Structural viscosity flow

PMDI also contains isocyanates with higher molar masses (triisocyanates, tetraisocyanates, polyisocyanates), whereby the structure and the molar mass depend on the number of phenyl groups. This distribution influences, to a great extent, the reactivity, but also the usual properties like viscosity, flowing and wetting behavior as well as the penetration into the wood surface. 

The Eyring flow shows a typical dependence upon stress. With increasing stress the viscosity r decreases (structural viscosity). The calculations are in good agreement with the experimental values. Figures 21 and 22 show the influence of shearing stress on the viscosity. 

The fact that the melt of the block copolymer above a critical shear stress shows the same structure-viscosity relations as found for linear melts in simple shear flow is taken as an indication of a monomolecular melt state. 

The critical analysis of the results on foam rheology, proposed by Heller and Kuntamukkula [16], has shown that in most of the experiments the structural viscosity depends on the geometrical parameters of the device used to study foam flow. This means that incorrect data about flow regime and boundary conditions, created at the tube and capillary walls, etc., are introduced in the calculation of viscosity (slip or zero flow rate). Most unclear remains the problem of the effect of the kind of surfactant and its surface properties on foam viscosity and on the regime of foam flow (cross section rate profile and condition of inhibition of motion at the wall surface). 

Structure The structure of the pigment black is an important parameter for the production of printing inks. A number of quaUty attributes depend on the structure dispersion properties, viscosity, flow properties, color density, gloss, mb resistance, and conductivity ... 

High-stmcture high yield value, high viscosity and reduction in gloss Low-structure good flow properties, low viscosity and increased gloss... 

From these positions different values of viscosity at the moment preceding the breakaway of the reactive system from the working surfaces of a viscometer become understandable and, moreover, the viscosity at this period increases with the concentration of the oligomer in solution. This is due to the fact that before the moment, when a continuous phase is formed from fragments of cured structures, the flow is determined by a dispersion medium whose viscosity depends on the solution concentration. 

FIGURE 8.7 Non-Newtonian flow behavior, a Structural viscosity (for high molecular solution). b Dilatant flow (suspension with high concentration), c Viscoplastic with flow limits 1, ideal plastic 2 or 3, nonlinear plastic flow, d 1, thixotropy flow 2, antithixotropy flow 3, viscoelastic flow e rheopexy flow. 

A limited amount of structural information can be obtained via viscometry of polymer solutions. Molecular dimensions have been mentioned as the province of dilute solution viscosity. Flow properties of more concentrated solutions provide information on whether solvent drains freely from the polymer coils and on how solvent attractions compete with the interactions among macromolecules that are bound to occur (37. 38). 

Table 17-2 shows some relations between structure and flow properties. The data for the substituted terphenyls are from the work of Schmidt-Colle rus, Krimmel and Bohner [2] those for the polyphenyl ethers are by Mahoney and his co-workers [7]. For the substituted terphenyls the location of the alkyl group rather than the linkage of the aromatic rings has the stronger influence on the viscosity behavior and the pour point. The viscosity at 310.8 K (100 F) for the n-heptylterphenyls substituted in the 2-position is 2.3 to 2.7 times that of the isomer substituted in another position, and the pour point for the 2-substituted isomer is also consistently higher. Unfortunately this trend in pour point is opposite to what is desired. The data for the polyphenyl ethers... 

Structural viscosity is the term applied to solutions whose rate of flow in capillaries is not proportional to the pressure, the viscosity decreasing with increase of pressure. Several workers have reported that gum arabic solutions do not show structural viscosity—e.g., Coumou (j ) for 20% gum arabic solution. Ostwald (17), on the other hand, showed that structural viscosity occurs in gum arabic sols at high concentrations (up to 45%) if the temperature is kept low enough (such as at 20 C.) and if the pressure is below 10 cm. of water. 

Since wet foams contain approximately spherical bubbles, their viscosities can be estimated by the same means that are used to predict emulsion viscosities. In this case, the foam viscosity is described in terms of the viscosity of the continuous hquid phase (j/g) and the amount of dispersed gas (0). In dry foams, where the internal phase has a high volume fraction, the foam viscosity increases strongly due to bubble crowding , or structural viscosity, becomes non-Newtonian and frequently exhibits a yield stress. As is the case for emulsions, the maximum volume fraction possible for an internal phase made up of uniform, incompressible spheres is 74%, but since the gas bubbles are very deformable and compressible, foams with an internal volume fraction of 99% or more are easy to make. As mentioned earlier, the structure of such a foam consists of irregular polyhedrons with a maximum of three lamella meeting at the plateau borders. Figure 6.23 shows an illustration of foam polyhedra being deformed in the presence of applied shear [86]. No flow occurs until there is so much distortion that four films meet at plateau borders. 

The phenomena described above are the basis of the structural viscosity and viscoelastic behavior mentioned in Sec. il. The region of elastic response and the region of viscous flow depicted in Fig. 33 correspond to the lirsi and to the second Newtonian plateau of Fig. 5, respectively. Further, the floe destruction region shown in Fig. 34 corresponds to the shear-thinning portion between both plateaus of Fig. 

In most cases of quasi-viscosity there is an upper limit of r, above which fj becomes independent of it again. This is shown, in the flow diagram of Fig. 12, borrowed from Philippoff where D and r are plotted on a logarithmic scale. Thus, at very small stresses one can speak of a viscosity coefficient (Vo), but at very high stresses one finds another coefficient 0/co). Between these two the region of quasi- viscosity or structural viscosity is encountered. The appearance of a constant value for be due... 

Departure from NEWTONian flow is often termed "" structural viscosity . According to J. Duclaux Viscositi, Paris 1934, p. 40), this terminology is physically incorrect and should not be maintained. 

The consistency of the non-vulcanised 2-component LSR is flow-capable and pasty. Although the pure polymer material has the characteristics of a Newtonian fluid in its flow behaviour (the viscosity is independent of the shear rate of the process), a mixture of polymer with extenders displays a structural viscous property. Structural viscosity means that the viscosity of the mixture decreases significantly with increasing shear rate. This may be observed in every thermoplastic. 

In this chapter we examine the flow behavior of bulk polymers in the liquid state. Such substances are characterized by very high viscosities, a property which is directly traceable to the chain structure of the molecules. All substances are viscous, even low molecular weight gases. The enhancement of this property due to the molecular structure of polymers is one of the most striking features of these materials. 

Next let us consider the differences in molecular architecture between polymers which exclusively display viscous flow and those which display a purely elastic response. To attribute the entire effect to molecular structure we assume the polymers are compared at the same temperature. Crosslinking between different chains is the structural feature responsible for elastic response in polymer samples. If the crosslinking is totally effective, we can regard the entire sample as one giant molecule, since the entire volume is permeated by a continuous network of chains. This result was anticipated in the discussion of the Bueche theory for chain entanglements in the last chapter, when we observed that viscosity would be infinite with entanglements if there were no slippage between chains. 

Determination of Controlling Rate Factor The most important physical variables determining the controlhng dispersion factor are particle size and structure, flow rate, fluid- and solid-phase diffu-sivities, partition ratio, and fluid viscosity. When multiple resistances and axial dispersion can potentially affect the rate, the spreading of a concentration wave in a fixed bed can be represented approximately... 

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