UV- Curable Powder Coating by linzhengnd


									UV- Curable Powder Coating
A new paint technology has been developed that is likely to change the way that many
products are manufactured in the future. UV-curable powder coating combines most of the
advantages of conventional powder coating, together with the advantages of radiation-
curable coatings. The result is a coating process with exceptionally low environmental
impact and which can be used on temperature-sensitive substrates.           For many
manufacturers it opens up the possibility of coating ready-assembled articles in-house
instead of using outside paint shops.

Coating Importance
The coating on the outside of a product is usually of vital importance. A good product can be ruined by
a poor coating; just as much as a poor product can be improved - at least in appearance - by a good
coating. It is not just a matter of applying an attractive coat of paint onto an article. Attention must be
paid to the physical properties required of the coating, such as substrate adhesion, resistance to
abrasion, corrosion and weathering. Mechanical properties, such as the ability to withstand bending,
and impact can be important, as well as chemical resistance towards attack by water, solvents, oils,
hydraulic fluids etc. In short, the coating enables an article to do the job it was designed to do and does
not just look good. This has its price, of course, particularly in the automotive industry, where the costs
of applying the different surface coatings make up approximately one quarter of the total manufacturing
cost of a vehicle. For general manufacturing industry, this figure is much less, but nevertheless a
factor for careful consideration.

Choice of the appropriate paint technology is of great importance too. On the one hand, environmental
legislation on the toxicity of coatings, emission of solvents and waste water treatment is becoming
increasingly severe. On the other hand, market forces dictate both fast fabrication times and a
minimisation of expenditure. Against this background, the new technology of uv-curable powder
coating has been developed, which should help some manufacturers find an attractive solution to their
coating problems.

How UV-Curable Powder Works
The article to be coated is first hung or placed onto a conveyor band and the uv-curable powder is then
sprayed electrostatically onto the object to give sufficient coverage in a single pass. So far this
process is the same as in conventional powder coatings.
The coated article then passes
out of the spray booth and into
an oven, where it is then briefly        Figure 1. Principle of UV-Curable Powder Coatings.
heated under IR lamps, just
enough to melt the powder
                                            IR heating
(Figure 1). Here temperatures
in the range 90 - 120°C are
sufficient.   The powder then
flows together to form a smooth                             IR heating

surface film and, while still hot,
                                           powder applied
is passed under a uv-lamp. The             to substrate,
                                                                           IR heating
uv-light        triggers          a        heating starts
photopolymerisation reaction in
the     molten     resin,   which                                                        UV irradiation
                                                           powder melts
toughens the film. On emer-                                flow starts
gence from the cure unit, the
painted object is fully hardened,                                       flow completed
but may need a few minutes to
cool down. The whole process,                                                            coating cures
including    spray application,
takes at most 5 minutes.
The chemistry of the system is a
little more complex. The resins
used to make the powders are            Figure 2. Schematic Diagramme of Cure Reaction.
typically unsaturated polyester
resins, often in combination with
                                                                              UV irradiation
other unsaturated compounds.
Key ingredients of uv-powders
are photoinitiators, which absorb
the uv-light to form highly
reactive free-radicals and which
in turn initiate the crosslinking of
the resin (Figure 2).          Flow         unsaturated resin
promoters are also included to                    +
assist the levelling of the coating
in the molten state. Pigments                                             photoinitiator
are added to give the required                                            breaks down
                                                                          to form free
colour shade and hiding power                                             radicals
(ability to cover white or black                                                                                 resin
substrates                                                                                                       crosslinks

without change of colour). This is only possible, to the extent that the absorption of light by the
photoinitiator is not unduly interfered with.

Powder Application
The first stage in the powder application process is the spray application of the powder. As with all
coating processes, it is important that the surface to be coated is clean and free of oils and here uv-
powder is no exception. Adhesion of the cured uv-powder to metals, such as steel, is much improved
if the surface has been phosphated prior to coating.

Conductive materials present no
problem for the electrostatic
powder application as long as              Figure 3. Principle of Powder Coating Guns
care is taken to ensure that the
object    to   be    coated    is
adequately earthed. Electrically
non-conducting        substrates,                  Tribo gun                                   Corona gun
however, are more difficult to
coat and best results are usually
achieved with a triboelectric
powder gun rather than the
corona guns more commonly                                                                                           Corona wire
                                                                    PTFE barrel                  Polypropylene
used in the powder coating
industry (Figure 3).
                                                     Powder / air feed line
 With       particularly    non-
conductive materials, build up of
                                          Friction of powder against PTFE                  Charged ions from corona attach
a powder film is difficult and            barrel causes powder charging                    to powder paint particles
warming the substrate before
application of the powder can
help. It is also possible to charge the surface of the object to be coated with a charge of opposite
polarity to that of the powder which is to be applied. Care must be given not to apply too much powder
to the substrate, as penetration of light to the bottom of the coating must be sufficient to ensure good
through-cure. Failure to do this will result in a cured coating surface that “floats” on an uncured layer of
resin and hence loss of the mechanical properties of the coating. This is an important difference to
conventional powder coatings, which are mostly not affected by excessive film build-up.
Although it is certainly easier to
coat and cure flat stock with uv-
powder, 3-D objects can be              Figure 4. Layout of a UV-Powder Coating Line.
painted with this technology if
care is given to the design of the
installation. It is necessary to
                                          hang up/ take             optional cleaning,
control both the residence time           down station              pre-treatment and
                                                                    blow dry
and the intensity of the infra-red
lamps (using pyrometers), so as
to ensure that sufficient heat is
given to the object to melt the
powder in all parts without
scorching (Figure 4).

Combination         IR-convection                         uv cure

ovens are usually the best
choice. To cure the powder, it is                                               IR-convection   powder spray
important that the lamps be so                                                  oven            application

positioned, that all parts of the coated surface are adequately exposed. It may also be advantageous if
the object is rotated during passage under the uv-lamps. The use of reflectors and careful positioning
and direction of the lamps helps to ensure a good cure without waste of energy.

Where UV-Powder fits in…
Conventional Powder Coating
Currently a manufacturer has to choose which of the available paint technologies is the most suitable
for the object that requires finishing. For metal articles that can withstand a bake cycle of ca. 20
minutes at temperatures in the range 160 - 200°C, conventional powder coating is usually the
technology of choice. The powder-coated article then passes into an oven where the coating
successively melts, then flows together to form a smooth, pinhole-free film and finally crosslinks to
form a tough, durable surface. On emergence from the oven the painted articles first have to cool
before they can be removed from the hangers. The process is environmentally friendly, as the powder
overspray can be mixed with fresh powder and resprayed and also because there are no solvent
emissions. The thermal energy requirements of the ovens are, however, considerable and the floor
space requirements can be prohibitive. Unfortunately only metal objects can withstand stoving in the

UV-Curable Liquid Coatings
Liquid radiation-curable coatings are usually free of solvents. They can be curtain coated or sprayed
using the same equipment as solvent paints, but are cured more or less instantaneously by passage
under a uv-lamp. Traditionally these lacquers find use mainly in the furniture industry and in specialised
clear coatings, such as on compact disks and as gloss coatings on packaging and printed papers. Until
recently uv-curable lacquers were restricted to clearcoats, but with new advances in the chemistry of
photoinitiators, coloured uv-lacquers can now be made. The very low energy requirement to cure the
paints and the low content of volatiles make this a clean technology. It is also well suited to use on
heat-sensitive materials. The principle drawback, however, is the problems involved in handling the
sticky lacquers and the reuse of overspray.

Solvent Coatings
To give a durable finish to articles such as electric motors, pumps and gearboxes etc. that contain
heat-sensitive components, like seals, wiring and electrical insulation is not straightforward. One
solution is to send off the fully-assembled article to a paint shop to be spray painted with a reactive
two-part polyurethane paint. An equally unattractive alternative is to paint each component individually,
for example with a conventional powder coating, and then assemble the finished product. Although
polyurethane paints have excellent and proven durability, the problems of treating the contaminated
water in the spray booths and the restrictions on the emission of solvent vapours greatly increases
coating costs. For small to medium sized manufacturers, such paint jobs are generally handled
externally and hence cause delay.
Benefits of UV-Curable Powder
With UV-powder, it is necessary
to apply enough heat only to
melt the powder and achieve            Figure 5. Advantages of UV-Curable Powder Coating
flow. The thermal exposure is
much less than in conventional
powder       coatings,     where
prolonged heating at higher
                                          =    Combined advantages of …...
temperatures is necessary to
bring about the cure process.                    UV-Curing                    Powder Coating
This enables uv-powder to be
used on a wide variety of                     • very fast                     • dry handling
substrates such as medium
density    fibreboard     (MDF),
                                              • low energy demand
                                              • ok on heat sensitive   +      • recyclable overspray
                                                                              • easily automated
                                                substrates                    • almost no emissions
paper,    cardboard,     plastics,            • low floor space               • thick coatings in one pass
leather and wood, in addition to                requirements
                                                                              • textured surfaces possible
pre-assembled objects contain-
ing heat sensitive materials.

For    uv-powder,    all   the
advantages of conventional

powder coating combine with the advantages of uv-curing technology, as summarised in the Figure 5.
For the manufacturer, this means the possibility to install a small coating line, that occupies little floor
space and to have in-house control of the coating process. The paint application step can be integrated
into the production process, so that immediately after assembly the products can be painted and
shipped out the same day .

Limitations of UV-curable Powder
At present uv-powder can be made in a variety of finishes and textures and in most colours. Although
uv-light is needed for the cure process, black is not the problem that most people would expect it to be.
Yellow shades are difficult to cure and are, at present state of the art, best avoided. The production of
fully flat matt finishes has also not as yet been demonstrated.

With regard to the curing line, it is important that the curing equipment be installed in such a way that
objects of different geometries can be cured, so as to ensure full flexibility of production. Here creative
solutions must be found if the technology is to realise its full potential in the manufacturing industry.

It is certainly difficult to put a value on a coating process that requires only minutes and which can be
integrated into a production line. Whereas the powder application equipment is today available at
competitive prices and from a number of manufacturers; the investment costs in an IR-UV line can be
expected to depend on the sizes and shapes and the diversity of the articles to be coated. In the
coming months more knowledge can be expected to emerge from pioneer users of uv-powder.

First Users
The first line for coating uv-powder was installed at the Baldor Electric Company at Westville,
Oaklahoma, USA and is used for coating electric motors. The radiators for some Dodge trucks are
now also being produced with a uv-powder coating in the USA. In Europe, the first uv-powder line is
now running in Wales at a factory that produces tables and stands for TV and HiFi equipment. Here
the ability of uv-powder to give exactly the same finish on both MDF and metal is considered to be an
advantage. Further installations are planned in applications in which uv-powder is to be applied to
MDF panels and mouldings.
For any company seriously
considering the installation of a
                                     Figure 6. Teamwork Requirement for an Integrated
uv-powder line, there is one
factor of overriding importance.     Solution for Coating with UV-Curable Powder.
It is essential that all the
suppliers of equipment and
powder - even down to the                                    Formulator
suppliers of the powder raw                                  • Resin supplier(s)
materials - are willing to work                              • Photoinitiator supplier
                                                             • Pigment supplier(s)
together    and     pool     their
knowledge in order to get the
total package to work properly
(see Figure 6). The supplier                 Powder
                                                               End User /
commitment must also extend                  Application       Customer
                                                                                UV-Cure Unit
to guaranteeing a full back-up               Equipment                          Constructor
service for future trouble-                  Supplier
                                                                             • UV-Lamp
shooting. Teamwork, more than                • Substrate                       supplier
any other aspect, is likely to
decide the future success of uv-
powder coating.

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