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UV Curing under Carbon Dioxide
Erich Beck
BASF AG Ludwigshafen
Abstract:
UV coatings are predominantly used in series production of flat substrates. UV curing in a
carbon dioxide atmosphere effectively reduces the inhibitory effect of oxygen. Improved curing
becomes possible with minimal expenditures of energy and extra equipment. Basic kinetic
research in this context demonstrates the expanded scope of procedural and processing
possibilities. The short distance between the lamp and the substrate, an essential reason for the
preferential use in flat-substrate applications, is overcome. Under carbon dioxide, it is possible to
cure with a minimum of extra equipment, with larger distances and reduced lamp power. This
allows for less complicated 3D-UV applications even for small production lots.
Introduction: UV coatings are ecologically efficient, but at present not universally useable
UV curing of acrylic-ester-based coatings is one of the fastest growing industrial coating
technologies due to fast and emission-less processing and the high degree of the resulting
quality. The method is especially well established for coating of flat substrates. Predominantly
acrylic-ester-based coating resins are used. They cure in seconds by means of radical chain
polymerization of their double bonds. In practical terms, this polymerization is started
instantaneously with the formation of the initiator radicals when UV light is turned on.
The most frequent application is the coating of paper, wood, and plastics. Further developments,
e.g., towards scratch proof automotive topcoats1are being intensively pursued, especially in
Germany.
On the occasion of a recent comparison of five different conventional industrial coating
technologies involving the coating of 1,000 doors, UV coating was found to be the most
ecologically efficient. UV coatings gain their ecological advantage above all from these factors:
energy required, reduced environmental impacts from evaporation and thermal drying processes,
material requirements, and space and equipment needed.
The share of UV-curable systems in the total industrial coatings market, at somewhat less than
5% in Europe in 20002, is rather small. The reasons for this are of course the technological
limitations.
In the USA, a survey3 concerning the limitations and possible improvements of UV technology
was carried out. The following factors were most often listed:
Resin cost, photoinitiators and equipment, as well as the curing speed
Processability of 3D substrates and curing in shaded regions and the
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use of low viscosity spray coating formulations
Formulations which are weather resistant
Pigmented UV coating formulations
Formulations with better adhesive properties
Formulations with improved toxicological properties, suitable for
food contact uses
Information about the UV technology
Economic factors, quality aspects and suitability for 3D applications are critical points on this
list. Possibilities and advantages for new applications, based on the process of UV curing under
carbon dioxide will be demonstrated below 4,5.
Atmospheric oxygen inhibits curing
The critical development factor for the UV curing of acrylic ester, and unsaturated polyester
formulations, by radical polymerization, was the reduction of the inhibiting effect of atmospheric
oxygen, the so-called oxygen-inhibition. Radicals involved with the curing process, especially
for the initiation of the polymerization, are intercepted by oxygen molecules, which are also
radicals, and converted to relatively unreactive products, such as peroxy-compounds. This can
interfere with or even completely prevent curing, especially at the surface exposed to air.
Oxygen-inhibition was eventually overcome by a combination of measures. A decisive factor is
an increase in the starting concentration of the radicals. This is achieved with high initiator
radical concentrations and high UV intensities. To this effect, high powered UV lamps, usually
mercury pressure lamps, are used. In order to reduce any irradiation losses, these are positioned
at a distance of a few centimeters above the coating surface. This is easy to implement for flat
substrates with lamps which are in fixed positions above a production conveyor belt. For 3D
substrates additional equipment is required, for example, several lamps may be distributed
around the substrate, or mechanical movement scenarios for either substrate or lamp at short
distances, e.g., robots6,7, may be used.
In addition, synergistic compounds are used, especially amines, which overcome the inhibiting
effect by further reacting with the peroxy-compounds to reactive radicals. Curing is also
enhanced by viscous binders, which feature several reactive double bonds per molecule.
Molecular weight increases rapidly by branching and cross-linking. The high degree of cross-
linking favors the hardness and chemical stability of the films, but reduces their flexibility.
In the absence of oxygen, the interfering reactions do not take place, and the countermeasures
mentioned above can be omitted. Higher curing rates and improved coating qualities are
obtained8,9,10,11. This was demonstrated on UV conveyor belt production lines, which were run as
partially enclosed facilities under an inert gas stream, usually nitrogen. Such UV production
lines with inert capacity for flat substrates have since been added to the product lines of most UV
plant manufacturers. However, no broad market penetration has yet been achieved.
The coating of car bodies by 3D-UV curing under nitrogen has recently been a hot topic for the
future12. Two different approaches were offered, a procedure involving an inert room, and a
2
process using blanketing. The inert room procedure involves flushing the vehicle in a closed
room with nitrogen followed by irradiation under reduced oxygen conditions. For the blanketing
approach, only the space between the lamp and the coated surface is to be rendered inert by a
stream of gas. The nitrogen costs per vehicle are estimated to be about an order of magnitude
lower for the blanketing procedure.
Carbon dioxide simplifies the 3D-UV Cure
Both nitrogen procedures consume the inert gas directly as a function of the number of bodies or
the total areas involved. Carbon dioxide is 1.5 times as dense as air or nitrogen, which allows for
the possibility of a more cost-effective and also simpler inert process. When CO 2 is added to
open vessels (Figure 1) for UV irradiation, a pool of inert gas is formed. Mixing and diffusion
with air is a slow process, i.e., the consumption of CO 2 is low. Significant CO 2 losses are
caused by turbulence, e.g., resulting from movement of the substrate, and also by thermal
convection originating at hot surfaces, e.g., lamps positioned inside the vessel. The interface
between CO 2 and air can be identified by smoke or with a flame. The oxygen content can be
determined with readily available instruments, which are now equipped with oxygen sensors13
specifically suited to carbon dioxide atmospheres. In open vessels residual oxygen values of less
than 1% by volume are possible. Carbon dioxide supply can be made available at low turbulence
by means of a screen inserted as the vessel-bottom (1). The supply source can be in-flowing gas
or slowly evaporating dry ice (2).
5
3
4
1
2
Figure 1: 3D-UV facility for a carbon dioxide atmosphere
Aluminum-clad walls (3) act as UV reflectors which can distribute the irradiation around the
substrates. Coated 3D substrates, which are suspended in the vessel (4), can be cured simply by a
lamp positioned above the vessel (5). For a vessel of 1 m3 capacity UV lamps rated at 0.4-2 kW
power are suitable. To prevent hazardous irradiation loads in the UVB and UVC-range, holding
devices must be provided.
Sample substrates for a 1 m3 pilot scale UV facility are chairs, plastic housing for household
items and automotive parts.
3
Production tests were carried out in a pilot facility14 with suspension type arrangement (Figure
2). 3D substrates with cross-sectional areas of about 1 m2 can be installed in a vat-like vessel
with carbon dioxide for irradiation.
Figure 2: Production 3D-UV facility for a carbon dioxide atmosphere
A coated Smart car body was recently produced by this facility as a UV sample for MCC15
(Figure 3).
Figure 3: Car body cured in CO2 , (Smart/MCC)
Curing under carbon dioxide is generally better than curing in air
We carried out basic research about the kinetics of curing in air and carbon dioxide16. This
allows us to optimize the curing conditions, and may provide clues for applications.
Real-time quantitative observation of the reactive double bond, using Real Time Infrared
Spectroscopy (RTIR), is especially suited to these kinetic investigations17. Data obtained by this
method allow the plotting of the disappearance of the reactive double bonds (%) versus time (s),
which provides excellent information about the speed of reaction, the rate, and any inhibiting
reactions. The results of these investigations are summarized below.
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A comparison of two cures on a film of 5 µm thickness, based on a urethane acrylate (Laromer®
LR 8987, BASF AG), with 3% by weight of a photoinitiator (Irgacure® 2959, Ciba SC) with a
medium pressure mercury lamp, at an intensity of 30 mW/cm2 (100 W EFOS Lamp) under pure
nitrogen and carbon dioxide (Diagram 1) is offered as an example for an RTIR-comparison. The
following conclusions can be made:
Both inert gases, CO2 and N2, behave identically.
Because of the absence of the reaction with oxygen, (inhibition), the curing reaction
under inert gases starts quicker (approx. 0.1 s).
The reaction proceeds faster, correspondingly the polymerization rate under inert gas
is faster.
The reaction proceeds to a higher degree of completion, i.e., the cure is better.
Conversion (%)
70
60
50
40 air
nitrogen
30
carbon dioxide
20
10
0
0 0,2 0,4 0,6 0,8 1
Time [s]
Diagram 1: RTIR comparison: Curing under nitrogen, carbon dioxide and air
Coating: Laromer® LR 8987, with 3% by weight of Irgacure® 2959
UV Intensity 30 mW/cm2, film thickness 5 µm
Under inert gas fewer inhibitory products (peroxides) are formed. Consequently, fewer double
bonds are lost and less photoinitiator is consumed, these are then additionally available to the
curing reaction. Altogether, from an energy point of view, the cure is more efficient, and the
resulting film is mechanically and chemically more stable.
Additional results derived from the RTIR-investigations are as follows:
Inhibition decreases as the oxygen concentration decreases. Even minor changes in
concentration, below the limit of 1% by volume of residual oxygen, have significant effects on
the cure.
Specifically the surface toward the oxygen-containing atmosphere is damaged. This can be
measured in investigations where the film thickness was a variable, using IR- and ATR-
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spectroscopy. Confirmation was also obtained from measurements of the distribution of the
double bond density as a function of film thickness, using confocal Raman Spectroscopy18.
Working under an inert gas makes it possible to use unusually low concentrations of
photoinitiators. Under pure CO2, even a concentration of 0.3% initiator is more effective than the
usually tenfold amount used with air.
Acrylates which react only weakly or not at all in air, especially low viscosity monomers, can
polymerize with high rates of transformation under CO2. This is also especially true for
monofunctional acrylates. For items with low processing viscosity, e.g., for spray coating
formulations, or for flexible coating films, this translates to a larger scope for formulations.
Reactivity
4
80°C
RT
3
50°C
6°C
2
CO2
-19°C
1 6°C
-19°C
RT
80°C
50°C air
0
0,1 1 10 100 1000 10000
Viscosity (Pa.s)
Diagram 2: Reaction rate as a function of temperature and viscosity
Coating: Laromer® LR 8987, with 3% by weight of Irgacure® 2959,
UV intensity: 15 mW/cm2, film thickness 5µm
From the cure as a function of temperature of the urethane acrylate (see above) between 19º C
and 80ºC (Diagram 2) the influence of the viscosity is also recognizable. At lower viscosity of
the coating formulation, the ability of oxygen to diffuse is greater, thus the inhibitory effect is
greater. Reaction rates in air and CO2 at -19º C are very much alike. The viscosity of the resin is
then so high that only little oxygen can diffuse into the film, i.e., the inhibitory effect in air is
also weak in this case. If the temperature is raised under CO2 atmosphere, the curing rate and the
total amount of transformation increase. In contrast, under air, both of these values decrease, due
to the dominating influence of the oxygen in the warmer, and thus less viscous resin.
At lower UV intensities under CO2, curing is faster and more complete, especially at the surfaces
of the films. Accordingly, inert-UV production lines can be run at higher through-put rates, or at
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lower lamp intensities, and also with larger substrate-lamp distances. Specifically the larger lamp
distances, as long as meters, as used for instance in CO2 -UV containers (see above), are a
critical precondition for 3D-UV concepts, and are easy to implement.
Under CO2, and using only UVA-radiation from medium pressure mercury lamps, better curing
results are achieved than with corresponding unfiltered radiation, i.e., radiation containing UVC,
UVB and UVA rays in air. UVA radiation under air impairs the cure to a considerable extent.
Thus, the toxicologically less critical UVA radiation, as used in tanning and sun lamps, is readily
useable under CO2.The ready availability of the commercially available tanning lamps is
especially attractive for small lot manufacture.
Summary
The main known advantage of UV polymerization of acrylates under inert gas conditions was so
far not successful for broadly based applications in the curing of coatings. With carbon dioxide,
it is possible to prepare reduced oxygen atmospheres over pools of inert gas in open vessels.
Under these conditions, 3D-UV cures can be implemented relatively simply, providing a means
for small lot manufacture and continuous processing. More favorable possibilities for processing
and formulations, as indicated from the basic research results obtained with RTIR, support the
promising perspectives for future growth of UV technology.
1
Neue Automobillacke und Lackaufbauten für PKW und Nutzfahrzeuge in Entwicklung und Anwendung,
“Automotive Circle International” Conference, Bad Nauheim , March 2003.
2
J. P. Howard, RadTech Europe, 1999, Conf. Proc. , Berlin, November 1999, 13-23.
3
K. Lawson, RadTech Asia 2001, Conf. Proc. , Kunming, May 2001, 1-13.
4
E. Beck, O. Deis, P.Enenkel, W. Schrof, Patent WO 01/39897 (1999).
5
E. Beck, RadTech Europe 2001, Basel, October 2001, 643-648.
6
P. Burger, besser lackieren, 15/2000.
7
M. Schneider, W.Klein, C. Schröder, RadTech Europe, 1999, 711-716.
8
F.R.Wight, Journal of Polymer Science: Polymer Letters Edition, 16, (1978), 121-127.
9
R. Müller, RadTech Europe 2001, Basel, October 2001, 149-152.
10
T. Henke, RadTech Europe 2001, Basel, October 2001, 145-148.
11
K. Menzel, H.-H.Bankowsky, P. Enenkel, M.Lokai, RadTech Europe, 1999, Conf. Proc., 165-170.
12
D. K. Ortlieb, New automotive top coatings and coating combinations for automobiles in development and
application, “Automotive Circle International” Conference, Bad Nauheim , March 2003, 68-84.
13
Oxygen Meter GNH3691, GOO 3691, Greisinger Electronic GmbH, Germany.
14
Pilot Plant: Rippert Luft- Und Anlagentechnik GmbH, Germany.
UV Lamps: IST Metz GmbH; Dr. Hoenle AG.
15
J. Ortmeier, New automotive top coatings and coating combinations for automobiles in development and
application, “Automotive Circle International” Conference, Bad Nauheim , March 2003, 54-67
16
K. Studer, C. Decker, E. Beck , R. Schwalm, Prog. Org. Coat. , in press.
17
C. Decker, K. Moussa, Makromol. Chem. 189, (1988), 2381-2394.
18
W. Schrof, L.Häusling, R. Schwalm, W. Reich, K. Menzel, R. Königer, E. Beck: RadTech Europe 1997 Conf.
Proc., 535-547.
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