Why can’t we recycle plastic in the same way we do for metal? Melt it and remold it?



Why can’t we recycle plastic in the same way we do for metal? Melt it and remold it?

In: Technology

Not all plastic melts. “Thermoplastic” ones melt when heated, but “thermosetting” ones are made strong by complicated bonds that don’t break down with heat. They will catch fire first.

This is a partial answer, naturally.

This is a great question and one that more people should be asking:)

It start with the fact that not all plastics are the same, there’s all types that can’t be mixed together.
So first off we need a better recycling system. For instance one where you would get money for the plastic waste you turn in. Like you get money for metals you recycle.
Another thing is that with metals, because of the price difference it is neatly sorted. You’ll get a lot more money for copper then for mild steel for instance.
Now to be able to do this we should get better at identifying plastics. You could start by learning how to identify the different types by looking at the recycling triangles!
they should be on every plastic part you have and they can help you id the plastic.
Now, you should also know that not all types of plastic are easily or even cleanly recyclable. But for the ones that are, yes we could melt them and remold them but, like you couldn’t melt copper and steel together, you cannot melt HDPE and PA together:)

If you’re interested in all this be sure to check out Dave Hakkens and the project ‘precious plastic’!

(HDPE is one of my favorites bc you can actually, safely, recycle that in your own oven:)

Well, for one, polymers degrade when melted, so you can’t just turn a PET water bottle into a new PET water bottle. It won’t have the same properties anymore. You can use post-consumer recycled PET in something *else*, like a CPET microwave dinner tray, but even then you can’t just use all of it, you can only use a small percentage of it. EDIT: That “something else” can, in fact, be a new bottle, but it will almost certainly be mixed with virgin material and won’t be 100% recycled.

For two, you can’t mix types of plastics, which means that either consumers have to be exceptional at separating their types of plastic (by which I mean the cap *and* ring on your water bottle has to be separated since they’re probably PP while the bottle itself is PET) or we have to have people sort through garbage bags full of recycling to separate it, which adds a hell of a lot to the cost. There’s also the fact that most plastics have some additives in them, for one reason or another, and plastics with certain additives can’t be mixed with others.

For three, any contaminants (adhesives from stickers, residue from food or drink, etc) have to be washed off to the point where they’re as close to completely gone as possible, which is basically impossible and also adds a ton of labor, again increasing the cost dramatically. (This is also true for paper/cardboard – STOP RECYCLING YOUR GREASY PIZZA BOXES, THEY CAN’T BE RECYCLED IF THERE’S FOOD OR GREASE STUCK TO THEM. *Edit: This may not be true, I’m not sure – my city explicitly says not to recycle pizza boxes with grease on them, but YMMV. Maybe someone with more experience in recycling, or paper/cardboard, can chime in on this one.*)

For four, not all plastics are thermoplastics – thermosetting polymers cannot be melted down, they’ll just burn up and release a ton of pollutants into the air. And obviously you can’t mix thermoplastic polymers and thermosetting polymers.

And for five, it’s largely just not feasible to *use* post-consumer recycled plastics because it’s 1) extremely expensive to buy and 2) can be extremely difficult to work into your existing process while maintaining quality and matching the needed performance without using so little that it’s basically irrelevant. You’re paying through the nose to use a tiny amount of the stuff you bought, just to be able to put a label on your product that says “Contains post-consumer recycled material!”

Basically, plastic recycling – while it *is* done to some extent – is ***much*** more limited than the public has been led to believe, and in an amount of cases that would shock the average person, is outright impossible. I guarantee that 80-90% of the plastic items people put in their recycling bin actually end up in a landfill or incinerated because they can’t actually be recycled for a myriad of reasons.

Source: I work in QA at a plastics manufacturer.

I saw this video a while back [Limitations of plastic recycling](https://www.youtube.com/watch?v=KXRtNwUju5g) (I am changing the title because I think the original one was slightly misleading).

The main idea is that plastic recycling is not a technological problem but an economic one. Either the technology to completely recycle plastic already exists, or if it does not, we can develop it. But, plastic recycling is not economically feasible. The world works such that if there is money to be made out of it, it will get done (even if it is selling children, or selling drugs, or selling drugs to children). But since there is no money to be made, we (as a country, or a specie) are not doing it.

Two large issues:

* There is no such material as “plastic” – there are gazillion different types of plastic, just like how there are a lot of types of metal.
So you have to work recycling out for every type individually.
And not mix multiple types together.
Just like with metals mixing random stuff together makes reusing it borderline impossible.
* Plastics are FAR TOO CHEAP TO MAKE.
They are made from ludicrusly cheap fossil fuel stuff.
Due to this its simply not economical to recycle it – thats an unavoidable issue, even if you want to be enviromentally conscious with your company. As the company that uses non-recycled stuff can do its thing for cheaper, price better, and drive you into bankruptcy.


Both issues can only (realistically) be solved by legistlation.


On top of these, ther are technological hurdles.
Stuff like plastics being made out of long chain like molecules, instead of “just atoms thrown in randomly”.
And with repeated reuse, molceular chains can break and thus shorten.
Shorter molecular chain touches and connects to fewr other molecules, thus your material gets weaker.

Ofc this issue can be circumvented by grading plastic – and designing the arts for appropriate strength. And when it becomes really useless you can still reprocess it chemically.

Technological issues are – in some sense – easier to overcome than legistlative ones.
As it can be done by a relatively small hard working group.
With legistlative problems you have to fight against large mutlinatinals and various other interests groups pushing back with all their ~~bribing might~~ lobby power

Have you ever tried unbaking a pie?

One way to think of plastic is like a weave of long ‘fibers’ similar to a cloth, but much smaller. When it’s originally made its quite strong because those fibers are so long and tangled together. Every time it gets recycled those fibers get pulled apart and many get broken into smaller and smaller pieces. Eventually those pieces of the original fibers are to small to weave together to hold a structure anymore and we don’t have great methods for decomposing them back into the original components to make fresh new plastic ‘fibers’ or a eco-friendly by-product.

First, let’s limit this to thermoplastics. That’s a type of plastic that can be melted. HDPE, ABS, PLA, PET, nylon, polyester, polycarbonate, acrylic are some types you’ve probably heard of.

A big part of the reason that metal is very forgiving when recycled is that the extremely high heat burns off contaminates, and most of what’s left turns into slag that’s easily skimmed off. It’s also not a huge deal if a little bit of other metals/alloys are in the mix. Sure, that changes the alloy a little, but that’s easily and quickly tested, and can be addressed. Metals can be recycled over and over, and aluminum is often described as being indefinitely 100% recyclable, and nearly 75% of all aluminum ever made is still in use in either its original or recycled form.

Plastic is tough to recycle because a miniscule amount of contaminate, including the wrong type of plastic, can irreparably ruin the entire batch.

It’s also tough to sort plastic. Bigger rigid items are much easier because automated systems can identify, sort and clean them. Shreds and sheets of plastic are basically impossible unless it comes from a manufacturing facility that already presorts it. Yeah, they might be technically recyclable, but identifying, sorting and cleaning it from a mixed batch of recyclables is incredibly expensive. Like how do sort a “polyester” t-shirt? The fabric may be polyester, with cotton or nylon thread, a nylon tag, and possibly some screen printing or embroidery.

Another problem is that the quality of plastic gets worse every time it’s recycled, which limits its applications.

Plastic processing engineer here.

Plastic parts can be ground and reused (regrind) but options for doing so are limited. Plastic has a heat history, the more you melt and reheat plastic, the more it degrades. As the plastic degrades you lose its physical properties. This means you have to use virgin material (no regrind) for many applications, such as automotove or medical parts.

Regrind also can be more difficult to process, as it is more prone to part defects. Visible burns in the final product or brittleness would be a few examples. Regrind typically needs to be mixed with virgin material during processing to avoid these defects.

The size of the regrind also matters. With virgin material the plastic comes in little plastic pellets, all the same size. When these are melted down they all melt at relatively the same time. With regrind you get bigger chunks and dust. The dust tends to burn while the bigger chunks may not melt evenly. There is equipment to remove dust and more evenly chop parts, but then there is now additional cost to reusing materials.

Then you get into contamination issues. Typically when doing regrind you don’t reuse consumer products, instead you are taking the scrap at the plant and regrinding it there. Even keeping it at the plant level, the chances for contamination is high. If a part is dropped on the floor and picks up dirt, now its in your regrind.

There are many different types of plastic and each may require different processing temperatures. In many cases you cannot even mix different grades of the same material. I may have 5 different types of polypropylene at the plant. each with different fillers or additives that should not be mixed. This creates a logistical problem at the plant, because now you need 5 different grinders and 5 different storage containers for you materials. All it takes is one person putting the wrong part in the wrong bin or grinder and now you have a lot of scrap.

Taking this a step further, this contamination could create machine damage in some cases. Incompatibile materials such as polycarbonate (melts at round 400F) and polypropylene (melts around 300F) mixed together could chew up your machines by running unmelted material through. Mixing materials such as PVC and Acetal can create explosive chemical reactions.

Many common plastics are thermoplastics and can be remelted. There is another group of materials called thermoset. These materials undergo a chemical reaction to harden them. (example epoxy) These materials cannot be reverted back to their original form and recycled.

TLDR: Some plastics can be reused, but process problems and costs make reusing plastics infeasible in many cases.

It’s like reheating and reusing oatmeal, if it was colored green and cinimmon flavor before, you can’t take that out. Not only will the texture not be quite the same as before from reheating, but you’ve got to find a product that it would blend well with (say apple spice flavor)

If your bottle is green, you can’t make a white bottle out of it, and it wont be the same as the fresh plastic anyway

ELI5 for an actual 5 year old:

Like metal, you can melt ice and freeze it in a different shape. Like plastic, you can’t un-fry and re-fry an egg. It’s also difficult to separate the quail egg from the Duck egg after stir-frying them together.

This is not an exact comparison, but I guess it will do for a 5 year old. Plastic is really quite complex on a molecular level. Metals are simple.

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.

I’m not happy with the ELI5 here.

Some plastics are like ramen noodles, overlapping each other and tangling up, but each noodle is distinct and separable. You might lose a lot of noodles by breaking them, but ultimately the end product is separated ramen noodles that you can remix together again. Keep doing this and eventually you don’t have strands of noodle anymore, just bits and pieces.

Other plastics are more like a sheet of lasagna, where everything is physically connected. You can’t separate it then mix it back together like the ramen, it’s just not the same thing anymore and would be more like fettuccine.

These are called thermoplastic and thermosets, respectively. Some thermoplastic can sorta be melted then remolded. 3D printers basically do this. Another issue is impurities. Impurities can completely ruin a plastic molecule from forming or binding the way it should. Metal doesn’t usually have as much of an issue with this, as the metal forms crystal structures rather than something like an amorphous structure (for ELI5 think random arrangement rather than orderly).