Laser-Induced Plasma Effects on Bond Breaking in High-Density Polyethylene Pyrolysis

Abstract

The conventional use of Laser-Induced Breakdown Spectroscopy (LIBS) for elemental analysis in high-density polyethylene (HDPE) limits the exploration of bond behavior in Physics and Chemistry. A suitable combination of process parameters, exceeding the bond dissociation threshold, enables LIBS to break HDPE bonds, facilitating laser-induced pyrolysis. However, understanding bond behavior post-breakage, yield formation pathways, and the role of plasma and ionization across laser harmonics is crucial. An experiment is conducted using three laser harmonics (1064, 532, and 266 nm) at 20 Hz with pulse energies ranging from 3 to 100 mJ. An intense Hα peak at 656.3 nm suggests bond breaking due to extensive C-H breaking and hydrogen production. Interestingly, lower photon energies of 1.17 and 2.3 eV for 1064 and 532 nm broke the bonds, attributed to plasma effects. Numerical models are used to calculate plasma temperatures and electron density, classifying plasma types. Plasma parameters such as cooling time, ionization rate, energy density, and expansion velocity are analyzed. Results show that all laser harmonics contributed to bond breaking: 1064 nm induced field-induced plasma, 532 nm favored intermediate multiphoton plasma, and 266 nm is dominated by photon-induced plasma. These findings help optimize laser-induced HDPE pyrolysis.