E. H. deBUTTS, J. A. HUDY, and J. H. ELLIOTT Research Center, Hercules Powder Co., Wilmington, Del.
Rheology of Sodium Carboxymethylcellulose Solutions
A . new theory of molecular arrangement in CMC solutions may explain their rheological behavior. As a burr will enmesh hair, poorly soluble crystalline material may entrap soluble CMC in gel centers he(d together b y electrostatic and hydrogen bonding forces. Aggregation and dispersion of these centers may account for many properties that figure importantly in selecting proper materials and methods for preparing solutions.
SODIUM
carboxymethylcellulose, known also as CMC, cellulose gum, and sodium CMC, is the water-soluble cellulose derivative of greatest commercial importance. Between 20,000,000 and 25,000,000 pounds per year are currently used in the United States. Its major uses are for detergents, pharmaceuticals, paper, adhesives, drilling muds, textiles, coatings and foods. CMC is prepared by the reaction of alkali cellulose with sodium chloro-
acetate (70) and the structural formula of a typical repeating unit is shown below. I n commercial types, degree of substitution (DS) ranges from 0.5 to 1.2 carboxymethyl groups per anhydroglucose unit and weight-average molecular weight ranges from approximately 50,000 to 500,000. Distribution of substituents in partially substituted CMC has been studied by Time11 and Spurlin (75) who found that unsubstituted, mono-, and disubstituted anhydroglucose units were present. But as expected from considering steric and electrostatic effects, none having substitutions in the 2,3- or 2,3,6positions were present. Samples of CMC similar in chemical composition and solution viscosity at the same concentration, may have widely different rheological properties in aqueous solution or dispersion. Research has been directed toward a better rheological characterization of CMC solutions and viscosity behavior of dilute solutions has been studied by a number of workers (4, 17, 13). This article however, is concerned with those
OH
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Hc2-OH
in the relatively high concentration range of 1 to 391, by weight. This range where intermolecular interactions play a significant role is extremely important under use conditions. I t was emphatically pointed out by Green and others (5) that a single viscosity measurement on systems showing complicated flow behavior is meaningless and in many cases misleading. For concentrated CMC solutions, such single or one-point values of apparent viscosity are a function of measurement conditions, method of preparing the solution, elapsed time during the determination, and shear and thermal history of the system before rheological measurements are made. Also, since CMC is a polyelectrolyte, other ions, particularly cations, in the solution exert a profound effect on the flow characteristics of the system. Non-Newtonian F l o w Properties
CMC solutions are non-NewtonianLe., shearing stress is not directly proportional to rate of shear. The most common type of non-Newtonian flow properties is pseudoplasticity. If a plot (rheogram), is made of rate of shear us. shearing stress, a curve concave toward the rate of shear axis is obtained (Figure 1). Apparent viscosity (shearing stress divided by the rate of shear) decreases as the rate of shear is increased and no time-dependent effects are observed. In thixotropic systems, however, time effects are of prime importance. Apparent viscosity diminishes with time if rate of shear is held constant. The structural breakdown of thixotropic material occurs over a measurable time; when shearing is stopped, the structure builds up again in a reversible manner. Depending upon the system, the time necessary for this structure build-up may range from a few seconds to several days. A method developed by Green and others (5) for characterizing thixotropic
CELLULOSE D E R I V A T I V E S systems, proved useful when applied to pigmented materials, and has been modified for CMC solutions. Briefly, this system involves use of a rotational viscometer whose rate of rotation may be rapidly changed in known increments. Readings are taken first with increasing then decreasing speed of rotation, and a rheogram such as Figure 2 is obtained. Unlike curves for pseudoplastic material, these are not coincident but form a hysteresis loop. Green's system does not give equilibrium values; hence, that area of the loop, which is a measure of thixotropy of the system, is a function of the time scale of the experiment. I n aqueous solutions of CMC, the times required for thixotropic breakdown and build-up are considerably longer than those generally found in pigmented systems. Early experiments with the Green system failed to demonstrate hysteresis loops for aqueous CMC solutions, although it was known from other measurements that these solutions were thixotropic. Such loops were obtained when a proper time scale was used. Since the measurements are not equilibrium measurements and since the area of the hysteresis loop is a function of the time scale, it is essential that experimental conditions be carefully standardized for valid comparisons. In gelation, another type of rheological behavior, a solution broken down by shear will set u p to a gel when allowed to stand. Both pseudoplasticity and gelation are special cases of thixotropy. In the former, time effects are not detected under the experimental conditions. In gelation, however, structure build-up, as occurs in the case of thixotropy, is sufficiently great for the resulting three-dimensional network to exhibit considerable mechanical strength. I n rheopexy, another type of rheological behavior, a system broken down by shear shows a significant increase in the rate of structure build-up when subjected to moderate shearing action. Experimental Measurements a t high shear rates were made with a recording concentriccylinder viscometer, the Hercules HiShear viscometer (74). Starting the determination with the bob at rest, the shear rate was increased at a rate of 21 sec.-l per second until the maximum of approximately 4400 sec.-Iwas reached; then it was quickly reduced to zero. Low shear-rate measurements were made with a modified Brookfield HBF Synchro-Lectric viscometer manufactured by the Brookfield Engineering Laboratories, Stoughton, Mass. The rate of gel build-up was studied with the instrument operated a t 1.5 r.p.m. The maximum scale reading observed during
HYSTERESIS
SHEARING STRESS,
TORQUE
SHEARING STRESS,
TOROUE
Figure 1. Rheogram of a nonthixotropic pseudoplastic CMC solution
Figure 2. Rheogram of a thixotropic CMC solution
one spindle revolution was taken as the measure of gel strength. Between readings the instrument was turned off. I t was found by calibration that four times the scale reading gives the shearing stress in dynes per square centimeter. All rheological measurements were made in a constant temperature room at 25' C. Intrinsic viscosities were determined in a 5% sodium hydroxide solution in dilution Ubbelohde viscometers (7). The results were extrapolated to zero concentration according to Martin's equation (8; 70, pp. 1212ff.). Centrifuge experiments were carried out in a Serval1 centrifuge developing a field of 20,000 g. Centrifugation required 8 hours. Solutions were prepared either with a Brookfield counter-rotating mixer which subjects the samples to relatively high shear rates, or by slow end-over-end tumbling where the shear rates involved are low. Various samples of CMC having two apparent viscosity ranges and a DS between 0.65 and 0.85, were used. One type had an apparent viscosity of 1300 to 2200 cp. in 1% aqueous solution at 25" C.; the other, 300 to 600 cp. in 2% solution a t the same temperature (6).
system which must be broken down before flow occurs. That these three types of rheograms represent markedly different CMC solution properties is apparent when appearance of the solutions is considered. Figure 4 showing a bottle of CMC solution that has just been inverted, is characterized by a rheogram for a pseudoplastic (Figure 1). The solution flows smoothly down the sides of the bottle and there is no evidence of structure. Figure 5 shows a similar picture for a thixotropic solution which gives a rheogram having a marked hysteresis loop (Figure 2). This solution does not flow smoothly but considerable structure is present. A microscopic examination shows highly swollen cellulosic fibers. If a solution of a thixotropic sample of CMC is prepared a t very high shear rates, a rheogram as shown in Figure 3 is obtained; this indicates the presence of gel structure which must be broken down before flow can occur. In such a solution, shown in the left jar of Figure 6 , the gel has sufficient mechanical strength to support its own weight. If, however, this system is shaken sharply, the gel structure is broken down (jar on right, Figure 6) ; there is still considerable structure present in the solution and its appearance after shaking is similar to that of the thixotropic solution shown in Figure 5. Since for some applications, a pseudoplastic, thixotropic, or gelled system may be desired, high-shear rheological characterization of CMC solutions can be useful in selecting the proper type of CMC.
Discussion of Results For measurements made at high shear rates, three types of rheograms are obtained : those for pseudoplastic (Figure l), thixotropic (Figure 2), and gel systems (Figure 3). The small spur a t the bottom of the up-curve in Figure 3 is a measure of the gel strength of the
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if it is then subjected to high shear (curve D),greater gel strength is developed. If this system is subjected to low shear (curves E, F ) the effects from previous high shear treatment are eventually obliterated. Two important conclusions may be drawn from this series o i experiments. First, the rheological properties of CMC solutions are remarkably dependent on the most recent event in their shear history; and second, a greater length of time (solutions E and F) is required for obliteration of previous shear history-a fact observed also in high shear experiments. The types of rheological measurements described in the foregoing are of practical importance in properly selecting types of CMC and methods of preparing solutions for a given application. They are also of scientific interest because the rheological properties of CMC solutions may be correlated with the molecular properties of this polymer in solution. Interpretation of Results
SHEARING STRESS? TORQUE
Figure 3. input
Rheograms of a thixotropic CMC solution prepared with high power
,41so, by such studies, quantitative comparisons of different samples can be made and correlated with manufacturing variables. The profound effect of previous shear history is clearly shown by the results of experiments on rate of gel strength build-up. These gel strengths were measured at low shear rates in the Brookfield viscometer, as described earlier.
The results on 2YGsolutions of CMC-70 medium are given in Figure 7. The solution prepared at a low shear rate develops little gel strength (curve A ) , whereas that prepared a t a high shear rate (curve B ) develops considerable strength. If the latter solution is subjected to tumbling (low shear), the rate and extent of development of gel strength (curve C) is markedly lower;
Figure 4. Flow of a pseudoplastic CMC solution
Figure 5. Flow of a thixotropic CMC solution
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Salt and others (12), Durig and Banderet ( Z ) , and Hoppler (7) have made rheological measurements on CMC solutions in the higher concentration range (1 to 3Y0)considered here. These workers did not observe thixotropy, probably because of the long time intervals required for thixotropic breakdown and build-up. Hoppler interpreted his results obtained at very low shear rates by peculiarities in etherification of cellulose. Many of his ideas, incorporated in the following description of CMC in solution, have been discussed more fully elsewhere (9). Cellulose from which CMC is prepared contains both amorphous and crystalline regions. During preparation of alkali cellulose and its subsequent
Figure 6. Thixotropic CMC solution prepared with high power input; left and right, before and after shaking, respectively
CELLULOSE D E R I V A T I V E S carboxymethylation, uniform distribution of substituents along the cellulose chain is not expected, since the interior of the highly crystalline regions would react more slowly than the remainder of the cellulose. Thus, there is a small percentage of CMC not molecularly dispersed but held together by crystalline remnants from the original cellulose. This crystalline, poorly soluble material acts as a gel center which entraps a relatively large amount of soluble CMC in a network (similar to a mass of hair held together by a burr) held together by electrostatic and van der Waals forces. Durig and Banderet ( 2 ) isolated such insoluble gels showing crystalline aggregates during x-ray examination. A schematic drawing of such a gel is shown on the left of Figure 8. The hypothesis can be proposed that thixotropy of CMC solutions results from groups of these gel centers in solution (center of Figure 8). When this system is subjected to shear, a fraction of gel centers and their associated entrapped molecules will be dispersed. This dispersion occurs over an appreciable time interval and hence, a t constant rate of shear, apparent viscosity of the system will decrease with time. When the solution is allowed to stand, gel centers will reaggregate and entrap molecularly soluble C M C molecules. Apparent viscosity of the system will therefore increase. For solutions prepared a t high power input-e.g., Brookfield mixer-dispersion of gel-center aggregates will be more complete, as shown schematically on the right of Figure 8. This leads to formation of a higher concentration of gel centers in the solution. O n standing, a three-dimensional network can be set up, giving a gel of appreciable mechanical strength (Figure 6). High shear rheograms of such a gel (Figure 3) show that structural breakdown is incomplete, for when rate of shear is carried to its maximum value a hysteresis loop is obtained. It should be considered now whether high power input disperses aggregates
C
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c E
7
6 v, LT
= 5
0 I
J 2 4
t-
3
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20 30 40 50 60 GEL STRENGTH, DYNES/SQ. CM.
0'
10
Figure 7. Effect of shear history on rate of developing shear strength Starting
Sol. A B C D E
Final Treatment Tumbler Brookfield mixer Tumbler Brookfield mixer Tumbler Tumbler
of gel centers as proposed above, or actually breaks up crystalline regions in the individual gel centers. Low shear experiments throw considerable light on this. The fact that CMC solutions subjected to high shear will revert to the same rheological properties as those
Time 16 hr. 20 min. 16 hr. 20 min. 24 hr. 48 hr.
Sol. A B
C D
E F
prepared a t low shear, if they are subjected to low shear for a sufficient time (Figure 7), strongly suggests that crystalline regions are not destroyed; it is unlikely that these, once destroyed, would reform in solution. The additional fact that solutions prepared a t high
\
Figure 8.
Schematic diagram of gel center and effect of shear on aggregates of such centers VOL. 49, NO. 1
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CENTRIFUGATE
Figure 9. Rheograms of original thixotropic CMC solution, centrifugate, and gel fractions
shear are rheopectic-Le., gentle shearing increases the rate and extent of structure build-up-is also consistent with the view that thixotropy arises from aggregates of gel centers (center of Figure 8). The view that thixotropy and gelation arise from regions of cellulose chain aggregation of limited solubility is confirmed by the following: 1. C M C for the solution having a pseudoplastic rheogram (Figure 1) and flow properties as shown in Figure 4, was prepared under conditions designed to give maximum uniformity in distribution of substituents. Few if any crystalline regions from the original cellulose persist in the final product. 2. When a thixotropic solution of CMC was centrifuged in a field of 20,000 g., the system separated into two phasesa clear fluid centrifugate and a gellike precipitate. The rheogram of the centrifugate is that of a pseudoplastic, whereas that of the gel fraction indicates high thixotropy (Figure 9). Evidence from Martin plots indicates that C M C is molecularly dissolved in the 5y0sodium hydroxide solution used for intrinsic viscosity determinations (Table I). Table I shows no significant difference between values for the original solution and its fractions. The small variations shown resulted from combining a number of centrifugations to obtain
sufficient material for study. Therefore, the amount of crystalline unsubstituted material in the gel phase is small because its effect on the analytical results is masked by entrapped soluble C M C having the same average composition as the centrifugate phase. Additional centrifuge studies are in process. 3. This hypothesis is further supported when multivalent cations are added to C M C solutions exhibiting no thixotropy. The rheological effects produced by adding iron(II1) to C M C are shown in Figure 10. When the ratio of iron(II1) to C O O - is 1 to 25 (curve B, Figure 10) there is a marked increase in viscosity but no thixotropy or gelation. The Fe(II1) links chains together and increases molecular weight, but concentration is not sufficiently high to form a three-dimensional network. Evans and Spurlin (3) found a similar effect for benzene solutions of ethylcellulose containing carboxyl groups having bound barium. When the ratio of iron(II1) to C O O - is raised to 1 to 15, however, a relatively strong three-dimensional structure is formed by the cross-linking action of iron(II1) (curve C, Figure 10). This system is a relatively strong gel, which breaks down on shearing to give a highly thixotropic solution. Other metal ions having a high coordination tendency such as copper(I1) and aluminum(II1) show similar effects. If a higher ratio of iron(II1) to C O O - is used, CMC is precipitated from solution. Thus, artificial formation of regions having borderline solubility leads to thixotropy in C M C solutions. Summary Rheological measurements of C M C solutions under both high and low
Table 1.
Data on Centrifuged CMC Solution Intrinsic Degree VisTotal of cosity Solids, Substi- 100 % ’ tution (Ml./G.) 0.64 3.8 Original solution 1.11 Gel phase 1.18 0.67 4.0 0.76 3.4 Centrifugate 1.04
SHEARING STRESS,
Curve
A B
C
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INDUSTRIAL AND ENGINEERING CHEMISTRY
TORQUE
Figure 10. Rheograms of the system, Fe(lll) and CMC Fe(lll)/COO0 1 /25 1/15
shear-rate conditions, are of practical value in selecting types of CMC and methods for preparing solutions to obtain optimum combination of properties for various applications. Also, these results, together with other information, give considerable insight into the molecular nature of CMC solutions. From rheological and other observations, a molecular picture of CMC solutions has been developed. Gel centers arising from crystalline areas in original cellulose produce thixotropy in CMC solutions; an important factor in determining if such centers occur appears to be uniformity of distribution of substituents along the cellulose chain. The observed rheological behavior of CMC solutions may be interpreted in terms of a molecular picture involving these gel centers and electrostatic and hydrogen bonding forces. The effect of multivalent cations in causing increased thixotropy, gelling, and precipitation plus the results of centrifugation experiments supports this molecular picture. Acknowledgment
The authors wish to express their thanks to H. M. Spurlin for many helpful discussions and suggestions during the course of this work, and Sheila Mulrooney, who carried out many of the rheological measurements. Literature Cited (1) Davis, W. E., Elliott, J. H., J . Colloid Sci. 4, 313 (1949). (2) Durig, G., Banderet, A., Helv. Chim. Acta 33, 1106 (1950). (3) Evans, E. F., Spurlin, H. M., J. Am. Chem. Soc. 72,4750 (1950). (4) Fujita, H., Homma, T., J . Colloid Sci., 9 , 591 (1954). (5) Green, H., “Industrial Rheology and Rheological Structures,” Wiley, New York, 1949. ( 6 ) Hercules Powder Co., “Properties and Uses of Hercules Cellulose Gum (CMC),” Wilmington, Del., 1953. ( 7 ) Hoppler, F., Kolloid-2. 98, 348 (1942). (8) Martin, A. F., 103rd Meeting, ACS, Memphis, Tenn., April, 1942. ( 9 ) Ott, Emil, Elliott, J. H., “Makromolekulare Chemie,” Staudinger Festband, in press, March 1956. (10) Ott, Emil, Spurlin, H. M., “Cellulose and Cellulose Derivatives,” 2nd ed., pp. 937ff., Interscience, New York, 1954. (11) Pals, D. T. F., Hermans, J. J., Rec. trau. chim. 71, 433 (1952). (12) Salt, D. L., Ryan, N. W., Christiansen, E. B., J . Colloid Sci. 6, 146 (1951). (13) Schneider, N. S., Doty, P. M., J . Phys. Chem., 58, 762 (1954). (14) Smith, J. W., Applegate, P. D., Paper Trade J . 126, 60 (June 3, 1948).
(15) Timeli,’ T. E., Spurlin, H. M., Suansk Papperstidn. 18, 1 (1952). RECEIVED for review April 19, 1956 ACCEPTEDSeptember 19,1956
