Getting a new car is always something exciting for me. Online configurators of car manufacturers enable me to turn my fantasy into a 2-dimensional or even 3-dimensional reality on my computer screen. Many questions come to my mind, among which the following seems particularly important to me: Which automotive paint should I choose? The color – an expression of my personality. Its quality – the assurance that the car body is protected against environmental influences. In this article you’re going to learn the basics of painting a car and how a rotational rheometer can help you with that.
The origin of rust
I can remember that my parents were discussing the rust on our car quite often. During my childhood, I naturally had no idea where this coppery appearance had its origin, leading to surface peeling in the worst case. Now, as a chemist, let me explain why the quality of automotive paint is that crucial for preventing rust. Rust originates once an anode, a cathode, and an electrolyte are present. The bodywork of our cars serves as the anode/cathode, water – such as simple rain water – can function as the electrolyte. As soon as the automotive paint levels off or gets a scratch, the protective effect of the car paint is lost and an electrochemical process occurs, leading to corrosion. Since the car body is mostly made of different kinds of steel, different iron oxides accrue, and rust becomes visible. Once the protective layer is damaged and rust forms, its progress occurs inconspicuously.
The process of painting a car’s body
The automotive paint is usually applied to the car body in several layers, each layer having a thickness of a few micrometers. There can be more than 20 process stages (including cleaning, degreasing, drying, etc.). In this article I will summarize the top 5:
- Preparation: The car body is cleaned and degreased thoroughly. A phosphate coat is applied as the first layer. It serves as a corrosion protector and later on provides the necessary layer to bond the paint.
- This step is followed by electro-coating. Solid paint particles are deposited onto the chassis via electrophoresis.
- A spraying process (e.g. via spray booth systems) is used to apply the primer. The primer serves as a leveling and protective layer.
- Afterwards, the base coat is sprayed onto the car body. The final finish of the base coat can be e.g. solid or metallic. While the solid one exhibits easier processability, the metallic one is more difficult to apply to the car body since solid particles need to be aligned in one direction to obtain a smooth surface.
- The last step is to apply the clear coat. The clear coat is used to protect the car body, which is usually made out of steel, against abrasion and UV light. Whereas most cars have a shiny, clear, reflective, and glossy appearance, a recent trend has been embraced by automotive car manufacturers – the matte finish. Matte finish is mostly applied within this process step and leads to a hazy appearance.
Painting a car: How a rotational rheometer can support the automotive paint application process
Knowing about the viscosity of the automotive paint is essential for its application process, its final quality, and long-term endurance. There are two main applications for a rotational rheometer to support you in getting the perfect processability and finish of the automotive paint:
- The paint is usually pumped through pipes to the spray nozzle. A rotational rheometer can help you study the flow behavior of the automotive paint at different shear rates. Thus, the shape of your pipes can be ideally designed.
- Studying the thixotropic behavior of the automotive paint is most important for its final appearance on the car. Of course, I want the surface of my car to be smooth and perfect. The three-interval thixotropy test (3ITT) simulates the behavior of the automotive paint during the spraying process:
- Low-shear interval: The behavior of the automotive paint at rest is evaluated.
- High-shear interval: Load is applied to the sample to cause structural decomposition. This step is equal to the stress applied to the automotive coat during the spraying process.
- Low-shear interval: How does the automotive paint regenerate and stick to the base coat? The structural recovery needs to be adjusted to guarantee optimal leveling behavior and prevent sagging.
This valuable information is highly relevant for the whole paints and coatings industry.
The application of automotive paint on the car body is one of the most demanding process steps when it comes to painting a car. A perfect, high-quality finish is necessary to prevent its abrasion and to ensure that a uniform protective layer is formed, for example against UV light or moisture. The rotational rheometer RheolabQC helps you reach perfect processability. It supports quality control steps before and during the application process – to ensure long-term corrosion protection and the perfect “finish” of the car.
Want to find out more about the rheology of paint and coatings?