- April 01, 2001, Richard M. Podhajny, Ph.D. Contributing Editor
Unlike solvent-based inks where binders are dissolved in the ink solvents, water-based inks comprise several binders, none of which are soluble in water.
Water-based inks use several different binders, but most are composed of a styrene acrylic alkali-soluble binder and a resin emulsion. The alkali-soluble binder dissolves in alkaline water, which contains ammonium hydroxide. This resin also is used to disperse the pigment. This binder has a high acid number due to the large number of carboxylic acid sites and a relatively low molecular weight.
Ammonium hydroxide, amines, and inorganic alkaline materials neutralize these acidic components and form salts. It's actually these salts that make it possible to “dissolve” these binders. When the ink is dried, the ammonium hydroxide and volatile amines are removed. However, any alkaline material that is part of the binder in the form of a salt will not be volatilized until there is sufficient heat to decompose the salt and liberate the volatile amine.
Therefore, it is essential that water-based inks dry with some thermal assistance to optimize their water-resistance properties.
The emulsion, on the other hand, is a high-molecular-weight resin, which has few carboxylic acid sites. It doesn't dissolve in the alkaline water, but it is stabilized by the charge-charge repulsion that keeps it from coagulating in the emulsion.
As the ink dries, the charge-charge repulsion is affected, since ammonia and volatile amines leave the ink, and the emulsion particles can “touch” and combine to form a continuous film. Coalescent solvents are added to facilitate this process. If the binder does not coalesce, then the ink cannot form a smooth film and provide the necessary resistance characteristics.
Waxes trapped in this continuous ink film will migrate to the surface and reduce the surface energy. The driving force of this surface migration is that lower energy components will be attracted to higher energy surfaces. The movement of these waxes, plasticizers, and other ingredients takes time in a “dried” ink film, and there are inherent barriers.
However, in about 12-24 hr most inks reach equilibrium, and further migration does not change the surface characteristics. Performance tests on package resistance should be carried out about 24 hr after ink drying. However, temperature and humidity affect the migration rate. Higher temperatures increase the rate of migration. High humidity will cause the water-based inks to “dry” more slowly.
Let's test these concepts by looking at the following problem.
You have water-based inks that show excellent adhesion to corona treated PE film. In addition, you have a water-based coating that also has good adhesion to the same corona treated PE film. Both ink and coating show good water resistance when tested separately. However, when the coating was applied over the ink and dried, the water resistance and ink adhesion were poor. Why?
When you apply the coating over the ink, you can trap the volatile ink components if the ink is left to dry without heat. In this case, as the coating coalesces and forms a moisture barrier, the coating entraps the volatile amines, which exist in the form of salts in the ink.
Although you can minimize this by placing this sample in the oven for some time, the best solution is to dry the ink and assure that it reaches a web surface temperature sufficient to decompose and remove the amine. Then, the coating can be applied and dried with excellent water resistance and ink adhesion to PE film.
The ink and the coating each has a composition that is quite different. Whereas you want the ink to stay open on the anilox roll, the coating should dry rapidly as it is typically applied with a coarser roll.
Since the ink can be formulated to dry somewhat more slowly, it is imperative that it be dried as thoroughly as possible.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314, e-mail: firstname.lastname@example.org.