Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Utility Patent
1999-01-14
2001-01-02
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C528S031000, C528S032000, C525S478000, C524S268000, C524S731000
Utility Patent
active
06169155
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a silicone gel composition and more particularly to an addition-curable silicone gel composition including a specific combination of alkenyl-containing polydiorganosiloxanes. This invention also relates to a silicone gel produced from such composition.
BACKGROUND OF THE INVENTION
Silicone gels have numerous commercial applications due to their unique properties, which include excellent dielectric properties, moisture resistance, thermal stability, vibration dampening properties, and optical transparency. For example, silicone gels are widely used as pottants and encapsulants for preserving and protecting delicate electronic devices in severe environments. Additional applications of silicone gels include optical joint sealants, optical waveguides, keyboard handrests, and clean room HEPA filter sealants. Addition-curable silicone gel compositions comprising an alkenyl-containing polydiorganosiloxane polymer, an organohydrogenpolysiloxane, and a hydrosilylation catalyst are known in the art. Illustrative of such compositions are U.S. Pat. No. 4,374,967 to Brown et al.; U.S. Pat. No. 4,529,789 to Kroupa; U.S. Pat. No. 5,332,795 to Fujiki et al.; U.S. Pat. No. 5,571,853 to Ikeno et al.; U.S. Pat. No. 5,371,163 to Wilson; European Patent Application No. 727462 to Hamada et al.; and European Patent Application No. 509515 to Enami et al. However, the preceding references do not teach the specific combination of polydiorganosiloxanes or the particular organohydrogensiloxane of the present invention.
Furthermore, Japanese Laid Open Patent Publication (KOKAI) SHO 62(1987)-39660 to Mogi et al. discloses a gel composition for optical joints comprising (A) a polyorganosiloxane containing an average of 0.1-2.0 vinyl groups per molecule and having a viscosity of 50-100,000 cP at 25° C.; (B) a siloxane expressed by the general formula R
1
Si(OSiR
2
2
H)
3
where R
1
is an alkyl group having 1 to 4 carbon atoms or a phenyl group, and R
2
is an alkyl group having 1 to 4 carbon atoms, in an amount such that the number of silicon-bonded hydrogen atoms in component (B) per vinyl group in component (A) is 0.1 to 0.8 or 1.5 to 3.0; and (C) a catalyst selected from the group consisting of platinum, palladium, and rhodium catalysts. According to Mogi et al., the composition forms a gel that has excellent adhesion with respect to base materials at ambient temperatures or moderately hot temperatures, does not peel from the base material or undergo cracking due to thermal shock, and undergoes only slight modifications in physical properties and color at high temperatures. However, Mogi et al. neither teach nor suggest the specific combination of polydiorganosiloxanes of the present invention.
Although silicone gels produced from the aforementioned compositions exhibit a wide range of adhesive properties, there is a continued need for silicone gels having improved tack. Importantly, the present inventors have discovered that a silicone gel composition containing a specific combination of alkenyl-containing polydiorganosiloxanes cures to form a gel having unexpectedly superior tack.
SUMMARY OF THE INVENTION
The present invention is directed to a silicone gel composition comprising:
(A) 100 parts by weight of a first polydiorganosiloxane containing an average of at least two silicon-bonded alkenyl groups per molecule and having a viscosity of from 0.2 to 10 Pa·s at 25° C.;
(B) at least about 0.5 part by weight of a second polydiorganosiloxane containing an average of at least two silicon-bonded alkenyl groups per molecule, wherein the second polydiorganosiloxane has a viscosity at 25° C. of at least four times the viscosity of the first polydiorganosiloxane at 25° C.;
(C) an organohydrogensiloxane having the average formula R
7
Si(SiOR
8
2
H)
3
wherein R
7
is an alkyl group having 1 to 18 carbon atoms or aryl, R
8
is an alkyl group having 1 to 4 carbon atoms, in an amount sufficient to provide from 0.1 to 1.5 silicon-bonded hydrogen atoms per alkenyl group in components (A) and (B) combined; and
(D) a hydrosilylation catalyst in an amount sufficient to cure the composition.
The present invention is also directed to a silicone gel comprising a reaction product of the silicone gel composition described above.
The present invention is further directed to a multi-part silicone gel composition comprising components (A) through (D) in two or more parts, provided neither component (A) nor component (B) are present with components (C) and (D) in the same part.
The present invention is still further directed to a kit for preparing a silicone gel composition, comprising a first package containing a portion of component (A), all or a portion of component (B), and all of component (D), and a second package containing the remainder of components (A) and (B), and all of component (C).
The silicone gel composition of the present invention has numerous advantages, including excellent flow, low VOC (volatile organic compound) content, and rapid low temperature cure. Moreover, the present silicone gel composition cures to form a silicone gel having unexpectedly superior tack, excellent thermal stability, and good dielectric properties.
With regard to flow, the present composition possesses the rheological properties required for a number of applications. For example the composition of the instant invention is easily dispensed using standard equipment and exhibits rapid flow around and/or under electronic components.
Further, the composition of the present invention, which does not require an organic solvent to achieve a workable viscosity, has a very low VOC content. Consequently, the present silicone gel composition avoids the health, safety, and environmental hazards associated with solvent-borne silicone compositions. In addition, the solventless composition of the present invention typically undergoes less shrinkage during curing than solvent-borne silicone compositions.
The silicone gel composition of the present invention also cures rapidly at room temperature or moderately elevated temperatures without either an exotherm or the formation of byproducts. In fact, the cure rate of the silicone gel can be conveniently adjusted by controlling the concentration of catalyst and/or optional inhibitor.
Importantly, the silicone gel composition of the present invention cures to form a silicone gel having unexpectedly improved tack relative to gels produced from similar compositions lacking the second alkenyl-containing polydiorganosiloxane, component (B), and/or containing a linear organohydrogensiloxane in place of component (C). Also, only a small amount of the higher viscosity, second alkenyl-containing polydiorganosiloxane is required to produce a significant improvement in tack. Consequently, the second alkenyl-containing polydiorganosiloxane has a minimal effect on the viscosity of the present silicone gel composition.
The excellent self-adhesive properties of the present silicone gel can facilitate repairs, particularly in electronic applications. For example, a section of a silicone gel pottant or encapsulant can be removed to permit repair of an underlying component. Subsequently, the silicone gel composition of the instant invention can be dispensed into the repaired area and cured in place. The repaired region will become an integral part of the original material. Also, in the absence of an optional filler, the silicone gel is transparent, allowing facile visual inspection of protected components and leads.
Furthermore, the silicone gel composition of the present invention cures to form a resilient gel having excellent thermal stability and flexibility in the temperature range from −50 to 150° C. The cured silicone gel retains much of the stress relief and self-healing healing qualities of a liquid, while developing much of the dimensional stability and non-flow characteristics of a solid elastomer. The silicone gel thus provides effective protection from stress and strain caused by thermomechanical shock and vibration.
Additionally, the silicone gel
Alvarez Khristopher Edward
Berry Paul Anthony
Stanga Michael Allen
Strong Michael Raymond
Dow Corning Corporation
Milco Larry A.
Moore Margaret G.
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