Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2001-10-25
2003-09-09
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S323000, C428S328000, C428S329000, C252S06251C, C252S062550, C524S440000, C524S441000, C524S442000, C524S588000
Reexamination Certificate
active
06617038
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electromagnetic wave absorbing silicone rubber compositions having a good electromagnetic wave absorbing ability and flexibility.
BACKGROUND ART
With the advance toward a higher density and higher integration of CPU, MPU, LSI and other components used in electronic equipment such as personal computers and mobile phones, the generation of electromagnetic noise now poses technical and social problems. The traditional countermeasure to electromagnetic disturbances is to use electromagnetic shields made of electroconductive materials to prevent electromagnetic waves from entering the equipment interior and from emanating from within the equipment interior. The electromagnetic shielding of this type can induce malfunctions because electromagnetic waves confined within the equipment interior give rise to electromagnetic interference.
In the prior art, an artisan with specialized knowledge and experience of noise suppression must be engaged in taking a countermeasure against disturbances by electromagnetic interference. It is a time-consuming task to find an effective countermeasure. Another drawback is that an electronic component in question must be previously given an extra space for mounting a shield.
To solve these problems, engineers are interested in electromagnetic absorbers which absorb electromagnetic waves for thereby reducing reflected and transmitted waves. Known electromagnetic absorbers include sintered soft ferrite and composite materials obtained by dispersing soft magnetic powder in matrices such as rubber and resins. The sintered soft ferrite is brittle and difficult to process, and the range of its application is limited because it suffers a sharp decline of its electromagnetic absorbing ability in a high frequency region. On the other hand, the composite materials having electromagnetic wave-absorbing soft magnetic powder dispersed in matrices such as rubber and resins are easy to process, but difficult to fill the soft magnetic powder to a high density, often failing to provide a high electromagnetic wave absorbing ability. Even if a high packing density is achievable, the resulting electromagnetic absorbers become hard and brittle and hence, very difficult to handle. Especially when the soft magnetic powder is of a metal base material such as iron or iron alloy, high packing is difficult because the powder is poorly wettable with silicone.
The same trend toward a higher density and higher integration of CPU, MPU, LSI and other components used in electronic equipment encounters the problem of increased heat release. Ineffective cooling will cause thermal runaway or undesired effects, giving rise to malfunction. One typical means for effectively radiating heat to the exterior is to dispose heat transfer media such as silicone grease and silicone rubber filled with heat conductive powder between CPU, MPU or LSI and heat sinks for reducing the contact thermal resistance therebetween. This means, however, cannot avoid the problem of electromagnetic interference within the equipment interior.
JP-A 2000-101284 discloses an electromagnetic absorber comprising an electromagnetic wave absorbing layer containing soft magnetic particles, a binder and an organic silane compound. It is described nowhere to use rubber as the binder. The composition described therein is effective for increasing the strength, but does not allow for high loading of soft magnetic particles.
When it is desired to have both an electromagnetic wave absorbing ability and a heat transfer ability, a soft magnetic powder and optionally, a heat conductive powder must be dispersed in a matrix such as rubber or resin. To impart a satisfactory electromagnetic wave absorbing ability and a satisfactory heat transfer ability, in particular, the increased loading of such powders is indispensable, but difficult with the state-of-the-art technology.
SUMMARY OF THE INVENTION
An object of the invention is to provide an electromagnetic wave absorbing silicone rubber composition having a satisfactory electromagnetic wave absorbing ability as well as improved workability and flexibility. Another object of the invention is to provide an electromagnetic wave absorbing silicone rubber composition having both a satisfactory electromagnetic wave absorbing ability and a satisfactory heat transfer ability as well as improved workability and flexibility.
It has been found that the above object is achieved by blending a soft magnetic powder, especially a soft magnetic powder of iron or iron alloy in silicone rubber and by further blending a specific surface treating agent therein for allowing the powder to be loaded in a larger amount. The same effect is achievable when a heat conductive powder is additionally blended in the silicone rubber. The surface treating agent used herein is selected from among (a) an organopolysiloxane containing at least one silicon atom-bonded alkoxy radical, silicon atom-bonded hydroxyl radical or functional organic radical in a molecule, (b) a titanate coupling agent, and (c) an aluminum coupling agent.
Specifically, when the surface treating agent selected from the above (a), (b) and (c) is blended in an electromagnetic absorber having a soft magnetic powder, especially a soft magnetic powder of iron or iron alloy, dispersed in silicone rubber, an electromagnetic wave absorbing silicone rubber composition is obtained which possesses a satisfactory electromagnetic wave absorbing ability and is easily workable and flexible.
Also, when both a soft magnetic powder, especially a soft magnetic powder of iron or iron alloy, and a heat conductive powder are dispersed in silicone rubber, and the surface treating agent selected from the above (a), (b) and (c) is blended therein, an electromagnetic wave absorbing silicone rubber composition is obtained which possesses both a satisfactory electromagnetic wave absorbing ability and a satisfactory heat transfer ability and is easily workable and flexible.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Briefly stated, the electromagnetic wave absorbing silicone rubber composition of the invention is arrived at by blending a soft magnetic powder in silicone rubber and further blending a surface treating agent. A preferred embodiment of the composition is arrived at by blending a soft magnetic powder and a heat conductive powder in silicone rubber and further blending a surface treating agent. In this embodiment, the composition in the cured state preferably has a thermal conductivity of at least 2.0 W/mK. The surface treating agent used herein is selected from among (a) an organopolysiloxane containing at least one silicon atom-bonded alkoxy radical, silicon atom-bonded hydroxyl radical or functional organic radical in a molecule, (b) a titanate coupling agent, and (c) an aluminum coupling agent.
The soft magnetic powder in the electromagnetic wave absorbing silicone rubber composition is preferably iron or an iron alloy. Soft magnetic materials are generally divided into ferrite base materials and metal base materials. The ferrite base materials exhibit a good electromagnetic wave absorbing ability only in a relatively low frequency region and so, their application is somewhat limited. Then the metal base materials are preferable. Among the metal base materials, iron and iron alloys are more preferable because they keep a good electromagnetic wave absorbing ability up to a relatively high frequency side and are inexpensive. Illustrative, non-limiting, examples of the iron alloy include Fe—Cr, Fe—Si, Fe—Ni, Fe—Al, Fe—Co, Fe—Al—Si, Fe—Cr—Si, and Fe—Si—Ni alloys. The soft magnetic powder may be of one type or a mixture of two or more types. The soft magnetic powder particles may be either of flat or granular shape or a mixture thereof.
The soft magnetic powder (particles) should preferably have a mean particle size of about 0.1 &mgr;m to about 100 &mgr;m and especially about 1 &mgr;m to about 50 &mgr;m. Particles with a particle size of less than 0.1 &mgr;m have too large a specific surface area, probably failing to achie
Sakurai Ikuo
Suzuki Akio
Birch & Stewart Kolasch & Birch, LLP
Dawson Robert
Keehan Christopher M.
Shin-Etsu Chemical Co. , Ltd.
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