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
1999-12-30
2001-08-28
Moore, Margaret G. (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...
C528S032000, C528S015000, C525S478000
Reexamination Certificate
active
06281279
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for producing micron sized silicone rubber powder. More particularly, this invention relates to a method for producing silicone rubber in the form of particles from a combination of vinyl terminated and hydride terminated liquid polysiloxanes.
BACKGROUND OF THE INVENTION
Methods are known in the art for producing silicone rubber powder. One such method involves pulverizing cured silicone rubber into a powder after it has been frozen with dry ice. See Japanese Kokai JP 60-81, 227; Soviet Union Patent No. 952895 and Silicone Rubber in Powder Form, by Swanson, Leicht and Wegener, American Chemical Society, Rubber Division, October 1974. Another method is pulverizing cured silicone rubber into a powder using a grinder at ambient temperature. Using these methods, it is difficult to obtain high productivity and uniform micron sized particles. It is also too costly for many applications.
There have also been direct methods developed for producing silicone rubber particles. European Patent Application Publication No. 025510, Method for Producing Silicone Rubber Powder, involves blending a curable liquid silicone rubber composition at temperatures from −60° C. to plus 5° C., maintaining the mixture at this temperature, and then spraying the mixture into hot air, at a temperature of from 80° C. to 200° C. This method uses a curable liquid silicone rubber composition containing a liquid, reactive-group-containing organopolysiloxane with at least two alkenyl groups per polymer, an organopolysiloxane crosslinker containing at least two hydride functionalities per polymer chain, and a catalyst. The technique is designed to produce spherical silicone rubber particles with diameters from several hundred microns to several hundred millimicrons.
Japanese Patent Application JP62-257939, discloses making silicone rubber particles having particle sizes below 20 microns. These particles are made by a process similar to that disclosed in U.S. Pat. No. 4,742,142 using spray drying to isolate the cured silicone rubber particles.
U.S. Pat. No. 4,743,670 is also directed to a method for producing silicone rubber powder. This method involves preparing a heat-curable, liquid silicone rubber dispersion in water at a temperature of from 0°C. to 25° C. in the absence of any surfactant.
U.S. Pat. No. 4,742,142 describes a method for making small silicone rubber powders of uniform shape without grinding or cryofracturing. This method uses a curable liquid silicone rubber composition containing a liquid, reactive-group-containing organopolysiloxane with at least two alkenyl groups per polymer, an organopolysiloxane crosslinker containing at least two hydride functionalities per polymer chain, and a catalyst. The ingredients are blended together at a temperature of from −60° C. to plus 5° C., preferably from −30° C. to 0° C., to prevent the mixture from curing prematurely. The mixture is then emulsified by blending with water and a surfactant at a temperature from 0° C. to 25° C. The emulsion is then dispersed into water and cured at a temperature of at least 25° C. Illustrative examples demonstrate the curing step, using water at 40° C. and 80° C., followed by isolating the particles to produce spherical crosslinked rubber particles of about 10 and 20 microns. This patent by itself and in conjunction with the earlier cited patents teaches that a lower temperature is preferred prior to dispersion.
Large scale commercial production of silicone rubber particles which requires maintaining a dispersion of a curable liquid organopolysiloxane at temperature below 25° C. to prevent premature curing, are a significant disadvantage. It would be more advantageous to be able to prepare cured silicone polymer particles without the need for such low temperature processing.
U.S. Pat. No 4,962,165 describes an ambient temperature process for producing micron sized particles by a somewhat tedious process beginning with small molecular weight materials (monomers) up to oligomers.
SUMMARY OF THE INVENTION
The present invention provides an improved method to prepare silicone-containing polymer particles of spherical shape with average particle size 6 microns or less. The process proceeds at ambient temperature and involves fewer steps than heretofore disclosed in the art. Particles of silicone rubber are fabricated by
(a) mixing a multifunctional vinyl terminated polysiloxane with a hydride terminated polysiloxane, surfactant, and polymerization catalyst under ambient conditions;
(b) dispersing the mixture in water under ambient conditions;
(c) heating the dispersion to cure the silicone;
(d) breaking the dispersion; and
(e) recovering the particles.
DETAILED DESCRIPTION OF THE INVENTION
This invention provides a method for preparing cured silicone particles having an average particle size 6 microns or less. The method consists of the following steps:
(a) mixing a multifunctional vinyl terminated polysiloxane with a multifunctional hydride terminated polysiloxane, surfactant, and polymerization catalyst under ambient conditions without the need for cooling the mixture. Each polymer chain has at least two endgroups and either the vinyl or hydride chains has at least three functional groups to crosslink the combination of vinyl terminated and hydride terminated liquid polysiloxanes;
(b) dispersing the mixture in water at ambient temperature;
(c) heating the dispersion to 60° C. to cure the silicone;
(d) breaking the dispersion with the addition of a salt such as sodium sulfate and washing the particles to remove the salt and the dispersing surfactant and recovering the particles. Depending on the desired application, the isolation of the particles can be carried to different extents up to and including drying the particles to produce a powder. By “ambient temperature” is meant 25-30° C.
The present method will now be explained in detail. The curable liquid silicone rubber composition used in the present invention is a liquid or a paste at ambient temperature and is based on a liquid, reactive group-containing multifunctional vinyl terminated polysiloxane (Component (A)) with a multifunctional hydride terminated polysiloxane (Component (B)) and a curing catalyst (Component C). Upon heating, the silicone rubber composition cures into a rubbery elastomer. With regard to the curing mechanism of the compositions, the present method can utilize addition-reaction types. Addition-reaction types are preferred due to their rapid curing rate and excellent uniformity in curing. At least one of Component (A) and component (B) must contain some molecules with greater than two functional groups per chain.
Component (A) is the principal component of the preferred liquid silicone rubber compositions, and it reacts with component (B) under the catalytic activity of component (C) to produce a cured silicone rubber. Component (A) must contain at least 2 vinyl functionalities in each molecule. In addition to vinyl functionality, component (A) can also be of allyl, propenyl, etc. functionality. The alkenyl groups can be present at any position in the molecule, but they are preferably present at least at the molecular terminals. Furthermore, the molecular configuration of component (A) can be straight chain, branch-containing straight chain. The molecular weight of this component is not specifically restricted.
Suitable vinyl-containing polysiloxanes include but are not limited to methylvinylpolysiloxanes, methylvinylsiloxane-dimethylsiloxane copolymers, dimethylvinylsiloxy-terminated dimethylpolysiloxanes, dimethylvinylsiloxyterminated dimethylsiloxane-methylphenylsiloxane copolymers, dimethylvinylsiloxy-terminated dimethylsiloxane-diphenylsiloxane-methylvinyl-siloxane copolymers, trimethylsiloxy-terminated dimethylsiloxane-methylvinyl-siloxane copolymers trimethylsiloxy-terminated dimethylsiloxane-methylphenylsiloxane-methylvinyl-siloxane copolymers, dimethylvinylsiloxy-terminated methyl(3,3,3-trifluoropropyl)polysiloxanes, dimethylvinylsiloxy-terminated dimethylsiloxane
Boulatnikov Nataly
Davis Stephen V.
Pickering Jerry A.
Kessler Lawrence P.
Moore Margaret G.
Nex Press Solutions LLC
Peng Kuo-Liang
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