Water-based varnishes are classified as eco-friendly products; the primary solvents used in their formulation are water and various alcohols, ensuring that their volatile components do not pollute the environment. The development of water-based varnishes has gained momentum in recent years, driven by the growing public demand for environmental protection.

The process of applying varnish essentially involves the coating material leveling out and drying on the printed substrate. Numerous factors influence the gloss level of water-based varnishes—including the surface quality of the substrate, the coating process itself, the equipment utilized, and the quality of the varnish formulation. This discussion focuses primarily on how the intrinsic properties of the water-based varnish affect its gloss. Factors inherent to the varnish that influence its gloss include the selection of polymer monomers for the resin, the blending ratio of water-soluble resins to emulsions, as well as the varnish's leveling properties, viscosity, pH value, and surface tension.

01 The Impact of Polymer Monomers on Water-Based Varnish Gloss

The distinct properties of the polymer monomers used have a profound impact on both the gloss and other performance characteristics of the resulting water-based varnish. Consequently, selecting the appropriate polymer monomers constitutes the most critical step in enhancing the gloss of water-based varnishes. In practical applications, it is common practice to employ a copolymerization process involving multiple monomers. When selecting monomers, it is essential to incorporate a substantial proportion of monomers known to boost gloss, while simultaneously ensuring the inclusion of monomers that serve to improve other key performance attributes of the varnish. Monomers commonly utilized in the acrylic resin formulations for water-based varnishes include methyl methacrylate, styrene, methyl acrylate, β-hydroxypropyl acrylate, and acrylic acid. Increasing the proportion of methyl methacrylate and styrene results in a coating with high hardness and excellent gloss; however, such coatings tend to be more brittle and exhibit reduced resistance to folding and solvents. Therefore, to achieve a high-gloss water-based varnish, the selection of appropriate polymer monomers is absolutely essential.

02 The Blending Ratio of Water-Soluble Resins to Emulsions

As previously noted, the specific ratio at which water-soluble resins are blended with emulsions exerts a significant influence on the gloss level of the water-based varnish. Increasing the proportion of emulsion can enhance glossiness, but it simultaneously diminishes the varnish's leveling properties and processing suitability. Conversely, increasing the proportion of water-soluble resin improves the varnish's leveling properties and processing suitability, yet results in a reduction in glossiness. During preparation, one may refer to the proportions specified in the preceding formulations.

03 The Impact of Leveling Properties on the Glossiness of Water-based Varnish

The leveling property of water-based varnish is another critical factor influencing its glossiness; it is governed by the varnish's viscosity, pH value, and surface tension. If the varnish possesses superior leveling properties—enabling it to spread out rapidly and form a smooth surface before the coating dries—the resulting film will exhibit high glossiness. Conversely, if the varnish dries before adequate leveling occurs, it may leave behind streaks, pinholes, or other surface defects, thereby compromising its glossiness. The key factors influencing leveling are the varnish's viscosity during the leveling process and its drying rate. If the viscosity during leveling remains relatively low—and exhibits a low rate of change during the initial stages—the coating will be able to level out more rapidly. This "leveling viscosity" is interrelated with the water-based varnish's inherent viscosity, pH value, thixotropy, and the absorbency of the substrate being coated. The drying rate also plays a significant role in leveling; if the drying rate is relatively slow—resulting in a slower rate of change in leveling viscosity—it allows for a longer window of time for the varnish to spread, thereby facilitating thorough and complete leveling.

04 The Impact of Viscosity on the Glossiness of Water-based Varnish

The viscosity of water-based varnish exerts a profound influence on various coating-related properties—such as leveling and wettability—during the application process. This is because the surface of a printed substrate possesses a certain degree of absorbency toward the varnish; furthermore, the rate at which the printed surface absorbs the varnish is directly proportional to the viscosity value of the varnish itself. If the viscosity of the varnish is too low, coating highly absorbent paper substrates can lead to excessive viscosity fluctuations during the leveling process. While the viscosity may be adequate to meet leveling requirements during the initial stages, it increases significantly during the intermediate and final stages, making it difficult to sustain proper leveling. Consequently, the leveling process terminates prematurely, resulting in an uneven coating on the printed surface. Certain localized areas may suffer from insufficient varnish coverage, thereby compromising the drying of the film layer as well as the smoothness and gloss achieved after calendering. Conversely, when coating printed materials with low absorbency, excessively low viscosity can lead to "sagging"—a phenomenon where the liquid coating runs or drips—resulting in an uneven coating and poor drying performance. If the viscosity is too high, it again fails to meet leveling requirements, causing the leveling process to end prematurely and resulting in diminished gloss.

05 The Impact of pH Value on the Gloss of Water-based Varnish

The pH value of water-based varnish is primarily regulated using ammonia solution; typically, the pH of water-based varnish should be maintained within the range of 8 to 9. If the pH value is too high—indicating excessive alkalinity—the viscosity of the varnish will decrease, negatively affecting gloss levels. Furthermore, this condition slows down the drying rate and reduces water resistance. Conversely, if the pH value is too low—indicating insufficient alkalinity—the viscosity will increase. In this scenario, the ammonia evaporates too rapidly during the leveling process, causing the leveling to cease prematurely and thereby compromising the final gloss.

06 The Impact of Surface Tension on Water-based Varnish

The surface tension of the varnish is a critical factor that influences not only the final gloss but also the overall quality of the varnish coating. Since varnish application is typically performed over solid ink areas or existing ink films—which generally possess relatively low surface free energy—the specific surface tension value of the varnish plays a pivotal role. Variations in surface tension significantly affect the wetting, adhesion, and penetration characteristics of the varnish on a given printed substrate, leading to substantial differences in the resulting film-forming quality after coating and calendering. If the surface tension of the varnish is low, it readily wets the surface of the printed material and, driven by surface tension forces, easily levels out to form a smooth, uniform film. Conversely, if the surface tension of the varnish is high, it becomes difficult to wet the ink film on the printed surface, making it challenging to form a uniform coating layer. Furthermore, when the varnish's surface tension exceeds that of the printed ink film, the applied coating may undergo shrinkage, potentially leading to localized defects such as pinholes.

The foregoing outlines the primary factors influencing the gloss of water-based varnishes; additionally, the solid content of the varnish also plays a role in determining its gloss level. The impact of solid content on gloss is analogous to that of viscosity; generally, the solid content of water-based varnishes should be maintained within the range of 30% to 40%.

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