WO1997031064A1

WO1997031064A1 - Use of polycarbosilanes for stabilising alkenyl polysilanes

Info

Publication number: WO1997031064A1
Application number: PCT/FR1997/000283
Authority: WO
Inventors: Nathan Bryson
Original assignee: Flamel Technologies
Priority date: 1996-02-23
Filing date: 1997-02-14
Publication date: 1997-08-28
Also published as: FR2745292A1; FR2745292B1

Abstract

The air and particularly oxygen and/or heat stabilisation of compositions containing polysilanes or polysilane mixtures as ceramic precursors or 'preceramics', is disclosed. The problem to be solved is that of providing a means for effectively stabilising preceramic polymers such as alkenyl polysilanes and polyvinylsilanes (PVS) in particular, while maintaining their cross-linkability and enabling their later use for producing ceramics with a high yield. The problem may be solved by using polycarbosilane (PCS) to stabilise a preceramic polymer such as an alkenyl and preferably vinyl polysilane. The PCS constitutes at most 50 wt % of the PCS/alkenyl preceramic mixture.

Description

USE OF POLYCARBOSILANES FOR THE STABILIZATION OF ALKENYL POLYSILANES

TECHNICAL AREA :

The general field of the present invention is that of compositions based on polysilanes useful, in particular, in the production of ceramics made of silicon carbide. More specifically, in the context of the invention, interest is focused on the stabilization in air, in particular with oxygen and / or at temperature, of compositions based on polysilanes or on mixtures of polysilanes, precursors. of ceramics and also called "pre-ceramics".

PRIOR TECHNIQUE:

Polysilanes are polymeric materials, the linear skeleton of which is made up of bonded silicon atoms. These silicon-rich materials have found applications in many technical fields. They are used in the electronics and composites industries, in particular as photoconductors, semiconductors, materials constituting the image in xerography and microlithography and, as indicated above, as precursors of carbide ceramics silicon (CHEM. REV. 1989, 89, 1410).

More specifically, polysilanes are thus mainly used:

- to make complex shapes and pyrolyze them to transform them into ceramics, - to spin the continuous fibers and transform them, by heat treatment, into ceramic fibers,

- to develop ceramic composite materials (to this end, the composition of polysilanes is used as a matrix with carbon or ceramic fibers or, as a binder, with ceramic powders), - to develop coatings resistant to oxidation (oxidizable materials, such as carbon-carbon composite materials or carbon fibers, are coated with said polysilane and pyrolysis thereof), as materials for filling the pores of a porous ceramic material, and this in the aim of improving its mechanical properties, - to produce thin ceramic films for use in electronics. Polysilanes can be obtained in various ways such as, for example, by condensation of halosilanes, such as chlorosilanes, with alkali metals, as described in US Patents 4,783,516 and US 4,537,942, as well as in publication CHEM REV 1989, 89 (6), 1359, or by coupled dehydrogenation reactions involving catalysts, as described in the publications J AMER CHEM SOC, 1986, 2079, CH or by redistribution reactions, as disclosed in the US patents 4,824,919 and US 4,310,651, or else by chemical modification of simple polysilanes as described, in particular, in the publication POLYM REPRINTS, 1987, 28, 222 The polysilanes considered in the context of the present invention are those of formula general average with the following reasons

- (R _x R ' _y Si) a- (I) in which

the groups R are identical or different from each other in the polymer and represent hydrogen, a linear and / or branched and / or cyclic C 1 -C ₂₀ , preferably C 1 -C ₁₀ alkyl, an aryl, a aralkyl, an alkylaryl, a silicic or organosilicon residue,

the groups R ^′ are identical or different from each other and represent a C, -C, ₀ , preferably C, -C ₆ alkenyl, vinyl (C ₂ ) being particularly preferred,

- x, y = 0 to 3 and the sum of x + y is equal to 1, 2 or 3,

- x and y can vary from one motif to another,

- a is equal to or greater than 5;

- And at least 5% of the silicon units carry a group R ′ The present invention relates, more particularly, to polysilanes having, on their backbone, hydrogen groups (Si-H) and olefinic groups preferably having from 1 to 10 carbon atoms and, more preferably still, 2 carbon atoms, which corresponds to the vinyl group (Vi -CH = CH ₂ ) These are, eg, polyvinylhydrogenosilanes or polyvinylhydrogenomethylsilanes, having hydrogen and vinyl groups in their skeleton and hereinafter referred to as PVS PVS are, in particular, described in US Patent 4,783,516

Insofar as we consider alkenylated polysilanes as preceramic polymers, it should be known that this application involves crosslinking of said polysilanes before their transformation into ceramics by pyrolysis. This crosslinking takes place by hydrosilylation, that is to say by a reticular reaction addition involving hydrogen groups (SiH) and olefinic groups (SiVi).

PVS, and more generally alkenylated polysilanes, which contain the hydrogen and vinyl groups, are endowed with this ability to crosslink by hydrosilylation. But the disadvantage directly related to this is that these alkenylated polysilanes (e. G. PVS) are sensitive to light and oxygen. They are thus likely to react to ambient air, intense light, and / or high temperatures by hydrosilylation, which can lead to premature and undesired solidification of the product, which prevents any further shaping of the product. This rapid oxidation and crosslinking can cause, in some cases, spontaneous combustion of these alkenylated polysilanes precursors of ceramics. This is also the reason why the few commercial products based on alkenylated polysilanes (PVS), which exist on the market, are subject to very strict safety instructions. Their handling is very delicate. It therefore appears that there is patently a problem of stability of alkenylated polysilanes, such as PVS, with respect to air and with respect to temperature. This major drawback represents a major obstacle to the development of techniques for preparing ceramics with silicon carbide, from alkenylated polysilanes. This drawback is reflected, in particular, by the fact that the manipulations of the pre-ceramic polymers must be carried out under an inert atmosphere, which undoubtedly constitutes a very annoying constraint on the economic level and in terms of ease of implementation. .

In any event, this is not a factor capable of encouraging those skilled in the art to undertake the manufacture of articles based on ceramics, from alkenylated polysilanes (PVS).

Patent FR 2 686 091, in the name of the applicant, aims to overcome this drawback and teaches the addition of stabilizers formed by phenolic, amino or olefinic antioxidants, optionally combined with sulfur or phosphorus antioxidants. The stabilization obtained is all the more important as the concentration of stabilizer is high. However, taking into account the intended application, the addition of stabilizers, of antioxidant type, in very high concentration, results in inhibition or retardation of crosslinking, which has the consequence of reducing the pyrolysis yield in carbide of silicon, during ceramization. A low pyrolysis yield is equivalent to a significant emission of gas at the heart of the material, which causes the appearance of macroscopic cracks in the ceramic and, concomitantly, a decrease in the mechanical properties of the matrix. Hence the interest of maintain the pyrolysis yield as much as possible and therefore minimize the concentration of stabilizer in such pre-ceramic polymers. The optimum stabilizer concentration must therefore be less than 10% of the mass of the composition and, preferably, between 0.01% and 1%. The technique of crosslinking the polymer with oxygen in the air is also known, as described in US Patents 4,052,430 and US 4,100,233 by YAJIMA, the pioneer in the field of pre-ceramic polymers. Indeed, YAJIMA teaches the preparation and the shaping of a polycarbosilane. This polymer, which contains Si-H functions, oxidizes in the presence of oxygen to form new Si-O-Si (siloxane) bonds between the polymer chains. The oxygen introduced into the polymer, in the form of siloxanes, has the perverse effect of being found in final ceramics, which considerably reduces their mechanical characteristics. It therefore appears that polycarbosilane is a polymer which reacts with air. Used alone, its crosslinking requires oxygen to allow the Si-H bonds to oxidize to siloxanes and thus create new Si-O-Si bonds between the chains.

The invention described in US Pat. No. 4,783,516 relates to PVS comprising, in the same polymer chain, the two groups: Si-H and Si-alkenyl capable of reacting with each other by hydrosilylation, which ultimately causes the crosslinking of the polymer, in the absence of air and without the addition of a catalyst These improved PVS constitute an improvement compared to the technique of crosslinking in air according to YAJIMA.

Likewise, US Patents 4,639,501, EP 0,251,678, US 5,171,722 teach the formulation of new compositions comprising polysilanes or polycarbosilanes carrying Si-H functions and crosslinking agents, of alkenene type. The three inventions, described in these patents, also aim to avoid the air crosslinking step, which has the disadvantage of introducing oxygen into the final ceramic. Their pre-ceramic compositions are obtained by mixing polyorganosilanes of the polymethylsilanes or polycarbosilanes type (methylated), on the one hand, and organic or organosilicon compounds of the type, polyvinylsilane, polyvinylsilazane or polyvinylsiloxane, on the other hand, as well as a radical catalyst for initiate crosslinking. In this way, a composition is obtained which makes it possible to make a polysilane or polcarbosilane infusible before proceeding to its pyrolysis. There is no question of stabilization of alkenylated polysilanes in any of these patents (or patent applications). The problem concerned rather relates to obtaining a composition which crosslinks without oxygen to obtain a hold. mechanical or rigidity, which allow the crosslinked resin to retain its shape during pyrolysis.

However, such compositions always suffer from instability, whether during handling in air or during prolonged storage. In addition, the inventors cited in these patents recommend the addition of a second component: a crosslinking catalyst, which constitutes one more impurity in the composition of the ceramic.

To date, there is therefore no really satisfactory means for stabilizing air and / or heat of alkenylated polysilanes, or of polycarbosilanes, which reference is made to the known documents already relating to stabilization. of these pre-ceramic polymers or even to other prior documents relating to pre-ceramics comprising, essentially, alkenylated polycarbosilanes and alkenylated polysilanes undifferentiated in reduced quantities.

SHORT STATEMENT OF THE INVENTION:

In this state of knowledge, one of the essential objectives of the present invention is to find an effective means of stabilization of pre-ceramic polymers, of the alkenylated polysilane type, in particular polyvinylsilanes (PVS).

Another essential objective of the invention is to stabilize the alkenylated polysilanes, while preserving their ability to crosslinking and subsequent ceramization, with a high ceramic yield.

Another essential objective of the present invention is to provide a process for the air and / or heat stabilization of alkenylated polysilanes, which is simple to implement and economical. Another essential objective of the present invention is to provide an air and / or heat stabilized composition of alkenylated polysilanes which is crosslinkable in the absence of a hydrosilylation catalyst, a reaction which is very vital for achieving a ceramic yield. high during subsequent ceramic processing. The absence of a catalyst has the advantage of eliminating a product which can be assimilated to an impurity in the final ceramic.

Another essential objective of the present invention is to give access, thanks to the stabilization of the alkenylated polysilanes, to new methods of manufacturing articles comprising ceramic materials obtained by heat-crosslinking / ceramization of said alkenylated polysilanes. On the basis of these objectives, and after long and painstaking research and experimentation, the applicant has highlighted, in a completely surprising and unexpected, that alkenylated polysilanes (eg PVS) can see their sensitivity to air and / or heat greatly reduced, or even suppressed, by association with at least one polycarbosilane (PCS), present in an effective amount which preferably remains lower than that of the alkenylated stabilized polysilanes. This is all the more surprising since PCS is itself a product which oxidizes in air. It is therefore particularly unexpected that this PCS allows the stabilization of another compound which is unstable in air, namely PVS.

In addition, the applicant has discovered that there is a synergy between the PCS used as stabilizer and other stabilizers of alkenylated polysilanes, such as antioxidants.

This compatibility and this combined efficiency of the alkenylated polysilanes and of the PCSs was all the less predictable since the two polymers are of very different structure, i. e. : Si-Si for the alkenylated polysilane (PVS), and Si-C-Si for the polycarbosilane. One would therefore have expected a phase separation, as is typical for most known organic polymers. And yet, against all expectations, the two polymers are perfectly miscible. No phase separation can be detected with the naked eye.

One can easily perceive all the interest of the present invention, in particular with regard to the synthesis, the storage and the implementation of these alkenylated polysilanes, which are thereby facilitated.

It follows that the subject of the present invention is the use of polycarbosilane (PCS) for the stabilization of a preceramic polymer of the alkenylated polysilane type, preferably vinyl.

The updating of this new PCS function by the applicant is at least as advantageous as it is unpredictable. It provides a composition based on alkenylated polysilanes and PCS, which is easy to handle in ambient air, without any particular precaution and without risk of spontaneous combustion, or even explosion, which, it will be agreed, n is not a small asset.

Another particularly advantageous property arising from the use of PCS as a stabilizer in accordance with the invention is the ability to hydrosilysation and therefore to the "heat-crosslinking" of the alkenylated polysilanes with the PCS. This gives this composition all the qualities to be a precursor of efficient ceramic.

In addition, it should be noted that the PCS, in its range of optimal stabilizing concentration, is not at the origin of a phenomenon of reduction in yield of pyrolysis On the contrary, the results achieved in this regard are entirely satisfactory, even improved.

DETAILED DESCRIPTION OF THE INVENTION:

With regard specifically to stabilizing concentrations, it is provided, in accordance with the use according to the invention, that the PCS represents, at most, 50% by weight of the PCS / alkenylated preceramic mixture. Preferably, the concentration of

PCS relative to this mixture is less than 40% by weight and, more preferably still, is between 20-30% by weight.

According to an advantageous method of use according to the invention, the alkenylated polysilane is chosen from polymers and mixtures of polymers of general average formula comprising the following units:

in which

the groups R are identical to or different from one another in the polymer and represent: hydrogen, a linear and / or branched and / or cyclic C 1 -C ₂₀ , preferably C _x -C ₁₀ , aryl alkyl, an aralkyl, an alkylaryl, a silicic or organosilicon residue; the groups R ′ are identical or different from each other and represent a C _r C ₁₀ , preferably C _r C ₆ alkenyl, vinyl (C ₂ ) being particularly preferred,

- x, y = 0 to 3 and the sum of x + y is equal to 1, 2 or 3,

- x and y may vary from one motif to another, - a is equal to or greater than 5;

- And at least 5% of the silicon units carry an R 'group.

Preferably, this alkenylated polysilane is a copolymer of the polyvinylsilane (PVS) type, essentially consisting of units (I), respectively

• of vinylhydrosilane type

C I H = CH 2,

- Si - CH ₃ and of the methylhydrosilane type

H

I -If—

Œ ₃ For more detail on the structure and on the synthesis of these PVS, reference is made to US Pat. No. 4,783,516, which is incorporated in the present description by reference. There are, moreover, commercial PVS, such as that sold by the company UNION CARBIDE under the reference Y-12044.

With regard to the polymer which serves as a support for the invention, through the discovery of a new of its functions, namely PCS, it is preferable to choose it from the polymers and mixtures of polymers of general formula medium comprising the following grounds (II):

1 ™ 2,

(If R _b 'R _c ² ) (CR _d ³ R _e ⁴ ) _f (U)

in which :

- R ¹ , R ² , R ³ and R ⁴ are identical or different in the same unit or from one unit to another and represent a radical corresponding to the same definition as that given above for R, the alkyls in C, -C _{| 0} and hydrogen being preferred and the case in point: R ¹ = CH ₃ ; R ² , R ³ , R ⁴ = H, being more particularly preferred,

- b, c = 0 to 3 with b + c <3;

- d, e = 0 to 3 and d + e ≤ 3;

- f = 1 or 2;

- b, c, d, e, may vary from one motif to another;

- n> 3.

Examples of patterns (II) are given by:

H-Si (CH,), (CH,) 4- or H-HSiCH— (CH,) -

US Patents 4,052,430 and / or 4,220,600, which are incorporated into the present application by reference, describe examples of polycarbosilanes and give information on their structure, as well as on their mode of production.

PVS and PCS are, respectively, liquid and solid under ambient conditions of temperature and pressure. It is therefore particularly unexpected that the mixture of these two products proves to be, not only possible, but moreover, leads to a homogeneous mixture having all the properties of stability, heat-crosslinking and pyrolysis performance, mentioned above.

In addition, another advantage of the use according to the invention is that the viscosity of the PVS / PCS mixture is variable over a wide viscosity range. To adjust it, it suffices to foil the quality and the quantity of PVS and PCS According to another characteristic of the use according to the invention, at least one antioxidant chosen from phenolic, olefinic or amino antioxidants and, preferably from phenolic antioxidants and mixtures thereof is used in association with the PCS. are those described in patent FR 2 686 091, which is in the name of the applicant and which is included in the present application by reference.

The preferred antioxidant concentration range compared to the alkenylated polysilane / PCS / antioxidant mixture is from 0.001% to 5%, but, more preferably still, from 0.01-1% Being able to be satisfied with an antioxidant concentration relatively low avoids the drawbacks associated with the use of the latter, as regards the pyrolysis yield

According to another of its aspects, the present invention relates to a process for stabilizing a preceramic polymer, of the alkenylated polysilane type, preferably vinyl, characterized in that it consists in mixing, in a solvent or not, said preceramic polymer with a stabilizer selected from the group comprising polycarbosilanes (PCS) - preferably those as defined above - antioxidants of the type of those also defined above and their mixtures The pre-ceramic polymers more particularly concerned by this process are alkenylated polysilanes of the type defined above, including in particular the PVS

According to an advantageous method of the process, the PCS represents at most 50% by weight of the mixture PCS / alkenylated polysilanes

All the other variants and advantages of the use according to the invention described above can also be transposed to the above process. The present invention also relates to a stabilized pre-ceramic composition of alkenylated polysilane, preferably of polyvinylsilane, characterized

In that it comprises at least one polycarbosilane (PCS) as stabilizer, said PCS being present in an amount of at most 50% by weight relative to the PCS / alkenylated polysilane mixture,

• and in that it is free of crosslinking catalyst

This new composition derives from the use presented above. It derives its advantages and exploits the new updated stabilizer function for

PCS One of these key advantages is the absence of crosslinking catalyst. This advantage is all the more interesting because it is associated with the three attributes which are endowed with this new composition, namely: stability in air and / or in heat, "heat-crosslinking" and / or high pyrolysis yield.

INDUSTRIAL APPLICATION:

The present invention has the advantage of opening up interesting prospects and outlets in the manufacture of ceramic materials including, in particular, composite materials comprising ceramics. For these purposes, a person skilled in the art is able to exploit the information contained in the technical literature and the patents which describe applications in which a ceramic precursor is used.

Also, it should be considered that the invention also relates to a method of manufacturing articles, at least partly made of ceramic, characterized in that use is made.

• alkenylated polysilane stabilized in accordance with the use and / or the process as described above,

• and / or an alkenylated TPCS / optionally antioxidant polysilane composition, which is also presented above.

By way of examples, mention may be made of the processes relating to the impregnation of fibrous supports, for example in the manufacture of composites and more particularly the impregnation of fabrics, powders or porous ceramics, made of mineral compounds

(SiO ₂ , AlO ₂ ..) or carbon, as described in the patent application

EP 0 550 305.

The present invention is particularly suitable for this application which requires a certain number of specifications: - to be handled at room temperature, the preceramic composition comprising polysilanes must be in semi-solid form, it must not flow;

- However, when hot under pressure, this polymer composition must be sufficiently liquid to interpenetrate the fibrous support, which can be multilamellar; a solid polymer is unsuitable for this application because it tends to crack and does not adhere well to fabrics; a liquid polymer is just as unsuitable because it cannot be fixed in the fabric;

the preceramic polymer must be able to thermo-crosslink, so as to be able to memorize the shape imposed on the support and to keep it during the pyrolysis of ceramic transformation, - the heat-crosslinked pre-ceramic polymer must offer high pyrolysis yields.

It has appeared that alkenylated polysilanes (eg PVS), stabilized using PCS, or even antioxidants, are also suitable for a spinning application for the preparation of ceramic fibers, as described in US Pat. No. 5,171 722.

The present invention can also be applied in the field of surface coatings, where the manufacture of articles coated with ceramic is proposed, using the methodology of coating a polymer solution on a surface to be protected (against abrasion, against high temperature oxidation, etc.).

The present invention will be better understood in the light of the examples below which highlight the advantages of stabilized PVS / PCS mixtures.

DESCRIPTION OF THE FIGURES

- Fig. 1 is an infrared spectrum of PVS according to Example 1 before exposure to air.

- Fig. 2 is an infrared spectrum of the PVS according to Example 2 after 10 min in the open air. In these figs. 1 and 2, the abscissa axis corresponds to the wavelengths in cm- ¹ and the ordinate axis to arbitrary units (ua) (peak Si-O = 1090 cm ⁴ , peak Si-Me = 1 245 cm- ').

- Fig. 3 shows the evolution of the IR absorbance signal corresponding to the SiO bonds at 1090 cm ⁻¹ , as a function of time in hours, for PCS concentrations of 0, 20, 30 and 50% by weight relative to the PCS mixture + PVS The ordinate axis is graduated in absorbance IR = AA _Q

EXAMPLES

EXAMPLE 1: PREPARATION AND CONTROL OF THE PVS POLYMER.

The PVS used is a polysilane of PVS 200 type (marketed by FLAMEL TECHNOLOGIES) resulting from a reaction of dichloromethylsilane, dichloromethylvinylsilane and trimethylsilane with sodium in the molten state, in a mixture of toluene-tetrahydrofuran solvents, such as such described in US Pat. No. 4,783,516. It is a SiC ceramic precursor polymer which is liquid at ambient temperature and thermoplastic in the range of temperatures from 25 to 100 ° C and heat-crosslinkable in the range of temperatures ranging from 101 to 300 ° C. It can be defined by the following formula

(MeHSi) o, i6 (MeSiVi) ₀ , 5 ₆ (Me ₃ Si) o, ₂₈ The stability of the precursor was determined by the deposition of a thin layer of the polymer on a KBr slide After evaporation of the solvent ( 10 min) A series of IR spectra were measured on the polymer layer during a period of one hour. The oxidation of the sample was determined by the evolution of the height of the signal corresponding to the Si-O bonds at 1. 090 cπv ¹ (internal reference Si-Me 1 245 cm ¹ ), and the appearance of a crosslinked polymer skin on the surface of the sample

In the absence of additional stabilizers, the oxidation of the sample is initiated immediately after exposure of the sample to the open air A skin of crosslinked polymer is formed on the surface of the sample in less than 10 minutes No signal due to Si-O bonds does not appear in the product before its exposure However, after 10 minutes in air, the signal increases rapidly The spectra of the polymer, before and after crosslinking, are provided in figs. 1 and 2 attached

In one of the tests, the reaction was so rapid that the polymer ignited spontaneously This test highlights the instability of the polymer in the open air The pyrolysis yield of this precursor under an inert atmosphere up to 1000 ° C is 66-68%

EXAMPLE 2: PREPARATION OF A STABILIZED COMPOSITION

PVS / BHT (ANTIOXIDANT).

To the polymer, 0.01% by weight of BHT (di-t-butylhydroxytoluene) is added according to patent application FR 2 686 091, the pyrolysis yield is 62%, ie a yield loss of 4-6% by report on the pyrolysis yield of the polymer alone This test highlights the loss of yield due to the addition of an antioxidant to the PVS

EXAMPLE 3 PREPARATION OF NEW STABILIZED COMPOSITIONS

BASED ON PVS / PCS.

A mixture of 19 g of PCS, 30 g of PVS, 50 mg of tris (nonylphenylphosphate), 50 mg of BHT (di-t-butylhydroxytoluene) in 20 ml of methylcyclohexane is prepared. The PCS polycarbosilane used is a polymer obtained from the MITSUI & Cie. It is a solid product at room temperature and thermoplastic at a higher temperature (up to 300 ° C). The mixture is stirred for 2 days, so as to obtain complete dissolution of all of its constituents. The viscosity of this mixture is approximately 50 mPa.s at 25 ° C (depending on the viscosity of the two starting polymers). Its viscosity can be controlled by adjusting the quantity of the various conventional constituents, by choosing their quality. Control of this viscosity will depend on the intended end application.

EXAMPLE 4: EVALUATION OF THE AIR STABILITY OF THE PCS / PVS COMPOSITIONS.

To illustrate the stability of the composition in air, tests were carried out under the following conditions: a very thin layer of the solution was deposited on a KBr slide. After evaporation of the solvent (10 min), a series of IR spectra were measured on the dried polymer layer during a period of 50 hours. The stability of the sample was determined by following the evolution of the signal corresponding to the Si-O bonds at 1090 cπr ¹ (internal reference Si-Me 1 245 cm ¹ ). Fig. 3 attached gives the results obtained from the growth of the Si-O signal, characteristic of oxidation, as a function of time. It is noted that, when the quantity of PCS in the composition is increased, the stability is increased (lower evolution of the mixture).

EXAMPLE 5: DRYING / CROSSLINKING AND PYROLYSIS OF A MIXTURE

PRE-CERAMIC PVS / PCS.

The mixture obtained in Example 2, with 60% PVS / 40% PCS, was dried in the open air for 3 days in an alumina pyrolysis boat. The resinous product, resulting from drying, is transparent and homogeneous to the naked eye, which confirms a homogeneous mixture (no macroscopic separation of the polymer phases). The transformation of this resin into ceramic is carried out by pyrolysis under an inert atmosphere from 25 ° C to 1300 ° C. Significant exotherm occurs at 210 ° C (measured by differential scanning calorimetry). This is indicative of the crosslinking of the mixture. The yield of the polymer / ceramic transformation is around 68%. Analysis of the ceramic is Si: 56% C: 44%, the oxygen level being undetectable (<2%), showing the absence of oxygen incorporation in the ceramic, therefore the efficiency of the mixtures PVS / PCS.

The following table shows the pyrolysis yields as a function of the proportion of PCS in the mixture.

* without addition of PCS, but stabilized by the addition of 0.1% of BHT according to the invention FR 2 686 091.

***

The preceding examples (Examples 1-5) clearly demonstrate all the advantages which the invention makes it possible to achieve, in addition to the air stability for the pre-ceramic. Mention will be made, in particular, of the modular nature of the viscosity of the mixture, from which derives its adaptability to a good number of applications, as well as the good ceramic yield obtained which is greater than 60%.

Claims

CLAIMS:

1 - Use of polycarbosilane (PCS) for the stabilization of a preceramic polymer, of the alkenylated polysilane type, preferably vinyl

2 - Use according to claim 1, characterized in that the PCS represents, at most, 50% by weight of the PCS / alkenylated ceramic mixture

3 - Use according to claim 1 or 2, characterized in that the alkenylated polysilane is chosen from polymers and mixtures of polymers of general average formula comprising the following units

in which

the groups R are identical or different from each other in the polymer and represent hydrogen, a linear and / or branched and / or cyclic C 1 -C ₂₀ , preferably C 1 -C ₁₀ alkyl, an aryl, a aralkyl, an alkylaryl, a silicic or organosilicon residue, - the groups R ′ are identical or different from each other and represent a C _r C ₁₀ , preferably C 1 -C ₆ alkenyl, vinyl (C ₂ ) being particularly preferred ,

- x, y = 0 to 3 and the sum of x + y is equal to 1, 2 or 3,

- and at least 5% of the silicon units carry an R 'group

4 - Use according to claim 3, characterized in that the alkenylated polysilane is a copolymer of the polyvinylsilane type (PVS), consisting of units (I), respectively • of the vinylhydrosilane type

CH = CH,

I ²

- If— i

CH,

• and of the methylhydrosilane type

H

I -If—

CH ₃

5 - Use according to any one of claims 1 to 4, characterized in that the PCS is chosen from polymers and mixtures of polymers of average general formula comprising the following units (H)

in which :

- R ¹ , R ² , R ³ and R ⁴ are identical or different in the same unit or from one unit to another and represent a radical corresponding to the same definition as that given above for R, the alkyls in C „C ₁₀ and hydrogen being preferred and the case in point: R ¹ = CH ₃ ; R ² , R ³ , R ⁴ = H, being more particularly preferred;

- b, c = 0 to 3 with b + c <3;

- d, e = 0 to 3 and d + e <3;

- f = 1 or 2; - b, c, d, e, may vary from one motif to another;

- n> 3.

6 - Use according to any one of claims 1 to 5, characterized in that one implements, in association with the PCS, at least one antioxidant chosen from phenolic, olefinic or amino antioxidants and, preferably from phenolic antioxidants and mixtures thereof.

7 - Method for stabilizing a preceramic polymer, of the alkenylated polysilane type, preferably vinylated, characterized in that it consists in mixing said preceramic polymer with a stabilizer selected from the group comprising: polycarbosilanes (PCS), preferably those as defined in claim 5, antioxidants, as defined in claim 6 and mixtures thereof.

8 - Method according to claim 7, characterized:

In that the preceramic polymer is an alkenylated polysilane, as defined in claim 3, preferably in claim 4,

• and in that the PCS represents at most 50% by weight of the PCS / alkenylated polysilane mixture.

9 - Stabilized preceramic composition of alkenylated polysilane, preferably of polyvinylsilane, characterized: • in that it comprises at least one polycarbosilane (PCS) as stabilizer, said PCS being present in an amount of at most 50% by weight per relative to the PCS / alkenylated polysilane mixture,

• and in that it is free of crosslinking catalyst.