Evaluation of mechanical properties of TiO₂-free tablet coatings

In early 2022, the European Union delisted titanium dioxide (TiO₂) from the list of approved food additives, creating a need to explore potential alternatives to titanium dioxide. Given the concerns regarding TiO₂ and the possibility of similar restrictions to their use in tablet coatings, there is a need to evaluate alternatives to TiO₂ opacifiers in tablet coatings. Specifically, the alternative opacifiers are expected to influence the mechanical and adhesion properties of tablet coatings. The current study performs a comprehensive evaluation of six different commercially available titanium dioxide-free (TF) tablet coatings. The TF coatings contain calcium carbonate, rice starch and magnesium carbonate as opacifiers, with polyvinyl alcohol (PVA) or hydroxypropyl methylcellulose (HPMC) as the base polymers. Detailed measurements of drying stress, mechanical strength, and adhesion testing were conducted to evaluate their suitability as pharmaceutical coatings. The TF coatings resulted in drying stresses that were comparable to those observed for TiO₂ coatings. However, the tensile strength and Young's modulus of all TF coatings, except one, were observed to be lower, indicating a relatively poor mechanical strength. Adhesive strength of TF coatings was similar to that of TiO₂ coatings, with waxy tablets exhibiting lower adhesion strength due to higher hydrophobicity. In summary, the HPMC-HPC-based TF coating showed superior mechanical strength and comparable adhesive properties when compared to TiO₂ coatings. This positions it as a viable alternative to TiO₂ among the coatings tested. In contrast, all other TF coatings did not achieve the performance levels of the TiO₂ coatings.