Eli5 why are so few plastics recyclable? Why cant you just melt it down and reuse it like glass?


Eli5 why are so few plastics recyclable? Why cant you just melt it down and reuse it like glass?

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Many materials can’t just be melted down the way glass and metals can. Take wood for example, or cotton. However, this is not what is responsible for so mnay plastics not getting recycled. It’s not that plastics CAN’T be recycled, it’s that it’s too expensive to do so. Sadly it’s purely a matter of matter of cost, and that virgin (new) plastic is so cheap it’s not worth recycling.

The plastics that *are* recyclable can be melted down because when they are made, they don’t undergo a chemical change. They simply get melted, formed into a shape, and then cool/harden. (thermoplastic polymer)

The plastics that *are not* recyclable *cannot* be melted down because when they are made they undergo a chemical reaction that causes them to hold their shape and harden. Once that chemical reaction has occurred, it can’t easily be reversed by simple reheating. (thermosetting polymer)

There are 2 broad categories of plastics. Thermoset plastics and thermoplastics. Thermoset plastics don’t melt, they burn so you can’t melt them down. When they’re made, they cure and are much stronger than thermoplastics. Thermoplastics melt but are much weaker, because they’re weaker, manufactures often put glass fiber in them to reinforce it which makes it much more difficult to recycle.

The major problem is among all the recyclable plastics is they need to be separated into their different types in order to be recycled, and the plastic degrades so virgin material needs to be added into each batch. You can mix different types, but the material you out of it is usually of lesser quality and used to make things like carpet and insulation for winter coats.


I’m not an expert so I can’t give a proper answer, but I’ll share what I know in case someone can fill in more (and teach me something):

A plastic object isn’t a uniform substance. A part made of, say, polyethylene will contain other chemicals. It might contain talc or other substances to make it stronger, or additives to absorb UV light. Each chemical added serves a purpose for that specific part. While it is easy to regrind waste from the production process back into that same part, it isn’t pure polyethylene (or whatever plastic) so it can’t be readily used in other parts if they need different properties.

If working with unknown plastics costs more than new plastic, then the manufacturer won’t buy recycled plastic.