Preparation of organopolysiloxane gum

Details Preparation of organopolysiloxane gum Preparation of organopolysiloxane gum

1999-12-02

2001-09-11

Dawson, Robert (Department: 1712)

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

Synthetic resins

From silicon reactant having at least one...

C528S023000, C528S021000, C528S037000, C556S460000

Reexamination Certificate

active

06288195

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 of the interaction between hydroxyl groups at the end of organopolysiloxane gum and hydroxyl groups on silica surface. Prior to use, the silicone rubber compound having undergone crepe hardening must be restored to the initial state by applying strong shear forces in a twin-roll mill or another kneader.
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 organopolysiloxane and a low molecular weight, linear organopolysiloxane as the terminal stopper. 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 or molecular sieve. 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 corresponding to U.S. Pat. No. 4,439,592 wherein part of cyclic organopolysiloxane is distilled off in the presence of a polymerization catalyst and at a temperature which is 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 corresponding to U.S. Pat. No. 4,563,513. 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 (or organopolysiloxane having a high degree of polymerization), 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 and of consistent quality in a simple and efficient manner.
The invention is directed to a method for preparing an organopolysiloxane gum by polymerizing a cyclic organopolysiloxane along with a low molecular weight, linear organopolysiloxane end-blocked with a triorganosilyl group in the presence of a polymerization catalyst of thermal decomposition type. The polymerization reaction step is followed by the step of heating the reaction product for decomposing or deactivating the polymerization catalyst of thermal decomposition type. Quite unexpectedly, by effecting the heating step under subatmospheric 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 does not invite 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 steps of polymerizing a cyclic organopolysiloxane with a low molecular weight, linear organopolysiloxane end-blocked with a triorganosilyl group in the presence of a polymerization catalyst of thermal decomposition type, and heating the reaction product under subatmospheric pressure for deactivating the catalyst.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the invention, a cyclic organopolysiloxane is polymerized together with a low molecular weight, linear organopolysiloxane end-blocked with a triorganosilyl group in the presence of a polymerization catalyst of thermal decomposition type to produce a high molecular weight organopolysiloxane gum.
Any well-known compounds may be used as the reactants. The preferred cyclic organopolysiloxane 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 at least 3.
R
1
and R
2
are preferably substituted or unsubstituted monovalent hydrocarbon groups of 1 to 12 carbon atoms and especially 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl and propyl, alkenyl groups such as vinyl and allyl, aryl groups such as phenyl and tolyl, aralkyl groups such as phenylethyl, alkaryl groups, and substituted ones of these groups in which some or all of the hydrogen atoms are replaced by halogen atoms, cyano and other groups, such as chloromethyl, trifluoropropyl, cyanoethyl, and &agr;-cyanophenylethyl. Preferably R
1
and R
2
are methyl, phenyl, vinyl or trifluoropropyl groups. Letter m is an integer of at least 3, preferably from 3 to 8, and most preferably equal to 4.
Illustrative examples of the cyclic organopolysiloxane of formula (1) include cyclized dimethylsiloxanes such as hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane, cyclized methylvinylsiloxanes such as tetramethyltetravinylcyclotetrasiloxane, cyclized diphenylsiloxanes such as octaphenylcyclotetrasiloxane, and cyclized methyltrifluoropropylsiloxanes such as trimethyltrifluoropropylcyclotrisiloxane. Of these, cyclized dimethylsiloxanes such as octamethylcyclotetrasiloxane are preferred because of the availability of starting reactants from which they are prepared. The use of cyclized dimethylsiloxanes in combination with cyclized methylvinylsiloxanes is also preferred from the standpoint of introducing alkenyl groups serving as the crosslinking site. These cyclic siloxanes may be used alone or in admixture of two or more. Preferred is a mixture of cyclic siloxanes containing at least 40%, and especially at least 60% of a cyclic compound wherein m=4.
These cyclic organosiloxanes can be prepared by well-known methods. For example, dimethyldichlorosilane is hydrolyzed to give a mixture of cyclized dimethylsiloxanes. By optional distillation, a simple substance ca

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