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Tablets & Capsules October 2019 29 film coating adhesion, but to a lesser extent than the lubricants (Figure 4). Discussion Mag nesiu m stearate has a lay- ering effect [3] that leads to lower overall bond abilit y bet ween par- ticles in a formulation and, there- fore, lower tablet tensile strength, higher f riabilit y (data not shown), and higher surface roughness for the tablets produced. Surface roughness, in combination with the hydropho- bicit y of MgSt, lead s to reduced film coating adhesion. This layering effect occurs to a lesser extent with SSF [4] and may even be reversed in some cases [5]. As a result, SSF yielded tablets with higher tensile strength than MgSt. Surface roughness. Tablets con- taining MgSt exhibited higher sur- face roughness than pure MCC tab- lets (Fig ure 1). Tablets containing SSF possessed a smoother tablet sur- face than pure MCC tablets, and the coprocessed excipient provided the smoothest tablet surface of all formu- lations tested, as shown in the SEM images in Fig ure 2. Disinteg rants increased the tablet cores' surface roughness compared to pure MCC. F i l m c o at i n g a d h e s i o n . T h e investigation of film coating adhe- sion (measured as stress of failure) showed that the lubricants strongly decreased the adhesion of the aque- ous HPMC coating (Figure 3). MgSt showed a stronger effect than SSF. T he d isinteg rants also decr eased Figure 2 SEM pictures of tablet surfaces c. Easytab a. MCC b. MCC + SSG Figure 3 Stress of failure of pure MCC tablet cores compared to tablet cores containing lubricants Stress of failure (megapascals) 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Easytab MCC MCC + MgSt MCC + SSF Figure 4 Stress of failure of lubricant-free tablet cores Stress of failure (megapascals) 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 MCC MCC + CCS MCC + PVPP MCC + SSG Within the group of tablets containing lubricants, the coprocessed excipient yielded the highest film coating adhesion values due to the tablets' high tensile strength and smooth surface.