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Shear Sensitivity
Melt index and density are two properties that
However, while higher MI resins lead to shorter help describe whether a polyethylene is useful cycle times as illustrated in Table 1, for for a given application. Density is the weight of reasons concerning both processing (parison the polymer per a given volume. Melt index (MI) melt strength) and bottle properties (drop is an indicator of the average molecular weight impact strength), polyethylene grades with high of a polyethylene and relates to both the melt indices are not always the best resins for processing and finished part performance of Polyethylene grades exhibit a phenomenon Melt index however, does not tell the entire called shear sensitivity. The resins in Table1 performance story. Grades of polyethylene with have different melt indices, but the same level of shear sensitivity. The resins in Table 2 all differently during the blow molding process. A have the same melt index but differ in shear third property – shear sensitivity –-provides TABLE2: EFFECT OF SHEAR SENSITIVITY ON EXTRUSION To see how important shear sensitivity is, you must first understand how the melt index alone According to the data in Table 2, the resin with polyethylene grades (HDPE) are given in Table high shear sensitivity produced less head 1. The data show the effect of melt index on the pressure while extruding 25 percent more best possible performance of three resins run material per hour at a lower melt temperature on the same blow molding machine under a than the other two resins. If the operator had constant extrusion pressure. The higher MI chosen a resin simply based on MI, he might resins have an advantage in both cycle time and have overlooked this highly productive grade. power usage. In fact, the 0.8 MI resin has a cycle time 6.2 percent shorter than that of the resin with a 0.3 MI. The power draw was The viscosity of a HDPE melt appears to change reduced by 7.5 percent, as well. As a result, depending on how rapidly it is undergoing more than 200 additional machine cycles can shear, i.e., how hard the resin is being pushed occur during an eight-hour shift if resin C is through processing equipment. The higher the shear rate, the lower the apparent viscosity. An extruder generally produces a moderate rate of TABLE 1: EFFECT OF MELT INDEX ON EXTRUSION decrease in apparent viscosity. Forced at high speed through a narrow die gap, the molten correspondingly large decrease in apparent viscosity. Once outside the die (in the form of a practically no shear and appears very viscous Shear Sensitivity (Continued)

Depending largely upon the molecular weight distribution of the resin, different HDPE grades exhibit different levels of shear sensitivity. Figure 1 shows that good shear sensitivity gives the best extrusion performance (lowest viscosity through the die) and best parison melt strength (highest viscosity at low shear). Some of the problems associated with low shear sensitivity are shown in A resin with poor shear sensitivity may require higher melt temperatures, resulting in longer cycles because of the extra cooling time needed. This resin may also exhibit low parison strength, Fig. 1: The importance of good shear sensitivity – leading to “pleated” parisons and webbing in the high viscosity in the parison, low viscosity A resin with poor shear sensitivity may require higher melt temperatures, resulting in longer cycles because of the extra cooling time needed. This resin may also exhibit low parison strength, leading to “pleated” parisons and webbing in the blown handle. Consistent shear sensitivity is critical for consistent performance. At Lyondell, our resins are constantly checked for consistently high shear sensitivity for successful blow molding. If you would like to learn more about shear Fig. 2: Low shear sensitivity with low Ml is bad sensitivity, please contact your Equistar sales or news for the molder - the cures are even worse. The information on this document is, to our knowledge, true and accurate. However, since the particular uses and the actual conditions of use of our products are beyond our control, establishing satisfactory performance of our products for the intended application is the customer's sole responsibility. All uses of Equistar products and any written or oral information, suggestions or technical advice from Equistar are without warranty, express or implied, and are not an inducement to use any process or product in conflict with any patent. Equistar materials are not designed or manufactured for use in implantation in the human body or in contact with internal body fluids or tissues. Equistar makes no representation, promise, express warranty or implied warranty concerning the suitability of these materials for use in implantation in the human body or in contact with internal body tissues or fluids. More detailed safety and disposal information on our products is contained in the Material Safety Data Sheet (MSDS). All users of our products are urged to retain and use the MSDS. A MSDS is automatically distributed upon purchase/order execution. You may request an advance or replacement copy by calling our MSDS Hotline at (800) 700-0946. Lyondell Chemical Company 1221 McKinney, Suite 700 P.O. Box 2583 Houston, Texas 77252-2583 (800) 615-8999



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