Variable-torque rheometers

Swelltest die heads also allow contactless estimation of die-swell to be obtained. A Planetarimixing head also allows the study of powdery materials (again useful in PVC processing). Temperatures up to 300°C can be studied. Current equipment of this type is produced by Brabender ( Plasticorder ), Haake ( Rheocord ) and Hampden Instruments (variable torque rheometer). 

6 Melt flow index (ASTM D 1238, BS2782 Method 720A, ISO R113) 

Melt flow index (MFI) or rate (MFR) remains one of the most popular quality-control tests for incoming raw thermoplastic materials [5]. 

The reasons for the continuing use of melt flow indexers are low cost (about 10% the price of a capillary rheometer), simplicity and speed of use. Improved PID heating controls and more precise linear barrel-bore machined from a nitride-hardened steel or specially engineered ceramic has led, with recent models, to more reproducible results. Melt indexers can be obtained in a range 

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Commercial instruments are available the Brabender Plastograph, and Plasticorder, this latter allowing use of an extruder head the RAPRA variable torque rheometer [5]. The advantages of these instruments are based on the similarity of their action to full-scale extrusion equipment and on the fact that they can be operated at shear rates appropriate for factory equipment. But because of the difficulty of matching exactly the range of shear rates which exist in full-scale plant, successful scaling-up is difficult. Elongation flow measurements have been reported by several workers, in which a sample is stretched in uniaxial tension at a constant strain rate [6]. 

Figure 9.7 Moving die rheometer (Monsanto). A pressurised rotorless curemeter for rubber. 9.6.5 Variable-torque rheometers...

Hellen L, Yliruusi J. Process variables of instant granulator and spheroniser III. Shape and shape distributions of pellets. Int J Pharm 1993 96 217-223. Chapman SR, Rowe RC, Newton JM. Characterization of the sphericity of particles by the one plane critical stability. J Pharm Pharmacol 1988 40 503-505. Rowe RC, Sadeghnejad GR. The rheology of mcc powder/water mixes-measurement using a mixer torque rheometer. Int J Pharm 1987 38 227-229. O Connor RE, Schwartz JB. Spheronization II Drug release from drug-diluent mixtures. Drug Dev Ind Pharm 1985 11 (9-10) 1837-1857. 

The amounts of HDPE in a blend and the shear rate were chosen as independent variables in this study. Experiments were carried out in the melt state at 190°C. A Goodrich method was used for determination of effective instrument dimensions, which allows one to employ the Daane et al procedure in order to relate torque-rheometer data to more fundamental rheological values. The same HDPE/LDPE ratios were used in the study of mechanical behavior of HDPE/LDPE system of injected specimens. As said previously, DSC thermograms were used to determine weight concentration of two crystalline components both in HDPE/LDPE blends and wastes. 

The TMS rheometer from Negretti is effectively a modification of the Mooney viscometer principle for use with elastomeric materials. The elastomer is introduced into a cavity containing a shearing disc by injection, and measurements of torque under fixed or variable conditions of temperature and rotor speed can be made. Effective simulation of a variety of process conditions can be made. The instrument has found particular use in the investigation of rubber-to-metal adhesion behaviour, a problem of importance to mould fouling and extrusion processing. 

Section 8.2 described how different rotary rheometers are designed to control and to measure rotation rate, angular position, torque, temperature, and other variables. Equally important is the analysis of these measurements, conversion of the raw millivolts to material functions. Twenty years ago this was all done by hand, but today commercial rheometers spit out materials functions like G and G" in real time. Data analysis software is becoming a more and more important part of rheometer design. We have already seen that the inertia correction algorithms illustrated in Figure 8.2.11 can significantly extend the performance of controlled stress rheometers. 

However, the software of advanced stress controlled instruments allows for running an experiment at variable strain amplitudes. In this operation mode, several iterative cycles have to be measured before the actual measurement. In these iterations, the applied torque is adjusted to produce the desired strain amplitude [27]. In contrast to the classical way of amplitude adjustment, new operating modes of stress controlled rheometers (termed Direct Strain Oscillation or Continuous Oscillation) use a feedback control to compare the current strain signal y(t) at time t to the desired pure sinusoidal signal yd t) = y o sin(control loop then adjusts the torque accordingly in order to minimize the difference Yd t + At) — y (t -I- At)I for the next step at t -I- At. This deformation control enables a stress controlled rheometer to mimic a strain controlled experiment [27]. This holds true even beyond the linear regime where nonlinear contributions to the strain wave are compensated for and are then transferred into the stress wave, as the control loop tries to make the appropriate adjustments to the torque within minimum time. 

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