Shanghai Institute of Optics and Fine Mechanics has made progress in the field of high-intensity laser cracking of high-density polyethylene

Recently, a team from the National Key Laboratory of Ultra strong Laser Science and Technology at the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, collaborated with the Arctic University of Norway (UiT) to make progress in the efficient cracking of high-density polyethylene (HDPE) using strong laser molecular bond breaking technology. The research results were published in Advanced Materials Interfaces under the title "Laser Induced Plasma Effects on Bond Breaking in High Density Polyethylene Pyrolysis".

Traditionally, laser-induced breakdown spectroscopy (LIBS) has been mainly used for elemental analysis of HDPE materials, limiting further exploration of their physical and chemical bond behavior. In the preliminary work, the team proposed a concept of using strong lasers to achieve plastic cracking [Sustainable Materials Technologies 41, e01074 (2024)]. However, existing research still has limitations in understanding the behavior after bond breakage, especially in terms of the mechanism of product formation and the plasma and ionization effects under different laser harmonics, which have not been systematically understood.

In this study, three types of nanosecond laser harmonics were used: 1064 nm (1.17 eV), 532 nm (2.34 eV), and 266 nm (4.6 eV) to investigate their interaction with HDPE. The cleavage of C-C and C-H bonds in HDPE requires energies of 3.6 eV and 4.2 eV, respectively, while the ionization potential of HDPE is approximately between 8-10 eV. The laser harmonics used in the experiment can simultaneously induce photothermal and photochemical effects. Among them, the 266 nm laser, due to its high photon energy, can directly achieve bond breaking and promote the formation of free radicals through multiphoton absorption, while the 1064 nm and 532 nm lasers mainly rely on avalanche ionization to achieve molecular bond breaking. These processes generate free radicals, which emit fluorescence at specific wavelength positions during recombination (e.g. C-C bond at 500 nm, C-H bond at 432.3 nm, C-N bond at 386.1 nm, and H α line at 656.3 nm). Among them, 266 nm showed a more efficient bond breaking efficiency, as shown in Figure 1 (a to c). This study successfully achieved efficient breaking of HDPE molecular bonds using strong laser pulses, revealing previously unexplored physical mechanisms and potential product generation pathways in the bond dissociation process. The behavior characteristics of photons and plasmas under the action of laser fields were analyzed in depth, providing key support for further understanding and optimization of laser efficient plastic cracking processes and technologies.

Figure 1. Spectral evidence of HDPE molecular bond breakage under different laser harmonics a) 1064 nm, b) 532 nm, c) 266 nm
Relevant research has been funded by NSAF Joint Fund, Shanghai Science and Technology Plan, Chinese Academy of Sciences International Cooperation Program and CSC International Student Scholarship.

Source: Opticsky