Automotive paint

Robotic arm applying paint on car parts.

Automotive paint is paint used on automobiles for both protection and decoration purposes.^[1][2] The paint layers total to around 100 µm. Modern automobile paint is applied in several layers, with a total thickness of around 100 µm(0.1mm).

History

In the early days of the automobile industry, paint was applied manually and dried for weeks at room temperature. As mass production of cars started, paint was dried in ovens. Nowadays, paint is usually applied by robotic arms and dried in just a few hours.

Until several decades ago lead, chromium and other heavy metals were used in automotive paint. Environmental laws have prohibited this, which has resulted in a move to water-based paints. Up to 85% of lacquer paint can evaporate into the air, polluting the atmosphere. Enamel paint is better for the environment and has become the most widely used paint.^[1]

Processes and coatings

Preparation

High-pressure water spray jets are directed to the body. Without proper pretreatment, premature failure of the finish system can almost be guaranteed.

A phosphate coat is necessary to protect the body against corrosion effects and prepares the surface for the E-Coat.

The body is dipped into the Electro-Coat Paint Operation (ELPO/E-Coat), then a high voltage is applied. The body works as a cathode and the paint as an anode sticking on the body surface. It is an eco-friendly painting process. In E-Coat, also called CED paint, utilisation is approximately 99.9% and has great SST life compared to other painting processes.

Primer

The primer is the first coat to be applied. The main functions of the primer are to act as a leveler and protector, and to make the base coat easier to apply to the component to which it is applied. The primer serves several purposes. It serves as a leveler, which is important since the cab often has marks and other forms of surface defect after being manufactured in the body shop. A smoother surface is created by leveling out these defects and therefore a better final product. It serves as a protector, the primer will protect from corrosion, heat differences, bumps, stone-chips, UV-light, etc. It also improves ease of application by making it easier for paints to stick to the surface, a more varied range of paints can be used.

Basecoat

The base coat is applied after the primer coat. This coat contains the visual properties of color and effects, and is usually the one referred to as the paint. Base coat used in automotive applications is commonly divided into three categories: solid, metallic, and pearlescent pigments.

• Solid paints have no sparkle effects except the color. This is the easiest type of paint to apply, and the most common type of paint for heavy transportation vehicles, construction equipment and aircraft. It is also widely used on cars, trucks, and motorcycles. Clear coat was not used on solid colors until the early 1990's.

• Metallic paints contain aluminium flakes to create a sparkling and grainy effect, generally referred to as a metallic look. This paint is harder to manage than solid paints because of the extra dimensions to consider. Metallic and pearlescent paints must be applied evenly to ensure a consistent looking finish without light and dark spots which are often called "mottling". Metallic basecoats are formulated so that the aluminium flake is parallel to the substrate. This maximises the "flop". This is the difference in the brightness between looking perpendicularly at the paint and that at an acute angle. The "flop" is maximised if the basecoat increases in viscosity shortly after application so that the aluminium flake which is in a random orientation after spraying is locked into this position while there is still much solvent (or water) in the coating. Subsequent evaporation of the solvent (or water), leads to a reduction in the film thickness of the drying coating, causing the aluminium flake to be dragged into an orientation parallel to the substrate. This orientation then needs to be unaffected by the application of the clear coat solvents. The formulation of the clear coat needs to be carefully chosen so that it will not "re-dissolve" the basecoat and thus affect the orientation of the metallic flake but will still exhibit enough adhesion between the coatings so as to avoid delamination of the clear coat. A similar mode of action occurs with pearlescent pigmented basecoats.
• Pearlescent Paints contain special iridescent pigments commonly referred to as "pearls". Pearl pigments impart a colored sparkle to the finish which works to create depth of color. Pearlescent paints can be two stage in nature (pearl base color + clear) or 3 stage in nature (basecoat + pearl mid-coat + clear-coat).^[3]

Clearcoat

Usually sprayed on top of a colored basecoat, clearcoat is a glossy and transparent coating that forms the final interface with the environment. For this reason, clearcoat must be durable enough to resist abrasion and chemically stable enough to withstand UV light. Clearcoat can be either solvent or water-borne.^[4] One part and two part formulations are often referred to as 1k and 2k respectively.^[5] OEM (original equipment manufacture) clear coats applied to the metal bodies of cars are normally 1K systems since they can be heated to around 140 deg C to effect cure. The clear coats applied to the plastic components like the bumpers and wing mirrors however are 2K systems since they can normally only accept temperatures up to about 90 deg C. These 2 K systems are normally applied "off line" with the coated plastic parts fixed to the painted metallic body. Owing to the difference in formulation of the 1K and 2K systems and the fact they are coated in different locations they have a different effect on the "redissolving" of the metallic base coat. This is most easily seen in the light metallic paints like the silver and light blue or green shades where the "flop" difference is most marked.

References

• Automotives Paints and Coatings, Streitberger & Dössel, 2008
• Hans-Joachim Streitberger; Karl-Friedrich Dossel (31 March 2008). Automotive Paints and Coatings. John Wiley & Sons. ISBN 978-3-527-30971-9.