Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
2000-07-27
2002-11-12
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...
C528S042000, C524S431000, C524S588000
Reexamination Certificate
active
06479610
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to fluorosilicone cross-linkers and their use in room temperature condensation curable organo(fluoro)polysiloxane compositions useful in the preparation of solvent resistant sealants.
Since the early 1950's when integral fuel tanks became a common structural feature of aircraft, fuel resistant sealing materials, such as polysulfide polymers, were used to contain the fuel. Initially, polysulfide polymers were employed because of their excellent fuel resistance. In addition to fuel resistance, elongation, flexibility, and tensile strength are additional properties needed in a fuel tank sealant. For example, upon cure, a typical fuel tank sealant may show at room temperature, typical values in a range between about 250 and about 300 elongation (%), a tensile strength (psi) of about 150, and a 100% modulus (psi) in a range between about 50 and about 100. An evaluation of available fuel resistant sealants showed that fluorinated organopolysiloxane polymers possess many desirable properties. However, commercially available fluorosilicone sealants are often based on a one-package moisture curable acetoxy cure system which results in the generation of corrosive volatiles, such as acetic acid.
A fluorosilicone composition having a neutral condensation cure system is shown by Fujiki, U.S. Pat. No. 5,236,997, who employs a fluorine containing polydiorganosiloxane base polymer having a reduced level of silicon bonded, fluorine containing substituent groups in the terminal position. Fujiki resorts to the synthesis of a special fluorosilicone base polymer substantially free of bulky fluorine containing organic groups in the terminal positions. The resulting chain-end modification to make curable polymers requires an additional step in the process. In addition, any reduction in the fluorine level in the base polymer typically results in a decrease in fuel resistance performance.
With the need for fuel tank sealants which do not corrode, neutral condensation cure systems for commercially available silanol terminated fluorine containing polydiorganosiloxane base polymers which do not require any alteration in the fluorine content are constantly being sought.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a neutral, room temperature condensation curable, fluorosilicone sealant composition comprising:
(A) a silanol terminated fluoroalkyl substituted polydiorganosiloxane,
(B) an oligomeric fluorosilicone crosslinker,
(C) filler, and
(D) an effective amount of a condensation catalyst,
A further embodiment of the present invention provides an oligomeric fluorosilicone crosslinker of the formula,
(R
2
O)
m
(R)
n
SiO—[R(R
1
)SiO]
x
—Si(R)
n
(OR
2
)
m
, (2)
where R is a C
(1-12)
organo radical, R
1
is a C
(3-8)
fluoroalkyl radical, R
2
is a C
(1-12)
alkyl radical, “m” is an integer having a value of 2 or 3, “n” is an integer having a value of 0 or 1, and the sum of “m+n” is equal to 3, and “x” is an integer in a range between about 3 and about 30 inclusive.
A further embodiment of the present invention provides a method for making an oligomeric fluorosilicone crosslinker having terminal polyalkoxysiloxy units which comprises effecting reaction between a silanol terminated polyfluoroalkylsilicone fluid and a polyalkoxysilane in the presence of an effective amount of a Platinum Group Metal catalyst.
DETAILED DESCRIPTION OF THE INVENTION
In preparing the neutral, room temperature condensation curable, solvent resistant sealant compositions of the present invention, it is preferred to initially prepare a “paste” in the form of a substantially uniform blend of a filler and a silanol terminated fluoroalkyl substituted polydiorganosiloxane. “Neutral” as used herein refers to a sealant composition which is substantially acid-free and substantially base-free. Additional components, such as a heat stabilizer can be added. Any number of methods for blending said components known in the art may be utilized, such as batchwise shearing in a double planetary, change-can type mixer. Preferably, the paste is prepared in a continuous fashion on a devolatilizing, counter-rotating, non-intermeshing twin screw extruder, as taught in U.S. Pat. No. 4,528,324, U.S. Pat. No. 5,354,833, and U.S. Pat. No. 5,514,749. Blending of the ingredients is typically carried out using external heating at temperatures in a range between about 50° C. and about 200° C., preferably in a range between about 100° C. and about 150° C. A vacuum also can be used on the paste to degas, deaerate, or combinations thereof to achieve a substantially uniform blend.
Commercially available silanol terminated fluoroalkyl substituted polydiorganosiloxanes having a viscosity at about 25° C. in a range between about 6×10
4
centipoise and about 1.6×10
5
centipoise can be used in the practice of the invention to make the neutral, room temperature condensation curable, solvent resistant sealant compositions. While the silanol terminated fluoroalkyl substituted polydiorganosiloxanes preferably include chemically combined trifluoropropylmethylsiloxy units, other alkylfluoroalkylsiloxy units also can be present, such as different C
(1-12)
alkyl radicals, for example, radicals, such as methyl, ethyl, propyl , and butyl and phenyl, and other C
(3-8)
fluoroalkyl units. The silanol terminated fluoroalkyl substituted polydiorganosiloxane comprises organofluorosiloxy units of formula (1),
R(R
1
)SiO—, (1)
where R is a C
(1-12)
organic radical, and R
1
is a C
(3-8)
polyfluoroalkyl radical. The silanol terminated fluoroalkyl substituted polydiorganosiloxane is typically present at about 80 parts per 100 parts by weight of the total sealant composition.
Some of the condensation catalysts which can be used in the neutral condensation curable room temperature fluorosilicone sealant compositions of the present invention, include but are not limited to, dibutyltin diacetate, dimethyltin neodecanoate, dibutyltin dilaurate, stannous octoate, dimethyltin hydroxyoleate, or combinations thereof. An effective amount of the condensation catalyst is in a range between about 0.1 part and about 5.0 parts by weight per 100 parts by weight of sealant composition, and preferably in a range between about 0.1 part and about 1.0 parts by weight per 100 parts by weight of sealant composition.
While fumed silica is preferably used in the sealant composition as a reinforcing filler, extending fillers, such as diatomaceous earth, precipitated silica, ground quartz, and calcium carbonate, also can be employed in particular instances. It is preferred to use fumed silica which has been pretreated with an effective amount of a cyclic siloxane, such as octamethylcyclotetrasiloxane, or a mixture thereof with an organosilazane, such as hexamethyldisilazane. There can be used in a range between about 0 parts and about 30 parts by weight of filler per 100 parts of the silanol terminated fluoroalkyl substituted polydiorganosiloxane and preferably, in a range between about S parts and about 15 parts by weight of filler per 100 parts of the silanol terminated fluoroalkyl substituted polydiorganosiloxane. In addition to reinforcing or extending fillers, heat stabilizers such as iron oxide, ceric oxide, and titanium dioxide, also can be employed in a range between about 0.1 parts and about 10 parts by weight per 100 parts by weight of the silanol terminated fluoroalkyl substituted polydiorganosiloxane.
The neutral room temperature condensation curable fluorosilicone sealant composition is prepared by blending the oligomeric fluorosilicone cross-linkers, shown by the following formula:
(R
2
O)
m
(R)
n
SiO—[R(R
1
)—SiO]
x
—Si(R)
n
(OR
2
)
m
(2),
which have terminal polyalkoxysiloxy units in combination with the above-described paste. The oligomeric fluorosilicone cross-linker is typically present in a range between about 1 part and about 20 parts by weight per 100 parts by weight of the silanol terminated fluoroalkyl substituted polydiorganosiloxane
Leman John Thomas
Singh Navjot
Whitney John M.
General Electric Company
Johnson Noreen C.
Moore Margaret G.
Vo Toan P.
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