Painting trends in the automotive industry Balancing optics and sustainability

By Doris Schulz

Painting provides more than just a protective skin: it
affects the vehicle’s appearance and thus the customer’s impression as well. At
the same time, paining is one of the most energy-intensive processes in vehicle
manufacturing. As a consequence, the painting technologies industry is working
on solutions which promote efficient use of resources, while simultaneously
optimising quality.

Whether the auto body, the interior or the exterior is
concerned – the optimisation of the qualitative properties of the paint coating
is one of the challenges faced by the automotive industry and its suppliers. On
the one hand, this involves improving resistance to corrosion, as well as to
other physical, chemical and mechanical stressing, to which the various vehicle
components are subjected.

On the other hand, emphasis is also placed upon enhanced
optics and haptics, which serve the purpose of vehicle individualisation. Rising
cost pressure due to global competition and demand for environmentally sound
production are making it necessary to lay out the painting process in a more
efficient, resource-conserving manner. A further aspect which influences the
painting process is the increasing mix of metals used in auto bodies and
components which results from the expanding use of lightweight construction
strategies.

More ecological pre-treatment

Certain prejudices still prevail – in particular where the
auto body is concerned – but the baths used to

pre-treat auto bodies and metallic parts are more and more
frequently “cold”. The reasons for the increasing popularity of nano-ceramic
pre-treatment processes, which can be implemented as immersion or spray
procedures, are obvious: as opposed to conventional zinc phosphating, the
heavy-metal-free alternative can be run at room temperature. This results in
savings in the areas of energy consumption and emissions. Minimal expenses for
wastewater treatment and disposal, as well as system cleaning and maintenance,
are a further advantage. And this pre-treatment technology has another trump up
its sleeve with regard to lightweight construction as well, because its
multi-metal compatible.

Ecologically beneficial alternatives are also available for
the initial coat based on cathodic dip painting. These alternatives contain
less the 1% solvent and are tin-free, which makes them compliant with future
European legislation, as well as other regulations. Dip painting is
distinguished by ideal coverage and good corrosion protection.

When auto bodies are transported through the pre-treatment
and cathodic dip painting processes, shuttle systems with freely programmable
axes for advancing, lifting and rotary motion allow for precise matching of
immersion curve and processing time to the respective body shape. This
coordinated motion sequence also provides for a uniformly even coating, as well
as fewer enclosures and less power input on bonded surfaces, thus assuring
optimised quality.

Skid-free conveyor technology is becoming more widespread,
not only in pre-treatment, but rather in the actual coating process as well.
This is not least due to the associated energy savings which result from the
fact that heat lost in warming up the large skids in the oven is eliminated.

More streamlined painting processes

For paint processes with filler coats, as well as for
so-called integrated systems, water-based paint alternatives are ecologically
more efficient than their solvent containing counterparts despite higher levels
of energy consumption and the resultant CO2 emissions. This is due to their
lower VOC emissions. In the case of conventional processes with filler coats,
wet-on-wet application (by means of which the primer, the base coat and the
clear coat are applied without intermediate drying) offers economic and
ecological advantages. These include shorter production time, reduced energy
consumption, shorter painting lines and lower emissions. These advantages are
even more significant where integrated painting processes are involved. In this
case, application of the filler coat is eliminated together with its associated
baking step. This results in reductions in energy consumption and CO2 emissions
of up to 20%. Last but not least, the entire filler coat section of the
painting system becomes superfluous, along with the associated investments. For
these reasons, integrated processes are becoming more and more popular, even
for painting plastic parts. Minimising paint loss also offers potential for
streamlining painting processes. Electrostatically assisted spray guns and
high-speed rotary atomizers contribute to the achievement of application
efficiency levels of greater than 90%. Consequently, these systems are being
used to an ever greater extent for interior auto body painting. This type of
painting is frequently automated with the help of robots. In addition to
reduced material consumption, robots also allow for better reproducibility of
the painting results and lower scrap rates, as well as easier changeover from
fresh air exhaust to recirculating air systems. This, in turn, results in
energy savings of between 60 and 70%.

Research is currently being conducted on new application
techniques, which are targeted at paint application with practically no
overspray at all. The simulation of paint application also supports the
minimisation of paint loss in this respect.

However, until it’s possible to apply paint without any
overspray, the most efficient way to remove paint mist must be found.
Electrostatic separator systems are one solution. As compared with conventional
wet scrubbing, up to ? of the required energy can be saved, and water
consumption can be reduced by more than 85%.

Energy-efficient drying

The industry is also working on energy efficient drying
processes. Approaches include optimised air routing through sluice and tunnel
areas, as well as optimised heating units and waste-heat utilisation systems.
Energy on demand is another approach. This solution focuses on the adaptation
of volumetric air flow and exhaust-air purification to zone-specific
requirements. Potential energy savings should also be exploited through the use
of highly efficient air distribution nozzles. They make it possible to heat up
thick-walled sections of the auto body more quickly, thus allowing for a
shorter drying zone. In addition to saving thermal and electrical energy,
overheating of thin-walled sections and deformation of components can also be
avoided in this way.

UV technology on the advance

UV paint systems make it possible to cure a layer of paint
in just a few seconds. Amongst other characteristics, the resultant paint layer
is very hard, as well as highly resistant to scratching, abrasion and
chemicals, after curing. However, coating and curing 3D components with these
environmentally friendly paints has proven problematic thus far, for which
reason they are used primarily for two-dimensional parts with simple shapes.
The problems are caused by the fact that the paint is not fully cured until all
of the component’s painted surfaces have received the required dose and
intensity of UV light. In the meantime, solutions have nevertheless resulted
from research projects, with the help of which three-dimensional metal and
plastic parts can be reliably coated with UV paints. Application, flash-off and
curing take place in an inert gas atmosphere with these solutions.

The inert gas, for example CO2 or nitrogen, reduces oxygen
content to a minimum, allowing adequate radiation energy to arrive at the
parts. The paint is cured in just a few seconds solely by means of UV
radiation, without any thermal process. This procedure, which is
extraordinarily short as compared with conventional drying, not only allows for
very high production throughput speeds. The typical, long and energy-intensive dryers
and downstream cooling zones become superfluous as well. This results in
advantage with regard to floor space requirements, investment and operating
costs, as well as energy consumption and CO2 emissions. Theoretically, it’s now
even possible to coat and cure an entire auto body with a UV clear coat, but
this is still associated with considerable expense.

Individuality in demand

The customer is king and his desires are fulfilled – with
regard to painting as well. The number of special paint finishes is increased
as a result. Amongst others, pearl effect paints are currently in fashion,
which place greater demands on painting and paint delivery systems. In the case
of clear coats for the high-gloss auto image, systems which offer increased
resistance to scratching are at the top of the priorities list. The paint
industry offers developments to this end which combine the advantages of “hard”
inorganic substances and “soft” organic substances, thus providing for a
certain self-healing effect.

Combination surfaces and plastic parts with chrome effect
represent a further trend. Plastic components are chrome plated by means of
either conventional electroplating or PVD
coating Conventional pigmented systems and UV-curing paints are available for
protecting the metal layer after the coating process, and for providing it with
an individualised appearance.

In the case of high quality interior components made of
wood, plastic or metal, an innovative curtain coating system is offering new
perspectives for reaction injection moulding (RIM). This modular paint system
is based on 2-component, solvent-free, aliphatic polyurethanes which are
processed in high pressure systems. Coating to a thickness of 0.1 to 2 mm
is accomplished in a single process step. The part to be painted is inserted
into an oversized mould to this end, and the remaining space is filled with paint. Depending on
part geometry and material type, coating and cross-linking take two to three
minutes. The part can then be removed from the mould and further processing is
possible after roughly 24 hours, although no grinding is necessary. In contrast
to multi-coat painting, this process not only offers time savings, it’s also
highly efficient thanks to minimal material loss. The surface fulfils the
requirements of the automotive industry, and is distinguished by good
resistance to heat and UV radiation. Surfaces with better scratch resistance
and a self-healing effect can also be produced in this way. If the surface is
scratched, the paint system’s self-healing function becomes active at room
temperature, or it can be triggered by applying a minimal amount of heat.
Second to the coating of interior parts, current developments are focused on
exterior use in piano black.