Influence of handling and storage conditions on morphological and mechanical properties of polymer-coated particles: characterization and modeling

Polymer-coated particles have been produced by top-spray fluid bed coater and both morphology and resistance to attrition have been analysed using Scanning Electron Microscope, SEM and Repeated Impact Tester, RIT respectively. Coating thickness, coated particle storage conditions and physical aging effects have been assessed. The coating thickness is found to be extremely relevant in raising the resistance to attrition. Thicker the coating and more resistant is the coated particles against attrition. This improvement is found to be more and more relevant while impact energy is increasing. The storage temperature is not influencing the morphology whereas is strongly affecting the resistance to attrition. Coated particles stored at −18 °C were found to be more resistant to attrition than ones stored at room conditions. Such differences, negligible at low energies (low numbers of impacts) increase as soon as the number of impacts and thus the energy rise.The coated particles, stored at ambient conditions, were subsequently aged in vacuum oven and the effect of aging steps was evaluated in terms of resistance to attrition. In aged coated particles were found a wasting in resistance to attrition directly proportional to the aging time. Moreover, the aging process was found to affect the breakage mechanism experienced by the coated particles during impact tests. The common attrition mechanism was found to be layer fatigue. Using the equation proposed by Tavares and King (2002) [48] as starting point, a new equation has been developed in order to fit to the resistance to attrition data. The equation takes into account the number of impacts, the velocity of the impacts, the coating thickness, the coefficient of restitution, e, of the coated particles and the mass specific fracture energy, Ef,m. This equation has been successfully applied for different coating materials and different coating thicknesses.

A fundamental analysis of the relationship between the strength of coated pellets and intrinsic characteristic of the coating shell, storage temperature and aging time that is required has been carried out. Moreover, a novel mathematical equation which is able to model the behavior of aged coated pellets has been successfully introduced.Figure optionsDownload as PowerPoint slide