Sliding friction coefficients of pharmaceutical tablets: influence of coating formulation and contact surface finish

The flow behavior of pharmaceutical tablets during conveying, coating, and packaging is strongly influenced by kinetic, or sliding, friction. Accurate measurement of friction coefficients can help guide coating formulation selection and support more reliable and efficient manufacturing and packaging processes. Kinetic friction coefficients were measured for a range of film-coated and uncoated pharmaceutical tablets using a pin-on-disk tribometer. Measurements were conducted for contacts between tablets and common equipment surfaces, including stainless steel with various surface finishes and polymeric materials, as well as tablet-tablet contacts. The results show that friction depends on both coating formulation and contact surface, and suggest that frictional behavior is not always simple or monotonic across test conditions, including variations in surface roughness and sliding speed. Specifically, tablet-tablet friction coefficients ranged from approximately 0 to over 0.6, with an ethylcellulose-based coating generally exhibiting the largest friction, and poly(vinyl alcohol)-based coatings exhibiting the smallest. Tablet-disk friction coefficients ranged from approximately 0 to nearly 0.9, and generally increased in the order polyoxymethylene, stainless steel, and polycarbonate. Among the formulation variables examined, friction coefficients appeared to be most closely associated with the choice of film-forming polymer. Accordingly, given knowledge of the film former chemistry and the contacting material, a sliding friction coefficient can be estimated from the present results. Both measured and estimated friction coefficients can inform coating formulation decisions, guide process design, and provide reliable inputs for computational models.