Regulatory bodies have put more pressure on the automotive industry to move toward more sustainable materials and processes. Throughout several decades, regulatory requirements, such as reducing organic solvent emissions, CO2 reduction, and increasing fuel efficiency, have prompted the coatings industry to develop ecologically sustainable coatings. These include high solids or waterborne materials and consolidated systems such as primer-less, two-wet, or three-wet systems. In order to reduce the overall weight of the car body, the OEMs are increasing the use of lower weight materials, including aluminum, magnesium, and polymeric substrates. Paint shop processes and materials must adapt accordingly – in the case of polymeric substrates, lowering the baking temperatures for coating hardening.
Lightweight materials help cut down CO2 emissions in the paint shop, as energy consumption can be reduced by using low-bake materials. These materials also offer enhanced styling capabilities due to new opportunities in part and vehicle geometry. The possibility of on-line and in-line coating of body and plastic parts offers an opportunity to improve color harmony and to simplify supply chain complexity.
However, these lightweight substrates will create challenges for coating technologies. For instance, new adhesion processes need to be developed, impacting baking of adhesives and coatings. Long-term durability and re-coatability of low-bake systems must also be ensured. Overall appearance of the final finish, which may cover various different substrates and lines, must be harmonized.
Engineers at Axalta Coating Systems, a global supplier of liquid and powder coatings, have been looking at ways to meet the challenges that these new lightweight materials are presenting.
Technologies for low-bake systems
Some solutions are already in place for low-bake coatings. These include well-established processes with technologies and substrates for:
- Automotive plastic parts
- Heavy-duty trucks
- Specialized OEMs using low bake mixing systems to provide small batches of customer-specific matching colors
Additional solutions and processes are still needed to meet the challenges presented by the newly created, diverse mix of substrates. Here are some of the various paint technologies for low-bake systems.
UV curable systems – The bake window can be brought down to ambient temperature, but there can be problems from radiation shadow zones. These may be avoided by mounting UV lamps on robots that can reach all areas and ensure proper curing. This solution could be used in combination with a three-wet or primer-less waterborne system, offering the opportunity to reduce overall clearcoat oven length by up to 70%.
Crosslinking technologies – Systems such as 2K epoxy acid, 2K acrylic/melamine with unblocked catalyst, or 1K acrylic/melamine with blocked catalyst, have the potential to reduce bake temperatures close to the target, but can have issues, such as stability or technological properties in curing. So far, the leading cross-linking technology for low-bake coatings is 2K polyurethanes.
(Poly) aspartate chemistry – Accessible through a straightforward synthetic procedure, shows promising results in achieving very good technological properties such as durability and chemical resistance, even at ambient curing temperature through very fast cross-linking reaction. This also could allow formulating higher-solids coatings to combine the benefits of lower bake and lower solvent emissions.
Another challenge is lowering the curing window for electrodeposition (ED) coating, as there will still be significant metal components in future body design. The route to lower bake ED coatings can be described in three developmental stages. The first is a target bake of Tmax on the exterior at 180°C, which would mean minimum required temperatures of 165°C to170°C. This could contribute to energy reduction in the paint shop. Axalta has had positive results based on current commercialized Aqua EC4027 technology with which it has achieved good solvent resistance and corrosion results down to 10 minutes at 150°C.
The next stage in development could reach the temperature zone (Tmax = 140°C) enabling on-line coating of specific plastic parts – so that metal and plastic pasts can be run through the ED bath and top coated on-line. Here, there is need for new cross-linking chemistry and significant development in process and materials, including higher temperature requirements for bake-hardening steel. The potential benefits of further reducing ED baking temperatures, down to 80°C to 125°C for instance, would need to be proven.
Lightweight body design will inevitably drive further innovation in the area of low-bake coatings, materials, and processes. And while some technology is already available, more is being developed that will address the challenges presented by new materials. The critical next step will be further integration of single solutions – materials, processes, and application – in real mass manufacturing environments.
Axalta Coating Systems
About the author: Dr. Klemens Bartmann is the global director of technology for original equipment manufacturing at Axalta Coating Systems. He can be reached at email@example.com or 220.127.116.119.0