Canadian Finishing & Coatings Manufacturing Magazine January/February 2022 - 42

INDUSTRIAL FINISHING: UV CURING
few of the things you'll find on gas station pump panels, as well as
natural and backlit advertisement and solar panels, automotive
interior trim and exterior assemblies for cars, rigid cosmetic
packaging, hardware embellishments, lighting louvres, polarised
sunglasses, and LCD screens for TVs.
Applying UV-curable polymer coatings is usually done with a
liquid that becomes a cross-linked solid when hit with ultraviolet
light. Photopolymerization is a process in which ultraviolet
energy drives chemical cross-linking reactions in specially made
materials. UV curing is one of these processes.
Polymer resins have been exposed to UV energy from solid
molecular bonds in less than a second. These bonds play a
significant role in making hardcoats have the surface properties
that make them unique. UV curing is suitable for processing
because it gives the coatings the energy to cross-link without
transferring too much heat, which can melt or warp the plastic
parts underneath. UV-cured coatings also have the advantage of
sticking to many different things.
Medium-pressure mercury vapour lamps, including electrode
arc and microwave, have been used for a long time to cure UV
hardcoats. UV LED curing has made a big difference in hardcoats,
like structural bonding adhesives and fibre optic coatings.
People who use hardcoats have been watching very closely. It
has led to many plastics manufacturers being very excited about
the availability of UV LED-curable surface coatings that they can
use in their processes. They are also increasingly asking material
and equipment suppliers for UV LED hardcore options updates.
UV LED curing technology isn't yet ready for most hardcore
applications because there aren't enough UVC LEDs, oxygen
inhibition at the surface, and insufficient irradiance at longer
working distances. However, there is a lot of development going
on in this space, which will make the technology more viable and
famous in the future.
UVC has the shortest and most energy wavelengths, while
UVV has the longest and least energy wavelengths. People tend to
absorb UVC wavelengths at the surface of paint or coatings, but
UVA and UVV wavelengths go much more profound. This type of
lamp is a vast spectrum. It emits ultraviolet energy in all four UV
bands and visible and infrared energy. Today's UV LED curing
lamps can only emit longer UVA and UVV wavelengths between
365 and 405 nm. The most recent development in UVC has been
between 275 and 285 nm.
It doesn't matter what kind of photoinitiator you use; it
will be able to absorb ultraviolet energy from a wide range
of wavelengths. Photoinitiators used in high-performance
industrial coatings were initially designed to absorb UV energy
most efficiently between 220 and 260 nm. Still, they also react to
longer UVC and UVB wavelengths. These PIs usually don't take
in any UVA light. A big problem with UV LED curing hardcoat is
that there isn't enough match between the long UVA waves that
commercial LEDs emit and the short UVC
waves that hardcore needs.
In non-hardcoat markets where UV LEDs
are used to cure inks, coatings, and adhesives,
formulators can only use photoinitiators
that work with longer UVA wavelengths. It is
hard to hide the yellowing caused by longer
wavelength-absorbing PIs with pigments or
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by mixing different photoinitiators with other chemicals in ways
that don't make them yellow.
New research has led to clear overprint varnishes (OPVs) for
graphics coatings that cure with LED at competitive line speeds,
are acceptable nonyellowing, and protect many printed labels
and some packaging. Unfortunately, it isn't possible to use better
photoinitiators that react to UVC, which means that many of
the more durable properties of hardcoats can't be achieved with
longer wavelength LEDs today.
When photoinitiators absorb ultraviolet light, they make free
radicals (RFree radicals bond with resin molecules to make long,
continuous polymer chains).
A process called " oxygen inhibition " can happen when oxygen
(O2) molecules in the air get in the way of the UV cross-linking
process. Most of the time, this interference is only on the surface
of the coating, where it weakens free radicals and reduces
their overall concentration. Formulations sensitive to oxygen
inhibition tend to have incomplete polymerization on the outside.
This can leave it to feel sticky, tacky, or greasy.
Many applications of mercury vapour curing can be made
without oxygen inhibition if there are enough UVC wavelengths
in the air to get rid of it. People often use UVC light because it has a
lot of energy and can be easily absorbed by free radical-producing
photoinitiators on the surface. This allows them to make enough
free radicals to replace those lost to oxygen (quenched) or
weakened by oxygen (scavenged). Changing the environment,
formulation, and lamp head configuration can help improve the
surface cure in cases where UVC wavelengths are not used or
have too little power.
For example, suppose you flood the chemistry surface with
a nonreactive gas, like nitrogen. In that case, it removes oxygen
from the area and stops it from interfering with the reaction.
Alternatively, thickening or making a coating more viscous
makes it harder for oxygen to get through, making free radicals
more likely to form inside the coating. This can result in the
outermost top surface curing the bottom up.
Photosensitizer and other reactive chemistry concentrations
can also be increased to make more free radicals. The irradiance
and energy density of the UV system can also be increased to
make more radicals.
It is usually easier to raise irradiance with UV LEDs than with
mercury lamps. Surface hardcoats also must deal with threedimensional
parts with very steep or deep part profiles; this is
another problem in itself. There must be a direct line of sight
between the lamp head and all parts that will cure - this is not a
big deal for webs and flat part profiles, but it could be under some
circumstances.
As part shapes become more complicated, lamp heads must be
moved away from the part's surface to provide enough clearance.
Multiple lamp heads at different angles may also be needed to get
UVC has the shortest and most energy wavelengths,
while UVV has the longest and least energy
wavelengths. People tend to absorb UVC wavelengths
at the surface of paint or coatings, but UVA and UVV
wavelengths go much more profound.
http://www.cfcm.ca
Canadian Finishing & Coatings Manufacturing Magazine January/February 2022

Table of Contents for the Digital Edition of Canadian Finishing & Coatings Manufacturing Magazine January/February 2022
