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
1999-10-12
2002-07-09
Seidleck, James J. (Department: 1711)
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...
C524S777000, C524S706000, C524S773000, C524S701000, C524S779000, C524S788000, C524S786000
Reexamination Certificate
active
06417263
ABSTRACT:
For which the following is a specification:
The present invention is a heat curable silicone rubber composition with high resistance to degradation by engine oils and coolants, and in particular, resistance to synthetic engine oils and long-life engine coolants.
Gaskets and packing materials formed from silicone rubber frequently suffer from poor resistance to hot hydrocarbon oils, for example, engine oil and gear oil, and from a poor resistance to radiator coolants. As a consequence, oil and coolant leaks may develop during the long-term use of silicone rubber as gasket materials in such applications.
Synthetic engine oils have as major components poly-alpha-olefins and esters which may break down to acids. Synthetic engine oils may also contain lesser amounts of additives such as oxidation inhibitors, rust inhibitors, anti-wear and extreme pressure agents, friction modifiers, detergents and dispersants, pour-point depressants, viscosity improvers, and foam inhibitors. These components of synthetic oils may interact with silicone rubber differently than do hydrocarbon oils, adversely impacting the sealing properties of the rubber.
Similarly, extended-life coolants may contain organic acids, such as aliphatic monobasic acids, hydrocarbyl dibasic acids, and the alkali metal, ammonium or amine salts of monobasic acids or hydrocarbyl dibasic acids as components that may interact with silicone rubber, in addition to conventional additives such as ethylene glycol, water, and corrosion inhibitors. See for example, U.S. Pat. No. 4,647,392 to Darden, et al. The acids and salts can attack the silicone rubber.
Fluorosilicone rubbers are generally known in the art for their resistance to fuel, oil, chemicals, and solvents. However, fluorosilicone rubbers are relatively costly materials, and not considered to be cost effective in many applications involving contact with engine oils and coolants. Therefore, there is a need to improve the performance of non-fluorinated silicone rubbers in contact with engine oils and coolants.
Inoue et al., in U.S. Pat. No. 4,689,363, teach compositions for room-temperature curable silicone rubber that are resistant to conventional engine oils. The compositions comprise 100 parts by weight of a hydroxy end-terminated polydiorganosiloxane having a linear molecular structure; from 1 to 25 parts by weight of an organosilicone having, in each molecule, at least two hydrolyzable groups bonded to the silicon atom or atoms; and from 1 to 50 parts of an alkali metal salt of a weak acid having a pK
a
in the range from 2.0 to 12.0 at 25° C. The polyorganosiloxane has a viscosity in the range of 25 to 500 Pa·s or preferably from 1 to 100 Pa·s at 25° C.
Koshii et al., in U.S. Pat. No. 5,013,781, teach compositions for room-temperature curable silicone rubber resistant to conventional coolants and hydrocarbon oils. A polyorganosiloxane composed of R′
3
SiO
0.5
and SiO
2
units or R′
3
SiO
0.5
, R
2
′SiO and SiO
2
units is included at 1 to 50 weight parts in a composition containing 100 parts polydiorganosiloxane, 5 to 300 weight parts inorganic filler, 0.1 to 10 weight parts alkoxysilane adhesion promoter, and a ketoxime silicon compound crosslinker. Koshii et al. teach that the polyorganosiloxane functions in combination with the alkoxysilane adhesion promoter to improve the hydrocarbon oil and coolant (chemical) resistance of room-temperature curable silicone rubber. The molar ratio of the R′
3
SiO
0.5
to SiO
2
in the polyorganosiloxane must be from 0.5:1 to 1.5:1. The polydiorganosiloxane is a flowable polymer, and has a viscosity within the range of 0.0001 to 0.1 m
2
/s at 25° C., and the chain terminals contain a silicon-bonded hydroxyl group or a silicon-bonded hydrolyzable group.
The approaches by Inoue et al. and Koshii et al. do not address the need for silicone compositions that are used in contact with synthetic engine oils or extended-life coolants. Furthermore, while these approaches are useful for room-temperature-vulcanizable compositions, they do not address the additional need for heat-curable silicone compositions with improved resistance to coolants and oils. Heat curable silicone rubbers are used in applications that experience much higher stress, and may possibly be exposed to higher temperature and pressure and harsher chemical environments. For example, a heat curable silicone rubber may be used in applications requiring tensile strengths of from about 60 to 106 kg/cm
2
, while room-temperature-vulcanizable compositions are more typically useful at lower tensile strengths from about 10 to 35 kg/cm
2
. More particularly, heat cured silicone rubber is used in engine and coolant system applications where such systems are under heat or pressure, and properties such as compression set and compression stress relaxation are of concern. Furthermore, heat curable silicone rubbers often have larger cross-sectional areas exposed to chemical agents, compared to room-temperature-vulcanizable silicone compositions, which are typically used in thin gaskets. This additional cross-sectional area exposes more surface to chemical attack. Further, as gaskets are exposed to temperature cycling, they tend to swell in oil or coolant when heated, and shrink when cooled. Thus, increased cross-sectional area of heat-cured silicone rubber gaskets significantly increases the chemical exposure of the entire gasket.
Therefore, a non-fluorinated, heat-curable silicone rubber composition is needed that is resistant to standard and long-term engine coolants, and standard hydrocarbon and synthetic motor oils.
DESCRIPTION OF THE INVENTION
This invention is a heat-curable silicone rubber composition comprising:
(A) 100 parts by weight of an organosiloxane polymer base comprising an organosiloxane polymer containing at least 2 silicon-bonded alkenyl groups in each molecule and about 1 to 65 weight percent reinforcing silica filler,
(B) curing component sufficient to cure the composition when heated, and
(C) an effective amount of at least one metal salt additive selected from the group consisting of
monobasic alkali metal phosphates, alkali metal oxalates, alkali metal tartrates, alkali metal tetraborates, alkali metal phthalates, and alkali metal citrates;
dibasic metal phosphates where the metal is selected from the group consisting of sodium, potassium, calcium and magnesium;
metal acetates, where the metal is selected from the group consisting of sodium, potassium, calcium, and magnesium;
metal sulfates where the metal is selected from the group consisting of sodium, potassium, calcium, magnesium, aluminum, and zinc; and
metal carbonates where the metal is selected from the group consisting of sodium, potassium, calcium, magnesium, aluminum and zinc.
The compositions of this invention provide superior compression set and compression stress relaxation results over other silicone compositions.
Component A, the organosiloxane polymer base (the base), comprises a mixture of an organosiloxane polymer with reinforcing silica filler. The organosiloxane polymer in the base has the average composition of R
a
SiO
(4-a)/2
. In the formula R is selected from substituted and unsubstituted monovalent hydrocarbon groups and is exemplified by alkyl groups such as methyl, ethyl, and propyl; alkenyl groups such as vinyl, allyl, butenyl, and hexenyl; aryl groups such as phenyl; and aralkyls such as 2-phenylethyl. The subscript a is a value from 1.95 to 2.05.
The organosiloxane polymer has at least 2 silicon-bonded alkenyl groups in each molecule. The alkenyl groups can be bonded in pendant positions, at the terminal positions, or at both positions. The molecular structure of the organosiloxane polymer generally has a degree of polymerization (dp) in the range of from 200 to 20,000, and preferably has a dp in a range of 1000 to 20,000. This dp range includes polymers which are thick, flowable liquids as well as those that have a stiff, gum-like consistency. The organosiloxane polymer can be a homopolymer or a copolymer or a mixture of such polymers. The siloxy units com
DeGroot, Jr. Jonathan Vierling
Fielder Lawrence Dale
Schulz, Jr. William James
Wright Antony Pope
Boley Willaim F.
De Cesare Jim L.
Dow Corning Corporation
Rajguru U. K
Seidleck James J.
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