Dampening Solution Ingredients

Dampening solutions, essential for offset lithography, are typically supplied as concentrated formulas that press operators dilute with water. In its concentrated form, this mixture is commonly termed fountain concentrate, fountain etch, or simply etch. One-step concentrates incorporate a natural or synthetic gum, an alcohol substitute, and other essential components, requiring only dilution. Two-step concentrates separate these ingredients; the first step usually contains all elements except the alcohol substitute, which is added in the second step. While this requires an extra procedure, it allows for finer control over the alcohol substitute concentration.

Several factors dictate the specific dampening solution required for a job, including ink type, plate characteristics, press speed, paper stock, temperature, and relative humidity. For instance, metallic or fluorescent inks often necessitate an alkaline dampening solution. Most solutions, however, are acidic, with a typical pH range of 3.5 to 4. The design of the dampening system itself also influences the solution's composition; some systems mandate the inclusion of a specific percentage of alcohol or alcohol substitute based on their method of application to the printing plate.

The precise chemical mixture is critical for achieving high-quality print results. While proprietary formulas vary by manufacturer, the following core ingredients are common to most dampening solutions.

Water. As the primary ingredient, water constitutes approximately 95~99% of the dampening solution by weight. It functions to repel oily ink from the non-image areas of the plate and aids in cooling press rollers and cylinders through evaporation.

Gum. Gum arabic, a natural substance, or a synthetic gum is a vital component. Few manufacturers continue to use natural Sudanese gum arabic due to its cost. The gum dissolves in water and coats the plate surface, continuously replenishing the desensitizing film. Without it, a plate may initially print cleanly but will soon experience scumming, where non-image areas begin to accept ink. Over time, gum can migrate into the inking system, coating rollers with a thin dried film. This causes the rollers to become stripped, meaning they are desensitized and can no longer properly carry ink.

Acid. Most dampening solutions are acidic, commonly utilizing citric or phosphoric acid. The gum only forms an effective desensitizing film in the presence of acid. A mixture of gum and water alone would not function correctly. When the pH is reduced to the optimum range of 3.5 to 4.5, gum molecules convert into their acid counterparts, enabling the formation of this critical layer on the plate. Maintaining the correct acid concentration is essential.

Buffers. The pH of a dampening solution can fluctuate during a pressrun. Alkaline papers may raise the pH, while chemicals from the plate can lower it. Buffers are added to prevent the acidity from exceeding a specific level. A properly mixed solution with a pH of 4.5 will maintain that acidity even if more acid is introduced, as long as buffers are present.

Wetting agents. These agents are added to lower the water's surface tension, allowing it to spread as a thinner film across the plate. Surface tension is easily understood through an analogy: a water droplet beads up on a freshly waxed car hood due to high surface tension. On an oxidized, unwaxed surface, the same droplet spreads out easily, indicating low surface tension.

Isopropyl alcohol was historically used as a wetting agent to reduce surface tension and increase viscosity, enabling cleaner printing with less water. However, its rapid evaporation releases volatile organic compounds (VOCs), which are now regulated by the U.S. Environmental Protection Agency. Consequently, alternative wetting agents, known as alcohol substitutes, have been developed. The specific considerations for using these substitutes are covered in a dedicated section on alcohol-free printing.