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Comment by TylerE | Hacker News Reader

The biggest problem is you can't _really_ recycle plastic. It would more accurately be called downcycling, as what you get out is of a much lower grade than what you put in. Contrast with aluminum where you get...aluminum back out.

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True.

But another perspective is that you're recycling petroleum. That is, we make plastic polymers:

   petroleum -> chemicals -> plastic polymers

So why can't we use plastic polymers like petroleum?

   old plastic polymers -> chemicals -> new plastic polymers

And generate worse chemical waste than the plastic was to begin with

Why any worse than waste from refining petroleum?

Just because we can go

    egg -> omelette

does not imply

    omelette -> egg

(Likely it can be done, but the energy involved will be a problem. Most polymerisation is exothermic, so reversing it requires energy input e.g. https://www.ewi.ca/reverse-polymerization.html )

Some plastics can be recycled by simple melting - thermoplastics like ABS. Unfortunately they're often mixed with additives like flame retardants that cause problems.

Ummm, "omelette -> egg" is a lot harder "plastic -> plastic monomers". And really, if you want to convert omelettes to eggs, you just feed them to chickens. And eventually, you feed unmarketable chickens to pigs.

Chemical reactions are typically driven by moving from a high potential energy state to a lower potential energy state. Polymers are incredibly stable because of their binding mechanisms. It takes a relatively large amount of energy to break down those bonds, and when you do, your end product is something far more stable than the original chemicals you started with. Which means it reacts less readily than the chemicals you started with.

Take for example polyethylene, a product we can recycle relatively well. The first reaction gives polyethylene chains of length N. Because they're as long as a polyethylene chain can be, they're as strong as polyethylene can be, and have the best performance characteristics you can expect. Each time they're heated up to be remolded or have contaminants removed, and to a lesser extent just from use and contact with sunlight, some of those chains break. This creates more N/(2^x) length chains. This causes the plastic to get weaker over time. The energy cost to recombine those chains would consume more petroleum than simply creating new plastics, if it could be done at all.

So it's "just" an energy problem?

In a hypothetical future with 100% cheap renewable energy, we could recycle plastic over and over?

If we have cheap/free energy, we could just synthesize hydrocarbons instead of digging them out of the ground.

As an imperfect analogy, we used to mine fertilizers. Now we use energy to create ammonia directly from nitrogen in the air.

No, we make ammonia from nitrogen and methane.

Assuming unlimited energy you could just take CO2 from the atmosphere and react it with hydrogen from water to synthesize all the hydrocarbon precursors you need to then make plastics. The plastics could be burned and you start over again.

Sure, we can burn plastics. But that can be problematic, because they often contain metals (either as filler, or in multiple layers) and are typically contaminated.

In a closed system, where you're just cracking plastics, it's arguably easier to deal with contaminants. And I also suspect that you'd use less energy overall.

I'm not talking about recombining polyethylene chains. I'm talking about thermally cracking polyethylene etc to some mix of small hydrocarbons, and likely some H2, CO and CO2. More or less how the industry now deals with low grade petroleum fractions and tar sands.

> It takes a relatively large amount of energy to break down those bonds, and when you do, your end product is something far more stable than the original chemicals you started with.

Sure, you could theoretically break down most plastics to CO2 and the like. They're a lower energy state than most plastics. The question is why expend the energy to get something we have a huge surplus of. You're not going to be able to get volatiles such as methane or ethylene out of that process efficiently, because it's a higher energy state than the plastics. Tar sands and the like are a mixture of volatiles and heavier compounds; they're not a homogeneous compound the way most plastics are. The compounds going into a thermal cracking system, such as decane, are actually higher in total energy content than the products, such as heptane. They're just more difficult to work with, and as such less valuable economically.

Really, if you're not satisfied with the previous explanation you're going to have to look into an entry level organic chemistry textbook to understand it.

Well:

Tsuji et al. (2003) Thermal cracking of oils from waste plastics https://sci-hub.tw/10.1007/s10163-003-0090-x

Walendziewski (2005) Continuous flow cracking of waste https://sci-hub.tw/10.1016/j.fuproc.2004.12.004

Das and Pandey (2007) Pyrolysis and catalytic cracking of municipal plastic waste for recovery of gasoline range hydrocarbons https://core.ac.uk/download/pdf/53188820.pdf

Like I said in the first response, if you're willing to throw a huge excess of energy at the problem, sure. Calculate out the energy expenditure on any of those reactions. You're talking about burning thousands of times more hydrocarbons than are being recovered. It's a bit like saying you don't need to worry about algorithmic efficiency with a big enough computer.

So they're just doing it as an exercise, I guess.

What then? Just landfill it? Recycling mixed plastics is nontrivial. And old-school incineration is a nightmare of toxics emissions.

What's wrong with landfills? The petroleum was taken from the ground now the plastic returns to where it came from. Unless Recycling is economical, it will release additional CO2 into the atmosphere.

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