Viscosity rheological instrumentation

Upon mixing and injection of the caprolactam monomer streams into the rheological instrument, polymerization was initiated and continued, whereas simultaneously monitoring the complex viscosity and other rheological parameters of the polymerizing system. The maximum measurable complex viscosity levels were achieved in about 100 s or less, depending on temperature. 

Noting that processability is an important factor in propellant formulation, a system is proposed for processability determination as a function of the rheology of the propellant. The instrumental analysis necessary to characterize the rheology by viscosity measurements, including the recommended viscosity measuring instruments, is suggested for various stages in propellant development and manufacture. 

Viscometers of relatively complex geometry, for example the Ostwald glass U-tube viscometer, can be used to measure the viscosity of Newtonian liquids, which is independent of shear rate and time, after calibration with a Newtonian liquid of known viscosity. Such instruments cannot be used for Theologically characterizing non-Newtonian liquids, and therefore cannot be classed as rheometers, as geometrical complexity prevents evaluation of shear stress and shear rate at a given location independently of sample rheological behavior. 

A rheological instrument such as a viscometer can be used to evaluate t and 7 and hence obtain a value for the shear viscosity, 17. Examples of Newtonian fluids are pure gases, mixtures of gases, pure liquids of low molecular weight, dilute solutions, and dilute emulsions. In some instances, a fluid may be Newtonian at a certain shear-rate range but deviate from Newton s law of viscosity under either very low or very high shear rates (2). 

A Brookfield viscometer with a helipath attachment (Stoughton, MA) is a useful rheological instrument for measuring the settling behavior and structure of pharmaceutical suspensions and for characterizing the properties and stability of flocculated suspensions. The viscometer should be properly calibrated to measure the apparent viscosity of the suspension at equilibrium at a given temperature to establish suspension reproducibility. Apparent viscosity, like pH, is an exponential term, and therefore the log-apparent viscosity is an appropriate way of reporting the results. 

Finally, there are industrial rheological instruments in common use that allow determination of viscosities for liquids to within a few percent, which is sufficient for many purposes. These involve measuring the torque required to maintain a given velocity for a fluid confined in the annulus between two concentric cylinders, one of which is rotated (Couette viscometer), or a fluid confined between a flat plate and a rotating cone (cone-and-plate viscometer). 

Instrumental quality control before, during, and after manufacture is one area to which food rheology makes important contributions. For example, the measurement of apparent viscosity and yield stress of ketchup helps to predict how well tomato ketchup drains from a bottle. A number of tests have been developed using either basic rheological instruments (rotational viscometer, capillary viscometers, etc.) or instruments simulating the situation in which the rheological properties are of importance (Bostwick... 

Rheometer Also called plastometer. A rheological instrument for determining the flow properties of a plastic, usually of high viscosity or in the molten condition of thermoplastics, by forcing the melt through a die or orifice of specific size at specific temperature and pressure. 

There are a number of techniques that are used to measure polymer viscosity. For extrusion processes, capillary rheometers and cone and plate rheometers are the most commonly used devices. Both devices allow the rheologist to simultaneously measure the shear rate and the shear stress so that the viscosity may he calculated. These instruments and the analysis of the data are presented in the next sections. Only the minimum necessary mathematical development will he presented. The mathematical derivations are provided in Appendix A3. A more complete development of all pertinent rheological measurement functions for these rheometers are found elsewhere [9]. 

Various methods are used to examine the viscosity characteristics of metallized gels. Two types that have received extensive application are the cone and plate viscometer and the capillary viscometer. Both instruments can measure rheological characteristics at high shear rates, and the former is useful for low shear rate measurements as well. 

We can distinguish between two types of stresses on an interface a shear stress and a dilatational stress. In a shear stress experiment, the interfacial area is kept constant and a shear is imposed on the interface. The resistance is characterized by a shear viscosity, similar to the Newtonian viscosity of fluids. In a dilatational stress experiment, an interface is expanded (dilated) without shear. This resistance is characterized by a dilatational viscosity. In an actual dynamic situation, the total stress is a sum of these stresses, and both these viscosities represent the total flow resistance afforded by the interface to an applied stress. There are a number of instruments to study interfacial rheology and most of them are described in Ref. [1]. The most recent instrumentation is the controlled drop tensiometer. 

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