Scale-up of a photochemical flow reactor for the production of lignin-coated titanium dioxide as a sunscreen ingredient

Lignin-protected TiO2 can reduce the release of free radicals photogenerated by TiO2. This can be achieved through the intrinsic absorption of lignin that acts as an antioxidant strategically located on the surface of TiO2 particles while retaining – and to some degree improving – the light absorbing and scattering properties of TiO2. A 1–3% loading of lignin is sufficient to achieve this protection, with the coating produced photochemically taking advantage of the intrinsic free radical photogeneration capacity of TiO2. The process has been scaled up to amounts around 50 g per day by developing a large (10–25 L) flow photoreactor. As industry tends to use larger particles, our studies show that radical generation is lower with 100 nm, compared with 25 nm TiO2 particles; further, rutile is less reactive than anatase, a desirable characteristic in the fields of sunscreen, cosmetic and health applications