Capillary rheometer measurement

A slit die is designed on the assumption that the material is Newtonian, using apparent viscous properties derived from capillary rheometer measurements, at a particular wall shear stress, to calculate the volumetric flow rate through the slit for the same wall shear stress. Using the correction factors already derived, obtain an expression for the error involved in this procedure due to the melt being non-Newtonian. Also obtain an expression for the error in pressure drop calculated on the same basis. What is the magnitude of the error in each case for a typical power law index n = 0.377... 

Capillary rheometers are in the form of a barrel where the operator puts the polymer sample. After heating to equilibrate its temperature, the sample is pushed by a piston through a die at chosen rates. Various sizes and shapes of dies are available. Capillary rheometers measure the rheological properties under broad ranges of conditions of temperature, pressure, stress, strain and time, allowing the adoption of parameters near to those for processing. 

The raw data from the capillary rheometer measurements are subject to four important corrections to obtain true viscosity and shear rate values. These corrections, which are fully described in Kwag (1998) and Kwag et al. 

Capillary rheometers measure the effect of pressure on volumetric flow through a cylindrical capillary. They are popular in practical work because shear rate and flow geometry are similar to conditions in extrusion and injection molding. They cover a wide range of shear... 

Capillary rheometer. The Monsanto capillary rheometer measures the viscosity properties of polymers, and provides a direct measure of viscosity and the change in viscosity with time and flow rate at plastica-tion temperatures. The capillary orifice simulates the gate and runner system of actual molding conditions, thus providing valuable flow information for molding compounds. 

ASTM Method 1238, Standard Test Method for Flow Rates of Thermoplastics by Extrusion Plastometer [6], completely defines the melt index, both the instrument and the experimental procedure. Temperatures and instrument configurations for common materials are given along with recommended calibration procedures and round-robin results. This method must be strictly followed if consistent melt index results are to be obtained. A melt index measurement is performed at low stress levels compared to capillary rheometers measurements (typical weights are in the range of 1.2-21.6 kg) and is sensitive to variations in operating procedure, instrument cleanliness (between tests), and operator bias (items such as packing and sample preparation). 

As a result of these difficulties, typical capillary rheometers measure pressure in or above the reservoir as indicated in Figure 6.2.1 or from the forces on a driving piston. (Different capillary rheometer designs are discussed further in Chapter 8.) To determine the true shear stress, a number of corrections must be considered. 

Fig. 2 shows the flow curve for the neat exact 5361 at loot). The instability problem of the metallocene based polymers with narrow molecular distribution is well known. Fig. 3 shows the photographs of the extrudate samples with varying Dechlorane concentration collected during the capillary rheometers measurement. It is interesting to see that while the severe instabilities such as slip-stick and gross-melt fracture were observed at the shear rate from 177.8 s to 3162.2 s for the neat Exact resin, the severe instability appeared at the shear rate between 177.8 s and 562.2 s but disappeared at the shear rate above 1000.1 s for Exact/10% Dechlorane suspension. The shark-skin like instabilities were observed above the Dechlorane concentration of 20% and the shear rate at which the instability started to appear was decreased as the Dechlorane concentration was increased. Since the viscosity of the Dechlorane-filled systems was higher than that of the neat resin at all rates of shear, the instabilities are expected to develop the melt fracture at even lower shear rates. The shear viscosity vs. shear rate relationships measured with plate-plate rheometers and capillary rheometers are shown in Fig. 4. In this figure it is seen that both sets of data are reasonably matched. It is observed that at low shear rate range the viscosity increment due to the increase in the filler concentration is more pronounced than that at high shear rate. Both plate-plate and capillary measurements were carried out with constant shear rate (CSR) mode. While the capillary rheometer could accurately follow the preset shear rate values the plate-plate rheometer couldn t keep up with the preset shear rate values. Above two observations are due to the yield stress developed at low shear rate. At low shear rate particle-particle interaction dominates the flow phenomena and the yield stress was observed. At high shear rate hydrodynamic effect dominates the flow phenomena.

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