“Could you describe the vehicle involved in the incident, please?”
“It was a light blue late model sedan.”
This exchange illustrates the point that the paint is one of the things people remember most about a car. Automotive coatings decorate and protect the exterior and interior of our cars. They also can identify a brand or personalize a car into a unique statement.
Automotive coatings fall into categories such as application (OEM or refinish) and chemistry (e.g. solvent-based or waterborne), and can fall into more than one category. Each category has its own set of performance requirements, formulation tricks and raw materials needed for success. This article will outline the definitions for the various categories, as well as provide a breakdown of the materials used in automotive coatings, and the categories where they are used.
The broadest division of automotive coatings comes between Original Equipment Manufacturer (OEM) and refinish applications. The former are used under well-controlled conditions in the “paint shops” of assembly plants while the latter may be applied in a clean, well-equipped repair facility or a DIYer’s garage and anywhere in between. While the technology used for automotive coatings is different based on global region as well as end use, this article will focus on applications within the NAFTA region.
OEM Automotive Coatings
That gleaming, durable finish on your new car is the result of years of coordinated development by coatings suppliers and the automotive manufacturers. Today’s OEM automotive coatings technology provides optimal levels of protection from the environment, durability and variety of appearance. Choice among OEM automotive coatings has greatly improved today, unlike the early days when Henry Ford reportedly commented that the customer could have a Ford in any color they liked as long as it was black.
Two main drivers of change in OEM coatings technology are environmental regulation and process simplification/cost reduction. Pollution reduction goals led the industry first to high solids and then to waterborne formulations. At the same time, customers’ expectations of improved appearance and long-term durability pushed the automotive manufacturers to use base coat/clear coat systems.
Recent OEM coatings development combines these advances with process simplification in “compact systems.” One such coating system, “3-Wet,” applies primer, base coat and clear coat without baking between each layer. This reduces the number of steps required to achieve a finish with the desired durability and appearance while meeting pollution and cost reduction goals.
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Once a car leaves the factory, it may need to be repainted for a variety of reasons. The most likely reason is repairing collision damage. Like the OEM coatings market, innovation in this category is driven by environmental regulations and process simplification/cost reduction.
Both waterborne and high solids repair coatings are commonplace, but waterborne technology is mostly utilized for base coats in regions with the strictest requirements while high solids prevail for clear coats and base coats in regions with less restrictive pollution control regulations.
Since labor is a significant component of collision repair, the development of “speed clear” coatings are becoming more popular. They reduce the time between clear coat application and final buffing of the finish, resulting in process simplifications and cost reductions.
Other markets for refinish coatings include vehicle restoration and custom paint jobs. The ability to match vintage and unique colors and ease of application are important in these markets. User skill levels may range from DIY hobbyists in a home garage to highly experienced and well-equipped specialty custom shops.
Automotive coatings raw materials
Like most coatings formulations, automotive coatings consist of five basic ingredients, each with unique properties:
- one or more polymers
Polymers are the foundation of the coating and help meet many requirements. Innovation in polymers (or binders) for use in automotive coatings centers around two technologies: waterborne polymers for base coats and high solids polymers for clear coats. The OEM and refinish coatings markets each demand different properties.
Important waterborne binder properties include:
- rheological (flow) properties allowing smooth application and proper orientation of effect pigments
- film formation that reduces time to clear coat application
- broad compatibility with pigment dispersions
- resistance to weathering degradation
- low levels of air pollution causing Volatile Organic Compounds (VOCs)
Acrylic latex and water soluble acrylic polymers are the primary binders for waterborne base coats; often these are proprietary latex polymers with complex architectures. The requirements for high solids base coats are essentially the same for waterborne base coats.
High solids polymers are often used in automotive coatings for clear coats in all applications and refinish base coats. Clear coats demand:
- low viscosity with low VOC content
- rheological properties allowing smooth flow out after application
- fast cure
- low shrinkage on curing
- the ability to be sanded and polished in refinish applications
Cross-linkers bridge the large polymer chains of the binder system to form high molecular weight films. These films are flexible, yet maintain hardness and resist the passage of water and oxygen to the underlying coating layers and substrate.
OEM coatings systems usually incorporate cross-linkers in both base coats and clear coats, while cross-linkers are usually found only in refinish clear coats. Although many chemistries can cross-link automotive coatings, isocyanates and amino resins are the most utilized.
Isocyanates are most common in refinish coatings due to their ability to react at lower temperatures. Amino resins are used almost exclusively in OEM coatings where higher temperature curing schedules are possible.
Pigments in automotive coatings
Pigments provide the color and flash to automotive coatings and are the key components of coatings styling. Innovations in the technology of effect pigments allow bold new looks for automotive designers. Most new developments are in effect pigments while color pigments are based on well-established chemistries.
Effect pigments can be classified into two broad categories: metallic pigments and pearlescent pigments. Metallic pigments are just that, flakes of metal (usually aluminum) incorporated into the base coat to provide color, sparkle and opacity.
Pearlescent pigment effects are based on a transparent substrate coated with an inorganic pigment layer which reflects light in a way that maximizes color and sparkle but is semi-transparent.
While used in small amounts in automotive coatings formulations, additives play important roles in meeting end users’ expectations. A thorough discussion of additives in automotive coatings would require its own article, so I will be brief. Additives perform a variety of functions, such as:
- Modify rheology
- Release air
- Incorporate and stabilize pigments
- Prevent damage UV radiation
- Improve scratch resistance
The use of solvents in automotive coatings has declined greatly over the past thirty years, moving more toward high solids and waterborne coatings, but they are still an important ingredient in coatings formulations. Solvents can:
- reduce viscosity
- assist in film formation
- allow the coating to flow smoothly over the substrate
- allow multiple coats to blend together during application
All of these materials combine together in modern automotive coatings to protect, beautify and differentiate vehicles. If our cars are an expression of our personality and style, then the outer coating is the face we present to the world.
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6 Responses to “Automotive Coatings: A Primer on Materials and Applications”
I would like to know your opinion on the use of air release additives that are based on organically modified silicones in waterborne automotive coatings. I know that the automotive industry is affraid of contamination when using silicone (PDMS) based additives.
In addition to melamine and isocyanate crosslinkers in automotive clearcoats, there is a significant volume of aliphatic epoxy – carboxylic acid crosslinking. Melamine crosslinking in the clearcoat has declined in the last couple of decades due to etching susceptibility by acid rain. Both of the other two technologies are resistant to etching.
Nice job Wally!!! Great article
Thanks Mike, Go Vikings
I believe these materials have a place and should not be rejected out of hand. That said, all possible unintended consequences need to be considered and eliminated before using PDMS based additives.
I agree, melamine cross-linked clearcoats exhibited acid etching and have been largely phased out in favor of isocyanate and other cross-linkers. Melamine is still a valuable cross-linker in basecoats.