Definition:

Viscosity describes the viscosity of substances and gases (fluids). The higher the viscosity, the thicker (less flowable) the fluid; the lower the viscosity, the thinner (more flowable) it is.

Example: Water low, honey high.

Source: https://en.wikipedia.org/wiki/Viscosity

The role of viscosity in film coating for solid dosage forms

How viscous should my coating suspension be? What happens if it is high or low viscous? What should I watch out for?

Too high a viscosity causes problems for the pump that sucks the suspension in for the coating process. If the product is too viscous, it cannot be processed.

When processing highly viscous polymers in suspensions, only small amounts of solids (e.g. 10 %) can be dissolved in the water. This has the consequence that the process time of the spraying process is extended, since only a small amount of solid material is applied to the end product (tablet, granulate, pellet).

If, on the other hand, low-viscosity polymers are used, a higher solids content of up to 20 % is achieved when suspending the coating solution, which also results in comparatively shorter process times. Because a lot of solid matter is applied to the tablet surface through the nozzle in a short time.
Coating suspensions with a viscosity of 200 – 700 mPa*s are easy to spray and apply to tablets. For polymer suspensions with a viscosity of more than 700 mPa*s, the spraying process is critical because pump problems can occur. While under 200 mPa*s insoluble components sediment in the suspension.

Examples of polymers with…

… low viscosity: polyvinyl alcohol-polyethylene glycol copolymer.
… high viscosity: hypromellose (HPMC)

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