Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-02-20
2001-10-02
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S134000, C523S218000, C523S219000
Reexamination Certificate
active
06297291
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid silicone rubber composition of low-specific gravity and in particular to a liquid silicone rubber composition of low specific gravity which produces low-specific-gravity rubber of excellent dimensional stability after heating, during heating, and during cooling.
BACKGROUND OF THE INVENTION
A silicone rubber of low specific gravity is characterized by light weight and excellent heat-resistant and weather-proof properties. Therefore this material finds wide use in the manufacture of items where specific properties of this material can be most efficiently utilized, i.e. various seals, packings, gaskets,
0
-rings, or similar automobile parts, as well as coatings for rollers used in copying machines, printers, and facsimile machines. A known low-specific-gravity composition of the aforementioned type comprises an expandable silicone rubber composition produced by incorporating thermoplastic hollow particles into a thermosetting liquid silicone rubber composition under heating (see Japanese Application Hei 5-209,080). A problem associated with this composition, however, is that a foam silicone rubber body obtained after curing the aforementioned composition has a large coefficient of thermal expansion and therefore heating leads to significant changes in its dimensions. For the above reasons, the aforementioned known composition is unacceptable as a material for items, such as coating layers on rollers of copying machines, printers, and facsimile machines, and as automobile gaskets, which require that the composition maintain dimensional accuracy during molding and that articles molded from the silicone rubber maintain dimensional stability during heating or cooling.
The authors have conducted a study aimed at finding a solution to the problems inherent in the prior art and arrived at the present invention. It is an object of the present invention to provide a low-specific-gravity liquid silicone rubber composition, which produces low-specific-gravity rubber that demonstrates excellent dimensional stability after heating, during heating, and during cooling.
SUMMARY OF THE INVENTION
The present invention is a low-specific-gravity liquid silicone rubber composition comprising
(A) 100 parts by weight of diorganopolysiloxane which is liquid at room temperature and contains at least two silicon-bonded alkenyl groups in each molecule,
(B) an organohydrogenpolysiloxane which has at least two silicon-bonded hydrogen atoms in each molecule at a mole ratio of silicone-bonded hydrogen atoms contained in this component (B) to silicon-bonded alkenyl groups contained in component (A) within a range of 0.3:1 to 5:1,
(C) a platinum catalyst providing metallic platinum atoms in an amount of 0.1 to 500 parts by weight per 1,000,000 parts by weight of component (A)
(D) 0.5 to 30 parts by weight of gas-filled hollow particles having shells made of a non-silicone thermoplastic resin, and
(E) 0.05 to 10 parts by weight of gas-filled hollow particles having shells made of a thermoplastic silicone resin.
DESCRIPTION OF THE INVENTION
The present invention is a low-specific-gravity liquid silicone rubber composition comprising
(A) 100 parts by weight of diorganopolysiloxane which is liquid at room temperature and contains at least two silicon-bonded alkenyl groups in each molecule,
(B) an organohydrogenpolysiloxane which has at least two silicon-bonded hydrogen atoms in each molecule at a mole ratio of silicone-bonded hydrogen atoms contained in this component (B) to silicon-bonded alkenyl groups contained in component (A) within a range of 0.3:1 to 5:1,
(C) a platinum catalyst providing metallic platinum atoms in an amount of
0
.
1
to
500
parts by weight per 1,000,000 parts by weight of component (A)
(D) 0.5 to 30 parts by weight of gas-filled hollow particles having shells made of a non-silicone thermoplastic resin, and
(E) 0.05 to 10 parts by weight of gas-filled hollow particles having shells made of a thermoplastic silicone resin.
The diorganopolysiloxane comprising component (A) contains in each molecule at least two silicon-bonded alkenyl groups, is liquid at room temperature, is a main component of the present composition, and cross-links to form silicone rubber. The diorganopolysiloxane is described by the following average unit formula
R
a
SiO
(4-a)/2
,
where R is a monovalent hydrocarbon group or a halogenated alkyl group. The aforementioned monovalent hydrocarbon group can be represented by methyl, ethyl, propyl, or a similar alkyl group; vinyl, allyl, or a similar alkenyl group; cyclohexyl or a similar cycloalkyl group; &bgr;-phenylethyl or a similar aralkyl group; and phenyl or a similar aryl group. The aforementioned halogenated alkyl group can be represented by 3-chloropropyl and 3,3,3-trifluoropropyl. In the above formula “a” is a number with a value between 1.8 and 2.3. Normally, organopolysiloxanes of the above type have a viscosity at 25° C. within a range of 100 to 1,000,000 mPa·s and their molecular structure is mainly linear. However, small molecular chains can be branched from the main chain.
Examples of the diorganopolysiloxane comprising component (A) include dimethylpolysiloxane end-capped with dimethylvinylsiloxy groups, a copolymer of methylvinylsiloxane and dimethylsiloxane end-capped with dimethylvinylsiloxy groups, a copolymer of methylvinylsiloxane and dimethylsiloxane end-capped with silanol groups, a copolymer of methylphenylsiloxane and dimethylsiloxane end-capped with dimethylvinylsiloxy groups, a copolymer of methylphenylsiloxane, methylvinylsiloxane, and dimethylsiloxane end-capped with dimethylvinylsiloxy groups, a copolymer of diphenylsiloxane and dimethylsiloxane end-capped with dimethylvinylsiloxy groups, a copolymer of diphenylsiloxane, methylvinylsiloxane, and dimethylsiloxane end-capped with dimethylvinylsiloxy groups, a copolymer of methyl (3,3,3-trifluoropropyl)siloxane and dimethylsiloxane end-capped with dimethylvinylsiloxy groups, and a copolymer of methyl (3,3,3-trifluoropropyl)siloxane, methylvinylsiloxane, and dimethylsiloxane end-capped with dimethylvinylsiloxy groups.
The organohydrogenpolysiloxane comprising component (B) is a cross-linking agent. In the presence of the platinum catalyst of component (C), silicon-bonded hydrogen atoms of component (B) are added through an addition reaction to silicon-bonded alkenyl groups of the organopolysiloxane of component (A), and the composition is thereby cured. It is required that at least two silicon-bonded hydrogen atoms be present in one molecule of component (B). The following organic groups, other than silicon-bonded hydrogen atoms, can be present in component (B): methyl, ethyl, propyl, and similar alkyl groups; and phenyl, tolyl, and similar aryl groups. Component (B) may have a linear, partially-branched linear, cyclic, or a resin-like structure.
Examples of the aforementioned organohydrogenpolysiloxane include methylhydrogenpolysiloxane having both molecular terminals end-capped with trimethylsiloxy groups, a copolymer of methylhydrogenpolysiloxane and dimethylsiloxane having both molecular terminals end-capped with trimethylsiloxy groups, a copolymer of methylhydrogenpolysiloxane and dimethylsiloxane having both molecular terminals end-capped with dimethylhydrogensiloxy groups. It is recommended that component (B) be used in such an amount that the mole ratio of silicon-bonded hydrogen atoms of this component to silicon-bonded alkenyl groups of component (A) be within a range of 0.3:1 to 5:1. If the above ratio is lower than 0.3:1, the density of cross-linking will be too low. On the other hand, the ratio exceeding 5:1 may cause either a reaction of dehydrogenation with the formation of bubbles or a decrease of heat-resistant properties in the cured silicone rubber.
The platinum catalyst of component (C) is used as a catalyst for curing the present composition. Examples of the aforementioned catalyst include a fine platinum powder, platinum black, chloroplatinic acid, platinum tetrachloride, an alcoholic solution of chloroplatinic
Oka Yutaka
Tsuji Yuichi
Cesare James L. De
Dow Corning Toray Silicone Company, Ltd.
Foelak Morton
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