Why aren’t all types of plastics recyclable?
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1: some plastics change irreversibly when heated and cooled multiple times, or when sunburned, and they stop being as good as new.
2: some plastics are mixed with other materials, and they can’t be separated again.
3: many times it’s cheaper to make new plastics than recycling them.
4: some plastics are contaminated and their use would be dangerous.
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.
Anything is recyclable with enough chemistry, the question is whether it’s economically recyclable.
With metals, the initial cost of extraction is higher, and the process of re-melting tends to burn off many impurities.
There are a lot of different kinds of plastic, and the time it takes to identify what a single piece of plastic is made of can easily be worth more than the plastic is worth. If you miss a piece of the wrong type of plastic, you can clog up the manufacturing equipment, when it chokes on a chunk of plastic that won’t melt.
ELI5. Plastics are a product that are “too good” in a sense. It is cheap, light, durable, inert, easily formed/molded, tough and easy to color. There are also many varieties which can be made for specific properties by using different materials and additives. All of these make plastics fantastically useful in a wide variety of applications.
The problem is that these very characteristics makes it very difficult to recycle economically. Many varieties means they can’t be mixed and have to be sorted. Cheap and light means that recycling has to be low cost to compete with new material. Durable and inert means that recycling is going to be difficult and expensive – needing high temperatures, pressures and/or chemicals.
Everything is recyclable. But some things aren’t _cheaply_ recyclable, and taxpayers (via sanitation departments) are only willing to pay for the cheap stuff.
The main thing that makes one plastic more expensive to recycle than another is it’s melting point. What exact variety of crude oil was used to make the plastic will determine the melting point, and if the melting point is too high, the electricity for the electric furnaces used to melt and remould it would be too expensive.
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