Electron Beam Curing in Flexible Packaging
- Published: September 01, 2001, By Anthony J. Bean, Sun Chemical Corp.
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Application: Recent advances in electron beam curing chemistry and equipment encourage the increased use of this type of material in flexible packaging applications.
Energy curing use in packaging decoration and protection has been in place since the first commercial run of UV (ultraviolet light) curing inks and coatings in 1969. The use of electron beam (EB) products for similar application took another ten years for commercialization due to economic considerations. The cost of EB units was almost prohibitive. The cost of these units has since decreased considerably. Reliability and efficient inerting have also improved.
The growth of EB technology for curing inks and coatings in the packaging market began with the decision by Tetra Pak to install EB curing units on web offset presses globally. These units cured inks and coatings used to decorate and protect various containers in the liquid packaging field. Several film manufacturers were using EB technology in the production of film for food packaging. The EB units crosslinked and modified the polymers used to produce the films. Cryovac is the leader in this area and is the largest EB user in the world.
Advantages
EB curing offers the advantage that the process controls necessary to run a production line are readily available. The beam is easily diagnosed for energy output and can periodically be checked to certify that it complies with desired specifications. The EB energy is not sensitive to variation in printing colors or the ink and coating thickness. The only other issue surrounding EB curing is the need for inerting the curing chamber. This normally uses nitrogen gas pumped into the chamber to sweep the incoming web to remove oxygen following the web. An oxygen meter documents this condition. Assuming proper choice of the chemistry of the inks and coatings and control to maintain the level of expectations for a given application, the resulting EB line can be an easily controllable process.
As with other process control issues, chemistry related issues of EB curing are possible to resolve if a converter and chemistry supplier will expend the necessary time, energy, and dollars. In the real world, EB curing often makes more sense since it is easier and a more robust process in areas such as food packaging. At the time of this paper, this is the reason only one commercial EB coating complies with FDA direct food contact issues.
The use of EB curing for flexible packaging seems like a natural selection since the technology provides many positive features:
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No VOCs or emissions
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Instant drying at high production speeds
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High chemical resistance
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High abrasion resistance
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High gloss
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No softening after curing
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Low odor and off-taste.
Deterrent Issues
With these obvious and desirable attributes, why has the technology not been adopted in flexible packaging? One answer is that many still believe that the cost of the equipment is in the million-dollar realm. A second answer is that many printers do not want to be the first to adopt a relatively new and radical drying method. A third and more important answer is that many people are under the impression that the chemistry is or can be harmful. This is actually far from the truth. In reality, the chemistry of both UV and EB curing is usually safer than existing solvent and even aqueous chemistry. Table 3 shows some typical health and safety data for solvents and UV/EB materials. The UV and EB materials offer a considerably better position for a converter.
| Issue | UV/EB | Solvent | Water | |
|---|---|---|---|---|
| VOC | Nil | Yes | Yes | |
| HMIS | ||||
| Health | 2-3 | 1 | 1-2 | |
| Flammability | 2-3 | 1-4 | 1-2 | |
| Reactivity | 2-3 | 1-2 | 1-2 | |
| SARA 313 | ||||
| Reportable | Not Usually | Maybe | Maybe | |
| CA Prop 65 | Maybe | Maybe | Maybe | |
| HAPs Permitting | No | Yes | Yes | |
| RCRA | Not Usually | Usually | Usually | |
If a user can move past these concerns, the “real” issues surrounding the actual application of EB materials to a flexible substrate emerge. With the actual application come real problems such as wetting a plastic substrate. This is not necessarily an easy task for EB materials since the surface energy of many preferred materials are higher than most of the substrate materials used for flexible packaging.
Wetting of the film is nevertheless possible. As with aqueous inks and coatings, treatment of the substrate surface is essential and greatly facilitates wetting and adhesion to the substrate.
As people evaluate EB curing for flexible packaging, areas that will require investigation are the impact of the substrate on the curing process and the impact of the curing process on the substrate. An important substrate in flexible packaging is polyethylene. Although most people regard this as a simple polymer of ethylene, the actual material is a complex mixture of many other items added to enhance the production and performance of the material. Their impact on the chemistry of EB inks and coatings can be dramatic as it is to many conventional inks and coatings. Solvented materials are generally not impacted by the addition of slip additives since the solvents dissolve the materials. These materials are actually often part of the solvent ink itself. With EB inks, these materials will interfere with the wetting of the film and the resultant adhesion and lay. Other chemicals added to stabilize the polyethylene polymer are excellent free-radical scavengers that can stop the curing mechanism of the EB polymerization. If low extractables is a desired final property, this could have devastating effects.
The impact of the EB energy on the substrate itself also requires consideration. Just as the energy will cause the ink and coating chemistry to crosslink, it can also cause changes within the substrate. Several film manufacturers use EB energy ito alter their products to obtain desired features. Further exposure of these films can result in degradation of those properties. Care is necessary to ascertain the impact of the curing before going to press with a new or altered substrate.
Future Use
The problems and potential problems will not deter the eventual adoption of EB curing for flexible packaging use. The benefits are too significant to stop this technology from finding use where it makes practical and economical sense. Since most flexible films are printed by flexography, we must consider how EB curing can have use in this printing process. Obviously, the cost and the size of the current EB units precludes their use as inter-station dryers — the norm for flexographic printing. If the inks cannot readily undergo EB curing, one possible use of EB curing would be to apply a coating or laminating adhesive that could use EB curing. Both offer advantages over the more conventional products.
Another possibility would be to use an EB unit at the end of a press to cure coating and minimally set UV inks. The power of the EB and the advantages over UV curing alone would ensure that the inks all had proper cure to maximum effectiveness. This type of application is in production on several central impression presses. Printing with inks that have a higher than normal viscosity and then “wet trapping” the inks may also be possible. An EB unit at the end of the press could then cure the “trapped” inks.
Conclusion
The cost of EB units has decreased significantly. As the technology evolves, development of an EB unit that is sufficiently compact to fit between the printing stations on a press may be possible. As EB curing becomes accepted in the flexible packaging industry, the needs and the demands of the converters will drive the development of new units. One must remember that the energy curing industry has made a business of doing the things that others have told them were not possible. Only time will tell.
