PVC Profiles & Decking: Colour Shift After Five Summers

Weathering benchmarks for window profiles

A 982 °C margin inside a 200 °C barrel

The PVC profile line runs its barrel at 170–210 °C; HDPE decking lines run 180–230 °C, and polypropylene pushes 250 °C. Chromium oxide green is thermally stable to 982 °C. That is a margin of more than 700 °C between the pigment's failure point and the hottest point of the process, and the margin is not a laboratory curiosity — it is the difference between a pigment that is a variable and a pigment that is a constant. Every organic green has a decomposition window that overlaps the barrel window. Degrade in the barrel and the pigment pays out in three ways: the colour shifts toward yellow or brown, the decomposition gases cause plate-out on die lips and calibration tools, and the chemistry of the degraded material starts a slow conversation with the stabilizer package that no formulator asked for. All three defects show up as production cost — die cleaning stops the line, colour shift stops the order, and stabilizer interference shows up years later as a profile that fails early.

Re-grind is where the 982 °C number earns its keep in money. Profile extrusion is a continuous process that runs on edge trim and startup scrap re-fed into the hopper, so the pigment in a profile has not one heat history but several. An organic green accumulates damage at every pass through the screw and drifts darker or yellower with each recycle; the compounder compensates, the lot diverges, and the line ends up managing colour instead of producing it. Chromium oxide has no chromophore to damage, so the fifth heat history looks like the first. The same logic covers hot spots: a worn screw, a misread thermocouple, a blocked screen pack — any excursion that cooks the melt for a few extra seconds at 20 °C over setpoint is an invisible event for Cr₂O₃ and a colour event for everything else. A pigment with a 700 °C safety margin is the cheapest insurance a line engineer can buy, because it is the only one that costs nothing to carry.

Warpage rating 1: what the number actually means

Warpage is the least discussed and most expensive pigment failure in PVC. Pigments interact with the polymer matrix in ways that show up only after the profile has left the die — as a 3-metre board that bows 2–3 mm, a window frame that twists in the pack, or a batch of trim that will not sit flat on the wall. The industry rates pigments for warpage on a scale where 1 means no measurable distortion and 5 means reject, and chrome oxide green sits at 1. The mechanism is worth understanding because it explains why the rating exists: anisotropic pigment particles align in the melt flow, seed differential crystallisation as the profile cools, and leave internal stress that releases as bowing. The test for it is brutally simple — extrude a pigmented strip, hang it, heat-cycle it, measure the deflection. Organic pigments with plate-like crystal habits fail it; titanium dioxide and chromium oxide, with compact, inert particles, pass it.

The deeper point is loading. Because Cr₂O₃ delivers tinting strength well beyond organics per unit weight, a profile maker achieves full colour at 0.8–2 % pigment rather than the higher loadings organic greens demand. Less pigment means less foreign material inside the polymer, less nucleation surface, less effect on the melt's cooling behaviour — and a profile that stays straight because there is less of anything inside it to make it bend. Micronized grades at D50 ≤0.5 μm push the same logic further for thin-wall sections: the finer the particle, the more uniform the distribution, the more even the shrinkage. A window profile is a precision extrusion — 6-metre lengths with millimetre tolerances and a 20-year weather warranty attached. A pigment that twists the profile by a millimetre has already cost more than a year of pigment price difference. Warpage rating 1 is not a data-sheet ornament; it is the line between a profile that sells and one that ships back.

Five summers: the ΔE ledger of a facade

Colour shift is measured in ΔE, and the numbers separate the honest greens from the brochure greens fast. After five years on a facade, a chromium oxide green profile typically shows ΔE below 3 — invisible to the eye except side by side against a brand-new lot. Organic greens in the same exposure run 8–15 ΔE, which in colour science is not a "slight shift" but a different colour. The ladder is worth memorising because it decides reorders: ΔE 1 is just perceptible to a trained eye, ΔE 3 is visible in direct comparison, and anything past 5 is visible on its own. A facade built over two seasons — one elevation fitted this year, one next year — becomes a laboratory for this. The chromium oxide elevation reads as one surface; the organic elevation reads as a patchwork, and the replacement profiles ordered three years later will not match either, because the original has kept moving. That is what "colour shift after five summers" costs: not a complaint, but a permanent mismatch that the building owner sees every morning.

The mechanism behind the difference is worth stating plainly. The green of Cr₂O₃ is a property of the crystal lattice — the colour exists because chromium(III) ions sit in a corundum structure and their d-electron transitions absorb red and violet light. Sunlight cannot bleach a lattice; there is no bond to break. An organic green is a molecule with a chromophore, and UV is a pair of scissors aimed at it. Weathering also separates the two on the surface: chromium oxide does not chalk, because chalking is a photocatalytic process that needs a pigment like titanium dioxide; the chromium particle stays put while the polymer skin around it erodes microscopically. The practical test is the one the profile buyer should demand: QUV 3000 hours with the ΔE read at the end, plus real Florida or Arizona exposure if the warranty is long. Five summers is a short experiment. A window is a 25-year product, and the pigment is the only component of it that the extruder has to defend alone, in its own warranty, in front of the building owner. The acceptance test writes itself from there: QUV 3,000 hours with the ΔE read at the end, a retained sample of the original lot kept in the dark as the reference, and a rule that every future reorder is judged against that sample — not against memory, and not against a freshly printed colour card.

Grade selection for profile extrusion

FAQ

Does chromium oxide green interfere with PVC stabilizers? No. Cr₂O₃ is chemically inert in the melt; it does not release ions or react with tin, Ca–Zn or lead stabilizer systems, and its 982 °C thermal stability means it introduces no decomposition by-products into the compound.

Will green profiles chalk over time? No. Chalking is a photocatalytic surface effect driven by pigments like titanium dioxide. Chromium oxide has no photocatalytic activity; the profile surface may weather microscopically, but the pigment does not powder out or lighten.

What loading gives full hiding in a 3 mm profile? Between 0.8 % and 2 % by weight depending on tone and grade. Micronized GREEN 1025/1061 reaches the same hiding at the low end of the range, which also helps keep warpage at rating 1.

Can re-grind containing chromium oxide green be re-used? Yes, and this is one of the pigment's quietest savings. Because there is no chromophore to degrade, re-grind keeps its colour through repeated heat histories instead of drifting darker or yellower like organic greens.

How do we match a profile extruded five years ago? Order against the certificate, not against memory. DELIKECOLORS lots ship with ΔE ≤1.0 measured against the reference standard, so a reorder matches the original production — which is the only way a mixed-era facade stays one surface.