Film-Forming Performance (Gloss, Transparency, Adhesion)

● Gloss: Oil-based varnish typically forms a film with very high gloss but often carries a warm amber tone and may yellow slightly over time. Water-based varnish can also achieve high gloss, forming a clear and transparent coating after drying, with almost no yellowing.

● Transparency: Oil-based varnish inherently offers high transparency, but its cured film may exhibit a slight yellowish tint. High-quality water-based varnish remains purely transparent (appearing milky before application but fully transparent after drying).

● Adhesion: Both demonstrate excellent adhesion to paper and cardboard. However, for plastic films (e.g., PET, BOPP), water-based varnish often requires surface pretreatment or the addition of wetting agents to reduce surface tension and enhance polar bonding with the substrate. Oil-based varnish, with its organic solvents, can slightly soften plastic surfaces, improving initial adhesion, but may cause film warping or blocking, requiring careful application.

The above summary indicates that both water-based and oil-based varnishes can achieve high-gloss protective films. However, water-based formulations focus more on transparency, non-yellowing, and strong adhesion (particularly through additives to enhance film adhesion).

Environmental Impact and VOC Emissions

● VOC Emissions: Oil-based varnish contains a large amount of organic solvents, with VOC content typically in the range of several hundred grams per liter, resulting in high emissions. Water-based varnish uses water as the primary solvent, with very low VOC content, complying with low-volatile organic compound coating standards (e.g., water-based varnish for cigarette packaging can be formulated to meet stringent VOC limits).

● Environmental Friendliness: Water-based varnish is free of highly toxic solvents like benzene and toluene, making it environmentally friendly. Industry experts note that water-based coatings significantly reduce VOC emissions, achieving low or near-zero VOC levels. In contrast, oil-based varnish has high VOC content, posing greater environmental and health risks.

Safety (Toxicity, Odor, Flammability)

● Toxicity and Odor: Oil-based varnish contains organic solvents (e.g., hydrocarbons, esters), which are highly toxic and have a pungent odor, requiring adequate ventilation. Water-based varnish primarily consists of acrylic emulsions or polyurethane dispersions, with low toxicity and high safety for humans and the environment. Its odor during application is significantly lower than that of oil-based coatings.

● Flammability: Oil-based varnish, due to its organic solvent content, is often flammable and requires explosion-proof safety facilities. Its use must comply with fire safety regulations. In contrast, water-based varnish uses water as a diluent and is non-flammable. Industry data indicates that solvent-based coating systems typically require explosion pressure relief devices and fire safety monitoring, while water-based systems pose no such high combustion risks, offering greater safety.

Suitability for Printing Processes (Gravure, Flexo, Offset, etc.)

● Offset Printing (Lithography): Traditional offset press towers often use oil-based varnish, such as solvent-based coatings on high-speed web or sheetfed presses. Modern offset presses can also be equipped with water-based coating units or post-processing coating units for inline application of water-based varnish layers.

● Flexographic and Gravure Printing: Water-based coatings or UV coatings are widely used in flexographic and gravure printing for packaging (e.g., plastic films, paper bags). Industry experts note that water-based coatings can be applied in flexographic and gravure presses, particularly for food packaging labels and film packaging. Oil-based coatings (solvent-based resins) can also be used in gravure/flexo printing on films but require organic solvent recovery and adequate drying systems. Coating manufacturers like Covestro state that their printable coatings are suitable for flexographic, gravure, or offset presses.

Cost Structure (Raw Material Costs, Equipment Compatibility, Drying Energy Consumption)

● Raw Material Costs: Raw materials for oil-based varnish, such as alkyd resins and solvents, are generally lower in cost. Water-based varnish uses polymer emulsions as film-forming agents, resulting in higher raw material costs. It is worth noting that water-based systems often require additional resins and additives to achieve high solid content. Overall, water-based systems have slightly higher raw material costs but eliminate solvent costs and solvent recovery needs.

● Equipment Compatibility: Oil-based systems require printing and coating equipment with explosion-proof and solvent containment facilities, along with solvent recovery or exhaust gas purification systems. Water-based systems require corrosion-resistant (rust-proof) materials and water for cleaning. Both can use traditional ink trays or coating devices, but water-based systems require additional efficient drying units (e.g., hot air or infrared ovens).

● Drying Energy Consumption: Due to the higher latent heat of evaporation of water compared to organic solvents, water-based varnish typically requires more thermal energy for drying. Data shows that solvent-based coatings require lower drying temperatures and shorter times than water-based coatings. In other words, oil-based systems can achieve drying with shorter drying tunnels and lower temperatures, while water-based systems require longer drying chambers and higher energy consumption for complete drying.

Drying Speed and Curing Methods

● Drying Speed: Generally, water-based varnish dries relatively quickly: it can cure within a few hours without heating and forms a film rapidly with hot air or infrared drying. Oil-based varnish dries slowly, relying on solvent evaporation and additive catalysis, often requiring 6–24 hours for each coat to fully cure. Manufacturers like Brigal note that oil-based varnish offers “high drying efficiency” (suitable for fast plate running in offset towers), while water-based varnish provides “rapid drying.”

● Curing Mechanism: Oil-based varnish curing relies on solvent evaporation and oxidative polymerization of oil-based resins (often accelerated by driers). Water-based varnish relies on water evaporation from the emulsion, with resin particles coalescing to form a film. Some water-based systems may require film-forming aids or cross-linking agents to enhance durability but fundamentally lack oxidative curing processes, typically eliminating the need for long curing times like oil-based paints.

Adaptability to Packaging Materials

● Paper Packaging (Cigarette Packaging, Cartons, etc.): Water-based varnish is increasingly used in high-end cartons and cigarette packaging to meet low-VOC requirements. Oil-based varnish can also be used for paper-based packaging coatings but is limited under strict environmental standards. Both generally adhere well to paper due to its high surface energy.

● Plastic Films (Plastic Membranes, Composite Films): Water-based varnish requires specialized formulations (e.g., adding silane coupling agents, reducing surface tension) to achieve good film formation on smooth substrates like PET and BOPP. Solvents in oil-based varnish may temporarily wet plastic film surfaces but can soften the substrate, causing curling or blocking. Therefore, when printing on plastic films, water-based varnish requires strict control of surface treatment and drying processes.

● Other Materials: For non-absorbent surfaces like metal, glass, and leather, both varnishes require selection based on substrate characteristics (e.g., epoxy, polyurethane, or UV coatings). Overall, water-based varnish offers good adaptability to various packaging materials due to its environmental benefits and adjustable surface tension.