Classifications

• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C14/00—Glass compositions containing a non-glass component, e.g. compositions containing fibres, filaments, whiskers, platelets, or the like, dispersed in a glass matrix
  • C03C14/002—Glass compositions containing a non-glass component, e.g. compositions containing fibres, filaments, whiskers, platelets, or the like, dispersed in a glass matrix the non-glass component being in the form of fibres, filaments, yarns, felts or woven material
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C2214/00—Nature of the non-vitreous component
  • C03C2214/02—Fibres; Filaments; Yarns; Felts; Woven material
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C2214/00—Nature of the non-vitreous component
  • C03C2214/20—Glass-ceramics matrix
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C2214/00—Nature of the non-vitreous component
  • C03C2214/32—Nature of the non-vitreous component comprising a sol-gel process

Definitions

• the invention relates to a process for the preparation by sol-gel route of a composite material comprising a vitroceramic matrix consisting essentially of lithium aluminosilicate or Li ⁇ 2-Al 2 0 3 -Si0 2 (LAS), and a fiber reinforcement.
• the technical field of the invention can, in general, be defined as that of the preparation, the manufacture, composite materials with ceramic matrix (CMC) or vitroceramic and with fibrous reinforcement, more particularly composite materials with ceramic matrix or. glass-ceramic based on silica reinforced by fibers. These composite materials are used in particular in the aeronautical and space industries because of their good behavior at medium or high temperature of the order of 600 to 2500 ° C.
• Composite materials with a ceramic or glass-ceramic matrix, in particular based on silica, reinforced with fibers can be prepared by various processes obeying different basic principles.
• these composites can be prepared by a process based on the principle of prepreg followed by hot pressing. This process initially involves impregnating the fibers in a powder suspension. After drying, the fibers are stacked according to a determined orientation sequence then introduced into a uni-axial press and hot compressed generally at more than 1000 ° C in order to carry out their sintering.
• the pressing-sintering process does not make it possible to obtain parts of complex geometry, for example parts comprising both positive and negative radii of curvature.
• the ceramic matrix and fiber composites can also be prepared from gaseous precursors by impregnation of fibrous preforms by the chemical vapor infiltration process ("Chemical Vapor Infiltration” or CVI in English). This technique is limited by the low density of the products obtained due to the closure of the porosities which prevents access of the gases to the core of the material and by the same to continue the internal densification. Therefore, to optimize the impregnation, it is necessary to choose very slow and therefore more expensive deposition kinetics. Composite materials with a ceramic matrix can also be obtained by sol-gel.
• the principle of these methods consists in gelling a soil - that is to say a suspension in a liquid, of particles of size less than 0.1 mm or a liquid consisting of an organic or inorganic precursor and a solvent - and gradually creating a three-dimensional network of oxide by a hydrolysis step followed by a step of polymerization or condensation of the precursor.
• This gives a solid and porous structure swollen with interstitial liquid, called gel, the skeleton of which is made up of more and more condensed species.
• a heat treatment for drying and densifying this gel then leads to a ceramic material.
• Processes for preparing ceramic matrix composite materials are described in particular in documents FR-A-2 655 327 and US-A-5 126 087.
• the document FR-A-2 655 327 relates to a process for preparing a glass-ceramic composition in which a gel is prepared by hydrolysis and polycondensation of precursor metal compounds in solution in a solvent; removal of the solvent; optional grinding of a gel thus obtained; dehydration and oxidation of the gel; re-grinding and sieving to obtain a powder; and finally densification and ceramization.
• the precursors used are tetraethyl orthosilicate Si (OC 2 H 5 ) 4 (TEOS); aluminum sec-butoxide Al (OCH (CH 3 ) CH 2 H 5 ) 3 (ASB), lithium nitrate L1NO 3 and optionally magnesium nitrate Mg (N0 3 ) 2 6H 2 0.
• Document US-A-5 126 087 describes a process for manufacturing a composite material formed of a fibrous reinforcement and a ceramic or vitroceramic matrix based on silica, in which a fibrous reinforcement is impregnated by means of a soil prepared from an aqueous solution of silica, a solution of a metal salt such as aluminum or lithium nitrate and a solution containing an agent that delays crystallization such as boric anhydride.
• the fibrous reinforcement thus impregnated is dried and the steps of impregnation and drying are repeated until a prepreg ("prepreg") having the desired portion of fiber by volume is obtained. Pyrolysis of the prepreg is carried out from 200 to 600 ° C for a sufficient time to remove the gases formed by chemical reaction.
• hot compression is carried out from 600 to 900 ° C. and at a pressure of less than 50 MPa, for example from 12.5 MPa to 25 MPa and cooled without pressure to ambient temperature.
• a thermal post-treatment called ceramization treatment can be carried out at temperatures above the hot compression temperature.
• the method has the advantage, compared to that described in document FR-A-2 655 327, of not requiring a drying and grinding phase of the gel since the reinforcement is directly impregnated with a LAS type soil and then densified .
• the densification of the charges at high temperature degrades certain fibers and this method of production also requires significant infrastructure.
• the aim of the present invention is to provide a process for the preparation of a composite material with a vitroceramic matrix of lithium aluminosilicate which meets inter alia the needs listed above.
• the object of the present invention is also to provide such a method which does not have the drawbacks, defects, limitations and disadvantages of the methods of the prior art and which solves the problems of the methods of the prior art. This object, and others still, are achieved, in accordance with the invention, by a process for the preparation of a composite material comprising a fiber reinforcement, and a vitroceramic matrix consisting essentially of lithium aluminosilicate.
• LAS said process comprising the following successive steps: a) preparation of a matrix precursor sol comprising a lithium salt, a binder reagent containing alumina, colloidal silica, and a solvent, and homogenization of said sol; b) impregnating the fiber reinforcement with the soil prepared in step a); c) drying the impregnated fiber reinforcement, whereby a gelled composite material is obtained comprising a fiber reinforcement and a gelled matrix; d) densification of the gelled composite material of step c) at a temperature less than or equal to 500 ° C.
• the final glass-ceramic matrix essentially consisting of lithium aluminosilicate has the composition xLi0 2 -yAl 2 ⁇ 3 -zSi0 2 , where x goes from 1 to 2, y goes from 1 to 2 and z goes from 1 to 4.
• the soil comprises by mass: from 1 to 4% of lithium salt, from 15 to 25% of reactive binder containing alumina, and from 30 to 50% of colloidal silica.
• the lithium salt is chosen from halides (fluorides, chlorides, iodides, bromides) of lithium, and lithium nitrate.
• the solvent for the soil is chosen from water, ethanol, and their mixtures.
• the soil also contains one or more additional precursors chosen from metal oxides and mica.
• said metal oxides are chosen from MgO, Zr0 2 and Ti0 2 .
• the said additional precursor (s) are added so as to each represent from 0.1 to 2% by mass of the matrix.
• the reinforcing fibers comprise one or more elements chosen from Si, B, 0, N and C.
• the reinforcing fibers are chosen from fibers of glass, carbon, silicon carbide, alumina-silica , alkaline earth silicates, and metallic wires sheathed with an electrical insulator, for example copper wires sheathed in glass.
• the fiber reinforcement is in the form of a fabric; a fiber paper, such as the product sold under the name "SUPER OOL PAPER X607" by the company SORED-UPM; or a nonwoven web of fibers.
• the fiber reinforcement is in the form of a stack of several layers, thicknesses, folds.
• the layers, thicknesses, folds differ in their composition, and / or their structure, and / or their properties, for example magnetic and / or electrical and / or optical and / or mechanical.
• the fibrous reinforcement is placed in or on a preform or mold.
• the impregnation of step b) is carried out with a brush or by dipping.
• this drying is carried out under vacuum or else under pressure. It can be carried out in an oven, a vacuum bag or in the open air.
• the drying is advantageously carried out at a temperature of 70 to 180 ° C.
• the drying temperature is advantageously maintained for a period of 1 to 4 hours.
• the drying temperature is advantageously reached by raising the temperature from room temperature at a rate of 1 to 4 ° C / minute.
• the gelled matrix represents from 15 to 25% by mass of the composite material.
• the impregnation and drying steps can be repeated from 1 to 3 times until the composite material has the desired mass content of gelled matrix.
• the densification of step d) is carried out at a temperature of 350 to 600 ° C.
• the densification temperature is advantageously maintained for a period of 1 to 5 hours and the densification temperature is advantageously reached by raising the temperature from room temperature at a rate of 1 to 5 ° C / minute.
• the process according to the invention is fundamentally different from the processes of the prior art implementing the sol-gel route such as those described in the documents mentioned above.
• the matrix is produced from widely available precursors, inexpensive, easy to use, which can be handled in the open air and not necessarily under a nitrogen sweep, as is the case for some alkoxide type precursors such as those cited in document FR-A-2 655 327 which hydrolyze and harden in the open air.
• use is not made of a uniaxial hot pressing step as in document US Pat. No. 5,126,087, which makes it possible to design parts of complex geometry.
• the densification temperature used in the process of the invention is low, namely less than or equal to 500 ° C., which is considerably lower than the 900 ° C., under pressure, of this document.
• the method according to the invention uses implementation means, apparatuses, simple and economical, it comprises a limited number of steps, themselves, simple, economical, easy to implement.
• the preparation of the soil is done as already mentioned above from precursors which do not require any precaution for use.
• the elaborate soil is directly ready to be injected into the reinforcement and it gels at the heart of the reinforcement during the drying stage.
• the step of dehydration and grinding of the gel, essential in the process of document FR-A-2 655 327 is not necessary, which greatly simplifies the process.
• the specific soil used in the process of the invention is very easy to use and the impregnation can be carried out by any known technique and in particular very simply with a brush or by dipping.
• the soil and the matrix prepared from it are compatible with fibers of very different composition and shapes. It is therefore possible with the method of the invention to develop, for example laminates consisting of plies, of thicknesses of different chemical nature.
• the fibers used can contain the elements Si, B, 0, N or C and the fibers used can be, for example, in the form of fabric or fibers.
• the specific matrix according to the invention is very reactive and the chemical adhesion between the matrix and the fibers and possibly between the different layers of the composite is also very rapid.
• a mechanical adhesion generated by uniaxial hot pressing is therefore, according to the invention, no longer necessary, which allows the preparation of parts of complex geometry which could not be prepared by the methods of the prior art.
• One of the essential characteristics of the process of the invention is the low densification temperature of the composite and more particularly of the matrix. Indeed, thanks to the process of the invention, it is possible to obtain a solid material at a temperature less than or equal to 500 ° C. This low densification temperature of a glass ceramic makes it possible to have shorter heat treatment cycles and thus to impose significantly less severe stresses on the fibrous reinforcements.
• the specific matrix according to the invention can be combined with fibrous reinforcements which would not have withstood the much higher densification temperature commonly used in the processes of the prior art.
• the method according to the invention essentially consists in preparing a specific soil comprising a specific composition based on commercial precursors, which gives a specific matrix, of high compatibility and reactivity with all kinds of fibers, and which ensures a densification at a low temperature.
• FIG. 1 is a schematic perspective view of a vacuum bag used in the drying step of the process of the invention.
• Figure 2 is a diagram showing the preparation by the method of the invention of a composite material comprising a laminated fibrous reinforcement, "sandwich", formed of n layers or plies.
• - Figure 3 is a schematic perspective view showing the preparation by the process of the invention according to Example 2, a laminate from a layer of fabric Nextel ® 312, a layer of metal sheathed son of Pyrex glass and a layer of carbon cloth.
• the method according to the invention firstly comprises a step of preparing a sol of the precursors of the matrix of the composite material which comprises a lithium salt, a reactive binder containing alumina, and a solvent .
• reactive binder it is generally understood that the constituents of this binder, other than alumina, are capable of reacting to form a polymer gel.
• the reactive binder may include reactive mineral monomers and the alumina filler. Such reactive mineral monomers are chosen, for example, from metal alkoxides.
• the sol is generally prepared by mixing an aqueous suspension of colloidal silica, an aqueous suspension of alumina acid, and lithium salts. These precursors are commercial precursors of lithium, aluminum and silica.
• the suspension of colloidal silica is for example a suspension marketed under the name LUDOX HS40 by the company DUPONT DE NEMOURS (composition: 40% by mass of Si0 2 , from 198 to 258 m 2. G "1 of specific area, dispersed in a basic aqueous solution).
• the alumina acid suspension is for example a suspension sold under the name binder 795 by the company COTRONICS CORPORATION, it it is an alumina suspension in an acidic aqueous medium.
• the lithium salt is generally chosen from lithium halides and lithium nitrate. A particularly preferred salt is lithium nitrate.
• a solvent generally chosen from water, ethanol and their mixtures. A preferred solvent is demineralized water.
• the precursors react with the added solvent, such as demineralized water, to give the sol of step a).
• the solvent promotes soil hydrolysis.
• the nature of the solvent and the dilution rate of the soil also play a role in the dilution of the salts.
• the solvent also makes it possible to modify the viscosity of the soil and therefore to play on the facilitating of impregnation of the soil in the fibrous reinforcement.
• the impregnated soil generally comprises, by mass, from 1 to 4% of lithium salt, from 15 to 25% of reactive binder containing alumina and from 30 to 50% of silica.
• the molar ratios of the various precursors are such that the final LAS matrix (after densification) has a composition xLi0 2 -yAl 2 ⁇ 3-zSi ⁇ 2 where x, y and z which represent the molar ratios in the matrix of the oxides formed are such that 1 ⁇ x ⁇ 2, 1 ⁇ y ⁇ 2 and 1 ⁇ z ⁇ 4. These molar ratios are adjusted according to the intended application.
• Other additional fillers or precursors can be added to the soil. These fillers are generally chosen from metal oxides such as MgO, Zr0 2 , Ti0 2 and mica. These additional charges or precursors make it possible to modify certain characteristics of the matrix.
• fillers such as mica improves the temperature resistance and limits the propagation of cracks in the composite.
• the additional fillers or precursors are generally added to the soil in proportions (for each) of 0.1 to 2% by mass (of the soil).
• the soil is finally homogenized by mechanical or magnetic stirring, for example with a suitable device, such as a magnetic stirrer? as the device marketed under the name
• the next step in the process of the invention is the impregnation of the fibrous reinforcement with the soil prepared in the step described above.
• the ground and the matrix resulting from this are compatible with fibers of different natures, for example these fibers can consist of the chemical elements Si, C, N, 0, B, alone or in mixtures.
• the chemical composition of the fibers used may differ depending on the intended use.
• fibers which can be used in the process of the invention mention may be made of glass fibers, carbon fibers, silicon carbide fibers and fibers of alumina-silica, alumina-silica-boron oxide or alkaline earth silicates.
• wires such as metallic wires such as copper wires sheathed with an electrical insulator, for example glass and in particular
• the fibers used can be unidirectional and continuous, or else they can be, as specified above, in the form of threads.
• the fiber reinforcement can be in the form of 2D or 3D fabrics; fiber paper, for example undirectional webs, for example unidirectional webs, or other non-woven fiber materials; or even multidirectional preforms.
• the fabrics can be in particular taffetas, satins.
• the fibers used in the process of the invention are generally so-called long fibers, that is to say that their length is generally from 3 to 100 cm.
• the fibrous reinforcement can be constituted by a stack, a superposition, a laminate of layers of the same nature, composition, chemical and of the same structure or else of layers of natures, compositions, chemical s and / or of structures and / or properties by different magnetic and / or electrical and / or optical and / or mechanical examples.
• the fibrous reinforcement before its use, generally undergoes a thermal or chemical desizing treatment.
• the sizing is an organic coating of the monofilament used for the manufacture of industrial fabrics which makes it possible to ensure a relative sliding of the fibers between them and to avoid premature wear of these fibers.
• a desensing heat treatment can consist, for example, of heating the fabric in air to from 500 to 700 ° C.
• a chemical sizing treatment can consist, for example, of soaking the fibers in a solvent or a mixture of solvents, for example an acetone / ethanol mixture, for for example a minimum duration of 10 hours.
• the fibrous reinforcement for example in the form of a fabric, a stack of fabrics or the like, is positioned on a preform, or placed in a mold, the shape of the preform or of the mold corresponding to the shape of the final part that we want to prepare.
• the preform or mold can, according to the method of the invention, take even very complex shapes, for example cylinder shapes.
• it is possible to manufacture large parts for example with a size of 1 to 3 m 2 .
• the next step of the process according to the invention consists in impregnating the fibrous reinforcement with the soil prepared above.
• the fibrous reinforcement can be impregnated by any suitable method. We can therefore achieve this impregnation, for example with a brush or by dipping. These methods are extremely easy to implement with limited equipment.
• the next step of the process of the invention is a step of drying the impregnated fibrous reinforcement.
• the drying step can be done in the open air or in an oven.
• the drying can also be done either at ambient pressure, or under a slight pressure, for example from 1 to 4 bars, by application of a mass on the impregnated fibrous reinforcement, or even in a vacuum bag at a pressure of 0 , 5 to 1 bar, in both cases, heating is carried out in an oven.
• the vacuum bag (1) comprises a vacuum film (2) forming its upper wall, a preform (3) forming the lower wall on which is placed the impregnated fibrous reinforcement (4), a separating film (5) placed above and below the impregnated fibrous reinforcement, a drainage fabric (6) placed between the evacuating film and the upper separating film, and finally a flexible sealant (7) defining side walls between said preform (3) and said vacuum film (2).
• the evacuating film (2) is traversed, for example in its center, by an orifice (8) connected to a suction nozzle (9) which makes it possible to create a vacuum in the vacuum bag. Drying is generally carried out by maintaining a temperature of 70 to 180 ° C for a period of 1 to 4 hours, for example a temperature of 180 ° C for a period of 4 hours. Preferably, the temperature rise, to reach the drying temperature is a slow rise, carried out for example at a speed of 1 to 4 ° C / minute. At the end of this drying step, a gelled composite material is obtained comprising the fiber reinforcement and a gelled matrix.
• the gelled matrix generally represents from 15 to 25% by mass of the gelled composite material resulting from the drying step carried out once.
• the composite generally has a fiber volume fraction of 0.5 to 0.7 after a drying operation.
• the steps of impregnation and then drying can be repeated for example from 1 to 3 times, until a composite having the desired content of gelled matrix, for example from 15 to 25% by mass, is obtained, or the desired fiber volume fraction, for example from 0.5 to 0.7.
• the gelled composite (impregnated and dried composite) is then densified by a heat treatment called densification treatment. Densification is generally carried out at a temperature of 350 to 600 ° C, for example 500 ° C, maintained for a period of 1 to 5 hours, for example 1 hour.
• the densification temperature is generally reached by raising the temperature from room temperature at a speed of 1 to 5 ° C / minute, for example from 1 ° C / minute, to the temperature of the densification stage. It should be noted that according to the invention this temperature of the final heat treatment or densification treatment is much lower than that reached in the processes of the prior art, therefore the energy expenditure achieved in the process of the invention is significantly less than that of the prior art methods.
• the densified composite material previously brought to room temperature, is removed from the mold or separated from the preform used for its preparation. The material is then ready for use.
• the material prepared by the process according to the invention has excellent properties, in particular, it generally has a less rigid, more flexible, less brittle texture than the materials of the prior art, which may be advantageous for certain uses, in particular in the form of a non-structuring coating.
• Figure 2 there is illustrated the preparation by the method of the invention of a composite material comprising a fibrous reinforcement which is a particular fibrous reinforcement, laminate, "sandwich" formed of n layers, folds or thicknesses.
• a composite material comprising a fibrous reinforcement which is a particular fibrous reinforcement, laminate, "sandwich" formed of n layers, folds or thicknesses.
• n layers or plies are stacked on the preform (mold) which, in FIG. 2, has been shown as having a tile shape.
• a “sandwich” (220) is thus obtained comprising n folds or layers placed on the preform (221).
• the impregnation of this sandwich is carried out with a floor (222) having the desired composition as described for example above.
• the drying treatment is then carried out, followed by the final heat treatment as described above.
• the composite materials produced by the process of the invention find numerous applications.
• the elaborate materials prepared according to the invention can be used in particular for the manufacture of parts which have to withstand high mechanical stresses, whether in bending-traction and / or in vibration, in a severe climatic environment where for example a high temperature prevails.
• the composite materials prepared according to the invention which have a sandwich structure, offer the possibility of combining elementary layers each having particular optical and / or magnetic and / or electrical properties. Because of its good temperature resistance, the composite can be used for example to produce engine cowls or light fireproof partitions.
• a piece of composite material is prepared which is in the form of a plate with dimensions 50 mm ⁇ 50 mm and the matrix of which consists of Si0 2 , A1 2 0 3 , Li0 2 .
• the fibrous reinforcement used is a silicon carbide (SiC) fabric desensitized by soaking in an equimolar acetone / ethanol mixture for 24 hours.
• SiC ribbons are cut to the dimensions of the part, then positioned on a mold having the shape of the part to match its contours (plan shape 50 mm x 50 mm and thickness 3 mm).
• the desensed tissue is impregnated with the soil prepared above which forms the ceramic matrix.
• the impregnated composite is then dried for 4 hours in an oven at 180 ° C. To obtain a mass fraction in matrix of 20%, it is necessary to repeat a cycle of impregnation-drying. After these two impregnation-drying cycles, the material is ceramized (fired) for one hour at 500 ° C.
• the composite material produced in this example is a coating which is in the form of a "sandwich” (34) comprising three layers, folds or thicknesses of different types.
• the matrix used for this sandwich is comparable to that used in Example 1, that is to say that it is LAS.
• the three folds, thicknesses or layers of different natures constituting the reinforcement are the following, the first fold or upper fold (31) is an alumina-silica-boron oxide fabric (Nextel 312) thermally desensitized at 700 ° C. for 1 hour, the second fold or median fold (32) is a sheet of metal threads sheathed in pyrex and finally the fold below is a carbon fabric.
• This carbon sublayer (33) is used to strengthen the chemical affinity between this part and the carbon substrate and therefore facilitate bonding of the composite.
• the different fabrics are impregnated by soaking in the ground, then are stacked to form the sandwich.
• This sandwich is placed in a vacuum bag (vacuum envelope) hermetically closed then introduced into an oven for the drying phase at 180 ° C for 4 hours.
• a permanent pumping is imposed on the envelope under vacuum in order to evacuate the solvent and to apply a constant compression force.
• the sandwich is removed from the envelope under vacuum and then densified by heat treatment comprising a step at an isothermal level of one hour at 500 ° C.
• a sol composed of 3.3 g of LiCl, 6.5 g of commercial ceramic binder containing alumina oxides, 20 g of colloidal silica and 10 g of mica is prepared.
• the viscosity of the soil is adjusted with 23 g of demineralized water.
• the soil is injected into a flexible fibrous reinforcement marketed under the name "alkaline earth silicate fiber paper" (SUPER OOL PAPER X607 from SORED-UPM). After having been dried for 4 hours at 180 ° C, then densified at 500 ° C for one hour, this composite can be used as an insulating joint.
• a soil is prepared with 3.3 g of LiCl, 6.5 g of commercial ceramic binder containing alumina oxides, 20 g of colloidal silica, 15.5 g of demineralized water r and 9 g of Ti0 2 powder. Titanium oxide gives the fabric a very white color.
• Nextel ® 312 (AF40) is impregnated with the soil thus prepared and then dried in a vacuum bag comparable to that presented in Example 2. After being dried, the stacks forming "sandwich” panels are removed from the vacuum bag and then introduced into an oven to undergo a two-hour heat treatment at 500 ° C.

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