Three years ago researchers from the Indian Institute of Technology (IIT) Madras observed silver slowly dissolving in a glucose solution when heated to 70 degree C. Now, the team has demonstrated an environment-friendly strategy to degrade the chemically inert and physically stable plastic fluoropolymer — polytetrafluoroethylene (PTFE) of which Teflon is made. All that is required to degrade the fluoropolymer into molecules is continuous stirring of it in water containing 1,000 ppm glucose and metal ions for about 15 days at 70 degree C.
Using the same strategy, the team led by T. Pradeep from the Department of Chemistry at IIT Madras was able to degrade polypropylene too. The results were published in the journal ACS Sustainable Chemistry & Engineering.
The researchers used a magnetic stirrer coated with Teflon to continuously stir for several days the water mixed with glucose in a glass beaker containing a gold foil. “After a few days we observed something unusual. We found tiny fragments with bright red luminescence floating on the surface,” Prof. Pradeep recalls. In earlier studies, they found that gold, too, corrodes slowly due to glucose. This phenomenon was observed when other sugars were tested.
Hunch that worked
The initial intuition that the bright red luminescent particles should be containing gold turned out to be correct. After all, compounds with gold-carbon bonds showing red luminescence are well known. But to their surprise, besides gold, they found that the tiny particles also contained fluorocarbon polymer. “We thought that Teflon impurities were being released due to mechanical action,” he says.
Teflon
So the team tested Teflon in different forms — pellets, tapes and plates. They repeated the experiment using a Teflon beaker and tried different metals too and still got the same result each time. The only difference was that the particles did not show bright red luminescence when copper, silver and iron were used instead of gold.
Glass beaker (top view) in which copper powder is stirred with glucose in water with a teflon-coated magnetic pellet causes polymer fragments to float on the surface.
“We then got a clue that the PTFE polymer might be breaking down into molecules through triboelectric degradation. An electric potential is produced at the interface of Teflon and water when the polymer is continuously stirred in water,” explains Prof. Pradeep.
Glucose added to water first leaches out ions from the metal surface. When the PTFE-coated magnetic pellet is continuously rotated, triboelectric charges get generated on the pellet. The PTFE gets negatively charged. The negative charge on the PTFE surface attracts the metal ions that have been leached out. The interaction between the metal ions and PTFE results in metal-polymer bonding, causing the carbon-carbon bonds to destabilise. This eventually results in PTFEs degrading into molecules.
No such degradation of PTFE was noticed in the absence of stirring, glucose or metal ions. The rate of degradation gets reduced at room temperature.
“The amount of triboelectric degradation depends on the amount of glucose dissolved in water. As the amount of glucose in water increases more metal ions get leached leading to more interaction between PTFE and the metal ions. As more metal ions bind to PTFE, there is enhanced PTFE degradation,” says Abhijit Nag from IIT Madras and the first author of the paper.
“Mass spectrometry signatures imply the presence of molecular fragments of PTFE polymer,” says Prof. Pradeep. “The fragments seen floating on the water surface might be due to aggregation of molecular fragments or due to fragmentation of the long polymer.”
“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 as the generation of triboelectic charge is less compared with PTFE,” says Nag.
According to the paper, similar chemistry can possibly lead to micro and nanoplastics in food during cooking as many modern cookware are coated with Teflon.
“Triboelectric degradation of PTFE, polypropylene and other polymers might be occurring in nature in large water bodies such as oceans where metal ions are found in abundance and waves provide constant agitation,” Prof. Pradeep says. “This must be one of the ways by which microplastics get generated.”
