Cohesive-Driven Particle Circulation in the Solids Conveying Zone of a Single-Screw Extruder

Aspects of heat transfer within the solids conveying zone of a single-screw extruder were studied by using a specially constructed drum testing apparatus. Experiments were conducted with linear low-density polyethylene, polystyrene (PS), and polypropylene (PP) samples by examining their transient temperature profile while the heated drum was stationary or moving. In accordance with classic solids conveying theory, the granular beds of PP and PS remained as plugs while the drum rotated. In such cases, the dominant modes of heat transfer for these systems are conduction through the contact area of a particle and conduction through the interstitial gas. An exception to this behavior was found with PE, in which the bed temperature increased more rapidly while the drum rotated. Visual observations of the bed showed that the particles circulated in the presence of shear and that this complex flow pattern increased in velocity as the drum temperature approached the onset temperature for melting the PE material. With strong correlation between the rate of circulation and the temperature rise in the bed, the movement of particles was assumed to act in a convective heat transfer mode bringing about more uniform heating of the polymer. The circulation phenomenon was attributed to dominant adhesive forces at the particle-drum interface overcoming the cohesive strength of the bulk. (C) 2009 Wiley Periodicals, Inc. Adv Polym Techn 27: 74-88, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20121