Laminated steel plates do not have high requirements for back paint, but compared with primers, they still have certain weatherability and flexibility requirements. The back paint is applied to the back of the coil, mainly for protection, while providing appearance and a certain durability. The paint film is required to have good corrosion, scratch, adhesion and processability, but no decorative and outdoor durability requirements. The commonly used back paints are epoxy type, alkyd type and polyester type. The first two are not widely used because of their poor durability. At present, most of them are amino polyesters. 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 is directly coated on the pre-treated substrate. Generally, no primer is applied. It can also be coated with a 5mm primer and then the back paint. From the perspective of construction and use, the back paint can be divided into three categories.
(1) For those with higher corrosion performance requirements, the coating structure on the backside of the substrate is a primer and a backcoat. At this time, the back paint mainly plays the role of shielding and protection, and can be of the same variety as the top paint, but the performance requirements are lower than that of the top paint, only scratching, adhesion and processability are required to meet the requirements.
(2) No primer is applied on the back of the substrate, only one back paint is applied. At this time, it plays the dual role of primer and backcoat. In addition to the scratching, blocking and processability of the backcoat, it must also have good corrosion properties. The film-forming material can generally be epoxy-amino, epoxy-polyester and polyester-polyurethane. For the corrosiveness of the back paint, in addition to the coloring pigments, corrosive pigments need to be added to the paint.
(3) When the coated metal coil is mainly used to prepare foam sandwich panels, in addition to good adhesion and mechanical properties to the primer or substrate, the back paint must also be adapted to the manufacturing process of the sandwich panel. For example, for the direct injection foaming process, the back paint is required to have good adhesion to the foam material; for the foam core board bonding process, the back paint is required to match the adhesive used. Film formers can generally be epoxy-polyester or polyester-polyurethane. In order to have sufficient adhesion to the sandwich material and the viscosity after winding in 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.
