A study using different types of fumed silica to modify the flowablity, wettability and surface free energy of a model cohesive powder

Abstract

The objectives of this study were to investigate the improvement on flowability of a model cohesive powder using two different types of fumed silica mixed by the dry coating method “mechanofusion” and to explore the relationship between flowability and surface free energy and/or wettability. The first aim was to find the optimum conditions for improving the flowability of a model cohesive powder, glass microspheres (DMV ~ 3.5 µm), by mechanofusion with fumed silica. Conventional blending using tumbling was done as a comparison. Mechanofusion was conducted at three different speeds 1500, 3000 and 4500 rpm and at three concentrations of hydrophobic Aerosil®R972 (1%, 3% and 5% w/w) for 10 minutes, while conventional blending was performed at 72 rpm for 30 minutes. It was revealed that the optimum concentration to improve flowability was 1% w/w Aerosil®R972 for all processes utilized. Increasing the concentration of the fumed silica decreased the flowability at all speeds. This indicates the enhancement of flowability of this model cohesive powder depends on the concentration of guest particles. The influence of different speeds to the flowability was observed upon the increasing concentration of Aerosil®R972 at 3% and 5% w/w. Mechanofusion was found to be a more effective mixing method compared to tumbling mixing presumably due to the higher energy applied that can de-agglomerate the cohesive microspheres. The second aim of this thesis was to determine the mechanism of action of the fumed silica in improving cohesive powder flow and to investigate the relationship between flowability and surface free energy and/or wettability. Hydrophilic fumed silica Cab-o-sil®M-5, which has the same particles size as Aerosil®R972, was used as a comparison. 1% w/w Cab-o-sil®M-5 failed to improve the flow of particles, whereas the same concentration of Aerosil®R972 did. Powder wettability and surface free energy were derived from contact angle measurements of uncoated and coated particles. The capillary rise technique was used to measure powder contact angles. The water contact angle increased when the glass microspheres were coated with hydrophobic Aerosil®R972, whereas it decreased when coated with hydrophilic Cab-o-sil®M-5. However, the 1-bromonaphthalene contact angle increased after coating with both types of fumed silica. The contact angle data was used to calculate the surface free energy of uncoated and coated particles. A decrease in dispersive and polar components of surface free energy was found in particles coated with hydrophobic fumed silica, while the surface free energy of particles coated with hydrophilic fumed silica reached the highest value. Particles with higher surface free energies had poorer flow than particles with lower surface free energies. Similarly, particles with a water contact angle less than 90° had lower flowability than particles with a water contact angle higher than 90°. Therefore, this study found that the improvement of flowability of micronized cohesive particles can be achieved by mechanofusion provided the chosen guest particle has relatively low surface free energy and/or is hydrophobic. The outcomes of this thesis present an enhanced better understanding of improving the flowability of a cohesive powder using fumed silica. These findings may have future value for the development of improved solid formulation of pharmaceutical preparations with the active pharmaceutical ingredients (APIs) in micron size.