The role of back paint on coated steel sheets

The coated steel plate does not have high requirements for the back paint, but compared with the primer, it still has certain weatherability and flexibility requirements. The back paint is applied to the back of the coil, mainly for protection, while providing appearance and certain durability. The paint film is required to have good corrosion resistance, scratch resistance, adhesion and processability, but there are no requirements for decoration and outdoor durability. Commonly used back paints are epoxy type, alkyd type and polyester type. The first two types have not been widely used because of their poor durability. At present, the amino polyester type is the most common. The composition of the polyester backcoat is basically the same as that of the topcoat. At present, most of the back paint of color-coated boards are directly coated on the pre-treated substrate, generally no primer is applied, and a 5¨m primer can also be applied and then the back paint can be applied. In terms of construction and use, back paint can be divided into three categories.
(1) For higher corrosion resistance requirements, the coating structure on the back of the substrate is primer plus back paint. At this time, the back paint mainly plays the role of shielding and protection, and can be of the same type as the top paint, but the performance requirements are lower than that of the top paint, and only scratch resistance, adhesion and processability are required to meet the requirements.
(2) There is no primer on the back of the substrate and only one coat of back paint. At this time, it plays the dual role of primer and back paint. In addition to the scratch resistance, adhesion and processability of the back paint, it must also have good corrosion resistance. The film-forming substances can generally be epoxy-amino, epoxy-polyester and polyester-polyurethane. In order to prevent the corrosion of the back paint, in addition to the coloring pigments, corrosion pigments need to be added to the paint.
(3) When the coated metal coil is mainly used to prepare foam sandwich panels, the back paint must be adapted to the manufacturing process of the sandwich panel in addition to good adhesion and mechanical properties to the primer or substrate. For example, for the direct injection foaming process, the back paint is required to have good adhesion to the foaming material; for the foam core board pasting process, the back paint is required to match the adhesive used. The film former can generally be epoxy-polyester or polyester-polyurethane. In order to have sufficient adhesion to the sandwich material and the viscosity after winding during production, additives and surface slip agents that reduce the surface tension of the paint film cannot be used in the back paint, and the hardness of the paint film itself should be used to stop the winding Adhesion.