Break

Understanding the mechanisms involved in alkali-depolymerization of poly(ethylene terephthalate) (PET) is essential in optimizing this polymer recycling process. Complete depolymerization of PET into disodium terephthalate (Na₂TPA) and ethylene glycol (EG) is achieved by ball milling it with sodium hydroxide (NaOH). This depolymerization can be conceptually described by the shrinking core model, where the NaOH reacts with the interface of the PET particle, achieving full monomer yield after 20 minutes.¹ To better understand this process, a detailed study of the single model collision was conducted by impacting pure PET and NaOH coated PET with a milling ball.

The experimental setup included a height-adjustable drop collision system which utilized an electromagnet to release the steel ball. PET films of varying thicknesses were coated with a suspension of NaOH in methanol and dichloromethane, leaving ~5 mg of NaOH on the film surface. The ball was dropped from different heights, where the standard height of 1.07 m correlated to a ball velocity of approximately 4.58 m/s. This value was within the range of collision velocities calculated for the upcycling of PET in a ball mill operating at 30 Hz.¹ The diameter and depth of the crater formed from the impact were examined with an optical microscope. Additionally, the extent of structural and chemical changes were analyzed using spatially-resolved Raman spectroscopy. Spectra were measured at random points on the unimpacted film and across the diameter of the crater of impact.

The first band analyzed in the spectrum was 1292 cm^-1, which is characteristic of the ester group in PET. The area reduction of this band was measured to determine the PET conversion from a single collision. For the NaOH coated film at the standard drop height, a conversion of roughly 25% was observed, with lower conversion in the outer regions of the crater. On the uncoated PET films, the conversion inside the crater did not exceed 4%.

Another important spectroscopic band was the carbonyl vibrational band at ~1730 cm^-1, which combines several subcomponents representing phases of the PET chain. The peak at 1726 cm^-1 represents the crystalline state, where the carbonyl groups in the terephthalate units are coplanar with the benzene ring and glycol units are in the trans conformation. The peaks at 1721 cm^-1 and 1733 cm^-1 correspond to the amorphous state of PET, where terephthalate units are completely disordered and glycol units are in gauche conformation.²

The peak areas were calculated for each film to establish a crystallinity basis ratio in PET before and after impact. In the uncoated films, there was a significant increase in the amorphous phase after the collision. Conversely, the NaOH coated films retained a higher crystallinity, implying that the NaOH layer prevents significant amorphization. This trend was observed across varying film thicknesses and different ball drop heights.

Overall, this characterization of the single collision allowed for a detailed understanding of the amorphization of the sample. This new insight into the process of depolymerization of PET is important in improving the efficiency of PET recycling at a commercial level.

References

1) W. Tricker, G. Samaras, K.L. Hebisch, M.J. Realff, C. Sievers, Hot spot generation, reactivity, and decay in mechanochemical reactors, Chemical Engineering Journal 382 (2020). https://doi.org/10.1016/j.cej.2019.122954

2) S. Fleming, K.L.A. Chan, S.G. Kazarian, FT-IR imaging and Raman microscopic study of poly(ethylene terephthalate) film processed with supercritical CO2, in: Vib Spectrosc, 2004: pp. 3–7. https://doi.org/10.1016/j.vibspec.2003.10.003.