VISCOMETERS AND THEIR USE Choice of a viscometer depends on fluid characteristics and viscosity range. This summaiy will guide the designer HENRY J .
KARAM
he measurement of viscosity and the science of T r h e o i o g y enter all phases of the chemical industry. Viscosity measurements may be used to solve the simplest practical problems of fluid flow or the most complex problems of determining molecular structure. The range of rheology is as wide as the range of materials that flow. It is not surprising that many methods of measuring viscosity have been devised to suit the materials and the purpose of the measurements. More and more interest is developing in the use of viscosity measurements for direct process control. Accuracy of the measuring device and efficiency of the
total measurement slatem have been improved to the degree where such an installation is often practical. I n such chemical processes as polymerization the advantages of a viscosity control over any other are obvious. I n its more everyday form, rheology is an old science. Ancient documents mention attempts to classify substances in rheological terms, in the writings of the Greeks, Egyptians, and Romans, and especially in Indian literature. The latter developed a very remarkable applied rheology school by the first century A.D. European studies were renewed by Leonard0 da Vinci, who studied the flow of water through orifices and channels.
Cabillary viscometers, available in many sizes to suit many materials, are used here for quality control of oil samples
Applications
The need for flow viscosity data for the design of processing equipment is common. The growth of the plastics industries has required extension of understanding of rheology for this purpose. Rheological data are used extensively to solve problems of measuring the ease of molding and extruding thermoplastic materials. Most of these materials are fabricated by techniques which apply heat and pressure. A thorough knowledge of their flow characteristics in terms of temperature and shearing stress is imperative. A knowledge of the latter variable is vitally important in that most thermoplastics are pseudo-plastic, Le., their viscosity is a function of shear stress. Rheological measurements assist in the study of degradation of high polymers. The experimental data gathering technique is quite simple. The results have much theoretical as well as practical significance. Other information, such as molecular weight and molecular shape of polymer molecules, can be derived from investigations of viscosity of dilute polymer solutions. A more complex field in rheological measurements is the study of time-viscosity behavior of materials. Materials exhibit many anomalies in flow behavior. People who study latex paints or printing inks are very much aware of the significance of this type of rheological measurement. Rheology has quite a few applications in the biochemistry field. The study of the flow of body fluids and properties of secretions, such as intra-ocular fluid and spinal fluid, is growing in importance. The theoretical prediction of viscosity of dilute solutions has received much recent interest. Einstein’s doctoral dissertation describes such prediction of the viscosity of dilute sugar solutions as a function of concentration. Much has been published on prediction of rheological properties of dilute solutions of high polymers. Problems in Viscosity Measurement
No single instrument should be chosen without careful consideration of the range of measurement needed and the type of flow of the material. Materials may range in viscosity from a fraction of a poise to billions of poises. Typical examples are: hydrogen, poise ; water and alcohol, oils, varnishes, and paints, 10 to lo2; fats and greases, 10 to 108; resins and gums, lo3 to 109; pitches, asphalts, thermoplastics, lo4to 10l2or greater. The range of viscosity encountered with a given material with temperature change, for example, creates another problem which necessitates making a series of measurements with several different instruments in order to sufficiently characterize the material. Choice of measuring instrument also depends on type of flow. Viscosity of some materials is independent of shear rate, that of others will vary-up or down-not only with shear rate but also as a function of duration of shear application at a given level. Still other materials require a finite force before they will flow. Viscosity, the change of viscosity with shear and time,
A sptcial semimicro dilution viscometer, also a ca;billary instrument, is used to determine molecular weight of polymers
PROCESS VISCOMETERS-FOR MEASUREM E N T AND CONTROL Name
Principle
Hallikainen
Measures the back pressure required to extrude material at constant rate
Rotational
Brookfield
Measures the torque required to rotate a disk or cylinder in a viscous medium
Falling slug
Nacross
Measures rate of fall of slug in a tube filled with a viscous medium
Capillary -
Fischer 8i Porter Measures the height a bob Rotameter rises in a rotameter tube. Material is pumped through the tube at constant rate
Vibrating type Bendix Ultra Viscoson
Dynatrol
Measures damping of an ultrahigh frequency signal applied to a probe Measures damping of a paddle moving in shear in the liquid. The paddle vibrates at 60 cycles
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and yield point are not the only parameters that characterize a fluid. Most complex fluids have elastic properties, that is, they are viscoelastic (2). This is especially true of polymer melts, polymer solutions, latex systems, elastomers, and disperse systems. The elastic properties of fluids result in two problems. First, they complicate the technique of determining the viscosity of fluids, and second, they change the type of measurement that is needed to characterize these properties adequately. Another important rheological phenomenon observed when shearing complex fluids is the M'eissenberg ( 4 ) or normal stress effect. These secondary stresses are a function of the applied shearing stress and further complicate the problem of measuring the rheological properties of fluids. Numerous practical problems also arise in viscometric measurements. One must consider calibration of the instrument, ease of operations, methods of accurately
measuring and recording shear and shear rate variables, temperature control, and sample size requirements. Viscometers which measure rheological properties of disperse systems must be made such that they prevent the separation of the two phases. Finally, a viscometer must be capable of operating over a wide range of shear and temperature if one wishes to correlate viscometric data with practical tests. Elastic effects and temperature rise, due to shearing, limit the range of instruments. The theoretical and practical problems associated with modern viscometry illustrate the complexity of the science. The various instruments available to handle the problems that confront researchers are discussed in the next section. It is hoped the lists given here will aid the researcher in selecting the proper instrument. Almost as important as selecting the proper instrument is that the researcher be acquainted hvith the rheological terms ( I , 3) describing fluid properties. References cited have excellent glossaries of rheological definitions.
G U I D E T O VISCOSITY MEASUREMENT Instrument
Viscosity Range, Pcise
Area of Use
10 -2-1 02 10 -2-1 0 2 10 -2-1 02
Control testing
CAPILLARY VISCOMETERS Bingham Absolute Ostwald Cannon Fenske Modified Ostwald Kinematic Ubbelohde Suspended Level Triple Capillary Bingham and Murry Capillary Rise
10 - ~ 1 0 4 10 - ~ 1 0 4 10-105
Bingham Hi Pressure Caplastometer Consistometer Instron Rheometer Melt Indexer
102-1 012 102-105 102-1 05 102-1 0'2 102-1 05
Thermoplastics
Pressure Viscometer Single Pass Capillary Viscometer
1-102 10 -2-1 04
Lubrication oil study and control testing
Capillary Viscometer with continuously varying pressure head
10 -2-1 02
Latex evaluation
Efflux Type Saybolt Redwood Engler Barbey Zahn Ford Cup
10-L10
Oils, paints, and varnishes.
10 -2-1 05 10 -4-1 O 5 10 -2-1 104-1 0 5 1-103
Where low shear viscosity is needed Study of effect of hydrostatic pressure on viscosity Gases, lubricating greases, polymer solutions and melts Adaptable to instream measurements
102-103
Study of biological fluids
Also for control testing
FALLING SPHERE VISCOMETERS Falling Sphere Hoeppler Rolling Ball Falling Cylinder Falling Coaxial Cylinder Forced Ball Viscometer SPECIAL INSTRUMENT
Falling Ball Micro Viscometer 40
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Instrument ROTATIONAL VISCOMETERS Garrison Stormer Precision Interchemical MacMichael Open End Rotating Viscometer Conicylindrical Viscometer Hi Shear Coaxial Cylinder Viscometer High Consistency Viscometer Cone and Plate Brabender Recording Viscometer
Viscosity Range, Poise
Area of Use
10 -2-1 0 1-108 10-103 1O"106 104-108 103-1 o g 103-109
Wide range Time viscosity effects Yield stresses of materials Study of viscoelastic properties
10-2-6 X lo6
SPECIAL INSTRUMENTS
Brookfield Synchro Lectin Ferranti Portable
10-2-106 10 -2-1 06
Instruments are portable
Epprecht Viscometer Haake Rotating Viscometer
10-2-105 1o -8-1 04
Wide temperature range
- 60
C. to 2000 C.
Kingsbury Tapered Plug Viscometer Hi Shear Low Viscosity Hi Shear Self Aligning Coaxial Cylinder Viscometer
Study of lubrication oils and paints
Double Cone Viscometer Mooney Viscometer
Rubber
Squibb Viscometer Precision Low Shear Viscometer
=IO-2 h.10-2
Liquids of low viscosity and yields (latex, blood)
Band Hi Shear Viscometer
5-1000
Printing inks
Sliding Plate Viscometer
102-1 0'2
Asphalts
Brabender Plastograph
IO"10'2
Plastic resin over the range of processing conditions
~
SPECIAL INSTRUMENTS
Rheogoniometer Viscodastomer Stress Relaxation Meter Coaxial Cylinder Viscometer
102-106 102-1 06 IO"106
VIBRATIONAL VISCOMETERS Bendix Ultra Viscoson Electrochemical Transducer
1-103 1-103
Weissenberg or normal stress effect Flow and time dependent elastic recovery (viscoelasticity)
Lend themselves to in-stream process instruments
Accurate measurements of low viscosity material
Torsional Crystal Viscometer Vibrating Plate
10 -8-1 03
Viscoelastic properties can be determined
Oscillating Sphere and Cylinder Oscillating Disk
10-7 10-7
Viscosity of gases
Surface Viscometers
Rheological properties of surface films
SPECIAL INSTRUMENTS
Double Transducer
Viscoelastic properties of gels and solids
COMPRESSION OR EXTENSION VISCOMETERS Parallel Plate Viscometer 104-109 Penetration Viscometer 104-1 09 Stretch viscometer 104-109
Thermoplastics and inorganic glass. Small sample. Wide temperature range. Viscoelastic properties of polymers
S P E C I A L INSTRUMENTS
Vicat Needle Flow 'Table
Cement and gypsum plaster
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TYPES OF VISCOMETERS Viscometers can be classified according to the principles employed in making the measurement. The six methods used in commercial units measure the following:
Rate of flow af fluids under pressure through tubas. Instruments employing this principle con measure a wide range of viscosities, IO-' to 101%poises, and are probably the most widely accepted type in the chemical industry. Typical examples of such instruments ore Ostwold, Ubbelohde, Bingham, and Caplastometer. Rate of molion of a solid article through a viscous medium. The falling boll viscometer, the Hoeppler Rolling Sphere viscometer, and the coaxial cylinder viscometer are typical. This type of instrument can measure viscosity at very low shear rote. Rate of rotation of a solid shape in a viscous medium when a known force is applied. The Open End and the Stromer Rotating Cylinder viscometer are examples. Advantages of this type of instrument ore simplicity and ease of cleaning, important in control testing.
Torque required ta rotate a solid shape at a definite angular velocity in a viscous medium. Cone-Plate viscometer, and lnterchemicol and MocMichoel rotating torsional viscometers ore typical of this method. These instruments are very popular with investigators who ore studying time-viscosity effects or determining the yield stress of fluids. They are used to measure Weissenberg or normal stresses and viscoelastic properties. Damping &et of a medium on a solid shape vibrating in the medium. These viscometers lend themselves to on-stream measurements of viscosity. In recent years, they have found new applications in t& study o f dynamic or viscoelastic properties of materials. The Bendix Ultro Viscoson i s an example.
Distoltion of a viscous medium under compression ar tension. These instruments lend themselves quite readily to measurement of the viscosity of very viscous materials. They con also be used t o determine viscoelastic properties of high polymers. Typical examples are the flat plate and plunger viscometers.
The table (page 40) summarizes important features of the most popular types of viscometen. The l i t is far from complete. Many labs employ viscometers which are not available commercially and which have not been described in the literature. The ranges and areas of use given here should guide the designer. Requirements ei a Pmceas Visromeier
Pmem viscometers must be chosen as carefully as the laboratory units, on the basis of the job to be done. Additional requirements of the plant make the instruments more complicated. The instrument must be reliable. This includes the usual requirements for reliability, simplicity, and low 42
INDUSTRIAL A N D ENGINEERING CHEMISTRY
maintenance cost. Provisions must be made within the instrument for periodic checking and adjustment. One must consider the range of viscosity to be measured in 'designing an instrument. This factor will largely dictate the type which will be used. There must also be sufficient flexibility to change the range of the instrument readily. The instrument must be of reasonable size. The designer must know the size of vessel in which the visulmeter will be installed and whether it will be a pressurized or an open reactor. Each of these has an important bearing on the final design. The last important consideration in design is the choice of material of construction. The viscometer must not
CONTROLLER
CONTROLLER
€3 I >-,“ POLYMERIZER
:
’
AI,
DEVOLATILIZER
‘b
PRODUCT
Example I: Controlling a polymerization reaction. The figure shows a schematic Jowsheet of a mass polymeritation process. The system consists of a reactor which is being fed with monomer continuously by a pump. The reactor must serve two purposes-it mixes and polymerizes the mixture. Partial polymer is removed from the reactor and fed into a devolatilizer. The unreacted monomer is removed, condensed, and fed back into the system. A fruid viscosity recorder controller is installed in the polymerization vessel. The controller controls the feed pump. I t is set to maintain the per cent solids in the reactor by continuously measuring the solution viscosity
1
TANK A
I
I
TANK B
I
I
Questions that frequently arise in viscometer instal lation are: - Is the sensing unit measuring material representative of production? -1s there a positive flow in the unit? -1s there hold-up in the instrument which might develop into a source of contamination? Contamination will impair the precision and usefulness of the instrument. An important consideration in installation of a viscometer is temperature control. The engineer is faced with three alternatives. He can accept the temperature at which the viscosity is measured and then correct to a datum temperature. Secondly, he can design temperature compensation into the instrument. Both of these choices imply a knowledge of the viscosity-temperature relationship. I n practice, this is not often the case-it may be very difficult to obtain such data. The last alternative, which is the most satisfying, is to condition the material to a predetermined temperature prior to making a measurement. A series of more minor considerations may spell success or failure : -Designer must predict viscosity from geometry of the instrument. -Linear relationship should exist between viscosity and chart reading. -Explosion hazards must be prevented in installation of instruments. -Measurement should be recorded remote from the source. -Readings must be continuous or at least made frequently. The requirements discussed are general. Each particular problem must be judged individually. LITERATURE CITED
L+i
(1) Brodkey, R. S., IND.ENG.CHEM. 54,44 (September 1962). (2) Ferry, J. D., “Viscoelastic Properties of Polymers,” Wiley New York, 1961. (3) Verde, C., “Glossary of Rheological Terms,” Drage Products, Box 330, Union City, N. J. (4) Weissenberg, K., Nature 159, 310 (1947).
VISCOMETER ~
PRODUCT
Example 11: Mixing operation.
A typical batch mixing operation involves the combination of two batches of material of dgsrent viscosities. The viscometer controller is set to maintain the viscosity of the final mix by opening or closing valves A and B
corrode or degrade or impart color to the material being processed. Installation of the instrument introduces additional considerations. The engineer must first decide whether he must have an in-stream or side-stream unit. The former has the advantage of reducing lag time and eliminating waste.
H. J . K a r a m is the Group Leader of Dow Chemical Company’s Plastics Fundamentals Group, in M i d l a n d , M i c h .
AUTHOR
BACKG ROU ND L ITERATU RE Alfrey, T., “Mechanical Bchavior of High Polymers,” Interscience, New York, 1948. Eirich, F., “Rheology,” Vols. I, 11,111, Academic Press, New York, 1956. Green, H., “Industrial Rheology and Rheological Structure,” Wiley, New York, 1949. Reiner, M., “Deformation and Flow,” H . K. Lewis, London, 1949.
A complete bibliography of 99 rcferences describing the instruments mentioned in this article has been developed by the author. For copies of this list, please address requests to : T H E EDITOR INDUSTRIAL AND ENGINEERING CHEMISTRY 1155 Sixteenth St., N.W. Washington 6, D. C.
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