The prepainted galvanized steel sheet liquid coating composition has four parts: resin, pigment, solvent, and auxiliaries. Although in exceptional cases, it is possible to omit one of them, as in varnishes for overcoating, the pigments can be omitted.
1. Resin. Resin is a substance that forms a film after the coating is applied and is a macromolecular organic polymer, also known as the aggregate in the coating. It can bond the particles of solid substances in the coating together to form a film layer, adhere well to the substrate, that is, the surface of the steel plate, and impart some physical or chemical properties to the coating according to its chemical properties.
2. Pigment. Pigments are solid components in coatings that have both decorative and protective effects. It is dispersed in the coating and the cured coating film in the form of particles. It is called extender pigment for some of the substances that do not provide color for the coating.
3. Solvent. In liquid coatings, the solvent is generally used as a general term for the liquid that can dissolve the resin and the liquid that can adjust the viscosity of the coating. It makes the coating fluid a dispersion medium for resins, pigments, and additives. It can also make the coating infiltrate the surface of the substrate, spread evenly on the surface, and evaporate and dissipate in the process of curing and forming a film.
4. Additives, some substances added to enable the coating to be stored, facilitate construction, and have or enhance specific properties after film formation, are called coating additives. Its characteristic is that although the amount added is minimal, the effect it plays is pronounced, which significantly influences the performance of the coating or coating film and has a clear pertinence. For example, adding a polymerization inhibitor is conducive to the storage of the coating, adding a catalyst can speed up the cross-linking reaction, adding a leveling agent and a wetting agent is conducive to coating construction, and adding a matting agent and an anti-aging agent can improve the gloss and anti-aging properties of the coating film.
The coating of prepainted galvanized steel sheet is required to have the following basic properties after construction or film formation:
1. It can be easily coated and cured by a particular construction method
2. The coating formed on the required substrate surface has good adhesion
3. The cross-linked and cured coating has the desired hardness, toughness, durability, corrosion resistance, and decorative properties
4. Good weather resistance and specific aging resistance
5. It has the performance of repairing and recoating. For example, when chalking, matting, scratches, etc., it can be polished, repaired, or repainted with supporting materials.
These properties are general requirements for coatings. However, higher requirements are placed on the coatings’ properties due to the particularity of the production process of color-coated steel sheets.