A UV coating comes off the line hard underneath and tacky on top. The instinct is to add lamp power or photoinitiator. Usually neither is the right answer, and understanding why saves both money and yellowing.
What is actually happening
UV curing works by free-radical chain polymerisation. The photoinitiator absorbs UV, fragments into radicals, and those radicals attack acrylate double bonds, propagating a chain reaction that crosslinks the film.
Molecular oxygen is a very effective radical scavenger. At the air-film interface, atmospheric oxygen reacts with the propagating radicals faster than they can add to the next acrylate group. The chain terminates.
The result is a thin surface layer — often only a few microns — that never reaches full conversion, sitting on top of a properly cured bulk. It feels tacky, marks easily and has poor scratch resistance, while every through-cure test says the coating is fine.
Why more lamp power is a poor fix
Increasing intensity does help, because generating radicals faster partially outruns the oxygen. But it is an inefficient fix:
- It costs energy continuously, for the life of the line
- It adds infrared heat, which is a problem on thin films and heat-sensitive substrates
- It does not address the mechanism, so at high line speed the problem returns
Increasing photoinitiator loading has the same partial effect, and brings its own costs: photoinitiator is expensive, residual photoinitiator is a migration concern in food packaging, and higher loadings contribute to yellowing.
What an amine synergist does instead
An amine synergist — an amine-modified acrylate, commonly called an amino acrylate — attacks the mechanism directly.
The tertiary amine functionality acts as a hydrogen donor. It reacts with the peroxy radicals that oxygen forms, converting them back into carbon-centred radicals that can continue the polymerisation. In effect, it consumes oxygen at the surface and regenerates the chain reaction where it was being terminated.
The practical outcomes are consistent:
- A fully cured, tack-free surface at the same lamp settings
- Or the same cure state at higher line speed
- Or the same cure state at lower photoinitiator loading
That last point is worth emphasising. Because the amine improves cure efficiency where it was worst, many formulations reach the same result with meaningfully less photoinitiator. That offsets the cost of the amino acrylate and reduces residual photoinitiator in the film — which matters if migration is a concern.
Why this matters more with LED UV
LED UV systems typically have lower peak intensity than mercury arc lamps, even where total dose is comparable. Lower peak intensity means slower radical generation, which means oxygen has more time to interfere.
The consequence is that oxygen inhibition is generally more pronounced under LED, and the value of an amine synergist correspondingly higher. Converters moving from mercury to LED who report surface tack that was not there before are usually seeing exactly this.
The alternative: nitrogen inerting
Flooding the cure zone with nitrogen eliminates oxygen and solves the problem completely. It is highly effective.
It is also capital equipment, ongoing gas cost, and an enclosure around the cure zone. For most converters, an amine synergist at a few percent of the formulation is the more sensible commercial answer.
The honest caveat
Amine functionality contributes to yellowing, particularly at higher loadings and over time.
In pigmented systems this is generally invisible. In clear coats over light substrates, and in white systems, it is worth evaluating at your intended loading before committing — measure b* on a cured panel and again after accelerated ageing.
Loading level is the lever. Enough to solve the surface cure, not more.
Practical starting point
If you have surface tack, before you touch lamp settings:
- Confirm the tack is surface-only by testing through-cure separately
- Trial an amino acrylate at the supplier's suggested starting loading
- If the tack resolves, try reducing photoinitiator and see how much you can recover
- Check colour on the final formulation, particularly for clears and whites
That sequence usually ends the problem in a week.