Abhijit Nag (sitting) and Pradeep perform degradation experiments.  

Three years ago, researchers at the Indian Institute of Technology (IIT) Madras observed that when heated to 70 degrees Celsius, silver slowly dissolves in a glucose solution. — Polytetrafluoroethylene (PTFE), made of polytetrafluoroethylene. All that is required to degrade the fluoropolymer into molecules is to continuously stir it in water containing 1,000 ppm of glucose and metal ions at 70 degrees Celsius for about 15 days.

Using the same strategy, the team led by T. Pradeep of the IIT Madras Department of Chemistry was also able to degrade polypropylene. The results were published in the ACS Sustainable Chemistry and Engineering Journal.

The researchers used a Teflon-coated magnetic stirrer to continuously stir the water mixed with glucose in a glass beaker containing gold foil for several days. "A few days later, we observed something unusual. We found tiny bright red luminous fragments floating on the surface," Professor Pradeep recalled. In an early study, they found that gold would also slowly corrode due to glucose. This phenomenon was observed when testing other sugars.

The initial intuition was that bright red luminous particles should contain gold, which turned out to be correct. After all, compounds having gold-carbon bonds that exhibit red luminescence are well known. But to their surprise, in addition to gold, they found that these tiny particles also contained fluorocarbon polymers. "We believe that Teflon impurities are released due to mechanical action," he said.

Therefore, the team tested different forms of polytetrafluoroethylene-pellets, tape, and plates. They repeated this experiment with a Teflon beaker and tried different metals, but they got the same results every time. The only difference is that when copper, silver and iron are used instead of gold, the particles do not show bright red luminescence.

A glass beaker (top view) in which copper powder and glucose are stirred in water, with magnetic particles coated with PTFE, so that polymer fragments float on the surface.    

"Then we got a clue that PTFE polymer may decompose into molecules through triboelectric degradation. When the polymer is continuously stirred in water, an electric potential is generated at the interface between Teflon and water," Professor Pradeep explained.

The glucose added to the water first leaches ions from the metal surface. When the magnetic particles coated with PTFE rotate continuously, frictional charges are generated on the particles. PTFE is negatively charged. The negative charge on the surface of PTFE attracts metal ions that have been leached. The interaction between metal ions and PTFE leads to metal-polymer bonding, resulting in instability of carbon-carbon bonds. This ultimately leads to the degradation of PTFE into molecules.

Without stirring, glucose or metal ions, no such degradation of PTFE was observed. The degradation rate will decrease at room temperature.

"The amount of triboelectric degradation depends on the amount of glucose dissolved in the water. As the glucose content in the water increases, more metal ions will be leached, resulting in more interactions between PTFE and metal ions. As more More metal ions combine with PTFE, and the degradation of PTFE will increase," said Abhijit Nag from IIT Madras and the first author of the paper.

"Mass spectroscopy features indicate the presence of molecular fragments of PTFE polymer," said Professor Pradeep. "Seeing debris floating on the water surface may be due to the aggregation of molecular debris or due to fragments of long polymers."

"We have now tested it on polyethylene and polyethylene terephthalate (PET) and got similar results. For a given concentration of dissolved glucose, the reaction is slow because compared to PTFE There is less triboelectric charge," Nag said.

According to the paper, since many modern cookware are coated with polytetrafluoroethylene, similar chemical reactions during the cooking process may cause food to contain microplastics and nanoplastics.

"The triboelectric degradation of PTFE, polypropylene, and other polymers may occur in nature in large water bodies such as the ocean, where a large number of metal ions are found and waves provide continuous agitation," said Professor Pradeep. "This must be one of the ways to produce microplastics."

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Printable version | November 20, 2021 at 10:21:14 AM | https://www.thehindu.com/sci-tech/science/iit-madras-finds-an-eco-friendly-way-to-degrade -plastics/article29417922.ece

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