Limitations of Blending: The relationship between viscosity and concentration can vary significantly, depending on the chemical composition as well as the molecular weight (viscosity type) of the polymers involved. The greatest accuracy is obtained from use of the equation or the blending chart Figure 9 if the following conditions are met. Departure from these conditions can result in deviation from the predicted value of viscosity.
- The chemical composition of the polymers must be similar—i.e., the type and level of chemical substitution must be the same.
- The solution viscosities of the polymers should be as close together as possible.
Figure 9
Chart for Blending Aqualon Water-Soluble Polymers
Figure 10
Effect of Concentration on Viscosity of Aqueous Solutions of Aqualon CMC
(Bands approximate the viscosity range for the types shown.)
Effect of Shear
CMC is often used to thicken, suspend, stabilize, gel, or otherwise modify the flow characteristics of aqueous solutions or suspensions. Preparation and use of its solutions involve a wide range of shearing conditions. It is therefore important that the user understand how rheological behavior can affect the system.
Pseudoplasticity—Small amounts of CMC dissolved in water greatly modify its properties. The most obvious immediate change is an increase in viscosity. Interestingly, a single CMC solution will appear to have a different viscosity when different physical forces are imposed on it.
These physical forces may be conveniently referred to as high, intermediate, or low shear stress. For example, rolling or spreading a liquid as if it were an ointment or lotion would be high shear stress. After the liquid has been applied, gravity and surface tension control flow. These forces are conditions of low stress. Intermediate stress is typified by pouring a liquid out of a bottle.
If a solution of high-viscosity CMC appears to be a viscous syrup as it is poured from a bottle, it will behave as a thin liquid when applied as a lotion, and yet when high shear stress is removed it will instantly revert to its original highly viscous state. This type of flow behavior is referred to as pseudoplasticity or time-independent shear-thinning—a form of non-Newtonian flow. It differs from the time-dependent viscosity change called thixotropy.
If shear stress is plotted vs. shear rate, as in Figure 11, a Newtonian fluid will produce a straight line passing through the origin. A pseudoplastic liquid, such as a CMC solution, will give a curved line. Plotting apparent viscosity against shear rate, as in Figure 12, produces a horizontal straight line for a Newtonian fluid and a curved line for a pseudo-plastic liquid.
Solutions of some medium- and high-viscosity types of CMC exhibit pseudoplastic behavior because their long-chain molecules tend to orient themselves in the direction of flow; as the applied force (shear stress) is increased, the resistance to flow (viscosity) is decreased. When a lower stress is imposed on the same solution, the apprent viscosity is higher because random orientation of molecules presents increased resistance to flow.
Figure 11
Shear Stress vs. Shear Rate for Newtonian and Pseudoplastic Liquids
Figure 12
Viscosity vs. Shear Rate
When viscosity (shear stress divided by shear rate) is plotted against shear rate, a Newtonian system gives a horizontal line. If viscosity decreases as shear rate is increased, the flow is pseudoplastic.
Generally, solutions of the medium- and high-viscosity types with a high DS (i.e., 0.9 and 1.2) and “S” types are pseudoplastic rather than thixotropic. In contrast to this, regular high- and medium-viscosity gums of DS 0.7 (slightly less uniformly substituted) show thixotropic behavior in solution. (See Thixotropy, below.)
Solutions of low-molecular-weight CMC—i.e., low-viscosity types—are less pseudoplastic than those of high-molecular-weight gum. However, at very low shear rate, all CMC solutions approach Newtonian flow. Figure 13 shows these relationships.
Figure 13
Effect of Shear Rate on Apparent Viscosity of Aqualon CMC Solutions
Figure 14A
Thixotropic Flow
Thixotropy—If long-chain polymers have a considerable amount of interaction, they will tend to develop a three-dimensional structure and exhibit a phenomenon known as thixotropy.
Thixotropy is a time-dependent viscosity change. It is characterized by an increase in apparent viscosity when a solu-tion remains at rest for a period of time after shearing. In certain cases, the solution may develop some gel strength, or even set to an almost solid gel. If sufficient force (shear stress) is exerted on a thixotropic solution, the structure can be broken and the apparent viscosity reduced.
Rheograms are helpful to illustrate the effect of thixotropy. A thixotropic solution will form a hysteresis loop when shear stress is plotted against shear rate, as shown in Figure 14A. The increased shear stress required to break the thixotropic structure has reduced the resistance to flow, or viscosity. If a solution has gel strength, a spur forms in the hysteresis loop; this is shown in Figure 14B. It is an indication of the stress necessary to break the gel structure and cause the solution to revert to its normal apparent viscosity.
Figure 14B
Extremely Thixotropic Flow With Gel Strength
Figure 15 illustrates thixotropy in another manner. At a constant shear rate (D = K), viscosity decreases with time. When shear is removed (D = zero), viscosity increases significantly with time.
Thixotropic solutions are desirable, or even essential, for certain uses of CMC, such as suspension of solids. High-and medium-viscosity types of regular Aqualon CMC (0.7 DS) generally exhibit thixotropic behavior. “S” types and high-DS types in medium and high viscosity have been developed for uses requiring clear, smooth solutions of little or no thixotropy. Figure 16 illustrates the difference in appearance between solutions of regular and “S”-type Aqualon CMC. “S” and high-DS types show the typical pseudoplasticity of long-chain molecules.
Figure 15
Thixotropic Flow Is a Time-Dependent Change in Viscosity
Figure 16
Thixotropic and Nonthixotropic Solutions of CMC
The solution of regular Aqualon CMC, left, is thixotropic; “S”-type Aqualon CMC, right, is essentially nonthixotropic.
Figure 17
Effect of Temperature on Viscosity of Aqualon CMC Solutions
Effect of Temperature
Viscosity of CMC solutions depends on temperature, as shown in Figure 17. Under normal conditions, the effect of temperature is reversible, so temperature variation has no permanent effect on viscosity. However, long periods of heating at high temperatures will degrade CMC and permanently reduce viscosity. For example, a 7L type held for 48 hours at 180°F lost 64% of its original viscosity.
Effect of pH
CMC solutions maintain their normal viscosity over a wide pH range. In general, solutions exhibit their maximum viscosity and best stability at pH 7 to 9. Above pH 10, a slight decrease in viscosity is observed. Below pH 4.0, the less soluble free acid carboxymethylcellulose predominates and viscosity may increase significantly. Figure 18 shows the effect of pH on the viscosity of typical Aqualon CMC grades.
Figure 18
Effect of pH on Viscosity of Aqualon CMC Solutions
Tests with Aqualon CMC Type 7M have shown that very little polymer degradation takes place if solutions are allowed to stand overnight at room temperature at a pH as low as 2. However, at pH values of 4-5 and temperatures of 150°F, most of the viscosity is lost in 24 hrs.
In acidic systems, the order in which CMC is added to the solvent is also important. If a CMC solution is prepared prior to the addition of acid, a higher viscosity is obtained than when dry CMC is dissolved in an acidic solution.
Aqualon cellulose gum Type 7HOF is a particularly efficient thickener for acidic systems. Clear, viscous solutions are obtained when it is dissolved in water and then acidified. Its stability in several organic acids, typical of those used in low-pH foods, is shown in Figure 19.
Figure 19
Stability of Aqualon Cellulose Gum in Organic Acids—1% Solution of Type 7HOF
Effect of Mixed Solvents
The behavior of highly substituted CMC in mixed-solvent systems, such as glycerin-water, is similar to its effect in water alone. In mixed systems, however, viscosity of the solvent affects viscosity of the solution. For example, if a 60:40 mixture of glycerin and water (which is 10 times as viscous as water alone) is used as the solvent, the resulting solution of well-dispersed CMC will be 10 times as viscous as the comparable solution in water alone. This behavior is shown in Figure 19 and is commonly referred to as the “viscosity bonus effect.”
Figure 20
Effect of Mixed Solvents on Viscosity of Aqualon CMC Solutions—1% Type 12M31
Stability
CMC is subject to microbiological attack and chemical degradation. However, corrective measures can be taken to prevent both from occurring.
Microbiological Attack
Although CMC is more resistant to microbiological attack than many other water-soluble gums, its solutions are not immune. Heat treatment can be used to destroy many microorganisms while having little effect on CMC properties. Heating for 30 min at 80°C, or for 1 min at 100°C, is generally sufficient.
When solutions are stored, a preservative should be added to prevent viscosity degradation. If cellulases (hydrolytic, viscosity-destroying enzymes) have been introduced by microbial action, even in trace amounts, addition of most preservatives will not prevent degradation; therefore, it is important to preserve solutions as soon as possible after preparation.
The preservatives shown below have proved effective for solutions of Aqualon CMC. The preservative manufacturer should be consulted regarding the kind and amount to be added.
Chemical Degradation
Under certain conditions, solutions of CMC are susceptible to chemical degradation. Permanent loss of viscosity can occur resulting from scission of the long-chain molecules. Such viscosity loss is accelerated by increasing the temperature and/or lowering the pH. Aqualon cellulose gum Type 7HOF provides improved resistance to viscosity degradation and precipitation in low-pH systems.
An oxidative type of degradation occurs under alkaline conditions in the presence of oxygen. The rate of viscosity loss is also increased by heat and/or ultraviolet light. Inclusion of an antioxidant, exclusion of oxygen, and avoidance of highly alkaline conditions are obvious preventive measures.
To obtain the best stability during prolonged storage of CMC solutions, users should:
- Protect against microbiological attack.
- Maintain solution pH as nearly neutral as possible (7.0 to 9.0).
- Avoid prolonged exposure to elevated temperatures.
- Exclude oxygen and sunlight.
