There are many factors to be considered in choosing materials for color coated sheet, including coating type, coating thickness, coating color and coating gloss. In addition, sometimes the requirements of primer and back coating are also considered.
一、Coating varieties
At present, the types of coatings used for color coated steel sheets are polyester coatings (PE), fluorocarbon coatings (PVDF), silicon modified coatings (SMP), high weather resistant coatings (HDP), propionic acid coatings, polyurethane coatings (PU), plastic sol coatings (PVC), etc.
For the choice of primer, there are two most important factors. One is to consider the adhesion of primer and top coat and substrate. The other is that primer provides most of the corrosion resistance of the coating. From this point of view, epoxy resin is the best choice. If flexibility and ultraviolet resistance are considered, polyurethane primer can also be chosen.
For the back coating, the most correct choice is to choose two layers of structure, one is the back primer, the other is the back paint, if the color coated steel sheet is single board. Primers are of the same variety as front paints. Topcoats are made of polyester with a light color (e.g. white). If the color-coated steel sheet is composite or sandwich plate in use, then coat the back with epoxy resin with excellent adhesion and corrosion resistance.
At present, there are many functional color coated steel plates, such as anti-bacterial, anti-static, thermal insulation, self-cleaning and so on. These products are developed to meet the special needs of users, but sometimes they can not take into account other properties of color coated products. Therefore, when users choose functional color coated steel plate, they must be clear about their real needs.
二、Selection of glossiness of coatings
Glossiness does not belong to the performance index of coatings. Like color, it is only a characterization. In fact, coatings (coatings) are easier to achieve high gloss.
However, the high reflectivity of high gloss surface to sunlight during the day will cause light pollution (many of the glass curtain walls are not used now because of light pollution). In addition, the high gloss surface has small friction coefficient and is easy to slip. It is easy to bring safety hazards during roof construction. The aging of color-coated steel plate when used outdoors first indicates that it loses light. If it needs to be repaired, the pole between the old and the new steel plate is very easy to slip. Easy to distinguish, resulting in poor appearance; if the back paint is high gloss, indoor lighting will easily produce halo and cause visual fatigue. Therefore, under normal circumstances, the color coated steel sheets used in construction are of medium and low gloss (30-40 degrees).
三、Selection of Coating Thickness
Microscopically speaking, the coating is a hollow structure. Water and corrosive media (chloride ions, etc.) in the air will penetrate through the weak part of the coating, causing corrosion under the film, and then the coating will foam 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, the front coatings above 20 microns can effectively prevent the invasion of corrosion media. Because of the different anti-corrosion mechanism of primer and top coat, not only the total film thickness, but also the thickness of primer ("5 micron") and top coat ("15 micron") are required respectively. Only in this way can the corrosion resistance of different parts of the coated steel plate be balanced.
PVDF products require thicker coatings. Because we need to provide a longer life guarantee. The requirements for back coatings depend on their use. Sandwich panels require only a layer of adhesive primer. Formed steel sheet is required to be coated with two layers because of indoor corrosion environment. Thickness is at least 10 um.
In addition, there are two misconceptions in the choice of the domestic market at present:
One is that there are a large number of white primers in China
The purpose of using white primer is to reduce the thickness of the top coat, because the corrosion-resistant primer used in normal buildings is yellow-green (so there is strontium chromate paint), it must have enough thickness of the top coat to have good covering power. In this way, the corrosion resistance is very dangerous. First, the corrosion resistance of primer is poor, and then the top coat is greatly thinned, less than 10 microns. Such color coated steel sheets look bright and will corrode in different places in two years (notches, bending points, under-film, etc.).
Secondly, different manufacturers and batches of color-coated steel sheets are used in the same project for construction projects, which seems to have the same color in construction. However, after several years of sunlight exposure, different manufacturers of different coatings have different color change trends, resulting in serious color difference.
There are too many examples. Even if the same supplier's products, it is strongly recommended that the same project be ordered once, because different batches of paint suppliers may use different products, increasing the possibility of color difference.
四、Colour Selection of Coatings
The choice of color mainly considers the matching with the surrounding environment and the preference of the owner, but from the point of view of using technology, the choice of light-color paint is wide, and the choice of inorganic coatings with superior durability (such as titanium dioxide) can be made. Moreover, the thermal reflectivity of the coatings is strong (the reflectivity is twice that of the dark coatings). The temperature of the coatings in summer is relatively low, which prolongs the life of the coatings. Beneficial. In addition, even if the coating is discolored or powdered, the contrast between the light color film and the original color is small, and the effect on the appearance is not great. Dark colors (especially brighter ones) are mostly organic colors, which tend to fade when exposed to ultraviolet radiation, and change color in the shortest three months.
Coating
At present, hot-dip galvanizing and aluminium-zinc plating are widely used in building color coatings. The greater the thickness of the coating, the better the corrosion resistance.
The thickness of coating mainly affects the incision corrosion performance of color coated steel plate. The incision corrosion is electrochemical corrosion. The influencing factors include pretreatment layer, primer, substrate thickness, coating thickness, etc. Normally, the pretreatment layer and primer are relatively fixed. The thickness of substrate and coating affect the corrosion performance of incision. The thinner the substrate and the thicker the coating, the better the resistance to incision corrosion. On this point, there are many studies in the world. At present, zinc ratio is generally accepted. That is, the number of grams per square metre of the weight of the coating on one side of the coating/the thickness of the substrate (in millimetres). It is considered that zinc ratio greater than 100 is an effective guarantee to prevent incision corrosion of color coated steel plate. For example, if the steel plate is 0.6mm, the galvanizing capacity on one side will reach 60g/m2. Although the corrosion resistance of aluminized galvanized sheet is better than that of hot galvanized sheet, the zinc ratio is also suitable for hot galvanized sheet for the spread of incision corrosion.
As for the choice of hot-dip pure zinc or hot-dip aluminium and zinc, it mainly depends on the acidity and alkalinity of the environment. Zinc and aluminium are amphoteric metals, but their performances in acid and base are different. The performance of pure zinc in alkaline medium is better than that of Al-Zn alloy. In acid pickling medium, the performance is opposite. Therefore, it is recommended to use aluminium-zinc coating in relatively acidic environment and hot-dip pure zinc coating in relatively alkaline environment. According to the use experience, wet wood, concrete, etc. should not contact the aluminum-zinc plated substrate for a long time.
Steel grade
In China, for the choice of color-coated steel plate, the relative less concern of users is steel grades. Because most color-coated steel production plants in China have fewer kinds of steel, most of them are DC51 (because they buy hot-rolled sheet or rolled coil is basically a kind of steel), some color-coated steel plants with hot-dip galvanizing production line produce all-hard steel (S550) or high strength steel (S350, etc.) through hot-dip galvanizing annealing, but it is very difficult because they can not control the quality and chemical composition of the substrate. Therefore, the quality stability is poor, resulting in large fluctuations in performance, resulting in worsening of profiled sheet metal.
For buildings, bearing weight (wind load, snow load, construction personnel and machinery), span of buildings, purlin distance design and so on all depend on the strength of steel plate. For building maintenance structure with color coated and aluminized galvanized sheet, there are corresponding standards abroad. Such as ASTM 653 and ASTM 792 in the United States, JISG3302 and JISG3321 in Japan, EN10215 and EN10147 in Europe, etc. The steel can be classified into CQ (general commercial grade), DQ (general stamping), HSS (high strength structural steel) and FH (all hard steel).