ABSTRACT
The term Rheology introduced by Eugine C Bingham in 1920, is currently used as a semi-quantitative tool in study of polymer melt studying
the deformation and flow under stress. The relationship between polymer structure and the measurable properties like viscosity, strain rate and visco-elastic properties are currently investigated by many researchers and remains a challenge to address the properties of commercially available polymers. (Dealy and Larson, 2006; Gahleitner, 2001). During the past two decades, several attempts are made to manufacture precise, reliable, simple to operate and economically attractive in-line and on-line rheometers (Mould et al., 2011). Melt Flow Index (MFI) is one of the most popular rheological measures for specifying the flow properties of polymer melt and its characterization. American Society for Testing and Materials specifies the standard procedure for measurement of melt flow rates for thermoplastics by extrusion (ASTM-D1238). The melt flow index represents the weight of polymer passing through a standard die (Inside bore-2.095 ± 0.0051 mm) at specified temperature and load in 10 minutes. The MFI of a thermoplastic depends on its molecular structure, degree of polymerization and branching, micro and nano particulate additives, temperature and shear rate. Commercially, a polymer blend procured for processing is specified by mentioning the density and MFI along with other parameters. The MFI measures the flowability of polymer melt and is broadly identified as the reciprocal of dynamic viscosity (η). The viscosity (ratio of shear stress
to shear rate) of polymer melt reduces with increased shear rate for
constant temperature as given in Fig. 4.1. This pseudoplastic behavior is
typical for thermoplastic polymers. The dynamic viscosity of polymer is represented as tan α where as tan
β represents the apparent viscosity. According to the theory of viscoelasticity, at very low shear rates the viscosity is independent of shear rate, and the limiting value of viscosity is called zero shear viscosity (η0). This is an important material constant and plays an important role in molecular rheology and serves as an indicator for molecular weight. The direct measurement of zero shear viscosity is often difficult, since the standard rheometers do not provide reliable data at sufficiently low shear rates to reach the region of Newtonian behavior. The zero shear viscosity has strong dependence on molecular weight.
