Viscosity of dispersion

Additives. Because of their versatility, imparted via chemical modification, the appHcations of ethyleneimine encompass the entire additive sector. The addition of PEI to PVC plastisols increases the adhesion of the coatings by selective adsorption at the substrate surface (410). PEI derivatives are also used as adhesion promoters in paper coating (411). The adducts formed from fatty alcohol epoxides and PEI are used as dispersants and emulsifiers (412). They are able to control the viscosity of dispersions, and thus faciHtate transport in pipe systems (413). Eatty acid derivatives of PEI are even able to control the viscosity of pigment dispersions (414). The high nitrogen content of PEIs has a flame-retardant effect. This property is used, in combination with phosphoms compounds, for providing wood panels (415), ceUulose (416), or polymer blends (417,418) with a flame-retardant finish. 

The process of flushing typically consists of the foUowing sequence phase transfer separation of aqueous phase vacuum dehydration of water trapped in the dispersed phase dispersion of the pigment in the oil phase by continued appHcation of shear thinning the heavy mass by addition of one or more vehicles to reduce the viscosity of dispersion and standardization of the finished dispersion to adjust the color and rheological properties to match the quaHty to the previously estabHshed standard. 

Other Factors Affecting the Viscosity of Dispersions. Factors other than concentration affect the viscosity of dispersions. A dispersion of nonspherical particles tends to be more viscous than predicted if the Brownian motion is great enough to maintain a random orientation of the particles. However, at low temperatures or high solvent viscosities, the Brownian motion is small and the particle alignment in flow (streamlining) results in unexpectedly lower viscosities. This is a form of shear thinning. 

Viscosity of dispersed phase = 0.5 to 100.0 Volume fraction of continuous phase = 0.1 to 1.0 Volume fraction of dispersed phase = 0.0 to 0.9... 

TLCP viscosity or rjo viscosity of dispersed phase rjo = 171) critical stress ratio Poisson s ratio of matrix density... 

In what way does the viscosity of dispersion depend on concentration of the solid phase ... 

At least, in absolute majority of cases, where the concentration dependence of viscosity is discussed, the case at hand is a shear flow. At the same time, it is by no means obvious (to be more exact the reverse is valid) that the values of the viscosity of dispersions determined during shear, will correlate with the values of the viscosity measured at other types of stressed state, for example at extension. Then a concept on the viscosity of suspensions (except ultimately diluted) loses its unambiguousness, and correspondingly the coefficients cn cease to be characteristics of the system, because they become dependent on the type of flow. 

Modification of filler s surface by active media leads to the same strong variation in viscosity. We can point out as an example the results of work [8], in which the values of the viscosity of dispersions of CaC03 in polystyrene melt were compared. For q> = 0.3 and the diameter of particles equal to 0.07 nm a treatment of the filler s surface by stearic acid caused a decrease in viscosity in the region of low shear rates as compared to the viscosity of nontreated particles more than by ten times. This very strong result, however, should not possibly be understood only from the point of view of viscometric measurements. The point is that, as stated above, a treatment of the filler particles affects its ability to netformation. Therefore for one and the same conditions of measuring viscosity, the dispersions being compared are not in equivalent positions with respect to yield stress. Thus, their viscosities become different. 

Viscosities of concentrated suspensions of carbon black in a white mineral oil (Fisher "paraffin" oil of 125/135 Saybolt viscosity) were measured with a Brookfield viscometer as a function of OLOA-1200 content. Figure 13 shows the viscosities of dispersions with 30 w%, 35 w% and 70 w% carbon black. In all cases the viscosity fell rapidly as the 0L0A-1200 content increased from 0 to 1%, then fell more gradually and levelled off as the 0L0A-1200 content approached 2%. In many respects the reduction in viscosity with increasing OLOA-1200 content parallels the conductivity measurements both phenomena are sensing the buildup of the steric barrier, and this steric barrier weakens, softens, and lubricates the interparticle contacts. As evidenced in foregoing sections, the particles are still flocculated but can be easily stirred and separated mechanically. The onset of electrostatic repulsion at OLOA-1200 contents in excess of 2.5% did not affect viscosities. 

There are numerous equations in the literature describing the concentration dependence of the viscosity of dispersions. Some are from curve fitting whilst others are based on a model of the flow. A common theme is to start with a dilute dispersion, for which we may define the viscosity from the hydrodynamic analysis, and then to consider what occurs when more particles are added to replace some of the continuous phase. The best analysis of this situation is due to Dougherty and Krieger18 and the analysis presented here, due to Ball and Richmond,19 is particularly transparent and emphasises the problem of excluded volume. The starting point is the differentiation of Equation (3.42) to give the initial rate of change of viscosity with concentration ... 

Fig. 25. Effect of viscosity of dispersed phase on drop volume.

Figure 12 Effect of degree of neutralisation of poly aery lie acid on viscosity of dispersed calcium carbonate 0.5 pm, 16 solids)...

The next topic is Einstein s theory of viscosity of dispersions of rigid, spherical particles. This theory is the starting point for most of the current approaches to flow properties of colloids and plays a practical, pedagogical, as well as historical role. 

Figure 4.10 shows the viscosity of dispersions of glass spheres of radius 65 ixm plotted in the manner suggested by Equation (43). Several conclusions are evident from the data replotted in this way ... 

FIG. 4.12 Viscosity of dispersions of some nonspherical particles (a) intrinsic viscosity as a function of the axial ratio a/b for oblate and prolate ellipsoids of revolution according to the Simha theory (redrawn with permission of Hiemenz 1984) (b) experimental values of relative viscosity versus volume fraction for tobacco mosaic virus particles of different a/b ratios (data from M. A. Lauffer, J. Am. Chem. Soc., 66, 1188 (1944)). 

Our objective in this chapter is modest, namely, to provide a general discussion of the electroviscous effects and to present a few equations that serve as guidelines for understanding the effects of colloidal forces on the viscosity of dispersions. The underlying theories are rather complicated and fall outside our scope. 

Other dimensionless groups that compare the thickness of the adsorbed polymer layer to the radius of the particle or the radius of gyration of the polymer to the particle radius in polymer/colloid mixtures can also be easily defined. We are mostly concerned with the volume fraction and the Peclet number Pe in our discussions in this chapter. However, the other dimensionless groups may appear in the equations for intrinsic viscosity of dispersions when the dominant effects are electroviscous or sterically induced. 

List some of the conditions under which the Einstein equation for viscosity of dispersions fails and how one can correct the situation. 

How does the viscosity of a polymer solution differ from the viscosity of dispersions What factors are important in the case of the former ... 

Viscosity of dispersion is known to follow Einstein s equation, in the extreme case where the particle concentration is so low that the particles do not interact with each other. 

Fig. 39. Apparent viscosity of dispersions of silica (2a = 2 nm) in 0.5 wt% solutions of polyacrylamide (Mw = 5.5 x 103 kg/mol) in 50/50 glycerin-water mixtures (Otsubo and Watanabe, 1987). Different symbols correspond to different weight percentages of silica.

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