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
2000-06-23
2002-04-09
Tucker, Philip (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...
C524S847000, C525S477000, C525S478000, C528S012000, C528S024000, C528S015000, C528S032000, C277S910000
Reexamination Certificate
active
06369155
ABSTRACT:
This invention relates to fluorosilicone rubber compositions curing into products having improved swell resistance upon immersion in fuel oil and lubricating oil as well as satisfactory compression recovery and mechanical strength and suitable for use as seals against such oils as fuel oil and lubricating oil.
BACKGROUND OF THE INVENTION
Because of heat resistance, freeze resistance, oil resistance, fuel oil resistance, and compression recovery, fluorosilicone rubber is widely used as parts for automobiles, aircraft and other transporting vehicles and parts for petroleum-related equipment.
When rubber parts molded into gaskets and O-rings are kept immersed in fuel oil or lubricating oil during their service as seals, they will swell to increase their volume. If this volume increase is substantial, the rubber part lies out of the sealing area and in an extreme case, is disengaged from the sealing area, failing to provide a sealing function. Also substantial swelling can cause further deformation of the rubber part which has been caulked for seal, and a loss of strength, leading to failure of the part.
It is then necessary to select, as the sealing material against fuel oil and lubricating oil, rubber of the type experiencing minimal swell with the oil to be sealed. For the sealing against lubricating oil, silicone rubber, acrylic rubber, chloroprene rubber, nitrile rubber, hydrin rubber and fluororubber are typically used. For the sealing against fuel oil, nitrile rubber, hydrin rubber, fluororubber and fluorosilicone rubber are typically used.
Of these sealing materials, fluorosilicone rubber is useful since it has improved oil resistance and fuel oil resistance as well as excellent heat resistance, freeze resistance and compression recovery. However, the fluorosilicone rubber has the problem that upon immersion in fuel oil, it swells to a somewhat greater extent than fluororubber used in the same application.
One common practice for reducing the swell of fluorosilicone rubber is to add a large amount of filler. More particularly, when reinforcing silica such as fumed silica or precipitated silica is added in a large amount, rubber hardness increases beyond the practical upper limit and the resulting rubber composition becomes less workable. When non-reinforcing silica such as quartz flour or diatomaceous earth is added in a large amount, the fluorosilicone rubber loses satisfactory mechanical strength such as tensile strength or tear strength.
Increasing a crosslink density is also effective for reducing the swell. With this approach, however, the fluorosilicone rubber increases its hardness beyond the practical upper limit and becomes brittle, losing mechanical strength such as tensile strength or tear strength.
U.S. Pat. No. 5,483,000 and JP-A 6-116498 disclose a further method of blending fluorosilicone rubber with fluororubber for reducing the swell. However, fluorosilicone rubber and fluororubber are rather less miscible with each other. Even when blended, cured parts of their blend do not have sufficient mechanical strength such as tensile strength or tear strength and will delaminate during service. In addition, although fluorosilicone rubber has excellent freeze resistance as compared fluororubber, their blend has as poor a freeze resistance as fluororubber.
None of the above-described methods are satisfactory in improving the swell resistance of fluorosilicone rubber upon immersion in fuel oil.
SUMMARY OF THE INVENTION
An object of the invention is to provide a fluorosilicone rubber composition which cures into a product having improved swell resistance upon immersion in fuel oil as well as satisfactory compression recovery and mechanical strength and which is suitable for use as seals against such oils as fuel oil and lubricating oil.
The invention pertains to a fluorosilicone rubber composition comprising an organopolysiloxane of the average compositional formula (1) to be defined below having a viscosity of at least 10,000 centistokes at 25° C., a microparticulate silica filler, and a curing agent. To the composition is added a specific amount of a linear organopolysiloxane oil of the general formula (2) to be defined below having trifluoropropylmethylsiloxy groups in its backbone and no crosslinked points in its molecule. The resulting composition cures into fluorosilicone rubber experiencing minimized swell upon immersion in fuel oil and having satisfactory mechanical strength. The invention is predicated on this finding.
Accordingly, the invention provides a fluorosilicone rubber composition comprising
(1) 100 parts by weight of an organopolysiloxane represented by the following average compositional formula (1):
R
1
a
R
2
b
R
3
c
SiO
(4−a−b−c)/2
(1)
wherein R
1
is trifluoropropyl, R
2
is a substituted or unsubstituted monovalent aliphatic unsaturated hydrocarbon group of 2 to 8 carbon atoms, R
3
is an unsubstituted, aliphatic unsaturation-free, monovalent hydrocarbon group of 1 to 8 carbon atoms, a is a number of 0.98 to 1.01, b is a number of 0.0001 to 0.01, c is a number of 0.98 to 1.01, and a+b+c is 1.98 to 2.02, and having a viscosity of at least 10,000 centistokes at 25° C.,
(2) 5 to 100 parts by weight of a microparticulate silica filler,
(3) 0.5 to 20 parts by weight of an organopolysiloxane represented by the following general formula (2):
wherein R
4
to R
6
are independently selected from substituted or unsubstituted, aliphatic unsaturation-free, monovalent hydrocarbon groups of 1 to 8 carbon atoms, p is a number of 0 to 50, q is a number of 4 to 100, p+q is from 4 to 100, and q/(p+q) is at least 0.7, and
(4) an effective amount of a curing agent.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A first component of the fluorosilicone rubber composition according to the invention is an organopolysiloxane represented by the following average compositional formula (1).
R
1
a
R
2
b
R
3
c
SiO
(4−a−b−c)/2
(1)
Herein R
1
is trifluoropropyl. R
2
stands for substituted or unsubstituted monovalent aliphatic unsaturated hydrocarbon groups of 2 to 8 carbon atoms, preferably 2 to 4 carbon atoms, for example, alkenyl groups such as vinyl and allyl, with vinyl being preferred. R
3
stands for unsubstituted, aliphatic unsaturation-free, monovalent hydrocarbon groups of 1 to 8 carbon atoms, preferably 1 to 6 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl and butyl, aryl groups such as phenyl and tolyl and aralkyl groups such as benzyl. Letter a is a number of 0.98 to 1.01, b is a number of 0.0001 to 0.01, c is a number of 0.98 to 1.01, and a+b+c is 1.98 to 2.02.
The organopolysiloxane of formula (1) should have a viscosity of at least 10,000 centistokes (cs) at 25° C., preferably at least 50,000 cs at 25° C., and more preferably at least 100,000 cs at 25° C., so that the silicone rubber obtained by curing the composition may maintain strength. The upper limit of viscosity is not critical, and the organopolysiloxane may be even gum-like.
The organopolysiloxane of the general formula (1) can be prepared, for example, by effecting ring-opening polymerization of tri(trifluoropropyl)trimethylcyclo-trisiloxane using a silisan oligomer shown below as an initiator, as disclosed in U.S. Pat. No. 5,059,668.
A second component is a microparticulate silica filler. For a practically acceptable mechanical strength, the silica should preferably have a specific surface area of at least 50 m
2
/g, and more preferably 100 to 400 m
2
/g as measured by the BET method. Exemplary silica fillers are fumed silica, fired silica and precipitated silica, alone or in admixture of two or more. These silica fillers may be surface treated with surface treating agents such as chain organopolysiloxanes, cyclic organopolysiloxanes, organochlorosilanes, and hexamethyldisilazane.
An appropriate amount of the microparticulate silica filler blended is 5 to 100 parts, preferably 10 to 50 parts by weight per 100 parts by weight of the first component, organopolysiloxane. Outside the range
Millen White Zelano & Branigan P.C.
Peng Kuo-Liang
Shin-Etsu Chemical Co. , Ltd.
Tucker Philip
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