- September 01, 2000, Richard M. Podhajny, Ph.D., Contributing Editor
The performance of water-based inks in flexographic printing is, to a large degree, based on additives that are incorporated into the ink to optimize specific properties.
In a typical ink formula, the ink is composed of a colorant, resin binder, solvent, and additives. Colorants, resins, and solvents constitute about 95% of a typical ink formulation. Additives make up the balance of the ink formula.
Let's take a closer look at these additives and their role in a water-based ink.
A typical ink formulation will contain lubricants that provide rub and mar resistance, as well as a low COF to the ink film. The level of these lubricants in an ink can vary depending on the process and end-use requirements. Laminating inks do not utilize lubricants, since the lamination bonds could be affected.
Among the common lubricants used in inks are PE and PP waxes, silicones, fatty acid amides, and fluorocarbon waxes such as tetrafluoroethylene (PTFE). These lubricants differ greatly in their effectiveness, rate of migration, and temperature resistance.
PTFE waxes provide very high temperature resistance as well as rub and low COF, but they are more expensive. Typically, the ink formulator will use several of these lubricants in a given ink formula to optimize ink lubricity.
Additives in the form of surfactants are used frequently to improve ink printability. Such surfactants lower the ink surface tension and provide greater latitude in ink wettability and press performance. Surfactants are used in very small amounts, in the range of 0.1%. Their use must be designed with care, as some surfactants can contribute to ink foaming.
Inks that must meet the toughest requirements will often utilize chemical crosslinking agents in the ink film. Crosslinking agents chemically "link" themselves to the binder, improving the binder's resistance properties to chemicals. Crosslinkers are used to improve block resistance as well as heat resistance.
Adhesion promoters, used to improve ink adhesion, can be exotic reactive materials or simple solvents. They can provide excellent adhesion characteristics, but they must be controlled carefully to prevent undesirable effects, such as odor or discoloration.
If you are using a water-based ink, then you are using anti-foam agents. These may constitute only 0.1% in the ink formulation, but if you don't have them, you may not be able to run your press.
Most water-based products require the use of biocides to prevent growth of fungus in your water-based system. They are added to the ink formulation in very small amounts, but they are essential if you store your product for extended periods.
These chemicals are added to dissolve some of the resin binders, maintain pH, and keep your ink open on press. Whereas ammonium hydroxide is very volatile, amines are used often, since they provide greater stability.
In water-based inks, the resin binders usually provide the necessary rheological properties, but it is not uncommon for the ink formulator to add a thickening agent if the process necessitates a higher application viscosity.
Some water-based inks are formulated with binders that can be polymerized by the use of oxidizing agents. Such systems are used where very high alkaline resistance is required.
Plasticizers are used to improve the flexibility of an ink film. Many are high boiling solvent esters and, as such, remain in the ink film layer even after drying. Their concentrations have to be minimized as they can lead to undesirable properties, such as higher COF and lower rub resistance properties.
Many other additives can be used, such as buffers, deodorants, UV absorbers, release agents, etc.
As you can see, water-based inks can contain many additives, and their role is often critical to the performance of your ink.