Discussion on Performance and Use Requirements of Offset Fountain Solution (Part Two)

(5) Antimicrobial effect of fountain solution

Plunge potions should be effective in preventing the growth of microorganisms in buckets and water systems. The spores present in the dampening solution are partly derived from air (2,000 to 3,000 spores per cubic meter of indoor air) but are infectable by the tap water solution (drinkables allow a certain spore content of 100) due to the weak acid pH. The addition of buffer salts and gum arabic for protection of printing plates, as well as constant temperature conditions, provide an ideal environment for yeasts, algae and bacteria. If these organisms divide once every 20 to 30 minutes, it is not difficult to understand that these organisms will be spread to the entire water delivery system in a very short period of time through free propagation. In this case, it may even cause downtime. Water pipes and valves on the water delivery device are very thin and should be kept particularly open. The problem of corrosion in the manufacture of offset presses has caused research on this issue. To this end, the German Printing Association developed a memorandum entitled "Proposals for Anti-corrosion of Offset Printing Machines".

In practical and theoretical studies, it has been shown that the fountain solution can be fed into the printing press by adding antiseptic bioactive substances to the fountain water. According to experiments, the dampening solution added with this preservative has little corrosion to the printing plate metal. Here, it should also be pointed out that the substances that can promote corrosion are not just tap water and fountain solution, but the whole offset process, including the mechanical manufacturing materials used, should also be taken into account when considering this issue.

(VI) Protection and requirements of fountain solution on printing plates

As we all know, the plate is not a smooth plate but consists of a multi-layered structure. Aluminum plate does not work for the actual printing process. What is important for the printing state of a plate surface is the hydrophilic oxide layer and the ink-receptive organic polymer layer.

The hydrophilic layer of the plate is covered with cracks (grains), which enlarges its surface and is prone to erosion. Whether it is the development of the printing plate after the glue, or water bucket syrup must be able to close the pores and grooves to prevent those who can react with aluminum intrusion.

When printing is interrupted, the plate cannot be continuously wetted. If the function of the water bucket protection printing plate is not enough, then the printing plate will be corroded, generally mistakenly referred to as plate oxidation. In this case, some of the substances infiltrated into the aluminum substrate through the capillaries, where they produced aluminum salt corrosion products.

Since the unit volume of salt is greater than that of alumina in the hydrophilic layer, it regularly “flowers”. The "mushrooms" produced in this way are not hydrophilic but pro-inking, so that there are many so-called oxidation spots (ie corrosion points) on the plate.

(7) Wettability of fountain solution to printing plate

The fountain solution must wet the plate so that the ink balance can be reached quickly at the start of printing, keeping the plate free in continuous printing. Surface tension and interfacial tension are used as measurable values ​​in this respect.

To explain the concept of these two tensions, we look at the internals of the liquid (see Figure 3). These individual water molecules exert forces on each other. This so-called cohesion counteracts each other when observing the molecules inside the liquid. However, one molecule of water on the surface is absorbed by the neighboring molecule, which means that the molecule has an inward force. Therefore, this molecule has greater energy than those inside the liquid.

Any liquid has the characteristic of reducing its own surface energy, that is, each liquid tries to occupy a minimum surface space in a larger volume. The geometry that meets this requirement is the sphere (see Figure 4 for mercury, water, and alcohol). The surface tension of ordinary tap water is about 72 mN/m (millinew Newton per meter). The fountain solution with this surface tension does not wet the plate surface well. Therefore, it is necessary to add a substance (wetting agent, alcohol substitute) that can break cohesive force in the water bucket medicine. This will better wet the surface of the plate.

If we look at two small water droplets on a flat plate, one with a large surface tension and the other with a small surface tension, one can clearly see the need for wetting the same size area with a liquid with a small surface tension. Droplet volume is small.

This condition indicates a good wetting state. It can be seen that the use of a water bucket syrup with reduced surface tension requires less water supply during the printing process. Therefore, the following problems can be avoided: too much moisture is absorbed by the paper, the ink absorbs too much moisture and emulsification occurs, and the ink in the ink delays ink penetration and drying. In addition to the previously mentioned wetting agents, isopropanol also has the property of reducing the surface tension. Therefore, in offset printing, isopropyl alcohol can be added to the water bucket to reduce the surface tension.

In addition, in order to avoid the occurrence of printing failure of the ink roller deinking, the water bucket syrup should prevent the formation of insoluble calcium salts, which are the root cause of the deinking failure of the ink roller.

Although the fountain solution often plays an unappealing role in offset printing, its positive effect cannot be ignored. Water has an indispensable cleaning and transport function, but also plays a certain role in cooling.

From the current point of view, water is indispensable for traditional offset printing, and it cannot survive without it. The technology of the douche potion has been widely used. However, we certainly have not yet reached the culmination of its development. The development of horizontal water-filled technology is still very large and deserves our attention.

Source: Journal Author: Yu Yong