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Capillary Rheometers

Melt Flow Index (MFI) equipment (left) and screw-driven capillary rheometer

Capillary rheometry is used to characterise a fluidís shear flow properties. It also allows an understanding of the way fluids flow in variable cross-section geometries. Fluid flow lies at the centre of all extrusion processes, whether batch or continuous. This includes, the dispensing of adhesives and other paste and fluid materials. Adhesive materials used in manufacture are becoming more complex in terms of their rheology and ever more complex dispensing processes are being introduced. Therefore, there is an increasing need to fully understand general flow in a tube and the specific case of capillary flow.

For steady flow within a constant circular section, it is relatively straightforward to derive equations describing the velocity profile and pressure drop from the fundamental constitutive equations [1, 18Ė20]. By measuring the pressure drop DP as a function of the fluid volumetric flow rate Q it is possible to determine the fluidís behaviour in terms of an apparent viscosity. The pressure drop DP can be expressed in terms of the volume flow rate Q, the capillary length L and radius r0:

Driven flow capillary rheometers [7] overcome some of the limitations of the hydrostatic head viscometers by providing sufficient pressures to measure higher viscosity adhesives. The simplest method of providing pressure is to use a mass and piston (e.g. melt flow index (MFI) test [21]). MFI, widely used in the plastics industry for batch quality control, has been little used for adhesives for various reasons:

A lack of reference data and no correlation with specific machine performance criteria.

Hot melt adhesives are typically lower in viscosity than most polymers at their application temperature, and hence are difficult to measure by MFI.

These basic systems, although cheap to purchase and simple to operate, do not have the capability to fully characterise visco-elastic or rate dependent adhesives.

Recent developments of the MFI test have concentrated on varying the geometry (length and diameter) of the capillary die, thus altering the flow regime in the instrument, to enable variable shear rate and flow condition data to be obtained from this simple test instrument. A more sophisticated type of capillary rheometer, uses a mechanical screw to apply the piston pressure (i.e. screw-driven capillary rheometer). The speed of the piston can be varied thus allowing variable flow rate tests. Shear rates in the fluid are controlled by the flow rate and the diameter of the exit die. An internal transducer measures the pressure drop. The temperature of the fluid in the barrel is controlled using the barrel heater. This measurement method most closely mimics the extrusion process.

This type of instrument is significantly more expensive than simple, non-mechanically driven instruments but benefits from having a wide range of shear rates available. The internal pressure transducer allows pressure measurement close to the capillary and the data can be logged to a chart recorder or PC. As part of the calibration routine for capillary rheometers, the dimensions of the capillary dies should be accurately measured, as the results are very sensitive to the die diameter [22].

Stretching flow external rheometer

In regions of the flow where the external diameter of the flow changes (e.g. entrance and exit of the die), the assumption of a shear flow regime is not valid. The convergence (or divergence) of the fluid adds an extensional (or tensile) component to the flow field. This is the situation in any extrusion process such as needle dispensing of adhesive. Work on polymer melts suggests that extensional rheological behaviour has a major impact on behaviour in extrusion processes and that for some polymer formulations the extensional viscosity can differ far more than the shear viscosity. Extensional flow can be characterised [23] using filament drawing equipment.

Extensional flow can also be characterised in capillary rheometers through the use of step changes in channel diameters to cause converging flow [24]. Recent results suggest that, with only minor modifications to procedures and equipment, extensional viscosity measurements can be made using MFI equipment. Such findings suggest that reasonably cheap and simple methods for characterising extensional flow of adhesives could be developed.

Next: Rotational Viscometers