Measurement of rheological and ultrasonic properties of food and synthetic polymer melts

A slit die viscometer has been used in conjunction with a co-rotating twin screw extruder to study the rheological behaviour of maize grits, potato powder and low density polyethylene, as a function of feed rate, screw speed and temperature. The shear viscosity of both maize and potato decreased with increasing feed rate. Increasing the temperature or screw speed at any given feed rate also reduced the viscosity. The ultrasonic velocity through the material has also been shown to be sensitive to the extruder operating conditions. Overall, the ultrasonic velocity decreased as screw speed and temperature increased and feed rate decreased.     

An automated dynamic viscometer with air bearing and inductive transducers for angular displacement detection

A dynamic viscometer is described, with which the dynamic moduli in the frequency range from 2 × 10^–3 to 10 Hz can be determined for liquids with 10^–3 Pa< |G ^* | <10² Pa. Due to the application of an air bearing and inductive transducers for the detection of the angular displacement of both the drive and the measuring cylinder a sensitive apparatus has been made. Very small strains (₀ 10^–3) can be applied and only a small amount of sample (4 ml) is needed. The operation of the apparatus is fully computer-controlled, thus, long runs at various frequencies and temperatures are possible without operator intervention. The theoretical background, calibration procedure, and operation window are described. A presentation of some measurements on two polyisoprene/polystyrene triblock copolymer solutions concludes the work.     

Elongational rheology of polyethylene melts — temporary network constitutive laws

The uniaxial elongational properties of various polyethylenes have been evaluated using an elongational rheometer and a melt-strength apparatus. It is possible to derive the data obtained in elongation from the distribution of relaxation times obtained from oscillatory shearing measurements (linear viscoelasticity), using a Wagner constitutive equation. The effects of the molecular parameters of the samples have been studied, in particular the effect of polydispersity on the shape of the damping function.     

The role of short chain molecules for the rheology of polystyrene melts.

Atactic polystyrenes of narrow molar mass distribution with average molar masses larger than the critical molar massM _c were mixed with similar polystyrenes of molecular masses lower thanM _c . Linear viscoelastic melt properties of these binary blends were measured with a dynamic viscometer of the concentric cylinder type. One of the experimental findings is that the time-temperature shift factorsa _T are dependent on the composition of the samples. This can be understood, if free volume due to chain-ends is taken into account. A computer-fitted WLF-equation being modified in a proper way leads to the following results: At the glass-transition-temperature the fraction of free volume in polystyrene of infinite molar mass is only 0.015. At a temperature of 180 °C the mean value of the free volume at a chain end is 0.029 nm³ for the polystyrene investigated.     

An experimental study of exit pressures for polymer melts

A slit die apparatus is used to measure exit pressures for five different polymer melts. Viscosity data obtained from the same apparatus agree well with values obtained from a cone-and-plate rheometer or a capillary rheometer. Except for a PVC sample where thermal degradation was found to occur, the exit pressures obtained by linear extrapolation of the measured pressure profiles are all positive, and increase with increasing shear stress. The values of the first normal stress difference calculated according to the exit pressure theory are of the right order of magnitude and in some cases correlate satisfactorily with values measured in a cone-and-plate rheometer. However, the high sensitivity of the exit pressure values to the method of extrapolation and the wild scatter of exit pressure data for some materials make it difficult to use the exit pressure method as a routine procedure for accurate determination of the first normal stress difference.     

Effect of processing and particulate fillers on the rheology of a nematic polymer melt

Processing of a nematic HBA/HNA polymer melt increases the capillary diameter dependence of the viscosity and induces structural changes which are evident in oscillatory shear, but cannot be characterized by DSC. The effect of 6-m calcium carbonate particulate fillers is to increase the viscosity uniformly. Low concentrations of sub-micron carbon black particles cause an unexplained viscosity minimum in a large (30-mil) capillary.     

The rheology of powders

This paper studies the slow flow of powders. It is argued that since powders can flow like liquids, there must be equations similar to those of liquids. The phenomenon of a variable density, dilatancy, is described by an analogue of temperature called the compactivity X. Whereas, in thermal physicsT = E/S, powders are controlled byX = V/S. The equations for, v, T of a liquid are replaced by, v, X. An analogy for free energy is described, and the solution to some simple problems of packing and mixing are offered. As an example of rheology, it is shown that the simplest flow equations produce a transition to plug flow in appropriate circumstances.Delivered as a Gold Medal Lecture at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.     

The concept of a rotational rheometer with helical screw impeller

The paper is concerned with the development of an on-line rheometer for suspensions with settling particles. A new concept of a rotational rheometer with helical screw impeller is discussed. On the basis of model considerations a procedure for the flow curve determination of the investigated fluid has been proposed. Contrary to previous attempts, in the developed procedure no calibration of the instrument is necessary. The preliminary experimental verification of the developed approach proved its applicability.     

Linear rheology of viscoelastic emulsions with interfacial tension

Emulsions of incompressible viscoelastic materials are considered, in which the addition of an interfacial agent causes the interfacial tension to depend on shear deformation and variation of area. The average complex shear modulus of the medium accounts for the mechanical interactions between inclusions by a self consistent treatment similar to the Lorentz sphere method in electricity. The resulting expression of the average modulus includes as special cases the Kerner formula for incompressible elastic materials and the Oldroyd expression of the complex viscosity of emulsions of Newtonian liquids in time-dependent flow.     

Generalized characteristic of polymer blend melt viscosity

A very simple reduction procedure is suggested for the blend viscosities of different polymer pairs. This procedure is based on the comparison of the blend viscosity, normalized either to the matrix or to the disperse phase viscosity, with the viscosities ratio of the initial polymers ( _m / _d ). We have obtained, for 13 different pairs containing 30% of the second component, the universal linear dependencies, mutual analysis of which allows connection of their special points with the stream morphology. The fibrillous morphology takes place in the range of _m / _d = 0, 1–5. Simultaneous, the thin skin consisting of the disperse phase polymers is formed. These results confirm the predominant role of the viscosities ratio in fibrillar composite material formation in comparison with the interphase tension phenomena.     

Rheological behaviour of low moisture molten maize starch

The rheological behaviour of a molten maize starch in low hydrated phase was investigated using a pre-shearing rheometer. This apparatus enables one to impose well-characterized thermomechanical treatment on the product before viscosity measurement. The influence of temperature, moisture content and intensity of the treatment were studied. The rheological behaviour of the maize starch may be described by a power law with exponential dependences on temperature, water content and mechanical energy provided to the product before measurement.     

On-line measurements of the rheological properties of fermentation broth

The paper is concerned with measurements of rheological properties of fermentation broth. An on-line rheometer, Rheohelix-1, based on the application of a helical screw impeller rotating in a draught tube has been constructed. The instrument was used for measurements of the rheological parameters of fermentation broth of Aspergillus niger in a submerged fermentation process. The results of rheological and some standard measurements have been compared and proved the applicability of the instrument.Presented at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.     

An experimental study of food melt rheology. II. End pressure effects

A twin-screw extruder-fed slit die viscometer (SDV) and a piston capillary rheometer have been used to measure the end pressure losses of a low density polyethylene, maize grits and potato powder. The entrance and exit pressure losses have been measured as a function of extrusion conditions. The entrance pressure losses were found to be less than 10% of the total pressure drop in the SDV for LDPE and maize grits. For the potato material, this loss was found to be as large as 58%. The exit pressures for the potato were between 10–20% of the total pressure drop compared to negligible values for the maize and LDPE. Various approaches due to Bagley, Han and Cogswell were used to investigate the elastic properties of these materials.     

Physiology,mechanics, and rheology

Biological systems possess rather specialized mechanical properties, acquired as part and parcel of the evolutionary development of the system as a whole. Their optimization permits the system to function physiologically in the context of a biologically essential, but mechanically often widely varying environment with adequate efficiency. The system's environment is its source of food and shelter; it represents the space in which it forages or preys on other creatures and in which it has to defend itself against still others. Thus, the system has to develop an adequately pliant, rheologically matched, energy-use efficient, mechanical interface between it and its surroundings. This must be an interface that both effectively excludes, but also effectively admits, the external. Internally, as well, it has to adapt the mechanical properties of cell and connective tissue to physiological function and the efficient performance of useful work. This will be illustrated by way of examples. Blood rheology is briefly discussed and put into the context of clinical hemorheology and epithelial protection; and function, by way of a mucus coating or a mucociliary clearance system, is reviewed in some detail. The importance of all aspects of rheological matching is demonstrated.Delivered as a Keynote Lecture at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.     

An investigation of the viscoelastic properties of molten polypropylene

Two series of polypropylene samples of different molecular weight, the first obtained directly from polymerization reactions and the second from controlled thermal degradation, were studied by dynamic testing in the melt state. Several viscoelastic parameters were determined, and correlated with weight-average molecular weightM _w . It is found that theM _w -dependence of the two series is rather different.     

Transient rheology in a new type of couette apparatus. Application to blood

By using our new air-bearing viscometer different types of transient flow can be studied; in the present work this viscometer was specifically applied to non-Newtonian blood suspensions. To observe the influence of both the aggregation and the deformation of red blood cells (RBCs), different concentrations of fibrinogen and dextran were used: the suspended RBCs were simply washed or rigidified with diamide. From the data three rheological parameters were determined, two of which are relative to the behavior at low and at high shear gradients, respectively. Their values were related to the theory of Taylor, and the internal viscosity of RBCs was estimated to be around 3 cp.     

Fragile networks and rheology of concentrated suspensions

Suspensions consisting of particles of colloidal dimensions have been reported to form connected structures. When attractive forces act between particles in suspension they may flocculate and, depending on particle concentration, shear history and other parameters, flocs may build-up in a three-dimensional network which spans the suspension sample. In this paper a floc network model is introduced to interpret the elastic behavior of flocculated suspensions at small deformations. Elastic percolation concepts are used to explain the variation of the elastic modulus with concentration. Data taken from the suspension rheology literature, and new results with suspensions of magnetic -Fe₂O₃ and non-magnetic -Fe₂O₃ particles in mineral oil are interpreted with the model proposed.Non-zero elastic modulus appeared at threshold particle concentrations of about 0.7 vol.% and 0.4 vol.% of the magnetic and non-magnetic suspensions, respectively. The difference is attributed to the denser flocs formed by magnetic suspensions. The volume fraction of particles in the flocs was estimated from the threshold particle concentration by transforming this concentration into a critical volume concentration of flocs, and identifying this critical concentration with the theoretical percolation threshold of three-dimensional networks of different coordination numbers. The results obtained indicate that the flocs are low-density structures, in agreement with cryo-scanning electron micrographs. Above the critical concentration the dynamic elastic modulus G was found to follow a scaling law of the type G ( _f - _f ^c ) ^f , where _f is the volume fraction of flocs in suspension, and _f ^c is its threshold value. For magnetic suspensions the exponent f was found to rise from a low value of about 1.0 to a value of 2.26 as particle concentration was increased. For the non-magnetic a similar change in f was observed; f changed from 0.95 to 3.6. Two other flocculated suspension systems taken from the literature showed a similar change in exponent. This suggests the possibility of a change in the mechanism of stress transport in the suspension as concentration increases, i.e., from a floc-floc bond-bending force mechanism to a rigidity percolation mechanism.     

Effect of secondary zone dimensions on melt fracture manifestations of high density polyethylene

An experimental study was performed on melt fracture phenomena in extrusion of high density polyethylene. The purpose of the work was to study the sensitivity of melt fracture driven roughness to the size of recirculation zones, viz. secondary zones. The experimental apparatus consists of a right angle die and a hypodermic needle used as a capillary. The position of the needle relative to the die was adjusted using a special fixture. The roughness of the extrudate was studied as a function of penetration depth. A developed procedure provides a comparison between profile lengths of extruded strands. The computed mean, median, and mode values for roughness were presented as a function of capillary position. A qualitative analysis was conducted for the force oscillations during extrusion with a separate set of dies, equipped with the fixed capillaries of identical lengths and different depths of penetration. It was observed that the oscillatory pattern is sensitive to the sizes of the secondary zones. This qualitative observation supports the conclusions from the quantitative analysis that the roughness of the extrudate can be controlled through an adjustment of the secondary zone sizes.Partly presented at the 58th Annual Meeting of the Society of Rheology, Tulsa, Oklahoma, October 20–23, 1986     

Shear and extensional flow of polyacrylamide solutions

As part of an EEC Science Stimulation programme on extensional viscosity two major conferences were organised on the subject. The second of these was devoted to the results obtained on a standard fluid, M 1. The data obtained in shear flow was remarkably consistent from laboratory to laboratory. Extensional flow results presented quite a different picture. Using a series of nonequilibrium techniques such as the spinline rheometer, opposing jet, falling drop and converging flow, extensional viscosity results were obtained which differed by as much as two to three orders of magnitude. Nevertheless, it was apparent that consistancy did exist between similar techniques. It is in the context of this information that the measurements described below have been made.The shear and extensional flow properties of partially ionised polyacrylamide in solution at concentrations ranging from 5 ppm were measured. The method of solution preparation was found to have a profound effect on the behaviour of the solutions in shear flow. The influence of salt concentration and pH was investigated and is discussed in the context of molecular shape in solution.Extensional flow measurements, using the spinline rheometer, show that the solutions are strongly strain thickening even at concentrations as low as 5 ppm. These results are considered in the light of polymer entanglement and association in the strong flow field.Delivered as a Keynote Lecture at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.