2Department of Chemical EngineeringThe University of British Columbia 221 6 Main Mall Vancouuer, BC 6VT 124, Canadu Froduct quality and, in general, processing windows and rates of production in the extrusion of molten polymm are limited by various flow instabilities (mainly melt fi-acture) that are observed at high production rates. While there still are unresolved issues in comprehending these phenomena, industry demands for process optimization dictate the employment of processing aids (PAS) for product quality improvement and energy requirement reduction. In this review paper, most commercially used processing aids (PAS) are discussed, namely, fluoroelastomers. stearates, small PAS on the rheologid properties and the instability (melt fracture] elimination is also discussed. The factors that determine the additive performance, such as concentration, dispersion quality. and interactions between additives, additive and polymer, and additive and die surface, are also examined. The various mechanisms by which Merent types of processing aids help eliminate the instabilities are reviewed as well. Section 2 discusses the various melt fi-acture phenomena in 'Dafieated tothe lnemoryof Dr. Charles w. Stewart, an imnstlgator* made *Author rrsponsiMe for conrspdmce. emaIk h a t z i k J & h~. u b c . c a 8orrTBl CXCCPUOIld eonttibutblU3 the -Of alds. & ADDITIVE TECHIVOLOGY, MARCH 2002, Yo/. 8, No. 1 7
JOURNAL OF VINYLwhere L is the length of the capillary, A P is the total pressure drop driving the flow, and AP,, is the end correction for the pressure drop. Figure 1 also represents typical behavior of a linear polymer in general. Polymers such as LDPE and PP do not exhibit the stick-slip flow regime as explained below. Four distinct flow regimes can be identified in Rg. 1. Initially, the flow is stable, and the shear stress increases "linearly" (log-log plot) with apparent shear rate (power-law behavior), while the extrudate has a smooth and glossy appearance. When the shear stress reaches a first critical value (crcl), small amplitude, periodic distortions m. 1. Qpicclrjbw curue for UnearpolyethyIene acbuded in apiston driven cqpillnry tllustmting the various regions of instability.