Effects of Sodium Carboxymethyl cellulose on the Performance of Ceramic Slurry
Sodium Carboxymethyl cellulose (referred to CMC) is a derivative with the ether structure obtained by chemical modification of natural cellulose. Due to the poor water solubility of acid form, the products are generally made into the sodium salt, i.e. sodium Sodium Carboxymethyl cellulose.
Sodium Carboxymethyl cellulose is a representative anionic cellulose ether, with the hydrophilic group introduced into the structural unit of cellulose, has such excellent properties as thickening, dispersion, suspension, emulsification, adhesion, film formation, protective colloids, and moisture protection, widely used in detergents, ceramics, oil recovery, textile, food, paper, tobacco, paints, medicine, cosmetics, building materials and other industrial sectors, and is a very important water-soluble polymer mostly widely used in the industrial production. Sodium Carboxymethyl cellulose is widely used in the ceramic industry as an additive. Its roles in the ceramic slurry and glaze are mainly dispersion, de-agglomeration, suspension, adhesion, and water retention; it has become an important auxiliary material in the industrial production of ceramics.
2. Effects of Sodium Carboxymethyl cellulose on the Ceramic slurry
Adding Sodium Carboxymethyl cellulose in the slurry can improve such properties of ceramic slurry as dispersion suspension, fluidity, water retention, viscosity, bond strength, and de-agglomeration.
2.1 Dispersed Suspension Effects
Sodium Carboxymethyl cellulose is a water-soluble long chain high polymer, and added into the slurry, the molecule long chains can form the the unique network structure, supporting the gravity of particles. The molecules or ions with Sodium Carboxymethyl cellulose stretch and occupy a certain space in the slurry like the ribbon, prevent particles from contacting with each other, and improve the spatial stability. And after the dissociation of Sodium Carboxymethyl cellulose and the absorption of negatively charged anionic groups with particles, making the particle surface potential increase, when particles get close the electrostatic repulsion will increase, thus playing the role of dispersed suspension.
Experiments have also proved that the slurry with the addition of Sodium Carboxymethyl cellulose has good dispersed suspension effects. The slurry without the addition of Sodium Carboxymethyl cellulose will produce precipitation when placed over 48 hours and the separation of water and the slurry will occur, while the slurry after adding Sodium Carboxymethyl cellulose will not produce precipitation even placed for a long time. Therefore, adding the dispersed suspending agent to the slurry can improve the dispersion stability of the slurry, benefit the uniformity and stability of the properties of green body after forming, and thus improve the performance of products.
2.2 Effects on the Fluidity of Slurry
The addition of Sodium Carboxymethyl cellulose can improve the fluidity of slurry. Studies have shown that with the increase of Sodium Carboxymethyl cellulose amount, the fluidity of slurry has an increase at first, then appears a downward trend, and its content between 0.5% and 1.5% can have better effects on improving the flow properties of mortar. In the very beginning, with the increase in the addition amount of Sodium Carboxymethyl cellulose, the fluidity of mortar improves, on one hand because the lubricating effect of Sodium Carboxymethyl cellulose between the particles enables the components of mortar to flow alone; on the other hand, it is due to the ball effect generated by the bubbles caused by the dispersed suspension effect and the induction effect on air. However, with the increase in the addition amount of Sodium Carboxymethyl cellulose, the fluidity of mortar will have a decrease, which is because its viscosity has hindered the flow of mortar. Therefore, Sodium Carboxymethyl cellulose has an optimum amount range in terms of improving the flow properties of mortar. The improved slurry with the addition of Sodium Carboxymethyl cellulose has a certain fluidity. If the fluidity is too high, the strength of green body after molding will not be enough, affecting the demolding and fettling quality. If the fluidity is poor, it will be inconvenient for the delivery of slurry, and the green body after molding will also be easily deformed. Therefore, controlling the fluidity of slurry is of great importance on meeting production needs, improving production efficiency and product quality.
2.3 Effects on the Water Retention Property of Slurry
When Sodium Carboxymethyl cellulose gets dissolved in water, the hydroxyl groups on the long chain and the oxygen atoms on the ether bond will be associated with the water molecules into the hydrogen bond, forming hydrated film and making water lose fluidity, and the free water is no longer “free”, thus producing the thickening and water-retaining effects on the slurry. With the increase in the amount of Sodium Carboxymethyl cellulose, more free water in the slurry will become the associated water and the water retention effects will be better. The excellent water retention property of slurry can not only prevent the moisture of slurry from prematurely diffusing into the plaster mold during the casting process and ensure the uniform water loss of green body, but also can avoid that too fast water diffusion process may reduce the strength of green body, which is of great importance on the formation of uniform, dense, smooth and defect-free grouting green body. However, if the addition amount of Sodium Carboxymethyl cellulose is too much, the water retention of slurry will be too high, so that the water loss of slurry will be too slow, resulting in the too long casting time and affecting the quality of casting green body, and even making the green body produce cracks in the drying process.
2.4 Effects on the Viscosity of Slurry
Sodium Carboxymethyl cellulose is a polymer molecule. When the addition amount into the glaze slurry is smaller, the free moving space of its molecular chain is greater, and several adsorbable groups on the chain may be absorbed by two or more solid particles, playing the bridging role, so the viscosity is larger. When the addition amount of Sodium Carboxymethyl cellulose is appropriate, the added polymer molecules can just be completely adsorbed by the solid particles, and almost all the interfaces of solid particles are covered with the molecular chains of polymer, appearing the steric stabilization (and the electrostatic repulsion), so the viscosity is the minimum. After adding an excessive amount, except for the polymers covered on the surface of solid particles, the excess polymers are distributed in the medium, and its long molecule chains and the solid particles with the adsorption layer will produce a bridging effect. A polymer chain molecule can bridge one pair of particles, or several particles. With the increase in the addition amount of Sodium Carboxymethyl cellulose, the number of bridged particles in the glaze slurry increases and the viscosity of suspension increases. When the added polymers make all the particles with absorption layers bridged up, then flocculation occurs. For the improvement of Sodium Carboxymethyl cellulose on the viscosity of slurry, it ensures that the slurry should have lower viscosity and good suspension stability, while ensuring that no flocculation phenomenon occurs in the slurry.
2.5 Effects on the Adhesion
The adhesion of Sodium Carboxymethyl cellulose in the slurry is shown by the firm network structure formed by the polymer macromolecules or the macromolecules depending on the hydrogen bonds and van der Waals forces. When Sodium Carboxymethyl cellulose is added to the slurry, water will penetrate inside the Sodium Carboxymethyl cellulose gum blocks. The part with less hydrophilic groups will combine with water and produce swelling, while the part with more hydrophilic groups will separate from gum blocks after swelling. Since the hydrophilic groups are not uniform in the production of Sodium Carboxymethyl cellulose, then the dispersed particle sizes of micelles are inconsistent. For the internal of the micelles, hydration swelling will occur, while the external will be combined with a layer of hydrates, i.e., the bound water layer. In the early stage of dissolution, the micelles are free in the colloid. Due to the asymmetry of size and shape, and regularly moving closer to each other by van der Waals forces, the bound water layer gradually forms the network structure. Since Sodium Carboxymethyl cellulose is fibrous, the network structure formed has a larger volume, and then stronger adhesion ability. Sodium Carboxymethyl cellulose can be added to improve the adhesion properties of slurry. If the slurry has poor bonding performance, the strength of green body will be low, easily leading to the waste products during the transport, kiln loading, and drying processes for such reasons as collision and rubbing; if the bonding properties of slurry are too strong, leading to the strong water retention but poor fluidity, it can not guarantee the product quality in the casting and drying processes, and can also easily lead to the appearance of waste products. Studies have shown that adding Sodium Carboxymethyl cellulose of 0.2 to 0.5% to the green body can increase the adhesive force of green body, is easy for molding, and can improve the mechanical strength of green body by 2-3.
As a water-soluble polymer, Sodium Carboxymethyl cellulose enables the slurry to have excellent dispersed suspension, fluidity, water retention, deagglomeration and other properties, also improves the viscosity of slurry, and has become an important additive in the ceramic industry.