Water based inks for printing have gained traction as an environmentally friendly alternative printing ink. Instead of emitting volatilized chemical solvents, water evaporates from ink leaving behind pigments. Use of water-based inks has been limited due to compatibility with substrate materials, but they are successfully used in a number of applications including printing on paper, fabric, and some plastics.
Water based inks perform differently from other inks that rely on evaporation for drying. For instance, water-based inks incorporate into fabric and paper fibers more easily, thus making for thinner printing and a breathable final product. Due to the differences in application, some effort is required to alter process and handling when switching from solvent-based inks. Inks are designed to stay wet during the printing process and be dry enough before the next process, therefore drying can neither be too fast nor too slow. Water-based inks, especially, require high control over drying speed since water evaporates more slowly than chemical solvents.
Ink Stability Depends on pH
Ink is comprised of extremely small particles and the stability of those particles in a solution are dependent on the solution pH. A larger gradient between the solution pH and the surface charge result in more stable particles. Most commercial water based inks are stable at an alkaline pH. Inks that have particles stable under neutral pH conditions exist, but are costlier.
Amines, a relative of ammonia, are an additive that works to address several ink control parameters; amines are added to increase and maintain pH for stability, to control ink drying time through amine choice, and to increase solubility of other ink additives such as resins. Different amines and different pH values result in different drying times; types of amines used for ink formulations are often referred to as faster and slower amines.
Viscosity Tuning via pH
Previous ink formulations required stringent pH monitoring and knowledge of chemistry to keep inks stable during a printing process. Modern inks are less subject to pH shift, however, monitoring is not only important for ink stability. In water based inks, pH is strongly tied to viscosity, which affects the color and texture of printed material. Color consistency relies on maintaining a constant viscosity because thicker ink will deliver more colorant and thinner ink less colorant. Viscosity also affects solvent retention and thus plays a role in drying rates.
Even small viscosity shifts can result in printing variation, thus pH monitoring can result in a more consistent printing run. Lower pH increases the rate of evaporation and thus can result in a thicker ink. Conversely, higher pH results in a thinner ink than expected due to less evaporation. Effects are significant when large volumes of ink are used. Intuitively, using large volumes may seem to ensure color consistency, but it may also leave the process at risk for chemical shifts.
Though small pH changes can affect viscosity, fortunately, pH adjustment and correction is relatively simple. While viscosity can be adjusted by the addition of water, tuning by water addition only helps after viscosity has shifted. Instead, by monitoring and maintaining pH, viscosity shifts can be avoided. The more monitoring and correcting, the better the printing result. 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