Why need paper coating process? Because paper fiber is no longer loose arrangement and it has some tightness after calendering. But the smoothness of the surface still does not meet the requirements of high-end fine print. The non-coated paper has poor surface uniformity, the printing ink is not uniform and low gloss, it cannot make the tiny dot restored, then it results in level loss and some uniformity in the field of the image decline, this affect the printing effect, thus fine printing paper needs paper coating processes to further improve the paper surface properties.
Lots of materials for paper, such as plant fibers and animal fibers, organic and inorganic, metal and so on. Coating additives can improve the quality of paint, give special properties or make coating operations normal. This series of chemicals mainly are: dispersant to reduce the high concentration of paint viscosity to make it has good rheological properties; lubricant can be used to build paint rheology, reduce the friction coefficient and reduce sticking phenomenon of of coated paper in super calendering; solvent resistance to increase water resistance of starch and casein, PVA and other coating adhesive; paint rheology modifiers used to adjust the viscosity of the paint to suit a variety of coating process. Viscosity reducing agent is to reduce the viscosity of paints, such as urea, dicyandiamide, etc. While thickener or super absorbent polymers is to increase viscosity of paints, such as acrylates copolymer, cellulose derivatives, and so on. Viscosity reducing agent can make paint maintain appropriate rheological properties, avoid coated paper peeling, streak or arching problems. Also, it needs to add defoaming agents, preservatives, fluorescent whitening agents and other additives during the coating process.
Coating adhesive can make the pigment bonding with each other in coating process. The traditional use of coating adhesive are casein, soybean protein, oxidized starch and polyvinyl alcohol and other natural or synthetic compounds, in the 1950s developed of styrene-butadiene LaTeX, acrylic emulsions, polyvinyl acetate, then is Carboxylated styrene-butadiene LaTeX which used as adhesive coating can improve mechanical stability and increased brightness coated paper and printing ink. It has taken the place of casein and used as the primary binder. After the oil crisis, mix use of synthetic LaTeX mixed and starch phosphate ester and other starch derivatives to ensure quality and reduce the cost of adhesives.
Purpose of paper coating is to cover a layer of paint consists of fine particles that has good absorption to inks on the surface formed by fibers of the rough and plain paper with large pores. This in order to obtain a good uniformity and smoothness of the paper. It also improves gloss, stability and opacity of the paper through coating, and these characteristics increase with the increase of coating. If you have interest, please follow OKCHEM.com now!