Sticking is often described as the Achilles’ heel of tablet production. It is one of the most common tablet tooling problems and can have a profound effect on production, leading to reduced output and increased costs. This article explains why sticking occurs and proposes several solutions.

Sticking can be confused with picking, which is another common manufacturing problem. However, they are fundamentally different. Sticking is when granule builds up on the punch tip face or die bore. Picking is compressed granule that has adhered to the embossing detail on the punch face, resulting in the “picking out” of parts from the tablet face.

Both issues have a negative effect on tablet appearance and can become so serious that production is interrupted. In extreme cases, punches need to be removed and cleaned. This is disruptive, labour intensive, reduces output and increases production costs.

There are many reasons why sticking can occur, and while it can feel like a challenge to resolve, there are solutions available.

Why Does Sticking Occur?

There is no simple answer to this question. It can be due to any number of reasons, from the physiochemical properties of the formulation to the surface characteristics of the punch face. It can also be related to the machinery, for example the compression force and speed, as well as the environmental conditions like temperature and humidity.

Sticky Granule: Problem

The granule within a formulation can be highly influential when it comes to sticking. Simply put, sticking is the battle between the cohesive forces holding a tablet together and the adhesive forces between the ingredients in the formulation. When the difference in force is significant and adhesive forces are higher, sticking occurs. There are three factors to consider when understanding the cohesive forces. The first is Van der Waals. This is the attraction of intermolecular forces between molecules and is affected by surface roughness and the effective contact area. Essentially, molecules can attract each other at moderate distances and repel at close range. Although these forces are very small, in mass they combine not only increasing cohesive forces within a tablet but also creating adhesive forces (when interacting with the elements in the tools steel or coating), which can cause sticking.

Capillary action is the second cohesive force created when moisture condenses into the gap between a particle and the surface, causing a capillary bridge. The strength of these forces can depend upon relative humidity, gap geometry and surface chemical condition. Capillary bridges increase the cohesive forces that bind a tablet together, and like Van der Waals, they can also be detrimental by increasing the adhesion forces between the granule and the punch tip faces, causing sticking. This is why it’s important to control the environment in all areas of tablet production and reduce humidity.

The third cohesive force is electrostatic. This is when tribo-charging occurs between contacting materials. It forms static electricity and is evident when the granule is very dry. The resulting force can be fairly strong and long-ranged, creating cohesive and adhesive forces. The environment is the biggest influence on adhesive forces causing sticking between the formulation and punch tip faces. Temperature can affect some active pharmaceutical ingredients (APIs). A good example of this is Ibuprofen, which has a very low melting point. For this reason, it is important to control the temperature in any area where the tabletting process takes place. Keeping the temperature at a constant low level will reduce sticking.

Humidity is also a problem. Moisture within a tablet can strengthen the adhesive force. This occurs with an increase in capillary action between the tooling surface and the granule. Capillary bridges form, causing high adhesion areas, creating sticking. Moisture can enter the process during wet granulation or from excess humidity in the compression chamber in a non-environmentally controlled area – the latter can even affect direct compression formulations.