Sequential Treatment of Mechanical and Chemimechanical Pulps with Light and Heat: A Raman Spectroscopic Study

Abstract

Raman spectroscopy was used to study the effects of heat and light treatments on unbleached and peroxide-bleached mechanical and chemimechanical pulps. For bleached mechanical pulp, spectral changes were associated with the removal of coniferaldehyde structures in lignin. In contrast, chemimechanical pulping not only degraded coniferaldehyde units but also partially degraded coniferyl alcohol groups. Furthermore, spectral evidence supported formation of chromophores during chemimechanical pulping : bleaching removed chromophores from chemimechanical pulp. Investigation of unbleached and bleached chemimechanical pulps at 514.5 and 647.1 nm excitation wavelengths revealed a decline in intensity upon the longer wavelength excitation for certain bands, indicating the presence of residual chromophores and suggesting the presence of coniferaldehyde structures. Spectra of light-and heat-treated pulps displayed intensity changes at 1120, 1595, 1620, and 1654 cm -1 , which were found to be due to the involvement of coniferaldehyde and/or coniferyl alcohol structures in lignin. The most informative Raman band was at 1654 cm -1 . Although newly formed chemical groups/structures due to heat and light treatments could not be identified, new Raman contributions were detected in the lignin aromatic-stretch region. The effects of light or heat were compared in single and sequential treatments. In most cases. the second-stage treatment caused spectral changes that were significantly different from those resulting from direct treatment of pulp, indicating that the effect of the second stage depended on the chemical changes induced in the first stage. For unbleached mechanical and bleached chemimechanical pulp, the order of the single light and heat treatments was found to be important. The sequence of light followed by heat (light-heat) caused more decay in the intensity of the 1654 cm -1 band than did the opposite sequence (heat-light). In contrast, for bleached mechanical and unbleached chemimechanical pulp, similar changes were detected in the 1654 cm -1 band intensity upon sequential treatment. Raman information on treated pulps was correlated with the results of a previous UV-VIS reflectance study. In general, similarity of spectral changes (in the 370nm region) among various pulps and treatments did not necessarily indicate similarity between chemical changes in the pulps. The results seem to suggest significant variation at the molecular level among the responses of pulps for a given treatment and among the treatments for a given pulp.