Here we will introduce for you the color steel sheet for the surface of sandwich panels.
color steel sheet composition
The mainstream color steel sheet is generally composed of a combination of base material steel plate + aluminum zinc plating / zinc plating + color coating.
When purchasing the color steel sheet, it should consider below:
the material and thickness of the base steel plate;
the amount of aluminum and zinc plating;
the composition of the color coating;
1. Base material steel plate;
• Material
• Thickness
The material of the base steel plate ranges from 300 Mpa to 550 Mpa in yield strength and from 0.3 mm to 0.8 mm in thickness.
The choice of material and thickness is to serve the bearing capacity of color steel plates. Due to the different plate types and purlin spacing requirements of various suppliers, it is not possible to provide a universal standard.
Mainstream applications: 0.6 mm thick roof panel, 0.5 mm thick outer wall panel, 0.35 mm thick inner liner (installed behind purlins)
2. Substrate plating
The content of aluminum zinc or zinc plating in the anti-corrosion coating ensures the corrosion resistance of the color steel sheet.
• Aluminum zinc plating
• Galvanizing
"The activity of steel is not as good as that of aluminum and zinc. Therefore, when aluminum and zinc are plated on the surface of steel plates, aluminum and zinc will be oxidized first, the resulting aluminum oxide and zinc oxide will form a dense protective film on the surface of the steel to protect the inner steel plate from corrosion. Therefore, the amount of aluminum zinc/zinc plating ensures the corrosion resistance of the color steel plate.
There are two types of anti-corrosion coatings: aluminum zinc plating (55% aluminum) and zinc plating. Aluminum zinc plating has better corrosion resistance. If it is used for roof panels, it is possible to directly use the primary color of aluminum zinc plating without the need for external color coating, and of course, it is also possible to add color coating to the aluminum zinc plating. The anti-corrosion performance of zinc plating is slightly poor, but because aluminum is expensive and zinc is cheap, after increasing the amount of zinc plating, the anti-corrosion performance of color coated steel plates is also a good choice.
Over the past few decades, aluminum zinc plating and zinc plating as anti-corrosion coatings have been confirmed by a large number of constructions worldwide. the amount of aluminum zinc plating (55% aluminum) of 150 g/m2 can basically guarantee the corrosion resistance life of color steel plates for more than 15 years (even without color coating coverage). The amount of zinc plating of 275 g/m2 can also guarantee a corrosion resistance life of over 15 years for the color coated plate (requiring color coating coverage). Of course, the prerequisite is that the internal environment and external natural environment of the building are free of corrosive gases. If there is frequent salt mist in the natural environment, metal dust in the building, acidic volatile gases, etc., it will greatly accelerate the corrosion of color steel plates. Under such harsh conditions, it should be considered to use aluminum zinc magnesium plated color steel plates or even stainless-steel plates with better corrosion resistance.
3. Coating type
The service life of color steel sheet largely depends on the type of coating. The price of different types of coating materials varies greatly.
• Type of finish paint and film thickness
• Primer type and film thickness
The surface color of the color steel sheet is achieved by using color coating paint. The surface represented on the outside of the color steel sheet is called the front, and the other side is naturally the back. Regardless of the front or back, the coating includes two layers of primer and topcoat. Generally, the thickness of the topcoat on the front side of the outer panel is 20 um and primer 5 um, the thickness of the finish paint on the front side of the inner panel is 15 um and primer 5 um.
A. Type of finish paint
On the market, mainstream topcoats are divided into:
• Polyester (PE)
PE coatings have good adhesion to materials, color coated steel plates are easy to process and form, inexpensive, and have a wide range of products. There is a wide choice of color and gloss. Under direct exposure to ordinary environments, the polyester coating has a corrosion resistance period of 7 to 8 years. The UV resistance and powdering resistance of the coating film are poor, and it begins to fade and pulverize within a few years.
• Silicon modified polyester (SMP)
Using silicone resin with excellent color retention and heat resistance as a denaturation reaction with polyester improves the UV resistance and pulverization properties of PE, extending the corrosion resistance life of SMP to 10-12 years, and the corresponding price is also higher than that of PE. However, due to the unsatisfactory adhesion and processability of Silicone resin to materials, coating peeling may occur during the cold pressing process of the plate, so SMP color coated steel plates are not suitable for plate types that require multiple cold pressing molding.
• High weather resistant polyester (HDP)
The high weather resistant polyester resin developed in early 2000 uses monomers containing cyclohexane structure to achieve a balance between flexibility, weather resistance, and cost of the resin. Aromatic free polyols and polyacids are used to reduce the absorption of UV light by the resin, achieving the high weather resistance of the coating. It can achieve 60% to 80% weather resistance of PVDF coatings, and outdoor weather resistance can reach 15 years. High weather resistant polyester coatings have been recognized by the market at home and abroad, with outstanding cost performance. Due to the fact that the price of high weather resistant polyester is not significantly different from that of silicon modified polyester, but its performance has improved significantly, high weather resistant polyester (HDP) is generally preferred, making silicon modified polyester (SMP) relatively rare in the market.
• KYNAR@500 : Contains 70% polyvinylidene fluoride (PVDF)
PVDF is one of the fluorocarbon coatings, which has the longest history and the most widely used among fluorocarbon coatings. Due to the strong bond energy between the chemical bonds of PVDF, coatings have excellent corrosion resistance and color retention. Among the color coated steel plate coatings used in the construction industry, PVDF is the most advanced product, with a large molecular weight and a straight bond structure. Therefore, in addition to chemical resistance, mechanical properties, UV resistance, and heat resistance are also prominent. In general environments, its anti-corrosion life can be as long as 20-25 years, and it hardly fades.
B. Type of primer
Generally, the primer is selected based on the product's purpose, application, processing level, and matching with the topcoat. The film thickness of the primer is typically 5 um. The mainstream primers are divided into:
• Epoxy resin (EP, Epoxy resin)
Easy to peel and short life.
• Polyester (PE, Polyester)
UV resistance and chalking resistance of the coating film are poor, but the primer for the inner liner is sufficient.
• Polyurethane (PU, Polyurethane)
Polyurethane in the primer has excellent chemical resistance, friction resistance, durability, and flexibility, making it the best primer in all aspects.
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