Prepainted Steel Sheets
The prepainted steel sheet for construction is an attractive substitute for wood and other materials, owing to its advantages, including fast color, good durability performance, chalking resistance, etc.It minimizes the maintenance cost. At the same time, various colors, surface structures and coatings provide such products with great flexibility and option. In China, more than 90% color coated sheets are applied in construction, and Baosteel's prepainted steel sheets are also widely used in such field.
Overview of Prepainted Steel Sheets
Brief Introduction
Prepainted steel sheet is coated with organic layer, which provides higher anti-corrosion property and a longer lifespan than that of galvanized steel sheets. The base metals for prepainted steel sheet consist of cold-rolled, HDG electro-galvanized and Hop-dip zinc-alu-mag base metal. The finish coats of prepainted steel sheets can be classified into groups as follows: polyester, silicon modified polyesters, polyvinylidene fluoride, high-durability polyester, etc. The production process has evolved from one-coating-and-one-baking to double-coating-and-double-baking, and even three-coating-and-three-baking. The color of the prepainted steel sheet has a very wide selection, like orange, cream-colored, dark sky blue, sea blue, bright red, brick red, ivory white, porcelain blue, etc. The prepainted steel sheets can also be classified into groups by their surface textures, namely regular prepainted sheets, embossed sheets and printed sheets. The prepainted steel sheets are mainly provided for various commercial purposes covering architectural construction, electrical household appliances, transportation, etc.
History of Prepainted Steel Sheet
The production of prepainted steel sheet originated in the United States in the mid 1930s. At the beginning, the technology was only adopted in producing prepainted steel strip. Till 1995, the first batch of coating equipment for broad strip steel was invented and produced in the United States in 1955. In the 1960s, the production of prepainted steel sheet made a rapid development in the United States, Europe and Japan. Besides, the prepainted steel products were also developed greatly all over the world.
By the end of the 1980s, the production of prepainted steel sheet in China had been almost blank. Until the 1980s, Wusteel and Baosteel established the double-coating-double-baking production lines of prepainted steel sheets in succession. With increasingly promotion and application of Baosteel's prepainted steel sheets at home, more and more people make themselves acquainted with the production of steel sheets. Thus, at the turning point of the 20th century the 21st century, a lot of production lines of prepainted steel sheets emerged like bamboo shoots after a spring rain and there was also a great demand for such products. The domestic annual output of prepainted steel is more than 10 million tons according to the incomplete statistics in 2020.
Coating Structure
1. Type of coating structure
2/1: Coat the top surface of the steel sheet twice, coat the lower surface once, and bake the sheet twice.
2/1M: Coat and bake twice for both top surface and undersurface.
2/2: Coat the top/lower surface twice and bake twice.
2. Usage of different coating structures
2/1: The anti-corrosion property and scratch resistance of the single-layer backside coating is relatively poor, however, its adhesive property is good. The prepainted steel sheet of this kind is mainly used for sandwich panel; 2/1M: Back coating has good corrosion resistance, scratch resistance and molding performance. Besides it has good adhesion and is applicable for single layer panel and sandwich sheet. 2/2: The anti-corrosion property, scratch resistance and processing property of the backside coating of prepainted steel sheet is better, so it is widely used for roll forming. But its adhesive property is poor, so it is not used for sandwich panel.
Introduction of Usage
Construction
The prepainted steel sheet for construction is an attractive substitute for wood and other materials, owing to its advantages, including fast color, good durability performance, chalking resistance, etc.It minimizes the maintenance cost. At the same time, various colors, surface structures and coatings provide such products with great flexibility and option. In China, more than 90% color coated sheets are applied in construction, and Baosteel's prepainted steel sheets are also widely used in such field. Baosteel's prepainted steel sheets satisfy the large-scale halls'rigorous requirements in construction in material color, shape, quality, property, etc. They have been recognized by many landmark buildings in China, covering hall centers, sports halls, aviation knobs, cultural facilities, etc. After 30 years'continuous research and improvement, Baosteel has been not only widely used but also been commonly recognized in auto, non-ferrous metal, power industry, coal mining, light industry, medicine, foodstuff, etc. Baosteel's construction steel has already substituted traditional building materials Introduction of Usage Technical Manual of Prepainted Steel Sheets with its safety, durability, esthetic quality and environmental-friendly property, for always building up a comfortable, harmonious and cozy living environment, from schools and residences to commercial buildings, and from villas to top-grade commercial apartments.
Household Appliances
The prepainted steel sheet for household appliances often takes electro-galvanized, hot-dip zinc-coated and cold rolled steel sheet as its base metal, which is widely applied in manufacturing refrigerators, large-scale air-conditioning systems, refrigerating cabinet, bread maker, furniture, etc.
The distribution of electrical household appliances is started from their good appearances.
Baosteel promises to guarantee the quality of its prepainted steel products. Its products can be applied to refrigerators, refrigerating cabinets, washers, air conditionings and other household appliances. Baosteel's production lines of prepainted steel products are equipped with first-grade equipment of production control, check and inspection, at the same time; its products also take the leading potion among fellow ones with Baosteel's abundant experience in the production of prepainted steel sheets. With excellent packing and convenient logistic system, Baosteel guarantees to deliver its customers the products with zero defect. Batches of prepainted steel products with good luster and high quality that have already delivered to all the providers of electrical household appliances offer them a powerful competitive force in quality.
Introduction of Base Metals
The base metals for prepainted steel sheet consist of cold-rolled, HDG electro-galvanized and Hop-dip zinc-alu-mag base metal.
Cold-rolled base metal
The prepainted steel sheet processed with cold-rolled base metal features in its smooth and beautiful appearance and the machinability of cold rolled steel sheet, however, in case of any tiny scratch on the top coating may expose the cold rolled base metal in the air and the exposed base metal may rust soon. Thus, the products of this kind can only be used as temporary isolation measures and indoor materials with low requirements.
Hot-dip zinc coated base metal
The prepainted steel sheet with hot-dip zinc-coated base metal as its base metal not only has the protective function of zinc coating but also has an isolation and protection function with its organic layer that prevents it from rusting and prolongs its service span longer, than that of hop-dip zinc steel sheet. The zinc content of hop-dip zinc base metal is generally 180 per 1 square meters(both sides), while that of the hot-dip zinc base metal for construction outdoors is 275 square meters per 1 square meters.
Hop-dip alu-zinc base metal
The hop-dip alu-zinc base metal adopts the hot-dip zinc-aluminum coated steel sheet ( 50%~60%AI-Zn) as its prepainted base metal. The corrosive resistance of the hot-dip alu-zinc coated steel sheet is twice to 5 times as much as that of the hot-dip zinc-coated base metal. It can satisfy the processing requirements of rolling, coiling and others processes alike. Neither serious oxygenation nor oxide scale might appear while continuously or intermittently working with an ambient temperature up to 490℃ . Its heat and the light reflected are respectively twice as much as those of hot-dip zinc steel sheet, and its reflectivity is more than 0.75, exceeding 0.65(required by the EPA Energy Star Standard), which is an ideal construction materials of saving energy.
Electro-galvanized base metal
The prepainted electro-galvanized steel sheet adopts an electro-galvanized base metal coated and baked with an organic coating. Owing to the thin zinc-coated layer of the electro-galvanized base metal, the zinc content of the prepainted steel sheet of electro-galvanized base metal is usually 20/20g/m2, and the products of this kind are not suitable for building walls or roofs outdoors. However, it has esthetic appearance and excellent machinability, which makes it suitable for electrical household appliances, acoustic devices, steel household appliances, interior decoration, etc.
Hop-dip zinc-alu-mag base metal
Hop-dip zinc-alu-mag base metal means that a certain proportion of trace elements such as Al/Mg are compounded into the hot-dip zinc coat or hot-dip al-zn coat. The purpose aims the promotion of the ability of anti-corrosion and cutting edge protection. The public accelerating and exposure experiments reveal the enhancement of ability in anti-corrosion with a certain proportion of Al and Mg will be significant as several times to more than ten times. Furthermore, another appreciable benefit of adding the element of Mg is the promotion in the anti-corrosion on the cutting edges. The remained film of Mg-Zn after the corrosion promises a better protection for the cutting edges.
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Type and usage of prepainted steel sheet
Mechanical Properties Follow the Regulations of the Substrate.
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Introduction of Coatings
Composition
Composition and function of coatings Every kind of coating consist of four kinds of ingredients, namely (1) resin, (2) pigment, (3) solvent, and (4) additives. Among them, the solvent is a kind of volatile organic compound.
(1) Resin Resin, namely the membrane forming matter, is the principal component and base of coating, also called base material, and the vital factor determining the property of the membrane. It is required that, being a membrane forming matter, resin should be very stable without any obvious physical change or chemical change in the storage period of coating. Under the specified condition, it can should be quickly dried and congealed into a membrane within the period of membrane formation. There is a great variety of resin products, among which the commonly used ones in the coil coating consist of acrylic resin, epoxy, polyester resin, polyurethane, etc. Different resin has different physical & chemical characteristics as well as different properties in weatherproof, and corrosion resistance.
(2) Pigment Pigment must match resin in coating, which mainly colors the membrane. Different proportion of pigment of the coating membrane has different rigidity, glossiness, and corrosion resistance, etc.
(3) Solvent The solvent is an important component of liquid coating, which is volatile while baking the coating. Generally, the solvent is also used to adjust the viscosity of the coating, and the solvent of this kind is also called diluent. The solvent has important influences on the production, storage and application of coating, the formation of the membrane and the quality of the membrane formation.
(4) Additives
The accessory ingredient is a small amount of additive for improving the performance of coating. Although the dosage of the accessory ingredient is little in the coating, its function is remarkable. For instance, some of them can improve the performance of coating and that of the membrane, some can adjust the baking time, and some can prevent the membrane from morbidity. There is a great variety of the accessory ingredients, including siccative, curing agent, leveling agent, defoaming agent, flatting agent, stabilizing agent, etc.
Classified
Category and film thickness of primers
The category of the primer includes epoxy, polyester, acrylic acid, polyurethane, etc.
In general, the primer is chosen in accordance with the usage, environment condition, processing stage and finish coat. The film thickness of the primer is usually 5-7 um.
(1) Epoxy primer
The epoxy primer has a strong adhesion to the base metal and good properties in waterproof, alkali-proof and chemical resistance, which is the original primer for the coiled steel, having a poorer flexibility than that of other primers.
(2) Polyester primer
The primer has strong adhesion to the base metal and excellent flexibility, which is relatively sensitive to damp environment and has a chemical resistance poorer than that of epoxy primer.
(3) Water-soluble acrylic acid primer
The primer of this kind has strong adhesion, excellent flexibility, lower organic solvent and low-temperature curing.
(4) Polyurethane primer
The primer of this kind has excellent chemical resistance, durability and flexibility.
Introduction of Coatings
Category and Characteristics of Finish Coats
Category of finish coats
Among all the ingredients of coil coatings, resin and pigment have the function of duration, and the primers commonly used consist of polyester, silicon modified polyester, polyvinylidene fluoride, high durability polyurethane, etc.
(1) Polyester
The finish coat of this kind has strong adhesive force, a variety of colors, wide scope of properties in formation and durability outdoors, moderate chemical resistance and low cost.
(2) Silicon modified polyesters
The membrane of the finish coat of this kind has excellent rigidity, abrasion resistance, thermal resistance, good outside durability, chalking resistance, high retention of color and luster, ordinary flexibility, moderate cost, etc.
(3) High-durability polyester
The finish coat of this kind has excellent retention of color, resistance of ultra-violet radiation, strong outdoors durability, chalking resistance, strong adhesion to the base metal, plenty of colors and relatively lower cost compared to the same quality.
(4) Polyvinylidene fluoride
The finish coat of this kind features in its excellent retention of color, resistance of ultra-violet radiation, outdoors durability, chalking resistance, resistance to solvent, formability, good dirt resistance, finite colors and high cost.
High-Durability Polyester
Reasons why HDP has longer durability
(1) Resin: HDP adopts the high molecular weight resin with little branched chain polymer, stable bond energy and resistance to photolysis, so HDP has a relatively good resistance to chalking and excellent retention of luster.
(2) Pigment: HDP adopts inorganic ceramic pigments, which makes it hard to fade in the sunlight. The supplier, one of the earliest and largest coating enterprises in the world, who provides
Polyvinylidene Fluoride
Usage of fluorocarbon prepainted sheet
The PVDF fluorocarbon coating is a masterwork among the existing construction coatings, which is generally acknowledged as an organic coating with the best protective performance, guarantees the metal building board not to be damaged for decades and always retains its beautiful color. Since entering the market in 1965, the fluorocarbon coatings world-widely applied to buildings have already successfully traveled through various poor weathers and still remained perfect in the past 30 years. PVDF denotes polyvinylidene fluoride. The fluorine-carbon bond formed by a fluorine atom with the largest electronegativity, together with its unique symmetry of molecule, lets the PVDF has an extra stability, unique resistance to photolysis of ultra-violate radiation, excellent insulation and mechanical properties.
Performance of coating
Baosteel has already accumulated more than 30 years' technical & production experience of the coatings of this kind. The coating adopts some patented recipes, Kynar 500 or Hylar 5000, and inorganic ceramic pigment, and each kind of new materials must be put into commercial uses upon Florida' s ten-year insolation certificate, which guarantees the product quality. The prepainted steel sheet for the workshop of Fitzpartick Nuclear Power Plant of the U.S.A. adopts the hot-dip zinc coated base metal that is coated with the fluorocarbon coating with 70% resin content. Since the workshop was built in 1971, while not only the surface and the base metal of the prepainted steel sheet are still perfect, but also its color is retained well.
Self-cleaning
1. Why does the prepainted steel sheet become dirt?
The pollutants produced either in a sunny day or in a rainy day may rest on the surfaces of the prepainted steel sheets applied to wall finishes or roofs, which is showed in Figure 1.
The pollutants can be classified into two groups, namely urban pollutants and suburban ones, according to the characteristics of pollution and environment.
Urban Pollutants: auto exhaust gas, industrial dust and fog, etc.
Suburban: dust.
2. What's the self-cleaning prepainted steel sheet?
The self-cleaning prepainted steel sheet is a new prepainted steel sheet for outdoors construction. Its organic coating not only has excellent properties of decoration and weatherproof that common prepainted steel sheets might possess but also has the special self-cleaning function. The factory buildings built by the prepainted steel sheets of this kind needn't any manually-operated cleaning or maintenance, which can retain the cleanness and glossiness of the prepainted steel sheets applied to the walls and roofs only by rainwater, even those in the industries and cities with poorer air quality, and can maintain the high decorative property of the prepainted steel sheets.
3. Different coating system might have different contamination and cleaning property
Fluorocarbon coating: at the medium and low contamination level, and easy to clean.
common polyester system: at high contamination level, and hard to clean.
Self-cleaning sheet: at medium and high contamination level, easy to clean.
4. Baosteel's self-cleaning prepainted steel sheet
Has excellent resistance to staining, which can prevent the pollutants from being penetrated into the coating membrane. Its good water solubility ensures that the rainwater can fully cover the surface of the coating and cleanse the surface well. Baosteel’s self-cleaning HDP and PVDF prepainted steel sheet provide 5-year quality assurance of self-cleaning performance.
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Adiabatic Products
Adiabatic prepainted steel also known as the cool roof means a superior kind of prepainted steel that decreases the temperature in the house by reflecting the sunlight to the air to decrease the thermal conduct into the interior space.The development of function mainly attributes to the revolution of the paint system.
The reflection are enhanced by reformation of pigments and additives.The decline of temperature contributes to the more comfortable life and less frequency use of air conditions. The cost will be reduced by higher efficient utility of energy, since the Heat-island Effect is weakened.
2000UV5%Visible46%Near
Antistatic
Antistatic prepainted steel sheet
1: Source and danger of static electricity
The electric charge may be generated instantaneously when different molecules and atoms are absorbing one another and interacting, which is also called static charge. Even very small static charge can generate a high static voltage. Generally speaking, the polymer with many polar groups is apt to carry positive charge; otherwise, it is apt to carry negative charge.
In our daily life, the static there are various static phenomena. For example, a voltage over 1000 volt may produce while a man is walking on the composite carpet in an air conditioning room, and our hand always feel an electric shock while we are taking off in winter, or opening doors of the auto car.
The surface of both conductor and semiconductor may also generate static electricity; however, because the accumulated charge can be conducted by the surface and the internal of materials, there is little static charge accumulated there. While the polyester or fluorocarbon on the surface of the prepainted steel sheet is a kind of high molecule polymer, which is an insulating material. However, as long as the surface generates electric charge, it may be really hard to discharge.
2: The danger of accumulation of static charge on the surface:
(1) Adsorbing dust and bacteria.
(2) Damage the performance and usage of the surface of the product.
(3) Cause the static electricity discharged, which may bring about fire disaster or explosion.
3: Working principle of the antistatic prepainted steel sheet
The working principle of the antistatic prepainted steel sheet is to add some conducting materials in the insulating polyester coating, which gets the former insulated coating, become a semiconductor. Being assorted with the earthing system of the construction, the accumulated static electricity on the surface of the prepainted steel sheet that is sourced from air convection or fabric friction is conducted into the earthing system and then disappears.
4: Usage of the antistatic prepainted steel sheet
Mainly used in antistatic, high cleanness and bacteria-free environments, covering: electronic semi-conductor industry, foodstuff processing industry, pharmaceutical industry, hospitals, etc.
5: Performance of the antistatic prepainted steel sheet
(1) Its machining property and durability match that of the prepainted steel sheet with common polyester as its base metal.
(2) Its resistance to chemicals and solvents is slightly higher than that of the common polyester prepainted steel sheet.
(3) Its surface resistance(Ω/cm2) is 106-109Ω(common polyester prepainted steel sheet: about 1013Ω
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Antisepitc
Antiseptic Color-coated Sheet
Definition:
Antisepsis is a long period germicidal action to control the activity and propagation of microorganisms to create a clean environment and then gradually kill all these microorganisms. As for bacteria growing in the living environment, the antiseptic effects may continue for more than several years and maintain long period (microbiology) hygiene for living environment.
Mechanisms:
Stripped metal ion antiseptic mechanism. Antiseptic agent slowly releases metal ions in application. Stripped metal ion can destroy the cell membrane of bacteria or activity of cellular protoplasm active enzyme, so it has antiseptic effects.
Antiseptic color-coated sheet normally adopts the inorganic antiseptic agent. To meet the requirements for long acting and board spectrum antisepsis, low toxicity and high efficiency and quick speed, the antiseptic agent combining stripped metal ions and active oxygen are used.
Application:
Used for hospital wall face, ceiling, food storing and processing area, ventilating system, etc.
Prepainted HYGISTEELTM sheet with antibacterial and antiviral white coat
A clean room is a room in which the concentration of airborne suspended particles is controlled. It can reduce the particles induced, generated and detained in the room, and other relevant indoor parameters such as temperature, humidity, pressure, etc. can be controlled according to the requirements. The main application fields are integrated circuits, chips and biomedicine. Classification: industrial clean room, biological clean room. Baosteel has recently developed Prepainted HYGISTEELTM sheet with antibacterial and antiviral white coat.
1: HYGISTEELTM is whiter than white gray
2: Lower gloss
More indoor lighting and glare caused by high light are easy to cause visual fatigue. The low gloss surface provides good diffuse reflectance and visual comfort. At the same time, the low gloss can enhance the scratch resistance of the organic coat. The white HYGISTEELTM is 15 units lower than the common white gray on aiming value.
Note: generally, the gloss of interior wall emulsion paint is less than or equal to 5.
3: Super flatness
Baosteel has developed ultra-high profile products for clean rooms on the basis of high flatness precision supply of all existing prepainted products, which are ordered through special APN O243 and O233.
The shape requirement is managed by the APN; Special steel grade is adopted with uniform strength.
4: Antiviral mechanism
Some special antiviral factors can inactivate all kinds of proteins, so as to play the role of killing the virus, and there is no selectivity. It also can kill bacteria. It is mainly through the complexation of antiviral factors with amide bond and sulfhydryl group of protein peptide bond.
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Super Thick Coated Prepaitned Steel
The coat thickness of the prepainted steel mainly means the top thickness including the top paint and the primer. Usually, 20-25 micron paint are applied on the top. For the acquisition of better protection to the alloy coat and the metal underneath, it is a brilliant way to increase the thickness of organic coat.
Baosteel develop series of prepainted steel products with super thick coat. They are 2 lays PVDF, 3 lays PVDF with pearly-lustre, super thick PVDF on both sides, super thick PU, PE with thick primer.
The demonstration of diagram compare the results on the scratch corrosion of different types of prepainted steel in the salt spray. It indicates that scratch of PVDF stretches most slowly during the 3000 hours' experiment comparing to the PE and HDP with regular thickness.Then we come to a conclusion that PVDF take the priority place in the corrosion resistance. And the performance get better when the coat thickness increases.
The weatherability of the super thick PVDF turn out to be appreciably excellent. The gloss loss of prepainted steel with different type and thickness after 3000 hours' QUVB is listed in the Figure 2. The gloss loss of PVDF perform apparently lower than the PE. The prepainted steel with the top thickness of 23 micron and 45 micron are similar in the gloss loss due to the similar mass of fluorocarbon.The PVDF with the thickness of 62 micron results in level 0, which is the best performance in the evaluation standard. The gloss loss of the prepainted steel with the thickness of 62 and 35 micron are the most excellent ones.
The series of thick prepainted steel:2 layers PVDF with thick primer(thick primer PVDF), 3 layers PVDF with pearly-lustre(3 layer PVDF), super thick PVDF on both sides(super thick PVDF on both sides), super thick PU(thick PU), PE with thick primer, etc. We illustrate the concepts of different types shown as the Figure 3. And Baosteel are willing to recommend the suitable products to customers on the consideration of the building locations and the designs of service period.
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Service Guide
A rational material choice can not only meet the customer's requirements but also lower the cost at the maximum level. If any improper choice of materials, the performance of materials may surpass the application requirements, as a result, just a waste of materials, otherwise it may not reach the requirements and lead to being degraded or discarded as a useless. However, customers should pay much attention to the proper choice of material. If any, please contact us.
The choice of the prepainted steel sheet is mainly determined by the mechanical property, type of base metal (type of coating), weight of coating, and the choices of the performances of front coating and back coating. Its usage, environmental corrosion, service time, durability, machining method and distortion level are the important factors that should be taken into consideration when choose relevant materials.
Choice of mechanical performance, type of base metal and weight of coating
The choice of mechanical performance should base on the factors, such as usage, machining method, distortion level, etc. For example, the prepainted steel sheet for the roofing of the building usually bears few weight and has few complex distortion while being machined, so it often adopts TDC51D. The prepainted steel sheet for any parts with relatively big distortion should adopt the materials with good machinability, just like TDC52D andTDC53D. While the prepainted steel sheet for any structure with weight bearing requirements should follow the corresponding design to adopt proper structural steel, like TS280GD and TS350GD. The machining methods commonly adopted by the prepainted steel sheet consist of cutting, coiling, rolling, etc., and the descriptions of the properties of each kind of these should be chosen when ordering. In addition, we should notice that we often substitute the mechanical properties of the prepainted steel sheet for those of the base metal in practice while the process of the prepainted steel sheet might change the mechanical properties of the base metal. The choice of the type of the base metal (type of coating) and the weight of coating should be determined in accordance with usage, environmental corrosion, service life, etc. Corrosion resistance is one of the main functions of the prepainted steel sheet. The type and the weight of the coating are the influential factors to the corrosion resistance of the prepainted steel sheet. Owing to being always exposed in the air, the prepainted steel sheet for construction usually selects the base metals with good resistance to corrosion and thick coating , like hot-dip zinc-coated metal and hot-dip alu-zinc coated metal. In addition, different kinds of coating have different resistances to corrosion. For example, the resistance to corrosion of the coating of the hot-dip alu-zinc coated metal is higher than that of the hot-dip zinc coated metal with a of identical thickness. Additionally, the resistance to corrosion usually increases with increase of the coating, so we can improve the resistance to corrosion of the prepainted steel sheet by using the base metal with high resistance to corrosion and/or increasing the weight of the coating. We should also pay attention that the steel sheet with different coating has different resistance to corrosion of the trimming die. Otherwise, the service life and durability are also the important factors for choosing materials. If a longer service life and high durability required, a base metal with high resistance to corrosion or big weight of coating should be chosen.
Aluminium can react to both acids and alkalis. And hot dip galvanized steel contain the aluminium with a proportion of 55%. Since it is relatively alkaline, the concrete will react with the aluminium.The aluminium dissolves and vanishes its ability to resist corrosion. That explains why the prepainted steel with the hot dip galvanized substrate is unable to support the direct contact with the concrete.
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Choice of performance of front coating
The performance choice of front coating is mainly determined by the type of coating material, and the thickness, color difference, coat gloss, rigidity, flexibility/adhesion, durability and other performances of coating.
Finish coat
The finish coats commonly used can be classified into four groups, namely polyester, silicon modified polyester, high-durability polyester and polyvinylidene fluoride. The finish coats have different performances in rigidity, flexibility/adhesive force, corrosion resistance, etc. The coating of polyester is the most popular coating for the time being, which has common durability, good rigidity and flexibility of coating and moderate price. The coating of silicon modified polyester has been already greatly improved in durability and retention of gloss and color, but the flexibility of its coating is still lower. The high-durability polyester coating not only possesses the merits of the polyester coating but also makes a great improvement in durability. Its quality is better. The coating of polyvinylidene fluoride has excellent durability and good flexibility of coating, while its rigidity is relatively poor, its color available is less and its price is high. For details of these finish coatings, please consult about relevant materials or contact experts. The finish coatings should be determined by the usage, environmental corrosion, service life, durability, machining method, distortion level, etc.
Primer
The primers commonly used consist of epoxy, polyester and polyurethane. Different primers can be differed by the adhesive force, flexibility, resistance to corrosion, etc. The epoxy primer has strong adhesive force to the base metal and higher resistance to corrosion, while its flexibility is poorer than those of other primers. The polyester primer has strong adhesive force to the base metal and excellent flexibility, while its resistance to corrosion is poorer than that of the epoxy primer. The polyurethane primer is a better primer with comprehensively good performances. For details of these primers, please consult about relevant materials or contact experts. The primer is usually determined by the manufacturer through matching the primer with corresponding process technology, purpose and environmental corrosion.
Thickness of coating
The corrosion resistance of the prepainted steel sheet has closed relationship with the thickness of the coating, and in general, the resistance to corrosion often increases with the increase of the thickness of the. The thickness of the coating should be determined properly by the environmental corrosion, service life and durability.
Color aberration of coating
Being influenced by the production batch, color change, service life, operating environment, purpose, etc., the prepainted steel sheet may have color aberration while being produced or using, so the color aberration of coating should be negotiated by both of the provider and the customer when ordering.
Rigidity of coating
Coat hardness is the ability to resist any damages from mechanical operation, such as scratch, friction, collision, and pressing, etc., which is close to the properties of the prepainted steel sheet, such as its resistance to scratch, durability, resistance to pressure, etc., and mainly determined by the usage, machining, storage & transportation, etc.
Flexibility/Adhesive force of coating
The flexibility/adhesive force of the coating is close to the machinability of the prepainted steel sheet, which is mainly determined by the machining way, distortion degree, etc. IncIn case of any fast and severer distortion, the prepainted steel sheet with high resistance to collision and small T-shape angle should be selected.
Durability of coating
The durability of the coating is a kind of performance that is indicated during the usage of the prepainted steel sheet, which is usually valued by its service life. The durability of the coating is usually influenced by the type of the coating, thickness of the coating, environmental corrosion, etc. The actual durability of the coatin can be determined via the air exposure test. In addition, it can also be assessed by the test of artificial aging test of the coating. The test of resistance to neutral salty mist is one of the simplest and commonest artificial tests; and the artificial aging test by ultra-violet lamp is also a relatively common method. In addition, the prepainted steel sheet may be used in a special environment with acid rain and humidity, so it should be assessed by corresponding artificial aging test prior to use. We should pay attention that the artificial aging test often fails to completely stimulate the actual operating environment.
Other performances
We should also notice that the prepainted steel sheet may have some requirements of better organic-soluble property, resistance to acid/alkali, resistance to pollution, etc. under peculiar circumstances.
Choice of performance of back coating
The back coating is often determined by the manufacturer in accordance with the purpose and operating environment. In case of poorer environmental corrosion, it just requires a layer of back coating, which mainly functions as a decoration. If lower back insulating materials required, please indicate in the order, so that the manufacturer can timely coat the materials with strong adhesive force to the base metal. In case of higher environmental corrosion, please coat two layer of coating to improve the resistance to corrosion.
