Factors affecting the gloss of water-based varnish
Water-based varnish is a green and environmentally friendly product. The solvents used are mainly water and some alcohols, and its volatiles will not pollute the environment. Water-based glazing oil has been gradually developed in recent years as people's environmental protection voices continue to rise. At present, its technology is not mature enough. The main problems are poor surface finishing and insufficient gloss.
The varnish coating process is essentially the process of varnish leveling and drying on the printed matter. There are many factors that affect the gloss of the water-based varnish, such as the appearance quality of the coated material, the coating process, the equipment and the quality of the varnish itself. The main discussion here is the performance of the water-based varnish on the gloss. influences.
Factors that affect the gloss of water-based varnish include the use of polymer monomers, the ratio of water-soluble resin and emulsion, the leveling of varnish, viscosity, pH and surface tension.
1 The effect of polymer monomer on the gloss of water-based varnish
The different properties of the polymer monomers have a very important influence on the gloss and other properties of the water-based glaze. Therefore, the selection of the appropriate polymer monomer is the most important part of improving the gloss of the water-based glaze. In actual use, a variety of monomers are usually used for copolymerization. When selecting monomers, it is necessary to add a large amount of additives with improved gloss, and also consider the use of monomers to improve other properties of varnish. Generally speaking, they can choose monomer copolymerization from Table 1.
The monomers that may be used in acrylic resins for water-based varnishes are: methyl methacrylate, styrene, methyl acrylate, β-hydroxypropyl acrylate, acrylic acid, etc. Increasing the amount of methyl methacrylate and styrene results in a coating with high hardness and good gloss, but it is brittle and has poor folding resistance and solubility resistance. Therefore, in order to obtain a high-gloss water-based varnish, a suitable polymer monomer must be selected.
2 Mixing ratio of water-soluble resin and emulsion
As mentioned earlier, the mixing ratio of water-soluble resin and emulsion has a great influence on the gloss of water-based varnish. Increasing the amount of emulsion can improve its gloss, but it reduces the leveling and processing suitability of varnish; increasing the amount of water-soluble resin can improve the leveling and processability of varnish, but its gloss will be reduced . When preparing, please refer to the proportion in the previous formula.
3 The effect of leveling on the gloss of water-based varnish
The leveling of water-based varnish is another important factor affecting its gloss, which is affected by the viscosity, pH and surface tension of the varnish.
If the varnish has good leveling properties, it can be quickly leveled before the coating is dried to form a smooth surface, and the coating will have a high gloss; if the varnish cannot be dried before leveling, Will leave some streaks, trachoma, etc., to reduce its gloss.
The key factors affecting the leveling are the leveling viscosity and drying speed of the varnish during the leveling process. If the leveling viscosity can be kept low, and the initial change rate is small, the coating can level faster. The leveling viscosity is related to the viscosity, pH value, thixotropy of the aqueous varnish and the absorption rate of the coated object. Drying speed also affects leveling. If the speed is low, the rate of change of leveling viscosity is small, and the time given to leveling is also large, which can fully level.
4 The effect of viscosity on the gloss of water-based varnish
The viscosity of the aqueous varnish has a very important influence on the coating suitability such as leveling and wetting during the coating process. This is because the surface of the printed matter has certain absorbency to the varnish, and the absorption rate of the varnish on the surface of the printed matter is proportional to the viscosity value of the varnish. If the viscosity value of the varnish is too small, when coating paper with high absorption rate, it will cause the viscosity of the varnish to change too much during the leveling process. At the initial stage, its viscosity can still meet the leveling requirements. However, in the middle and late stages, the viscosity of the varnish increased significantly, and it was difficult to meet the leveling requirements. As a result, the leveling ended prematurely, causing uneven coating on the surface of the printed matter, and some local under-varnishes affected Smoothness and gloss after drying and calendering. When the printed matter with low absorption rate is coated, the viscosity is too low, and sagging will occur, resulting in problems such as uneven coating and poor drying. If the viscosity is too high and the leveling requirements cannot be met, the leveling ends prematurely and the gloss becomes poor.
Effect of 5PH value on gloss of water-based varnish
The pH value of the water-based glazing oil is mainly adjusted by ammonia water. Generally, the pH value of the water-based glazing oil should be controlled between 8-9. If the pH value is too high and the alkalinity is too strong, the viscosity of the varnish will decrease, affecting the gloss, and at the same time the drying speed will be slower, and the water resistance will be poor; when the pH is too low, the alkalinity will be too weak, the viscosity will increase, and ammonia Too fast to evaporate in the leveling process, so that the leveling ends too soon, affecting the gloss.
6 Effect of surface tension on water-based varnish
The surface tension of the varnish is not only an important factor affecting the gloss, but also the overall quality of the varnish. Because the coating of varnish is usually achieved on the ground or ink film, and their surface free energy is usually small, the surface tension value of the varnish is very different from the wetting, adhesion, and penetration of the same printed matter. Large, the film forming effect after coating and calendering is also very different. If the surface tension value of the varnish is very small, the varnish is easy to wet the surface of the printed matter, and under the effect of the surface tension, it is easy to level into a smooth and uniform film surface; if the surface tension value of the varnish is large, Then there are difficulties in wetting the ink film on the surface of the printed product, and it is not easy to form a uniform coating layer; when the value is greater than the surface tension value of the ink film surface of the printed product, the coated varnish coating will shrink to a certain extent, or even Trachoma and other phenomena appear in some areas.
The above are the main factors affecting the water-based varnish. In addition, the solid content of the water-based varnish also has an effect on its gloss. The effect of the solid content on the gloss is similar to the viscosity. Generally, the solid content of water-based varnish should be controlled at 30% to 40%.
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