Extrusion of Mica-Reinforced Polypropylene: Surface Texture

Abstract

Surface roughness of polypropylene/mica extrudates was related to the presence of entrapped air in the melt. Orientation of mica flakes inside the extruder hinders the escape of air from within the molten polymer. Upon exiting from the die escaping air bursts out through the sheet skin creating surface discontinuities which are responsible for surface roughness. Direct extrusion of polypropylene powder/mica dry blends also produces rough surfaced extrudate, but the pores are significantly smaller. In single screw extrusion, feeding at degrees of starve feeding of 15% and more permit the production of air-free smooth extrudates. In addition, operation in the starve feeding mode, results in a reduced specific power consumption by as much as 50%. Flake orientation in the extrudate is controlled by extrusion conditions: smaller gap, higher convergence ratio and shear rate favor the flake alignment which translates into smoother surfaces. In thin extrudates, the flake orientation is so developed that the surface had sufficient cohesion to prevent the air from escaping at the die exit giving extrudates which, though very porous inside, were smooth surfaced.Brought to you by | Purdue University Libraries Authenticated Download Date | 5/30/15 7:39 AM