- February 01, 2008, By David Argent Contributing Editor
In the next three columns, we will cover the basic mechanisms of ink drying. This month we look at inks drying primarily by evaporation and absorption as related to gravure and flexo printing.
- Solvent-Based Inks
The fast solvent release and strong adhesion of solvent inks make them ideally suited to printing on nonabsorbent surfaces, and they are the primary ink type for this market segment. On absorbent substrates solvent-based inks dry by a combination of penetration of the ink into the substrate and evaporation of the ink solvent from the ink film. Normally, there is no chemical interaction with the substrate. Solvent inks are very resoluble, and for this reason, they are often favored in gravure printing of paper and board due to lower tendency to plug the image cells.
- Water-Based Inks
These inks are based on a combination of acidic resin held in solution by amine and water-borne resin dispersions known as emulsions. Solution resins aid print quality, and the emulsion is used to provide resistance properties in the dried ink. During drying, several mechanisms are happening at the same time. On absorbent surfaces such as paper and board, the ink components will start to penetrate into the substrate. Evaporation of water and amine takes place at the same time, often aided by dryers on the press.
Loss of the liquid part of the binder due to evaporation and penetration has several effects. The ink starts to gel. The acid resin starts to precipitate due to amine removal. The rate of drying depends on the absorbency of the substrate. Acid substrate components can help set the ink resin by neutralizing the amine and precipitating the soluble ink resin. On absorbent stocks the combination of precipitation and coalescence produces a drying rate often as fast as that of solvent-based inks.
Formulation of water-based inks requires a balance of ingredients that are often in conflict. A more fugitive amine assists drying speed, but print quality suffers. Loss of amine in low-coverage decks leads to ink bodying and print defects, but monitoring and maintaining pH in the specified range will keep the ink in balance.
In many cases, such as corrugated box printing, the presses are not equipped with dryers. Due to penetration of the binder, there is a positive mechanical anchor of ink to substrate, providing strong adhesion. Water-based flexo inks ideally are suited for printing on paper and board and dominate this segment.
Water-based inks do not dry or adhere as well to nonabsorbent surfaces. The primary reason is that there is no absorbency factor to assist the drying. So these substrates are printed more frequently with solvent-based inks.
Due to loss of volatile components in liquid inks during drying, the dry ink film always is substantially thinner than the wet ink film. Factors that increase drying speed include the use of more volatile ink components, increased web temperature, and higher air velocity in the dryers. Not so obvious is that stronger inks dry more quickly due to metering of a thinner ink film to achieve a specific color.
Next month this column will explore some of these factors in a computer simulation.
Process improvement expert David Argent has 30+ years of experience in process analysis with particular emphasis on ink and coating design and performance. Contact him at 636-391-8180; firstname.lastname@example.org.