Most plastic recycling produces low-value materials – but we've found a way to turn a common plastic into high-value molecules
Bales of plastic waste intended for recycling. Koron / Getty Images
If you thought that these weak, single-use plastic bags were the largest part of our plastic waste problem, think again. The volume of plastic that the world throws away each year could rebuild the Ming Dynasty Great Wall of China - roughly 3,700 miles long.
In the six decades that plastic was made for commercial use, more than 8.3 billion tons were produced. Plastics are light, versatile, cheap, and almost indestructible (as long as they don't get too hot). These properties make them incredibly useful in an enormous range of applications including sterile food packaging, energy efficient transportation, textiles, and medical protective equipment. But their indestructible nature comes at a price. Most of them decompose extremely slowly in the environment - on the order of several hundred years - causing a global epidemic of plastic waste there. The consequences for human and ecosystem health are not fully understood, but potentially significant.
I am a chemist with experience in developing processes for the manufacture of plastics and I was interested in the use of plastics as a large, untapped resource for energy and materials. I wondered if we could turn plastic waste into something more valuable to keep it out of landfills and the natural environment.
A new way of using plastic waste
Plastics are made by linking large numbers of small carbon-based molecules together in almost infinite ways to form polymer chains.
To reuse these polymers, recycling plants could in principle melt and reshape them, but the properties of plastics tend to deteriorate. The resulting materials are almost never suitable for their original use, although they can be used to make inferior materials such as plastic wood. The result is a very low effective recycling rate.
A new approach is to break down the long chains back into small molecules. The challenge is to do this in a precise way.
Since a lot of energy is primarily released in the manufacture of the chains, a large amount of energy has to be added back when reversing. In general, this means the material is being heated to a high temperature - but when plastic is heated, the material turns into a nasty mess. It also wastes a lot of energy, which means more greenhouse gas emissions.
My team at UC Santa Barbara, working with colleagues from the University of Illinois, Urbana-Champaign and Cornell, came up with a clean way to turn polyethylene into useful smaller molecules.
Polyethylene is one of the most useful and widely used types of plastic in the world. It's also one of the biggest contributors to plastic waste. It represents a third of the nearly 400 million tons of plastic the world makes each year. These range from sterile food and medical packaging to waterproof films and coatings, cable and wire insulation, building materials and water pipes, to wear-resistant hips and knee replacements and even bulletproof vests.
How the new process works
The process we developed does not require high temperatures, but relies on tiny amounts of a catalyst containing a metal that removes some hydrogen from the polymer chain. The catalyst then uses this hydrogen to cut the bonds that hold the carbon chain together into smaller pieces.
The key is to use the hydrogen as soon as it forms so that the chain cutting provides the energy to make more hydrogen. This process is repeated many times for each chain, turning the solid polymer into a liquid.
Crushing naturally slows down when the molecules reach a certain size, so it is easy to prevent the molecules from becoming too small. We can reclaim the valuable liquid before it turns into less useful gases.
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A majority of the molecules in the recovered liquid are alkylbenzenes, which are useful as solvents and can easily be converted into detergents. The global market for this type of molecule is approximately $ 9 billion per year.
The conversion of plastic waste into valuable molecules is known as upcycling. Although our study was a small demonstration, preliminary economic analysis suggests that it could easily be adjusted over the next several years to become a much larger process. It is a win-win situation to keep plastic out of the environment by reusing it economically.
This article was republished by The Conversation, a non-profit news site dedicated to exchanging ideas from academic experts. It was written by: Susannah Scott, University of California Santa Barbara.
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Susannah Scott has received funding from the US Department of Energy, Mitsubishi Chemical, and Dow Chemical for her work in polymer upcycling. She is the co-founder of a U.S. patent application related to this discovery, filed by the University of California.
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