The 3M Company, a long-time leader in new technology, was developing a new ink hardening system. Normally, inks dry by solvent evaporation or they are hardened with heat.

The new approach would use ultraviolet radiation, or UV. UV has the proper photonic energy to cause chemical reactions, and the idea was to use this source of power to polymerize, or harden inks. Instead of taking minutes to harden, the inks could be finished in just seconds.

The UV system would represent a breakthrough in productivity and energy savings. The R&D lab at the St. Paul headquarters was happy with results. They had an ink vehicle, a fluid that could be colored with dye or pigment, that was working quite well. It tested fine on aluminum sheet stock. The first target market was aluminum beer can inks.

The addition of colorant would add complexity to the process since the colorants would interfere with the hardening polymerization process. Many common pigments were ruled out since they prevented hardening by UV. Dark colors were particularly a problem. Blue pigments had not worked so soluble blue dyes were tested.

The traditional pigments would have been preferred since they have more intense colors that typically don't fade. A series of blue dyes were evaluated for compatibility, and one was found that had no inhibiting effect on the hardening mechanism.

A small batch of blue ink was run through the small UV processor in the lab. The resulting ink came out hard and had good adhesion to the aluminum test panels. Other tests confirmed that the ink properties were satisfactory. The batch was scaled up for an initial production run at the American Can Company.

American Can had been developing UV processing capabilities over the past year in their Chicago plant and had been working with 3M. The factory UV line was much bigger than the 3M lab, but had the same wavelength thus no problems were anticipated.

While the 3M lab had a single UV lamp, the American Can factory line had 12. The rationale was that product could be run at a much faster rate by simply adding more lamps. The total exposure time remained the same at the higher speed. The tests started off with the clear coating that was applied to aluminum panels using a hand roller.

The product performed as expected. The blue ink was tested next. The ink was rolled onto the same type of blue panels that were placed on the conveyor belt that would take them under the bank of lamps.

A technician from American Can was at the other end of the long UV line. "Everything is looking good, so send down the blue ink", he shouted over the factory noise. He repeated the request about another minute later.

The 3M guys answered back that blue had been running for the last dozen samples. "OK, just mark a B on the next panel", replied the tech at the back of the line. Out came the aluminum panel marked with a "B" but no blue ink. What was happening?

The Rest Of The Story

After further tests and some thinking, they all realized that the UV was bleaching out the blue color so that the ink looked like a clear coating. Why didn't this happen in the lab?

It seems that the extra lamps and the additional ozone was enough to chemical modify the blue dye molecule to one that had no light absorption. Take note, scale-up can have surprises.