Flexographic printing uses water-based inks, which do not contain petroleum hydrocarbons or toxic substances such as heavy metals like lead and cadmium, making it the most environmentally friendly printing method available today. After years of effort, domestic flexographic printing has now been established.
Water-based ink is a low-carbon and environmentally friendly printing material with good stability. It is a non-flammable ink and is also inexpensive. The printing process does not require the addition of volatile solvents, making water-based ink healthier and more environmentally friendly. With the promotion of low-carbon concepts, environmental regulations are becoming increasingly strict, and the requirements for inks are also rising. Therefore, research on water-based inks is becoming more in-depth. It is well known that the initial application of water-based inks was in flexographic printing. However, due to various demands, water-based inks began to be used in gravure printing starting in the early 1980s. We deeply realize that water-based ink is the future trend of gravure printing. With the rapid development of technology, the difficulty of transitioning from solvent-based inks to water-based inks has gradually decreased.
Today's water-based inks have seen significant technological improvements compared to the past. The current inks can be diluted directly with water (without alcohol) and contain almost no volatile organic compounds.
1. Binder
Binders are adhesive fluids that serve as carriers for pigments, mixing and connecting solid particles such as powdered pigments. Additionally, they act as adhesives to ensure that pigments ultimately adhere to the substrate surface, achieving the purposes of displaying text, images, markings, and decorations.
The quality of the binder directly affects the performance of the ink. The choice of binder largely determines the ink's viscosity, tackiness, drying properties, and fluidity. The main components of binders are oils, resins, solvents, and auxiliary materials. Different printing processes, substrates, and drying requirements necessitate different types of binders. Ink binders can generally be classified into: a. Dry binders; b. Non-drying oil types; c. Solvent-resin type binders; d. Resin-oil type binders; e. Water-based binders; f. Light-curing types.
Among them, solvent-resin type binders are generally used mainly for gravure inks. These inks rely on volatile drying, containing easily volatile solvents. After printing on the substrate surface, the solvent evaporates into the atmosphere, and the remaining resin in the ink, together with the pigment, forms a solid ink film that adheres to the printed surface.
Inks made from water-based binders are referred to as water-based inks. Water-based binders can typically be divided into two categories: one type uses water as a solvent, containing water-soluble resins to adjust ink viscosity and fluidity, stabilize dispersion effects, and impart inherent pigment properties to the ink film; the other type is water-in-oil emulsions, where water serves as the external phase, and adjusting the quality ratio of the internal and external phases can regulate ink viscosity. The resin dissolves in the internal solvent or oils, giving the ink film strength. Water-based binders meet environmental and low-carbon requirements, posing no harm to the environment or worker health, making them a reliable printing material with long-term development prospects.
How to address the differences between water-based inks and solvent-based inks for gravure printing has become the main issue for the application of water-based inks in gravure printing.
2. Additives
Additives in inks are mainly divided into: plasticizers, diluents, de-tackifiers, drying agents, thinning agents, anti-gelling agents, anti-drying agents, anti-soiling agents, surfactants, preservatives, defoamers, UV absorbers, etc.
The use of additives is primarily to enable printers to actively adapt to changing printing conditions. These additives are mainly used as additives during the actual printing process to improve print quality.
3. Pigments and Dyes
The colored materials used in printing inks are usually pigments, with some dyes also being used. Pigments and dyes are both finely divided colored substances. Pigments are generally insoluble in water and binders, mostly remaining in a suspended state in solutions; dyes are generally soluble in binders.
The relative density, transparency, lightfastness, heat resistance, and chemical resistance of inks are all directly related to the properties of the pigments. The choice of pigment determines whether the resulting ink is suitable for a particular printing process or meets the final use of the printed product.
The pigments in water-based inks mainly consist of phthalocyanine blue and lithol red from organic pigments, and carbon black and titanium dioxide from inorganic pigments. The advantages of phthalocyanine pigments include bright colors, high tinting strength, high chemical stability, and insolubility in common solvents. Phthalocyanine blue is bright and transparent, typically used for cyan ink in four-color printing.
The pH range of carbon black varies widely, from 2.5 to 11, which makes controlling the pH value in water-based ink printing quite important. It is necessary to constantly monitor and ensure that the pH value of the ink remains above 6.8. Titanium dioxide is the main component of white ink in water-based ink printing. It has strong tinting strength, good opacity, is non-toxic and stable, has high dispersibility, and good lightfastness and alkalinity, and is insoluble in acrylic acid. Its pH value is between 6.5 and 7.5, making it a neutral substance that does not significantly affect the drying of the ink.
Changes in pH significantly affect the stability of water-based ink colloids. When the pH is 3, the water-based ink colloid undergoes significant coagulation, indicating that lower pH values can disrupt the stability of the water-based ink colloid. As the pH increases, the stability of the water-based ink colloid gradually improves, reaching a stable state around a pH of 6.8. Therefore, in water-based ink printing, controlling the pH value of the water-based ink and using pH stabilizers are crucial, as they directly impact the printing stability of the water-based ink.
Based on the above, policies should favor this environmentally friendly ink by providing production enterprises with a special subsidy for each flexographic publication for a period of time; offering tax incentives for flexographic printing and publishing enterprises; re-evaluating the pricing methods for flexographic water-based printed publications; and calling on socially responsible entrepreneurs in the country to pay attention to the development of water-based printing and to apply water-based printing as much as possible.