Originally, when lithography was invented , Alois Senefelder had used an aqueous solution of gum arabic to etch a lime surface into a letterpress shape. During his experiments, he noticed that he only had to etch briefly so that no color was accepted in these areas. The planographic printing process with dampening solution (stone printing , lithography) was thus invented. To date, gum arabic, a natural polysaccharide , is the most suitable protective substance for the non-printing areas. It can sit very firmly on the modern plate surface made of aluminum oxide and eagerly attract water there. A simple solution of gum arabic and acid in water would, however, no longer suffice in modern offset printing to maintain the sensitive printing process over a run or even an entire work shift. Today, dampening solutions are finely tuned aqueous solutions that are printed together with the ink in offset.
Treatment of the domestic water
Tap water can be used directly in some areas if the water hardness and impurities are within certain limits. If it has to be processed first, this is done by means of ion exchangers or reverse osmosis . It is then “conditioned”, ie adjusted to a water hardness between 8 and 12 ° dH through controlled additives.
Functions of the dampening solution
In addition to keeping the pressure-free plate parts free, there are further requirements:
Stabilize printable emulsion
The emulsion formation with the ink should guarantee a stable, printable emulsion , whether there is a slight or a strong decrease in ink . This emulsion must consist of water droplets that are distributed in color (water-in-color - emulsion). To do this, it has to continuously and dynamically form and disintegrate (be dynamic) in order to adapt to the temporally and locally changing conditions. On the contrary, a “paint-in-water emulsion” would collect in a pudding-like consistency on rollers or other machine parts, interrupt the transport of paint and spray it by being thrown away. And above all, it behaves outwardly like color. This means that it is trapped in the pressure-free areas of the plate and transferred by it. It brings a variety of clay phenomena and can even contaminate the dampening system with paint sludge . A pure color (not as an emulsion) would tend to tone, spread more poorly over the rollers, print out more grainy (blotchy) and rasp the paper or cardboard. All these problems are typical for the start-up phase of the machine, in which " start-up waste" is printed until the emulsification equilibrium has been established. In the extreme case of newspaper printing, that can mean 200-300 copies that end up in the trash.
The favorable pH range is between 4.7 and 5.3. It has to be buffered in between because impurities from the printing material usually move it upwards. Lower pH values bring risks through loosening of particles from the paper coating , delays in the oxidative filming of the inks up to complete blockage, corrosion on machine parts made of metal such as cylinders and rollers and some other special printing problems. Higher pH values, on the other hand, worsen the free running of the plates. They cause clay phenomena such as blocking of grid areas and emulsification problems, as emulsion formation also depends on the pH value. Cases are known in which printing is intentionally done outside the classic pH range, but they need remedies in other ways against these expected risks. And such medicines have side effects again.
Decreased surface tension
The surface tension of water is 72 mN / m, which is unfavorable for dynamic emulsion formation in a complex manner. In the past, 5 - 25% isopropanol helped. Because of the associated emissions and physiological problems, it is replaced by non-volatile (higher vapor pressure) surface-active substances ( surfactants , wetting agents), e.g. B. Glycols .
Viscosity of the dampening solution
All aqueous solutions and suspensions that are moved mechanically can tend to foam. Fine-tuned surfactants help here as defoamers.
No sludge formation
All technical systems in which aqueous solutions work can be clogged by microorganisms ( algae , slime , mold, etc.). Appropriate antidotes ( biocides ) are necessary here, especially in large-scale systems that operate one or more shifts a day.
No tendency to corrosion
Slightly acidic aqueous solutions are predestined to attack metal parts if they are not completely protected. In printing machines, this mainly affects steel components such as rollers and cylinders, but also other parts such as bearer rings (a kind of spacer on the edge of the roller). Solutions containing halogens (e.g. chloride ions) in particular can have a corrosive effect. There are additives, so-called corrosion inhibitors , which can significantly increase the service life of steel parts. For this z. B. used certain phosphates. Corrosion occurs more strongly under mechanical stress than at rest. This makes it understandable that newspaper printing presses and heatset presses are particularly affected. Machine manufacturers check the dampening solution concentrates that can be used there with sophisticated analyzes and, after approval, demand that the recipe be kept constant. But corrosion also occasionally occurs in sheetfed offset.
Depending on the machine and production conditions, additional additives may be necessary, for example decalcifying agents, mineral additives or adhesive for the rubber blankets.
- Helmut Kipphan: Handbook of the print media . 1st edition. Springer-Verlag, Berlin Heidelberg 2000, ISBN 3-540-66941-8 , pp. 220 .
- Helmut Teschner: Print and media technology . Dr.-Ing Paul Christiani GmbH & Co. KG, Konstanz 2010, ISBN 978-3-86522-629-7 , p. 718 .
- RH Leach, RJ Pierce: The Printing Ink Manual . 5th edition. Blueprint, London 1993, ISBN 978-0-948905-81-0 , pp. 19 .
- Company publication Profi Tip 1 dampening solution in offset printing, 2008