system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why aren't all types of plastics recyclable?
gpt
Theoretically, they are. In practice it is a lot more difficult.

Material issues:

1. There are **many** different types of common plastics (HDPE, PET, PP, etc.) and also engineering plastics (PEEK, PMMA, Nylon); within these there are also many different grades of plastics (high molecular weight, branching, isotactic, and other confusing terms). This makes separating them very hard.
2. There are many different fillers and additives in plastics, these affect how the plastics respond to the environment they are intended to be used in. You'd want improved thermal stability for engineering plastics in high temperature environments, for example. These eventually in recycling can mix together to make a 'soup' of additives that make controlling the properties of recycled plastics difficult, so fewer organisations want to buy them as it will impact their product quality.
3. Many plastics simply cannot be mixed together, some like HDPE and PP can as they are polyolefins (simple chain or branched chain carbon-hydrogen plastics), but PVC contains chlorine, this will not mix well and can release HCl acid, which degrades the other chains more.
4. Degradation over time - plastics do indeed breakdown. Their molecular weight and other properties will diminish, this is exacerbated by reprocessing where thermal degradation occurs. Plastics *could* all be (mechanically) recycled, but eventually they will have to be disposed of via some other means (such as incineration or landfill). Chemical recycling is not yet at scale.


Systemic issues:

1. Poor investment in infrastructure (collection, separating & sorting, reprocessing) limits how well and how much we can achieve.
2. Legislation - sometimes this can directly actively impede expansion, but sometimes it relates to safety. You wouldn't want the additives used for some applications to become mixed in with food contact plastics, so there is legislation against this.
3. Incorrect disposal/waste management - from consumers and then illegitiamte or poorly operating companies.
4. Exportation of waste from technically able countries to those that are not able. This is a relatively simple one, and is based on cost/reward ratio of processing domestically vs sending it elsewhere.

There's plenty more to it, but these are some of the basics of the material and system issues.