Common Defects in Film Coating Process: Causes and Possible Solutions

6. Film Peeling and Flaking

On occasion (particularly during application of aqueous-coating formulations), if cohesive failure (cracking) of the coating occurs, that coating may subsequently peel back from the surface of the substrate. While both cohesive and adhesive failure are implicated here (both phenomena being linked to internal stress), appropriate solutions typically involve addressing the initial cracking problem by increasing the mechanical strength of the coating.

7. Infilling of Logos

While visually similar to logo bridging, infilling of logos typically occur during the spray application of aerated aqueous film-coating solutions. Unlike logo bridging, infilling is caused by a buildup of dried coating solution within the logo. This may be as a result of partially spray-dried material gradually building up within the logo or, the buildup of dried foamy material within the logo.

Normally, dried material that is deposited on the tablet surfaces would breakdown as tablets slide over each other but because the intagliation form a protected area, this will not occur for material that resides in the logo. Once the foam/material has accumulated to a level approaching the outer contour of the tablet surface, normal attrition can occur, allowing the structure to be covered with a continuous film.

The general buildup of prematurely dried material can be corrected by optimizing the atomization process to give a more uniform distribution of droplet sizes. Once this has been achieved, further improvement can be achieved by adjusting coating process conditions (from the thermodynamic standpoint to balance spray rates and drying rates) to ensure that most droplets arrive at the tablet surface in a suitably solvated state to facilitate droplet spreading.

In the case in which foamy material tends to build up within the logo, it is interesting to note that the addition of conventional antifoam agents does not decrease the incidence of this defect. However, the addition of alcohol, the use of spray nozzles capable of finer atomization, optimizing the mixing process either by setting mixer height correctly, choosing mixer blade of correct size (about 30% of tank diameter), choosing optimal mixer speed and incorporation powder into water as fast as possible to avoid excessive viscosity build-up before all powder has been wetted out have been found to be effective.

8. Tablet Surface Erosion

This is another mechanical defect that occurs as tablets tumble during the coating process. In addition to unsuitable punch design, core surface erosion can be facilitated by over-wetting (which causes the tablet surface to soften and hence become less resistant to attritional effects), presence of centrally placed logo, insufficient film strength (to provide resistance to edge damage) and too high pan speed.

This problem can be eliminated by simply addressing over-wetting conditions (reducing spray rates, increasing processing temperatures, or both). Additional solutions might include:

• Reformulating tablet core
• Increasing compaction forces
• Optimizing pan speed
• Changing punch design to reduce curvature on upper and lower faces and
• Changing logo location from the centre of the tablet faces to the periphery.

9. Tablet Pitting

Tablet pitting is the presence of small pits (indentations) in the surface of the coated substrate. This problem is caused when agglomerates of lubricants such as stearic acid and polyethylene glycol melt under the influence of the coating process conditions used. Although this defect does not interrupt the film coat, it does diminish the pharmaceutical elegance of the dosage form.

Tablet pitting can also occur when large particles of super-disintegrant present in the tablet surface on contact with water from the film coating solution being applied swell to an extent at which they become loosened from the tablet surface, again leaving pits in the tablet surface so that, after the coating has been applied, they still remain visible.

In both cases, the remedies are relatively simple and include:

• optimizing blending process (prior to tabletting) to minimize the chance of producing agglomerates.
• optimizing coating process conditions to minimize use of excessively high temperatures (the key factor here will be tablet bed temperatures) when low melting point ingredients are used in the tablet core and
• preventing over-wetting conditions (potentially offset by reducing spray rates and/or increasing process temperatures) when the core contains very hydrophilic materials, especially super-disintegrants.

10. Tablet Breakage

This is one of the most troublesome defects caused by mechanical stress, primarily because it cannot be easily resolved by minor adjustment of the coating process. In this type of defect, tablets break apart during loading of coating pans, during the coating process, or during unloading of coating pan.

Tablet breakage is primarily caused by

• Mechanical deficiencies in the tablet cores (tablets are not robust enough to withstand film coating process)
• Variations in die fill uniformity during the tabletting process, possibly due to variations in hopper fill.
• Mishandling of tablets during loading/ emptying of coating pans.
• Excessive attritional conditions used during application of the applied coating.

Problems of this nature may be alleviated by

• Improving tablet core robustness, either by addressing the core formulation or examining the tabletting process (slowing down press speeds to increase dwell time, increasing compaction forces, and possible use of precompression).
• Ensuring that the coating process is optimized with respect to
  1. Spray rate (increased spray rate provides protection for tablets)
  2. Pan loading (underloaded pans potentially create more attritional problems, especially because tablets can strike the exposed edges of baffles)
  3. Pan speed, which can be reduced to minimize mechanical stress

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