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DESIGN ENGINEERING

UV curing

FASTENERS AND ADHESIVES

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. HERE
MATT BASELEY OF INTERTRONICS, EXPLAINS HOW TO AVOID A TACKY SURFACE AFTER CURING
ost UV light curing adhesives cure with a free
radical polymerisation process. When the
correct wavelengths of UV light are absorbed
Mby 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.
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 bringing the lamp closer to the adhesive surface, curing for “
tacky surfaces when cured with the correct equipment, as a longer period or using a more powerful UV curing lamp.
chemists have become clever in mitigating it, so it is more Oxygen inhibition can be prevented by using short and THE MOST OPTIMAL SCENARIO
likely with older products or softer adhesives. Bear in mind medium wavelength UV because the higher energy causes WOULD BE TO SPECIFY AN
that some soft adhesives may be formulated with a quicker breakdown of the photo-initiators, meaning ADHESIVE FORMULATED TO
additives that remain sticky when cured and can appear there is less time for oxygen to penetrate the exposed
to be signs of oxygen inhibition when they are deliberate adhesive surface. But to get a good depth of cure, the CURE TACK FREE WITH LED
properties of the cured adhesive. manufacturer must combine shorter wavelengths with UV CURING LAMPS
longer wavelength UV light. Intertronics supplies metal
Overcoming oxygen inhibition halide UV bulbs that usually generate the right balance of
More intense UV light is one way of tackling tackiness curing light (approximately 300-450 nm). In some cases, If tackiness cannot be avoided by making
because it has the chance of completing the cure before the use of a bulb with higher amounts of short-wave UV improvements to the process, the manufacturer may want
oxygen inhibition can take effect. In addition, (a mercury UV bulb) will overcome oxygen inhibition. to consider removing the tacky layer with an isopropyl
manufacturers will usually achieve more optimal cures If a manufacturer is using an LED UV curing lamp, one alcohol (IPA) wipe. Alternatively, the manufacturer can
using higher intensity light, along with shorter cure times. thing to consider is their ability to cure tack free. As change adhesives, opting for one less sensitive to oxygen
To increase intensity, manufacturers can check their discussed, oxygen inhibition can be prevented by short inhibition.
bulb outputs to see if they are due for replacement and if and medium wavelength UV curing light. LED UV curing Perhaps the most optimal scenario would be to specify
so, try a new bulb. Other ways to achieve this include lamps emit narrow spectrum light, primarily in the long an adhesive formulated to cure tack free with LED UV
wave zone, and so may struggle to achieve a tack free cure curing lamps, so that the considerable benefits of modern
with some adhesives that are prone to oxygen inhibition. LED UV curing technology can be realised.
“YGEN INHIBITION CAN BE If you are experiencing sticky or tacky surfaces on your
OX
Exclude oxygen from the surface adhesive or coating, our recommendations are to use a
PREVENTED BY USING SHORT A third option is to exclude oxygen from the adhesive higher intensity UV light, use more short wave UV light,
AND MEDIUM WAVELENGTH UV surface, such as to place a thin film of plastic over the exclude oxygen from the surface or change the adhesive to
BECAUSE THE HIGHER ENERGY adhesive and cure through the plastic. Many adhesives one less sensitive to oxygen inhibition. Partnering with an
will have poor adhesion to plastics like polypropylene or experienced adhesives supplier will help you to specify a
CAUSES QUICKER BREAKDOWN polyethylene and so they can be removed easily post cure. suitable adhesive and understand the curing process
OF THE PHOTO-INITIATORS Another way to do this is to cure under an inert gas like including wavelengths, intensity and exposure time.
nitrogen or argon. MORE INFORMATION: www.intertronics.co.uk

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