Rheometry is historically associated with viscosity measurement, and whilst this is an important rheological parameter it constitutes only one of a number that are required to yield a comprehensive characterisation. Listed below are some of the other essential parameters:
Shear Viscosity - Bagley and Rabinowitsch corrected, essential for result integrity and pre-requisite for most die design and flow simulation packages.
Melt Fracture - a very real problem in film blowing, sheet and pipe extrusion manifesting itself as a surface distortion or sharkskin. Melt fracture is easily detected as pressure fluctuations in capillary rheometry, thus enabling the onset of fracture to be predicted off-line using a small quantity of material.
Stress Relaxation - specifically for the measurement of yield stress t yield or the no-flow stress and useful in the characterisation of multi- phase suspensions (ceramics) where green strength is of importance; yield stress is indicative of green strength.
PVT - often used in Injection Moulding simulation software for mould fill prediction.
Flow Visualisation - rheological characterisation is increasingly being used in flow simulation software such as Compuplast for die\screw design, multi-layer extrusion and film blowing optimisation.
Elongational Viscosity - determined via the Cogswell convergent flow model. Elongational viscosity, or the resistance to elongation is known to be sensitive to material structure (MWD and chain branching).
Dissimilar MWD can often be resolved in radioactive and peroxide cross-linked LLDPE's that other techniques (shear viscosity and MFI) are blind to. Branching differences, such as those seen in HDPE and LDPE, can also be differentiated as can changes due to catalyst type using this measurement.
Wall Slip - often found in PVC, filled and multi-phase suspensions.
Visit the Fleming PTC (polymer testing & consultancy) website for more information on Viscosity Measurement