Summary: White-coat yellowing comes mainly from aromatic isocyanates, certain amines/UV absorbers and lignin bleed. A non-yellowing system uses aliphatic hardeners, phenol-free (non-phenolic-yellowing) additives and a blocking sealer.
Mechanism
Aromatic isocyanates form chromophores under UV and yellow; aliphatic (e.g. HDI) resist far better; packaging phenols cause 'phenolic yellowing'.
Key data
- Aliphatic isocyanates resist yellowing far better than aromatic (TDI-type).
- Avoid phenolic packaging/films in long contact to prevent phenolic yellowing.
- A sealer blocks lignin/tannin bleed that also causes colour.
System
| Type | Yellowing |
|---|---|
| Aromatic (TDI-based) | Poor, yellows |
| Aliphatic (HDI etc.) | Good |
Defects & fixes
- Window-side chalk/yellow: strong UV + wrong topcoat — aliphatic PU/fluoro.
- Yellow after packing: phenolic — use phenol-free packaging/film.
- Local yellow spots: lignin bleed — sealer first.
FAQ
Why does white paint yellow over time?
Aromatic hardeners and some additives photo-oxidise into chromophores; lignin bleed adds colour.
Is waterborne less yellowing?
Usually better than aromatic oil systems, but pigments/additives and bleed still matter.
How to improve?
Aliphatic hardener, UV-stable topcoat, a sealer, and avoid phenolic packaging contact.
Ref: PU yellowing mechanism and non-yellowing selection (public).
Kexin New Materials (Guangdong) Co., Ltd. · 2026-09-18