Inorganic sunscreens attenuate UV light by a combination of absorption and scattering. For more detail on this topic, see the following reference:-
M.W.Anderson et al, “Broad-Spectrum Physical Sunscreens: Titanium Dioxide and Zinc Oxide”, in “Sunscreens: Development, Evaluation, and Regulatory Aspects”, 2nd edn., edited by N.J.Lowe, N.A.Shaath and M.A.Pathak; published by Marcel Dekker, New York, 1997; pages 353-397).
This can cause much confusion. Particle size can mean the size of the fundamental crystals, or the size of aggregates or agglomerates. All inorganic particulates aggregate to some degree, and in use, it is the size of the aggregates which determines the optical properties of the material. When trying to interpret particle size data, it is important to be aware of whether the data is referring to crystal size or the size of aggregates.
There are a number of different particle size measurement techniques available, but few which can be applied to inorganic sunscreens. Most techniques are not sensitive to particles which are smaller than about 100 nm, and therefore tend to overestimate the true particle size. Also, different techniques give different results, and the preparation of samples for measurement can further influence the results. So particle size data on different materials can only be compared if measured by the same method and under the same conditions. We have found that the Brookhaven X-ray disc centrifuge gives reliable and reproducible results for inorganic sunscreen materials.
Titanium dioxide exists in 3 main crystal forms : rutile, anatase and brookite. Rutile (used in all Croda titanium dioxide products) has a higher refractive index than anatase and is therefore more effective as a UV attenuator. Rutile is also more thermodynamically stable and more photostable than anatase. The brookite form is of little interest commercially and is not used at all in UV-attenuating grades.
SPF is the recognised method for measuring protection from burning or erythema, and is primarily influenced by UVB protection. UVA protection must be measured in a different way. Various methods exist (for example, PPD= persistent pigment darkening, or IPD = immediate pigment darkening) but none of them are globally accepted. Spectroscopically the UV attenuation can be measured across the whole of the UV spectrum and this gives a good guide to the protection offered in a final formulation.
No. Croda sunscreen dispersions are designed to give good attenuation in the ultra violet region of the electromagnet spectrum and do not absorb infra-red radiation (heat). Particulates could be designed to do so but they would be white or coloured.
The bright white appearance of Tioveil™ and Spectraveil™ dispersions arises because the particles scatter visible light to some extent, as well as UV light. The particles in Solaveil™ Clarus dispersions, however, are very ineffective in scattering visible light (hence the much greater transparency). Therefore the dispersion appears off-white in colour. This effect is not apparent in a final formulation, which will usually appear white, due to scattering of light by the emulsion droplets.
Titanium dioxide can develop a slight colour (greyness) on exposure to UV light due to the photoreduction of a very few Ti4+ atoms to Ti3+(which has a purple colour). The reaction is reversible on exposure to air (oxygen) and does not affect the efficacy of the actives. This “photogreying” does not occur if the formulation is stored in opaque packaging.
Titanium dioxide is particularly effective as a UV attenuator because it is capable of absorbing UV light. The flip side of such absorption is exploited in photocatalytic applications. However titanium dioxide used for cosmetic applications is coated so as to minimise or eliminate any photocatalytic action. In any case ongoing reactions are very short lived and have never been found to cause phototoxic effects.