Preparation of organoplysiloxane gum

Details Preparation of organoplysiloxane gum Preparation of organoplysiloxane gum

1999-11-12

2001-04-24

Dawson, Robert (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From silicon reactant having at least one...

C528S014000, C528S023000, C556S462000

Reexamination Certificate

active

06221996

ABSTRACT:

This invention relates to a method for preparing a organopolysiloxane gum which has a minimized content of hydroxyl in the terminal unit so that the gum is suitable in various silicone rubber compositions.
BACKGROUND OF THE INVENTION
As is well known in the art, organopolysiloxane polymers are prepared, for example, by polymerizing a cyclic organosiloxane in the presence of an alkaline catalyst, optionally neutralizing the catalyst for deactivation, and distilling off low volatiles from the reaction product. The degree of polymerization of such a polymer is controlled by the amount of a low molecular weight organopolysiloxane serving as a terminal stopper. Also the terminal unit structure of the resulting organopolysiloxane depends on the structure of the terminal stopper.
However, the above-described method for preparing organopolysiloxane has the drawback that a trace amount of water in the reactant can also function as the terminal stopper. The resulting organopolysiloxane has hydroxyl groups introduced into the terminal unit, departing from the desired terminal unit. When such a high molecular weight organopolysiloxane gum having terminal hydroxyl groups is mixed with a reinforcing agent such as silica to formulate a silicone rubber compound, the compound gives rise to a crepe hardening phenomenon with the lapse of time because the bonding reaction between hydroxyl groups at the end of organopolysiloxane gum and a silica surface is accelerated. Prior to use, the silicone rubber compound having undergone crepe hardening must be restored to the initial state by applying strong shear forces.
In the application where organopolysiloxane gum is used as a base component of a silicone rubber compound, it is desired to design the organopolysiloxane gum such that its terminal unit consists of a triorganosilyl group. Nevertheless, in the currently available gums, hydroxyl groups are introduced in the terminal unit owing to incidental factors as mentioned above. For the preparation of organopolysiloxane gum, it was needed to reduce the content of hydroxyl groups.
In this regard, the conventional method for preparing organopolysiloxane gum is generally preceded by a pretreatment for removing a trace amount of water from the starting reactants such as a cyclic organosiloxane and a low molecular weight, linear organopolysiloxane. A typical pretreatment is by blowing an inert gas such as nitrogen into the reactants or drying the reactants in the presence of a desiccant such as silica gel. However, such pretreatment adds to the number of steps and renders the overall process complicated.
One known means for reducing the influence of water in the reactants is disclosed in JP-A 58-69228 wherein part of cyclic organosiloxane is distilled off in the presence of a polymerization catalyst and at a temperature at least 10° C. lower than the polymerization starting temperature. This method is also cumbersome because the complex pretreatment of the reactant is necessary.
Another technique of reducing hydroxyl groups in organopolysiloxane terminal units is by adding a triorganohalosilane and a hexaorganodisilazane for neutralizing the alkali catalyst as disclosed in JP-A 60-49033. This technique is successful in reducing hydroxyl groups, but raises the problem of metal equipment corrosion due to the use of halosilane.
Therefore, for the preparation of organopolysiloxane gum, it is desired to have a technique capable of effectively reducing the content of hydroxyl groups in the terminal unit.
SUMMARY OF THE INVENTION
An object of the invention is to provide a novel and improved method for preparing an organopolysiloxane gum having a minimized content of hydroxyl groups in the terminal unit in a simple and efficient manner.
The invention is directed to a method for preparing an organopolysiloxane gum by polymerizing a cyclic organosiloxane with a low molecular weight, linear organopolysiloxane end-blocked with a triorganosilyl group in the presence of an alkaline catalyst. This polymerization reaction is carried out under atmospheric pressure in the prior art. Quite unexpectedly, by effecting the polymerization reaction under a reduced pressure below atmospheric pressure, preferably 500 mmHg or lower, an organopolysiloxane gum substantially free of hydroxyl groups in the terminal unit can be prepared in a simple manner without a need for pretreatment and in an industrially advantageous manner without concern about equipment corrosion. The organopolysiloxane gum obtained by this method rarely invites a crepe hardening phenomenon when it is blended with a reinforcing agent such as silica to formulate a silicone rubber composition. The gum is useful as the base component in a variety of silicone rubber compositions.
Accordingly the invention provides a method for preparing an organopolysiloxane gum comprising the step of polymerizing a cyclic organosiloxane with a low molecular weight, linear organopolysiloxane end-blocked with a triorganosilyl group in the presence of an alkaline catalyst, characterized in that the polymerization reaction is effected under a reduced pressure below atmospheric pressure.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the invention, a cyclic organosiloxane is polymerized together with a low molecular weight, linear organopolysiloxane end-blocked with a triorganosilyl group in the presence of an alkaline catalyst to produce a high molecular weight organopolysiloxane gum.
Any well-known compounds may be used as the reactants. The preferred cyclic organosiloxane used herein is of the following formula (1).
Herein, R
1
and R
2
are substituted or unsubstituted monovalent hydrocarbon groups and may be identical or different, and m is an integer of 3 to 8. R
1
and R
2
are preferably substituted or unsubstituted monovalent hydrocarbon groups of 1 to 10 carbon atoms and especially 1 to 6 carbon atoms, for example, alkyl groups such as methyl, ethyl and butyl, cycloalkyl groups such as cyclohexyl and cyclopentyl, alkenyl groups such as vinyl and allyl, aryl groups such as phenyl and xylyl, aralkyl groups such as benzyl, and substituted ones of these groups in which some or all of the hydrogen atoms are replaced by halogen atoms, mercapto, glycidoxy, (meth)acryloxy and amino groups. Preferably R
1
and R
2
are methyl, vinyl or phenyl groups.
Illustrative examples of the cyclic organosiloxane of formula (1) include hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, hexaphenylcyclotrisiloxane, octaphenylcyclotetrasiloxane, and tetramethyltetravinylcyclotetrasiloxane. These cyclic organosiloxanes may be used alone or in admixture of two or more.
The low molecular weight, linear organopolysiloxane serves as a terminal stopper for adjusting the degree of polymerization of the resulting organopolysiloxane gum. It is end-blocked with a triorganosilyl group and is preferably of the following formula (2).
Herein, R
3
to R
8
are substituted or unsubstituted monovalent hydrocarbon groups and may be identical or different. The monovalent hydrocarbon groups represented by R
3
to R
8
are preferably those having the same number of carbon atoms as R
1
and R
2
in formula (1), with illustrative examples thereof being the same as listed above. In particular, R
7
and R
8
are preferably methyl or vinyl. The letter n is an integer of 1 to 200, preferably 10 to 100, and more preferably 20 to 80.
Preferably the low molecular weight, linear organopolysiloxane of formula (2) has a viscosity of about 1 to 500 centistokes at 25° C., and more preferably about 20 to 100 centistokes at 25° C.
The amount of the low molecular weight, linear organopolysiloxane varies with the desired degree of polymerization of the end organopolysiloxane gum and the degree of polymerization of the linear organopolysiloxane used although it is preferably about 0.01 to 10 parts, more preferably about 0.1 to 5 parts by weight per 100 parts by weight of the cyclic organosiloxane.
The alkaline catalyst is preferably selected from among alkali metal hydroxides such as sodium hydro

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