As an environmentally friendly alternative to printing inks, water-based inks for printing have gained widespread attention. The water does not give off the chemical solvents that evaporate, but evaporates from the ink leaving the pigment behind. Water-based inks have limited use due to compatibility with substrates, but have been used successfully for many applications, including printing on paper, fabric and some plastics.
Water-based inks behave differently than other inks that rely on evaporation to dry. For example, water-based inks blend more easily into fabric and paper fibers, allowing for thinner prints and breathable final products. Switching from solvent-based inks requires some effort to change processes and handling due to differences in application. The ink is designed to stay wet during the printing process and dry well before the next process, so it can neither dry too fast nor too slow. Especially for water-based inks, since the evaporation of water is slower than that of chemical solvents, strict control of the drying speed is required.
Ink stability depends on pH
Ink is composed of extremely small particles whose stability in solution depends on the pH of the solution. The larger the gradient between solution pH and surface charge, the more stable the particle. Most commercial water-based inks are stable at alkaline pH. Inks with particles that are stable at neutral pH conditions exist, but are more expensive.
Amines (as opposed to ammonia) are additives that address several ink control parameters. Amines are added to increase and maintain pH stability, to control ink drying times by selecting amines, and to increase the solubility of other ink additives such as resins. Different amines and different pH values result in different drying times. The types of amines used in ink formulations are often referred to as fast and slow amines.
Viscosity adjustment by pH
Previous ink formulations required rigorous pH monitoring and chemical knowledge to keep the ink stable during printing. Modern inks are less affected by pH changes, however, monitoring is not only important for ink stability. In water-based inks, pH is closely related to viscosity, which affects the color and texture of printed materials. Color consistency depends on maintaining a constant viscosity, as thicker inks will deliver more colorant, while thinner inks will provide less colorant. Viscosity also affects solvent retention and thus plays a role in drying rate.
Even small changes in viscosity can cause printing variations, so pH monitoring can make printing runs more consistent. A lower pH increases the rate of evaporation and therefore causes the ink to thicken. Conversely, higher pH results in thinner inks due to less evaporation. Great results when using a lot of ink. Intuitively, using large volumes seems to ensure color consistency, but may also expose the process to chemical shift risks.
Although minor pH changes can affect viscosity, fortunately, pH adjustment and correction is relatively simple. Although it is possible to adjust the viscosity by adding water, it is only useful to adjust the viscosity by adding water after the viscosity has changed. Instead, viscosity changes can be avoided by monitoring and maintaining pH. The more you monitor and correct, the better your prints will be. The result of improved ink stability and improved process control means that printing today is easier than ever to maintain consistent print and color quality
