Manufacture 1381 Particle Coating

Particles of size 50 mm to 2 mm and mini tablets can readily be coated in conventional fluid bed equipment. The Wurster process is particularly suited to such layering and is frequently preferred setup due to its superior air streams providing longer residence of the coat in its "plastic phase" on the particles' surfaces and hence more uniform film quality. In principle, top spray setups with two-chamber filter cleaning are also feasible. However, coats applied using this technology may be less dense. Hence higher coating levels may be required. Coating in perforated pan coaters may be feasible with specific drum design but is not preferred due to suboptimal product movement patterns.

Various innovative fluid bed designs have been recently developed to optimize coating efficiency and process performance and to simplify scale up. Examples are the GEA Precision coater™ with FlexStream™ processor, the Oystar Huettlin Disk Jet® technology or the Innojet Ventilus®, and Aircoater® systems.

Films are formed by evaporation of solvent from polymer solutions or dispersions. Processing conditions must reflect the solvent and equipment. With aqueous polymer dispersions a homogeneous film is generated through coalescence of the polymer particles. Product temperature needs to be at least 10 K above the MFT (Minimum Film forming Temperature) of the formulation for acceptable film formation. Product temperature is the key process parameter, being a composite of contributions from spray rate, inlet air temperature, and inlet air capacity. It is measured either directly in the product bed or indirectly as exhaust air temperature. Usually before starting the spraying the product is gradually heated to the target temperature to ensure optimal film formation at the outset. Too high product temperatures increase the risk of spray drying of the polymer particles leading to insufficient film structure and level, thereby altering release. Figure 13.10 exhibits cross sections of coatings comparing good quality with a coating that shows enclosures of spray dried particles. If the product temperature is too low, the particles may stick together, the fluidization dynamics become dysfunctional, possibly leading to loss of the batch.

It is crucial at process startup to keep mechanical stress low as particles are not yet stabilized by applied film. Spray rates should also be low at startup to avoid penetration of water or solvent into the cores which may be subsequently difficult to remove due to the barrier in place, consequent to coat application.

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Fig. 13.10 Coating qualities: left: homogeneous and dense, right: enclosure of spray dried particles which may compromise film functionality

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Fig. 13.10 Coating qualities: left: homogeneous and dense, right: enclosure of spray dried particles which may compromise film functionality

Some polymers applied as aqueous dispersions may require thermal treatment after spraying to enhance coalescence and film formation. This can be achieved by conventional tray drying or more efficiently by fluidization in the coating equipment. Polymer manufacturers usually provide polymer-specific recommendations for such processing.

Large-scale coating operations are usually more efficient than at laboratory scale. At the same time particles may encounter greater attrition at large scale due to higher material load and longer processing time.

In 2009, Vector corp. US launched a dry polymer coating fluid bed process [34] using the Granurex® conical rotor technology. Polymers that are available as micron-ized powders can be applied without organic solvents or other vehicle. An aqueous plasticizer solution is sprayed onto the pellets moving in a conical rotor. At the same time, the dry polymer powder is fed at a controlled rate from the powder feeder and dispersed onto the surface of the pellets via a powder spray gun. The dry polymer sticks to the surface of the pellet and the presence of the plasticizer in the spray causes the dry polymer to coalesce into a homogeneous film. Advantages for this process are much shorter process times, reduced or eliminated organic solvent usage, minimal exposure of the product to moisture, and decreased material preparation steps from the elimination of the need to prepare the polymer solution.

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