Both UV and EB curing have been around for some time and are used in a wide range of industrial applications but are just beginning to break into packaging applications in a big way. This article focusses on their use in conversion of packaging materials using processes like printing, coating and lamination or combinations thereof. They are used in conversion of continuous webs as well as in sheet-fed operations.
The need for curingPrinting inks, coatings and adhesives need to be cured after they are applied on to substrates to enable them to “set” or harden and bond securely to the respective substrate surfaces – typically paper or board, flexible or rigid plastics, films or multilayered laminates, metallic foils or sheets and nonwoven structures. Conventionally, they are applied wet using liquid, pasty or thixotropic formulations or as dispersions/emulsions. To be applied on to substrates, they have to use a solvent like water or organic chemicals as a carrier or vehicle and this solvent has to be fully evaporated to enable the applied substance to dry and set with good adhesion to the substrate(s) involved. Also, many inks, varnishes, coatings or adhesives need to be hardened through a chemical reaction and this is achieved by polymerising them or developing chemical bonds via a cross-linking mechanism. This process is called curing and is conventionally achieved using heat. If necessary, a chemical ingredient like a hardener or initiator is incorporated into the formulation that sets the chemical reaction in motion. (Some processes like extrusion coating/lamination or hot-melt coating/ lamination use cooling as opposed to heating.) Normally, industrial organic solvents are easier to evaporate than water since they vapourise at lower temperatures; this also enables equipment to run faster and for machinery to be simpler and occupy less space (no extended tunnels or complex heating systems required). Hence, traditional formulations of liquid inks, coatings and adhesives are all solvent-based systems using low boiling point organic solvents like alcohols, toluene, MEK, ethyl acetate etc.
Problems with conventional curing
The use of heat and organic solvent-based formulations give rise to several problems and impose limitations since:
– The energy requirement is high and expensive.
– The equipment needs to incorporate extended and complex heating systems.
– The solvents are expensive, inflammable and unsafe for handling or inhalation.
– The solvents emit volatile organic compounds (VOC’s) on drying that cannot be exhausted or released directly into the atmosphere.
– The speed of the process is constrained by the rate at which the solvent evaporates.
– Heat sensitive substrates like oriented films and some specialty polymeric films require special handling.
As a result, a major development focus has been on the replacement of organic solvent based formulations with more environment and operator friendly aqueous formulations, wherever possible. However, this meant using even more heat energy and the problem is exacerbated as webs have tended to go wider and operating speeds have gone up to boost productivity.
Radiation curing or energy curing has been developed to address these problems. There are presently two technologies available – one using ultraviolet (UV) light radiation and the other using electron beam (EB) systems. In both cases, the inks, coatings and adhesives are chemically cured and are so formulated as to cure or harden instantaneously by polymerisation of the ingredients on exposure to either UV radiation or a stream of electrons.
UV curable systems incorporate a photo-initiator like ITX or 4MBP in the formulation as hardener. It absorbs UV light from UV lamps and produces free radicals by breaking of the carbon-carbon double bonds. These radicals cause the ingredients to react chemically and polymerise. In the process, the formulation hardens and becomes “set”. The polymerisation continues until the ink, coating or adhesive formulation is fully hardened and adheres firmly to the substrate surface. The whole process is very rapid and takes place in just a fraction of a second.
The UV source is normally a medium to high pressure mercury bulb that emits radiation in the 200 – 400 nm range. As the reaction speed is high, the curing speed increases and the energy consumption is normally reduced as compared to organic solvent or water borne systems.
The UV curing system has been around in the printing industry for a while and is quite used extensively for printing and coating on paper and board as well as synthetic substrates especially on sheet-fed offset litho presses. UV curing is also used to litho print metal sheets for manufacture of cans and closures. The print quality, gloss, scuff resistance, chemical resistance and fade resistance are superior to those obtained from traditionally cured offset inks. The curing is instantaneous and the substrates can be subjected to downstream operations (cutting and creasing, folding and gluing, press work, slitting and rewinding etc.) immediately or even in-line.
Problems with UV systems
There are several problems and limitations with UV curable systems and adequate safety precautions are necessary.
Photo-initiators are very expensive and significantly raise the costs of UV curable formulations. They also leave a strong residual odour and tend to migrate from the substrate surface. This is why they are not approved for food-grade applications that require direct contact with the product. UV curable inks also mostly use acrylate monomers which are an irritant to the skin and have to be carefully handled.
The UV radiator produces ozone from atmospheric oxygen. Ozone is a strong irritant and suspected carcinogen. Oxygen also interferes with the polymerisation and absorbs UV light thus decreasing the UV radiation reaching the ink or coating layer necessitating an increase in the amounts of initiator to be used. Hence, exhaust ventilation close to the UV radiator is absolutely essential. In fact, some specialised equipment use a nitrogen blanket to create an inert atmosphere. UV radiation is also harmful to the skin and the eyes and the lamp has to be properly shielded for safety.
Another problem is that dark colours and heavier pigments in the printing design compete with photo-initiators to absorb UV radiation and do not easily permit the light to pass through and penetrate the film layer; the higher the film thickness, the more the UV energy requirement will be. (The UV radiation needs to penetrate the entire film thickness to fully cure it – this also limits printing speeds.) This is why one does not get consistent cure with UV systems and adjustments in settings need to be made for each separate production set-up. This problem is compounded by the fact that, with use, UV lamps and reflectors lose their efficacy and need to be constantly monitored and adjustments made. Indeed, they need to be periodically replaced. This makes the technology operationally very demanding. The UV lamps cannot be turned on and off during the run or during input change-overs/stoppages because they need time to stabilise, warm up and cool down; so, when the line is not running or being adjusted, the lamps need to be shielded and kept on in stand-by mode.
Yet another drawback of UV systems is that the lamps also deliver a lot of IR radiation that generates considerable heat. Further, in a UV lamp system, the mercury plasma is contained in a quartz envelope (the lamp) and the core has to be maintained at temperatures up to 6,000 degrees Celsius to maintain the correct mercury vapour output spectrum. So, the surface temperature of the lamp runs at over 800 degrees Celsius during curing. Therefore, UV curing of heat-sensitive substrates like oriented films and flexible PVC is problematic as they start distorting at the temperatures encountered. While a lot of development has taken place to reduce the heat reaching the substrate surface, the problem still exists to some degree. The solution is to use cooling; for example, in CI flexo printing, the entire CI drum needs to be adequately cooled so that substrate temperatures do not exceed critical values.
In printing using UV curable inks, UV curing stations need to be located after each colour station to cure the ink before the next colour can be applied (even conventional printing processes using standard or liquid inks require inter-colour drying).
New developments in UV curing
One of the most exciting new developments in UV curing is the use of LED curable technologies. Here, the UV radiation source is a LED lamp instead of the traditional mercury vapour lamp. At drupa 2008, several manufacturers displayed presses using LED curable inks both for sheet-fed and wide web formats. The lamps themselves are much more expensive but they offer much less electricity consumption, lower temperatures (no IR radiation), zero ozone generation, vastly increased lamp life and they can be switched on and off instantly. However, the energy output of LED lamps is still low and their longer wavelengths make them suitable only for niche applications at the moment until the technology is developed to juice them up. One can safely predict that this will certainly be the UV curing technology of the future.
EB curable systems generate a stream of electrons which are accelerated till they reach a velocity that is almost two-thirds the speed of light. This stream of electrons is made to impinge on the coated web and the energy at impact causes the breaking of the double bonds in the ingredients of the ink/coating/adhesive formulation generating free radicals which then attack the other double bonds setting up a polymerisation reaction that causes the coating to solidify and harden. This process is also very rapid and produces instantaneous cure. The reaction is much faster than that in UV curing. These systems do not require initiators as the electrons are sufficiently energy-rich to produce free radicals themselves. That is why EB curable formulations are much cheaper than their UV curable counterparts and run at much higher speeds. Electrons can also produce X-rays but this is not a serious problem as adequate shielding can be provided in the equipment design.
Here, again as in UV curing, oxygen interferes with the polymerisation process and needs to be excluded. This is imperative for EB systems, which need to be purged with nitrogen to maintain an inert atmosphere.
EB curable inks and coatings are approved for food contact and are suitable even for critical applications like food and pharmaceutical packaging. They do not leave any residual odour or migrate from substrate surfaces.
Although EB systems have been in use for some time for curing of adhesives and coatings and cross-linking of polymeric structures, it is just beginning to find widespread acceptance in printing, especially on paper and board. Printing of polymeric films and laminates with EB curable inks is also a very recent development although EB curable overcoats on conventionally printed webs have been in use for a longer time. As of now, EB curable systems have not been established yet for sheet-fed applications and rotogravure printing; they are being used only for CI flexo printing (see schematic diagram on page 15). The use in rotogravure printing has been constrained only by the availability of suitable EB curable inks but this limitation should soon be overcome as the liquid inks used in this process are essentially the same as those used for flexography (for which EB curable inks already stand established).
Historically, EB curing equipment has been very expensive and bulky making it very difficult and unaffordable to incorporate it into web conversion systems. However, recent innovations have brought down costs and sizes considerably and they have now become eminently viable.
The biggest advantage of EB curable systems is that one requires only a single EB curing station at the end of a CI flexo printing line because it is possible to print wet-on-wet without inter-colour curing or drying, something that is not possible with conventional processes using liquid or UV curable inks.
Another significant advantage of EB curing is that the cure is consistent and not dependent on the colours or ink/coating film thickness. The curing is not affected by the printing design, the colour shades or even the sequence of colours, giving the same efficacy for both surface and reverse printing. This, combined with the much more rapid cure, makes it possible for printing to be carried out at speeds of up to 500 meters per minute regardless of the substrate, print design or ink film thickness.
EB curing is also a relatively “cold” process and the increase in web temperature is only between 5 and 12 degrees Celsius. This makes it suitable even for heat-sensitive substrates like oriented films, PVC and specialty plastic films.
The surface properties of EB curable inks and coatings are excellent print quality, scuff resistance, heat resistance, chemical resistance, fade resistance, gloss and COF properties much superior to those obtained by any other systems. In fact, the COF can be very precisely tailored for specific slip or anti-slip requirements. The ink consumption and coating weights are also usually lower than those for other curing processes.
EB curing is also much more energy efficient than either heat curing or UV curing and the operation is much more safe, environmentally sustainable and operator friendly. It is also easier to control and saves on consumables and maintenance downtime.
UV curing vs EB curing
EB curing has several advantages as compared to UV curing and is probably the way to go in the future, especially for food-grade applications. The comparative situation is summarised in the chart on page 18. It’s only disadvantage as of now, if it could be termed as such, is that availability of suitable flexographic ink systems is restricted to only a couple of major international suppliers while rotogravure inks still have to be developed. These limitations should, however, be overcome soon as more process development takes place and the technology matures commercially.