1. Technical Field of the Invention
The present invention relates to a process for photostabilizing, with respect to UV radiation, at least one dibenzoylmethane UV-screening agent with at least one arylalkyl benzoate compound.
The present invention also relates to novel photoprotective compositions, in particular cosmetic compositions for topical application.
2. Description of Background and/or Related and/or Prior Art
It is known that light radiation with wavelengths of from 280 nm to 400 nm allows tanning of the human epidermis and that light rays more particularly ranging from 280 to 320 nm, known as UV-B rays, cause skin burns and erythema which can harm the development of a natural tan. For these reasons, as well as for aesthetic reasons, there is a constant demand for means of controlling this natural tanning so as to thus control the color of the skin; this UV-B radiation should thus be screened out.
It is also known that UV-A rays, with wavelengths of from 320 to 400 nm, which cause tanning of the skin, are liable to induce adverse changes therein, in particular in the case of sensitive skin or skin which is continually exposed to solar radiation. UV-A rays cause in particular a loss of elasticity of the skin and the appearance of wrinkles leading to premature aging of the skin. They promote triggering of the erythemal reaction or amplify this reaction in certain individuals and may even be the cause of phototoxic or photoallergic reactions. Thus, for aesthetic and cosmetic reasons such as the conservation of the skin's natural elasticity, for example, an increasingly large number of individuals wish to control the effect of UV-A rays on their skin. It is therefore desirable to also screen out the UV-A radiation.
For the purpose of protecting the skin and keratin materials against UV radiation, anti-sun/sunscreen compositions comprising organic screening agents that are active in the UV-A range and in the UV-B range are generally used. The majority of these screening agents are liposoluble.
In this respect, a particularly advantageous family of UV-A screening agents currently consists of dibenzoylmethane derivatives, and in particular 4-tert-butyl-4′-methoxydibenzoylmethane, which are liposoluble and in fact have a high intrinsic absorbing power. These dibenzoylmethane derivatives, which are products that are now well known per se as screening agents that are active in the UV-A range, are described in particular in FR-A-2,326,405 and FR-A-2,440,933, and also in EP-A-0,114,607; 4-tert-butyl-4′-methoxydibenzoylmethane is, moreover, currently marketed under the trademark “Parsol 1789” by Roche Vitamins.
Unfortunately, it has been found that dibenzoylmethane derivatives are products that are relatively sensitive to ultraviolet radiation (especially UV-A), i.e., more specifically, they have an annoying tendency to be degraded more or less rapidly under the action of this UV. Thus, this substantial lack of photochemical stability of dibenzoylmethane derivatives towards ultraviolet radiation, to which they are by nature intended to be subjected, does not make it possible to ensure constant protection during prolonged exposure to the sun, and so the user must make repeated applications at regular and frequent time intervals in order to obtain effective protection of the skin against UV rays.
Dibenzoylmethane derivatives are oil-soluble solid screening agents. Among the oils capable of effectively solubilizing these UV-screening agents, alcohol benzoates are known, in particular C12/C1-5 alkyl benzoates, for instance the commercial products Finsolv TN or Witconol APM manufactured and marketed by Witco.
However, the alcohol benzoates known to date do not make it possible to solve the problem of the photostability of dibenzoylmethane derivatives with respect to UV radiation.
WO 2005/009341 discloses screening compositions based on organic screening agents in which an aryl phenylethyl ester compound is used as a solvent for the screening agents in oils. That document teaches that this type of compound makes it possible to effectively solubilize the dibenzoylmethane derivative: butyl methoxydibenzoylmethane. It describes solutions of butyl methoxydibenzoylmethanes in 2-phenylethyl benzoate, 2-phenylethyl o-toluate, 2-phenylethyl p-toluate or in a 2-phenylethyl o-toluate/2-phenylethyl p-toluate (1/1) mixture. It also describes anti-sun compositions containing butyl methoxydibenzoylmethane in the presence of octyl methoxycinnamate. That document does not, however, mention the problem of the photoinstability of dibenzoylmethane derivatives.