Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-02-08
2001-12-25
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...
C521S088000, C521S134000, C521S154000
Reexamination Certificate
active
06333364
ABSTRACT:
This invention relates to silicone rubber compositions having blended therein an organic resin fine hollow filler or an organic resin fine filler adapted to expand into a hollow filler upon heating.
BACKGROUND OF THE INVENTION
Heat curing type liquid silicone rubber compositions are employed in a variety of applications since they are effectively moldable and exhibit good heat resistance, weather resistance and electrical insulation after molding. On the other hand, silicone rubber sponge is lightweight while maintaining the heat resistance, weather resistance, electrical insulation inherent to silicone rubber. By virtue of the inclusion of gas in the molded part, the silicone rubber sponge serves the function of a shock absorber or cushioning member by utilizing its volume contraction and the function of a thermal insulating or thermal storage material by utilizing its low heat transfer, finding a wide variety of applications. One version of the silicone rubber sponge is a silicone rubber foam which is prepared by several methods, for example, by adding a thermally decomposable blowing agent, or by molding and curing while generating hydrogen gas by-product. In the method of adding a thermally decomposable blowing agent, the toxicity and odor of decomposed gases are problems. In the system wherein a platinum catalyst is used as the curing catalyst, the retardation of cure by the blowing agent is a problem. The method of utilizing hydrogen gas by-produced during the curing step suffers from such problems as the potential explosion of hydrogen gas and the careful handling of uncured composition during shelf storage. Further, the gas generating method encounters difficulty in forming controlled uniform cells when the silicone rubber composition is a liquid one. One known approach addressing the above-mentioned problems is to incorporate hollow particles of inorganic material, typically ceramic material into rubber. This method provides little contribution to weight reduction because of a high specific gravity and achieves only a little thermal conductivity decline and a weak cushioning effect because of the inorganic nature. It is also known from JP-A 5-209080 corresponding to U.S. Pat. No. 5,246,973 and JP-A 9-137063 corresponding to U.S. Pat. No. 5,750,581 to add a hollow filler of organic resin and an organic resin filler encapsulating a volatile material so that it may expand upon heating. Since the heat resistance of the organic resin filler is poor, the physical properties of molded parts are largely altered, failing to take advantage of the heat resistance of silicone rubber. In particular, the compression set increases to considerably large values at elevated temperature, especially above 80° C. due to the influence of hollow filler. This negates the application to sealants, gaskets, cushions and rolls which are used at elevated temperature.
SUMMARY OF THE INVENTION
An object of the invention is to provide a silicone rubber composition capable of forming a low specific gravity rubbery elastomer having a minimal compression set even at elevated temperature.
The invention is directed to a silicone rubber composition comprising a heat curable organopolysiloxane composition and a thermally expandable, unexpanded organic resin fine filler or expanded organic resin fine hollow filler. It has been found that when a polyhydric alcohol or derivative thereof is blended in this silicone rubber composition, the polyhydric alcohol or derivative thereof effectively functions as an improver capable of improving the compression set of a hollow filler-loaded, low specific gravity silicon rubber elastomer even at elevated temperature. There is obtained a hollow filler-loaded, low specific gravity silicon rubber elastomer which undergoes a reduced compression set at elevated temperature, and thus finds effective application as sealants, gaskets, cushions and rolls which are used at elevated temperature.
Accordingly, the invention provides a silicone rubber composition comprising
(A) 100 parts by weight of a heat curable organopolysiloxane composition,
(B) 0.1 to 30 parts by weight of a thermally expandable, unexpanded organic resin fine filler or expanded organic resin fine hollow filler, and
(C) 0.5 to 50 parts by weight of at least one of polyhydric alcohols and derivatives thereof.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Component (A) is a heat curable organopolysiloxane composition which may be either an addition reaction curing type organopolysiloxane composition or an organic peroxide curing type organopolysiloxane composition.
The addition reaction curing type organopolysiloxane composition is preferably defined as comprising (1) 100 parts by weight of an organopolysiloxane having on the average at least two alkenyl groups in a molecule, (2) 0.1 to 50 parts by weight of an organohydrogenpolysiloxane having on the average at least two hydrogen atoms attached to silicon atoms in a molecule, and (3) a catalytic amount of an addition reaction catalyst.
The organic peroxide curing type organopolysiloxane composition is preferably defined as comprising (i) 100 parts by weight of an organopolysiloxane having on the average at least two alkenyl groups in a molecule, and (ii) a catalytic amount of an organic peroxide.
In the addition reaction curing type organopolysiloxane composition, the organopolysiloxane having on the average at least two alkenyl groups in a molecule as component (1) is typically represented by the following average compositional formula (1):
R
1
a
SiO
(4−a)/2
(1)
wherein R
1
, which may be the same or different, is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms, and “a” is a positive number of 1.5 to 2.8, preferably 1.8 to 2.5, and more preferably 1.95 to 2.05.
Examples of the substituted or unsubstituted monovalent hydrocarbon groups attached to silicon atoms, represented by R
1
, include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, cyclohexyl, octyl, nonyl and adecyl; 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 substituted ones of the foregoing groups in which some or all of the hydrogen atoms are replaced by halogen atoms (e.g., fluoro, bromo and chloro), cyano groups or the like, such as chloromethyl, chloropropyl, bromoethyl, trifluoropropyl and cyanoethyl.
At least two of the R
1
groups must be alkenyl groups, preferably of 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms. It is noted that the content of alkenyl groups is preferably 0.001 to 20 mol %, especially 0.01 to 10 mol % based on the entire organic groups attached to silicon atoms (that is, the substituted or unsubstituted monovalent hydrocarbon groups included as R
1
in formula (1)). The alkenyl groups may be attached to silicon atoms at ends of the molecular chain and/or silicon atoms midway the molecular chain. When the cure rate of the composition and the physical properties of cured parts are taken into account, the organopolysiloxane should preferably have at least alkenyl groups attached to the silicon atoms at molecular chain ends.
With respect to the structure, the organopolysiloxane is generally a diorganopolysiloxane of a basically straight chain structure whose backbone is comprised of recurring diorganosiloxane units and which is blocked with a triorganosiloxy group at either end of the molecular chain. However, it may have a partially branched or cyclic structure. The alkenyl group-containing organopolysiloxane may have any desired degree of polymerization or viscosity, and encompasses from ones having a low degree of polymerization which are liquid at room temperature (25° C.) to gum-like ones having a high degree of polymerization. Often, the organopolysiloxane used herein has an average degree of polymerization (weight average degree of polymerization)
Meguriya Noriyuki
Tomizawa Nobumasa
Foelak Morton
Shin - Etsu Chemical Co. Ltd.
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