25er liquid black coloration issue

[DK_one]

Hey, does anyone have any ideas about the following problem, specifically regarding the cause or reason for following problem?

It concerns a 25ml bottle of HDPE liquid that was stored for about 5 years in a cool, dark, and sealed container. Upon inspection, a contact of mine noticed a black discoloration in a small corner at the bottom of the bottle. The solvent is unknown.

Does anyone have any ideas why this happened? How should this discoloration be assessed in relation to the contents, assuming it's even possible to say for sure? Of course, I'm also grateful for any explanations that have context for not making any use of it.

 

[Transform]

HDPE is oxygen-permeable, not much but enough to be significant over the course of five years.

Without knowing what the solvent is - and even to a large extent even with, it's hard to be specific about the identity of the black discoloration, but the main candidates are decomposition product, bacteria or mould.

 

[DK_one]

Ok, big thanks