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Feb 1, 1995 12:00 AM
Water pickup is probably one of the most controversial and misunderstood ink characteristics. The numbers obtained on water pickup and the resulting curves plotted from this data are open to a wide range of interpretations.
Nevertheless, printers and inkmakers insist on conducting this test and trying to predict an ink's performance based on water pickup values. The reason? The idea of a water pickup value seems to make sense.
In lithography, whether web or sheet-fed, water or dampening solution wets a plate's non-image areas, preventing ink from transferring. Meanwhile, image areas repel water but attract ink, thus producing the image. To ensure sufficient wetting, a surplus of water is supplied to the plate surface. The more skilled the printer, the less the amount of water applied; nevertheless, water always is left over and must go somewhere. Paper absorbs some, and a certain amount of evaporation occurs, but a large quantity also is taken up by ink.
The surplus dampening solution works back into the ink train, and the rollers' rotating action "mills" ink and water together to form an emulsion.
One would expect the greasy nature of inks to repel the action of water - but, just as mayonnaise is a stable mixture of olive oil and water/vinegar, oils in ink allow a certain amount of water to become suspended within the ink's structure.
This phenomena isn't new, but has existed since lithography began. The key factor is what happens to ink once water is emulsified into it.
Under ideal circumstances, ink should continue to transfer and print well, even though particles of water are suspended in it. In addition, the material should retain its viscosity and "exhibit good flow and length. Once printed, it also should maintain its gloss-producing characteristics.
Problems occur when an ink's rheological properties are broken down or destroyed.
In the early stages of water absorption, certain inks retain all their favorable printing qualities. Then, as water absorption reaches a certain point, ink breaks down, loses flow and no longer transfers to the plate. Worse, it may transfer to non-image areas, causing tinting and scumming.
Laboratory observations show that during the early stages of water addition, a water-in-ink emulsion is established. However, as the amount of water increases, at some point the ink no longer takes up any more and breaks down, creating an ink-in-water emulsion. Once this situation exists, ink loses print density (graying out), and dampening solution carries ink particles to the plate's non-image area, creating scumming.
The industry needed a way to predict this phenomena. Enter the water pickup test.
In the early days, ink and water were run together on the famous litho break tester. This item consisted of a couple of inked rollers rotating in a reservoir of dampening solution, which enabled experts to determine if the ink would break down over time and discolor the fountain solution in the reservoir. The test had some merit. If the ink and fountain solution were incompatible or if the ink pigment bled into the fountain solution, as rhodamine reds do in solutions containing 25 percent of isopropyl alcohol, then this test would detect such problems before they occurred on press. However, there was no way to put a number on the results.
Other early attempts to mix ink and water included a pestle and mortar and Sunbeam Mix Master. The latter led to a refined test called the Duke water pickup tester. It was named for its developer, who worked at that time for Nashville printer Baird Ward.
The Duke has mixing arms that protrude into a vessel. Operators add a measured amount of ink and fountain solution, which are mixed together under a constant shear rate for a given time period. Next, operators turn off the machine, and pour out and measure the surplus water the ink did not absorb, as well as note the percentage absorbed. They then return the water to the vessel and mix it for a further time period. This technique can be repeated several times, each time recording the amount of water absorbed. Operators next create a plot on a graph showing how much water is absorbed over what time period.
If the ink continues to pick up more water, it most likely would break down and cause scumming. On the other hand, an ink may take up some water initially, then resist further absorption - this is the preferred condition.
Inks should absorb some water; if they're too water resistant, the dampening solution wouldn't remove any ink that contacted non-image areas. Also, the water would remain on the ink surface, creating transferring problems.
The big question always raised with this test becomes the optimum percent water pickup. Should it be 20 percent or 30 percent? What happens if it goes as high as 70 percent?
In practice, inks have worked fine at all these percentages, but they also have failed, hence the confusion over interpreting results.
The key to understanding the significance of water pickup values? It is not so much the percentage of water that is picked up as it is the condition ink remains in once it has reached its saturation point. In other words, whether the percentage is 30 percent or 70 percent, how the ink body looks and behaves as well as how it transfers, are the important factors.
This means other tests should accompany the water pickup test.
For instance, viscosity and yield value tests on ink can be conducted before and after emulsification. A transfer test using a wedge printing plate before and after also can determine what degree of emulsification will interfere with ink performance.
Finally, there is nothing like conducting these tests and then observing the ink's behavior on press to help operators compile a log on ink characteristics versus on-press performance.
Bear in mind different ink vehicles behave differently when they absorb water. Therefore, water pickup numbers applying to one vendor's inks may not apply to another's, or even to material from the same vendor if the firm changes ink formulation.
An ink's tack, viscosity and yield value (body) also influence how much fountain solution it will absorb. The lower the tack and the softer the body, the more water it will absorb. Level of pigmentation and ink film thickness on the rollers also have an effect.
Variations in fountain solution formulation also can have a major impact on how much solution absorbs into inks. Fountain solutions with varying degrees of alcohol added, or wetting agents, as well as their degree of acidity or alkalinity, can change their reactions with ink.
With all these variations, it can seem difficult to put the proper interpretation on water pickup values.
But, by studying the considerations outlined and conducting the tests discussed, along with the water pickup test, and carefully evaluating the press performance of the inks tested, operators can obtain meaningful, predictive results. These findings can help inkmakers formulate the optimum running ink and allow printers to perform a test to predict ink's behavior prior to going to press - which is, after all, the object of the exercise.
TERRY SCARLETT Contributing editor and president of Burntwood Industries (Marco Island, FL), a consulting company specializing in inks and coatings