Water-based ink pH test

Aqueous solutions can be decomposed into equal amounts of hydrogen ions and hydroxide ions. The amount of these ions is very small, but they can determine the pH of aqueous solutions. When the water temperature is 25 degrees Celsius, the hydrogen ion [more precisely, the amount of hydronium ion (H3O+)] is 10-7 mol/l, and the pH is -log [H3O+] at this time.

When the water temperature is 25 degrees Celsius, the pH is -log 10-7 or 7. If the amount of hydronium ion is more, the aqueous solution is acidic, and if the concentration of hydronium ion is less than 10 -7 mol/l, the aqueous solution is alkaline.

Many components of water-based inks affect the pH value. For example, acrylic resins commonly used in aqueous inks are acidic. This type of substance does not dissolve in water but becomes soluble after adding some basic substances.

When these acidic resins are dissolved in a basic solution, they undergo a neutralization reaction to form soluble salts. The solubility depends on the pH value. If the pH is lowered, the resin will become less soluble and may also be separated out in solution. The pigment itself is insoluble, but the pH value of the solution also has an influence on it, and the stability of the pigment can be improved by repulsion between particles. Since many pigments are themselves salts, it is essential that they be tested for pH compatibility.

Similarly, one can choose a more stable resin emulsion under alkaline conditions. Most aqueous inks use ammonium hydroxide and volatile amines to control and maintain the ink's alkalinity. Because these substances are volatile, people often use other alkaline additives or buffers to help maintain the pH of the ink.

In fact, all the components of the ink must be tested so that the pH range of the ink is optimal.

Most aqueous inks have a pH of between 8-9.5 (basic). Since ammonium hydroxide is highly volatile, the pH of the ink will decrease if some proper method is not controlled, and the viscosity of the ink will increase and accumulate on the printing plate, creating an unpleasant bubble. . In addition, the heavy use of ammonium hydroxide also produces a very pungent smell of ammonia.

It is often the case that problems arise due to the lower pH. When the pH value decreases, the viscosity of the ink increases, and the printing effect deteriorates. The ink on a flexo plate dries prematurely, causing the printed edges to be untidy or stained, and “dirty printing” is a term used specifically to describe such printing.

A few years ago, Eastek/Eastman Chemical introduced a new type of water-based ink that can be used under neutral conditions. This technology is based on a sulfonated polyester that can be separated in water.

Since the resins are soluble in alkaline solutions, they can be formulated in approximately neutral inks. These sulfonated polyesters are actually resinous isolates and are generally less viscous. Although this technology has many advantages, it still has some limitations. It has only a small breakthrough in the traditional water-based ink market.

At present, ink chemists are working hard to develop inks used at low Ph values ​​into inks that can be used under neutral conditions. New resin binders containing small amounts of acids and volatile organic compounds are also being developed. These inks are very similar to high-Ph acrylics but can be formulated with resins containing small amounts of acids.

The use of low pH inks to balance the water resistance of prints is a thorny issue. Despite the use of resin binders, which substantially reduce the amount of ammonium used, these inks must still have the same viscosity and water resistance. These new neutral aqueous inks are soluble in alkaline solutions and can be used in a wide variety of applications in a low pH range.

Our goal is to eliminate the need to adjust the pH when printing inks. Another advantage is that the press only produces a small amount of foam when it is working, and the cleaning of the press is easier.

Reducing the odor of ammonia when printing, improving the stability of the printing press and the printing effect are the direct impetus for people to develop neutral inks.

Author Dr Richard M. Podhajny