Developments from consumer expectations to new paint processes and current environmental considerations
Today, paint finishes are playing an increasingly important role in determining customer choice, whereas automotive paint originally served simply to protect vehicle body elements from corrosion.
The quality of the coating must meet strict requirements in order to ensure that the relevant visual properties are achieved. The painting process is also influenced by the expanding range of materials used for automotive bodies and other components as a result of lightweight construction strategies. Furthermore, rising cost pressure due to global competition and tighter regulations for environmental protection necessitate more efficient, resource-conserving paint processes.
It is well-known that more than half of the energy required for automotive body fabrication is currently consumed by the painting process. Potential savings in spray painting have been investigated by a German project called Green Carbody Technologies – Energy-Efficient Painting, which is summarised elsewhere in this issue of APS (see article on pages 26-27).
However, it is worth mentioning some of the underlying motivations here. For example, the trend towards vehicle customisation through the use of multi-coloured paint finishes provides an incentive to conduct research on how to achieve minimal paint loss.
Another area of focus for the project was energy-efficient drying; primary energy consumption can be reduced by roughly 30% in existing painting systems through the use of innovative drying nozzles, a reduced exhaust air rate and control of volumetric flow for recirculating air. In new projects, opportunities are presented by skidless auto body transport, partial exhaust air purification concept, heating units with integrated exhaust air purification and cyclic operation instead of continuous feed.
Elsewhere, a growing trend is for nano-ceramic processes to provide corrosion protection and optimised paint adhesion instead of conventional zinc phosphating when pre-treating automotive bodies and metallic components. This is based on the ecological and economic advantages of systems which are compatible with numerous metals, not least that products which can be applied by means of dip and spraying processes will be free of toxic heavy metals.
In the meantime, environmentally friendly alternatives to cathodic dip painting are also available. These alternatives contain less than a single percentage point of solvent and are tin-free, which makes them compliant with likely future European legislation, as well as other relevant regulatory frameworks. Dip painting is distinguished by excellent coverage and good corrosion protection. While destructive testing is still the established practice for quality control of cathodic dip painted components, this costly quality control process can be reduced or replaced by means of simulation technologies.
Also now dispensable is the traditional power washing system for the pre-treatment of plastic parts with downstream retained-water drying. This cost, space and energy intensive variant is being replaced with increasing frequency by alternative processes such as CO2 snow-jet cleaning, plasma processes or the steam cleaning method.
Due to their requirement for a drying process, water-based paint systems consume more energy and emit more CO2 than solvent-based counterparts, but on the other hand have a better overall ‘ecological balance sheet’. The reason for this is their minimal VOC emissions. Thanks to wet-on-wet application of the primer, the basecoat and the clearcoat without intermediate drying, their economic and ecological advantages can be exploited with conventional painting processes.
Less energy is consumed and fewer emissions are produced in this way; compressed production times and shorter painting lines also strongly favour solutions that dispense with the need for intermediate drying. So-called ‘integrated processes’, in which filler coat application and drying are eliminated, allow for reductions in energy consumption and CO2 emissions by up to 20%.
One approach to minimising paint loss involves increasing transfer efficiency by using electrostatically supported paint guns and high-speed rotary atomisers. Transfer efficiencies of greater than 90% can be achieved through the latter method. The increasing degree of automation through the use of robots is also leading to reduced paint consumption for interior as well as exterior painting of automotive bodies. Painting robots make it possible to switch over from fresh air exhaust to recirculating air systems, resulting in energy savings of 60-70%.
Conventional wet scrubbing of overspray consumes a great deal of energy and water; savings in this area are possible with the use of the dry scrubbing alternative. Electrostatic precipitation systems represent one way of achieving this. They enable overspray to be fed to the precipitation system over the entire length of the painting area, while the air charged with paint particles flows through the alternately arranged active and passive elements of the precipitation system. The paint particles are thereby effectively precipitated and removed from the process. Due to the fact that no mechanical filter systems are required, constant flow conditions prevail in the painting booth without any pressure fluctuation, thus resulting in ideal conditions for good painting quality.
In addition, air conditions are altered only minimally due to precipitation. Consequently, most of the purified air can be fed back to the painting booth. Depending on which type of paint is being used, this allows for up to 95% use of recirculated air. Furthermore, as compared with conventional wet scrubbing, up to 75% of the required energy can be saved, while water consumption can be reduced by more than 85%.
A simpler option for dry scrubbing is a mechanical overspray separation system of modular design. This solution, which can be operated with recirculating air, as well as with supply and exhaust air, functions entirely without water, chemicals or any additives. The air charged with paint particles flows through a hybrid filter, which is a combination of surface and depth filters. The individual filters are composed of receptacle and chamber-like structures. This provides for effective coarse and fine filtering.
A second, separate filter stage for ensuring that the desired degree of filtration is reached is located downstream from the separation modules. The modules can be set up and replaced easily and quickly by untrained personnel. This system, the modules of which consist primarily of recycled materials, can also be retrofitted to existing painting systems.