Patent Application
20080085986
A method of preparing a water soluble silicone is provided. The method comprises the reaction between a polydialkoxysiloxane (the alkoxy usually being either methoxy or ethoxy) and an alcohol, such as glycerol, Bronopol, or mixtures of glycerol and Bronopol. In the inventive process, the alkoxy group of the silicone undergoes a reaction with an aliphatic hydroxyl group of the alcohol in order to form an —Si—O—CH2‘ configuration with the alkoxy group of the alcohol. This reaction has been referred to in the past as “transetherification.”
To carry out this reaction, the polydialkoxysiloxane is mixed with the alcohol, with or without the addition of a catalyst. Progress of the reaction is monitored by sampling the reaction mixture and testing for the appearance of water-soluble material. If the reaction velocity is inconveniently low, it may be increased by heating the reaction mixture. When a satisfactory amount of water-soluble material has been produced and recovered, the catalyst, if any, is removed or neutralized and the water-soluble fraction of the reaction mixture is purified, preferably by means of chromatography.
In general, what has been discovered is that certain alcohols react under mild conditions with alkoxy groups situated on a polysiloxane chain in order to undergo what could be termed an “exchange reaction” or a “transetherification”; for example, the methoxy or ethoxy groups of the polyalkoxysiloxane are lost as methanol or ethanol while the alkoxy moiety of the reacting alcohol (usually of higher boiling point than that of methanol or ethanol) is bound to the polysiloxane. The inventive reaction thus provides a simple and rapid methodology for the modification of the chemical and physical properties of “silicones” having alkoxy groups in order to produce new chemical structures suitable for numerous applications.
The reaction with alkoxy groups requires a free alcohol group which has been in the past a primary, aliphatic group. It seems likely that reaction can be obtained also with secondary or tertiary groups, or with phenols, but probably with greatly decreased rates. The chemical reaction involved can be represented by the following equation:
In theory, the above reaction is probably readily reversed so that, in a closed system, equilibrium would likely be reached in which all the species indicated would be present at concentrations not drastically different from each other. However, if the reactive groups on the silicone are methoxy or ethoxy, while R2 is somewhat larger, the above reaction will easily go to completion by allowing the more volatile product to escape. In the present invention, two alcohols (R2OH), namely, glycerol and Bronopol are preferred, but other alcohols can be used for the inventive reactions without departing from the scope of the invention.
Modifying silicones in accordance with the invention produces materials that are suitable as bases for numerous dermatological preparations, both human and veterinary. In addition, possible uses may be found as materials of construction, paints and coatings for home, agricultural and industrial products where some affinity for water is desirable. The inventive material is both water soluble and compatible with human skin, where it is able to form an extremely thin film on it. This film resists washing with water and imparts a feeling of softness, while protecting the skin from bacterial and fungal invasion by virtue of added protective agents.
The inventive reaction is carried out on a practical scale by mixing the alkoxypolysiloxane, usually methoxy or ethoxy, with the alcohol to be coupled to the silicone and then heating at between about 140°-300° F. for 20 hours or more. The addition of an acidic catalyst, such as hydrochloric acid or trichloroacetic acid, or a basic catalyst, such as N-ethyldisopropylamine or triethylamine, to the reaction may be found useful in achieving desired reaction rates. If the substituting alcohol has, in addition to the hydroxyl group consumed in the coupling reaction, additional hydrophilic groups, the polysiloxane product may be water soluble. After heating the reaction mixture and then cooling to room temperature, the water soluble portion can be obtained by mixing with water and recovering the water soluble portion by centrifugation, decantation or filtration. The residue left after the extraction with water may be expected to contain polysiloxane species that have reacted with a number of alcohol molecules too few to give water solubility but, nevertheless, with a sufficient number to become very hydrophilic. Such material may prove to be well suited for incorporation into dermatological salves and creams or other products where some wettability is desirable.
The water soluble material produced by the inventive process is then preferably fractionated, either by size exclusion chromatography (SEC) or high performance liquid chromatography (HPLC). Fractionation by HPLC could serve to separate reaction produced products into many categories with slightly varying properties. This would reflect the enormous number of ways in which the polysiloxane structure may be substituted, both with respect to the degree of substitution and with respect to the arrangement of substituent groups along the polysiloxane chain.
Silicones substituted with glycerol or Bronopol, or combinations of the two, can be quite water soluble and, when applied in aqueous solution to the skin, leave a very thin film on the skin. They can impart also an hydrophobicity and an exceptionally smooth texture, which remains despite repeated exposures to water. The water solubility of such modified silicones strongly suggests that hydrophobic interactions between the polysiloxane and the skin are taking place. The modified silicones of the invention may promote healing in burn cases, both in terms of accelerating the healing process and in moderating pain.
The inventive silicone products could also be used as additions to finishes for many manufactured products since such additions discourage the accumulation of dust or other airborne contaminants. They are also potentially useful as a special class of lubricants since they could be attached covalently to surfaces using the reaction described in the equations set forth above, provided that the surface to be treated has free alcoholic —OH groups to act as an anchor for the polysiloxane.
Experimental results showing the feasibility for the synthesis of hydrophilic silicones by reacting polyalkoxysiloxones with aliphatic alcohols are given in Table 1 below along with a brief indication of some of the properties of these products:
It will thus be seen that the objects set forth above, among those made apparent by the description, are attained by the inventive reaction and the silicone products produced thereby.
The inventive scope is now defined in the following claims.
