Direct UV cross-linking opens up the possibility of curing suitable varnish, paint and coating systems without photoinitiators or with a reduced photoinitiator content. The key to this is the targeted use of short-wavelength, high-energy UV radiation.
In combination with a formulation tailored to this, the radiation can directly support photochemical cross-linking reactions. Whether completely photoinitiator-free curing is possible depends on the formulation used, the substrate and the specific process conditions.
In conventional radical UV curing, photoinitiators absorb UV radiation and form radicals. These initiate the polymerisation of the reactive components and thus the cross-linking of the coating.
Direct UV cross-linking takes a different approach: with suitable formulations, short-wavelength, high-energy UV light can directly trigger photochemical reactions, thereby enabling cross-linking without photoinitiators or with a reduced proportion of photoinitiators.
Whether and to what extent this is possible depends on the specific formulation and the process conditions. The radiation spectrum, UV dose and other process parameters must therefore be specifically tailored to the application.
The effect of UV radiation on a coating depends largely on its wavelength. Longer-wavelength UVA radiation has a comparatively high penetration depth and therefore plays an important role, particularly in the cross-linking of deeper layers.
Short-wavelength UV radiation, on the other hand, is absorbed more strongly near the surface. At the same time, photons at shorter wavelengths have higher energy. These properties make the UVC spectral range particularly interesting for certain photochemical cross-linking processes.
However, which spectral distribution is suitable for a particular application depends largely on the specific formulation and the desired properties of the coating.
Photoinitiators play a central role in conventional radical UV curing: they absorb UV radiation and form radicals that initiate the polymerisation of the reactive components.
At the surface, oxygen from the ambient air can react with these radicals, thereby inhibiting polymerisation. This so-called oxygen inhibition can lead to insufficient surface curing and compromise the desired properties of the coating.
For reliable UV curing, the formulation, the UV spectrum used and the process conditions must therefore be coordinated. This applies in particular to photoinitiator-free or photoinitiator-reduced systems, where the composition and reactivity of the formulation play a decisive role.
Not every UV-curing formulation can be processed without photoinitiators. For successful direct UV cross-linking, the radiation source, formulation and production process must be considered as an integrated system.
Key influencing factors include:
In the case of aqueous UV systems, physical drying – or the removal of the water content – must also take place prior to the actual UV curing.
With suitable formulations and process conditions, direct UV cross-linking can offer various technological and economic benefits:
Which of these potential benefits can be realised in a specific application must be assessed on the basis of the respective formulation and process conditions.
Shorter wavelengths are also increasingly coming into focus in LED UV technology. Whilst industrial UV LED systems currently operate predominantly in the UVA range, UVC LEDs could open up additional possibilities for targeted surface curing in the future.
UVA radiation can penetrate comparatively deep into coatings, whilst UVC radiation is absorbed more strongly near the surface. In the long term, therefore, the combination of different wavelengths could be of particular interest in order to specifically tailor depth and surface curing to one another.
However, UVC LEDs have not yet reached the level of technological maturity and performance of established UVA LED systems. Furthermore, which wavelengths and combinations are appropriate depends heavily on the specific formulation and application.
Blog post: UVC LEDs in UV curing: current status and potential
With FREEcure, IST METZ offers a UV system solution for photoinitiator-free or photoinitiator-reduced UV curing of suitable formulations.
FREEcure utilises a specially designed medium-pressure UV lamp with an increased proportion of short-wave UV radiation. The radiation spectrum is designed to support the photochemical processes relevant to direct cross-linking.
This means that FREEcure can be used for both appropriately formulated 100% UV systems and aqueous UV systems. Whether photoinitiators can be dispensed with entirely or their proportion reduced depends on the specific application and formulation.
