Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
2000-08-21
2002-01-29
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C556S459000, C556S442000, C528S017000, C528S018000, C528S029000, C528S034000, C528S901000
Reexamination Certificate
active
06342575
ABSTRACT:
This invention relates to an acetic acid removal type room temperature curable organopolysiloxane composition which remains stable during shelf storage and readily cures in the presence of air-borne moisture into a rubbery elastomer having improved heat resistance and adhesion during water immersion, and a method for preparing the same.
BACKGROUND OF THE INVENTION
In the prior art, acetic acid removal type room temperature curable organopolysiloxane compositions which readily cure with moisture into rubbery elastomers are used in a wide variety of applications including adhesives, coating materials, electrical insulating seals, and building sealants. In the preparation of such compositions, methyltriacetoxysilane is often used as the curing agent. Methyltriacetoxysilane, however, has a problem that it must be admitted into a drying oven prior to use because its melting point is about 30° C. Another problem is that in winter, the once melted curing agent crystallizes again upon metering.
To avoid crystallization, use is often made of modified products of methyltriacetoxysilane whose methyl group is replaced by another group such as ethyltriacetoxysilane, propyltriacetoxysilane and vinyltriacetoxysilane. The dimer and trimer of methyltriacetoxysilane are also used.
However, the use of curing agents other than methyltriacetoxysilane can exacerbate adhesion and cause discoloration upon heating. The dimer and trimer of methyltriacetoxysilane are described in U.S. Pat. No. 4,116,935. The product is undesirable in quality because the ratio of monomer/dimer/trimer often varies from lot to lot. U.S. Pat. Nos. 3,886,118 and 4,672,004 disclose to add methoxysilanes or higher alcohols of at least 6 carbon atoms to methyltriacetoxysilane. The addition of such methoxysilanes or higher alcohols is effective for preventing crystallization, but can exacerbate the adhesion to metallic aluminum. Since methyltriacetoxysilane alone fails to improve the adhesion to metallic aluminum, adhesion improvements are made by using as the adhesion promoter di-t-butoxydiacetoxysilane as disclosed in U.S. Pat. No. 4,296,228 or aminosilanes as disclosed in U.S. Pat. No. 4,220,748, which leads to an increased cost.
SUMMARY OF THE INVENTION
An object of the invention is to provide an inexpensive, room temperature curable organopolysiloxane composition which has eliminated the crystallization problem of methyltriacetoxysilane used as the curing agent and firmly adheres to metallic aluminum.
The invention pertains to an acetic acid removal type room temperature curable organopolysiloxane composition comprising a both end hydroxyl group-blocked organopolysiloxane having a viscosity of about 10 to 1,000,000 centistokes at 25° C. and methyltriacetoxysilane or a partial hydrolyzate thereof as a curing agent. It has been found that adding a selected amount of methanol to methyltriacetoxysilane or a partial hydrolyzate thereof, and especially adding a premix of methyltriacetoxysilane or its partial hydrolyzate and methanol to the organopolysiloxane is effective for improving the adhesion of the composition to metallic aluminum and also for preventing the crystallization of methyltriacetoxysilane or its partial hydrolyzate which becomes a problem during manufacture. Additionally, the resulting composition has satisfactory physical properties and is inexpensive.
The invention provides a room temperature curable organopolysiloxane composition comprising
(1) 100 parts by weight of an organopolysiloxane of the following general formula (I):
wherein R is a monovalent hydrocarbon group, and n is such an integer that the organopolysiloxane may have a viscosity of about 10 to 1,000,000 centistokes at 25° C.,
(2) 2 to 10 parts by weight of methyltriacetoxysilane or a partial hydrolyzate thereof, and
(3) 0.01 to 0.5 part by weight of methanol per part by weight of component (2).
In another aspect, the invention provides a method for preparing the room temperature curable organopolysiloxane composition, comprising the steps of premixing components (2) and (3), and mixing the premix with component (1).
DESCRIPTION OF THE PREFERRED EMBODIMENT
The base polymer in the room temperature curable organopolysiloxane composition of the invention is an organopolysiloxane of the general formula (I):
wherein R is a monovalent hydrocarbon group, and n is such an integer that the organopolysiloxane may have a viscosity of about 10 to 1,000,000 centistokes at 25° C. The R groups may be the same or different.
In formula (I), R stands for substituted or unsubstituted monovalent hydrocarbon groups, preferably having 1 to 10 carbon atoms, more preferably 1 to 8 carbon atoms. Examples include alkyl groups such as methyl, ethyl, propyl, butyl and hexyl, aryl groups such as phenyl and tolyl, alkenyl groups such as vinyl, allyl and butenyl, aralkyl groups such as benzyl and 2-phenylethyl, and substituted ones of these groups in which some or all of the hydrogen atoms attached to carbon atoms are replaced by halogen atoms or cyano groups, such as chloromethyl, trifluoropropyl and cyanoethyl. Of these, methyl, phenyl, vinyl and trifluoropropyl are preferred.
Letter n which corresponds to a degree of polymerization is such an integer that the organopolysiloxane may have a viscosity of about 10 to 1,000,000 centistokes (cs) at 25° C. and especially about 500 to 100,000 cs at 25° C. from the working standpoint.
Illustrative examples of the organopolysiloxane of formula (I) are given below.
In the formulae, Me is methyl, Ph is phenyl, r and s are positive integers, and r+s is equal to the integer of n which is as defined above. Of these, the compounds of formula (Ia) are preferred.
Apart from the above-described structure, the polymer may be blocked at one end with 0 to 10 mol % of trimethylsilyl groups. Alternatively, the polymer may have introduced in its structure a branched structure of the following formula in an amount of 0 to 3 mol % based on the molecular weight.
The second component in the inventive composition is methyltriacetoxysilane of the following formula (II) or a partial hydrolyzate thereof.
An appropriate amount of methyltriacetoxysilane or its partial hydrolyzate (2) blended is about 2 to 10 parts, and preferably about 3 to 6 parts by weight per 100 parts by weight of the organopolysiloxane (1). A composition containing less than 2 parts of methyltriacetoxysilane becomes short curable and unstable during storage. With more than 10 parts of methyltriacetoxysilane, the cured composition becomes hard and brittle and fails to perform well as a sealant, and the cost is increased.
The third component (3) is methanol. Methanol is blended in an amount of 0.01 to 0.5 part, and preferably 0.03 to 0.3 part by weight per part by weight of methyltriacetoxysilane or its partial hydrolyzate (2). A less amount of methanol is insufficient to prevent the crystallization of methyltriacetoxysilane and improve the adhesion to metallic aluminum whereas a larger amount of methanol causes the composition to thicken or gel.
In the inventive composition, an inorganic filler may be blended as a fourth component. Useful inorganic fillers include silica, carbon black, titanium oxide, calcium carbonate, colloidal calcium carbonate, clay and talc, which are surface treated if desired. These reinforcing fillers may be used alone or in a combination of two or more since a suitable combination of such fillers can impart satisfactory durability to the composition. When used alone, the preferred fillers are silica, carbon black, and colloidal calcium carbonate. When the fillers are used in admixture, suitable combinations include a combination of silica with colloidal calcium carbonate, a combination of silica with carbon black, and a combination of colloidal calcium carbonate with carbon black.
More specifically, the preferred filler used alone is silica. Dry silica which has been hydrophobicized by surface treatment with suitable agents such as silanes, siloxanes and silazanes is especially preferable since its zero water content allows the amount of the curing age
Arakawa Masaya
Miyake Masatoshi
Dawson Robert
Peng Kuo-Liang
Shin-Etsu Chemical Co. , Ltd.
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