Automotive Paint Technology: From Past to Present

Ford, in 1908, determined that black was the only practical automotive paint color for the Model T because it provided both durability and low cost. Of course, the black automobile paint that Ford applied to the Model T was not actually "automotive" paint, but rather the paint technology that existed at that time; a paint based on natural linseed oil resin as a binder. Oil resins harden through oxidative cross-linking, which requires a long time for the paint to dry. Ford's black paint was applied to the Model T by brush, and the process took a week to complete. Even though black paint dried faster than all other available colors, this timeframe created a terrible production bottleneck for Ford's innovative mass production process. Model T vehicles passing through the paint operation at the end of the assembly line began to accumulate in the automotive factory warehouse. This bottleneck was the primary motivation for the development of the first paint specifically formulated for automotive coatings, namely DuPont Company's "Duco" paint. This new coating technology made an extremely important change in productivity by reducing painting time and drying time from several days to several hours. DuPont chemists, who had used nitrocellulose chemistry to manufacture explosives and cinema film, discovered that when they altered the molar ratio of NO2 groups in the cellulose backbone, they obtained a low-viscosity resin at approximately 15% solids by weight, and they realized this could be applied as a coating. As a lacquer, this coating dried in approximately two hours (through solvent evaporation alone). Formulation development work conducted by paint chemists found that this new synthetic lacquer resin provided an excellent foundation for a paint with improved appearance, toughness, and durability compared to natural oil-resin paints, and that it could also be easily pigmented with a wide range of color pigments in addition to black. After several years of testing, General Motors reported in 1924 that Duco was used on nearly all automotive production lines. This was the first example of how industry needs have guided the advancement of automotive coating technology. In this case, the need to increase productivity in the automotive factory led to the invention and development of a new coating chemistry. Since the first technological innovation of the 1920s, continuous innovation has characterized automotive coating technology. In fact, we see that many new technologies and chemistries in the science of coatings have originated from developments pioneered in the automotive coatings field. This article has been prepared to conduct a brief historical review of the evolution of automotive coating technology.

Early Automotive Coating Chemistry: From Alkyds to Acrylic Lacquers

Nitrocellulose paints were extremely productive, but required polishing to achieve high gloss on the final coat. In the 1930s, paint chemists began working to develop a paint binder system that somehow provided the productivity of natural oil resin and naturally superior appearance. The result of this work was the development of the first alkyd paint system. Synthesized using three monomers, it was the first "polymer" made for coatings: phthalic anhydride, glycerol, and linoleic acid. The chemistry, combining both synthetic monomers and natural products, provided a coating resin system exhibiting intermediate performance between synthetic finishes and natural oils. Given that this technology provided extraordinary film properties, this new alkyd paint was first commercialized as an automotive primer. It is important for the science of coatings to note that in this case, a resin technology was selected for a specific coating layer of the total coating system, a fundamental concept used in coatings. Moreover, alkyd chemistry continues as the primary foundation of existing coating technology. The next major development in automotive coating technology was not reached until the 1950s; the use of thermoplastic acrylic varnishes. At this time within American society, the automobile was no longer merely a means of transportation. Cars had become a personal status symbol that owners wanted to display to their friends. This meant that coatings needed to provide better appearance and emphasize the new curved design shapes of the era. Rohm and Haas Co. developed a new synthetic polymer based on polymethyl methacrylate, and the coatings industry investigated whether this technology could be used in coatings. This chemistry, of course, was based on the controlled polymerization of various acrylic monomers to obtain a polymer resin with the desired molecular weight and glass transition temperature. This would be the first example of man-made resin technology for use in automotive coatings. Thermoplastic acrylic resin technology dominated the automotive topcoat market for approximately 20 years, from the 1950s to the 1970s. The reason was the excellent appearance that could be achieved with these coatings. Given the high molecular weight (80-100k) of the acrylic resin binder and high Tg (approximately 70°C), the viscosity is high. For this reason, coatings based on this technology had to be sprayed at relatively low solids levels of approximately 20%. Achieving the desired approximately 25-micron film thickness meant multiple topcoats. Of course, by today's standards this appears to be a disadvantage. However, at that time this acrylic lacquer technology possessed a critical advantage over previous automotive paints: it provided an excellent binder system for new pigment coloring technology—metallic pigments. Metallic effect pigments provide bright, lustrous car colors that respond to the curvature of the vehicle body. These pigments took automotive color styling to a new level. However, to achieve the maximum visual effect of flat, plate-like metallic pigments, the pigments must be aligned parallel to the painted surface. The acrylic lacquer paint's rheological profile was perfect for achieving this effect; low initial viscosity (considering low solids) to allow metallic flakes to flatten, followed by a rapid increase in viscosity (given high molecular weight and Tg) to keep the flakes in place. This coating technology was an advantage for General Motors' styling approach by the 1960s, coloring nearly every automobile with acrylic varnish topcoat paint.

Protection of the Automotive Body

While topcoat technology continued to advance for automotive coating systems, automobiles still faced a major problem on the automotive body. A major coating technology development in the 1970s solved this problem: electrocoat primer, commonly known as "e-coat." The first automotive electrocoat was an anodic product developed by Dr. George Brewer at Ford in 1957, but this technology had certain disadvantages. Subsequently, PPG Industries introduced the first cathodic e-coat system for automotive bodies in 1973. Because these coatings stopped automotive bodies from rusting, this new primer technology was one of the greatest advances in automotive coating technology. Modern electrocoat automotive primers are applied by completely immersing the assembled automobile body in a large tank containing water-based e-coat, and the coating is achieved through cathodic electrodeposition. This covers approximately 100% of all metal surfaces by the primer. When this resin technology is combined with the excellent coverage provided by electrodeposition, it provides one of the most effective coatings known for corrosion protection. Nearly all automobiles today still use e-coat technology as the foundation of their coating system.   While e-coat provides excellent corrosion protection, an automotive coating system has two weak points: insufficient appearance and inadequate photostability. To address these problems, new enamel automotive primers were developed in the 1980s. These primer surfacers were designed to be applied over hardened e-coat to achieve a smoother surface to provide improved topcoat appearance while providing opacity to protect e-coat primers from UV radiation. The combination between electrocoat and primer-surfacer provided a complete automotive primer system with excellent corrosion protection and an outstanding surface for the topcoat. This formed a stepping stone to the next major advance in automotive coating technology: basecoat/clearcoat coating.

Basecoat/Clearcoat Automotive Topcoats

As mentioned earlier, thermoplastic acrylic lacquer automotive coatings were the most important automotive topcoat paints used in the 1950-70s given their excellent appearance. However, these lacquer topcoats had a significant disadvantage; they had poor exterior durability. After approximately one to two years, the coatings began to deteriorate and aggressive polishing was required to "restore the gloss" of these systems. By the 1980s, automotive manufacturers wanted better durability for automobile topcoats because consumers wanted their cars to maintain their original appearance and luster for at least five years. At the same time, the U.S. Environmental Protection Agency began announcing new volatile organic compound (VOC) regulations that limited the amount of solvent an automotive facility could emit to the atmosphere. The high VOC content and poor durability of acrylic varnish coatings were no longer acceptable in the automotive market. How did automotive coatings formulators improve coating appearance while achieving harder, better durability? The answer was the next step that occurred in automotive coatings: basecoat and clearcoat enamel topcoat technology. Now, instead of a single-layer topcoat, formulation specialists designed a two-layer system consisting of a basecoat containing pigments to provide beautiful color effects, followed by a clear polymer coating layer that protects the basecoat below. Both basecoat and clearcoat were enamels based on hydroxyl-functional acrylic resins cross-linked using melamine chemistry. These basecoat/clearcoat systems provided previously unattainable balance for automotive coating systems, delivering striking visual appearance and long-term durability.

Water-based basecoats, new cross-linking chemistry and new application processes

In the 1990s, another major advancement occurred in automotive coating formulation: the use of water-based basecoats. The chemical characteristics of these primers can vary from water-reducible acrylics and polyesters to acrylic latexes, polyurethane dispersions, but the common factor is the use of water as one of the main volatile components. Typically, the reason for using water-borne technology is to achieve lower VOCs and reduce the environmental footprint of the coating process; however, this is not the only benefit of using water-based basecoats in automotive applications. Given their low-formulated solids and unique rheological profiles, water-based automotive basecoats have been found to typically provide improved appearance and metallic effects. Many new automotive coatings using new cross-linking chemistry have also been developed over the past twenty years. New chemical technologies have focused on clearcoats; therefore, in addition to the original acrylic resin/melamine systems, acrylic/silane/melamine, acid/epoxy, carbamate/melamine and acrylic/isocyanate systems are now available. Important new properties for clearcoats can be summarized as improved appearance and durability, and better resistance to acid etch and scratching. The final major step change in automotive coatings technology occurred in the 2000s, and this advancement focused on process efficiency. In a typical automotive assembly plant, the paint operation can occupy nearly half of the entire facility, constitute a large portion of the assembly plant's capital costs, consume 80% of energy, and generate the vast majority of CO2 and VOC emissions released at the facility. OEM manufacturers asked paint suppliers to find a way to reduce this area that nearly occupies half the facility and reduce application costs. This request led paint formulators to develop coatings that could be applied more efficiently, in fewer steps, and with lower energy requirements. There are now many new processes at automotive facilities meeting these criteria. The typical process of conventional water-based primer, water-based basecoat, and solvent-based clearcoat system includes two oven bakes and a heated flash, each requiring time and energy. This is now beginning to give way to new technology—a new "3-Wet" process in which solvent-based primer, solvent-based basecoat, and solvent-based clearcoat are applied successively, with all three layers cured in a single oven after application. This 3-Wet application method reduces the carbon footprint of the coating line, shortens the total time for the painting operation, and saves energy costs. Obviously, these changes in the application process have required reformulation of the primer/topcoat coating system to tolerate wet-on-wet processes. It would not be wrong to predict that new technologies will emerge in the periods ahead.

Prepared by: B. Serhat Cengiz

References: • John Pfanstiehl (1998). Automotive Paint Handbook: Paint Technology for Auto Enthusiasts & Body Shop Professionals. Penguin. • Kimio Toda; Abraham Salazar; Kozo Saito (21 December 2012). Automotive Painting Technology: A Monozukuri-Hitozukuri Perspective. Springer Science & Business Media. • "A Guide to Seals in the Automotıve Industry, Pre-Treatment & Paınt Plants" arthomson.com. • DuPont Automotive: Paint & Coatings for Metal Exterior. • http://www.paint.org/article/brief-history-automotive-coatings-technology/