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.
What is claimed is:
1. A process for removing water contamination from a cyclic diorganosiloxanes
(a) contacting the cyclic diorganosiloxane monomer with a silica
(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
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
5. The process of claim 1 wherein said drying step is conducted
under an atmosphere of nitrogen.
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:
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.