Measurement viscosity

These experiments are great at picking out mechanical properties of materials subjected to single axial stresses. To study the effect of cyclic stresses, a dynamic mechanical analyzer (DMA) is recommended (next section). 

As noted in Chapter 5, viscometry can be used to estimate polymer molecular weight,M3 but those solutions were required to be dilute. For polymer melts and more concentrated solutions, viscosity measurements require viscometers configured somewhat differently from the capillary viscometers introduced in Chapter 5. Details of such instruments and procedures have been reviewed [2,3]. 

Example 16.2 Obtain the expressions for shear rate and shear stress in a cone-and-plate viscometer in terms of the rate of cone rotation (o, the measured torque M, and the geometry. 

Note Here, t is constant, and can be taken outside the integral, because t = t (y.), and y. is constant, as shown by Equation 16.4. 

Cone-and-plate viscometers of the type shown here are usually limited to fairly low shear rates. At higher shear rates, solutions tend to be flung from the gap by centrifugal force, and melts tend to ball up (like rubbing a finger over dry mbber cement). These problems can be overcome by enclosing the fluid around a biconical rotor, giving, in effect, two cone-and-plate viscometers back-to-back [6]. The flow curves earlier in this chapter were obtained with such a device. 

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Certain calibrated orifice instruments (Engler-type) provide viscosity measurements at temperature lower than pour point. This is possible because the apparatus agitates the material to the point where large crystals are prevented from forming whereas in other methods, the sample pour point is measured without agitation. 

Emsley, in the same paper referred to in Computer Project 3-1, presents viscosity measurements T] for solutions of KE in HOAc as a function of molality m with the following results... 

Strauss and Williamst have studied coil dimensions of derivatives of poly(4-vinylpyridine) by light-scattering and viscosity measurements. The derivatives studied were poly(pyridinium) ions quaternized y% with n-dodecyl groups and (1 - y)% with ethyl groups. Experimental coil dimensions extrapolated to 0 conditions and expressed relative to the length of a freely rotating repeat unit are presented here for the molecules in two different environments ... 

A number of technically important viscosity-measuring devices may be thought of as variants of this basic apparatus. 

Figure 2.5 shows some actual experimental data for versus 7, measured on a sample of polyethylene at 126°C. Note that the data are plotted on log-log coordinates. In spite of the different coordinates. Fig. 2.5 is clearly an example of pseudoplastic behavior as defined in Fig. 2.2. In this and the next several sections, we discuss shear-dependent viscosity. In this section the approach is strictly empirical, and its main application is in correcting viscosities measured...

Once the value of the constant and the a value in Eq. (2.36) have been evaluated for a particular system, viscosity measurements constitute a relatively easy method for determining the molecular weight of a polymer. Criticize or defend the following proposition Since viscosity is so highly dependent on molecular weight for M > M, a 10% error in 17 will result in a 34% error in M above M, but only a 10% error in M below M, . 

Rheology. Flow properties of latices are important during processing and in many latex appHcations such as dipped goods, paint, inks (qv), and fabric coatings. For dilute, nonionic latices, the relative latex viscosity is a power—law expansion of the particle volume fraction. The terms in the expansion account for flow around the particles and particle—particle interactions. For ionic latices, electrostatic contributions to the flow around the diffuse double layer and enhanced particle—particle interactions must be considered (92). A relative viscosity relationship for concentrated latices was first presented in 1972 (93). A review of empirical relative viscosity models is available (92). In practice, latex viscosity measurements are carried out with rotational viscometers (see Rpleologicalmeasurement). 

Molecular Weight. The molecular weight of polypropylene is typically determined by viscosity measurements. The intrinsic viscosity [Tj] of the polymer in solution is related to the molecular weight, Af, by the Matk-Houwink equation ... 

Analytical and test methods for the characterization of polyethylene and PP are also used for PB, PMP, and polymers of other higher a-olefins. The C-nmr method as well as k and Raman spectroscopic methods are all used to study the chemical stmcture and stereoregularity of polyolefin resins. In industry, polyolefin stereoregularity is usually estimated by the solvent—extraction method similar to that used for isotactic PP. Intrinsic viscosity measurements of dilute solutions in decahn and tetraHn at elevated temperatures can provide the basis for the molecular weight estimation of PB and PMP with the Mark-Houwiok equation, [rj] = KM. The constants K and d for several polyolefins are given in Table 8. 

Resoles. Like the novolak processes, a typical resole process consists of reaction, dehydration, and finishing. Phenol and formaldehyde solution are added all at once to the reactor at a molar ratio of formaldehyde to phenol of 1.2—3.0 1. Catalyst is added and the pH is checked and adjusted if necessary. The catalyst concentration can range from 1—5% for NaOH, 3—6% for Ba(OH)2, and 6—12% for hexa. A reaction temperature of 80—95°C is used with vacuum-reflux control. The high concentration of water and lower enthalpy compared to novolaks allows better exotherm control. In the reaction phase, the temperature is held at 80—90°C and vacuum-refluxing lasts from 1—3 h as determined in the development phase. SoHd resins and certain hquid resins are dehydrated as quickly as possible to prevent overreacting or gelation. The end point is found by manual determination of a specific hot-plate gel time, which decreases as the polymerization advances. Automation includes on-line viscosity measurement, gc, and gpc. 

PPS is well-recognized for its exceptional chemical resistance. There are no known solvents for PPS below 200°C. A comprehensive survey of solvents for PPS has been published (115). Extreme conditions are required to dissolve PPS in both common and exotic solvents. Solution viscosity measurements are made difficult by this high temperature requirement. Inherent viscosity measurements are performed in 1-chloronaphthalene at 206°C at a concentration of 0.4 g of polymer per deciliter of solution. The inherent viscosity of PPS solutions shows a usefiil response to increa sing molecular weight. Table 2 shows a correlation of inherent viscosity measurements with melt flow measurements. 

Effect of Temperature. In addition to being often dependent on parameters such as shear stress, shear rate, and time, viscosity is highly sensitive to changes in temperature. Most materials decrease in viscosity as temperature increases. The dependence is logarithmic and can be substantial, up to 10% change/°C. This has important implications for processing and handling of materials and for viscosity measurement. 

Extrapolation to infinite dilution requites viscosity measurements at usually four or five concentrations. Eor relative (rel) measurements of rapid determination, a single-point equation may often be used. A useful expression is the following (eq. 9) (27) ... 

Cone—Plate Viscometer. In a cone—plate viscometer (Fig. 28), alow angle (<3°) cone rotates against a flat plate with the fluid sample between them. The cone—plate instmment is a simple, straightforward device that is easy to use and extremely easy to clean. It is well suited to routine work because measurements are rapid and no tedious calculations are necessary. With careful caUbration and good temperature control it can also be used for research. Heated instmments can be used for melt viscosity measurements. 

Controlled Stress Viscometer. Most rotational viscometers operate by controlling the rotational speed and, therefore, the shear rate. The shear stress varies uncontrollably as the viscosity changes. Often, before the stmcture is determined by viscosity measurement, it is destroyed by the shearing action. Yield behavior is difficult to measure. In addition, many flow processes, such as flow under gravity, settling, and film leveling, are stress-driven rather than rate-driven. 

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. 

Eig. 34. Diagram of the Rheometric Melt Elongational Rheometer (RME) for elongational viscosity measurements on polymer melts. 

Viscoelastic Measurement. A number of methods measure the various quantities that describe viscoelastic behavior. Some requite expensive commercial rheometers, others depend on custom-made research instmments, and a few requite only simple devices. Even quaHtative observations can be useful in the case of polymer melts, paints, and resins, where elasticity may indicate an inferior batch or unusable formulation. Eor example, the extmsion sweU of a material from a syringe can be observed with a microscope. The Weissenberg effect is seen in the separation of a cone and plate during viscosity measurements or the climbing of a resin up the stirrer shaft during polymerization or mixing. 

Procedures for determining ash, moisture, solution preparation, and viscosity measurements can be found in manufacturers product bulletins (40,41) andin ASTMD2364-69 (65). 

Procedures for determining the ash content and moisture level, solution preparation, and viscosity measurement techniques are given in the manufacturer s hterature (50). 

When viscometric measurements of ECH homopolymer fractions were obtained in benzene, the nonperturbed dimensions and the steric hindrance parameter were calculated (24). Erom experimental data collected on polymer solubiUty in 39 solvents and intrinsic viscosity measurements in 19 solvents, Hansen (30) model parameters, 5 and 5 could be deterrnined (24). The notation 5 symbolizes the dispersion forces or nonpolar interactions 5 a representation of the sum of 8 (polar interactions) and 8 (hydrogen bonding interactions). The homopolymer is soluble in solvents that have solubility parameters 6 > 7.9, 6 > 5.5, and 0.2 < <5.0 (31). SolubiUty was also determined using a method (32) in which 8 represents the solubiUty parameter... 

Miller, G.H., and Ahrens, T.J. (1991), Shock-Wave Viscosity Measurement, Rev. Modern Phys. 63, 919-948. 

ISO viscosity number. Table 12.1 compares typical correlations between number and weight average molecular weights with ISO numbers, AT-values and ASTM viscosity measurements. 

The results of viscosity versus shear rate are reported in Fig. 11 for the two pure components and their blend, respectively. The temperatures were the same for the viscosity measurements and for the injection molding. At temperatures of 280°C and 320°C, the viscosities of the blend are found to be values between the limits of the two pure components. In both cases, the TLCP still... 

ASTM D-2502 is one of the most accurate methods of determining molecular weight. The method uses viscosity measurements in the absence of viscosity data, molecular weight can be estimated using the TOTAL correlation. 

ASTM, Standard Test Method for Estimation of Molecular Weight (Relative Molecular Mass) of Petroleum Oils from Viscosity Measurements, ASTM Standard D-2502-92, 1992. 

Estimation of Molecular Weight of Petroleum Oils from Viscosity Measurements... 

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