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How to prevent tablets from sticking and twinning

Review of the two most common headaches in coating processes and several variables to consider when dealing with overwetting problems. Insight from our specialists after many years of performing tests and archiving of numerous recipes.

September 21, 2016
Caterina Funaro
Massimo Malavolti

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The answers of the coating experts

Is there any particular problem in coating triangular or bizarre shaped tablets? Is this leading to more sticking possibilities?
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In the absence of flat surfaces/edges, it is possible to coat also unusually shaped tablets. IMA has hundreds of different case histories on coating of triangular tablets and other unique shapes on file in the IMA database. With the good process parameters and if the tablets are hard enough to be moved in the coater, it is possible to coat them successfully.

Is the recommendation of anti-tacking agent valid for both aqueous and organic solvent based formulation?
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These are two different situations:

  • In the organic solvent formulation, the viscosity is normally higher and it is necessary to be careful in increasing the amount of anti-tacking agent to avoid viscosity which is too high.
  • In the aqueous solvent formulation, the viscosity is lower but the tendency of coating dispersion to wet the tablet is higher.
    The quantity of the anti-tacking agent could be different in both situations, while the plasticizer concentration should not be different since it is calculated compared to the polymer amount.
Can we increase the number of guns to minimize twinning?
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Theoretically, the maximum number of guns is the one that optimizes the exposure of the cores to the spray coating solution and that avoids the risk of over wetting the cores. The risk in increasing the number of guns is that you lose more product and dirty the pan faster. This would increase also the washing steps required to clean the machine. Within IMA instruction manuals, the right number of guns is always suggested according to the batch size to be handled.

As far as sticking is concerned, is a formulation containing api in solution/suspension a worst case?
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Yes, but only in part. During drug layering the coating formulation normally provides a higher amount of water with solids concentration of about 10%. Additionally the process is managed with short gun distance and lower atomization to minimize losses and improve API distribution. This is the worst case for sticking issues. On the other hand, the process must be slow, the spray rate is kept very low and this helps avoid twinning and sticking.

Is glycerol mono stearate a good alternative to talc in coating formulations as anti-tacking agent?
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Yes, but it provides both advantages and disadvantages. GMS has a high efficacy and requires only low amounts in formulation (2 to 15% based on dry polymer). When added in low quantities, the GMS helps to achieve more flexible films than conventional talc formulations do. It provides a more glossy coating compared to talc and the sedimentation of pigments is reduced or avoided. On the other hand, GMS must be dispersed in water separately and in hot condition. This is for sure a limitation for industrial production batches preparation.

Which type of alarms/surveillances can be set in the coating recipe and controlled by HMI for a process challenging sticking issues?
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The cores temperature can be fixed in a safer mode (especially the minimum value) by modulating spray rate and inlet air temperature. This keeps tablets in the best range and avoids sticking caused by a too low/high value. A cores temperature probe is mandatory. A sensor for in line measurement of gun distance and the possibility to keep it constant during the modification of batch height/flow is also useful. It provides a standardization of process conditions and prevents the guns getting closer to the bed during the development of the spray. A camera for in line monitoring of cores quality can be also an option.

Meet our webinar speakers

Caterina Funaro
Process R&D Laboratory Manager
Caterina Funaro got her degree in Chemistry and Pharmaceutical Technology at the University of Bologna in 1998. She is actually employed at IMA Active Division as process laboratory manager and her main responsibilities are technical assistance to sales for all solid dose equipment, after sales process assistance, R&D, training and cleaning support for solid dose manufacturing equipment.
Massimo Malavolti
Product Manager for Coating Equipment
Massimo Malavolti got his degree in Technological Institute specialized for electronical and mechanical applications in 1990 in Bologna. He is employed at IMA since 1990. At the beginning he supported validation development and calibration strategy for coating equipment, actually he is Product Manager for coating equipment and his main responsibilities are sales technical support and projects management.
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