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
2001-09-27
2002-09-03
Moore, Margaret G. (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...
C524S433000, C524S588000, C524S788000, C428S447000, C427S387000
Reexamination Certificate
active
06444740
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention provides silicone compositions, with cured elastomers thereof demonstrating improved resistance to oil and improved adhesion retention.
2. Brief Description of Related Technology
Silicone compositions have excellent sealing and adhesive properties, and have many commercial uses. For instance, they can be formed into gaskets which are used extensively in the automotive industry or applied as conformal coatings in electronics applications.
In use, silicone compositions may become exposed to a variety of conditions, and should continue to function without compromised integrity. One such condition includes exposure to oil.
Oil resistant silicone sealant compositions generally are known. In particular, U.S. Pat. No. 4,514,529 (Beers) generally discloses a low modulus, high elongation RTV silicone composition having oil resistance. This composition may include a devolatilized silanol-terminated diorganosiloxane of 2,000 to 250,000 Cst, a devolatilized diorganosiloxane fluid plasticizer terminated with triorganosiloxy groups, a cross-linking agent, a catalyst and a filler. Articles formed from such a composition can be used as, e.g., gasket sealants, as well as formed-in-place gaskets for use on internal combustion engines.
U.S. Pat. No. 4,673,750 (Beers), U.S. Pat. No. 4,735,979 (Beers) and U.S. Pat. No. 4,847,396 (Beers) generally disclose adhesion promoter compositions for use in auto-adhering, one-component RTV silicone sealant systems having oil resistance. The adhesion promoters set forth in these patents include glycidoxyalkyl substituted mixed alkoxy-oxime silanes and di-substituted mixed oxime-alkoxysilylalkyl ureas, respectively.
The RTV silicone compositions which contain these oxime adhesion promoters generally include polydimethylsiloxanes, trimethylsilyl terminated polydimethylsiloxanes and various other fillers, additives and catalysts. Such compositions may be used to make, e.g., formed-in-place gasket materials.
International PCT Publication No. WO 93/19130 discloses a one-part RTV silicone rubber composition as a formed-in-place gasket having oil resistant properties. Such a composition includes a diorganopolysiloxane, an M-stopped plasticizer, such as a trimethyl-terminated non-reactive silicone composition, gamma-aminopropyltriethoxysilane, a catalyst, a crosslinker and various fillers.
One drawback to the RTV silicone compositions above is their rate of cure, which is commercially unacceptable for certain applications, such as sealing electronic modules, where high volume production may depend upon cure rate. Accordingly, silicone compositions with improved cure rates are desirable.
In addition, inclusion of certain grades of metal oxides to silicone elastomers is known to result in silicone rubber compositions having a certain degree of oil resistance. For example, European Patent Publication No. 572 148 assigned to General Electric Company discloses the incorporation of mixed metal oxides into heat cured silicone elastomeric compositions containing MQ resins (M represents R
3
SiO
1/2
monofunctional groups; Q represents SiO
2
quadri-functional groups). In the '148 publication, such compositions are formed into engine gaskets which are reported to display a certain degree of oil resistance. Magnesium oxide is disclosed as one component of a mixture of metal oxides from group (IIa) and (IIb). The '148 publication, however, is silent as to the benefits, if any, conveyed by the use of a single metal oxide on the oil resistance of the final elastomer.
U.S. Pat. No. 5,082,886 also assigned to General Electric Company describes liquid injection molded (“LIM”) silicone compositions containing magnesium oxide to allegedly impart oil resistance to the elastomeric product. The use of magnesium oxide in the LIM system, however, adversely affected the compression set imparted by the platinum catalyst. Thus, to counteract this affect, cerium hydroxide or tetramethyldivinyldisilane was also added. This additional reagent not only adds another step and ingredient to the process but also increases the cost of the final product.
U.S. Pat. No. 4,052,357 assigned to Dow Corning Corporation describes a silicone rubber composition used as a seal or gasket. This composition includes a polydiorganosiloxane gum, a reinforcing silica filler, a hydroxylated diorganosilicone, fiberized blast furnace slag fibers and an alkoxy silicone. It is suggested by the '357 patent that the addition of magnesium oxide to this composition imparts a certain degree of oil resistance thereto. Such a composition is inconvenient and more expensive to produce because of its requirement for fiberized blast furnace slag fibers. Moreover, the presence of such fibers decreases the tear strength of the end product.
These patent documents describe the use of magnesium oxide to impart a certain level of oil resistance to various types of silicone elastomers. These silicone elastomers, however, suffer from the draw back that the oil resistance conveyed by the magnesium oxide is of marginal utility because the physical characteristics of the magnesium oxide used is not optimized for the desired oil resistant property.
To date, it is not believed that a filler has been included in such addition curable silicone composition for the purpose of conferring oil resistance to the cured elastomer.
In addition, it would be desirable for a sealant composition to be imparted with improved oil resistance by optimizing the physical characteristics of the magnesium oxide particles dispersed therein.
SUMMARY OF THE INVENTION
The present invention provides silicone compositions, with cured elastomers thereof demonstrate improved resistance to oil and improved adhesion retention. The compositions include broadly a reactive silicone component, where the silicone component cures by way of an addition cure mechanism or a condensation cure mechanism; and a basic filler component to confer oil resistance to the cured elastomer. Where the reactive silicone component cures by way of an addition cure mechanism, an addition cure catalyst is also provided.
In one aspect of the invention, therefore, the compositions include an addition curable silicone component, a basic filler component to confer oil resistance to the cured elastomer and an addition cure catalyst.
In one embodiment of this aspect of the present invention, it has been found surprisingly that addition curable silicone compositions may be rendered resistant to oil by including basic fillers, such as inorganic basic fillers like certain carbonates with or without certain oxides, such as calcium carbonate and magnesium oxide.
In another embodiment of this aspect of the invention, the compositions includes a hydroxy-terminated diorganopolysiloxane, a precipitated calcium carbonate, at least about 5% by weight of a composition including magnesium oxide particles having a mean particle size of about 0.5 &mgr;M to about 1.5 &mgr;M and a mean surface area of about 50 M
2
/g to about 175 M
2
/g and a crosslinker.
In this regard, it has also surprisingly been found that by incorporating a particular grade of magnesium oxide within a particular range into a silicone composition that remarkably superior oil resistance can be imparted to the cured elastomer formed therefrom compared to known silicone compositions having magnesium oxide particles with physical characteristics which do not optimize oil resistance in the cured elastomer. Such compositions are especially useful as gear oil resistant articles.
The invention also provides a method of applying silicone compositions as described above to a surface which would be expected to be exposed to oil during its intended use.
The invention also provides methods of improving oil resistance in a silicone sealant. These methods include providing the silicone sealant, and incorporating into the sealant a filler component to confer oil resistance to the cured elastomer.
The invention also provides elastomeric silicone articles having improved oil resistanc
Bennington Lester D.
DeCato Alfred A.
Hoffmann & Baron , LLP
Loctite Corporation
Moore Margaret G.
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