1. Field of the Invention
The invention relates to metal oxide particles coated with silicon dioxide, a process for their production, and their use.
2. Description of the Background
Metal oxides, such as titanium dioxide or zinc oxide are widely used in sun screening agents. Their action is substantially based on the reflection, scattering and absorption of harmful UV radiation and is substantially dependent upon the primary particle size of the metal oxides.
The drawback of metal oxides such as titanium dioxide or zinc oxide is their photocatalytic activity through which reactions are triggered which may lead to changes to components of a sun screening agent.
Attempts are being made to reduce the photocatalytic activity of these metal oxides without reducing the UV-screening properties by, for example, surrounding them with a coating.
EP-A-0 988 853 describes metal oxide particles coated with silicon dioxide, plus their production and use as a component in sun screening agents. Here, the drawback is that these coated metal oxide particles have low surface functionality and a high degree of particle coalescence. On the one hand, this hampers the incorporation of the particles in a cosmetic formulation and, on the other hand, it restricts their resistance with regard to sedimentation. Another drawback is the fact that during the production of these particles, in addition to water, an organic solvent is essential to enable a coating to form. The organic solvent requires high safety precautions. In addition, the organic solvent, preferably miscible with water, necessitates large economic expenses in order to separate the organic solvent from the water after the reaction and in order to dispose of the organic solvent. Finally, according to EP-A-0 988 853, only tetraalkoxysilanes may be used, while halogen-substituted silanes, for example, are explicitly excluded.
Therefore, an object of the present invention is to provide coated metal oxide particles that avoid the drawbacks of the prior art. They should, in particular, be easy to incorporate in cosmetic formulations, be stable therein and have a low level of photocatalytic activity.
Another object is to provide a process that avoids the drawbacks of the prior art. In particular, no organic solvents should be used. Furthermore, the silicon dioxide precursors should not be restricted to only one class of compounds.
The present invention provides coated oxide particles, comprising a metal oxide core and a silicon dioxide coating surrounding the core, characterized in that the coated oxide particles have a low structure, defined by the absence of an endpoint during the dibutyl phthalate absorption.