Eli5 why are so few plastics recyclable? Why cant you just melt it down and reuse it like glass?
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TLDR: Some plastics reach thermal decomposition point before they reach melting point.
There are two types of plastics:
Thermoplastic polymers, which become solid when “cold” and liquid when “hot”, but remain the same chemical compound. They can be repatedly melted and reformed and, thus, recycled. But not infinitely, because their molecules still break down (gradually, each melting cycle), so mechanical (and other) properties of resulting material become less and less useful. To alleviate that degradation recycled plastics can be merged with newly-produced plasctics. The quality of the product still would not be exactly the same as all-new plastics.
Thermoreactive (thermosetting) polymers, which initially have one chemical structure (and are liquid, and can be formed into whatever), and then undergo a chemical reaction (or reactions), their structure change and becomes solid, and if heated, generally, reaches point of thermal decomposition (or burning) before melting. They, obviously, can not be recycled into the “same” kind of a material*. They can be recycled into something else though.
EDIT: * – in an economically feasible way.
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