Ink and Substrates, like Oil and Water

You need to know the dyne level of a substrate to know your chance of getting good print adhesion.

To help understand the concept of dyne measurements, think of rain water on a freshly waxed car. Rain water (or water in general) has a dyne level around 72 (high surface tension) and a good wax coating on a car will bring the surface energy to around a dyne level of 23. So, we have water with a high surface tension dropping onto a waxed car with very low surface energy, and the result is the water (or think of ink) does not spread out and wet the car, instead it forms round droplets that roll off. Ideally there should be a gap around 10 dyne between surface energy and surface tension, with the substrate having a higher number than the ink for good wetting and therefore adhesion.

In most digital printing environments, there is only one ink available, so the variable comes down to substrate and primers/adhesion promoters needed to develop a good bond. Some manufacturers will have a couple of different inks that they recommend for their printers depending on the end-user prevalent workflow. For instance, is the printer and ink being used primarily for flexible or rigid applications. Other manufacturers have taken the step of incorporating primer in as one of the ink channels. This makes it easier to apply the primer when and where it is needed, reducing the need to apply the primer manually prior to printing. I would keep in mind that most adhesion promoters comes in multiple solutions for different substrates – meaning that the universal jettable primer needs to be tested thoroughly.

Keeping in mind the need for a 10-dyne difference for good wetting out of the inks (and using the dyne levels of the inks above in the 25-35 dyne level range), that means I can expect to print reasonably well on a substrate with a surface energy in the 35-45 dyne level. As an example, untreated Polyethylene is typically in the 30-32 dyne range, so I wouldn’t expect to get good results from these inks without doing something to treat the substrate. One method used is corona or flame treatment. Exposure to high voltage corona oxidizes the surface of the material, which increases the surface energy. Flame (or corona) treated polyethylene is typically in the 37-38 dyne level range, which would mean I am okay if my inks are in the 25-30 dyne level range, but if the inks are more in the 30-35 dyne level range I might need to consider chromic acid treatment, which will raise the dyne level to 39-40.

And that's a quick primer on dyne levels and printing!