UV inkjet inks
UV curable inks primarily contain reactive monomers, photo initiators, oligomers, pigments and additives. When the ink reacts to UV light in the curing process, the monomers polymerise, forming a tough film. UV curable inks have been prevalent in traditional printing methods such as flexographic, lithographic, rotogravure and screen for over a decade. In some of these sectors, UV inks account for 20% or more of the ink market. UV was not implemented using inkjet until advances in printhead technology and materials made it viable. Despite the longer wait for UV to become established, as customer demands expanded, many companies began to introduce UV inks to their product lists. These challenging customer requests often included rapid processing, high abrasion and chemical resistance and irregular/hard to print on substrates.
The advantages of UV curable inks are numerous. Some of the most beneficial include: no VOCs (volatile organic compounds), little or no air pollutants, high print quality, low odour after curing, reduced ink wastage, constant ink composition, no print-head clogging, low energy requirements and the ability to print on a wide range of substrates.
UV inks are typically high viscosity, which limits flow on the substrate. This, combined with the instant or spot fixation offered by UV curing lamps, leads to excellent print quality and fast overall operating speeds. Using a solvent based system, high-speeds are usually only possible with extremely large dryers present, increasing energy and operation costs. Since no dryers are required, costs are reduced and floor space is saved. With a UV system, there is no evaporation of volatile solvents in the printhead, meaning less waste and time spent on maintenance. UV ink also has good colour value; highly optimised UV inks also produce a very clean colour gamut. Additionally, since the ink does not cure until exposed to UV light, there is no risk of the ink drying prematurely which prolongs the life of the printhead and ensures less downtime (provided the inks are stored and managed correctly).
UV inks are typically 100% solids, meaning everything that is printed remains on the substrate after curing. Environmentally unfriendly VOCs which are emitted from solvent inks are also no longer an issue. The reduction of air pollution is a benefit for those wishing to take a more ‘green’ approach to production. This is also particularly relevant since most countries have stringent laws that are constantly being updated regarding VOC levels. Using UV inks is one way to avoid production pitfalls which may occur due to legislation. However, there are some circumstances where UV inks are just not suitable.
Since UV inks are typically highly viscous, they are often less desirable for printing smooth, consistent films or for applications where a very thin film thickness is required. Heat from UV lamps can also damage thin or delicate substrates, although the introduction of UV LED technology is helping to combat this issue. Another potential problem can occur with over-curing; adhesion may be reduced causing the ink film to flake. Staff proficient with solvent based systems may not be aware of the issues at hand with UV ink and may require retraining which can cost time and money. In addition to this, handling uncured materials carries potential health hazards that staff need to be aware of.
It is beneficial that waste and maintenance costs are diminished using UV, however setup and operation costs are often higher. While the cost of raw materials for UV curable inks have decreased in recent years they are still significantly more expensive than solvent based alternatives. The curing of UV inks will also require the purchase of special UV lamps (which consume energy rapidly), driving up setup costs significantly. Ancillary equipment may also be required in order to dissipate heat from the printing machinery. It is a matter of debate whether these extra costs are justified by the potential benefits UV has to offer.
While UV inks can be used on a wide variety of non-porous substrates, they do not always show good adhesion and may need modification for more difficult substrates such as polypropylene. Likewise, since UV light must reach all the way through the ink layer in order for it to polymerise, care must be taken to ensure that the combination of film thickness, ink absorption and UV intensity is optimised to ensure full curing.
While UV inks have their limitations, the future is bright as improvements in technology continue to bring UV to wider markets. For instance, UV digital inkjet is now the dominant method of direct to rigid substrate for the POP market and continues to increase its market share in areas such as outdoor signage and film packaging. Provided suppliers are well informed of the correct usage and benefits of UV, it is clear that the technology will continue to expand.
Tim Phillips, Catenary Solutions