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The right chemistry for the UV cure

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Commercial printing was one of the very first industries to undergo the transition from classical solvent-based chemistries to UV.

The inks and varnishes contain no solvents but are based on close to 100% solids. Unlike conventional oxidative drying where heat is often used to drive solvent from the wet ink or varnish, UV curing changes the liquid (wet) to solid (dry) state almost instantaneously as the print is passed at high speed under the UV lamp system.

UV solutions are being used increasingly for flat-sheet carton, paper & board litho printing applications and metal decorating. All types of presses from the major manufacturers have been successfully converted to run UV.

The need for technological change became apparent as run-lengths have become shorter and press make-ready has greater impact on line utilisation time.

More designs are now being printed with standard process colours and specials in-line with follow-on UV varnish. The number of ink changeovers and the need for wash-ups between colours can therefore be minimised.

UV has the advantage of non-drying whilst left on press and may be left in the ducts without skinning. The use of inter-deck UV curing to provide dry trapping between colours also eliminates colour contamination often associated with conventional wet on wet printing and improves print quality.

The process also extends to other parts of the industry including flexible and formed plastic container for industrial, toiletries and food packaging where other traditional printing methods such as flexo, screen and dry-offset are used.

Application extends beyond that of decoration: the stand diameter of a two-piece beverage can is often coated with a clear UV rim coat to provide base protection whilst being transported along the track work and give added mobility during the filling operations.

Latest developments

Today the Graphic Arts and Printing Industries have undergone rapid evolution with many of the traditional analogue processes being replaced by digital methods. This extends from the front end, whereby all pre-press operations are now almost exclusively digital, through to direct to plate or press.

There is a perceived growth in digital printing in those markets requiring print on demand, variable print or coding. Ink jet has been proven to offer advantages as a non-contact printing process: UV ink providing superior material performance for more demanding applications encountered within the industrial sector

Motivators for converting to UV

UV offers more cost effective solutions, environmental compliance and improved material performance, as summarised in Figure 1.

Capital cost savings of up to 70% may be made in comparison to installing a thermal oven without incineration equipment. The reduction in floor space can be as high as 200%: the footprint of the UV curing system being very small.

The UV process is clean, efficient and highly controllable. There is no large thermal mass to heat as with a conventional oven and the UV system can be powered-off or set to operate at a low standby level during idle or line stoppage.

An example of the relative cost of UV compared to conventional printing as based on a simple cost model is shown graphically in Figure 2.

Material cost alone should not be treated in isolation: applied costs must be considered in relation to the total process.

In terms of energy savings alone, electrical power to operate a UV system is approximately the same as that to supply the electrical drive and fans for a thermal oven.

Furthermore individual UV power levels can be stored to memory according to the UV energy requirements for curing a specific UV material and individually set or ramped with line speed.

As legislation moves ever forwards towards VOC reduction, many industries are compelled to seek alternative types of chemistries which contain minimal amounts of hydrocarbon based organic solvents.

Although water-based chemistries offer compliance, performance may be less good particularly as evaporation during thermal drying reduces film thickness.

UV Chemistry

The classic arc lamp, as manufactured by Nordson is available in a wide range of sizes and end-fittings.

A high voltage is applied between the electrodes to cause the inert gas within the lamp to ionise and form the arc.

Due to the rapid increase in temperature, the small amount of mercury also contained within the lamp then vaporises and enters the arc to form a plasma.

Inter-atomic collision with the high speed moving electrons stimulates photon emission in both the UV and visible parts of the spectrum.

The UV chemistries contain special components which chain or cross-link and harden on exposure to UV light through a process of photopolymerisation.

As there are no solvents to evaporate, there is no change in film thickness.

UV light may be sub-divided into UV-A, UV-B, UV-C and UV-V according to wavelength (nm). The short wave UV within the UV-C region has highest energy.

The photo-initiator which triggers the reaction must absorb UV energy at specific wavelengths, ideally corresponding to the peak spectral outputs of the UV source used to cure the material. Different initiator types have different absorption spectra and often a compromise has to be reached as more than one initiator can be contained within the curing chemistry.

UV customised engineering solutions

Essentially all UV curing systems consist of a lamp, reflector and cooling mechanism together with power drive.

Different reflector designs are available and often a hybrid elliptical shape is used to focus the light. This gives combined high UV intensity together with some IR which increases the reaction kinetics and assists cure.

Not all UV systems offer the same efficiency in converting electrical energy into useful UV - see Figure 3. This depends on the optical design, type of reflector system and the light gathering capability.

A continuous single span lamp is normally used for curing inks and varnishes on flat-sheet presses. Often a number of lamps are arranged one after another in parallel to provide the total UV dose required for curing the UV material chemistry at maximum line speed.

Certain colours such as dark blues, black, whites and metallic are more difficult to cure due to the pigment, titanium dioxide or aluminium flake either absorbing or reflecting the UV. The energy available to the photo-initiator may, therefore, be reduced. However, high power lamps are now manufactured with ratings up to 240 watts per cm. that have reflectors that are both air and water cooled such as the Quadcure as shown in Figure 4.

One or two Inter-deck lamps may be required depending on the press speed, ink film thickness and type of pigmentation. Set-stack or end-of-press curing units often contain three or four lamps to give final through cure of all inks and follow-on varnish. Plate coaters are available which offer full sheet or spotcoating if necessary. IR assist may also be used to improve flow-out and gloss, prior to UV curing.

An example the latest generation lamphead technology from Nordson for inkjet application is shown in Figure 5. The golf ball used in the illustration gives an indication of scale.

The technological challenge is to provide a high intensity UV light source with effective heat management that is sufficiently compact to be readily integrated within the printing machine. Alternative types of UV source including LED technologies are also being developed for ink jet, although reduced intensity is a factor limiting their use to mainly “pinning” rather than full curing at this time.

A new generation of Microwave powered UV (Figure 6) has recently become available from Nordson which offers step change improvements over other designs as the system uses non-conductive dichroic-coated borosilicate glass rather than metal reflectors.

These types of UV systems use microwave energy rather than electrical current to stimulate photon emission. As the bulb contains no electrodes, lamp life is greatly extended. The smaller bulb diameter can also give better light penetration due to the higher focal intensities. However, as the microwave powered UV bulb is restricted to a maximum length of 250mm, lamps have to be configured end to end to cover the total curing width.

Microwave powered UV is gaining increasing acceptance for narrow web and formed component application where space restrictions within the printing or coating machine is not a limitation.

Future UV materials and processes

Currently UV materials for Food Packaging application are restricted to external decoration rather than internal protection. Given UV is being used in many other applications including medical and dentistry, the chemistry could evolve.

Also some UV materials cannot provide sufficient flexibility. One potential solution is to partially cure the material such that it can be handled to allow forming and then 3D cure the final component.

At this time, the base coat for UV flat sheet metal decorating is still dried by means of conventional thermal curing. Similarly, a clear size has to be applied and thermally cured to provide adhesion when UV printing transparent metallic colours.

Development UV white base coats and UV sizes for metal decorating is ongoing that could provide a total UV solution thereby eliminating thermal curing altogether. To provide sufficient opacity, the UV white base coat maybe applied as two separate passes with partial UV curing between each. The chemistry is often specially formulated to operate with gallium doped UV lamps which extends the UV output to longer wavelengths. This assists UV curing thicker sections containing high white loading of titanium dioxide

* Dr Maitland is special applications manager for Nordson. He is also the head of research for Nordson UV head office in UK. Nordson UV is represented in Australia and New Zealand by Nordson Aust/NZ .

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