Overview
Polyurethanes and polyureas built on aromatic isocyanates such as MDI absorb UV light. Photo-oxidation then breaks bonds near the surface, producing colored products and eroding the film. Research has traced which parts of the polymer fail first and how fast.
Key findings
- In an aromatic poly(urethane-urea) elastomer, urea bonds broke down rapidly even at low UVA doses, followed by urethane chain scission and photo-Fries rearrangement; polyether soft segments oxidized, and carbon black gave only moderate protection.[2]
- An MDI-based polyurethane/polyurea coating yellowed within its first week of UV exposure and later browned, with infrared spectroscopy showing carbonyl and peroxide changes. The authors cautioned that UV-A-only accelerated tests may not predict field durability.[4]
- After 150 days at a coastal marine site, an MDI/polyetheramine polyurea lost about 92% of its gloss and became much more hydrophilic with growing micro-cracks, yet its tensile strength fell only about 7%. The soft segment degraded more than the urea hard segment.[3]
- Aromatic polyurethane pipeline coatings in accelerated UV testing yellowed and lost gloss mostly in the first weeks and kept slowly losing mass, while impedance stayed high and no corrosion appeared: appearance failed early while protection continued.[5]
- UV absorbers such as hydroxyphenyl-triazines convert UV to heat, while HALS trap radicals in a regenerating cycle; combining the two is standard in clearcoats, and basic HALS need special grades in acid-catalyzed systems.[6]
- A comprehensive review covers photo-, thermal, hydrolytic and oxidative degradation of polyurethane elastomers and how isocyanate, polyol and chain-extender choices affect stability.[1]
What it means in the field
For specifiersExpect aromatic polyurea and polyurethane to yellow and lose gloss quickly outdoors, even when they keep protecting the substrate. Where color and gloss matter, specify a UV-stable aliphatic or polyaspartic topcoat with a UV absorber and HALS package, and compare products using the same weathering test conditions.
References
- Xie F, Zhang T, Bryant P, et al. (2019). Degradation and stabilization of polyurethane elastomers. Progress in Polymer Science, 90, 211–268. doi:10.1016/j.progpolymsci.2018.12.003
- Zhang T, Xie F, Motuzas J, et al. (2018). Early-stage photodegradation of aromatic poly(urethane-urea) elastomers. Polymer Degradation and Stability, 157, 181–198. doi:10.1016/j.polymdegradstab.2018.09.020
- Che K, Lyu P, Wan F, Ma M (2019). Investigations on aging behavior and mechanism of polyurea coating in marine atmosphere. Materials, 12(21), 3636. doi:10.3390/ma12213636
- Mayer-Trzaskowska P, Robakowska M, Gierz Ł, et al. (2024). Observation of the effect of aging on the structural changes of polyurethane/polyurea coatings. Polymers, 16(1), 23. doi:10.3390/polym16010023
- Croll SG, Buratto M, Calzone T, et al. (2016). Changes during the weathering of polyurethane water pipeline coatings. In: Pipelines 2016, ASCE, 44–56. Author PDF
- Leppard D, Hayoz P, Schäfer T, et al. (2002). Light stabilisers. CHIMIA, 56(5), 216. doi:10.2533/000942902777680559
Citations were checked against publisher, DOI or agency records. Summaries are paraphrased; read the original papers for full methods and data.