01. Properties of sodium carboxymethylcellulose

Sodium carboxymethyl cellulose is an anionic polymer electrolyte. The degree of substitution of commercial CMC ranges from 0.4 to 1.2. Depending on the purity, the appearance is white or off-white powder.

1. The viscosity of the solution
The viscosity of CMC aqueous solution increases rapidly with the increase of concentration, and the solution has pseudoplastic flow characteristics. Solutions with a lower degree of substitution (DS=0.4-0.7) often have thixotropy, and the apparent viscosity will change when shear is applied or removed to the solution. The viscosity of CMC aqueous solution decreases with increasing temperature, and this effect is reversible when the temperature does not exceed 50 °C. At higher temperature for a long time, CMC will degrade. This is the reason why the bleed glaze is easy to turn white and deteriorate when printing thin line pattern bleed glaze.
The CMC used for glaze should choose a product with a high degree of substitution, especially the bleeding glaze.

2. The effect of pH value on CMC
The viscosity of CMC aqueous solution remains normal in a wide pH range, and is most stable between pH 7 and 9. With pH
The value decreases, and CMC turns from salt form to acid form, which is insoluble in water and precipitates. When the pH value is less than 4, most of the salt form turns into acid form and precipitates. When the pH is below 3, the degree of substitution is less than 0.5, and it can completely transform from the salt form to the acid form. The pH value of complete transformation of CMC with high degree of substitution (above 0.9) is below 1. Therefore, try to use CMC with a high degree of substitution for the seepage glaze.

3. Relationship between CMC and metal ions
Monovalent metal ions can form water-soluble salts with CMC, which will not affect the viscosity, transparency and other properties of the aqueous solution, but Ag+ is an exception, which will cause the solution to precipitate. Divalent metal ions, such as Ba2+, Fe2+, Pb2+, Sn2+, etc. cause the solution to precipitate; Ca2+, Mg2+, Mn2+, etc. have no effect on the solution. Trivalent metal ions form insoluble salts with CMC, or precipitate or gel, so ferric chloride cannot be thickened with CMC.

There are uncertainties in the salt tolerance effect of CMC:
(1) It is related to the type of metal salt, the pH value of the solution and the degree of substitution of CMC;
(2) It is related to the mixing order and method of CMC and salt.
CMC with a high degree of substitution has better compatibility with salts, and the effect of adding salt to CMC solution is better than that of salt water.
CMC is good. Therefore, when preparing osmotic glaze, generally dissolve CMC in water first, and then add osmotic salt solution.

02. How to recognize CMC in the market

Classified by purity
High-purity grade — the content is above 99.5%;
Industrial pure grade — the content is above 96%;
Crude product — the content is above 65%.

Classified by viscosity
High viscosity type – 1% solution viscosity is above 5 Pa s;
Medium viscosity type – the viscosity of 2% solution is above 5 Pa s;
Low viscosity type – 2% solution viscosity above 0.05 Pa·s.

03. Explanation of common models

Each manufacturer has its own model, it is said that there are more than 500 kinds. The most common model consists of three parts: X—Y—Z.

The first letter represents the industry use:
F – food grade;
I——industrial grade;
C – ceramic grade;
O – petroleum grade.
The second letter represents the viscosity level:
H – high viscosity
M——medium viscosity
L – low viscosity.

The third letter represents the degree of substitution, and its number divided by 10 is the actual degree of substitution of CMC.

Example:
The model of CMC is FH9, which means CMC with food grade, high viscosity and substitution degree of 0.9.
The model of CMC is CM6, which means CMC of ceramic grade, medium viscosity and substitution degree of 0.6.

Correspondingly, there are also grades used in medicine, textile and other industries, which are rarely encountered in the use of ceramic industry.

04. Ceramic Industry Selection Standards

1. Viscosity stability
This is the first condition for choosing CMC for glaze
(1) Viscosity does not change significantly at any time
(2) Viscosity does not change significantly with temperature.

2. Small thixotropy
In the production of glazed tiles, the glaze slurry cannot be thixotropic, otherwise it will affect the quality of the glazed surface, so it is best to choose food-grade CMC. In order to reduce costs, some manufacturers use industrial-grade CMC, and the glaze quality is easily affected.

3. Pay attention to the viscosity test method
(1) The CMC concentration has an exponential relationship with the viscosity, so attention should be paid to the accuracy of weighing;
(2) Pay attention to the uniformity of the CMC solution. The strict test method is to stir the solution for 2 hours before measuring its viscosity;
(3) Temperature has a great influence on viscosity, so attention should be paid to the ambient temperature during the test;
(4) Pay attention to the preservation of CMC solution to prevent its deterioration.
(5) Pay attention to the difference between viscosity and consistency.