The color-coated board does not have high requirements for the back paint, but it still has certain requirements for weather resistance and flexibility compared with the primer . Back Luan coating 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 resistance 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. 9 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 the back paint can also be coated after Sμm primer. In terms of construction and use, back paint can be divided into three categories.
(1) If the anti-corrosion performance is high, the coating structure on the back of the substrate is primer and back paint. At this time, the back paint mainly plays the role of shielding 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, anti-blocking and processability to meet the requirements. From economic considerations - amino alkyd resins can generally be used as film-forming resins for back paints, such as soybean oil modified short oil alkyd resins and melamine resin back paints.
(2), No primer is applied on the back of the substrate, only one back coat is applied. At this time, it plays the dual role of primer and back paint. In addition to the scratch resistance, anti-blocking and processability of the back paint , it also needs to have good corrosion resistance. The film-forming substances are generally epoxy -amino, epoxy-polyester and polyester-polyurethane. In order to improve the corrosion resistance of the back paint, in addition to coloring pigments, anti-corrosion pigments need to be added to the paint.
(3) When pre-coated metal coils are mainly used to prepare foam sandwich panels, the back paint needs to adapt to the sandwich panel manufacturing process 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 ensure sufficient adhesion to the sandwich material and anti-adhesion 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 should be prevented by the hardness of the paint film itself. Rewinding adhesion.