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...
Reexamination Certificate
2000-09-11
2002-10-15
Metzmaier, Daniel S. (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, C524S588000, C516S058000, C516S066000, C516S924000
Reexamination Certificate
active
06465568
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
REFERENCE TO A MICROFICHE APPENDIX
Not applicable.
FIELD OF THE INVENTION
This invention is directed to a method of emulsion polymerization of siloxane oligomers in which the particle size of siloxane polymer formed in the emulsion can be decreased by decreasing the amount or concentration of ionic surfactant, i.e., anionic surfactant or cationic surfactant, used in preparing the emulsion.
In particular, the invention is directed to a method of emulsion polymerization of siloxane oligomers in which the size of the siloxane polymer particles formed in the emulsion is controlled by the aqueous phase concentration of ionic surfactant and electrolytes, and by the reaction temperature. Narrow uniform particle size distribution emulsions and microemulsions are produced. According to the method, decreasing the aqueous phase concentration of ionic surfactant results in decreased size of siloxane polymer particle formed during emulsion polymerization. Contrary to current understanding, the method of the invention enables one skilled in the art to decrease the particle size of siloxane polymer formed in the emulsion by decreasing, rather than increasing, the amount or concentration of ionic surfactant used in preparing the emulsion.
BACKGROUND OF THE INVENTION
The production of aqueous silicone emulsions is commonly practiced by one of three general methods. One method is the emulsification of previously formed organosiloxane polymers by use of surfactants, and the application of shearing forces by a mechanical means, i.e., mechanical emulsification. A second method is the suspension polymerization of reactive oligomeric organosiloxanes that involves the mechanical emulsification of the organosiloxane oligomers, followed by polymerization of the oligomer in the emulsion particles to higher molecular weight organopolysiloxanes. In this second method, often referred to as suspension polymerization, the organosiloxane oligomers are not capable of diffusion into or through water, because they are too high in molecular weight to have any solubility in water. A characteristic of suspension polymerization is that the emulsion particle size is achieved during the mechanical emulsification step and does not change during the polymerization process.
The third method is known as emulsion polymerization, and it utilizes organosiloxane precursors, typically cyclosiloxanes or alkoxysilanes, which are compositions capable of diffusion into or through water in their original form or when hydrolyzed. In the process of silicone emulsion polymerization, siloxane polymers are formed from the starting siloxane precursors, and new emulsion particles are formed which contain the siloxane polymers formed in the polymerization process. The new particles are characteristically smaller than the starting droplets of organosiloxane precursor.
A key component enabling reactions to occur in silicone emulsion polymerization is a surface active catalyst which has both the properties of a surfactant and a catalyst, described in the literature generally as a surfactant-catalyst. It is sometimes described as surfcat for the sake of brevity. Surfcats may be formed in situ in the emulsion polymerization process by ion exchange of a strong acid or base catalyst and an ionic surfactant that is the salt of a surface active strong acid or base, respectively. They can also be prepared beforehand by ion exchange of a strong acid or base catalyst and an ionic surfactant that is the salt of a surface active strong acid or base, respectively, in an aqueous solution.
However, little is known about factors that may control particle formation, size, and size distribution in silicone emulsion polymerization. For example, the
Journal of Polymer Science,
Part C, No. 27, Pages 27-34, (1969), in an article entitled Anionic Emulsion Polymerization of Siloxanes, Weyenberg et al describe the process in some detail, using cyclosiloxanes or alkyltrimethoxysilanes with dodecylbenzene sulfonic acid (DBSA) as the surfcat. The authors document that increasing the concentration of DBSA results in smaller size particles of methylsilsesquioxanes when using methyltrimethoxysilane, but the effect when using cyclosiloxanes is not described.
In this regard, however, it is generally believed by those skilled in the art, that increasing surfactant concentration relative to the material being emulsified, by whatever means, generally results in formation of smaller size particles. This view is also held by those skilled in emulsion polymerization in particular. For example, in organic free-radical emulsion polymerization, it has been shown that particle number is directly proportional to surfactant concentration, and reference may be had to standard texts such as
Emulsion Polymerization and Emulsion Polymers,
John Wiley & Sons, Page 46, (1997), Edited by Peter Lovell and Mohamed El-Aasser. As particle number increases for a given amount of dispersed phase, particle size consequently decreases.
Organic free radical emulsion polymerization and silicone emulsion polymerization are similar in many respects, although one significant difference is that the former process uses a free radical initiator which is gradually consumed during the polymerization, and the latter process uses a surfcat as described above which is not consumed.
In organic free radical emulsion polymerization, there are three process intervals, each having different characteristic mechanisms, and reference may be had, for example, to such standard texts as
Emulsion Polymerization—A Mechanistic Approach,
Robert G. Gilbert, Academic Press Limited, Pages 51-55, (1995). As noted in such texts, the three process intervals are (1) a period of particle formation (Interval I), (2) a period of increasing particle size with no formation of new particles (Interval II), and (3) a period of polymer growth with no change in particle size and number (Interval III). Generally, Interval I is brief and most of the polymerization occurs in Intervals II and III. During Interval I, surfactant concentration in the aqueous phase is greater than the critical micelle concentration (CMC) of the surfactant in water. Interval I ends when the surfactant concentration becomes less than the CMC. Interval II ends when the starting reactant droplets are completely consumed.
There is limited knowledge in organic free-radical emulsion polymerization about the factors that control particle formation, size, and size distribution in Interval I. Although the reaction chemistry is different, the three Intervals I-III also occur in silicone emulsion polymerization. Typical silicone emulsion polymerizations pass through the three periods, and the particle formation period of Interval I is generally brief. Control of the size of particles being formed is not well understood, however.
It is believed that in the present invention, a method has been discovered whereby the duration of the particle formation period, Interval I, is extended to be the entire or predominate process during emulsion polymerization of cyclosiloxanes, and that the size of particles formed is controlled by certain operating parameters. Accordingly, in terms of the present invention, it is possible to control the size of silicone polymer particles being formed during silicone emulsion polymerization.
It should be noted that silicone emulsion polymerization methods were first described by Hyde and Wehrly in U.S. Pat. No. 2,891,920 (Jun. 23, 1959), and then by Findlay and Weyenberg in U.S. Pat. No. 3,294,725 (Dec. 27, 1966). Both US Patents specify that the organosiloxane precursor be emulsified to carry out the process, except in the case of alkoxysilanes which become very water soluble upon hydrolysis. Neither patent discloses a method to make polydiorganosiloxane microemulsions.
U.S. Pat. No. 4,999,398 (Mar. 12, 1991) discloses a method of making polydiorganosiloxane microemulsions by sequentially adding at an effective rate a
Gee Ronald Paul
Wrolson Burt Michael
Zimmerman Brett Lee
De Cesare Jim L.
Dow Corning Corporation
Metzmaier Daniel S.
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