IR-change and yellowing of polyurethane as a result of UV irradiation

In this investigation IR-change and yellowing of polyurethane as a result of UV radiation were studied. In the presence of UV radiation (200 h, λ > 300 nm), the synthesized aromatic polyurethane undergoes photodegradation with gradual change of its colour. The photochemical degradation of the polyurethane is associated with the scission of the urethane group and photooxidation of the central CH2 group between the aromatic rings. These reactions are combined with the yellowing of the polyurethane surface. Analysis of the colour changes in PU surface during photodegradation was carried out by measuring CIEL∗a∗b∗ colour components (L∗,a∗,b∗ and ΔEa,b∗). FT-IR spectroscopy was used to study the chemical changes caused by UV irradiation. The colour difference of yellowing Δa,b∗ exhibits a systematic tendency to higher values with increasing irradiation time. Overall, ΔEa,b∗ colour change correlates well with photodegradation of polyurethane by relative increase of the concentration of carbonyl group. Our results are in agreement with the quinone (yellow colour) formation as the chromophoric reaction product of polyurethane degradation.