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
2001-08-17
2004-05-11
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...
C524S439000, C524S440000, C528S015000, C528S024000, C528S032000, C252S511000, C252S514000, C428S405000, C428S630000, C428S570000
Reexamination Certificate
active
06734250
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrically conductive silicone rubber compositions having improved adhesion, which compositions are suitable for use in fields that require high electrical conductivity, for example, as current contact parts in electrical components and mobile electrical units, and sealants in electromagnetic shields, office equipment rollers, antistatic members, and connectors.
2. Prior Art
In the fields of electrical components and mobile electrical units requiring high electrical conductivity in their current contact portions and electromagnetic shields or the like requiring high electrical conductivity in their seals, such parts are often formed of silicone rubber compositions loaded with high-conductivity fillers. Where bond to resins or metals is additionally required, an adhesive primer must be applied to the bond surface before the rubber is molded thereto.
Undesirably, many primers contain solvents which give rise to environmental and working problems and also prolong the working time. Also, to lower the electrical resistance of silicone rubber, a large amount of highly conductive filler must be added. This increases the surface area of filler exposed on the rubber surface, with a possibility that mere application of the primer fails to provide sufficient adhesion.
SUMMARY OF THE INVENTION
An object of the invention is to provide an electrically conductive silicone rubber composition which cures into a part having a stable high conductivity and improved self adhesiveness.
It has been found that by blending an organopolysiloxane having aliphatic unsaturated groups with finely divided silica, a metal powder or an electrically conductive powder in the form of metal-plated inorganic filler or resin particles, and an adhesive aid, there is obtained an electrically conductive silicone rubber composition which cures into a part having a low volumetric resistivity, a stable high conductivity and improved self adhesiveness and is thus suitable as electrically conductive parts and electrical contacts in electrical equipment.
The invention provides an electrically conductive silicone rubber composition having improved adhesion, comprising
(A) 100 parts by weight of an organopolysiloxane having at least two aliphatic unsaturated groups in a molecule,
(B) 0.1 to 100 parts by weight of finely divided silica,
(C) 30 to 700 parts by weight of a metal powder or an electrically conductive metal-plated powder,
(D) 0.1 to 20 parts by weight of an adhesive aid, and
(E) a curing agent in an amount sufficient to cure the organopolysiloxane.
Also contemplated herein is an article comprising a substrate of a metal or resin and a coating of the conductive silicone rubber composition applied and cured to the substrate without a primer, the cured coating of the composition being integrated with the substrate.
DETAILED DESCRIPTION OF THE INVENTION
Component (A) of the electrically conductive silicone rubber composition of the invention is an organopolysiloxane at least two aliphatic unsaturated groups, especially alkenyl groups in a molecule. Preferably, the organopolysiloxane has the average constitutional formula (1):
R
1
a
SiO
(4−a)/2
(1)
wherein R
1
is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 12 carbon atoms, 0.001 to 20 mol % of the R
1
groups are alkenyl groups, and “a” is a positive number from 1.5 to 2.8.
In the alkenyl-bearing organopolysiloxane of formula (1), exemplary R
1
groups are substituted or unsubstituted monovalent hydrocarbon groups having 1 to 12 carbon atoms, and preferably 1 to 8 carbon atoms, including alkyl groups such as methyl, ethyl, propyl, isopropyl, isobutyl, butyl, tert-butyl, pentyl, neopentyl, hexyl, cyclohexyl, octyl, nonyl and decyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl, isobutenyl, hexenyl, cyclohexenyl, and octenyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyl and phenylpropyl; and halo- and cyano-substituted hydrocarbon groups such as chloromethyl, bromoethyl, 3,3,3-trifluoropropyl, 3-chloropropyl and cyanoethyl.
The substituent groups may be the same or different. The preferred content of alkenyl groups is 0.001 to 20 mol %, especially 0.01 to 10 mol % of the R
1
groups. As noted above, it is essential for the organopolysiloxane to have at least two alkenyl groups in a molecule. The R
1
groups may be any of the above-exemplified ones although the preferred alkenyl group is vinyl and the preferred other substituent groups are methyl and phenyl. In formula (1), “a” stands for a positive number from 1.5 to 2.8, and preferably from 1.8 to 2.9.
The organopolysiloxane of formula (1) may have a linear molecular structure or a branched molecular structure including R
1
SiO
3/2
units and SiO
4/2
units. Preferably it is a linear diorganopolysiloxane having 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 groups. Most preferably, the diorganopolysiloxane is capped with trivinylsilyl, divinylmethylsilyl or vinyldimethylsilyl groups at both ends of the molecular chain.
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. 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 cure of the composition and the cured physical properties.
The alkenyl-bearing organopolysiloxane preferably has an average degree of polymerization in the range of about 100 to 3,000, especially about 200 to 2,000 when the composition is liquid, and in the range of about 3,000 to 100,000, especially about 4,000 to 20,000 when the composition is of millable type.
The alkenyl-bearing organopolysiloxane may be prepared by known methods. According to one suitable method, preparation may involve carrying out equilibrium reaction between an organopolysiloxane and a hexaorganodisiloxane in the presence of an alkali or acid catalyst.
Component (B) of the silicone rubber composition is finely divided silica. Its type is not critical and any silica powder that is used in conventional silicone rubber compositions, with the exception of metal-plated silica powder, may be employed. Illustrative examples include precipitated silica, fumed silica and fired silica having a specific surface area of at least 50 m
2
/g, and especially 50 to 400 m
2
/g, as measured by the BET method. Other suitable examples include ground quartz and diatomaceous earth having an average particle size of up to 50 &mgr;m, and preferably within a range of 0.1 to 20 &mgr;m.
The silica powder may be used directly without modification. Alternatively, the silica powder may be used in the form of a hydrophobic silica powder after surface treatment with a silazane such as hexamethyldisilazane, a silane such as trimethylchlorosilane or an organosilicon compound such as polymethylsiloxane. The silica powder may also be rendered hydrophobic by blending it with a surface treatment agent during compounding.
The finely divided silica (B) is incorporated in an amount of 1 to 100 parts by weight, and preferably 2 to 50 parts by weight, per 100 parts by weight of the organopolysiloxane (A). With less than 1 part of silica, the mechanical strength of the cured product may become low. The use of more than 100 parts of silica tends to obstruct proper loading of the conductive filler (C), adversely affecting the workability of the composition.
Component (C) is a metal powder or a conductive metal-plated powder (conductive powder having a metal plating surface layer) whose purpose is to confer the silicone rubber composition of the invention with electrical conductivity.
No limitations are imposed on the particle size of the metal powder, although an average particle size within a range of 0.05 to 100 &mgr;m, and especially 0.1 to 10 &mgr;m, is pr
Azechi Syuuichi
Nakamura Tsutomu
Birch & Stewart Kolasch & Birch, LLP
Dawson Robert
Shin-Etsu Chemical Co. , Ltd.
Zimmer Marc S.
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