UV varnish adhesion on hot foil

[Tomas]

Hello everyone.
Has anyone encountered the problem of UV varnish adhesion on hot foil?

What happened:
1. Printing was done on cellulose cardboard with oxidizing paints with a water primer.
2. Then hot stamping with gold foil was done
3. Then UV varnishing was done

The defect appeared almost immediately.
No material substitutions were made for this order
The order is repeated, in previous editions the defect appeared from strong mechanical impact, but this time the varnish peels off from the slightest deformation and scratching. It peels off like a cracked film.

What could have gone wrong?

 

[Asures]

There’s a lot of reasons why you had job failure.

-It cold be the dyne level of the foil applied. If it was too low, the adhesion of the UV coating may be been compromised.

-Over exposure of the UV Coating? That would make it brittle and easy to scratch.

-Not a flexible UV Coating was used. Was it applied screen or roller UV?

Here's just a few things to look at.

 

[Tomas]

I think you are right about these options:

1. Today I tested the hypothesis about overexposure - it turned out that even with the smallest dose of UV radiation,
scratching occurs with the same ease as with maximum doses.
Therefore, in this matter, everything is normal.

But when I checked the quality of the surface tension of the material (foil), the markers showed the presence of 30 Dyne.

As far as I understand, there should be at least 38 Dyne?

Tomorrow I will test this hypothesis.
I will put several other brands of foil under equal conditions and see if any other foil gives more than 30 Dyne.

And I am confident in the varnish itself - I have long tested it in different editions and it has enough flexible.

We apply the varnish with a roller system. Not with a screen.

 

[Tomas]

Next, I did a test with six types of foil (six different manufacturers).

1. First, I applied the foil to the paper under equal conditions
2. I checked the amount of Dyne on each foil with markers.
3. Then I applied UV varnish over the foil and tried to scrape the varnish off the foil. Here's what I got in the end:

It's clear that one foil (#5) clearly differs in the quality of Dyne and the degree of adhesion.
It has 40 Dyne, while the others have about 32 Dyne. You can scratch any of them if you try, but one is especially difficult to catch and scratch.

By the way, marker #38 doesn't work correctly for me. It's from a different series, so you need to track
the dynamics of Dyne on the rest from 30 to 42 numbers.

The foil that has the best adhesion is also the most expensive. Too expensive.

From all of the above, I have two questions for you, dear colleagues:
1. What is the minimum required surface energy (Dyne) on the foil to work with UV varnish on it.
Are my foils in the test with 32 Dyne defective or is this the industry norm?

2. If most foils have a low surface tension of 30-32 Dyne, then how can I increase adhesion with UV varnish?
Adhesion additives in UV varnish? or another application technology.
(Let me remind you that I did not make the mistake of overpolymerizing the varnish)

I'd be grateful for the advice

 

[Tomas]

Well, I'll answer myself since I've sorted out my problem a bit.

In the end, I replaced the foil to make the print run. This is foil number "6" (and it's the same price as my old foil). It's a little different from numbers 1-4 in terms of dyne - I think 35-36 Dyne remains on it after embossing. This is much more than numbers 1-4 where only 32 dyne is visible. The varnish on foil #6 held up much better than on my old working foil. This was confirmed on print runs with embossing and UV varnish on top. Yes, I could still scratch the varnish off, but the varnish didn't peel off on the creasing channels after folding - this was a big difference and success.

I think it's possible to solve the problem from a strictly varnish selection perspective, but it's more complicated. The UV glossy varnish itself must contain components that lead to the formation of gloss after polymerization - it is unlikely that you will be able to choose a very tenacious, sliding varnish that will adhere to a surface with low surface tension.

In addition, if the adhesion problem can be solved with the help of varnish, then this will already be a varnish for the Drip-Off effect without silicones, and this is very expensive.

Therefore, I advise you to test the foil and varnish for their mutual compatibility - adhesion.