Glass viscometer

Gla.ss Ca.pilla.ry Viscometers. The glass capillary viscometer is widely used to measure the viscosity of Newtonian fluids. The driving force is usually the hydrostatic head of the test Hquid. Kinematic viscosity is measured directly, and most of the viscometers are limited to low viscosity fluids, ca 0.4—16,000 mm /s. However, external pressure can be appHed to many glass viscometers to increase the range of measurement and enable the study of non-Newtonian behavior. Glass capillary viscometers are low shear stress instmments 1—15 Pa or 10—150 dyn/cm if operated by gravity only. The rate of shear can be as high as 20,000 based on a 200—800 s efflux time. 

In some cases, the selection of SEC for the particular application seems to be illogical. A typical example is the determination of the intrinsic viscosities of polymer samples by SEC. Here a simple and cheap glass viscometer is substituted by a rather expensive sophisticated SEC instrument. However, the advantage of SEC is evident when value of manpower, as well as speed and precision of determinations is taken into account. 

This technique is by far the easiest for the characterization of polymers in solution. This can be seen from the simplicity of the typical (glass) viscometer shown in Figure 2.7. It is used to obtain the viscosity of a liquid by the use of Poiseuille s equation, which is... 

The viscosity method of molecular weight determination requires very little investment in apparatus and can be carried out quite rapidly by flow time measurements with a simple glass viscometer (see p. 291) set up in a constant temperature bath. As a result, this is the most widely used method for measuring a polymer molecular weight average. 

This determination is performed very easily with simple glass viscometers. Since the viscosity of a liquid depends markedly on temperature, viscosity measurements must be made at a carefully controlled temperature (within 0.1°C). Before a measurement, the viscometer is therefore equilibrated in a carefully controlled thermostatic bath at the required temperature. 

Figure 4.21 Glass viscometers, (a) Ostwald viscometer (b) Ubbelhode suspended-level viscometer (c) A modified suspended-level viscometer (see text for description).

The MW obtained in this way, M, is higher that and lower than sometimes My can reach values very close to My,. An advantage of obtaining this average MW with capillary viscosimetry is that the equipment used (the viscometer) is very inexpensive in comparison to those used in other sophisticated techniques, and the measurements of flow time are very simple the only drawback is the time consumed to prepare the samples at different concentrations and to run the samples in the glass viscometer, repeating the measurements a certain number of times. 

The viseosity of solvents can be determined by one of three methods glass viscometer, Saybolt viseometer, and bubble time method. Glass viscomelry is applicable to Newtonian, transparent liquids which because of volatility cannot be measured in conventional capillary viscometers. The viscometer uses a purge gas whieh helps to transfer the measuring liquid from a lower reservoir to the sample bulb. The time of flow is measured for a fixed volume of liquid at a temperature eonlrolledwifliapreeisionof 0.01"C. A set of liquids is available as viscosity standards in order to seleet the standard having closest viscosity to the measured sample. 

KNO3, LiNOs, NaOH, KOH, and LiOH solutions at temperatures up to 573 K and at pressures near the vapor pressure. They used nickel and glass viscometers. The uncertainty of the measured values of viscosity quoted is 1.5% for the glass viscometer and 1.0 to 1.5% for the nickel viscometer. The internal radius and length of the metallic capillary was 0.15 mm and 13.5 cm, respectively. 

Solution Polymers. Acryflc solution polymers are usually characterized by their composition, solids content, viscosity, molecular weight, glass-transition temperature, and solvent. The compositions of acryflc polymers are most readily determined by physicochemical methods such as spectroscopy, pyrolytic gas—liquid chromatography, and refractive index measurements (97,158). The solids content of acryflc polymers is determined by dilution followed by solvent evaporation to constant weight. Viscosities are most conveniently determined with a Brookfield viscometer, molecular weight by intrinsic viscosity (158), and glass-transition temperature by calorimetry. 

Fig. 24. (a) Ostwald glass capillary viscometer, (b) Cannon-Fenske viscometer, and (c) Ubbelohde viscometer. 

The Cannon-Fenske viscometer (Fig. 24b) is excellent for general use. A long capillary and small upper reservoir result in a small kinetic energy correction the large diameter of the lower reservoir minimises head errors. Because the upper and lower bulbs He on the same vertical axis, variations in the head are minimal even if the viscometer is used in positions that are not perfecdy vertical. A reverse-flow Cannon-Fen ske viscometer is used for opaque hquids. In this type of viscometer the Hquid flows upward past the timing marks, rather than downward as in the normal direct-flow instmment. Thus the position of the meniscus is not obscured by the film of Hquid on the glass wall. 

The Ubbelohde viscometer is shown in Figure 24c. It is particularly useful for measurements at several different concentrations, as flow times are not a function of volume, and therefore dilutions can be made in the viscometer. Modifications include the Caimon-Ubbelohde, semimicro, and dilution viscometers. The Ubbelohde viscometer is also called a suspended-level viscometer because the Hquid emerging from the lower end of the capillary flows down only the walls of the reservoir directly below it. Therefore, the lower Hquid level always coincides with the lower end of the capillary, and the volume initially added to the instmment need not be precisely measured. This also eliminates the temperature correction for glass expansion necessary for Cannon-Fen ske viscometers. 

For accurate and precise measurement the glass capillary must be clean. The viscometer must be cleaned thoroughly after each series of operations. Samples being tested and cleaning solvents should be filtered to remove particles that can clog the capillary. 

AH glass capillary viscometers should be caUbrated carefully (21). The standard method is to determine the efflux time of distilled water at 20°C. Unfortunately, because of its low viscosity, water can be used only to standardize small capillary instmments. However, a caUbrated viscometer can be used to determine the viscosity of a higher viscosity Hquid, such as a mineral oil. This oil can then be used to caUbrate a viscometer with a larger capillary. Another method is to caUbrate directly with two or more certified standard oils differing in viscosity by a factor of approximately five. Such oils are useful for cahbrating virtually all types of viscometers. Because viscosity is temperature-dependent, particularly in the case of standard oils, temperature control must be extremely good for accurate caUbration. 

More recent developments in the rolling ball area include an automated micro viscometer, the Paar AMV 200, from Paar Physica. The specimen to be measured is introduced into a glass capillary down which a gold-covered steel ball roUs. The rolling time is measured automatically. The shear stress may be varied by changing the inclination angle of the capillary tube. The shear rate range is 10 1000, which makes the instmment useflil for... 

In the Irvine-Park falling needle viscometer (FNV) (194), the moving body is a needle. A small-diameter glass or stainless steel needle falls vertically in a fluid. The viscous properties and density of the fluid are derived from the velocity of the needle. The technique is simple and useflil for measuring low (down to lO " ) shear viscosities. The FNV-100 is a manual instmment designed for the measurement of transparent Newtonian and non-Newtonian... 

Viscosity can also be determined from the rising rate of an air bubble through a Hquid. This simple technique is widely used for routine viscosity measurements of Newtonian fluids. A bubble tube viscometer consists of a glass tube of a certain size to which Hquid is added until a small air space remains at the top. The tube is then capped. When it is inverted, the air bubble rises through the Hquid. The rise time in seconds may be taken as a measure of viscosity, or an approximate viscosity in mm /s may be calculated from it. In an older method that is commonly used, the rate of rise is matched to that of a member of a series of standards, eg, with that of the Gardner-Holdt bubble tubes. Unfortunately, this technique employs a nonlinear scale of letter designations and may be difficult to interpret. 

Fig. 19.7. A rotation viscometer. Rotating the inner cylinder shears the viscous glass. The torque (and thus the shear stress aj is measured for a given rotation rate (and thus shear strain rate y).

Capillary viscometers are simple and inexpensive. They are normally constructed from glass and resemble a U-tube with a capillary section between two bulbs. The initial design originated with Ostwald and is shown as part A in Figure 3.2-1. The Cannon-Fenske type, a popular modification of the Ostwald design that moves the bulbs into the same vertical axis, is shown as part B in Figure 3.2-1. 

Calculations. For determination of the intrinsic viscosity [ti] the prepared pectins were solved in an 0.1 M phosphate buffer with pH 6.0. The relative viscosity was determined by a glass. Ubbelhode viscometer at 25 0.1 °C. The flow time of solvent (L) was 81.8 seconds. At least six pectin solutions with different concentrations were measured in a way that their flow times (ts) comply the order 1.2to[Pg.528]

The glass surface is then washed with buffer and finally an appropriate amount of buffer is added. The time of flow, as a function of amount of buffer in the viscometer is then determined. 

Oligomer and Film Characterization. Brookfield viscosity measurements were taken on a Model RVTD digital readout viscometer. Samples for Instron testing were prepared on glass plates using 25 or 75 pm (1.0 or 3.0 mil) Byrd film applicator. Coatings for cure speed and MEK double rub (MEKDR) studies were prepared on aluminum Q-Panels using a 40 wire wound rod (100 pm or 4.0 mil). 

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