Drawdown force, defined as the force needed to stretch a polymer melt uniaxially at capillary extrusion, is investigated using isotactic polypropylene. In this study, we focus on the effect of die geometry on the drawdown force. As similar to the swell ratio, which decreases with increasing die length, the drawdown force is found to increase with the die length. A prolonged residence time in a long die leads to the marked orientation. Consequently, crystallization occurs rapidly after passing through a long die, which is confirmed by wide-angle X-ray measurements. Moreover, the rapid crystallization by the addition of a nucleating agent also enhances the drawdown force. C 2014 Wiley Periodicals, Inc. Adv Polym Technol 2015, 34, 21477; View this article online at wileyonlinelibrary.com.
Rheological properties and flow instability at capillary extrusion of a random terpolymer composed of vinyl butyral, vinyl alcohol, and vinyl acetate, that is denoted as PVB in this article, are studied. It is found that the rubbery plateau modulus G 0 N is 1.3 MPa at 100 C from the oscillatory shear modulus. Furthermore, the average molecular weight between entanglement couplings M e is found to be 2670. Because of the relatively high value of G 0 N , it shows rubbery region in the wide temperature range (90 C-180 C). At the capillary extrusion, the surface instability (shark-skin failure) appears prior to volumetric melt fracture. The onset stress of the shark-skin failure, ca. 0.18 MPa, is similar to that of polyethylene, although PVB used in this study has narrow molecular weight distribution. Moreover, the apparent slippage is not detected, presumably due to good adhesion to the die wall. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40337.
Effect of applied processing history on flow instability at capillary extrusion is studied using a commercially available low-density polyethylene (LDPE) having long-chain branches. It is found that processing history in an internal mixer in a molten state depresses long-time relaxation mechanism associated with long-chain branches, which is known as ''shear modification.'' Consequently, the onset of output rate for melt fracture increases greatly. Furthermore, it should be noted that the sample having intense shear history shows shark-skin failure without volumetric distortion, although it has been believed that LDPE exhibits gross melt fracture at capillary extrusion. The reduction of elongational viscosity by the alignment of long-chain branches along to the main chain is responsible for the anomalous rheological response. As a result, the sample shows shark-skin failure like a linear polyethylene at a lower output rate than the critical one for gross melt fracture.
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