the following mass proportions:

80 to 90% of thermosetting matrix 16,

- 10 to 20% of substrate 15.

The second step 12 of dehumidification of the impregnated substrate 23 consists, before the expansion step 13, to bring the water content of the impregnated substrate 23 to a lowest possible value such that the mass loss of the substrate is between 55 % and 65%.

In a preferred embodiment of this dehumidification step 12, the impregnated substrate 23, previously placed on a porous support, a metal grid for example, is maintained for a period of time adapted to a fixed temperature within an enclosure that it can be assimilated to an oven, an enclosure whose atmosphere will be continually renewed in order to evacuate the humidity. As illustrated in Figure 3, a preferred embodiment of this step is to maintain the impregnated substrate 23 at a temperature maintained between 27 and 33 ° C for a period of between 25 and 30 hours.

At the end of the dehumidification step 12, the impregnated substrate is characterized in that the loss of mass caused is between 55 and 65% of the initial mass, which makes possible the implementation of the next step 13 .

It should be noted that, generally, it is preferable to dehumidify the impregnated substrate 23 in the desired shape for the final material. In this case the impregnated substrate is dehumidified while it is mounted on the template, corresponding to this form, the assembly being placed in the thermally controlled enclosure mentioned above. The high efficiency of the dehumidification step 12 as proposed in the invention advantageously makes it possible to dispense with a step of evaporation of volatile compounds such as envisaged by the known methods.

The third step 13 of the process for manufacturing a composite material according to the invention concerns the expansion of the dehumidified impregnated substrate. This step is intended to ensure the activation of the agent expansion and crosslinking of the thermosetting resin. It is preferably carried out by heating equipment, by raising the temperature of the substrate, to a temperature greater than or equal to the expansion temperature of the blowing agent, for example, at a temperature generally between 75 ° C. and 180 ° C. ° C, preferably between 90 ° and 150 ° C. Alternatively, it can be performed by exposing the impregnated and dehumidified substrate to low or high frequency electromagnetic radiation.

According to a preferred embodiment, illustrated by FIGS. 4 and 5, this step is carried out by means of a heating press, the heating temperature preferably being between 135 ° C. and 145 ° C., which temperature allows the both the activation of the blowing agent and the crosslinking of the resin. The heating press advantageously makes it possible to apply a pressure limiting the expansion of the material caused by the heating.

In this preferred embodiment, the duration of the setting in press is further defined according to the thickness (and therefore the density) of the material to be produced for the application in question. In this respect, the use of a heating press advantageously makes it possible to control the final thickness, after expansion, of the composite material 17 produced.

According to this preferred embodiment, the step 13 of expanding the composite material according to the invention comprises carrying out the following operations:

- preheating the press;

- Establishment of equipment, means to control the thickness and the geometry of the composite material element to achieve. In the case of a planar element 51, these means are for example constituted by metal shims 52 interposed between the upper tables 53 and lower 54 of the press, whereas in the case of a part 61 having a given volume these means are for example constituted, as illustrated in FIG. 5, by indentations 62 and 63 conforming to the shape of the part, the imprints used to form the impregnated substrate, for example:

- Removal of a release agent (not shown in Figures 4 and 5) on the lower table of the press, as well as on the upper face of the impregnated substrate. This release agent is for example made of parchment paper;

- Removal of the impregnated substrate on the lower table of the press;

pressurization of the press, the pressure values to be applied depending on the thickness that the element must have after expansion being, for example, previously recorded in the control and control system of the press.

Thus, the third stage 13 of expansion is advantageously carried out by means of a heating press allowing to apply on the material a pressure value able to limit the expansion caused by the heating; the pressure value applied being determined as a function of a crosslinking temperature of the material, and / or geometric characteristics of the material expected at the end of this step.

Advantageously, the pressure value applied by the press is a slave value on a pressure level generated by the blowing agent.

Advantageously, the pressure applied by the press is between 65 and 75 bars. Before using the material thus obtained at the end of the three manufacturing steps proper, 1 1, 12 and 13, it is subjected to a final stabilization step 14 of leaving the composite material element 17 to rest. on a horizontal plane for a sufficient time to return naturally to room temperature. According to a preferred implementation of this final step, the resting of the element of material is of a minimum duration of 2 hours. This final stabilization step notably allows the relaxation of the mechanical stresses within the material, relaxation which guarantees its integrity, that is to say the absence of internal defects, as well as the maintenance of its dimensional characteristics. This produces a composite material element 18 ready to be used to produce the desired object or structure.

Furthermore, the material according to the invention thus produced may, depending on the use for which it is intended, undergo complementary operations such as painting, removal of a surface coating forming a mechanical reinforcement or conferring on the material certain aesthetic characteristics.

Figure 6 illustrates a preferred embodiment of a complementary step of draping the material, of depositing on the surface of the composite material 51, a surface reinforcement 71, more commonly called skin. According to the invention the surface reinforcement 71 is constituted by a prepreg such that the fiber reinforcement is made of basalt fibers and the resin is a phenolic resin. In a preferred case of the invention, the fibrous reinforcement of the prepreg is geometrically organized. One thinks in particular of organized surface reinforcements of type 2D fabric or twill.

Thus, the method according to the invention advantageously comprises a complementary step of draping a reinforcement 19 on the upper and / or lower surface of the material.

Advantageously, the reinforcement consists of a composite material based on basalt fibers and phenolic resin. The composite material reinforcement may advantageously have a geometry of the 2D woven or twill type. Typically, the reinforcement has a density of between 90 and 10 kg / m 3.

In a preferred embodiment of the invention, the draping step of depositing on the material a reinforcement of pre-impregnated type is followed by a heating step capable of permitting the crosslinking of the composite material constituting the pre-impregnated reinforcement. impregnated. This heating step intended, by crosslinking the resin of the prepreg, to adhere said prepreg on the core composite material previously produced, is preferably applied between 1 10 ° C and 140 ° C over a range of time between 1 and 5 minutes. A reinforced end product 72 is thus obtained.

Note also that in an alternative embodiment, the layup step may be performed after the dehumidification step 12 and before the expansion step 13. Thus, the crosslinking of the composite constituting the pre-impregnated reinforcement is carried out by the step 13 of expanding the material. The process is limited to a single heating step, to limit the energy requirements of the process.

Thus, by carrying out the process according to the invention with the ingredients described in the present application, it is advantageously obtained a composite material having characteristics of composition and structure, as well as physical characteristics (mechanical, thermal and acoustic) likely to constitute a solution to the problems mentioned above, problems which existing composite materials do not provide a satisfactory solution.

The following exemplary embodiment is presented for purposes of illustration of the present invention. Example 1: Production of a composite material with basalt fibers for aeronautical application. This material is called ROXALTE® by the applicant.

The material is a composite material according to the invention, produced by following the steps of the method according to the invention, which are recalled below:

The material in question is obtained from a mixture 16 of resin, blowing agent and water, the whole being mechanically stirred within a vertical mixer of the emulsifier type, or on a device of the return type barrel. over a range of 25 to 35 minutes.

The phenolic resin / blowing agent mixture is here produced so that the proportions of the final mixture are as shown in Table 2 below.

Table 2: example of formulation ROXALTE® for aeronautical application

The mixture described above is mechanically stirred in a vertical mixer over a range of between 25 and 35 minutes (time of attaining the homogeneity of the mixture). The above mixture is used to perform the 1 1 double-face impregnation by capillarity of a felt 1 5 whose grammage is 780 g / m ² , this felt being commercially distributed by Basaltex under the reference 6/1 30 of the range. BCF Fibers Needlefelts / Mats, the final mass ratio reinforcement / resin, adjusted by capillarity, to be 1 0/90.

After impregnation with the phenolic resin the felt 1 1 undergoes a dehumidification step 12 of about 25 hours.

Finally, the expansion step 1 3 is here carried out by means of a press with a machine pressure of 75 bar / m ² for the production of a surface plate equivalent to 1 m ² .

A sample of ROXALTE® material thus manufactured, the characteristics of which are recalled below, was subsequently characterized in mechanical strength tests. The main results of these tests, analyzed in accordance with the standards in force, are summarized in the following table.

Table 3: Main performances of the lightweight composite material Roxalte

Claims

1. Lightened composite material (17) characterized in that it consists of:

- A substrate (15) made of natural fiber having the structure of a felt, the felt being needle-punched on these two surfaces by providing a complementary fiber polyethylene;

a thermosetting matrix (16), integrated into the substrate (15) by impregnation, formed of an aqueous base resin and an expansion agent dispersed in the matrix whose expansion is initiated by bringing it to a given temperature ; the mass proportion of expansion agent in the matrix (16) being between 10% and 15%, and in that the mass proportions of substrate (15) and thermosetting matrix (16) are defined so as to obtain a final composite material having the following mass proportions:

between 10% and 20% of substrate (15) made of natural fiber,

between 80% and 90% of thermosetting matrix (16);

2. Material according to claim 1, characterized in that the natural fibers used in the implementation of the substrate are short basalt fibers, having a mean diameter of between 13 and 17 μιη.

3. Material according to claim 2, characterized in that the short basalt fibers have a Weibull modulus between 3.9 and 5.2 m.

4. Material according to one of claims 2 to 3, characterized in that the short basalt fibers have a sizing rate of between 0.1 and 0.5%.

5. Material according to claim 4, characterized in that the short basalt fibers comprise a silane sizing agent.

6. Material according to one of the preceding claims, characterized in that a reinforcement (71) is fixed at the upper and / or lower surface of the material.

7. Material according to one of the preceding claims, characterized in that the reinforcement (71) is a composite material based on basalt fibers and phenolic resin.

8. A method for manufacturing the composite material according to any one of the preceding claims, characterized in that it comprises the following steps:

a first step (1 1) for impregnating the substrate (15) with the thermosetting matrix (16), the impregnation being carried out by capillarity by at least one face of the substrate (15),

a second dehumidification step (12) of the impregnated substrate (23), the dehumidification being carried out by steaming and forced ventilation, the baking being carried out at a controlled temperature of between 27 ° C. and 33 ° C. for a duration of between and 30 hours

a third expansion step (13) capable of activating the blowing agent and then crosslinking the resin by raising the temperature of the impregnated substrate

(23); the substrate being under pressure stress.

9. A method according to claim 8 characterized in that it comprises a prior step of preparing the thermosetting matrix (16), performed in a controlled thermal environment to control the rheology of the thermosetting matrix (16).

10. Method according to one of claims 8 or 9, characterized in that the second step (12) of dehumidification provides a loss of mass of the impregnated substrate (23) between 55% and 65%.

1 1. Process according to one of Claims 8 to 10, characterized in that the third expansion stage (13) is carried out by means of a heating press which makes it possible to apply to the material a pressure value capable of limiting expansion. caused by heating; the value of applied pressure being determined as a function of a crosslinking temperature of the material, and / or geometric characteristics of the material expected at the end of this step.

12. The method of claim 1 1, characterized in that the pressure value applied by the press is a slave value on a pressure level generated by the blowing agent.

13. Method according to one of claims 1 1 or 12, characterized in that the pressure applied by the press is between 65 and 75 bar.

14. Method according to one of claims 8 to 13, characterized in that it further comprises a complementary step of draping (19) of a reinforcement (71) on the upper surface and / or lower material (51).

15. The method of claim 14, characterized in that the reinforcement (71) is made of a composite material based on basalt fibers and phenolic resin.

16. The method of claim 15, characterized in that the reinforcement (71) is made of a composite material having a geometry of 2D woven or twill type.

17. Method according to one of claims 14 to 16, characterized in that the reinforcement (71) has a density of between 90 and 1 10 kg / m3.

18. Method according to one of claims 15 to 17, characterized in that it further comprises a heating step at a temperature between 1 10 ° C and 140 ° C, for a period of between 1 and 5 minutes, performed after draping the material, able to allow the crosslinking of the composite material constituting the reinforcement (71).