Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
2000-05-10
2001-12-04
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C528S010000, C556S450000, C524S837000
Reexamination Certificate
active
06326452
ABSTRACT:
The present invention relates to a fundamental principle of silicone chemistry, namely the hydrolysis of halosilanes and more particularly of organohalosilanes to obtain poly(organo)siloxanes. This synthesis is based on the very high reactivity with water of ≡Si-Cl bonds and, to a lesser extent, of ≡SiOR bonds (R=alkyl).
Organohalosilanes are thus converted by hydrolysis and polycondensation (homo- and heterocondensation) into intermediate polyorganosiloxanes (oligoorganosiloxanes) of linear or cyclic structure, which may themselves be polymerized and crosslinked into silicone oils or into silicone elastomers of higher molecular weight.
Dimethyldichlorosilane is an organohalosilane which is well known as a starting material for this hydrolysis/condensation reaction.
Technical processes involving the said hydrolysis/condensation reaction of Me
2
SiCl
2
operate continuously and are fully established industrially. This is described, for example, by H. K. Lichtenwalner and M. M. Sprung in Encyclopedia of Polymer science Vol. 12 Wiley and Sons, New York, 1970.
The hydrolysis of Me
2
SiCl
2
leads to a complex mixture of cyclic and linear oligoorganosiloxanes. Conventionally, the source of reaction water required for the hydrolysis is a hydrochloric acid solution. To make this process as viable as possible, the hydrochloric acid formed by hydrolysis is recovered and may thus be upgraded, for example, by being reacted with methanol to form chloromethane, which is a starting material in the synthesis of dimethyldichlorosilane, according to the direct synthesis process.
Various proposals for the hydrolysis of organohalosiloxanes, e.g. Me
2
SiCl
2
, find their place in the prior art.
Thus, a process is known, from French patent application Ser. No. 2,512,028, for the hydrolysis of organohalosilanes, in particular of Me
2
SiCl
2
, in which the source of water for hydrolysis of the organohalosilane is an aqueous hydrochloric acid solution having at least 35% by weight of hydrogen chloride, the said solution being used in an amount such that the H
2
O/organochlorosilane molar ratio ranges between 10 and 30. The hydrolysis takes place in a single step. The aims targeted here are, on the one hand, to reduce significantly the weight percentage of halogen (chloride) in the polyorganosiloxane-based hydrolysate obtained, and, on the other hand, to obtain better cyclopolyorganosiloxane yields. For reaction temperatures of between 20 and 30° C., the concentration of the aqueous HCl solution is less than or equal to 37% by weight. In any case, irrespective of the temperature, this HCl solution is never saturated in the examples of that patent application Ser. No. 2,512,028.
The drawback of such a hydrolysis in the presence of an excess of water leads to large amounts of aqueous hydrochloric acid. Now, given that the most easily upgradeable KCl is HCl gas, it is necessary to carry out a distillation of the aqueous HCl, which is particularly expensive.
In addition, hydrolysis in the presence of an excess of water is extremely exothermic, inevitably entailing operating difficulties.
The invention covered by French patent application No. 2,518,099 has the aim of overcoming these drawbacks, by using a virtually stoichiometric amount of water, for a one-step hydrolysis of dimethyldichlorosilane. This makes it possible to obtain anhydrous hydrochloric acid (gas), a saturated aqueous hydrochloric acid solution and a polyorganosiloxane hydrolysate. Although the aqueous HCl at the end of the hydrolysis is saturated, it emerges from the examples that this is in no way the case for the initial HCl solution. Cf. Example 3:37% by weight at 60° C. It may therefore be concluded that the HCl produced in the reaction is not entirely released in gaseous form.
The invention according to FR-A-2,518,099 is aimed at avoiding the excesses of water without, however, creating conditions of a water deficit, which are known to give rise to linear polyorganosiloxanes that are not fully hydrolysed and comprise halogens (chlorines) in terminal positions. This is entirely penalizing since the hydrolysis must then be completed using large amounts of water, thereby leading to aqueous HCl effluents that are particularly burdensome and awkward to deal with insofar as it is difficult to recycle them and/or to reprocess them at a lower cost.
It is not possible to carry out this additional hydrolysis with small amounts of water since, under such conditions, a polycondensation of the oligoorganosiloxanes takes place, the direct consequence of which is to increase the viscosity prohibitively.
More recently, U.S. Pat. No. 5,169,970 discloses a process for the hydrolysis of organochlorosilanes, according to which the hydrolysis is carried out in two stages:
the first hydrolysis step consists in hydrolysing the organochlorosilane by using a substantially stoichiometric amount of water and thus producing a hydrolysate consisting of polyorganosiloxanes,
in the second hydrolysis step, the hydrolysate obtained from step 1 is subjected to this same treatment, but this time using an amount of water in stoichiometric excess, the source of water consisting of an HCl solution having a predetermined HCl concentration.
This process is supposed to make it possible to obtain a polyorganosiloxane hydrolysate without suffering the drawbacks of the known techniques involving a hydrolysis step, and which include either an excess of water or a deficit of water, or a stoichiometric amount of water, relative to the organohalosilane. This process would also make it possible to regulate the viscosity of the polyorganosiloxane obtained.
This process is flawed in that it produces HCl gas at low pressure, which complicates the recovery and upgrading of this gas.
The principle of a two-step hydrolysis is also adopted in the process for the manufacture of polydimethylsiloxane described in European patent application No. 0,658,588. According to this process, dimethyldichlorosilane is reacted in a first step with water, supplied by an aqueous hydrochloric acid solution with a concentration of 25% by weight, in order to obtain a crude hydrolysate consisting of cyclic and linear &agr;&ohgr;-dichloro polydimethylsiloxanes and hydrogen chloride gas, with the exclusion of an aqueous HCl solution.
It should be noted, furthermore, that under the reaction conditions of step 1, the solution is not saturated with HCE.
In a second step, the crude hydrolysate is treated with steam to reduce its chlorine content by additional hydrolysis, which also results in the formation of aqueous hydrochloric acid. The latter is recycled into the first step of the process.
The pressure of hydrogen chloride gas in the first step is between 0.15 and 0.5 MPa, in particular between 0.25 and 0.35 MPa. The temperature of step 1 is room temperature, whereas for step 2, reference is made to a temperature of between 110 and 160° C. At the end of the first hydrolysis step, the polydimethylsiloxane obtained comprises 50% cyclic oligomers and 50% linear &agr;, &ohgr;-chloro oligomers.
The use of such an HCl concentration in the hydrolysis step 1 and of a high temperature in step 2 makes it difficult to stabilize and to control the viscosity of the final products.
Patent application DD-227,145 describes a process for the preparation of neutral dimethyldichlorosilane hydrolysate having a low viscosity and stability on storage, as well as a residual content of SiCl bonds of less than 10 ppm.
This process comprises a treatment for the removal of the remaining SiCl groups and aqueous HCl contained in a dimethyldichlorosilane hydrolysate by passing this hydrolysate, after washing and/or neutralization, through a porous material in sheet form. This material is a fibrous material of the paper or textile type (wool, cellulose, polyester, glass fibre). Such a filtration is carried out at room temperature and makes it possible to obtain a hydrolysate having a residual SiCl content of 2 ppm and an undetectable HCl concentration.
This process for the treatment of dichlorodimethylsilane hydrolysate d
Berrier Roger
Doguet Loic
Burns Doane Swecker & Mathis L.L.P.
Chimie Rhodia
Moore Margaret G.
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