Upcycling – loosely defined as creating a higher-value product from waste – has long been confined mostly to Pinterest boards and questionable home hacks. The term, and its counterpart downcycling, has not been a substantial part of policymaking or the discourse around recycling. In the EU, the waste hierarchy has been in place since 1970s; it defines and prioritizes reuse and recycling but does not define either up- or downcycling. However, the growing interest in chemical recycling and increased focus on plastic waste has propelled these terms into the mainstream.
Chemical companies like BASF, Dow, and Eastman have begun to use the term "upcycling" in their marketing, often in connection with mass balance-based pyrolysis approaches. More importantly, the Break Free From Plastic Pollution Act introduced in the U.S. Congress would exclude downcycling from the definition of recycling, defining downcycling as "a method … that does not preserve the original material quality, and, as a result, the aggregated material is no longer usable for its initial purpose or a substantially similar product." This would be a massive disruption to existing plastic recycling; mechanical recycling of bottles into fibers (the dominant plastic recycling method) would be likely to fall under this definition.
This definition of up- and downcycling is based purely on the utility of the produced materials; recycling processes that generate a product that's less useful than the original material are downcycling; the opposite is true for upcycling. The definition of "utility" here is a bit squishy – a recycled product could, potentially, be useful for an equal- or higher-value purpose, even if it's not reusable for the initial purpose. Still, the general thrust of it gets at real potential concerns: It's possible that a major expansion of mechanical recycling could lead to an oversupply of low-grade plastics while demand for food-grade plastics remained underserved. However, this is clearly not the only thing that matters.
A recycling process that consumes an enormous amount of energy to produce an identical quality product is obviously not sustainable. The established aluminum recycling space makes both a value-driven argument (recycled aluminum is just as good as primary aluminum) and an energy-driven argument (recycled aluminum consumes far less energy in production than primary aluminum) to justify recycled aluminum's advantages. The energy costs or advantages of different recycling processes are often baked into the price of recycled materials. However, for regulators deciding which processes to allow, or companies deciding between different "sustainable" materials, it's important to be able to understand the energetic underpinnings of different processes without complicating things with the economics of energy prices or plastic markets for various material grades. A basic energetic evaluation of these processes can help contextualize claims of up- or downcycling.
Evaluations of process energy are a core component of life cycle assessments (LCAs); if we had comparable LCAs for all of the various recycling approaches, finding the most sustainable option would be relatively straightforward. Unfortunately, LCAs have limitations – there are many ways to perform one, making comparison difficult; the complex nature of the LCA makes it very difficult to perform for early-stage technologies and often cost-prohibitive for ideas early in the innovation funnel, when investment is limited. We propose a simpler approach: Comparing the energy of combustion of inputs and outputs, along with a basic estimation of process energy, allows for an estimation of energy retention.
For example, consider mechanical recycling of polyethylene (HDPE): HDPE has a heat of combustion of 44 MJ/kg and a specific heat of 1,900 J/K/kg. Mechanically recycling HDPE requires raising its temperature by 110 °C over room temperature, and the process has a yield of around 68%. Roughly 67% of the energy (material and process) is retained; the process energy is relatively low compared to the material energy, so the outcome is overwhelmingly determined by the yield.
This simple assessment allows for useful comparisons: Pyrolyzing the same material by raising its temperature by 380 °C over room temperature gives you a 75% yield of pyrolysis oil by mass (using process specs from Vadxx) but only a 60% energetic retention, as pyrolysis oil is less energetically dense than HDPE. This result is intuitive – it makes sense that a high-temperature process retains less energy than a low-temperature process. Still, it's interesting to note that mechanical recycling of HDPE only retains a little more energy than pyrolysis. If HDPE mechanical recycling had the same yield as PET mechanical recycling (80% or higher), it would be far more favorable. Of course, by stopping with the pyrolysis step, this analysis implicitly assumes that you burn the pyrolysis oil as fuel. If instead you invest further energy into the oil (via hydrotreating) and suffer further yield reductions to turn oil into plastic – as the case for considering pyrolysis to be "upcycling" requires you to do – the overall yield (and energy retention) will be much worse.
This approach gives us some practical answers. Is pyrolysis of plastic waste to produce new plastic upcycling? Clearly not in energetic terms. But it also helps show that pyrolysis of plastic is not useless; it can be used to produce low-quality liquid fuels for sectors like marine transport with energy retention that's relatively close to that of mechanical recycling.
Of course, this energy-based perspective is not the only way to evaluate these different approaches. Emissions are also important: Pyrolysis is far more CO2-intense than mechanical recycling, due to the combustion of fuel and noncondensable gases. The endpoints and assumptions used drive the outcomes, so this energetic evaluation is closer to a heuristic than an ironclad law. Those interested should incorporate these types of simple tests, along with measures of value produced relative to energy used or emissions produced, into their scouting efforts and evaluation of early-stage technologies and try to educate policymakers (and the public) on the other aspects of up- and downcycling.