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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for producing a multi-layer lacquer
coating with a mechanically stable quick-drying clear-lacquer coating
based on systems curable by radiation.
2. Description of Related Art
Coatings as applied in the series production of automobiles nowadays mostly
consist of a surface lacquer of basecoat and clear lacquer which is
applied to bodywork that has been electrophoretically primed and coated
with filler. In this process basecoat and clear lacquer are preferably
applied wet-on-wet, i.e. after a flash-off period optionally subject to
heating. After subsequent application of a clear lacquer the basecoat is
stoved together with this lacquer, as described for example in EP-A-38 127
and EP-A-402 772. In this connection suitable clear lacquers are
described, for example, in EP-A-38 127 and EP-A-184 761. The stoving
process in industrial production lacquering requires long drying phases,
and naturally a certain time passes before the lacquer is no longer tacky,
so that special measures have to be taken in order to avoid incorporating
dust in the surface.
Both in the case of the use of one-component (1C) and also of two-component
(2C) clear lacquers the lacquering process is associated with emissions of
environmentally harmful solvents or dissociation products of the
crosslinking reaction. In the case for example of isocyanate-crosslinking
2C clear lacquers, e.g. according to DE-OS 33 22 037 or DE-PS 36 00 425,
recycling of overspray is by its nature not possible.
In JP-A-6213 2570 clear UV lacquers are described which serve to protect
electrical instruments used in domestic appliances and in the automobile
industry. They are applied in a thin film; multiple precoating does not
take place.
In EP-A-0 118 705 and GB-A-2 226 566 UV-curable layers are described for
protecting automobile underbodies from the impact of stones. The layers
are applied with a thickness of up to 1500 .mu.m. They are formulated so
as to be soft and elastic and are not capable of being ground.
In EP-A-0 247 563 coatings are described which by way of surface lacquer
have a coating which in addition to an isocyanate-hydroxyl-group
crosslinking reaction is also subjected to crosslinking by UV radiation.
The overspray accruing during application of the coating agent can in view
of the chemical reaction no longer be subjected to recycling.
SUMMARY OF THE INVENTION
The object of the invention is to make available a lacquering process for a
multi-layer lacquer coating, in particular for the automobile industry, in
which a clear lacquer enabling fast crosslinking is used as
surface-lacquer coating, in which process the overspray following
application can be recycled, and in which a shiny or matt, hard and clear
surface lacquer is produced by way of substrate coating.
It has been shown that this aim can be achieved by a process for producing
a multi-layer lacquer coating in which a liquid clear lacquer which can be
crosslinked exclusively by radicalic and/or cationic polymerization is
applied to a previously dried basecoat layer. Application of the clear
lacquer is effected while daylight is screened off, optionally during
illumination with visible light having a wavelength of over 550 nm. The
overspray accruing during application of the clear lacquer is collected
and can optionally be re-used for spraying after recycling. Curing of the
clear-lacquer layer is subsequently effected by irradiation with
high-energy radiation or is initiated by irradiation with high-energy
radiation.
An advantage of the process according to the invention consists in the fact
that substrates which are sensitive to temperature can also be provided
with a durable layer of surface lacquer. In addition, as a result of short
reaction and drying times, pollution of the freshly lacquered surface can
be avoided. The surfaces obtained in this way have good optical
characteristics and a high degree of resistance to scratching.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The lacquer systems which can be used according to the invention all make
use of coating agents which are curable by radiation and which crosslink
exclusively as a result of radicalic or cationic polymerisation or
combinations thereof. Aqueous systems rich in solids and occurring as
emulsions constitute a preferred embodiment of the invention. But coating
agents containing solvents can also be used. Particularly preferred are
100% lacquer systems which can be applied without solvent and without
water. The clear lacquers curable by radiation can be formulated as
unpigmented or transparently pigmented surface lacquers, optionally
coloured with soluble dyestuffs.
The clear-lacquer coatings can be applied to conventional basecoats. These
may contain solvents or be of an aqueous or powdery type. The basecoats
contain conventional physically drying and/or chemically crosslinking
binding agents, inorganic and/or organic colouring pigments and/or
pigments producing special effects, such as metallic pigments or those
giving a pearly-lustre, as well as other auxiliary substances which are
customary in lacquering, such as catalysts, levelling agents or
anti-cratering agents. These basecoats are applied to conventional
substrates either direct or on pre-coated substrates. Prior to application
of the basecoat the substrates can, for example, be provided with
conventional primer, filler and intermediate layers such as are customary
for, e.g., multi-layer lacquer coatings in the automobile industry. Metal
or plastic parts are suitable as substrates.
Prior to coating with radiation-curable lacquers the layers of primer are
dried or stoved under such conditions as to ensure that they only contain
small amounts of volatile substances. In particular, at the time of the
radiation-induced crosslinking reaction of the applied layer of
clear-lacquer coating, substantial amounts of volatile components should
no longer be present in the basecoat layer. Such components can impair
gloss and adhesion in the clear-lacquer film. Drying of the basecoat layer
can be effected at room temperature or at temperatures up to 150.degree.
C. This does not exclude the possibility of a chemical crosslinking
reaction.
In the particularly preferred case of solvent-free radiation-curable
clear-lacquer systems, the process according to the invention enables a
particularly good metal effect to be achieved on metallic basecoats by way
of basecoat layer.
After application and drying of the basecoat the workpiece is provided with
the radiation-curable surface lacquer. Until the workpiece is discharged
from the coating unit the coating process is carried out subject to
illumination with visible light having a wavelength of over 550 nm or
subject to the exclusion of light. To this end necessary measures for
screening off other light sources are optionally employed, e.g. light
traps at the entrances and exits of the lacquering plant, filters in front
of light sources or measures for preventing reflection. The only light
sources used have an emission spectrum starting at above 550 nm. Such
sources are, e.g., lamps provided with UV filters or yellow filters.
Illumination optionally also can be effected from outside by the use of
windows. During stages of the process which run automatically and need no
optical control it is of course possible to proceed subject to the
exclusion of light, so that the above-stated light sources only have to be
switched on if a fault occurs. In the case of pure electron-ray curing
with suitable lacquer systems work can also proceed under normal lighting
conditions.
Application of the radiation-curable lacquer can be carried out by all
conventional spray-application methods, such as, e.g., compressed-air
spraying, airless spraying, high-speed rotation, electrostatic spray
application (ESTA), optionally coupled with hot-spray application such as
hot-air spraying, at temperatures not exceeding 70.degree.-80.degree. C.
In this manner suitable application viscosities are achieved and no change
occurs in either the lacquer material or the overspray to be recycled
during the short time that the thermal treatment is applied. In this way
hot spraying can be organized in such a way that the lacquer material is
only heated for a short time in the spray jet or a short distance upstream
of it.
The spraying booth may optionally be a circulation-type booth of adjustable
temperature, operated with an absorption medium suitable for the
overspray, e.g. the lacquer material. The spraying booth consists of
materials which preclude the possibility of contamination of the material
to be recycled and which are not affected by the circulating medium.
Examples are high-grade steel or suitable plastics.
By avoiding light with a wavelength below 550 nm the lacquer material used
and the overspray are not affected. This enables direct reprocessing. The
recycling unit essentially comprises a filtration unit and a mixing device
which maintains an adjustable ratio of fresh lacquer material to be
reprocessed and optionally circulates lacquer material. In addition,
storage containers and pumps as well as control devices are present. When
non-100% lacquer material is used a mixing device is necessary for
maintaining a constant level of volatile components such as organic
solvent components or water.
Application is performed in such a manner that dry layer thicknesses of
preferably 10-80 .mu.m, and in particular 30-60 .mu.m, are achieved.
Application of the clear lacquer can optionally be effected in several
layers.
After application of the clear-lacquer coating agent the coated substrate
is optionally subjected after a rest period to the crosslinking process.
The rest period serves for example to enable levelling, degassing of the
lacquer film or evaporation of volatile components such as solvents, water
or CO.sub.2 if the lacquer material has been applied using supercritical
carbon dioxide as solvent, as described for example in EP-A-321 607. It
can optionally also be supported by increased temperatures of up to
80.degree. C., and preferably up to 60.degree. C.
The actual radiation-curing process can be carried out either by UV
radiation or electron-ray radiation or with actinic radiation emitted from
other radiation sources. In the case of electron-ray radiation it is
preferable to work in an atmosphere of inert gas. This can be achieved for
example by supplying CO.sub.2, N.sub.2 or a mixture of both directly to
the surface of the substrate.
Use may also be made of an atmosphere of inert gas in the case of UV
curing. If a protective gas is not used, ozone may be generated. This can,
for example, be extracted by suction.
Preferred radiation sources are UV emitters or electron-ray sources. UV
radiation sources having emissions in the wavelength range 180-420 nm, and
preferably 200-400 nm, are, for example:
optionally doped high-pressure, medium-pressure and low-pressure mercury
emitters, gas discharge tubes such as low-pressure xenon lamps, pulsed and
unpulsed UV lasers, and UV spot-type emitters such as UV-emitting diodes.
Particularly suitable radiation sources emitting in the longwave UV
spectrum are so-called black-light tubes. Measures can optionally be taken
to counter the heat of the radiation source, e.g. by cooling with water or
air.
Cathode-ray sources include, spot-type emitters working according to the
electron-ray principle (i.e., made by Polymerphysik, Tubingen) or linear
cathodes which work according to the Electrocurtain.RTM. principle (i.e.,
made by Energie Science Inc). They have a radiation output of 100 keV to 1
MeV. Combinations of these radiation sources are also possible.
Both the electron sources and the UV radiation sources can also be designed
to work discontinuously. Particularly suitable then are laser light
sources or electron sources. Another possibility with regard to provision
of UV sources capable of being rapidly switched on and off (pulsed
operation) includes interposing, e.g., moveable shutters.
By way of auxiliary units, conventional light-control systems may be used
which are customary in the sphere of optics technology, such as absorption
filters, reflectors, mirrors, lens systems or light-wave conductors.
According to the invention irradiation can be carried out in such a way as
to ensure that thorough crosslinking of the layer of clear lacquer is
effected in one step. It can however also be advantageous to bring about a
prior gelling of the coating film by UV-induced crosslinking, e.g. in a
first zone in which black-light irradiation takes place, and then to
continue crosslinking in a second step or several steps, for example by
renewed UV irradiation or by irradiation with electron rays.
The arrangement of the radiation source is in principle well-known and can
be adjusted to suit the conditions of the workpiece and the parameters of
the process.
For example, the workpiece can be irradiated as a whole, or a radiation
curtain can be used which moves in relation to the workpiece. In addition,
by the use of an automatic device a spot-type radiation source can be
passed over the substrate to initiate the crosslinking process. In order
to achieve a crosslinking reaction on all sides of the workpiece, movement
of the substrate in front of the radiation sources about the longitudinal
or transverse axes is also possible.
The distance of the radiation source can be fixed or it can be adapted to a
desired value according to the form of the substrate. The distances of the
radiation sources from the wet-lacquer surface preferably lie in the range
from 2 to 25 cm, and in particular 5-10 cm. If a UV laser is used, a
greater distance is possible.
Of course, the process steps listed as examples can also be combined. This
can be effected in a single stage of the process or in process stages
temporally or spatially separated from one another.
The duration of irradiation lies for example in the range from 0.1 seconds
to 30 minutes, according to lacquer system and radiation source. A
duration of less than 5 minutes is preferred. The duration of irradiation
is chosen in such a way as to achieve total curing so the formation of the
required technological characteristics is ensured.
The process according to the invention can be used to particular advantage
in the production of multi-layer lacquer coatings in the automobile
industry, e.g. in the manufacture of car bodies or their parts.
A problem with the coating of automobile bodies with radiation-curable
lacquer systems lies in the curing of areas not directly accessible to
radiation such as shadow zones, e.g. cavities, folds and other undercuts
resulting from manufacture. This problem can be solved by, e.g., using
spot-type, small-area or omnidirectional emitters with an automatic
movement device directed to irradiating interiors, engine compartments,
cavities or edges.
It is also possible to apply a thermal activation in order to bring about
crosslinking of the coating agent on surfaces which can not be subjected
to the radiation-crosslinking process adequately. When using coating
agents capable of polymerization in radicalic manner it can be
advantageous in this connection to use radical initiators which can be
activated thermally, so that subsequent to irradiation or simultaneously
with irradiation thermally activated radicalic polymerization can be
achieved. When using cationically polymerizing coating agents it is not
necessary to use special initiators which can be activated thermally. The
cationic polymerization initiated by the radiation energy also spreads to
the shadow zones, i.e., the unirradiated or only slightly irradiated
surfaces. It is however also advantageous in this case to apply heat in
order to support polymerization in the shadow zones.
According to the invention radiation-curable clear-lacquer coating agents
can be used which are well-known in principle and described in the
literature. This involves either systems which are curable in radicalic
manner, i.e. by the effect of radiation on the coating agent radicals are
formed which then trigger the crosslinking reaction, or systems which are
curable in cationic manner, in which by irradiation of initiators Lewis
acids are formed and serve to trigger the crosslinking reaction.
Systems which are curable in radicalic manner make use of, e.g.,
prepolymers, such as polymers or oligomers which have olefinic double
bonds in the molecule. These prepolymers can optionally be dissolved in
reactive diluents, i.e. reactive liquid monomers. In addition, coating
agents of this type can also contain conventional initiators,
light-ray-absorbing agents and, optionally, transparent pigments, soluble
dyestuffs and additional auxiliary lacquering agents.
Examples of prepolymers or oligomers are (meth)acrylic-functional
(meth)acrylic copolymers, epoxide resin (meth)acrylates which are free of
aromatic structural units, polyester(meth)acrylates,
polyether(meth)acrylates, polyurethane(meth)acrylates, unsaturated
polyesters, amino(meth)acrylates, melamine(meth)acrylates, unsaturated
polyurethanes or silicon(meth)acrylates. The molecular weight (number
average Mn) lies preferably in the range from 200 to 10000, and in
particular from 500 to 2000. Here and in the following, (meth)acrylate
denotes acrylate and/or methacrylate, and (meth)acrylic denotes acrylic
and/or methacrylic.
If reactive diluents are employed they are generally used in quantities
between 1 and 50% by weight, and preferably 5-30% by weight, relative to
the total weight of prepolymers and reactive diluents. They can be mono-,
di- or polyunsaturated. Examples of such reactive diluents are:
(meth)acrylic acid and its esters, maleic acid and its semi-esters, vinyl
acetate, vinyl ether, substituted vinyl carbamides, alkylene glycol
di(meth)acrylate, polyethylene glycol di(meth)acrylate, 1,3-butanediol
di(meth)acrylate, vinyl(meth)acrylate, allyl(meth)acrylate, glycerine
tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, styrene, vinyl
toluene, divinyl benzene, pentaerythritol tri(meth)acrylate,
pentaerythritol tetra(meth)acrylate, dipropylene glycol di(meth)acrylate
and hexanediol di(meth)acrylate, as well as mixtures thereof. They serve
to influence viscosity and technical lacquering characteristics, such as,
e.g., the crosslinking density.
Photoinitiators for systems curable in radicalic manner can, e.g., be used
in amounts from 0.1 to 5% by weight, and preferably 0.5-3% by weight,
relative to the total quantity of prepolymers which may be polymerized in
radicalic manner, in addition to the reactive diluents and initiators. It
is advantageous if their absorption range is within 260-450 nm. Examples
of photoinitiators are benzoin and derivatives, benzil and derivatives,
benzophenone and derivatives, acetophenone and derivatives, e.g.,
2,2-diethoxyacetophenone, thioxanthone and derivatives, anthraquinone,
1-benzoylcyclohexanol, and organophosphorus compounds such as
acylphosphine oxide. The photoinitiators can be used on their own or in
combination. In addition, other synergistic components, e.g. tertiary
amines, can be used.
In addition to the photoinitiators, conventional photosensitisers such as
anthracene can also be used, if necessary, in the usual quantities, for
example with a view to irradiation with black-light tubes. Additionally,
radicalic initiators which can be activated thermally can optionally be
used, so that between 80.degree. and 120.degree. C., radicals are formed
which then start the crosslinking reaction. Examples of thermolabile
radicalic initiators are: organic peroxides, organic azo compounds or
C-C-dissociating initiators such as dialkyl peroxides, peroxocarboxylic
acids, peroxodicarbonates, peroxide esters, hydroperoxides, ketone
peroxides, azodinitriles or benzpinacolsilyl ethers. C-C-dissociating
initiators are particularly preferred, since with thermal dissociation no
gaseous reaction products are formed which can cause faults in the lacquer
coating. The preferred quantities to be used are between 0.1 and 5% by
weight relative to the total quantity of prepolymers which may be
polymerized in radicalic manner, in addition to the reactive diluents and
initiators. The initiators can also be used in a mixture.
Binding agents for cationically polymerizing coating agents are for example
polyfunctional epoxy oligomers which contain more than two epoxy groups in
the molecule. It is advantageous if the binding agents are free from
aromatic structures. Such epoxy oligomers are, for example, described in
DE-OS 36 15 790. They are, for example, polyalkylene glycol diglycidyl
ethers, hydrated bisphenol-A glycidyl ethers, epoxy urethane resins,
glycerine triglycidyl ethers, diglycidylhexahydrophthalate, diglycidyl
esters of dimeric acids, epoxidated derivatives of (methyl)cyclohexene
such as 3,4-epoxycyclohexyl-methyl-(3,4-epoxycyclohexane)carboxylate or
epoxidated polybutadiene. The number average molecular weight of the
polyepoxide compounds preferably lies below 10000.
If low viscosities are needed for application they can be adjusted by the
use of reactive diluents, i.e. reactive liquid compounds such as
cyclohexene oxide, butene oxide, butanediol diglycidyl ether or hexanediol
diglycidyl ether. Examples of additional reactive solvents are alcohols,
polyalkylene glycols, polyalcohols, hydroxy-functional polymers, cyclic
carbonates or water. These can also contain solid constituents in
solution, for example solid polyalcohols such as trimethylolpropane.
Photoinitiators for cationically curable systems are used in amounts from
0.5 to 5% by weight, on their own or in combination, relative to the total
quantity of cationically polymerisable prepolymers, reactive diluents and
initiators. There are substances known as onium salts which when
irradiated give rise photolytically to Lewis acids. Examples are diazonium
salts, sulfonium salts or iodine onium salts. Particularly preferred are
triarylsulfonium salts.
Non-reactive solvents for systems which are curable in radicalic and
cationic manner are conventional lacquer solvents such as esters, ethers,
and ketones, for example butyl acetate, ethylene glycol ether, methylethyl
ketone, and methylisobutylketone, as well as aromatic hydrocarbons. For
systems which are to be polymerized in radicalic manner C.sub.2 -C.sub.4
-alkanols, and preferably water, are also suitable as solvents.
The clear lacquers used according to the invention preferably have
light-ray-absorbing agents added to them. Examples of these are phenyl
salicilates, benzotriazole and derivatives, and HALS compounds, as well as
oxalanilide derivatives, optionally also used in combination. Customary
concentrations amount to 0.5 to 5% by weight, preferably 1-2% by weight,
relative to the total quantity of clear lacquer. When choosing the
light-ray-absorbing agent, attention must be given to ensuring that the
initiation of crosslinking is not impaired by the light-ray-absorbing
agent and that such agents that are used are stable when irradiated during
the radiation-curing process.
Further additives may include, for example, elastifying agents,
polymerisation inhibitors, defoamers, levelling agents, anti-oxidation
agents, transparent dyestuffs or optical brightening agents.
Transparent colourless fillers and/or pigments can optionally be added to
the coating agent. The amount used is up to 10% by weight, relative to the
total amount of clear lacquer. Examples are silicon dioxide, mica,
magnesium oxide, titanium dioxide or barium sulphate. The size of the
particles preferably lies below 200 nm. With UV-curable systems attention
should be given to ensuring that the coating film in the layer thickness
used remains transparent to UV radiation. Additional useable additives
are, for example, conventional inorganic or organic delustering agents.
These can be added in conventional amounts, for example up to 10% by
weight. Examples of delustering agents are silicates, pyrogenic silicic
acids such as aerosil, bentone or condensed and crosslinked urea
formaldehyde resins, and natural and synthetic waxes. The particle sizes
of such delustering agents lie generally in a range up to 100 .mu.m, and
preferably up to 30 .mu.m.
The stages of the process for producing suitable radiation-curable
clear-lacquer coating agents are well-known. It is possible to combine
systems with different radiation-induced chemical crosslinking mechanisms.
These can be various crosslinking systems curable in radicalic manner, or
cationically curable crosslinking systems, or radically and cationically
curable crosslinking combined with one another. Attention should be given
to choosing the composition in such a way as to ensure long storage life.
Likewise different reaction-initiating processes can be combined, for
example UV with UV curing, UV with thermal initiation or electron-ray
curing with UV curing.
The various crosslinking reactions can be started with mixtures of suitable
initiators. For example, mixtures of UV initiators with differing maximum
absorption characteristics are possible. In this way various emission
maxima of one or several radiation sources can be utilized. This can be
effected simultaneously or in sequence. For example, curing can be
initiated with radiation from one radiation source and continued with that
from another. The reaction can then be carried out in two or more stages,
and separated spatially if desired. The radiation sources used can be the
same or different.
According to the invention it is possible to carry out firstly a
radiation-induced, and either sequentially or simultaneously, a thermally
induced crosslinking reaction. To this end, and in addition to one or
several photoinitiators, one or several thermally dissociating initiators
can optionally be used. The use of photoinitiators is not necessary when
curing by electron rays.
Two- or multi-stage operation can be advantageous, in order, for example,
to achieve initial gelling, whereby for example runs on lacquered vertical
surfaces can be avoided. Gelling is also advantageous in the case of
solvent-based systems to allow evaporation of the solvent.
The photoinitiators are preferably chosen in such a way that they do not
decay in light having a wavelength of over 550 nm. With the use of
thermally dissociating initiators these should be chosen in such a way as
to ensure that they do not decay under the conditions of application of
the lacquer material. In this way it is possible to recycle the overspray
of the coating agent directly and to re-use it, since a chemical reaction
is avoided during application.
The crosslinking density of the lacquer films can be adjusted by the
functionality of the components of the binding agent employed. The choice
can be made in such a way as to ensure that the crosslinked clear-lacquer
coating has sufficient hardness and that too high a degree of crosslinking
is avoided, in order to prevent the film from becoming too brittle.
By means of the process according to the invention multi-layer coatings are
obtained which constitute a clear-lacquer surface with high resistance to
scratching and also a high degree of gloss, as well as a high degree of
mechanical durability. As a result of the process parameters and the
chosen crosslinking mechanism, the overspray of the coating agent to be
applied likewise can be made available for immediate re-use. The process
according to the invention is particularly suitable for use in series
production lacquering in the automobile industry; for example, for the
lacquering of car bodies and their parts.
In all the examples described below, application of the radiation-curable
clear lacquers was performed in a room illuminated exclusively by
red-light sources (light wavelength greater than 600 nm).
EXAMPLE 1
By mixing the following components a radiation-curable clear-lacquer
coating agent was formed:
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% by weight
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44.5 Novacure 3200
(aliphatic epoxy acrylate made by
Interorgane)
32.2 Ebecryl 264
(aliphatic urethane acrylate made by
UCB)
3.0 Irgacure 184
(photoinitiator made by CIBA)
10.0 dipropylene glycol
diacrylate
10.0 trimethylolpropane
triacrylate
0.3 Ebecryl 350
(silicon acrylate made by UCB)
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Subsequently a lacquer structure was produced as follows:
A metal plate with a primer composed of KTL (20 .mu.m) and pre-coated with
filler which is customary in the trade (35 .mu.m) was coated in one case
with conventional water-based lacquer, in a second case with
solvent-containing basecoat (15 .mu.m dry layer thickness), and then in
both cases stoved for 20 min at 140.degree. C. Subsequently the above
lacquer system was applied with a layer thickness of 35 .mu.m.
Given a belt velocity of 9 m/min, curing of the horizontal metal test plate
was effected by irradiation by two medium-pressure mercury emitters, each
of which having an output of 100 W/cm and placed at a distance of 10 cm
from the surface to be cured (duration of irradiation 1-2 sec). A shiny
and hard surface with good adhesion was obtained on both the aqueous
basecoat and the conventional basecoat.
EXAMPLE 2
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% by weight
______________________________________
40.5 Novacure 3200
27.5 Ebecryl 264
2.0 C--C-dissociating initiator
(tetraphenylethane derivative according to DE-A-1 219 224)
2.0 Irgacure 184
10.0 dipropylene glycol diacrylate
10.0 tripropylene glycol diacrylate
0.3 Ebecryl 350
7.7 vinyl toluene
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A metal test plate was produced in a similar way to that described in
Example 1. In this case, however, the test plate was coated on both sides,
and after application of the above radiation-curable clear lacquer it was
irradiated on just one side while freely suspended, the side to be
irradiated being moved evenly, at a distance of 10 cm within 5 sec, past a
medium-pressure mercury emitter as stated in Example 1.
The tacky rear side which was only partially crosslinked by irradiation was
stoved for 15 min at 110.degree. C. in an air-circulating furnace.
Surfaces were obtained on both sides of the metal test plate with
characteristics as described in Example 1.
EXAMPLE 3
(radiation-induced cationically curable clear lacquer)
______________________________________
% by weight
______________________________________
60.0 Degacure K 126
(cycloaliphatic epoxide made by
DEGUSSA)
25.0 Araldit DY 026
(hexanediol diglycidyl ether made by
CIBA)
4.5 Degacure KI 85
(sulfonium salt made by DEGUSSA)
0.5 Dynasilan Glymo
(glycidyl-functional silane made by
Dynamit Nobel)
10.0 cyclohexanol
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With this formulation the procedure was completely analogous to that in
Example 1. A similar lacquered surface was obtained.
EXAMPLE 4
Example 1 was repeated, with the same lacquer result. The only difference
being that the basecoat layers here were stoved for 30 min at 120.degree.
C. and pre-coated polycarbonate sheets were used.
EXAMPLE 5
To 100 parts of the clear-lacquer coating agent from Example 1, two parts
of anthracene were added as photosensitiser. Application was effected as
described in Example 1. Then irradiation was effected at a belt velocity
of 1 m/min, lying flat, with 10 black-light tubes at a distance of 10 cm
from the wet-lacquer surface (duration of irradiation 90-120 sec). A
tacky, partially crosslinked surface was obtained. The metal test plate
was suspended for 5 min and then, hanging free, irradiated, the still
tacky surface being moved uniformly, at a distance of 10 cm within 5 sec,
past a medium-pressure mercury emitter as stated in Example 1. A lacquer
result as stated in Example 1 was obtained. The surface was free from
runs.
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