Innovation and challenge: lightweight multi-substrate body design and low bake coatings
Dr. Klemens Bartman, Global Director OEM, Axalta Coating Systems reports on ways how to meet the challenges that new lightweight materials present. Increasing pressure from regulatory bodies has been put on the automotive industry to move towards more sustainable materials and processes. Over several decades, regulatory requirements, such as reduction of organic solvent emission, overall CO2 reduction and increased fuel efficiency, have prompted the coatings industry to develop new, ecologically sustainable coatings.
In order to reduce the overall weight of the car body, the OEMs want to increase the usage of lower weight materials, including aluminium and magnesium and polymeric substrates. Paint shop processes and materials need to be adapted accordingly. In the case of polymeric substrates, the baking temperatures for the coating hardening processes need to be lowered.
The implications of lightweight body design
The lightweight materials help to cut down CO2 emissions in the paint shop, as energy consumption can be reduced when 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 means there is an opportunity to improve colour harmony and to simplify the supply chain complexity. However, these lightweight substrates will create challenges when it comes to coating technologies. For instance, new adhesion processes need to be developed, which can impact on the baking of adhesives and coatings. Long-term durability and re-coatability of lower bake systems must also be ensured. Overall appearance of the final finish, which may cover various different substrates and lines, must be harmonised.
Existing and potential paint technologies for low bake systems
However, there are already some solutions in place for low bake coatings. These include well-established processes with a multitude of technologies and substrates used for: automotive plastic parts, heavy duty trucks that require an increasingly demanding appearance and high quality levels from low bake primers and topcoats, and specialised OEMs where low bake mixing systems provide small batches of customer-specific ‘tie colours’. Nevertheless, additional solutions and processes are still needed to meet the challenges presented by the newly-created, diverse mix of substrates. Of the various paint technologies feasible for low bake systems, each has advantages and challenges. With 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%.
Other crosslinking technologies, 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 other issues, such as stability or technological properties with these curing conditions. So far, the leading cross-linking technology for low bake coatings is 2K Polyurethanes.
Another option is the (Poly) Aspartate chemistry, which is accessible through a straight-forward synthetic procedure. It shows promising results in achieving very good technological properties like durability and chemical resistance, even at ambient curing temperature through very fast cross linking reaction. This also gives the possibility of formulating higher solids coatings to combine the benefits of lower bake and lower solvent emissions.
Last but not least, there is another challenge in lowering the curing window for electrodeposition 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 T max on the exterior at 180oC, which would mean minimum required temperatures of 165oC -170oC. This could contribute to energy reduction in the paint shop. We had positive results based on current commercialised “Aqua EC4027” technology with which it has achieved good solvent resistance and corrosion results down to 10 minutes at 150oC.
The next stage in development could reach the temperature area (T max = 140°C) enabling on-line coating of specific plastic parts – in other words, metal and plastic pasts can be run through the ED bath and top coated on-line. Here there is the need for new cross-linking chemistry and a significant development of process and materials. The issue of higher temperature requirement for bake hardening steel would also need to be addressed.
The potential benefits of a further reduction of ED baking temperatures, down to 80oC -125oC for instance, would need to be proven – this would, however, be a longer term development process.
Lightweight body design will inevitably driver 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 that the new materials present. The critical next step will be further integration of single solutions – materials, processes and application – in real mass manufacturing environments.
Ask your questions
Meet Dr Klemens Bartmann at the European Coatings Conference “Automotive Coatings” on 24/25 September in Dusseldorf. He is going to present “Between differentiation and standardisation: How advanced coating solutions meet OEM market trends”
