Viscosity characterisation

Viscosity characterisation191 involves measuring the flow, rate from the t st cell jn the 

DIERS bench-scale apparatus and comparing, it with the calculated value for 

The measured value of G (the mass vent capacity per unit area) is obtained from  

This value of G should be used only for comparison with the calculated turbulent G in order to determine whether flow is laminar or turbulent. If flow is found to be laminar, then the measured G would need correction for scale effects before being used for relief sizing purposes (see 10.2). 

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In the case of polymer molecules the situation is much more complex. Polymers cannot be evaporated since they decompose before boiling. In the solid state a polymer is only exceptionally purely crystalline (so-called single crystals), but generally it is partially or totally amorphous. Furthermore a very high viscosity characterises the liquid state. 

To specify the agreement between theoretical and experimental data, adduced in Figure 8.3, one can use the variable 2 in Equation 4.18. An increase in z with growth in melt viscosity, characterised by melt flow index MFI reduction (Figure 8.4), was found. It is significant that at zero melt viscosity, z = 0 and crystallisation also ceases. Hence, the crystallisation proceeds at polymer structure border state - at its transition from melt to solid-phase state. Therefore both characteristics of forming the solid phase (dimension and melt (MFI) influence the value of K,... 

The intrinsic viscosity characterises the capacity of the polymer to increase the viscosity of the system when unaffected by the presence of other polymer molecules, that is at zero concentration (when C = zero). 

Evidence from the viscosities, densities, refractive indices and measurements of the vapour pressure of these mixtures also supports the above conclusions. Acetyl nitrate has been prepared from a mixture of acetic anhydride and dinitrogen pentoxide, and characterised, showing that the equilibria discussed do lead to the formation of that compound. The initial reaction between nitric acid and acetic anhydride is rapid at room temperature nitric acid (0-05 mol 1 ) is reported to be converted into acetyl nitrate with a half-life of about i minute. This observation is consistent with the results of some preparative experiments, in which it was found that nitric acid could be precipitated quantitatively with urea from solutions of it in acetic anhydride at —10 °C, whereas similar solutions prepared at room temperature and cooled rapidly to — 10 °C yielded only a part of their nitric acid ( 5.3.2). The following equilibrium has been investigated in detail ... 

For commercial purposes the molecular weight is usually characterised from measurements of the viscosity of dilute solutions. It has been shown that, for dilute solutions, the relation between the viscosity and the molecular weight (in this case the viscosity average molecular weight) may be given by the relationship... 

Organosols (Mix B, Table 12.5) are characterised by the presence of a volatile organic diluent whose function is solely to reduce the paste viscosity. After application it is necessary to remove the diluent before gelling the paste. Organosols are therefore restricted in use to processes in which the paste is spread into a thin film, such as in the production of leathercloth. Because of the extra processes involved, organosols have not been widely used, in Europe at least. 

Initially fermentation broth has to be characterised on the viscosity of the fluid. If the presence of the biomass or cells causes trouble, they have to be removed. Tire product is stored inside the cells, the cells must be ruptured and the product must be freed. Intracellular protein can easily be precipitated, settled or filtered. In fact the product in diluted broth may not be economical enough for efficient recovery. Enrichment of the product from the bioreactor effluents for increasing product concentration may reduce the cost of product recovery. There are several economical methods for pure product recovery, such as crystallisation of the product from the concentrated broth or liquid phase. Even small amounts of cellular proteins can be lyophilised or dried from crude solution of biological products such as hormone or enzymes.2,3... 

If monomers which have functionalities greater than 2 are used for step polymerisation the product that forms consists of an infinitely large three-dimensional network and the polymerisation is characterised by sudden gelation at some point before the reaction is complete. The gel point is observed readily as the time when the mixture suddenly loses fluidity as viscosity rises sharply... 

Table 1. Characterisation data and viscosities, ri0, of polystyrene (molar mass distribution Mw/Mn<1.3) in toluene at 25 °C. Theoretical viscosities, ri0(theor), were calculated from Eq. (14). In the last column A represents the relative deviation of ri0(theor) from T 0(exp) ...

That particular combination of properties possessed by high polymers, characterising the rubber-like state. Depending on the temperature and the time of stressing, a high polymer may show viscous flow or high elasticity. See Elasticity, Glass Transition, Thixotropy and Viscosity. 

The usefulness of viscosity as a measure of polymer Molecular weight was recognised in the early work of Staudinger (1930). Solution viscosity is a measure of the size or extension in space of polymer molecules. It is empirically related to Molecular weight for linear polymers the simplicity of the measurement and the usefulness of the viscosity-Molecular weight correlation are so great that viscosity measurement constitutes an extremely valuable and simple tool for the molecular characterisation of polymer. 

Most characterisation of non-linear responses of materials with De < 1 have concerned the application of a shear rate and the shear stress has been monitored. The ratio at any particular rate has defined the apparent viscosity. When these values are plotted against one another we produce flow curves. The reason for the popularity of this approach is partly historic and is related to the type of characterisation tool that was available when rheology was developing as a subject. As a consequence there are many expressions relating shear stress, viscosity and shear rate. There is also a plethora of interpretations for meaning behind the parameters in the modelling equations. There are a number that are commonly used as phenomenological descriptions of the flow behaviour. 

Conditions of flow relative to a spherical particle are similar to those relative to a cylinder, except that the flow pattern is three-directional. The flow is characterised by the Reynolds number Re (= udp/p) in which p is the density of the fluid, p, is the viscosity of the fluid, d is the diameter of the sphere, and u is the velocity of the fluid relative to the particle. 

Rheological properties of filled polymers can be characterised by the same parameters as any fluid medium, including shear viscosity and its interdependence with applied shear stress and shear rate elongational viscosity under conditions of uniaxial extension and real and imaginary components of a complex dynamic modulus which depend on applied frequency [1]. The presence of fillers in viscoelastic polymers is generally considered to reduce melt elasticity and hence influence dependent phenomena such as die swell [2]. 

Polypropylene compositions containing magnesium hydroxide, with and without magnesium stearate surface treatment, were characterised at low and high shear rates using dynamic and capillary measurement techniques [36]. A significant reduction in viscosity was observed when surface treatment was present, particularly at low shear rates. In addition, with this system, the yield stress for the onset of flow was markedly reduced (Compare magnesium hydroxide variants A and E in Fig. 9). 

Perhaps the most important and striking features of high internal phase emulsions are their rheological properties. Their viscosities are high, relative to the bulk liquid phases, and they are characterised by a yield stress, which is the shear stress required to induce flow. At stress values below the yield stress, HIPEs behave as viscoelastic solids above the yield stress, they are shear-thinning liquids, i.e. the viscosity varies inversely with shear rate. In other words, HIPEs (and high gas-fraction foams) behave as non-Newtonian fluids. 

A number of peculiar properties are displayed, including rheology characterised by viscoelasticity. Viscosities are far higher than that of either bulk phase this is a result of the large amount of energy required to deform the network of thin films of the continuous phase. A yield stress is observed, below which HIPEs behave as elastic solids and will not flow. Resistance to flow occurs from the inability of compressed droplets to easily slip past each other. Above the... 

The characterisation of the viscosity is difficult for non-Newtonian fluids because the viscosity changes as a result of the flow process, which increases the shear rate. This is further complicated for two-phase fluids because the presence of bubbles will also affect the viscosity. The simpler methods to obtain G for high viscosity fluids make the simplifying assumptions that the fluid viscosity is equal to the liquid viscosity and that the fluid is Newtonian. 

An alternative order of introducing acylating reagents onto the bis[3-amino-4-(p-aminophenoxy)]-benzophenone, that is, the addition of first bis(phthalic anhydride) and then phthalic anhydride to the tetramine, yields polyimides containing predominantly N-phthalimide o-substituents (Scheme 3.7). These polyimides are characterised by higher viscosities and softening temperatures (Table 3.8) but poorer solubilities as compared to those polymerised according to Scheme 3.6. 

Many modifications of the basic Ostwald geometry are employed in different situations. One example is the Cannon-Fenske routine viscometer (Fig. 6.37b) which is used in the oil industry for measuring kinematic viscosities of 0.02 m2/s and less(4<). As viscosity is sensitive to variations in temperature, these types of viscometer are always immersed in a constant temperature bath. They are not normally suitable for non-Newtonian fluids although FAROOQI and Richardson(47) have employed a capillary viscometer to characterise a power-law fluid. 

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