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Letters to the editor

Sunscreen and coatings industry

I spent 35-plus years in the R&D of the coatings industry, where we used TiO₂ and ZnO as pigments for a wide variety of coatings. Extensive research conducted over about a century in the coatings (and related) industries taught the coatings industry that while TiO₂ is an excellent opacifier and ultraviolet-blocking pigment, it is hardly inert for the polymeric binder that carries it. Separately, ZnO is considered a poor opacifier and UV-blocking pigment and is used primarily for its biostatic properties. It too is far from inert. So I wonder about the US Food and Drug Administration’s proposal to label both as generally recognized as safe and effective (C&EN, March 4, page 14).

TiO₂ is the primary source of UV-catalyzed degradation of the polymeric binders employed in white coatings (gloss loss; chalking). The anatase form is a potent catalyst, while the rutile form (used primarily as the pigment of choice) is far less so but still far from zero. But even commercial exterior-grade rutile pigments are completely encapsulated with a SiO₂ layer to seal off the TiO₂ core from interactions with the polymeric binder. An additional alumina layer is on top of the silica layer to provide dispersibility in the various media employed in making liquid paints (water, various solvents).

ZnO is a far less efficient UV screen than TiO₂ and is so deficient in opacity that it has not been used as a white pigment in coatings since the onset of widespread use of TiO₂ in the 1940s. It is slightly soluble in water, so it causes package instability in water-based coatings that must be separately combated. The bioactivity in coatings (mildew resistance for exterior house paints) is both direct and from the soluble Zn²⁺ ions in solution when it rains or forms dew.

I wonder if the people working on UV screens for human skin are aware of, concerned for, and researching the relevance of the coatings industry’s knowledge as it might relate to the issue of whether they are safe when used directly on human skin for prolonged periods to screen out UV radiation (sunburn). For example, are coated versions of rutile TiO₂ being used, or other types? It would be helpful if authors included this line of inquiry in future articles. Or perhaps fellow readers could share their knowledge on this concern.

Since TiO₂ and ZnO manufacturers worldwide were the ones that educated the coatings industry on these issues over the decades, they should have an abundance of information along these lines, if authors would only ask.

Lou Floyd
Independence, Ohio