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Benefits of UV-curable coatings
Typically flexible fluoropolymer PUR coatings are spray
applied directly after vulcanisation of the rubber during the
continuous extrusion process and then thermally cured in a
series of conventional, microwave or infrared ovens. This
conventional cure process is costly, inefficient and results in
high overhead costs. In the age of continual demands for
cost reduction, improved efficiency and reduction in VOC
levels, a range of waterborne 1-pack UV curable coatings
meeting market and technical requirements have been
developed. Realizing the benefits of a UV curable coating
system in a continual extrusion process means at least an
enormous increase in speed of cure, waste reduction, lower
solvent emissions, lower energy costs and huge space
savings. In some cases it is even the only way to implement
the application of a coating into to the manufacturing
process due to limitations in line speed and/or in the
mechanical properties of the rubber substrate.
Kevin Bruen, Kurt Davidson, Daniel F. E. Sydes, Peter M.
Siemens.
Nowadays more and more flexible substrates for industrial
applications, e.g. in the automotive industry or the building
industry, are equipped with flexible fluoropolymer PU
coatings to improve or modify the end-use properties of a
given substrate (most often EPDM). Typical applications are
weatherstrip seals and glass run channels of a car, where
low friction, non-stick, freeze release, noise reduction as
well as chemical or abrasion resistance are important. More
recently coloured or pearlescent flexible fluoropolymer
coatings became design elements in the automotive and
building industry, where UV-resistance and easy-clean
properties are important.
Rubber seals play an important role both in the construction
of vehicles and buildings. Without them it is currently
impossible to make a windows wind and water tight. So
without rubber we have no dry, calm and pleasant interiors,
unless we want to abandon sunlight from our everyday life.
Modern automotive rubber door and boot seals for example,
including primary and secondary seals for weatherstrip and
glass run applications, are commonly manufactured from
extruded EPDM. There is a requirement to apply a coating
to the seal to improve the end-use properties. These include
low friction, non-stick, freeze release, noise reduction,
chemical and abrasion resistance properties specified by the
automotive manufacturers. A complete range of flexible
coating systems is produced by Whitford. These principally
fluoropolymer-based coatings have been used extensively
to coat all kinds of seals for automotive applications for
several years.
A new field of application are architectural sealing systems.
Modern office buildings and houses often have facades with
many windows or are even nearly completely made of
window panes. All these windows in principle consist of
window glass fitted into a frame made of metal (typically
aluminium), plastic (e. g. PVC) or wood using multiple
rubber seals of various shapes (see Figure 1). Most of these
rubber seals are invisible to the beholder when the window
is closed, but some parts of them are visible as a black
border between the glass and the window frame. In modern
architecture windows and window frames became a design
element several years ago. Nowadays these visible rubber
seals get more in focus. The aim is to have coloured window
seals, that either match the colour of the window frame or
the facade, or form a pleasant contrast that catches the eye.
On top of acting as a design element flexible fluoropolymer
coatings are used as an assembly aid for window profiles.
The rubber seals have to be lubricated before they can be
fitted into the window frame. Currently silicone oils are used
for this, but this solution suffers from disadvantages. As
silicone oils are liquids they tend to contaminate the window
glass during the assembly. This leads to extra costs for
extensive cleaning. In the case of modern self-cleaning
facades the use of silicone oils is not possible at all as they
destroy the self-cleaning property of the glass by attacking
the coating on the glass.
Before the curing step the coating has no abrasion or
chemical resistance and is often tacky
Typically conventional polyurethane dispersion (PUD) based
coatings contain a lubricant and are spray applied directly
after vulcanisation of the rubber during the extrusion
process. After application of the coating a secondary
(coating) cure cycle is required and usually involves a series
of convection 'tunnel', microwave or IR ovens (see Figure 2).
Before the curing step the coating has no abrasion or
chemical resistance and is often tacky, even when the
solvent (either water or organic solvents) has evaporated.
The heat driven cure facilitates the reaction of the
crosslinker with free OH-groups to form a fully crosslinked
polyurethane.
Although the heat-cured coating systems show very good
performance and are therefore well accepted in the industry
and often specified by the automotive manufacturers, the
cure process itself implies a number of significant
disadvantages:
Facilitating the cure process is a costly and inefficient
process overhead. The tunnel ovens for the curing process
cause high energy costs and high investment in machinery.
They also occupy a significant amount of expensive space
in production: Typically at least 30 % of the space for an
extrusion line with online coating application is occupied by
the ovens facilitating the heat cure of the fluoropolymer
coating.
UV-curable coating systems cure in less than a second
UV-curable coatings are well known within the industry. A
significant amount of the coatings used e. g. in the furniture
or window industry is cured by UV radiation. One of the
most important advantages of UV-curable coatings over
conventional air-drying or heat-cured coatings is the speed
of cure. Typically a UV-curable coating is cured in less than
a second. A conventional air-drying wood-coating needs
several minutes or even hours to dry completely. This
difference in speed of cure allows much higher line speeds,
an enormous economic advantage.
The different cure mechanism also gives the cured film a
much higher chemical resistance as a rigid network of
covalent bonds is formed. Typically these UV-curable wood
coatings are 100% monomer systems. That means they
contain no additional solvents, but only UV-curable
monomers and/or oligomers and solids like pigments, fillers
etc. (see Figure 3).
UV-technology for continuous extrusion processes
coating systems
Existing heat-cured fluoropolymer coatings for rubber
substrates are usually chemically cross-linked. Therefore
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the reason for implementing UV-technology into this
business area is not improved chemical resistance. The aim
is to develop a product that offers the benefits of the well
established heat-cured coatings, but with the short, almost
immediate, curing time of a UV-curable coating.
Transferring these benefits of UV curable coating systems to
a continuous extrusion process leads to:
Space saving
The UV drying system requirements are in the region of 5m
in length, the equivalent to the space occupied by the
spray-booth configuration in contrast to typically 30 m for a
conventional hot air oven. The heat source used in the
coating process is a short set of IR-lamps. This is necessary
to ensure complete water removal before the UV-cure takes
place (see Figure 4), as all presented coating systems are
water-based.
Easy installation of UV curing equipment
The required UV curing equipment can easily be installed on
to the existing extrusion/coating lines, at the same time
freeing up expensive manufacturing space occupied by the
defunct 'tunnel' oven confi- guration.
Lower energy costs
As stated previously the use of 'tunnel' ovens requires the
pre-heating of the oven before the coating of the extrusion
profile can commence and maintaining this required
'working' temperature when the extrusion line is idle. UV
lamp systems allow for standby, shuttered operation at low
power when not in use and incorporate rapid, de facto
instantaneous, lamp warm-up when needed.
Reduced waste
As with any continuous process there is always a degree of
wastage until the required running conditions are achieved.
In this process obtaining the correct wet film/dry film
thickness of the coating is required. Presently for the heat
cured PUD coatings dry film measurements are obtained
after the 1,5 - 3 min cure cycle, combined with a typical
extrusion speed of 15 m/min this equates to waste coated
profile until the correct coating thickness is obtained and
may require several spray gun adjustments. For the UV
coating process, since the cure cycle is greatly reduced the
corresponding amount of scrap profile is also greatly
reduced.
Increased line speed
During the last couple of years many modern extrusion
processes have been developed. State of the art extrusion
lines for EPDM profiles, especially those types of profiles
used for windows, can run at up to 50 m/min. With the
current PUD coating systems it is not possible to cure the
coating at the high line speeds because
- The required oven capacity, approximately 100 m in
length, would require too much investment in machinery,
consume huge amounts of energy for heating and would
require too much space or would not fit in typical production
facilities.
- As PUD coatings are typically tacky till they are fully cured,
the extruded profile must run though the 'tunnel' oven
without touching any parts of it with a coated surface. This
can be achieved at the current oven lengths, but can not be
managed at oven lengths needed for these high line speeds.
Coatings are based on different types of waterborne
PUR-dispersions
The coatings used in this set of experiments, code-named
as "Xylan 2525", are based on different types of
water-based polyurethane dispersions. A wide range of
UV-curable PUD's is available from various manufacturers.
One that has been used in this special application is
"Neorad R440" (NeoResins), a UV-curable aliphatic
urethane oligomer dispersion with 40 % solids. As these
UV-curable PUD's are not flexible enough for this kind of
application (car manufacturers usually specify 100-150 %
elongation) the UV-curable PUD is blended with a
non-reacting aliphatic polyurethane dispersion, e.g. "Neorez
R600" (NeoResins). This gives the resulting PUR-coating
enhanced adhesion to the rubber substrate and better
elongation. The fluoropolymer component is either
Polytetrafluoroethylene (PTFE) or a perfluoropolyether. To
improve the dispersibility of the fluoropolymer component a
fluorosurfactant has been added. Other additives are
silicone polyester acrylates and polyether siloxane
copolymers which have been used as wetting agents and
slip and flow additives. A mixture of two α-Hydro-xyketones
and Bisacylphosphine (BAPO) is used as a photoinitiator
blend. The coating has been pigmented to give a matt black
finish. See Table 1 for an example of a standard formulation
used.
UV-curing equipment with very stable UV output over
life time
A standard lab-scale conveyor UV-oven (Fusion UV
Systems) is used for curing the coatings (see Figure 5).
Different types of microwave powered lamps have been
chosen as the UV-source. The reason for this is that
microwave powered lamps have a very stable UV output
over their life time (>6000 h). The UV-spectra of the different
bulb types used in the experiments are shown in Figure 6.
As only one slot for UV-lamps was available, longer
exposure times were simulated by multiple passes trough
the oven. In case of curing schedules with multiple lamp
types the bulbs were changed in between the passes.
Parameters tested: solvent resistance, abrasion
resistance and flexibility
The wet coating has been spray-applied on both EPDM
swatches from automotive weatherstrip seals as well as
aluminium panels. Before the application of the coating
material both types of substrates have been treated with a
primer for adhesion promotion ("Xylan 4016"). The primer
has been spray-applied and then been flashed-off for 2 min
at 150°C (if not stated differently) to simulate the IR flash-off
which is typically much faster. Immediately afterwards the
coating is sprayed onto the substrate, flashed-off 2 min at
150°C and then cured in the UV-oven at a conveyor speed
of 15 m/min. The different curing schedules used are 2D
(two passes under a D-lamp), 2V, 2H, 2V+2H (two passes
under a V-lamp followed by two passes under a H-lamp)
and 2D+2H.
The parameters tested for the cured coating are solvent
resistance, abrasion resis-tance and flexibility. Solvent
resistance was determined by a solvent rub test. For this
test a cotton swab is immersed in the specified solvent. In
most cases isopropanol, toluene or N-methylpyrrolidone
(NMP) are used, but any other liquid is possible, depending
on the end use of the coated parts. Immediately afterwards
you begin rubbing the test panel in a back and forth motion
in a straight line with a stroke at least 8 cm long using
moderate pressure. Approximate weight of pressure should
be around 50g. The rate of rubs is approximately 100 double
rubs per minute. The rubs are counted (one forward and one
backward to be counted as one double rub) and continued
until the coating film is attacked. The resulting number of
double rubs is the measuring unit for the solvent resistance
of a coating on a given substrate. If the solvent used is very
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volatile (e. g. toluene) the cotton swab is immersed in the
solvent every 50 double rubs to avoid abrading the coating
with a dry swab.
Abrasion resistance is measured by a modified Crock test.
The standard Crock test as specified by Ford (Ford BN
107-01) demands a given result for the discolouration of the
cloth after a specified number of cycles (e.g. 500 cycles)
with a specified load (900 g). We have modified the
standard wet crock test with soapy eater (Ford specifies an
addition of a mixture of soap : water 1:5 (w/w) in
specification WSB M2D 49-A2, sector 3.8) to make it more
severe by adding 1 mL of a mixture of liquid detergent
(standard washingup liquid) and water 1:2 (w/w) onto the
cloth. Instead of running the test for a given number of
cycles the test is continued until the coating fails (i.e. the
substrate is visible).
For elongation tests flat EPDM samples are elongated till
cracks appear in the coating film. At this point the elongation
in percent (%) is recorded.
Water removal stage is essential
To evaluate the impact of the water removal stage on the
performance of the cured coating several different time and
temperature pairs have been examined. As a means to
measure the quality of the cure solvent double rubs with
toluene have been carried out. It can be seen (see Table 2)
that the water removal stage is essential for the cure of the
coating. If water is still present during the curing step, it gets
entrapped and the network formed by the cured PUR's is
most likely to loose to give the coating the necessary solvent
resistance. It can also be seen that too extensive flash-off
times or temperatures have a negative effect on solvent
resis-tance, too. This can be attributed to thermal
degradation of the photoinitiators used.
Combining UV-output of a V- and a D-bulb respectively
of a H and a D-bulb results in superior solvent
resistance
The experiments show that the curing conditions have a
huge impact on the solvent resistance of the finished coating
on aluminium panels (see Table 3).
Using only a single UV source results in very limited solvent
resistance, especially in the case of the H-bulb. D- and
V-bulbs show no significant difference solvent resistance.
This is quite surprising as V-bulbs are usually recommended
for white coatings, not for black ones. Even when a white
coating formulation for architectural sealing systems was
cured, this correlation was confirmed (see Table 4).
Combining the UV-output of a V- and a D-bulb respectively
of a H- and a D-bulb results in superior solvent resistance of
the cured coating. As the combination of D- and H-UV gives
better results in all experiments done, this combination is
recommended for the curing of these types of black "Xylan
2525" coatings and will be used in all future experiments
with these materials.
The coating passed 5000 cycles in the modified crock test
mentioned above. This means the abrasion resistance is far
better than usually specified for flexible coatings. Other
results for standard tests as specified by the automotive
industry are summarized in Table 5.
approached these problems and developed UV curable
coatings, "Xylan 2525", for automotive applications. These
coatings meet major car ma-nufacturers' specifications, are
environmentally friendly and satisfy VOC legislation as well
as all other present legislation.
- For the first time UV-curable flexible coatings for
architectural sealing systems are presented. These coatings
can act both as coloured design elements as well as
technical assembly aids.
- Applying UV-curing technology to continuous extrusion
processes gives major economic benefits. It also makes
coating technology for flexible materials applicable to a
wider variety of substrates. In the course of the conversion
of the materials used to build cars to increase their
recyclability other kinds of rubber substrates have become
more and more important.
- At the moment EPDM is widely used, but it lacks any
recyclability. Therefore more and more thermoplastic
elastomers (TPE's) and thermoplastic vulcanisates (TPV's)
are used for automotive applications as they can be
recycled more easily. But as they are thermoplastic they can
not be coated with the existing, heat-cured coating systems
due to the high cure temperature. The presented UV-curable
coatings overcome this problem by eliminating the need for
high temperatures from the curing process.
The authors:
> Kevin Bruen, project chemist for flexible finishes and a
Chartered Chemist, is responsible for coatings application
and technical support in the Flexible Finishes industry. He
has 25 years experi ence in formulating and applying
speciality coatings, including a spell in the extrusion
industry.
> Kurt Davidson, B.S. in Chemistry from the University of
Pittsburgh (1994) and M.S. in Chemistry from Lehigh
University (2002), worked from 1995-1999 at Air Products
and Chemicals, Inc. as a research technician mainly
performing emulsion polymerization and formulating
polyurethane coatings for the automotive industry. Then
from 2000-present he is working as a development chemist
fluoropolymer coatings for various applications for Whitford
Corp.
> Daniel Sydes, a graduate engineer, joined Whitford in
1993 and has worked in the coatings industry for 15 years.
As the business manager for Flexible Finishes he is
responsible for his division of Whitford Worldwide.
> Peter Siemens, Dr. rer. nat. in Physical Chemistry from
University of Bielefeld (2000), joined Whitford as Technical
Manager for Whitford GmbH (Germany) 2 years ago. The
development part of his work is mainly focussed on Flexible
Finishes and Textile Coatings.
This paper was presented at the European Coatings
Conference in Berlin, March 17-18 2004
ACKNOWLEDGMENTS
We would like to thank Fusion UV Systems GmbH for their
technical assistance and advice and Dätwyler AG for their
cooperation.
Result at a glance
- In the age of continual demands for cost reduction,
improving efficiency and reduction in VOC levels we have
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Figure 1: Modern facades built with state of the art architectural design consist almost
only of glass and rubber seals. Today these architectural sealing systems get into the
designer's focus. (Source: Dätwyler AG).
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Figure 2: Simplified representation of an EPDM-extrusion line with online coating
application and heat curing (not to scale).
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Figure 3: Comparison of curing mechanisms of physically drying coatings versus
conventional UV curable coatings..(Source: Fusion UV Systems GmbH).
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Figure 4: Simplified representation of an EPDM-extrusion line with online coating
application and UV-curing (not to scale).
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Figure 5: UV-oven used in the experiments. The UV-source is mounted on top of an
adjustable conveyor system.
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Figure 6: Spectral output of microwave-powered D-bulbs, H-bulbs and V-bulbs in the
UV-region of the spectrum.
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