JPS6220531A - Production of diorganosiloxane polymer-finely powdery silica mixture - Google Patents

Abstract

PURPOSE:To obtain the titled mixture suitable as chief agent for silicone rubber composition with sufficiently high polymerization degree attainable, by polymerization of a diorganosiloxane oligomer in the presence of each specific precipitated silica and polymerization catalyst. CONSTITUTION:The objective mixture can be obtained by polymerization, while mixing, between the following three components: (A) 100pts.wt. of either diorganosiloxane oligomer of formula HO(R2SiO)xH or diorganocyclosiloxane oligomer of formula which may contain another diorganosiloxane oligomer of formula R3SiO(R2SiO)ySiR3 or R3SiO(R2SiO)zH (R is monovalent hydrocarbon; x is 3-200; y is 3-10; z is 0-50), (B) 2-150pts.wt. of precipitated silica with the content of alkali metal <0.35wt% on its oxide basis, and (C) a polymerization catalyst selected from (i) sulfuric acid or sulfonic acids of formula XSO3H (X is halogen, etc.), (ii) perfluoroalkanesulfonic acids, and (iii) mixtures of quaternary ammonium carboxylate and carboxylic acid.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field 1] The present invention relates to a method for producing a diorganosiloxane polymer/finely powdered silica mixture by polymerizing a diorganosiloxane oligomer in the presence of precipitated finely powdered silica.

[Prior Art and its Problems] A mixture of Diol 7y/siloxane polymer and finely powdered silica is used as a main ingredient in various silicone rubber compositions, silicone compounds, silicone greases, and the like. The customary method for producing these mixtures has been to first produce diorganosiloxane polymers of the required viscosity by polymerizing diorganosiloxane oligomers and then mechanically incorporating finely powdered silica into the diorganosiloxane polymers. . According to this method, two different
This method requires two steps, and requires separate polymerization equipment and mixing equipment, making the process complicated and disadvantageous in terms of cost. Furthermore, when the viscosity of the silicone polymer becomes high, it becomes difficult to mix and disperse fillers, and a large amount of energy is required to obtain a good mixture. This drawback becomes particularly noticeable when the molecular weight of the diorganosiloxane polymer is high enough to be called raw rubber.

In contrast, a method for producing a silicone polymer-filler mixture by polymerizing organosiloxane oligomers in the presence of finely powdered silica is disclosed in British Patent No. 1,325,654.
Patent Application Publication No. 1988-176322, Patent Application Publication No. 1987-176323, Patent Application Publication No. 1981-
Publication No. 176324, patent application published 17632, 1982
It is disclosed in Publication No. 5.

In these methods, fumed silica (dry silica) and precipitated silica are mentioned as fine powder silica, and fumed silica is preferred, and the properties of fumed silica are explained and examples related to polymerization in the presence of fumed silica are given. However, regarding precipitated silica, Patent Application Publication No. 59-176322 only mentions preferable specific surface area, acidity, and particle size, and there are no examples regarding polymerization in the presence of precipitated silica. .

Therefore, the inventors of the present invention conducted additional experiments on the polymerization of diorganosiloxane polymers in the presence of precipitated silica, and found that the polymerization of diorganosiloxane polymers using the usual precipitated silica used in silicone rubber compositions was unexpected. If the diol produced is not sufficiently reduced, the nosiloxane polymer/precipitated silica mixture may be unsuitable as a main ingredient for silicone rubber compositions, silicone compounds, silicone greases, etc., especially as a main ingredient for silicone rubber compositions. Il+ revealed. Therefore, as a result of intensive studies on the properties of precipitated silica, we found that if the alkali metal content is less than 0.35% by weight as an alkali metal oxide, the degree of polymerization of the diorganosiloxane polymer will be sufficiently high, and the resulting diorganosiloxane polymer will be 1-1 The siloxane polymer/precipitated silica mixture is useful as a main ingredient for silicone rubber compositions, silicone powder, silicone grease, etc., especially as a main ingredient for silicone rubber compositions.
As a result, we have arrived at the present invention.

[Means for solving the problem and its effects 1 That is, the present invention provides the following: (8) (a) Formula 110(R2Siθ)xll
diorganosiloxane oligomer.

and (c) a diorganosiloxane oligomer of the formula R=SiO(R2SiO)zSiR3.

(d) A diorganosiloxane oligomer optionally containing an organomesiloxane oligo'mer of the formula R15iO(R2SiO)zll or a mixture thereof (wherein each R is an unsubstituted or substituted hydrogenated hydrogen group) , X has an average value in the range of 3 to 200, and y has an average value of 3
-10, Z is in the range of average value () -50) 100 parts by weight, (B) 2 to 150 parts by weight of precipitated silica and (C) (a) sulfuric acid or the formula X5O311 (wherein (b) a perfluorinated alkanesulfonic acid; (C) a mixture of a quaternary ammonium carboxylate and a carboxylic acid; Polymerization catalyst selected from the group
In a method for producing a diorganosiloxane oligomer/fine powder silica mixture by polymerizing the diorganosiloxane oligomer while mixing a catalytic amount, the alkali metal content in the precipitated silica is alkali metal oxide. The present invention relates to a method for producing a mixture of diol-nosiloxane polymer and finely powdered silica, characterized in that the amount of silica does not exceed 0.35% by weight.

To explain this, the diorganosiloxane oligomer of component (A) is a raw material for diorganosiloxane polymer. The diorganosiloxane oligomers include diorganosiloxane oligomer (a) only, diorganocyclosiloxane oligomer (b) only, and the above diorganosiloxane oligomer (a) and the above diorganocyclosiloxane oligomer (b). ) may be used as a mixture.

In the above formula, each R is an unsubstituted hydrogen group, and examples of unsubstituted hydrogen groups include alkyl groups such as methyl group, ethyl group, propyl group, butyl group; phenyl group, Examples include heptaryl groups such as tolyl group, xylyl group, and ethylphenyl group; and fulkenyl groups such as hinyl group, allyl group, and 1-propenyl group. As a substituted hydrogen group,
Chloromethyl group, bromomethyl group, 2-chloroethyl L
2-iodoethyl i, 2-bromoethyl group, 2-fluoroethyl i, 3-chloropropyl group, 3+3+3-)
Examples include substituted alkyl groups such as lJ fluoropropyl group, chloroisobutyl L2-phenylpropyl L2-phenylethyl-L3-i captoprobyl group, mercaptoisobutyl group, and 2-cyanoethyl group.

In both the diorganosiloxane oligomer (a) and the diorganocyclosiloxane oligomer (b), one R in each siloxane unit is a methyl group, and Ikekata's R is a monovalent hydrocarbon group other than the methyl group. is common.

In the diorganosiloxane oligomer (a), the average value of X is in the range of 3 to 200, but preferably in the range of 3 to 100. Diorganocyclosiloxane oligomer (
In b), the average value of y is in the range of 3-10, preferably in the range of 3-6.

The diorganosiloxane oligomer used in the present invention may further contain the nosiloxane oligomer (d). In the formula, each R is as defined above, and Z is the average value 0
~50. These diorganosiloxane oligomers (C
), (d) are used. The amount of components (c) and (d) added depends on the average molecular weight required of the diornosiloxane polymer in the final organosiloxane polymer-finely powdered silica mixture.Component (c) in the starting material.
The higher the concentration of component (d), the lower the molecular weight of the diol-nosiloxane polymer in the final product. The optimum amounts of component (c) and component (d) to be used to obtain the desired result are most advantageously determined by experimentation, but are usually small amounts, for example, the sum of component (a) and component (b) 1
It is less than 5 parts by weight per 00 parts by weight.

Component (B), precipitated silica, functions as a thickener, reinforcing agent, etc. Precipitated silica, which is used in ordinary silicone rubber compositions, is produced by, for example, using an aqueous solution of a silicate containing sodium silicate as the main component, such as water glass, with sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, carbon dioxide gas, or sulfur dioxide gas. It can be easily manufactured commercially by decomposing it with an inorganic acid or its anhydride such as 5.2 to precipitate finely powdered silica and collecting it. However, these precipitated silicas contain significant amounts of alkali metals such as sodium and potassium.

However, neutralization with excess inorganic acid and sufficient washing with water can reduce the alkali metal content to less than 0.35% by weight as alkali metal oxide. The specific surface area of such precipitated silica is preferably 50n+2/g or more,
More preferably 150 m2/g or more. Further, when the slurry is made into a 2% by weight slurry with distilled water, the pH is preferably 7.S or less.

(B) The amount of In added is 2 to 150 parts by weight per 100 parts by weight of (^) diorganosiloxane oligomer, but when a mixture of diorganosiloxane polymer and finely powdered silica is used in a millable type silicone rubber composition, In this case, the preferable addition amount is 15 to 75 parts by weight from the viewpoint of processability and physical properties. When used in liquid silicone rubber compositions or silicone sealants, the preferred addition amount is 7 to 40 parts by weight in terms of extrudability and physical properties. When used in silicone grease or silicone compounds, the preferred addition amount is 5 to 20 parts by weight from the viewpoint of increasing consistency. Two or more types of precipitated silica having different specific surface areas etc. may be used together. Incidentally, since precipitated silica generally contains 6 to 8% by weight of water, care must be taken in controlling the water content depending on the use of the diorganmesiloxane oligomer/fine powder silica mixture.

Component (C) is a catalyst for polymerizing diorganosiloxane polymers into diorganosiloxane polymers, (,) sulfuric acid or the formula XSO,H (wherein, (b) perfluorinated flucansulfonic acid; (c) mixtures of quaternary ammonium carboxylates and carboxylic acids;

Regarding sulfuric acid or formula X5O3H (wherein X is as defined above) in (a), the preferred sulfonic acid is of formula
It is an alkylbenzenesulfonic acid having C6H4S03H (in the formula, R1 is an alkyl group having 6 or more carbon atoms). A preferred alkylbenzenesulfonic acid is dodecylbenzenesulfonic acid. Concentrated sulfuric acid or fuming sulfuric acid is preferable as the sulfuric acid. The preferred amount of sulfuric acid added is 0.1 to 1.0 parts by weight per 100 parts by weight of diorganosiloxane oligomer, and the preferred amount of sulfonic acid added is 0.5 to 5.0 parts by weight.
It is 0 parts by weight.

The perfluorinated alkanesulfonic acid (b) preferably has the formula CnF2n + 1sO311 (where n is 1 to 30), and specific examples include trifluoromethanesulfonic acid and perfluoro-n-butane. There are sulfonic acids and perfluoro-11-octanesulfonic acids. A preferable addition amount is 0.02 to 1.0 parts by weight per 100 parts of diorganosiloxane oligomer. A large amount is preferable in order to increase the polymerization rate, and a small amount is preferable in order to obtain a high molecular weight polymer.

To explain the mixture of quaternary ammonium carboxylate and carboxylic acid (C), preferably the formula % formula % (wherein R2 is a monovalent aliphatic group having 1 to 20 carbon atoms, and R3 is a carbon atom selected from the group consisting of monovalent aliphatic hydrocarbon groups of up to 5, phenyl groups and benzyl groups, at least one R2 having 4 or more carbon atoms), and is soluble in diorganesiloxane oligomers. Quaternary ammonium carboxylate 0.05~
5.0 parts by weight and 0.05 to 50 parts by weight of a carboxylic acid having the formula R'COO11, where R3 is as defined above. Specific examples of R3 are methyl group, butyl group,
These are lauryl group, γ-hydroxypropyl group, β-phenylethyl group, and hexenylethyl group. Specific examples of quaternary ammonium carboxylates are tetra-n-butylammonium acetate, lauryltrimethylammonium acetate, and dilauryltrimethylammonium acetate. The amount of carboxylic acid used is not critical as long as it is present during the polymerization, but is preferably from 0.5 to 10 parts by weight per 100 parts by weight of siloxane oligomer.

The polymerization catalysts (,), (b), and (c) above may be used alone or in combination.

The order of addition of component (^), component (B), and component (C) is arbitrary; when these three components are mixed, component (^) becomes (C).
Polymerizes due to the catalytic action of the components. The polymerization temperature is preferably room temperature to 150°C. It is preferable to continue mixing during polymerization. Examples of mixing devices include planetary mixers, Niegoo mixers, two-wheel continuous mixing extruders, colloid mills, sulters, and dry mixers.

After reaching a predetermined degree of polymerization, the diorganosiloxane polymer/finely powdered silica mixture is preferably heat treated at a high temperature or neutralized with a Lewis base to inactivate the catalyst. When using a diorganosiloxane polymer/finely powdered silica mixture as the main ingredient of a silicone rubber composition, it is preferable to neutralize it with an organic amine, hexamethyldisilazane, magnesium oxide, etc. When used as a main component of a composition, it is advantageous to use a crosslinking agent having a silicon atom-bonded oxime group or a silicon-bonded amine group as a crosslinking agent, as these will also act as a neutralizing agent and deactivate the catalyst. There is.

The diorganosiloxane polymer/finely powdered silica mixture thus produced is useful as a base ingredient for silicone rubber ML compositions, silicone foam bound, silicone grease, and the like. For use as a milling-type silicone rubber composition, an organic peroxide is added to a mixture of raw rubber-like diorganosiloxane polymer and finely powdered silica, and the mixture is kneaded until uniform. Suitable organic peroxides are benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, 1-butyl peroxide, 2,5-bis(butylperoxy)-2,5-1 methylhexane, Examples include dicumyl peroxide, tert-butyl perbenzoate, and parachlorobenzoyl peroxide.

In order to use the diorganosiloxane polymer/finely powdered silica mixture in the condensation reaction curable organopolysiloxane 2 composition, a diorganosiloxane polymer with both hydroxyl groups endblocked and finely powdered silica.

A crosslinking agent and, if necessary, a curing catalyst are added to the mixture and mixed. As the crosslinking agent, any one of trifunctional silane, tetrafunctional silane, and polyfunctional siloxane oligomer may be used. Typical silicon-bonded functional groups include an acetoxy group, an alkoxy group, a diorganoamide group, a ketoxime group, and an acylamine group. Curing catalysts used as necessary include metal carboxylates, alkyl metal carboxylates, alkyl metal alphoxides, organotitanates, and the like. Among these, stannous octoate, dibutyltin diacetate, dibutyltin dilaurate, tetrabutyl titanate, dibutyltin dimethoxide and tetraisopropyl titanate are preferred. In order to use the diorganosiloxane polymer finely powdered silica mixture in an addition reaction curable silicone rubber composition, diorganosiloxane polymers containing two or more monovalent unsaturated aliphatic groups such as vinyl or allyl groups per molecule must be used. An organohydrogenpolysiloxane containing more than two silicon-bonded hydrogen atoms per molecule and a trace amount of a platinum-containing catalyst are mixed into a mixture of organosiloxane polymer and finely divided silica. Add a small amount of additives to the diorganosiloxane polymer/finely powdered silica mixture,
Thermal stability, workability, storage stability, compression set, oil resistance, flame retardance, etc. may be improved, or the material may be colored.

Examples of such additives include cerium oxide, cerium hydroxide, iron oxide, silicone oil, higher fatty acids or their metal salts, alkynes, benzotrisol, barium zirconate, carbon black, zinc carbonate, fumed titanium dioxide, and Examples include zinc and alumina.

[Example 1] Next, the present invention will be explained by examples, but the present invention is not limited in any way. In the examples, "part" means part by weight, "%" indicating content means "% by weight", and "viscosity" means the value at 25°C. Further, the molecular weight of the diorganosiloxane polymer in the obtained diorganosiloxane polymer/fine powder silica mixture was measured by the following method. That is, about 1 g of the above mixture was mixed with 12 g of toluene and 12 g of aqueous ammonia.
i? After shaking for 24 hours and removing the filler from the toluene layer by centrifugation, the toluene layer was collected by decantation and the toluene was evaporated. This residue was dissolved in toluene to a concentration of 1% by weight, and the weight average molecular weight was determined by gel permeation chromatography.

In addition, the physical properties of silicone rubber test pieces were measured by JI
Followed S-6301.

Example 1 100 parts of dimethylsiloxane oligomer endblocked at both hydroxyl ends and having an average degree of polymerization of about 35 (weight average molecular weight of about 2600) and 2 parts of 40% active lauryl trimethylammonium acetate were placed in a small Nigu mixer and mixed. Next, 40 parts of precipitated silica shown in Table 1 was added, and an additional 30 parts of
Mixed for a minute. Next, 2 parts of glacial acetic acid was added, and mixing was continued under nitrogen gas at 70° C. while maintaining the internal temperature of the Nigu mixer at 120° C. Note that 2 parts of glacial acetic acid were also added after 1 hour and 2 hours.

After 3 hours, heat the Nigoo mixer to 200'C,
The polymerization catalyst was deactivated.

Then, the Niegoo mixer was cooled to room temperature, and the bulk diorganosiloxane/fine powder silica mixture was taken out.

The weight average molecular weight of the trimethylsiloxane polymer in these three mixtures was measured. 3 parts of 2,4-dichlorobenzoyl peroxide per 100 parts of the above mixture.
The mixture was mixed using a main roll, press-cured at 120°C for 10 minutes, and then oven-cured at 200'C for 4 hours to obtain a silicone rubber sheet. The physical properties of these silicone rubber sheets were measured.

The results are shown in Table 1.

Table 1 Example 2 Dimethylsiloxane oligomer 10 used in Example 1
0 parts, specific surface area 130n+27g, pH (2% slurry) 7.10. 30 parts of precipitated silica with a Na2O content of 0.29% is put into a planetary mixer and mixed, and the mixture is dispersed by a colloid mill to make a planetary mixer.
I put it back in the mixer. Next, dodecylbenzenesulfonic acid 2
A portion of the mixture was placed in a planetary mixer and mixed for 60 minutes at room temperature to obtain a paste. The weight average molecular weight of the diol-nosiloxane polymer endblocked with silyl groups at both ends in this paste was 50,000. In the absence of substantial moisture, a mixture 15 consisting of 98% by weight of methyltris(methylethylketoxime)silane and 2% by weight of dibutyltinnolaurate was added to 100 parts of the above paste.
It was converted into a room-temperature curable silicone rubber composition by adding 50% of the mixture. This room temperature curable silicone rubber composition was coated to a thickness of 2. It was spread to a thickness of Smm and left in the atmosphere at 25° C. and 50% humidity for 7 days to harden. The physical properties of cured silicone rubber are hardness 35 and tensile strength 22 kg/
cm2, and the elongation was 280%.

Instead of the above precipitated silica, the specific surface area is 230m2/
Pond 6 using 30 parts of precipitated silica with g, pll (2% slurry) '5.70 and Na2O content of 0.48% was tested under the same conditions, but the molecular weight hardly increased.

Example 3 Dimethylcyclosiloxane 1 with an average degree of polymerization of about 4
00 parts and 50 parts of the three types of precipitated silica shown in Table 2 were placed in a Nigu mixer and mixed at 15()°C for 2 hours to remove moisture. Next, the temperature inside the Nigoo mixer was set to 120.
Instead of 'C, 0.2 part of concentrated sulfuric acid was added as a polymerization catalyst.

The mixture was mixed with cooling for 2 hours under a nitrogen purge, and when the temperature reached 30° C., 1 part of magnesium oxide was added to inactivate the polymerization catalyst.

The weight average molecular weight of the dimethylsiloxane polymer in these three mixtures was measured. 3 parts of 2°4-dichlorobenzoyl peroxide was mixed with 100 parts of the above three types of mixture using a two-roller, and a silicone rubber sheet was prepared under the same conditions as in Example 1, and its physical properties were measured. The results are shown in Table 2.

Table 2 Example 4 99.5 parts of a dimethylsiloxane oligomer with an average degree of polymerization of 40 (weight average molecular weight about 2900) and end-blocked with hydroxyl groups at both ends and 085 parts of an IA methylvinylsiloxane oligomer with an e= group ff at both ends and a degree of polymerization of 12. Pour into a Nigu mixer and mix, then add various precipitated silicas shown in Table 3.
0 part was added and mixed at 170° C. for 2 hours while degassing to remove water in the system. Next, the temperature inside the Nigu mixer was cooled to room temperature, 0.2 part of trifluoromethanesulfonic acid was added, and the mixture was mixed for 2 hours at 170°C under nitrogen purge, and immediately 2 parts of diethylamine was added to inactivate the polymerization catalyst. . Next, the Nigu mixer was cooled to room temperature, and a lumpy diorganosiloxane polymer/fine powder silica mixture was taken out. The weight average molecular weight of the trimethylsiloxane/methylvinylsiloxane copolymer in these three mixtures was measured. Using two rolls, mix 3 parts of dicumyl peroxide with 100 parts of the above three types of mixture, and mix 15 parts of dicumyl peroxide.
Press vulcanization was performed at 0°C for 110 minutes, followed by oven vulcanization at 200°C for 14 hours to obtain a silicone rubber sheet. The physical properties of these silicone rubber sheets were measured. The results are shown in Table 3.

Table 3 Example 5 100 parts of dimethylcyclosiloxane oligomer with an average degree of polymerization of 4.5 and 1 part of bis(7;nylmethylvinyl)disiloxane were charged into a Nigu mixer and mixed.
Sample No. used in

30 parts of precipitated silica from No. 7 was added and mixed.

Trifluoromethane sulfonic acid 0. IS part was added and mixed at 70° C. for 2 hours under nitrogen burn. Next, 0.5 part of hexamethyldisilazane was added to inactivate the polymerization catalyst, and the mixture was mixed at 150° C. for 2 hours while degassing to remove volatiles.

The thus obtained mixture of dimethylsiloxane polymer and precipitated silica endblocked with phenylmethylvinylsilyl groups at both ends was in the form of a lump, and the polymerization average molecular weight of the dimethylsiloxane polymer was 120,000. 100 parts of this mixture, 1 part methylhydrogen polysiloxane with a viscosity of 10 cs
．． A silicone rubber sheet was prepared by mixing 5 parts of chloroplatinic acid and 0.001 part of chloroplatinic acid under the same conditions as in Example 1, and its physical properties were found to be 28 in hardness and 35 kg/cm in tensile strength.
The elongation was 250%.

Example 6 100 parts of the dimethylsiloxane oligomer endblocked at both hydroxyl ends used in Example 1 and trimethylsiloxy endblocker dimethylsiloxane having an average degree of polymerization of 7.

Roxane oligomer 1.3 parts and specific surface area 150+n2
/ [? , ptl (2% slurry) 5.55, Na, 0
6.0 parts of precipitated silica containing 0.07% was placed in a planetary mixer and mixed. Next, 1.0 part of dodecylbenzenesulfonic acid was added to a planetary mixer.
After mixing for 30 minutes at room temperature, 1.7 parts of diethylamine was added to neutralize the polymerization catalyst. The weight average molecular weight of the dimethylsiloxane polymer in the thus obtained mixture of dimethylsiloxane polymer endblocked with trimethylsilyl groups at both ends and precipitated silica was 83,000.

The consistency of the mixture measured according to JIS-2220 was 290, and the degree of oil separation at 200°C for 24 hours was 9.
．． It was 2% by weight.

This greasy mixture was useful as a mold release grease.

[Effect of the invention 1] In the present invention, when producing a mixture of silica/silica polymer and fine powder silica by polymerizing a diorganosiloxane oligomer with an acidic catalyst such as sulfuric acid in the presence of precipitated silica, the precipitation method is used. Since the alkali metal content of the silica does not exceed 0.35% by weight as the alkali metal oxide, the degree of polymerization of the diorganosiloxane polymer is sufficiently extraordinary.

Therefore, the diorganosiloxane polymer/finely powdered silica mixture according to the present invention can be used in various silicone rubber compositions.
Useful as a main ingredient in silicone compounds, silicone greases, etc.

Claims (1)

[Claims] 1(A) Diorganosiloxane oligomer of formula (a) HO(R_2SiO)_xH, diorganocyclosiloxane oligomer of formula (b) ▲There are mathematical formulas, chemical formulas, tables, etc.▼, and their (c) a mixture of the formula R_3SiO(R_2SiO)_2SiR_3;
(d) a diorganosiloxane oligomer of the formula R_3SiO(R_2SiO)_2H, or a mixture thereof, in which each R is an unsubstituted or substituted monovalent hydrocarbon; group, x has an average value in the range of 3 to 200, and y has an average value of 3
~10, and z is in the average value range of 0 to 50) 100 parts by weight, (B) 2 to 150 parts by weight of precipitated silica and (C) (a) sulfuric acid or the formula XSO_3H (wherein, (b) a perfluorinated alkanesulfonic acid; (c) a mixture of a quaternary ammonium carboxylate and a carboxylic acid; A method for producing a diorganosiloxane polymer/fine powder silica mixture by polymerizing the diorganosiloxane oligomer while mixing a catalyst amount selected from A method for producing a mixture of diorganosiloxane polymer and finely powdered silica, characterized in that the content of oxides does not exceed 0.35% by weight. 2. The method for producing a diorganosiloxane polymer/fine powder silica mixture according to claim 1, wherein the polymerization catalyst is inactivated after the polymerization of the diorganosiloxane oligomer is completed.