Rheological parameters, determining

Rheological parameters, such as relaxation time, shear modulus, and stored elastic energy, are determined from the extrudate swell and stress-strain data as previously described. Representative examples of the variation of these parameters with blend ratios for both blends are shown in Figs. 16-18. Figure 16 shows that relaxation time for both preblends without heating and... 

The dependence of Vs on rheological parameters-shear stress on the wall and /notion coefficient — as far as the author knows, for filled polymers was not investigated somewhat completely, though its determination is necessary for a specific solution of hydrodynamic problems related to the flow of filled polymers. 

The most useful parameters commonly measured to assess the effect of stress conditions on emulsions include phase separation, rheological property determination, electrical property measurements, and particle size analysis. 

To determine rheological parameters such as the yield stress and effective viscosity of a foam, commercial rheometers are available rotational and conlinuous-lfow-tubc viscometry are most commonly employed (See also Rheology). However, obtaining reproducible results independent of the sample geometry is a diflicull goal which arguably has not been achieved in most of the experiments reported in the scientific lileralure... 

M Ciaffi, L Tozzi, D Lafiandra. Relationship between flour protein composition determined by size-exclusion high-performance liquid chromatography and dough rheological parameters. Cereal Chem 73 346-351,1996. 

Thus, adequate determination of nonlinear rheological parameters can be obtained, using industrial polymer processing-relevant flows, albeit with very substantial computational efforts, virtually assuring the relevance of the use of the constitutive equation for solving other complex processing flows. 

If the yield stress of a sample is known from an independent experiment, ATh and can be determined from linear regression of log a — ctoh versus log()>) as the intercept and slope, respectively. Alternatively, nonlinear regression technique was used to estimate ctoh> and h (Rao and Cooley, 1983). However, estimated values of yield stress and other rheological parameters should be used only when experimentally determined values are not available. In addition, unless values of the parameters are constrained a priori, nonlinear regression provides values that are the best in a least squares sense and may not reflect the true nature of the test sample. 

Interfacial rheological parameters can be determined in various ways. Preferably experiments should be carried out either in pure dilation or in pure shear but in practice it is difficult to avoid interference between these two. All techniques have in common that the area is in some fashion deformed (l.e. sheared, compressed or dilated) and the rheological response measured. Interpretation requires accounting for momentum transport which can take place in the monolayer itself or by transfer to (or from) the adjacent bulk phases. 

For practical purposes and technical inspection, it may be convenient to use methods which do not provide well defined rheological quantities. The main advantage is simplicity complete determination of rheological parameters is difficult and not always necessary because the relations between rheological parameters and the practical working properties are not always known. 

It should be noted that the various forming methods require somewhat different rheological parameters, so that the way of determining the optimum workability described above is just a rough approximation. In addition, one should also consider plastic Viscosity, which characterizes the resistance to the acting stress offered by the body being formed. 

The complex viscosity as a function of frequency, maximum strain and temperature is generally determined with one rheometer. Standard ASTM 4440-84/90 defines the measurement of rheological parameters of polymer samples using dynamic oscillation. This standard reiterates the importance of determining the linear viscoelastic region prior to performing dynamic frequency sweeps. 

The gel structure is determined by the volume fraction of particle material, the size of the building blocks, and the fractal dimensionality. Simple scaling laws are derived for the permeability and for rheological properties as functions of particle concentration. The rheological parameters also depend on those of the particles, especially the extent of the linear range. 

Table 1 Determination of rheological parameter as fiinction of time during initial 4-h enzymatic hydrolysis.

Table 2 Determination of rheological parameter as function of time during the initial SFF process.

Casson models were used to compare their yield stress results to those calculated with the direct methods, the stress growth and impeller methods. Table 2 shows the parameters obtained when the experimental shear stress-shear rate data for the fermentation suspensions were fitted with all models at initial process. The correlation coefficients (/P) between the shear rate and shear stress are from 0.994 to 0.995 for the Herschel-Bulkley model, 0.988 to 0.994 for the Bingham, 0.982 to 0.990 for the Casson model, and 0.948 to 0.972 for the power law model for enzymatic hydrolysis at 10% solids concentration (Table 1). The rheological parameters for Solka Floe suspensions were employed to determine if there was any relationship between the shear rate constant, k, and the power law index flow, n. The relationship between the shear rate constant and the index flow for fermentation broth at concentrations ranging from 10 to 20% is shown on Table 2. The yield stress obtained by the FL 100/6W impeller technique decreased significantly as the fimetion of time and concentration during enzyme reaction and fermentation. 

Several applications can be found in literature regarding the use of NIR for the paediction of the main physical and rheological parameters of pasta and bread. De Temmerman et al. in 2007 proposed near-infrared (NIR) reflectance spectroscopy for in-line determination of moisture concentrations in semolina pasta immediately after the extrusion process. Several pasta samples with different moisture concentrations were extruded while the reflectance spectra between 308 and 1704 ran were measured. An adequate prediction model was developed based on the Partial Least Squares (PLS) method using leave-one-out cross-validation. Good results were obtained with R2 = 0,956 and very low level of RMSECV. This creates opportunities for measuring the moisture content with a low-cost sensor. 

Basic rheological parameters of polymer blends determined by the properties of the polymer matrix, so the properties of melts are generally defined by PHB. Lower viscosity of the melt blend composition (due to the formation of a continuous matrix PHB) in this case will significantly simplify the processing of the investigated materials. Also, the data in Figure 2.6 are consistent with the position that the greater the difference in viscosity of mixed polymers, the earlier the formation of a continuous matrix less viscous component. 

In the following the major test methods for determination of the extrusion behaviour of ceramic bodies are introduced. The most important property for extrusion as a rheological parameter is the plasticity or ductility. Definitions of the term plasticity, or ductility, have been published amongst others by Haase, Moore and Astbury [7-9]. 

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