The requirement of color coated board for back paint is not high, but compared with primer, it still has certain requirements of weatherability and flexibility. The back paint is applied on the back of the coil, which mainly protects the coil and provides
the appearance and durability. It is required that the paint film has good corrosion resistance, scratch imitation resistance, adhesion resistance and processability, but no decorative and outdoor durability requirements.
There are three kinds of common back coatings for color coated panels: epoxy type, alkyd type and polyester type. Because of the poor durability of the first two, they are not widely used now. At present, most of them are amino-polyester type. The composition of polyester backcoat is basically the same as that of topcoat.
At present, most of the back coatings of color coated boards are directly coated on the pretreated substrates, usually without primer, or after 5um primer, back coatings can be applied. Back paints can be classified into three categories according to their construction and use.
(1) The anticorrosion performance is required to be high. The structure of the back coating of the substrate is primer and back paint. Back paints at this time mainly play a protective role and can be used with the same variety of topcoat, but the performance requirements are lower than topcoat, as long as scratch resistance, adhesion resistance and processability meet the requirements. Amino alkyd resins can be generally 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 color-coated board, only one back paint is applied. At this time, it plays a dual role of primer and backcoat. In addition to scratch resistance, adhesion resistance and processability of backcoat, it must also have good corrosion resistance. Film-forming substances can be generally used in epoxy amino, epoxy-polyester and polyester-polyurethane, etc. In order to improve the anticorrosive property of back paint, besides coloring pigments, anticorrosive pigments should be added to the paint.
(3) when the pre coated metal coils are mainly used to prepare foam sandwich panels, the backing paint must also be adapted to the manufacturing process of the sandwich panels in addition to requiring good adhesion and mechanical properties to primers or substrates. For example, the direct injection foaming process requires the backing paint to have good adhesion to the foamed material; the foam core board bonding process; the required backing paint is matched with the adhesive used. Film-forming materials can be generally used in epoxy-polyester or polyester-polyurethane. In order to ensure enough adhesion to sandwich material and anti-adhesion after winding in production, no auxiliaries and surface sliding enhancers can be used to reduce the surface tension of paint film in backcoat, and the hardness of paint film itself should be used to prevent winding adhesion.