Oil dispersions of zinc oxide

Abstract: An oil dispersion which is substantially transparent to visible light comprises an oil, particles of zinc oxide having an average size of from 0.005 to 0.15 micron and an organic dispersing agent. The solids content of the dispersion is at least 30 weight per cent, its extinction coefficients at 308 nm (E(308)) and 360 nm (E(360)) wavelengths are both at least 9 litres per gram per centimetre and the ratio E(360):E(308) is in the range 0.75:1 to 1.5:1. A method of preparing the dispersion is also described in which the ingredients are milled together in the presence of a particulate grinding medium until the required particle size and optical properties are obtained. The dispersions are useful in sun-screening preparations and especially those products designed to provide a balanced protection from UVA and UVB radiation.

Description: This invention relates to dispersions of zinc oxide particles and particularly to dispersions of these particles in an oil. It is known from, for example, JP60-231607 that ultrafine powdery zinc oxide can be used in a cosmetic to provide protection for the skin from ultraviolet light without introducing a whiteness due to reflection of visible light. It is also known from GB 2 184 356A that the use of ultrafine zinc oxide together with ultrafine titanium dioxide can provide balanced protection against UVA and UVB light. It is an object of the present invention to provide a convenient form of ultrafine zinc oxide which gives a balanced attenuation of UVA and UVB radiation. According to the present invention an oil dispersion comprises an oil, particles of zinc oxide having an average size of from 0.005 to 0.15 micron and an organic dispersing agent for said particles, the amount of said particles being such that the dispersion has a solids content of greater than 30 per cent by weight and said dispersion being substantially transparent to visible light and having an absorbance for ultraviolet light such that the extinction coefficient (E(308)) at a wavelength of 308 nm is at least 9 litres per gram per centimetre, the extinction coefficient (E(360)) at a wavelength of 360 nm is at least 9 litres per gram per centimetre and the ratio E(360):E(308) is in the range 0.75:1 to 1.5:1. According to the invention also a method for the manufacture of an oil dispersion comprises milling particulate zinc oxide in an oil in the presence of a particulate grinding medium and an organic dispersing agent for said zinc oxide in said oil in which the amount of zinc oxide is such that the dispersion has a solids content of greater than 30 per cent by weight and continuing said milling for a period of time such that the particulate zinc oxide has an average size of from 0.005 to 0.15 micron and that the dispersion obtained is substantially transparent to visible light and has an absorbance for ultraviolet light such that the extinction coefficient (E(308)) at a wavelength of 308 nm is at least 9 litres per gram per centimetre, the extinction coefficient (E(360)) at a wavelength of 360 nm is at least 9 litres per gram per centimetre and the ratio E(360):E(308) is in the range 0.75:1 to 1.5:1. It is preferred that the dispersions of the current invention have a high solids content since this enables the dispersions to be used in a wide range of final products. Particularly useful dispersions are those in which the solids content is at least 40 per cent by weight and dispersions in which the solids content is at least 60 per cent by weight are especially useful since they exhibit increased stability and allow the formulator using the dispersions increased flexibility in formulation. The invention is a dispersion of the fine particle sized zinc oxide dispersed in a high amount in an oil and is particularly directed to the use of an oil which is suitable for inclusion in a cosmetic product useful as a preparation to protect skin against sun-burning or other damage associated with exposure to ultra violet radiation. In particular the dispersions are useful in products designed to provide a balanced protection from both UVA radiation (wavelengths of 320-400 nm) and UVB radiation (wavelengths of 290-320 nm). Such materials are of increasing importance and use now since a greater number of people than ever are enjoying leisure activities in the outdoors and increasing their exposure to sunlight. The particles of zinc oxide in the oil dispersion have an average size of from 0.005 to 0.15 micron and where the particles are substantially spherical then this size will be taken to represent the diameter. However since the invention also encompasses the use of non-spherical particles then in such cases the size refers to the largest dimension. Preferably the particles of zinc oxide have an average size within the range 0.01 to 0.1 micron and most preferably within the range 0.03 to 0.07 micron when they are substantially spherical in shape. For particles having an acicular shape then the average largest dimension preferably is within the range 0.05 to 0.12 micron. The particles to be used to form the dispersions of the present invention may be uncoated or coated as is desired with one or more oxides or hydrous oxides of e.g. aluminium, silicon, titanium, zirconium, magnesium or zinc. One preferred coating is formed of an oxide or hydrous oxide of aluminium and an oxide or hydrous oxide of silicon in a weight ratio of Al₂O₃:SiO₂ of at least 0.5 and not greater than 4.5 and preferably the ratio is within the range 0.5 to 3.5. Most preferably the ratio is within the range 1.0 to 3.5. When such a coating containing aluminium and silicon oxides is employed then the actual amount of coating present is usually such that the amount of oxide or hydrous oxide of aluminium when expressed as Al₂O₃ is from 1.0 to 30.0 weight per cent based on the weight of solid (zinc oxide), and preferably from 3.0 to 15.0 weight per cent Al₂O₃ on weight of solid (zinc oxide). Consequently the amount of oxide or hydrous oxide of silicon will be that necessary to maintain the ratio of the amounts of coating oxides or hydrous oxides within the specified range and generally speaking the weight of oxide or hydrous oxide of silicon will be within the range 0.2 to 20.0 weight per cent SiO₂ based on solid (zinc oxide) and preferably from 1.5 to 7.0 weight per cent. If desired the particulate material may carry a coating of one or more organic materials such as an organic silicon compound e.g. a polymeric organic silicon compound. Other organic coating agents which may be present are the polyols, amines or alkanolamines. The particulate material of the present invention may be formed by any suitable process and typical processes are the French Method in which metallic zinc is melted and evaporated before being oxidized in the gas phase, the American method in which zinc ores are sintered and reduced with cokes and the zinc thus obtained is oxidised to zinc oxide and a wet method in which a water soluble zinc salt such as zinc chloride or zinc sulphate is crystallised and then converted to zinc oxide by sintering. The products of the present invention have the ability to transmit visible light but are partially or completely impermeable to UV light. This means that the products can find use in a wide variety of applications wherein it is important to maintain transparency to visible light while substantially preventing transmission of UV light to a surface. Cosmetics, sun-creams, plastics films and wood coating and other coating compositions are just a small number of applications for the products. The particle absorbancy for UV light is expressed as a function of the amount of the uncoated particle and when expressed as an extinction coefficient is substantially independent of the medium in which the particles are dispersed. The dispersion of particles of the present invention is partially or completely impermeable to a broad range of wavelengths of UV light and the extinction coefficients for UVA radiation (e.g. E(360)) and for UVB radiation (e.g. E(308)) are approximately equal. E(360) is greater than 9 litres per gram per centimetre and preferably greater than 10 litres per gram per centimetre. E(308) is greater than 9 litres per gram per centimetre and preferably both E(360) and E(308) are greater than 12 litres per gram per centimetre. Most preferably both E(360) and E(308) are between 12 and 30 litres per gram per centimetre. The ratio E(360):E(308) is in the range 0.75:1 to 1.5:1 and, preferably, in the range 0.9:1 to 1.2:1. The oil dispersions of the present invention are prepared by milling the zinc oxide product, coated or not, with a particulate grinding medium in the chosen oil and with a dispersing agent in the desired amount until the dispersion has the desired absorbance for ultraviolet light as hereinbefore described and the zinc oxide product has the particle size within the range stated hereinbefore. The oil can be any oil which is desirably present in the resultant dispersion but usually will be an oil which finds value in a cosmetic preparation. Such oils usually are the vegetable oils, for example, fatty acid glycerides, fatty acid esters and fatty alcohols with typical examples being sunflower oil (fatty acid triglyceride), castor oil, oleic and linoleic glycerides, oleyl alcohol, isopropyl palmitate, pentaerythritol tetracaprylate/caprate, propylene glycol di-esters of coconut fatty acids and pentaerythritol tetraisostearate. The mill which is employed to effect the grinding of the zinc oxide product in the oil is one which uses a particulate grinding medium to grind the product. Such mills are bead mills equipped with one or more agitators and using sand, glass beads, ceramic beads or other particles as the grinding medium. Particularly useful are those mills which operate at a high speed and depending on the size of mill a speed of the order of 2500 rev per minute (r.p.m) is not unusual. For instance mills operating at a speed of from 1000 r.p.m. to 6000 r.p.m are suitable. Agitator mills in which the tip speed of the agitator is up to and can exceed 10 metres/sec are of use. If desired the mill can be cooled. Also the ingredients of the dispersion can be pre-mixed using a high speed stirrer or the oil can be added to the mill initially and then the zinc oxide and the organic dispersant co-added to the oil subsequently. After milling has been carried out for the required time the dispersion is separated from the grinding medium by screening through a narrow gap. The dispersions of the present invention include an organic dispersing agent to promote the dispersion of the particulate zinc oxide in the chosen oil. Many types of organic dispersing agent have been developed and are available for use in promoting the dispersion of particles in oily media. Typically the dispersing agent can be one having a formula X.CO.AR in which A is a divalent bridging group, R is a primary secondary or tertiary amino group or a salt thereof with an acid or a quaternary ammonium salt group and X is the residue of a polyester chain which together with the -CO- group is derived from a hydroxy carboxylic acid of the formula HO-R'-COOH. As examples of typical dispersing agents are those based on ricinoleic acid, hydroxystearic acid, hydrogenated castor oil fatty acid which contains in addition to 12-hydroxy-stearic acid small amounts of stearic acid and palmitic acid. Dispersing agents which are polyesters or salts of one or more hydroxycarboxylic acid can also be used. These polyesters may also be formed from hydroxy carboxylic acids and carboxylic acids free from hydroxy groups. Compounds of various molecular weight can be used. Other suitable dispersing agents are those monoesters of fatty acid alkanolamides and carboxylic acids and their salts based on 6-22C (un)saturated fatty acids. Alkanolamides are based on ethanolamine, propanolamine or aminoethyl ethanolamine for example. Alternative dispersing agents are those based on polymers or copolymers of acrylic or methacrylic acids e.g. block copolymers of such monomers. Other dispersing agents of similar general form are those having epoxy groups in the constituent radicals such as those based on the ethoxylated phosphate esters. The dispersing agent can be one of those commercially referred to as a hyperdispersant specifically available as such and a particularly useful form of hyperdispersant is polyhydroxy stearic acid. When the dispersions of the invention are to be used in cosmetic or skin care preparations then it is desirable that the ingredients should have an acceptable level of toxicity and irritancy. The quantity of the dispersing agent used depends on various factors but generally an amount of from 5 per cent to 35 per cent, preferably 5 to 20 per cent by weight based on the weight of zinc oxide particles will be used. The invention is illustrated in the following Examples. EXAMPLE 1 50g uncoated spherical zinc oxide having an average particle size of 0.05 micron was added with 59.9g of a mixture of equal parts by weight of a mineral oil sold under the Trade Name Ondina L and a triglyceride of caprylic/caprinic acid sold under the Trade Name Myritol 318 R and 4.37 grams of a dispersant being a polyhydroxy stearic acid known as Solsperse 3000 to a high speed bead mill (Eiger M-50-VSE) with 60 grams of 1 mm glass beads as grinding aid. The dispersion was milled for 2 hours. The solids content of the dispersion was 43.8% by weight. After separation from the grinding aid a portion (0.1 gram) of the milled dispersion was diluted with n-hexane (100 ml). This diluted sample was then further diluted with n-hexane in the ratio sample:n-hexane of 1:19. The total dilution was 1:20,000. The diluted sample was then exposed in a Perkin Elmer Lambda 2 spectrometer with a 1 cm path length and the absorbance of UV and visible light measured. Extinction coefficients at several wave lengths were calculated from the equation A = E.c.l where A = absorbance, E = Extinction Coefficient in litres per gram per cm, c = concentration in grams per litre and l = path length in cm. The absorbance of UV and visible light was measured as described above and the extinction coefficient at various wavelengths calculated as follows:- E (308 nm) E (360 nm) E (524 nm) E (max) λ(max) 12.7 12.7 0.6 13.0 364 EXAMPLE 2 175g uncoated spherical zinc oxide having an average particle size of 0.05 micron was added with 249.4g of a mixture of equal parts by weight of a mineral oil sold under the Trade Name Ondina L and a triglyceride of caprylic/caprinic acid sold under the Trade Name Myritol 318 R and 13.1g of a dispersant being a polyhydroxy stearic acid known as Solsperse 3000 to a high speed bead mill (Eiger M-250-VSE) with 200g of 1 mm glass beads as grinding aid. The dispersion was milled for 45 minutes. The solids content of the dispersion was 40.0% by weight. After separation from the grinding aid, a portion (0.02g) of the milled dispersion was diluted with cyclohexane (100 ml). The total dilution was 1:5,000. The diluted sample was then exposed in a Perkin Elmer Lambda 2 spectrometer as previously described in Example 1. The absorbance of UV and visible light was measured and the extinction coefficient at various wavelengths calculated as follows:- E(308 nm) E(360 nm) E(524 nm) E(max) λ(max) 11.5 11.5 0.6 11.6 362 EXAMPLE 3 200g uncoated spherical zinc oxide having an average particle size of 0.05 micron was added with 185g of a mixture of equal parts by weight of a mineral oil sold under the Trade Name Ondina L and a triglyceride of caprylic/caprinic acid sold under the Trade Name Myritol 318 R and 15g of a dispersant being a polyhydroxy stearic acid known as Solsperse 3000 to a high speed bead mill (Eiger M-250-VSE) with 200g of 1 mm glass beads as grinding aid. The dispersion was milled for 45 minutes. The solids content of the dispersion was 50.0% by weight. After separation from the grinding aid, a portion (0.02g) of the milled dispersion was diluted with cyclohexane (100ml). The total dilution was 1:5,000. The diluted sample was then exposed in a Perkin Elmer Lambda 2 spectrometer as previously described. The absorbance of UV and visible light was measured and the extinction coefficient at various wavelengths calculated as follows:- E(308 nm) E(360 nm) E(524 nm) E(max) λ(max) 12.3 11.9 0.7 11.9 361 EXAMPLE 4 210g uncoated spherical zinc oxide having an average particle size of 0.05 micron was added with 124.25g of an octyl palmitate oil and 15.75g of a dispersant being a polyhydroxy stearic acid known as Solsperse 3000 to a high speed bead mill (Eiger M-250-VSE) with 200g of 1 mm glass beads as grinding aid. The dispersion was milled for 45 minutes. The solids content of the dispersion was 60.0% by weight. After separation from the grinding aid, a portion (0.02g) of the milled dispersion was diluted with cyclohexane (100ml). The total dilution was 1:5,000. The diluted sample was then exposed in a Perkin Elmer Lambda 2 spectrometer as previously described. The absorbance of UV and visible light was measured and the extinction coefficient at various wavelengths calculated as follows:- E(308 nm) E(360 nm) E(524 nm) E(max) λ(max) 12.8 12.3 0.5 12.3 361 EXAMPLE 5 210g uncoated spherical zinc oxide having an average particle size of 0.05 micron was added with 124.25g of a triglyceride of caprylic/caprinic acid sold under the Trade Name Myritol 318 R and 15.75g of a dispersant being a polyhydroxy stearic acid known as Solsperse 3000 to a high speed bead mill (Eiger M-250-VSE) with 200g of 1 mm glass beads as grinding aid. The dispersion was milled for 45 minutes. The solids content of the dispersion was 60.0% by weight. After separation from the grinding aid, a portion (0.02g) of the milled dispersion was diluted with cyclohexane (100ml). The total dilution was 1:5,000. The diluted sample was then exposed in a Perkin Elmer Lambda 2 spectrometer as previously described. The absorbance of UV and visible light was measured and the extinction coefficient at various wavelengths calculated as follows:- E(308 nm) E(360 nm) E(524 nm) E(max) λ(max) 12.8 12.4 0.8 12.4 360 EXAMPLE 6 250g of coated (2.0% SiO₂/8.0% Al₂O₃) spherical zinc oxide was added with 350g of a mixture of equal parts by weight of a mineral oil sold under the Trade Name Ondina L and a triglyceride of caprylic/caprinic acid sold under the Trade Name Myritol 318 R and 25g of a dispersant being a polyhydroxy stearic acid known as Solsperse 3000 to a high speed bead mill (Eiger M-250-VSE) with 200g of 1 mm glass beads as grinding aid. The dispersion was milled for 45 minutes. The solids content of the dispersion was 40.0% by weight. After separation from the grinding aid, a portion (0.1g) of the milled dispersion was diluted with cyclohexane (100ml). This diluted sample was then further diluted with cyclohexane in the ratio sample:cyclohexane of 1:19. The total dilution was 1:20,000. The diluted sample was then exposed in a Perkin Elmer Lambda 2 spectrometer as previously described. The absorbance of UV and visible light was measured and the extinction coefficient at various wavelengths calculated as follows:- E(308 nm) E(360 nm) E(524 nm) 10.6 9.5 2.7

Claims: An oil dispersion comprising an oil, particles of zinc oxide having an average size of from 0.005 to 0.15 micron and an organic dispersing agent for said particles characterised in that the amount of said particles is such that the dispersion has a solids content of greater than 30 per cent by weight and said dispersion is substantially transparent to visible light and has an absorbance for ultraviolet light such that the extinction coefficient (E(308)) at a wavelength of 308 nm is at least 9 litres per gram per centimetre, the extinction coefficient (E(360)) at a wavelength of 360 nm is at least 9 litres per gram per centimetre and the ratio E(360):E(308) is in the range 0.75:1 to 1.5:1. An oil dispersion according to claim 1 characterised in that the solids content is at least 60 per cent by weight. An oil dispersion according to claim 1 or 2 characterised in that the particles of zinc oxide are substantially spherical and have an average diameter within the range 0.01 to 0.1 micron. An oil dispersion according to claim 1 or 2 characterised in that the particles of zinc oxide are acicular and have an average largest dimension within the range 0.05 to 0.12 micron. An oil dispersion according to any one of the preceding claims characterised in that the extinction coefficient E(360) is greater than 10 litres per gram per centimetre. An oil dispersion according to any one of the preceding claims characterised in that both the extinction coefficients E(308) and E(360) are greater than 12 litres per gram per centimetre. An oil dispersion according to any one of the preceding claims characterised in that the oil is a fatty acid glyceride, a fatty acid ester or a fatty alcohol. An oil dispersion according to any one of the preceding claims characterised in that the dispersing agent is based on ricinoleic acid, hydroxystearic acid or a hydrogenated castor oil fatty acid. An oil dispersion according to any one of the preceding claims characterised in that the organic dispersing agent is present in an amount of from 5 to 35 per cent by weight with respect to zinc oxide. An oil dispersion according to any one of the preceding claims characterised in that the particles of zinc oxide are coated with an oxide or hydrous oxide of aluminium, silicon, titanium, zirconium, magnesium or zinc. An oil dispersion according to claim 10 characterised in that the particles of zinc oxide are coated with an oxide or hydrous oxide of aluminium and an oxide or hydrous oxide of silicon in a weight ratio of Al₂O₃:SiO₂ between 0.5 and 4.5. An oil dispersion according to claim 11 characterised in that the amount of oxide or hydrous oxide of aluminium, expressed as Al₂O₃, is from 1.0 to 30.0 weight per cent based on weight of zinc oxide. An oil dispersion according to claim 11 or 12 characterised in that the amount of oxide or hydrous oxide of silicon, expressed as SiO₂, is from 0.2 to 20.0 weight per cent based on zinc oxide. An oil dispersion according to any one of the preceding claims characterised in that the particles of zinc oxide are coated with an organic material. A method for the manufacture of an oil dispersion comprising milling particulate zinc oxide in an oil in the presence of a particulate grinding medium and an organic dispersing agent for said zinc oxide in said oil in which the amount of zinc oxide is such that the dispersion has a solids content of greater than 30 per cent by weight and continuing said milling for a period of time such that the particulate zinc oxide has an average size of from 0.005 to 0.15 micron and that the dispersion obtained is substantially transparent to visible light and has an absorbance for ultraviolet light such that the extinction coefficient (E(308)) at a wavelength of 308 nm is at least 9 litres per gram per centimetre, the extinction coefficient (E(360)) at a wavelength of 360 nm is at least 9 litres per gram per centimetre and the ratio E(360):E(308) is in the range 0.75:1 to 1.5:1. A method according to claim 15 characterised in that the particulate zinc oxide is milled in a mill operating at a speed of from 1000 to 6000 revolutions per minute. A method according to claim 15 or 16 characterised in that the zinc oxide, oil and organic dispersing agent are mixed using a high speed stirrer before being added to a mill.

Inventors: TAPLEY CAROLE ALLYSON MARIA, TAPLEY, CAROLE ALLYSON MARIA
Applicants: TIOXIDE SPECIALTIES LTD, TIOXIDE SPECIALTIES LIMITED
Application year: 1992
Main classification: A61K7
IPC codes: A61K8, B01J13, G02B5, C09D5, B01J19, A61Q17, C09C1, B02C17, C01G9, B02C19, B01J2