system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't microplastics keep breaking down?

It's my understanding that as pieces of "stuff" dissolve or disintegrate into smaller pieces the process accelerates as the surface area/volume ratio changes. It seems like plastics in the ocean have broken down into "micro" sized pieces then just... stopped? Is there some fundamental unit of plastic which plastic products are breaking down into that have different properties to the plastic product as a whole, and don't disintegrate the same way?

Bonus question I only thought of while trying to phrase this question correctly - what is the process that causes plastics to disintegrate in the ocean? Chemically dissolving? Mechanically eroding like rocks into sand?
gpt
There's 2 different but related types of "breaking down". One is the type of breaking down which converts plastic polymers into water, CO2 and biomass which are relatively harmless, usually this is biodegradation done by microbes. 

Then there's degradation which is usually physical wear and tear and breaking down of larger polymer chains to smaller ones with relatively similar properties through abiotic physical/cheimcal processes (UV breakdown, heat, chemical reasons, etc). 

Both these processes exist for plastics, but for the plastics we don't call "biodegradable plastics" or bioplastics etc, the biodegradation process is extremely slow. So they deteriorate mostly using the latter process, still maintaining their plastic properties and due to their resistance to biological processes and to an extent physical processes, they accumulate. 

Because they are resistant and accumulate while breaking into smaller pieces, they disrupt animal functions, develop large effective surface area to transport presistent organic pollutants and eventually come back to us.