Functions of Sodium Carboxymethyl cellulose in Pigment Coating
Sodium Carboxymethyl cellulose has been widely used pigment coating of paper and paperboard, including the ingredients of the coatings in bar coating, air knife coating, blade coating, and metering film coating. It is all very important that Sodium Carboxymethyl cellulose plays a role in controlling and regulating the rheology and water retention of coatings, and the runnability of coating operation, as well as other supplementary roles. The functions of Sodium Carboxymethyl cellulose in pigment coating mainly include: improving the stability of coatings, regulating the water retention of coatings, adjusting the viscosity of coatings, acting as the binder and dispersant, and being the carrier of brightener and lubricant of blade.
2.1 Improving the Stability of Coatings
After the extensive use of blade coating, the coater speed is increasingly growing, and the requirements for the stability and operability of coatings have also been increased. Sodium Carboxymethyl cellulose can improve the mechanical and chemical stability, both of which would affect the performance of coater.
2.2 Regulating the Water Retention of Coatings
The water retention of coatings usually means that the coatings of paper have the ability to maintain the free water without overflow. The water retention of coatings is a crucial indicator in the production of high-quality coated paper and paperboard, and is the key to improving the efficiency and quality of coatings. Paper is a kind of porous absorbent material. Therefore, paper can absorb some moisture from coatings containing soluble adhesives. The higher the water retention of coating is, the less water paper will absorb and the less the migration of adhesives will be; if the water retention is too low, a series of problems will occur, such as the occurrence of coating streaks, the migration of adhesives, poor control of coating rheology, and the increase in the number of sheet breaks, etc.
Water-soluble adhesives can improve water retention, and in this regard Sodium Carboxymethyl cellulose is very effective. Other common adhesives can also improve water retention, but the amount should be large enough to achieve the same effect as that of Sodium Carboxymethyl cellulose . In general, Sodium Carboxymethyl cellulose of low to medium viscosity types has higher water-retaining time than that of high viscosity type.
2.3 Rheological Properties
The coatings containing Sodium Carboxymethyl cellulose are pseudoplastic (shear thinning), which is just suitable for the requirements of high-speed blade coating, with excellent water retention and favorable operating performance in the coating machine. When Sodium Carboxymethyl cellulose is used as an auxiliary adhesive, these properties can be obtained and improved. For the coatings with the use of Sodium Carboxymethyl cellulose , the rheological properties are usually not affected by the type and amount of latex. When the type and amount of Sodium Carboxymethyl cellulose are changed, the viscosity of coatings can be controlled and regulated.
Comparison of Application Effects of Sodium Carboxymethyl cellulose and Soy Protein
In the coating of paperboard, the soy protein auxiliary adhesive has been applied in North America for decades. It can be seen from the results that CMC has a higher brightness and gloss than soy protein.
Sodium Carboxymethyl cellulose has a higher brightness and gloss than the coating of soy protein, while other properties such as smoothness, GIT, K & N, print gloss and adhesive force are similar. Since soy protein itself is brown, and the addition amount is high, it has affected the brightness and gloss of coated paperboard; the total adhesive amount of Sodium Carboxymethyl cellulose coating is lower, also making the brightness and gloss better. Since Sodium Carboxymethyl cellulose has very good adhesive force, 1 share of Sodium Carboxymethyl cellulose can replace 3-4 shares of soy protein; with the use of Sodium Carboxymethyl cellulose , the total adhesive amount has been reduced and the cost can be reduced; in place of soy protein, the optical properties can be improved; the high shear viscosity is lower, improving the operating performance, especially in high-speed coating; Sodium Carboxymethyl cellulose is convenient for dissolution, with no need to add NaOH or NH4OH to help dissolve like soy protein, improving the operating environment.
- Light Weight Coated Paper (LWC)
LWC is the main variety of coated paper containing mechanical pulp, mainly used for magazines, catalogs and advertisements, and it is one of the paper grades with the highest annual growth.
Experimental results show that the optical properties of Sodium Carboxymethyl cellulose coatings are better and the opacity is also better. The opacity is very important to LWC. The total adhesive amount of Sodium Carboxymethyl cellulose coating has been reduced and Sodium Carboxymethyl cellulose has prevented fine pigment particles penetrating into paper, so that more pigment particles will be retained in the coating layer. Therefore, in such aspects as gloss and smoothness, the coatings with the use of Sodium Carboxymethyl cellulose are better than that with starch.
The biggest difference between Sodium Carboxymethyl cellulose and starch, the two kinds of coatings, lies in printability. Due to the migration of adhesives, the coatings with starch almost inevitably generate printing spots, while the coatings with Sodium Carboxymethyl cellulose have very small migration phenomenon.
The coatings prepared by Sodium Carboxymethyl cellulose have better operating performance than those by starch, especially for high-speed coaters. The coatings prepared by CMC are pseudoplastic and get thinned by shear, which is suitable for the requirements of high-speed blade coating. Sodium Carboxymethyl cellulose also has the role of lubricant. The experimental result of LWC by gravure printing is similar to the above conclusion.