Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2001-01-17
2002-10-22
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C428S548000, C524S493000, C524S588000, C524S440000, C528S015000, C528S024000, C528S031000, C528S032000
Reexamination Certificate
active
06469090
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to heat-curable electrically conductive liquid silicone rubber compositions capable of being molded by such techniques as injection molding or insert molding, which compositions are suitable for use in fields that require high electrical conductivity of the sealants employed in the current contact portions of electrical components and mobile electrical units, and the sealants used in association with electromagnetic induction (EMI) shielding, office equipment rollers, antistatic materials, and connectors.
2. Prior Art
Silicone rubber compositions loaded with high-conductivity fillers are employed in fields that require sealants used in the current contact portions of electrical components and mobile electrical units and in association with EMI shielding and related materials to have a high degree of electrical conductivity.
Silicone rubber compositions composed largely of high-viscosity uncured rubber-like organosiloxanes require a pre-forming step and are thus troublesome to use. Production costs can be reduced by employing a molding process such as injection molding or insert molding that does not require pre-forming, but there are upper limits on the viscosity of compositions which can be used in such processes. To obtain good-quality molded articles from one of these molding processes, the composition generally should have a viscosity at 25° C. of not more than 100,000 poise.
However, lowering the electrical resistance of a silicone rubber largely composed of high-viscosity uncured rubbery organosiloxane requires the addition of a large amount of highly conductive filler to the organosiloxane. Such addition increases the viscosity of the composition, making injection molding or insert molding impossible to carry out.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an electrically conductive liquid silicone rubber composition which has a low volumetric resistivity, is capable of exhibiting stable electrical resistance, and can be injection-molded or insert-molded.
We have discovered that an organopolysiloxane having a viscosity of 10 to 100,000 centipoise (cp) at 25° C. in which specific amounts of a finely divided silica powder and either a metal powder or a metal-coated silica powder have been compounded and to which a curing agent has been added provides a heat-curable electrically conductive liquid silicone rubber composition which has a high electrical conductivity that is stable and which can be molded by such techniques as injection molding or insert molding.
Accordingly, the invention provides an electrically conductive liquid silicone rubber composition comprising (A) 100 parts by weight of an organopolysiloxane having a viscosity of 10 to 100,000 centipoise at 25° C., (B) 1 to 100 parts by weight of a finely divided silica powder, (C) 30 to 700 parts by weight of a metal powder or an electrically conductive metal-plated powder, and (D) a curing agent in an amount sufficient to cure the organopolysiloxane.
DETAILED DESCRIPTION OF THE INVENTION
Component A of the electrically conductive liquid silicone rubber composition of the invention is an organopolysiloxane having a viscosity of 10 to 100,000 cp at 25° C.
Preferably, the organopolysiloxane serving as component A has the average constitutional formula:
R
1
a
SiO
(4-a)/2
(1),
wherein R
1
is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 12 carbons, with from 0.001 to 20 mol % of the R
1
groups being alkenyl groups, and the letter “a” is a positive number from 1.5 to 2.8. An organopolysiloxane bearing at least two alkenyl groups per molecule is more preferred. The use of one or more such organopolysiloxane that is capped at both ends of the molecular chain with trivinylsilyl, divinylmethylsilyl or vinyldimethylsilyl groups is especially preferred.
Exemplary R
1
groups in the above formula are substituted or unsubstituted monovalent hydrocarbon groups having 1 to 12 carbons, and preferably 1 to 8 carbons, including alkyl groups such as methyl, ethyl, propyl, isopropyl, isobutyl, butyl, tert-butyl, pentyl, neopentyl, hexyl, octyl, nonyl and decyl; cycloalkyl groups such as cyclohexyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl, isobutenyl, hexenyl and octenyl; cycloalkenyl groups such as cyclohexenyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyl and phenylpropyl; and any of the foregoing groups in which some or all of the hydrogen atoms attached to carbon atoms have been replaced with substituents such as halogen atoms or cyano groups, including chloromethyl, bromoethyl, 3,3,3-trifluoropropyl, 3-chloropropyl and cyanoethyl. The R
1
moiety preferably has an unsaturated aliphatic group (referred to hereinafter as “alkenyl group”) content, based on all the organic groups, within a range of 0.001 to 20 mol %, and especially 0.01 to 10 mol %. As noted above, it is advantageous for the R
1
moiety to have at least two alkenyl groups. Vinyl is preferred as the alkenyl group. The introduction of other substituents such as methyl and phenyl is also preferred. In the above constitutional formula (1), the letter “a” stands for a positive number from 1.5 to 2.8, and preferably from 1.8 to 2.5.
The organopolysiloxane of formula (1) is preferably a linear diorganopolysiloxane which may include R
1
SiO
3/2
units and SiO
4/2
units within a range that does not compromise the rubber elasticity of the cured product thereof, which has a backbone that is basically composed of recurring R
1
2
SiO
2/2
diorganosiloxane units, and which is capped at both ends of the molecular chain with R
1
3
SiO
1/2
triorganosiloxy units insofar as this does not compromise the rubber elasticity of the cured product. The alkenyl groups in the molecule may be bonded to silicon atoms either at the ends of the molecular chain or at the middle of the chain, or both, although the presence of alkenyl groups bonded to at least the silicon atoms at both ends of the molecular chain is preferable in terms of the curability of the composition and the properties of the cured product.
The alkenyl group-bearing organopolysiloxane may be prepared by a known method. According to one suitable method, preparation may involve carrying out an equilibrium reaction between an organopolysiloxane and a hexaorganodisiloxane in the presence of an alkali or acid catalyst.
The organopolysiloxane serving as component A has a viscosity at 25° C. within a range of 10 to 100,000 cp, and preferably 50 to 50,000 cp. At a viscosity below 10 cp, component C tends to settle out of the composition and the cured conductive silicone rubber is often brittle, whereas a viscosity greater than 100,000 cp often makes it difficult to carry out injection molding or insert molding.
Component B of the silicone rubber composition of the invention is a finely divided silica powder. Any such powder that is used in conventional silicone rubber compositions, with the exception of metal-plated silica powder, may be employed without particular limitation as component B. Illustrative examples include precipitated silica, fumed silica and fired silica having a specific surface area, as measured by the BET method, of at least 50 m
2
/g, and especially 50 to 400 m
2
/g. Other suitable examples include pulverized silica having an average particle size of up to 50 &mgr;m, and preferably within a range of 0.1 to 20 &mgr;m. Any one or combinations of two or more of these silica powders may be used.
The finely divided silica powder may be used directly without modification. Alternatively, it may be used in the form of a hydrophobic silica powder prepared by surface treating the finely divided silica powder with a silazane such as hexamethyldisilazane, a silane such as trimethylchlorosilane or an organosilicon compound such as polymethylsiloxane, or it may be rendered hydrophobic by blending it with a surface treatment agent during compounding.
Component B is incorporated in an amount of 1
Azechi Syuuichi
Fukushima Motoo
Nakamura Tsutomu
Birch & Stewart Kolasch & Birch, LLP
Dawson Robert
Shin-Etsu Chemical Co. , Ltd.
Zimmer Marc S.
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