Rub Out Your Rub-off
by Peter Ford
Your customers shouldn't have to wash their hands or clothes after reading your newspaper--and that's why it pays to know about ink rub-off.
Some of the key factors influencing rub-off are:
Newsprint. Figure 1 shows the effects on rub-off of varying the newsprint while keeping the ink constant. Changing the newsprint can cause rub-off to vary by over 50 percent.
Print density. Most pressmen attempting to print a low-rub newspaper try to keep their black densities at around 1.00 or 1.05. That's very important because, as you can see in figure 2, rub-off increases with density. When you get to the 1.2-to-1.25 range, you're producing a 30 or 40 percent increase in rub-off. Density also influences ink mileage and, therefore, ink expense (see figure 3). In other words, if you're laying down too much ink, then you're spending too much money.
It is important to understand that the density scale is logarithmic. In going from a density of 0.9 to 1.3, as we're illustrating in figure 3, you might think the ink-film thickness increases by about 50 percent. Actually it increases by nearly 100 percent--you're nearly doubling the ink film.
Timing. Figure 4 shows how all categories of low-rub black ink improve with time after printing. At around three hours, most of them begin to plateau, and that's why it's an industry standard to test rub-off at three hours after printing.
Ink formulation. Figure 4 also shows the influences on rub-off of different categories of low-rub black ink. In our product range they're called "Super Standard," "Low Rub" and "RubPruf." Notice that even the medium-range "low-rub" product is really only giving you about a 35-to-45 percent improvement in rub-off. Those improvements can be totally lost if you use the wrong newsprint or print at the wrong density.
What is the difference between a low-rub formulation and a conventional one? There's quite a bit to it, but the most essential difference is that low-rub inks contain a high level of varnish. Varnish is a solution of hard resin that serves as the binder to prevent rub-off--to actually hold the pigment down on the newsprint.
To carry that resin in the ink, you have to cut it in some form of low-viscosity oil, and the oil you choose can have a huge influence on volatile organic compounds. The oil that has traditionally been used to cut the resin and carry it in a low-rub ink formulation is low-viscosity mineral oil, which has a VOC content of between 50 and 70 percent. So you can understand why some low-rub inks have tremendously high VOCs, maybe as high as 30 or 40 percent. It's because they carry so much of that low-viscosity mineral oil.
An alternative oil is soybean oil, which has very low VOCs but is quite costly and very different chemically from mineral oil. If you want to use soybean oil, you must change other components in the ink formulation.
US Ink has developed inks using alternate grades of mineral oil that have VOC numbers similar to those of soybean oil but prices closer to those of traditional mineral oils. We hope this will open up an opportunity for people who want to have low-rub newspapers and yet stay at the lowest VOC levels.
Peter Ford is technical director for US Ink. Phone, (201) 935-8666, ext. 366; fax, (201) 933-2291.