Quelle/Publication: European Coatings Journal
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05/2008
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Low viscosity without VOCs
Solventborne UV coatings are still widely used
over plastics, because physically drying waterbornes
cannot smooth out the slight roughness of the
basecoats, giving lower gloss. A specially formulated
UV-PUD coating combining high crosslink density with
improved flowout has produced both clearcoats and
a single-coat metallic finish with excellent gloss and
resistance properties.
Waterborne UV coatings for plastics match properties
of solventbornes
Along with roller coating and printing, spraying represents
the third major application method for UV-curable coatings.
Spray application demands the lowest viscosity from the
coating formulation. In some applications, this low viscosity
can be achieved by diluting UV oligomers with monomers.
However, if a high amount of monomer is needed to achieve
spray viscosity, this limits the quality achievable.
It is also often more economic to use solvents or water
as diluents, accepting the additional processing step of
flashing-off before UV cure while achieving a lower coating
thickness and less consumption of paint solids for a given
coated area.
Price rises in organic solvents and more and more stringent
VOC legislation are shifting the industry’s attention to
waterborne UV coatings. Dispersions of urethane acrylates
(UV-PUDs) often combined with physical drying acrylic
dispersions are being used widely in the European furniture
coatings industry, and are also finding growing acceptance
in the North American market.
Dispersion technology opens up new possibilities in the
chemistry of UV coatings. It is no longer necessary to look
for the lowest viscosity oligomers since the viscosity of any
dispersion is independent of the viscosity of the dispersed
substance. High molecular weight urethane acrylates can
be synthesised using hydroxy-functional acrylic monomers
or oligomers such as polyester or epoxy acrylates, resulting
in polymers with high acrylate functionality.
In fact, the ever-important oligomer functionality in 100%
solids or solventborne formulations is a parameter of much
less relevance in waterborne UV systems. For the same
reason, monomers are not really needed in waterborne UV
formulations. However, certain monomer types, depending
on their compatibility with waterborne systems, can be
added either by the manufacturer of the oligomer/polymer
or by the formulator to adjust the properties of the cured
coating.
The effect of these differences in chemistry on the UV
curing parameters of waterborne UV systems has been
studied previously [1, 2]. Especially noteworthy is the
pronounced influence of the temperature at which UV curing
is accomplished on the degree of double bond conversion.
Plastics pose gloss challenges for UV waterbornes
Solventborne UV coatings are still predominantly used
for application on plastics such as mobile phones. These
and other high-value equipment from the computer,
communications and consumer electronics industry are
typically coated with physical drying solventborne (or in
some cases waterborne) pigmented thermoplastic acrylic
(TPA) basecoats and solventborne UV clearcoats. The UV
clearcoats are based on high-functionality oligomers and
monomers, so that the required high resistance properties
are achieved by a high crosslink density.
One of the challenges for waterborne UV systems in this
application is to achieve high gloss over basecoats. The
physically drying basecoats form a surface with a certain
micro-roughness which other physically drying systems are
not able to cover. The roughness is thus also found at the
surface of the clearcoat and the surface does not show the
high gloss and brilliance that can be achieved with the nonphysically drying solventborne UV systems.
In wood coatings, the primed wood is usually sanded before
application of the topcoat, hence high gloss is possible with
physical drying UV systems. This is one of the reasons why
the simple transfer of wood coatings to plastic coatings in
waterborne UV technology is not usually possible.
Physically drying systems, however, have advantages in
pigmented systems. Even if deep curing is incomplete, the
combination of partial UV crosslinking and physical drying
is sufficient to achieve a high property profile. In furniture
coatings, waterborne UV pigmented topcoats are already
widely used, e.g. for kitchen cabinets. In most solventborne
UV coatings, however, the high amount of monomers used
leads if undercuring is caused by the pigmentation to
bleeding of uncured monomers from the coating. Singlelayer pigmented solventborne UV spray coatings for plastics
are consequently rare on the market.
Experimental materials and methods
UV-curing polyurethane dispersions were synthesised
using polyester or epoxy acrylates by procedures described
elsewhere [3] incorporating high-functionality monomers
through a proprietary dispersing process. For comparison,
a commercial UV-curing solventborne formulation based
on a blend of oligomers and monomers for mobile phone
coatings was used ("Desmolux B175X" in combination with
the urethane hexaacrylate "Desmolux U400", both from
Bayer MaterialScience). Commercial photoinitiators and
standard additives for waterborne coatings were used.
Flash-off of water and solvents and UV curing was
accomplished by a combination of a convection/IR dryer
and a UV oven. After application by spray coating, the
films were left for five minutes at room temperature.
Subsequently, flash-off was accomplished by treatment
with hot air at 60°C for two minutes and, where noted, by
additional simultaneous irradiation with infrared light.
The surface temperature of the pre-dried coatings could be
determined by a temperature sensor immediately before UV
cure and was varied between 40 and 80°C. UV cure was
accomplished by a
single 120 W/cm mercury or gallium doped lamp. Unless
noted otherwise, a dose of 1000 mJ/cm2 was applied.
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Glass transition temperatures were determined by
differential scanning calorimetry (DSC) after heating the
sample briefly to 150°C. Double bond conversion was
measured by Fourier transform infrared (FTIR), analysing
the C=C-band at 810 cm-1 and using an uncured sample
as reference for 0% conversion.
Determining the optimum curing conditions
The route for the development of waterborne UV hardcoats
is based on UV-PUDs used in wood coatings technology.
Two effects are achieved by incorporation or blending of
high-functionality UV monomers. The double bond density
of the coating formulation is increased to a greater degree
than that of solventborne formulations of oligomers and
monomers (Figure 1) and secondly, physical drying of the
coating is reduced by the low molecular weight monomers,
enabling the waterborne system to flow out and yield
clearcoats over basecoats with high gloss similar to that of
solventborne UV systems (Figure 2).
In further experiments, the influence of the curing
parameters on the properties of the cured film of the
new waterborne UV hardcoat was investigated. A first
screening of available photoinitiators for waterborne UV
systems pointed towards bis-HDMAP ("Irgacure 127") as
the photoinitiator that yields the best performing coatings
in combination with the new UV-PUD. It should be noted
that this type of photoinitiator can lead to increased initial
yellowing compared to other types. The yellowing, however,
bleaches within a few hours.In a statistically designed
experiment, photoinitiator concentration, temperature at UV
curing and UV dose were varied, studying the effects on film
hardness (pencil hardness and pendulum hardness) and
the glass transition temperature of the cured film.
At low photoinitiator concentrations, the pendulum
hardness increases with temperature at UV cure and with
higher UV dose. This dependency is less pronounced at
higher photoinitiator concentration, which was thus selected
as the optimum concentration. Even at high photoinitiator
concentration, the highest pencil hardness of H-2H is only
achieved if curing takes place at high surface temperature
and high UV dose (Figure 3a).
The previously reported positive influence of the
temperature at UV curing
[1, 2] is obviously also valid for the combination of a
high molecular weight PUD with a high functionality acrylic
monomer. The same behaviour as for the pencil hardness is
also found for the glass transition temperature of the cured
film (Figure 3b). Within a range between 60 to above 100°C,
the highest values are found for high temperature before UV
curing and high UV dose.
Coatings pass standard mobile phone casing tests
Once these optimum curing parameters were established,
a formulation of the new UV-PUD hardcoat was subjected
to the testing protocols set out by several mobile phone
producers. These protocols vary from one OEM to another,
but essentially similar characteristics are required by
all. The tests were performed on polycarbonate mobile
phone shells coated with silver-metallic solventborne
thermoplastic acrylic or waterborne acrylic or PU basecoats
(10 µm dry film thickness) and WB UV hardcoat (20 µm dry
film thickness). The following parameters are included in
almost any testing protocol by the mobile phone producers:
- High gloss;
- Pencil hardness H or higher;
- Excellent adhesion in crosshatch/tape test, which must
be retained after subjecting the coated part to immersion in
boiling water;
- Resistance against prolonged exposure to high
temperature and high humidity;
- Chemical resistance against solvents and also against
suntan lotion at elevated temperature and high humidity,
cosmetics and fluids that simulate human sweat;
- Abrasion resistance in RCA and/or Taber test;
- Low yellowing after irradiation with lamps that emulate
sunlight;
- Resistance to abrupt temperature changes (hot-coldcycles);
If good adhesion of the basecoat-clearcoat build-up could
be achieved and the temperature at which UV curing
was performed was high, all other tests gave results
on the same level as the established solventborne TPA
base and solventborne UV clearcoat formulations i.e. the
specifications of several mobile phone producers could be
passed.
Metallic hardcoats can be produced
Exploring further the possibilities of the new UV-PUD
for hardcoats, a metallic formulation that could be used
as single-layer coating was developed. Targets for the
development were good gloss and resistance properties as
well as excellent adhesion. The question of adhesion is
related to the through-cure that is achievable, which in turn
depends on the layer thickness and pigmentation level.
A phosphorous-organic treated metallic effect pigment
that is stable in waterborne formulations was chosen.
Photoinitiators were studied, following the pigment
supplier’s formulation recommendations. In this case, a
blend of benzophenone and an alpha-hydroxyketone was
used for surface cure, while through cure was improved
when BAPO was added to the formulation.The influence
of curing parameters (IR dose or temperature at UV cure,
lamp type, UV dose and layer thickness) on the surface and
through cure were studied by FT-IR spectroscopy on the
front and reverse side of free coating films.
For surface cure, the variations in double bond conversion
were marginal. In all cases, conversion was well above 90%
and could thus be considered as complete. [Statistically,
for a tetrafunctional monomer, curing of 80% double
bond conversion corresponds to 0.24 = 0.16% of wholly
unpolymerised molecules, while curing of 90% corresponds
to 0.14 = 0.01% of unpolymerised molecules.]
Good through-cure even at double layer thickness
The greatest effect on through-cure is the influence of the
layer thickness. Figure 4 illustrates the model statistically
derived from the data for two series of dry films, with
thicknesses of 13 µm and 26 µm. For the thin films,
good through-curing was achieved for large areas of the
experimental space. The thicker layer, however, requires
optimised conditions. Nonetheless, a conversion of 80%
of the double bonds is possible at high temperature and
high UV dose. The high temperature (7080°C) at UV curing
is best induced by hot air and infrared irradiation. The
influence of the lamp type (Hg or Ga) was not statistically
significant.
Measurements were carried out on the reverse side of the
coatings to determine through cure rather than surface cure.
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The IR dose affects surface temperature, 0% IR resulting
in 50°C and 50% IR in 80°C. Curing was by one Hg lamp
at 120 W/cm. The pigmentation level of 19% on formulation
solids gave full hiding power at 16 µm dry film thickness.
The findings of good through-cure could be confirmed
by application-related testing of the metallic coating on
polycarbonate mobile phone shells. The optimised but still
basic formulation has good hiding power and gloss of
approximately 70% (60° angle), good adhesion, even after
immersion in boiling water for one hour, and excellent
resistance against solvents. Resistance against suntan
lotion is almost at the same level as the non-pigmented
formulation.
Both single and two-coat systems are practicable
With a new generation of waterborne UV-curing
polyurethane dispersions, it is possible to formulate
coatings for plastics with performance characteristics
that could hitherto only be achieved by two-component
polyurethane systems or solventborne UV systems.
The UV coating of mobile phones and other equipment
with similar high demands on the performance properties
is for the first time possible without the use of organic
solvents. Waterborne UV technology offers the possibility
to convert the conventional dual-layer coating system from
solventborne to waterborne. The high quality single-layer
pigmented systems are a new and very economical option.
Using water as the solvent is the more favourable
alternative, economically as well as ecologically. The
decision to convert to waterborne UV technology has
been taken by many companies in the European furniture
industry. It is now up to the manufacturers of high value
coated goods made from plastics to look, test, calculate and
make their choice.
References
[1] J. Weikard et al., Proc. of 6th Nürnberg Congress
"Creative Advances in Coatings Technology" (2001), 125
144. [2] F. Masson et al., Progress in Organic Coatings 39,
(2000), 115 126. [3] EP-B 0753531, Bayer MaterialScience
AG.
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Figure 1: Calculated double bond density of the new waterborne UV system for
mobile phone coatings (UV-PUD including a high-functionality acrylic monomer)
in comparison to a WB UV system for wood coatings and a commercial
solventborne UV coating for mobile phones
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Bild zu Low viscosity without VOCs
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Figure 3: Statistical model for the
dependence of a) the pencil hardness
(Mitsubishi pencils, 1 kg load, 45°
angle, flat tip) and b) the glass
transition temperature on temperature
at UV cure and UV dose for new UVPUD hardcoat (photoinitiator BisHDMAP at 2.5% on solids)
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Figure 4: Statistical model for
the dependence of double bond
conversion (IR spectroscopy data)
on the reverse side of the coating
on UV dose and temperature for
a waterborne metallic pigmented
formulation based on the new hard
UV-PUD. a) dry film thickness 13 µm,
b) dry film thickness 25 µm
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