The requirements for the back paint of coated steel plate are not high, but compared with the primer, it still has certain weather resistance and flexibility requirements. The back paint is applied on the back of the coil, mainly for protection, while providing appearance and certain durability. The paint film is required to have good corrosivity, scratch, adhesion and processability, but there is no requirement for decoration and outdoor durability. There are three types of commonly used back paint: epoxy type, alkyd type and polyester type. Because of poor durability, the first two are not used now. At present, most of them are amino polyester. The composition of polyester type back paint is basically the same as that of finish paint. At present, the back paint of most color coated boards is directly applied on the pretreated substrate. Generally, the primer is not applied, but the back paint can also be applied after the primer of 5 ° m. According to the construction and use, the back paint can be divided into three categories.
(1) For those with high corrosion performance requirements, the coating structure on the back of the substrate is primer plus back paint. At this time, the back paint is mainly used for shielding and protection. It can be the same type as the finish paint, but its performance requirements are lower than that of the finish paint, as long as the scratch, adhesion and processability meet the requirements.
(2) The back of the base plate is not painted with primer, but only one layer of back paint. At this time, it plays a dual role of primer and back paint. In addition to scratch, adhesion and processability of back paint, it must also have good corrosion performance. The film forming materials can generally be epoxy monoamine, epoxy polyesters and polyester polyurethanes. For the corrosiveness of the back paint, in addition to coloring pigment, corrosion pigment shall also be added to the paint.
(3) When the coated metal coil is mainly used to prepare the foam sandwich panel, the back paint must adapt to the manufacturing process of the sandwich panel in addition to the requirements for 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 pasting process of foam core board, the back paint is required to match with the adhesive used. The film-forming material can generally be epoxy polyester or polyester polyurethane. In order to have sufficient adhesion to the sandwich materials and the viscosity after winding during production, the additives and surface plasticizers that can reduce the surface tension of the paint film shall not be used in the back paint, but the hardness of the paint film itself shall be used to stop the winding adhesion.
