Colour-coated coils, formally known as colour-coated steel sheet coils, utilise cold-rolled steel sheets, galvanised steel sheets, or metal-coated steel sheets such as aluminium-zinc or zinc-aluminium-magnesium as substrates. These undergo surface pre-treatment to remove oil residues and oxide layers, followed by the application of one or multiple layers of organic coatings through a continuous roll-coating process. Finally cured through high-temperature baking, it forms a composite metal coil. Combining the strength of the metal substrate with the weather resistance and decorative properties of the organic coating, it is a widely used ‘high-aesthetic + high-performance’ material in industrial and architectural applications.

I. Core Structure: The ‘Five-Layer Protection System’ from Core to Surface

The performance of colour-coated coils is determined by the synergistic interaction of multiple layers, each fulfilling distinct functions. From core to surface:

Substrate Layer: The core structural layer determining the material's mechanical strength (tensile, compressive) and fundamental corrosion resistance, serving as the ‘skeleton’ of the coil. Mainstream substrate types and characteristics:

Galvanised substrate (GI): High cost-effectiveness with robust basic corrosion resistance, suitable for standard outdoor applications (e.g., industrial roofing);

Galvalume substrate (GL): Contains 55% aluminium and 43.5% zinc, offering 2-3 times the corrosion resistance of GI, ideal for medium-to-high corrosion environments (e.g., coastal construction);

Zinc-Aluminium-Magnesium Coated Substrate (ZM): Contains zinc, aluminium, and magnesium alloy, offering 3-5 times the corrosion resistance of GI. High scratch resistance makes it suitable for highly corrosive environments (e.g., chemical plants, coastal facilities);

Cold-Rolled Substrate: Used exclusively for indoor applications (e.g., appliance casings). Uncoated, it relies entirely on the applied coating for corrosion protection.

Chemical conversion coating (pretreatment layer): Acts as a ‘bridge’ between substrate and coating, with a thickness of just 0.5-2μm. Its function is to enhance adhesion between substrate and coating while further isolating moisture and salts, thereby improving overall corrosion resistance (common processes include phosphating or chromate passivation; chromium-free passivation is used in environmentally sensitive applications).

Primer layer: The ‘foundational defence’ of the coating system, typically 5-10μm thick and composed of epoxy or polyurethane coatings. Characterised by strong adhesion and excellent chemical resistance, it effectively blocks corrosion medium penetration to shield the substrate from erosion.

Topcoat layer: The functional and aesthetic core of the colour-coated coil, with a thickness of 10-25μm. This layer determines the material's weather resistance, colour, gloss, and stain resistance. Selection depends on the application environment:

Standard Polyester (PE): Cost-effective with moderate weather resistance (5-8 years outdoor lifespan), suitable for low-corrosion inland settings with modest durability requirements (e.g., temporary structures, interior decoration);

Silicon-modified polyester (SMP): Incorporates silicon elements, offering superior weather resistance to PE (outdoor lifespan 8-12 years) with enhanced resistance to chalking and fading. Suitable for standard outdoor structures (e.g., residential roofing, commercial facades);

High-Durability Polyester (HDP): Weather resistance approaches PVDF while costing less than PVDF (outdoor lifespan 12-15 years). Resists UV radiation and humid heat, suitable for high-sunlight, rainy regions (e.g., southern coastal buildings);

Polyvinylidene fluoride (PVDF, commonly known as ‘fluorocarbon coating’): Offers the highest weather resistance (outdoor lifespan 20-25 years), with excellent UV resistance, acid/alkali resistance, and pollution resistance. Suitable for high-end buildings (e.g., airport terminals, sports stadiums, landmark structures).

Backing Coating (optional): Used only where rear-side protection is required (e.g., preventing condensation corrosion on roof interiors), with a thickness of 3-8μm. Typically an epoxy or polyester coating, its primary function is rust and scratch prevention.