Evolving EB curing technology for flexible package printing

Electron beam (EB) curing for wide web flexible package printing continues to evolve. Flexographic ink systems have been optimised to allow the wet trapping of ink colours cured with a single EB unit after the last station in a central-impression press configuration.

Long runs with stable printing performance are enabled by new press technology that includes temperature-controlled print stations. Web offset flexible package printing is also attractive given advances in variable repeat press technology. This includes the development of presses with offset print stations arranged in a central impression configuration. Both of these technologies allow flexible package printing with near-zero volatile organic compounds (VOC) and low-migration inks that are well suited for food packaging applications.

EB curing uses the high energy of accelerated electrons to directly cause the cross-linking of inks and coatings. The energy of this process ensures that a high degree of conversion from oligomer to polymer takes place. The use of this actinic radiation occurs under an inert atmosphere, such as nitrogen, to reduce ozone and other species generated under ionising radiation. The non-oxidative atmosphere promotes a high level of cross-linking between the ingredients in the formulation. This reduces free monomer and oligomer fragments to a level where the cross-linked product is acceptable for food packaging applications.

The pros and cons

Some key characteristics of EB curing include fast curing (up to 400mpm); a relatively ‘cold’ process; electrons that can penetrate deep into printed structures; curing inhibited by oxygen (nitrogen inerting required); and reduced odour when cured under nitrogen. This curing is ideally suited to web processes, including wet-on-wet printing with a single curing unit at the end of press.

Some advantages include electronically controlled dose rates (GMP/assurance of cure), the ‘sterile’ nature of the cured end product, low energy consumption, low heat (IR) radiation on substrate or central impression cylinder, low maintenance (meaning less downtime) and no need for photoinitiators.

In comparison, disadvantages are primarily related to the influence on the substrate, such as the discoloration of some polyamide (PA), PVC and OPP formulations. For issues regarding polyethylene (PE), the heat seal temperature of the thermoplastic can be increased, and for OPP the hot-tack window can be altered. Some plastic films may be subject to chemical breakdown products under EB radiation, and some chlorinated substrates suffer from odour effects.

EB installations require a higher investment cost but are less maintenance and have lower operating costs, which balances out the equation. EB curing on lithographic offset presses has been around for about 30 years, predominately for folding carton and multiwall aseptic packaging applications. The applications are roll-to-roll or roll-to-die-cut. It is wet-on-wet lithographic printing, similar to conventional web offset heat-set or conventional sheet-fed printing, except that the inks and coatings are curing via EB rather than heat or oxidative drying.

There is no interstation curing (although some presses have one to two UV curing units at the front of the press to perform additional converting). The substrate tension control can be a challenge. Newer in-line presses are equipped with variable repeat length designs, such as variable sleeves, to accommodate the requirements needed for the flexible packaging printing market.

One new central impression (CI) offset press is the first of its kind to use the film-handling capabilities that are commonplace in solvent flexographic presses. In general, the cost of offset plates is lower than the cost of flexo plates, making lithographic offset EB curing an attractive alternative to flexography for the flexible packaging printing market. This has been a growing area since 2006.

EB traditional flexographic printing presses come in three formats: in-line, stack and CI. High-quality graphics on wide web flexible film for food packaging are traditionally associated with large central impression flexo presses. Some printers and converters have modified or removed dryers and replaced them with UV or EB curing units to remain flexible in their operations. Changing from solvent-based or water-based inks to UV/EB presents challenges, yet many are interested in the quality and rapid curing of newer technology.

Some CI flexo presses with overhead dryers can be converted to EB flexo by replacing the dryer with an EB curing unit. Other changes, such as ink delivery and web handling, also are possible. This is a wet-on-wet application with no interstation drying. Several suppliers offer technology with EB curing, and others are offering technologies with and without interstation drying. The benefits are seen in the high-quality graphics and chemically resistant inks that are suitable for high speed, and the wide web flexible film for food packaging applications.

Food packaging applications

For applications where extractables or low odours are essential, such as food and drug packaging, this process provides a high margin of safety. EB inks and coatings generally do not contain volatiles, so emission control equipment that is used for water or solvent-based systems is not necessary. Local regulatory standards should be followed. Although EB inks and coatings can be formulated to have very low extractables, they do not have FDA approval for direct food contact. The products, like many conventional inks, can be used where a barrier exists between them and the food.

Despite the above mentioned cautions, EB ink curing is used to print on individual juice boxes and products containing milk in a folding carton format. Many are produced on web presses running commercially at 250–300mpm. For many customers, folding carton applications for food packaging are a large part of their businesses. The converted or polycoated board is usually die cut in line using a platen or a rotary die-cut device, with the latter enabling rapid finishing of the product. Several converters print on plastic films or paper/PE extrusion laminates for products such as in-mould labels, label wrap stock, shrink sleeves or flexible food packaging.

Different requirements for food packaging protection will impact the choice and combination of inks, coatings, packaging form and material used:

• Folding cartons: predominantly used in the US to store dry foods such as cereals and pasta.
• Liquid and ice cream packaging: made from coated board with EB inks in conjunction with extrusion lamination or UV/EB overprint varnishes.
• Pet food bags: multiwall paper bag/film construction used for dry pet food (UV and EB).
• Shrink sleeve labels: a range of films (PP, PE, PET, PVC and OPS) with reverse-printed EB inks and a gravure applied solvent.
• Wrap-around labels: a range of films (PP, PE, PET, PVC and OPS) with surface printed EB inks.
• In-mould labels: used for items such as yogurt lidding and cold seal snack foods, for example.

Non-sensitive packaging is used when a functional barrier exists between the ink and the packaging, or if the contents are not edible, with one prevalent example being multiwall fruit juice containers. Sensitive ‘indirect’ packaging is used when the food is not in direct contact with the packaging material, but there is no safe barrier between the packaging and the contents (examples include cereal and snack food boxes). Sensitive ‘direct’ packaging is used when food comes into direct contact with the inner, unprinted side of the packaging. Examples include orange juice and milk containers, such as gable-top polyboard boxes.

An understanding of the packaging material and how it is processed is important when considering the effects of ink and coating migration:

• Penetration: migration from the printed side of the package through the substrate and onto the unprinted side.
• Contact: migration from the printed side to the unprinted side of another sheet, stack or roll.
• Evaporation: migration due to the evaporation of volatile materials by heating (cooking or boiling the frozen product within its original packaging).
• Distillation: migration through steam distillation by baking, cooking or sterilisation.

The first two forms of migration are the most commonplace. The latter are less so, but the construction of the package and decoration are vital for avoiding any negative organoleptic effects. Judicious selection of inks and coatings along with the substrate is an important cooperative opportunity between suppliers and converters.

Understanding the limits of EB curing is important when considering flexible package printing. Namely, is the final package printed/ coated with wide web EB curing suitable for downstream converting, filling, packaging, transporting, warehousing, display, consumer handling and storage?

Trends and developments

Technology developments in energy curing include small electron beam units for narrow web label printing and converting, and the use of EB curing for inkjet, screen and gravure printing. The shifting marketplace for shorter runs and faster printing is coupled with the desire for high-quality graphics. Digital high-speed inkjet and other toner-based digital technologies are lagging behind the modern flexible packaging market with its 250–300mpm EB curing capabilities, but they will catch up eventually, and maybe even with the assistance of EB curing. The flexo and gravure wide web presses will not go away. In fact, the flexo to gravure ratios across the globe are US 80:20, Europe 40:60 and Asia 10:90. This technology is used for wide web flexible film printing for common food packing.

EB cured litho and flexo are making their way into applications such as common food packaging, high-end laminates and lamination-look packages (using a high-gloss coating to replace lamination). There will be more in-line EB laminate structures as the technology evolves to control bond strength using novel chemistry and EB curing.