Case Study: How Rolls Can Help Save Energy In Converting
Films are generally metallized and brightly printed before they are combined together to make the packaging we see all around us. In the manufacture of these films, it is necessary to change the surface so that the ink and aluminum layers will stick to them. Without this film surface treatment, the films could not be made into the packaging that we use and take for granted. The treatment is often carried out by passing the film through a corona discharge, where the film surface is burned (i.e., oxidized) by the intense corona produced by the treater. This process is called “corona treatment” and is used widely throughout the world to treat plastic films.
The corona is produced by passing a high voltage discharge through a thin layer of air between a metal bar and an insulating roller. This process consumes electricity to create the corona discharge. This project was designed to determine how much electricity is necessary to corona treat the film. It turns out that the amount of electricity needed to create the corona is controlled by the insulating roller, which has to have a tough, durable insulating surface. Usually the roll is made of metal so that it is strong enough to use. The coating is applied to the outer surface of the roller to protect it from the high voltage. The coating must have excellent electrical properties and be tough. Many different types of coatings can be used, such as epoxy plastic coatings, silicone rubber sleeves, ceramic coatings and glass coatings. In practice, the epoxy and silicone sleeves are prone to damage because they are relatively soft and are easily scratched and cut, which leads to an electrical failure of the roll. The ceramic and glass rollers are much more scratch resistant and have been favored in recent years.
This project was designed to compare the improved electrical efficiency of the glass coatings relative to ceramic coatings in a controlled way. In this study, we partnered with NYSERDA, Empire, ExxonMobil Chemical Films Division and EMMOUNT.
The study showed a clear reduction in electrical power consumption as well as improved treatment levels when using the glass coated roller. In fact, when the glass roll was installed, the lowest power setting tested yielded a greater treatment level than the highest power setting used with the ceramic roller. When the results are compared at constant treatment level, the glass roll requires a watt density (Watts-sec/in² ) of 0.77, while the ceramic roll required a watt density of 1.31. That’s a savings of 0.54 Watts-sec/in² of film treated, a reduction of about 40 percent.
At higher film treatment levels, the glass roll is even more efficient than the ceramic roll. The glass roll gives a lower receding contact angle at all power levels tested with the corona treater. The power to the glass roll can be reduced significantly to obtain the same treatment level. An alternative is that the current line can run faster at the same power lever to increase film productivity with acceptable treatment.
In reviewing the results for treating polypropylene films, it’s clear the glass treater roll uses less energy to achieve any level of treatment relative to the ceramic roll which it replaced. Assuming that the receding contact angle of 40 degrees achieved by the ceramic roll is an acceptable target, the equivalent watt density for the glass roll can be estimated from the linear, or the second order regression result, for the glass roll.
Extrapolating from the second order estimate, we find for a receding contact angle of 40 degrees, an equivalent Watt density value of 0.8 Watts-sec/in² for the glass roll compared to the approximate value of 1.1 Watts-sec/in² found for the ceramic roll. This represents an energy savings for the glass roll of approximately 0.3 Watts-sec/in², or approximately 27 percent relative to the ceramic roll. Use of the liner regression would yield an even higher savings.
Therefore, using these Watt density values we can approximate the energy savings expected for a typical treatment process where we calculate the energy requirement for the area treated from the line speed and treated film width times the Watt density needed to treat that film area with each treater roll. Using the typical converting treater roll, the expected process conditions would be:
- Treated roll width 60 inches
- Line speed 500 ft/min
This yields a treated area of 6,000 in²/sec, which would require 6,600 Watts (6.6 KW) for the corona treater. In comparison, the glass roll would consume 4,800 Watts (4.8 KW) to achieve the same level of film.
Empire Rollers & Coatings