1. Introduction
A variety of coating methods and materials have been introduced for different coating applications for protecting parts and structures against mechanical and chemical damage. A review [
1] has been published. An article for coating on flexible substrates [
2] and an article for coating to bio-applications [
3] are also published.
For automobile bodies, in a conventional manufacturing process, paint is applied to the steel sheets by electrodeposition followed by baking. Then, a primer surfacer is applied and baked, followed by the deposition of a basecoat and clearcoat by a wet-on-wet process. Subsequently, baking is performed again. However, in recent years, car manufacturers have been required to reduce the environmental burden of their manufacturing processes, which mainly involves reducing carbon dioxide emissions [
4]. Currently, the standard coating method is called 3-coat-1-bake (3C1B), in which the primer surfacer, basecoat, and clearcoat are applied by a wet-on-wet-on-wet process without baking the primer surfacer.
Coating and baking processes generate heterogeneous wavelengths on the coating film surface. Unfortunately, the extent of this waviness is larger in the 3C1B process than in the conventional 3-coat-2-bake painting, which degrades the appearance of the coating surface. Variety of methods have been proposed to resolve this problem, and many fundamental studies on automotive paint appearance have been performed. For example, Peters et al. evaluated texture evolution during the curing process [
5]. Ellwood et al. have evaluated the effect of particle droplet size on the appearance of coating surface [
6]. Domenico et al. have studied the effect of bell application parameters on the appearance of coating surface [
7]. Osterhold et al. have characterized the surface structure of plastics coatings [
8] and appearance of the surfaces [
9]. Lin et al. have reviewed several factors that affect the appearance of coating surface and studied the effect of crosslinking chemistry on substrate telegraphing [
10]. Recently, the authors suggested a 3C1B coating system by adopting base resins with a low glass transition temperature to promote levelling and by reducing the melamine content of the paint to minimize shrinkage during baking [
11]. Then, Hayashi et al. developed a new 3C1B coating system with waviness equivalent to a conventional 3-coat-2-bake coating system for economy-class automobiles [
12]. However, the authors seek novel concept of another 3C1B coating system which is applicable to luxury-class automobiles which coating appearance has less waviness than economy-class. Carbon dioxide emissions can be more reduced during painting if 3C1B coating system is applied to luxury-class automobiles, too. Therefore, car manufacturers also seek another 3C1B coating system which is environmentally friendly for the luxury-class automobiles.
In this study, we focused on the shrinkage during bake and examined whether the appearance is improved by reducing the difference in the shrinkage percentage between the primer surfacer and the basecoat and the clearcoat after flow/leveling stops in the clearcoat while baking.
Figure 1 and
Figure 2 show a schematic representation of the telegraphing of interface waviness.
Figure 1 shows that the shrinkage percentage is different between the upper and lower layers after flow/leveling stops in the upper layer, and
Figure 2 shows that the shrinkage percentage is the same between the upper and lower layers after flow/leveling stops in the upper layer. Although
Figure 1 shows that telegraphing occurs on the surface,
Figure 2 shows that telegraphing does not occur. Thus, it is considered that the waviness of the interfaces does not correspond to the waviness of the surface. Consequently, if the difference in the shrinkage percentage can be reduced to nearly zero, the waviness of the interfaces will not be telegraphed or reduced, which minimizes the waviness of the surface. So far there is some literature provided that minimizing whole shrinkage percentage of multilayer coating films can improve appearance, but no literature or patent provided that minimizing shrinkage percentage difference between those layers also can improve appearance. In this study, we examined whether this consideration is correct.
Based on the coating film, which consists of a melamine-curable primer surfacer, basecoat, and isocyanate-curable 2K-clearcoat, the shrinkage percentage after flow/leveling stops in the clearcoat follows basecoat > primer surfacer > clearcoat. This occurs because:
The crosslinking reaction is faster for isocyanate than for melamine;
Isocyanate crosslinking is an addition reaction, and melamine crosslinking is a condensation reaction.
The amount of melamine resin follows basecoat > primer surfacer. We examined the relationship between the appearance and melamine resin amount in the basecoat or blocking agent amount for isocyanate in the clearcoat.