Insufficient water resistance of water-based pre-printed ink is a common problem in the printing process, especially in humid environments or when in contact with liquids, easily leading to color fading, mottling, or ink layer peeling. The core issue lies in the insufficient water resistance of the resin in the ink formulation, the density of the film-forming structure, and the synergistic effect of additives. Water resistance can be significantly improved by adjusting key components in the formulation, specifically through resin selection, crosslinking modification, additive optimization, and improvements to the pigment dispersion system.
The resin, as the film-forming material of water-based pre-printed ink, directly determines the ink's water resistance. Traditional linear thermoplastic resins (such as ordinary acrylic resins) have weak intermolecular forces, making them prone to water absorption and swelling after film formation, resulting in poor water resistance. Using self-crosslinking acrylic resins or polyurethane resins can effectively solve this problem. Self-crosslinking resins can form a three-dimensional network structure during drying, reducing water penetration channels; polyurethane resins, containing urethane bonds, have strong intermolecular forces, resulting in a dense film and excellent water resistance. In addition, epoxy resin-modified acrylic copolymer emulsions are also an effective choice for improving water resistance, as their epoxy groups can participate in crosslinking reactions, enhancing film hardness and chemical resistance.
Crosslinking modification is the core method for improving water resistance. Through chemical crosslinking agents (such as aziridine and isocyanates) or physical crosslinking techniques (such as hydrogen bond strengthening), the binding force between resin molecules can be increased, forming an irreversible crosslinking network. For example, adding waterborne silane coupling agents can form silicon-oxygen bonds at the resin-pigment interface, enhancing both adhesion and water resistance; introducing functional monomers (such as fluorinated monomers) can reduce the surface energy of the film, reducing water adsorption. Physical crosslinking, by regulating the resin molecular structure (such as introducing crystalline segments), utilizes hydrogen bonding to form physical crosslinking points, improving film density.
The selection and compatibility of additives significantly affect water resistance. Excessive defoamer can lead to increased film porosity and reduced water resistance; therefore, non-silicone defoamers with good compatibility should be selected. Leveling agents containing hydrophilic groups may weaken water resistance; therefore, hydrophobic leveling agents should be chosen. Furthermore, adding external crosslinking agents (such as waterborne isocyanate curing agents) can react with the hydroxyl or carboxyl groups in the resin to form additional crosslinked structures, significantly improving water resistance and chemical resistance. The addition of wax emulsions can also enhance the hydrophobicity of the film layer, with PE wax emulsions showing the best effect, forming a hydrophobic protective layer.
The stability of the pigment dispersion system directly affects water resistance. Poor pigment dispersion can easily lead to flocculation, disrupting the film's smoothness and causing water penetration. Dispersants with good resin compatibility should be selected to ensure uniform and long-term stable pigment particle dispersion. Simultaneously, the pigment particle size should be controlled within a suitable range (e.g., ≤20μm) to prevent coarse particles from damaging the film's continuity. For high water resistance requirements, inorganic pigments (such as iron oxide) can be used to replace some organic pigments due to their higher chemical stability.
Formulation balance is key to improving water resistance. The ratio of resin, additives, pigments, and solvents must be comprehensively considered. For example, increasing the resin content can enhance film-forming properties, but excessively high content can lead to increased viscosity, affecting printability. Adding an appropriate amount of fast-drying co-solvent (such as ethanol) can accelerate drying and prevent prolonged moisture retention, but its evaporation rate must be balanced with environmental requirements. Furthermore, controlling the pH value within a suitable range (e.g., 8.5-9.5) can maintain resin stability and prevent a decrease in water resistance due to excessive alkalinity.
Post-treatment processes supplement water resistance. Applying a water-based varnish overprint after printing can form an additional protective layer; the dry film thickness should be ≥3μm to ensure complete coverage. If using UV curing, sufficient curing energy (e.g., ≥300mJ/cm²) must be ensured to allow for full cross-linking of the film layer, preventing poor water resistance due to insufficient curing. For high-requirement applications, chemical cross-linking and physical protection can be combined; for example, first using a cross-linking agent to enhance film cohesion, then using a water-based varnish to enhance surface hydrophobicity.
Improving the water resistance of water-based pre-printed ink requires collaborative improvements in multiple aspects, including resin selection, crosslinking modification, additive optimization, pigment dispersion, formulation balancing, and post-processing. By selecting highly water-resistant resins, strengthening the crosslinking structure, optimizing additive compatibility, ensuring stable pigment dispersion, balancing formulation components, and perfecting post-processing techniques, the water resistance of inks can be significantly enhanced, meeting the requirements for use in humid environments or when in contact with liquids.
