JPH08259816A - Expandable silicone rubber composition - Google Patents

Abstract

PURPOSE: To obtain an expandable silicon rubber composition, consisting essentially of an organopolysiloxane, a reinforcing silica, a specific azo-based foaming agent and a curing agent, having low toxicity of the decomposed foaming agent and high safety and capable of providing a silicone rubber sponge having fine bubbles (cells). CONSTITUTION: This composition consists essentially of (A) 100 pts. wt. organopolysiloxane of the average compositional formula Rm SiO(4-n)/2 [R is a (substituted) monovalent hydrocarbon; (n) is 1.98-2.02], (B) 10-100 pts.wt., preferably 30-80 pts.wt. reinforcing silica having >=50m<2> /g, preferably 100-400m<2> /g specific surface area, (C) 0.5-30 pts.wt., preferably 1-20 pts.wt. azo-based foaming agent of the formula [R<1> to R<6> are each an alkyl, a nitrile or NH2 -CO] (except azobisisobutyronitrile) [preferably 2,2'-azobis(2-methylbutyronitrile] and (D) a curing agent (e.g. a combination of an organohydrogenpolysiloxane with a platinum group metallic catalyst) in an amount effective in curing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a foamable silicone rubber composition which gives a silicone rubber sponge having fine cells (cells) with low toxicity and high safety of decomposed products of a foaming agent.

[0002]

2. Description of the Related Art Generally, it is known that silicone rubber sponges can be roughly classified into the following two methods. That is, one is a method of using a low boiling point substance by-produced by a condensation reaction of siloxanes represented by the following formula, and the other is to add a curing agent and a foaming agent to a silicone rubber composition. Is the way. ≡SiOH + HSi≡ → ≡SiOSi≡ + H ₂ ↑ In the former method, when the silicone rubber composition has few functional groups involved in the condensation reaction of siloxanes, the expansion ratio is remarkably reduced, resulting in a sponge-like silicone. May not give rubber. This is often seen when using a high degree of polymerization siloxane (silicone raw rubber). Further, since the gas generated by the chemical reaction is used for foaming, it is difficult to control the reaction rate, the size of the cell, etc., and the silicone rubber sponge obtained by this method has extremely weak strength and its usable applications are limited.
Further, in the latter method, azobisisobutyronitrile (abbreviated as AIBN) is usually used as a foaming agent (Japanese Patent Publication No. 44-461). The silicone rubber sponge obtained by this method has a relatively dense and uniform cell, but when AIBN is used, the decomposition product tetramethylsuccinonitrile has a high toxicity of LD ₅₀ = 39, and this treatment method is complicated. The working conditions are also limited. Therefore, research on the production of silicone rubber sponges not using AIBN has been conducted for a long time (Japanese Patent Publication Nos. 47-14592 and 53-16025).
No. 5-59207).

However, in all of these proposals, the working time and the curing conditions are restricted by using a step cure or curing by a hydrosilylation reaction.
It was not a sponge with dense and uniform cells, and there were many practical problems.

The present invention has been made in order to solve the above-mentioned disadvantages, and it is highly safe when cured and foamed without using AIBN, and a silicone rubber sponge having easily dense and uniform cells. It aims at providing the silicone rubber composition used as this.

[0005]

Means and Actions for Solving the Problems As a result of intensive studies for achieving the above-mentioned object, the present inventor has found that (A) the following average composition formula (1) R _n SiO _{(4-n) / 2} ( 1) 100 parts by weight of an organopolysiloxane represented by the formula (wherein R is the same or different, unsubstituted or substituted monovalent hydrocarbon group, and n is a positive number of 1.98 to 2.02), ) Reinforcing silica having a specific surface area of 50 m ² / g or more 10-
A foamable silicone rubber composition containing 100 parts by weight of (C) a foaming agent (D) a curing agent is represented by the following general formula (2) as the foaming agent, provided that azobisisobutyronitrile (AIBN) is used. It was found that when the azo type foaming agents other than the above are used, a harmful silicone by-product as in the case of using AIBN is not produced, and a silicone rubber sponge having a uniform and fine texture and good texture can be obtained. The invention was completed.

[0006]

Embedded image

The present invention will be described in more detail below. The expandable silicone rubber composition of the present invention comprises (A) an organopolysiloxane, (B) a reinforcing silica, and (C) an azo foam of the above formula (2). Agents (excluding AIBN),
(D) The curing agent is an essential component.

Here, the organopolysiloxane of the component (A) has the following average compositional formula (1) R _n SiO _{(4-n) / 2} (1) (wherein R is the same or different and is not substituted or substituted). A monovalent hydrocarbon group, n is a positive number of 1.98 to 2.02.).

In this case, R is an alkyl group such as a methyl group, an ethyl group, a propyl group or a butyl group, a cycloalkyl group such as a cyclohexyl group, an alkenyl group such as a vinyl group, an allyl group, a butenyl group or a hexenyl group, A phenyl group, an aryl group such as a tolyl group, or a chloromethyl group in which a part or all of hydrogen atoms bonded to carbon atoms of these groups are substituted with a halogen atom, a cyano group, or the like,
The same or different, preferably selected from a trifluoropropyl group, a cyanoethyl group, etc., preferably having 1 to 10 carbon atoms,
More preferably, it is an unsubstituted or substituted monovalent hydrocarbon having 1 to 8 carbon atoms, and n is a positive number of 1.98 to 2.02. This organopolysiloxane has a trimethylsilyl group at the molecular chain terminal, It may be blocked with a dimethylvinyl group, a dimethylhydroxysilyl group, a trivinylsilyl group, or the like.

The organopolysiloxane of this type is obtained by cohydrolytically condensing one or more selected organohalogenosilanes, or a cyclic polysiloxane (such as a trimer or tetramer of siloxane). Can be obtained by ring-opening polymerization using an alkaline or acidic catalyst, which is basically a linear diorganopolysiloxane,
It may be a mixture of two or more different molecular structures. The viscosity of this organopolysiloxane is
It is preferable that the viscosity at 25 ° C. is 1,000 cs or more. More preferably 100,000 to 10,000,
It is 000 cs. The degree of polymerization is preferably 300 or more,
It is more preferably 2,000 to 100,000, and even more preferably 5,000 to 10,000.

Next, the reinforcing silica as the component (B) is indispensable for obtaining a silicone rubber having excellent mechanical strength. For this purpose, the specific surface area is 50.
m ² / g or more, preferably it should be those of the 100 to 400 m ² / g. Typical examples of the reinforcing silica include fumed silica, precipitated silica, and silica whose surface is hydrophobized. If the amount of the silica powder added is less than 10 parts by weight with respect to 100 parts by weight of the organopolysiloxane (A), a sufficient reinforcing effect cannot be obtained, and if it is more than 100 parts by weight, workability is improved. Since it deteriorates and the physical strength of the obtained silicone rubber decreases, it is 10 to 100 parts by weight, preferably 30 to 80 parts by weight.

In the present invention, an azo type foaming agent represented by the following general formula (2) is used as the foaming agent of the component (C) (provided that in the formula (2), R ¹ and R ⁴ are nitriles, respectively). Azobisisobutyronitrile, which is a group and R ² , R ³ , R ⁵ , and R ⁶ are each a methyl group).

[0013]

Embedded image

The unsubstituted alkyl group represented by R ^{1 to} R ⁶ preferably has 1 to 6 carbon atoms.

As such an azo type foaming agent, 2,
2'-azobis (2-methylbutyronitrile), 2,
2'-azobis (2,4-dimethylvaleronitrile),
Dimethyl-2,2-azobisisobutyrate, 2,2 '
-Azobis (2,4,4-trimethylpentane), 2,
2'-azobis (2-methylpropionamide) dihydrate, 4,4'-azobis (4-cyanovaleric acid), 1,1'-azobis (cyclohexane-1-carbonitrile), dimethyl-2,2'- Azobis (2-methyl propionate) etc. are mentioned.

Of these, preferred are 2,2'-azobis (2-methylbutyronitrile), 2,2'-azobis (2,4-dimethylvaleronitrile) and dimethyl-.
2,2-azobisisobutyrate, 2,2'-azobis (2,4,4-trimethylpentane), 2,2'-azobis (2-methylpropionamide) dihydrate,
It is 4,4'-azobis (4-cyanovaleric acid). Further most preferred is 2,2'-azobis (2-methylbutyronitrile). The component (C) is added in an amount of 0.5 to 30 parts by weight based on 100 parts by weight of the component (A). If the amount is less than 0.5 part by weight, sufficient foaming is not performed and thus it becomes difficult to form a sponge. If the amount exceeds 30 parts by weight, the foaming is sufficient, but the silicone sponge obtained is cracked or deformed because the expansion ratio is too high. Easily occurs and the quality of the sponge deteriorates, so 0.5-
It should be in the range of 30 parts by weight. More preferably, it is 1 to 20 parts by weight.

The method of curing the foamable silicone rubber composition of the present invention to form a sponge is either a conventionally known method utilizing a hydrosilylation reaction or a method of vulcanizing an organic peroxide as a catalyst. Can be cured. Therefore, the curing agent of the component (D) is a combination of an organohydrogen polysiloxane and a platinum group metal-based catalyst when curing using a hydrosilylation reaction,
Or it becomes an organic peroxide.

When a sponge is formed by utilizing a hydrosilylation reaction, the organopolysiloxane of component (A) must have at least two alkenyl groups bonded to silicon atoms. Is two or more Si in one molecule
Any organopolysiloxane having an H group may be used and may be linear, cyclic or branched. Also,
Such SiH group may be at the end of the polysiloxane chain or may be in the middle thereof. Such an organohydrogenpolysiloxane has 0.5 to 3 SiH groups per 1 mol of alkenyl groups of the organopolysiloxane (A).
It is used in an amount ratio of 1 mol to 2 mol. The platinum group metal-based catalyst used at the same time acts as a catalyst for the hydrosilylation reaction between the alkenyl group in the organopolysiloxane (A) and the SiH group in the organohydrogenpolysiloxane (A). It is used in the range of 0.1 to 1,000 ppm, preferably 1 to 100 ppm (as platinum metal), based on the total amount of the organopolysiloxane and the organohydrogenpolysiloxane. As such a platinum group metal-based catalyst, for example, a chloroplatinic acid catalyst, a platinum-hydrocarbon complex compound, a chloroplatinic acid-olfine complex compound, a platinum-vinylsiloxane complex or the like can be used. Further, when curing by hydrosilylation reaction, in order to maintain good storage stability at room temperature and to maintain a suitable pot life,
A reaction control agent such as methylvinylcyclotetrasiloxane and acetylene alcohols can also be added.

When an organic peroxide is used as a catalyst, usually 0.01 to 10 parts by weight, particularly 0.05 to 5 parts by weight, is added to 100 parts by weight of the organopolysiloxane as the component (A), and the mixture is heated. By doing so, a cured sponge can be obtained.

The organic peroxide can be used without particular limitation as long as it is one usually used for curing an organic peroxide-curable silicone rubber composition. For example, benzoyl peroxide, bis ( 2,4-dichlorobenzoyl) peroxide, bis (o-chlorobenzoyl) peroxide, di-t-butylperoxide, 2,5-dimethyl-di-t-butylperoxyhexane, t-butylperbenzoate, t -Butyl peroxyisopropyl carbonate, dicumyl peroxide, etc. may be mentioned.

The organic peroxide curing type is preferable to the hydrosilylation type because it can make the cells of the sponge uniform and dense. In particular, in order to make the cells of the silicone sponge fine and stabilize the expansion ratio, it is preferable to use a combination of a low temperature decomposition type organic peroxide catalyst and a high temperature decomposition type organic peroxide catalyst. The low temperature decomposition type organic peroxide catalyst can thicken the silicone rubber before the foaming agent is decomposed at the initial stage of heat curing of the silicone rubber and can efficiently take in the gas generated during decomposition of the foaming agent into the silicone sponge, resulting in a dense cell. become. Further, since the surface of the silicone sponge is vulcanized well, the surface adhesion of the molded sponge is reduced. On the other hand, the high temperature decomposable organic peroxide catalyst works to complete the vulcanization and curing of the sponge after decomposing the blowing agent, improving the dimensional stability of the sponge during molding,
Improves mechanical properties such as compression set and tensile strength of sponge after molding.

Among the specific examples of the organic peroxides mentioned above, the low temperature decomposition type organic peroxide catalysts are bis (2,4-dichlorobenzoyl) peroxide, bis (o-chlorobenzoyl) peroxide and bis ( p-methylbenzoyl) peroxide and bis (o-methylbenzoyl) peroxide, and the high temperature decomposition type organic peroxide catalyst is di-t-butyl peroxide, 2,5-dimethyl-di-t-butylperoxyhexane. , T-butyl perbenzoate, t
-Butyl peroxyisopropyl carbonate, dicumyl peroxide.

That is, the low temperature decomposition type organic peroxide catalyst is an organic peroxide having a half-life of 1 hour or less at a temperature of 100 ° C., and the high temperature decomposition type organic peroxide catalyst is a temperature of 100 ° C.
It is an organic peroxide having a half-life of 0 hours or more, and it is desirable to mold with a combination of these two types of organic peroxides.

Among them, when bis (o-chlorobenzoyl) peroxide is used as the low temperature decomposition type organic peroxide catalyst, a molded sponge having a uniform cell size is preferable.

The weight ratio of the low temperature decomposition type organic peroxide catalyst to the high temperature decomposition type organic peroxide catalyst is 1:10 to 5: 1, more preferably 1: 5 to 2: 1. Preferably there is. Further, in this case, the low temperature decomposition type organic peroxide catalyst is 0.01 to 2 with respect to 100 parts by weight of the component (A).
Parts by weight, the high temperature decomposition type organic peroxide catalyst is (A) component 10
The amount is preferably 0.1 to 8 parts by weight with respect to 0 parts by weight.

The silicone rubber composition of the present invention is prepared by uniformly mixing the above-mentioned components (A) to (D) using a rubber kneader such as a two-roll, Banbury mixer, dough mixer (kneader), It can be obtained by subjecting it to a heat treatment as necessary, but a filler such as crushed quartz as an extender or calcium carbonate may be added to this if necessary. Further, if necessary, various additives such as colorants, heat resistance improvers, flame retardancy improvers, etc., and diphenylsilane diol, various alkoxy silanes, carbon functional silanes as release agents or dispersants for fillers. It is optional to add a silanol group-containing low-molecular-weight siloxane or the like.

The foamable silicone rubber composition thus prepared can be easily foamed and cured to obtain a silicone rubber sponge. The curing and foaming method may be any method as long as it can apply sufficient heat to the decomposition of the foaming agent and the vulcanization of the silicone rubber, and the molding method is also particularly limited, such as continuous vulcanization by extrusion molding, pressing, and mold molding by injection. It is not something that will be done.

[0028]

EFFECT OF THE INVENTION The expandable silicone rubber composition of the present invention has low toxicity and high safety of decomposed products of the foaming agent, and
A silicone rubber sponge having uniform cells can be obtained.

[0029]

EXAMPLES The present invention will be described in more detail below by showing Examples and Comparative Examples, but the present invention is not limited to the following Examples.

[Examples 1 to 7, Comparative Examples 1 and 2] Dimethyl siloxane unit 99.825 mol%, methyl vinyl siloxane unit 0.15 mol%, dimethyl vinyl siloxane unit 0.025 mol% and average degree of polymerization Is about 8.0
100 parts by weight of organopolysiloxane of 00, 45 parts by weight of fumed silica [manufactured by Nippon Aerosil Co., Ltd.] having a specific surface area of 200 m ² / g, having silanol groups at both ends as a dispersant, and having an average degree of polymerization of 10 parts by weight of dimethylpolysiloxane having a viscosity of 15 cs at 13 and 25 ° C. was kneaded with a kneader and heat-treated at 160 ° C. for 2 hours to prepare a base compound.

To this base compound, the foaming agent and the curing agent listed in Tables 1 to 3 were added and mixed with a two-roll mill to give 250
The hardness and foaming ratio of the product cured for 10 minutes with a dryer at ℃ and the cell size were observed with a microscope. The results are shown in Table 1.
~ 3.

In the table, the platinum catalyst and the cross-linking agent are
The following is shown. The addition amount is parts by weight based on 100 parts by weight of the base compound. Platinum catalyst: platinum catalyst with octyl alcohol modified solution of chloroplatinic acid in an amount of 1% by weight of platinum Crosslinking agent: 17 methylhydrogen siloxane units with both ends of the molecular chain having a viscosity of 12 cs blocked by dimethylhydrogensiloxy groups. Methylhydrogen polysiloxane containing mol%

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

Front page continuation (72) Inventor Masaharu Takahashi 1 Hitomi, Osamu Matsuida-cho, Usui-gun, Gunma Shin-Etsu Chemical Co., Ltd. Silicone Electronic Materials Research Laboratory

Claims (3)

[Claims] 1. (A) The following average composition formula (1) R _n SiO _{(4-n) / 2} (1) (wherein, R is the same or different unsubstituted or substituted monovalent hydrocarbon group, and n is 100 parts by weight of organopolysiloxane represented by 1.98 to 2.02), (B) 10 to 100 parts by weight of reinforcing silica having a specific surface area of 50 m ² / g or more, and (C) the following general formula. (2) [Chemical formula 1] 0.5 to 30 parts by weight of an azo type foaming agent (excluding azobisisobutyronitrile) represented by the formula (D) and a curing effective amount of the curing agent (D) as essential components. Rubber composition. 2. The foaming agent of component (C) is 2,2′-azobis (2-methylbutyronitrile), 2,2′-azobis (2,4-dimethylvaleronitrile), dimethyl-
2,2-Azobisisobutyrate, 2,2'-azobis (2,4,4-trimethylpentane), 2,2'-azobis (2-methylpropionamide) dihydrate or 4,4'-azobis (4 -Cyanovaleric acid), The expandable silicone rubber composition according to claim 1. 3. The curing agent as component (D) is 1 at 100 ° C.
The foamable silicone rubber composition according to claim 1 or 2, which is a combination of an organic peroxide having a half-life of not more than 10 hours and an organic peroxide having a half-life of not less than 10 hours at a temperature of 100 ° C.