Spatially Resolved Characteristics of Pharmaceutical Sprays Ariel R. Muliadi and Paul E. Sojka Maurice J. Zucrow Laboratories, School of Mechanical Engineering, Purdue University, IN 47907 DOI 10.1002/aic.12775 Published online in Wiley Online Library (wileyonlinelibrary.com). Spatially resolved drop size, velocity, and volume flux data for five different spray coating guns were described in this study. Such spatially resolved measurements show how sprays respond to changes in operating conditions and gun design in ways that less complete measurements can not provide. Data for instance, allow us to recognize the unique drop size distribution of one of the sprays tested, which ultimately was an important factor in determining the dual roles of the shaping air flows: depending on drop size, viscosity, and the magnitude of the shaping air velocity, the shaping air can either pinch or induce secondary atomization to the sprays. When the former outweighs the latter, a dumbbell-shaped spray develops; a more uniform spray results when the opposite occurs. Volume flux data from the different sprays also suggest that a more robust and consistent tablet coating process can likely be designed by utilizing multiple overlapping round sprays. V V C 2011 American Institute of Chemical Engineers AIChE J, 00: 000–000, 2011 Keywords: spray tablet coating, spray pattern, phase Doppler analyzer, pharmaceutical, multiphase flow Introduction Most pharmaceutical tablets have a thin (10–20 lm) film coating on their surface. The coating is often of significant importance as it may mask taste, improve tablet mechanical properties, separate reacting ingredients within the tablet, ease swallowing, seal the tablet from moisture to improve shelf-life, and control drug release rate and location within the patient (enteric coating). A film coat is usually applied by subjecting tablets to an atomizer-produced spray while tumbling them inside a rotat- ing cylindrical drum. Current implementation of this process, however, is such that film finish defects (e.g., cracks/splits in films, film peeling off tablet surfaces, poor film-to-tablet ad- hesion, etc.) and inter/intratablet coating nonuniformities of- ten occur. Finish defects are the result of several factors. Tablet-to- tablet and tablet-to-drum surface attrition are the most obvious explanations. Not often explored are the roles that the spray and its characteristics play on film coat quality. Studies that do relate film coat quality to spray characteris- tics are: • Kim et al. 1 and Rowe and Forse, 2 who reported that incidences of logo infilling (bridging) increased as liquid supply rate increased. • Twitchell, 3 Twitchell et al., 4 and Reiland and Eber, 5 who found that increasing the atomizing air pressure or decreasing the gun-to-target distance resulted in a smoother film coat. • Twitchell, 3 who noted that a decrease in gun-to-target distance decreased film surface roughness. • Ruotsalainen et al., 6 who determined that film surface roughness increased with an increase in liquid supply rate. • Fisher and Rowe 7 and Rowe, 8 who observed that film- to-tablet adhesion decreased as liquid viscosity increased. • Twitchell 3 and Rowe and Forse, 9 who found that increasing the local spray mass flux at the tablet bed, achieved by increasing the liquid supply rate or changing the spray pattern from elliptical to circular, decreased the inci- dence of film splitting. In addition to finish defects, spray characteristics can also contribute to inter/intratablet coating nonuniformities. Rele- vant literature includes that by: • Twitchell et al., 10 Ruotsalainen et al., 6,11 and Tobiska et al., 12 all of whom confirmed that increasing atomizing air pressure decreased intratablet film thickness variations. • Rege et al. 13 and Porter et al., 14 who noted that a greater intertablet film thickness variation resulted when atomizing air pressure, 13,14 liquid viscosity, or liquid supply rate was increased. 14 While finish defects may reduce only the aesthetic appear- ance of the tablets, inter/intratablet coating nonuniformities pose a more serious problem; measurements show that film thickness varied, on average, by as much as 57% from one tablet to another. 15 This is significant, especially when the film coat controls the release rate and/or release location (en- teric coating) of the active ingredient(s). In such cases, the drug may be released too early—which happens when a por- tion of the film coat is too thin, is cracked/split, or when it is missing altogether—or too late, which happens when the film coat is too thick. The behavior described earlier can also be explained by droplet wetting characteristics, 3,16 and by understanding how mechanical stresses are developed in film coatings. 2,16,17 Coating nonuniformities, on the other hand, likely arise from nonuniformities in sprays. 15 Accordingly, spray data that Correspondence concerning this article should be addressed to P. E. Sojka at sojka@purdue.edu. V V C 2011 American Institute of Chemical Engineers AIChE Journal 1 2011 Vol. 00, No. 0