Melacoll 2021-01-25 02:19
Cellulose is a natural polymer compound of polysaccharide macromolecules, and its chemical structure is based on anhydrous β-glucose. Each base ring has one primary hydroxyl group and two secondary hydroxyl groups. A series of cellulose derivatives can be obtained through chemical modification, such as cellulose ether. Cellulose ethers are widely used in many industries, and cellulose ether in the coating industry is one of the main application areas of cellulose ethers.
Cellulose ether is a white or milky white, odorless, non-toxic, fluid fibrous powder, easy to absorb moisture, and dissolves in water into a transparent viscous stable colloid.
MC is soluble in cold water, insoluble in hot water, and soluble in some solvents; MHEC is soluble in cold water but insoluble in hot water and organic solvents. HPMC solubility is the same as MHEC.However, MC and MHEC will precipitate when the aqueous solution of MC and MHEC is heated. MC precipitates at 45~60℃, while the precipitation temperature of mixed etherified MHEC increases to 65~80℃.
Cellulose ether has a certain degree of delayed swelling in neutral pH water, but it can overcome this delayed swelling in alkaline pH water.
Cellulose ether is dissolved in water in colloidal form, and its viscosity depends on the degree of polymerization of cellulose ether. The solution contains hydrated macromolecules. Due to the entanglement of macromolecules, the solution’s flow behavior is different from that of Newtonian fluid, but it shows behavior that changes with shear force. Due to the macromolecular structure of cellulose ether, the solution’s viscosity increases rapidly with increased concentration and decreases rapidly with the increase of temperature.
Cellulose ether is used in the water phase. As long as there is water, bacteria will grow. The growth of bacteria leads to the production of enzyme bacteria. Enzyme bacteria break the bond of adjacent unsubstituted anhydroglucose unit of cellulose ether, which reduces the molecular weight of the polymer. Therefore, if the cellulose ether aqueous solution is to be stored for a long time, it is necessary to add preservatives. This is true even when using antibacterial cellulose ether.
Cellulose ether in the coating, such as HPMC, HEC plays a significant role. Adding 0.2% to 0.5% of the total formula can thicken, retain water, prevent precipitation of pigments and fillers, and increase adhesion and bonding strength.
About cellulose ether viscosity grade for coating, you can watch the following video:
The viscosity of the aqueous solution of cellulose ether changes with the shearing force. Coatings and slurries thickened with cellulose ether also have this characteristic. To make the coating easy to apply, the type and quantity of cellulose ether must be carefully selected. For coatings, when cellulose ether is used, medium viscosity products can be used.
Cellulose ether can prevent moisture from quickly entering the porous substrate so that during the entire construction process, it can form a uniform coating without making it dry too quickly. When the emulsion content is high, the water retention requirement can be met using less cellulose ether. The water retention of coatings and slurries depends on the concentration of cellulose ether and the temperature of the coating substrate.
Pigments and fillers have a tendency to precipitate. To keep the coating uniform and stable, the pigment filler must be suspended. Cellulose ether in the coating can make the coating have a specific viscosity and will not produce precipitation during storage.
Due to the excellent water retention and adhesion of cellulose ether, good adhesion between the coating and the substrate can be ensured. MHEC and NaCMC have excellent dry adhesion and adhesion, so they are especially suitable for paper reinforced pulp, while HEC is not suitable for this purpose.
Because of cellulose ether’s hydrophilicity, it can be used as a protective colloid for coatings.
Cellulose ether is widely used in latex paint as a thickener to adjust the construction viscosity. Medium and high viscosity hydroxyethyl cellulose and methyl hydroxyethyl cellulose are mainly used in emulsion paint. Sometimes cellulose ether can be used together with synthetic thickeners (such as polyacrylate, polyurethane, etc.) to improve specific properties of latex paint and give the latex paint uniform stability.
Cellulose ethers have excellent water retention and thickening properties, but some properties and application are different. We have to choose viscosity grades according to different applications. Anionic cellulose ethers easily form water-insoluble salts with divalent and trivalent cations. Therefore, compared with methyl hydroxyethyl cellulose (HEMC) and hydroxyethyl cellulose (HEC), sodium carboxymethyl cellulose has lower scrub resistance. But HPMC and CMC also have many differences, you should choose according to your application.
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