Avoiding plasticky coating feel

[Davino]

A general technical question on matte UV varnish application:

We've settled on matte varnish flood coat with image knockouts on Arctic Volume White 130gsm, LED-UV press. The stock's pleasing natural tactile surface has already been significantly altered by the press process itself it seems – perhaps the nip pressure, dampening, UV cure, or calendering – so we're not starting from pristine mill samples. But we'd still prefer the lightest effective coat possible.

Two questions:

1. Is the "plasticky" feel associated with some varnish coats primarily a function of coat weight, or varnish formulation independent of weight? In other words, can a lighter coat of a standard formulation suppress gloss effectively while feeling less like a polymer skin?

2. Is there a minimum effective coat weight below which matte UV varnish gives incomplete or patchy gloss suppression on coated matte stock? I need enough to reliably kill text glare on body text and fine 8.5pt annotation text, but no more than necessary.

Has anyone run tests at different coat weights on similar stock, or successfully run two zones at different weights on a proof sheet for comparison?

 

[Davino]

Is it that UV energy polymerizes anything it hits, and paper coatings and surface sizings can be affected? That would explain the transformation of these exquisite-to-touch tactile, lightly-coated stocks (Sappi Magno Volume, Magno Matt, Arctic Volume White) being hammered by such a process into generic, more hardened surface feel. I'm inferring this may be what's happening, but after striving for answers for so long, it seems a reasonable hypothesis. Expert voices needed. If that's the case, then the printer's decision to do this job as a UV LED print job (without relaying these impacts to us beforehand) is the underlying cause of trouble. Traditional offset wouldn't have caused this change to the paper, and we wouldn't have had to contend with applying a matte varnish to spare us the UV LED ink sheen on text and losing all the original tactility and desired sheen free matte text.

I'm raising this point of a global UV polymerization to the sheet here, and would appreciate experts' reactions, but if that's not what's happening, then we're certainly going ahead with the matte varnish. The initial question of whether light or medium matte varnish coatings are possible in order to achieve the matte effect without overdoing the plasticky coated feel, still remains. Thank you in advance.

 

[SteveSuffRIT]

Is it that UV energy polymerizes anything it hits, and paper coatings and surface sizings can be affected?
NO, UV radiation (LED or older Hg mercury) will not affect the paper coating or fibers.
It only effects the ink or coating since they contain photo-initiators that are reactive to UV.

 

[Davino]

Is it that UV energy polymerizes anything it hits, and paper coatings and surface sizings can be affected?
NO, UV radiation (LED or older Hg mercury) will not affect the paper coating or fibers.
It only effects the ink or coating since they contain photo-initiators that are reactive to UV.

It's understood that the system is not intended to affect the paper in this way, but we still haven't an answer as to what is degrading the original tactile feel of the lightly coated paper that it is engineered to have. Is it a thermal reaction from the heat of the lamps, fountain solution, nip/roller pressure...

I'll give it a second try contacting someone from technical (not sales) departments at Sappi and Arctic.

But separately, when using a matte varnish, is there a range of thicknesses to apply it in?

 

[SteveSuffRIT]

LED UV is often referred to as "cold" UV because it emits much less thermal (infrared IR) heat.
Conventional mercury (Hg) lamps emit 70% IR heat, about 212'F.
LED UV has almost no IR, about 100'F. Think about the difference between an incandescent tungsten filament bulb (hot) and a LED bulb (cold).

There is a technical difference between a "varnish" and a "coating".
Varnish is normally applied through the ink train rollers like an ink. Therefore, will have a higher tack, #7, (viscosity) and thinner thickness (1 micron, um).
Coating is applied by a special flooded nip rubber roller or engraved metal/ceramic anilox (flexo) roller.
Therefore, will have a lower tack (viscosity) and be thicker (taller in height), maybe 15 microns.

Since both varnish and coating are transparent/clear, there is no easy optical way to measure their thickness/height. With colored inks, you can use a densitometer. The typical QC method used is weight per surface area, measured on an aluminum foil test strip. Water base-Aqueous (AQ) might be 1.0 lb per 1,000 sq ft.

 

[Davino]

Given LED UV's lower IR output, do you have any experience on whether nip/roller pressure or fountain solution penetration are more likely culprits for surface texture change on lightly-coated stocks specifically, independent of the heat question?

Does cumulative multi-pass UV LED exposure — even at low per-pass temperature — have a documented additive thermal or mechanical effect on lightly-coated stock surface texture, distinct from single-pass exposure?

 

[SteveSuffRIT]

I know you are desperately searching for answers to your technical questions about the change in papers tactile feel and water absorbency/holdout.
I think you will only get satisfactory answers if you conduct some test/experiments on press, if the printer is willing to accommodate that request. They should if you offer to pay for the time & materials. To reduce costs, maybe you can do this on your next press proof (you call it a wet proof). Below are 3 simple, quick tests, that should only take 5 min each on press.

1) Run sheets through press under impression only, no printing (no ink or water applied). The mechanics is the paper is squeezed under pressure between the soft rubber offset blanket and the rigid metal back or impression cylinder. This isolates the effect of impression pressure.

2) Run sheets through press with LED UV light exposure only, no printing, no impression. If needed, back off on impression pressure to eliminate that effect. This isolates the effect of LED UV lights.

3) Run sheets through press with impression and with LED UV lights on but still no printing (no ink, no water). This shows the effect of both impression and LED UV lights but now isolates the effect of printing with ink and water.

My prediction is there will be no difference or change in tactile feel between the before and after samples for each test.
Applying the LED UV ink and coatings, that are 100% solids, will stiffen the sheet and make it more rigid because they act like a thin plastic layer. This is not noticeable on thicker paper boards (12-24 point) used in folding carton packaging or cover stock but may be noticeable on thinner book/text paper (0.004").
I still can't explain why the nonprinted areas (dampened by fountain solution) are repelling water droplets and becoming hydrophobic (repel) instead of hydrophilic (attract).