Chemical Considerations for Automotive Applications

By Brandon Lloyd

As the automotive industry evolves with a focus on lightweighting, increasing utilization of alternative substrates, and expanding the electric vehicle (EV) market, powder coating has kept pace through formulation of new powder types and low temperature curing technologies. But how have these changes impacted the chemistries used in support of the powder coating process?

In its early days, automotive lightweighting was a somewhat straightforward initiative—reduce the amount of steel in a car to increase gas mileage and decrease greenhouse gas emissions. This was to be accomplished by replacing steel with aluminum in car bodies. What was a seemingly simple endeavor brought about a number of challenges for automakers such as material strength considerations and the welding of dissimilar materials, amongst others. Today, lightweighting trends are being driven by factors beyond basic weight reduction, including sustainability, cost, performance, and the ever-growing EV market.

A Shift in Substrates and Chemistries
In preparing components for coating, shifts in the automotive industry have increased the need for multi-metal safe cleaning solutions. Each metal, whether aluminum, zinc, steel, or to a lesser extent, magnesium, can encounter a variety of manufacturing processes like stamping, forming, casting, etc. Each of these processes introduces different types of soils and contaminants to the parts that must be removed before coating. Therefore, a robust multi-metal compatible cleaning solution is necessary. Such solutions enable a variety of components to be processed together and minimize investment by requiring just one line rather than two separate lines for metal and part types. They also reduce floor space requirements and can increase production throughput.

When it comes to conversion coatings, the increased use of aluminum has allowed OEMs and tier suppliers to transition away from traditional chemistries like phosphates to move toward more environmentally friendly options like zirconium-based pretreatments. This naturally supports a more sustainable powder coating pretreatment solution that directly supports decarbonization efforts. The majority of transitional metal coatings advancements across the industry are focused on operating at low or ambient temperatures, which result in energy usage reduction and immediate carbon footprint improvement. This can significantly improve the total cost of operation/ownership for the applicator without sacrificing performance.

Depending on the specification for a particular application or production piece, the coating characteristics or requirements can vary and impact the pretreatment process. For example, an automotive OEM could specify a conversion coating weight or thickness for thin film/zirconium coatings that differs from what is typical in other industries. This would require a greater level of control to ensure the pretreatment process is creating a consistent coating.

Operating parameters like concentration, contact time, and temperature can impact the coating weight and deposition uniformity. Part cleanliness also plays a role in coating characteristics which means cleaning and other surface preparation steps must be closely observed as well.

The Impact of EVs
With nearly every automotive manufacturer offering at least one EV model, the growth of this market segment has had a major impact on the powder coating industry. EVs have introduced entirely new components to auto manufacturing, both within and outside of the battery system, expanding powder coating applications.

Parts within or in close proximity to the battery system present unique challenges and require special consideration due to high energy storage and voltage inherent in electric vehicle battery cells. Components such as the battery enclosure and thermal management systems may require a dielectric resistant coating to help reduce the risk for expansive battery failure. Part preparation and coating application are crucial, as a defect, void, or even a powder coating that is too thin, can increase the risk of current passing through those components and potentially proliferating throughout the battery system.

There are a variety of applications where new specifications are being implemented. As noted, dielectric resistance is an important requirement in EVs, and while existing thin film pretreatments are capable of meeting the specified requirements of the coating, the component construction or composition may dictate a more specialized surface preparation to adequately prepare the part. In some cases, a component may require the application of powder in some areas while other areas remain passivated only (no powder), meaning the same conversion coating will be used but must satisfy varying requirements within the component. For the powder coated area there will be the typical requirements of excellent adhesion and corrosion resistance, while the passivated-only areas may need to have a low contact resistance for welding or high bond strength for structural adhesive applications. With the expansion of adhesive bonding to be used in conjunction with or as a direct replacement for welding or fastening, the integrity of the pretreatment is critical to ensure the bond between the two components will not be compromised. In these instances, existing thin film pretreatment may not meet all of the requirements, creating a need for new processes developed specifically for these applications.

While automotive components are of course still tied to specifications, OEMs are more open to different technologies based on their technical or commercial benefits. This is particularly the case when it comes to new components in EVs for which a history of production experience is lacking or where there are differing technical demands. There is a level of open-mindedness to consider new technologies and/ or push the limits on existing processes as OEMs look for ways to improve profitability and increase production volumes in a highly cost-competitive market, while striving to make EVs more affordable for consumers.

Other Chemical Considerations
In the powder coating world, chemistry as it relates to pretreatment is certainly a focus. However, another treatment method also plays an important role—paint removal or stripping. All powder coaters know, or should know, that grounding is a crucial fundamental to an efficient powder coating operation. If grounding is ignored, transfer efficiency is reduced, resulting in higher powder use and ultimately a higher cost of operation. Rack maintenance is critical for a solid ground and the removal of coating buildup must be top of mind.

For many years, processes like media blasting, molten salt, and even burn-off ovens have been used for rack maintenance. However, in most scenarios these processes are not feasible for part reclamation because of the impact they can have on metallurgical properties of parts, especially for light metals.

These conventional methods also carry with them negative effects like long strip times, secondary cleaning processes to remove ash and dust, as well as rack damage from thermal distortion, resulting in costly repairs or replacement.

Advancements in chemical paint removal methods minimize or eliminate typical negative effects while improving the efficiency of the stripping process. In contrast to early chemistries that utilized chlorinated solvents, concentrated acids, and other hazardous processes, modern paint removal chemistries are free of the concerns associated with previous commodity-based processes. They are also highly applicable for multi-metal processing, with no negative impact on metallurgical properties, including light metals such as aluminum and zinc, making them an ideal fit for today’s auto manufacturing trends.

While chemistries used to create cleaners, conversion coatings, and strippers are changing to align with new trends and technologies, ultimately all are intended to support the same end goal—adhesion and coating resistance for safer, longer-lasting vehicles.

Brandon Lloyd is global product manager paint support and technology at MKS Instruments, Inc. | Atotech USA LLC.