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Author index ►Donald G Baird, Tung W Chan, Christopher McGrady, Syed M Mazahir

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Evaluation of the use of a semi-hyperbolic die for measuring elongational viscosity of polymer melts

Appl. Rheol.20:3 (2010) 34900 (12 pages) ►

The semi-hyperbolic (SHPB) die with and possibly without wall lubrication has been proposed as a device for measuring the elongational viscosity of polymeric fluids. Using numerical simulation under the condition of complete wall slip, it was found for two polyethylenes (LDPE and LLDPE) that the calculated elongational viscosity values agreed well with strain-averaged values, < η_{e}>, obtained from independent measurements in stretching type rheometers. This is in agreement with the original hypothesis of Everage and Ballman (E-B). Numerical simulations showed that the Baird and Huang (B-H) approach for calculating < η_{e}>, which accounts for the shear stress due to geometric considerations in the presence of complete slip, agreed with data better than did the E-B approach. Numerical simulations using varying degrees of wall slip indicated that reasonable values of < η_{e}> could be obtained using the B-H approach with wall slip levels which could be most likely reached using a coating such as a flouroelastomer. The numerical simulations provided an explanation as to why the elongational viscosity values determined in the SHPB die for resins such as LDPE, which are extensional-strain hardening, are less sensitive to wall slip than non-strain-hardening resins such as LLDPE.► Cite this publication as follows:

Baird DG, Chan TW, McGrady C, Mazahir SM: Evaluation of the use of a semi-hyperbolic die for measuring elongational viscosity of polymer melts, Appl. Rheol. 20 (2010) 34900.

The lubricated semi-hyperbolic die has been proposed as a technique for generating uni-axial extensional flow and, hence, as a device for measuring elongational viscosity. Two methods for extracting extensional viscosity data for polymer melts in laminar flow from this device have been proposed and are evaluated here. Following the approach proposed by Collier and coworkers, values of the transient extensional viscosity, η_{c}^{+}, obtained from a non-lubricated semi-hyperbolic (SHPB) die for several polyethylene (PE) melts were found to be considerably higher than values obtained by means of the Münstedt type device. Furthermore, the values of η_{c}^{+}obtained from the SHPB die were considerably higher than the strain averaged values of η_{c}^{+}which Everage and Ballman proposed would be obtained from a lubricated SHPB. The pressure drop across a SHPB die was estimated assuming resistance was all due to wall shear (using the lubrication approximation) for two PE resins. In the case of low density PE (LDPE) the values agreed to within 20% of the measured values suggesting that shear effects at the die wall were dominating the pressure drop and not extensional stresses. An analysis was carried out which showed that in the presence of lubrication the conditions for which the values of η_{c}^{+}obtained from the SHPB would be relatively accurate (Hencky strains > 5.0).► Cite this publication as follows:

Baird DG, Huang J: Elongational Viscosity Measurements Using A Semi-Hyperbolic Die, Appl. Rheol. 16 (2006) 312.

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