Twinning Defects in Tablet Coating: Causes, Prevention, and Solutions

Twinning in tablet coating refers to a defect where two or more tablets stick together during the coating process, forming “twins” or clusters. This issue can lead to incomplete or uneven coating, impacting the tablets’ appearance, effectiveness, and durability. Twinning is especially common with round or oval tablets, as their flat surfaces are more likely to adhere when exposed to moisture and pressure during the coating process.

Main Causes of Twinning in Tablet Coating

Several factors can contribute to twinning during coating, including:

1. Tablet Shape and Surface Characteristics: Tablets with larger, flat surfaces are more prone to sticking since their contact area is greater, making it easier for them to bind under the moist conditions of the coating process. Round tablets, in particular, are more susceptible.
2. High Moisture and Humidity Levels: The moisture introduced during the coating process can make tablets sticky. When moisture content is too high or drying isn’t sufficient, tablets are more likely to stick together.
3. Formulation Ingredients: Certain formulations contain water-attracting ingredients that retain moisture during coating, making them more likely to experience twinning.
4. Coating Pan Speed and Mixing Settings: If the coating pan speed or mixing level is too low, tablets may remain in contact for too long, increasing the chance of them sticking together.
5. Lack of Anti-Adherent Agents: Anti-adherents, like talc, help reduce stickiness during coating. Without these, tablets are more likely to stick to each other and create twins.

How Twinning Affects Tablet Quality

Twinning can impact tablet quality in several ways:

• Causes uneven or incomplete coating, which can affect appearance and impact the controlled release of active ingredients.
• Leads to a decrease in production yield, as tablets that stick together may need to be discarded.
• Increases the risk of tablet damage during the de-twinning process, potentially leading to broken or chipped tablets, which compromises overall product quality.

Effective Strategies to Prevent Twinning During Coating

1. Adjust Tablet Shape and Edges:

   Tablets designed with beveled edges or rounded shapes are less likely to stick together, minimizing twinning.

2. Control Moisture and Drying Conditions:

   Optimize drying within the coating pan to efficiently remove excess moisture, reducing the chance of tablets sticking together.

3. Optimize Coating Parameters:

   Increase Pan Speed: Higher pan speed allows tablets to move freely, reducing prolonged contact.Adjust Spray Rate and Atomization: Maintain a controlled spray rate and atomize correctly to keep moisture levels balanced and prevent sticking.

4. Use Anti-Twinning Agents:

   Adding talc, magnesium stearate, or other anti-adherents to the coating formulation creates a barrier, reducing tablet stickiness and preventing twinning.

5. Modify Coating Solution Composition:

   Adjusting the coating solution’s viscosity can help control moisture interaction with the tablets. For tablets with moisture-sensitive formulations, lower-moisture coating methods may be beneficial.