BOYA TURK Magazine August / September 2008
OEM coatings are used for both functional and aesthetic requirements. Functional performance requirements include corrosion and stonechip performance, durability against environmental factors such as chemical / acid spillage from many sources, high loads of salt and humid air in coastal regions, high UV radiation, temperature differences etc. In order to serve both purposes a multilayer application of different functionality coatings are used as shown in Figure-1. Each layer is compatible with above and below layer and functions for the overall quality. The top layer can be divided into two groups as: (a) One layer called solid topcoat (b) Wet on wet applied two coats consisting of basecoat and clearcoat.
Manufacturing process of passanger cars and commercial vehicles starts with building the metal car body by welding the metal sheets. After building the car body, the subsequent processes shown in Figure-2 are carried out. Finalizing the coating processes, the coated car body is now ready for the assembly.
Figure-2: Passenger car paintshop basic processes.
The basic properties of the coating layers applied in these processes can be summarized as follows
Inorganic Phosphate Coatings Applied to Metal
Aim: To contribute corrosion resistance and to improve paint adhesion
Car body which is a combination of several metal parts welded together, is first dipped into alkaline degreasing baths. After rinsing, the car body is coated by solution of Zinc, Manganase, Nickel, and phosphate salts. Passivated and phosphate treated metal surfaces have better corrosion resistance and improved paint adhesion.
CED Primer application
Aim: To have outstanding corrosion resistance and provide an excellent surface preparation for the subsequent coats
Cathodic e-coating (CED) is realized in a cathodic e-coat bath where coating particles are deposited on the steel panels with the aid of attractive forces produced by electric current. Body is connected to an electric circuit as cathode and current is applied. Inside the bath there are positively charged coating particles dispersed in water. With the aid of voltage gradient, the charged coating particles are attracted to the car body, through interaction with water electrolysis byproducts they neutralizes and deposited on the body. The body is then rinsed off before baking (to realize crosslinking) at approx. 160-180 degrees Celsius.
Although initial investment cost is high for CED, its use is growing continuously since 1977 because e-coat application offers some significant advantages such as, the ability to form a highly corrosion resistant and mechanically strong film, a homogenous film thickness on body, low emissions of VOC, the lower risk of fire accident and lower paint consumption.
Products: Waterborne Polyester-Urethane Modified Epoxy CED primer
Aim: To improve stonechip performance, and provide smooth surface before topcoat application.
The use of only CED primer is not sufficient for best paint performance, because, in the first place, it can not hide defects coming from body surface, and second UV resistance is poor. Therefore, primer, which is usually applied by bell application, has to be used on top of CED primer. Applied primer film is cured at high temperature ovens between 150°C-170ºC.
The colors of primers for passenger cars are designed so that they are close to the colors of topcoat layers’. By this way the top layer containing expensive pigments cover the primer surface at lower film thicknesses and during the lifespan of car defects made by stones can not be easily seen.
In order to decrease VOC’s there is a tendency to switch from solventborne primers to waterborne types.
Products: Waterborne Modified Polyester, 1K Modified Polyester, 1K Solventborne PO (For PP bumpers), stonechip and air drying repair primers
Aim: To improve environmental resistance of paint film, to create a long lasting, attractive and brilliant films.
The basic properties of topcoats can be summarized as outstanding film appearance, high gloss, high scratch resistance, and outdoor durability (resistance against UV, humidity, oxidation, alkaline, solvent, bird droppings etc.).
One layer solid topcoat application
Solid topcoats are applied to primer coated bodies via bell or spray application. After a few minutes of flash off body moves on to oven whose temperature may change according to the system between 80°C-140°C, to complete crosslinking reactions.
Products: 1K Polyester, 1K Acrylic, 2K epoxy, 2K Acrylic-NCO
Wet on wet ‘basecoat+clearcoat’ application
When topcoats containing metallic and/or pearlesent pigments is applied as single layer, some disadvantages like application difficulties, low gloss and poor exterior resistance of paint films arise. Because of these reasons metallic and pearlescent paints are applied as two layers called as basecoat+clearcoat.
Colored coat called basecoat is applied on top of primer by means of bell or spray guns. After giving a flash off time a colorless clear coat is applied. After a short flash off body is moved on to the oven to cure at temperatures between 130°C-150°C. In order to decrease VOC’s during paint application there is a shift from solventborne basecoats to waterborne basecoats.
Products: Waterborne Acrylic, 1K/2K Modified Polyester-Melamine, 1K/2K Acrylic-melamine and 2K Acrylic repair basecoats, and 1K Acrylic-melamine, 2K Acrylic-NCO and air drying repair clearcoat.