The water droplets on a waxed car illustrate an important principle of surface tension. The water beads because its surface tension is higher than the finish it rests on. This same principle applies to printing: When the ink's surface tension is higher than the surface energy of its substrate, the ink won't wet out.
Fortunately, there are ways to alter the surface energy of substrates to achieve proper wet out. Measured in dynes/cm, surface energy is raised in label substrates primarily through corona treating.
"A corona treater is essentially a device that takes 10,000 volts of high frequency high voltage energy and ionizes the air in the air gap between the electrode and the ground," says Tom Gilbertson, VP application engineering for Enercon, Menomonee Falls, WI, USA.
The result is a substrate with a raised dyne level. As a general rule, the substrate's surface energy should be 10 dynes higher than the ink's surface tension in order to produce desirable labels. Corona treating also has an added advantage: "It increases the polarity of the film surface, which creates a stronger bond to the ink itself," says Craig Potter, films technology leader for Avery Dennison's process, technology and innovation group, in Painesville, OH, USA.
Corona treatment is generally done at extrusion by substrate manufacturers and in-line prior to printing. The technology has been around for a while, and corona treater marketers give mixed reactions on the state of the corona industry as it pertains to narrow web.
Ken Klein, president of QC Electronics in Protage, WI, USA, says the corona treater market began showing signs of maturity 10 years ago. Today, "the entire corona treater market in general is very mature. I honestly believe that in a few years you are going to see mergers and acquisitions occur with the corona treater companies," he said.
Others believe that trends in the narrow web market-place are pushing the corona treater market to further growth. "We're still seeing increased demand," says Bruce Stobbe, president of Corotec in Farmington, CT, USA.
"What we attribute it to is increasing speeds of the presses, wider webs, and more and more people branching out into material substrates that require treatment. Some of them may also be moving from certain types of inks over to inks that require higher levels of surface treatment," he adds. Printing at higher speeds or on wider webs requires more treating power.
Types of treaters
There are two main types of treaters: bare roll and covered roll. There are also hybrids, namely convertible and dual dielectric treaters. The hybrids are sometimes referred to as universal treaters.
Although a bare roll system can be used for non-conductive materials, its primary draw is its ability to handle conductive substrates. As far as system composition, "the bare roll is bare steel. The electrode has a ceramic over it so that it doesn't are directly to the metallic substrate," says Rob Hablewitz, sales manager, surface treatment for Pillar Technologies, a division of ITW, in Hartland, WI, USA.
A covered roll system is only used for non-conductive substrates. In this system, the electrodes are bare and the roll has a dielectric covering over it, such as ceramic.
Each system has benefits and drawbacks. As previously mentioned, bare roll treaters are advantageous because of their ability to print on both conductive and non-conductive substrates. They are less efficient, however, than covered roll systems. Covered roll systems cannot be used for conductive substrates, but they offer greater efficiency.
Both systems are used extensively within the narrow web industry, but companies disagree about which system is the best for this market.
"We found that the narrow web business would be better off with a bare roll and that's what we are building for this moment," says Hablewitz. "The majority of narrow web...