Causes of excessive foaming of PS plate developer and its elimination method

When users of the PS version in the southeast coastal areas of China use the Huaguang PD-type developing machine for PS-based development, they often encounter a very problematic headache--a lot of foam is generated, which affects the normal development of the PS plate. This phenomenon is rarely encountered in the north. Why is it easy to produce bubbles on the southeastern coast, and how do you actually solve this headache problem?

In terms of classification of hydrochemical regions, the southeastern coastal areas of China belong to a wet area, with abundant precipitation and small evaporation, resulting in large runoff; therefore, the soil layer is thin, there are more hard granite rocks, and the soil is leached for many years. Most of the soluble salts have been lost. Therefore, the water contains less salt, lower salinity, and correspondingly lower hardness. It is extremely soft water (water hardness < 1 meq/l). In addition, the tap water supplied to the southeastern coastal areas is mostly surface water. Generally speaking, surface water is softer than groundwater. In northern China and inland areas, the supply of tap water is mostly groundwater with high hardness (the hardness of deep well water in the north is >300 meq/l).
Both PS coating and developer contain surfactants. The coating generally contains macromolecules and non-ionic surfactants, the main role of which is to solubilize and disperse in the formulation of the coating solution. Anionic surfactant is added to the developer to facilitate penetration of the developer. At the same time, they can also reduce the surface tension of the developer.
When a liquid forms a foam, its surface area increases. Smaller surface tensions tend to foam more easily, but the stability of the bubble is independent of the surface size of the liquid. The formed bubbles, due to the action of gravity, liquid in the liquid film automatically flows downwards. The liquid molecules flowing down during the liquid film discharge process have greater free energy than the liquid molecules at the bottom, so the air bubbles are continuously discharged. The liquid thins the membrane wall and breaks it, causing the foam to disappear. When molecules of the surface activator are adsorbed on the surface of the liquid film, the foam is stabilized because the molecular film of the surfactant inhibits the flow of the liquid. In addition, the surfactant molecules attached to the foam film act as a surface "repair" to the liquid film, resulting in good stability of the foam. Anionic surfactants are much more stable to foam than nonionic surfactants. Therefore, the foaming material during development processing is mainly an anionic surfactant in the developer.
The type of surfactant is the main factor that determines the foaming force, but the different environmental conditions, such as the humidity, the hardness of the water, the pH value of the developer and additives, have a certain impact on the foaming power.
Due to the presence of hard water softener in the development, the water softens and the water content is not reduced due to the softening of the water. When the Huaguang PD developer is diluted 1:4 by water, it is calculated according to the ideal condition. The concentration of anionic surfactant in the working fluid is within its critical micelle concentration. In the eastern coastal areas, when Huaguang PD developer is diluted with tap water with low salinity and low salt content, water has little or no effect on the critical micelle concentration of the anionic surfactant in the developer. Therefore, in the critical micelle concentration range of the anionic surfactant in the working fluid, the surface tension of the working fluid is the smallest and the foaming force is the largest, resulting in a large number of bubbles in use and affecting the normal progress of the development processing.
However, in northern China and in inland regions, there is not much groundwater for use, and the degree of mineralization is high and the content of metal ions is high. Although softening agent is added to make water soft, the total number of ions does not change. With this type of water, the metal ions ionize the double layer of the micelles, causing the anionic surface active ion groups to repel each other, and more surfactant ion groups enter the micelles to cause micelle aggregation. The number increases. At the same time, since the counter ions are immobilized on the surfactant-associated body, the repulsive force between the ions of the same charged surfactant is reduced and the micelles are easily formed. Thus the critical micelle concentration of the anionic surfactant is reduced. In general, divalent metal salt ions such as (Cu, Zn, Mg, etc.) have a greater effect on lowering the critical micelle concentration than monovalent metal salt ions such as (K, Na). It is precisely because the critical micelle concentration decreases that the concentration of anionic surfactant in the working fluid is greater than the critical micelle concentration. The surface tension of the working fluid becomes larger and the foaming force becomes smaller. Therefore, excessive bubbles do not occur and the normal development is affected.

Since the foaming is related to the critical micelle concentration, the defoaming is also started with the critical cementing concentration. Theoretically, the objective can be achieved as long as the critical micelle concentration of the anionic surfactant in the working fluid can be reduced. So, how to avoid excessive bubbles in the development? The easiest and most practical method is to add high-priced metal salts. Another method is to add another anionic surfactant solution such as soapy water, etc. to reduce the critical micelle concentration of the surfactant in the working fluid and prevent bubble generation. The most practical method is to add alcohol. Because the alcohol molecules can penetrate the micelles and reduce the repulsive force between the surfactant ions, and because the addition of the alcohol molecules into the micelles is easy to form and increase, so that the critical micelle concentration is reduced to avoid the generation of bubbles, so as to achieve defoaming The role. The amount of alcohol is: in 25L: Huaguang PD developing working solution is added ethanol: butanol = 1:1 (volume ratio) mixture of 50ml (depending on the water quality, the defoaming effect can be increased or decreased as appropriate). In this way, a good defoaming effect can be achieved, and the problem of foam flaps when the developer is used can be solved.

Source: Print Magazine