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Method and apparatus for desiccant drying of fluorosilicones

Abstrict

A method of polymerizing organosiloxane monomers to produce high molecular weight polymers having a relatively narrow range of molecular weights, which includes drying the monomers to a selected moisture content of 10 ppm or less by moisture transfer contact with a desiccant at temperatures below the polymerization temperature prior to polymerization.

Claims

What is claimed is:

1. A process for removing water contamination from a cyclic diorganosiloxanes monomer, comprising:

(a) contacting the cyclic diorganosiloxane monomer with a silica gel; and

(b) drying the monomer by absorbing the water contamination onto the silica gel, wherein the silica gel is present in a quantity sufficient to adsorb water whereby the concentration of water present in the monomer after removal of the water from the monomer is less than about 10 ppm and wherein the adsorption of water is conducted at a temperature ranging from about 30.degree. to about 40.degree. C. for a period of about four hours, whereby the monomer is not polymerized by said adsorption and whereby when said monomer is polymerized a high molecular weight polymer having a narrow range of molecular weights results wherein said molecular weight range of said high molecular weight polymer has a dispersion index below about 1.8.

2. The process of claim 1 wherein the ratio of silica gel to monomers is about at least 3%.

3. The process of claim 2 wherein the ratio is about 10%.

4. The process of claim 1 wherein the diorganosiloxanes are mixed with chain stoppers comprising hydroxyl, methyl, chlorofluorocarbons and vinyl.

5. The process of claim 1 wherein said drying step is conducted under an atmosphere of nitrogen.

Description

FIELD OF THE INVENTION

The present invention relates to a process for drying organosiloxane monomers prior to polymerization and, more specifically, to a method and apparatus for desiccant drying of cyclic diorganosiloxane and halogen bearing diorganosiloxane monomers prior to polymerization.

BACKGROUND OF THE INVENTION

The polymerization of cyclic diorganosiloxanes to high molecular weight polymers requires that the monomer first be dried. It is preferred that the moisture level be reduced to a level which minimizes the amount of silanol end groups formed on the polymer chain ends. Evans, U.S. Pat. No. 4122247 discloses a process for the polymerization of cyclic diorganosiloxanes with cation-complex catalysts in which it is preferred that there be present less than 10 ppm of water in the composition of cyclic siloxanes which is to be used with the catalyst to make the polymers. Removal of all but traces of water is accomplished by heating to 100.degree. C. or above with a nitrogen purge. According to the patent, this effectively reduces the water content of the cyclic siloxane composition to less than 10 ppm. If there is substantially more than this amount of water present in the cyclic siloxanes, then the desired low molecular weight oil or high molecular weight diorganosiloxane homo or copolymer gum will not be formed in commercially attractive yields. In essence, Evans raises the temperature of the material above the boiling point of water to drive off the excess water by distillation.

Saam et al., U.S. Pat. No. 4780519 discloses a method of producing polydiorganosiloxane gum in which azeotropic drying is employed to reduce the water level. This is accomplished by controlling the water vapor pressure in the reaction container. In particular, the reaction container is sealed so that the atmosphere can be controlled at a desired water vapor level. According to Saam, water vapor pressure is regulated by means of water vapor control of a nitrogen sweep during polymerization. A dew point meter placed at the exit stream of the reaction container is used to determine the amount of water vapor pressure present in the reaction container. By control of the dew point, that is, by the control of the amount of water carried by the nitrogen into the vessel, the amount of water in the reaction can be controlled. Saam also mentions that distillation techniques are available but not satisfactory.

The various methods discussed in Saam are deficient in a number of ways. Drying and polymerization occur simultaneously. Thus, the heating step may cause premature polymerization which can result in poor product yield. The various methods require additional apparatus to achieve the desired results. Further, unless the variables such as, for example, time, temperature and dew point are strictly controlled, such known processes may not achieve satisfactory results.

The moisture content in the reaction is controlled to minimize the amount of silanol end groups and to control the desired or target molecular weight range. The nature of the end groups and the molecular weight are controlled by the addition of chain stoppers such as:

or

where

R.sub.1 is CH.sub.3 ;

R.sub.2 is CH.sub.3 or CF.sub.3 --(CH.sub.2).sub.2 --; and

R.sub.4 is CH.sub.2 =CH--.

It is known that high molecular weight (HMW) polymers can be derived without drying from cyclic siloxanes, in particular from 135-trimethyl-135-tris(333-trifluoropropyl) cyclotrisiloxane to provide gums with a Williams Plasticity (WP) (3') of <200. The value can be increased by azeotropically drying the polymer prior to polymerization. However, azeotropic drying is inefficient and does not result in the desired degree of control of the resulting high molecular weight range. A narrow range of molecular weight has a positive effect upon the producibility and quality of resulting compounds, for example heat cured rubbers. Distillation and azeotropic drying, while effective to produce a water level thought to be necessary in this art, nevertheless result in shortcomings, both in terms of cost and quality of the product.

SUMMARY OF THE INVENTION

The present invention is directed to a method of polymerizing organosiloxane monomers to produce high molecular weight polymers. In a particular embodiment the method includes drying the monomers to a selected moisture content of 10 ppm or less by moisture transfer contact with a desiccant at temperatures below the polymerization temperature of the monomers, thereby minimizing the formation of silanol end groups. The dry monomers are combined with selected amounts of chain stoppers for determining molecular weight, and the monomers and chain stoppers are heated to an elevated temperature above the polymerization temperature so as to result in formation of high molecular weight polymers having a relatively narrow range of molecular weights with a dispersion index less than about 1.8 relative to a target value, preferably less than 1.5 and more preferably about 1.2.

Various apparatus may be employed to implement the process including batch processing by continuous circulation from a holding tank through the desiccant or once through apparatus which include reaction columns containing a sufficient quantity of desiccant.


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