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

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

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Anonymous 0 Comments

Okay, kiddo! So you know how sometimes you have a toy that’s made out of plastic and you want to make it into something else? Well, it’s a little bit tricky with plastic because it’s not like glass, which is easy to melt down and turn into something new.

Plastic is made of lots of different little pieces called polymers, and they don’t always play nice together when we try to melt them down. They kind of get all mixed up and make a big old mess, and it’s hard to turn that mess back into something useful.

But don’t worry, we’re working on it! Some types of plastic can be recycled, like the kind they make soda bottles out of. And we’re trying to make more and more types of plastic that are easier to recycle. But until then, we have to be extra careful with our plastic toys and make sure they don’t end up in the trash where they’ll never turn into anything new. Got it?

Anonymous 0 Comments

That plastic is rarely recycled (because of the many reasons in this discussion) should be more widely understood. If recycling services almost always send it to the landfill then this means the illusion of recycling plastic adds to the cost of real recycling of metal and glass.

I would bet that given a finite recycle bin most people would favor putting plastic in instead of metal and glass because plastic has the reputation of breaking down into “micro particles” and finding its way into everything and everywhere. Recycling is probably perceived as a better way to prevent this even though plastics going to a landfill are not the cause of the “plastic particles everywhere” problem.

Plastics in the recycling mindspace potentially push out less stigmatized materials that are actually energetically recycled.

A lot of people think the deliberately misleading three arrows and a number on a plastic bottle mean the plastic is assuredly recyclable.

The plastics industry knows what would happen if the public caught on.

Anonymous 0 Comments

Off subject somewhat but I’ve done a little experimentation with melting and shaping #2 HDPE by hand in a toaster oven at 350 degrees F. I’ve made some clave (click stick percussion) and plan to try making a walking stick in the big oven. There are a few people on YouTube with videos of people doing this.

Many plastic bags are also made of this. I braid them together and then melt them. Some are #4 LDPE, often it’s written in the folds of the bag.

If mass recycling doesn’t work, I wonder if there is any localized recycling that could work.

Anonymous 0 Comments

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.

Anonymous 0 Comments

Plastics are often made with fossil fuels and can be melted down an burned in internal combustion engines.

Anonymous 0 Comments

Some things can be formed, melted, reformed, melted, reformed etc.

An example would be aluminium, which can be melted and reformed. Or water > ice cubes > water > ice cubes in and endless cycle

But some things cannot be reformed. Once you’ve changed it, you can’t change it back.

An example would be an egg. Once you’ve cooked/fried/scrambled an egg, you can’t undo that. Or if you use the egg to bake a cake, you can’t unbake it and get the egg back so you can use it in something else.

Some plastics behave like aluminium. Some plastics behave like eggs.

Anonymous 0 Comments

So I’m actually a chemical engineer at a polypropylene plant. When we make plastic for customers there’s very specific characteristics of the same polypropylene plastic depending on application. Some plastic it’s okay for it to shatter/fragment on impact – think a rigid plastic tub. Other plastic you want to “bend” or take an impact: ie your car bumper. This plastic – though all polypropylene have different molecular weight distributions, crystallinity, etc that we control during the reactor process (when we take propylene and react with a catalyst = polypropylene). Now you’re left with a bunch of fluffy non uniform plastic with given specs. We then take that, extrude it, add a bunch of additives depending on the customer – customer wants white pellets? Customer wants plastic that’s see through? Cusomter doesn’t care? All different additive packages that get added into the extruder. The extruder then mixes it all up in one gooey mixture, pushes it through holes like play dough and cuts it into perfect little round pellets for our customers to use for whatever application.

Now imagine trying to recycle this plastic. Even with it all being polypropylene, if you mix all this plastic together – car bumpers, pill bottles, etc. it’s very difficult to get a product that have the same specific characteristics that the virgin polymer has. Sorting it to even get close (i.e. all at least the same plastic type) is expensive and way pricier than the virgin stuff and lesser quality. We’re struggling now to get customers to pay the extra premium for our recycled plastic (my company is doing it in specific markets). But to do it we have to find for example a company that recycles just PE (polyethylene). This company does all the sorting for us. We pay them for that plastic. Then we could feed it into an extruder with virgin polymer or whatever and produce something new. This product is pricier than just straight virgin. Our customers aren’t biting yet. It’ll take consumer pushing for certain percentages of recycled plastic in products to incentivize companies to pay the extra dollar for recycled goods.

Not speaking for industry or anything. Just work for a big petrochemical company and this is my unique perspective actively working as a chemical engineering manager at the polypropylene plant.

Anonymous 0 Comments

Non thermoset plastics are recyclable like that if, and only if they’re pure, not contaminated (such as food oils soaked into surface) or chemically damaged (like sun damage). Technology can handle some of this, but too much of any of these problems can mean that the final product of this kind recycling isn’t useful or is at best not as good. Notice that recycled plastic products can feel flimsier for example.

If plastics are highly mixed with other plastics they simply can’t be recycled as a mixture. If this is tried one plastic might get damaged at the others melting temperature, or one might melt while the other stays solid.

Problem is, waste is, by its very nature, mixed. So unless plastic is sorted into pure(ish) states this isn’t possible. It’s hard to tell the difference between some plastics (e.g. PVC and PE films) so sorting cant fix it all. Automatic sorting technology for plastics isn’t that great and there comes a point where the cost of the energy to sort is high enough to prevent profit making or environmental harm of recycling is higher than landfilling it. At that point it’s unrecyclable.

Glass doesn’t suffer from these issues because it is more chemically stable, doesn’t absorb things, and sorting is simply by colour. The only issue is that that making colourless glass is hard due to contamination with green/brown glass.

Anonymous 0 Comments

I was reading that just one wrong type mixed in with a batch can turn an entire vat into useless goo that will take hours if not days to clean out.

but there’s just no money in it. except for some minor amounts used to make fleece and some lumber decking substitutes, it costs far more to carefully separate, to truck it over, to clean it well enough…. than to get some petroleum waste product to make fresh new plastic.

Even glass isn’t profitable, our local refuse won’t take it anymore.

Anonymous 0 Comments

Those non recyclable plastics might be turned into something else like an alternative aggregate material for concrete rather than sand. The advantages, to name two, would be disposal of something unwanted and reduced weight.