Particle Properties of Spray-Dried and Agglomerated Products of Salicylic Acid and Sodium Salicylate

Abstract

The aqueous slurries of salicylic acid and sodium salicylate containing various kinds of binder, e.g. gum arabic, gelatin, polyvinyl alcohol, carboxymethylcellulose, methylcellulose, polyvinylpyrrolidone, tragacanth and sodium alginate, were spray-dried using a centrifugal wheel atomizer. Almost all of the agglomerated products are fine granules having a diameter of 15-100μ, compared with 1-8μ for the original salicylic acid, and are fairly free flowing. It is the case with polyvinylpyrrolidone and gum arabic products.The particle properties, such as diameter, true density, sphericity and surface area were measured, all of these being strongly influenced by the concentrations of sodium salicylate and binder. The surface topography of the products was studied with a scanning electron mciroscope. Gum arabic and polyvinylpyrrolidone products had smooth surfaces without holes or craters. Granules borne with other binders showed characteristic surfaces having craters of 5-10μ. The analysis of the X-ray diffraction patterns and the infrared absorption spectrum of products showed that gum arabic and polyvinylpyrrolidone granules still contained both salicylic acid and sodium salicylate, in contrast to the cases where other binders were used, where only sodium salicylate remained, the salicylic acid having sublimed. Gum arabic and polyvinylpyrrolidone products are microcapsules containing salicylic acid and sodium salicylate of grain size of about 200-800Å, amorphous contents of about 50-80% and grain size of about 130-170Å, polymorphism including original crystalline form of 3-20% respectively. The thickness of the encapsulating film is 1-3μ. Finally, the change in appearance of a drying slurry droplet is discussed and two typical drying processes, namely agglomerated crust and encapsulating film formation, are described.