UV curing and tack free cures

Here, Matt Baseley, Senior Technical Salesperson of Intertronics, explains how to avoid a tacky surface after curing. 

Most UV light curing adhesives cure with a free radical polymerisation process. When the correct wavelengths of UV light are absorbed by the photo-initiators in the adhesive, the
photo-initiators generate chemically reactive-free radicals. These induce cross-linking, or polymerisation, of the oligomers and monomers in the adhesive – resulting in a polymer of cured material.

Oxygen inhibition

If the surface of the adhesive is exposed to atmospheric oxygen during the cure, oxygen can penetrate
the very top layer and inhibit polymerisation. This causes an incomplete surface cure, leaving unreacted oligomers and monomers – the tacky residue. We would define tackiness as when you feel a tack as you rub your finger across the surface and get traces of wet residue
on your gloved hand. Of course, if your bond is completely interfacial and between two surfaces, then oxygen inhibition will not occur, since the adhesive is not exposed to oxygen.

In situations where the joint design has exposed adhesive fillets and a sticky surface occurs,
the bulk of the adhesive is cured and what the manufacturer is detecting is a very thin layer of adhesive constituent. Structurally, the bond is likely to be quite sound. However, it can be undesirable from the perspective of contamination or aesthetics.

Read the full article in the April issue of DPA.