2023-08-11 08:00:27
Plastics and soaps often have little in common in terms of texture, appearance and, above all, in terms of their utility. But there is one curious connection between the two at the molecular level: the chemical structure of polyethylene –one of the most widely used plastics– is similar to that of a fatty acid that is used as soap chemical precursor.
Both materials are made of long carbon chainsalthough fatty acids they have an extra group of atoms at the end of the chain.
Hence, now, a team led by researchers from Virginia Tech (USA) has been able to develop a new method to recycle plastics and turn them into substances of high value, known as surfactants (also called surfactants), which are used in soaps, detergents, and other products. The results of the study are published this week in Science.
The key: shorten carbon chains
For Guoliang ‘Greg’ Liu, Associate Professor of Chemistry in the Virginia Tech College of Science, this similarity of materials implied that it must be possible to convert polyethylene into fatty acids. Although the dilemma was how to break a long chain of polyethylene into many short chains efficiently, Liu hoped to come up with a method of upcycling (repurposing materials for a second life) in order to reuse low-value plastic waste.
The answer came from reflecting on fire and combustion, as Liu began to wonder what would happen if polyethylene could be burned in a safe laboratory.
Incomplete combustion of polyethylene would produce smoke that, if someone were able to capture it, might contain “broken” polymers into short chainswhich would then be transformed “into small gaseous molecules before their complete oxidation to carbon dioxide (CO2),” according to Liu’s reasoning.
At the bottom, the furnace-shaped reactor is hot enough to break the polymer chains, and at the top it cools to stop decomposition
“If we break down the synthetic polyethylene molecules that way, but stop the process before they break down completely into small gaseous molecules, we should get short-chain polyethylene-like molecules,” he adds.
With the help of Zhen Xu and Eric Munyaneza, two doctoral students in his lab, the chemist built a small reactor in the shape of a oven in which they could heat polyethylene in a process called thermolysis of thermal gradient. At the bottom, the oven is one high enough temperature to break the polymer chains and, at the top, cools at a temperature low enough to stop decomposition.
Guoliang Liu holds a water pitcher in his Hahn Hall South lab. /Steven Mackay, Virginia Tech.
After thermolysis, they collected the remains – something equivalent to cleaning soot from a chimney – and discovered that, indeed, this residue was made up of short chain polyethylene or, more exactly, of waxes.
That was the first step, to which some more procedures were added, such as the saponificationto get to manufacture the first soap made from plastics.
So, to continue the process, the team enlisted the help of experts in computational modeling y economic analysisamong others, to improve the recycling process and be able to share it with the scientific community.
Although polyethylene was the plastic that inspired this project, the ‘upcycling’ method can also work with polypropylene
“Our research shows a new route to recycle plastics without using catalysts novel or complex procedures. In this work we have demonstrated the potential of a joint strategy for the recycling of plastics”, highlights Xu, co-author of the article.
“This will help people develop more creative procedures for upcycling in the future,” he says.
cheap war against pollution
Although polyethylene was the plastic that inspired this project, the method of upcycling can also work with polypropylene. And together, these two materials make up much of the plastic that consumers encounter on a daily basis, in food packaging or fabrics.
In this sense, another of the interesting characteristics of this way of reusing is that it can be carried out with these two plastics at the same time, which means that no need to separate them.
For a recycling method to be effective on a large scale, the final product must be valuable enough to cover the costs of the process.
This is a great advantage compared to some current recycling methods, which require a careful classification of plastics (even some very similar ones) to avoid contamination.
It is, in effect, a technique with very simple requirementssuch as the availability of plastic and heat.
While it is true that later steps in the process require some additional conditions to convert the wax molecules into fatty acids and soap, the initial transformation of the plastic is a straightforward reaction. This contributes to the profitability of the method, as well as to a environmental impact comparatively small, according to those responsible for the article.
A vial filled with waxes generated from polyethylene and polypropylene waste is heated in an oil bath, and the waxes are oxidized by a stream of air to produce fatty acids by catalytic oxidation. /Steven Mackay, Virginia Tech.
Otherwise, for the transformation method to be effective on a large scale, the final product must be as valuable enough how to cover costs of the process and make it economically attractive. And while soaps may not seem like a particularly expensive product at first, they can actually be worth double or triple that of plastics when compared by weight.
According to Liu, this research lays the foundation for a new way to reduce waste by channeling used plastics into the production of other useful materials.
For his part, Xu points out that “plastic pollution is a global challenge and not a problem of a few dominant countries”, so “a simple process can be more accessible to many other countries around the world”.
Finally, his expectation is that this constitutes a “good start for the war against plastic pollution”.
Reference:
Liu, G. et al. “Chemical upcycling of polyethylene, polypropylene, and mixtures to high-value surfactants”. Science (2023).
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