Short answer
Aromatic polyurethanes (TDI and MDI based) yellow under UV whatever the pigment, and some organic pigments fade on their own. Whites and pastels show it most. Fix it with light-fast pigments, a UV-stable lacquer or in-mould coating carrying UV absorbers and HALS, or an aliphatic system for demanding exterior use.
Likely causes
Photo-oxidation of aromatic PU
The polymer itself yellows, independent of the colorant.
Pigments with poor light-fastness
Certain organic reds and yellows, and any dye-based colour.
No UV protection at the surface
Bare aromatic PU in daylight and heat.
How to diagnose it
- Compare an exposed area with a covered area: a hue change points to pigment, a yellow cast points to the polymer.
- Run accelerated weathering (xenon-arc) with an agreed ΔE limit.
How to fix it
- Specify light-fast pigments for the shade.
- Add a UV-stable lacquer or in-mould coating.
- Move to an aliphatic top coat or system where appearance is critical.
How to stop it coming back
- Define exposure conditions and acceptance (for example ΔE after a set number of hours) in the brief.
- Test before production.
Where this problem appears most
Send us your batch number, PU system and a photo of the defect and our technical team will help you trace the cause. Ask a technical question.
