Evolution of the Automotive Body Coating Process

At the beginning of the automotive industry about 100 years ago, cars were painted with a varnish-like product that was brushed onto the vehicle surfaces; this coating was sanded and smoothed, and then the varnish was reapplied and refinished to establish several layers of the coating.

After multiple layers of varnish were applied, vehicles were polished to produce shiny surfaces. Some manufacturers, including Ford in the Model T line, employed a combination of brushing, dipping, and even pouring to fully cover and protect various parts of a car [1]. All of these coating steps were implemented manually, and it was not uncommon for the period between the beginning of the coating process to the end, when the coating was dry and the car was ready to sell, to take up to 40 days. Between the 1920s and 1940s, automotive coating technologies transitioned to using spray equipment and “stoving enamels” that were based on alkyd resins; these two advancements decreased application and drying times to a week or less. Because of the newly developed spray coating technologies, the surface finishes were more even and less sanding was needed.

In 1923, E.I. DuPont De Nemours developed nitrocellulose lacquer systems that included many choices of color and offered improved applicability for the use of spray paint guns [2]. These lacquer systems required the application of 3–4 coats to achieve the desired surface properties and, because of their chemical makeup, had relatively poor resistance to chemical solvents like hydrochloric acid. This disadvantage hindered the ability of coatings to endure acidic environments with various chemicals.

Another significant enhancement in paint technology was the development of “alkyd” enamel paints that were introduced on some car models in the early 1930s [3]. These enamels formed a very durable film because of molecular bonding reactions that occurred after the paint was sprayed onto the vehicles and then cured or baked in an oven. Each of the cured paint films was about 0.002 inches (51 μm) thick, and had high resistance to chemicals and solvents; another advantage was that enamel paints had shorter application times that included 2–3 steps instead of the 3–4 steps for lacquers. Furthermore, organic pigments were available in many different colors, the selection of which pleased customers. However, alkyd enamel paints were degraded by oxidization in sunlight, which caused colors to fade slowly or dull.

The durability of enamel finishes was improved considerably by the 1960s with the introduction of acrylic stoving enamels [4]. They were applied using a paint spray gun and then oven-baked, resulting in a resilient, glossy finish. However, the paint spray was applied manually, which could lead to uneven coating thicknesses during application of the multiple coats that were required. Nevertheless, distinct layers were applied for different reasons by this time, including corrosion protection associated with the primers, smoothness and chip resistance associated with the primer surfacers that were often applied at the front ends and exposed areas of the automobiles, color, and weather resistance associated with the final topcoat layer.

In the 1950s, application of the dip coating process was plagued with safety, environmental, and processing issues. The use of solvent-borne or water-borne dip tanks was beset by explosions and fire hazards [5]. These issues drove the introduction of anodic electrodeposition paints, which were introduced by the mid-1960s and were mostly based on maleinized natural oils. However, by the mid-1970s, cathodic deposition coatings replaced anodic electrodeposition because better corrosion protection was offered by the modified epoxy resin backbones and reactive polyurethane-based crosslinkers of these resins; this process also provided an increased throwing power, resulting in higher coating deposition at lower currents, and higher process reliability [6].

To further improve coatings’ appearance and durability, a new type of wet-on-wet finish was developed and introduced in the late 1970s, consisting of a thin basecoat and a thicker clearcoat [7]. The topcoat painting process was split into a pigmented enamel basecoat, followed by a clear enamel finish. A key to the success of this new technology was the development of a clearcoat material with superior durability in all climates. Even though the cost of the basecoat/clearcoat paint process was prohibitive for the less expensive automobile lines, it was used on more expensive, high-end automobiles. Then, refinements in the material and processing technologies reduced costs, and by the late 1980s the use of basecoat/clearcoat processing had become widespread [2], such that only a small fraction of cars manufactured today do not use this painting process. Additionally, the first water-based basecoats were introduced at Opel in Germany in the 1980s, followed by water-based primer surfacers in the 1990s [8]. Hence, in a relatively short period, automobile coating processes had evolved to be compatible with the high throughput needs of the industry with the ability to apply even coatings with thicknesses near 100–140 μm; this thickness implies an average of 9–16 kg of paint used per automobile; paints had also evolved to be very effective and durable. Importantly, it is now estimated that the corrosion protection and durability of color and gloss are about double what was typical 25 years ago [9]. Today, most clear coats in Europe are based on a two-component (2K) formulation. This formulation, incorporates an acrylic resin with OH-functionalities and a reactive polyurethane crosslinker. The rest of the world mostly uses a one-component formulation based on acrylic resins and melamine crosslinkers [10].

Novel developments in paint pigments have been accomplished simultaneously with improved processing and paint chemistries. For example, flake-based pigments based on aluminum and interference pigments that change color depending on the angle at which they are viewed (otherwise known as the “flip flopping” effect) have enhanced the brilliance, color, appearance, and customer satisfaction of automotive coatings [11]. At first, these new pigments were challenging to use with spray gun technology; however, new spray guns and spray gun configurations have been developed to meet these challenges.

Manual spray painting required significant craftsmanship because of the need to apply enough of a coating with an even thickness independent of whether the surfaces were relatively flat or highly curved. Now, with computer-controlled spray guns, the need for spray painting craftsmanship has been dramatically reduced. Furthermore, these automated processes have undergone improvements that ensure worker safety and increase the ratio of deposited paint-to-paint sprayed [12]. Nevertheless, automobile paint shops are still a major energy-consuming area and the most expensive operational aspect of an automobile assembly plant, consuming 30%–50% of the total costs of the manufacturing of automobiles [13]. These costs are wrapped into the energy used for air handling and conditioning (HVAC), as well as for paint drying and treatment of emissions generated by paint droplets that are not deposited on automobile surfaces; the painting booths must be purged to remove evaporated solvent, overspray paint particles, and regulated pollutants (like VOCs). Hence, the energy associated with only booth ventilation is significant [14]. In general, up to 70% of the total energy costs in assembly plants is within the painting operations [15]. Although the energy used to dry a 200-μm film on an automobile surface could be calculated to be (and is) not significant, it must be realized that paint drying includes heating of the paint and underlying automobile body, and the dollies and the carriers on which automobiles are moved through a painting process.

Today, automobile painting processes are more standardized than they have ever been because of the benefits of inorganic pretreatments, cathodic electrodeposition, liquid or powder primer surfacers, liquid base coats, and one or two component solvent-borne clear coats. For example, the development of new and highly reliable powder coatings has attained a point where many car manufacturers have decided to use them; as an added advantage, powder coatings introduce the capacity to aggressively meet environmental regulations [16]. Powder coatings are now used throughout primer surfacer operations in North America at Chrysler in all actual running plants, at GM for their truck plants, and in all new paint shops. In Europe, in some plants at BMW, powder coatings are also used for the clearcoat process [17]. This expansion of powder coating applications has coincided with a dramatic shift in the type of materials used in automobile body construction. Formerly made mostly of steel, today’s automobile bodies consist typically of up to 30% of aluminum and high-strength steel. Other lightweight materials are also finding application, including magnesium and polymer composites made of glass and carbon-fiber-reinforced thermosets and thermoplastics [18].

Automotive coatings continue to evolve as they either satisfy or are anticipated to meet customer expectations and environmental regulations while also lowering manufacturing and ownership costs. One of these evolutions is in the use of smart coatings because they offer the potential to significantly improve surface durability while adding additional functionalities or properties like self-healing, super-hydrophobicity, self-stratifying, self-sensing, sound proofing, and vibration damping. For example, a smart coating could respond to its environment to enhance the coating life; a smart coating with self-healing properties would be useful in response to an abrasive, mechanical trigger or to a corrosive event in which the coating is self-healing as a result of UV, heat, or mechanical activation [19]. Self-healing can also be achieved by employing shape memory polymers that are triggered with temperature and humidity manipulations, or with UV radiation; self-healing associated with the swelling of special clays such as montmorillonite is also possible [20]. Other smart coatings include those with internal sensing capabilities that entail the passive or active triggering of fluorescent molecules or quantum dots [21]. In the former, the sensing system signals and activates changes in or repair of the coating by sending data to an external detector; in the latter, the sensing system itself would be responsible for outputting the response signal.

Another development is the eventual introduction of self-stratifying coatings that are formulated with a compatible combination of liquid and powder coatings and based on partially compatible polymer blends that produce micro-heterogeneous structures [22].

To read the full copy of this article, pleaee visit https://www.mdpi.com/2079-6412/6/2/24/pdf