Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
2002-07-22
2003-11-11
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...
C528S036000, C528S037000
Reexamination Certificate
active
06646090
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to copolymer compositions useful in the preparation of fuel tank sealants. More particularly, the present invention relates to condensation curable poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer compositions and the use of polyalkoxysilylorganic compounds as cross-linkers to facilitate the condensation cure of poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymers.
As shown by Smith, U.S. Pat. No. 3,109,826, bis(alkoxysilyl) hydrocarbons, such as 1,2-bis-(triethoxysilyl)ethane, can be used as cross-linkers in combination with a metal salt to effect the neutral condensation cure of hydroxy end-blocked polydiorganosiloxanes. However, the resulting cured silicone compositions have been found to be problematic as aircraft fuel tank sealants, as they do not have the required solvent or fuel resistance. It is known that fluorosilicones, for example, made by polymerizing tris[(trifluoropropyl)methyl]cyclosiloxane, can provide excellent fuel resistance. However, fluorosilicones often do not meet the wide temperature stability requirements needed in aircraft sealants, such as temperatures in a range between −54° C. and 177° C. over an extended period of time. In addition, fluorosilicones are subject to depolymerization which can result in the formation of low molecular weight cyclics.
In an effort to enhance the thermal stability of fluorosilicones, non-siloxane groups, such as p-silphenylene, have been inserted into the polyfluorosiloxane backbone, as shown by Grassie and Beattie, “The Thermal Degradation of Polysiloxanes: Part 7”, Polymer Degradation and Stabilization 8:177-193 (1984). It is also reported by Dvornic and Lenz, Macromolecules, 25, 3769 (1992), that copolymers having a glass transition temperature (Tg) of −51° C. can be made by reacting methyl(3,3,3-trifluoropropyl)silanediol and 1,4-bis(dimethylhydroxysilyl)benzene.
While fluorosilicones having improved thermal stability have been made by inserting non-siloxane groups, such as p-silphenylene into the polyfluorosiloxane backbone, such copolymers have been found to have a glass transition temperature which does not satisfy the minimum −54° C. Tg flexibility requirements of aircraft fuel tank sealants.
Experience also has shown that in addition to being sensitive to depolymerization, condensation curable fluorosilicone compositions often suffer from an incomplete cure using a conventional curing catalyst, such as a tin salt, and a standard neutral condensation curable cross-linker, for example a polyalkoxysilane. One possible explanation, as discussed by Fujiki, U.S. Pat. No. 5,236,997, is that the steric hindrance or electronic effects of bulky terminal trifluoropropyl groups inhibit crosslinking of the network.
Accordingly, depolymerization resistant silicone base copolymers which could be compounded to a fuel resistant condensation curable silicone composition convertible to the elastomeric state upon cure exhibiting stability over an operable temperature in a range between about −54° C. or below and at least about 177° C. over an extended period of time are constantly being sought which also exhibit low temperature flexibility, in addition to high temperature stability.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a condensation curable poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer exhibiting a glass transition temperature not exceeding about −54° C.
A further embodiment of the present invention provides a method for making a poly(fluoroorgano)siloxane-poly(silarylene)siloxane copolymer comprising effecting reaction between a bis(diorganohydroxysilyl)arylene and a poly(fluoroalkylorgano)cyclopolysiloxane
In yet a further embodiment of the present invention, there is provided a neutral condensation curable poly(fluoroorgano)siloxy-poly(silarylene)siloxane block copolymer sealant composition comprising
(a) a poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer,
(b) a cross-linker, and
(c) a condensation catalyst.
In yet another embodiment of the present invention, there is provided a method for making a neutral condensation curable poly(fluoroorgano)siloxy-poly(silarylene)siloxane block copolymer sealant composition which comprises
(a) effecting reaction between bis(diorganohydroxysilyl)arylene and poly(fluoroalkylorgano)cyclopolysiloxane to form a condensation curable poly(fluoroorgano)siloxane-poly(silarylene)siloxane block copolymer,
(b) shearing the copolymer, and
(c) blending a cross-linker and a condensation catalyst with the copolymer to form a sealant.
DETAILED DESCRIPTION OF THE INVENTION
The poly(fluoro)organosiloxane-poly(silarylene)siloxane block copolymers, or “block copolymers” within the scope of the present invention, can be used to make aircraft fuel tank sealants having property profiles which include a glass transition temperature (Tg) of about −54° C. or below and can be made by the ring opening polymerization of a poly(fluoroalkylorgano)cyclopolysiloxane in the presence of a bis(diorganohydroxysilyl)arylene.
As used hereinafter, the term “bis(diorganohydroxysilyl)arylene”, or “bis(hydroxysilyl)arylene” is shown by the formula,
(HO—(R)
2
Si)
2
—Q
1
, (I)
and the term “silarylenesiloxy” can be represented by the formula,
—(R)
2
SiQ
1
Si(R)
2
O—, (II)
where Q
1
is a C
(6-12)
divalent aromatic organic radical, and R is a C
(1-4)
alkyl radical. Preferably, the bis(diorganohydroxysilyl)arylene is 1,4-bis(dimethylhydroxysilyl)benzene.
“Poly(fluoroalkylorgano)cyclopolysiloxane”, sometimes expressed as “poly(fluoroalkyl)cyclic siloxane” is shown by the following formula:
[(R
1
)(R
2
)SiO]
a
, (III)
where R
1
is a C
(3-8)
polyfluoroalkyl radical, R
2
is a C
(1-12)
organic radical, and “a” is an integer in a range between about 3 and about 8 inclusive, and preferably 3 or 4. Poly(fluoroalkylorgano)cyclopolysiloxane can be used in a proportion in a range between about 0.5 and about 4 moles, and preferably in a range between about 1 and about 2 moles, per mole of bis(diorganohydroxysilyl)arylene. Reaction to form the copolymer is typically effected with agitation under neat conditions or in the presence of an inert organic solvent, for example, toluene, at temperatures in a range between about 60° C. and about 150° C. Reaction is typically effected for a time period in a range between about 30 minutes and about 2 hours, and preferably, in a range between about 45 minutes and about 1.5 hours. It is preferred to operate within an inert atmosphere, for example, under a nitrogen blanket. Typically, there is also present an initiator, for example, an alkali hydroxide (e.g. sodium hydroxide) or an alkali fluorosilanolate (e.g. sodium fluorosilanolate) in a range between about 5 parts per million (ppm) and about 50 ppm, based on the weight of reaction mixture, and a quencher, for example, phosphoric acid or silyl phosphate in a range between about 10 ppm and about 60 ppm, based on the weight of reaction mixture.
C
(1-4)
alkyl radicals which are included within R are, for example, methyl, ethyl, propyl and butyl; C
(3-8)
polyfluoroalkyl radicals which are included within R
1
, include but are not limited to, for example, trifluoropropyl units, tridecafluoro-1,1,2,2-tetrahydrooctyl units, nonafluoro-1,1,2,2-tetrahydrohexyl units, and pentafluoro-1,1,2,2-tetrahydrobutyl units. Radicals included within R
2
include, but are not limited to, for example, methyl, ethyl, propyl, butyl, and phenyl.
Among the poly(fluoroalkylorgano)cyclopolysiloxanes there is preferably included tris[(trifluoropropyl)methyl]cyclosiloxane.
The sealant composition of the present invention includes a bis(polyalkoxysilyl)organo cross-linker having the formula,
[(RO)
2
(X)Si]
2
Q, (IV)
where Q is a C
(2-12)
divalent organic radical, R is as previously defined, and X is a member selected from the group consisting of R and RO. Some of the C
(2-12)
divalent hydrocarbon radica
Krabbenhoft Herman Otto
Leman John Thomas
Singh Navjot
Whitney John M.
Bennett Bernadette M.
General Electric Company
Moore Margaret G.
Patnode Patrick K.
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