(328g) Influence of Temperature on the Packing Dynamics of Powders

Numerous industrial processes involve granular materials and fine powders. However, due to the lack of complete theoretical descriptions, the understanding of powder behavior has to rely on the characterization of the macroscopic properties (flow, packing dynamics, agglomeration, segregation, ...). Powder behavior is known to be influenced by particles shape and size distributions, friction forces and cohesive interactions. Cohesive interactions are due to the presence of liquid bridges, electrostatic charges, van der Waals interactions or more rarely magnetic dipole-dipole interactions. For powders, the behavior is usually driven by these cohesive interactions due to the small particle sizes.

Environmental conditions (relative humidity and temperature) can alter the powder properties changing the predominance of these grain interactions. Indeed, moisture is known to influence both surface grains conductivity and capillary bridges formation with as a consequence a modification of the flowing properties. Furthermore, empirical observation seems to indicate that the interactions between the grains as well as grains breakage behavior are affected by temperature. However, precisely controlling the powder temperature during a process requires costly equipment and is almost not feasible for large production facilities. Seasonal changes and production in different sites over the globe induce unavoidable variations of the environmental conditions at which the powder is processed. Therefore, understanding the influence of temperature on the powder behavior is of great interest to predict the degradation of process performance and establish optimal processing conditions. Investigations on the effect of environmental conditions as mainly focused on relative humidity [1-2], temperature has only be considered in processes that involve powder heating at high temperature, such as polymer and metal powders in additive manufacturing [3].

In the present study, we emphasized the importance of even low variations of temperature on common food and lactose powders. Changes in powder properties have been assessed by a packing dynamics characterization method which provides useful information on the powder properties that can be related to its flowability. An improved version of the classical tapped density method has been used (GranuPack, GranuTools)[4] and modified to allow continuous powder heating during the measurement. After each tap, the density is measured accurately with the use of an inductive sensor, allowing to extract useful information on the packing kinematics. Temperature is found to slow down the packing dynamics at temperature variations that are usually considered as normal room fluctuations (20 – 40 °C). The importance of evaluating the effect of temperature even for processes that do not involve powder heating is thus highlighted. We also demonstrate the inability of the classically used Hausner ratio analysis to properly describe this behavior. These new results evidence the need to take into account the environmental conditions at which the powder will be processed at the beginning of the development and characterization stage.