Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-11-15
2002-05-14
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S096000, C521S154000
Reexamination Certificate
active
06387971
ABSTRACT:
This invention relates to silicone rubber sponge compositions which cure into silicone rubber sponges suitable for use as building gaskets, sponge rolls, thermal insulating sheets and the like.
BACKGROUND OF THE INVENTION
Silicone sponges possess physical properties characteristic of silicone rubber, namely excellent properties including heat resistance, freeze resistance, electrical insulation, flame retardance and compression set. The silicone rubber sponge is generally formed by adding a curing agent and a blowing agent to a thermosetting silicone rubber compound and heating the composition for blowing and curing into sponge. It is important that the composition be effectively blown into a silicone rubber sponge having a uniform fine cell structure and a smooth, tack-free skin layer, without detracting from the physical properties characteristic of silicone rubber.
For prior art silicone rubber sponges, azobisisobutyronitrile (AIBN) is a typical blowing agent. AIBN, however, requires use of a sophisticated vulcanizing apparatus and a duration of post-cure therein since decomposed products of AIBN contain organic cyan compounds.
It was then proposed to use organic blowing agents such as azodicarbonamide (ADCA), p,p′-oxybis(benzenesulfonyl hydrazide) (OBSH) and dinitropentamethylenetetramine (DPT). These blowing agents have a higher decomposition temperature than AIBN and induce little blowing in the atmospheric hot air vulcanization (HAV) process which is a common molding technique of silicone rubber sponges. Even if sponges are obtained, they have large uneven cells.
JP-A 8-259816 discloses an organic azo blowing agent which decomposes into less toxic products. This proposal, however, is not fully satisfactory with respect to the reduction of toxicity and the uniformity of cells.
U.S. Pat. No. 6,096,792 discloses an organic blowing agent, 1,1′-azobis(1-acetoxy-1-phenylethane), which gives off malodor inherent to acid compounds when decomposed.
SUMMARY OF THE INVENTION
It would be desirable to have a blowing agent which is decomposable at an appropriate temperature to mold silicone rubber sponge and does not give off malodor. Among cyano group-free organic azo compounds, a focus is placed on a specific azo compound free of nitrile group as a blowing agent which is effective for foaming a silicone rubber composition and reducing a post-cure time.
Therefore, an object of the invention is to provide a silicone rubber sponge composition comprising a specific azo compound which allows the composition to be effectively blown and cured, without a need for long-term post-cure, into a sponge having a uniform fine cell structure and a smooth skin layer.
We have found that a silicone rubber sponge composition comprising (A) an organopolysiloxane of formula (1), (B) finely divided silica having a specific surface area of at least 50 m
2
/g, and (C) an organic azo compound of formula (I), preferably dimethyl 1,1′-azobis(1-cyclohexane carboxylate), as essential components, can be blown, without a need for the blowing agent of azobisisobutyronitrile, into a silicone rubber sponge having a uniform fine cell structure and a smooth, tack-free skin layer.
According to the invention, there is provided a silicone rubber sponge composition comprising as essential components,
(A) 100 parts by weight of an organopolysiloxane of the following average compositional formula (1):
R
1
a
SiO
(4-a)/2
(1)
wherein R
1
is independently a substituted or unsubstituted monovalent hydrocarbon group and “a” is a positive number of 1.95 to 2.04,
(B) 1 to 100 parts by weight of finely divided silica having a specific surface area of at least 50 m
2
/g, and
(C) 0.01 to 50 parts by weight of an organic azo compound of the following formula (I):
wherein R
2
and R
3
are independently alkylene groups, R
4
and R
5
are independently hydrogen or substituted or unsubstituted monovalent hydrocarbon groups.
In a preferred embodiment, the composition further includes a curing agent which is an organic peroxide, more preferably a mixture of a diacyl organic peroxide and another organic peroxide, or an addition reaction curing agent, or a combination of an organic peroxide with an addition reaction curing agent.
Also contemplated herein is a silicone rubber sponge obtained by blowing and curing the silicone rubber sponge composition.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Component (A) of the silicone rubber sponge composition according to the invention is an organopolysiloxane of the average compositional formula (1).
R
1
a
SiO
(4-a)/2
(1)
In formula (1), R
1
which may be the same or different is a substituted or unsubstituted monovalent hydrocarbon group, preferably of 1 to 12 carbon atoms, more preferably 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl, butyl, hexyl and dodecyl, cycloalkyl groups such as cyclohexyl, alkenyl groups such as vinyl, allyl, butenyl and hexenyl, aryl groups such as phenyl and tolyl, aralkyl groups such as benzyl and &bgr;-phenylpropyl, and substituted ones of the foregoing groups in which some or all of the hydrogen atoms attached to carbon atoms are replaced by halogen atoms, cyano groups or the like, such as chloromethyl, trifluoropropyl and cyanoethyl. Of these, methyl, phenyl, vinyl and trifluoropropyl are preferred. It is more preferred that methyl be included in an amount of at least 50 molt, especially at least 80 molt. The letter “a” is a positive number of 1.95 to 2.04, preferably 1.98 to 2.02.
The organopolysiloxanes may be ones whose molecular chain is end blocked with trimethylsilyl, dimethylvinylsilyl, dimethylhydroxysilyl, methyldivinylsilyl, trivinylsilyl or similar groups, although it is preferred to have at least one alkenyl group at the end. The organopolysiloxanes should preferably have at least two alkenyl groups attached to silicon atoms in a molecule. The content of alkenyl groups is preferably 0.001 to 5 molt, especially 0.01 to 0.5 mol % based on the R
1
groups. Of the alkenyl groups in R
1
, vinyl is most preferred.
The organopolysiloxanes can be prepared, for example, by (co)hydrolytic condensation of one or more organohalogenosilanes, or ring-opening polymerization of a cyclic polysiloxane (trimer or tetramer of siloxane) in the presence of a basic or acidic catalyst. They are basically linear diorganopolysiloxanes which may be partially branched. Also a mixture of two or more organopolysiloxanes having different molecular structures or degrees of polymerization is useful. The organopolysiloxane should preferably have a viscosity at 25° C. of at least about 100 centistokes (cs), more preferably about 100,000 to 100,000,000 cs, and especially about 5,000,000 to 20,000,000 cs. The degree of polymerization is preferably at least about 100, more preferably at least about 3,000 and up to about 100,000, more preferably up to about 20,000.
Finely divided silica (B) is essential for producing silicone rubber sponge having mechanical strength. To this end, finely divided silica should have a BET specific surface area of at least 50 m
2
/g, preferably 100 to 400 m
2
/g. Examples of the finely divided silica include fumed silica (or dry silica) and precipitated silica (or wet silica), with the fumed silica (or dry silica) being preferred. Prior to use, the silica may be hydrophobized by surface treatment with organopolysiloxanes, organopolysilazanes, chlorosilanes, alkoxysilanes or the like. Such silicas may be used alone or in admixture of two or more. A mixture of fumed silica and precipitated silica is also useful.
The amount of finely divided silica added is 1 to 100 parts, preferably 5 to 100 parts, more preferably 10 to 90 parts, and even more preferably 30 to 80 parts by weight, per 100 parts by weight of the organopolysiloxane (A). Less than 1 part of silica is too small to achieve reinforcement whereas more than 100 parts of silica renders the composition less workable and can compromise the physical properties of silicone rubber.
The organic azo compound (C) is a component re
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