Effect of Coated Paper Coating Structure on Printability

The structure of the coated paper plays an important role in the performance of the coated paper, and the properties of the coated paper have a great influence on the final printing performance.

The purpose of paper coating is to give the paper a good appearance and good printability. Coating can improve some of the paper's surface strength, whiteness, smoothness, opacity, ink absorption and other properties, but in the end we want to achieve a better printability, so that printing products printed on paper The quality meets our requirements. These properties are very much related to the coating structure.

The formation process of the coating is very complicated. Generally, the amount of coating needed for calculation is applied to the original paper. The water in the coating penetrates and evaporates to the base paper to form a three-dimensional particle stacking structure. Then the drying stage causes the coating to deform and then passes through the pressure. Light further compresses the coating. Therefore, the main components of the coating structure are pigments and adhesives as well as a small amount of additives. The effect of pigments on the coating is the most important.

Effect of Pigments on Coating Structure

The pigment is the main component of the coating structure. It mainly refers to a white pigment, and its volume accounts for more than 70% of the coating structure. At present, the most used pigments in the country are kaolin, calcium carbonate and titanium dioxide. The difference in the crystal shape and particle size distribution of these pigments can make the coating structure greatly different.

1. The effect of different pigment types on printability
The use of kaolin should be relatively extensive. It has good fluidity and maneuverability, high smoothness of the coating formation, and its price is relatively affordable, the use effect is good, the shape of the particles is hexagonal, and it can be better. Cover the surface of the paper. However, due to its low solids content and high shear viscosity, its use is limited to less than 20%.

The calcined kaolin has a void-filled structure that provides optimal light scattering and opacity, increases the coating's elasticity, imparts higher whiteness and gloss to the coated paper, but it increases coating The viscosity and hardness of the layer increase the wear, so it is not used alone.

Although the coating of amorphous calcium carbonate is inferior in smoothness and gloss, it has high brightness and good ink absorption. Irregular particle morphology also gives very good rheological properties to calcium carbonate. Properly processed light calcium compensates for some of the disadvantages of ground calcium carbonate, with higher whiteness, scattering coefficient and purity, low abrasion value and narrow particle size distribution, giving the resulting coating a fine microporous structure and high Absorption of ink, improve the looseness of coated paper. Gloss and opacity improve print printability. Due to the porous nature of the calcium carbonate particles, the coating structure has a large porosity, and it is easy to produce a relatively open coating surface. Titanium dioxide has the highest refractive index in white pigments, and its use in coatings can significantly increase the hiding power of coatings.

2. Effect of pigment combinations on coating properties

Appropriate pigment combinations for optimal coating performance

Because these pigments can not only coexist, they can also interact with each other to improve and complement each other's performance. In the best combination of pigments to achieve a high performance coating, it is possible that the selected pigment itself is not the best, but it plays a key role as part of the coating. Fine Kaolin ~JJD. Calcium carbonate is one of the recipes. Although fine kaolin clay itself is not optimal, fine kaolin clay can provide good coating properties as part of the best system for calcium carbonate coatings. The traditional kaoline of fine-grained particles is poor in gloss, and fine-grained kaolin alone can increase the gloss of the paper sheet, but it also reduces the △ gloss. This is due to the fact that this pigment produces fine micropores. Similarly, fine ground calcium carbonate itself has low gloss and print gloss, but the use of ground calcium carbonate, which is poor in performance, and fine-particle kaolin, which produces fine pores, can increase Δ gloss. After the two pigments are mixed, they complement each other in some form to form a superior coating. In particular, the narrow particle size distribution of kaolin and calcium carbonate mix provides the possibility to control the pore size of the coating in a wide range. This gives it the best pore volume, keeping the coating smooth and optical, while at the same time achieving the desired printability.

Effect of Different Kinds of Adhesive on Coating Structure

1. Effect of Different Kinds of Adhesive on Coating Structure

The type of adhesive has a certain influence on the performance of coated papermaking. Different adhesives have different effects on the coating. Natural adhesives (such as starch, casein, etc.) than synthetic adhesives (latex, PVA) Produce a larger coating void volume on the more open coating surface, so that the air permeability, K&N value of the coated paper is larger. PVA and latex have strong film-forming properties and interact strongly with pigment particles. The void volume of the resulting coating is small, the surface is closed, the air permeability of the coated paper is small, and the K&N value is low. Water-soluble adhesives (starch, casein, PVA) produce coarser microscopic surfaces, lower gloss, and larger K&N values ​​than latexes. Its print gloss is also lower than that of latex.

2. Effect of Adhesive Migration on Coating Structure
The migration of the adhesive can cause uneven distribution of the internal structure of the coating. Migration can occur in two directions, namely internal migration to the base paper and migration to the coating surface. Application and drying are two very important stages in the coating process and are two stages in which the adhesive migrates. At present, there are not many reports on the migration of adhesives during the application phase, and the results have yet to be discussed. During the drying process. Due to the large amount of moisture in the paint and base paper is evaporated. The movement of water greatly affects the migration of adhesives. When dry intense. The moisture in the coating is rapidly evaporated. The adhesive in the coating migrates less to the interior of the base paper.

The paint surface has a relatively high concentration of adhesive. Makes the coating's micropore volume decrease. Micro hole becomes smaller. Printing gloss decreases. on the contrary. When the drying is too mild, the adhesive migrates to the interior of the base paper. The coating surface concentration of the adhesive is too low and the micropores become larger. The ink absorption during printing is too large, and the surface strength is low and the gloss is poor. therefore. Proper control of adhesive migration is the key to improving paper surface strength and gloss.

3. Effect of Adhesive Content on Printing Performance
Effect of Additives in Coatings on Coating Structure
Some pigment particles such as precipitated calcium carbonate are difficult to disperse. Easy to produce particle flocculation. And because many pigments have smaller particle sizes. It is susceptible to secondary flocculation under the influence of attractive forces such as molecular forces. therefore. Must use dispersants and ensure the dispersion effect and stability. To avoid flocculated particles affect the optical properties of the coating structure and the paper. Many synthetic water-soluble polymers can be used as thickeners.

Through the interaction with pigments to improve the water retention properties of the paint. Some foreign experts have studied the four polymeric thickeners: It is believed that the thicker the polymeric thickener has on the pigment particles, the greater the effective thickening effect. Adsorption depends on the composition of the polymer and the surface structure of the pigment. The thickener has a greater adsorption capacity for china clay than calcium carbonate, and the addition of a thickener can make the surface of the coated paper smoother and more uniform. CMC (Carboxymethyl Cellulose) can effectively reduce the migration of adhesives, improve coating coverage, and increase coating strength due to its excellent water retention. And can reduce the porosity of the coating. It is easy to produce a thicker and more open coating.

In the coating composition of coated paper, each aspect will have more or less influence on the various properties of coated paper. In order to improve printability, we need to consider factors such as pigments, adhesives, additives, and coating methods. On this basis, choose the best combination. The coated paper meets the required printing performance.