From a view point of dermatology, ultraviolet ray in sunlight is divided to long wavelength ultraviolet ray (UV-A) in a range from 400 nm to 320 nm, medium wavelength ultraviolet ray (UV-B) in a range from 320 nm to 290 nm, and short wavelength ultraviolet ray (UV-C) in a range of 290 nm or less. Of these, the ultraviolet ray having a wavelength of 290 nm or less is absorbed by the ozone layer, and does not reach the surface of the earth.
Ultraviolet rays reaching the surface of the earth gives various effects to skin of a man. Of the ultraviolet rays which reach the surface of the earth, UV-B forms erythema or blister on human skin and also promotes formation of melanin. On the other hand, UV-A changes color of human skin to brown, and promotes rapid aging of human skin including decrease of elasticity thereof or generation of wrinkles on it. Also UV-A promotes start of the erythema reaction or actives this reaction in certain types of patients, and furthermore it may becomes a cause for light toxicity or allergy against light. To protect human skin from the harms of UV-A as described above, various types of ultraviolet ray absorbent have been developed.
Conventional types of UV-A ultraviolet ray absorbent such as benzophenone derivative, dibenzoyl methane derivative, and benzotriazole have been used as a UV-A absorbent, and compounded in external preparations for skin.
On the other hand, in recent years, such silicone-based bases as dimethyl siloxane having excellent water resisting property which are not easily flown away by sweat or in swimming have been widely used in external preparations for skin in which a ultraviolet absorbent is compounded. These silicone-based bases have been used largely because, in addition to their excellent water resisting property, they can easily be extended on human skin and are very comfortable and are not sticky when used.
However, the conventional types of UV-A ultraviolet ray absorbent as described above have a remarkably low compatibility with silicone-based bases.
Also, an absorbent is generally a crystalline material having a dark color, and is easily crystalized under a low temperature, or often contaminates clothes, so that a quantity of absorbent to be used is limited to a very small level and functions of the UV-A absorbent have not been fully realized.