By R.W. Stowe, UV applications engineering consultant, Heraeus Noblelight America LLC
It is generally accepted that short-wavelength UV has a major influence on surface cure, but the photochemical mechanisms are a combination of the effects of exposure, composition and optical properties. The most common symptom of a deficiency of surface cure is a tacky, sticky or greasy surface. Poor mar or rub resistance also might be symptomatic of insufficient surface cure.
Surface cure of a free-radical system can be affected by the following:
- spectral absorbance of the curable material
- photoinitiator blend and its spectral absorbance
- relative UVC content of spectral exposure
- photon flux rate (irradiance)
- oxygen inhibition
- functionality of component oligomers and monomers
A dominant factor affecting surface cure in a free-radical system is oxygen inhibition. The effect occurs owing to atmospheric oxygen. Oxygen reacts with the free radical and forms peroxy radicals whose reactivity is insufficient to continue the free radical polymerization process and reduces the rate of propagation, resulting in inadequate surface cure.
The effect of oxygen inhibition can be eliminated by displacing atmospheric oxygen by either (a) nitrogen flooding, (b) covering the surface, as in laminating or (c) in bonding applications where most of a curable adhesive is not exposed to air. However, UV curing in air is the most common, preferred – and less costly – arrangement.
Overcoming or reducing oxygen inhibition can be addressed by (a) managing the UV exposure or (b) modifying the formulation – usually both. For example, the formulation may include tertiary amines that have the disadvantage of yellowing, or incorporation of higher functionality monomers or oligomers.
The spectral absorbance of the ink or coating causes the shortest UV wavelengths (UVC) to be absorbed at and near the surface. The significance is that short wavelengths are limited in their penetration into the curable film, and short wavelength photoinitiators are effective only at the surface. Historically, UV-curable inks, coatings and paints contain a blend of two or more photoinitiators – one of which will invariably be of short (UVC) wavelength response, enhancing the reactivity of the ink or coating at the surface. The longer-wavelength photoinitiator will affect both surface and deep cure.
The higher photon flux rate (irradiance) of short wavelength (UVC), the greater the number and rate of photon-photoinitiator molecule reactions that occur. Combined with the two factors – wavelength and irradiance (“intensity”) – the rate of initiation and the polymer chain formation rate of reaction can be enhanced at and near the film surface, overcoming the molecular affinity for oxygen by literally beating the oxygen molecule to the free radical site.
Measuring the UVC presents some difficulties. The designated UVC range is 200 to 280 nm. However, almost universally, most portable UV radiometers measure only the sub-band of 250 to 260 nm. This sub-band was originally keyed to the strong mercury emission at 254 nm, and typically reported as “UVC.” This means that some of the important energy for surface cure is unobserved. For discussion purposes, we have labeled the “missing” sub-band as “UVC2” (200 to 240 nm). This range can be measured with a spectroradiometer, which – although not commonly employed in the production environment – is available in most UV labs.
It has been found that enhancing the UVC2 irradiance, by selection of engineered medium pressure bulbs and reflectors, can have a substantial positive impact on surface cure.
What about LEDs?
The game changes. Currently, LED arrays with sufficient power for UV curing are narrow-band emitters, falling roughly between the UVA and UVV bands. Without short wavelength (UVC and UVC2) energy, typically generated by medium-pressure mercury lamps, good surface cure is more difficult to achieve.
The remedy is in the formulation. A thorough discussion of formulating for LEDs can be found in “Mitigation of Oxygen Inhibition in UV LED, UVA, and Low Intensity UV Cure,” by Jo Ann Arceneaux, in UV+EB Technology, Q3 2015. The changed playing field stimulates new studies of formulation such as “Photoinitiator Selection for LED-Cured Coatings,” by Michael Wryostek and Matthew Salvi, UV+EB Technology, Q2 2017.
We have observed that many of the newer formulations for monochromatic (LED) cure have a surprisingly high concentration of photoinitiator. This appears to be the result of the fact that only a narrow portion of the photoinitiator action spectrum is involved in initiation. The higher concentration results in increased absorption of the principal wavelength, potentially impeding depth of cure. This, in turn, demands higher irradiance (UV power) to achieve full cure and required speed and performance.
Over the years, UV curing technology has brought new developments in equipment and applications, from low- and medium-pressure arc lamps, microwave lamps, excimers, pulsed xenon lamps, RF lamps and now LEDs. These have generated a perpetual need for chemists and formulators to devise materials to address these changing drivers.