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
1999-04-29
2001-05-29
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...
C524S282000, C524S588000
Reexamination Certificate
active
06239202
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to elastomers. In particular, the present invention relates to an additive for improving the heat age properties of elastomers, including hardness, tensile strength, elongation, and modulus.
Elastomer compositions are known in the art. For example, heat-cured elastomers, particularly silicone elastomers, are very useful in high temperature sealing applications such as electrical wires and cables, automotive and oven gaskets, hot air ducts, window hoods, and the like. In these applications the heat age properties of the elastomer are of primary importance for optimal function. As used herein, “heat age properties” refers to the degree to which high or low temperature affects the elastomer's physical properties, including compression set, elongation, hardness, modulus, and tensile strength. For many applications significant changes in these properties with temperature or in response to temperature could result in failure of the material to meet the requisite performance specifications.
Heat age properties of silicone elastomers are generally dependent on at least three factors: the composition and quantity of the filler; the composition and quantity of any processing aids present in the formulation (processing aids affect the heat age properties of the elastomer by modulating the polymer-filler interactions), and the crosslink density or the state of cure of the polymer phase.
Heat age properties are particularly important in gasket and other sealing applications. Prior art attempts to improve silicon-based gasket materials have focused on improving compression set or sealing force retention. For example, U.S. Pat. No. 5,346,940 to Brassard et al. and U.S. Pat. No. 5,623,028 to Fitzgerald et al. disclose a method for controlling the sealing force retention. There are other examples in the literature describing heat age additives for silicone elastomers, e.g., TiO
2
, iron octoate, Ce(OH)
4
and rare earth octoates. Nonetheless, there remains a need in the art for methods for further improving the heat age properties of elastomers.
SUMMARY OF THE INVENTION
In one of its aspects the present invention is a pre-elastomer composition for the manufacture of an elastomer having improved heat age properties, comprising: (a) a curable polyorganosiloxane composition; (b) a catalyst in an amount effective for promoting cure of the pre-elastomer composition; (c) a filler composition; and (d) ammonium carboxylate. Typically, ammonium carboxylate is present in an amount in the range from about 5 ppm to about 1 weight percent of total polymer composition. In another of its aspects the invention is an elastomer prepared from said pre-elastomer composition. In yet another of its aspects the invention is a method for enhancing the heat age properties of an elastomer which comprises combining a pre-elastomer composition with ammonium carboxylate.
DETAILED DESCRIPTION OF THE INVENTION
The present composition is a curable pre-elastomer composition comprising an ammonium carboxylate, and the elastomer formed therefrom. Use of ammonium carboxylate in curable pre-elastomer compositions results in significant improvement in the heat age properties of the elastomer, which is suitable for both high- and low-temperature sealing applications.
Siloxane-based pre-elastomer compositions for forming elastomers are well known in the art, generally comprising at least one polyorganosiloxane with either vinyl-on-chain moieties or vinyl end-group moieties, or both. Vinyl-on-chain moieties refer to those vinyl groups present on polyorganosiloxanes at a monomer position other than an end-group. In especially preferred embodiments vinyl-on-chain moieties are vinylmethylsiloxy groups, and vinyl end-group moieties are dimethylvinylsiloxy groups. The term end-group moieties within the context of the present invention is synonymous with the term end-stopped moieties.
Preferred siloxane pre-elastomer compositions comprise at least one member selected from the group consisting of:
(a) at least one vinyl end-stopped and vinyl-on-chain substituted polyorganosiloxane having the following formula:
M
vi
D
x
vi
D
y
M
vi
wherein x and y are independent integers greater than zero and the sum of x and y has values whereby the viscosity of (a) is between 200,000 and 200,000,000 centipoises (cps), and the alkenyl level varies from about 0.005 weight percent to about 14 weight percent based on (a);
(b) at least one vinyl end-stopped polyorganosiloxane gum having the following formula:
M
vi
D
z
M
vi
wherein z is an integer greater than zero having a value whereby the viscosity of (b) is between 10,000 and 150,000,000 cps, and the alkenyl level varies from about 8 to about 2000 parts per million based on (b); and
(c) at least one vinyl-on-chain polyorganosiloxane gum having non-reactive end groups with the following formula
MD
vi
q
M
wherein q is an integer greater than zero whereby the viscosity of (c) is between 200,000 and 200,000,000 cps, and the alkenyl level varies from about 0.01 weight per cent to about 14 weight percent based on (c).
Pre-elastomer compositions of the present invention may also include as an optional component:
(d) a diluent polyorganosiloxane gum having the following formula:
MD
w
M
wherein w is an integer greater than zero whereby the viscosity of (d) is between 200,000 and 200,000,000 cps.
All of the polyorganosiloxane gum components utilized in the present invention have a viscosity ranging from about 10,000 to about 200,000,000 centipoises at 25° C. The amounts of the above components (a)-(c) (optionally in combination with (d)) may be widely varied to produce the pre-elastomer composition, and any particular gum (a), (b), (c), (d) may be completely absent from the polymer system. Any of components (a), (b), and (c), when they are present, typically comprise from about 1 to about 140 parts, preferably from about 1 to about 120 parts, and more preferably from about 2 to about 80 parts by weight of the composition. In especially preferred embodiments the combination of (a)'s, (b)'s, and (c)'s, taken together comprise from about 40 to about 240 parts, preferably from about 60 to about 160 parts, and more preferably from about 80 to about 120 parts by weight of the composition. Component (d) is typically used in an amount from 0 to about 80 parts by weight of the composition. Preferably, the total alkenyl level of the gum blend is in the range from 0.01 to about 0.5 weight percent of the composition.
In the foregoing formulae,
M is R
1
3
SiO
0.5
wherein R
1
is selected from the group consisting of alkyl groups having from 1 to 8 carbons, phenyl, and trifluoropropyl;
M
vi
is R
2
(R
1
)
2
SiO
0.5
wherein R
1
is selected from the group consisting of alkyl groups having from 1 to 8 carbons, phenyl, and trifluoropropyl, and R
2
is selected from the group consisting of linear or cyclic alkenyl groups having from 2 to 10 carbons;
D
vi
is R
2
R
1
SiO
wherein R
1
and R
2
are as previously defined; and
D is (R
3
)
2
SiO
wherein each R
3
is independently selected from the group consisting of alkyl groups having from 1 to 8 carbons, phenyl, and trifluoropropyl. Polyorganosiloxanes containing mixtures of D groups, such as, but not limited to, a combination of (dialkyl-SiO) and (alkyl-trifluoropropyl-SiO), are also within the scope of the invention.
In addition to the above components, the pre-elastomer compositions may comprise at least one optional process aid, generally a lower molecular weight polyorganosiloxane of the general formula
M
k
D
p
M
k
(e)
wherein p is an integer greater than zero whereby the viscosity of (e) is between about 4 and about 300 cps, wherein (e) is used in an amount of from 0 to about 30 parts, and preferably from about 0.1 to about 10 parts by weight of the composition. In the above formula, D is as defined above, and
M
k
is R
4
(R
1
)
2
SiO
0.5
wherein R
1
is as defined above, and R
4
is selected from the group consisting hydrogen, hydroxyl, or alkoxy radicals such as methoxy or ethoxy.
Mor
Doin James Edward
Evans Edwin Robert
Osaheni John Aibangbee
General Electric Company
Johnson Noreen C.
Moore Margaret G.
Stoner Douglas E.
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