A novel method for the detection of sticking of tablets

The purpose of this work was to develop an instrumented upper punch to measure the adhesion force which occurs when the punch detaches itself from the upper surface of the tablet after compression. A specially designed adhesion force sensor instrumented with semiconductor strain gauges was inserted into an upper punch with a 25-mm punch face diameter suitable for a Korsch EK II eccentric press. Sorbitol, microencapsulated acetylsalicylic acid (ASA), and a formulation of a new active ingredient resulted in characteristic pull-off signals, providing a quantitative measure of the adhesion force. With "sticking-free" substances such as microcrystalline cellulose, tension signals could not be obtained; only Starch 1500 showed small adhesion force signals that indicated a sticking tendency. The compression force had a specific influence on the extent of the adhesion force; increasing the compression force caused an increase (sorbitol) or a decrease (ASA) of the adhesion force signals due to the plastic and elastic behavior of the substances. Depending on running time, ASA showed an increase in the adhesion force, reaching a plateau after 150 tablets. The addition of lubricants such as magnesium stearate resulted in smaller adhesion forces. The instrumented upper punch is a new helpful tool for the quantification of sticking and a valuable instrument in the development of formulations.