Authors: A. Morfesis and D. Fairhurst
Affilation: Malvern Instruments, Inc., United States
Pages: 800 - 803
Keywords: sunscreen agents, UV protection, particle size, zeta potential, ZnO, TiO2
The physical (particulate) sunscreen agents, ZnO and TiO2, are increasingly used to provide UV protection in a wide variety of cosmetic and personal care products. The particle size distribution (PSD) of the ZnO and TiO2 directly impacts product performance in two critical ways. First is the UV attenuation efficiency, as measured by the ratio of the measured sun protection factor (SPF) per % active. Second, cosmetically acceptable and aesthetically pleasing products must also be transparent (i.e. non-whitening) when applied to the skin; this is a function of the scattering efficiency for visible light. All sunscreen actives are FDA monographed drugs. The grade of oxide currently used for sunscreens differs from standard USP grade (as used in diaper rash ointments) in that the particle size requirements are much smaller, typically around 100nm for ZnO and 80nm for TiO2. Whilst the advent of particle size improvements, smaller particle sizes, would further enhance customer products. The actual PSD in the final formulation is directly dependent upon the method of preparation and especially the quality of the oxide dispersion in the particular cosmetic vehicle. ZnO and TiO2 have their own chemical compatibility issues that must be recognized to obtain maximum performance. All these factors will be reviewed and illustrated by measurements comparing and contrasting both the particle size, scattering efficiency and zeta potential of a variety of first - and (newer) second-generation nanosize, transparent, ZnO and TiO2.