The automotive industry is going through seismic changes. One of the key megatrends that is impacting this industry is sustainability and the drive toward a circular economy. At the heart of sustainability are the regulations in different regions of the world on fuel efficiency and exhaust emissions. Some vehicle manufacturers and governments are mandating bans on sales of petrol and diesel-powered vehicles. This is leading to greater technology innovation for electric vehicles (EVs) to achieve cost parity with internal combustion engine (ICE) vehicles and to increase consumer confidence in EV range.
Car manufacturers are making changes for success in the circular economy by lightweighting vehicles to reduce emissions, in case of ICE vehicles, and for achieving higher miles per charge in EVs. Additionally, the entire automotive industry is on a mission to reduce Scope 1, Scope 2 and Scope 3 greenhouse gas emissions, which impacts the choice of materials and processes?
“To protect the planet from climate change and the population from harmful effects of volatile organic compounds [VOCs], regulatory requirements and manufacturer initiatives are compelling the automotive industry to reduce emissions and waste, to improve energy consumption and to support higher fuel efficiency standards,” said Dr Sanghamitra Sircar, global product manager for Axalta Mobility Coatings. “In response, the automotive industry is developing lighter weight materials for vehicle bodies and structures, and using more sustainable raw material in the factories.”
The coatings industry is playing its part by supporting these changes. The industry has come a long way from the manual application of varnishes with limited colour palettes and longer drying times of the 1900s. Today, the industry has several primerless applications, waterborne coatings, high-solids waterborne and solvent-borne coatings and harmonised coating systems – along with robotic bell or one bell applications with a range of metallic and pearlescent colours.
This has already resulted in significant reduction of organic solvent emissions and energy usage in the paint shop. For example, energy usage has gone down from about 2 500KWh/car in 2010 to less than 700Kwh/car now, based on publicly available OEM CSR reports. Equally, VOC emissions in several parts of the world are trending down from >600g/l just a few years ago to 420g/l in waterborne basecoats. And now, manufacturers’ use of multi-substrate, lightweight automotive body designs are aimed at reducing the overall weight of the car body.
Lightweight materials such as high-strength steel, aluminium, magnesium, carbon fibre reinforced plastics, sheet moulding composites and thermoplastic polyolefins, help increase mileage in EVs and reduce exhaust emissions. However, these lightweight substrates create challenges for coating technologies in the following areas:
- Coatings uniformity and colours harmony over multiple substrates.
- Heat distortion of plastic substrates.
- Adhesion and sealing processes, which can impact the baking of adhesives and coatings.
“As technological advancements due to regulations and customer demands have increased over the last eight decades, Axalta has continued to leverage our extensive expertise in the automotive, refinish and industrial segments to develop and to implement suitable coating systems for car manufacturers’ new substrates,” Sircar said.
Low-bake systems are designed for varying substrates, usually comprised of plastics and composites, which cannot withstand high temperatures. There are already some solutions in place for low-bake coatings. These include well-established processes for a multitude of technologies for automotive plastic parts and heavy-duty trucks, which require increasingly demanding appearance and quality levels from low-bake primers and topcoats – and specialised VMs where low-bake mixing systems provide small batches of customer specific special edition colours.
Nevertheless, additional solutions and processes are still needed to meet the challenges presented by the diverse mix of substrates. Newer coating formulations have to adhere to the high-performance surfaces and to maintain superior appearance and durability.
“Lightweight vehicle construction is definitely driving low-bake coatings innovation,” Sircar said. “Over the next decade, automotive manufacturers will accelerate the replacement of high-strength steel with advanced high-strength steel, aluminium, plastics and composites. They will also lower the weight from safety, communication and electrical components in an effort to make vehicles lighter to get higher mileage per charge in EVs and to reduce emissions and fuel consumption in ICE vehicles.”
Axalta and other paint manufacturers already provide low-bake systems which cure at lower temperatures than traditional systems to help car manufacturers reduce full-chain energy costs by painting metallic and non-metallic parts together at the same time on the same line. Axalta continues to develop lower-temperature topcoat systems and continues to work with customers to make low temperature coatings a success. The company also has a broad-bake electrocoat for e-coat temperatures from 140°C to 200°C to allow uniform coating of EVs without slowing the line speeds.
Currently, the leading technology for low bake coatings is 2K polyurethanes. Several non-isocyanate technologies like 2K acrylic/melamine with unblocked catalyst, or 1K acrylic/melamine with blocked catalyst have the potential to reduce bake temperatures to the target range. Other cross-linking strategies that have traditionally been used in industrial applications have shown interesting results in automotive low-bake conditions. In all cases, the challenge is to blend the reactivity of the cross-linking technology with system stability and film properties at these restricted curing conditions.
One promising option is (poly) aspartate chemistry, which is accessible through a straightforward synthetic procedure. It shows encouraging results in achieving very good technological properties like durability and chemical resistance, even at ambient curing temperature through very fast cross-linking reaction.
Sircar explained: “This also gives the possibility of formulating higher solids coatings to combine the benefits of lower bake and lower solvent emissions.”
Lightweight body design will inevitably drive further innovation in the area of low-bake coatings, materials and processes for both manufacturers and repairers. And while a certain amount of technology is already available, more is being developed that will address the challenges that the new materials present.
The next decisive step to enable maximum reduction of CO2 emissions from coatings is for all parts to be painted together on the same line. This will mean lowering the bake temperature at the clearcoat bake stage. In the case of EVs, with heavy battery carriages underneath the body, this will also require a broad-bake electrocoat so that the higher thermal-mass – containing battery system gets coated evenly and at the same time as the other sections of the car body.