Sodium CarboxymethylCellulose, CMC for short, is a typical kind of cellulose ether and belongs to macro-molecule anionic electrolyte.
Sodium carboxylmethyl cellulose is odorless, tasteless and nontoxic white or yellowish powder.
Sodium carboxylmethyl cellulose can easily absorb water and has favorable water solubility. Indeed, it is soluble in cold or hot water to become colloidal solution, but insoluble in organic solvents such as methanol, ethanol, acetone, chloroform and benzene.
The viscosity also has a great effect on the water solubility and the viscosity is usually 25mPa.s-8000mPa.s (1% aqueous solution)
When the substitution degree is about 0.3, the solution shows alkaline solubility. With the rise of substitution degree, the transparency of solution will improve accordingly.
The solution of Sodium carboxylmethyl cellulose is the same as other macro-molecule electrolytes. In the solution, it first produces swelling phenomenon and then gradually dissolve. Therefore, in the preparation of solution, make sure each particle is homogeneously wetted and then it can rapidly dissolve. Otherwise, after it swells in the aqueous solution, mutual adhesion between particles helps to produce very strong membrane or micelle, so the particles are difficult to dissolve and then the solution is also difficult.
Sodium carboxylmethylcellulose equilibrium water content will increase with the rise of air humidity but decrease with the rise of temperature. At room temperature and average humidity of 80-85%, the equilibrium water content is more than 26% but moisture content in the products is lower than 10%, lower than the former. As far as its shape is concerned, even if the water content is about 15%, there seems no difference in appearance. However, when the moisture content reaches above 20%, inter-particle mutual adhesion can be perceived and the higher the viscosity is, the more evident it will become.
4. Solution Viscosity
4.1 Brief Introduction
The viscosity of carboxylmethyl cellulose aqueous solution is a concrete manifestation of DP (degree of polymerization) of cellulose. It depends on the average DP of cellulose raw materials, degradation degree of cellulose DP during alkalization and etherification process, as well as homogeneity of reaction. The viscosity of Sodium carboxylmethyl cellulose aqueous solution rises nearly linearly with the increase of concentration. Apart from DP of cellulose and the distribution, the solution viscosity is also affected by the solution concentration, pH value, temperature, velocity gradient and substitution degree.
4.2 Influencing Factors on Viscosity
A. Influence of Solution Concentration
For Sodium carboxylmethyl cellulose with whether high, medium or low viscosity, the viscosity will rise with the increase of solution concentration and it is nearly linear with the solution concentration.
B. Influence of pH Value
The viscosity of 1% Sodium carboxylmethyl cellulose solution reaches the maximum and is also the most stable in pH value 6.5-9.0. Generally speaking, the viscosity won’t have big changes when pH value is within the range of 9.0-11.0. However, when pH value is lower than 6, the viscosity will rapidly decrease, then begin to form Sodium carboxylmethyl cellulose acid, and the latter is complete in pH≌2.5. If the pH value is more than 9, the viscosity will also decrease but relatively slowly in the beginning; if the pH value is more than 11.5, it will begin to rapidly decrease. This is because the association between unsubstituted hydroxyl group and alkaline molecules helps to promote the dispersion of cellulose.
C. Influence of Temperature
The viscosity of carboxylmethyl cellulose solution will decrease with the rise of temperature. When it cools down, the viscosity will immediately rise again, but permanent viscosity decrease will arise when the temperature rises to a certain extent. It must be pointed out that the viscosity decrease has a close relation to the substitution degree of carboxylmethyl cellulose. The higher the substitution degree is, the smaller the influence of temperature on the viscosity will be.
D. Influence of Salts
The existence of various kinds of inorganic salt plasma will reduce the viscosity of Sodium carboxylmethyl cellulose solution. The influence of salts on the viscosity almost depends on the valence number of cation. When meeting salts of monovalent cation, the solution shows water solubility; when meeting salts of trivalent cation, it shows water insolubility; when meeting salts of bivalent cation, it is in between.
5. Sodium carboxylmethyl cellulose will generate sedimentation with heavy metallic salts including PbSO4, FeCl3, FeSO4, CuSO4, K2Cr2O7, AgNO3, and SnCl2.
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