Why can’t we recycle plastic in the same way we do for metal? Melt it and remold it?
In: Technology
Here’s a real attempt at an ELI5 answer.
The “simple” answer is “chemistry”.
When you melt some things, like water, by applying heat, they just become different versions of the same thing. In fact, for those substances, we tend to purify them with heat. Different things melt at different temperatures, so if we heat substances to very specific temperatures we can separate the stuff we want from the stuff we don’t. For example, gold melts at a different temperature than the rocks it’s usually found in, so when we throw giant batches of rock into a furnace we get pure gold with still-solid “slag” on top that’s easy to separate.
This only tends to be true with very simple chemical compounds. Since gold is an element, it’s as simple as a compound can get. Given any kind of material, we can create a process that will extract gold from it if any gold exists using our knowledge of chemistry.
Or, think about paper. If all you did was write on it with normal ink, we can shred it, throw it in water, bleach it, and produce new paper based off those clean wood fibers. The bleach destroys the chemicals that make the ink visible, then the remnants evaporate. However, if instead the paper was used as the wrapper for a greasy cheeseburger, we have to add a step to our recycling machine to deal with the fat related to the food it soaked up. Fat doesn’t evaporate, so we have to work harder to remove it, hard enough that it’s easier and more efficient to plant new trees so many recycling plants won’t deal with paper that has food residue on it.
However, plastics are extremely complicated chemicals. To be stable, they require very specific ratios of materials to be brought to very specific temperatures. Too hot or too cold, and they don’t make plastics, or they don’t make the kind of plastic we were trying to make.
That makes recycling plastics very difficult. Some plastics, when melted, cannot reform into the same kind of plastic they were before melting. In theory we could force this to happen, but it involves adding so much heat and so many new materials that it is more wasteful than just letting the plastic go to a landfill. Imagine if you have a $5 bill, but to make another one you have to spend $20 of materials. That’s not going to make money.
However, it’s usually true that we can take a “complex” plastic and recycle it as a “simpler” plastic without spending as much energy or material as it takes to make the “simple” plastic from scratch. So some of these plastics can be recycled, but they don’t end up being recycled to the same kind of plastic as they started.
This is overall the conflict with recycling: some forms of material recycling cost us more energy and pollution than just manufacturing a new copy of the old thing. If our goal is to reduce pollution, we have to be pragmatic and admit that we just can’t recycle some things in a way that helps the planet. However, this leads to other tradeoffs. For example, a milkshake that costs $2 in a styrofoam cup that is impossible to recycle might cost $6 if offered in a completely reclaimable glass container. A lot of people argue it’d be a shame if we lost money like that and found out the only benefit is a cleaner planet.
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