Water-based inks can be diluted with water or ethanol and cleaned up with tap water; however, they form a water-resistant film upon drying on the substrate. Their primary advantage lies in significantly reduced VOC (Volatile Organic Compound) emissions, thereby minimizing risks to human health and reducing contamination of packaged goods. As a result, they are particularly suitable for printing packaging for products with strict hygiene requirements, such as food, beverages, pharmaceuticals, and cigarettes.
However, due to the nature of their components, water-based inks exhibit a notable tendency to foam. Foam interferes with the smooth operation of the printing process and compromises print quality. Foam is a dispersion system in which gas is dispersed within a liquid. It forms when gas pockets are encapsulated and separated by liquid films, while foam destruction occurs when these separating liquid films thin and eventually rupture.
The stability of foam depends primarily on the rate at which the liquid film thins (i.e., the rate of liquid drainage from the film) and the mechanical strength of the film itself. When foam forms, the total liquid surface area increases, raising the system's free energy; conversely, when foam collapses, the surface area decreases, lowering the system's energy. From an energetic perspective, lower surface tension facilitates foam formation, whereas liquids with higher surface tension tend to resist foaming. However, low surface tension does not necessarily promote foam stability; it contributes to stability only when the liquid film possesses sufficient strength to form polyhedral bubbles.
As liquid inks, water-based inks have much lower viscosity than offset inks, making them more prone to air entrapment and foam generation during the ink transfer process. The binder system in water-based inks consists of solution-type or emulsion-type resins, water, small amounts of ethanol, and various additives. Resins must be neutralized with ammonia to become water-soluble; these water-soluble resins exhibit a soap-like structure that readily generates foam. Furthermore, surfactants are added to improve adhesion to substrates by lowering surface tension-a reduction that simultaneously promotes bubble formation.
Therefore, the addition of antifoaming agents is often necessary in water-based ink printing. The use of antifoaming agents counteracts measures that stabilize foam. While some antifoaming agents act through chemical reactions (such as acids or calcium salts disrupting soap films), most function by spreading at the liquid surface and disrupting the foam film structure.
Antifoaming agents include a wide range of substances. In addition to alcohols, fatty acids, phosphate esters, and amides, silicone oil is one of the most commonly used and highly effective defoamers. It is suitable for both aqueous and non-aqueous systems. Silicone oil has extremely low surface tension; when added to an aqueous system, it preferentially adsorbs onto the liquid film, forming a surface layer with lower surface tension than the original foam film.
Although this may appear to promote bubble formation, silicone-based defoamers consist of low-surface-energy molecular chains that produce a weak interfacial film. This film lacks sufficient mechanical strength, causing bubble rupture and thereby effectively eliminating foam.
