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
2001-02-28
2002-09-10
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...
C528S015000, C528S012000, C528S031000, C528S032000, C524S858000, C524S437000, C525S478000, C525S474000
Reexamination Certificate
active
06448329
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a silicone composition and more particularly to an addition-curable silicone composition containing an alumina filler and a polyether. The present invention also relates to a thermally conductive cured silicone product formed from the silicone composition.
BACKGROUND OF THE INVENTION
Silicones are useful in a variety of applications by virtue of their unique combination of properties, including high thermal stability, good moisture resistance, excellent flexibility, high ionic purity, low alpha particle emissions, and good adhesion to various substrates. For example, silicones are widely used in the automotive, electronic, construction, appliance, and aerospace industries.
Addition-curable silicone compositions comprising an alkenyl-containing organopolysiloxane, an organohydrogenpolysiloxane, an alumina filler, and a hydrosilylation catalyst are known in the art. Illustrative of such compositions are U.S. Pat. No. 4,444,944 to Matsushita; U.S. Pat. No. 5,008,307 to Inomata; U.S. Pat. No. 6,025,435 to Yamakawa et al.; U.S. Pat. No. 6,169,142 to Nakano et al.; and EP 496419 to Fujiki et al.
However, conventional silicone compositions containing an alumina filler exhibit little or no thixotropy, rendering them unsuitable for some applications, such as fabrication of certain electronic packages that require formation of silicone deposits on well-defined regions of a substrate. Under the shear conditions typically encountered during application, viscous silicone PSA compositions exhibit a pronounced tendency to adhere to the surface of the dispenser or coating device, forming a continuous string of adhesive between the device and the deposit. This condition can result in contamination of the substrate. Also, viscous silicone compositions typically have lower dispense rates relative to compositions having moderate or low viscosities. On the other hand, low viscosity silicone compositions have a tendency to sag or flow beyond the initial boundaries of the deposit. Furthermore, relatively thick films cannot be readily prepared in a single application from low viscosity silicone compositions.
Although silica having a high surface area, such as fumed or precipitated silica, is typically added to a silicone composition to increase thixotropy, the addition of even small amounts of silica to a silicone composition containing an alumina filler can cause a significant increase in viscosity and attendant decrease in dispense rate.
Consequently, there is a need for a thixotropic addition-curable silicone composition that cures to form a thermally conductive silicone product.
SUMMARY OF THE INVENTION
The present inventors have discovered that an addition-curable silicone composition containing an alumina filler and a polyether has unexpectedly high thixotropy. Specifically, the present invention is directed to a silicone composition for preparing a cured silicone product, the composition comprising:
(A) an organopolysiloxane containing an average of at least two silicon-bonded alkenyl groups per molecule;
(B) an organohydrogenpolysiloxane containing an average of at least two silicon-bonded hydrogen atoms per molecule in a concentration sufficient to cure the composition;
(C) an alumina filler in a concentration sufficient to impart thermal conductivity to the cured silicone product;
(D) an effective amount of a polyether; and
(E) a catalytic amount of a hydrosilylation catalyst.
The present invention is also directed to a cured silicone product comprising a reaction product of the above-described composition.
The present invention is further directed to a multi-part silicone composition for preparing a cured silicone product, the composition comprising component (A) through (E) in two or more parts, provided components (A), (B), and (E) are not present in the same part.
The silicone composition of the present invention has numerous advantages, including adjustable thixotropy, low VOC (volatile organic compound) content, and adjustable cure. Moreover, the silicone composition cures to form a silicone product having good thermal conductivity.
The silicone composition of the instant invention is particularly useful for filling the gap between a heat sink and an electronic device or for attaching a heat sink to an electronic device. The silicone composition can also be used for encapsulating the wire windings in power transformers and converters.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.
DETAILED DESCRIPTION OF THE INVENTION
The silicone composition of the present invention is “thixotropic” or has “thixotropy”, meaning that the composition exhibits a reduction in viscosity when a shearing action is applied and an increase in viscosity upon subsequent rest.
As used herein, the term “thixotropy index” is defined as the ratio of the viscosity of the silicone composition at a shear rate of 0.01 rad/s to the viscosity of the composition at a shear rate of 0.1 rad/s, wherein each viscosity is measured at 23±2° C.
The present invention is directed to a silicone composition for preparing a cured silicone product, the composition comprising:
(A) an organopolysiloxane containing an average of at least two silicon-bonded alkenyl groups per molecule;
(B) an organohydrogenpolysiloxane containing an average of at least two silicon-bonded hydrogen atoms per molecule in a concentration sufficient to cure the composition;
(C) an alumina filler in a concentration sufficient to impart thermal conductivity to the cured silicone product;
(D) an effective amount of a polyether; and
(E) a catalytic amount of a hydrosilylation catalyst.
Component (A), also referred to herein as the “polymer,” is at least one organopolysiloxane containing an average of at least two silicon-bonded alkenyl groups per molecule. The organopolysiloxane can have a linear, branched, or resinous structure. The organopolysiloxane can be a homopolymer or a copolymer. The alkenyl groups typically have from 2 to about 10 carbon atoms and are exemplified by, but not limited to, vinyl, allyl, butenyl, and hexenyl. The alkenyl groups in the organopolysiloxane may be located at terminal, pendant, or both terminal and pendant positions. The remaining silicon-bonded organic groups in the organopolysiloxane are independently selected from monovalent hydrocarbon and monovalent halogenated hydrocarbon groups free of aliphatic unsaturation. These monovalent groups typically have from 1 to about 20 carbon atoms, preferably have from 1 to 10 carbon atoms, and are exemplified by, but not limited to alkyl such as methyl, ethyl, propyl, pentyl, octyl, undecyl, and octadecyl; cycloalkyl such as cyclohexyl; aryl such as phenyl, tolyl, xylyl, benzyl, and 2-phenylethyl; and halogenated hydrocarbon groups such as 3,3,3-trifluoropropyl, 3-chloropropyl, and dichlorophenyl. Preferably, at least 50 percent, and more preferably at least 80%, of the organic groups free of aliphatic unsaturation in the organopolysiloxane are methyl.
The viscosity of the organopolysiloxane at 25° C., which varies with molecular weight and structure, is typically from 0.005 to 100 Pa·s, preferably from 0.05 to 50 Pa·s, and more preferably from 0.05 to 5 Pa·s.
A preferred organopolysiloxane according to the present invention is a polydiorganosiloxane having the formula R
2
R
1
2
SiO(R
1
2
SiO)
a
SiR
1
2
R
2
wherein each R
1
is independently selected from monovalent hydrocarbon and monovalent halogenated hydrocarbon groups free of aliphatic unsaturation, as defined above; R
2
is alkenyl, as defined above; and subscript a has a value such that the viscosity of the polydiorganosiloxane at 25° C. is from 0.005 to 100 Pa·s. Preferably, R
1
is methyl and R
2
is vinyl.
Examples of organopolysiloxanes useful in the silicone composition include, but are not limited to, polydiorganosiloxanes having the following formulae: ViMe
2
SiO(Me
2
SiO)
a
SiMe
2
Vi, ViMe
2
SiO(Me
2
SiO)
0.25a
(MePhSiO)
0.75a
SiMe
2
Vi, ViM
Hirschi David Dean
Lutz Michael Andrew
Dawson Robert
Dow Corning Corporation
Milco Larry A.
Peng Kuo-Liang
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