General information on coil coating

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Since pre-coated coils appeared in the United States in the 1930s, coil coatings used for industrial electrical equipment and building structures have seen vigorous development thanks to the strong promotion by two pioneering organizations: the National Coil Coating Association (NCCA) in the United States and the European Coil Coating Association (ECCA). They have then been rapidly spread around the world. 　　The coil materials used for pre-coating are usually galvanized steel sheets, aluminum, and cold-rolled steel sheets. These materials are pre-coated with solvent-based coating solutions or covered with plastic films via heat pressing, and then sold to users. They can be directly cut and shaped without the need for additional painting as a protective layer. This **reduces the production time required to manufacture the final products and lowers the costs associated with re-coating them». As a result, solvent-based coatings have seen rapid development in the field of large-scale industrial products. Here, let’s take a look at the general characteristics of solvent-based coating solutions: Pre-coating rolled steel sheets not only offers economic advantages in terms of efficiency during reproduction but also plays an important role in enhancing the aesthetic appearance of the finished products and providing corrosion protection, thereby extending their lifespan and increasing their utility value. The topcoat for coil coatings offers excellent decorative properties as well as protection and durability. Typically, such topcoats are made from polyester resins, acrylate resins, fluorocarbon-modified polyesters, and silicone-modified polyester resins that possess outstanding light resistance, weather resistance, and impact resistance. Crosslinking agents can be high-weather-resistant amino resins with good hardness and flexibility, or aliphatic capping isocyanate resins, which ensure high performance even after subsequent processing. 　　To improve the corrosion protection and adhesion of the substrate, as well as to enhance the adhesion between the topcoat and the base layer, there are two additional transition layers between the topcoat and the substrate: the primer and the conversion coating layer on the metal surface. The conversion coating layer provides chemical protection for the substrate while also improving the adhesion of the primer to the metal surface (by changing the polarity of the original metal surface and forming a very thin film with certain gaps). The primer is typically an epoxy ester, a high-molecular-weight epoxy resin containing OH groups with a molecular weight of 10,000–30,000, or a polyester resin; these are combined with amino crosslinking agents or capping aromatic isocyanate polymers to form the main film-forming material. This material exhibits excellent resistance to stamping processes and offers superior adhesion to both the underlying layer and the topcoat. 　　Coiled steel sheets have two sides, and it is not possible to provide protective coating only on one side of them; the side that is on the inside of the finished product also needs to be coated for protection. This refers to the paint applied to the back side of the coil. Given the lower requirements regarding weather resistance and the higher demands for decorative and protective properties, special attention is paid to ensuring proper protection as well as ease of further processing. The back layer usually consists of two film layers: an inorganic conversion film layer that is in direct contact with the metal sheet and serves to provide chemical protection as well as to improve the adhesion between the topcoat and the base material (it performs the same function as the chemical conversion film layer at the bottom of the topcoat); and a topcoat layer. To ensure both protective properties and processability, and to enhance the scratch resistance of the surface layer, polymeric epoxy resins are used as the main components of this layer, thereby increasing the mechanical strength, hardness, and scratch resistance of the paint film. 　　The general performance requirements for the film of solvent-based coil coating are as follows:
1. Film thickness: Backcoat: 10 um; Primer: 5–10 um; Topcoat: 10–15 um.
2. Oven temperature: 215–240°C.
3. Curing time: Backcoat and primer: 80–90 seconds; Topcoat: 40–45 seconds.
4. Bendability: <3T.
5. Resistance to stamping: 1/2” punch, 9 joules.
6. Hardness: H-2H.
7. Resistance to wiping with MEK: ≥100 times/1Kg (back and forth).
8. Adhesion: 1mm×1mm×100×100%.
9. Resistance to scratching by coins: Grade 1.
10. Resistance to boiling water: 100°C for 2 hours, Grade 1.
11. Salt spray resistance: 360 hours, Grade 1 (for galvanized steel).
12. Artificial aging (QUV): 1000 hours, Grade 1 (for topcoat).

The composition of the backcoat paint that meets these performance requirements is as follows:
Light gray backcoat paint:
60% Polyester resin 311 (a product from Miki Company)
50
10% Bentone
2
Special baking solvent
10
Dispersant
0.3
Carbon black
0.1
Rust-inhibiting pigment A
10
Rust-inhibiting pigment B
8
Rutile titanium dioxide
15
600-mesh talc
3
100% HMMM amino resin
10
50% Epoxy607
20
Leveling agent
0.3
K-2500
0.3
Total
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