Investigating laser-induced bond breaking in high-density polyethylene pyrolysis

Abstract

A particular laser harmonic with an effective combination of process parameters can break plastic bonds by surpassing the dissociation threshold. Exploring this direction is imperative to uncover new physics and chemistry related to laser-high-density polyethylene (HDPE) interactions and to understand the phenomena occurring during laser induced degradation. An experiment was conducted in an open-air environment using the first (1064 nm), second (532 nm), and fourth (266 nm) harmonics at a 20 Hz repetition rate, with pulse energies of 5–40 mJ for the first and second harmonics, and 3–10 mJ for the fourth harmonic. The findings revealed that all laser harmonics broke HDPE bonds, with the fourth harmonic being the most effective in directly breaking bonds, particularly C–H bonds, which was evident from a prominent Hα peak at 656.3 nm. The results demonstrate initial, partial, and post-bond breaking, which can address deficiencies in laser-HDPE recycling and enable laser-induced HDPE pyrolysis, which has not been investigated. Optical microscopic analysis showed that craters formed by the fourth harmonic were wider and exhibited more efficient photon absorption with minimal ablation. Additionally, a high confidence interval (R²) value of 0.9758 with increased electron density and plasma temperatures, further supports the efficiency of bond breaking.