Color Coated Steel Coil: Substrates, Paint Systems and Specification
Feb 23, 2024
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Substrate and Coating Build Up
Color coated steel coil is strip steel that has been given a metallic protective coating and then painted on a continuous coil coating line. The substrate decides the corrosion reserve: hot dip galvanized strip carries a zinc coating, designated for example Z275 meaning 275 g/m² total zinc on both faces, while aluminium-zinc coated strip carries a 55 % aluminium and 45 % zinc alloy coating, designated AZ150 for 150 g/m². These designations come from the continuously hot dip coated flat product standards EN 10346 and, for the zinc version, ASTM A653/A653M, with the aluminium-zinc counterpart in ASTM A792/A792M. Organic coating of the finished strip is covered by EN 10169.
On the coating line the strip is cleaned, chemically pre-treated, primed and then top coated, each layer cured in an oven. The pre-treatment is the interface that carries most of the corrosion performance, so it is specified together with the paint system rather than treated as a separate step.
Paint Systems Compared
| System | Resin | Typical topcoat dry film | Strength | Relative cost |
|---|---|---|---|---|
| Polyester (PE) | polyester | 18-25 µm | Formability, colour range, adhesion | low |
| Silicon modified polyester (SMP) | polyester with silicone | 20-25 µm | Hardness, abrasion and heat resistance | low to medium |
| High durability polyester (HDP) | polyester, modified | 20-25 µm | Colour and gloss retention, UV resistance | medium |
| PVDF | polyvinylidene fluoride, usually 70 % or more of the resin | 23-27 µm | Best UV and chalking resistance, solvent resistance | high |
Coatings are normally applied two sides, with the reverse face either the same system or a thinner protective coat. Primer dry film is typically 5-7 µm and sits between the metallic coating and the topcoat. Total dry film thickness, pencil hardness, adhesion after bending, T-bend and impact resistance are the routine release properties.
Forming and Service Behaviour
Coil coated strip is roll formed, profiled, bent and deep drawn, so elongation and T-bend are specified together with adhesion, because a coating that cracks at the bend exposes the metallic layer to the weather. Over time the failure mechanisms are edge creep from cut edges, chalking and colour change from ultraviolet exposure, and blistering where the pre-treatment or primer bond fails. Colour change is best judged with an accelerated weathering test plus outdoor exposure, not from colour alone. Thermal reflectivity of light colours keeps roof surface temperature lower than bare galvanized steel, which is why light shades dominate in building cladding.
Where the Product Is Used
Building cladding, roofing and sandwich panels take the largest tonnage, followed by domestic appliances, electrical cabinets, air conditioning housings, advertising panels, furniture and automotive interior parts. The practical selection sequence is to fix the expected service environment, choose the metallic coating mass and paint system from that, then confirm the forming route against T-bend and hardness before settling the colour.
Frequently Asked Questions
Q: What is the difference between PPGI and PPGL?
A: PPGI is color coated strip on a hot dip galvanized substrate with a zinc coating such as Z275, while PPGL uses a 55 % aluminium-zinc alloy coating such as AZ150, which performs better in high temperature and marine exposure.
Q: Which paint system lasts longest outdoors?
A: PVDF gives the best UV and chalking resistance and is normally specified for architectural facades, while high durability polyester is the usual compromise for roofing and general cladding.
Q: Does colour affect corrosion resistance?
A: Colour affects heat build up and fade rather than corrosion, but light colours reflect more solar radiation and keep the substrate cooler, which slows coating degradation.
Q: Why does coating mass matter more than paint thickness?
A: The zinc or aluminium-zinc layer is the sacrificial corrosion reserve at cut edges and scratches, so a heavier metallic coating extends edge life even when the paint film is unchanged.
Q: How is the paint bond tested?
A: Adhesion is checked after bending and by tape or cross hatch methods, and formability is expressed through T-bend and impact tests defined in the organic coated flat product specification.
Q: What thickness of steel is usual?
A: Building panels normally use 0.40-0.60 mm substrate, appliances 0.40-0.80 mm, and the strip is supplied as coils, slit coils or cut sheets according to the forming line.