Rheometry, oscillation

In another study, oscillating rheometry was used to examine the effect of adding various simple metal salts to glass-ionomer cements (Crisp, Merson Wilson, 1980). It was found that cement formation for certain glasses which react only slowly with poly(acrylic acid) could be accelerated significantly by certain metal salts, mainly fluorides such as stannous fluoride and zinc fluoride. Some non-reactive glasses could be induced to set by the addition of such compounds. 

As a further example of the use of the technique of oscillating rheometry, the work of Crisp et al. (1979) can be cited. This study was of the formation... 

Oscillating rheometry continues to be useful in the study of AB cements, and has recently been used to give further insight into the role of (-1-)-tartaric acid in glass-ionomer cements (Hill Wilson, 1988). Further examples of its use are described in earlier chapters of this book. 

Rotational Rheometry. Two instruments were used for these measurements. An Instron model 3250 rheometer was used for measuring rotational viscometry and forced oscillation as a function of frequency with strains in the region of 0.5. A Bohlin VOR rheometer was used for dynamic measurements at smaller strains (i.e., 0.1). 

Figure 1. Cure response by oscillating disc rheometry at 177 °C of a compound optimized for use In o-rlngs.

The most important measurements for characterizing emulsions are forced oscillation rheometry studies. The presence of significant elasticity clearly defines whether or not a stable emulsion has been formed. The viscosity by itself can be an indicator of the stability of die emulsion, although it is not necessarily conclusive, unless one is fully certain of the viscosity of the starting oil. Color is an indicator, but may not be definitive. This laboratory s experience is that all table emulsions were reddish. Some mesostable emulsions were also reddish and unstable emulsions were always the color of the starting oil. Water content is not an indicator of stability and is error prone because of the excess of water that may be present. It should be noted, however, that stable emulsions have water contents greater than 70% and that unstable emulsions or... 

Figure 5 (a) Elastic and viscous moduli as functions of shear stress obtained from oscillatory shear dynamic rheometry (see text and Ref 80 for details) performed at the oil-water interfece. The system consisted of 40 ml of water on to which 35 ml of an oil phase was layered, consisting of 0.75% (w/v) B6 asphaltenes dissolved in 50% (v/v) toluene in heptane. The asphaltenes were allowed to adsorb for 8 h at which point the rheometrical measurements were performed. The frequency of oscillation of the biconical hob was 1 rad/s. (b) Elastic and viscous moduh as functions of frequency. The same system and experiment as described for Fig. 5a were used with the exception that the shear stress was fixed at 1 mPa and the frequency was varied. Asphaltenes were allowed to adsorb for a period of 8 h. 

Elastic Modulus. The elastic shear modulus of swollen superabsorbent polymer is commonly measured by means of oscillatory rheometry, using any of a variety of commercially available instruments (31). The sample to be analyzed is screened to the desired particle size cut such that the swollen particle size is smaller than the gap between the measuring plates. The sample is swollen in the desired liquid, usually 0.9 wt% NaCl solution or synthetic urine. Excess liquid is removed from the swollen hydrogel by blotting to minimize interparticle slippage. The hydrogel typically is placed onto the lower circular plate. An upper conical plate is positioned at the proper sample gap, which is packed with the gel. An oscillating torque is applied at a known frequency to the upper plate and the... 

DQM Craig, S Tamburic, 0 Buckton, JM Newton, An investigation on the structure and properties of Carbopoi 934 gels using dielectric spectroscopy and oscill.itory rheometry. J. Controlled Release 30 213-22.3, 1994,... 

Oscillating disc rheometry. Full analysis of vulcanisation curing Rheocheck lOOC Barco Rheomicro Computer package BS ASTM ISO Negretti Automation ... 

In addition to the rotational shear tests with the rheometer, oscillation tests, like amplitude and frequency sweeps, are done. Viscoelastic behavior of the solutions, meaning a higher storage modulus, is only measurable at a mass fraction of 15 % in the solution. Extensional rheometry has also been done by project partners with different K90 solution and a CABER by Haake, but no reliable results have been generated. 

Beyond a critical rate of flow in cores the viscoelasticity of polyacrylamide solutions is reflected by shear thickening behaviour. Practical information is provided on the critical flow rate and magnitude of shear thickening and their dependency on core permeability, temperature, molecular weight and concentration of commercial polymers. To describe the onset of shear thickening, critical Deborah numbers were calculated using fluid relaxation times obtained by oscillation rheometry. Using three core-flow parameters, viz. the critical flow rate and two power-law exponents, the viscoelastic effects are quantified. 

This paper presents results obtained with oscillation rheometry, which allow separation and assessment of the elastic and viscous responses. The derived fluid relaxation time is used to convert the onset of shear thickening - observed in consolidated and unconsolidated sandstones - into critical Deborah-numbers. 

From oscillation rheometry, separating viscous and elastic effects, a practical fluid relaxation time can be derived, which is indicative of the critical flow rate of non-ideal polymer solutions. 

The organization of this chapter parallels that of its predecessors in Part II, describing first the design of drag flow rheometers, then pressure-driven ones. Separate sections are devoted to analysis of data, particularly sinusoidal oscillations, and to special designs for process-line rheological measurements. The section on exten-sional rheometry is brief because most of the design issues were discussed in Chapter 7. 

A typical study by Simon and Suverkrup [ 170] compared techniques used to characterise the crystallisation behaviour of fatty acid bases. In addition to DSC, isothermal DSC, NMR, oscillation rheometry and thermorheography were used. The crystallisation of hard fats was a biphasic process [170]. The formation of crystal nuclei was controlled by the partial glyceride content the degree of super-cooling determined the formation of crystal nuclei the final solidification level depended on the crystallisation temperature. Similar studies [171] showed that the recrystallisation rates and the degree of supercooling before crystallisation occurred depended on the molecular weight of PEG. 

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