No.78: Rules of thumb for coating systems and adhesion

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Key Notes 

An increase in film thickness can lead to increased internal stress, but using paint with a low α value can prevent a decrease in adhesiveness.
Balancing Tg and α values is a key point in paint design.
The history of repainting structures serves as evidence for painting rules of thumb and is useful in selecting paint.

As mentioned on another page, there is a principle in the painting process that suggests “increasing the amount of pigment (solid particles) in the base layer.” Steel structures such as bridges and plant facilities are maintained and managed through regular repainting. The concern here is that the increase in film thickness might lead to increased internal stress, potentially causing the paint to peel off. 

1. Causes of Increased Internal Stress

The increase in internal stress due to the thickness of the film is mainly due to the difference in the coefficient of thermal expansion (α value) between the paint film and the substrate. Recent studies have reported that using paint with a lower α value for repainting can prevent a decrease in adhesiveness, even with increased film thickness. Here are some data on α values: compared to steel, the α value of a clear paint film is about ten times higher, but it decreases to less than half when pigment is added. The addition of plate-like particles, such as talc or glass flakes, further reduces the α value.

2. Points on Paint Design

In paint design, attention must also be paid to the glass transition temperature (Tg) of the paint film. The internal stress caused by the difference in α values is known as “thermal stress.” Thermal stress generated above the Tg does not remain in the paint film, but the strain generated below the Tg becomes internal stress. Therefore, while lowering the Tg could potentially reduce internal stress, a low Tg paint film has reduced water and oxygen resistance, compromising its weather resistance. Thus, designing paint that maintains Tg while lowering the α value is crucial. 

Historical steel structures in Japan are often repainted with the same paint as used at the time of construction. This practice of repainting over the years without problems has accumulated as a rule of thumb in the painting process. 

Performance is critical in painting, and good results are passed down as a rule of thumb to future generations.