What's the reason that alkanolamines change their color at storage?

What's the reason that alkanolamines (e.g. monoethanolamine, diethanolamine, triethanolamine) of technical purity change their color (from colorless over yellowish to red-brown) at storage?


Is it because of oxidation, or UV-light?




Oxidative degradation by oxygen certainly occurs and has been studied [1](https://ntnuopen.ntnu.no/ntnu-xmlui/bitstream/handle/11250/2461998/Vevelstad_Oxidative+degradation+of+2-ethanolamine.pdf?sequence=1), [2](https://www.sciencedirect.com/science/article/abs/pii/0043135494902275). I can't access the papers so I have no information on possible mechanisms. I've seen no mention of UV degradation, but that doesn't mean it does not happen.


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