In the last month before Christmas, two instances of problems with UV coating adhesion over pre-printed inks came to the NPL for assessment, and having viewed them I mused that after 30 years surely all the tricks of UV coating would be very well known (see March 2003 column), but it would appear that this is not the case.
We need to consider the cases of UV coating both wet on wet (WoW), and wet on dry (WoD) over UV and conventional inks, as each set of circumstances is different.
Generally the application of UV curable coating materials over UV inks whether WoW or WoD goes without drama. A notable exception is the application of cationic coatings over radical cure UV inks (normal UV inks) in which case, any trace of amine in the ink will quite dramatically shut down the curing process of the coating. It is possible to formulate inks that will work OK with cationic coatings, but discussion with the ink supplier is an absolute must before this is attempted.
With regard to the general curing conditions for UV inks and coatings, it is most important that UV curable materials receive the correct amount of UV energy, neither too much nor too little.
The more common case of under-cure is easily recognised, as the ink and/or coating film simply remains wet or tacky, although a less obvious situation is where a coating (or another colour) is applied WoW over a dark colour (especially black), and then under exposed.
This situation may lead to the top coating layer being adequately cured, but the ink layer underneath being still soft, such that although the film may feel dry, a tape test will remove both the topcoat and under layer completely. (If this situation should occur, no amount of extra UV exposure will cure the under layer).
A more subtle set of circumstances is involved in over-cure.
Particularly in the instance of fast curing clear coatings or varnishes, too much UV energy can lead to the clear coat becoming brittle, and easy to scratch off as a solid film resembling fish-scales. This may also be exacerbated by excess heat from inefficiently cooled lamps.
However, obtaining a reliable result from the over-coating of conventional offset inks with UV curable coating certainly requires close control of materials, but of equal or even greater importance, is control of process.
In the post-processing of conventional lithographically printed work with UV curable coatings, inter-film adhesion is principally affected by the level of drying of the ink film, and the degree of cure of the UV coating.
UV coating will adhere well when printed wet on wet over oil inks, as the coating and ink will partially meld together. In this case, the gloss level of the coating will be much reduced compared to the WoD result that may be achieved from the same combination.
The reason is simple enough, when the coating is exposed to UV light, virtually all of the coating is immediately cured, however the conventional ink first sets by phase separation, then dries by oxidation of the vegetable oils in the ink vehicle before a stable film is formed.
This stabilisation process can take several hours to occur, during which time the ink is slowly sinking into the stock, or undergoing chemical reaction. It is not then difficult to visualise the semi-rigid coating "roof" being stressed by movement in the ink "foundation", leading to unevenness in the UV coating film, and hence loss of gloss (this also explains why the coating is glossy over any unprinted areas of the sheet, and why the loss of gloss often takes some time to appear).
However, UV coatings will not give good adhesion to oil inks when applied on partially dried ink.
The setting process I mentioned above is based on the separation of low viscosity mineral oils from the ink vehicle, and these oils are intended to soak into the substrate, but some will invariably make their way to the surface of the print and prevent the UV coating from adhering (whether by physical separation or by incompatibility is yet to be proven).
The same effect is frequently seen when trying to UV over-coat metallic inks, as the metal particles in the ink have a slightly ‘greasy’ treated surface to prevent the metallic powder clumping.
If poor adhesion of UV coating to conventional oil ink for this reason is suspected, it can be proven by removing the UV coating film with adhesive tape; if the ink "foundation" is also removed along with the coating, then the ink was coated prematurely.
About two days ink-drying time is a reasonable target to avoid this kind of problem.
Another variable in the drying process of conventional litho inks is water, and although high levels of water (fountain solution) in the ink may slightly speed setting, it will certainly inhibit drying. The addition of drying stimulator to the fount is only a partial answer; it is far better to run the bare minimum of water on the plate that will avoid catchup.
Finally, the adhesion of UV coatings to hard dried conventional inks can also range from variable to downright awful.
Particularly in the case of oil inks that are formulated to be very tough or scuff resistant with high levels of drier and wax, if the prints are allowed to dry hard ("crystallise" in old printers parlance), when UV coated they can give very poor inter-film adhesion with a similar "shelling" or "candling" effect to that seen with over-cured coatings.
Although running out of space, a few other points are worthy of mention.
o avoiding the heavy use of spray powder on oil ink prints that are to be subsequently UV coated WoD,
o not running high IR on prints and ‘cooking’ them,
o keeping the coating film thickness to recommended levels,
o checking the success of the combination of substrate, ink, and coating on a short trial run before starting a big job (an obvious step, but nevertheless it is sometimes overlooked).
As is clearly apparent, there is not even a hint of rocket science in this subject, but as evidenced by those pre-Christmas jobs, problems do still occur!
Rod Urquhart works at Monash University where many aspects of the industry are being studied. He is based in Australia.
Contact Rod Urquhart via email: firstname.lastname@example.org