Pyrolyzed plastic ash is worthless, but it may not last for long.
Rice University scientists have turned their attention to the Joule heating of the material, a by-product of plastic recycling processes. A strong burst of energy makes it flash in graphs.
The technique of the laboratory of the rechemist James Tour creates turbostratic graphene flakes that can be added directly to other substances such as polyvinyl alcohol (PVA) films that better withstand water in packaging, cement paste and concrete and dramatically increase their compressive strength.
The research appears in the journal carbon.
Like the flash graphene process introduced in 2019, pyrolyzed ash turns into turbostratic graph. This has weaker attractive interactions between the flakes, making them easier to mix into solutions.
Last October, the Tour Laboratory reported on a process for converting waste plastic into graphene. The new process is even more specific and turns plastic that is not recovered through recycling into a useful product.
"This work improves the circular economy for plastics," said Tour. "So much waste plastic goes through pyrolysis to turn it back into monomers and oils. The monomers are used in repolymerization to make new plastics, and the oils are used in a variety of other applications. But there is always a residue." 10% to 20% ash that is worthless and is usually sent to landfill.
"Now we can turn that ashes into flash graphene, which can be used to improve the strength of other plastics and building materials," he said.
Pyrolysis is the process of heating a material to break it down without burning it. Products made from pyrolysed, recycled plastic include high-energy gases, heating oils, waxes, naphtha and pure monomers from which new plastic can be made.
However, the rest – an estimated 50,000 tons per year in the US – is discarded.
"Recyclers don't make big profits from cheap oil prices, so only about 15% of all plastic is recycled," said Kevin Wyss, lead author of the study. "I wanted to fight these two problems."
The researchers conducted two experiments to test the flashing ash. First, they mixed the resulting graphene with PVA, a biocompatible polymer that was being investigated for medical applications, fuel cell polymer electrolyte membranes, and environmentally friendly packaging. It was held back by the poor mechanical properties and the water susceptibility of the base material.
The addition of just 0.1% graphene increases the pre-failure stress on the PVA composite by up to 30%. It also significantly improves the material's resistance to water permeability.
In the second experiment, they observed significant increases in compressive strength by adding graphene from ash to Portland cement and concrete. Stronger concrete means that less concrete has to be used in structures and roads. This reduces energy consumption and reduces pollutants from its production.
The National Science Foundation, the Air Force Office of Scientific Research, and the Department of Energy supported the research.