300 Agarwal et al.: Raman Spectra of Light- and Heat-Treated Pulps Holzforschung 49 (1995) 300–312 Sequential Treatment of Mechanical and Chemimechanical Pulps with Light and Heat: A Raman Spectroscopic Study By Umesh P. Agarwal 1 , Rajai H. Atalla 1 and Ingegerd Forsskåhl 2 1 Chemistry and Pulping Research Work Unit, USDA Forest Service, Forest Products Laboratory 3 , Madison, U.S.A. 2 Paper Science Center, Finnish Pulp and Paper Research Institute, Espoo, Finland Keywords Raman spectroscopy Mechanical pulp Coniferyl alcohol Coniferaldehyde Yellowing Bleaching Light Heat Introduction Summary 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, chemi- mechanical pulping not only degraded coniferaldehyde units but also partially degraded coniferyl alcohol groups. Furthermore, spectral evidence supported formation of chromophores during chemi- mechanical pulping; bleaching removed chromophores from chemimechanical pulp. Investigation of unbleache 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 1654cm-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 1654cm-1. Although newly formcd chemical groups/structures due to heat and light treatmets 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 sewqucntitd 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 1654cm-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 betwecn 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. Chemical changes caused by light exposure (photo- yellowing) and heat treatment in lignin-rich pulps are not wel -characterized. This situation is a result of the complexity of lignocellulosics and the limited capa- bilities of analytical techniques. We have used diffuse reflectance UV-VIS spectroscopy, diffuse reflectancc infrared Fourier transform (DRIFT) spectroscopy, and Raman spectroscopy to better characterize such treatment-induced changes. We anticipate that using 3 The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. a multi-technique approach will lead to improved understanding of the phenomena of heat- and light- induced color reversion. Results of diffuse reflectance UV-VIS and DRIFT spectroscopic studies were reported previously (Forsskåhl and Janson 1991, 1992). The UV-VIS and infrared (IR) techniques have traditionally been used in color reversion research. Raman spectroscopy, on the other hand, is a relatively new technique in ligno- cellulosic research (Atalla et al. 1992) and, thus far, has been applied in only a few instances. Earlier work showed that the technique is highly sensitive to aro- matic ring-conjugated structures in lignin (Agarwal and Atalla 1993a); the coniferyl alcohol (CALC) and Holzforschung / Vol. 49 / 1995 / No. 4 1995 Walter de Gruyter • Berlin • New York