Introduction

According to The Future of UV Inkjet Printing to 2018, a market report recently published by a PIRA, the global market for ultraviolet (UV) inkjet printing will grow by more than 10% annually and have an estimated value of $15.9 billion by 2018. The report also points out that flexible packaging and labels applications are currently growing at 20% CAGR and will cumulatively represent nearly 60% of all UV inkjet printing applications by 2018.  The ability to cost effectively print shorter runs with inkjet fits well with shorter product lifecycles and industry trends towards regionalization and personalization.  But, there are several barriers to the adoption of digital technology, which include VOCs in solvent-based inks, the limited substrates addressed by water-based inks and the safety of ultraviolet curable inks in food packaging.

This article compares UV to low-energy electron beam (ebeam) curing for inkjet inked food packaging. UV curing is characterized by light wavelength exposure (energy per unit area measured in Joules/cm2) delivered to the surface of the substrate via UV arc lamp.  Ebeam curing is characterized by dose (energy per unit mass measured in kilograys) deposited in the substrate via electrons. With ebeam curing, no photoinitiators are required. In lieu of photoinitiators, ebeam electrons produce sufficient energy to instantaneously break chemical bonds and enable photopolymer curing.  Ebeam is already well known as an option for curing varnishes, adhesives and web offset inline printing.

Low-Viscosity Photopolymer Chemistry and Inkjet Printing 

Typically, lower viscosity resins (below 40 cPs at 25 °C) dominate the majority of UV inkjet ink formulations. Monomers such as tetrahydrofurfurylacrylate, isobornyl acrylate and propoxylated neopentyl glycol diacrylate are used to modify viscosity, promote substrate adhesion and carry pigments. Polyester acrylates  and urethane acrylates oligomers are used on a limited basis to impart substrate adhesion, scratch resistance and flexibility .  Photoinitiators that create reactive species (free radicals) when exposed to UV or visible radiation are key components in UV inkjet formulations.  Photoinitiators that fail to react with photopolymer resins remain a part of any given UV inkjet ink formulation after curing occurs. Depending on the construction of the printed substrate, unreacted photoinitiator fragments along with monomers and oligomers can migrate into the substrate.  If UV inkjet surface printing is not appropriately protected - either through reverse printing, lamination or overprint - finishing migration can occur either through off-set or transfer of unreacted inks during stacking, winding or final product handling.  With respect to inkjet printed food packaging and contrary to UV curing, ebeam provides a consistent, safe approach for meeting low migration requirements. UV lamps degrade in wavelength output over time. If not regularly maintained, UV bulbs and reflectors can as well degrade in performance as they get dirty.

Electron beam surface penetration depends upon the mass, density and thickness of the material being cured.  Compared with UV penetration, electrons penetrate deeply through both lower and higher density materials. It is for this reason that ebeam is ideal for curing high opacity white, black and metallic ink layers without the need for photoinitiators. Titanium dioxide (Ti0₂) first-down whites are a strong absorber of UV light.  Depending on the thickness of the first-down white, the curing power of UV diminishes as it travels through the film substrate to its interface. Failure to get sufficient UV light through to the surface of a higher density Ti0₂-rich ink prevents full ink cure and results in poor substrate adhesion. It can also lead to significant photoinitiator usage in order to achieve fuller cure.

But, due to their relatively low viscosity, inkjet inks have lower pigment levels compared with conventional printing systems. Lower pigment levels are necessary to allow inkjet inks to jet properly at <20 cPs viscosities and prevent nozzle clogging and damage. As shown in Table 1 below, UV cured flexographic printing inks are typically as thin as four microns with pigment loads greater than 50%. UV cured inkjet inks may be as thick as 14-15 microns and typically do not present opacity (optical density) challenges. However, opaque UV curing inkjet inks still do contain partially unreacted photoinitiators and monomers, which remain a migration concern in printed food packaging applications.     

Source: UV Inkjet Inks for Single Pass Label Applications, Xaar PLC

First ebeam Inkjet Commercialization

During the IMI Ink Jet Printing 2016 conference, Collins Inkjet Corp. publically affirmed its commitment to adopting ebeam as a curing technology. In interviews leading up to the Anaheim, CA, conference Collins revealed that over the past several years, it has been formulating and validating ebeam inkjet inks as a low-migration niche solution that current UV/LED curable inkjet inks do not offer. According to Collins’ Marketing Manager Kristin Adams, “Concerns regarding material variability, potential migration, print-integrity, complete and consistent cure are obsolete with EB technology.”

DRUPA 2016 ebeam Collaboration

During DRUPA 2016, Collins Inkjet in collaboration with Prototype and Production Systems Inc. (PPSI) and COMET ebeam Technologies demonstrated a single-pass ebeam inkjet printing using a 280mm COMET ebeam curing lamp mounted on a six-color DICElab™ test bed integrated with DIMATIX Star-Fire 400 dpi heads. The ebeam curing lamp has a penetrating voltage of 80kV and is capable of delivering a dosage of 30 kGy at 100 m/min, which is sufficient for most inline inkjet curing applications. The advantage of PPSI’s DICE™ technology for single-pass high-speed printing is its ability to continuously circulate ink through its print heads along with automatic head-cleaning, which minimizes the incidence of “jet-outs” and related print defects.

PPSI’s inkjet test bed with ebeam curing capabilities was developed for ink companies and printers interested in developing ebeam curing as an approach for meeting low-migration requirements for printed food packaging. The 280mm wide ebeam lamp used on the DICElab below is one of several system configurations available in widths up to 1.5 meters that could be used in order to achieve ebeam curing performance with inkjet. The ebeam curing system shown on the test bed (Figure 2 below) includes an eight-inch shield roll, which also serves as a chilling system for heat-sensitive substrates and nitrogen inerting capabilities to displace oxygen, which inhibits free-radical polymerization.  Early tests of inkjet printing with EB-curable inks have shown excellent wet-out and adhesion to a white multi-layer film, and low migration of ink components.

80 kV ebeam curing unit

DICElab system with ebeam curing unit

Conclusion

For printers and converters of flexible packaging who are considering UV inkjet technology and seek added safeguards against indirect food contact and migration, ebeam curing should be considered. Ebeam is photoinitiator-free and ideal for curing high-opacity white, black and metallic inkjet inks and coatings. UV and ebeam inkjet formulations share the same resin, pigment and specialty additive ingredients. For narrow to intermediate web inkjet applications, compact 280mm and 400mm sealed 80kV lamps and 762mm vacuumed ebeam systems are an acceptable replacement for mercury vapor lamps. Ebeam has been successfully demonstrated as an alternative to UV curing with 100% solid systems for flexible food packaging.

Anthony Carignano is a Sales and Marketing Specialist at COMET ebeam Technologies. Carignano is an active member of RadTech International North America and has spoken globally on issues ranging from chemicals of concern to bio-renewable building block chemistries for consumer product packaging applications. Contact Carignano at awcarignano@teampct.com.

ACKNOWLEDGEMENTS:

The author would like to acknowledge Chris Lynn, consultant to Prototype & Production Systems Inc., Steve Lapin, PhD of COMET ebeam Technologies, and Collins Inkjet for their contributions in this article.

REFERENCE DOCUMENTS

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