Information - UV Resistance

When used for indoor applications such as industrial screeds, aromatic systems are ideal. Their suitability for indoor/screed applications is largely due to the fact that they are not clear and can be formulated with additives such as pigments/fillers (titanium and zinc oxides) that protect them from UV radiation. 

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When formulated into a clear binder for outdoor surfacing applications, aromatic isocyanate systems suffer from discolouration however this is a symptom of a more complex problem. The discolouration is a result of oxidation reactions in the backbone of the polymer as it degrades. In simple terms, both aliphatic and aromatic resin bound surfaces are made by reacting a polyol, an isocyanate and trace amounts of water (airborne moisture).

Unlike UV resistant, aliphatic isocyanates, the aromatic isocyanates will form an amine which will oxidize to a quinone if subjected to UV radiation. Quinones are yellow and their formation will make the resin surface appear increasingly yellow as the oxidation progresses over time. Since the quinone structure is part of the backbone of the polymer, the resin surface will appear discoloured before there is any degradation however once the oxidation starts breaking chemical bonds, the degraded polyurethane will lose strength.