Plastic waste pyrolysis ash converted to flash graphene
Chemists from Rice University in Texas have converted pyrolysed ash from plastic recycling into graphene using a Joule heating process. The graphene could be used to strengthen concrete and toughen plastics used in medicine, energy and packaging applications.
In the lab of Rice chemist James Tour, a strong jolt of energy flashes pyrolysed ash into turbostratic graphene. Weak attractive interactions between the graphene flakes makes it easier to mix them into solutions such as films of polyvinyl alcohol (PVA) — which better resist water in packaging — and cement paste and concrete, dramatically increasing their compressive strength.
The research appears in the journal Carbon.
The Tour lab reported on a process to convert waste plastic into graphene last October, but the new process is even more specific, turning plastic that is not recovered by recycling into a useful product.
“This work enhances the circular economy for plastics,” Tour said.
“So much plastic waste is subject to pyrolysis in an effort to convert it back to monomers and oils. The monomers are used in repolymerisation to make new plastics, and the oils are used in a variety of other applications. But there is always a remaining 10% to 20% ash that’s valueless and is generally sent to landfills.
“Now we can convert that ash into flash graphene that can be used to enhance the strength of other plastics and construction materials,” he said.
Pyrolysis involves heating a material to break it down without burning it. The products of pyrolysed, recycled plastic include energy-rich gases, fuel oils, waxes, naphtha and virgin monomers from which new plastic can be produced.
But the rest — an estimated 50,000 metric tons in the United States per year — is discarded.
“Recyclers do not turn large profits due to cheap oil prices, so only about 15% of all plastic gets recycled,” Rice graduate student and study lead author Kevin Wyss said.
“I wanted to combat both of these problems.”
The researchers ran a pair of experiments to test the flashed ash, first mixing the resulting graphene with PVA, a biocompatible polymer being investigated for medical applications, fuel cell polymer electrolyte membranes and environmentally friendly packaging. It has been held back by the base material’s poor mechanical properties and vulnerability to water.
Adding as little as 0.1% of graphene increases the amount of strain the PVA composite can handle before failure by up to 30%, the authors reported. The addition of graphene also significantly improves the material’s resistance to water permeability.
In the second experiment, the team observed significant increases in compressive strength by adding graphene from ash to Portland cement and concrete. Stronger concrete means less concrete needs to be used in structures and roads. That curtails energy use and cuts pollutants from its manufacture.
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