The main component of color-coated steel coating is resin, which is the base material in the coating formulation. In addition, most coatings require solvents and can also contain various additives. In addition, many coatings also contain a cross-linking agent. After the coating is applied to the substrate, it chemically reacts with the resin during the curing stage to form a network-like cross-linked paint film. This network structure is very necessary to produce good paint film performance. After the coil coating is applied at high speed, it will cure quickly under high temperature (15-30 seconds). The solvent must evaporate during the curing stage to form a coating film.
Many properties are important for the coating film, including hardness, flexibility, weather resistance, chemical resistance, solvent resistance, corrosion resistance, adhesion to various substrates, impact resistance, and so on. These properties depend on many factors, including the type, molecular weight, monomer composition, and glass transition temperature (Tg) of the resin; the type and amount of crosslinking agent; curing conditions; curing catalyst; and auxiliaries. Changing these parameters can result in large changes in paint film properties to meet the requirements of many different applications.
Color coated steel coating types
According to different coatings, coatings can be divided into three categories: primer, topcoat and backcoat:
The primer is divided into polyester primer, polyurethane primer, epoxy primer, etc. according to the film-forming resin. Different primers can be selected according to the needs of the user. Commonly used are epoxy primers and polyurethane primers, which are manufactured with anti-rust pigments such as zinc chrome yellow and silver yellow.
Generally use polyester resin, high weather resistance polyester resin, silicon modified polyester resin, acrylic resin, polyvinylidene fluoride resin, polyurethane, heat curing cross-linking curing system using methyl etherified amino resin or closed isochlorate Resin, and choose high temperature and weather resistance pigment.
At present, it can be divided into 2/1, 2/2, and 2 / 1M systems in China. The back of the 2/1 system can be coated with a layer of primer or only a layer of back paint. It is mainly used for sandwich panels, and the coating must have good adhesion and foaming properties. The 2/2 system uses a two-coat system of a primer and a back paint. Generally, foaming performance is not required, and it is used as a veneer. 2 / 1M system, generally coated with 3 micron primer and 6 micron back paint, can replace 2/1, and requires foaming on the back.
Corrosion resistance mechanism of color coated steel plate
In fact, the so-called anti-corrosion mechanism of color-coated steel sheet depends more on the coating type of color-coated steel sheet. At present, the coatings with better corrosion resistance on the market are mainly PVDF fluorocarbon coatings, HDP high weather-resistant polyester coatings, SMP Silicone modified polyester, etc.
PVDF fluorocarbon coating
PVDF fluorinated resin coating (≥70%) began in the 1960s. After more than 60 years of practical use, it has been proven to have excellent weather resistance. It is currently the most excellent weather resistance in coil coatings. It also has extremely high chemical stability.
PVDF resin has excellent UV resistance, but it can’t block UV by itself. The pigment in the coating must be used to block UV and protect the primer from UV. Therefore, the pigment used must have excellent weather resistance and resistance Chemical, able to withstand outdoor exposure for 20-30 years.
Most organic pigments will be degraded or discolored under the action of sunlight and the atmosphere. The chemical properties of inorganic high temperature burned metal oxide pigments are relatively stable and suitable for long-term protection.
In recent years, fluoro resins copolymerized with trifluorochloroethylene and vinyl ester monomers have been widely used in building exterior walls and metal panels. Due to the use of easily hydrolyzed vinyl ester monomers and fluorine, the content is lower than PVDF. Compared with PVDF, its weatherability has a certain gap.
HDP high weather resistance polyester coating
At present, the color plates used in construction mainly use ordinary polyester coatings. After 8-10 years of use, the coatings will gradually lose light, lose protection and decoration. High weather-resistant polyester (HDP) resins are synthesized using monomers containing cyclohexane structure to achieve a balance of flexibility, weather resistance and cost of the resin, and aromatic-free polyols and polyacids are used to reduce the resin’s exposure to UV. Absorption of light to achieve high weatherability of coatings. Add UV absorber and hindered glue (HALS) to the coating formulation to improve the weatherability of the film.
SMP silicone modified polyester
Conventional silicone-modified polyester coil coatings are modified by cold-working with silicone resin and polyester resin to ensure outdoor weather resistance for 10 years. An effective method is to directly modify the polyester resin with the silicone monomer to further improve the weatherability of the silicone-modified polyester resin, so that the coating can reach the weather resistance of the outdoor for 20 years, and has become a hot spot for development. When modifying polyester resins, methoxy organic silicone intermediates are usually used. When the silicone content reaches 30% of the resin ratio and high-temperature calcined metal oxide pigments are used, the coating can reach 25 years of weather resistance. The performance requirements are second only to fluororesin coatings.
Color-coated steel plates used in construction are mainly affected by ultraviolet rays in sunlight, moisture in the air, and harmful substances caused by industrial pollution to reduce the service life. The anticorrosion of color-coated products for construction is a combination of coatings, pretreatment films and coatings (primer, topcoat and backcoat). This combination is very important and directly affects its service life. In order to improve the field performance of the color plate, especially the deformation area of the steel plate, the regulations in the use guide should be observed.
The failure process of color-coated steel plates, that is, coating failure, coating failure and even perforation of steel plates are the key corrosion occurrence processes. Therefore, the thickness of the coating layer is increased, and the weather-resistant and corrosion-resistant coating layer is the most effective method to prevent the corrosion failure of the color-coated steel plate.
Coating loss of light → Coating discoloration → Coating differentiation → Coating cracking → Coating blistering → White / red rust at the scratch → Peeling of the incision → Large area of the coating peeling off → Large area of white embroidery on the coating and red rust locally Corrosion and perforation of steel sheet → steel sheet failure
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