Introduction: The “sandwich” three-layer synergistic system of coil coatings, where each layer performs its distinct function
Color-coated steel sheet (PPGI/Prepainted Galvanized Steel)——widely used in construction (roofing/wall panels) >60%, home appliances (refrigerators/washing machines/microwaves) >25%, and automotive (interior parts) >10%——is one of the globally most consumed
coating types (accounting for >8%/>1.5 million tons/year of total global coatings). The coil coating’s “three-layer system” (1) primer (20μm/color + weather resistance + aesthetics); (3) backcoat (>10μm/corrosion resistance + appearance) each layer is an independent formulation + independent baking (>200°C PMT/30-60s)
——completed continuously under extremely high line speed (>100m/min)
——the quality of coil coatings (weather resistance 20-30 years/adhesion/OT bend no cracking) is the coating industry’s “efficiency + quality” dual extreme
benchmark.
I. Functions and Formulations of the Three-Layer Coil Coating System
| Coating | Resin Type | DFT(μm) | PMT(°C) | Core Function | Key Tests |
|---|---|---|---|---|---|
| Primer | Epoxy/Polyester/Polyurethane | 5-10 | 200-232 | Anti-corrosion (chromate/chromate-free) + topcoat adhesion (chemical bond + mechanical anchoring) | Salt spray >1000h + OT bend — no cracking between primer/topcoat layers |
| Topcoat | PVDF(70%+30% acrylic)/FEVE/Polyester | 18-25 | 224-249(PVDF)/216-232(Polyester) | Color + gloss + super weather resistance (>20 yrs PVDF / 5-15 yrs polyester) | QUV >3000h gloss retention >80% + OT bend no cracking |
| Back Coat | Epoxy/Polyester | 5-15 | 200-216 | Anti-corrosion + appearance (gray/protection) | Salt spray >500h + workability |
FAQ
Q1: “Zero T-bend” (OT Bend / zero-radius 180° bend) of coil-coated steel — why is it the most severe test of coating workability?
OT bend — fold the coated steel sheet 180° in half — bend radius = sheet thickness (zero radius)
— the coating on the outer rounded corner of the bend is stretched >50% (strain >0.5)
— the coating does not crack / does not peel off under such large strain
— indicates the ductility of the coating and steel sheet is synchronized — the coating’s adhesion and flexibility simultaneously meet extremely high requirements
— this is the “ultimate test” of coil-coated steel workability.
Q2: Why must PVDF (70% + 30% acrylic) be >70% PVDF—will 69% work?
The >70% threshold for PVDF—(1) >70%—PVDF forms a continuous phase (crystalline network of PVDF) in the coating
—acrylic is the dispersed phase—the PVDF continuous phase provides ultra-weather resistance (>20 years); (2) <70%—acrylic becomes the co-continuous phase or continuous phase
—degradation of acrylic under UV (>5-10 years)—weather resistance of the coating decreases significantly
—drops from >25 years to >10-15 years—approaching the level of polyester topcoat. PVDF’s >70% is the “phase transition threshold” for weather resistance
—the difference between 69% and 70% is >10 years of weather resistance lifespan difference.
Q3: Back paint is easily overlooked—why is it the “weak link” of the coil coating system?
The back side of the steel sheet—in building applications not exposed to UV
—but exposed to condensate (back side of roof panels—day-night temperature difference—condensation) + corrosive atmosphere (industrial zones/coast)
—if the back paint’s corrosion protection is insufficient—the steel sheet corrodes from the “back side first” and rusts through to the front—topcoat intact—but the steel sheet is already perforated
—this is the “hidden failure” in the coil coating system, often overlooked. Back paint corrosion protection—epoxy (salt spray >500h) is better than polyester (>300h)—epoxy back paint is recommended (even if slightly more costly).
Related Reading
Summary
The three-layer coating system for coil steel—primer (corrosion protection) + topcoat (weather resistance + color) + backcoat (backside corrosion protection)—on continuous production lines operating at >100 m/min—with each layer independently baked (>200°C/30–60 s)—achieves >20–30 years of outdoor weather resistance. The OT bend (zero-radius 180° bend) is the ultimate test of coating processability. Kexin New Materials provides customers with a full range of three-layer coil coating products and process technical support.