Carboxymethyl Cellulose (CMC) is a derivative of natural cellulose that has been chemically modified to have an ether structure. Due to its poor acid-type water solubility, the product is generally made into sodium salt, namely carboxymethyl cellulose sodium.
Adding CMC additive to the slurry can improve the performance of the ceramic slurry in terms of dispersion, flowability, water retention, viscosity, adhesive strength, and deflocculation.
Dispersion and flocculation effects: CMC is a water-soluble long-chain polymer, which can form a unique network structure in the slurry by adding the long-chain molecules, supporting the gravity of the particles. The molecules or ions of sodium carboxymethyl cellulose are like strips stretching in the slurry, occupying a certain space to prevent particles from contacting each other, improving the spatial stability. After the carboxymethyl cellulose CMC is dissociated, the negatively charged anion group is adsorbed by the particle, making the surface potential of the particle increase. When the particle approaches, the electrostatic repulsion increases, thus playing a role in dispersion and flocculation.
Experiments have also proven that the slurry with added carboxymethyl cellulose CMC has good dispersion and flocculation effects. Slurry without carboxymethyl cellulose sodium added will produce precipitation after being placed for more than 48 hours, separating water and slurry, while the slurry after adding carboxymethyl cellulose sodium will not produce precipitation even if it is placed for a long time. Therefore, adding dispersion suspending agents to the slurry can improve the slurry's dispersion stability, which is conducive to the uniformity and stability of the properties of the green body after molding, thus improving the performance of the product. CMC gum ceramics is a good example.
Impact on slurry water retention: Carboxymethyl Cellulose CMC dissolves in water, and the hydroxyl and oxygen atoms on the ether bond of the long chain combine with water molecules to form a hydration film, making the water lose its mobility, and free water is no longer "free," thereby increasing thickening and water retention in the slurry. With the increase in the amount of carboxymethyl cellulose CMC, more free water in the slurry becomes bound water, and the water retention effect is better. Excellent water retention in the slurry can not only prevent the water from diffusing into the gypsum mold too early during casting, ensuring that the green body loses water evenly, but also avoid the rapid water diffusion process from reducing the strength of the green body, which is of great significance for forming uniform, dense, smooth, and defect-free grouting green body. However, if the amount of carboxymethyl cellulose CMC added is too much, the slurry's water retention will be too high, causing the slurry to lose water too slowly, resulting in a long casting time, which affects the quality of the cast green body and even causes cracking during the drying process.