(230f) Combined Effects of Temperature and Moisture on Polymer Powders

Polymer powders are found in numerous industrial applications. Especially, with the emergence of powder bed-based additive manufacturing, polymers are one of the most used materials in processes such as selective laser sintering (SLS) or binder jetting. In both cases, thin layers of powder have to be successively deposited on the powder bed, with the requirement to produce flat and homogeneous layers. The spreadability of the powder is thus an essential parameter for these applications and directly depends on the powder properties.

The cohesion is an important property of fine powders and is usually the main driving parameter for flowability. Cohesive forces are mostly due to the presence of capillary bridges between the particles and/or the setting of electrostatic charges arising from tribocharging mechanisms. Exposure of the powder to higher relative humidity will lead to modifications of the moisture content that directly influence the strength of the capillary bridges. Also, electrostatic properties will be influenced by the moisture content, the higher the moisture content the lower the charge buildup inside the powder. Indeed, the presence of the capillary bridges helps to set a low resistivity network that helps to dissipate the charges through the material. Therefore, two cohesive regimes are linked to the moisture: at low moisture content, the cohesion is driven by electrostatic, and at high moisture content the cohesion is driven by capillary forces.

In addition, temperature elevation leads to a modification of the surface properties of the particles, contributing to a change in surface friction as well as the global elasticity of the grains. Also, the moisture content will decrease when the powder is exposed to a higher temperature. Therefore, there are combined effects of temperature and moisture on cohesion that cannot be decoupled.

In this study, the effect of temperature and moisture has been investigated for different polymer powders. The moisture content has been varied by preconditioning the samples in a controlled atmosphere for a wide range of relative humidity. Influence on powder behavior is assessed with an improved tapped density analysis allowing to measure accurately the evolution of bulk density after each tap (GranuPack, Granutools, Belgium). The packing analysis is complemented by a flowability analysis performed in a rotating drum (GranuDrum, Granutools, Belgium). To evaluate the effect of temperature a special version of the GranuPack, the GranuPack High Temperature, allowing to heat the powder up to 200°C during the measurement has been used. The results gathered during this study allow a better understanding of the combined effects of temperature and moisture on polymer powders behavior.