- October 01, 2002, by Dr. Richard M. Podhajny, Ph.D. Contributing Editor
Since the earliest attempts at transferring ink from one surface to another, the characteristics of a printing ink have been an important factor in print quality and ink performance.
One of these characteristics is the tack. “Ink tack” can be defined as the “stickiness of an ink” similar to what you would feel if you try to pull your fingers apart with ink between them.
This “adhesive ink cling” is important to its transfer. The ink cohesive strength and tack are due to similar forces. The ink must have enough tack to transfer but not enough to “drag the substrate” to the extent it can interfere with the printing process or damage the substrate.
Tack is particularly important in the press performance of heavy-bodied inks, which we refer to as “paste inks.” Usually, these include letterpress, dry offset, and lithographic, as well as some intaglio inks. Flexographic and rotogravure inks are low-viscosity inks and are referred to as “liquid inks.” Whereas liquid inks have viscosities of less than 100 centipoise, paste ink viscosities can be in the range of several hundred poises.
Tacks of liquid inks usually are quite low and normally are not important to these printing processes. Tack is a complex property of an ink formulation that depends on its composition. The ink formulation is a careful balance of pigment, vehicle, solvents, and additives.
Oil and solvents added to the ink tend to lower the tack. Higher levels of binder will raise the tack values. As a result, ink tack measurements are conducted as standard quality control tests for ink consistency.
Instruments are available that can make tack measurements under dynamic conditions at known ink film thickness, temperature, and speeds.
The basic principle is that several rollers, one of which is driven, rotate with an ink film of known thickness between them. The second roller permits the measurement of the force required to hold it in a stationary position relative to the driven roller.
There are at least five different tack-measuring devices. The ink tack commonly is measured on an “Inkometer” by means of a torque measurement. The configuration involves two rubber rollers with a brass roller in between.
Speeds of the Inkometer are set to approximate the printing process. That is, typically 800 rpm for sheet-fed and 1,200 rpm for web offset. Temperature is set at 90 deg F. Tack is measured 1 min after inks are distributed evenly on the rollers. Ink stabilities or “ink length” are compared at 10 min. Typically, sheet-fed inks have tacks of 15, whereas those of web offset inks are 10.
High quality process printing requires good trapping of the ink colors with predictable trapping to attain the desired colors. As a result, the inks are wet trapped in descending order from highest tack down to lowest tack.
As new presses reach higher speeds, ink tacks require modification to keep pace with high quality print performance. Balancing the ink tack, viscosity, and misting behavior is a challenging and important task.
The measurement of tack is carried out routinely by all ink manufacturers. It can be argued that its theoretical relationship is not mathematically rigorous. However, it has been found that this empirical method of looking at ink consistency is a key parameter that allows for high quality ink press performance.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314; firstname.lastname@example.org.