| After four years of development work, an innovative
new clear lacquer went into series production at Mercedes-Benz at the
end of 2003. Ground-breaking nanotechnology ensures that the new product
is substantially more scratch-resistant than conventional paint. The E,
S, CL, SL and SLK-Class model series are the first cars in the world to
be available with this new paint system, whilst customers of other Mercedes
models can look forward to the increased scratch resistance of nano-paintwork
from this spring.
The newly developed clear lacquer, which contains microscopically
small ceramic particles, hardens in the paintshop oven, forming an extensively
cross-linked network. The paint is therefore more effectively protected
against scratches caused by mechanical car washes, for example. The nano-particles
provide a three-fold improvement in the scratch resistance of the paintwork
and ensure visibly enhanced gloss over an extended period of time. Following
extreme tests in a laboratory car wash, Mercedes engineers noted an around
40% improvement in paint gloss compared to conventional clear lacquers.
Mercedes-Benz carried out extensive testing on the nano-particle clearcoat,
both in the laboratory and under everyday conditions. Even after several
years of use, the 150 plus test cars involved in the long-term testing
programme displayed significantly greater scratch resistance and enhanced
paint gloss compared to vehicles with conventional paintwork. Added to
which, the newly developed paint system also meets the stringent Mercedes
standards in terms of the protection it offers from chemicals in the environment.
Remarkable advances in the area of nano-technology have allowed tiny ceramic
particles – each less than a millionth of a millimetre in size –
to be integrated into the molecular structure of the binding agent. These
particles float around freely at first in the liquid clearcoat, before
cross-linking as the drying process takes effect. The particles link in
with one another in such a way as to create an extremely dense and smoothly
structured network at the paint surface. This provides a protective layer
and ensures that the new nano-particle clearcoat is considerably more
scratch-resistant than conventional paintwork.
The effectiveness of the new technology was borne out by the results of
an extreme test conducted in a laboratory car wash according to DIN standards.
The water used in the test contains a precisely measured concentration
of fine particles and is spread over the paintwork by the rotating washing
brushes, leaving behind scratches. After 10 wash cycles in the laboratory
car wash – reproducing the degenerative effect of some 50 to 100
regular car washes – the nano-painted sheet metal emerged with around
40% greater gloss than samples with conventional clear lacquer.
Mercedes-Benz is the world’s first vehicle manufacturer to offer
this more scratch-resistant clear lacquer. Nano-particle clearcoat serves
as an early indicator of the huge potential of nano-technology for the
future, techniques that allow scientists to reach into and alter the atomic
structure of materials. Indeed, it will also be possible to give materials
in other areas of automotive development new properties that allow them
to carry out particular functions.
The paintwork on the latest cars consists of several exceptionally thin
layers, which each fulfil different tasks. The complex painting procedure
begins with the phosphating process, in which the car body is sealed in
an extremely fine but highly effective zinc phosphate coating. This protects
the sheet metal from corrosion and at the same time forms a sound basis
for the cataphoretic dip priming – whose primary function is also
to provide a shield against corrosion – which is next to come. The
car body is submerged in a tank of water-thinnable paint, which coats
every cavity, corner, groove and edge through an electrophoretic reaction.
Together, phosphating and cataphoretic dip priming form a layer only some
22 micrometres thick.
After a 25 micrometres thick layer of filler, a layer of paint is applied
after that of the base coat – approximately 15 micrometres thick
– which contains not only the customer’s choice of colour
in pigment form but also, if a metallic finish has been ordered, the tiny
aluminium flakes which provide the elegant metallic effect. As with the
filler, an electrostatic charge increases the effectiveness of the paint
application in this process as well. The base coats used by Mercedes-Benz
are water-soluble and contain as much as 80% less organic solvent than
conventional paint finishes.
The top layer, which is around 40 micrometres thick, is formed by the
transparent clear lacquer, which provides the gloss and weatherproof properties
of the paintwork. This lacquer is put under particular stress in the everyday
life of a passenger car, having to withstand environmental elements such
as acid precipitation, tree resin, bird droppings, dust and soot, as well
as a considerable physical battering – from stone chipping, sunlight,
abrasion and fluctuations in temperature, among other factors.
The paintwork of a Mercedes-Benz car thus consists of five layers with
a combined thickness of some 100 micrometres – approximately 0.10
millimetres. Each of these coats is the result of a complex development
process spread over several years that has seen a team of experts fulfil
a number of requirements. In addition to refining the process technology
involved, they have addressed key issues relating to environmental protection,
quality and durability.
The innovative nano-particle clearcoat, which Mercedes-Benz became the
world’s first carmaker to offer at the end of 2003, is a case in
point, with over four years of development and testing behind it. The
new clear lacquer has helped the engineers resolve the technical conflict
of interests between scratch resistance and chemical resistance that has
plagued the development of clear lacquer until now. This new and innovative
paint system meets Mercedes’ stringent standards in both respects.
A large proportion of all paintwork scratches are caused by mechanical
car washes. Minute particles of hard materials, such as road dust and
sand, become lodged in the rotating brushes and etch scratches into the
paint surface. These “hair scratches” are particularly noticeable
in darker paint shades.
With the help of the nano-technology developed at the beginning of the
1980s, scientists have been able to alter the molecular structure of the
binding agent and integrate tiny, microscopic ceramic particles. These
each have a diameter of less than 20 nanometres, which makes them tens
of thousands times thinner than a human hair.
During the electrostatic paint application process, the binding agent
particles float around freely at first in the liquid paint. It is not
until the car body is placed inside the paintshop ovens at a temperature
of some 140°C that the particles cross-link into a dense network.
This allows the lacquer to provide much more effective scratch protection
than conventional paints, whose binding agent and cross-linking agent
form comparatively long molecular chains. Tests confirm that the tiny,
microscopic ceramic particles do indeed enhance the scratch resistance
of this clear lacquer several times over.
In other words, nano-particle clearcoat offers considerably greater and
longer-lasting resistance to paint scratches – such as those caused
by car washes – than conventional paint finishes. However, even
this new clear lacquer developed on the basis of nano-technology cannot
provide effective protection against vandalism such as when the paintwork
is deliberately scratched using keys, tools or other objects.
Mercedes-Benz began by testing the impressive scratch and chemical resistance
of nano-particle clearcoat on individual body parts in the laboratory,
before sending out more than 150 test cars as part of a practical testing
programme in the summer of 2001. The results were extremely encouraging.
After several years of testing, the nano-painted Mercedes test cars stood
out from vehicles with conventional clear lacquer with their significantly
higher level of paint gloss. This conclusion was backed up by standardised
tests carried out in a laboratory car wash according to DIN standards,
in which a precisely calculated measure of fine quartz sand is mixed in
with the water. This test provided an accelerated simulation of the processes
in a regular car wash – according to the amount of dirt on the car,
10 cycles in the lab car wash reproduce the degenerative effect of 50
to 100 regular car washes.
After the wash programme, the engineers use a special gloss measuring
device to examine the scratch marks on the painted metal plates and assess
the residual gloss. The results showed that whilst the residual gloss
of conventional paintwork stood at around 35% after 10 wash cycles in
the laboratory car wash, the more scratch-resistant nano-particle clearcoat
gave a reading of 72%.
The Research Institute for Pigments and Paints (FPL) in Stuttgart, which
worked closely with Mercedes-Benz in the testing programme for the new
clear lacquer, supplied a nano-scratch testing device which allowed a
precise examination of the nano-structure of the paint surface. The equipment
is fitted with a diamond point measuring two micrometres, which is passed
over the paint sample according to a pre-defined scale of graduallyincreasing
force and leaves behind an extremely fine scratch mark on the paint.
Experts then analyse the paint surface under the atomic force microscope
and take a series of measurements, including the force required for the
first cracks to appear in the paint structure.
Cracks begin to form in the nano-particle clearcoat when a force of 20
millinewtons (mN) is exerted, whilst the diamond point breaks through
conventional paint with only 7.4 mN of force. This scientific test thus
provided further evidence of the significantly greater scratch resistance
of Mercedes-Benz’ nano-particle clearcoat.
Mercedes engineers also assessed the chemical resistance of the innovative
clear lacquer using standardised procedures. As part of the testing programme,
samples of four different substances – sulphuric acid, pancreatin
(simulating bird droppings), tree resin and fully desalinated water –
were dripped onto the paint surface in rows and left to work their way
in over a period of half an hour and in temperatures of 30° to 75°C.
The engineers then assessed from what temperature the paint surfaces began
to show signs of lasting damage, with target values pre-calculated for
each substance. In this test, the newly developed nano-particle clearcoat
fulfilled the same stringent Mercedes quality criteria as conventional
paint systems.
The new nano-particle clearcoat has also passed other extreme tests –
such as extended exposure to ultraviolet light mirroring the intensity
of the sun in the US state of Florida – which has proved its readiness
for series production in every respect.
Weathering the tests
In the future, new automotive coatings will be introduced to the market
considerably faster due to an accelerated test method for determining
resistance to acid damage that has been jointly developed by BASF and
Q-panel Lab Products. This method allows a significant acceleration and,
at the same time, more testing cycles for the weathering tests required
in paint development.
The new test method – BASF accelerated acid test – simulates
the so-called Jacksonville weathering test. This special type of outdoor
exposure test can be performed only from mid-May to the end of August
in the Jacksonville, Florida area and provides information regarding the
resistance of coatings in case of acidic ambient conditions. To perform
the new accelerated test method in the laboratory, BASF Coatings in Münster-Hiltrup,
Germany uses a specially modified device provided by Q-panel Lab Products,
with which the first tests have already been performed on clearcoats.
The stress due to acid rain and dew, in combination with the subsequent
solar radiation, is a really severe test for automotive coatings. The
cocktail of sulphuric acid, nitric acid, hydrochloric acid, and ammonia
occurring during natural weathering is an aggressive enemy of car finishes.
“Acid etching” causes paintwork defects that look like dried
water drops, but cannot be removed by washing and polishing. The new accelerated
test method effectively supports the development of new coating systems
with improved acid resistance. In the modified Q-Sun xenon testing device,
panels are stressed by acids, different temperatures, increased humidity,
and the filtered radiation of a xenon burner in consecutive steps. During
a 420-hour testing period, the panels are subjected to stress comparable
to that of the 14-week-long Jacksonville test. As a result, instead of
a test that can be performed only once a year, approximately 20 weathering
tests can be performed for developmental purposes each year.
Last year, BASF Coatings invested around €3 million for a large-scale,
highly advanced paint testing unit, the so-called Robot Application Centre
(RACE), at its Münster plant in Germany. It allows line coating of
original-size body panels to be simulated. In the past, bodies passed
through a simple coating unit with fixed paint atomisers. A quick-change
coupling now allows for the fast and flexible deployment of high-rotation
atomisers and conventional pneumatic spray guns.
For BASF , the opening marked the end of an 18 month planning and construction
phase. Apart from the single-arm robot, the Robot Application Centre also
comprises a ventilation facility, drying ovens and the control system.
Tailor-made programme control allows a specific succession of coating
programmes.
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