Study of pulverization of low density polyethylene using solid state shear extrusion

Solid State Shear Extrusion (SSSE) is a size reduction process method by which polymeric material may be turned into powder. Such powders may be used as fillers and reinforcing agents in polymer, composite, and thin layer coatings on metals and wood.; This study pursues two major objectives. The first objective of this work is to develop and optimize the SSSE process for low density polyethylene (LDPE) in an extruder. The second objective of this study is to obtain fundamental understanding of pulverization and agglomeration of Low Density Polyethylene at high shear normal forces.; From the extruder experiments it was determined that LDPE with melt flow index of 55 pulverizes at temperature of 85{dollar}spcirc{dollar}C. The produced powder had mass average diameter of 460 {dollar}mu{dollar}m, with about 50% of the powder has size smaller than the average. Reprocessing this powder resulted in 10% reduction in mass average with 90% of the powder less than average. In order to obtain optimum extruder conditions, experiments were performed in which operation window for prescribed particle size distribution was determined. Furthermore, it has been shown that recycling of produced particles may be used to control the particle size distribution.; It is hypothesized that pulverization process has two stages. In the first stage simultaneous action of compression and shear leads to propagation of an excessive number of cracks, and consequently, results in disintegration of low density polyethylene sample. The difference between stored strain energy due to shear and frictional forces is available for crack propagation. In the second stage agglomeration of the newly formed particles takes place. This agglomeration depends on the magnitudes of shear force and the contact force between two particles. This hypothesis is tested with the Bridgman anvil, in which a sample (circular disc) of low density polyethylene is placed between two metal pins. A prescribed torsion and compression can be simultaneously applied to the sample. Experiments with a Bridgman anvil reveals that, magnitude of compression and the residence time are both important for initiation of pulverization; moreover, there is an optimum value of compression that needs to be applied to the sample for initiation of the pulverization. Bridgman anvil result qualitatively confirms our hypothesis. Numerical analysis of the Bridgman anvil showed the magnitude of the stored energy is maximized at the edge of the circular disc. This numerical analysis is in line with the Bridgman anvil experimental observation.