The Formulation of Water-Based Varnish and Three Key Points to Master

1. Water-soluble overprinting varnish/overprinting coating, commonly referred to as water-based varnish, is used for full or partial coating. On printed materials, it can protect the prints and increase their gloss. Its advantages include being non-toxic, environmentally friendly, free of solvent evaporation (low residual odor), which can improve the production environment, benefit the health of operators, and meet the environmental import standards of Europe and America.

Water-based varnish dries quickly, has strong wear resistance, is not prone to yellowing, and has high gloss, making it widely applicable to various types of paper. Specifically formulated water-based varnish can be used universally on non-absorbent materials, such as gold and silver paper, PET, PP, PVC, etc.

1. Water-based varnish generally consists of 80-90% water-based resin and 10-20% other additives. After coating and overprinting, it leaves a layer of polymer coating on the surface of the printed material. Different formula combinations endow the coating with various properties, such as high gloss, matte finish, and wear resistance.

Among these, water-based resin is the most crucial component of water-based varnish. It determines and influences the properties of the varnish, such as gloss, adhesion, wear resistance, and drying speed. Therefore, the correct selection of water-based resin is the key to the successful formulation of water-based varnish.

There are many types of water-based resins, including water-based rosin-modified maleic resin, water-based polyurethane resin, water-based acrylic resin, water-based alkyd resin, and water-based amino resin. For water-based varnish, the selected resin should have good salt-forming ability, excellent water release, high gloss after film formation, good heat resistance, excellent chemical resistance, and fast drying speed. Water-based acrylic copolymer resin can meet the requirements of printing and overprinting to the greatest extent. Therefore, when formulating water-based varnish, water-based acrylic copolymer resin becomes our best choice.

Water-based acrylic copolymer resin can be divided into water-based acrylic resin solution, water-based acrylic dispersion, and acrylic emulsion. Acrylic emulsion can be further classified into film-forming acrylic emulsion and non-film-forming acrylic emulsion.

The properties of water-based acrylic copolymer resin vary due to the influence of monomer composition and synthesis process. Some monomers can improve gloss and hardness, while others provide chemical resistance and adhesion.

Numerous experiments have proven that selecting the appropriate water-based acrylic resin and acrylic emulsion, and scientifically compounding them, can result in water-based varnish with ideal performance.

1. Additives also play an important role in improving and enhancing the performance of water-based varnish, such as leveling agents, wetting agents, defoamers, and wear-resistant agents.

• Leveling agents improve flowability, extend leveling time, and help the coating form a smooth film after drying, avoiding water ripples and streaks.
• Wetting agents mainly reduce the surface tension of the water-based varnish system, enhancing the coating's wetting ability on the surface of the printed material.
• Defoamers can suppress and eliminate the foam generated in the circulation system of water-based varnish, avoiding excessive foam that affects operation.
• Wear-resistant agents can enhance the wear resistance, scratch resistance, and smoothness of water-based varnish.
• Additionally, adding a small amount of auxiliary solvent (such as alcohol) in the formula can allow the components of water-based varnish to mix better, reduce the internal cohesion of the system, and improve the overprinting adaptability.

1. Regardless of the overprinting method chosen, to achieve satisfactory overprinting results, in addition to selecting the appropriate water-based acrylic resin and acrylic emulsion, it is essential to master three key points in the use of water-based varnish:

• Coating amount: A uniform and moderate coating, after film formation, results in high gloss and good wear resistance. If the coating amount is too small, the water-based varnish cannot form a complete and continuous film, leading to poor gloss and failing to provide the desired protection and decoration. However, if the coating amount is too large, the film will be thick, increasing the overprinting cost and affecting the drying speed. In severe cases, it may cause sticking. Generally, we recommend a wet coating of 4-10g/㎡. The specific coating amount should be adjusted according to the characteristics of the printed material and the equipment.

• Viscosity: For offline roller application, the recommended viscosity is generally 18-25 seconds (DIN4, 25℃), and for inline overprinting, the viscosity is generally recommended to be 40-70 seconds (DIN4, 25℃). If the viscosity is too low, it will not provide protection and decoration; if the viscosity is too high, it may result in poor leveling and slow drying.

The diluent for water-based varnish is usually water, but a mixture of isopropanol (1:1) can also be used as a diluent according to specific conditions. The specific viscosity should be adjusted according to the characteristics of the printed material and the equipment.

• Drying time and temperature: Water-based varnish mainly dries through penetration and also involves cross-linking curing. Common drying devices include hot air drying and infrared drying. Generally, infrared drying is more effective because the main solvent of water-based varnish is water, which has a high heat of vaporization, making the drying speed slower than oil-based varnish. Similarly, the drying temperature will be higher than that of oil-based varnish. It is generally recommended to dry at 80-100℃ with a drying channel of about 2m. If the temperature is insufficient or the drying channel is too short, the coating will not dry, and severe cases may result in sticking and scuffing. However, if the temperature is too high or the drying time is too long, the paper fibers may lose too much water, making the paper brittle and prone to breaking and cracking during post-processing.

The above briefly describes the three key points of using water-based varnish, but the use of water-based varnish also needs to be considered comprehensively according to different printing methods, printing equipment, types of printed materials, ink characteristics, and overprinting equipment to ultimately bring out the excellent performance of water-based varnish.