eli5: Why does plastic take so much longer than metal to degrade?

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eli5: Why does plastic take so much longer than metal to degrade?

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Metals rust. They naturally oxidize due to the oxygen in the air. Eventually, they degrade down to just a pile of rust.

Plastics interact with sunlight, mostly, but it takes a long time, by design, for them to break down. We make them to be stable and long-lasting by design.

It’s really only aluminum and iron that decompose. Gold nuggets can sit in a stream for 50K years, that’s how we find them.

Plastics take so long to decompose because they are brand new molecular arrangements never before seen on the planet. Because of this, there are no organisms on the planet that have the ability to break these new molecules down (because why would they?). So, the only way plastic can decompose is through the slow degradation of the bonds that happens naturally over thousands of years, which can be sped up a bit with UV light. But, basically there’s nothing that eats them.

Interestingly, this has happened before. In the beginning of the Carboniferous period plants first evolved the ability to produce lignin, the polymer that makes plants tall and rigid, there was nothing in the planet that could break it down. No fungi or bacteria could digest the lignin. When trees died, then fell over and just layed there until another tree fell on top of it tens or hundreds of years later. This led to dense piles of dead trees stacked impossibly high, which were then submerged and exposed to heat deep in the earth. These dense masses of undegeaded trees later became our coal deposits (or oil, I can’t remember which).