Pseudoplastic paste

These examples serve to illustrate the ability of soluble polymer, interacting in a controlled fashion with colloidal particles, to transform both the equilibrium state and the mechanical properties of dispersions. All states are possible, from low viscosity fluids to pseudoplastic pastes with high yield stresses. 

With added fillers - typically 70 wt% for early materials - the dimethacrylate monomers form very viscous pseudoplastic pastes (Watts et al., 1980). The manipulation of these materials was very technique-sensitive. Thorough mixing of the two pastes was essential, yet this was difficult to ensure, given that both pastes were of the same color. 

Thus soluble polymer, interacting in a controlled fashion with colloidal particles, can transform both the equilibrium state and the mechanical properties of dispersions. The possibilities range from equilibrium, low viscosity fluids to nonequilibrium, pseudoplastic pastes with high yield stresses. However, substantial ga( still exist in the ability to, for example, (i) create high viscosity equilibrium fluids with prescribed relaxation spectra, (ii) impart a sol-gel transition at prescribed conditions, or (iii) connect explicitly macromdecular structure with rheological behavior. 

Due to the high viscosities of the pseudoplastic paste, the separation tendency and the diluted... 

DSC revealed that the XG and starch did not interact synergistically and hence did not promote the formation of three-dimensional network structures. However, the hydrocolloid significantly decreased the retrogradation and syneresis of the starch paste, particularly in blends with a starch/XG ratio of 8.5/1.5. Mixing 1% or 2% tamarind XG with 9% cornstarch resulted in an increase in the paste viscosity from 385 to 460 and 560 BU (Brabender units), respectively [298]. The XG is associated with starch, as was evident from the lowering of the pasting temperature and the synergistic increase in pseudoplasticity and yield value of the blend pastes. However, carboxymethylated and hydroxypropylated XGs showed a diminished interaction. 

Xanthan gum shows a good solubility in water, giving a highly viscous solution with a pseudoplastic appearence and a temperature independent viscosity. Xanthan gum is used in pharmaceuticals for its excellent emulsifying and suspending properties. The pseudoplastic properties of this gum enables tooth pastes and ointments both to hold their shape and to spread readily. 

Since n is less than unity, the apparent viscosity decreases with the deformation rate. Examples of such materials are some polymeric solutions or melts such as rubbers, cellulose acetate and napalm suspensions such as paints, mayonnaise, paper pulp, or detergent slurries and dilute suspensions of inert solids. Pseudoplastic properties of wallpaper paste account for good spreading and adhesion, and those of printing inks prevent their running at low speeds yet allow them to spread easily in high speed machines. 

The apparent viscosity of PVC pastes varies with the shear rate applied, so that they rarely, if ever, exhibit truly Newtonian behaviour. The rheological properties of plastisols can range from pseudoplastic ( shear thinning ) to dilatent ( shear thickening ), as illustrated in Figure 107. 

A dilatant flow is characterized by the opposite type of pseudoplastic flow in which the apparent viscosity increases with the increase in shear stress (i.e., shearthickening). The empirical equation described for the dilatant flow is similar to Equation (4.84) but the exponent n is greater than 1. This behavior is not common for all pharmaceutical solutions and dispersions but it is exhibited by pastes of small, deflocculated particles (solid content > 50%). There is only a limited amount of fluid that can till the interparticulate voids. 

An understanding of the rheological behaviour is necessary as PVC pastes are classified as non-Newtonian liquids and can be dilatent (shear thickening), pseudoplastic (shear thinning) or thixotropic (viscosity reduces with time under constant shear). Each process requires specific rheological characteristics and this is achieved by formulation of appropriate PVC grades and knowledge of the influence of shear rate and time under constant shear. 

In PVC coating formulation fillers play a role. Filler choice mostly depends on the way the filler affect viscosity. The filler should not absorb the plasticizers nor interfere with the pseudoplastic behavior of the paste which is determined by the resin properties and by the choice of plasticizers. Fillers must be completely dispersed, since the gaps between the coated substrate and the knife are very small. There must be no lumps. Fillers should not interfere with deaeration which is... 

If in non-Newtonian liquids the structure of the liquid is destroyed upon increasing y, hysteresis curves are observed as shown in Fig. 1.29. The behaviour of these liquids depends not only on the time of shear but also on the past shear and thermal history. Pseudoplastic liquids of this kind are named thixotropic, and dilatant liquids are referred to as rheopectic. The longer the duration of shear, the stronger is the destruction of the liquid structure, and the longer it takes to restore it. 

Polymer solutions and melts, residual oils, rubber solutions, many petroleum products, paper pulps, biological fluids (blood, plasma), pharmaceutical compounds (emulsions, creams, and pastes), various food products (fats and sour cream) can serve as examples of pseudoplastic fluids. Dilatant properties are mainly exhibited by high-concentration or coarse-disperse systems (such as... 

Because the ceramic porous structure depends on the shape of individual grains and the way they are packed, different factors can affect the two major characteristics of membrane supports, mechanical strength and porosity. The pseudoplastic behavior of the paste during extrusion is responsible for an exponential dependence of extrusion velocity versus applied pressure. High pressures to increase support permeability and strength have been emphasized in the literature [14]. These can be linked to a better (more dense) particle pack-... 

The hydrolysis of starches by amylases Is most conveniently considered In terms of sugar production (for saccharification) or viscosity reduction (for liquefaction). Starch pastes are characteristically pseudoplastic (shear thinning), and thus satisfy, over an Intermediate range of shear rates, a power law relationship between shear stress, T, and shear rate, y, of the form (2.4) ... 

PVC pastes exhibit complex rheological behavior with the viscosities showing dependence on the shear rate and on the time of shear. A paste viscosity may increase with shear rate (dilatancy) or decrease shear thinning or pseudoplasticity). Some pastes may show dilatant tendencies over one range of shear rates but... 

In paste processing, particle size distribution influences the properties of paste. The amount of free plasticizer in the total amount of plasticizer depends on the particle size distribution of the PVC resin. This is related to packing density, which is improved by bimodal particle size distribution (combination of smaller and larger particles) in which smaller particles fill interstices between the larger partieles. The broad partiele size distribution was found to cause pseudoplastic behavior of PVC pastes. The dynamic viscosity and storage modulus depend on particle size distribution. ... 

As it was aforementioned, the rheological properties of cement pastes are dependent on many factors. In Fig. 5.7 the evolution of the properties of pastes from the pseudoplastic fluids through the Bingham fluid to the showing dilatancy, at higher... 

B and b— are the experimental constants. This formula relates to the c lower than the value corresponding to the transformation of paste finm the dilatancy showing fluid to the pseudoplastic one. The constant b depends on the shear rate, type of material and shape of grains but not on the specific surface area and the grain size distribution. The constant B depends on the shear rate and specific surface area of cement. 

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