SEQUENTIAL TREATMENT OF MECHANICAL AND CHEMIMECHANICAL PULPS WITH LIGHT AND HEAT - A RAMAN-SPECTROSCOPIC STUDY

Raman spectroscopy was used to study the effects of heat and light tre atments on unbleached and peroxide-bleached mechanical and chemimechan ical pulps. For bleached mechanical pulp, spectral changes were associ ated with the removal of coniferaldehyde structures in lignin. In cont rast, chemimechanical pulping not only degraded coniferaldehyde units but also partially degraded coniferyl alcohol groups. Furthermore, spe ctral evidence supported formation of chromophores during chemimechani cal pulping: bleaching removed chromophores from chemimechanical pulp. Investigation of unbleached and bleached chemimechanical pulps at 514 .5 and 617.1 nm excitation wavelengths revealed a decline in intensity upon the longer wavelength excitation for certain bands, indicating t he presence of residual chromophores and suggesting the presence of co niferaldehyde structures. Spectra of light-and heat-treated pulps disp layed intensity changes at 1120, 1595, 1620, and 1653 cm(-1), which we re found to be due to the involvement of coniferaldehyde and/or conife ryl alcohol structures in lignin. The most informative Raman band was at 1654 cm(-1). Although newly formed chemical groups/structures due t o heat and light treatments could not be identified, new Raman contrib utions were detected in the lignin aromatic-stretch region. The effect s 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 treatme nt of pulp, indicating that the effect of the second stage depended on the chemical changes induced in the first stage. For unbleached mecha nical and bleached chemimechanical pulp, the order of the single light and heat treatments was found to he important. The sequence of light followed by heat (light-heat) caused more decay in the intensity of th e 1654 cm-1 band than did the opposite sequence (heat-light). In contr ast, for bleached mechanical and unbleached chemimechanical pulp, simi lar changes were detected in the 1654 cm(-1) band intensity upon seque ntial treatment. Raman information on treated pulps was correlated wit h the results of a previous UV-VIS reflectance study. In general, simi larity of spectral changes (in the 370nm region) among various pulps a nd treatments did not necessarily indicate similarity between chemical changes in the pulps. The results seem to suggest significant variati on at the molecular level among the responses of pulps for a given tre atment and among the treatments for a given pulp.