Effect of shear rate

As the viscosity falls with increasing shear rate, does this mean that the polymer flows more easily No just the reverse, in fact. Remember that when a rubber 

Since the sample has both viscosity and elasticity, it is said to be viscoelastic, and the phenomenon is called viscoelasticity. As the name impHes,in this region the polymer liquid exhibits flow characteristics which are reflected in the term viscosity and rubber-like behaviour reflected in elasticity . 

If the material is placed on a flat surface, it slowly flows to cover the surface. Flowever, if it is moulded into a spherical shape and thrown on the floor, the impact time is only a fraction of a second, and the material bounces like a rubber ball. 

We now examine two examples of the consequences of this viscoelasticity in technology. In both cases, the phenomena are concerned with the elasticity that arises at high shear rates, sometimes ca]lcd elastic memory . 

When the polymer is forced through the die at high speed, there is not time for full flow to take place, and the elasticity makes the extrudate spring back towards its original thickness. This can be overcome by increasing the melt temperature so as to speed up the flow molecular motions. Of course, the temperature cannot be raised beyond the point at which the polymer starts to decompose. 

As discussed in previous sections, most polymer fluids exhibit shear-thinning behavior, i.e., the viscosity decreases with increase in shear rate. The shear-thinning 

Based on the discussion in sections 8.6.1 and 8.6.2, the following guidelines can be used if the polymer viscosity needs to be reduced during fiber formation  

For polymers with flexible chains and weak intermolecular interactions, it is more effective to increase the shear rate. 

In real applications, many other factors can affect the actual viscosity of a polymer. As a result, these guidelines may not woik for some polymers. In addition, there are limitations while increasing the temperature or shear rate. For example, the processing temperamre cannot be too close to the polymer degradation 

The intrinsic viscosity is defined in terms of the viscosity in the limit of zero shear-rate, but the viscosity of a polymer solution depends on the shear-rate. Factors that enhance the shear-rate dependency of [77] are high solvent power, large molecular weight and broad molecular weight distribution [66,67]. To avoid consideration of this parameter, measurements must be made at sufficiently low shear rates that the viscosity is essentially equal to its low-shear rate limiting viscosity. 

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Viscosity of surfactant solutions depends on the kind of solution, shear rate, temperature and concentration. Figure 2.51 shows the effect of shear rate U5 on shear... 

S. Vitthal and J. M. McGowen. Fracturing fluid leakoff under dynamic conditions Pt 2 Effect of shear rate, permeability, and pressure. In Proceedings Volume, pages 821-835. Annu SPE Tech Conf (Denver, CO, 10/6-10/9), 1996. 

There are two important issues concerning the factor that gives the excluded volume 1 /shear rate (ii) what is the effect of polydispersity ... 

In the in situ consolidation model of Liu [26], the Lee-Springer intimate contact model was modified to account for the effects of shear rate-dependent viscosity of the non-Newtonian matrix resin and included a contact model to estimate the size of the contact area between the roller and the composite. The authors also considered lateral expansion of the composite tow, which can lead to gaps and/or laps between adjacent tows. For constant temperature and loading conditions, their analysis can be integrated exactly to give the expression developed by Wang and Gutowski [27]. In fact, the expression for lateral expansion was used to fit tow compression data to determine the temperature dependent non-Newtonian viscosity and the power law exponent of the fiber-matrix mixture. 

Solans et al. [80] studied the effect of shear rate on shear stress, x, for w/o HIPEs of shear rate, the shear stress was seen to rise to a maximum then quickly fall to zero, with time. This was attributed to the breakdown of the emulsion at higher rates of shear. 

Ismail, A. F., Ng, B. C., and Abdul Rahman, W. A. W. (2003), Effects of shear rate and forced convection residence time on asymmetric polysulfone membranes structure and gas separation performance, Sep. Purif. Technol, 33,255-272. 

Garbolino et al. (65) studied the effects of shear rate on crystallization of a confectionery coating fat (hydrogenated and fractionated mixture of soybean and cottonseed oils) using ultrasonic sensors. They hypothesized that primary nucleation is less likely to be affected by shear and suggested that crystal nuclei probably form from heterogeneous nucleation sites (dust particles or other suspended insoluble materials and imperfections in the container walls). They also suggested that... 

Weiss HJ, Turitto VT, Baumgartner HR Effect of shear rate on platelet interaction with subendothdium in cilrated and native blood. I. Shear dependent decrease of adhesion in von Willebrand s disease and the Bemard-Soulier syndrome. J Lab Clin Med 92 750-764,1978. 

The stress in viscoelastic liquids at steady-state conditions is defined, in simple shear flow, by the shear rate and two normal stress differences. Chapter 13 reviews the evolution of both the normal stress differences and the viscosity with increasing shear rate for different geometries. Semiquantitative approaches are used in which the critical shear rate at which the viscosity starts to drop in non-Newtonian fluids is estimated. The effects of shear rate, concentration, and temperature on die swell are qualitatively analyzed, and some basic aspects of the elongational flow are discussed. This process is useful to understand, at least qualitatively, the rheological fundamentals of polymer processing. 

Jaffrin et al. [57] have made a hydrodynamic comparison between the rotating disk and the VSEP system based on the flux achieved under similar maximum shear rates for baker s yeast microfiltration with an 0.2 p,m ME and skim mUk UF at 50 kDa. They found that the flux variation with time in these two modules was nearly identical when they were operated at the same maximum shear rate, suggesting the dominant effect of shear rate on the filtration performance. 

In the first part of this work, static, dynamic, and intermediate dynamic-static crystallization kinetics of cocoa butter measured under isothermal conditions are reported. The effect of shear rate and time is analyzed using a kinetics theory previously developed for polymers. In the second part, these isothermal experimental... 

Figure 8.16 Effect of shear rate on the viscosity of gum solutions grouped according to their rheological behaviour.

Recent work has included tests that examine the effects of shear rate and cure simultaneously (which will also be discussed below). The models derived from these tests are recombined to provide an overall chemoviscosity model to be used in processing applications. The effect of pressure on chemoviscosity has not been studied extensively however, system pressure may be relevant to high-pressure injection-moulding and transfer-moulding processes. 

Table 4.3. Chemorheological models for the effect of shear rate ...

The effects of shear rate on the viscosity of thermosets are usually measured on uncatalysed resins or resins that have undergone minimal curing. This is because the effects of shear rate will be most prevalent in the early stages of the process, during transfer or injection of the resin. 

The most common measuring systems used for the effects of shear rate on the chemo-viscosity of thermosets have been the parallel-plate rheometer and the capillary rheometer however, the choice of rheometer is dependent on the type of system to be studied. The advantages and limitations of these systems are presented in Table 4.6. 

The work of several authors (Peters et al, 1993, Halley et al, 1994) has demonstrated the use of non-isothermal dynamic sweep tests to examine the combined effects of shear rate and curing on the chemoviscosity of a highly filled epoxy resin simultaneously. These tests use a selected temperature ramp with repeated dynamic rate sweeps to investigate the effects on the chemoviscosity. The advantage of these tests is that the effects of shear rate and cure are not separated, which is similar to processing conditions. 

Simple chemoviscosity models have been used to examine the effects of shear rate on the chemoviscosity = T) as shown in Table 4.3. For example, Sundstrom and Burkett (1981) and Hartley and Williams (1981) found that polyesters and polyurethanes exhibit essentially Newtonian behaviour, whereas epoxies and phenolics show marked shear thinning. The power-law model is the most extensively used shear-rate model for thermosets and has been used for unfilled (Ryan and Kamal, 1976, Kascaval et al, 1993,... 

Quemada (1978a, 1978b) examined the rheology and modelling of concentrated dispersions and described simple viscosity models that incorporate the effects of shear rate and concentration of filler and separate effects of Brownian motion (or aggregation at low shear) and particle orientation and deformation (at high shear). The ratio of structure-build-up and -breakdown rates is an important parameter that is influenced by the ratio of the shear rate to the particle diffusion. A simple form of viscosity relation is given here ... 

Wessel and Ball " and Kanai et al. studied in detail the effects of shear rate on the fractal structure of flocculated emulsion drops. They showed that the size of the floes usually decreases with the increase of the shear stress often the floes are split to single particles at high shear rates. As a result, the viscosity decreases rapidly with the increase of the shear rate. 

F. Combined effects of shear rate and polydispersity on melt viscosity... 

F. Combined Effects of Shear Rate and Polydispersity on Melt Viscosity... 

The effects of shear rate (y) and polydispersity (the ratio Mw/Mn, where Mn denotes the number-average molecular weight) on melt viscosity have been reviewed by van Krevelen [7] from the viewpoint of fitting experimental data by empirical relationships, and by Kumar [5] from the viewpoint of fundamental theoretical treatments. It is clear from these reviews that the effects of y and polydispersity are inextricably intertwined and must be considered at the same... 

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