We have compared the light-induced yellowing of three peroxide-bleached mechanical pulps: softwood TMP (spruce/balsam fir mixture), softwood CTMP (spruce), and aspen CTMP. We attribute dissimilarities in the absorption difference spectra of softwood TMP and softwood CTMP to photobleaching of residual coniferaldehyde groups present in peroxide-bleached TMP. We also interpret an earlier report of photobleaching by ascorbic acid as coniferaldehyde photobleaching. Coniferaldehyde-free softwood pulps showed a broad absorption maximum at 360 nm and a shoulder at about 416 nm. The 360 nm peak resulted from the coalescence of two absorption peaks with #x39B;max#x2248; 330 nm and 360 nm. Irradiated aspen CTMP had an absorption maximum at 360 nm, but no shoulder at wavelengths above 400 nm. The difference spectra of the irradiated pulps were compared with those for pulps where quinone groups were introduced by oxidation with either Fremy's salt or sodium periodate. These experiments suggested that even if one attributes all the absorption increase above 400 nm to formation of simpleo-quinones, these quinones cannot account for the magnitude of the peak at 360 nm. The kinetics for increases in absorption coefficient were fit by a sum of two exponential terms. Within the error of the fitted parameters, all three pulps had identical rate constants.
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