Adjusting dissolution time and flowability of salicylic acid powder in a two stage plasma process

Many industrial products and intermediates in powder form suffer from low flowability and poor dissolution behavior. Current techniques to overcome such problems often include time-consuming admixture of additives or wet-chemical processes that are harmful to the environment. Therefore, the objective of this work was to develop a continuous non-equilibrium plasma process to modify the flowability and dissolution behavior of fine-grained, cohesive and temperature sensitive powders. As test substance salicylic acid was used due to its low melting point, cohesive nature and similarity to many pharmaceutical products. proposed process the powders were passed through a low-pressure discharge to modify the single particle surfaces. In a first process step SiOx nanostructures and thin films were formed in the plasma and deposited on the salicylic acid particles. The created nanostructures acted as spacers and reduced the prevailing interparticle van der Waals forces. Hence, the flowability of the dry powder was improved from the cohesive to the easy-flowing regime. The flowability was higher the more nanoparticles were deposited per surface area. On the other hand, the deposition of nanostructures retarded the dissolution, which was attributed to the partial coverage of the particle surfaces. Therefore, these already coated powders were surface activated in a second plasma-assisted process step in the same reactor. The particle surfaces were enriched with polar groups, which increased the surface free energy. This improved the powder wettability and the dissolution time was reduced even below the measured value of uncoated powder. with the introduced process both the flowability and dissolution time of the cohesive, temperature-sensitive and fine-grained test substance salicylic acid could be adjusted within a very short process time in the order of 1 s.