PVC Profile Weathering: TiO2, Stabilisers and Colour Retention

Why white PVC stays white, and why it sometimes does not

Unplasticised PVC is inherently susceptible to ultraviolet light. The polymer chain contains allylic chloride structures — defects formed during polymerisation — and UV energy is enough to break the carbon-chlorine bond, releasing hydrogen chloride. Once that starts, it propagates: the liberated HCl catalyses further dehydrochlorination, and the result is a conjugated polyene sequence that absorbs visible light. That is why degraded PVC turns yellow, then brown, then black.

Two things stop it. The first is titanium dioxide, which absorbs UV and scatters it before it reaches the polymer. The second is a stabiliser system, which intercepts the HCl and the radical species. Both have to be present at the right level, and both are the first things a cost-reduced formulation cuts.

TiO₂: grade matters as much as loading

Table 1 — TiO₂ grades. Anatase is cheaper and is the wrong choice — it photo-catalyses the PVC it is supposed to protect.

The mechanism matters: untreated TiO₂ generates free radicals on its surface when it absorbs UV, and those radicals attack the polymer. A surface coating — alumina, silica, or a combined treatment — passivates the particle so it absorbs UV without catalysing degradation. A cheap formulation using untreated or lightly treated TiO₂ will chalk badly, and the chalk is not dirt: it is degraded PVC leaving the surface.

Loading interacts with wall thickness. A thin wall (2.5 mm) needs the higher end of the range because there is less material to screen the UV; a thick wall can work at the lower end. This is why two profiles with the same formulation but different wall classes do not weather identically.

Stabilisers: lead, calcium-zinc and tin

Table 2 — Stabiliser systems. CaZn replaced lead for regulatory reasons, not performance ones.

The shift from lead to calcium-zinc is the biggest formulation change in the PVC window industry in thirty years, and it is not a drop-in substitution. Lead stabilisers are excellent lubricants as well as stabilisers, so removing lead changes the fusion behaviour of the dry blend. A CaZn formulation needs a carefully balanced package of internal lubricant, external lubricant and co-stabiliser to get the same fusion and the same surface. Formulations that simply swapped the stabiliser and adjusted nothing else produced profiles with poor fusion, plate-out and — in the field — early yellowing.

Colour options and what each costs you

Table 3 — Colour options. The dark-colour note is the one that causes field failures.

Dark colours are the fastest-growing segment and the most demanding. A black or anthracite profile in direct summer sun reaches 65 – 75 °C surface temperature, against 40 – 45 °C for white. That is above the heat deflection temperature of standard rigid PVC, so the profile softens, creeps and can bow — and the differential between the sunlit face and the shaded one produces thermal stress that a white profile never sees.

The industry answer is a cap layer or a laminate over a white or light-grey core. The dark surface gives the appearance; the light core gives the thermal and structural performance. A through-coloured dark profile requires a formulation specifically stabilised for it, and even then it is usually limited to lighter shades.

How weathering is actually tested

Table 4 — Weathering tests. 4,000 hours of xenon arc is roughly two years of Florida sun.

The acceleration factor is worth knowing so you can read a test report critically: 4,000 hours of xenon arc approximates 18 – 24 months of Florida exposure, which is a small fraction of a 25-year claim. That is why serious profile systems run extended tests and combine them with natural exposure data. A supplier claiming 25-year colour stability on the basis of 1,000 hours of QUV is extrapolating far beyond what the test supports.

Field failures and what causes them

Table 5 — Field failures. The pink staining is specific to lead stabilisers and is a good diagnostic.

The pink and brown staining deserves a note because it is misdiagnosed constantly. Lead-stabilised PVC reacts with hydrogen sulphide and other sulphur compounds in the atmosphere — from industrial emissions, from rubber gaskets, even from some sealants — to form lead sulphide, which is brown-black. Customers report it as "mould" or "dirt". The cure is not a cleaner; it is a change of stabiliser system.

Cleaning and maintenance: what damages a profile

Table 6 — Cleaning agents. Abrasive cleaning is the most common cause of customer-induced damage.

Abrasive cleaning is worse than it looks. The TiO₂ is distributed through the wall, but the surface 0.1 mm carries the highest concentration and the least degraded polymer. Scouring that layer off removes the UV protection and starts a yellowing process that no subsequent cleaning will reverse. Windows that "went yellow after the builders cleaned them" were usually scrubbed with an abrasive pad.

What to put in a specification

Table 7 — A weathering specification. The cleaning instruction is the cheapest item and the one most often missing.

The cleaning instruction is worth insisting on because most colour complaints are not manufacturing failures. A window that has been scrubbed with an abrasive, or wiped with a solvent, has been damaged by its owner — and the supplier absorbs the complaint because nobody told the owner what not to use. A one-page instruction in the handover pack prevents it.

Frequently asked questions