Ashish M. Sukhadia

Wall slip of HDPEs: Molecular weight and molecular weight distribution effects

The slip behavior of several high-density polyethylenes (HDPEs) is studied as a function of molecular weight (Mw) and its distribution for broad molecular weight distribution metallocene and Ziegler–Natta catalyst resins. It is found that slip depends strongly on Mw and its distribution. First, the slip velocity increases with decrease of molecular weight, which is expected to decay to zero as the Mw approaches a value with characteristic molecular dimension similar to surface asperities. For HDPEs that exhibit stick–slip transition, the slip velocity has been found to increase with increase of polydispersity. The opposite dependence is shown for HDPEs of wider molecular weight distribution that do not exhibit stick–slip transition. A criterion is also discussed as to the occurrence or not of the stick–slip transition which is found to depend strongly on Mw and its distribution.

Trade-Offs in Blown Film LLDPE Type Resins from Chromium, Metallocene and Ziegler-Natta Catalysts

In this paper, the processing-structure-property (P-S-P) behavior and trade-offs observed with three LLDPE resins made from chromium, metallocene and Ziegler-Natta catalysts were examined and critically compared. The basic molecular and rheological properties were examined. The blown film performance of these resins was examined with respect to the effects of film thickness and molecular orientation on film properties. Also, the processability of these resins was compared through an examination of the typical extrusion and film blowing processing parameters. Finally, an attempt was made to highlight the advantages and disadvantages of each resin type as they apply to blown film applications. The results demonstrate that LLDPE resins from these various catalysts are dramatically different in nature, thereby resulting in different trade-offs with respect to their P-S-P behavior in film blowing.

The Influence of Solid-State Morphology on the Dart Impact Strength of Linear Low-Density Polyethylene Blown Films

In this study, we report on the influence of lamellar morphology and orientation on the dart impact strength of linear low-density polyethylene (LLDPE) blown films. Evidence confirming the importance of lamellar anisotropy on the impact strength performance of such films is presented. Specifically, lower degree of lamellar orientation (more random orientation) is shown to favor higher impact strengths. We also note that the films stretch considerably during the measurement of dart impact strength with the initial morphology influencing the stretching process.

Worm melt fracture and fast die build-up at high shear rates in extrusion blow molding of large drums

In the extrusion blow molding process of high density polyethylene (HDPE) for making of large size drums, string-like defects, which are referred to as worm melt fracture in the industry, are often observed on the extrudate surface. Such string-like defects in various shapes and sizes are observed in capillary extrusion at very high shear rates after the slip-stick transition. The HDPE resin with broader molecular weight distribution (MWD) exhibits a greater degree of worm melt fracture while the narrow MWD PE resin, which has higher slip velocity and a uniform slip layer, shows a lesser degree of worm melt fracture. It is hypothesized that the worm melt fracture is related to fast die build-up and cohesive slip layer, a failure within the polymer melts at an internal surface. If the cohesive slip layer at an internal surface emerges out from the die, it can be attached on the surface of extrudate as string-like defects, the worm melt fracture. The resin having more small chains and lower plateau modulus ...