Other Viscometers

A number of other methods are occasionally used for viscosity measurements. The most common are the parallel plate viscometer, used in the 10 -10 Pa s range, the penetration viscometer, used in the 10 -10 Pa s range, and the torsion viscometer, used in the lO -lO Pas range. Although each of these methods has advantages under specific conditions, none have gained wide acceptance in the glass community. 

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Haake has introduced other viscometers, including the RheoStress RSlOO, which offers controlled stress as well as controlled shear rate and oscillatory modes over a temperature range of —50 to 350°C (ambient to 500°C is also possible). This versatile viscometer covers a shear rate range of 10 ... 

Although the viscometers discussed above are used in laboratories, there are other viscometers in the supply system that are available for local use. These viscometers can be... 

A number of other viscometers were also used, including Haake CV100 and RV3 models. The latter was coupled with a D40/300 measuring head and oil bath circulator for measurements above 100 C. Back pressures up to 4 bar were used and measurements made up to about 160°C. 

Today, the use of the sliding plate viscometer is very limited and mainly for comparative studies with other viscometers. For further information, see Griffin et al. (1957) and Shell Bitumen (2003). 

Viscometers used for silicone fluids, fluorocarbons, and other liquids which are difficult to remove by the use of a cleaning agent, shall be reserved for the exclusive use of those fluids except during their calibration. Subject such viscometers to calibration checks at frequent intervals. The solvent washings from these viscometers shall not be used for the cleaning of other viscometers. 

Allow the charged viscometer to remain in the bath long enough to reach the test temperature. Where one bath is used to accommodate several viscometers, never add or withdraw a viscometer while any other viscometer is in use for measuring a flow time. 

Even though eommereial instruments are available for making extensional viscosity measurements on polymer melts, this is not a routine measurement. The stretch-rate range of these extensional viscometers is such that the maximum stretch rate that can be achieved is of the order of 1 sec in polymer processing operations, a stretch rate of 100 sec is commonplace. Also, not every polymer stretches uniformly, and, even when it does, steady-state stress levels are not always attained. For all of these reasons, extensional viscometry is an area of current research. Additional details regarding extensional and other viscometers may be found in the book by Dealy [16]. 

The concentric cylinder viscometer described in Sec. 2.3, as well as numerous other possible instruments, can also be used to measure solution viscosity. The apparatus shown in Fig. 9.6 and its variations are the most widely used for this purpose, however. One limitation of this method is the fact that the velocity gradient is not constant, but varies with r in this type of instrument, as noted in connection with Eq. (9.26). Since we are not considering shear-dependent viscosity in this chapter, we shall ignore this limitation. 

Slurry Viscosity. Viscosities of magnesium hydroxide slurries are determined by the Brookfield Viscometer in which viscosity is measured using various combinations of spindles and spindle speeds, or other common methods of viscometry. Viscosity decreases with increasing rate of shear. Fluids, such as magnesium hydroxide slurry, that exhibit this type of rheological behavior are termed pseudoplastic. The viscosities obtained can be correlated with product or process parameters. Details of viscosity deterrnination for slurries are well covered in the Hterature (85,86). 

To solve a flow problem or characterize a given fluid, an instmment must be carefully selected. Many commercial viscometers are available with a variety of geometries for wide viscosity ranges and shear rates (10,21,49). Rarely is it necessary to constmct an instmment. However, in choosing a commercial viscometer a number of criteria must be considered. Of great importance is the nature of the material to be tested, its viscosity, its elasticity, the temperature dependence of its viscosity, and other variables. The degree of accuracy and precision required, and whether the measurements are for quaUty control or research, must be considered. The viscometer must be matched to the materials and processes of interest otherwise, the results may be misleading. 

Viscometers may be separated into three main types capillary, rotational, and moving body. There are other kinds, usually designed for special apphcations. For any given type there usually is a choice of several different instmments. The choice depends on the particular requirements of the investigator and the price range. 

Absolute viscosities are difficult to measure with capillary viscometers, but viscosities relative to some standard fluid of known viscosity, such as water, are readily determined. The viscometer is caHbrated with the reference fluid, and viscosities of other fluids relative to the reference sample are determined from their flow times. 

Piston Cylinder (Extrusion). Pressure-driven piston cylinder capillary viscometers, ie, extmsion rheometers (Fig. 25), are used primarily to measure the melt viscosity of polymers and other viscous materials (21,47,49,50). A reservoir is connected to a capillary tube, and molten polymer or another material is extmded through the capillary by means of a piston to which a constant force is appHed. Viscosity can be determined from the volumetric flow rate and the pressure drop along the capillary. The basic method and test conditions for a number of thermoplastics are described in ASTM D1238. Melt viscoelasticity can influence the results (160). 

A number of instmments are based on the extmsion principle, including sHt flow and normal capidary flow (Table 6). These instmments are useful when large numbers of quahty control or other melt viscosity test measurements are needed for batches of a single material or similar materials. When melt viscosities of a wide range of materials must be measured, rotational viscometers are preferable. Extmsion rheometers have been appHed to other materials with some success with adhesives and coatings (10,161). 

Coaxial (Concentric Cylinder) Viscometer, The eadiest and most common type of rotational viscometer is the coaxial or concentric cylinder instmment. It consists of two cylinders, one within the other (cup and bob), keeping the specimen between them, as shown in Figure 27. The first practical rotational viscometer consisted of a rotating cup with an inner cylinder supported by a torsion wire. In variations of this design the inner cylinder rotates. Instmments of both types ate useful for a variety of apphcations. 

OtherRota.tiona.1 Viscometers. Some rotational viscometers employ a disk as the inner member or bob, eg, the Brookfield and Mooney viscometers others use paddles (a geometry of the Stormer). These nonstandard geometries are difficult to analy2e, particularly for an infinite bath, as is usually employed with the Brookfield and the Stormer. The Brookfield disk has been analy2ed for Newtonian and non-Newtonian fluids and shear rate corrections have been developed (22). Other nonstandard geometries are best handled by determining iastmment constants by caUbration with standard fluids. 

Another type of rotational viscometer is the hehcal-screw rheometer (176). This iastmment is basically a screw-type metering pump that does not pump. The measure of force is the pressure difference resulting from the rotational motion. It is possible to use a bank of pressure transducers of different sensitivities to measure viscosity over a wide range. The iastmment can be used for high temperature rheometry and to foUow polymerkation, shear and heat degradation, and other developments. 

In the Mooney shearing disk viscometer, a serrated disk is rotated ia a sample fixed ia a pressuri2ed cavity. The instmment was developed for mbber and other elastomeric materials and is a standard quaUty control iastmment ia the mbber iadustry (ASTM D1646). It is used to measure high viscosities givea ia arbitrary Mooaey units, but usually ca 7.5 x 10 mPa-s atlow(ca 1.5 ) shear rates. 

Recently, patented ethoxylation catalysts have become available that can significantly narrow the ethylene oxide distribution of the alcohol ethoxylates used to obtain alcohol ether sulfates. These products are termed peaked alcohol ether sulfates whereas all others are termed conventional alcohol ether sulfates. Peaked alcohol ether sulfate solutions thicken more than those with a conventional ethylene oxide distribution [78]. Peaked alcohol ether sulfate solutions also exhibit behaviors different from those of conventional sulfates [79]. Smith [78] studied the viscosities of 15% sodium dodecyl ether sulfate solutions of both families with NaCl content between 2% and 10% at 25°C using a Brookfield model DVII viscometer at a shear rate of 2 s 1. The results are shown in Fig. 5 where the very different viscosities achieved are clearly observed. 

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