The higher the degree of substitution, the more rapidly CMC dissolves. The lower the molecular weight, the faster the rate of solution.
Particle size has a pronounced effect on the ease of dispersing and dissolving CMC. “C,” or coarse, types were developed to improve dispersibility of the granules when agitation is inadequate to produce a vortex on the liquid surface. Solution time, on the other hand, is extended considerably with a coarse material.
For applications requiring a rapid solution time, CMC of fine particle size (X grind) is best. However, special dissolving techniques, such as prewetting the powder with a non-swelling liquid, mixing it with other dry materials, or using an eductor-type mixing device, are necessary to obtain dispersion.
Shear Rate
Preparing solutions by extremely low shear agitation, such as shaking by hand, is generally not recommended. Properties of the resulting solution are quite different from those prepared by higher shear methods.
Dispersion Methods
CMC particles have a tendency to agglomerate, or lump, when first added to water. To obtain good solutions easily, the dissolving process should be considered a two-step operation:
- Dispersing the dry powder in water. Individual particles should be wet and the dispersion should not contain lumps.
- Dissolving the wetted particles.
When the proper technique is used, good dispersion is obtained, and CMC goes into solution rapidly. To prepare lumpfree, clear solutions, a variety of methods can be used:
Method 1
Add CMC to the vortex of vigorously agitated water. The rate of addition must be slow enough to permit the particles to separate and their surfaces to become individually wetted, but it should be fast enough to minimize viscosity buildup of the aqueous phase while the gum is being added.
Method 2
Prior to addition to water, wet the powder with a water-miscible liquid such as alcohol, glycol, or glycerol that will not cause CMC to swell. Two to three parts of liquid per part of CMC should be sufficient.
Method 3
Dry-blend the CMC with any dry, nonpolymeric material used in the formulation. Preferably, the CMC should be less than 20% of the total blend.
Method 4
Use a water eductor (Figure 4) to wet out the polymer particles rapidly. The polymer is fed into a water-jet eductor, where a high-velocity waterflow instantly wets out each particle, thus preventing lumping. This procedure speeds solution preparation and is particularly useful where large volumes of solutions are required. For users wishing the convenience of an automatic system, a polymer solution preparation system (PSP), which is used in conjunction with a water eductor, is shown in Figure 5.
Special, fast-dissolving fluidized polymer suspensions of CMC are available to give very rapid dissolution where it is required or where agitation is substandard.
Users are encouraged to contact their technical representative for information on PSP units or fluidized suspensions of CMC.
Figure 4
Typical Installation of Eductor-Type Mixing Device
Figure 5
Automated Polymer Solution Preparation (PSP) System
Theory of Polymer Dissolution
When a polymer is dispersed in a solvent, the degree of disaggregation—i.e., separation of polymer molecules— is affected by the:
- Chemical composition of the polymer.
- Solvating power of the solvent.
- Shear history of the resulting solution.
Figure 6 shows how these states of disaggregation may affect viscosity of the liquid. If CMC is added to a liquid and its degree of disaggregation reaches equilibrium, the polymer may:
- Remain as a suspended powder, neither swelling nor dissolving (1).
- Swell to a point of maximum viscosity without completely dissolving (2).
- Reach maximum disaggregation (3).
- Exist in an intermediate state (1a, 1b, 2a).
Depending on choice of polymer, solvent, and mechanical means of preparing the solution, the user of CMC can alter its state of disaggregation to suit his needs. Table IV shows the effect of these factors on the disaggregation of CMC as measured by solution viscosity.
Increasing DS makes CMC more hydrophilic, or “water-loving”; hence, types having high DS are more readily disaggregated in water. Plotting solution viscosity at constant shear against increasing DS (Types 7 through 12) produces a curve similar in shape to that shown in Figure 6.
Increasing electrolyte concentration reduces disaggregation, as evidenced by the lower viscosity in saltwater of Type 7. The viscosities listed in Table IV were measured under quality control conditions—that is, two hours after solution was complete. At this point, CMC dissolved in an electrolyte solution is probably in the Stage 1 section of the disaggregation curve. CMC dissolved in distilled water under quality control conditions is at Stage 3 of the curve. Viscosities of CMC/salt solutions measured at this point will be lower than the viscosities of corresponding CMC solutions prepared in distilled water. Since disaggregation is a time-dependent phenomenon, if CMC/salt solutions are allowed to stand, it is very possible that the final stage of disaggregation will be Stage 2 and the equilibrated viscosity will be higher than that of CMC in distilled water. Hence, one cannot assume that addition of salt will lower equilibrated solution viscosity, only that it will inhibit polymer disaggregation. With Types 9 and 12, the slight viscosity increase in saturated salt is caused by the “viscosity bonus effect” discussed on page 20.
Figure 6
Idealized Curve Showing Effect of Degree
of Disaggregation on Viscosity of Polymer Solution
