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Tablets & Capsules September 2020 19 Adhesive forces Adhesive forces cause tablets to stick to the punch faces after compression. Adhesive force is affected by the surrounding environment, the chemistry at the interface between the formulation and the punch, the formulation's spacial heterogeneity, the formulation's deformation mechanics, and the tooling's surface characteristics. Environment. The environment around the tableting process and even around the formulation preparation process are critical. Temperature has a huge influence on some active pharmaceutical ingredients (APIs); ibuprofen, for example, has a very low melting point. Lowering the temperature in the tableting area can help reduce sticking during compaction. Excess humidity in and around the tablet press or for- mulation processing equipment or in the area where the formulation is stored prior to tableting can also introduce capillary bridging between the granules and the tablet tooling, significantly increasing the chance of sticking. During the TSAR project, researchers used a sessile droplet test to measure the influence of capillary forces for each individual surface. This test determines the static contact angle between a liquid (in this case, water) and a solid surface and should always be used when developing anti-stick coatings or surfaces for tablet tooling because it establishes the surface's level of hydrophobicity or hydro- philicity. Hydrophobic surfaces will have a static contact angle greater than 90 degrees, while hydrophilic surfaces will have an angle less than 90 degrees. Figure 2 shows the results of the sessile droplet test for different steels and coated surfaces. The yellow columns represent three pharmaceutical-grade steels (HPG-S, HPG-P, and HPG-SS), which have varying contact angles because of different compositions and polishes. The red columns represent tooling coated with two types of hard-chromium (HC and HC+). Hard-chromium-based coatings have been used in tableting for many years to decrease sticking by creating a less hydrophilic surface, but they are less common today due to the carcinogenic waste produced during manufacture. Also, when hard In 2012, I Holland began a two-year project to gain detailed insight into the problem. The Tableting Science Anti-Stick Research (TSAR) project investigated the root causes of sticking and looked at solutions through a pre- dictive mathematical model. The results pointed to sev- eral causes that can be classified as either cohesive or adhesive forces, as shown in Figure 1. Cohesive forces Cohesive forces hold a tablet together and are largely affected by three elements: Van der Waals forces, capil- lary action, and electrostatic force. Van der Waals forces. Van der Waals forces are attractive forces between molecules. They act over a short range but, in mass, combine to increase the cohe- sive force within a tablet and can also cause sticking. They are relatively weak and, during tablet compression, are affected by the surface roughness and effective con- tact area of the punch. Capillary action. Capillary action can be linked to tablet formulations with high moisture content and is generated when moisture condenses in the gap between particles or between a particle and a surface to create a liquid bridge. The strength of this cohesive force can depend on the relative humidity, gap geometry, and sur- face chemical condition. Capillary bridges increase the cohesive forces that help bind a tablet together. How- ever, they can also increase the adhesion forces between the granules and the punch tip face, leading to sticking. Electrostatic forces. Static electricity is a major cause of sticking and often occurs when tableting dry granules. Electrostatic forces can also arise when tribo-charging (a transfer of electrical charges) occurs between contacting materials. The resulting force can be relatively strong and long ranged, with the potential to create both cohesive and adhesive forces. Manufacturing equipment can also influence the presence and strength of the electrostatic force. To minimize electrostatic force generation and protect press operators from shock, tablet presses are typ- ically connected to a grounding cable. Non-conductive tool coatings or treatments can also help to reduce elec- trostatic force generation. Figure 1 Cohesive and adhesive forces Van der Waals Capillary Electrostatic Cohesive forces Adhesive forces Roughness and morphology of contact Deformation mechanics Chemistry at interfaces and spatial heterogeneity Environment Figure 2 Sessile droplet test Static contact angle (degrees) Steel or coating type 80 70 60 50 40 30 20 10 0 HPG-S HPG-P HPG-SS HC HC+ ECxtra CN CN+ CT DN