Volume 6, Number 3
Sep 1993

Printing on Alkaline Paper

This paper was originally presented at the 1992 International Printing and Graphic Arts Conference, Pittsburgh, Pennsylvania, October 1821, 1992. Copyright TAPPI 1992. The author is Supervisor of the Chemical Division, Graphic Arts Technical Foundation, 4615 Forbes Ave., Pittsburgh, PA 15213.

Successful printers have learned to please their customers at the lowest cost. Thus, they must be flexible and adjust their printing processes quickly to get the best results with changes in their materials. The paper industry's rapid conversion from acid to neutral, or alkaline, paper has caused problems for many offset printers.

Overall, alkaline papers are superior to most acid papers. For example, the alkaline stock has much better permanence, so it will not disintegrate after 30 or 40 years (like many acid papers). Alkaline papers tend to be brighter and more opaque than comparable acid papers, so the prints look better. They also tend to be more friendly to the environment and offer economic advantages to the paper companies and to the printers.

Problems

Alkaline papers have caused some problems for printers. Many uncoated alkaline papers contain significantly more filler than comparable acid papers. With less absorptivity, alkaline papers tend to have better ink holdout, so colors are brighter and the prints look better. But there is a greater tendency for drying and setoff problems. Any such drying problem, however, will be less important than the problems experienced with coated papers. So, most quality printers can readily adapt to this if they submit samples of the stock to their ink companies and modify the inks as needed for the stock.

A more serious problem involves size reversion and lower moisture resistance on some alkaline papers. Unless the paper has adequate moisture resistance, calcium carbonate from fillers or pigments may build up or "milk" on the blanket. This buildup then contacts the plate surface, neutralizes the acid fountain solution, and/or abrades the surface of the plate. When milking occurs, the pH on the plate surface may be more than I pH higher than that of the solution in the fountain. For example, on a single-color press test, run in Pittsburgh, with an alkaline bond paper, the pH of the fountain solution on the plate surface was 5.7 vs. a pH of 4.5 in the fountain.

Since most acid fountain solutions will not properly desensitize nonimage areas of the plate unless the pH is below 5.0, the plate starts to scum. The printer then increases the water, which results in more milking, further neutralizes the fountain solution and causes even more scum until the printer gives up.

This problem occurs on both web and sheetfed presses with both coated and uncoated paper. I observed this problem with an uncoated, lightweight stock on a 38-in. sheetfed press. The press had been running satisfactorily for three hours with a good alkaline stock. Then, when a similar alkaline stock with inadequate water resistance was used, the blankets turned white within 20 minutes.

How Printers can Avoid Problems

Milking occurs on both acid and alkaline papers, but it tends to be worse on alkaline papers where size reversion may be a problem. Although milking results when the paper does not have adequate water resistance, the printer can eliminate milking problems by reducing the amount of water getting to the paper. On a web press, for example, a coated alkaline paper with inadequate water resistance caused one of the worst milking problems I have ever seen. However, we were able to run this paper on our heatset web offset press without any sign of milking by running the minimum amount of fountain solution needed to keep the nonimage areas of the plate clean.

When printing alkaline paper, it is important to use a highly buffered fountain solution that keeps the plate clean at low dampener settings. Then, increase the concentration slightly above normal to be sure that there is enough desensitizer to keep the plate clean with the least amount of fountain solution. Running very low amounts of water keeps the paper dry and prevents milking. Of course, a well-buffered fountain solution resists pH changes from the calcium carbonate.

The most common mistake most press operators make is to run too much water. With continuous dampeners, the press operator can note the potentiometer readings to numerically express the relative amount of fountain solution required. Printers who use nonabsorptive surfaces such as aluminum foil or polycoated board have learned to keep the fountain solution to a minimum since these stocks will not tolerate any excess water. Printers with milking problems should do the same.

Quick-release blankets may help reduce milking or blanket piling. If milking is a problem, printers may have to wash blankets frequently using some water. Automatic blanket washers can be a big help.

With better ink holdout, drying may be more troublesome on some alkaline papers. A low pH fountain solution, however, may also increase drying problems. Many printers, therefore, attempt to keep the fountain pH high. This is a mistake, because increasing the pH in acid fountain solutions generally results in increased scum so the press operator has to run more water. It is not the acid in the fountain solution that causes trouble. It is the acid that is emulsified in the ink. Thus, by reducing the water to a minimum, ink drying problems will be minimized.

Calcium carbonate on the blanket can be abrasive to the plate. Most metal plates resist this reasonably well, but many paper plates are very susceptible to abrasion. Electrostatic paper plates that might run 10,000 impressions with acid papers can become sensitive and scum very fast running inadequately sized alkaline papers. For proper paper plate life, it is important to keep the water to a minimum so that the calcium carbonate does not accumulate on the blanket. Surface dirt can result in dusting on the blanket, but this is generally much less severe than milking.

Another problem with an alkaline paper containing more ash than an acid paper is a lower caliper for the same basis weight. Paper of lower caliper is not as stiff and "feels" different. Some buyers have objected to this difference and have rejected entire print jobs because the "paper was different"--even though the prints looked significantly better on the alkaline stock. Thus, the printer should be sure that the customer will accept the "different" paper before printing the job.

The reduced stiffness may also cause some feeding problems on sheetfed presses. Where a 60 lb. acid paper runs fine at high speeds, it might be necessary to use a 70-lb. alkaline paper and /or slow down to avoid feeder problems. It is always good to try some of the "new" stock at the end of a job before buying large amounts of paper and committing to a customer without a print test.

Both alkaline and acid papers develop wavy or tight edges when not in equilibrium with ambient conditions. This can cause misregister, slur, doubling, and creases or draws in the prints. The reduced stiffness of the alkaline stock may cause more troubles with wavy or tight edges. To avoid this, printers should control ambient conditions in the pressroom, allow the paper to come to room temperature, not unwrap the paper until just before printing, and cover printed skids with plastic.

Summary

There are some 4,500 different printing papers available in the United States. Unfortunately, quality varies with different papers, whether they are acid or alkaline. The quality of the paper is more important than the type of paper. It is the responsibility of the printer and the buyer to carefully select the proper stock for any print job. Then the printer should adjust print conditions to obtain the best results with the least amount of trouble.