Dampening Solution Ingredients
Dampening solutions are usually sold as concentrated solutions that are diluted with water to the proper concentration. In concentrated from, it is commonly referred to as fountain concentrate, fountain etch, or just etch. Most one-step concentrates already contain a natural or synthetic gum, an alcohol substitute, and other essential ingredients, and simply require being diluted with water. With two-step concentrates, the first step generally contains all of the ingredients except the alcohol substitute, with the alcohol substitute added as part of the second step. Although this extra step might be an inconvenience, it permits the press operator to control the alcohol substitute concentration better.
Ink, plate, press speed, paper, temperature, and relative humidity are the principal factors that influence the need for various dampening solutions. For example, a metallic or fluorescent ink may require an alkaline dampening solution. Most dampening solutions, however, are acidic with a pH of 3.5~4 being typical. The dampening system itself also influences the composition of the dampening solution. For example, some dampening systems require the of a percentage of alcohol or alcohol substitute due to the method of applying the solution to the printing plate.
The proper mixture of chemicals making up the solution is critical for quality printing. Though there may be many chemicals that make up a given manufacturer's dampening solution concentrate, the general ingredients common to most are described below.
Water. The primary ingredient in dampening solution is water, which makes up about 95~99% of the solution in weight. Water serves to repel the oily ink from the nonimage areas of the plate surface and to help cool roller and cylinder surfaces of the press by evaporation.
Gum. Gum arabic (a natural substance) or synthetic gum is a critical ingredient in dampening solution. Very few manufacturers still use natural Sudanese gum arabic be cause of its cost. The gum dissolves in water and coats the plate surface, replenishing the desensitizing film on the plate. Without gum, the plate might print clean for a short time, but soon the nonimage areas would begin to pick up ink, a problem called scumming. In fact, gum can work its way into the inking system over time, coating the inking rollers with a thin coating of dried gum. When this occurs, rollers become stripped, meaning that they are desensitized and no longer accept ink well.
Acid. Most dampening solutions are acidic in nature. Citric and phosphoric acid are common types of acid added to dampening solution. Gum will become a desensitizing film only when in the presence of acid. A gum and water mixture would not work to desensitize the plate surface if the solution were not acidic. When pH (the measure of acidity) is reduced to optimum levels of about 3.5 to 4.5, the gum molecules are converted into their acid counterparts, allowing the formation of a desensitizing layer on the plate surface. The proper concentration of acid in the solution is critical.
Buffers. The pH of dampening solution may change over the course of a pressrun. Alkaline papers may act to raise pH levels, while plate chemicals applied during the run may reduce it . Buffers act to keep the acidity of the dampening solution from exceeding a certain level. For example, a dampening solution that is properly mixed might have a pH (acidity level) of 4.5. As long as a buffer is present, more acid may be added to the solution but the pH will still remain at 4.5.
Wetting agents. Wetting agents are added to dampening solution to lower the surface tension of water and allow it to spread as a thinner film on the surface ofthe plate. The principle of surface tension can best be understood by considering an example. If a drop of water were placed on the hood of a freshly waxed car, the drop would form a bead. This means that the surface of the wax coating is not allowing the edges of the drop to spread out and thus, the surface tension is very high. Over time the hood of the car loses the wax coating and becomes oxidized. The same drop of water will now spread out on the surface of the hood very easily. Thus, the surface tension is low.
For many years isopropyl alcohol was used in dampening solutions to reduce surface tension and increase viscosity so that the plate would stay clean with less water. Isopropyl alcohol evaporates quickly to form volatile organic compounds (VOCs), the release of which is now limited by U.S. Environmental Protection Agency laws. Because of potential environmental damage, alternate wetting agents have been developed, most commonly referred to as alcohol substitutes. Because of the special considerations of printing with alcohol substitutes, a section entitled "Alcohol-Free Printing" is devoted to that topic later in this chapter.
Antifoaming agents. Defoaming agents are added to reduce problematic foam buildup. Dampening solution acts a bit like soapy water in that the solution tends to foam when worked. Foam can adversely affect the even transfer of solution in the dampening system.
Fungicides. Fungicides help prevent the fomation of fungus and bacteria in the dampening system. Fungus and bacteria can form very quickly in moist environments including dampening system pans (fountains), recirculation tanks, and water lines.
Other ingredients. Some dampening solutions include corrosion inhibitors to prevent the dampening solution from reacting with the plate. Magnesium nitrate is sometimes used; it also acts as a scratch desensitizer and buffer. Some also include a drying stimulator, which is a substance--e.g., chloride--that complements the drier in the ink. Drying stimulator is used only if ink is not drying fast enough. Typical concentrations are 1~2 oz. of stimulator per gallon (8~16 ml per liter) of dampening solution.
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