SK57196A3 - Elastomer organopolysiloxane composition heat stabilized by metallic nanoparticles and gained transparent elastomer - Google Patents

Abstract

Compsn. polyorganosiloxane exhibiting thermal stability when crosslinked comprises organophilic particles dispersed throughout the compsn. made from at least one metallic oxide in the form of polycrystalline particles of max. size 50 nm (made from 4 - 6 nm crystals) and a treated surface of BET between 250 and 300m<2>/g, pref. 250 mg<2>/g. Also claimed is a transparent polyorganosiloxane elastomeric material obtd. by hardening the claimed compsn. Pref. the metal oxide is chosen from alumina, silica, transition metal oxides, esp. TiO2, and rare earth oxides, esp. CeO2. The organophile particles are prepd. by reacting a hydroalcoholic suspension of polycrystalline particles with a soln. of alcoholic anhydride consisting of at least one alkoxysilane of formula Si(OR)xR'4-x (I). R = 1-6C alkyl esp. methyl or ethyl; R' = alkyl, cycloalkyl, aryl, alkylaryl, arylalkyl, alkenyl or alknyl, esp. octyle, dodectyl or octadectyl; and x = 1 to 3, esp. 3. The polycrystalline particles are prepd. by reacting metal oxide polycrystalline particles with at least one tetra-alcoxysilane of formula Si(OR'')4 (II). R" = 1-5C alkyl. The compsns. contains between 0.1 and 5% organophilic particles pref. 0.3 to 1%. The organophilic particles comprise 60 to 90% TiO2 and 40 to 10% SiO2.

Description

Technical field

The invention relates to organopolysiloxane compositions which, when crosslinked, provide elastomers having improved thermal stability as well as vulcanized elastomers obtained therefrom. The invention also relates to methods for their preparation.

BACKGROUND OF THE INVENTION

Organopolysiloxane compositions providing elastomers are well known commercially available materials. Compared to organic elastomers, organopolysiloxanes or silicones have a higher heat resistance. However, at temperatures in excess of 200 ° C and in particular in the range of 220 to 300 ° C, the organopolysiloxane elastomers can also lose their elastomeric properties and become hard and brittle after a period of exposure to heat.

Attempts have been made for a long time to improve the thermal stability of silicones and have consistently consisted of adding compounds of certain types to the matrix of organopolysiloxane compositions.

The British patent thus describes the addition of at least 3% by weight of fuming titanium dioxide to the base elastomeric composition, while also describing improved behavior (compression and reversal resistance) of the cured elastomer at temperatures of 232 and 315 ° C maintained for 16 and 30 ° C respectively. 24 hours.

According to French patent FR-A-2 308 664, the improvement of the thermal stability of the cured siloxane-based elastomeric compositions is achieved by incorporating titanium dioxide (which has a particle size in particular between 15 and 40 microns) and cerium oxide and / or cerium hydroxide. These two additives should be able to be more effective than titanium dioxide alone and to allow the behavior of the cured elastomer to be improved at temperatures ranging from 220 to 270 ° C.

European Patent Application EP-A-0 595 078 discloses as a thermal stabilizer a mixed titanium dioxide containing 1 to 30% by weight of alumina or silica, the mixed oxide having a specific BET surface area of between 10 and 150 m ³ / g. Said oxides have a heat-stability of their specific surface area higher than that of titanium dioxide P25 alone (BET surface area of 50 m ³ / g).

However, none of the solutions known to date can guarantee significant stability when exposed to high temperatures, in particular temperatures in excess of 250 ° C and in particular in the range of 275 to 300 ° C, for a relatively long period of time. Therefore, there remains a need to further improve the thermal stability of said elastomers in a manner that allows substantially at least their elastomeric properties and thus their ability to be used for the purposes for which they are intended, even if repeated or / and prolonged exposure to heat is maintained. .

A further disadvantage of the solutions proposed so far is that the incorporation of titanium dioxide-type stabilizers makes it impossible to obtain transparent elastomers, since it is not only necessary to maintain the original transparency but also to maintain this property of the cured elastomer substantially even after the elastomer has been exposed to high temperatures.

It is therefore an object of the present invention to provide organopolysiloxane compositions from which, after crosslinking, transparent elastomers with high thermal stability are obtained.

SUMMARY OF THE INVENTION

It has now been found that the above object can be achieved by dispersing in the elastomeric composition organophilic metal oxide particles onto which organosilicon groups are grafted. This measure makes it possible to obtain particles with a high specific surface area which are highly dispersible in the base elastomeric composition and which allow the final elastomer to be stabilized against heat (by inhibiting thermal oxidation by deactivating CH ₂ radicals and preventing oxidation bridges). The composition according to claim 1, characterized in that the base elastomer composition does not contain compounds or pigments which are intended to later obtain the opaque nature of the elastomer.

Accordingly, the present invention provides an elastomeric organopolysiloxane composition providing, after crosslinking, a thermally stable elastomer and containing in dispersed form organophilic particles of at least one metal oxide in the form of polycrystalline nanoparticles, preferably consisting of 4-6 nm crystallites, of at most 50 nm and having a BET specific surface area of m ² / g, preferably between 250 and 300 m ² / g, which would be subjected to a surface treatment to obtain an organophilic character by grafting the organosilicon groups.

In the context of the invention, a thermally stable elastomer is understood in particular to mean an elastomer which retains its elastomeric properties and which does not become hard or brittle when subjected to a temperature of more than 200 ° C, in particular a temperature between 275 and 300 ° C days, especially for 3 days. Preferably, the term thermally stable elastomer refers to elastomers which are thus resistant to a temperature of greater than 250 ° C, in particular a temperature of between 275 and 300 ° C, maintained for more than 3 days, especially for 10 days or longer.

In fact, for example, an elastomer that has retained acceptable elastomeric properties and which does not become brittle after being exposed to a temperature of 275 ° C for 10 days has been remarkably obtained in the context of the invention. In doing so, the hardness, elasticity, tensile strength, elongation at break and tear strength of the elastomer remained within acceptable limits, i.e. within the limits that allow the intended use of the elastomer, during said exposure. In addition, the elastomer, which was initially transparent, surprisingly retained to a significant extent the original trasparence, both after the addition of the stabilizer and during aging at this temperature.

Preferably, organophilic metal oxide particles selected from the group consisting of alumina, silica, transition metal oxides, especially titanium dioxide, and rare earth metal oxides, especially cerium oxide, are used.

The organophilic particles are preferably obtained by a so-called compatibilization or grafting reaction of organosilicon groups, in which an aqueous / alcoholic suspension (metal oxide particles) is reacted with an anhydrous alcohol solution b) containing at least one alkoxysilane of the formula I

Si (R) R (1) in which

R represents an alkyl group having 1 to 6 carbon atoms,

R 'represents a hydrocarbon group selected from the group consisting of alkyl, cycloalkyl, aryl, alkylaryl, arylalkyl, alkenyl and alkynyl containing at least one carbon atom and x is an integer from 1 to 3, said alcohols which are the same or different, containing 1 to 3 5 carbon atoms.

The polycrystallite nanoparticles of the metal oxide may be subjected to a surface pretreatment in order to obtain said basic polycrystallite nanoparticles, in particular the treatment with a tetraalkoxysilane of the general formula 2. Said oxide particles are preferably suspended in an aqueous / alcoholic medium, the alcohol preferably containing 1 to 5 carbon atoms, the suspension thus obtained c) is reacted with at least one tetraalkoxysilane of formula 2

Si (OR (2) in which R '' represents an alkyl group containing 1 to 5 carbon atoms.

Said tetraalkoxysilane is preferably added in small amounts to said suspension. At the end of the treatment, a stable aqueous / alcohol suspension of a) particles is obtained, which suspension can then be directly reacted with an alkoxysilane of formula 1 as mentioned above in connection with self-compatibility.

Said alkoxysilane of formula 1 hydrolyzes in aqueous / alcoholic medium suspensions a) to a silanol capable of condensing with one or more hydroxyl functional groups present on the metal oxide surface.

The overall compatibilization (grafting) reaction can thus be described as a sequence of two reactions:

1) hydrolysis of alkoxysilane = SULF + H2O → SiOH + ROH

2) condensation of the obtained silanol with a metal oxide surface = MOH + = SiOH -> = M-O-Si = + H 0

Many silicon compounds are suitable for carrying out such grafting reactions by the mechanism of hydrolysis and subsequent concentration, but not all of them can be used in the compositions of the invention.

Highly suitable for this purpose are the alkoxysilanes of formula (1) because they are soluble in the grafting medium and do not hydrolyze too rapidly in the medium.

A silicon compound that exhibits a considerable tendency towards hydrolysis would prefer condensation of its own molecules rather than condensation with surface hydroxyl groups of minerals. This occurs in the case of disilazane.

This drawback is eliminated by the choice of alkoxysilanes of general formula (1) which have less reactivity.

Preferred compounds are those compounds of formula (I) wherein R is methyl or ethyl.

It is likewise preferred that in the general formula 1 R 'represents an octyl group, a dodecyl group or an octadecyl group.

In addition, preferred compounds are trialkoxysilanes, i. the compound of formula 1 wherein x is 3.

Preferred examples are octyltrimethoxysilane (OTMS), dodecyltrimethoxysilane (DTMS) or n-octadecyltrimethoxysilane (ODTMS).

Among the metal oxides, mention may be made in particular of titanium dioxide, used in the form of rutile or anatase. In particular, it may be lenticular or platelet-shaped anatase. Titanium dioxide described in patent applications EP-A-0 335 773 and EP-A-0 351 270 can also be advantageously used.

The ratio of compounds 1 and 2 is preferably between 4 and 40% by weight. Preferably, the organophilic particles comprise 60 to 90% titanium dioxide and 40 to 10% silica.

In the following, a preferred method for preparing the organophilic oxide particles that can be used in the present invention will be described.

In order to carry out this process, an aqueous / alcohol suspension of a) metal parent particles is prepared. Said oxide particles are readily dispersed in said medium. The weight ratio of water to alcohol in this group is preferably between 0.2 and 0.8. Preferably, a dilute suspension is used, the particle concentration by weight of the suspension preferably being between 5 and 200 g / l. The aqueous / alcoholic suspension a) may be an acid or a base and the pH of the suspension will vary according to the metal oxide used.

When operating in a basic environment, the pH is generally between 9 and 12. The base used for adjusting the pH may be a base selected from alkali metal hydroxides such as, in particular, sodium hydroxide, lithium hydroxide and potassium hydroxide, alkali metal carbonates such as, in particular, sodium carbonate and potassium carbonate, and other substances from the group consisting of ammonia and primary, secondary and tertiary amines such as monoethylamine or diethylamine. It is particularly preferred that the pH is higher than IEP + 2, where IEP is the isoelectric point of the oxide used.

When operated in an acidic environment, the pH is generally preferably between 1 and 5. The acid used to adjust the pH may be selected from the group consisting of nitric acid, hydrochloric acid, acetic acid, and other organic acids. In particular, it is δ

preferably, the pH is less than IEP-2, wherein the IEP is the isoelectric point of the oxide used.

An anhydrous alcohol solution b) containing at least one alkoxysilane of the formula I is then slowly poured, taking care that the solid is still dispersed in the reaction medium. This makes it possible to avoid the risk of self-condensation of the alkoxysilane or alkoxysilanes.

The alcohol contained in solution b) is preferably the same as the alcohol contained in aqueous-alcohol suspension a).

The amount of alkoxysilane added varies according to the size of the treated particles, the weight ratio of the total amount of alkoxysilane 1 to the amount of oxide being from 1 to 60%, preferably from 2 to 30 I.

The temperature of the reaction medium during said addition is preferably 15 to 70 ° C and preferably equal to the ambient temperature.

During the addition, flocculation occurs in the suspension, indicating a change in the interfacial properties of the oxide particles. Upon completion of the addition, it is preferred to allow the reaction to continue for one or two hours.

After completion of the reaction, the compatibilized particles can be isolated in powder form.

To obtain a powder, the reaction mixture is subjected to separation of the liquid phase from the solid phase, preferably by centrifugation. The separated solid is washed with alcohol and then dried at a low temperature, for example 60 ° C.

If the grafting of the alkoxysilane of formula (1) is carried out in an ammoniacal environment, the reaction mixture is preferably subjected to a low-temperature distillation step to remove ammonia and alcohol before separating the solid from the liquid.

According to a specific embodiment, the hydrophobic particles can be dispersed in the organic composition by phase transfer. In this case, the organic composition is added to the aqueous or aqueous / alcoholic suspension. Conversion of the particles from the aqueous phase to the organic phase can be accomplished by simple agitation followed by separation or distillation.

Without departing from the scope of the invention, the organophilic particles can be directly converted into alpha, omega-dihydroxylated polydiorganosiloxane oil, which is subsequently used in the silicone elastomer composition.

In the organopolysiloxane composition, it is possible to disperse from 0.1 to 5%, in particular from 0.2 to 3% and preferably from 0.3 to 1% of the organolithium particles of the invention (e.g. titanium dioxide particles treated according to the invention), based on the total weight of the composition. .

In general, the dispersed form of the organophilic particles according to the invention have in the composition a specific BET surface area of between 50 and 160 m ³ / g and / or a size of between 10 and 50 nm.

It will be appreciated that the particles of the invention may in particular have 0.5 ppm, a vanadium and tin content of less than 5 ppm.

that the organophilic compositions typically have a high degree of purity, a chromium and antimony content less than 1 ppm nickel, and a lead content

A further object of the invention is a process for stabilizing organopolysiloxane elastomers or the preparation of such stable elastomers, which is based on the above-described incorporation into the basic elastomeric composition of the stabilizer described by its characteristics, properties and / or its preparation.

Yet another object of the invention is a transparent organopolysiloxane elastomer obtainable by curing the above composition of the invention.

The last object of the invention is a transparent and thermally stable organopolysiloxane elastomer, which means that it retains the basis of its elastomeric properties after exposure to a temperature between 275 and 300 ° C, for several days, especially for 3 days, and especially for at least 3 days, for 10 days or more.

In the following, the invention will be described in more detail using exemplary and non-limiting compositions to be used in the present invention, meaning that the organophilic particles of the invention can be dispersed in these compositions. Non-limiting examples of the preparation of these nucleophilic particles will also be described in this section, as well as an example of compositions stabilized according to the invention and an example of compositions stabilized by hitherto known commercially available compounds, where the compositions are subjected to considerable thermal stress.

Basic elastomeric compositions

In particular, the invention applies to a thermosetting organopolysiloxane composition comprising:

(A) 100 parts of at least one organopolysiloxane gum having a viscosity of more than 1 000 000 mPa · s at 25 ºC,

B) 5 to 150 parts of reinforcing filler,

C) 0.1 to 7 parts of organic peroxide; and

F) an anti-structural compound.

The abovementioned gums A are well known products marketed by silicone manufacturers. These gums can be produced by methods already known.

Diorganopolysiloxane gums A having a viscosity higher than 1 000 000 mPa · s at a temperature of 25 ºC, preferably higher than

000,000 mPa · s at 25 ° C are high molar linear polymers in which the diorganopolysiloxane chain consists mainly of units of the formula R 2 SiO. This chain is blocked at each end by units of formula R ₃ SiO _qb or by a radical of formula OR '. In the above formulas, R and R 'are organic groups, especially alkyl groups. It is not excluded that in the organopolysiloxane chain will be no small amount of units other than the units of formula R ^ SiO, for example units of the formula RSiO ^ _p and / or SiO _2, in the amount of not more than 2%, based on the number of units of formula R ^ SiO. While the meanings of R and R 'will be described in more detail below, it should be noted that an alkyl group is understood to mean an alkyl group having 1 to 4 carbon atoms and in particular a methyl group, an ethyl group and an n-propyl group.

As specific examples of units of formulas R SiO and

The R ₃ SiO _axes and OR 'groups may be represented by groups of formulas;

(CH ₃ ) ₂ SiO,

CH ₃ (CH ₂ = CH) SiO,

CH (CH) SiO, (C _e H _s ) ₂ SiO,

CH (CH) SiO, (CH CH CH) CH SiO, CH (nc H) SiO, (CH) SiO, (CH) CH = CHSiO 0, s' (CH ₃ ) (C ₆ H _s ) (CH _a = CH) SiO CF CH CH Si

2 2 O, S '

OH,

-OCH, ’

-OC H,

-O-n-C H,

-O-iso-C H,

-O-n-C H,

0,5 '

-OCH CH OCH

2 3

Preferred rubbers and the rubber formed by the sequence siloxyjednotiek formula R ^ SiO, blocked at the ends of the chains of siloxy units R ₃ SiO _q and / or a radical of formula R ', in which formulas the symbols R, which are identical or different, are methyl ethyl, n-propyl, phenyl, vinyl and 3,3,3-trifluoropropyl, and at least 60% of the total is methyl and at most 3 mol% of the siloxy units are organovinylsiloxy units , and R 'represents a hydrogen atom, a (1-4C) alkyl group or a beta-methoxyethyl group.

Particularly within the scope of the present invention is the use of vinyl vinyl dimethyl polysiloxane rubber, i.e., vinyl-containing rubber, attached to the silicon atoms in the chain and / or to the chain ends, wherein the molar content of vinylmethylsiloxy units is at most 1%.

Fillers B, preferably reinforcing silica B, are used in an amount of 5 to 150 parts, preferably in an amount of 8 to 100 parts, per 100 parts of diorganopolysiloxane gums A. These silicas are selected from the group consisting of silica derived from combustion and precipitated silica. These oxides have a BET and CTAB specific surface area of at least 50 m ^a / g, preferably greater than 70 m ^a / g, a mean primary particle size of less than 80 nanometers and a bulk density of less than 200 g / l.

These silicas can be incorporated as such, i.e. without treatment, or after they have been treated with organosilicon compounds which are usually used for this purpose. Among these compounds may be mentioned: methylpolysiloxanes such as hexamethyldisiloxane and octamethylcyclotetrasiloxane, methylpolysilazanes, such as hexamethyldisilazane and hexamethylcyclotrisilazane, chlorosilanes such as dimethyldichlorosilane, trimethylchlorosilane, or Methylvinyldichlorosilane dimetylvinylchlórsilán, or alkoxysilanes, as dimetyldimetoxysilán, dimetylvinyletoxysilán or trimetylmetoxysilán. During this treatment, the silicon oxides may increase their starting weight by up to 20%, preferably by approximately 18%.

In addition to the reinforcing silicas B, semi-reinforcing inorganic fillers B may be introduced. These fillers B are thicker and have a mean particle diameter of greater than 0.1 micrometer. These fillers are represented by ground quartz, calcined clay, diatomaceous earth, calcium carbonate and iron, titanium, magnesium and aluminum oxides, zinc sulfate and barium sulfate. These fillers are introduced in an amount of 5 to 120 parts, preferably 10 to 50 parts, per 100 parts of rubber A. These inorganic fillers can be used as such, i.e. without any treatment, or after treatment with the organosilicon compounds mentioned above. connection with reinforcing silicas B.

The organic peroxides are used in an amount of from 0.1 to 7 parts, preferably from 0.2 to 5 parts, per 100 parts of rubber A. These organic peroxides are well known products for technically skilled persons and include, in particular, benzoyl peroxide, 2,4 -dichlorobenzoyl peroxide, dicumylperoxide, 2,5-bis- (t-butylperoxy) -2,5-dimethylhexane, tert-butylperbenzoate, tert-butylperoxyisopropyl carbonate, di-tert-butyl peroxide or 1,1-bis- (tert-butylperoxy) - 3,3,5-trimethylcyclohexane. These may be directly cross-linked with platinum (a platinum compound).

These individual peroxides generally decompose at different temperatures and at different speeds. Their use is governed by the desired curing conditions. In the case of vinyl rubbers 2,5-bis- (tert-butylperoxy) -2,5-dimethylhexane is a particularly suitable peroxide.

The compositions of the invention may additionally contain 0.1 to 10 parts, preferably 0.3 to 5 parts, of at least one diorganopolysiloxane oil (F) having a viscosity of at least 5000 mPa · s at 25 ° C, consisting of a sequence of units of formula R ' SiO and blocked at each end of its chain by a group of the formula OR ', wherein in the formulas R' ', which are the same or different, mean a methyl group, a phenyl group or a vinyl group and at least 40% of the total is made up of methyl groups, and R 'has the meaning given above for diorganopolysiloxane gums A.

The meanings of the general symbols R ' ¹ and R · have already been explained above.

Specific examples of units of the formula R 1 → SiO and groups of the formula OR · include:

(CH) SiO, '3 ^of 2'

CH ₃ (CH ₂ == CH) SiO,

CH (CH) Si0, (C ₆ H _{s) 2} SiO,

C H (CH = CH) SiO,

5 '2 ^of ''OH,

-OCH, '

-OC H,

-O-n-C H a

IN

-OCH CH OCH.

2 3

The following oils are preferably used:

- dimethylpolysiloxane oils blocked at each end of their chain by a hydroxyl, methoxy or beta-methoxyethoxy group having a viscosity of 10 to 200 mPa · s at 25 ºC,

methylphenylpolysiloxane oils consisting of CH (C H) -SiO 2 65

- units blocked at each end of their chain by a hydroxy and / or methoxy group having a viscosity of 40 to 2000 mPa · s at 25 25 C.

The purpose of using oils F is to prevent the compositions of the invention from undergoing any change during storage, and in particular to prevent the compositions from structuring and curing them. These substances are therefore anti-structural agents. Their use is particularly recommended when a considerable amount of B reinforcing silicon is used, for example 30 to 40 parts per 100 parts of rubber A.

Oils F may be replaced, in whole or in part, by other anti-structural agents, such as diphenylsilandiol and silanes of the general formulas:

(CH ₃ ) ₂ -C — (CH ₃ ) ₂ -C 0-3 '

(CH ₃ ) ₂ -C

I (CH ₃ ) ₂ -C

However, these agents are often more expensive than the F oils and / or are more difficult to incorporate into the compositions of the invention than the F oils.

Other additives may be added to the compositions of the invention, in particular:

- platinum or platinum compounds as disclosed in US-A-3,514,424 to improve the fire resistance of the compositions,

- a fatty acid (stearic acid, lauric acid, oleic acid or palmitic acid) or a metal salt thereof to improve the processability of the compositions according to the data given in Japanese Kokai Application 58/027 749,

a heat stabilizer selected from the group consisting of metal salts of organic acids, for example ferrous octoate, in an amount of 20 to 50 ppm of iron, based on the total weight of the composition, titanium dioxide according to the teachings of US-A-3,647,741 and US-A -3 692 737, as well as compounds D and / or E according to French patent document FR-A-2 592 656.

The compositions of the invention are prepared using known mechanical means such as paddle mixers, roller mixers and screw mixers. The individual components of the composition can be introduced into said devices in any order. However, it is advisable to introduce rubber A and reinforcing silicon dioxide into the above devices first and to introduce peroxides C at the end of the mixing process.

The compositions obtained are stable during storage. These compositions are particularly suitable for molding and extrusion molding.

These compositions can be cured to elastomers by heating at elevated pressure or atmospheric pressure at a temperature of about 100 to 350 ° C and / or by irradiation with gamma irradiation at doses between 8 and 20 Mrad. The heating time of the composition for curing usually depends on the temperature used, the pressure used and the nature of the peroxide used. Generally, this time is a few minutes at about 100 to 200 ° C and a few seconds at 250 to 350 ° C. Exposure to radiation also makes it possible to improve the behavior of elastomers against oils, as described in French patent application FR-A-2 057 433.

The elastomers thus formed can then be heated, in particular the elastomers obtained by molding, to a temperature of 200 to 280 ° C for at least one hour to complete their cross-linking and remove the volatiles which still contain these elastomers.

Of course, the invention can also be used in the case of cold-curable compositions which crosslink by polyaddition or polycondensation.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

The synthesis of the heat stabilizer, which in this case consists of a titanium dioxide stabilizer, is carried out in two stages.

a) Synthesis of a titanium dioxide colloidal suspension

In this case, a colloidal suspension is understood to be a system consisting of fine particles of colloidal size of titanium dioxide and / or titanium hydroxide. The elemental titanium dioxide particles of the invention in the form of agglomerates or individual particles can be obtained by any suitable known method.

These particles can be obtained by the method described in EP-A-0 335 773, which consists in hydrolyzing a titanium compound and isolating the solid formed during this hydrolysis.

The titanium compound suitable for said hydrolysis may be selected from the group consisting of halides, oxyhalides, nitrates, alkoxides and sulfates of titanium.

Said hydrolysis can be carried out by heating a solution of the titanium compound. In some cases, the hydrolysis can be carried out in the presence of germinated titanium dioxide particles of high purity, as described in EP-A-0 351 270.

The product obtained can also be isolated by washing and re-dispersing in an acidic medium to obtain a colloidal suspension.

b) Surface treatment of nanomer particles (size 40 nm and specific surface 290 m ² / g) of colloidal suspension

250 g of the colloidal suspension (50 g of titanium dioxide) are diluted with 250 ml of water and introduced into a beaker.

To adjust the pH to 10, approximately 100 mL of 20% NH 4 OH was added and 1250 mL of absolute ethanol was added to dilute the solution.

The resulting solution is sonicated due to deagglomeration of the particles. The controlled rate Ethylsilikat introduced 61 g (35% _SiO2) in 395 ml of absolute alcohol.

Thereafter, 25.6 g of trimethoxyoctylsilane (38.2% relative to titanium dioxide and silica) were introduced in a controlled flow rate (feed rate of 2 ml / min) in 75 ml of ethanol. The mixture was stirred for an additional 4 hours. The product is left to rest, then washed with alcohol and collected by centrifugation, and then dried at low temperature and ground.

Example 2

In this example, the particles to be treated are titanium dioxide particles in a lenticular anatase form (obtained by thermal hydrolysis of titanium oxychloride in the presence of citric acid). These polycrystalline particles (crystallite size = about 6 nm) have a diameter of 40 nm, a specific BET surface area of 290 m ² / g and are well individualized.

In a 500 rpm stirrer, a slurry was prepared with stirring at 25 ° C by stirring 240 ml of deionized water, 500 ml of pure ethanol, 260 ml of aqueous ammonia containing 25% NH ₃ and 10 g of titanium dioxide.

Surface pretreatment is performed by adding (at a rate of 15 ml / h) a solution containing 12 g of tetraethyl orthosilicate in 50 ml of ethanol. Upon completion of the addition, a stable suspension is obtained.

Subsequent compatibilization is then performed by adding (at a rate of 15 ml / h) a solution containing 5 g of octyltrimethoxysilane in 10 ml of ethanol. Upon completion of the reaction, a flocculated suspension is obtained whose solid is deposited at the bottom of the reactor. The product is washed, collected by centrifugation, dried at low temperature and ground.

Example 3

In this example, the particles to be treated are titanium dioxide particles in a lens-like anatase form (obtained by thermal hydrolysis of titanium oxychloride in the presence of citric acid). These polycrystalline particles (crystallite size = about 6 nm) have a diameter of 40 nm, a BET surface of 290 m ² / g and are very well individualized.

In a 500 rpm stirrer reactor, a suspension is prepared with stirring at 25 ° C by stirring 240 ml of deionized water, 500 ml of pure ethanol, 260 ml of aqueous ammonia solution containing 25% NH 4 and 10 g of titanium dioxide.

Surface pretreatment is performed by adding (addition rate of 15 ml / h) a solution containing 12 g of tetraethyl silicate in 50 ml of ethanol. Upon completion of the addition, a stable suspension is obtained.

Subsequent compatibilization is then carried out by adding (addition rate 15 ml / h) a solution containing 5 g octyltrimethoxysilane in 10 ml ethanol. Upon completion of the reaction, a flocculated suspension is obtained whose solid is deposited at the bottom of the reactor.

The resulting suspension is aged for 2 hours with stirring.

Thereafter, distillation is carried out under vacuum to remove ethanol and ammonia at a temperature of 70 to 80 ° C at a pressure of about 8 kPa. When 200 ml of distillate were recovered, the reactor was allowed to cool, normal pressure was set and water was added to the reactor to restore the original volume. These operations are then repeated to remove residual alcohol and residual ammonia.

When the volume of liquid in the reactor reaches only 500 to 600 ml, the mixture starts to foam and the dispersion has a soap and pearlescent appearance. The reactor contents are allowed to cool.

The entire contents of the reactor are then separated by centrifugation. The cake obtained at the bottom of the centrifuge tube is resuspended in 100 ml of absolute ethanol, mixed well and centrifuged again. The obtained cake at the bottom of the centrifuge tube is sufficiently compact and solid. Allow to air dry to give a powdered product.

Example 4

In this exemplary embodiment, cerium oxide particles with a particle size of 8 nm and a specific surface area of 200 m ² / g are compatible.

In a 500 rpm stirrer, a suspension is prepared with stirring at 25 ° C by stirring 500 ml of deionized water, 500 ml of pure ethanol, and 5 g of colloidal cerium hydroxide, the pH of the aqueous / alcohol suspension is adjusted to 3.5 with the necessary amount of sodium acetate.

A solution containing 1 g of octyltrimethoxysilane and 50 ml of ethanol is then added (addition rate of 15 ml / h) to the slurry in the reactor. A flocculated suspension is obtained in which a solid is deposited on the bottom of the reactor.

Example 5

Composition with stabilizer - comparative example

0.3% by weight of titanium dioxide from various sources is introduced into the conventional HVE (hot-vulcanizable elastomer) of the composition, the silica of the invention and the two commercially available silicas.

Said conventional HVE composition comprises:

- 100 parts by weight per chain of vinyl-cured polyvinyl siloxane rubber having a viscosity of 20 x 10 ⁶ mPa.s,

- 2,5 parts by weight of alpha, omega-dihydroxylated polydimethylsiloxane oil with a viscosity of 50 mPa · s, parts by weight of fumed silica for the purpose of hydrophobising the surface silanol sites, and

0.7 parts by weight of 2,4-dichlorobenzoyl peroxide.

This whole mixture, consisting of a conventional HVE composition and titanium dioxide, is then crosslinked for 8 minutes at 115 ° C.

The obtained elastomers are then heated for 4 hours at 200 ° C, then subjected to a test exposure comprising a treatment of 275 ° C for 3 days and the same temperature for 10 days.

The results obtained are summarized in the following table, which clearly shows that only the HVE sample no. 2, which has been stabilized with the titanium dioxide of the invention (prepared in Example 1), is temperature resistant and retains its elastomeric properties. It should also be noted that, despite slight yellowing after annealing at 200 ° C, the elastomer of the invention retains its transparency even after exposure to 275 ° C for 10 days.

Conversely, after incorporation of commercial titanium dioxide particles, elastomers (s. 3 and 4) are obtained which are white and non-transparent and which, like the transparent base elastomer no. 1 becomes brittle after a three-day exposure to 275 ° C.

The abbreviations used in the table have the following meanings:

SAH hardness (DIN standard 53 505),

RZ elasticity (DIN standard 53 512, values in%),

T / S tensile strength (ISO standard R 37, values in MPa),

E / B elongation at break (ISO standard R 37, values in%),

R / T tear resistance (ASTM standard D 624 / A,

0.5 mm, values in kN / m).

s

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Claims (14)

PATENT An elastomeric organopolysiloxane composition, characterized in that, after crosslinking, it provides a thermally stable elastomer and comprises, in dispersed form, organophilic particles consisting of at least one metal oxide in the form of polycrystalline nanoparticles consisting of 4 to 6 nm crystallites having a size of at most 50 nm and a specific BET surface area of at least 250 m ² / g, preferably 250 to 300 m ² / g, which have been subjected to a surface treatment to obtain an organophilic character by grafting the organosilicon groups. Elastomeric organopolysiloxane composition according to claim 1, characterized in that the metal oxide or metal oxides are selected from the group consisting of alumina, silica, transition metal oxides, in particular titanium dioxide, and rare earth metal oxides, in particular cerium oxide. * Elastomeric organopolysiloxane composition according to claim 1 or 2, characterized in that the organophilic particles can be obtained by reacting an aqueous / alcoholic suspension of a) nanocrystalline nanoparticles with an anhydrous alcohol solution b) containing at least one alkoxysilane of the general formula SifORJJR · ^ (1) in which R represents an alkyl group having 1 to 6 carbon atoms, R 'represents a hydrocarbon group selected from the group consisting of alkyl, cycloalkyl, aryl, alkylaryl, arylalkyl, alkenyl and alkynyl group containing at least one carbon atom and x is an integer from 1 to 3; alcohols, which are the same or different, contain 1 to 5 carbon atoms. 4. An elastomeric organopolysiloxane composition wherein R is methyl or ethyl. Elastomeric organopolysiloxane composition according to either of Claims 3 or 4, characterized in that R 'represents an octyl group, a dodecyl group or an octadecyl group. Elastomeric organopolysiloxane composition according to one of claims 3 to 5, characterized in that x has a value of 3. Elastomeric organopolysiloxane composition according to one of Claims 3 to 6, characterized in that the basic polycrystallite particles can be obtained by reacting an aqueous / alcoholic suspension of c) polycrystalline nanoparticles of a metal oxide with at least one tetraalkoxysilane of the formula 2 Si (OR) ₄ (2) in which R '' represents an alkyl group containing from 1 to 5 carbon atoms, in order to obtain an aqueous / alcohol suspension of a), basic polycrystalline nanoparticles. Elastomeric organopolysiloxane composition according to one of claims 3 to 7, characterized in that the metal oxide is titanium dioxide. Elastomeric organopolysiloxane composition according to claim 7, characterized in that the ratio of the compounds of formulas 1 and 2 is between 5 and 40% by weight. Elastomeric organopolysiloxane composition according to one of Claims 1 to 9, characterized in that it comprises 0.1 to 5%, preferably 0.3 to 1%, of incorporated organophilic particles, based on the weight of the composition. Elastomeric organopolysiloxane composition according to one of claims 3 to 10, characterized in that the organophilic particles comprise 60 to 90% of titanium dioxide and 40 to 10% of silica. Elastomeric organopolysiloxane composition according to one of claims 1 to 11, characterized in that the organophilic particles dispersed in the composition have a specific surface area of between 50 and 160 m ² / g and a size of between 10 and 50 nm. A transparent organosiloxane elastomer obtainable by curing the composition of any one of claims 1 to 12. 14. A transparent and thermally stable organopolysiloxane elastomer retaining at least the basis of its elastomeric properties after exposure to a temperature above 200 ° C, in particular about 275 ° C, for at least 3 days.