Six major factors affecting the gloss of waterborne varnish
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Water-based varnish is a green product. The solvent used is mainly water and some alcohols, and its volatiles will not pollute the environment. Water-based glazing oil has gradually developed in recent years with the increasing voice of people's environmental protection. At present, its technology is not mature enough. The main problems are that the surface finishing effect is not good, and the gloss can not reach the ideal effect.
The coating process of varnish is essentially the process in which the varnish is leveled and dried on the print. There are many factors affecting the gloss of waterborne varnish, such as the appearance quality of the coated material, the coating process, the quality of the equipment and the varnish itself. The main discussion here is the performance of the waterborne varnish on the gloss. influences.
The factors affecting the gloss of the water-based varnish include the polymer monomer, the mixing ratio of the water-soluble resin and the emulsion, the leveling property of the varnish, the viscosity, the pH value and the surface tension.
1. Effect of polymer monomer on the gloss of waterborne varnish
The different properties of the polymer monomer have an important influence on the gloss and other properties of the prepared water-based varnish. Therefore, selecting the appropriate polymer monomer is the most important part to improve the gloss of the water-based varnish. In actual use, a plurality of monomers are usually used for copolymerization. When the monomer is selected, a large amount of additives having an improved glossiness and a use of other properties of the varnish are also required. Generally they can be selected from Table 1 for monomer copolymerization.
The monomers which may be used for the acrylic resin of the aqueous varnish are methyl methacrylate, styrene, methyl acrylate, β-hydroxypropyl acrylate, acrylic acid and the like. Increase the amount of methyl methacrylate and styrene to form a coating with high hardness and good gloss, but it is brittle, and has poor folding resistance and solubility. Therefore, in order to obtain a high gloss aqueous varnish, it is necessary to select a suitable polymer monomer.
2. Mixing ratio of water-soluble resin and emulsion
As already mentioned, the blending ratio of the water-soluble resin to the emulsion has a great influence on the gloss of the aqueous varnish. Increasing the amount of the emulsion can increase the gloss, but the leveling property and processing suitability of the varnish are lowered; increasing the amount of the water-soluble resin can improve the leveling property and processing suitability of the varnish, but the gloss thereof is lowered. . When preparing, refer to the ratio in the previous formula.
3. Effect of leveling on the gloss of waterborne varnish
The leveling of waterborne varnish is another important factor affecting its gloss. It is affected by varnish viscosity, pH and surface tension.
If the varnish has good leveling property, it can be quickly leveled before the coating is dried to form a smooth surface, and the coating will have high gloss; if the varnish cannot be dried before leveling, Will leave some streaks, blisters, etc., so that its gloss is reduced.
The key factor affecting the leveling property is the leveling viscosity of the varnish during the leveling process and its drying speed. If the leveling viscosity is kept low and there is a small rate of change in the initial stage, the coating can be leveled faster. The leveling viscosity is related to the viscosity, pH value, thixotropy of the aqueous varnish and the absorption rate of the coated object. The drying speed also affects the leveling property. If the speed is small, the rate of change of the leveling viscosity is small, and the time for leveling is also large, and the leveling can be sufficiently leveled.
4. Effect of viscosity on the gloss of waterborne varnish
The viscosity of the aqueous glazing oil has an important influence on the coating suitability such as leveling property and wettability in the coating process. This is because the surface of the printed matter has a certain absorption to the varnish, and the absorption rate of the surface of the printed matter to the varnish is proportional to the viscosity of the varnish. If the viscosity of the varnish is too small, the coating of high absorbency paper will cause the viscosity of the varnish to change too much during the leveling process. In the initial stage, the viscosity can still meet the leveling requirements. However, in the middle and late stages, the viscosity of the glazing oil is obviously increased, and it is difficult to meet the leveling requirements. As a result, the leveling ends prematurely, causing uneven coating on the surface of the printed matter, and some parts are owed to the varnish. Smoothness and gloss after drying and calendering of the film.
When the printed matter having a low absorptivity is applied, the viscosity is too low, and sagging occurs, resulting in problems such as uneven coating and poor drying. If the viscosity is too high and the leveling requirements are not met, the leveling of the leveling is prematurely terminated, and the gloss is deteriorated.
5. Effect of PH value on the gloss of waterborne varnish
The pH of the aqueous glazing oil is mainly regulated by ammonia water. Generally, the pH value of the aqueous glazing oil should be controlled between 8 and 9. If the pH is too high, 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 worse; when the pH is too low, the alkalinity will be too weak, the viscosity will increase, and the ammonia will be Too fast to volatilize during the leveling process, so that the leveling ends too quickly, affecting the gloss.
6. Effect of surface tension on waterborne varnish
The surface tension of the varnish is not only an influence on the gloss but also an important factor affecting the overall quality of the varnish. Because the coating varnish is usually realized on the ground or on the ink film, and their surface free energy is usually small, the difference in the surface tension value of the varnish is very different for the wetting, adhesion and soaking effect of the same print. Large, the filming effect after coating and calendering is also very different. If the surface tension value of the varnish is small, the varnish easily wets the surface of the printed matter, and under the action of the surface tension, it is easy to level into a smooth and uniform film surface; if the surface tension of the varnish is large, Then there is difficulty in wetting the ink film on the surface of the printed matter, and it is not easy to form a uniform coating layer; when the value is greater than the surface tension value of the printed ink film, the coated varnish coating will have a certain shrinkage, even In some parts, there are phenomena such as blisters.
The above is the main factor affecting the aqueous varnish. In addition, the solid content of the aqueous varnish also has an effect on the gloss. The effect of the solid content on the gloss is similar to the viscosity. Generally, the solid content of aqueous varnish should be controlled at 30% to 40%.