Surface Preparation and its Effect on Sticking

This study examined how controlled punch surface preparation and subsequent atmospheric aging influence adhesion between punch metal and formulation powder during tablet compaction. We aimed to clarify how the evolving surface state of S7 tool steel affects sticking for representative excipients and active pharmaceutical ingredients (APIs), and to establish a reproducible approach for surface state control in material-sparing sticking studies.

Removable S7 tool steel punch tips were mechanically polished, chemically cleaned, and rinsed to create a defined "as-cleaned" surface. Punches were aged at 55% RH for 10 min to 72 h. Sticking of mannitol, Starch 1500, acetylsalicylic acid (ASA), and ibuprofen (IBU) was quantified using a non-contact laser reflectivity sensor and measuring adhered mass after single compactions. Static contact angle measurements tracked surface evolution, and X-ray photoelectron spectroscopy (XPS) characterized chemical changes.

Freshly cleaned punches showed strong adhesion for mannitol and starch. Starch lost its sticking tendency within minutes of aging, while mannitol remained adhesive until several hours of exposure. ASA and IBU exhibited moderate sticking that declined slightly with aging. Contact angle and XPS indicated rapid formation of a carbonaceous/oxide overlayer, reducing polar surface energy and altering adhesion pathways.

Punch surface chemistry evolves quickly under ambient conditions, strongly impacting sticking behavior. Cleaning to testing time can alter adhesion outcomes within minutes, highlighting the need to standardize surface preparation and define the punch-surface state. This framework offers a practical basis for surface control in pharmaceutical compaction studies.