High-shear

Corrosion and anti-wear protection Anti-corrosion and anti-wear power High viscosity at high shear rates... 

Pseudoplastic fluids have no yield stress threshold and in these fluids the ratio of shear stress to the rate of shear generally falls continuously and rapidly with increase in the shear rate. Very low and very high shear regions are the exceptions, where the flow curve is almost horizontal (Figure 1.1). 

Polyolefin melts have a high degree of viscoelastic memory or elasticity. First normal stress differences of polyolefins, a rheological measure of melt elasticity, are shown in Figure 9 (30). At a fixed molecular weight and shear rate, the first normal stress difference increases as MJM increases. The high shear rate obtained in fine capillaries, typically on the order of 10 , coupled with the viscoelastic memory, causes the filament to swell (die swell or... 

Retention aid polymers are used in a very high shear environment, so floe strength and the abiHty for floes to reform after being sheared (43) is important. The optimum floe size is a compromise. Larger floes give better free drainage, but tend to produce an uneven sheet due to air breakthrough in... 

Although there is no official ASTM test to measure the water handling properties of gasoline, there are a number of widely used industry tests. One uses a Waring Blender to simulate high shear (57), whereas another uses a wrist action shaker to simulate mild shear conditions (58). 

Inks. Basic raw materials for letterpress inks, such as mineral oils, soya bean oil, resins, and pigments, are essentiaHy the same as those used in web offset inks. Inks are tinctoriaHy weak, relatively fluid, and their low and high shear viscosities are low. 

Many methods for the conversion of acid copolymers to ionomers have been described by Du Pont (27,28). The chemistry involved is simple when cations such as sodium or potassium are involved, but conditions must be controlled to obtain uniform products. Solutions of sodium hydroxide or methoxide can be fed to the acid copolymer melt, using a high shear device such as a two-roU mill to achieve uniformity. AH volatile by-products are easily removed during the conversion, which is mn at about 150°C. A continuous process has been described, using two extmders, the first designed to plasticate the feed polymer and mix it rapidly with the metal compound, eg, zinc oxide, at 160°C (28). Acetic acid is pumped into the melt to function as an activator. Volatiles are removed in an extraction-extmder which follows the reactor-extmder, and the anhydrous melt emerges through a die-plate as strands which are cut into pellets. 

In the post-dispersion process, the soHd phenoHc resin is added to a mixture of water, cosolvent, and dispersant at high shear mixing, possibly with heating. The cosolvent, frequently an alcohol or glycol ether, and heat soften the resin and permit small particles to form. On cooling, the resin particles, stabilized by dispersant and perhaps thickener, harden and resist settling and agglomeration. Both resole and novolak resins have been made by this process (25). 

Dispersions. In phenoHc resin dispersions, the continuous phase is water or a nonpolar hydrocarbon solvent. The resin exists as droplets that have particle sizes of 1—20 p.m and are dispersed in the continuous phase. Aqueous dispersions are prepared either in situ during the preparation of the resin itself or by high shear mixing (25,35). 

Polymer Suspensions. Poly(ethylene oxide) resins ate commercially available as fine granular soHds. However, the polymer can be dispersed in a nonsolvent to provide better metering into various systems. Production processes involve the use of high shear mixers to disperse the soHds in a nonsolvent vehicle (72—74). 

Another technique to reduce the problems caused by stickies is to use additives to reduce the tackiness of these particles. This prevents their later reagglomeration and attachment to paper machine surfaces. These additives are usually added to the pulper. The most common is talc (17) usually added to the pulper in repulpable bags. Emulsified talc is also sometimes added to the pulp just before the pulp encounters high shear. Organic polymers (18) such as a polyvinylpyrrohdinone (PVP) copolymer (19) have also been reported to reduce the tackiness of stickies. 

The other models can be appHed to non-Newtonian materials where time-dependent effects are absent. This situation encompasses many technically important materials from polymer solutions to latices, pigment slurries, and polymer melts. At high shear rates most of these materials tend to a Newtonian viscosity limit. At low shear rates they tend either to a yield point or to a low shear Newtonian limiting viscosity. At intermediate shear rates, the power law or the Casson model is a useful approximation. 

The Williamson equation is useful for modeling shear-thinning fluids over a wide range of shear rates (15). It makes provision for limiting low and high shear Newtonian viscosity behavior (eq. 3), where T is the absolute value of the shear stress and is the shear stress at which the viscosity is the mean of the viscosity limits TIq and, ie, at r = -H... 

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