FT-Raman investigation of bleaching of spruce thermornechanical pulp

Spruce thermomechanical pulp was bleached initially by alkaline hydrogen peroxide and then by sodium dithionite and sodium borohydride. Near-infrared Fourier-transform-Raman spectroscopy revealed that spectral differences were due primarily to coniferaldehyde and p-quinone structures in lignin, new direct evidence that bleaching removes p-quinone structures. In unbleached pulp, the contribution of p-quinones was detected in the 1665-1690 cm(-1) region. This contribution was removed to varying degrees in bleached pulps, depending on the extent to which the pulps were brightened. The relative change in the post-colour number of pulp was correlated linearly with the Raman intensity decline in the 1665-1690 cm-1 region, indicating that p-quinone structures were largely responsible for determining pulp brightness. The C-13 nuclear magnetic resonance spectra of milled softwood lignins, obtained to further support the Raman findings, exhibited a peak at similar to180 ppm due to quinones. This peak was not detected in the spectra of milled hardwood lignins.