UV CURING
OVERCOMING
OXYGEN
INHIBITION
Sometimes cured UV adhesives have a tacky
or sticky surface, even after being exposed
to the correct amount of UV light, due to a
phenomenon called oxygen inhibition. However,
with a full understanding of the process,
manufacturers can achieve tack free cures. Here
Matt Baseley, Senior Technical Salesperson of
adhesives specialist 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 photoinitiators
in the adhesive,
the photo-initiators generate
chemically reactive free
radicals. These induce crosslinking,
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.
Modern adhesive
formulations are less likely
to have tacky surfaces when
cured with the correct
equipment, as chemists have
become clever in mitigating
it, so it is more likely with
older products or softer
adhesives. Bear in mind that
some soft adhesives may be
formulated with additives
that remain sticky when
cured and can appear to be
signs of oxygen inhibition
when they are deliberate
properties of the cured
adhesive.
OVERCOMING OXYGEN
INHIBITION
More intense UV light is one
way of tackling tackiness
because it has the chance
of completing the cure
before oxygen inhibition
can take effect. In addition,
manufacturers will usually
achieve more optimal cures
using higher intensity light,
along with shorter cure
times.
To increase intensity,
manufacturers can check
their bulb outputs to see if
they are due for replacement
and if so, try a new bulb.
Other ways to achieve
this include bringing the
lamp closer to the adhesive
surface, curing for a longer
period or using a more
powerful UV curing lamp.
BROAD SPECTRUM UV LIGHT
Oxygen inhibition can be
prevented by using short
and medium wavelength UV
because the higher energy
causes a quicker breakdown
of the photo-initiators,
meaning there is less time
for oxygen to penetrate the
exposed adhesive surface.
But to get a good depth of
cure, the manufacturer must
combine shorter wavelengths
with longer wavelength UV
light Intertronics supplies
metal halide UV bulbs that
usually generate the right
14 Issue 2 2020