Calculation of Complex Viscosities from Slow-Speed Transient Torque Measurements with the Mooney Rheometer

It has been customary to assign certain time scales to given polymer processes. For example, the extrusion process is said to occur at a shear rate in the order of some hundred reciprocal seconds and injection molding at some thousand reciprocal seconds or higher. These statements are usually accompanied by instructions that the viscosity of a material is to be measured at the respective shear rate in order to characterize its processability. However, the above argument is only partially valid, and a single-point viscosity measurement is only a part of the processability evaluation. Inadequacy of the above rationale has been recognized by industry for a long time. With the rapid growth of plastics production in the 1960's, plastic processing went through a technological evolution. In the early stage of evolution of various fabrication techniques, development of suitable grades of material for the respective processes was the major effort of the plastic producers. Soon it became clear that resins which had the same viscosity at the so-called processing shear rate often behaved differently in the actual process. This led to the measurement of the steady-state flow properties at lower shear rates than the so-called processing shear rate, which was representative of the highest shear rate involved in the process. The significant observation was that the viscosity differences of resins often were magnified at the lower shear rate. Sometimes, a subtle difference in processability corresponded to a viscosity difference observable only at very low shear rates. Thus, acquisition of the steady-state flow curve from the low-shear-rate limit (i.e., the Newtonian viscosity) to the high shear rate limit (i.e., the limiting power-law region) became a subject of practical interest. The characterization of such flow curves and their relation to molecular weight distribution (MWD) became a subject of intense study for commercial plastics having a large variation in MWD.