In this article, we discuss the coater in the paper manufacturing process used to enhance the quality of printed images. Blade and roll coaters are the two most commonly used methods, with starch being the preferred binder for blade coatings and synthetic products used for roll coatings. Starch is commonly used as a binder for paper coatings, but its limitations include low strength, water sensitivity, and high surface energy. Starch ethers and esters are widely used as coating binders due to their resistance to retrogradation. Cationic binders have been attempted to render the coating color cationic, but they are not widely used. All coating colors containing starch as a binder must also contain a biocide to prevent spoilage.
The Coater in the Paper Machine
In the paper manufacturing process, applying a coating to the paper surface can enhance the quality of the printed image. On-machine and off-machine coating are the two methods used for this purpose. The most commonly used coaters are blade coaters which apply and remove excess coating fluid from the paper surface. Roll coaters, on the other hand, print a metered film of coating fluid onto the paper surface. The coating fluid consists of pigments in water, binders, and functional adjuncts.
Starch is the preferred binder for blade coatings, while synthetic products are used for roll coatings. The quantity of coating applied by a blade coater is determined by various factors such as the roughness of the paper surface, the solids content and viscosity of the coating color, the speed of the coating operation, the geometry of the system, blade mechanical characteristics, and the pressure applied at the blade. The coating color will release water and form a filter cake after application to the paper, which facilitates holdout on the surface.
Starch Selection for Paper Coating
Starch is commonly used as a binder for paper coatings, either on its own or combined with synthetic binders. However, its low strength, water sensitivity, and high surface energy limit its use as a coating binder. Starch can be adjusted in molecular weight to obtain the desired rheological properties for uniform coating distribution and controlled water retention at high application speeds. Starch ethers and esters, which are resistant to retrogradation, are widely used as coating binders. Excessive reaction of the substituting reagent at the surface of starch granules can impair their dispersion and may require high-temperature cooking to obtain a uniform paste. Starch esters are sensitive to alkalinity, which will break the ester bond. Various attempts have been made to render the coating color cationic using cationic binders, but there has not been widespread use of this process. All coating colors containing starch as a binder must also contain a biocide to prevent spoilage.