The Formation of ''Microfoam'' in Water-Based Flexo Inks
- Published: November 01, 2000, By Richard M. Podhajny, PhD, Contributing Editor
If you bubble air into water, the foam bubbles produced rupture rapidly, and the water surface layer becomes flat. However, if you add a small amount of soap to the water and introduce air, a large number of foam bubbles will be generated.
In this soap example, the air bubble is surrounded by soap molecules. The nonpolar ends face into the air bubble, while the polar or ionic ends will face into the water. The soap distributes itself at the air surface in a similar way. It's important to note that this creates charge stabilization to the foam. Since each air bubble can be surrounded by a series of ionic charges, these "charged bubbles" will repel each other and stabilize their existence.
Unlike water, alcohols have low surface tensions. Foaming is rarely a problem in solvent-based inks, since the low surface tension does not support foam stabilization. Water-based inks, however, are prone to foaming since many of their ingredients can stabilize foam.
Water-based flexo inks have come a long way since their introduction in the early 1970s. Press speeds then were about 450 fpm, and solvent-based inks were still the products of choice. In the late 1970s, environmental pressures to reduce solvent emissions accelerated the use of water-based inks, and many new hurdles had to be cleared.
Problems encountered by these early users of water-based inks included slow drying rates, poor ink printability, low gloss, variable adhesion, pH instability, and, of course, foaming.
By the early 1980s, some of these problems were diminished. Inks were still slow, but printability was improved as well as gloss and ink adhesion. Foaming was reduced. However, press speeds were still slower than with conventional solvent-based inks.
By the late 1980s, water-based inks were formulated that could be flexo printed at 800 fpm on paper as well as on corona treated PE films. Introduction of the chamber doctor blade and enclosed ink feeding systems significantly improved the quality of flexographic printing. However, some new problems were being encountered, such as "microfoam" formation at high press speeds.
It was in the late 1980s that I first came across microfoam. This foam differs from other foam-generating processes in that the air bubbles are extremely small. Hence, the term "microfoam." At this point in time, a water-based flexo ink would perform satisfactorily at speeds up to 800 fpm, but above this press speed microfoam would be generated in the ink, which would not dissipate readily.
Unlike other foam processes that generate foam on the surface of the liquid layer, microfoam is more akin to what you observe in a milk shake. It creates large amounts of entrapped air.
Analysis as to the cause of this microfoam formation suggests that the air bubbles become entrapped as the anilox roll comes in contact with the ink inside the chamber doctor blade assembly. The microfoam formation may be due to the high shear at these high press speeds, the high-speed entrapment of the air from the anilox cells, or, possibly, air carried on the land areas of the anilox rolls under the doctor blades.
The generation of these small air bubbles into the ink puts great demand on the ink anti-foam additives. So much area is created that there is insufficient anti-foam to effectively control the microfoam formation.
Anti-foaming additives are intended as "preventive medicine," whereas defoamers break existing foam bubbles. Most of these materials cannot withstand high shear and high heat conditions. A common denominator associated with the formation of a milk shake or the microfoam generated on a flexo press is the high shear.
Some flexo printers are using water-based inks at speeds well over 1,200 fpm and are struggling with this microfoam formation. New defoamers and anti-foam additives that are effective at high press speeds (high shear) are needed to move water-based ink technology to the next level of high-speed performance.
