JPH09268007A - Hydrated silicic acid suitable as filler for silicone rubber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a hydrated silicic acid useful as a filler for a silicone rubber capable of stably producing a silicone rubber compound having excellent workability and processability when compounded to a silicone rubber. SOLUTION: This hydrated silicic acid useful as a filler for a silicone rubber has a pH of 5.0-7.0 as a 4% aqueous suspension and has a weight ratio (1)/(2) of 0.2-0.9 and that of (2)/SiO2 of 0.002-0.02 wherein (1) is the sum of sodium, potassium, magnesium and calcium in terms of Na2 O, K2 O, MgO and CaO, respectively, and (2) is the sum of aluminum, iron and titanium in terms of Al2 O3 , Fe2 O3 and TiO2 , respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filler for silicone rubber, and more particularly, by controlling the amount of a specific chemical substance in a solid, it is possible to give a very stable viscosity when compounded in the silicone rubber. The present invention relates to hydrous silicic acid for silicone rubber prepared as described above.

[0002]

2. Description of the Related Art Hydrous silicic acid is produced by a wet method, and is generally called white carbon together with dry silicic acid anhydride. It is a carrier for agricultural chemicals, a dispersant, or a paint, an ink, an adhesive, a synthetic resin, and papermaking. In addition to being used in a wide variety of fields as a filler and coating agent for white rubber, it is known as a white rubber reinforcing agent with performance comparable to carbon black, and silicone rubber is one of the important applications as this rubber reinforcing agent. Fillers for use. That is, by adding this hydrous silicic acid to silicone rubber, it is possible to impart appropriate hardness and viscosity to the kneaded material of the silicone rubber to provide appropriate workability and processability of the kneaded material of the silicone rubber, and This is because the vulcanized silicone rubber exhibits appropriate tensile strength, elongation, tear strength, impact resilience, and other reinforcing properties depending on the application.

By the way, when hydrous silicic acid is blended with silicone rubber as a reinforcing filler, it is desirable that the blending amount necessary for imparting an appropriate viscosity to the kneaded material of the silicone rubber can be set to a constant ratio. However, in reality, the viscosity of the kneading dough and the hardness of the silicone rubber may change greatly even if the physical properties of the hydrous silicic acid change slightly.In such a case, the compounding amount must be changed. Has become. The details of the reason why such variations appear are not necessarily clear, but due to the selection of the raw materials used for the production of hydrous silicic acid, the production method, etc., the effect of different particle surface activity of the produced hydrous silicic acid It is believed to be greatly affected by the above.

[0004] Conventionally known methods for producing hydrous silicic acid are roughly classified into two methods, one of which is an aqueous solution of alkali silicate adjusted to a predetermined concentration in advance or a mineral acid. It is a one-sided dropping method in which a mineral acid or an alkali silicate aqueous solution is dropped for a certain period of time while maintaining a constant concentration in the reaction tank, and the other is a reaction tank in which a fixed amount of warm water is pre-filled. Constant p
This is a simultaneous dropping method in which an aqueous solution of alkali silicate and a mineral acid are added for a certain period of time while maintaining H and temperature. The fine hydrous silicic acid deposited by such a method is the alkali ion remaining in the reaction slurry (hydrous silicic acid slurry),
It is filtered to remove acidic ions, washed with water to obtain a hydrous silicic acid cake, which is then dried, pulverized, filtered, and produced by classifying as necessary. Due to different raw material components of mineral acid, different neutralization reaction conditions, and different grinding and classification conditions, different hydrated silicic acids are produced as secondary particles with extremely complex structure in which primary particles are combined and condensed. The surface activity of the particles is greatly influenced by the types and amounts of the acids, bases, salts, etc. remaining in the hydrous silicic acid, and this is considered to affect the viscosity when compounded in the silicone rubber.

As described above, when the silicone rubber is blended with hydrous silicic acid at a constant ratio, unless the kneaded dough has an appropriate viscosity and hardness, workability and workability are deteriorated, and further, Since vulcanized silicone rubber does not have appropriate tensile strength, elongation, tear strength, and reinforce properties such as impact resilience, the compounding and filling amount must be adjusted each time, and the production efficiency of silicone rubber, There was a problem that productivity was significantly impaired.

For the purpose of solving such problems and providing hydrous silicic acid suitable for use as a reinforcing filler for silicone rubber, the content of metal oxides having a valence of 2 to 4 other than SiO ₂ is set to a predetermined value. Of controlling within the range of
No. -1879), or a proposal for highly controlling the surface state of hydrous silicate particles (JP-A-6-171922).

[0007]

However, even in the case of hydrous silicic acid produced based on the above-mentioned proposal, if the washing condition changes or the water quality of the washing water changes, a silicone compounded with the silicic acid may be used. It was discovered that the viscosity of rubber changes.

The inventor of the present invention has made various studies on various factors for changing the viscosity of the kneaded dough when blended with silicone rubber in order to solve such a problem, and has accomplished the present invention. That is, the viscosity changes depending on the blending, the degree of rinsing at the time of rinsing and rinsing water when filtering and rinsing to remove alkaline ions and acidic ions remaining in the reaction slurry (hydrous silicic acid slurry). I thought that it might be caused by the water quality of
The total amount of residual alkali metals (mainly sodium and potassium) among the remaining chemical substances, or the content of sodium, potassium, magnesium, and calcium among the chemical substances derived from the wash water and adsorbed by hydrous silicic acid. It was found that the ratio of the total content of aluminum, iron and titanium affects the viscosity of the kneaded dough when blended with silicone rubber.

The present invention has been made on the basis of such findings, and an object thereof is to enable stable production of a silicone rubber compound having excellent workability and processability when compounded and filled in a silicone rubber. It is intended to provide hydrous silicic acid as a filler for rubber.

Another object of the present invention is to provide a hydrous silicic acid which can reduce defective lots of products by providing a silicone rubber having a stable viscosity.

[0011]

The characteristic feature of the hydrous silicic acid suitable for the silicone rubber filler of the present invention that achieves the above object is that the pH of a 4% aqueous suspension is 5.0 to 7.0. , And the total amount when the contained sodium, potassium, magnesium, and calcium are converted into Na ₂ O, K ₂ O, MgO, and CaO, respectively (that is, = Na ₂ O + K ₂ O +
MgO + CaO) and the contained aluminum, iron, and titanium are converted into Al ₂ O ₃ , Fe ₂ O ₃ , and TiO ₂ , respectively (ie, = Al ₂ O ₃ + Fe ₂ O).
₃ + TiO ₂ ) weight ratio (/) is 0.3 to 0.9
And the weight ratio to SiO ₂ (/
It is where the SiO ₂₎ is to be in the range of 0.002 to 0.02.

The hydrous silicic acid of the present invention has a p-concentration of 4% water suspension.
It is necessary that H is 5.0 to 7.0, preferably 5.5 to 6.5.

If the pH is lower than this range, the viscosity of the silicone rubber compound compounded with the pH tends to be too low, and the tackiness of the rubber becomes strong and the workability is significantly impeded in many cases. On the contrary, if the pH exceeds this range,
The viscosity of the compounded silicone rubber compound tends to be too high, and the winding on the roll becomes poor and the workability is often impaired.

When the pH is out of this range,
Probably because the water-containing silicic acid slurry is not sufficiently washed with water, it is difficult to set the above-mentioned (/) ratio within the range of 0.3 to 0.9, and the obtained silicone rubber has a poor color tone. Cause the problem of.

The above ratio (/) of the present invention, that is, (Na ₂ O + K ₂ O + MgO + CaO) / (Al ₂ O ₃
+ Fe ₂ O ₃ + TiO ₂ ) needs to be 0.3 to 0.9, and preferably 0.5 to 0.7.

When the (/) ratio is lower than 0.3, the viscosity of the silicone rubber compound containing the same tends to be too low, and on the contrary, the (/) ratio is 0.9.
If it exceeds, the viscosity of the compounded silicone rubber compound tends to be too high, which often impairs the processability of the rubber as described above.

The above (/ SiO ₂ ) ratio of the present invention, that is, (Al ₂ O ₃ + Fe ₂ O ₃ + TiO ₂ ) / SiO ₂
Is required to be 0.002 to 0.020 (0.2 to 2.0 parts by weight with respect to 100 parts by weight of SiO ₂ ), and preferably 0.002 to 0.006. .

Aluminum, iron, contained in hydrous silicic acid,
If the weight ratio of the total amount of titanium to the oxide-converted amount of SiO ₂ is out of the above range, the reason is not clear, but the processability of the silicone rubber compounded with this and the reinforcing property of the rubber are unsatisfactory. Sufficient to achieve the object of the present invention.

Among the chemical substances specified in the present invention, the above (Al ₂ O ₃ + Fe ₂ O ₃ + TiO ₂ ) is contained by being derived from silica silicate, silica sand, siliceous earth, etc., which are alkali silicate raw materials. . In addition, the above (Na ₂ O + K ₂ O + MgO + C
aO) is used in a large amount when dissolving these silica stones, silica sand, and siliceous earth as an alkali silicate raw material, or is supplied from natural water when washing the hydrous silicic acid slurry in the process of producing hydrous silicic acid. Will be included.

The hydrous silicic acid of the present invention is not particularly limited in other characteristic values as long as it satisfies the above characteristic values, and can be in a general range.

For example, the amount of adsorbed water of hydrous silicic acid is normally 3 to 8% and stable, and the bulk density is usually 30 to 100 g / l (JIS K-5105-18). It should be the one.

The BET specific surface area of hydrous silicic acid is 130 to 250 in consideration of dispersibility in silicone rubber.
m ² / g, preferably in many cases it is appropriate to use of about 180~220m ² / g. Furthermore, the average particle size of the secondary particles measured by the Coulter Counter method is preferably 1 to 12 μm, and more preferably 4 to 10 μm, for the hydrated silicic acid in consideration of dispersibility, reinforcing property and the like.

The method for producing hydrous silicic acid for silicone rubber according to the present invention is not particularly limited in terms of production method and other characteristic values as long as the above-mentioned characteristics are satisfied.

For example, in the case of reacting an aqueous solution of an alkali metal silicate with a mineral acid to precipitate hydrous silicic acid, ordinary commercially available sodium silicate and sulfuric acid can be used, and the production method is as described above. Various known reaction treatment methods such as a one-side dropping method and a simultaneous dropping method can be adopted.

Various methods can be used as a method for controlling the amount of the above chemical substance contained in the hydrous silicic acid of the present invention so as to be within the above numerical range. For example, aluminum, iron, and titanium in hydrous silicic acid can be added in advance to the raw material aqueous alkali silicate solution or mineral acid, or can be added during the neutralization reaction. Alternatively, a method of rinsing the hydrous silicic acid slurry with wash water containing these chemical substances in advance and adsorbing it to the hydrous silicic acid can be adopted, but the method is not limited to these.

The method of controlling the amounts of sodium, potassium, magnesium, and calcium contained in the hydrous silicic acid so as to fall within the above numerical range is not limited in the same manner, and these substances remaining in the hydrous silicic acid slurry are not limited to the same. The amount of washing water may be adjusted so that the amount falls within the above numerical range and washing may be performed, or washing water containing these chemical substances in advance may be used. When these chemical substances (sodium, potassium, magnesium, calcium) are contained in the wash water, it is a compound with a substance usually called weak acid such as carbonic acid, bicarbonate, boric acid, organic acid, etc. The method of containing several tens of ppm or containing it as hydroxide in water is preferable because it facilitates control of the viscosity of the silicone rubber. The method of adding the above-mentioned chemical substances (sodium, potassium, magnesium, calcium) as a compound with a strong acid such as hydrochloric acid, sulfuric acid, nitric acid is not clear for the reason, but is not effective for controlling the viscosity of the silicone rubber.

As a preferred method for producing the hydrous silicic acid of the present invention, the above (/) ratio in the production process, that is, (Na ₂ O + K ₂ O + MgO + CaO) / (Al ₂
O ₃ + Fe ₂ O ₃ + TiO ₂ ) can be used to control the washing conditions while measuring the (/) ratio during production by measuring the electrical conductivity of the filtrate of the washing water of the hydrous silicic acid slurry. It can be easily performed by detecting.

A commonly used filter press or the like can be used for filtration / washing of the hydrous silicic acid slurry, and the drying method can also be a general method such as air stream drying or stationary drying, and further pulverization A general method such as an impact type or a jet type crusher can be used as the method, but the method is not limited to any of them.

[0029]

EXAMPLES Examples and comparative examples will be described below to specifically describe the present invention. In addition, each physical property value etc. were implemented by the method shown below.

(Chemical substances in hydrous silicic acid) Using a fluorescent X-ray analyzer (type system 3080; manufactured by Rigaku Corporation), the contents of sodium, potassium, magnesium, calcium, aluminum, iron and titanium are determined to be oxides, that is, Na
₂ O, K ₂ O, MgO, CaO, Al ₂ O ₃ , Fe ₂ O
₃ and TiO ₂ were quantified. The silica content (SiO ₂ ) in hydrous silicic acid was also quantified in the same manner.

(PH value of 4% water suspension of hydrous silicic acid) A sample of 4 g was placed in a beaker, and distilled water having a pH of 5.5 or more was added to make a total of 100 g. This was thoroughly stirred for about 5 minutes and the stable value of the indicated value was measured with a commercially available glass electrode pH meter (model; D-14 Co., Ltd., Hikiba Seisakusho).

(Evaluation of Workability of Silicone Rubber Compound (Viscosity)) For the adjustment of the unvulcanized silicone rubber compound, only the raw silicone rubber and hydrous silicic acid were blended in order to clarify the difference in viscosity.

The kneading of the raw silicone rubber and the silicic acid hydrate was carried out at room temperature for 10 minutes using a 6-inch roll. The viscosity of this rubber compound was measured for 24 hours with a Mooney viscosity meter (model SMV-200 manufactured by Shimadzu Corporation) at a temperature of 100 ° C. using an L rotor, and the value was ML _{1 + 10} . Indicated by.

The content of hydrous silicic acid was 40 parts by weight based on the raw silicone rubber. When the Mooney viscosity value (ML _{1 + 10} ) is 20 to 40, the workability evaluation is ◯,
If it exceeds 40, it is judged that the silicone rubber compound becomes harder and harder, and the workability is evaluated as x. If it is less than 25, it is softly gripped and there is winding around the roll.

(Evaluation of Rubber Reinforcing Property (Tensile Strength / Hardness)) The test was carried out in accordance with the method for testing physical properties of crosslinked rubber of JIS K-6301.

Tensile strength of 70 or more and hardness of 5
If 0 or more, it was judged that the vulcanized silicone rubber had sufficient reinforcing property, and the reinforcing property was evaluated as ◯, and if not, x.

Example 1 A sodium silicate aqueous solution having a Na ₂ O concentration of 4.2% and a SiO ₂ / Na ₂ O molar ratio of 3.2 was placed in a reaction tank equipped with a baffle plate having a stirrer having an internal volume of 240 liters (commercially available). Dilute sodium silicate and add aqueous solution of sodium aluminate to obtain the total content of aluminum, iron and titanium (Al ₂ O ₃ + Fe
₂ O ₃ + TiO ₂ ) -converted aqueous solution adjusted to 0.3 wt% relative to the amount of SiO ₂ in hydrous silicic acid: 8 liters of aqueous solution and 100 liters of warm water heated to 75 ° C. were charged. .

This was heated to 75 ° C., and then 82 liters of an aqueous solution of sodium silicate and 9 liters of sulfuric acid having a concentration of 49% were simultaneously added for 90 minutes to carry out a neutralization reaction. Thereafter, sulfuric acid was further added to adjust the pH of the hydrous silicic acid slurry to 3.0, and the whole reaction was completed. During this period, stirring was performed in the reaction tank so that a constant stirring force was always obtained.

A portion of the hydrous silicic acid slurry thus obtained was filtered with a filter press to obtain (N
a ₂ O + K ₂ O + MgO + CaO) / (Al ₂ O ₃ + F
e ₂ O ₃ + TiO ₂ ) ratio is 0.4, pH of 4% water suspension
Was washed with water so that the ratio was 5.5 to 6.5.

After washing with water, drying, crushing and classification were carried out to obtain hydrous silicic acid for silicone rubber.

The physical properties of the obtained silicic acid hydrate are shown in Table 1 below.
(The same applies to the following Examples and Comparative Examples).

The evaluation of the workability at the time of compounding the silicone rubber and the measurement of the tensile strength and hardness of the crosslinked silicone rubber were carried out according to the above-mentioned evaluation methods, and the results are shown in Table 1 below (the following examples and comparative examples). The same).

Example 2 A portion of the hydrous silicic acid slurry obtained by the neutralization reaction of Example 1 was filtered to obtain (Na ₂ O + K ₂ O + Mg) in hydrous silicic acid.
_{O + CaO) / (Al 2} O 3 + Fe 2 O 3 + TiO 2)
It was washed with water so that the ratio was 0.6, and the pH of the 4% aqueous suspension was 5.5 to 6.5.

Comparative Example 1 Part of the hydrous silicic acid slurry obtained by the neutralization reaction of Example 1 was filtered to obtain (Na ₂ O + K ₂ O + Mg) in hydrous silicic acid.
_{O + CaO) / (Al 2} O 3 + Fe 2 O 3 + TiO 2)
Washing with water was carried out so that the ratio was 1.0, and the pH of the 4% aqueous suspension was 5.5 to 6.5.

Comparative Example 2 Part of the hydrous silicic acid slurry obtained by the neutralization reaction of Example 1 was filtered to obtain (Na ₂ O + K ₂ O + Mg) in hydrous silicic acid.
_{O + CaO) / (Al 2} O 3 + Fe 2 O 3 + TiO 2)
It was washed with water so that the ratio was 1.0, and the pH of the 4% aqueous suspension was 8.5. Example 3 In Example 1, aluminum in the hydrous silicic acid, iron,
The total content of titanium is (Al ₂ O ₃ + Fe ₂ O ₃ +
In terms of TiO ₂ ), 0.7 wt% relative to the amount of SiO _{2 in} the hydrous silicic acid, (Na ₂ O + K ₂ O + M in the hydrous silicic acid)
gO + CaO) / (Al ₂ O ₃ + Fe ₂ O ₃ + TiO
₂ ) A hydrous silicic acid for silicone rubber was obtained in exactly the same manner as in Example 1 except that the ratio was adjusted to 0.4.

Example 4 A part of the hydrous silicic acid slurry obtained by the neutralization reaction of Example 3 was filtered to obtain (Na ₂ O + K ₂ O + Mg) in hydrous silicic acid.
_{O + CaO) / (Al 2} O 3 + Fe 2 O 3 + TiO 2)
It was washed with water so that the ratio was 0.6, and the pH of the 4% aqueous suspension was 5.5 to 6.5.

Comparative Example 3 Part of the hydrous silicic acid slurry obtained by the neutralization reaction of Example 3 was filtered to obtain (Na ₂ O + K ₂ O + Mg) in hydrous silicic acid.
_{O + CaO) / (Al 2} O 3 + Fe 2 O 3 + TiO 2)
It was first washed with hydrochloric acid so that the ratio became 0.2, and then washed so that the pH of the 4% aqueous suspension became 5.5 to 6.5.

Comparative Example 4 Part of the hydrous silicic acid slurry obtained by the neutralization reaction of Example 3 was filtered to obtain (Na ₂ O + K ₂ O + Mg) in hydrous silicic acid.
_{O + CaO) / (Al 2} O 3 + Fe 2 O 3 + TiO 2)
Washing with water was performed so that the ratio was 0.4 and the pH of the 4% aqueous suspension was 4.5. Example 5 An aqueous sodium silicate solution having a Na ₂ O concentration of 4.2% and a SiO ₂ / Na ₂ O molar ratio of 3.2 (commercially available sodium silicate) was placed in a reaction tank with a baffle plate having a stirrer having an internal capacity of 240 liters. 8 liters of aluminum, iron, and titanium content of 0.05 wt% or less with respect to SiO ₂ ) and 100 liters of warm water heated to 75 ° C. were charged.

This was heated to 75 ° C., then 82 liters of an aqueous solution of sodium silicate and Fe ₂ O were added.
₃ in terms of a concentration of 49% of the dissolved iron ions so that 0.5 wt% to SiO ₂ in the hydrous silicate and sulfuric 9 l added simultaneously over 90 minutes was carried out neutralization reaction.

Thereafter, sulfuric acid in which iron ions were not dissolved was added to adjust the pH of the hydrous silicic acid slurry to 3.0, and the whole reaction was completed. During this period, stirring was performed in the reaction tank so that a constant stirring force was always obtained.

After the neutralization reaction, the pH of the hydrous silicic acid was adjusted to 5.5 to 6.5 by the method according to Example 1, (Na ₂ O + K ₂ O).
+ MgO + CaO) / (Al ₂ O ₃ + Fe ₂ O ₃ + Ti
It was washed with water so that the O ₂ ) ratio was 0.4, dried, pulverized and classified to obtain hydrous silicic acid for silicone rubber.

Example 6 A sodium silicate aqueous solution having a Na ₂ O concentration of 4.2% and a SiO ₂ / Na ₂ O molar ratio of 3.2 (commercially available) was placed in a reaction tank with a baffle plate having a stirrer having an internal capacity of 240 liters. 8 liters of sodium silicate; the content of aluminum, iron and titanium is 0.05 wt% or less with respect to SiO ₂ ) and 100 liters of warm water heated to 75 ° C. were charged.

This was heated to 75 ° C., then 82 liters of an aqueous solution of sodium silicate and the iron content of TiO ₂ were added.
A neutralization reaction was carried out by simultaneously adding 9 liters of sulfuric acid having a concentration of 49% in which iron ions were dissolved so as to be 0.5 wt% with respect to SiO ₂ in hydrous silicic acid for 90 minutes.

Thereafter, sulfuric acid in which iron ions were not dissolved was added to adjust the pH of the hydrous silicic acid slurry to 3.0, and the whole reaction was completed. During this period, stirring was performed in the reaction tank so that a constant stirring force was always obtained.

After the neutralization reaction, the pH of the hydrous silicic acid was adjusted to 5.5 to 6.5 by the method according to Example 1, (Na ₂ O + K ₂ O).
+ MgO + CaO) / (Al ₂ O ₃ + Fe ₂ O ₃ + Ti
It was washed with water so that the O ₂ ) ratio was 0.4, dried, pulverized and classified to obtain hydrous silicic acid for silicone rubber.

Comparative Example 5 In Example 1, aluminum, iron,
The total content of titanium is (Al ₂ O ₃ + Fe ₂ O ₃ +
2.5 in terms of TiO ₂ ) based on SiO ₂ in hydrous silicic acid
wt%, (Na ₂ O + K ₂ O + MgO + in hydrous silicic acid
A hydrous silicic acid for silicone rubber was obtained in exactly the same manner as in Example 1 except that the ratio (CaO) / (Al ₂ O ₃ + Fe ₂ O ₃ + TiO ₂ ) was adjusted to 0.4.

In this case, addition of sodium aluminate to the aqueous solution of sodium silicate causes the sodium silicate to become cloudy. Therefore, sodium aluminate was added at the same time as sodium silicate and sulfuric acid during the neutralization reaction.

Comparative Example 6 In Example 1, aluminum, iron,
The total content of titanium is (Al ₂ O ₃ + Fe ₂ O ₃ +
Calculated as TiO ₂ ) by 0.1 with respect to SiO ₂ in hydrous silicic acid
2 wt% of (Na ₂ O + K ₂ O + MgO in hydrous silicic acid
+ CaO) / (Al ₂ O ₃ + Fe ₂ O ₃ + TiO ₂ ) ratio was adjusted to be 0.4 to obtain a hydrous silicic acid for silicone rubber in exactly the same manner as in Example 1.

[0059]

[Table 1]

[0060]

EFFECTS OF THE INVENTION According to the hydrous silicic acid suitable for the silicone rubber filler of the present invention, it is possible to stably provide a silicone rubber compound having excellent workability and processability by blending the silicic acid. Played.

Further, since the silicone rubber viscosity can be stably obtained by using the hydrous silicic acid of the present invention,
There is also an effect that the defective lot of the product can be reduced and the product can be provided at low cost.

Claims (1)

[Claims] 1. A 4% aqueous suspension having a pH of 5.0 to 7.0, and containing sodium, potassium, magnesium and calcium contained in Na ₂ O, K ₂ O, MgO and C, respectively.
Total amount when converted to aO (that is, = Na ₂
O + K ₂ O + MgO + CaO) and aluminum, iron, and titanium contained in Al ₂ O ₃ , Fe ₂ O ₃ , and TiO, respectively.
The total amount when converted to ₂ (that is, = Al ₂ O
_{3 + Fe 2 O 3 + TiO} 2) and the weight ratio of (/) in the range of 0.3 to 0.9, the range of weight ratio of SiO ₂ above (/ SiO ₂₎ is 0.002 to 0.02 A hydrated silicic acid suitable for a silicone rubber filler, characterized in that