Polystyrene secondary flows

Ide and White W studied the viscoelastic effects in agitating polystyrene solutions with a turbine. At concentrations below 50% PS, flow was normal. Abovfe 35%, the viscoelastic forces caused the flow to reverse, moving away from the impeller along the axis. At 30 to 35% PS, both occurred, causing a segregated secondary flow around the turbine. 

Figure 10.15. Flow of a high-impact polystyrene resin (Dow Styron 484) through a 61-cm channel with a square cross section that is 0.9525 cm on a side. A special feedblock brings in concentric layers, which are colored for contrast, (a) At the channel entrance, (b) At the channel exit, (c) Computed secondary flow. Reprinted with permission from J. Dooley, Viscoelastic Flow Effects in Multilayer Polymer Coextrusion, Ph.D. dissertation. Technical University of Eindhoven, the Netherlands, 2002.

Many polymers exhibit neither a measurable stick-slip transition nor flow oscillation. For example, commercial polystyrene (PS), polypropylene (PP), and low density polyethylene (LDPE) usually do not undergo a flow discontinuity transition nor oscillating flow. This does not mean that their extrudate would remain smooth. The often observed spiral-like extrudate distortion of PS, LDPE and PP, among other polymer melts, normally arises from a secondary (vortex) flow in the barrel due to a sharp die entry and is unrelated to interfacial slip. Section 11 discusses this type of extrudate distortion in some detail. Here we focus on the question of why polymers such as PS often do not exhibit interfacial flow instabilities and flow discontinuity. The answer is contained in the celebrated formula Eqs. (3) or (5). For a polymer to show an observable wall slip on a length scale of 1 mm requires a viscosity ratio q/q equal to 105 or larger. In other words, there should be a sufficient level of bulk chain entanglement at the critical stress for an interfacial breakdown (i.e., disentanglement transition between adsorbed and unbound chains). The above-mentioned commercial polymers do not meet this criterion. 

Butyl stearate (butyl octadecaonoate) n. C17H35COOC4H9. A mold lubricant and plasticizer, compatible with natural and synthetic rubbers, chlorinated rubber, and ethyl cellulose. It can be used in vinyls in very low concentrations as a non-toxic, secondary plasticizer and lubricant. In the production of polystyrene, butyl stearate is added to the emulsion polymerization to impart good flow properties to the resin See image). 

Since the discovery of proline-catalyzed enantioselective aldol reactions, an extensive research program to explore chiral secondary amine catalysts has been pursued. Several polymer-supported chiral amines have been synthesized for aldol, Mannich, and related reactions. Polystyrene is a popular solid phase for use in place of silica gel in the proline-based organocatalysis. In contrast, silica gel displays a slightly acidic character and has a hydrogen-bond donor or acceptor, which may change the catalytic activity and chiral space of the organocatalyst. Flow enantioselective aldol [158-161], Mannich [162], Michael [163], and related reactions... 

Typical Experimental Results Consider first an amorphous polymer polystyrene [134]. This first example has been chosen to show the effect of flow on the final orientation. The molecular orientation in the part thickness has been characterized by shrinkage measurements (Fig. 15.33). It is maximum at the surface, decreases, passes through a secondary maximum, and finally tends to zero at the core. 

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