What PPGL Coil Is and How the Coating Is Built
PPGL is the trade shorthand for prepainted Galvalume steel coil: a cold-reduced steel substrate that is first hot-dip coated in a 55% aluminium, 43.4% zinc, 1.6% silicon bath and then coil-coated with an organic paint system. The metallic layer is the corrosion barrier and the sacrificial anode; the paint layer supplies colour, gloss and ultraviolet resistance. Neither layer is decorative only, so a coil that meets a paint thickness target but misses the metallic coating mass target will fail long before its nominal service life.
Coil coating runs continuously: alkaline cleaning, chemical conversion (chrome-free or chromate passivation), primer application and curing, topcoat application and curing, water quench, drying and recoiling, with a strippable film where specified. Line speed, oven temperature profile, conversion coating weight and dry film thickness are logged per coil. Those logs, together with the mill certificate for the substrate, are the practical evidence a buyer should hold for each delivery.
Substrate and Grade Selection
Typical substrates are DX51D+AZ commercial forming grades, the S250GD+AZ to S550GD+AZ structural range, or commercial quality carbon steel such as Q195. Under EN 10346 the designation follows the coating: DX51D+AZ, DX52D+AZ and DX53D+AZ indicate increasing formability, and the +AZ suffix identifies the aluminium-zinc coating rather than the +Z pure zinc coating. EN 10346 lists DX51D+Z with a tensile strength of 270-500 MPa and A80 elongation of at least 22%, DX52D+Z at 270-420 MPa and at least 26%, and DX53D+Z at 270-380 MPa and at least 30%. Because profiling happens before painting, the grade must be selected against the actual bending radius and profile depth, not against the finished roof appearance alone.
Metallic coating mass is stated in grams per square metre over both faces. AZ150 therefore means 150 g/m2 total, that is 75 g/m2 per face, which at a coating density of about 3.7 g/cm3 corresponds to roughly 20 micrometres per side. The 1.6% silicon addition suppresses growth of an excessively thick brittle intermetallic layer, which is why the coating tolerates roll forming without flaking. ASTM A792/A792M covers 55% aluminium-zinc coated sheet with designations such as AZ50 and AZ55, and ASTM A924/A924M is the general requirements document that governs coating mass determination by the triple-spot method.
Paint Systems and Film Build
| Paint system | Typical top film | Typical back film | Service profile |
|---|---|---|---|
| Polyester (PE) | 11-25 um | 5-14 um | General roofing and walling, economy class |
| Silicone modified polyester (SMP) | 18-25 um | 5-14 um | Improved chalk and colour retention |
| High durability polyester (HDP) | 20-25 um | 5-14 um | Exterior colour retention over many years |
| PVDF | 23-35 um | 5-14 um | Architectural facades, coastal and high UV exposure |
Primer chemistry decides adhesion as much as topcoat chemistry does. Epoxy primers give the most reliable bond to the aluminium-zinc surface and are the default for roofing; polyester primers suit lower bake temperatures; water-borne acrylic primers are used where solvent emission limits are tight. Topcoat and backcoat are not interchangeable. The backcoat exists to protect the reverse face during handling and to prevent coil-to-coil marking in storage, and GB/T 12754 formalises the colour coated product family in the same terms.
Performance in Service Environments
The aluminium-zinc coating performs best where humidity, salt and industrial sulphur are present simultaneously, because the aluminium-rich surface oxide is more stable than zinc oxide and the zinc phase still provides sacrificial protection at cut edges and fastener holes. Two practical limits govern lifetime: cut-edge exposure and coating damage during installation. Coils should be cut with shears rather than abrasive discs, and any site cut should be treated with a compatible repair coating, otherwise the edge becomes the first corrosion site regardless of the original coating mass.
Roof pitch also matters. Low-pitch and concealed-fix roofs hold water and debris for longer periods, so they call for a higher coating mass and a heavier top film than a steep exposed profile. Wood grain and matt finishes use the same substrate and film build as plain colours, but their textured surface can retain airborne contaminants, so cleaning intervals are shorter in industrial atmospheres.
Common Defects and How They Are Avoided
Coating delamination after profiling is normally traced to insufficient conversion coating, over-baked primer, or a profile radius below the grade's forming limit. Edge cracking is a substrate problem, not a paint problem. Colour drift between batches is controlled by retaining the approved colour master and by verifying colour difference after every coil change, while interleaved marking comes from storing coils at excessive stacking pressure or in damp conditions.
Frequently Asked Questions
Q: Which environments suit prepainted aluminium-zinc coated coil?
A: Coastal zones, heavy rainfall areas and industrial atmospheres with combined salt and sulphur exposure. The aluminium-rich oxide resists humid corrosion better than pure zinc, and the zinc phase still protects cut edges by sacrificial action.
Q: Is AZ150 the same as Z275?
A: No. AZ150 is 150 g/m2 of aluminium-zinc over both faces, and Z275 is 275 g/m2 of pure zinc over both faces. They are not interchangeable by mass: the aluminium-zinc coating is roughly half the density of zinc, so AZ150 is about 20 micrometres per side while Z275 is about 19 micrometres per side of pure zinc.
Q: How is the grade selected for a profiled roof panel?
A: Match the forming demand to the grade family. Commercial forming grades suit shallow corrugations, while deeper or tighter profiles require a higher forming grade with at least 26% or 30% A80 elongation under EN 10346, checked against the roll former's radius limit.
Q: How thick should the paint be?
A: Standard practice is a 5 to 14 micrometre backcoat and an 11 to 25 micrometre topcoat for polyester, rising to 23 to 35 micrometres for PVDF architectural work. Film thickness alone is not sufficient evidence: adhesion after bending and after impact must also be tested on the finished profile.
Q: Can the coil be welded after painting?
A: Painting interrupts electrical continuity and releases fumes, so the paint is normally stripped from the weld zone first. Spot welding, seam welding and mechanical fastening are all used on the finished panel, and every stripped or welded area needs a repair coat.
Q: What should appear on the inspection document?
A: Substrate grade and heat number, metallic coating mass per unit area, paint system and specified dry film thickness, colour reference, mechanical properties and the adhesion or bending test result. These items allow a receiving inspection to confirm the delivery before the coil is uncoiled.