Hercules Polymers Viscosity

BULLETIN VC-453C(Supersedes VC-453B)

Rheology ofAQUALON® Water-Soluble Polymers in Solution

AQUALON® water-soluble polymers are used to thicken, suspend, stabilize, gel, solidify, or in otherways modify the flow characteristics of water or other solvents or solutions. For most established uses,detailed information on these effects is available in other Aqualon bulletins specific to the product and/orthe use.

For newer uses, however, where development and formulation are still in process, a broader under-standing of the behavior of these polymers in solution is needed. Viscosity, thixotropy, dilatancy, elasticity,pseudoplasticity, and viscoelastic behavior become important areas for study.

The purpose of this bulletin is to define and characterize these areas and to illustrate their interrelation-ships, using specific data on selected Aqualon water-soluble polymers. And, since so many of the uses forthese hydrocolloid polymers involve a wide range of shearing conditions in both preparation and applica-tion, which has a marked rheological effect on behavior, it is important for the user to know how the systemwill respond.

Rheology is the science of the deformation and flow of matter when subjected to an applied force. Themagnitude of this applied force may range all the way from the gravitational force on a single, small, suspended particle to the very high shear rates encountered in high-speed mixing or homogenization.

For water itself, for the common solvents, and for noninteracting liquid systems and solutions wherethe dissolved material is low in molecular weight, nonassociating, and with limited solute-solvent inter-action or solvation, the characterization of flow is simple. Flow is directly proportional to the force applied,and the system is said to be Newtonian.

More complex solutions, however, tend to respond in a nonlinear manner to applied stress. Here, thedissolved or solvated molecules are large, the tendency to entangle and/or reassociate is high, and the sol-vent must exert some solvating force to maintain the polymer in solution. Such solutions are classified asnon-Newtonian. Since solutions of Aqualon water-soluble polymers are of this latter type, with nonlinearflow response, their rheological characterization has become an important part of Aqualon technology.

Hercules IncorporatedAqualon DivisionHercules Plaza1313 North Market StreetWilmington, DE 19894-0001(800) 345-0447www.aqualon.com

Technical Information

The products and related information provided by Hercules are for manufacturing use only. Hercules makes no express, implied, or other representation, warranty, or guarantee concerning (i) the handling, use,or application of such products, whether alone, in combination with other products, or otherwise, (ii) the completeness, definitiveness, or adequacy of such information for user’s or other purposes, (iii) the quality of such products, except that such products are of Hercules’ standard quality. Users are advised to make their own tests to determine the safety and suitability of each such product or product combination for theirown purposes. Read and understand the Material Safety Data Sheet (MSDS) before using this product.Hercules does not recommend any use of its products that would violate any patent or other rights.

HER. 41281 PRINTED IN U.S.A.

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CONTENTS

Page

I. BASIC CONCEPTS OF FLOW AND DEFINITION OF TERMS . . . . . . . . . . . . . . . . . . . . . . 3Newtonian Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Non-Newtonian Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

II. HOW VISCOSITY IS MEASURED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Capillary Viscometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Rotational Viscometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Falling-Sphere and Bubble Viscometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Vibrational Viscometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

III. FLOW CHARACTERISTICS OF AQUALON®

WATER-SOLUBLE POLYMER SOLUTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

IV. THIXOTROPY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

V. ELASTICITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figures

Figure 1. Basic Concepts of Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Figure 2. Different Ways of Plotting Newtonian Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Figure 3. Types of Non-Newtonian Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Figure 4. Viscosity of Several Aqualon® Polymers at Various Concentrations . . . . . . . . . . . . 9Figure 5. Shear Stress vs. Shear Rate for Aqualon® CMC and Natrosol® HEC . . . . . . . . . . . 10Figure 6. Effect of Shear on Viscosity of Natrosol Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 10Figure 7. Aqualon® CMC in Various States of Aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . 11Figure 8. Effect of Polymer Disaggregation on Viscosity of the System . . . . . . . . . . . . . . . . 12Figure 9. Effect of Solutes on Viscosity of Aqualon® CMC in Solution . . . . . . . . . . . . . . . . . . 12Figure 10. Rheograms for Thixotropic Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Figure 11. Effect of High Power Input on Thixotropic Structure . . . . . . . . . . . . . . . . . . . . . . . . 14Figure 12. Rheograms of a Thixotropic Aqualon® CMC Solution Containing Some

Gel Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Figure 13. Viscoelastic Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

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BASIC CONCEPTS OF FLOW AND DEFINITION OF TERMSWhen a force is applied to a system, the system may respond in a number of ways. One of these

responses is to relieve the strain by flowing, in which case the system is said to be a liquid. The system willresist this imposed flow to a greater or lesser degree, or it would have no original form at all. Viscosity isthe internal friction of a flowing material and measures the tendency of the liquid to resist the appliedshearing force. Thus, viscosity is a property of all material capable of flowing.

The basic concepts of flow are best understood by reference to Figure 1.

Figure 1Basic Concepts of Flow

This drawing shows a cross-section normal to two parallel planes (1 and 2) separated by distance (x),the space in between the planes being filled with liquid. A constant force (F) is applied to the top layer havingarea (A), not shown, sufficient to maintain the top layer moving with velocity (Y). The unit force applied isthe shear stress (S) and may be defined as S = F/A, in dynes/cm2 (dyne is the force necessary to give anacceleration of 1 cm/sec/sec to 1 g of mass). The liquid between the two planes obviously flows at variousspeeds, depending on its distance from plane 2. Thus, the layer next to 2 at point (b) has zero velocity,while the layer next to 1, at point (a), moves with the velocity of (Y). Intermediate layers move at interme-diate velocities. The velocity gradient across the liquid is Y/x. This is the response of the system and iscalled the shear rate or rate of deformation (D). Thus, YD=—(sec-1).(1)

xThe ratio of the shear stress (S) to the shear rate (D) is the coefficient of viscosity (η). Thus, η = S/D,

in poises, where 1 poise = 1 dyne-sec/cm2. Thus, the coefficient of viscosity, usually referred to simply asviscosity, may be regarded as a measure of the force per unit area required to maintain a given rate of flow.As will be discussed in the next section, shear stress and shear rate are not measured directly, but arederived from measurable quantities such as seconds of flow or fall, rpm, scale readings, etc.

(1)This expression is true only for fluid flows with a linear velocity gradient.


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Typical examples of expressed viscosity are:

Matter Viscosity, poises

Air 0.0001Water 0.01Oils 1-1,000Resins 1,000-1,000,000

Newtonian FlowAs pointed out in the introduction, viscous flow can be either one of two types, Newtonian or non-

Newtonian. When Newtonian flow occurs, the coefficient of viscosity (S/D) is constant even though theshear stress and the shear rate vary. Another way of expressing this is: When the viscosity is measured atvarying shear rates and shear stresses and the results are plotted, the straight line thus formed passesthrough the origin. The slope of the line is constant, as shown in Figure 2. Examples of Newtonian liquidsare dilute salt and sugar solutions, glycerin, and light oils.

Figure 2Different Ways of Plotting Newtonian Flow

Non-Newtonian FlowIn many systems, however, the response to shear stresses is not linear. The viscosity for these systems,

then, is not a constant value. It varies, depending on the shear stress and the rate of application of stress.This results in non-Newtonian flow; different types are shown in Figure 3, page 5.

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Figure 3Types of Non-Newtonian Flow

Pseudoplastic (associated with the presence of swollen particles, solvation, and aggregation such asemulsions and polymer solutions whose long-chain molecules are oriented by the flow).

Dilatant (occurs in suspensions containing large amounts of solids and associated with deflocculationand close packing of these solids, such as icings).

Plastic (associated with the properties of a system requiring the application of at least a minimumamount of shear stress before flow begins. Catsup is an example).

As is shown, the viscosity of pseudoplastic materials decreases, and that of dilatant materials increases,with the rate of shear (D). Consequently, for the viscosity of non-Newtonian systems to be meaningful, therate of shear must be specified. The viscosity of pseudoplastic and dilatant materials is generally referred toas the apparent viscosity (ηapp.). Plastic systems have a yield point—that is, a certain amount of stress mustbe applied to the system before flow occurs.

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II. HOW VISCOSITY IS MEASUREDAs has been mentioned, shear stress and shear rate are not measured directly, but are derived from

measurable quantities such as rate of flow, torque, rpm, etc. Numerous instruments exist for making these measurements.

Capillary ViscometersIn this method a liquid is forced, or allowed to flow, through a fine-bore tube, and the viscosity is deter-

mined from the flow rate, pressure applied (gravitational or otherwise), and tube dimensions. The mainadvantages of this type of viscometer are simplicity, ease of operation, wide range, and precision. Examplesare Ubbelohde, Instron, Saybolt, and Zahn viscometers.

Rotational ViscometersA rotating body experiences a viscous drag, or retarding force, the amount of which varies with the

speed of rotation. In rotational viscometers, the viscosity is determined by measuring the drag on a spindlerotating in the material. The chief advantages of these instruments(2) are:

• They are simple to use.• Continuous measurements can be made at a given rate of shear or stress.• The dependency of viscosity on time can be readily determined.• Yield stresses can be determined.

Examples are the Brookfield Synchro-Lectric, Rheometrics, Stormer, MacMichael, Bohlin, Haake, andBrabender viscometers.

Falling-Sphere and Bubble ViscometersIn these instruments, the time required for a sphere of some sort to pass through a liquid is measured.

The sphere may be a falling ball or a rising bubble. This method is particularly good for low-shear measure-ments. Examples are the Hoeppler rolling-ball viscometer and the Gardner-Holdt comparative-bubble tubes.

Vibrational ViscometersIn these instruments, the damping of a blade that vibrates at high frequency is measured. They are useful

in measuring low viscosities and changes in rheological properties during processing, as well as determin-ing viscoelastic properties. The shear rate, however, cannot be readily changed. An example is the BendixUltra-Viscoson.

MiscellaneousOther such instruments include compression or extension viscometers, penetrometers, and parallel-

plate viscometers.

(2)Other rotational viscometers include bob-and-cup, cone-and-plate, and parallel plate.

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III. FLOW CHARACTERISTICS OFAQUALON® WATER-SOLUBLE POLYMER SOLUTIONSOne of the important characteristics of pseudoplastic systems is the variation of viscosity with concen-

tration. Figure 4, page 9, shows the effect of concentration on the viscosities of solutions of a number ofcellulosics measured with a rotational viscometer (Brookfield Synchro-Lectric). The same types of curvesare obtained for all these pseudoplastic materials.

One way of studying the flow behavior is to determine shear stress at various shear rates, as shown inFigure 5, page 10. Here the shear stress (S) is plotted versus the shear rate (D). If these solutions had beenNewtonian, straight lines would have resulted. However, these solutions deviate from a straight line, thedeviation being greater at high shear stresses and shear rates.

A more useful way of picturing the flow behavior of water-soluble polymers is shown in Figure 6, page 10,where the apparent viscosity of hydroxyethylcellulose is plotted versus shear rate. Remember that shearrate is the response to an applied stress. The data are taken from a plot similar to that shown in Figure 5.Since a Newtonian system would give a straight line of zero slope, it is seen that the low-viscosity materialis less pseudoplastic than the high. This is true of other cellulose derivatives as well. This figure also showshow the shear rate varies with use. Normally, the viscosity is measured at an intermediate shear-rate rangeusing, for example, a Brookfield rotational viscometer. However, as seen in Figure 6, the differentiationobtained by “Brookfield measurements” may not hold under use conditions. Thus, if high viscosity at highshear rate is wanted, it might be desirable to use a type of gum designated as “low viscosity” when measuredin the intermediate shear range.

As discussed earlier, the performance of pseudoplastic solutions is associated with the presence ofswollen particles (gel centers), solvation, and aggregation. Cellulosics in solution may exist in variousstates of aggregation, and the possible states are illustrated in Figure 7, page 11.

When Aqualon cellulose gum or, in fact, any solid polymer is added to a liquid and brought to equilib-rium, the powder may do several things: remain as a suspended powder (Figure 7A), swell to a point whereall the liquid is imbibed into the particles (7D), or dissolve completely into its individual molecules (7F).

The effect on viscosity of these various states is illustrated in Figure 8, page 12. The capacity of the liquid (which can be a single substance or a mixture) to act as a solvent is one of the important factors indetermining the state of the cellulose gum—that is, its position on this aggregation-disaggregation curve.Thus, if the cellulose gum neither swells nor dissolves, it is in State I. In State II, we have maximumswelling or maximum solvent imbibition, and thus maximum viscosity. Here the discrete gel particlesoccupy almost the enire volume of the system. In going from State I to State II, some, but not all, of theinternal associations are broken. As we go from State II to State III, more of the internal associations arebroken; the particle becomes less readily deformed and begins to approach molecular dispersion. In StateIII, the molecule is completely dispersed. The flow properties of cellulosics will thus vary with their degreeof aggregation. Aggregation, in turn, is affected by the nature and composition of the solvent, the composi-tion of the polymer, and the shear history of solution (see Section IV, page 13).


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A number of factors enter into the performance of Aqualon® cellulose gum when it is added to a solvent.The solvent can attach itself to the cellulose gum through hydrogen bonding or through association withpolar or ionic groups of the polymer. The greater the solvation, the greater the tendency of the cellulosegum to disaggregate. In addition, the solvent may associate the mobile counterions derived from an ioniccellulose gum, thereby reducing the energy required to separate the counterion from the vicinity of thepolymeric ion and promoting dispersion. The use of poor solvents or the presence of salts maintains theseareas of interpolymer association (the so-called “crystalline areas”) and inhibits dispersion.

The effects of solutes such as salts or polar nonsolvents on a type of carboxymethylcellulose (CMC)containing many of these areas is shown in Figure 9, page 12. If the CMC is thoroughly dissolved in waterand then the solute is added, the solute has only a small effect on the viscosity. However, if the solute is dis-solved before the CMC, the solute inhibits breaking up of the crystalline areas and lower viscosities areobtained. This effect of solutes is less noted with more uniformly substituted material containing fewercrystalline areas.

Solution rate and molecular dispersion of cellulosics are enhanced by several factors:

Degree of Substitution (DS): The higher the degree of substitution, the more readily a cellulosic willdissociate or disaggregate. Thus, CMC of DS 1.2 will disaggregate more completely than CMC of DS 0.4,since it contains fewer crystalline areas and also a larger number of strongly solvating groups.

Molecular Weight: The lower the molecular weight, the faster the rate of solution.

Uniformity of Substitution: The more uniformly the material is substituted, the fewer will be the crystalline areas and the more readily it will disaggregate. Solutions of cellulosics that have a high DS andlow molecular weight and that are uniformly substituted are therefore the most nearly Newtonian in theirflow characteristics. The viscosity characteristics of their solutions are less affected by the presence ofother solutes.

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Figure 4Viscosity of Several Aqualon® Polymers

at Various Concentrations


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Figure 5Shear Stress vs. Shear Rate for Aqualon® CMC and Natrosol® HEC

Figure 6Effect of Shear on Viscosity of Natrosol Solution



Figure 7Aqualon® CMC in Various States of Aggregation


Figure 8Effect of Polymer Disaggregation

on Viscosity of the System

Figure 9Effect of Solutes on Viscosity of Aqualon® CMC in Solution



IV.THIXOTROPYSome fluids exhibit a time-dependent flow pattern.

Fluids that thin under fixed flow conditions are termed“thixotropic.” Thixotropy arises when a reversible solgel system exists. Some sort of three-dimensionalstructure, albeit weak, is necessary. In thixotropic solu-tions, this internal structure is temporarily broken downby shaking or stirring, but reforms upon standing.

Typical flow curves for a thixotropic system,Aqualon® CMC-7H, are shown in Figure 10, at right.As the shear stress is increased, the shear rate varies intypical pseudoplastic fashion. However, when theshear stress is rapidly decreased, the flow curve is lessnon-Newtonian in character (10A). The reason is thatoriginally the CMC was not molecularly dispersed—that is, it contained “structure” (Figure 11A). As theshear stress is increased, the aggregates, or structuralunits, become dispersed—that is, the gel structure isbroken (Figure 11B). The area of the loop between thetwo curves (10A) is a measure of the thixotropic break-down. Upon standing, structure will re-form with time,and the viscosity will increase (10B). Depending onthe material, the temperature, and the shear history, thefinal viscosity may be the same as, greater than, or lessthan the original viscosity. (Figure 10C shows a typicalcase.) If the cellulosic polymer has a low DS and is notuniformly substituted, gels may form. Thixotropic structureis also enhanced by the presence of multivalent cations.

The presence of gel structure in a solution of CMCis reflected in the appearance of a spur at the bottom ofthe ascending curve, as shown in Figure 12A, page 14,or in very high viscosity at low shear rate, as shown inFigure 12B.

Also, shearing stresses below the yieldpoint areconsidered to be in the elastic region—below the yieldvalue all deformation is completely recoverable. Thisis discussed in Section V, page 15.

Figure 10Rheograms for Thixotropic Flow


Figure 11Effect of High Power Input on Thixotropic Structure

Figure 12Rheograms of a Thixotropic Aqualon® CMC

Solution Containing Some Gel Structure


V. ELASTICITYWhen some systems (doughs, plasters, cements) are subjected to continued stress, they exhibit (1) a

small, instantaneous, reversible deformation and (2) a slower, reversible deformation exponentially relatedto time. These reversible deformations, which can show up as changes in volume or shape, are referred toas elastic deformations and are always a function of the applied stress. The ratio of stress to strain, in thisarea of full recovery to original shape, is called the elastic modulus.

Thus, the properties of many systems are not characterized adequately if only the viscosity is mea-sured; it is necessary to measure the combined viscoelastic properties. This is particularly true of systemssuch as doughs, plaster, putty, and asphalt. In general, measurements of the viscoelastic properties are morecomplex than are measurements of the viscosity only.

The behavior of a simple viscoelastic system is shown in Figure 13, page 16. A force is applied to asystem that causes the system to elongate and is maintained from time T1 to T2. If the system were purelyelastic it would stretch immediately, maintain this elongation for time T1 to T2, and at time T2 would returnto its original length. This behavior is shown in Figure 13A, and by the broken line in Figure 13B.However, if the system is viscous as well as elastic, response of the system would be gradual, the viscosityslowing down the elastic response. When the applied force is removed, the system gradually returns to itsoriginal length. This viscoelastic behavior is shown by the solid line in Figure 13B. We can consider thepurely elastic system to perform as a spring that stretches as soon as a force is applied and relaxes as soonas the force is removed (13C). The performance of the viscoelastic system shown can be considered as aspring in parallel with a dashpot, the dashpot representing the viscosity and slowing down the elasticresponse (13D). The combination of the two effects gives curvature to the response.

In general, it is not necessary to determine the viscoelastic properties of cellulosics in simple solutionto approximate their characteristics. However, in order to determine their effect on complex systems, it isoften necessary to determine the viscoelastic properties of the system in which they are used. Viscoelasticproperties of fluid materials can be measured under oscillatory shear on rotational rheometers such asRheometrics RFS or Bohlin VOR.



Figure 13Viscoelastic Flow

10-01

© Aqualon, 2001.

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