Preparation of organopolysiloxane emulsion

Details Preparation of organopolysiloxane emulsion Preparation of organopolysiloxane emulsion

1999-08-12

2001-06-12

Dawson, Robert (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...

C524S745000, C524S858000, C524S860000, C528S023000, C516S058000

Reexamination Certificate

active

06245852

ABSTRACT:

This invention relates to a method for preparing an organopolysiloxane emulsion having improved stability.
BACKGROUND OF THE INVENTION
Organopolysiloxane emulsions are useful as parting agents for synthetic resins, rubber, paper and molds, coating agents on synthetic resin films and sheets and paper, water-repellents, softeners or lubricating agents for organic fibers (e.g., natural fibers and synthetic fibers) and inorganic fibers (e.g., glass fibers), emulsion paint, and anti-foaming agents. Of these organopolysiloxane emulsions, those emulsions of a small particle size prepared by emulsion polymerization are sometimes preferred from the considerations of stability and a high degree of polymerization. Such emulsion polymerized products are prepared by various methods. For example, JP-B 34-2041 discloses polymerization using strong acids or strong alkalis as the polymerization catalyst. JP-B 41-13995, Belgian Patent No. 686812 and U.S. Pat. No. 3,360,491 disclose polymerization using alkylbenzene sulfonic acids, alkyl-naphthalene sulfonic acids, aliphatic sulfonic acids, silylalkylsulfonic acids, aliphatically-substituted diphenyl ether sulfonic acids or alkylhydrogensulfates as the polymerization catalyst. JP-B 54-19440 discloses a polymerization process involving emulsifying and dispersing an oligomer of diorganosiloxane (consisting of 3 to 6 monomer units) in a salt type surfactant aqueous solution, adding an ion exchange resin to the dispersion, and effecting ion exchange of the salt type surfactant. Since all these processes take a long time for polymerization, a rapid process is desired.
SUMMARY OF THE INVENTION
An object of the invention is to provide a novel and improved method for preparing an organopolysiloxane emulsion having improved stability within a short time.
Regarding a method of preparing an organopolysiloxane emulsion by emulsifying and dispersing a low molecular weight organopolysiloxane (A) in water in the presence of at least one anionic surfactant (B) selected from organic sulfonic acids and organic sulfates to form an initial emulsion and effecting polymerization, the inventors have found that by reducing the mean particle size of the initial emulsion to 300 nm or less, the polymerization time is reduced and the resulting organopolysiloxane emulsion is improved in stability.
Accordingly, the invention provides a method for preparing an organopolysiloxane emulsion comprising the steps of emulsifying and dispersing a low molecular weight organopolysiloxane (A) in water (C) in the presence of at least one anionic surfactant (B) selected from organic sulfonic acids and organic sulfates to form an initial emulsion having a mean particle size of up to 300 nm, subjecting the initial emulsion to polymerization, and neutralizing the emulsion.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Component (A)
The method for preparing an organopolysiloxane emulsion according to the invention starts with a low molecular weight organopolysiloxane. Any desired low molecular weight organopolysiloxane may be used although it is preferably a cyclic organopolysiloxane, a linear organopolysiloxane end-blocked with a triorganosilyl, diorganomonohydroxysilyl or diorganomonoalkoxysilyl group, or a mixture thereof.
The cyclic organopolysiloxanes used herein are preferably those of the general formula (I):
wherein R
1
and R
2
are independently hydrogen or monovalent hydrocarbon groups of 1 to 8 carbon atoms including an alkyl group such as methyl, ethyl and propyl, an alkenyl group such as vinyl and allyl or an aryl group such as phenyl, and m is a number of 3 to 8 on the average. Illustrative examples include hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, 1,1-diethylhexamethylcyclotetrasiloxane, phenylheptamethylcyclotetrasiloxane, 1,1-diphenylhexamethylcyclotetrasiloxane, 1,2,3,4-tetramethyl-1,2,3,4-tetravinylcyclotetrasiloxane, 1,2,3,4-tetramethylcyclotetrasiloxane, dodecamethylcyclohexasiloxane, and 1,2,3,4-tetramethyl-1,2,3,4-tetraphenylcyclotetrasiloxane.
The end-blocked linear organopolysiloxanes used herein are preferably those of the general formula (II):
wherein R
3
is hydrogen or a monovalent hydrocarbon group of 1 to 8 carbon atoms, for example, an alkyl group such as methyl, ethyl or propyl, an alkenyl groups such as vinyl or allyl or an aryl group such as phenyl; R
4
is as defined for R
3
or a hydroxyl group or an organoxy group of 1 to 8 carbon atoms including an alkoxy group such as methoxy, ethoxy, propoxy, butoxy, pentoxy and hexoxy or an aryloxy group such as phenoxy; and n is a number of 0 to 40 on the average. Illustrative examples include hexamethyldisiloxane, octamethyltrisiloxane, decamethyltetrasiloxane, hexadecamethylheptasiloxane, hexaethyldisiloxane, tetramethyldiethyldisiloxane, tetramethyldivinyldisiloxane, tetramethyldihydroxydisiloxane, tetramethyldimethoxydisiloxane, octamethyldihydroxytetrasiloxane, and octamethyldimethoxytetrasiloxane.
In one preferred embodiment, a mixture of a major proportion of a cyclic organopolysiloxane and a minor proportion of an end-blocked linear organopolysiloxane is used as the low molecular weight organopolysiloxane because this enables to control as desired the number of siloxane units in the organopolysiloxane at the end of emulsion polymerization. Although the blending proportion of the two organopolysiloxanes is not critical, it is preferred to blend at least 70% by weight, especially at least 90% by weight of the cyclic organopolysiloxane. A blending proportion within this range facilitates to adjust the molar ratio of siloxane units in the final organopolysiloxane. The amount of the low molecular weight organopolysiloxane used is not critical although it is preferred to establish an organopolysiloxane concentration of 10 to 60% by weight, especially 20 to 50% by weight in the emulsion. A concentration of less than 10% by weight may make the emulsion industrially inefficient whereas an initial emulsion having an organopolysiloxane concentration of more than 60% by weight may be too viscous to work with.
In preparing an organopolysiloxane emulsion according to the invention, branching units may be introduced, if desired, in order to improve the durability of a protective film which is formed from the organopolysiloxane emulsion. Such branching units may be introduced, for example, by adding a trifunctional silane (such as methyltrimethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, vinyltrimethoxysilane or ethyltriethoxysilane) or a hydrolytic condensate thereof, or a tetrafunctional silane (such as tetramethoxysilane or tetraethoxysilane) before polymerization is effected.
The above-mentioned trifunctional silane, hydrolytic condensate thereof or tetrafunctional silane is preferably added in an amount of up to 10% by weight, more preferably up to 1% by weight of the component (A). Although these components are not essential, it is preferred to add them in an amount of 0.01% by weight or more, especially 0.1% by weight or more of the component (A) when added.
Further in the practice of the invention, a hydrolyzable silane having an organic functional group, a hydrolytic condensate thereof or an organosiloxane oligomer having organic functional group-bearing siloxane units may be added to the organopolysiloxane prior to polymerization, insofar as the objects of the invention are not impaired. By adding such a component, organic functional groups can be introduced into the resulting organopolysiloxane. Exemplary organic functional groups are organic groups having acryloxy, methacryloxy, mercapto, carboxy, amino and epoxy. The organic functional group-bearing siloxane units are preferably introduced in an amount of up to 10 mol %, especially up to 5 mol % based on the low molecular weight organopolysiloxane (component (A)). Although these components are not essential, it is preferred to add them in an amount of 0.01 mol % or more, especially 0.1 mol % or more of the component (A) when added.
Illustrative examples of the hydrolyzable silane having an organic functi

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