1. Technical Field of the Invention
The present invention relates to the synthesis of hydroxylated polyorganosiloxanes, in particular of linear polydiorganosiloxanes comprising silanol endgroups (≡SiOH) of relatively low molar mass. These silicones (e.g., α,ω-diOH polydimethylsiloxane) are useful in particular as dispersants in the context of the manufacture of silicone elastomers, such as are found in crosslinked silicone coatings (e.g., release paper), or else in room temperature vulcanizable silicone emulsions (polycondensation) to form mastics or other sealants.
The present invention thus relates to a process for the hydrolysis/condensation of halosilanes in a multiphase (preferably two-phase) reaction medium, this process being of the type of those entailing mixing the halosilanes, water and buffer together with stirring and in then separating the aqueous phase and the organic phase, which comprises the hydrolysis/condensation products, namely hydroxylated polyorganosiloxanes.
The present invention also relates to apparatus for carrying out this process for the hydrolysis/condensation of halosilanes.
2. Description of Background and/or Related and/or Prior Art
As is indicated in the scientific and technical literature, for example in the text by Noll, “Chemistry and Technology of Silicones” (published by Academic Press, 1968), two well known routes exist for the synthesis of polydiorganosiloxanes having ends carrying SiOH units:
1) ring opening of cyclic polyorganosiloxanes, octamethyltetrasiloxane (D4) or heptamethyltrisiloxane (D3), and/or redistribution of D2 siloxyl units: —R2SiO—O2/2, catalyzed by acids or bases;
2) hydrolysis of ≡SiZ units (Z=halogen or alkyl) in an aqueous phase, followed by condensation.
Generally, long chain oils comprising silanol ends (low level of ≡SiOH (w/w), that is to say <1%) are manufactured industrially by redistribution of short chains starting from cyclic polyorganosiloxanes of D3 and/or D4 type.
Short chain oils with a high concentration of ≡SiOH are obtained conventionally by hydrolysis/condensation of silanes comprising ≡SiZ units (Z=halogen or alkyl) in an organic/aqueous two-phase medium.
The present invention more especially relates to the technology for the hydrolysis/condensation of halosilanes, for example of chlorosilanes.
This technology has long been known.
By way of illustration, mention may be made of U.S. Pat. No. 2,661,348, which discloses a process for the preparation of a polysiloxane resin which entails making use of a solution comprising an organotrihalosilane (trichloromethylsilane) in an inert halogenated organic solvent (trichloroethylene, tetrachloroethane, tetrachloroethylene, and the like). This organic solution of halosilanes is contacted with water in an amount sufficient to permit the complete hydrolysis of the organotrihalosilane, in order, finally, to extract the nonaqueous organic phase comprising the polysiloxane resin.
This process is intended to make it possible to solve a recurrent technical problem during the hydrolysis/condensation of halosilanes, namely that, with polyorganosiloxanes substituted by lower alkyls (methyls), the hydrolysis and the condensation occur simultaneously. This results in uncontrolled polycondensation, leading to the formation of long linear polysiloxane entities and of cyclic polysiloxane entities. Insoluble silicone gels thus appear in the reaction medium, which gels render the silicone products obtained by hydrolysis/condensation of halosilanes unsuitable for any subsequent use in any industrial and commercial application. The fact of employing an inert organic solvent, as provided in U.S. Pat. No. 2,661,348, makes possible better dissolution of the hydrolyzed silicone product in the organic phase, while the acid (HCl) which promotes hydrolysis and all the hydrophilic products remain in the aqueous phase. Success is thus more or less achieved in preventing the reaction from becoming uncontrolled. In addition, the choice of a solvent with a density of greater than 1.1 (halogenated solvent) promotes and facilitates the separation of the two phases without the formation of an oil-in-water emulsion.
According to U.S. Pat. No. 2,661,348, the reactor is equipped with a stirrer but the stirring parameters are not presented as critical.
As indicated above, the starting materials for the hydrolysis/condensation can be alkoxysilanes instead of halosilanes. Thus, EP-A-1-052,262 discloses the hydrolysis/condensation of one mole of dimethoxydimethylsilane by two moles of water, the reaction medium being adjusted to a pH of 3.4 using hydrochloric acid. This stoichiometric two-phase system is vigorously stirred at ambient temperature. Stirring is continued for a period of time. The reaction mixture becomes homogeneous and a KH2PO4/A2HPO4 buffer system is subsequently added thereto so as to adjust the pH to 6.8. After stirring for a few minutes, the reaction medium is subjected to a vacuum distillation operation to remove the water and the volatile substances (methanol).
It will be seen from this document that the hydroxylated polyorganosiloxanes obtained have degrees of polymerization which are slightly greater than 2 and which have a tendency to increase on storage (instability).
In addition, this document does not particularly emphasize the stirring conditions, any more than the silane/aqueous phase ratio, from the viewpoint of the performance expected for the hydrolysis/condensation process under consideration.
In reality, the process according to EP-A-1-052,262 does not guarantee good control of the reaction for the hydrolysis/condensation of silanes comprising hydrolyzable units (≡-Si-halogen, ≡-Si-alkoxy or ≡-Si—OH).
Finally, this process is not the most economic and also presents the disadvantage of being ecotoxic.