Abstract
The roll-to-roll converting industry is under constant pressure to balance productivity, energy efficiency, and regulatory compliance. UV curing has long been a proven method for instant crosslinking of coatings and inks, yet traditional systems rely heavily on photoinitiators and inertization. FREEcure, developed by IST METZ GmbH & Co. KG in collaboration with BASF SE, represents a paradigm shift by enabling direct UVC-induced polymerization with drastically reduced or even eliminated photoinitiator content. This article examines the scientific principles, technological innovations, and industrial benefits of the FREEcure system, demonstrating its potential to redefine sustainable roll-to-roll curing.
The Challenge: Photoinitiators and Oxygen Inhibition
Conventional UV curing systems depend on photoinitiators to absorb UV light and generate radicals that trigger polymerization by converting carbon-carbon double bonds (C=C). However, oxygen inhibition at the coating surface limits reaction efficiency, causing slower line speeds and incomplete curing. Traditional countermeasures include inertization with nitrogen or excessive photoinitiator loading—both costly and environmentally problematic. Regulatory restrictions and supply constraints, particularly under REACH and more and more restrictions on Specific Migration Levels e. G. German Ink Ordinance and Swiss Ordinance, have further challenged the ink and coating industries to find photoinitiator-free alternatives.
The Physics Behind FREEcure
The development team at IST METZ investigated whether UV polymerization could be achieved through direct bond cleavage, bypassing photoinitiators entirely. Theoretical calculations indicated that photon energies exceeding 6.15 eV—corresponding to wavelengths near 202 nm—are sufficient to crack the C=C double bond directly. Hence, the objective became clear: develop a UV source capable of producing intense radiation around 200 nm without requiring an inert atmosphere.
Excimer and LED sources were evaluated but ruled out due to low penetration depth, ozone formation, and insufficient optical power. The medium-pressure mercury lamp, however, offered a broadband UV spectrum starting near 200 nm and could be modified for higher UVC emission. IST METZ engineers therefore based the FREEcure development on their proven BLK platform, shifting the spectral output toward the short-wave UVC range and increasing emission below 220 nm heavily.
This was achieved through optimization of plasma technology, improved reflector materials, specialized lamp materials, and refined electronic control systems. The resulting UV system exhibits extraordinary photon density in the UVC region, ideal for direct double-bond cleavage.
From Physics to Process: The Chemistry of Direct Curing
BASF SE contributed its expertise in binder chemistry to evaluate for UVC reactivity. Binder selection, functionality of binder, and amine modification proved critical for suppressing oxygen inhibition and achieving high conversion rates. In laboratory FTIR analysis conversion levels above 80–90 % were achieved without photoinitiators. Both 100 % UV and water-based UV dispersions were successfully cured.
The cooperation project thus bridged lamp engineering and chemical formulation, demonstrating that targeted synchronization between raw material and light source can yield process-ready systems. In many cases, FREEcure coatings displayed chemical resistance, scratch hardness, and gloss levels similar or better to conventional systems employing standard photoinitiator concentrations.
Implementation for Roll-to-Roll Production
In continuous coating and printing lines, production speed, space requirements and system configuration play an important role. With suitable formulations, FREEcure enables high curing speeds. Depending on the application, only a few FREEcure units may be sufficient, potentially supplemented by LED UV systems to meet additional requirements for through-cure.
Due to the high UVC output, the number of UV units required for the curing process can be reduced in suitable applications. This can provide benefits in terms of space requirements as well as energy, maintenance and operating costs.
The actual reduction in the number of units and any potential savings depend on factors including the formulation, production speed, coating thickness, substrate, required degree of cure, system configuration and the respective reference system.
Operator Perspective: 'It Cures Like Hell!'
Early trials with industrial users quickly confirmed the system’s power and reliability. During initial curing experiments, one operator famously remarked, “It cures like hell!”—a fitting description for a technology delivering radical performance without radicals from photoinitiators. Despite its humor, the phrase captures the excitement around FREEcure’s rapid surface polymerization and immediate handling strength.
Requirements for modern web printing processes
With suitable formulations, FREEcure enables photoinitiator-free or reduced-photoinitiator UV curing. This can be particularly relevant for applications where photoinitiators and their by-products need to be considered as potential sources of migration.
However, actual migration and suitability for a specific packaging application depend on the entire ink, coating, substrate and process system.
Depending on the application, FREEcure can also be used without inerting. This can simplify system integration. Whether inerting is required depends on the respective formulation and process requirements.
Conclusion
FREEcure combines a UVC-rich lamp spectrum with formulations designed accordingly, expanding the possibilities of UV curing. In suitable applications, coatings can be cured without photoinitiators or with a reduced photoinitiator content.
For web printers, this opens up additional possibilities in terms of process design, the number of UV units and system integration. The benefits for a specific production line must be evaluated based on the respective application and process conditions.
For more information or evaluation of feasibility in your process please contact us.