25 Common Practical Questions and Suggested Answer for Tablet Coating

Question 17: What pan speed should I be aiming for?

Answer: In some ways pan speed is a compromise between adequate tablet bed mixing and considerations of abrasion of the tablet cores. All manufacturers will give pan speed suggestions for various loadings but occasionally this will have to be modified for a particular need. An example would be a tablet core where edge attrition could be prevented by slowing the pan. This measure can often be helped by increasing spray rate, if that is possible.

Question 18: What are the advantages of an airborne over an airless spray?

Answer: Basically an airless spray atomizes a liquid stream by the use of a high hydrostatic pressure through a small orifice nozzle. Its benefits include lack of ‘spray bounce’. However, its relatively high throughput and lack of droplet size control make it generally unsuitable for aqueous-based spraying. It also blocks easily. Here the versatility of the airborne spray,  where droplet characteristics are more independent of spray rate considerations, is more appropriate with aqueous spraying.

Question 19: What can I do about poor mixing in my tablet-coating pan?

Answer: Apart from suspicions arising from observations of the tablets revolving in a pan, this will be apparent from variable color coverage if a colored coating is being used. It will also be apparent from intra-batch variability in performance observed with a functional coating.

The tablet bed should flow evenly. Underloading or overloading a pan will cause poor tablet bed rotation and poor mixing.

It is also worthwhile with old equipment to check the manufacturer’s latest recommendation as improvements are often introduced periodically with new models.

Some high solids coating compositions are capable of being applied very rapidly, occasionally so quickly as to ‘run ahead’ of the mixing ability of the pan. Under these conditions, when coating times are crucial, the mixing ability of the pan must be upgraded with the assistance of the equipment manufacturer.

Question 20: How do I know that I have achieved the correct rate of application?

Answer: Within a given set of drying conditions, the ‘correct’ rate of application will be the one which neither causes overwetting on one hand nor spray drying on the other. This simplistic picture may be explained further. An excessive spray application rate will be marked by tablet picking and possibly by adherence of tablets to the pan. Spray drying is characterized by an excessively ‘dusty’ coating process where the window or sight glass is obscured by powder deposits. Any horizontal surfaces such as the gun supports will also tend to collect powder under these conditions.

Read Also: Film Coating Process: Polymers Used in Immediate-Release Film Coating

Question 21: Should I expect repeated nozzle blockage with aqueous spray procedures?

Answer: Repeated nozzle blockage should not happen during a coating run. The following should be investigated if this occurs:

• Coating suspension. Poorly dispersed pigment agglomerates are a common cause. Also the polymer itself may not have been subject to adequate dispersion to fully solubilize it. When using a commercial latex or pseudolatex dispersion, it should be confirmed that coagulation of the coating suspension is not taking place for some reason. Common causes are excessive temperature (both processing and suspension temperature) and unsuitable additives to the formula, causing polymer coagulation.
• Process consideration. The atomizing air pressure may be too low. Alternatively, nozzle blockage may be exacerbated by an unnecessarily small nozzle. A diameter of 1 mm is typical for a standard aqueous process.

Question 22: How can film-coated tablets be polished?

Answer: It is quite feasible to polish film-coated tablets. However, it is also advisable to consider whether this is really necessary. An aqueously coated tablet may appear matt compared with an organic solvent-coated tablet or even a sugar-coated tablet, but nonetheless the final appearance can be aesthetically pleasing.

On the other hand, if ‘house’ requirements or marketing dictate a polished appearance, then there are many possibilities. The following should be taken into consideration:

• Acrylic polymer formulations are usually inherently quite shiny but the smoothness of cellulosic systems can be enhanced by a final application of spray suspension without the pigment.
• Attention to process conditions is nearly always capable of producing improvements.  Spray conditions should be ‘wet’ with a relatively low bed temperature and a higher rate of spray than normal. Extreme caution should be exercised in the initial validation of those conditions as they are conducive to verwetting.
• Generally it is possible to use the waxes, polishes, and glazes normally utilized for sugarcoated tablets. Nowadays totally aqueous polish mixes are commercially available. Another effective method is to use an aqueous solution of a high molecular weight—PEG, e.g. 20000 grade—sprayed on at the completion of coating. The use of dry carnauba wax added to the completed batch of tablets in a cylindrical pan and rolled for a period until shine develops is also an effective method.

Should a lustrous appearance be required, the use of talc in the coating formula should be considered. Sometimes, polishing may be completed in the same pan utilized for coating, providing it is not too contaminated with dried spray. The shape of the tablet bed and the change in noise emitted from the pan can be used as indicators as to when polish and shine has been imparted onto the tablets.

Question 23: How can I cure variable dissolution results with controlled-release coatings?

Answer: Assuming that the dissolution methodology and analytical testing are satisfactory, the process should be examined with regard to the following features:

• Is the pan design and product loading appropriate to enable sufficient mixing to take place?
• Is the process constant and optimal regarding overwetting or spray drying of coating material?
• Has sufficient coating been applied?
• In particular with an aqueous dispersed commercial coating for modified release, have the manufacturer’s recommendations regarding processing been followed?

The coating formula should be examined to see if it is appropriate for the task, e.g. ethylcellulose will not give an enteric effect. Is the quality of the materials adequate?

If changes in dissolution performance of, for instance, modified release coated beads alters on storage then the coating itself is ‘maturing’ or possibly there are interactions between the coat and the core material.

Question 24: How can I cure metallic marks on white-coated tablets?

Answer: This is a problem most often seen with new pans and is especially noticeable with white or pale-colored tablets. First, the pan should be thoroughly cleaned. If necessary a thin application of spray material to the pan itself will cure the problem. Ensuring that unduly dry spray conditions are not used will also aid the resolution of the problem.

Question 25: How can I optimize the smoothness of a film coating?

Answer: Occasionally smoothness and elegance of a film coating is of paramount importance over other factors such as speed of operation and batch throughput. The viscosity of the coating suspension has a major part to play since, generally, smoother coatings result from low-viscosity suspensions/solutions.

Certain types of spray-gun can be a contributing factor to the overall effect. Other process parameters of importance are:

• reduction of the gun-to-bed distance
• increase in atomizing air pressure
• use of an unmodified spray cone

These measures will combine to produce a ‘controlled overwetting’.

Source

• Cole, G. (1995). Pharmaceutical Coating Technology. UK: Taylor & Francis Ltd.