Many people don't know how to choose high-quality color-coated steel sheets because the surfaces among them are similar, and you can find no problems in a short time.

Many factors matter when considering the coating layers. The factors affecting the coating layers mainly include their types, thickness, color and gloss. In addition, sometimes also consider the requirements of the primer and back coating of the coating.

The types of coatings currently used for color-coated steel plates include polyester (PE), PVDF, silicone modified polyester (SMP), high durable polyester (HDP), acrylic coating, polyurethane (PU), polyvinyl chloride (PVC), etc.

01

Common coating types

Polyester (PE)

PE coating has good adhesion, so the color-coated steel plate with PE coating is easy to process and shape. The price of such steel plates is low and we can have a lot of choices when considering the color and gloss., while polyester coatings are not ideal for UV resistance and chalking resistance of the coating film. Therefore, there are still some restrictions for PE coating applications. It is generally used in areas with less air pollution or products that require multiple molding processes.

• Applicable industries

Ordinary industrial plants and logistics warehouses do not corrode color panels and require high corrosion resistance and aging resistance so what we need to consider more is the practicability and cost-effectiveness of plant construction.

Silicone Modified Polyester (SMP)

Because polyester contains reactive groups -OH/-COOH, it is easy to react with other polymer compounds. SMP provides better durability of steel plate, and its anti-corrosion period can be as long as 10-12 years. And of course, its price is also higher than that of PE.  Since silicone resin adhesion to materials and processing and formability is not ideal, color-coated steel plates with SMP coating are not suitable for occasions that require multiple forming and processing. Most of them are used for building roofs and exterior walls.

High durable Polyester(HDP)

Regarding the shortcomings of PE and SMP,  HYDRO in Britain (now acquired by BASF), BECKER in Sweden has developed HDP polyester coatings in 2000 that can reach 60-80% weather resistance of PVDF coatings. It is better than ordinary silicon-modified polyester coatings, and its outdoor weather resistance can up to 15 years.

The high weather resistance polyester resin uses monomers containing cyclohexane structure during synthesis to achieve the balance of flexibility, weather resistance and cost of the resin, and uses non-aromatic polyols and polyacids to reduce the resin’s absorption of UV light to achieve the high weather resistance of the coating. Adding a UV absorber and HALS to the coating formulation can improve the weather resistance of the paint film. High weather-resistant polyester coil coatings have been recognized by the market abroad, and the cost-effectiveness of the coatings is very outstanding.

• Applicable industry

Metallurgical, Non-ferrous metal smelters (copper, zinc, aluminum, lead, etc.) in the power industry are the most challenging for the lifespan of color-coated plates. Steel plants and power plants will also produce corrosive media which require high corrosion resistance for color-coated plates.

PVDF

Due to the bond energy between the chemical bonds of PVDF, the coating layer has very good corrosion resistance and color retention. Among the color-coated steel plate coatings for the construction industry, PVDF is the most advanced coating with a large molecular weight and straight bond structure. In addition to its excellent chemical resistance, its mechanical properties, UV resistance and heat resistance are also outstanding.
Its anti-corrosion period can be as long as 20-25 years in general conditions.

• Applicable industry

Products in the chemical industry have volatile characteristics and are more likely to produce acidic or alkaline and other corrosive volatile substances. When exposed to water, they are easy to form dewdrops and adhere to the surface of the color plate, which will corrode the coating of the color plate and may further corrode to the zinc layer or even the steel plate.

02

Coating thickness

From a microscopic point of view, the coating is a porous organization. Water in the air and corrosive media (chloride ions, etc.) will invade through the weak part of the coating, causing corrosion under the film, and then the coating layer will bubble and peel off.

In addition, even with the same coating thickness, the secondary coating is denser than the primary coating. Since the anti-corrosion mechanism of the primer and the finish coating is different, not only the total coating thickness must be specific, but also the thickness of the primer (>5μm) and finish coating (>15μm) have to. Only in this way can the different parts of the color-coated steel plate be guaranteed. The corrosion resistance is balanced.

PVDF products require a thicker coating film because of the extended warranty period. The requirements for the back coating depend on the specific application, and the sandwich panel only requires a layer of bondable primer. The formed steel plate requires two coating layers due to the corrosive environment interior with a thickness of at least 10μm or more.

03

Coating color

How to choose color mainly depends on matching with the surrounding environment and the owner’s preferences. From the perspective of the technology, there are more choices of pigments for the light color paint. And we can choose the inorganic paint with excellent durability. Besides, the reflection ability of the paint is more strong (its reflectivity is twice that of the dark painting), the temperature of the coating itself is lower in summer, which is beneficial to the extension of the life of the coating layers.

In addition, even if the coating is discolored or chalked, the difference of the light-colored coating film with the original color after the change is small, and the effect on the appearance is not heavy. The dark colors mostly use organic pigments which are more likely to fade when exposed to ultraviolet light and can have a color difference in even three months.

According to relevant test data, when the outside temperature is highest at noon in summer, the temperature of the white surface is 10 degrees lower than the blue surface and 19 degrees lower than the black surface. The reflectivity of different colors to sunlight is different.

04

Coating gloss

The degree of gloss is not a performance index of the coating. It is just a sign like the color. And the coating layer is more likely to achieve high gloss.

However, the high reflectivity of the high-gloss surface during the day will cause light pollution (many glass curtain walls are not promoted now because of their light pollution). In addition, the high-gloss surface has a low coefficient of friction and is easy to slip, which may cause safety hazards during roof construction.

The aging of the color-coated steel plate, when used outdoors, is the first sign of loss of gloss. If maintenance is required, it is easy to distinguish between the old and the new steel plate, resulting in poor appearance; if the back paint is high-gloss, it is easy to produce a halo when there are lights in the room and cause personnel visual fatigue. Therefore, under normal circumstances, color-coated steel sheets for construction use medium and low gloss (30-40 degrees).

At present, there are many functional color-coated steel plates, such as antibacterial prepainted sheets, antistatic prepainted sheets, thermal control prepainted sheets, self-cleaning prepainted sheets, pattern prepainted sheets, aluminum-magnesium-manganese prepainted sheets and animal husbandry prepainted sheets.

The development of these products aims to meet the requirements of end customers. But sometimes they cannot take into account other properties of color-coated products. Therefore, end-users must consider their actual requirements when choosing functional color-coated steel sheets.