In any coating process, a variety of problems may be encountered. Often such problems may be related to formulation issues that have been compounded by those associated with processing.
This article will center on various sugar coating defects, causes, and possible remedies.
This is caused by excessive use of insoluble fillers and pigments. It can be resolved by addition of small quantities of polymers (such as cellulosics, PVP, acacia, or gelatin) to one or more of the various coating formulations.
Tablet cores that expand, either during or after coating, are likely to cause the coating to crack. Such expansion may result from stress-relaxation of the core after compaction (a phenomenon which is known to occur, for example, with ibuprofen) or moisture absorption by the tablet core.
Expansion due to post compaction stress relaxation can be resolved by extending the time between the compaction event and commencement of sugar coating, whereas moisture sorption can be minimized by appropriate use of a seal coat.
This is the inability of sugar coatings (especially those based on sucrose) to dry properly. It is often an indicator that excessive levels (greater than 5%) of invert sugar is present. Inversion of sucrose is worsened by maintaining sucrose syrups at too high a temperature under acidic conditions for a long period of time.
Such conditions occur when sugar-coating solutions containing aluminum lakes are kept hot for too long; or such sugar-coating formulations are constantly being reheated to redissolve sugar that is beginning to crystallize out. The remedy, of course, is to avoid the excess heating of the sucrose syrup under acidic conditions.
Read Also: Steps Involved In Sugar Coating
By their very nature, sugar-coating formulations are very sticky, particularly as they begin to dry, and allow adjacent tablets to stick together. Buildup of multiples really becomes a problem when the tablets being coated have flat surfaces which can easily come into contact with one another.
This can be particularly troublesome with high-dose, capsule-shaped tablets that have high edge walls. Appropriate choice in tablet punch design can be effectively used to minimize the problem.
Because color-coating stage of the sugar-coating process has a major impact on final tablet appearance, the process is critical to ultimate tablet quality. Uneven distribution of color particularly with the darker colors is often visually apparent and thus a major cause of batch rejection. Many factors may contribute to this type of problem, including:
1. Poor distribution of coating liquids during application – This may be caused by poor mixing of tablets in the coating process or failure to add sufficient liquid to coat completely the surface of every tablet in the batch. This can be resolved by adding sufficient coating liquid and ensuring that it is properly mixed.
2. Color migration of water-soluble dyes while the coating is drying – This can be minimized by changing the solvent system or optimizing the drying condition.
3. Unevenness of the surface of the subcoat when using dye-colored coatings – This unevenness causes a variation in thickness of the transparent color layer that is perceived as different color intensities. It can be resolved by achieving the required smoothness during the subcoat application.
4. “Washing back” of pigment-colored color coatings – While pigments do not migrate on drying if excessive quantities of coating liquid are applied during the coloring process, there is a tendency for the previously applied (and dried) color layers to be redissolved and distributed nonuniformly; thus, giving rise to non-uniform appearance. This problem is particularly noticeable for formulations predominantly colored with aluminum lakes where the level of opacifying pigments (such as titanium dioxide) is low (i.e., dark colors). This can be prevented by avoiding excessive use of coating liquid, replacing aluminum lake, or using a combination of dyes and pigments.
5. Excessive drying between color applications – This can cause erosion of the color layer and contributes to unevenness in the color coat. It can be corrected by reducing drying rate or temperature.
Residual moisture (in finished sugar-coated tablets) can often be a problem. Over a period of time, this moisture can diffuse out and affect the quality of the product. Moderate levels of moisture egress cause the polish of the product to take on a fogged appearance, a phenomenon often termed blooming. At higher levels (of moisture egress), the moisture may appear like beads of perspiration on the tablet surface. This second phenomenon, often called sweating, can be much more serious since tablets stored in closed containers will ultimately stick together.
Obtaining appropriate levels of moisture in the sugar coating is conducive to good polish characteristics and avoidance of sweating and blooming. Thus, great care has to be taken with the drying stage at the end of each application of coating liquid as well as to selection of appropriate racking/ drying of tablets prior to polishing.
One of the secrets to achieving a high-quality sugarcoated product is to ensure that color is uniformly distributed in the color layer and that a smooth coating surface is obtained at the end of the application of the color coating prior to polishing. Failure to achieve the requisite smoothness often results in a marbled appearance on polishing. This problem occurs as the result of the collection of wax in the small surface depressions of a rough coating and is particularly evident with darker colors. It can be resolved by ensuring that a smooth surface is achieved at the end of color coating.