Challenging the paradigm of tablet relaxation in continuous coating: analysis of tablet dimensional changes after continuous direct compression

Tablet dimensional changes are one hurdle in continuous coating. The aim of the present study was to evaluate whether a time period of tablet core relaxation is needed prior to coating. The conditions for the experiments were chosen such that maximum tablet core dimensional changes were achieved as challenge for subsequent coating. Three powder blend compositions containing the (super)disintegrants Starch 1500® and/or crospovidone resulting in tablet cores with different relaxation tendency were coated immediately or 24 h after compaction with three immediate release Opadry® coatings of variable flexibility to show a potential impact on the coating quality. ‘Dry’ and ‘moist’ coating conditions were applied by variation of the coating suspension spray rates. The tablets increased significantly in tablet dimensions depending on their formulation. Despite these dimensional changes, the quality of the coatings was satisfactory as no defects were found. In addition, no differences were observed in the coating thickness of the three coated tablet formulations. The expected defects in the coatings were only found under high humidity storage conditions. In conclusion, the presented results revealed that no relaxation time interval between compaction and coating was necessary, indicating only minor relevance of the elastic relaxation in (continuous)