Stress bentonite

Rheopectic behavior is the opposite of thixotropy. Shear stress increases with time at constant shear rate. Rheopeclic behavior has been obsei ved in bentonite sols, vanadium pentoxide sols, and gypsum suspensions in water (Bauer and Colhns, ibid.) as well as in some... 

Mix D is a typical plastigel. The incorporation of such materials as fumed silicas, certain bentonites or aluminium stearate gives a paste which shows pronounced Bingham Body behaviour (i.e. it only flows on application of a shearing stress above a certain value). Such putty-like materials, which are also... 

Montmorillonite clays absorb water readily, swell greatly and confer highly plastic properties to a soil. Thus soil stress (Section 14.8) occurs most frequently in these soils and less commonly in predominantly kaolinitic types. Similarly, a soil high in bentonite will show more aggressive corrosion than a soil with a comparable percentage of kaolinite. A chalky soil usually shows low corrosion rates. Clay mineralogy and the relation of clays to corrosion deserves attention from corrosion engineers. Many important relationships are not fully understood and there is need for extensive research in this area. 

The use of a rotating vane has become very popular as a simple to use technique that allows slip to be overcome (33,34). Alderman et al (35) used the vane method to determine the yield stress, yield strain and shear modulus of bentonite gels. In the latter work it is interesting to note that a typical toique/time plot exhibits a maximum torque (related to yield stress of the sample) after which the torque is observed to decrease with time. The fall in torque beyond the maximum point was described loosely as being a transition from a gel-like to a fluid-like behavior. However, it may also be caused by the development of a slip surface within the bulk material. Indeed, by the use of the marker line technique, Plucinski et al (15) found that in parallel plate fixtures and in slow steady shear motion, the onset of slip in mayonnaises coincided with the onset of decrease in torque (Fig. 8). These authors found slip to be present for... 

The influence of addition of sodium bentonite (a commonly used antisettling system) on the rheological behaviour of a pesticide suspension concentrate (250 g dm ) has been investigated. Steady state shear stress-shear rate curves were carried out to obtain the yield value and viscosity as a function of shear rate. The shear modulus was also measured using a pulse shearometer, and the residual viscosity was obtained in afew cases from creep measurements. The rheological parameters Tg (Bingham yield value),... 

Steady State Measurements Fig. 1 shows the shear rate-shear stress curves at various bentonite concentrations (calculated on the basis of the continuous phase) Hysteresis in the shear rate-shear stress curves was insignificant and the correlation between the ascending and descending curves was within experimental error. The results shown in Fig. 1 were therefore, the mean value of the ascending and descending curves. In the absence of any bentonite the suspension was Newtonian, whereas all suspensions containing bentonite at concentrations > 30 g dm were all pseudoplastic. This is illustrated from a plot of viscosity versus shear rate (Figure 2) which shows an exponential reduction of h with increase in shear rate. 

Figure 1, Shear rate - shear stress curves at various bentonite concentrations...

Creep Curves Fig. 4 shows the creep curves for the formulations containing 30 and 45 g dm bentonite. Suspensions containing less than 30 g dm bentonite did not give a measurable creep curve. The creep curves shown in Fig. 4 are typical of those obtained with viscoelastic systems. They consist of three regions (a) directly after the application of the stress one observes a rapid elastic deformation resulting in an elastic compliance (instantaneous shear modulus = T/y =... 

The relationship between lEPs and mechanical properties of dispersions of clay minerals is more complex than for oxides. Specific results for bentonites can be found in [394] (Tables 3.1317, 3.1318, and 3.1321). Eor kaolin, tlie maximum in the yield stress was at pH 5.3, while the potential was negative at pH > 4. This lack of coincidence of the IEP and the maximum in the yield stress is due to different charges of faces and edges [1019]. 

At lowest shear stresses the behavior of bentonite clays may be the same as that of a solid-like system with high viscosity, which is consistent with the Kelvin model and corresponds to region I. The investigation of relaxation properties of coagulation structures forming in these moderately concentrated dispersions of bentonite clays revealed the existence of an elastic aftereffect at low shear stresses. This aftereffect is related to mutual coorientation of anisometric particles that are capable of taking part in rotational Brownian motion without any rupture of contacts. Consequently, the nature of elastic aftereffect is entropic. In such systems high viscosities are related... 

The full rheological curve of such thixotropic system may be presented as a graph showing the effective viscosity, ry- = x/y, as a function of the shear stress, x, (Fig. IX-25). In this Figure r min corresponds to viscosity of the system with completely disintegrated structure. For the above mentioned bentonite... 

Figure 16. Variation of Casson high shear viscosity (kf) and yield stress (k02) on polymer concentration for simple bentonite-polymer (CMC) drilling fluids. Bentonite content fixed at 20.6 g/L. (Reproduced with permission from reference 32. Copyright 1993 Society of Petroleum Engineers.)...

Heath and Tadros (75) showed that the addition of poly (vinyl alcohol) (PVA) to bentonite suspensions increased the yield stress and plastic viscosity for polymer concentrations up to about 5 g/L. At this PVA concentration only about 25% of the surface of the bentonite was covered with polymer. For higher polymer concentrations, the plastic viscosity and yield stress decreased with increasing polymer concentration. Heath and Tadros explain the increased yield stress and plastic viscosity by the flocculation of the bentonite by the PVA via a bridging mechanism. 

Figure 20. Dependence of Bingham plastic viscosity (rjp) and yield stress (t ) of bentonite suspensions (40 g/L) on pH at various ionic strengths. (Reprinted with permission from reference 75. Copyright 1983 Academic.)...

Despite the low levels of adsorption, the anionic polymers have a significant effect on the rheology of the bentonite suspensions. Figure 22 shows the dependence of the Bingham yield stress rG on bentonite concentration with and without added dispersants. Rabaioli et al. found the dependence of rG on the volume fraction of bentonite 0 to be... 

The polyacrylate polymers and a derivative of a vinyl acetate maleic anhydride copolymer cause V30 to decrease monotonically with increasing polymer concentration, similar to the CMC polymers (Figure 46). The polymers PVA and poly(vinyl pyridinium) (PVP) hydrochloride markedly increased V30 at low concentration at concentrations above 1 g of polymer per gram of added bentonite PVA functions as a static fluid loss additive. The maximum in the API fluid loss at low PVA concentrations approximately coincides with the maximum in the yield stress and plastic viscosity found by Heath and Tadros (75). The increased static fluid loss is consistent with Heath and Tadros s conclusion that bentonite is flocculated by low concentrations of PVA. The concentration of PVA required to decrease V30 below that of the neat bentonite suspension is significantly larger than the concentration of CMC, where effective static fluid loss control can be achieved at polymer bentonite weight ratios of about 0.1 g/g. More effective fluid loss control has been achieved with other synthetic polymers such as poly(vinyl sulphonate)-poly(vinyl amide) copolymer (40) and other sulphonated polymers (39). 

Sodium silicates are also used to provide the fluid with a yield stress large enough to hold the particles at high water content. The mechanism is completely different from that of bentonite platelets that, having opposite charges on the faces and on the edges, gel the fluid by forming card house structures. Here, sodium silicate reacts with lime or calcium chloride to form a calcium silicate gel. It is this gel that provides the yield stress required to hold the particles. 

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