Melt Fracture of Two Broad Molecular Weight Distribution High-Density Polyethylenes Mahmoud Ansari, 1 Savvas G. Hatzikiriakos, 1 Ashish M. Sukhadia, 2 David C. Rohlfing 2 1 Department of Chemical and Biological Engineering, The University of British Columbia, Vancouver, British Columbia, Canada 2 Chevron Phillips Chemical Company LP, Bartlesville, Oklahoma The melt fracture instabilities of two broad molecular weight distribution (MWD) high-density polyethylenes (one Ziegler–Natta and one metallocene HDPEs) are studied as functions of the temperature and geometri- cal details and type of die (cylindrical, slit, and annular). It is found that sharkskin and other melt fracture phe- nomena are distinctly different for these resins, despite their almost identical rheology. It is also found that the critical conditions for the onset of various melt fracture phenomena depend significantly on the type of die used for their study. For example, sharkskin melt fracture in slit and capillary extrusion was obtained at much small critical shear stress values compared with those found in annular extrusion. Moreover, the metallocene HDPE shows significant slip at the die wall in the sharkskin flow regime. On the other hand, the Ziegler–Natta HDPE has shown no sign of slip. These differences are dis- cussed on the basis of differences in their MWDs that influence their melt elasticity. POLYM. ENG. SCI., 52:795– 804, 2012. ª 2011 Society of Plastics Engineers INTRODUCTION Extrusion is one of the most important routes of pro- duction plastic articles such as pipes, films, and sheets. From the economical point of view, it is desirable to increase the rate of production without sacrificing product quality. However, this is limited by the occurrence of flow instabilities at flow rates greater than a critical value. These instabilities manifest themselves as surface defects on the surface of extrudates and are collectively known as melt fracture phenomena [1–4]. These include sharkskin (small amplitude periodic distortions) or surface melt fracture, slip-stick or oscillating melt fracture (alternating relatively smooth and distorted portion on the surface), and gross melt fracture (large amplitude periodic and/or nonperiodic, chaotic distortions). Melt fracture has been observed in a number of polymers (mainly linear), more frequently on all types of linear polyethylenes including high-density polyethylenes (HDPEs), which is the subject of this work [5]. It has been reported that melt fracture phenomena depend strongly on the molecular characteristics of poly- ethylenes such as molecular weight and its distribution and levels of long chain branching [1, 6]. To assess their processability and correlate it with their molecular struc- ture, rheological methods are frequently employed [7]. In particular shear, extensional, and capillary rheometry have been proven to be indispensable methods in assessing the processing behavior of polyolefins and relate it to rheol- ogy [8]. Many attempts have been devoted to understand and resolve these instabilities from different points of view. A thorough review can be found in Refs. 5 and 9. There are several studies that relate these instabilities with the mo- lecular structure as assessed by the molecular weight and its distribution [10–12]. Based on these studies, it is gen- erally agreed that by increasing molecular weight and decreasing the breadth of molecular weight distribution (MWD), these instabilities become more pronounced, which means they are obtained at smaller apparent shear rates in capillary rheometry [13, 14]. In particular, Kazatchkov et al. [14] have found a dramatic improve- ment in processability of linear low-density polyethylenes with polydispersity index (PI) roughly greater than 9. However, it is not known how these phenomena are affected for very broad MWDs typically with PI greater than about 20. In a previous article, we studied the rheological proper- ties of two series of ZN and m-HDPEs of broad molecu- lar weight with PI greater than 20 [15]. In particular, it was reported that the broadness of the MWD affects the power-law exponent in the relation between the zero shear viscosity and the weight average molecular weight. How- ever, once these effects are accounted for, the power Correspondence to: Savvas G. Hatzikiriakos; e-mail: hatzikir@interchange. ubc.ca Contract grant sponsor: Chevron Phillips Chemical Company LP. DOI 10.1002/pen.22144 Published online in Wiley Online Library (wileyonlinelibrary.com). V V C 2011 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—-2012