If you change one thing in a UV formulation and want to understand what will happen, change functionality. It moves everything, and it moves things in directions that are not always obvious.
What the number means
Functionality is simply the count of reactive acrylate groups a molecule carries. A difunctional oligomer has two; a hexafunctional one has six. Monomers used as reactive diluents range from monofunctional to hexafunctional.
Higher functionality means more points at which a molecule can join the crosslinked network, which means a denser network at full cure.
The predictable consequences
Cure speed rises with functionality. More reactive sites means a higher probability of chain propagation, and the gel point arrives sooner. A hexafunctional system cures noticeably faster than a difunctional one at the same photoinitiator level.
Hardness and chemical resistance rise with functionality. A dense network resists penetration by solvents and resists mechanical deformation. This is why epoxy acrylates, which are relatively high functionality, dominate hard glossy coatings.
Elongation and flexibility fall with functionality. The same dense network that resists solvent also resists bending. A highly crosslinked film has nowhere for stress to go, so it cracks.
None of that is surprising. The next part is where formulations go wrong.
Cure shrinkage: the non-obvious consequence
When an acrylate double bond converts to a single bond during polymerisation, the carbon atoms move closer together. Across a whole film, that adds up to volumetric contraction — typically several percent.
The higher the functionality, the more double bonds per unit volume, and the more the film shrinks.
That shrinkage does not happen freely. The film is bonded to a substrate, so the contraction generates stress at the interface. On a well-treated substrate with good adhesion, the bond holds. On a marginal substrate, it does not.
This produces a failure mode that confuses people: a formulation that wets the substrate perfectly, cures to a beautiful hard film, and then lifts or delaminates. The wetting was fine. The adhesion was adequate for a static film. It was not adequate for a film contracting against it.
If you see adhesion failure on a high-functionality formulation that wets well, reduce crosslink density before you do anything else.
Practical positioning
A rough map of where to sit:
| Requirement | Functionality direction |
|---|---|
| Hard, glossy, chemically resistant, rigid substrate | High (4-6) |
| Flexible substrate, film packaging, formed parts | Low (2-3) |
| Difficult adhesion substrate | Low, to limit shrinkage stress |
| Maximum line speed | High, but watch shrinkage |
| Low shrinkage, dimensional stability | Low |
| Deep-section or heavily pigmented cure | Lower, to reduce surface skinning |
Blending is normal, not a compromise
Almost no production UV formulation uses a single oligomer at a single functionality.
The standard approach is to build on a primary oligomer that supplies the dominant property — urethane acrylate for flexibility, epoxy acrylate for hardness and cost — and then adjust with reactive diluents of chosen functionality to land on the required balance of viscosity, cure speed and film properties.
Monofunctional diluents thin effectively, improve flexibility and adhesion, and reduce shrinkage, at the cost of slower cure and potential extractables. Multifunctional diluents cure fast and add hardness, at the cost of shrinkage and brittleness.
The formulation is the blend, not the oligomer.
A useful working method
When a UV formulation is not doing what you need:
- Identify which property is failing
- Decide whether the fix moves functionality up or down
- Make the change through the diluent package first — it is faster and cheaper than changing the oligomer
- Re-check adhesion after any change that raised functionality, because shrinkage moved too
That last step is the one most often skipped, and it is the one that produces the surprising failures.