Paint Professional Cars and coatings - a short history

Paint Professional

Cars and coatings - a short history

Jamie Panter, technical manager at BASF Coating Australia turns back the clock to trace the fascinating history of automobile coatings.
Since the beginning of the automobile industry at the turn of the twentieth century, coatings have been used on automobiles. The original coatings were the same as those used on carriages at the time. They were produced by grinding pigments in a varnish made from natural resins and oils.
These early varnishes were very slow to dry and required many coats to be brushed on for coverage. Therefore, they needed large storage bays for the automobiles while they dried, with the entire process taking up to six to seven weeks. Furthermore, the finishes had poor durability and would only last for several months of exposure before chalking. Consequently, they needed to be polished often to keep up their gloss. These coatings did have advantages - they were easy to repair, readily available to the owner and could be easily brushed on. In the early 1920s, a major breakthrough was made when a process for making lower viscosity nitrocellulose was developed. Subsequently, the nitrocellulose was formulated into lacquer finishes. A lacquer is a coating that forms its film by evaporation of the carrier solvent and, by nature, can be redissolved by solvent. Nitrocellulose lacquers are very fast drying and the durability and appearance were much better than the prior generation of coatings. Also, sufficient film thickness could be achieved with several coats by spraying or brushing. The lacquer topcoat had to be hand buffed, but within six hours of force drying, a hard and glossy finish could be attained
By 1925, the entire finishing process dropped to about 50 hours. This breakthrough allowed for the production line manufacture of automobiles. Nitrocellulose lacquers remained a major component of automobile finishes for more than 30 years. Limited colours were available in the very early finishes. Customers’ demand for more colours, and a willingness to pay for them, spurred the development of new pigments during the 1920’s
The next significant development occurred in the mid to late 1930s with the advance of alkyd resins. Coatings formulated from these resins were higher solids than nitrocellulose lacquers and did not need to be rubbed to achieve high gloss. Alkyds coatings form films by crosslinking via the chemical process of oxidation.
In addition to topcoats, undercoats were also formulated around these two technologies. The OEM coating process consisted essentially of three steps ‘ primer, surfacer, and topcoat. With some exceptions in the use of combination primer-surfacers, it remained that way until the advent of colour coat/clear coat technology in the late 1960s.
By the 1940s, the auto refinishing industry had grown significantly. This was due to several factors: automobiles were widely owned and a large trade in used vehicles had built up that required the renewal of OEM finishes. Nitrocellulose lacquers were used for their ability to facilitate quick, quality repaints. The refinish technology was the same as the OEM technology.
Diverging Technologies
As much as any factor influencing the drive to shorter finishing schedules, was forced drying in place of ambient drying. As early as the 1920’s, forced drying was in use as a way to shorten the production time. This led to coatings that could respond to force drying to chemically speed up the finishing process.
The capability to force dry paints has contributed to a divergence in OEM and refinishes coatings technology. Numerous curing technologies have been employed that are effective at 120° C to 175° C or higher. This technology would not work in the 60°C force dry temperature of a normal refinish application.
OEM Direction
Over the last 50 years, two main technologies have dominated the OEM market ‘ amino resins and blocked isocyanate resins. Both cross-linking technologies offer a one-component (1K) system. The paint is stable until sprayed or dipped then baked, at which time the elevated temperature triggers the chemical reaction. The 1950’s saw the advent of acrylic lacquer topcoats, with improved gloss and weathering, acrylic lacquers replaced nitrocellulose lacquers. In the 1960’s, acrylic enamels provided improved weathering over alkyd enamels. With the introduction of colour coat/clear coat in Europe in the 1970s, the customer was given a leap forward in finish appearance. The advantage of the two-stage process was improved durability and the ability to incorporate new, more colourful pigments and metallic and other effects pigments. The use of colour coat/clear coat finishes has increased to where today virtually all automobiles are coated in this manner.
For colour coats, both solvent borne and water borne technology is employed. A ‘wet-on-wet’ process is used whereby the colour coat is applied, followed by the clear coat and
then both layers are baked. When the colour coat is water borne, it goes through a flash-off tunnel to remove water before the clear coat is applied.
Another important development in undercoat was electrodeposition (E’coat). In 1962 Ford introduced anodic electrodeposition on its paint line. The advantages of this to the normal solvent borne spray or dip primers of the time were complete coverage including recesses and sharp edges, reduced fire hazard, and high material utilisation. In 1977, this technology was taken one step further with the introduction of cathodic electrodeposition. This Technology gave coatings with much improved corrosion resistance compared to the previous process. Cathodic electrodeposition was revolutionary and is in use in almost every automobile plant today.
Regulatory pressure began in the late 1960’s to reduce the volatile organic compounds (VOC) in coatings and is a major driver in coatings development. It has led to the use of water borne, powder and higher solids solvent borne finishes.
Refinish Direction
By 1960, refinish technology consisted of two main technologies - alkyds and lacquers. As acrylics became more prevalent, topcoats moved from nitrocellulose lacquer to acrylic lacquer. Acrylics were incorporated into alkyds to make better weathering acrylic enamels. Primers and primer-surfacers were also alkyds and lacquers.
The 1960’s saw the first use of isocyanate crosslinkers in refinish. However, it was not until the late 1970’s that isocyanate use began to accelerate to where it has become a major component of topcoat and primer lines today. The growth was spurred by the advent of the base coat/clear coat process, in which the clear coat is a 2-component (2K) urethane. Most repairs today are done with base coat/clear coat systems. Solvent borne base coat dominates, but water borne technology is making inroads. The majority of primer compositions today are either 2K urethanes or epoxy/polyamides. The epoxy systems distinguish themselves by superior adhesion, corrosion protection and pot life. The urethanes have advantages in fast dry time and better sanding characteristics.
New Technology - UV curing

The nature of the refinish shop environment, with its potential for worker exposure, precludes the use of the high irradiance, broad-spectrum UV lamps employed in controlled industrial settings. This has led to the introduction of small size lower irradiance, UVA lamps.
A growing focus of the refinish industry in recent times has been on increasing productivity. UVA cured coatings are a possible addition to the refinish paint repair process, by offering faster cure than conventional coatings. Additionally, if the UV coatings are 1K, added time savings could be realized in mixing and measuring, cleaning equipment, and disposing of materials. Compared to heating up a booth to cure, UVA curing would offer lower energy costs.
In OEM, UV cure finishes would have some of the same advantages such as energy savings and lower VOC’s. Additionally, they are of interest for potential performance enhancements in areas like scratch resistance and environmental etch.
Uniting Technology’
After years of divergence, just as refinish and OEM coatings were very similar in the early years of the industry, the use of UV coatings may result in a convergence of coatings technology based on 1K, UVA curing. There would be numerous advantages to this. Currently, a significant amount of manpower is used in selecting, testing and approving of refinish materials in OEM approved body shops. If the coatings used in the original finishing were the same or similar enough to the refinish coating, this approval process could be streamlined. Other advantages would be if similar equipment and raw materials were used industry wide. Currently, repairs on the line and post line are done with materials of varying quality, depending on the particular situation. All repairs could conceivably be done with the same original material, maintaining a consistency in quality.
For all of these advantages to be realised, the development of high quality UVA cured coatings will be the factor to lead refinish and OEM together into this technology.
BASF Coating is a market leader with new coatings technology. For more information contact technical services on 1800 010 010