Note: Descriptions are shown in the official language in which they were submitted.
CA 02347868 2008-03-25
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Multilayer lacquer coating process
This invention relates to a process for providing
multilayer lacquer coatings on substrates using radiation
curable coating compositions. The process may
advantageously be used for automotive and industrial
lacquer coating, preferably in automotive repair lacquer
coating.
It has been known for a relatively long time to use UV
technology in coating and curing, particularly in the wood
coating industry. It is, however, also known in other areas
of application, such as for.example in automotive lacquer
coating, to use coating compositions which are curable by
means of high-energy radiation. These applications also
enjoy the advantages of radiation curable coating
compositions, such as for example the very short curing
times, low solvent emissions from the coating compositions
and the very good hardness of the coatings obtained
therefrom.
In addition to suitable radiation curable binders and
photoinitiators, various types of radiation sources have
also become known.
DE-A-196 35 447, for example, accordingly describes a
process for the production of a multilayer repair lacquer
coating, wherein a coating composition solely containing
binders free-radically polymerisable by UV radiation is
applied as the clear lacquer or pigmented topcoat lacquer.
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Once applied, the coating composition is exposed to UV
light with UV flash lamps.
EP-A-0 540 884 describes a process for the production of a
multilayer coating for automotive original lacquer coating
by application of a clear lacquer layer onto a dried or
cured base lacquer layer, wherein the clear lacquer coating
composition contains binders curable by free-radical
polymerisation and the clear lacquer layer is cured by
means of UV radiation. Application of the clear lacquer
proceeds with illumination with light of a wavelength of
greater than 550 nm or with exclusion of light.
Coating compositions curable by means of high-energy
radiation have also been described which contain binders
which may be cured by means of high-energy radiation and
additionally by means of a further crosslinking mechanism.
DE-A-28 09 715, for example, discloses binders curable by
means of high-energy radiation which are based on an NCO-
and acryloyl-functional urethane compound, which is
produced from a (meth)acrylic acid hydroxyalkyl ester and a
polyisocyanate, and on a polyfunctional hydroxyl compound.
EP-A-0 000 407 describes coating compositions curable by
means of high-energy radiation based on an OH-functional
polyester resin esterified with acrylic acid, a vinyl
compound and a polyisocyanate. In a first curing step,
irradiation is performed with UV light and, in a second
curing step, final curing proceeds at temperatures of 130
to 200 C.
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U.S. Patent No. 6,332,291 proposes coating
compositions curable by means of high-
energy radiation, which contain as binder compounds A)
having free-radically polymerisable double bonds and
further functional groups reactive for the purposes of an
addition and/or condensation reaction together with
compounds B) having free-radically polymerisable double
bonds and further functional groups reactive for the
purposes of an addition and/or condensation reaction,
wherein the latter should be reactive towards the
additional reactive groups of the compounds A). The
resultant coatings may be completely cured after UV
irradiation by exposure to relatively high temperatures of
for example 30 to 120 C.
However, the coatings obtained with the above-stated
processes for multilayer automotive lacquer coating using
binders curable by means of high-energy radiation are still
in need of further improvement in certain respects. The
coatings still exhibit weaknesses with regard to resistance
to weathering and chemicals and exhibit unsatisfactory
.sandability. Moreover, the curing process for the coating
compositions curable by means of high-energy radiation
brings about a shrinkage in volume of the applied coating,
which gives rise to stresses and cracking in the film. This
may result in detachment from the substrate. The problem of
cracking and deficient interlayer adhesion has not hitherto
satisfactorily been solved.
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The object of the invention was accordingly to provide a
multilayer automotive lacquer coating process, in
particular for multilayer automotive repair lacquer
coating, using at least partially radiation curable coating
compositions, by means of which process coatings are
obtained which exhibit no cracking and exhibit good
adhesion to the substrate. The resultant coatings should
exhibit very good resistance to chemicals and weathering
together with good sandability. The coatings should also
exhibit adequate flexibility even at an elevated crosslink
density. The coatings should moreover exhibit a perfect
optical appearance.
This object is achieved by the multilayer lacquer coating
process provided by the invention which comprises the
application of one or more surfacer layers and/or further
layers, which may, for example, comprise conventional
interlayers, onto an optionally precoated substrate, and
subsequent application of a topcoat lacquer layer of a base
lacquer/clear lacquer structure or of a pigmented single-
layer topcoat lacquer, wherein at least one of the layers
of the multilayer structure is prepared from a coating
composition at least partially curable by means of high-
energy radiation, which process is characterised in that,
once the coating composition(s) at least partially curable
by means of high-energy radiation has/have been applied,
irradiation is performed first with infrared radiation (IR
radiation) and then with high-energy radiation, preferably
ultraviolet radiation (UV radiation), wherein irradiation
with IR radiation may at least in part overlap with the
subsequent irradiation with high-energy radiation.
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The high-energy radiation used may in particular comprise
UV radiation, but may also be electron beam radiation.
5 Once the coating composition(s) at least partially curable
by means of high-energy radiation have been applied, a
flashing-off phase is preferably provided. This may, for
example, comprise flashing-off for 5 to 15 minutes,
preferably for 5 to 10 minutes, at room temperature. Only
thereafter is irradiation with IR radiation performed.
The coating compositions at least partially curable by
means of high-energy radiation used in the process
according to the invention may be aqueous, diluted with
solvents or contain neither solvents nor water. The coating
compositions may comprise those which are completely or
only partially curable by means of high-energy radiation,
preferably by means of UV radiation. Coating compositions
curable by means of high-energy radiation in particular
comprise cationically and/or free-radically curing coating
compositions known to the person skilled in the art. Free-
radically curing coating compositions are preferred. The
action of high-energy radiation on these coating
compositions gives rise to free-radicals in the coating
composition which initiate crosslinking by free-radical
polymerisation of olefinic double bonds.
The preferably usable free-radically curing coating
compositions contain conventional prepolymers, such as
polymers or oligomers, the molecules of which comprise
free-radically polymerisable olefinic double bonds, in
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particular in the form of (meth)acryloyl groups. The
prepolymers may be present in combination with conventional
reactive diluents, i.e. reactive, liquid monomers.
Examples of prepolymers or oligomers are (meth)acrylic-
functional (meth)acrylic copolymers, epoxy resin
(meth)acrylates, polyester (meth)acrylates, polyether
(meth)acrylates, polyurethane (meth)acrylates, unsaturated
polyesters, unsaturated polyurethanes or silicone
(meth)acrylates having number average molecular weights
(Mn) preferably in the range from 200 to 10000,
particularly preferably from 500 to 3000, and with an
average of 2 to 20, preferably 3 to 10 free-radically
polymerisable, olefinic double bonds per molecule.
(Meth)acrylic should here be taken to mean acrylic and/or
methacrylic.
If reactive diluents are used, they are used, for example,
in quantities of 1 to 50 wt.%, preferably of 5 to 30 wt.o,
relative to the total weight of prepolymers and reactive
diluents. These reactive diluents comprise defined, low
molecular weight compounds which may be mono-, di- or
polyunsaturated. Examples of such reactive diluents are:
(meth)acrylic acid and the esters thereof, maleic acid and
the semi-esters thereof, vinyl acetate, vinyl ethers,
substituted vinylureas, ethylene and propylene glycol
di(meth)acrylate, 1,3- and 1,4-butanediol di(meth)acrylate,
vinyl (meth)acrylate, allyl (meth)acrylate, glycerol tri-,
di- and mono(meth)acrylate, trimethylolpropane tri-, di-
and mono(meth)acrylate, styrene, vinyltoluene,
divinylbenzene, pentaerythritol tri- and
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tetra(meth)acrylate, di- and tripropylene glycol
di(meth)acrylate, hexanediol di(meth)acrylate. Reactive
diluents may be used individually or as a mixture.
Diacrylates, such as for example dipropylene glycol
diacrylate, tripropylene glycol diacrylate and/or
hexanediol diacrylate, are preferably used as the reactive
diluents.
The free-radically curing coating compositions contain
photoinitiators, for example in quantities of 0.1 to 5
wt.o, preferably of 0.5 to 3 wt.%, relative to the total of
free-radically polymerisable prepolymers, reactive diluents
and photoinitiators. Conventional photoinitiators are
suitable, such as for example benzoin and the derivatives
thereof, acetophenone and the derivatives thereof,
anthraquinone, 1-benzoylcyclohexanol, organophosphorus
compounds, such as for example acyl phosphine oxides. The
photoinitiators may be used individually or in combination.
Further synergistic components, for example tertiary
amines, may also be used.
The coating compositions at least partially curable by
means of high-energy radiation usable in the process
according to the invention preferably contain, apart from
the binder system curable by means of high-energy
radiation, one or more further binders. The further binders
which may additionally be present preferably comprise
conventional binder systems curable by means of addition
and/or condensation reactions. They may, however, also
comprise conventional physically drying binder systems or
combinations of both stated binder systems. It is also
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possible for the binder systems which are per se curable by
means of high-energy radiation to have groups capable of
crosslinking by addition and/or condensation reactions in
addition to the free-radically polymerisable double bonds.
The addition and/or condensation reactions for the present
purposes comprise lacquer chemistry crosslinking reactions
known to the person skilled in the art, such as for example
ring-opening addition of an epoxy group onto a carboxyl
group with formation of an ester and a hydroxyl group, the
addition of a hydroxyl group onto an isocyanate group with
formation of a urethane group, the reaction of a hydroxyl
group with a blocked isocyanate group with formation of a
urethane group and elimination of the blocking agent, the
reaction of a hydroxyl group with an N-methylol group with
elimination of water, the reaction of a hydroxyl group with
an N-methylol ether group with elimination of the
etherification alcohol, the transesterification reaction of
a hydroxyl group with an ester group with elimination of
the esterification alcohol, the transurethanisation
reaction of a hydroxyl group with a carbamate group with
elimination of alcohol, the reaction of a carbamate group
with an N-methylol ether group with elimination of the
etherification alcohol.
The binder system preferably contains functional groups
which permit crosslinking at low temperatures, for example
at 20 to 80 C. These particularly preferably comprise
hydroxyl and isocyanate groups. The functional groups, in
particular the hydroxyl groups and isocyanate groups, may
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in each case be present either in the binder curable by
means of high-energy radiation and/or in a separate binder.
Polyurethane (meth)acrylates, polyester (meth)acrylates
and/or (meth)acryloyl-functional poly(meth)acrylates may
preferably be used in the clear lacquer, base lacquer or
single-layer topcoat lacquer, while epoxy (meth)acrylates
are preferably used in the surfacer or further layers, such
as interlayers. Particularly good results are obtained if
the above-stated (meth)acryloyl-functional binders are
combined with binders based on a crosslinking mechanism
between hydroxyl and isocyanate groups. The hydroxyl and/or
isocyanate groups may here also be present in the
(meth)acryloyl-functional binder(s). It should merely be
noted that those components comprising hydroxyl groups and
those components comprising isocyanate groups must be
stored separately and may be mixed together only shortly
before application. Particularly preferably and
advantageously used binder systems are those which contain
(meth)acryloyl- and OH-functional components and
polyisocyanates, wherein the (meth)acryloyl and OH groups
may be present in a single and/or different binder
components, and also binder systems containing A) compounds
comprising one or more free-radically polymerisable double
bonds, which additionally contain at least one further
functional group reactive for the purposes of an addition
and/or condensation reaction and B) compounds comprising
one or more free-radically polymerisable double bonds,
which additionally contain at least one further functional
group reactive for the purposes of an addition and/or
condensation reaction, wherein the additional reactive
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functional group is complementary to or reactive towards
the additional reactive functional groups of component A).
In the latter case, one or more monomeric, oligomeric
and/or polymeric compounds having at least one functional
5 group reactive for the purposes of an addition and/or
condensation reaction towards the functional groups from
component A) or component B) present in addition to the
free-radically polymerisable double bonds may optionally
also be present.
The coating compositions which are at least partially
curable by means of high-energy radiation and are usable in
the process according to the invention may contain
additional components conventional in lacquer formulation.
They may, for example, contain conventional lacquer
additives. The additives comprise those conventional
additives usable in the lacquer sector. Examples of such
additives are levelling agents, anticratering agents,
antifoaming agents, catalysts, coupling agents, rheological
additives, thickeners, light stabilisers and emulsifiers.
The additives are used in conventional quantities familiar
to the person skilled in the art.
The coating compositions usable in the process according to
the invention may contain proportions of organic solvents
and/or water. The solvents comprise conventional industrial
lacquer solvents. These may originate from the production
of the binders or are added separately. Examples of such
solvents are mono- or polyhydric alcohols, for example
propanol, butanol, hexanol; glycol ethers or esters, for
example diethylene glycol dialkyl ethers, dipropylene
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glycol dialkyl ethers, in each case with Cl to C6 alkyl,
ethoxypropanol, ethylene glycol monobutyl ether; glycols,
for example ethylene glycol, propylene glycol and the
oligomers thereof, esters, such as for example butyl
acetate and amyl acetate, N-methylpyrrolidone and ketones,
for example methyl ethyl ketone, acetone, cyclohexanone;
aromatic or aliphatic hydrocarbons, for example toluene,
xylene or linear or branched aliphatic C6 to C12
hydrocarbons.
The coating compositions usable in the process according to
the invention may contain pigments and/or extenders. These
comprise the conventional extenders usable in the lacquer
industry and organic or inorganic coloured and/or effect
pigments and anticorrosion pigments. Examples of inorganic
or organic coloured pigments are titanium dioxide,
micronised titanium dioxide, iron oxide pigments, carbon
black, azo pigments, phthalocyanine pigments, quinacridone
and pyrrolopyrrole pigments. Examples of effect pigments
are: metal pigments, for example made from aluminium,
copper or other metals; interference pigments, such as for
example metal oxide coated metal pigments, for example
aluminium coated with titanium dioxide or with mixed oxide,
coated mica, such as for example mica coated with titanium
dioxide and graphite effect pigments. Examples of extenders
are silicon dioxide, aluminium silicate, barium sulfate and
talcum. In addition to the conventional additives, the
coating compositions may advantageously contain specially
coated extenders for increasing scratch resistance.
Extenders which may be considered for this purpose are, for
example, micronised aluminium oxide or micronised silicon
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oxides. These extenders are coated with compounds which
contain UV-curable groups, for example with acrylic-
functional silanes and consequently also participate in the
radiation curing of the coating composition. Such
transparent extenders which are particularly suitable for
clear lacquers are available as commercial products, for
example under the name AKTISILO.
The overall composition of the usable coating compositions,
for example the nature of the pigment, is determined by
which layer of the multilayer structure is to be produced
with the particular coating composition, i.e. whether, for
example, it comprises a clear lacquer, a base lacquer or a
surfacer or another conventional interlayer.
The coating composition may be applied onto various
substrates in the process according to the invention.
Preferred substrates are metal or plastics substrates.
Application in the multilayer structure proceeds by
conventional methods, preferably by spraying. The
substrates may be precoated, for example provided with a
conventional priming coat.
Once the coating composition(s) at least partially curable
by means of high-energy radiation has/have been applied,
irradiation with IR radiation is performed. IR light
sources known to the person skilled in the art and
conventional for lacquer drying may be used. The IR light
source is positioned in front of the substrate surface to
be irradiated, for example at a distance of 20 to 70 cm.
The duration of irradiation with IR radiation may be, for
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example 1 to 20 minutes. Depending upon the duration of
irradiation and the power of the radiation source,
substrate surface temperatures of for example 40 to 200 C
may be achieved. The settings are favourably arranged such
that temperatures of for example 40 to 100 C at the
substrate surface are achieved. Particularly good results
are achieved if irradiation with IR radiation is not
performed directly after application, but instead a
flashing-off phase intervenes. Flashing-off may last, for
example, for 5 to 15 minutes, preferably 5 to 10 minutes,
at room temperature.
When the desired substrate surface temperature is achieved
by means of IR irradiation or the planned duration of
irradiation has elapsed, irradiation may be performed with
high-energy radiation, preferably with UV radiation.
Curing of the coating at least partially curable by means
of high-energy radiation, preferably UV radiation, may
preferably proceed with UV radiation sources emitting in
the wavelength range from 180 to 420 nm, in particular from
200 to 400 nm.
Examples of usable UV radiation sources are, for example,
high-, medium- and low-pressure mercury lamps. Lamp length
may vary. Lamps of between 5 and 200 cm in length are
usual. Lamp and reflector geometry may be tailored relative
to each other in the conventional manner depending upon the
particular application and the required radiation energy.
Lamp power may, for example, vary between 20 and 250 W/cm
(Watts per cm of lamp length). Lamps of a power of between
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80 and 120 W/cm are preferably used. The mercury lamps may
optionally also be doped by the introduction of metal
halides. Examples of doped light sources are iron- or
gallium-mercury lamps.
Further examples of UV radiation sources are gas discharge
tubes, such as for example low pressure xenon lamps, UV
lasers, W point light sources, such as W-emitting diodes,
and black light tubes. In addition to these continuously
operating W radiation sources, however, discontinuous UV
radiation sources may also be used. These preferably
comprise so-called high-energy flash installations
(abbreviated to UV flash lamps). UV flash lamps may contain
a plurality of flash tubes, for example quartz tubes filled
with inert gas, such as xenon. The UV flash lamps exhibit,
for example, an illuminance of at least 10 megalux,
preferably of 10 to 80 megalux, per flash discharge. The
energy per flash discharge may, for example, amount to 1 to
10 kJoules.
The W radiation sources are generally incorporated into a
UV installation, which normally consists of the Uv
radiation sources, the reflector system, the power supply,
electrical controls, screening, cooling system and ozone
extractor. Other arrangements are, of course, also possible
and certain of the stated components may also be omitted.
When UV flash lamps are used as the UV radiation source,
the duration of irradiation with W radiation may, for
example, be in the range from 1 millisecond to 400 seconds,
preferably from 4 to 160 seconds, depending upon the
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selected number of flash discharges. The flashes may, for
example, be triggered approximately every 4 seconds. Curing
may proceed, for example, by 1 to 40 successive flash
discharges.
5
When continuous UV radiation sources are used, the duration
of irradiation may, for example, be in the range from a few
seconds to approx. 5 minutes, preferably below 5 minutes.
10 The distance between the W radiation sources and the
substrate to be irradiated may be, for example, 5 to 60 cm.
Screening of the W radiation sources to prevent escape of
radiation may be achieved, for example, by using a suitably
lined protective housing around a transportable lamp unit
15 or by means of other safety measures known to the person
skilled in the art.
The multilayer lacquer coating process according to the
invention, which is characterised in that, once the coating
composition(s) at least partially curable by means of high-
energy radiation have been applied, irradiation is
performed first with IR radiation and then with high-energy
radiation, may be performed in various embodiments.
It is, for example, possible to perform the UV irradiation
phase once the IR irradiation phase is complete or tJV
irradiation may begin while IR irradiation is still under
way. In the latter case, the IR and UV irradiation phases
may partially or completely overlap, i.e. the IR
irradiation phase may be ended before or simultaneously
with the completion of the UV irradiation phase.
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It is also possible to perform a further IR irradiation
phase once the UV irradiation phase has ended. The
subsequent IR irradiation phase may amount, for example, to
0.5 to 30 minutes. Otherwise, the statements already made
above with regard to IR irradiation apply. In the case of
an IR irradiation phase following the W irradiation phase,
irradiation may be performed in succession in the order IR,
UV, IR irradiation, or the IR irradiation phase may extend
over the entire irradiation time, i.e. IR irradiation is
performed before, during and after the UV irradiation
phase.
The IR irradiation phase and subsequent UV irradiation
phase may, if required, also be repeated several times in
succession.
In either of the stated embodiments, the duration of
irradiation per irradiation period and the overall duration
of irradiation may be varied.
It is furthermore also possible to apply the associated IR
and UV irradiation periods in conjunction with the
performance of two or more spray passes, two or more
operations or in conjunction with the radiation curing of
two or more successive layers of the multilayer structure.
For example, once the at least partially radiation curable
coating composition has been applied in one spray pass,
intermediate hardening may be performed first with IR
irradiation and subsequent UV irradiation, whereafter the
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coating composition is applied in one or more further spray
passes and then first IR irradiation and subsequently W
irradiation are performed in turn. This mode of operation
is particularly advantageous for applying surfacer layer
films which are desired in relatively high thicknesses, for
example of up to 400 m.
It is also possible in the multilayer structure first to
apply an at least partially radiation curable base lacquer
and to subject it first to IR and then UV irradiation. An
at least partially radiation curable clear lacquer may then
be applied and in turn subjected first to IR and then UV
irradiation. In both cases, further IR irradiation may
optionally be performed after the UV irradiation. Radiation
curing of the individual layers of the multilayer structure
and of the layers applied by means of two or more spray
passes may here be performed in each case with differing
radiation intensity and differing duration of irradiation
for each layer individually or jointly for two or more
layers.
The lacquer-coated substrate surfaces may, for example, be
irradiated according to the invention by positioning IR
light sources and UV light sources next to each other and
to switch them appropriately or optionally to position the
light sources alternately in front of the substrate surface
to be irradiated. It is also possible to use a so-called
combined light source which contains the IR and UV
radiation source in a single device. In the latter case, IR
and UV lamps may, for example, be arranged alternately next
to each other in the device.
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One or more layers at least partially curable by means of
high-energy radiation of a conventional multilayer
structure in automotive lacquer coating may be cured using
the process according to the invention. Said layers may,
for example, comprise a multilayer structure comprising
primer, surfacer, base lacquer and clear lacquer or
comprising primer, surfacer and single-layer topcoat
lacquer. One or more layers of the multilayer structure may
here be prepared from at least partially radiation curable
coating compositions.
Crack-free coatings having very good adhesion to the
substrate and very good interlayer adhesion are obtained by
the process according to the invention. The applied
coatings exhibit adequate sag resistance and, once cured,
perfect optical appearance. Resistance to chemicals and to
weathering is very good. The resultant coatings
simultaneously combine an elevated crosslink density with
adequate flexibility. Surfacer coatings produced using the
process according to the invention are very readily
sandable.
The following Examples are intended to illustrate the
invention in greater detail.
Example 1
A clear lacquer curable by means of UV radiation was first
produced. To this end, the following components were mixed
together and homogenised for several minutes using a high-
speed stirrer:
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55 g of a conventional commercial OH- and acryloyl-
functional binder (Jagalux 5154)
10 g of a conventional commercial polyisocyanate
(Desmodur N 75)
3.8 g of a conventional aryl phosphine oxide based
photoinitiator (Lucirin TPO)
0.5 g of a conventional commercial levelling agent
(Byketol OK)
2.5 g of butyl acetate.
Production of a multilayer structure
A surfacer layer (binder based on 2-component polyurethane,
solvent-based) was applied to a resultant dry film
thickness of approx. 80 m onto a cathodically
electrocoated metal sheet and, after a brief flashing-off
period at room temperature, cured for 30 minutes at 60 C.
An aqueous base lacquer (produced according to DE-A-196 43
802, production example 4) was applied to a resultant dry
film thickness of 13 to 15 m onto the surfacer layer.
After flashing-off for 20 minutes at room temperature, the
clear lacquer curable by means of UV radiation described
above was applied to a resultant dry film thickness of
40-50 m.
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After flashing-off for 5 minutes at room temperature, IR
irradiation of the applied clear lacquer was performed.
Irradiation time was 5 minutes. UV irradiation was then
performed with a W flash lamp (power 3500 Ws). Irradiation
5 was performed with 30 flashes, which were triggered at
approx. 4 s intervals, at a distance of approx. 20 cm from
the object.
Example 2
10 A similar method was used as in Example 1, except that the
UV irradiation was followed by a further IR irradiation (5
minutes' irradiation).
Comparative Example 1
15 A similar method was used as in Example 1, except that,
once the clear lacquer had been applied and after a
flashing-off phase of 30 minutes at room temperature, W
irradiation was performed directly with a W flash lamp
(power 3500 Ws). UV irradiation was performed with 30
20 flashes, which were triggered at approx. 4 s intervals, at
a distance of approx. 20 cm from the object.
30
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Comparison of resultant lacquer properties
Example Example Comparative
1 2 Example 1
Humid heat test (1) (2) 1/1 1/1 1/3
Adhesion (3) 0-1 1 1-2
Adhesion (3) after humid 2 2-3 3
heat test (1)
Optical properties OK OK slight
microtexture
(1) Humid heat test to DIN 50017
5(2) Evaluation of blistering to DIN 53209
(3) Crosshatching according to DIN 53151
