Note: Descriptions are shown in the official language in which they were submitted.
CA 02482325 2004-10-26
Process and apparatus for curing a
radiation-curable coating, and an irradiation chamber
Description
The present invention relates to a process for curing a
radiation-curable coating according to the preamble of
claim 1 as well as to an apparatus for carrying out
this process according to the preamble of claim 12 and
to an irradiation chamber provided in such an
apparatus.
Such irradiation chambers are part of an apparatus for
curing radiation-curable coatings. The irradiation
chamber is provided with one or more W radiation
sources. It is possible thereby, in particular to treat
two-dimensional or three-dimensional substrates by
providing them with a radiation-curable coating that is
then cured by means of W radiation.
It is known to cure radiation-curable coatings by means
of high-energy W radiation, for example by using
medium-pressure mercury radiators or W Excimer
radiators (R. Mehnert et al., W & EB Technology and
Application, SITA-Valley, London 1998). The specific
electric power of these radiators is typically between
50 and 240 W per cm radiator length. For a radiator
length of 1 m, the converted electric power is thus
between 5 and 24 kW. These powerful radiators are used
chiefly for curing coatings on planar substrates.
Typical illuminances of 100 to 1000 mW/cm2 are measured
on the layer to be cured. It is possible thereby to
achieve curing times of 100 ms and less . Such a system
is known, for example, from DE 24 25 217 Al. A
comparable apparatus is also known, for example, from
WO 96/34700 A1 and FR 2 230 831 A1.
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It is to be borne in mind when applying medium-pressure
mercury radiators that approximately 50% of the
electric power is converted into heat. An arrangement
5 in which radiators of this type are situated close to
one another fails not only for reasons of overheating,
but also because of the high-voltage supply required at
the ends (electrodes) of the radiators. Admittedly, in
the case of W Excimer radiators the heat is dissipated
10 by cooling the lamp surface, but the distance between
neighboring tubes and their geometric arrangement is
likewise limited by the required high-voltage supply.
Because of the biological effects of W rays, extensive
15 screening measures and other protective measures are
required when use is made of these W radiators. In
order to cure coatings on three-dimensional objects,
individual W radiators are, for example, fitted in
closed spaces such that it is possible to ensure
20 adequate radiation protection. An adequate homogeneous
irradiation of the coatings to be cured on three-
dimensional substrates is, however, impossible in
practice. The energy outlay for curing is therefore
determined by the outlay for curing layer regions, that
25 can be achieved only by obliquely incident radiation or
scattered radiation.
A further problem arises, in particular, with the
polymerization and crosslinking of radically curing
3o compounds such as, for example, monomers and oligomers
of acrylates or methacrylates. Radical polymerizations
are inhibited by oxygen due to the action of oxygen as
a radical interceptor. Oxygen can prevent both the
initiation and the chain propagation, and also the
35 crosslinking. This takes place principally at the
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interface between air and coating at the region of a
depth of a few Vim. Here, during the irradiation
operation the consumption of oxygen that takes place in
the layer owing to the reaction of radicals with oxygen
5 can be balanced by rapid rediffusion from the air. The
consequence of this process is incomplete curing at the
surface which leads to sticky, completely useless
coatings.
10 This inhibition effect can be reduced or avoided by
measures such as covering the surface to be cured with
waxes or films, by using very high dosing powers (>2000
mW/cm2) during irradiation, by raising the photo
initiator concentration, and by using special amines as
15 co initiators (R. Mehnert et a.: UV&EB Curing
Technology, SITA-Wiley, London 1998).
The most frequently applied measure for avoiding
inhibition is, however, to carry out the radiation
20 curing in an oxygen-reduced protective gas atmosphere.
The literature describes apparatuses and processes in
which nitrogen (for example EP 540 884 A1) or carbon
dioxide (DE 199 57 900 A1) are used as protective gas.
The reduction of the residual oxygen concentration in
25 the irradiation chamber is decisive for the avoidance
of inhibition of the radiation curing by oxygen.
Inhibition of the surface of the coating is reliably
avoided during radiation curing whenever the number of
the initiator radicals, produced by the irradiation,
30 for polymerization and crosslinking during the entire
irradiation operation clearly exceeds the number of the
radicals deactivated by the reaction with oxygen.
Since the concentration of the initiator radicals in
35 the coating is directly proportional to the dosing
power (in mW/cmz) of the radiation source, the upper
boundary of the oxygen concentration required in the
inert gas to avoid inhibition reliably is determined by
the dosing power of the radiation source.
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German patent application DE 102 42 719.4 discloses an
apparatus in the case of which a plurality of W
radiation sources are arranged close to one another and
S interconnected to form one or more irradiation modules,
the illuminance inside an irradiation module and/or
between at least two irradiation modules being
spatially variable. This apparatus is therefore built
up from geometrically suitable arrangements of a
10 plurality of radiation sources situated close to one
another. Each of these arrangements is denoted as an
irradiation module. Thus, an irradiation module is
understood here as a planar arrangement of radiation
sources arranged close next to one another (for example
15 with a common electric supply) . The enveloping surface
of the radiation sources of each module can be flat or
curved. It is possible to construct irradiation modules
that focus light into a selected, including curved,
irradiation plane, and which permit the substrate
20 surfaces to be irradiated in a geometrically largely
homogeneous fashion.
The construction is therefore performed in such a way
that a spatially variable illuminance is set up in the
25 interior of the irradiation chamber, in which the
radiation-curable coatings are cured, such that the
coating to be cured is cured homogeneously without
there being a disturbing input of heat into the coating
and/or substrate. The variation can be performed, on
30 the one hand, by setting the enveloping surfaces of the
radiation sources of a single module and, on the other
hand, by the spatial arrangement of the irradiation
modules relative to one another in the apparatus, it
being possible to realize a multiplicity of geometric
35 arrangements. Owing to the modular construction, the
apparatus can thus be adapted to the geometry of the
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substrate to be treated such that the energy outlay is
reduced. The radiation modules can therefore have
fluorescent tubes with a specific electric power of,
for example, 1 W/cm. The dosing power of these
irradiation units is therefore only between 10 and 100
mW/cmz. This also has the consequence that biological
radiation protection is simplified, that is to say may
be limited, for example to measures which apply to the
use of tanning lamps.
In order, however, to be able to use the advantages,
named in this patent application, of these irradiation
modules for radiation curing of coatings such as, for
example, reduced energy outlay for curing, low thermal
input into the substrate and the coating, as well as
simple radiation protection, it is necessary to provide
apparatuses that produce a residual oxygen
concentration of preferably below 500 ppm in the
corresponding irradiation chamber during the
irradiation operation.
This concentration is also to be maintained during the
continuous passage of substrates to be coated, which
are fed at short spaces over lengthy times by means of
a conveyor unit. However, during continuous conveyance
of the substrates air is continuously entrained into
the inert gas by turbulence. The tolerable oxygen
concentration in the inert gas can thereby be exceeded
rapidly.
It is therefore the object of the present invention to
provide an apparatus that permits an adequate
inertization of irradiation chambers which are, for
example, equipped - although not necessarily - with
fluorescent tube modules even if coated substrates are
continuously conveyed through the irradiation chamber
at short spaces.
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The solution consists of a method having the features
of claim 1, and of an apparatus having the features of
claim 12, and of an irradiation chamber having the
features of claim 8.
Thus, it is provided according to the invention that
the substrate is guided through a closed channel that
is formed by an inlet chamber, the irradiation chamber
and an outlet chamber, and that an inert gas is fed
10 into the irradiation chamber in such a way that an at
least slight inert gas overpressure is produced in the
irradiation chamber.
The apparatus according to the invention is
distinguished in that the irradiation chamber is
assigned an inlet chamber and an outlet chamber, the
irradiation chamber, inlet chamber and outlet chamber
forming a closed channel, and wherein the irradiation
chamber has an apparatus for feeding in inert gas which
20 produces an at least slight inert gas overpressure in
the irradiation chamber.
The subject matter of the present invention is also an
irradiation chamber having a frame inside which an
25 irradiation space is arranged whose walls are
transparent to W light and in which an apparatus for
feeding in inert gas is provided.
The apparatus for feeding in inert gas serves for
30 filling the volume of the chambers with inert gas by
displacing air. The substrates can pass continuously in
this case through the irradiation chamber at short
spaces. The low concentration of residual oxygen
required for radiation curing is achieved by
35 continuously feeding inert gas into the irradiation
chamber such that an inert gas volumetric flow is
produced in the direction of the inlet chamber and/or
of the outlet chamber. The inert gas must be fed in
such that the required minimum oxygen concentration in
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the irradiation chamber is maintained during the
process of conveying the substrates_ No undesired
suction of air may be allowed to happen. For this
purpose, an at least slight overpressure is set up in
5 the irradiation chamber by appropriately feeding in
inert gas. The effect of this is that the inert gas
flows out continuously in the direction of the inlet
chamber and of the outlet chamber. The outflowing inert
gas therefore removes from the closed channel the
atmospheric oxygen entrained by the conveyance of a
substrate.
Advantageous developments follow from the subclaims.
15 It is possible for the irradiation chamber to be
composed of a plurality of elements interconnected in
an airtight fashion, that is to say to be of modular
configuration. The apparatus according to the invention
can therefore be designed for different capacities. If
20 the capacity is to be increased, the transit time of a
substrate through the irradiation chamber is usually
shortened in conjunction with a given length of the
irradiation chamber with increasing quantity of the
substrate to be coated. In order then to ensure the
25 required irradiation time for curing the substrate
surfaces, it is possible to use a longer irradiation
chamber with additional W light sources. By contrast,
it is advantageous to lengthen a modularly configured
irradiation chamber by one or more elements with W
30 light sources. This allows an improved flexibility for
the apparatus according to the invention with regard to
its capacity and permits a particularly economical
operation.
35 The same holds mutatis mutandis for the inlet chamber
and the outlet chamber.
A particularly preferred development provides that
inert gas is also fed either into the inlet chamber or
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into the outlet chamber, or into both in such a way
that an inert gas pressure is produced that is lower
than the inert gas pressure in the irradiation chamber.
The inflow of oxygen during the conveying operation of
the substrates is thereby additionally impeded.
Furthermore, the inert gas in the irradiation chamber
can be fed in as a volumetric flow running in the
conveying direction, and maintained. The orientation in
the conveying direction effects an additional
orientation of the inert gas in the direction of the
inlet or outlet chamber, in order to impede the
entrance of oxygen into the irradiation chamber.
In this case, the inert gas is preferably fed into the
irradiation chamber via an arrangement of one or more
tubes that are arranged parallel to the conveying
direction of the substrate and have outlet openings
which can, for example, be configured such that the
inert gas can flow out essentially with a low level of
turbulence. This is achieved, for example, by suitable
rows of nozzles and/or bores that are narrow and/or
arranged close to one another. Instead of tubes,
however, it is also possible, for example, to use
plates made from a porous material, such as sintered
plates, for example. The porous material is intended to
be permeable to the inert gas and preferably to have
small pores situated close to one another so that the
inert gas can flow out essentially without turbulence.
Another development provides also that the inlet
chamber or the outlet chamber, preferably however every
chamber has a dedicated apparatus for feeding in inert
gas. The flow rate of the feeding of inert gas is
advantageously set such that the inert gas pressure is
highest in the irradiation chamber. This ensures that
the inert gas flows continuously in the direction of
the inlet chamber and outlet chamber, that is to say in
the direction of both outlets of the closed channel.
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The volumetric inertization thus implemented can be
supplemented by feeding inert gas into the inlet
chamber and/or outlet chamber, the inert gas flow
thereof being overlaid at the side on the volumetric
flow running in the conveying direction. Consequently,
further apparatuses for feeding in inert gas can be
fitted on the walls of the inlet chamber and/or outlet
chamber perpendicular to the conveying direction of the
substrate. The flow rate of the inert gas, here flowing
out perpendicular to the conveying direction, is
preferably set such that an inert gas flow that removes
entrained air and displaces it into the outgoing
volumetric flow is applied to the substrate (which is
being let in or out) obliquely to the conveying
direction. In the outlet chamber, this way of feeding
in inert gas advantageously additionally prevents air
from being back-mixed into the irradiation chamber.
Use is made in general of inert gas volumetric flows of
15 to 1000 Nm3/h, preferably between 30 and 400 Nm3/h.
The flow rate of the inert gas volumetric flow fed in
perpendicular to the conveying direction is preferably
approximately 10 to 80% by volume of the entire inert
gas volumetric flow, preferably 15 to 60% by volume of
the volumetric flow. In a closed channel with a cross
section of 500 x 500 mm, for example, the flow rate for
the volumetric flow can typically be 200 Nm3/h, while
the flow rate of the inert gas fed in perpendicular to
the conveying direction is preferably approximately
25-50% by volume of the volumetric flow.
In order to keep the consumption of inert gas as low as
possible, it is not only possible to adapt the cross
section of the closed channel to the cross section of
the substrate to be coated, but the inlet and outlet of
the closed channel can be sealed, for example via
fitted plates, baffles or doors to such an extent that
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the substrate can pass and yet inert gas can still flow
out with a low level of turbulence.
As radiation sources, consideration is given to lamps,
preferably fluorescent tubes, of low electric power,
for example from 0,1 to 10 W per cm radiator length,
which have, for example, a continuous emission spectrum
between 200 and 450 nm, preferably between 300 and
450 nm. Since the development of heat is lower than in
the case of high-power UV radiators, it is sufficient
to cool merely the surface thereof, for example with
the aid of an air current.
Such lamps are known per se and are used, for example,
as tanning lamps in solaria. With a specific power of,
for example, 1 W per cm radiator length and the low
illuminance resulting therefrom, these lamps are not
suitable as such in and of themselves for technical
applications for curing radiation-curable coatings.
Such lamps, which are typically provided with
reflectors with emission angles of, for example,
approximately 160°, generally have standardized
dimensions (diameter of the tubes approximately 25 to
45 cm, luminance length up to approximately 200 cm) and
are operated at an operating voltage of 220 V, are very
well suited as radiation sources for the irradiation
modules mentioned. This relates, in particular, to the
reflectors that simplify focusing into the desired
irradiation plane. Also advantageous is their high
photon yield of approximately 30% of the electric
power.
At a distance of, for example, 10 cm from the radiation
source, irradiation modules of this design yield
illuminances of typically approximately 20 mW/cm2.
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Admittedly, these illuminances are smaller by a factor of
to 50 than those that can be achieved with conventional
W radiators, but they suffice to cure coatings given
radiation times of approximately 30 to 300 s.
5
A further advantageous development consists in that at
least one irradiation module is arranged in the
apparatus in a fashion capable of movement about at
least one of its three spatial axes. This facilitates
the geometric adaptation to the substrate and the
focusing of the rays in the desired radiation plane.
In order to improve the adhesion of radiation-cured
coatings on some substrates such as, for example,
15 polypropylene, polycarbonate and polyamide, it is
advantageous also to vary the illuminance temporally.
If the irradiation is begun with a low illuminance, for
example, the layer, which always shrinks during curing,
can relax more effectively than in the case of immediate
20 irradiation with a high illuminance. Stresses between the
layer to be cured and the substrate can be balanced out
more effectively. The consequence is a better adhesion of
the cured layer on the substrate. A temporal control of
the power of the individual irradiation modules is
25 possible in a simple way, and so it is possible to
exploit this advantageous irradiation regime.
Exemplary embodiments of the present invention are
explained in more detail below with the aid of the
30 attached drawings, in which:
figure la shows a schematic illustration, not true to
scale, of an embodiment of the irradiation
module according to the invention in the view
35 from below;
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figure lb shows the irradiation module from figure la
in a side view in accordance with arrow B;
5 figure lc shows the irradiation module from figure la
in a side view in accordance with arrow C;
figure 2 shows a section along the line II - II in
figure la;
figure 3 shows a schematic side view, not true to
scale, of an exemplary embodiment of the
apparatus according to the invention for
discontinuous irradiation;
figure 4 shows a schematic side view, not true to
scale, of an exemplary embodiment of the
apparatus according to the invention for
continuous irradiation;
figure 5 shows a longitudinal section through an
exemplary embodiment of an apparatus
according to the invention in a schematic
illustration, not true to scale;
figure 6 shows a longitudinal section through an
irradiation chamber of an apparatus according
to the invention;
30 figure 7 shows a section along the line vII-VII in
figure 6; and
figure 8 shows a longitudinal section through an inlet
or outlet chamber in an illustration as in
35 figure 7.
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The first step is to describe the structure of an
irradiation module 10 by way of example with the aid of
the exemplary embodiment illustrated in figures la, b,
c and 2. The components are mounted on a baseplate 11.
The baseplate 11 preferably consists of a metal such as
aluminum or steel, or of a metal alloy, and has on its
rear side the required electric terminals 13 and, if
appropriate, a holder 12. Furthermore, devices can be
provided there for installing the irradiation module 10
in irradiation systems and devices for moving the
irradiation module 10. Also mounted on the baseplate
are the starters and terminals for W radiation sources
18. Inlet and outlet for a ventilation system 16 of the
radiation sources 18 are also located here. Cross-flow
fans, for example, are suitable for this purpose
Also provided on the front side of the baseplate 11 is
a frame 14 inside which the ventilation system 16 and
the W radiation sources 18 are installed. Suitable W
radiation sources 18 are, for example, fluorescent
tubes such as are used as tanning lamps in solaria.
Such fluorescent tubes generally have standardized
dimensions, for example a luminance length of 2 m in
conjunction with a diameter of 25 to 45 cm. They can,
furthermore, be provided with reflectors that have an
emission angle of approximately 160°, for example.
These fluorescent tubes are operated at an operating
voltage of 220 V.
The frame 14 with the ventilation system 16 and the W
radiation sources 18 is surrounded on three sides in an
airtight fashion by a W-transparent plate 15, for
example made from plastic, such as, for example,
polymethyl methacrylate or polycarbonate. The surface
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of the plate 15 forms the front side of the irradiation
module 10, as illustrated by the arrow A symbolizing
the direction of radiation.
5 One or more irradiation modules 10 are installed in a
sealed irradiation vessel. The irradiation vessel
surrounds an irradiation chamber that is illuminated by
at least one irradiation module.
Figure 3 shows schematically an exemplary embodiment of
10 an apparatus 20 for discontinuous irradiation of
substrates. A rectangular container, provided with
supporting feet 21, of length 2.10 m, width 80 cm and
height 80 cm was equipped with four irradiation modules
10 of length 1.50 m and equipped with 10 fluorescent
15 tubes 18 arranged in a planar fashion. The irradiation
modules 10 were fastened to the frame of the container
on the base, the sides and the cover. The upper
irradiation module can be raised with the cover of the
container. The fluorescent tubes 18 in the irradiation
20 modules 10 were cooled by means of cross-flow fans.
The upper sides of the plates 15 of the irradiation
modules define and surround a rectangular irradiation
space 22 of length 1.60 m, width 60 cm and height
25 40 cm. Furthermore, four laterally arranged tubes 23
each having 40 bores for letting in nitrogen are
located in the irradiation space 22.
Such a device 20 can be operated as follows. The coated
30 substrates are introduced into the irradiation space
22. Thereafter, the irradiation space 22 is flooded
with inert gas. When an oxygen concentration of 5%,
preferably 1%, with particular preference 0.1%, is
reached, the irradiation is started, and it is
35 terminated after curing of the layer. The duration of
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the irradiation is typically approximately 30 to 300 s.
In this embodiment, the apparatus according to the
invention is particularly suitable for curing coatings
on substrates. It renders possible the application of
radiation curing, for example in the handicraft sector
for production and repair. The moderate electric supply
power of the modules, which is typically 1 to 2 kW, is
advantageous in this case.
In a test, a motor vehicle wheel rim as substrate was
coated on all sides with a radiation-curing spray
lacquer. The wheel rim was provided with a holder at
the valve hole and suspended in the irradiation space
22. After closure of the irradiation space 22, the
latter was flooded with nitrogen. The concentration of
the oxygen was measured with the aid of a sensor in the
irradiation space 22, and displayed. An oxygen
concentration of below 0.1% was achieved after flooding
for 2 minutes given a nitrogen current of 60 m3/h. Once
this value was reached, the nitrogen current was
reduced to 10 m3/h and irradiation was started. After an
irradiation time of 2 minutes, the nitrogen was
switched off and the apparatus 20 was opened. The
lacquer on the wheel rim was cured at all points and
could also not be damaged by manual pressure.
However, the irradiation modules 10 described can also
be used to construct an irradiation tunnel 30 as it is
illustrated diagrammatically in figure 4. In such an
irradiation tunnel 30, the irradiation modules 10 are
arranged on the sides and on the top side such that
they define and surround a tunnel-shaped irradiation
chamber 32. Coated substrates passing through via
conveying appliances, for example, can be cured therein
during the transit. If, for example, two irradiation
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modules are arranged in a row, the luminance length of
the irradiation chamber 32 can be up to 4 m. If the
curing is performed within approximately 30 to 300 s,
transit speeds of 0.8 to 8 m/min are possible. It is to
5 be borne in mind in this case that the residual oxygen
concentration should be sufficiently low during the
transit and the irradiation. The atmospheric oxygen
introduced into the irradiation zone by the movement of
the molded part to be irradiated should not exceed the
10 limiting value of 5%. Consequently, locks and/or
suitable nozzles are advantageously provided, chiefly
upstream of the irradiation zone, as seen in the
conveying direction, for the purpose of feeding in
inert gas, preferably nitrogen, which prevents the
15 entrainment of air.
In a continuation of this exemplary embodiment, the
subject matter of the present invention will be
explained below with the aid of figures 5 to 8.
Figure 5 shows schematically an exemplary embodiment of
an apparatus 40 according to the invention. This
apparatus comprises an irradiation chamber 50 that is
assigned an inlet chamber 60 and an outlet chamber 70.
25 The substrate to be coated is transported in a
direction of the arrow A by means of a conveying
device, for example a conveyor belt, firstly through
the inlet chamber 60, then through the irradiation
chamber 50 and, finally, through the outlet chamber 70.
30 The actual curing of the coating of the substrate by W
rays takes place in the irradiation chamber 50. The
inlet chamber 60, the irradiation chamber 50 and the
outlet chamber 70 form a channel closed to the outside.
The inlet and the outlet of the closed channel, that is
35 to say the end of the inlet chamber 60 and of the
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outlet chamber 70 averted in each case from the
irradiation chamber 50, can be sealed, for example, via
fitted flaps, doors or plates to such an extent that
the substrate can pass. It is thereby possible for less
inert gas to emerge to the outside, and so the
consumption of inert gas is reduced.
Figures 6 and 7 show schematically the structure of the
irradiation chamber 50. Just like the inlet chamber 60
and/or the outlet chamber 70, the irradiation chamber
50 can also be of modular configuration, that is to say
be composed of two or more elements that are preferably
of the same design, are interconnected in an airtight
fashion and form a closed channel. Proceeding from
figure 4, the frame of the irradiation chamber 50 is
now denoted by 51. Fitted inside the frame 51 is a
second frame 52 that is constructed, for example, from
plates that are preferably sealed from one another in
an airtight fashion and are transparent to W light. W
radiators 53 are arranged between the frames 51 and 52.
These radiators can, but need not necessarily, consist
of the irradiation modules described. The frame 52
stands, for example, inside the frame 51 on stilts 52'
that are reinforced with cross-struts 52 " .
The frame 52 surrounds an irradiation space 54. The
substrate is conveyed through the irradiation space 54
by means of a conveying device, for example by means of
a conveyor belt. In this process, the substrate is
firstly transported at the start of the apparatus 40
into the inlet chamber 60, and it moves further through
the irradiation chamber 50 and the outlet chamber 70
until it leaves the apparatus 40 at the end of the
outlet chamber 70. Fitted inside the irradiation space
54 is an apparatus 55 for feeding in inert gas, for
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example nitrogen or carbon dioxide. This apparatus 55
has, for example, two tubes 56a, 56b running along the
inner longitudinal sides of the irradiation space 54.
Of course, it is also possible for a plurality of such
tubes to be arranged under or next to one another, or
to be combined otherwise with one another in any
desired way, for example whenever the irradiation space
has particularly long or high dimensions. Those tube
sections that run parallel to the conveying direction
(arrow A) of the substrate have outflow openings 57,
situated close to one another, for an inert gas, for
example nitrogen or carbon dioxide. The outlet openings
can be configured, for example, as nozzles or bores.
Provided on the underside of the tubes 56a, 56b,
outside the irradiation space 54 but inside the
irradiation chamber 50 are valves 58 for controlling
manually or electronically the volumetric flow of the
inert gas fed through them.
Instead of tubes with outflow openings, it is also
possible to use porous materials, for example, the
inert gas entering the irradiation space 54 through the
pores. It would also be conceivable, for example, for
the inner walls of the irradiation space 54 to be
partially clad with plates made from porous material,
for example sintered material, ceramic, etc, and for
the inert gas to be introduced into the interspace
between the irradiation space 54 and the plates.
Particularly suitable are porous materials in which the
pores are small and arranged regularly close to one
another. The effect of this is that the inert gas flows
out essentially without turbulence.
One or more apparatuses 55 of identical design can also
be provided in the inlet chamber 60 or in the outlet
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chamber 70, or in both chambers, in order to further
reduce the entrance of air into the closed channel
during the entrance and/or exit of the substrate.
Instead of this, or in addition, it is also possible to
provide an apparatus 65 such as is illustrated in
figure 8 with reference to the example of an inlet
chamber 60, the following statements being valid
correspondingly for the outlet chamber 70. The inlet
chamber 60 (or the outlet chamber 70) also has a frame
61 inside which there is fitted a second frame 62 that
surrounds a conveying space 63. This frame 62 can be
made from any desired material; all that is important
is for it to be configured such that the conveying
space 64 of the inlet chamber 60 (or the outlet chamber
70) and the irradiation space 54 of the irradiation
chamber can be combined to form a closed channel. The
frame 62 stands, for example, inside the frame 61 on
stilts 62' that are reinforced with cross-struts 62 " .
The substrate is conveyed by means of a conveying
device through the conveying space 64 and the
irradiation space 54, for example by means of a
conveyor belt. In the process, the substrate is firstly
transported at the start of the apparatus 40 into the
conveying space 64 of the inlet chamber 60, and it
moves further through the irradiation space 54 of the
irradiation chamber 50 and the conveying space of the
outlet chamber 70 until it leaves the apparatus 40 at
the end of the outlet chamber 70. Fitted inside the
conveying space 64 is either an apparatus 55 for
feeding in inert gas, for example nitrogen or carbon
dioxide, as has already been described with reference
to the example of the irradiation space 54. Instead of
or in addition to this, an apparatus 65 for feeding in
inert gas can be provided. This apparatus 65 has a tube
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66 running along the cross section of the conveying
space 64. Of course, it is also possible for a
plurality of such tubes to be arranged next to one
another or otherwise combined with one another in any
desired way, for example whenever the conveying space
64 has particularly long or high dimensions. The tube
64 has outflow openings 67 situated close to one
another for an inert gas, for example nitrogen or
carbon dioxide. The outflow openings 67 can be
configured, for example, as nozzles or bores. Provided
on the underside of the tube 66, outside the conveying
space 64 but inside the inlet chamber 60 are valves 68
for manually or electronically controlling the
volumetric flow of the inert gas fed through them.
Instead of tubes with outflow openings, it is also
possible to use porous materials, for example, the
inert gas entering the conveying space 64 through the
pores. It would also be conceivable, for example, for
the inner walls of the conveying space 64 to be wholly
or partially clad with plates made from porous
material, for example sintered material, ceramic, etc,
and for the inert gas to be introduced into the
interspace between the conveying space 64 and the
plates. Particularly suitable are porous materials in
which the pores are small and arranged regularly close
to one another. The effect of this is that the inert
gas flows out essentially without turbulence.
The apparatus 40 according to the invention operates as
follows:
The substrates transit the closed channel individually
or continuously. The low concentration of residual
oxygen required for radiation curing is achieved by
CA 02482325 2004-10-26
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continuously feeding inert gas into the irradiation
space 54 of the irradiation chamber 50, the inert gas
flowing out of the tubes 56, 66 essentially with a low
level of turbulence. This produces an inert gas
5 volumetric flow in the direction of the conveying space
64 of the inlet chamber 60 or the outlet chamber 70.
For this purpose, an at least slight overpressure is
set in the irradiation space 54 by appropriately
feeding in inert gas. The effect of this is that the
inert gas flows out continuously in the direction of
the conveying spaces 64. In addition, the inert gas is
fed into the irradiation space 54 as a volumetric flow
running in the conveying direction, and maintained. The
orientation in the conveying direction effects an
15 additional orientation of the inert gas in the
direction of the conveying spaces 64 in order to impede
the entrance of atmospheric oxygen into the irradiation
space . The inert gas f lowing out thus removes f rom the
closed channel the atmospheric oxygen entrained by the
conveyance of the substrate.
Apparatuses 65 for feeding in inert gas are
additionally provided in the exemplary embodiment, both
in the conveying space 64 of the inlet chamber 60 and
25 in the conveying space of the outlet chamber 70. The
inert gas pressure in the conveying spaces is, for
example, set such that it is lower than the inert gas
pressure in the irradiation space 54. In this way, the
inert gas flow from the irradiation space 54 is
30 maintained in the direction of the conveying spaces 64,
but the inlet and outlet of the closed channel are
additionally shielded against the penetration of
atmospheric oxygen.
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- 22 -
In the exemplary embodiment, the apparatuses 65 for
feeding inert gas into the conveying spaces 64 from the
inlet chamber 60 and outlet chamber 70 are designed
such that their inert gas flow is overlaid at the side
5 on the volumetric flow running in the conveying
direction. This is achieved by virtue of the fact that
the tubes 66 are arranged with their outflow openings
67 perpendicular to the conveying direction of the
substrate. Furthermore, in the exemplary embodiment the
10 outflow openings are set such that an inert gas flow
that removes entrained air and displaces it into the
outgoing volumetric flow is applied to the substrate
(which is being let in or out) obliquely to the
conveying direction. In the conveying chamber of the
15 outlet chamber 70, this way of feeding in inert gas
advantageously additionally prevents air from being
back-mixed into the irradiation space 54 of the
irradiation chamber 50.
20 Use is made in general of inert gas volumetric flows of
15 to 1000 Nm3/h, preferably between 30 and 400 Nm3/h.
The f low rate of the inert gas volumetric f low fed in
perpendicular to the conveying direction is preferably
approximately 10 to 80% by volume of the entire inert
25 gas volumetric flow, preferably 15 to 60% by volume of
the volumetric flow. In a closed channel with a cross
section of 500 x 500 mm, for example, the flow rate for
the volumetric flow can typically be 200 Nm'/h, while
the flow rate of the inert gas fed in perpendicular to
30 the conveying direction is preferably approximately
25-50% by volume of the volumetric flow.
