How to Make Automotive Paint Shops More Sustainable

Q. Our paint shop is the place of highest energy consumption and highest VOC and CO₂ emissions in the car manufacturing process. What measures can be taken to reduce them?

A. Sustainability combines ecological, economic and social components. These objectives have been given a high priority in production technology. They link profit-seeking to questioning how the profits are generated. Practically all OEMs have set the goal of ensuring sustainability and at the same time, efficient production.

For all of these considerations, paint shop efficiency is the focus. It is still by far the biggest VOC emission source and the biggest energy consumer. Painting accounts for almost 100 percent of all VOC emissions in an automotive plant and well over 50 percent of total energy consumption.

Where can improvements in efficiency be achieved?

Process efficiency: high plant availability, high first run rate and high flexibility
Emission efficiency: lower VOC, CO₂ and particulate emissions, less noise and malodorous emissions
Energy efficiency: innovative products and systems with minimal power consumption
Material efficiency: high paint application efficiency and low flushing losses of the application technology, minimal water consumption
Space efficiency: intelligent layouts and space-saving plants

Significant progress has already been made in recent years. An important contribution to the reduction of VOC emissions is the introduction of water-based paints, which has now reached a high level worldwide. Further advances are achieved by the increased use of primer-less and compact painting processes. Low-temperature painting processes and so-called low-bake painting processes could make an important contribution to further reducing emissions and energy consumption in the future.

Improvement in Application Technology

In addition, both the plant and application technology offer considerable potential for reducing the environmental impact and conserving resources. In the application technology, the high-speed rotary atomizers were specifically tailored for the individual tasks. Their compact design made it possible to use atomizers with external charging in car interior painting, as well. Application efficiency was increased further and the associated reduction in overspray is clearly recognizable. The compact design results in lower atomizer contamination. This means not only less cleaning effort with the associated production interruptions, but also less cleaning agent consumption and thus lower VOC emissions. It is, of course, impossible to completely avoid contamination of the atomizers, especially when it comes to interior painting. The housing and the shaping air ring must be cleaned from time to time. Fully automatic cleaning devices are available for this purpose. The latest generations of atomizer cleaners consume only 50 mL of cleaning agent per cycle, compared to 550 mL for previous devices. The extraordinary feature is that cleaning can be carried out in the available cycle time.

Dry Separation of Overspray

One example on the plant side is the dry separation of paint overspray. The overspray is thereby bound with powdered limestone. The advantage is that there is no paint sludge as in conventional wet scrubbing. The powdered limestone is replaced automatically at a certain degree of saturation and can be recycled, for example, in cement plants. With a special fluidized bed furnace, the powdered limestone can be re-utilized directly in the paint shop. The organic paint components of the saturated binding agent are burned and thus energetically recycled. The recycled powdered limestone regains its paint-binding properties. This process can be repeated several times without degrading the quality of the binding agent. In other words, the repeated use of the powdered limestone reduces consumption and thus also the cost. Through the use of this technology, which allows for recirculated air in the paint spray booth, the annual energy consumption of a fully automated paint shop is reduced by up to 16 million kWh. At the same time, energy reduction is also associated with up to 5,200 metric tons less CO₂ emissions.

Improved Control Technology

Intelligent process air conditioning, i.e. optimum control for air supply and air recirculation units, can also help to ensure stable and energy-efficient operation of the paint shop. Special software thereby controls the temperature and humidity level in the paint spray booth. The set point value is automatically adjusted ten times per second to the external conditions. The different control variables are optimally coordinated just as often.

Innovative Oven Concepts

The biggest energy consumer after the spray booth is the oven. In order to reduce the energy requirement, fresh and exhaust air can be controlled according to the number of car bodies in the oven. This means a process-optimized volume flow and thus reduced energy consumption during breaks or in the case of different capacity utilization. In the latest designs, the car bodies no longer pass lengthwise through the oven, but rather in cycles perpendicular to the conveying direction.

This operation revolutionarily permits heating from the inside. Through the windshield opening, nozzles are used to blow hot air with high discharge ranges into the vehicle interior. The result is a significant improvement of the leveling characteristics with up to 30-percent shorter heat up times and by up to 25-percent reduced energy consumption.

Innovative Plant and Application Technology

Today, a painted car body ideally needs only around 400 kWh of power. A few years ago, even more than three times that amount was sometimes needed. VOC emissions can now be pushed well below the permissible value of 35 g/m². Water consumption has also been reduced by almost 60 percent.

This was achieved mainly by dry overspray separation and the use of rotation dip technology in pretreatment. In the end, paint consumption too could be reduced by almost 50 percent. Besides lower costs, less paint consumption also means less residual materials and solvent emissions.

Sustainable automotive paint finishing is not a just a vision; it has become a reality. Numerous individual solutions, products and systems contribute to this achievement. It would nevertheless appear that an integrated concept has a greater impact than the sum of the individual components.

Dr. Svejda is the key account manager for Application Technology for Dürr Systems AG. Visit durr.de.