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
2002-08-09
2004-03-16
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...
C524S424000, C524S435000, C524S496000, C524S588000, C428S402000, C428S403000, C528S024000
Reexamination Certificate
active
06706799
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an organic peroxide vulcanization type conductive silicone rubber composition, and in particular to a conductive silicone rubber composition suitable for use in electromagnetic interference sealed gasket materials, conductive contact members, connectors, and roller members in office equipment and the like.
DESCRIPTION OF THE PRIOR ART
Conventionally, methods for producing silicone rubber with a low electrical resistance have involved the addition of a metallic powder with a high electrical conductivity to known silicone rubber compositions such as addition reaction curing type silicone rubber compositions, condensation reaction curing type silicone rubber compositions and peroxide vulcanization type silicone rubber compositions. However, in cases in which a metal such as silver powder or nickel powder or the like is used as the conductive powder, problems arise in that the degree of agglomeration of the metallic powder is high, meaning the powder is not uniformly dispersed within the silicone rubber, and the environmental stability is poor, meaning the metal surface oxidizes and deteriorates, particularly under conditions of high temperature and high humidity.
Furthermore, although conductive silicone rubbers in which carbon such as conductive carbon black or graphite has been dispersed display comparatively good heat resistance, producing a material of superior conductivity with a volume resistivity of no more than 1×10
−3
&OHgr;·m has been difficult.
An example of the conventional technology is an addition reaction curing type conductive silicone resin composition containing nickel coated carbon, disclosed in Japanese Laid-open publication (kokai) No. 59-199756 (JP59-199756A). However, addition reaction curing type compositions typically use platinum based catalysts, and suffer from problems including susceptibility to catalyst poisoning and a short pot life.
As a result of intensive research aimed at resolving the problems described above, the inventors of the present invention discovered that a silicone rubber composition using a radical generating organic peroxide compound enabled the production of a conductive silicone rubber composition which was in no way inferior, and were hence able to complete the present invention.
SUMMARY OF THE INVENTION
In other words, an object of the present invention is to provide an organic peroxide vulcanization type conductive silicone rubber composition which has an adequate pot life, is unlikely to be affected by catalyst poisoning, displays excellent dispersibility, is suitable for normal pressure hot air vulcanization (HAV), and displays superior mechanical properties such as tensile strength and elongation, a low volume resistivity, and excellent electromagnetic interference sealing, as well as providing an electromagnetic interference sealed material comprising a conductive silicone rubber produced from such a composition.
As a result of intensive investigations aimed at achieving the aforementioned object, the inventors of the present invention discovered that by combining nickel coated carbon with an organopolysiloxane, and vulcanizing the mixture using an organic peroxide, a superior conductive silicone rubber could be obtained.
In other words, the present invention provides a conductive silicone rubber composition comprising:
(a) 100 parts by weight of an organopolysiloxane represented by an average composition formula of R
a
SiO
(4−a)/2
wherein, R is an unsubstituted or substituted monovalent hydrocarbon group which may be the same or different, and a is a positive number of 1.90 to 2.05,
(b) 50 to 500 parts by weight of conductive particles of carbon coated with Ni, and
(c) 0.1 to 10 parts by weight of an organic peroxide,
and also provides an electromagnetic interference sealed material produced from the composition.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As follows is a detailed description of each of the constituents of a silicone rubber composition according to the present invention.
(a) Organopolysiloxane
In the present invention, an organopolysiloxane used as the base constituent is represented by an average composition formula (I) shown below.
R
a
SiO
(4−a)/2
(1)
In this formula, R represents an unsubstituted or substituted monovalent hydrocarbon group of preferably 1 to 10 carbon atoms, and even more preferably 1 to 8 carbon atoms, which may be the same or different, selected alkyl groups such as methyl groups, ethyl groups, propyl groups and butyl groups, cycloalkyl groups such as cyclohexyl groups, alkenyl groups such as vinyl groups, allyl groups, butenyl groups and hexenyl groups, aryl groups such as phenyl groups and tolyl groups, as well as groups in which a portion of, or all of, the hydrogen atoms bonded to carbon atoms within the above groups are substituted with halogen atoms or cyano groups or the like, such as chloromethyl groups, trifluoropropyl groups and cyanoethyl groups. Methyl groups, phenyl groups, vinyl groups and trifluoropropyl groups are particularly preferred, and at least 80 mol %, and preferably at least 95 mol %, of the R groups should preferably be methyl groups.
Furthermore, the rate of incorporation of alkenyl groups within the total amount of R groups should preferably be 0.001 to 20 mol %, and even more preferably from 0.025 to 5 mol %. At values less than 0.001 mol %, the vulcanization may be unsatisfactory, whereas at values exceeding 20 mol %, the workability and mechanical characteristics of the rubber may deteriorate.
Furthermore, a is a positive number from 1.90 to 2.05.
The organopolysiloxane of the formula (1) should preferably be basically a straight chain structure, although may also be a mixture of two or more compounds with different molecular structures. The molecular chain terminals should preferably be blocked with vinyl group-containing silyl groups such as dimethylvinylsilyl groups, methyldivinylsilyl groups or trivinylsilyl groups. In addition, the average degree of polymerization of the above organopolysiloxane should be from 100 to 20,000, and preferably from 3,000 to 10,000. At values less than 100, good mechanical characteristics may be unachievable, whereas at values exceeding 20,000, the addition and mixing of conductive particles may become difficult.
The conductive particles of Ni coated carbon of the constituent (b) are a particularly important constituent in imparting good conductivity and electromagnetic interference sealing to the silicone rubber. Suitable examples of the carbon material for coating with Ni include normal carbon black, artificial graphite, natural graphite or granulated carbon, although artificial graphite is particularly preferred.
The average particle diameter of the carbon should preferably be from 10 to 300 &mgr;m, with particles of 20 to 150 &mgr;m being particularly preferred. If the average particle diameter is less than 10 &mgr;m, then the Ni coating process may become difficult, whereas if the average particle diameter exceeds 300 &mgr;m, then the mechanical strength of the conductive silicone rubber may deteriorate. Furthermore, the amount of Ni used in the coating is from 20 to 90% by weight, and preferably from 50 to 80% by weight, relative to the weight of carbon. At values less than 20% by weight, the initial conductivity may be unsatisfactory, whereas at values exceeding 90% by weight, the variation in the conductivity over time at high temperature may increase.
Examples of suitable Ni coating methods include normal chemical plating methods, electrolytic methods and carbonyl methods, although carbonyl methods are particularly preferred from the viewpoint of preventing the introduction of impurities.
The amount of the constituent (b) added is from 50 to 500 parts by weight, and preferably from 150 to 300 parts by weight, per 100 parts by weight of the organopolysiloxane of the constituent (a). At amounts less than 50 parts by weight, the conductivity and electromagnetic interference sealing achieved is unsatisfactory, wherea
Igarashi Minoru
Koike Yoshiaki
Nakamura Tsutomu
Yaginuma Atsushi
Dawson Robert
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Shin-Etsu Chemical Co. , Ltd.
Zimmer Marc S
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