Band 20, Heft 3

There have been many publications on the measurement and use of yield stress as a means of determining the ability of a system to suspend. Although in theory it is a useful predictive tool, in reality, it will often be found to give erroneous results, particularly when attempting to draw comparisons between dissimilar systems. Alternative techniques can be used which, whilst not being perfect, will give results which are closer to the reality. Several of these methods are evaluated and compared.

Two types of starch gels made with various starch/water concentrations were studied in terms of their mechanical behaviour. Indentation tests were performed which revealed a rate independent load-deflection response. An inverse analysis based on the Marquardt-Levenberg optimisation algorithm and Finite Element Analysis was used to derive the stress-strain behaviour from the indentation data. The inverse predictions for the stress-strain curves are in good agreement with the direct measurements from uniaxial compression and shear tests up to high values of strain. The validity of the method was proven for both self-supporting and non self-supporting gels, with initial moduli ranging from a very small 60 Pa to 55 kPa. Thus the indentation characterisation method is proven as a powerful, fast and efficient way of evaluating and/or monitoring the behaviour of gels.

We propose an exhaustive experimental characterization of a series of poly(n-butyl acrylate) samples that were synthesized by controlled radical polymerization and have different molecular weights. We focus on the rheological behavior of these polymers and propose a model of their rheological behavior using a molecular model based on the reptation concept. We report the principal rheological parameters for these homopolymers and demonstrate good agreement between model predictions and experimental data.

A rheological characterization and extrusion of aqueous suspensions of natural zeolites were carried out in this work. Preparation of suspensions followed similar routes to those used for the colloidal processing of ceramic powders. The suspensions were prepared at different volume fractions (v/v%) ranging from 5 to 61 v/v% for a pH value of 7. The suspensions showed a Newtonian behavior for solid contents up to 20 v/v% and shear thinning at higher solid loads. For solid contents above 35 v/v%, the suspensions exhibited hysteresis and a yield stress that growth exponentially with the solid load. The appearance of a yield stress and its fast growing at relatively low solid concentration, as compared to other type of ceramic suspensions, is attributed to several factors as inter-particle interactions, the presence of relatively large particles and to the influence of their irregular morphology. Also, a breaking stress was measured for solid contents above 35 v/v%, which characterizes a failure of the structure of the suspensions after yielding. This breaking stress determines the onset of slip flow of the suspensions, which is interpreted in this work as a signal of good extrusion characteristics. Finally, inexpensive and free of surface defects tubes were obtained from natural zeolites.

The present article describes and analyzes different calibration methods for a screw type process rheometer, Searle type, having a die hole at the downstream of a barrel. The work also quantifies the effect of time dependent flows due to the screw on the measurement performance. Time variations in torque and pressure become more notorious at increased resistances to flow (higher fluid viscosities and smaller die diameters). Screw speeds seem to do not affect these variations. Shear stress in the system is related to pressure and torque, and by using any of them, is possible to predict an average viscosity. Similar prediction errors were found when using torque or pressure. A section of practical applications is added to understand the use of a screw type process rheometer better.

The semi-hyperbolic (SHPB) die with and possibly without wall lubrication has been proposed as a device for measuring the elongational viscosity of polymeric fluids. Using numerical simulation under the condition of complete wall slip, it was found for two polyethylenes (LDPE and LLDPE) that the calculated elongational viscosity values agreed well with strain-averaged values, <ƞ e >, obtained from independent measurements in stretching type rheometers. This is in agreement with the original hypothesis of Everage and Ballman (E-B). Numerical simulations showed that the Baird and Huang (B-H) approach for calculating <ƞ e >, which accounts for the shear stress due to geometric considerations in the presence of complete slip, agreed with data better than did the E-B approach. Numerical simulations using varying degrees of wall slip indicated that reasonable values of <ƞ e > could be obtained using the B-H approach with wall slip levels which could be most likely reached using a coating such as a flouroelastomer. The numerical simulations provided an explanation as to why the elongational viscosity values determined in the SHPB die for resins such as LDPE, which are extensional-strain hardening, are less sensitive to wall slip than non-strain-hardening resins such as LLDPE.