Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
1999-12-02
2001-04-10
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, C528S037000, C366S301000, C556S460000
Reexamination Certificate
active
06214962
ABSTRACT:
This invention relates to a process for continuously preparing an 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
Because of its improved characteristics including weather resistance, electrical properties, compression set, heat resistance, and low-temperature resistance, silicone rubber is widely used in a variety of fields including electronic equipment, automobiles, buildings, medical and food fields. Illustrative applications include rubber contacts used as rubber contact keys in remote controllers, type writers, computer keyboards, and musical instruments, building gaskets, rolls in copiers and printers (e.g., fixing rolls, developing rolls, transfer rolls, charging rolls, and paper feed rolls), vibration dampers in audio equipment, and compact disc packing in computers. As the demand for silicone rubber is increasing, the high productivity manufacture of silicone rubber at a low cost is desired.
The continuous preparation of organopolysiloxane is effective to meet such needs. JP-A 53-99300 corresponding to U.S. Pat. No. 4,128,568 discloses a process for continuously preparing a highly viscous organopolysiloxane in the presence of a basic and/or acidic reaction catalyst. It is described that viscous organopolysiloxanes having a viscosity of 10 to several millions of centipoises can be prepared, but no reference is made to organopolysiloxanes having a higher viscosity, that is, in the gum region. The catalysts used therein require a post-treatment for neutralization, which is cumbersome.
Another process for continuously preparing an organopolysiloxane gum is disclosed in JP-A 60-202124 corresponding to U.S. Pat. No. 4,551,515, which uses a neutralizing agent in a sufficient amount to neutralize the polymerization catalyst. This process is complicated and yields as a by-product the neutralization salt which must be disposed of and can have a detrimental influence on the product quality if left behind.
The terminal unit structure of organopolysiloxane depends on the structure of a terminal stopper used in the polymerization process. However, the process for preparing organopolysiloxane generally 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.
One process for reducing the hydroxyl group content is disclosed in the above-referred JP-A 60-202124. For removing a trace amount of water from the starting reactants such as a cyclopolysiloxane and a low molecular weight, linear organopolysiloxane as the terminal stopper, the reactants are previously dried using a desiccant such as molecular sieve. However, such pretreatment adds to the number of steps and renders the overall process complicated.
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 increases the number of steps and 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 more efficient 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 process for continuously preparing an organopolysiloxane gum having a minimized content of hydroxyl groups in the terminal unit and of consistent quality and low cost through simplified steps and in a highly productive manner.
The inventor has found that an organopolysiloxane gum can be continuously prepared by the steps of (I) mixing (A) at least one cyclopolysiloxane of the formula (1) to be defined below with (B) an end capping agent, (II) feeding the mixture of components (A) and (B) to a self-cleaning continuous polymerization reactor, continuously mixing (C) a polymerization catalyst of thermal decomposition type with the mixture, and effecting polymerization at a temperature of 80 to 130° C., and (III) heating the reaction mixture of step (II) at a temperature of 130 to 200° C. under subatmospheric pressure for deactivating the catalyst, thereby terminating the polymerization reaction. Since the reaction product resulting from step (III) contains the cyclopolysiloxane and volatile components as polymerization residues, step (III) is preferably followed by the step (IV) of continuously removing the cyclopolysiloxane and volatile components from the reaction product. In this way, an organopolysiloxane gum of consistent quality and substantially free of hydroxyl groups in the terminal unit can be prepared by a simplified process without a need for pretreatment and without a need for a cumbersome neutralization step. The organopolysiloxane gum can be prepared at a high productivity and low cost without concern about equipment corrosion.
Accordingly the invention provides a process for continuously preparing an organopolysiloxane gum, comprising the steps of:
(I) mixing (A) at least one cyclopolysiloxane of the following general formula (1):
wherein R
1
and R
2
are independently substituted or unsubstituted monovalent hydrocarbon groups, and n is an integer of at least 3, with (B) an end capping agent,
(II) feeding the mixture of components (A) and (B) of step (I) to a self-cleaning continuous polymerization reactor, continuously feeding and mixing (C) a polymerization catalyst of thermal decomposition type with the mixture, and effecting polymerization at a temperature of 80 to 130° C., and
(III) heating the reaction mixture of step (II) at a temperature of 130 to 200° C. under subatmospheric pressure for deactivating the catalyst, thereby terminating the polymerization reaction.
REFERENCES:
patent: 4128568 (1978-12-01), Büchner et al.
patent: 4551515 (1985-11-01), Herberg et al.
patent: 4739026 (1988-04-01), Reiderer et al.
patent: 0 492 662 (1992-07-01), None
patent: 1 174 219 (1969-12-01), None
patent: 60-202124 (1985-10-01), None
Patent Abstracts of Japan vol. 1999, No. 14, Dec. 22, 1999 &JP 11 246663 (Shin Etsu Chem Co. LTD Sep. 14, 1999.
Hagiwara Yutaka
Igarashi Minoru
Shibata Keiji
Takahashi Masaharu
Tawara Kenji
Dawson Robert
Millen White Zelano & Branigan P.C.
Peng Kuo-Liang
Shin-Etsu Chemical Co. , Ltd.
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