Introduction of Prepainted Steel Sheets

The prepainted steel sheets are prepared by coating one or more layers of organic coating on hot-dip galvanized sheet, hot-dip Al-Zn steel sheet, electrolytic galvanized sheet, cold rolled sheet, etc, and then cured by baking, which provides higher anti-corrosion property and a longer lifespan than that of galvanized steel sheets. Compared with traditional concrete and other traditional building materials, prepainted steel sheet meets the requirements of green building materials. The high strength (yield strength up to 550 MPa), good waterproof performance, good toughness, excellent seismic performance of prepainted steel sheet, made it as an irreplaceable candidate in construction industry. The production process of prepainted steel sheet has evolved from one-coating-and-one-baking to double-coating and double baking, and even three-coating-and-three-baking.

Selection for base metal

Type and coating weight of base metal: Type and coating weight of base metal are important factors that influence the corrosion resistance of prepainted coils. The choice of base metal type and weight of coating should be determined in accordance with usage, environmental corrosion, service life and durability. The prepainted steel sheet with hot-dip galvanized / aluminized substrates are usually selected as candidate for outdoor use, and the corrosion resistance increase with the coating weight. For example, in areas of severe industrial pollution or coastal wetness, it is recommended to use the substrate with a zinc coating weight of more than 180 g / m2. Interior decoration or home appliance, prepainted steel sheet with ordinary galvanized substrate is suggested. Category of coating: Coating is the key factor which influence the usage of prepainted coil. Appropriate type of coating should be determined according to usage, service environment, and service life requirement. Taking the coastal wet area as an example, it is recommended to use the prepainted steel sheet with high durability coatings (such as PVDF, HDP, etc.). In severe industrial pollution area, not only take the better corrosion resistance of the prepainted steel sheet into account, but also the pollution resistance. Here, PVDF or self-cleaning

prepainted steel sheet is a reasonable choice.

Types of prepainted plates

Category of finish coats

Among all the ingredients of coil coatings, resin and pigment play the role of duration. The primers commonly used consist of polyester, silicon modified polyester, polyvinylidene fluoride, high durability polyurethane, etc.

1、（PE）Polyester

This kind of finish coat has strong adhesive force, a variety of colors, wide scope of properties in formation and durability outdoors, moderate chemical resistance and low cost, with a warranty period of 7-10 years in the normal environment.

2、（SMP）Silicon modified polyesters

The membrane of the finish coat of this kind has excellent rigidity, abrasion resistance, thermal resistance, good outside durability, chalking resistance, high retention of color and luster, ordinary flexibility, moderate cost,etc.

3、（HDP）High-durability polyester

High molecular weight resin is used in the finish coat, which has fewer branches, stable bond energy, and photolysis resistance. Therefore, the product possesses excellent color retention and UV resistance, excellent outdoor durability and anti-pulverization, good film adhesion, rich color, and high cost-performance.

4、(PVDF)Polyvinylidene fluoride

The finish coat features in its excellent weatherability, chemical resistance and corrosion resistance. It shows good stability in acid-base and polluted environment, excellent machining performance, high flexibility, and impact resistance. These outstanding properties make the product meets the requirements of major public building which need to maintain super-long-lasting. At present, some prepainted coil production lines, choose fluorine resin content < 70 % or even lower content of fluorine resin coating in order to reduce costs. The fluorine resin content of PVDF coating used in our prepainted steel sheet lines is ≥ 70 %. Reasons for selecting coatings with ≥ 70 % fluororesin: High crystallinity: Microstructure of PVDF / acrylic resin mixture is closely related to the proportion of PVDF and acrylic resin. When the content of PVDF is reduced to less than 70 %, the crystallinity of PVDF mixture decreases significantly, and the MEK wipe performance is far less than the one content ≥ 70 %. PVDF.

High gloss retention: The most critical performance of PVDF coating is the long-term weather resistance of paint. Due to the high stability and protection of C-F bond, PVDF resin has excellent chemical and optical resistance. However, the contents affect gloss retention of PVDF coating and polyester coating obviously.

In order to ensure the excellent weather ability, corrosion resistance and gloss retention of fluorocarbon coatings, Kynar500 and Hynar5000 fluororesin PVDF coatings, which fluororesin content was ≥ 70 %, were selected for fluorocarbon prepainted coils in Shougang. Because the high content of fluorine resin and relatively low content of acrylic resin, Shougang products coating is relatively soft. It is suggested that the scratch of the coating should be avoided in the process of processing.