Finishing in the fast lane

Transcription

Quelle/Publication: European Coatings Journal
11/2005
Ausgabe/Issue:
36
Seite/Page:
Finishing in the fast lane
Thiol-isocyanate UV system speeds up automotive
repair coating.
Nazire Dogan, Huig Klinkenberg, Liesbeth Reinerie, Danny
Ruigrok, Peter Wijnands.
A novel UV curing system has been developed which
eliminates the problems of 'oxygen inhibition' and lack of
cure in shadow areas found in free-radical UV curable
coatings. Clearcoats for vehicle refinishing based on this
thiol-isocyanate chemistry have been evaluated. Initial test
results show that they meet all the main performance
requirements and greatly speed up curing.
UV-initiated free radical curing is a well accepted technology
in certain parts of the coatings industry and its benefits have
been published widely [1-4]. It reduces curing times to a few
seconds or minutes. However, the free radical technology
commonly used for UV-curing in paint systems also has a
number of drawbacks. These include oxygen inhibition and
the inability to cure in non-irradiated (shadow) areas.
UV cure has many benefits in vehicle refinishing
UV technology can offer major advantages within the car
refinish industry. Changing the curing process from the
standard thermal cycle of 30-60 minutes at 60°C to
UV-curing at room temperature offers many opportunities to
increase bodyshop profitability. However, the limitations
already mentioned need to be overcome. Other important
requirements are:
- Providing a safe work environment, i.e. employing low
energy UV-A radiation;
- Easy application, comparable with conventional paint
systems with existing spray equipment;
- Improved product properties;
- Cost efficiency.
In the last few decades, two-component (2K) polyurethane
(PU) chemistries have been widely employed in car refinish
applications [5]. Chemically crosslinked systems provide
highly durable films with good mechanical and solvent
resistance. Variations in resin properties provide various
routes to the desired coating performance.
Akzo Nobel Car Refinishes has successfully developed a
UV-A curable clearcoat based on novel chemistry [6]. A
comparison between this new clearcoat and a conventional
2K thermally cured PU clearcoat system has been made in
terms of raw materials, technical properties and
environmental aspects and is discussed below.
Meeting the challenges of thiol-isocyanate chemistry
The reaction between thiol and isocyanate functional resins
proceeds very rapidly when they are combined with a basic
compound [7]. Directly after mixing of these three
constituents, a gel will be formed, which makes this reaction
unsuitable for application in practical paint formulations.
However, in the absence of any basic compound the
reaction speed is slow.
Thus in order to use this chemistry for practical paint
formulations, the challenge was to develop a latent catalyst
which becomes active upon (e.g.) UV-A irradiation. Such a
system would give coatings with mixing, potlife and
application properties at a practical level, while upon
deblocking of the catalyst a very fast reaction would occur.
A novel photolatent amine has been developed [6] and the
technology behind these coating compositions has been
patented by Akzo Nobel [8]. Figure 1 shows the reaction
between thiol and isocyanate functional resins.
With the blocked catalyst the resins react with each other
only slowly. After UV-A irradiation the catalyst deblocks, with
rapid formation of a thio-urethane network. Hence, the
crosslinking mechanism used here is essentially a regular
catalysed 2K curing chemistry, comparable with other car
refinish systems. The key characteristic from a UV point of
view is that the specific catalyst used can be activated by
UV-A irradiation.
Compared to free radical curing UV-systems this new
approach has some clear advantages. It shows no oxygen
inhibition and a slow but complete cure occurs in shadowed
areas.
New UV cure chemistry offers application benefits
The two clearcoat systems are prepared by mixing the
different components. While the reference clearcoat is a
straightforward 2K formulation, the UV-A clearcoat has a
third component, a solution of the blocked amine catalyst,
added separately from the base and crosslinker.
Clearcoat samples were conventionally spray-applied with
HVLP or LVLP spray guns providing a high transfer
efficiency. The actual application details are specified in
Technical Data Sheets [9]. Both clearcoats are VOC
compliant, though the 2K UV curable formulation has a
lower VOC content of only 350g/l or 2.9 lbs/gal (according to
ASTM D3960).
There are no substantial differences in application properties
(wetting, flow etc) and preferred application conditions
(temperature, relative humidity) between the two clearcoats;
both are easily applied over solventborne or waterborne
basecoats.
Extended potlife may improve economy
The viscosity of a 2K paint mixture is important for its
application properties (sprayability) and for the properties of
the resulting film. The potlife is defined as the time the
viscosity of the ready-to-spray mixture remains low enough
for application [10].
The viscosity increase of the ready-to-spray mixtures is
measured with DIN cup #4 (according to DIN 53211, 23°C)
and is shown in Figure 2. The reference clearcoat starts to
react immediately at ambient temperatures, and viscosity
increases rapidly. Its practical potlife is about two hours.
Provided the UV clearcoat is not exposed to UV irradiation,
the resins react slowly and potlife in a closed can is at least
four times longer (> 8 hours) than that of the conventional
clearcoat. A positive side effect of this is that the material
remaining after a paint job can be (re)used for the next one,
reducing material waste.
Curing is fast and flexible
The drying of conventional refinish clearcoats is usually
carried out at about 60°C. After about 30 minutes at 60°C,
the coating can be handled freely. The spraybooths in a
bodyshop environment are designed with heating and
cooling modes to accommodate this. Curing at elevated
temperatures and the subsequent cooling down step greatly
increase the overall process time for car repairs, but the
curing of a 2K clearcoat at room temperature would take a
few hours.
A key requirement for a safe UV-curing process within a
bodyshop is the use of a light source which produces only
low energy UV-A light. (However, adequate precautions for
eye and skin protection are recommended to avoid
overexposure to UV-A.)
Once the mixed two-pack UV-A clear is applied and
exposed to the light source, the latent amine catalyst is
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deblocked and crosslinking occurs rapidly at room
temperature. By using a mobile unit for spots or larger areas
(Figures 3) or a handheld HID (high intensitiy discharge)
lamp for spot and panel repairs (Figure 4), the coating can
be handled after approximately 5 minutes.
Different applications need different curing systems
The two UV-A curing units differ in irradiation time and
distance between the lamps and object. The mobile
fluorescent unit is directed at the painted substrates and
switched on for five minutes, whereas the handheld UV-A
lamp can be moved over the painted area, in a method
similar to the earlier application of the paint. Both drying
options result in the same free-to-handle time of the UV-A
clearcoat.
For the curing of larger areas or a complete vehicle, several
options, with appropriate safety considerations, can be
evaluated. A possible option could be the use of (fixed)
UV-A lamps in the spraybooth walls. Due to the different
shapes and geometry of cars it is difficult to irradiate the
total surface with the same intensity and so curing of a
complete car will take a few minutes longer than a smaller
repair.
The new chemistry results in a slower cure of shadow spots,
but one which is still acceptable from a practical point of
view. The reduction of drying time gives higher throughput
and thus more flexibility to manage production peaks.
Additionally, because it is no longer necessary to heat up
the spraybooth for rapid curing, energy costs are
significantly reduced.
Dry film properties are evaluated
Both clearcoat systems were applied at 22°C ± 2°C and a
relative humidity of 40% ± 5%. The conventional clearcoat
was cured for 30 minutes at 60°C and the UV-coating was
dried with the fluorescent tube system for 5 minutes, at a
measured intensity of approximately 2 mW/cm2.
The topcoats were then evaluated according to the tests
listed in Table 1. Measurements were carried out at ambient
temperatures (23°C), with a dry film layer thickness of
around 70 micrometres.
Gloss levels match those of standard coatings
A key requirement of a clear coating is to provide a smooth
appearance, since the aesthetic appeal of an 'invisible'
repair is the first property to be judged by the customer.
Table 2 compares gloss values of the two clearcoats over
two different types of waterborne basecoats. Both yield good
optical properties and no significant differences in gloss
were observed.
Scratch resistance test imitates carwash
Small scratches adversely influence the appearance of
clearcoats, so a high scratch and mar resistance is
desirable. Several test methods are available, and a
practical approach within the automotive industry is to
simulate a carwash.
Samples are brushed with soapy water containing small
particles (e.g. aluminium oxide) to represent dirt. Scratch
resistance is evaluated by measuring the difference in gloss
before and after the test (see Table 2). Compared to the
reference clearcoat, the UV cured coating has substantially
better scratch resistant properties.
Chemical resistance surpasses reference system
A clear coat must also protect the (multilayer) paint system
against chemicals such as gasoline, antifreeze or aliphatic
volatiles present in waxes and polishes. The chemical
resistance was evaluated directly after curing by applying
cotton wool soaked with a chemical to the substrate and
immediately covering it with a lid.
The effects were assessed visually. For each combination of
chemical and topcoat, the resistance to exposure was
scored as an overall result, on a scale from 1 (severely
attacked) to 10 (unaffected).
Table 3 shows that the UV coating was unaffected by
immersion in automotive fluids for up to 24 hours. The
appearance of the conventional system changed slightly
with water, gasoline and motor oil, indicating that this film is
less highly crosslinked. Tests with the solvents acetone,
xylene and methyl ethyl ketone were performed for 1
minute. The chemical resistance is better for the UV-A
clearcoat than for the conventional system.
Resistance to accelerated weathering appears good
The resistance to weathering of the two clearcoats (both
containing light stabilisers) was tested by exposing the paint
systems and measuring amongst other properties the
decrease in 20° gloss at different time intervals. Long term
exterior durability tests are currently running.
Both topcoats were also subjected to accelerated
weathering tests. After 2500 hours of exposure, both
clearcoats retained more than 90% of their gloss. No crack
formation was observed. The gloss retention during
weatherometer exposure is shown in Figure 5; both systems
were applied over a blue metallic waterborne basecoat.
Positive results will lead on to other applications
The UV-A curable clearcoat based on a novel (patented)
thiol-isocyanate technology catalysed with a specially
developed photolatent amine, meets the technical
requirements for car refinish applications. Very fast curing
takes place after deblocking of the catalyst. Final properties
are better than those of commonly used clearcoats, while
the insufficient curing of shadow areas and oxygen inhibition
found with free radical UV systems are eliminated.
Changing the curing process in a bodyshop environment
from elevated temperature cure to UV-curing increases
productivity and flexibility, reduces energy requirements,
VOC emissions and waste. This technology offers great
potential in many other applications and its further
expansion is anticipated.
ACKNOWLEDGEMENTS
The authors are indebted to all colleagues from Akzo Nobel
who contributed to this project. The specific contributions of
Peter van Kesteren, Suzanne Frings, Rob van der Krogt,
Edward Marinus, Fred Rous, Rosienne Steensma, and
Heert Andringa are gratefully acknowledged.
REFERENCES
[1] P. K. T. Oldring, (Ed.), Chemistry and Technology of UV
and EB Formulation for Coatings, Inks, and Paints, SITA
Technology, London, 1991
[2] S. P. Pappas, (Ed.), Radiation Curing: Science and
Technology, Plenum Press, New York, 1992
[3] D. R. Randell, (Ed), Radiation Curing of Polymers, The
Royal Society of Chemistry, Bristol, 1991
[4] J. V. Koleske, Radiation Cured Coatings, in Coatings
Technology Handbook, D. Satas, (Ed.), Marcel Dekker, New
York, 2001
[5] M. Bock, Polyurethanes for Coatings, Vincentz Verlag,
Hannover, 2001
[6] K. Dietliker, K. Misteli, T. Jung, P Contich, J. Benkhoff, E.
Sitzmann, Novel Chemistry for UV Coatings, ECJ, October
2005
[7] G. L. Linden, GB 2176197, 1986
[8] H. Klinkenberg, J. C. v. Oorschot, PCT Pat. Appl WO
11/2005
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01/92362 (2001); US patent 6,579,913
[9] http://www.sikkenscr.com/sikkens/corporate
[10[ Z. W. Wicks, F. N. Jones, S. P. Pappas, Organic
Coatings, John Wiley & Sons, 1999
Results at a glance
- UV curable coatings have been developed using a
thiol-isocyanate chemistry which eliminates the problem of
oxygen inhibition and provides full curing in shadow areas.
- Since these coatings will cure under UV-A light, they are
potentially suitable for use in vehicle refinishing applications.
- A refinish clearcoat based on the new chemistry has
properties at least equal to those of standard thermally
cured coatings.
- The change from thermally cured 2K systems to UV curing
reduces energy costs and increases throughput.
The authors:
-> Nazire Dogan is group leader for UV curable systems at
the international product development department of Akzo
Nobel Car Refinishes, Sassenheim.
-> Huig Klinkenberg is group leader for new business
technologies, Akzo Nobel Car Refinishes, Sassenheim.
-> Liesbeth Reinerie is a junior specialist at the international
product development department of Akzo Nobel Car
Refinishes, Sassenheim.
-> Danny Ruigrok is a specialist at the international product
development department of Akzo Nobel Car Refinishes,
Sassenheim.
-> Peter Wijnands is a technician at the international product
development department of Akzo Nobel Car Refinishes,
Sassenheim.
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Figure 1: Crosslinking of thiol and isocyanate functional resins .
Figure 2: Changes in viscosity of 2K PU and 2K UV ready-to-spray mixtures over time
(DIN 4 cup at 23°C).
Figure 3: Mobile UV-A curing unit in use on horizontal repair.
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Figure 4: Handheld lamp used for UV-A curing.
Figure 5: Gloss retention of 2K UV-A versus 2K PU clearcoats after weathering in
xenon weatherometer (WOM).
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Finishing in the fast lane

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