Lithography is a very simple process. Water and ink are metered to the plate simultaneously, at high speed, and with great precision. The ink and water mix with each other in a controlled way on the form rollers and then deposit separately, each to its own designated area on the plate. Experienced pressmen know that the secret of a good impression is a minimum of ink and a minimum of water.
Surface Tension
The lithographic process relies on the equilibrium of the ink-water mixture. One of the most difficult variables to control is the ever-changing relationship between ink and water on the plate surface. To ensure a minimal use of ink and water and to maintain ink-water balance throughout the run, a fountain solution with a surface tension varying between 32 and 45 dynes/cm, according to the type of the press used, is necessary.
Besides the ink-water balance, there is also a second important element contributing to the success of a good impression—the cleanliness of the non-image area of the plate.
A fountain solution containing a natural arabic gum ensures the continuous formation of a protective film, which desensitizes the plate’s non-image area. A good control over the protective film will ensure a minimal use of water in the process and allow a good ink-water balance.
The importance of a good fountain solution is now well understood. Its surface tension has important consequences for the printing process, since it permits the formation of a thin, uniform, and reproducible water film before each application of ink.
These two requirements—fast plate wetting and the ability to form thin water film—are critical for lithographic dampening systems. They are largely determined by the surface tension of the solution.
Pure water has a surface tension of 72 dynes/cm. In a dampening solution containing an alcohol concentration of between 10% and 25%, the surface tension will be reduced to 35-45 dynes/cm, enabling the solution to spread over the plate rapidly in a thin continuous film.
In general, a fountain solution will consist of water, acid and gum (as detailed above), corrosion inhibitors to prevent the fountain solution from reacting with the plate, fungicide, antifoaming agent to prevent build-up, and a drying stimulator that complements the drying of the ink.
Last but not least, we find in the fountain solution wetting agents, such as isoproponol or alcohol substitutes, which decrease the surface tension of water and water-based solutions.
When alcohol or substitutes are mixed in with a fountain solution on a press using a dampening system with alcohol, the surface tension is reduced. The result is that the alcohol (or its substitute) helps the water to wet the dampener form roller evenly, requiring less dampening solution. A thinner film of solution will help keep the non-image area of the plate clean and also help to properly spread water over the ink on rubber bareback form rollers.
A thinner solution film also helps to prevent water spotting, because the fountain has finer drops that are dispersed in the ink. It also suppresses foaming.
The addition of alcohol to the fountain solution increases the viscosity of water, which allows a thicker layer of dampening solution to be applied to the ink and non-image area and improves the performance of the ink, paper, and printing plates. When using substitutes, the fountain solution is metered by the squeezed rolls used in the majority of continuous contact-type dampeners. The immediate effect is that the dampener speed must be turned up, which leads many press operators to conclude erroneously that they must use more water to print with a substitute.
To partially offset the loss in the viscosity produced by eliminating alcohol, the dampening solution can be cooled (50-55° F). Substitutes differ from alcohol in several key properties, including how they affect viscosity, surface tension, pH, and conductivity of dampening solutions.
In alcohol-free dampening solutions, surface tension is reduced by additives called surfactants. Surfactants are organic chemicals that tend to concentrate at interfaces because of their polar molecular structures. They travel to the interfaces between the dampening solutions, the air, and the ink on the image areas of the plate. At higher press speeds, interfaces are used and reformed rapidly. Surfactants may diffuse rapidly to replenish the new interfaces. Too much surfactant can contribute to excessive ink emulsification.
Alcohol substitutes offer an excellent opportunity to dramatically cut the amount of VOC emissions from a press. Because alcohol substitutes do not have a diluting effect on ink when used in proper concentrations, less ink and water are needed for acceptable colour. Sharper dots are produced and there is less of a tendancy for dot gain.