A friend of mine was involved in a recycling project for a major chemical company (decades ago).

Just a small fraction - way less that 1/3 - can be used in recycling and it makes the resulting plastic a lot weaker and limited in uses.

I would imagine for something like plastic for food or water bottles might need to be new.

and then there are prions:

https://blogs.scientificamerican.com/artful-amoeba/prions-ar...