Abstract:
A method for protecting and stabilizing dihydroxyacetone (DHA) is described. The method is characterized in that at least a part of the dihydroxyacetone is enclosed in dimer form in a watertight composition which releases the dihydroxyacetone when applied. The protected and stabilized DHA composition has application in the cosmetics industry.

Description:
FIELD OF THE INVENTION 
     The present invention relates to dihydroxyacetone (DHA), and particularly to its cosmetic applications. 
     BACKGROUND OF THE INVENTION 
     The dihydroxyacetone has been used for many years in cosmetology for its skin pigmentation properties and has become a tanning agent used in what is usually called suntan creams. 
     Such a use of the dihydroxyacetone is faced with many difficulties due to its solubility in water, its oxidizing capacity, its weakness when subjected to heat, the instability of its pH, its sensitivity to microbial and bacterial agents, etc. These difficulties require using the product containing dihydroxyacetone very quickly after its preparation, the portion of dihydroxyacetone which remains active diminishing very rapidly with time. Moreover, the product has to be stored in optimum conditions of temperature and hygrometry in order to avoid the decomposition and/or dehydration of the aqueous solution of dihydroxyacetone. 
     In order to facilitate the use of dihydroxyacetone in the cosmetology industry, it is therefore necessary to remedy at least some of these disadvantages. The works carried out in this respect have resulted into the present invention. 
     SUMMARY OF THE INVENTION 
     To this effect, the invention relates to a method for the protection and stabilization of the dihydroxyacetone (DHA), characterized in that a portion at least of the dihydroxyacetone in the dimer form imbedded in a watertight composition which releases the dihydroxyacetone when applied. 
     The watertight composition protects the DHA from water and oxidation and forms an envelope ensuring a certain thermal insulation. 
     According to a preferred embodiment of the method of this invention, the watertight composition is soluble in a solvent. 
     Preferably, said watertight composition contains a mixture of polymers insoluble in water such as: cellulose acetophthalate, vinylidene polychloride, styrene-acrylonitrile copolymers, copolymers of dimethylaminoethylmethacrylate and other neutral esters of methacrylic acid, cellulose acetate, nitrocellulose, ethylhydroxyethyl cellulose and ethyl cellulose. 
     Advantageously, the watertight composition contains ethyl cellulose in a concentration of 0,1% to 10%, preferably of about 2%, with respect to the weight of DHA. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to facilitate the release of DHA when applied on the skin, one has to ensure that the watertight composition contains oily fats having a set fusion point such as a mixture of monoglycerides, diglycerides or triglycerides of saturated or unsaturated fatty acids having a chain with a length between C 9  and C 22 , preferably between C 12  and C 18 , and one controls the fusion point and the hardness by adding hydrocarbons derived from petroleum, from silicones and derivatives thereof, from mineral, vegetable or animal waxes and/or from fatty alcohols, from their long chain esters and from saturated or unsaturated fatty acids, for example lauric, myristic, palmitic, oleic or stearic acid, and particularly stearic acid in a concentration of 1% to 15%, preferably of about 7%, with respect to the weight of DHA. 
     Advantageously, in order to stabilize the pH in a zone between acid and neutral inside of and in the immediate vicinity of the watertight composition, the invention provides that the composition contains an organic acid or one of its salts such as acids-phenols, acids-alcohols, or a precursor of such an acid in the form of lactone, particularly the glucuno-delta-lactone in a concentration of 0,01% to 2%, and preferably of about 0,15% with respect to the weight of DHA. 
     According to a preferred embodiment of the method according to the invention, and to avoid the bacterial degradation of DHA, one sees that said watertight composition contains a bacteriostatic and/or a bactericide and/or a fungicidal agent compatible with the use in cosmetology, such as the derivatives of isothiazolone, the formaldehyde generating products, the organic and mineral derivatives of mercury, bromine, chlorine, the acids, the alcohols, the phenols and derivatives thereof, particularly the derivatives of parahydroxybenzoic acid, of its esters, of its salts or of its solution in phenoxyethanol, in a concentration of 0,05% to 0,5%, preferably of about 0,15%, with respect to the weight of DHA. 
     According to the invention, the watertight composition is applied in a single application or in several applications containing each at least some of the components of said watertight composition, by a granulation, pelliculation or encapsulation operation by a fluidized bed or by paring in a turbine. These methods are such that they allow a lesser portion of the DHA to remain in a monomer form immediately usable from the cosmetologic viewpoint. 
     The invention relates also to the intermediate industrial product, in the cosmetologic industry, formed by the DHA protected and stabilized by the hereabove method, as well as the cosmetologic products containing this intermediate product. 
    
    
     EXAMPLE 
     By way of example, the composition of two protection layers applied according to the method of the invention to 1 kg of DHA will be described hereafter. The percentages mentioned are by weight. 
     
         ______________________________________1st layer______________________________________Ethyl cellulose        (sealing agent)  1,25%Stearic acid controlled sealing,                         4,00%Triglycerides        fusion and crushing                         5,00%        agents, cosmetic        softener______________________________________ 
    
     A plastifier of the ethylcellulose dissolved in alcohol or methylene choride. 
     
         ______________________________________2nd layer______________________________________glucuno-delta-lactone           (pH stabilizer)                        0,10%stearic acid    (as in layer 1)                        2,00%triglycerides   (as in layer 1)                        20,50%hydrophobic silica           (tightness, control                        0,11%           of fusion point)mixture of esters of           (antibacterial)                        0,10%parahydroxybenzoic aciddissolved in alcohol ormethylene choride.______________________________________ 
    
     The DHA thus protected has turned out to be without the faults of the non protected DHA, thereby allowing a reliable, large scale and stable application in the cosmetologic industry.