JP2000086894A - Reinforcing filler for silicone rubber and its preparation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reinforcing filler for silicone rubbers which has good rubber-reinforcing properties and dynamic fatigue resistance, improved workability and high bulk density and which comprises hydrous silicate. SOLUTION: A hydrous silicate is obtained by spray drying a suspension obtained by hydrolytic conditioning a hydrous silicate cake obtained by the reaction of an alkali metal silicate and a mineral acid, to form a slurry having a concentration of 150 g/l or less. After spray drying, the BET specific surface area of the resulting product is 150-250 m2/g, the average particle diameter of the secondary aggregated particle (weight distribution) measured by the coulter counter method is 14 μm or less and its bulk density is over 150 g/l.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reinforcing filler for silicone rubber, and more particularly to a silicone rubber having excellent silicone rubber properties, especially excellent dynamic fatigue durability such as repeated load, and high bulk specific gravity. The present invention relates to providing hydrated silicic acid as a reinforcing filler for silicone rubber exhibiting workability, and to providing a method for producing the same.

[0002]

2. Description of the Related Art Hitherto, hydrated silica has been used as a reinforcing filler for silicone rubber. In particular, relatively inexpensive hydrated silica (which can provide excellent elasticity (click characteristics) and good compression set) is used. Wet silica) is widely used in silicone rubber for keyboard materials (rubber contacts) in calculators and personal computers. At present, silicone rubber for such applications is required to have excellent dynamic fatigue durability, and there is an increasing demand for improving the physical properties of hydrous silicic acid.

[0003] Hydrous silicic acid is obtained by filtering and washing a reaction solution containing particles obtained by a neutralization reaction between an aqueous solution of an alkali metal silicate and a mineral acid, drying and, if necessary, pulverizing and classifying. Various conditions have been conventionally obtained depending on the conditions described above. In particular, Japanese Patent Publication No. Hei 9-91463 discloses a hydrated silicate having an average particle diameter of 5 μ or less, and Japanese Patent Application Laid-Open No. 9-302230 discloses an average particle diameter of the secondary particles. 10μ or less and 16
It discloses a hydrous silicic acid in which coarse particles of μ or more account for 15% by weight or less of the entire hydrous silicic acid.

However, since such hydrous silicic acid has a low bulk specific gravity, workability is poor due to generation of dust and dust, and it is difficult to penetrate even when compounded with silicone rubber, requiring long-time kneading. However, there is a problem that the transportation cost is high.

As an improvement, the present applicant has proposed a hydrated silica in which coarse and fine powders are cut to improve workability (Japanese Patent Application No. 10-39940).

[0006]

As described above, hydrous silicic acid having a small average particle size and no coarse particles is suitable for improving the dynamic fatigue durability. However, when the average particle size is small, the bulk specific gravity decreases, so that the above-described problems of deterioration in workability, extension of kneading time, and increase in transportation cost occur, and for improvement thereof, such as dynamic fatigue durability. A hydrous silicic acid having a high bulk specific gravity while maintaining excellent properties is further required.

By the way, as a method of increasing the bulk specific gravity, a method of simply granulating by vacuum degassing, compression, or the like can be considered, but on the other hand, dispersion becomes difficult depending on the degree of granulation, and if only fatigue durability is used, Therefore, there is a possibility that the rubber reinforcing property is reduced. That is, it is not easy to provide hydrated silica having a high bulk specific gravity and improved workability while maintaining excellent rubber reinforcing properties, compression recovery properties, rubber elasticity, and fatigue durability.

Although it is difficult to realize hydrated silicic acid having excellent physical properties as described above without increasing the number of steps and decreasing the yield as compared with the conventional method, the present invention solves such a difficult problem. It was done to do so.

[0009]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, the workability when blending with silicone rubber according to the drying conditions when producing hydrous silicic acid is good. In addition, the present inventors have found a new finding that hydrous silicic acid with improved dynamic fatigue durability can be obtained. That is, as a condition for drying, the BET specific surface area is 150 to 250 m ² / g by adjusting the slurry concentration of the hydrous silicate cake to 150 g / l or less by adding water, and by combining the operation of spray drying the slurry. The average particle size of the secondary aggregated particles is 14 μm, and the bulk specific gravity is 150
They have found that hydrous silicic acid exceeding g / l can be obtained, and have completed the present invention. A typical feature of the present invention is that hydrous silicic acid as a reinforcing filler for silicone rubber having the above-mentioned excellent physical properties can be obtained without pulverizing and classifying dried hydrous silicic acid.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The reaction formula of an alkali metal silicate and a mineral acid for obtaining a hydrous silicic acid of the present invention is not particularly limited.
As long as the reaction is carried out so that the ET specific surface area falls within a target numerical range, a known reaction formula can be arbitrarily adopted. For example, as a method of precipitating hydrated silicic acid by reacting an aqueous solution of alkali silicate (commercially available sodium silicate) with a mineral acid, a predetermined amount of a sodium silicate solution having a predetermined concentration is poured into a reaction tank, and mineral acid is added under predetermined conditions. There is no particular restriction on the method of adding one side or the simultaneous addition of sodium silicate and mineral acid for a certain period of time while controlling pH and temperature in a reaction tank previously filled with a certain amount of warm water. Can be adopted.

The hydrated silica provided by the present invention is BE
The T specific surface area is preferably in the range of 150 to 250 m ² / g, more preferably 160 to 230 m ² / g. When the BET specific surface area is lower than this range, the reinforcing properties of the silicone rubber are reduced and the transparency is low. Decreases. Conversely, if it is higher than this range, the cohesiveness of the primary particles increases,
It is not preferable because poor dispersion tends to cause a decrease in reinforcement and fatigue durability. Therefore, the hydrous silicic acid of the present invention needs to be within the range of the BET specific surface area.

In order to produce the hydrous silicic acid of the present invention, the end solution of the neutralization reaction obtained by the above method is generally filtered and washed with a filter press to remove impurities.

The hydrated silicate cake thus obtained is generally about 160 to 250 g / l, which is usually dried by a static drying method, pulverized and classified to obtain a predetermined particle size. It is regarded as hydrous silicic acid having a particle size distribution.

On the other hand, in the present invention, water is added to the hydrated silicate cake obtained above to re-suspend it so that the silica concentration becomes 150 g / l or less, followed by spray drying (flash drying). It is extremely remarkable that the hydrous silicic acid intended for the present invention can be obtained only by performing the re-suspension and spray drying.

That is, in general, when a hydrated silicate cake is obtained by drying a hydrated silicate cake, it is generally considered that it is advantageous to dry a hydrated silicate cake having a high concentration from the viewpoint of energy cost. Therefore, performing the above-mentioned water addition and re-suspension of the present invention is a disadvantageous operation according to common sense, but if the method of re-suspending and re-suspending in this way is not followed by a spray-drying method, the present invention The hydrous silicic acid having the desired properties cannot be obtained. Silicone rubber using the hydrous silicic acid obtained according to the present invention as a reinforcing filler has the property of being excellent in fatigue durability, which indicates that the hydrous silicic acid of the spray-dried product obtained according to the present invention has a weak particle aggregation. This is presumed to be due to the fact that it is easily dispersed.

The water and re-suspension performed for the above-mentioned spray-drying are, in view of the dispersibility in a rubber compound when hydrous silicic acid is used as a reinforcing filler, those having a lower silica concentration are more agglomerated. Although it is preferable because the particles are small and excellent in dispersibility, the lower (thinner) the concentration of the slurry to be spray-dried is, the higher the energy cost of drying is, so there is an industrial limit, and generally 50 g / l to 50 g / l. Fifteen
0 g / l, preferably 70 g / l to 100 g / l.

The re-suspension by adding water to the above-mentioned hydrated silicate cake is not particularly limited, and a known agitator,
The dispersing power is not particularly limited as long as it can be performed using a mixer or the like and can be sufficiently suspended so that there is no clumping.However, it is sometimes preferable to pass through a strainer to remove large defective dispersion. Many.

Next, this silica suspension is spray-dried. Immediately after drying, the secondary agglomerated particles of hydrous silica have an average particle size of 1%.
It is preferably 4 μm or less, more preferably 5 to 13 μm. The average particle size of the secondary aggregated particles is 14μ
If it exceeds m, it becomes difficult to uniformly disperse the hydrated silica in the silicone rubber, and the reinforcing effect of the silicone rubber or the dynamic fatigue durability cannot be sufficiently obtained. On the other hand, if the average particle size is too small, the bulk specific gravity is small even if the average particle size is not greatly affected, and there is a problem in workability because dusting occurs.

The average particle size of the secondary agglomerated particles of hydrous silicic acid obtained after the above-mentioned spray drying can be adjusted to the above range by controlling the size of droplets at the time of spraying. That is, if the type of spray drying is, for example, a nozzle type, it can be obtained by using a nozzle having a small nozzle diameter,
Similarly, if the type is a disk type, it can be obtained by controlling the number of rotations. In the present invention, it is not necessary to grind and classify the hydrated silica dried as described above.
Hydrous silicic acid having a bulk specific gravity exceeding 50 g / l can be easily obtained. Practically, 150 to 200 g / l is preferable. If it is 200 g / l or more, the dispersibility tends to be poor. The hydrous silicate obtained by drying under the above conditions,
Since the silicone rubber is easily dispersed at the time of kneading, it is not necessary to pulverize and pulverize the silicone rubber. As a result, hydrated silica having a high bulk specific gravity can be obtained. In addition, it is also possible to classify lightly as needed.

The hydrated silica of the present invention thus obtained is not particularly limited in other characteristic values and the like. For example, the adhering moisture is usually 2 to 9%, and the pH value is 4.0 to 4.0.
9.0, more preferably 5.5 to 7.0.
Further, since residual salts and the like have an adverse effect on the electrical properties of the silicone rubber, it is preferable that the residual salts be as small as possible, and the electrical conductivity of the 4% aqueous suspension is desirably 150 μS / cm or less. According to the method of the present invention, pulverization and classification are not required, so that the process can be labor-saving.

[0021]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. In addition, the measurement of each physical property value etc. was implemented by the method shown below.

(1) Measurement of BET specific surface area The BET specific surface area was measured in accordance with JISZ-8830 (method for measuring the specific surface area of powder by gas adsorption), and a fully automatic powder specific surface area measuring device AMS-8000 (Okura Riken Co., Ltd.) Was measured by the one-point method. About 0.05 g of the sample was measured after pretreatment at 200 ° C. for 30 minutes.

(2) Measurement of Bulk Specific Gravity (Apparent Density) The bulk specific gravity (apparent density) was measured by a bulk test method specified in JIS K5101 (pigment test method) 18. In other words, level the cask measuring instrument, transfer about 10 g of the sample onto a sieve with an opening of 0.5 mm, gently and lightly rub the entire surface of the sieve with paint brush, drop the sample, drop it into a 30 ml stainless steel cylinder, , Repeat this operation until the peak, then
After a mountain part was dropped with a spatula having a straight side, the sample in the cylinder was weighed and calculated.

(3) Measurement of Average Particle Size of Secondary Aggregated Particles Using a Coulter Multisizer II (manufactured by Coulter Electronics Limited), which is commonly used in the art for measuring the particle size of secondary aggregated particles of hydrous silica, 50 ml was used. 20 ml of Isoton-II solution (0.85% NaCl aqueous solution) attached to the apparatus as a dispersion medium was placed in the beaker, and a small amount of a sample was charged therein, and ultrasonic (20 W) dispersion was performed for 40 seconds. The dispersed sample was measured using a 70 μ aperture tube of the same apparatus.

(4) Measurement of Dynamic Fatigue Durability 100 parts of silicone raw rubber, 40 parts of hydrous silicic acid, and 2 parts of wetter (dimethylethoxysilane) were kneaded for 10 minutes using a 6-inch roll, and the resulting compound was subjected to kneading. Heat treatment was performed at 200 ° C. for 5 hours in a gear oven. After kneading the compound, 0.5% of a crosslinking agent (2,5-dimethyl-2,5di (t-butylperoxy) hexane) was added.
After adding and mixing again using a 6-inch roll, press vulcanization was performed at a molding temperature of 165 ° C. for 10 minutes, and then
The sheet was subjected to secondary vulcanization at 00 ° C. for 4 hours to prepare a sheet having a thickness of 2 mm, which was cut into JIS No. 3 dumbbell pieces. This dumbbell piece was subjected to repeated elongation of 0 to 100% using a dematcher bending fatigue tester (manufactured by Toyo Seiki Co., Ltd.), and the average value of the number of times until breakage by five test pieces was determined. Was defined as dynamic fatigue durability. The measurement results were obtained as relative values, with the result obtained using a commercially available hydrous silicic acid (Nip Seal LP: manufactured by Nippon Silica Industry Co., Ltd.) as 100.

(5) Measurement of physical properties of rubber (hardness: JIS-
A, Tensile strength) The sheet used for the dynamic fatigue durability was JIS K-630.
The test was carried out in accordance with the method for testing physical properties of crosslinked rubber of No. 1.

(6) Evaluation of workability From the above values of bulk specific gravity, those with good workability were evaluated as “○”, those with intermediate workability as “△”, and those with poor workability as “X”.

Example 1 A 240 liter (l) stainless steel reaction vessel was charged with 80 liter (l) of water and sodium silicate (SiO ₂ / Na ₂ O molar ratio 3.3) so as to have a pH of 10 ± 0.5. ,
After the temperature was raised to 80 ° C, 49% sulfuric acid was added at 4 liter (l) / h.
r, At the same time, the same sodium silicate as described above was added for 120 minutes at a temperature in the range of 80 to 82 ° C. so as to maintain the pH at 10 ± 0.5, and then only the same sulfuric acid as above was added until pH 3 was reached. A precipitate was obtained.

Thereafter, the obtained reaction precipitate is filtered, and p
It was washed with water until H 5.5-7.0 and the electric conductivity became 150 μS / cm or less to obtain a hydrous silicate cake (silica concentration 180 g / l). The BET specific surface area was 205 m ² / g.

Water was added to the cake until the silica concentration became 150 g / l, emulsified with a reciprocating stirrer for 30 minutes, and the suspension was disc-type spray-dried (model: LT-8 type spray dryer, Okawara Kakoki Co., Ltd.). (Manufactured by K.K.) and dried at an atomizer rotation speed of 28,000 rpm to obtain a sample. The following table shows the measurement results of the direct physical properties ((1) to (3) and (6) above) of this sample and the reinforcing properties when blended with silicone rubber ((4) and (5) above). 1 (the same applies to the following Examples and Comparative Examples).

Example 2 The suspension of Example 1 was used by rotating the atomizer at 36,000 rpm.
The sample was dried at pm.

Example 3 Water was added to the suspension of Example 1 to reduce the silica concentration to 75 g /
After that, the sample was dried at an atomizer rotation number of 36000 rpm to obtain a sample.

Example 4 The hydrous silicate cake obtained in Example 1 was subjected to a silica concentration of 94 g.
/ L, and emulsified with a reciprocating stirrer for about 3 hours, and dried at an atomizer rotation speed of 36000 rpm to obtain a sample.

Comparative Example 1 The hydrous silicate cake obtained in Example 1 was dried by standing and ground and classified to obtain a sample. Comparative Example 2 The dried product obtained in Example 1 was pulverized and classified so that the average particle size of the secondary aggregated particles was 4 μm or less, to obtain a sample.

Comparative Example 3 The suspension obtained in Example 1 was dried by setting the number of revolutions of the atomizer to low (16,000 rpm) so that the average particle size of the secondary aggregated particles was large, and dried. And

Comparative Example 4 Commercially available hydrous silicic acid (Nip Seal LP (BET specific surface area 2
10 m ² / g): Nippon Silica Industry Co., Ltd.) was used as a sample.

Comparative Example 5 A commercially available hydrous silicic acid (nip seal LP) similar to the above was pulverized and classified to obtain a sample.

[0038]

[Table 1]

As can be seen from Table 1, each of the examples has a high bulk specific gravity, so that it is excellent in workability, and also has good reinforcing properties such as tensile strength and dynamic fatigue durability. confirmed. On the other hand, those of Comparative Examples 1, 2, 4, and 5 were inferior in workability due to low bulk specific gravity, and those of Comparative Example 3 were high in bulk specific gravity and excellent in workability, but were large in average particle size. In particular, the dynamic fatigue durability was insufficient.

[0040]

The reinforcing filler for silicone rubber according to the present invention maintains excellent rubber reinforcing properties and dynamic fatigue durability by controlling the spray-drying conditions without the need for pulverization and classification. However, there is an effect that hydrated silica having a high bulk specific gravity and improved workability can be obtained.

Claims (4)

[Claims] 1. A hydrated silicate cake obtained by a reaction between an alkali metal silicate and a mineral acid is adjusted to a slurry concentration of 150 g / l or less, and the suspension is spray-dried hydrated silicate. Physical properties after drying, BET specific surface area is 1
Reinforcement filling for silicone rubber characterized in that the secondary aggregated particles have a mean particle size (weight distribution) of not more than 14 μm and a bulk specific gravity exceeding 150 g / l by a Coulter counter method, which is 50 to 250 m ² / g. Agent. 2. The reinforcing filler according to claim 1, wherein the average particle size (weight distribution) of the secondary aggregated particles is 5 to 13 μm. 3. The bulk specific gravity after spray drying is from 150 g / l to 2 g / l.
The reinforcing filler according to claim 1, wherein the amount is 00 g / l. 4. A step of obtaining a hydrous silicate cake by a reaction between an alkali metal silicate and a mineral acid and subsequent dehydration, and adding a slurry to the obtained hydrous silicate cake by adding a slurry concentration of 50 to 1
A step of preparing a 50 g / l suspension and a step of spray-drying the slurry adjusted to the above-described concentration, wherein the average particle diameter (weight distribution) of secondary aggregated particles of the hydrous silicic acid after spray drying is measured by a Coulter counter method. ) Is controlled to be 14 μm or less, the BET specific surface area is 150 to 250 m ² / g, and the bulk specific gravity exceeds 150 g / l without pulverizing treatment. 3. The method for producing hydrated silicic acid as a reinforcing filler for silicone rubber according to any one of 3.