Elongational rheometry of coextruded polymer melts.

摘要

In most key polymer processing operations such as fiber spinning, blow molding and extrusion through a converging die, the dominant mode of flow is elongational. Rheological characterization of polymeric materials in elongational flow is critical for material quality control as well as for process modeling applications. Unlike shear flows, which have been studied extensively, elongational flows are not well understood, primarily due to the experimental difficulties involved in generating a pure and steady elongational flow field. There is a dearth of commercially available elongational rheometers in the present rheological literature; those that are commercially available, are useful only at elongational strain rates at least an order of magnitude lower than those encountered in typical industrial processes.; Use of lubricated skin/core flow of polymer melts and a hyperbolic converging die is shown to result in essentially pure elongational flow at a constant elongational strain rate in the core. Experimental measurements on a laboratory scale coextrusion system in a planar slit die using tracer particles and an image analysis system confirm the predicted behavior and demonstrate the ability to achieve a constant elongational strain rate in the core layer. The technique is implemented on a commercial capillary rheometer by replacing the capillary die with a hyperbolic converging axisymmetric die to determine the uniaxial elongational viscosity of several polymer melts. A two layered billet for placement in the rheometer barrel is prepared by completely encapsulating the core polymer with a polyethylene skin. Elongational viscosity at high extensional rates can be determined with this method; values in excess of 500 s{dollar}sp{lcub}-1{rcub}{dollar} have already been achieved.; Agreement with the data available in the literature at equivalent elongational strain rates was found with polypropylene melts. Both polypropylene and syndiotactic polystyrene melts were observed to be strain softening in the range of strain rates studied. At low strain rates Nylon-66 melts showed a strain hardening behavior, followed by an apparent peak and a subsequent strain softening at higher strain rates.