I keep reading about how most plastic can’t be recycled and goes to the landfills instead.
With our technology, why can’t we melt it down, using some sort of scrubbers/filters to capture the emissions, and add it to asphalt or other industrial uses?
At the very least it seems like we could reuse that plastic for non food items that are typically used once and tossed.
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A lot of plastics are set in a way where they can no longer be melted down the way you expect. Basically the melted form and the solid form are different and you cannot easily go backwards.
I can’t think of a good analogy off the top of my head but imagine if:
You have oil and you have very easy way to change it into coal. You now have coal which you can’t just melt into oil as it will burn. You have to do some extra work. And this extra work is more expensive and complicated than changing oil into coal.
Noted I don’t know much about the science and engineering around roads or plastics, but my first thought is asphalt to a certain degree absorbs water and I question how adding plastic to that could impact that on roadways, causing more flooding and hyrdoplaining. Maybe it’s a suitable idea for different climates though??
There are some pilot projects attempting this, but not all plastics are equal and a lot are really one-use only. Part of the problem is that many plastics are only completely forming while they are being made and once they are hardened, you cannot do anything else with them.
You also have to consider that many plastics were designed around making materials that were resilient, if not impervious to environmental influences. Hence why getting rid of them is such a bitch.
Lots of nasty methyl ethyl formaldehydes and even worse things that make some plastic. It just burns off and turns into a small pile of death goop
What would happen if we shoved a bunch of it into a volcano?
Lots of people are already trying it. They’re considering things like microplastics and recycling and full lifecycle cost. It might take a couple of years until they know the results.
https://en.m.wikipedia.org/wiki/Plastic_road
https://www.forbes.com/sites/jeffkart/2021/09/17/single-use-plastic-used-to-make-longer-lasting-asphalt-in-missouri
https://austroads.com.au/infrastructure/pavements/incorporating-recycled-plastics-into-asphalt
https://i.stuff.co.nz/taranaki-daily-news/news/113269206/new-plymouth-trials-putting-recyclable-plastic-into-roads
Okay, let’s imagine you have a big toy box with lots of toys inside. Some of these toys are made of a special material called plastic. Now, plastic is like playdough, but it’s very, very tough and strong.
You know how we can melt chocolate to make yummy things? Well, people can melt plastic too. But when they do that, it can create some stuff in the air that is not very nice. It’s like when you cook and sometimes there’s smoke that comes out of the oven.
To make the air nice and clean, grown-ups use special things called scrubbers and filters. These are like magic tools that catch the not-so-nice things and make them disappear before they go into the air. But sometimes, even with these magic tools, it’s still a bit tricky to make the air super clean.
Now, when we melt plastic and try to add it to things like asphalt (which is like the road stuff), it’s like putting a new ingredient into your special playdough. Sometimes, the plastic can change how the road stuff works, and it might not be as safe or strong anymore.
But guess what? Some smart grown-ups are trying to figure out the best way to do this! They’re working on new recipes and ideas to use plastic in things like roads or other stuff. It’s a bit like how you try different ways to build your towers with your blocks.
So, even though it’s a bit tricky right now, the grown-ups are working hard to find ways to use plastic in a super cool and safe way, just like you make your cool creations with your toys!
I am a polymer chemist who works in the asphalt industry (roofing, specifically) on sustainable additives to improve function. I don’t know how I found this post, but you asking this question has validated my existence.
There are two parts to your question. In regards to “why can’t we melt plastic down,” I think some other users have given you good answers so far. Many commercial polymers, like rubber and glue, are what are called “thermosets.” When they form, they do what is called “curing” where each of the individual polymer strands forms chemical bonds to adjacent strands in a process called crosslinking. This makes the polymer very tough, but also means it can’t be melted down. Polymers don’t melt in the same way that ice does, where the material goes from one phase to another in a clean transition. Instead, when heated, the many individual strands of polymer are given more energy and mobility to slide past each other, and we see this as the polymer “melting” and starting to flow. If there are bonds connecting all these strand together, then they will never flow past each other no matter how hot it gets. They would have to break the chemical bonds tying them together, which is the process we know as burning. Once you’ve burned your polymer, it loses all of its useful features like its toughness and plasticity.
But, not all polymers are thermosets. As you know, many can be reheated and then cooled, theoretically many times in some cases. These polymers are called thermoplastics and include polypropylene, polystyrene, and other extremely common plastics. As other users have pointed out, though, we rarely use only one type of polymer when making a plastic product. Any kind of contamination will often degrade the thermoplastic ability of a polymer, and this effect is cumulative with every time the polymer is reprocessed. It is almost always easier to just get some new polymer instead of spending a lot of time and effort to make old polymer work at a lower level.
This brings us to asphalt, the second part of your question. Using recycled fillers in asphalt is a big market that is growing even bugger by the day. But, in order for recycled materials to be effective, they must improve some target property of the asphalt in some way. Otherwise, you’re just making you product worse for not much cost savings, since asphalt is already a dirt cheap material (it is a byproduct of oil processing – imagine how easy that must be to acquire with all the oil processing we do). Many companies include polymeric additives to augment a specific property of their asphalt based on a need. For example, many roofing shingles have SBS (styrene-butadiene-styrene block copolymer) as a way to improve their impact resistance in hail-stricken areas. The polymer improves the plasticity of the asphalt while also maintaining processability.
This only really works when you have a good polymer, though. Some reprocessed, contaminated polymer blend will not contribute as much to targeted properties as fresh polymer which is, again, dirt cheap in many cases. Not just that, but polymers have drawbacks of their own. They can interfere with the stability of asphalt microstructures, make asphalt more vulnerable to UV degradation, and ruin the fine-tuned physical characteristics of current commercial asphalt.
There is also a consideration that not many people think of. We think of asphalt as one common material, but the reality is that it is an EXTREMELY complex blend of uncountably many types of chemicals. Asphalts that come from different oil sources and refineries all have different properties, so when you add polymer of one type and concentration to a certain asphalt from a certain place, there is no guarantee that same ratio will work elsewhere. Putting additives into asphalt while maintaining properties across a market is actually an incredibly time-consuming and expensive task, which disincentivizes companies from throwing stuff willy-nilly into their asphalt products.
So, to recap: many polymers can be reprocessed, but doing so degrades the physical characteristics every time the polymer is reprocessed, due to contamination and natural heat-related breakdown. We certainly can add recycled polymer to asphalt, and this is a very promising and important area of research going forward. However, currently it would amount to spending a lot of money to reprocess polymers into a usable (but inferior) state, and then putting that polymer into asphalt where it would degrade the quality of the product. From both a cost and logistics standpoint, it doesn’t make a lot of sense right now. But hopefully with continuing research it will become more feasible, and eventually become profitable to use recycled materials to augment asphalt.
Obviously I am quite interested in this area of research and I would be happy to provide further explanation or clarification on any of the things I discussed here.
The first issue is you have to sort it first which is more expensive than its worth. The second thing is that plastic is very hard to recycle, and most of it can’t be. When you melt it down and reform it, it doesn’t form the same structure easily.
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