Why PPGI Coils Show Color Difference: Causes, Measurement and Prevention in Coil Coating

What PPGI Coil Is and Why Colour Consistency Matters

PPGI (pre-painted galvanized iron) coil is produced by continuously coating a zinc-coated steel substrate with an organic paint system on a coil coating line. The strip is uncoiled, cleaned and chemically treated, primed where required, painted on both faces, cured in an oven and recoiled. The result is a finished surface that goes directly into roofing sheets, wall cladding and appliance panels without any further painting step.

Because the surface is visible, colour difference is the defect that customers notice first. Two coils that satisfy the same nominal colour number can still look mismatched when they are installed next to each other on the same roof. Uniformity therefore has to be built into the process, not inspected into the coil at the end of the line.

Surface Pretreatment Before Coating

Paint adhesion and corrosion resistance depend on what happens before the paint head. PPGI coil needs efficient degreasing and chemical treatment to produce a clean, chemically active surface.

Alkaline degreasing. A hot alkaline solution is normally used, for example pressure spraying at about 0.1 MPa with a 0.5-1.5% sodium hydroxide solution at 60 °C, containing a small amount of sodium polyphosphate. Electrolytic degreasing is used on some lines for faster soil removal.

Mechanical brushing. Nylon brushes remove adhering particles and prepare the surface for chemical treatment. They must run below 60 °C because nylon loses its shape and stiffness at higher temperatures.

Chemical conversion treatment. A conversion film is formed by pressure spraying or dipping. The conversion fluid is selected according to the substrate, with iron-oxide or phosphate types used for cold-rolled sheet.

Passivation. For improved corrosion resistance, passivation with a chromate solution at 0.1-0.5% chromium content is applied. The rinse water contains chromium and must be purified before discharge, and chromium-free passivation is the preferred route for markets with restricted substance rules.

A conversion film that is too heavy, uneven, or insufficiently dried is one of the most frequent root causes of colour problems, because the film colour and its light scattering differ from point to point along the coil.

Main Causes of Colour Difference

Cause	Mechanism	Typical appearance
Coating process defects	Conversion film too thick, drying temperature too low, or a damaged coating roller	Yellow spots after drying, most visible on light colours
Paint condition	Incomplete mixing, large viscosity differences between batches, or incomplete cleaning during a colour change	Colour-difference bands running across the coil width
Spangle size variation	Different spangle sizes create pits of different size at the grain boundaries, which scatter light unevenly	Patchy, mottled appearance on light colours

Among these three, painters most often overlook the substrate. The zinc coating of a hot-dip galvanized sheet is made of visible spangles whose size and orientation vary with coating weight and cooling practice. Under a thin, light-coloured paint film the spangle pattern remains partly visible, so the coil appears to change shade along its length.

How Colour Difference Is Measured

Colour difference is evaluated with a spectrophotometer against an agreed reference standard and expressed as ΔE in the CIELAB colour space. The measured value combines lightness, chroma and hue deviations into a single number, which makes it a practical acceptance criterion for coil shipments.

Common test methods for colour-coated products include GB/T 13448 for colour coating test methods and ASTM D2244 for calculation of colour tolerances from instrumentally measured colour coordinates. The acceptance tolerance is normally agreed between supplier and buyer before production and typically lies at ΔE of 1-2 for visible shade matching within one batch. Buyers with critical architectural facades may ask for a tighter limit and should state it in the purchase specification.

Prevention Measures on the Coating Line

Hold conversion film weight and drying temperature inside the validated process window, and maintain coating rollers on a fixed replacement cycle.

Stir paint thoroughly before use and standardise viscosity between batches; record viscosity at the paint head for every coil.

Clean the coating machine completely when changing colour, with particular attention to the change between high-contrast shades.

Select a no-spangle or small-spangle substrate for light colours, so that the zinc structure cannot show through the paint film.

Run continuous production with the same paint supplier and batch where colour matching matters, and keep traceability records linking each coil to its paint batch and process parameters.

Traceability is what makes prevention repeatable. When a deviation appears, the production record should identify the paint batch, viscosity, line speed, oven profile and substrate lot for the affected section, so the cause can be isolated instead of guessed.

Substrate, Paint System and Passivation Choices

Three specification decisions strongly influence how tolerant a coil is to colour variation. The substrate choice sets the visible texture of the surface: a regular spangle galvanized sheet, a skin-passed sheet and a zinc-aluminium coated sheet all reflect light differently under the same paint. The paint system defines the film build and the colour stability, with polyester, silicone-modified polyester and fluorocarbon systems offering different balances of formability, hardness and exterior durability. The passivation type determines corrosion behaviour during transport and service, and chromium-free systems are increasingly specified for environmentally restricted markets.

Zinc coating mass also matters. Heavier coatings give longer corrosion protection but create a coarser spangle pattern, which makes light shades harder to keep uniform; the specification should therefore balance the required service life against the achievable colour consistency.

Questions Buyers Ask About PPGI Colour Difference

Q: What ΔE value is acceptable for a PPGI coil shipment?
Tolerance is a commercial agreement, not a fixed figure. A limit of ΔE of 1-2 within one batch is common for visible shade matching, and tighter values can be agreed for exposed architectural surfaces.

Q: Why does colour difference appear only on light colours?
Light paint films are translucent enough for the spangle pattern and the conversion film beneath them to influence the reflected light, so small variations in the substrate become visible as a shade change.

Q: Can colour difference be corrected after the coil is produced?
No. The coating is cured on the line, so a coil that falls outside the agreed tolerance cannot be reworked in place. The control must be applied to the process parameters before and during coating.

Q: Is chromium-free passivation worse for colour consistency?
Not necessarily. Chromium-free systems change the passivation chemistry rather than the paint film, but any change of passivation type should be trialled because the film appearance and the subsequent paint adhesion may differ.

Q: Which standard should be quoted when a ΔE limit is agreed?
Quote the measurement method together with the limit, since GB/T 13448 and ASTM D2244 define the test procedure, and the instrument geometry and illuminant must also be stated for the value to be comparable.

Q: Does paint viscosity variation really change the colour?
Yes. Viscosity alters wet film thickness, and film thickness changes the hiding power of the pigment, so viscosity drift between batches appears as a colour difference band across the coil.