(429c) Influence of Storage and Application Conditions On Morphological and Mechanical Properties of Polymer-Coated Particles Under Repeated Impacts and Nanoindentation: Characterization and Modelling

Polymer-coated particles have been produced by top-spray fluid bed coater and then stored at different conditions, namely ambient conditions (23°C, 55% RH) and in the freezer (-18°C, 25% RH). Surface morphology has been first analysed using Scanning Electron Microscope, SEM and Atomic Force Microscope, AFM and then compared in terms of storage conditions effect. Resistance to attrition and viscoelastic properties have been measured by Repeated Impact Tester, RIT and Nanoindentation respectively. Coating thickness, coated particles 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. Such influence, negligible at low energies (low numbers of impacts) increase as soon as the number of impacts and thus the energy rise.

Quasistatic and dynamic nanoindentation were found to agree very well with each other. In these cases the storage-dependence behaviour was not as clear as for repeated impact tested particles. Comparison of the nanoindentation results and the RIT results showed that tan d is strongly related to the attrition behaviour.

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 impacts tests. The common attrition mechanism was found to be layer fatigue. Using the equation proposed by Tavares and King [1] 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.

[1] Tavares, L.M., King, R.P., 2002. Modeling of particle fracture by repeated impacts using continuum damage mechanics, Powder Technology 123, 138-146.