Mechanistic comparison of pulse laser induced phase separation of particulates from cellulose paper at 213 nm and 532 nm

摘要

The laser-induced phase separation of charcoal particles on additive-free cotton linters cellulose paper was investigated by electron and optical microscopy, colorime-try, and diffuse reflectance FT-IR. The fibre bundles were vaporised in depth of several 10 μm above destruction flu-ence thresholds using visible 532 nm radiation. This is in contrast to mid-ultraviolet 213 nm radiation, where only the top fibre bundles were modified and partially evaporated. The colorimetric lightness results generally represented the cleaning status, whereas the colorimetric yellowing data represented irreversible chemical and/or photochemical changes. Charcoal-contaminated paper treated with visible and mid-ultraviolet radiation exhibited yellowing, whereas uncontaminated did not. This suggests that the electron-rich plasma generated by the evaporation of the particles heats the adjacent substrate and also excludes oxygen. Mid-ultraviolet, in contrast to visible radiation, shows particle removal always accompanied by paper destruction. IR spectroscopy results suggest cross-linking by ether bonds near the destruction threshold, but do not prove the formation of oxidation products and double bonds as the basis of the yellowing. A "cleaning window" between the cleaning threshold (0.1 J/cm~2) and the paper destruction threshold (2.9 J/cm~2) with a pulse number of 2 is provided by visible 532 nm laser treatment.