Shear field induced diffusion and molecular weight fractionation during polymer processing

Abstract

It has been hypothesized that a high shear gradient induced during polymer processing could cause an increased migration of oligomer to the surface, thereby leading to "plateout" on mold and roll tooling. To test this hypothesis, poly(ethylene terephthalate) (PET) and poly(ethylene-co-cyclohexylenedimethylene terephthalate) (PETG) were extruded into film, and the molecular weight probed as a function of the depth from the surface by incremental milling. GPC analysis showed an18% and14% higher concentration of cyclic oligomer on the surface of these films as compared with the center for PET and PETG respectively. These results compare favorably with predictions from a theoretical shear field model based on a Convected Maxwell fluid. Results from this study should lead to a better understanding of how oligomer buildup at the die wall may relate to roll plateout, die drooling, die lubrication, and possibly even melt fracture.

Geometrical Confining Effects in Compression Molding of Co-continuous Polymer Blends

“…It has been reported that the low viscosity component in a polymer blend during shear flow tends to migrate to the mold-blend interface to minimize the total system free energy. 7,10,11,30,39,40 In this PLA/PS blend, PLA was the low viscosity component during compression and, according to the energy minimization theory, it has the tendency to flow to the high shear rate region near the mold-blend interface. This possible mechanism for phase migration may be considered in a simple extreme case involving a liquid soaked sponge.…”

Migration of gluten under shear flow: Influence of process parameters on separation behaviour

“…There are numerous publications on the effect of simple shear on the migration of polymers (MacDonald & Muller, 1996;Shelby & Caflisch, 2004) and macromolecules (Agarwal, Dutta, & Mashelkar, 1994;Ponche & Dupuis, 2005). Shelby and Caflisch (2004) mentioned that a gradient in shear rate causes a gradient in the elastic strain energy for HMW molecules.…”

“…There are numerous studies describing shear-induced migration of macromolecules and polymers in rotating cone and plate geometries (Del Castillo, Criado-Sancho, & Jou, 2000;Ma & Graham, 2005;MacDonald & Muller, 1996;Shelby & Caflisch, 2004;Usta, Butler, & Ladd, 2006;Zheng & Yeung, 2003). However, there is to our knowledge no work published that uses these principles for concentrated biopolymer systems such as dough.…”

Historical review of die drool phenomenon during plastics extrusion

“…Further, in some research works [20,31,92,93], authors speculate about flow induced molecular weight fractionation (diffusion of short molecules under highly inhomogeneous stress profile towards to die wall) of polymer melt flowing inside the extrusion die as probably one of the basic formation mechanism of die drool phenomenon. This idea was experimentally proved in the recently published works focused on die drool during unfilled, well-stabilized and virtually linear HDPE melts extrusion at very low processing temperatures [16,17].…”