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Ultraviolet light screening compositionsRelated Patent Categories: Drug, Bio-affecting And Body Treating Compositions, Topical Sun Or Radiation Screening, Or Tanning PreparationsUltraviolet light screening compositions description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060039875, Ultraviolet light screening compositions. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATIONS [0001] This application is a continuation of U.S. application Ser. No. 11/054,188, filed Feb. 9, 2005, now pending, which is a continuation of U.S. application Ser. No. 09/701,213, filed Apr. 16, 2001, now U.S. Pat. No. 6,869,596 B1, issued Mar. 22, 2005, which is a national stage filing under 35 U.S.C. .sctn. 371 of PCT International application PCT/GB99/01685, filed May 27, 1999, which was published under PCT Article 21(2) in English. BACKGROUND OF THE INVENTION [0002] The present invention relates to UV screening compositions, methods for their preparation and their use. The invention in particular relates to, for example, compositions comprising particulate oxides, their preparation and their use as, for example, paints, plastics, coatings, pigments, dyes and compositions for topical application, in particular, for example, sunscreens. [0003] The effects associated with exposure to sunlight are well known. For example, painted surfaces may become discoloured and exposure of skin to UVA and UVB light may result in, for example, sunburn, premature ageing and skin cancer. [0004] Commercial sunscreens generally contain components which are able to reflect and/or absorb UV light. These components include, for example, inorganic oxides such as zinc oxide and titanium dioxide. [0005] Titanium dioxide in sunscreens is generally formulated as "micronised" or "ultrafine" (20-50 nm) particles (so-called microreflectors) because they scatter light according to Rayleigh's Law, whereby the intensity of scattered light is inversely proportional to the fourth power of the wavelength. Consequently, they scatter UVB light (with a wavelength of from 290 to 320 nm) and UVA light (with a wavelength of from 320 to 400 .mu.m) more than the longer, visible wavelengths, preventing sunburn whilst remaining invisible on the skin. [0006] However, titanium dioxide also absorbs UV light efficiently, catalysing the formation of superoxide and hydroxyl radicals which may initiate oxidations. The crystalline forms of TiO.sub.2, anatase and rutile, are semiconductors with band gap energies of about 3.23 and 3.06 eV respectively, corresponding to light of about 385 nm and 400 nm (1 eV corresponds to 8066 cm.sup.-1). [0007] An incident photon is absorbed by titanium dioxide if its energy is greater than the semiconductor band gap Eg shown in FIG. 1. As a result an electron from the valence band (vb) is promoted into the conduction band (cb) (transition [1]). If the energy of the incident photon is less than Eg it will not be absorbed as this would require that the electron be promoted to within the band gap and this energy state is forbidden. Once promoted, the electron relaxes to the bottom of the conduction band (transition [2]) with the excess energy being emitted as heat to the crystal lattice. [0008] When the electron is promoted it leaves behind a hole which acts as a positive particle in the valence band. Both the electron and the hole are then free to migrate around the titanium dioxide particle. The electron and hole may recombine emitting a photon of energy equal to the band gap energy. However, the lifetime of the electron/hole pair is quite long due to the specific nature of the electronic band structure. Thus there is sufficient time (ca. 10.sup.-11 s) for the electron and hole to migrate to the surface and react with absorbed species. [0009] In aqueous environments, the electrons react with oxygen, and the holes with hydroxyl ions or water, forming superoxide and hydroxyl radicals:TiO.sub.2+h.nu..fwdarw.TiO.sub.2(e.sup.-/h.sup.+) .fwdarw.e.sup.-(cb)+h.sup.+(vb)e.sup.-(cb)+O.sub.2..fwdarw.O.sub.2..fwdar- w.HO.sub.2h+(vb)+OH.sup.-.fwdarw..OH [0010] This has been studied extensively in connection with total oxidation of environmental pollutants, especially with anatase, the more active form [A. Sclafani et al., J. Phys. Chem., (1996), 100, 13655-13661]. [0011] It has been proposed that such photo-oxidations may explain the ability of illuminated titanium dioxide to attack biological molecules. Sunscreen titanium dioxide particles are often coated with compounds such as alumina, silica and zirconia which form hydrated oxides which can capture hydroxyl radicals and may therefore reduce surface reactions. However, some TiO.sub.2/Al.sub.2O.sub.3 and TiO.sub.2/SiO.sub.2 preparations exhibit enhanced activity [C. Anderson et al., J. Phys. Chem., (1997), 101, 2611-2616]. [0012] As titanium dioxide may enter human cells, the ability of illuminated titanium dioxide to cause DNA damage has also recently been a matter of investigation. It has been shown that particulate titanium dioxide as extracted from sunscreens and pure zinc oxide will, when exposed to illumination by a solar simulator, give rise to DNA damage both in vitro and in human cells [R. Dunford et al, FEBS Lett., (1997), 418, 87-90]. SUMMARY OF THE INVENTION [0013] The present invention provides UV screening compositions which address the problems described above and are less liable to produce DNA damage on illumination than conventional sunscreen compositions. [0014] The present invention accordingly provides UV screening compositions comprising particles which are capable of absorbing UV light, especially UV light having a wavelength below 390 nm, so that electrons and positively charged holes are formed within the particles, characterised in that the particles are adapted to minimise migration to the surface of the particles of the electrons and/or the positively charged holes when said particles are exposed to UV light in an aqueous environment. It is believed that under these circumstances the production of hydroxyl radicals is substantially reduced. Thus the production of hydroxyl radicals may be substantially prevented. [0015] The minimisation of migration to the surface of the particles of the electrons and/or the positively charged holes may be tested by, for example, looking for a reduction in the number of strand breaks inflicted on DNA by light in the presence of particles or UV screening compositions according to the present invention, as compared with the number of strand breaks observed in DNA on treatment with particles used in conventional sunscreen compositions and light, or light alone. [0016] The compositions according to the present invention may find application as paints, plastics, coatings, pigments, dyes and are particularly favoured for use in compositions for topical application, especially, for example, sunscreens. [0017] The average primary particle size of the particles is generally from about 1 to 200 nm, for example about 50 to 150 nm, preferably from about 1 to 100 nm, more preferably from about 1 to 50 nm and most preferably from about 20 to 50 nm. For example, in sunscreens the particle size is preferably chosen to avoid colouration of the final product. For this purpose particles of about 50 nm or less may be preferred especially, for example, particles of about 3 to 20 nm, preferably about 3 to 10 nm, more preferably about 3 to 5 nm. [0018] Where particles are substantially spherical then particle size will be taken to represent the diameter. However, the invention also encompasses particles which are non-spherical and in such cases the particle size refers to the largest dimension. [0019] In a first embodiment the present invention provides UV screening compositions comprising particles which contain luminescence trap sites and/or killer sites. By luminescence trap sites and killer sites will be understood foreign ions designed to trap the electrons and positively charged holes and therefore inhibit migration. [0020] These particles may be, for example, reduced zinc oxide particles, especially reduced zinc oxide particles of from about 100 to 200 nm in size or smaller, for example, from about 20 to 50 nm. [0021] Such reduced zinc oxide particles may be readily obtained by heating zinc oxide particles which absorb UV light, especially UV light having a wavelength below 390 nm, and reemit in the UV in a reducing atmosphere to obtain reduced zinc oxide particles which absorb UV light, especially UV light having a wavelength below 390 nm, and reemit in the green, preferably at about 500 nm. It will be understood that the reduced zinc oxide particles will contain reduced zinc oxide consistent with minimising migration to the surface of the particles of electrons and/or positively charged holes such that when said particles are exposed to UV light in an aqueous environment the production of hydroxyl radicals is substantially reduced as discussed above. 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