Tablet Coating Defects and their Remedies
Coating tablets with a thin polymer film can be an effective way to give your products a professional edge. High quality tablets can be quickly and easily produced using a tablet coating machine and the correct excipients . Unfortunately several defects can arise with coatings. The following list provides helpful remedies for common issues that may be encountered.
Definition: Blistering of a surface film occurs when its elasticity or adhesive properties are compromised. The result is that the film becomes detached from the tablet’s substrate.
Cause: Blistering is usually a result of high temperatures that may occur during the drying process, during the spraying stage or at the end of the coating process
Remedy: Use mild drying conditions, and ensure moderate temperatures at other stages of the coating process.
Definition: Chipping occurs when the film becomes dented and chipped and this is most notably visible on the edges of the tablet.
Causes: Deformity in the tablet cam occur when there is a decrease in the rotation speed in the machinery during the coating process. Another cause would be a poor polymer or coating solution – e.g., an incorrect amount of plasticizer is used in the coating solution,.
Remedy: Increase the hardness of the film by adjusting the proportion of plasticizer in the coating solution or selecting a polymer with a higher molecular weight.
Definition: Cratering happens when a defect on the film’s coating results in craters appearing on the tablet which in turn results in the exposure of the tablet’s surface.
Causes: Cratering can occur in certain instances where there is insufficient drying time to seal the film or a high volume of coating solution is applied. In these cases excess polymer solution can penetrate to the surface of the tablet, especially in the crown area, causing the disruption of the coating and degeneration of the tablet’s core.
Remedy: Check the efficiency of the drying process and optimise drying conditions.
Definition: Picking happens when part of the film sticks to the pan resulting to some of the tablet pieces being detached from the core.
Causes: Picking occurs when there is overwetting of tablets by the polymer solution, making the film become tacky which results to the tablets sticking to one another.
Remedy: Overwetting can be avoided by increasing the efficiency of the drying process e.g, by increasing the air inlet temperature. Alternatively, the rate of applying coating solution can be decreased, or the solution viscosity increased.
Definition: Pitting is the deformation of the core of the tablet without any visible signs of disruption of the film coating .
Causes: Pitting can occur when the tablet core becomes hotter than the melting point of the materials used in its preparation. .
Remedy: Dispense with preheating procedures at the start of coating and modify the drying (inlet air) temperature such that the temperature of the tablet core does not become greater than the melting point of the batch of additives used.
Definition: Blooming is the fading or dulling of a tablet colour after a prolonged period of storage at a high temperature.
Causes: The tablet colour can become dull as a result of changes in the composition of the surface film. It is usually the result of using too much plasticiser or of using a plasticiser with a low molecular weight.
Remedy: Decrease the concentration and increase the molecular weight of the plasticiser in the polymer.
Description: Blushing is a haziness or appearance of white specks in the film.
Causes: Haziness or white specks are particles of polymer that has precipitated in the film. It usually forms as a result of an excessively high coating temperature. Alternatively it may be formed by gelation of the polymer when used in certain combinations with other materials.
Remedy: Decrease the drying temperature to avoid precipitation of polymer. Avoid the use of sorbitol with polymers such as hydroxy propyl cellulose, hydroxy methyl cellulose, methyl cellulose and cellulose ethers.
Description: Variation in the colour of tablets within a batch.
Causes: Colour variations may occur by a number of different faults in the preparation e.g., poor mixing, uneven spray patterns of the machinery, insufficient coating, migration of soluble dyes-plasticizers and other additives during drying.
Remedy: Aim for even geometric mixing, reformulate with different plasticizers and additives and/or use mild drying conditions.
Description: This refers to the filling of intagliations - i.e., the distinctive words or symbols formed on the tablet.
Causes: Infilling is caused when a polymer solution that is sprayed onto the table is unable to disperse. When sprayed with air, bubbles can form in the solution leading to a foam. Unlike a liquid, foams may accumulate within the intagliations rather than dispersing over the whole tablet. The result is that droplets of liquid become concentrated in the intagliations. This leads to a higher concentration of polymer within the intagliations. If the droplets build up, they can coat the whole pellet giving rise to an uneven polymer film.
Remedy: Add alcohol to the polymer solution to improve dispersion, or use a spray nozzle capable of finer atomization.
Orange Peel (Roughness)
Description: The tablet has the appearance of an “Orange Peel” on account of having a rough surface, which may also have a matt rather than glossy texture.
Causes: Orange peel can be the result of poor tablet composition causing it to become soft. It can also be caused by too high a spray pressure combined with a fast spray rate, leading to uneven coating of the tablet.
Remedy: Use mild drying conditions or use additional solvents to decrease the viscosity of the polymer solution so that spraying rate can be reduced.
Description: Cracking occurs when the film coating the tablet cracks in the crown area or splits around the edges.
Causes: Cracking occurs when the film’s internal stress exceeds the tensile strength of the film. This is common with higher molecular weight polymers or polymeric blends.
Remedy: Use lower molecular weight polymers or polymeric blends. Also adjust plasticiser type and concentration.
During tablet manufacture, an industrial pharmacist may encounter many problems. Solving these problems requires an in-depth knowledge of tablet-formulation as well as machine-operating processes. Some of the more general issues that may be encountered are:
- Capping and Lamination arising from air-entrapment in the granular material.
- Chipping of very dry granules.
- Cracking due to rapid expansion of tablets, when deep concave punches are used.
- Sticking, picking and binding resulting from too much binder in the granules.
- Mottling is an imperfection that arises due from more than one factor, e.g., in a coloured drug, dirt in granules or the use of an oily lubricant.
- Double-Impression which is the result of a defective machine: it is caused by the free rotation of punches that have some engraving on the punch-faces.
In addition to such general issues are those that arise from applying polymer coatings. These are summarised again below.
- Blistering is caused by entrapment of gases in or underneath the polymer film due to overheating either during spraying or at the end of the coating run. Use of mild drying conditions can solve this problem.
- Chipping is related to higher degree of attrition associated with the coating process. Increase in hardness of the film by increasing the molecular weight of the polymer can solve this problem.
- Cratering is related to penetration of the coating solution into the surface of the tablet, often at the crown where the surface is more porous, causing localized disintegration of the core and disruption of the coating. Decrease in spray application rate and use of optimum and efficient drying conditions can solve this problem.
- Pitting is a defect in which the temperature of the tablet core is greater than the melting point of the materials used in tablet formulation. Dispensing with preheating procedures at the initiation of coating and modifying the drying (inlet air) temperature can solve this problem.
- Blooming or dull film is generally a result of using of high concentrations and lower molecular weight of plasticizer. To avoid blooming, use lower concentrations and a higher molecular grade of plasticizer.
- Blushing/Whitish specks/Haziness of the film is related to precipitation of polymer exacerbated by the use of high coating temperature at or above the thermal gelation temperature of the polymers.
- Colour variation arises through poor mixing, uneven spray pattern, insufficient coating or migration of soluble dyes during drying. Address problem this by opting for geometric mixing, using mild drying conditions and reformulate with different plasticizers.
- Infilling arises through bubble/foam formation during air spraying of a polymer solution. Addition of alcohol or use of spray nozzle capable of finer atomization can solve this problem.
- Orange peel/Roughness is related to inadequate spreading of the coating solution before drying. A decrease in viscosity of coating solution can reduce this defect.