For any film coating process, a balance must be obtained between achieving a fast coating time and a high quality coated product. Film coating formulations have recently been introduced, which may significantly increase the spray rates possible in production processes. Indeed, the inclusion of formulation additives such as lactose1 and microcrystalline cellulose2 should not only increase spray application rates but may also improve product quality by promoting tablet-film adhesion.

Although much recent work has investigated methods of improving film coating process times and product quality by formulation techniques,3,4 relatively little work has examined the characteristics of the spray applied to the tablets.

Equation 1.

The purpose of this study was to investigate the factors influencing atomization from two spray guns. In particular, consideration was given to spray rates and air pressures used during the coating of small development-scale batches on interchangeable drum coating equipment and how the spray conditions used here compare with those typically used in a production environment.

Figure 1: The relationship between atomizing air pressure, fan air pressure and median droplet diameter for the Manesty spray nozzle and Figure 2: The relationship between atomizing air pressure, fan air pressure and median droplet diameter for the Schlick 930/7-1 spray nozzle.

A 12% w/w suspension of Opadry OY 35018 (Colorcon, Dartford, UK) was used as the spraying medium for all the droplet size measurements. This suspension was pumped and atomized using a Flowtab system (Manesty, Knowsley, UK).

For droplet size determination, the gun was mounted on a manifold and positioned to spray through the laser beam. The gun was activated and the spray aimed through the laser. The laser/spray guns were protected with a Perspex cylinder to prevent any residual spray swirl recirculating back through the laser beam, which could produce atypical results. The spray was extracted by exhaust to prevent spray bounce back.

A 40 g/min spray rate and a measuring distance of 15 cm was used - both intended to mimic typical process conditions used in a 610 mm diameter drum (16 L working capacity).

Figure 3: Comparison of fan air mass flow rate at different fan air pressures for the Manesty and Schlick spray guns and Figure 4: Comparison of atomizing air mass flow rate at different atomizing air pressures for the Manesty and Schlick spray guns.

The results for droplet size are expressed as the median volume diameter (Dv 50). Droplet size distribution is expressed as the span (Equation 1).

Phase doppler droplet velocity measurement. A phase doppler particle analyser (PDPA) from Aerometrics Inc. (Sunnyvale, California, USA) was used to measure droplet velocity from the Manesty spray gun. The gun was mounted on a manifold and the spray activated such that it was aimed directly downwards. The laser was set up so that the measuring zone (the point where the two horizontal laser beams crossed) was directly in the centre of the spray cone. The gun-to-measurement distance was set at 10 cm and the coating solution used for the droplet velocity measurements was 9% w/v Pharmacoat 606 (Shin Etsu Chemical Co., Tokyo, Japan). A range of atomizing and fan air pressures (0-200 kPa) was studied.

Figure 5: Manesty spray gun.

Spray shape/area measurement. The spray area of a series of gun configurations was determined using a 12% w/w Opadry OY35018 suspension. The appropriate spray gun was mounted on the manifold of a Premier 200 coating machine (Manesty) such that the guns sprayed directly downward. Upon activating the spray, the gun was covered with a polythene bag until the desired spray rate, atomizing and fan air pressures were achieved. At this point, the bag was removed from the gun and the spray pattern built up on a sheet of A3 paper. Spraying time was set at 2 s. The wet spray patterns were left to dry overnight and the dimensions and area of each spray pattern were determined.

Spray gun air consumption measurement. The air consumption at a series of atomizing and fan air pressures, and liquid flow rates, was determined during the spraying of batches of tablets in an XL coater (Manesty). Air flow readings were taken with an online air flow meter (Dwyer Instruments Inc., High Wycombe, UK). Volumes were converted to air masses using appropriate pressure and temperature conversions.