When many people buy color-coated boards, they don’t know the specific differences between really good color-coated boards and poor color-coated boards, because the surfaces are similar, and there will be no problems if they are not used for a period of time. There are many factors to consider when choosing a coating, which mainly include coating type, coating thickness, coating color and coating gloss. In addition, sometimes the requirements for the primer and back coating of the coating must be considered. The types of coatings currently used for color-coated steel plates include polyester coating (PE), fluorocarbon coating (PVDF), silicon modified coating (SMP), high weather resistance coating (HDP), acrylic coating, Polyurethane coating (PU), plastisol coating (PVC), etc.
Polyester (PE, Polyester)
PE coatings have good adhesion to materials. Color-coated steel plates are easy to process and shape. They are cheap and have many products. There is a wide choice of colors and glosses. Polyester coatings are not ideal for ultraviolet light resistance and powdering resistance of the coating film. Therefore, the use of PE coatings still needs to be subject to some restrictions. It is generally used in areas where air pollution is not serious or for products that require multiple molding processes.
▲ Applicable industries
Ordinary industrial plants and warehousing and logistics warehouses do not cause corrosion to the color plates themselves, and do not have high requirements for the corrosion resistance and anti-aging of the color plates. More consideration is given to the practicality and cost-effectiveness of the factory construction.
Silicone Modified Polyester (SMP, Silicone Mobified Polyester)
Since polyester contains active groups -OH/-COOH, it is easy to react with other polymer compounds. In order to improve the sunlight resistance and powdering properties of PE, silicone resin with excellent color retention and heat resistance is used for denaturation reaction. , and the denaturation ratio of PE can be between 5% and 50%. SMP provides better durability of steel plates, and its corrosion protection life can be as long as 10-12 years. Of course, its price is higher than that of PE, but due to the silicone resin The adhesion and processing formability of the material are not ideal, so SMP color-coated steel plates are not suitable for occasions that require multiple forming processes, and are mostly used for building roofs and exterior walls.
High weather-resistant polyester (HDP, High durable Polyster)
Regarding the shortcomings of PE and SMP, British HYDRO (now acquired by BASF), Swedish BECKER and others developed HDP polyester coatings that can achieve 60-80% weather resistance of PVDF coatings in early 2000, and are better than ordinary silicon modified ones. Polyester coating, its outdoor weather resistance reaches 15 years. Highly weather-resistant polyester resin uses monomers containing cyclohexane structure during synthesis to achieve a balance between flexibility, weather resistance and cost of the resin. Non-aromatic polyols and polybasic acids are used to reduce the absorption of UV light by the resin. , to achieve the high weather resistance of the coating.
UV absorbers and hindered amines (HALS) are added to the paint formula to improve the weather resistance of the paint film. Highly weather-resistant polyester coil coatings have been recognized by the market abroad, and the coatings are very cost-effective.
▲ Applicable industries
Non-ferrous metal smelters (copper, zinc, aluminum, lead, etc.) in the metallurgy and electric power industries are the most challenging to the service life of color plates. Steel plants, power plants, etc. also produce corrosive media, which require higher corrosion resistance for color plates.
PVC plastisol (PVC Plastisol)
PVC resin has good water resistance and chemical resistance. It is usually painted with high solid content. The coating thickness is between 100-300μm. It can provide smooth PVC coating or light embossing treatment for embossed coating. ; Since the PVC coating film is a thermoplastic resin and has a high film thickness, it can provide good protection for the steel plate. However, PVC has weak heat resistance. It was mostly used in Europe in the early days, but due to its relatively poor environmental properties, it is currently used less and less.
Due to the strong bond energy between the chemical bonds of PVDF, the coating has very good corrosion resistance and color retention. Among the prepainted steel plate coatings used in the construction industry, it is the most advanced product and has a large molecular weight. It has a direct bond structure, so in addition to chemical resistance, it also has outstanding mechanical properties, UV resistance and heat resistance. Under normal circumstances, its corrosion protection life can reach 20-25 years. In recent years, fluorine-containing resins copolymerized with chlorotrifluoroethylene and vinyl ester monomers have become popular in China and are widely used in building exterior walls and metal panels. Due to the use of easily hydrolyzable vinyl ester monomers and fluorine content, they are 30% lower than PVDF. About %, so there is a certain gap between its weather resistance and PVDF. The PVDF content of the fluorocarbon coating produced by Baosteel is not less than 70% (the rest is acrylic resin).
▲ Applicable industries
Products in the chemical industry are volatile and are prone to produce highly corrosive volatile substances such as acids or alkalis. When exposed to water, dewdrops can easily form and adhere to the surface of the color plate, corroding the coating of the color plate and possibly further corroding it. to the zinc layer or even the steel plate.
02Performance comparison table of different coatings
There are two most important factors for the selection of primers. One is to consider the adhesion of the primer, topcoat and substrate, and the other is that the primer provides most of the corrosion resistance of the coating. From this perspective, epoxy resin is the best choice. If you consider flexibility and UV resistance, you can also choose polyurethane primer. For the back coating, the most correct choice is to choose a two-layer structure if the color-coated steel plate is used as a single plate, that is, one layer of back primer and one layer of back topcoat. The base paint is the same as the front paint, and the top coat is a layer of light-colored (such as white) polyester. If the color-coated steel plate is used as a composite or sandwich panel, it is enough to apply a layer of epoxy resin on the back with excellent adhesion and corrosion resistance.
03Coating gloss selection
❖Glossiness is not a coating performance indicator. Like color, it is just a representation. In fact, paint (coating) is relatively easy to achieve high gloss. However, the high-gloss surface is glaring and the high reflectivity of sunlight during the day can cause light pollution (many people do not use glass curtain walls now because of light pollution). In addition, the high-gloss surface has a small friction coefficient and is easy to slip, which can easily cause safety hazards during roof construction. ; The first sign of aging of color-coated steel plates when used outdoors is loss of gloss. If repairs are needed, it is easy to distinguish between old and new steel plates, resulting in poor appearance; if the back paint is high-gloss, halo will easily occur when there is light indoors. Personnel visual fatigue. Therefore, under normal circumstances, color-coated steel plates for construction use medium and low gloss (30-40 degrees).
04Coating thickness selection
Microscopically, the coating is a porous structure. Water and corrosive media (chlorine ions, etc.) in the air will invade through the weak parts of the coating, causing corrosion under the film, and then the coating will blister and peel off. In addition, even with the same coating thickness, the secondary coating is denser than the primary coating. According to foreign reports and relevant corrosion test results, a front coating of 20 μm or more can effectively prevent the intrusion of corrosive media. Since the anti-corrosion mechanisms of primer and topcoat are different, not only the total film thickness must be specified, but also primers must be separately required (》 5μm) and topcoat (》15μm). Only in this way can the corrosion resistance of different parts of the color-coated steel plate be ensured to be balanced.
PVDF products require thicker coatings. Because it needs to provide a longer service life guarantee. The requirements for back coating depend on the application, with sandwich panels requiring only a bondable primer. The formed steel plate also requires two layers of coating due to the indoor corrosive environment. The thickness is at least 10μm.
Coating color selection (emphasis added!)
The choice of color is mainly based on matching with the surrounding environment and the owner’s hobbies. However, from a technical point of view, light-color paints have a large selection of pigments. Inorganic paints with superior durability can be selected (such as titanium dioxide, etc.), and the paint’s The heat reflection ability is strong (the reflection coefficient is twice that of dark paint). The temperature of the coating itself is relatively low in summer, which is beneficial to extending the life of the coating. In addition, even if the coating changes color or powders, the contrast between the light-colored coating and the original color will be small, and the impact on the appearance will not be significant. Dark colors (especially bright colors) are mostly organic colors, which are easy to fade when exposed to ultraviolet rays, and can change color in as little as 3 months. According to relevant test data, when the outside temperature is the highest at noon in summer, the white surface is 10 degrees cooler than the blue surface and 19 degrees cooler than the black surface. Different colors have different abilities to reflect sunlight.
05 color reflectivity reflection effect
For color-coated steel plates, usually the thermal expansion rates of the coating and the steel plate are different, especially the linear expansion coefficients of the metal substrate and the organic coating are very different. When the ambient temperature changes, the bonding interface between the substrate and the coating will change. Expansion or contraction stress occurs, and if not properly relieved, coating cracking will occur. Baosteel conducted an 8-year exposure test in Hainan of the same paint type, the same paint supplier, and different colors. The results also confirmed that light-colored paints have less discoloration.
06 gloss color difference original thickness now thickness
In addition, here we would like to explain two misunderstandings about selection in the current domestic market:
First, there are currently a large number of white primers in China. The purpose of using white primer is to reduce the thickness of the topcoat, because the normal corrosion-resistant primer for construction is yellow-green (hence the strontium chromate pigment), and there must be sufficient topcoat thickness to have good hiding power. This is very dangerous for corrosion resistance. First, the primer has poor corrosion resistance, and secondly, the topcoat is greatly thinned, less than 10 microns. Such color-coated steel plates look bright, but will corrode in different places (cuts, bends, under the film, etc.) in less than two years.
The second is the color-coated steel plates used in construction projects. The same project uses color-coated steel plates from different manufacturers and different batches. The colors seem to be consistent during construction, but after several years of sunlight exposure, the colors of different coatings and manufacturers change. There are too many examples of different trends leading to serious color differences. Even if the products are from the same supplier, it is strongly recommended to place an order for the same project at once, because different batch numbers may use products from different coating suppliers, increasing the possibility of color difference.
Reasonable material selection can not only increase the service life of the building, but also reduce costs, thereby truly being environmentally friendly and resource-saving.
Taishan Industrial Development Group Co., Ltd.
We will always adhere to the service tenet of quality first and customer first, provide customers with the best products, and do our best to save costs for customers. Depending on the use environment of the material and the cost of architectural design, we recommend the use of more cost-effective Taishan Inc color coating, Maanshan Iron and Steel color coating and Shougang color coating. Ordinary PE products can be used for at least 10 years, and PVDF products can last for 20 to 25 years. Beautiful and durable, it makes your factory more beautiful. Our company provides one-stop production, processing, and sales services, serving customers throughout the entire process from customer inquiry to later application.
Post time: Sep-21-2023