Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-06-29
2003-04-22
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...
C521S154000, C523S218000
Reexamination Certificate
active
06552096
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to silicone rubber which contains one or more types of hollow organic resin filler and have at the interior an open-cell state. The invention also relates to a method of producing such silicone rubber.
2. Prior Art
Heat-curable liquid silicone rubber compositions are used in many different applications since they are effectively moldable and once molded, they provide cured products (silicone rubbers) having excellent qualities, including heat resistance, weather resistance and electrical insulating properties. One distinctive type of silicone rubber having a broad range of potential applications is sponge-like silicone rubber. In addition to possessing the above outstanding performance features of silicone rubbers (heat resistance, weather resistance, electric insulating properties, etc.), silicone rubber sponges can be made lightweight. Moreover, the inclusion of a gas in the molded material provides volumetric shrinkage qualities that enable the silicone rubber sponge to be used as an shock-absorbing, or cushioning, material. The low heat conductivity resulting from the incorporation of a gas also allows silicone rubber sponges to be used as a heat-insulating or heat-storing materials.
Such silicone rubber sponges are produced from silicone rubber compositions to which a blowing agent is added. The blowing agent may be incorporated in a number of ways, such as by adding a heat-decomposable blowing agent or by using hydrogen gas generated as a by-product during curing. Drawbacks of adding a heat-decomposable blowing agent include the toxicity and odor of the decomposition gases. When a platinum catalyst is used as the curing catalyst, an additional problem has been the inhibition of curing by the blowing agent. The use of hydrogen gas generated as a by-product of curing also presents a number of difficulties, such as the explosive nature of hydrogen gas and the special care required when handling the uncured product during storage. Another problem with using hydrogen gas generated during curing as the foaming agent, particularly when the silicone rubber composition is a liquid, is the difficulty of obtaining uniformly controlled cells. One method that provides some improvement in these respects involves incorporating within the silicone rubber composition a hollow powder made of an inorganic material such as glass or ceramic (U.S. Pat. No. 5,580,794). Yet, such inorganic materials have a high specific gravity and thus are not conducive to a sufficient reduction in the weight of the silicone rubber. Moreover, as inorganic materials, they do not adequately lower the heat conductivity and confer on the sponge only poor cushioning properties. According to another prior-art technique, described in JP-A 5-209080 corresponding to U.S. Pat. No. 5,246,973, an organic resin filler which swells under heating is included in the silicone rubber composition. Expansion is induced during curing to give a cell-containing molded product. However, this approach has a number of molding-related drawbacks, including an inability to reliably mold articles of a predetermined size and poor uniformity of curing. Another known method, described in JP-A 9-137063 corresponding to U.S. Pat. No. 5,750,581, provides for the addition of a hollow organic resin filler. However, the resulting molded material contains discrete and independent cells, and thus fails to fully overcome such shortcomings as insufficient cushioning property and a high compression set.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide hollow organic resin filler-containing silicone rubber which has an internally communicating state, good cushioning properties, and a low compression set. Another object of the invention is to provide a method of producing such silicone rubber.
It has been found that a silicone rubber having an internally communicating structure composed of cells in an open-cell state (that is, the structure in which each cell is connected to at least one other cell, rather than the structure in which cells are independently distributed in the silicone rubber matrix) created by inducing a gas at the interior of a hollow filler to diffuse can be produced by a method which involves curing a silicone rubber composition of (A) a curable organopolysiloxane composition and (B) at least two hollow organic resin fillers of different softening points at a temperature at which the lower softening point hollow filler collapses and the higher softening point hollow filler substantially maintains its shape.
Accordingly, the present invention provides a cured silicone rubber produced by curing a silicone rubber composition that includes (A) a curable organopolysiloxane composition and (B) at least one hollow organic resin filler. In the silicone rubber, cells formed by the filler are present in an open-cell state.
DETAILED DESCRIPTION OF THE INVENTION
Component A of the silicone rubber composition used to produce the inventive silicon rubber is a curable organopolysiloxane composition, which may be either an addition-curable or an organic peroxide-curable one.
Preferred addition-curable organopolysiloxane compositions are made of:
(1) an organopolysiloxane having at least two silicon-bonded alkenyl groups per molecule,
(2) an organohydrogenpolysiloxane having at least two silicon-bonded hydrogen atoms per molecule, and
(3) an addition reaction catalyst.
The organopolysiloxane having at least two silicon-bonded alkenyl groups serving as component (1) of the addition-curable organopolysiloxane composition may be a compound 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 represents substituted or unsubstituted C
1-10
, and preferably C
1-8
, monovalent hydrocarbon groups. The letter a is a positive number in a range of 1.5 to 2.8, preferably 1.8 to 2.5, and most preferably 1.95 to 2.05.
Illustrative examples of the substituted or unsubstituted monovalent hydrocarbon groups bonded to silicon represented by R
1
include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, cyclohexyl, octyl, nonyl and decyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyl and phenylpropyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl, hexenyl, cyclohexenyl and octenyl; and any of the above groups in which some or all of the hydrogen atoms are substituted with cyano groups or halogen atoms such as fluorine, bromine or chlorine, including chloromethyl, chloropropyl, bromoethyl, trifluoropropyl and cyanoethyl.
At least two of the R
1
groups in the above organopolysiloxane must be alkenyl groups which have preferably 2 to 8 carbons, and most preferably 2 to 6 carbons. The alkenyl group content of the organopolysiloxane is preferably 0.01 to 20 mol %, and most preferably 0.1 to 10 mol %, of all the organic groups bonded to silicon atoms (that is, of the substituted or unsubstituted monovalent hydrocarbon groups represented by R
1
in above average compositional formula (1)). The alkenyl groups may be bonded to silicon atoms at the ends of the molecular chain, silicon atoms located at intermediate positions along the molecular chain, or both. However, to provide a good composition curing rate and a cured product having good physical properties, it is advantageous for the organopolysiloxane used in the invention to include alkenyl groups bonded to at least the silicon atoms at the ends of the molecular chain.
The organopolysiloxane is generally a diorganopoly-siloxane having a basically linear structure in which the backbone is composed of repeating diorganosiloxane units and both ends of the molecular chain are capped with triorganosiloxy groups, although the structure may be partially branched or cyclic. The degree of polymerization, or viscosity, of the alkenyl group-bearing organopolysiloxane is not critical. Use can be made
Birch & Stewart Kolasch & Birch, LLP
Foelak Morton
Shin-Etsu Chemical Co. , Ltd.
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