Should You be “Bump” Treating?

By Pillar Technologies

The increased demand for corona treater products is apparent by the greater volume of materials being handled by plastics processors and the growing number of applications. The trend is for even greater reliance upon corona treating to prepare increasing amounts of film and foil webs for printing, gluing and other converting steps that require a surface that will readily accept inks or adhesives. This is true for both materials currently in use as well as for new materials under development for manufacturing products and new packaging Systems.

A major factor in forming a satisfactory bond is the interaction between the contacting surfaces of plastic film and adhesive or ink. In the absence of purely mechanical bonding, which is the case of an ink or glue penetrating into a porous surface like paper or wood, an adequate surface interaction is necessary. The surface interaction has two components: the wettability of the film and the chemical adhesion strength between the film and the ink or adhesive.

Until the 1980's, corona treating was a process primarily used by extruders and film suppliers to improve the adhesive characteristics of their films. The trends towards waterborne inks and the more frequent use of PP, BOPP and high slip films have changed all of that. The tendency for treat levels to degrade faster with these films and the need to treat to higher dyne levels for water-based inks no longer allow film suppliers to treat to 38 - 40 dyne, store the film until the converter needs it, then expect the customer to get good adhesion. Although many film suppliers have been improving and upgrading treating equipment, in order to obtain higher treat levels, treat degradation, slip additives and over treatment still work against them.

Printers using solvent based inks were often able to get sufficiently treated films from their supplier, since the required treat levels were not as high. However, with the switch to waterbased inks, many suppliers were faced with having to treat the materials to 50+ dynes/cm, in order to allow for decay in treatment. These higher treat levels frequently resulted in faster decay rates, blocking, and degradation of the film's properties.

It is now common for printers to re-treat material on the press, even though the film supplier has treated on the extruder, already. It is also common for film suppliers to treat to 38-40 dynes/cm, and expect the printer to "bump treat" the material to the necessary 42-45 dynes/cm for waterbased inks.

The conversion to waterbased inks has had a significant impact on the flexographic printer. The use of corona treaters and the need to understand surface tension measurement has become necessary for most waterbased ink applications. The increased use of polyethylenes and polypropylenes with high amounts of slip additives, will not only cause treaters to be necessary, they will become larger, as well.

Starting in the mid 1980's, more and more converters began adding corona treaters to coaters, laminators and printing presses. The equipment was only intended for simple "bump treating" to bring the treat level up to the originally treated levels. Oftentimes a converter or printer will purchase pretreated film. Since treatment does tend to decay and because of a drop off in the required dyne level during storage, they will have to “bump treat” the film with their own on-line corona treating system prior to the printing or converting process it was originally intended.

Corona bump treating requires less power than the system originally used to treat the films stock since they are basically used to “regenerate or rejuvenate” the original treat levels. The most common situation for bump treating is the printer who buys “pretreated” rolls of film. Many factors including time, weather and even storage conditions can alter the state of originally treated corona. Printers find that purchasing a corona treater for “bump treating” saves them from having to discard roll stock that may have lost it’s original treat level.

Once the responsibility of the film supplier, plastic film users are finding that in conjunction with using waterbased inks they must bring the surface treatment process in house in order to produce an acceptable print quality. As a result, it has become necessary for printers to be familiar with the surface treatment process, as well as a basic understanding of surface science and the procedures for testing treat levels.

Corona treaters used specifically for bump treating are designed for compactness, ease of operation and low maintenance for lower power, simple treating applications. Bare roll treaters were perfect for these applications since they are easy to operate, low maintenance and compact enough to fit on most converting lines. In addition, since bare roll was capable of treating either conductive or non-conductive materials, they were perfect for the converter who was experimenting with water based inks or coatings, despite it's being less effective than the covered roll systems used by the film producers.

However, as the harder to treat materials gained in popularity and the waterbased inks and coatings began being employed on more and more jobs, the need for higher treat levels and more flexibility became necessary. Bare roll systems got larger and power requirements dramatically increased, partly due to the inherent inefficiencies of the systems. In some situations, treat levels were unattainable with the

bare roll design. Lane treating and skip treating, where only specified sections of the film were required to be corona treated, was also limited if using bare roll systems.

Converters found that in order to obtain the treat levels needed, they had to have covered roll systems. The introduction of more reliable roll covering materials such as ceramic, has helped to reduce the maintenance issues and improve treat levels. However, converters were unable to use these systems for treating metalized materials. Thus, the converter was usually forced to depend on the metalized film supplier.

It is for these reasons that Pillar Technologies introduced their Universal Corona Treater and Universal Split Box Treater at the recent NPE 2000 show in Chicago. Universal Corona treaters offer convenience of bare roll with the efficiencies of covered roll by simply changing a magazine. It is now possible to treat conductive or non-conductive materials, lane treat and/or skip treat all with the same corona treater. Unlike the Universal, earlier designs were only capable of fixed treat widths and were more difficult to change over. These earlier designs were not intended to be sold as a convertible system and were only converted when the bare roll configuration proved incapable of achieving the results obtainable with a covered roll system.

The patented modular interchangeable electrode magazine system enables the operator to clean the electrode off line, thus reducing maintenance downtime to just minutes. An external gap adjust makes set up quick and easy. Roll changes are done in a snap because of the unique roll design. Also patented are the pneumatics which enable the electrodes to easily swing out of the way when passing a knot or splice, reducing web kinks and broken electrodes. The microprocessor based diagnostics make troubleshooting effortless.

Ceramic holder blocks and inorganic baffles are found in the bare roll version to reduce arcing and carbon treating. The unique construction is designed to maintain an even gap. Dissimilar materials are allowed to expand or contract without bowing or warping due to the unique magazine design.

The covered roll version features segmented style electrodes which can be adjusted in 1/2" increments. Roll coverings include ceramic, epoxy, glass, hypalon, silicone and silicone sleeve.

The Universal Split Box corona treater was specifically designed for blown film extrusion lines. Pillar’s new Universal Split Box is a more economical system that promotes an easy-thread, “drop-through” design. This system opens up horizontally so you can thread substrates directly into the system. The Universal Split Box also eatures a removable electrode for “off-line” cleaning. Other features include easy gap adjust; stainless steel segmented electrode; and easy roll removal.

For more information on the issue of corona “bump” treatment or Pillar’s line of corona treating equipment and power supplies, please contact Pillar Technologies via phone at (262) 367-3060; fax (262) 367-3206; e-mail to treaters@pillartech.com.