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Near-infrared laser-induced plastic (polyethylene terephthalate (PET)) degradation with photosensitizers based on resonant energy transfer mechanism
Summary
Scientists found a way to break down PET plastic (the kind used in water bottles) using infrared light plus a special light-absorbing mineral additive, breaking off 60% of the plastic's mass in lab tests. This matters because PET plastic is a major source of the microplastics increasingly found in our food, water, and even bloodstreams, and this energy-efficient, low-heat method could offer a greener way to break it down instead of it lingering in the environment for centuries. That said, this is early-stage lab research, so it's not yet a solution you'll see used on plastic waste anytime soon.
We developed and validated a rapid method for degrading plastic into smaller molecular fragments using light. In this method, near-infrared (NIR) laser light is used to degrade particles with minimal energy consumption. Polyethylene terephthalate (PET) was selected as the model polymer for this study. PET exhibits no absorption in the near-infrared region; therefore, a photosensitizer is necessary to absorb it. Materials must have sufficient energy levels to absorb near-infrared light to meet the requirements for selecting photosensitizers. PET degradation is initiated when the photosensitizer’s absorbed energy is transferred to the bond dissociation limit of the polymer’s targeted chemical bonds. In this investigation, we used three different photosensitizers: magnesium diboride (MgB 2 ), graphite, and titanium diboride (TiB 2 ). Using cyclic voltammetry, we determined the lowest unoccupied molecular orbital (LUMO) levels of the photosensitizers. Among three photosensitizers, the LUMO level of MgB 2 nearly matches the PET’s bond dissociation limit. Hence, if we mix PET with MgB 2 , polymer degradation will occur even at ordinary temperature and pressure for a modest level of light intensity if the energy transfer between MgB 2 and PET is resonant. Using a 3 W NIR laser, we demonstrate 60% mass loss of PET with MgB 2 photosensitizer. The degradation of the PET with MgB 2 photosensitizer was verified by gas chromatography-mass spectrometry (GC-MS) analysis. Thus, the demonstrated NIR laser-assisted PET degradation using photosensitizers serves as one of the green photonics solutions to the PET plastic problems.