Photoinitiators – the cure all for inks?
Some swear LED-UV is a more efficient technology, others still aren’t ready to invest. Here is a closer Here look at the pros and cons of conventional UV and UV-LED curing technologies.
Since its introduction in the 1960s UV or ultraviolet curing has been widely adopted for curing or drying products which are used in a wide range of industries. It’s easy to see why its use has become widespread as it has many benefits over traditional ‘air’ drying techniques, including increased production speeds, reduced rejection rates, higher print quality and a glossier finish. It also provides much better adhesion to substrates that are notoriously difficult in this regard, such as plastics and other non-absorbent substrates.
Unlike conventional air drying inks, energy curing inks contain relatively small molecules with molecular weights which are incapable of producing a non-tacky film without further processing. This is achieved by applying energy to convert these materials to a polymeric, non-tacky network.
This change is made with the help of photoinitiators, chemical substances which are added to UV inks, to allow them to dry. They do this by reacting with UV light, which breaks them down rapidly, forming new, highly reactive substances, which attack the acrylate groups in the inks causing them to react with further acrylate bonds and so on, to produce an extensive network of ‘linkages’, so drying the ink.
UV and UV-LED
The main difference between UV lamps and UV-LED’s is the way in which the UV light is generated. UV-LED’s, using a variant of the LED technology currently used in domestic lighting, are specially tuned to emit UV light in a very narrow bandwidth. Conventional medium pressure mercury lamps produce UV light across a much wider wavelength range, based on emissions produced by an energy change in the mercury inside the lamp.
UV-LED’s produce a narrow band of power output, so there are only a limited number of photoinitiators that are currently suitable for use with them. However this may change, as ongoing development work leads to LED’s emitting a broader range of wavelengths, which will allow the use of a wider range of initiators, providing a better curing profile for inks and varnishes.
In order to accommodate the power output and bandwidth of current LED technology, inks need to be more reactive than those formulated to cure with standard mercury lamps.
This presents ink formulators with a number of challenges. Particularly with white inks and varnishes, where yellowing can be an issue, due to the chemistry of the photoinitators, best suited to wavelengths LED’s emit. The formulation style required for UV-LED inks also results in a higher cost structure for the products, compared to those used with mercury lamps. In addition, the output wavelengths of UV-LED’s are currently close to that of visible light, which is why presses running these products must exclude ambient light as much as possible, to prevent drying on the press.
However, mercury lamps are effective with a wider range of photoinitiators and are not as susceptible to ambient light curing, making this technology easier to use on the press, overall.
One issue which could significantly affect the adoption rate of UV-LED’s is the Restriction of Hazardous Substances (RoHS 2) Directive in relation to the ongoing use of mercury halide lamps. Introduced in January 2013, this legislation restricts the levels of use of a range of heavy metals in new electrical and electronic equipment. It isn’t clear at present how this will specifically relate to mercury in UV lamps, so the industry is poised to discover the final outcome.
The pros and cons of UV-LED
Enthusiasts claim that UV-LEDs have advantages over mercury lamps, including longer lifespans. Additionally, they emit very little heat, in the direction of the substrate that’s being printed compared to UV lamps.
Typically, UV-LEDs have a lower power consumption than UV lamps, are mercury free and do not generate ozone, hence they are generally seen as more environmentally-friendly alternatives to conventional technology. They also do not require a warm up period, so can be used immediately as the press starts up and is switched on and off, as required. They are particularly suitable for use with heat-sensitive substrates, such as shrink sleeve films.
One potential disadvantage is that the distance between LED units and substrate needs to be carefully maintained to make full use of UV output. This distance is quite small and so web movement needs to be carefully controlled to prevent print marking and ink deposition on the LED units.
Lifetimes quoted for LEDs are often well in excess of those for mercury lamps, initially 10 times the life or more, although this is under review as field experience is longer-term. However, UV-LEDs are currently more expensive and often represent a capital expenditure item which, for some printers, may be a barrier to adoption, especially when mercury lamps are treated as a consumable item, typically lasting up to 1,000 hours.
There has not been a major shift to the new technology, possibly as a consequence of the issues explained so far, especially the cost. Yes, there are some early adopters, but the UV-LED technology has yet to gain significant traction in the print market.
Many in the industry believe that the ‘green’ credentials and operational advantages promoted for UV-LED’s will drive market development as the technology’s use extends beyond the digital market, where it is already well-established. Progress has nevertheless been slow to date. The question that remains for the technology is not if, but when, it will gain a major foothold in the print market.