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
2002-01-18
2004-09-28
Sellers, 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...
C523S210000, C523S212000, C524S730000, C524S780000, C524S862000, C525S474000, C525S478000
Reexamination Certificate
active
06797772
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to electrically conductive organosiloxane elastomers. More particularly, this invention relates to storage-stable silver-filled organosiloxane compositions yielding cured electrically conductive elastomers that retain their electrical properties for extended periods of time. The reduced variation of contact resistance and volume resistivity with time exhibited by the elastomers are attributed to the manner in which the silver particles are processed prior to being incorporated into the curable organosiloxane composition.
Elastomers, gels and resins prepared from curable organosiloxane compositions containing finely divided silver particles exhibit high levels electrical conductivity and are therefore used in specialized applications that require materials exhibiting heat resistance, flex resistance, and electrical conductivity.
Japanese Patent Application Laid Open [Kokai or Unexamined] Number Hei 3-170581 [170,581/1991] teaches an electrically conductive silicone rubber composition comprising an organopolysiloxane containing at least 2 alkenyl radicals in each molecule, an organohydrogensiloxane containing at least 2 silicon-bonded hydrogen atoms in each molecule, a platinum-containing hydrosilation catalyst, and silver particles.
The silver particles used to prepare electrically conductive silicone rubber are typically classified as chemically reduced silver, electrolytically reduced, and atomized silver. Chemically reduced silver is prepared by reacting an aqueous silver nitrate solution with a reducing agent such as hydrazine, formaldehyde or ascorbic acid. Electrolytically reduced silver is prepared from aqueous silver nitrate solutions by electrolytic deposition at the cathode. Atomized silver particles are prepared by spraying molten silver maintained at a temperature of at least 1,000° C. into water or an inert gas.
Silver particles are available in the form of granules, flakes, dendrites or amorphous particles. Silver flake is preferably used because it yields silicone rubbers with particularly high electrical conductivities.
Japanese Laid Open Patent Application No. 59/170,167 discloses a method for preparing gold- or silver-filled inks by blending an organic solvent with a powdered form of gold or silver that has been treated with the combination of a methylhydrogen polysiloxane and an amino-functional silicone oil.
U.S. Pat. No. 5,227,093, which issued on Jul. 13, 1993 teaches increasing the electrical conductivity of elastomers and other products prepared from curable organosiloxane compositions containing finely divided silver particles by treating the silver particles with a fatty acid ester prior to blending the silver particles with the other ingredients of the curable composition.
Japanese Laid Open Patent Application No. 03/49,105 describes electrically conductive particles suitable for use with adhesives. The particles exhibit diameters of from 1 to 20 microns and comprise a core of a high polymer on which is deposited a layer of silver followed by a layer of gold. During blending with the ingredients of a curable organosiloxane composition the particles are treated with a silane coupling agent selected from the group consisting of gamma-methacryloxypropyltrimethoxysilane, gamma-mercaptopropyltrimethoxysilane and gamma-chloropropyltrimethoxysilane.
Several problems are associated with the electrically conductive silicone rubber composition taught in Japanese Patent Application Laid Open Number Hei 3-170581 and the other publications mentioned in the immediately preceding paragraphs. When silver in a flake form is used as a filler, it has been found that during storage of the composition not only do the silver particles separate from the composition, but the curability of the composition declines with the passage of time to the point that the composition ultimately becomes uncurable.
Another problem associated with the prior art conductive silicone rubber compositions that the large variations in contact resistance and volume resistivity that occur with the passage of time in cured elastomers prepared using the curable compositions described in this patent publication. This phenomenon renders the rubber unsuitable for use for the continuous electrical connection of electrically conductive elements.
The present inventors have been able to determine that one cause of the variation of electrical properties of silver-filled organosiloxane elastomers with time is the low affinity of silver particles for the other ingredients of the curable composition used to prepare the elastomer.
The present inventors have also been able to confirm that the curability of electrically conductive silicone rubber compositions declines with elapsed time due to the presence of residues of the lubricant that is present during grinding of the silver particles. These lubricant residues remain on the surface and/or in the interior of the particles.
One or more of the following lubricants typically have been used with silver particles during grinding of the particles: saturated and unsaturated higher fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, arachidic acid, and behenic acid; metal soaps such as aluminum laurate, aluminum stearate, zinc laurate, and zinc stearate; higher aliphatic alcohols such as stearyl alcohol; the esters of higher aliphatic alcohols and carboxylic acids; higher aliphatic amines such as stearylamine; higher aliphatic amides; and polyethylene waxes.
The present inventors attempted to remove the lubricant remaining on the surface of lubricant-treated silver flake by subjecting the silver particles to repeated washings with organic solvents. The washings did not provide a satisfactory suppression of the timewise variation in the curability of electrically conductive silicone rubber compositions containing this type of silver as a conductive filler.
The present inventors have also confirmed that a decline in adhesion and affinity between the cured silicone elastomer and the silver particles contribute to the timewise variations in, respectively, the contact resistance and volume resistivity of electrically conductive silicone rubbers prepared by curing silver-filled electrically conductive silicone rubber compositions.
As the result of extensive investigations directed at solving the problems described in the preceding paragraphs of this specification, the present inventors discovered that the timewise variation in curability can be suppressed by treating the finely divided silver particles with an organosilicon compound prior to combining the particles with the other ingredients of the curable organosiloxane composition.
The present inventors also discovered that the presence in the curable organosiloxane composition of an organosilicon compound containing silicon-bonded alkoxy groups that is in addition to the compound used to treat the silver particles, further reduces the timewise variations in contact resistance and volume resistivity of the cured elastomer. The present invention is the result of these two discoveries.
One objective of the present invention is to provide silver-filled silicone rubber composition that exhibits excellent values of electrical conductivity in combination with little timewise variation in curability and electrical conductivity. A second objective is to provide a method for treating silver particles intended for use in electrically conductive organosiloxane compositions.
SUMMARY OF THE INVENTION
The objectives of the present invention can be achieved by pretreating the silver particles intended for use in the present organosiloxane compositions with an organosilicon compound prior to combining the particles with the other ingredients of these compositions.
The variation in electrical properties with time exhibited by the curable composition can be further reduced by the presence in the curable composition of an alkoxy-containing organosilicon compound as an additive. This compound is in addition to an
Mine Katsutoshi
Nakayoshi Kazumi
Tazawa Rikako
Brown Catherine U.
Dow Corning Toray Silicone Co. Ltd.
Sellers Robert
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