JPH0643554B2 - Introduction of silica filler into polysiloxane rubber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of incorporating a silica filler into a polysiloxane rubber. More specifically, the present invention relates to a method of incorporating a silica filler into rubber as described above and performing surface treatment in situ.

BACKGROUND OF THE INVENTION Curable silicone gums are usually compounded with reinforcing fillers such as finely divided silica to maximize their physical properties when cured into rubber. To eliminate or control crepe hardening during shelf aging, namely polymer-
To prevent filler interactions, the filler is surface treated with an octamethylcyclotetrasiloxane agent or fluorosilicone hydrolysis product to reduce the number of free silanol groups on the surface of the finely divided silica.

One surface treatment method for silica fillers (US Pat. No. 4,
529,774), the filler is exposed to a fluorosilicone hydrolysis product in the gas phase. This hydrolysis product is a mixture of silanol-terminated linear siloxane oligomers and mixed cyclic siloxane compounds. The linear to annular ratio is 14 to 36% by weight for linear
Then, the cyclic one may be 64-86% by weight. The hydroxy end group content of such a hydrolysis product is 1.0 to
It will vary over a range of 2.5% by weight.

In the above gas phase filler treatment, silica is first heated in a fluidized bed reactor to a temperature of 110 ° C. or higher to remove adsorbed moisture. The hydrolysis product is then pumped in under pressure, while raising the temperature to 280-300 ° C. to achieve the desired grafting to the surface silanols. Usually this method gives a good reinforcing filler,
Spherical agglomerates of polymeric gums may form in the filler depending on acidity and the presence of contaminants such as iron.
Also, depending on the processing conditions, particulate contamination due to corrosion of the container may occur. As a result, the reproducibility of this process is poor. Further, treating the filler in this manner and incorporating it into the polysiloxane rubber is a cumbersome process that requires both processing equipment and blending equipment.

The object of the present invention is therefore to simplify the treatment of silica fillers and their incorporation into polysiloxane rubbers.

Yet another object of the present invention is to produce a silica-filled polysiloxane rubber that exhibits the desired degree of physical properties at low levels of silica filler content.

Yet another object of the present invention is to integrate the method of surface treatment of silica filler and the method of incorporating silica filler into polysiloxane rubber in a single step.

DETAILED DESCRIPTION OF THE INVENTION In summary, the present invention provides a method of incorporating a silica filler into a polysiloxane gum. The method includes the following steps. That is, (a) mixing a curable non-condensable polysiloxane gum, a silica filler and a condensable diorganopolysiloxane; and (b) combining the silica filler and the condensable diorganopolysiloxane. Heating the mixture to a temperature of up to about 210 ° C. for a time sufficient to complete the condensation reaction in between. The curable polysiloxane gum may be molded and cured immediately or may be stored and cured after some time.

The curable polysiloxane gum used in the present invention should not condense or react when simply heated below about 210 ° C. That is, in the absence of a catalyst or other curing agent, the reaction should not be carried out in such a substantial amount as to hinder the surface treatment under heating or prevent the subsequent curing. The curable polysiloxane gums preferred for the present invention are diorganopolysiloxane gums having silanol, vinyl, alkoxy or methyl functional groups.

In general, suitable diorganopolysiloxane gums have the general formula:

R _a SiO _{4-a / 2} (1) Here, R is an alkyl group having 1 to 8 carbon atoms, a vinyl group, a phenyl group, a haloalkyl group having 3 to 10 carbon atoms, a halophenyl group, a hydroxy group, It is selected from among alkoxy groups, aryloxy groups, cyanoalkyl groups and combinations thereof, and a varies from about 1.98 to about 2.01. It should be mentioned here that the invention does not lie in the type of gum used, nor in the curing process or the functional groups involved in the curing. As already mentioned, all that is required is that the gum used is neither condensable nor reactive as defined above.

The diorganopolysiloxane gum preferably has the formula:

The viscosity is about 500,000 to about 30 at 25 ° C.
Preferably, it varies from about 1,000,000 centipoise, about 1,000,000 to about 200,000,
More preferably up to 000 centipoise. Equation (2)
Wherein R ¹ may be vinyl, methyl, hydroxy or alkoxy, R ² is vinyl, phenyl, having 1 to 8 carbon atoms.
Alkyls, fluoroalkyls of 3 to 10 carbon atoms or combinations thereof, and y is 2,500 to 1
Varies up to 1,000.

Gums conforming to formula (1) or (2) above and methods of curing or vulcanizing them are well known to those skilled in the art. Usually, a catalyst is required for rational reaction rates and cure times. In the case of free-radical curing, ie curing involving cross-linking between the methyl or vinyl groups of the free radical, the catalysts often used are organic peroxides such as dialkyl peroxides, dialkyl alkyl peroxides, alkyl alalkyl peroxides and the like. Catalysts, metal catalysts such as platinum, organometallic catalysts such as organoplatinum complexes,
And an organosilicon catalyst.

Crosslinking agents such as low molecular weight M-terminated polysiloxanes and vinylpolysiloxanes may be used during rubber cure. However, hydride crosslinkers are not recommended because they are reactive at the temperature and conditions of surface treatment. Therefore, in the case of a hydride cross-linking agent, it is desirable to add it immediately after the heat treatment of silica or to make it a two-component system.

The diorganopolysiloxane gum is manufactured by methods well known in the art and cured into a rubber. 196
8 Walter, Academic Press, New York, NY, “Chemistry and Technology of Sil
iconses) ", No. 11, which is hereby incorporated by reference.

Silica fillers suitable for use in the present invention include finely divided silicas which may have free hydroxyl groups, either in the form of Si-bonded functional groups or in the form of adsorbed moisture, depending on their method of preparation. It is a divided reinforcing filler. This S
The hydroxyl group attached to i may also have been converted into another functional group during its preparation, eg alkoxy.

These silica fillers are reinforcing fillers, as opposed to other fillers of the non-reinforcing non-structural forming type, such as titanium dioxide and calcium carbonate. Examples of such silica fillers are US Pat. Nos. 2,541,137, 2,610,167 and 2,657,149,
And French Patent No. 1,025,837 (195
3 years) and 1,090,566 (issued 1955). Such a structure-forming filler may be slightly acidic or alkaline (that is, pH slightly lower or higher than 7) depending on the manufacturing method,
It can be obtained by an aerosol-aerogel method, a smoke generation method such as vapor phase combustion of silicon tetrachloride or ethyl silicate, a precipitation method, or the like. Commercially available fumed silica includes Cabot Corp.'s CABOSIL (CA
BO-SIL®) and Degussa, Inc.'s AEROSIL®. Fumed silica is preferred.

The condensable diorganopolysiloxane used in the present invention must be liquid at certain temperatures below 210 °,
Moreover, it must have a hydroxy or alkoxy functionality that readily reacts with the surface of the silica. In the case of a diorganopolysiloxane having insufficient condensation ability, the reactivity with the silica surface can be increased by adding a condensation accelerator such as stannic acid, stannous salt and Lewis acid. These condensation accelerators can be added as long as they do not promote undesired side reactions such as polymerization which results in the formation of gum spheres. The hydroxy or alkoxy functionality can be anywhere on the polymer, but the diorganopolysiloxane is hydroxy terminated and has no condensable substitutions on the chain to prevent the formation of crosslinked gum spheres. Is preferred. Preferred condensable diorganopolysiloxanes are of the formula:

Here, x has a value of about 1 to 20 and R ³ is a monovalent substituted or unsubstituted hydrocarbon group. The condensable diorganopolysiloxanes are preferably all hydroxy-terminated diorganopolysiloxanes of the above formula. The rest of the condensable diorganopolysiloxane can be, for example, a hydroxy substituted siloxane side chain. In the preferred case, x in the above formula is from about 2 to 10, most preferably about 2 or 3.
Is. R ^{3 in the} above formula is usually numerically at least about 5
0% is methyl, the remainder is alkyl such as methyl, ethyl, propyl, butyl, hexyl, etc., alkenyl such as vinyl, aryl such as phenyl, cycloalkyl such as cyclopentyl, cyclohexyl, cycloheptyl, etc., Alkyl halides such as 3-chloropropyl, 4-chlorobutyl, 3,3-difluoroallyl, 3,3,3-trifluoropropyl and the like, 3
-Aryl halides such as fluorophenyl,
It is selected from among cycloalkyl halides and the like.
Preferred R ³ is —CH when R ³ is halogenated
A ₂ CH ₂ R ^4, the ^{R 4} is perfluoromethyl,
Perfluoroalkyl such as perfluoroethyl, perfluorohexyl and the like. R ³ has up to about 10 carbon atoms.

The hydroxy-terminated diorganopolysiloxanes described above can be prepared by methods well known in the art. In one method, the diorganodihalosilane is partially hydrolyzed to form a mixture of cyclic and linear diorganopolysiloxanes. For further details regarding this method, see Hurd et al., US Pat. No. 2,737,506, Razzano, US Pat.
No. 37,684, No. 4,341, of Razzano
888 and Evans, et al., 4,529,7.
No. 74, all of which are hereby incorporated by reference.

In another method, the cyclic diorganopolysiloxane is heated in an organic solvent in the presence of a clay catalyst treated with H ⁺ to open the ring and form a hydroxy-terminated diorganopolysiloxane. In this method, the composition of the diorganopolysiloxane product can be adjusted by adjusting the purity of the cyclic diorganopolysiloxane raw material and the size of the ring. This second method is
It is preferable in terms of purity and kind of cyclic polydiorganosiloxane that can be used.

Other additives may be present in the uncured mixture, such as pigments, stabilizers, plasticizers, additional fillers and the like. Those skilled in the art will know what additives are necessary and suitable for achieving a given purpose.

According to the method of the present invention, the curable non-condensable polysiloxane gum, the silica filler and the condensable diorganopolysiloxane are mixed in a container with gentle heating to reduce viscosity if necessary. To do. If the gum is very viscous, silica must be added in stages or in bulk to agglomerate the batch with mixing.

The amount of silica filler added to the batch varies over a very wide range. It would be desirable to masterblock polysiloxane gums that could subsequently be cured with a filler-free gum. In this case, for each 100 parts by weight of curable polysiloxane gum, about 10 parts of silica filler is used.
Up to about 400 parts by weight can be added. Usually about 10 to about 100 parts by weight of silica filler is used. of course,
The amount of condensable diorganopolysiloxane added to the batch depends on the amount of silica added. This amount is about 5 to about 50 parts by weight of the condensable diorganopolysiloxane, and more preferably about 17 parts by weight, for each 100 parts by weight of silica.
To about 30 parts by weight, and most preferably about 19 to about 25 parts by weight.

Once the batch is fully agglomerated, treatment of the filler is accomplished by simply raising the temperature of the batch up to about 210 ° C. with mixing. At temperatures above 210 ° C, polysiloxane gum begins to decompose and prematurely cure. Of course, no curing catalyst for this gum was added at this point, and gums typically at temperatures below about 210 ° C. would not cure without the catalyst. This treatment temperature may be lowered to about 110 ° C. while maintaining an acceptable treatment time. Reasonable processing time, that is, reaction time is about 1 to about 4 hours, and practically about 2 hours. The preferred reaction temperature is about 130 to about 180 ° C., the optimum reaction temperature is about 140 to about 160.
It is in the range of ° C.

During heating and processing, volatiles may be removed by purging or vacuum. Condensation produces water, and non-condensable or non-curable volatiles may have been added to the batch, but it is desirable to remove these as well. For example, if the condensable diorganopolysiloxane has been added as the hydrolysis product, the cyclopolysiloxane can be removed at this point by vacuum or nitrogen purge.

After the treatment or heating step, catalysts, stabilizers, as well as additional gums or crosslinkers can be added to impart curability and long-term storage capacity to the curable polysiloxane gum. As explained above, the gum may be a thermoset gum, a room temperature set gum, or even a cold set gum. Careful selection of materials and simple experimentation should be considered to achieve optimal results in a given situation.

The following examples are provided for purposes of illustration and not limitation so that those skilled in the art may better understand how to practice the present invention.

Examples Examples 1 and 2 In a clean 1420 ml dough mixer, a condensable diorganopolysiloxane treating agent [silanol-terminated methyl-containing 6.8 wt.% Hydroxy end groups was used.
20 g of 3,3,3-trifluoropropylpolysiloxane fluid] and 401.5 g of non-condensable polysiloxane gum and additives. This 401.5 g of gum and additives had a Williams plasticity (25
380 g of vinyl-terminated methyl-3,3,3-trifluoropropylsiloxane gum, having a 3'value at 200 ° C.) of 200 ± 20 and a vinyl end group content of 0.01 to 0.20 wt. vinyl content of _(CH 2 = CH- as) Williams 1.6 wt% (Williams) formula plasticity (3 'value at 25 ° C.) is 190 ± 20, methyl and endblocked vinyl-3,3, 20 g of 3-trifluoropropylsiloxane gum, M with 4.2% by weight of vinyl on the chain
It was composed of 0.5 g of polydimethylsiloxane gum stopped at 1 and 1.0 g of vinyl-terminated polydimethylsiloxane plasticizer. 20-45r of this mixture
After mixing for 30 minutes at a shear rate of pm, it was heated to 50 ° C. under a nitrogen atmosphere. Finely divided untreated fumed silica with a surface area of 200 ± 20 m ² / g was added in increasing amounts at a given time between each addition so that the batch solidified. The total weight of fumed silica added is shown in Table I. When the addition of filler was complete and the batch was completely solidified, the temperature was raised to 120-160 ° C. and the nitrogen purge rate was 1-6 ft ³ / hr and maintained for 1-3 hours. Then increase the nitrogen purge rate to 10-15 ft ³ / hr
Maintained for 6 hours. The batch temperature eventually dropped below 80 ° C. Thereafter, 0.7 g of a stabilizer, that is, cerium hydroxide having a purity of 90.5% and a sieve size of +250 mesh was added. Lupersol 101 hardener, 2,5-dimethyl-2,5-di (t-butylperoxy) hexane, added to silica and diluting gum in a total amount of 100 g.
0.75g per gram, then 15 at 177 ° C
It was press-cured for 1 minute and post-baked at 204 ° C. for 4 hours to cure the gum.

Control A clean 1420 ml dough mixer was charged with 412.5 g of non-condensable polysiloxane gum and additives. This 412.5 g of gum and additives are
ams) vinyl-terminated methyl-3,3,3-trifluoro having a plasticity (3 'value at 25 ° C) of 200 ± 20 and a vinyl end group content of 0.01 to 0.20% by weight. Propyl siloxane gum 380g, vinyl content on the chain is (CH
₂ = CH-) at 1.6 wt% Williams (Willi
ams) plasticity (3 'value at 25 ° C) of 190 ± 20,
20 g of vinyl-terminated methyl-3,3,3-trifluoropropylsiloxane gum, vinyl on the chain 4.
Consists of 0.5 g of 2% by weight M-terminated polydimethylsiloxane gum and 12 g of processing aid, ie, silanol-terminated polydimethylsiloxane telomer, the chain of which contains on average 5 siloxane units. It was This mixture is stirred at a shear rate of 20-45 rpm for 30
After mixing for minutes, it was heated to 50 ° C. Under a nitrogen atmosphere,
120 g of finely divided treated fumed silica with a surface area of 200 ± 20 m ² / g before treatment were added in increasing amounts at given times between each addition so that the batch solidified. The treatment of fumed silica was performed according to US Pat. No. 4,529,774. Specifically, fumed silica was dried and contacted with a fluorosilicone hydrolysis product (which is a mixture of fluorosilicone telomer silanols and fluorosilicone cyclics) for 8 hours in the gas phase at 280-300 ° C. . The residual fluorosilicone hydrolysis product was then released and the treated fumed silica was volatile removed by purging with nitrogen at 300 ° for 10 hours. When the addition of fillers is complete and the batch is completely set, the temperature is 120-1
Raise the temperature to 60 ℃ and set the nitrogen purge rate to 1-6ft ³ / hr.
Maintained for ~ 3 hours. The batch temperature eventually dropped to 80 ° C. Thereafter, 0.7 g of a stabilizer, that is, cerium hydroxide having a purity of 90.5% and a sieve size of +250 mesh was added. The gum and fumed silica mixture was cured as in Examples 1 and 2.

As is apparent from Table 1, the fumed silica incorporated into the gum according to the present invention provides a polydiorganosiloxane rubber with a characteristic profile that is better than that of the prior art pretreated fumed silica.

Examples 3 and 4 In a clean 1420 ml dough mixer, US Pat.
529,774 according to dichloromethyl-3,3,3
A hydrolyzate fluid (see Table 2) containing a non-condensable cyclopolysiloxane in addition to a condensable telomeric silanol, prepared from trifluoropropylsilane;
Non-condensable polydiorganosiloxane gum and additives 4
01.5 g was added. This 401.5 g of gum and additives had a Williams plasticity (3 'value at 25 ° C.) of 200 ± 20 and a vinyl end group content of 0.01.
~ 0.20% by weight of vinyl-terminated methyl-
3,3,3-Trifluoropropylsiloxane gum 38
0 g, _(as a CH 2 = CH-) Kusarijo vinyl content 1.
20 g of vinyl-terminated methyl-3,3,3-trifluoropropylsiloxane gum, Williams formula plasticity (3 'value at 25 ° C) of 190 ± 20 at 6% by weight, vinyl on chain Consisted of 0.5 g of M-terminated polydimethylsiloxane gum of 4.2% by weight, and 1.0 g of vinyl-terminated polydimethylsiloxane plasticizer. The mixture was mixed for 30 minutes at a shear rate of 20-45 rpm and then heated to 50 ° C. Under a nitrogen atmosphere, 112 g of finely divided untreated fumed silica with a surface area of 200 ± 20 m ² / g was added in increasing amounts at a given time between each addition so that the batch solidified. . When the addition of filler was complete and the batch was completely solidified, the temperature was raised to 120-160 ° C. and the nitrogen purge rate was 1-6 ft ³ / hr and maintained for 1-3 hours.
Then increase the nitrogen purge rate to 10-15 ft ³ / hr and
Maintained for ~ 6 hours. The batch temperature eventually dropped below 80 ° C. Then stabilizer, ie purity 90.5%
Then, 0.7 g of cerium hydroxide having a sieve size of +250 mesh was added. Most of the non-condensable cyclopolysiloxane was removed during the nitrogen purge. LUPERSOL
SOL) 101 hardener, ie 2,5-dimethyl-2,5
-Di (t-butylperoxy) hexane was added at a rate of 0.75 g per 100 g total of silica and diluting gum,
Then press-cure at 177 ° C for 15 minutes, then 204
The gum was cured by post-baking at 4 ° C for 4 hours.

As shown in Table 2, prior art hydrolyzate treating agents can be used in the process of the present invention to obtain treated silica having at least comparable properties.

Examples 5-7 In a clean 1420 ml dough mixer, US Pat.
529,774 according to dichloromethyl-3,3,3
A hydrolyzate fluid containing a non-condensable cyclopolysiloxane in addition to a condensable telomeric silanol prepared from trifluoropropylsilane (see third)
And non-condensable polydiorganosiloxane gum and 416 g of additives. This 416 g of gum and additives was added to 400 g of Williams plasticity (25
(3 'value in ° C) is 200 ± 30 and vinyl content on the chain is 0.
06-0.07% by weight silanol-terminated methyl-3,3,3-trifluoropropylsiloxane gum, and a Williams formula plasticity of 90 ± 2.
0 chain on vinyl content with the _(CH 2 = CH- as) 4.2
It comprised 16% by weight of trimethylsiloxy-terminated polydimethylsiloxane gum. The mixture was stirred under a nitrogen atmosphere at 50 ° C. with a shear of 20-50 rpm. After mixing for 30 minutes, 92 g of finely divided silica was added in increasing amounts at a given time between each addition to set the batch. When the addition of fillers is complete and the batch is completely solidified, the temperature is raised to 140-160 ° C and the nitrogen purge rate is 1-6 ft ³
/ Hr was maintained for 1 to 3 hours. The nitrogen purge rate was then increased to 10-15 ft ³ / hr and maintained for 2-6 hours. The batch temperature eventually dropped below 80 ° C. Iron octoate stabilizer, ie 6% by weight in mineral spirits
1.0 g of titanium dioxide was added together with 0.2 g of iron 2-ethylhexanoate. Most of the non-condensable cyclopolysiloxane was removed during the nitrogen purge. Caddox (C
ADOX) TS-50 hardener, ie 2,4-dichlorobenzoyl peroxide, added to silica and diluent gum in a total of 10
Add at 1.6g per 0g, then 1 at 124 ° C
Press cure for 5 minutes and postbake at 204 ° C. for 4 hours to cure the gum.

Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display part C08L 83:08) (72) Inventor Michael John Roscoco Box 31, Box 31, Shagti Cork, New York, USA Day No. 2 (No address) (56) References Special Table Sho 58-500369 (JP, A)

Claims (16)

[Claims] 1. (a) heated to a temperature of 210 ° C. or lower,
Curable non-condensable polysiloxane gum, 10 to 40 parts by weight of silica filler with 100 parts by weight of said gum, having Si-bonded hydroxy or alkoxy functional groups, and said filler 100 Mixing 5 to 50 parts by weight of the condensable diorganopolysiloxane with respect to parts by weight, wherein the condensable diorganopolysiloxane has the formula R ¹ RSiX ₂ and R ² ₂ SiX. Is a hydrolysis product of a diorganodihalosilane represented by ₂ , wherein R ¹ is 3,3,3-trifluoropropyl, R is methyl or ethyl, R ² is methyl or ethyl. X is halogen, and the hydrolysis product is a fluoroalkyl-functional cyclic diorganopolyene having 3 to 10 siloxy units. A mixture of roxane, a silanol-terminated, low molecular weight linear diorganopolysiloxane terminated with silanol, which readily reacts with the silica filler at temperatures below 210 ° C., (B) the mixture of the gum, the filler and the diorganopolysiloxane at a temperature of up to 210 ° C. for a time sufficient to complete the condensation reaction of the silica filler and the condensable diorganopolysiloxane. A step of mixing the silica filler in the gum, the method comprising: 2. The method of claim 1 including the step of curing the polysiloxane gum after the heating step. 3. The method of claim 1 including the step of adding a polysiloxane gum curing catalyst after said heating step. 4. The method according to claim 1, wherein the mixing step and the heating step are performed simultaneously. 5. The method according to claim 1, wherein the heating temperature is in the range of 110 ° C. to 210 ° C. 6. The method according to claim 5, wherein the heating temperature is in the range of 130 ° C. to 180 ° C. 7. The method according to claim 6, wherein the heating temperature is in the range of 140 ° C. to 160 ° C. 8. The method according to claim 1, wherein the viscosity of the polysiloxane gum is 500,000 to 300,000,000 centipoise at 23 ° C. 9. The polysiloxane gum has the general formula: R _a SiO _{4-a / 2} [wherein R is an alkyl group having 1 to 8 carbon atoms, a vinyl group, a phenyl group, or 3 to 10 carbon atoms]. Selected from a haloalkyl group, a halophenyl group, a hydroxy group, an alkoxy group, an aryloxy group, a cyanoalkyl group, and combinations thereof, and a varies from 1.98 to 2.01]. The method according to claim 1. 10. The polysiloxane gum has the general formula: [Wherein R ¹ may be vinyl, methyl, hydroxy or alkoxy, and R ² is vinyl, phenyl, 1 to 1 carbon atoms.
It may be 8 alkyls, fluoroalkyls of 3 to 10 carbon atoms or combinations thereof and y is 2,500 to 1
Variable up to 1,000]. 11. The method of claim 10 wherein the polysiloxane gum is M-terminated. 12. The method of claim 10 wherein the polysiloxane gum contains vinyl functional groups. 13. The polysiloxane gum containing hydroxy functional groups according to claim 10.
The method described in the section. 14. The polysiloxane gum containing alkoxy functional groups according to claim 10.
The method described in the section. 15. The method according to claim 1, wherein there is a step of adding a crosslinking agent after the heating step.
The method described in the section. 16. (a) 100 parts by weight of said gum having a curable non-condensable polysiloxane gum heated to a temperature below 210 ° C. and having Si-bonded hydroxy or alkoxy functional groups. And 10 to 40 parts by weight of a silica filler, an effective amount of a cross-linking agent, and 5 to 50 parts by weight of a condensable diorganopolysiloxane based on 100 parts by weight of the filler. Wherein the condensable diorganopolysiloxane is a hydrolysis product of a diorganodihalosilane represented by the formulas R ¹ RSiX ₂ and R ² ₂ SiX ₂ , wherein R ¹ is 3, 3,3 - a trifluoropropyl, R is methyl or ethyl, R ² is methyl or ethyl, X is halogen, and further, the hydrolysis products, 3 to 1 A fluoroalkyl-functional cyclic diorganopolysiloxane having 6 siloxy units and a silanol-terminated low molecular weight linear diorganopolysiloxane having a hydroxy functionality and a temperature below 210 ° C. (B) a sufficient amount of time to complete the condensation reaction of the silica filler with the condensable diorganopolysiloxane, the gum, the filling Heating the mixture of wood and the diorganopolysiloxane to a temperature of up to 210 ° C., and mixing the silica filler into the gum.