Terahertz spectroscopy for polyethylene terephthalate (PET)

Abstract

Plastics present a major challenge of the 21st century as they are prolific but produced from a non-renewable source (crude oil) and can cause many environmental problems. While reducing the global reliance on plastics is part of the solution, plastics are still necessary in some fields, for example in preserving sterile medical supplies. Thus, improving current recycling methods is still a valuable endeavour. Terahertz spectroscopy is a maturing technique for probing the vibrational modes of materials in a non-ionising, non-destructive manner that may be used to discern different plastic samples. Firstly, characteristic features need to be identified that can be used as markers to distinguish different plastics. This work focuses on identifying these characteristic features in polyethylene terephthalate (PET) by studying the terahertz spectrum.

Spectra have been collected on the terahertz beamline at the Australian Synchrotron. Fourteen samples of PET plastic from everyday household items have been studied, discerned by their plastics identification code, along with a commercially purchased biaxially orientated sample (BO-PET). The BO-PET sample is more crystalline than standard PET and acts as a reference point for the other samples studied. The samples have been investigated in a variety of ways to determine the suitability of terahertz spectroscopy in their spectroscopic identification. Firstly, the most intense terahertz source of radiation has been used. Next, low temperatures and polarisation dependence have been investigated for additional insight into the spectra. The results, summarised in Fig. 1, show that while there is much variety in commercial PET, there are common absorptions. These features can be used to identify PET amongst other plastics.