Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
2000-03-24
2001-10-30
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Free metal containing
C428S447000, C439S086000
Reexamination Certificate
active
06309563
ABSTRACT:
This invention relates to electrically conductive silicone rubber compositions, and more particularly, to conductive silicone rubber compositions which cure into silicone rubber with a stable resistance. It also relates to low-resistance connectors suitable for connection between liquid crystal displays and circuit boards or between electronic circuit boards.
BACKGROUND OF THE INVENTION
Owing to its high electrical conductivity, silver powder is widely utilized as a conductive filler in a variety of silicone rubber compositions including addition reaction curing type silicone rubber compositions, condensation reaction curing type silicone rubber compositions, and peroxide vulcanizing type silicone rubber compositions. Since silicone rubber compositions having silver powder blended therein cure into silicone rubber with a low electrical resistance, they are used in the application where electrical conductivity and heat resistance are required. The silver powder blended in silicone rubber compositions usually takes the form of particles and flakes.
As a general rule, silver powder has a strong tendency to agglomerate. The silver powder which is stored for a long term is unsuitable to add to silicone rubber compositions because it is difficult to disperse the agglomerated silver powder during compounding. An improvement in this regard is desired. Another problem is that the cured silicone rubber has a volume resistivity which is unstable.
In particular) flake silver powder is utilized in forming low-resistance (or high conductivity) silicone rubber. In order to facilitate compounding, silver powder is often treated with a chemical agent during commutation. Such chemical agents are saturated or unsaturated higher fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, and oleic acid, metal soaps, higher aliphatic amines and polyethylene wax. This treatment, however, has a possibility to retard vulcanization of silicone rubber compositions to which treated silver powder has been added.
It has recently been considered to use silver powder-loaded connectors instead of U-shaped metal wire connectors for providing connection between electronic circuit boards. The silver powder-loaded connectors include a plurality of layers of a conductive elastomer and an insulating elastomer which are stacked in a zebra pattern, and provide a stable contact, avoiding a point contact and display failure.
However, silver powder used as the conductive element tends to agglomerate and becomes difficult to added to elastomers after a long-term storage as mentioned above. If agglomerated silver powder is compounded, dispersion becomes poor, resulting in a resistance instability and variation.
When elastomers are stacked in alternating layers to construct a zebra connector, the poorly dispersed silver powder can cause a puncture phenomenon that upon pressing in a block form for vulcanization, tearing occurs within conductive layers or at the interface between a conductive layer and an insulating layer. It is then very difficult to consistently manufacture such connectors on a mass scale.
SUMMARY OF THE INVENTION
A first object of the invention is to provide a conductive silicone rubber composition containing silver powder which is prevented from agglomeration and thus highly compatible with the remaining components, the composition curing into a silicone rubber having a stable volume resistivity.
A second object of the invention is to provide a low-resistance connector which establishes a stable conductive path when used between a liquid crystal display and a circuit board or between circuit boards and which can be mass produced at a low cost.
We have found that by admixing silver powder with at least 0.2% by weight of fine powder selected from the group consisting of inorganic fillers and spherical organic resins, there is obtained a conductive powder which is effectively dispersible. This conductive powder eliminates the above-mentioned problems of silver powder by itself.
More particularly, when silver powder is admixed with at least 0.2% by weight of fine powder selected from the group consisting of inorganic fillers and spherical organic resins, the resulting conductive powder (silver powder) does agglomerate little with the lapse of time and is effectively dispersible in silicone rubber compounds. By blending an organopolysiloxane having at least two aliphatic unsaturated groups with an appropriate amount of the conductive powder, there is obtained a silicone rubber composition which has a stabilized volume resistivity. This composition can be cured with an organic peroxide or an organohydrogen-polysiloxane/platinum base catalyst alone or with a combination of an organic peroxide with an organohydrogen-polysiloxane/platinum base catalyst. The composition is molded and cured into a silicone rubber product which has a stable low resistance (or stable high conductivity) and performs well during long-term service and is thus suited as conductive contact members, connectors, roll members in business machines, and electromagnetic shield gaskets.
In a first aspect, 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
is independently a substituted or unsubstituted monovalent hydrocarbon group and n is a positive number of 1.98 to 2.02,
(B) 100 to 800 parts by weight of a conductive powder comprising a silver powder premixed with at least 0.2% by weight of fine powder selected from the group consisting of inorganic fillers and spherical organic resins, and (C) a sufficient amount to cure component (A) of a curing agent.
In a second aspect, the invention provides a low-resistance connector comprising a plurality of alternating layers of a conductive elastomer and an insulating elastomer, at least one elastomer being flexible, which are alternately disposed to form a multilayer structure such that their juncture surfaces are parallel to each other, each conductive elastomer layer comprising as a conductive element a cured product of a silicone rubber composition according to the first aspect.
Since the conductive powder which can be stored for a long term and effectively dispersed in silicone rubber compounds is used as a steady-resistivity conductive element, the connector can be mass produced at a low cost. The low-resistance connector offers a stable conductive path when used between a liquid crystal display of the COG or TAB type and a circuit board or between circuit boards.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The silicone rubber composition according to the invention includes as a first essential component (A) an organopolysiloxane represented by 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. 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 organopolysi
Aoki Kazuhiko
Iino Mikio
Nakamura Tsutomu
Kopec Mark
Millen White Zelano & Branigan P.C.
Shin-Etsu Chemical Co. , Ltd.
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