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
2000-10-06
2002-07-02
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...
C428S450000, C428S548000, C428S448000, C524S403000, C528S031000, C528S032000
Reexamination Certificate
active
06414078
ABSTRACT:
This invention relates to a conductive silicone rubber composition which cures into a conductive part having a low and stable volume resistivity.
BACKGROUND OF THE INVENTION
Prior art compositions for producing elastomers having a low resistivity include silicone rubber compositions of the addition reaction curing type, condensation reaction curing type, and peroxide vulcanizing type wherein powdered silver having a high electric conductivity is generally added to a polymer. The use of silver as the conductive powder has several problems because of a high agglomerating tendency and the lack of environmental stability. Silver particles do not uniformly disperse in silicone rubber and are readily oxidized or degraded on the surface in a hot humid atmosphere.
SUMMARY OF THE INVENTION
An object of the invention is to provide a conductive silicone rubber composition which cures into a conductive silicone rubber part having a low and stable volume resistivity.
It has been found that useful conductive particles are obtained by plating a metal on base particles of inorganic material or organic resin to form a plated metal layer on their surface. When the surface-metallized inorganic particles or organic resin particles are added to an organopolysiloxane, the resulting silicone rubber composition has a stable volume resistivity because of ease of dispersion of the particles. This silicone rubber composition can be cured with an organic peroxide or organohydrogenpolysiloxane/platinum group catalyst alone or a combination thereof. The cured part or silicone rubber has a constantly low electrical resistance or high conductivity and withstands a long term of service, finding best use as conductive contact members, business machine roll members, electromagnetic shielding gaskets, etc.
The invention provides a conductive silicone rubber composition comprising
(A) 100 parts by weight of an organopolysiloxane having at least two aliphatic unsaturated groups represented by the following average compositional formula (1):
R
1
n
SiO
(4−n)/2
(1)
wherein R
1
, which may be the same or different, is a substituted or unsubstituted monovalent hydrocarbon group and n is a positive number of 1.98 to 2.02,
(B) 90 to 800 parts by weight of conductive particles comprising base particles of an inorganic material or organic resin coated on their surface with a plated metal layer, and
(C) a sufficient amount to cure component (A) of a curing agent.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The conductive silicone rubber composition of the invention includes as a first essential component (A) an organopolysiloxane of the following average compositional formula (1):
R
1
n
SiO
(4−n)/2
(1)
wherein R
1
is independently a substituted or unsubstituted monovalent hydrocarbon group and n is a positive number of 1.98 to 2.02.
The substituted or unsubstituted monovalent hydrocarbon groups represented by R
1
, which may be identical or different, are preferably those of 1 to 10 carbon atoms, more preferably 1 to 8 carbon atoms. Examples include alkyl groups such as methyl, ethyl, propyl, butyl, hexyl, and octyl; cycloalkyl groups such as cyclohexyl; alkenyl groups such as vinyl, allyl, propenyl, butenyl, and hexenyl; aryl groups such as phenyl and tolyl; aralkyl groups such as benzyl and phenylethyl; 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 or cyano groups, such as chloromethyl, trifluoropropyl, and cyanoethyl. Of these, methyl, vinyl, phenyl and trifluoropropyl are preferable. At least two of the R
1
groups must be aliphatic unsaturated groups (i.e., alkenyl groups). The content of aliphatic unsaturated groups is preferably 0.001 to 20 mol%, more preferably 0.025 to 5 mol% of the R
1
groups. The letter n is a positive number of 1.98 to 2.02. Preferably, the organopolysiloxane of formula (1) basically has a linear structure, although a mixture of two or more organopolysiloxanes of different molecular structures is acceptable.
The organopolysiloxane should preferably have an average degree of polymerization of 100 to 20,000, and more preferably 3,000 to 10,000.
A second essential component (B) is conductive particles in the form of base particles coated on their surface with a plated metal layer. The base particles used are inorganic particles (also referred to as inorganic filler, hereinafter) or organic resin particles. By plating a metal on surfaces of the base particles, the conductive particles are obtained.
The inorganic fillers used herein include silica, titanium dioxide, alumina, mica, barium sulfate and carbon black, but exclude glass powder. Of these, silica and alumina, especially spherical silica and spherical alumina, are preferred as well as carbon black. Useful carbon black species include Ketjen Black, acetylene black, furnace black and channel black, with Ketjen Black and acetylene black being especially preferred. The organic resin particles include polyolefins such as polyethylene, polyvinyl chloride, polypropylene, and polystyrene, acrylic resins such as styrene-acrylonitrile copolymers and polymethyl methacrylate, amino resins, fluoro-resins, and nitrile resins. Of these, spherical particles of polymethyl methacrylate are preferred.
Although the base particles (inorganic fillers or organic resin particles) may have any appropriate particle size, they preferably have a mean particle size of 0.01 to 1,000 &mgr;m, and especially 0.01 to 10 &mgr;m. Too small a mean particle size corresponds to a too large specific surface area, which may need a more amount of plating metal and hence, a more expense. Particles with too large a mean particle size may be less dispersible in the silicone rubber composition.
Examples of the metal to be plated on the base particles include gold, silver, nickel, palladium, copper and alloys thereof. The plating layer may be a single layer of such metal or a multilayer structure of two or more plating layers of such metals. Of these metals, nickel and gold are preferred. More preferred are double metallized particles obtained by first plating nickel on the base particles and then plating gold on the nickel layer. Most preferred are metallized particles having a four-layer structure of base particle-silicon compound-nickel-gold in which a silicon compound intervenes between the base particle and nickel for improving the adhesion between the base particle and the plated metal layer. The silicon compounds used herein are silane coupling agents having a bonding ability such as aminopropyltriethoxysilane and aminopropyltrimethoxysilane and silicon polymers having a reducing ability.
The method of depositing a metal on the base particles is not critical. Either wet plating or gas phase deposition may be used. In the case of wet plating, any of well-known electroless or electric plating solution compositions may be used while any well-known pretreatment and any well-known plating method are used. In particular, a method comprising the following steps (1) to (4) is employed. Although the following description refers to the preparation of metallized silica using silica as a typical example of the base particles, it is understood that when other inorganic fillers or organic resin particles are used, metallized particles can be similarly prepared therefrom by following steps (1) to (4).
Step (1): Silica particles are treated with a silicon compound, preferably a silicon compound having a reducing ability to form a layer of the silicon compound on the silica surface.
Step (2): The particles resulting from step (1) are treated with a solution containing a salt of a metal having a standard oxidation reduction potential of at least 0.54 V to deposit the metal colloid on the silicon compound layer on the silica surface.
Step (3): While the metal colloid serves as a catalyst, electroless nickel plating is effected to deposit a metallic nickel layer on the silicon compound layer.
Step (4): Gold plating is effected to deposit a gold layer on the nickel layer.
Fukushima Motoo
Iino Mikio
Nakamura Tsutomu
Birch & Stewart Kolasch & Birch, LLP
Dawson Robert
Shin-Etsu Chemical Co. , Ltd.
Zimmer Marc S.
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