There are the following pigments commonly used in the coating paper industry at home and abroad:
   1. Kaolin Clay: It is a kind of clay composed of kaolin minerals and minerals. It is a water-containing silicate generated by weathering or sedimentation of medium and acidic igneous rocks, volcanic rocks, sedimentary rocks, metamorphic rocks, etc. Its particle structure includes hexagonal lamellar structure, tubular and rhombohedral hexahedron, etc. Among them, hexagonal lamellar structure is more suitable for papermaking coating. The hexagonal flake kaolin has a better specific surface area, and has better coverage, smoothness and gloss after coating compared with other kaolin structures.
       2. Calcium carbonate: Calcium carbonate used in coatings is usually divided into heavy ground calcium carbonate and light precipitated calcium carbonate. Heavy grinding calcium carbonate is to grind high-quality limestone with hammer crusher and other equipment, and then grind it with zirconia beads or ceramic beads as abrasive until the particle size reaches the grade required for paint [15]. Its advantages are better coverage than kaolin, high whiteness, good ink absorption, and good adaptability to other components in the coating. The demand for adhesives is lower than kaolin, and the dispersed solid content is higher than kaolin, especially the calcium carbonate reserves in China. The disadvantage is that the wear is higher than kaolin, and too much will reduce the water retention performance of the coating and reduce the gloss of the coated paper and the gloss of the ink.
   3. Titanium dioxide: Titanium dioxide has the highest refractive index of all pigments, which can give the paint excellent opacity and whiteness, and its hiding power is also very strong. There are usually three forms, rutile, anatase and plate titanium, etc., general coatings use rutile and anatase type titanium dioxide. It is mainly produced in Australia. However, the high price limits its application in the coated paper industry. It is usually used when other pigments do not provide the desired opacity, or when a small amount is used in the coating of cardboard to shield the darker base paper. In the coating of European coated paper and white cardboard, the amount of titanium dioxide is generally not more than 5 parts, even if the ultra-low quantity coated paper, the amount of titanium dioxide is not more than 15 parts.
   4. Plastic pigments: Plastic pigments are new pigments developed in the 1970s, which belong to synthetic resins. The main components are styrene and styrene and acrylate copolymer emulsion. There are hollow balls and solid balls. Hollow sphere is now used more. The particle size can be controlled as needed to provide the best light scattering effect. The specific gravity is close to the water, can be evenly dispersed in the paint, in the coating and drying, the water inside the hollow ball diffused through the porous shell, the particles are filled with air to remain dispersed without forming a film, after soft finishing, the particles are flattened, so that the paper can be given a good gloss and ink gloss. Because the particles are easily flattened, the pressure of soft calendering is not high, reducing the loss of the loose thickness of the coating, and the need for adhesives is less than that of traditional mineral pigments. However, its high price limits its application, and it is generally used in some high-end coated paper industries. It should be noted that because the specific gravity of plastic pigments is much lower than that of mineral pigments, the viscosity of the coatings will increase with the increase of the proportion of plastic pigments.