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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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.
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