Powder Flow Cell Characterization of Powder Behavior in Manufacturing Processes of Polymers
The manufacturing of powders with consistent behavioral characteristics can be difficult, especially on an industrial scale. Parallel manufacturing lines often generate products with different characteristics despite using the same processing parameters, which can cause problems in the manufacturing process itself and it is also an issue for the final product quality. Therefore fine tuning of processing parameters can help to establish and hold a well-defined output quality. In this report, various characteristics are measured, and their impacts on processes are analyzed. The addressed characteristics are cohesion and abrasion of the tubing, as well as bridging in powder processes.
Cohesion of a bulk solid is a mechanism leading to an enhanced mechanical resistance to shear of the powder. Various physical characteristics are involved in cohesion, such as particle size distribution, particle shape, particle elasticity, presence of humidity or chemical reactions on the surface. The cohesion in uncompressed powders can be estimated in the Anton Paar powder flow cell by monitoring the torque needed to rotate a blade inside the bulk at a given speed (see the application report: Introduction to Powder Rheology). However, some applications need to measure the cohesion with a given pre-compaction, which can be achieved with a variation of the Warren Spring Method. Abrasion between the tubing and the powder particles can be compared quantitatively between samples by using wall friction measurements. The tubing material can be mounted onto a specific measuring tool and will yield the friction coefficient at the applied normal force. Bridging can occur when both friction inside the bulk (internal cohesion) and the wall friction reach high values. The internal cohesion of the bulk is a powder bulk parameter which can be measured in the powder flow cell with the Warren-Spring geometry. The approach is similar to the method for cohesion measurements; but it takes place on a precompressed bulk. The Warren-Spring tool allows one to measure shear strength within the powder itself, thus defining internal cohesion. This work exemplifies how this set of physical parameters can be used to compare and quantify the behavior of two similar polymer powders.
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