11. Tablet-to-Tablet Colour Variability
Tablet-to-tablet colour variability corresponds to inter-tablet variation of coating. Although variation in the amount of coating applied on each tablet within the batch will, on a weight basis, always exist, the challenge is to ensure that enough coating is applied to prevent visible differences in amount of coating applied or even affecting final product functionality.
Excessive tablet-to-tablet colour variability may be attributed to;
- Inadequate uniformity of coating distribution
- Inadequate hiding power
These problems can be solved by
- Increasing pan speed to increase the number of presentations to the spray zone during the coating process.
- Increasing the number of spray guns to expand the spray zone and give better surface coverage.
- Optimizing pan loading to make sure the tablets are tumbling effectively, with no intermittent stalling of the tablet bed, and to ensure that a consistent gun-to-bed distance is maintained.
- Balancing both spray rates and drying conditions to achieve good coating process efficiency (typically > 90% and ideally > 95%).
- Reducing coating solids to facilitate more even coating distribution.
- Increasing the amount of coating applied to allow better and more consistent tablet coverage.
- Optimizing spray gun setup so that there is no mis-spraying, that all spray guns are spraying at the same rate, and that each is providing the same area of coverage over the tablet bed surface.
- Selecting a film coating formulation with increased hiding power (lighter colours tend to exhibit better “hiding power” than darker ones).
12. Tablet Twinning
This is the term for two tablets that stuck together, usually after becoming aligned along flat surfaces. Twinning of tablets during coating operation is commonly seen with capsule-shaped tablets and may be due to use of excessively tacky coating formulation or poor suspension evaporation.
While there are certain coating process changes that can be made to obviate this problem, including increasing pan speed to reduce the dwell time in the spray zone (thus reducing over-wetting), improving the drying conditions (such as increasing process air temperatures or volumes and/or decreasing spray rates) or expanding the spray zone (either by increasing gun-to-bed difference, pattern air pressure or the number of spray guns) to minimize the risk of localized over-wetting, proactively dealing with the issue during product development provides a more effective solution.
One approach that can be very effective deals with core design. For example, if a capsule-shaped tablet is required, changing from the more traditional design to one in which there is a small degree of curvature on the edges of the tablets can reduce the area of contact between tablets and greatly minimize the risk of twinning.
Additionally, while all film coatings will develop some degree of adhesiveness during the drying process, not all coating formulations are equal in this regard. For example, film coatings based on poly (vinyl) alcohol as well as delayed-release coating based on acrylic polymers are well known for their increased tackiness behaviours. Thus, when these kinds of polymers are required, the use of anti-adhesive agents (such as lecithin, talc, glyceryl monostearate, magnesium stearate) in the coating formulation can be quite effective in reducing their tackiness.
13. Tablet Discolouration
This defect is commonly seen with nutraceutical products and may be expressed in many ways. For example, interaction of ingredients in the core, often in the presence of moisture derived from the coating process, can create dark spots that remain visible through the coating. In a similar vein, migration/ bleeding of ingredients from the tablet core, through the coating can also be a factor. Finally, poor dispersion of pigment ingredients within the coating formulation can also be a source of tablet discolouration; in which case, this must be dealt with at the time of preparation of the coating suspension to make sure pigments are effectively dispersed into the final coating suspension. For coating formulations prepared from powdered premixes made by a third party vendor, the issue of ineffective pigment dispersion must be dealt with by those vendors.
Tablet Discolouration may be solved by minor processing remedies, such as increasing processing temperatures, reducing spray rates, or increasing pan speed to reduce dwell times in the spray zone can reduce over-wetting and thus prevent interaction between tablet core ingredients (such as that between iron salts and ascorbic acid).
Similarly, when ingredients that become liquid and bleed through the coating do so as a result of high processing temperatures, an increase in spray rate and/or reduction in processing temperature can be helpful. However, potential discolouration issues are more effectively dealt with during product development, especially when past experience suggests that this kind of problem is likely to occur in the current product being developed. For example, designing coating formulations (such as high solids coating formulations) that can minimize moisture penetration into the tablet core during application of the coating can effectively reduce or eliminate the problem. Alternatively, if discolouration occurs on storage, use of a moisture barrier coating can also be effective.
14. Scuffing of Film-Coated Tablets
Tablet scuffing involves the generation of grey-to-black marks on the surface of white (or lightly coloured/ pastel) film-coated tablets. Although this problem has been attributed to many causes, it is generally thought to be an abrasion interaction between titanium dioxide in the coating formulation and the surfaces of the stainless steel coating equipment.
Rosoff and Sheen have commented on this phenomenon, linking it to polymorphism that exists with different forms of titanium dioxide. Rowley, on the other hand, confirmed that the main issue is related to the level of titanium dioxide used.
While ensuring that the surfaces of the coating equipment are perfectly clean, an approach that can be augmented by pan passivation is a relatively simple approach that can help reduce scuffing problems, a more proactive formulation approach can provide a long-term solution. Reduction in the levels of titanium dioxide used in the coating formulations (as suggested by Ogasawara et al.) can be quite effective; however, this approach will also reduce the opacity of the coating formulation, necessitating the use of higher levels of applied coating to achieve complete tablet core coverage.
Tablet scuffing has, at various times, been associated with the use of coating formulations based on polyvinyl alcohol (PVA). This may be alleviated by using hybrid polymeric film coatings based on combinations of Hydroxypropyl methylcellulose (HPMC) and copovidone.
15. Coated Tablet Dissolution Issues
Generally, unless it is deliberately intended (as in the case of modified release dosage forms), the application of a film coating to pharmaceutical dosage forms should not have any influence on drug release and bioavailability. Dissolution problems occur when there is a statistically significant change in dissolution behaviour after film coating (comparing film-coated tablets to uncoated ones). Although a number of factors may be involved here, the most prominent one is the impact of coating process conditions on tablet core behaviour, rather than the impact of the applied coating itself (through chemical interaction with an ingredient in the core).
For example, for an immediate-release product, ibuprofen tablets (because of the relatively low melting point of this active substance) are well known to exhibit dissolution problems when exposed to high processing temperatures.
In the case of modified-release dosage forms, coating process conditions can influence the degree to which phase separation can occur with a resulting impact on the size of pores created within the film structure (and hence drug release) when ethylcellulose is used as primary insoluble polymer and hydroxypropyl cellulose as the water-soluble pore former.
Coated tablet dissolution problem can be solved by
- Using high solids film-coatings systems (such as Aquarius Preferred HSP) that facilitates the use of lower processing temperatures
- Adjusting core formulation to improve dissolution behaviour and offset sensitivity to processing conditions
- Adjusting coating formulation to allow reduction in coating process temperatures.
Recovery of Film-Coated Tablets
Unlike with sugar-coated tablets, film-coated tablets cannot be readily recovered (should a batch be rejected) by simply washing off the coating.
Recovery of film-coated tablets thus typically involves milling of the tablets to produce “granules” that can be introduced (at a predetermined level) into other batches of compression mix (of the same product). Depending on the fracture characteristics of the tablets, and to some extent the adhesion of the coating to the tablet surface, the milling process may cause the coating to flake off so that it can be removed from the tablet core material by sieving.
Usually, however, a portion of the coating will remain in the compression mix. Thus, any rework procedure must be suitably validated so as to ensure that recompressed tablets perform within the specifications of the original product.
Shah, in one of his publications, discussed the use of the Crackulator for recovery of film-coated tablets. Using such equipment, a better separation of tablet and coating fragments was achieved.
Despite advancements and innovations in tablet manufacture, film-coating processes continue to produce many challenges. Assessing the extent of these problems can be difficult because no objective criteria are currently available for this purpose. The development of suitable decision criteria based on objective and quantitative evaluation methods is therefore highly desired. Also, implementation of a proactive approach that acknowledges such problems focuses on problem avoidance during product and process development and provides the most effective means of dealing with a subject that for so long has plagued the production of film-coated dosage forms.
- Avis, K., Shukla, A. and Chang, R. (1998). Pharmaceutical Unit Operations: Coating London, Taylor & Francis Group, LLC.
- Cole, G. (2002). Pharmaceutical Coating Technology. UK, Taylor & Francis Ltd.
- Felton, L. (2017). Aqueous Polymeric Coatings for Pharmaceutical Dosage Forms (4th ed.). New York: Taylor & Francis Group, LLC.
- Lieberman, H., Lachman, L. and Schwartz, J. (1990). Pharmaceutical Dosage Forms: Tablets. New York: Marcel Dekker, Inc.
- Rosoff, M. and Sheen, P. (1983). Pan abrasion and polymorphism of titanium dioxide in coating suspensions. Journal of Pharmaceutical Science, 72(12): 1485.
- Rowley, F. A. Toward a greater understanding of the scuffing defect observed on Film-coated tablets. Tablets & Capsules (April 2012).
- Ogasawara, Y., Steffenino, R. and Cunningham, C. Scuffing measurement methodology and improved film coating systems. Poster presented at Annual AAPS Convention November 2008.
- Photo credit: www.filmcoating-troubleshooting.com
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