Assignment of the photoyellowing-related 1675 cm-1 Raman/IR band to p-quinones and its implications to the mechanism of color reversion in mechanical pulps

Using FT (Fourier-transform) Raman and FT infrared (IR) spectroscopies, a new band was detected at 1675 cm(-1); this was most likely to have come from the yellow chromophores in photoexposed thermomechanical pulps (TMPs). On the basis of spectroscopic studies that involved both o- and p-quinone models and Fremy's salt-oxidized TMP, the 1675 cm(-1) band is assigned to the p-quinone functional group. Moreover, in the presence of known photoyellowing inhibitors, the photoyellowing behavior of methyl hydroquinone was similar to that of TMPs. Another important finding was that the molecular oxygen sensitivity of the laser-induced fluorescence (excited at 514.5 nm) of p-quinone and hydroquinone models was similar to what had been previously observed for yellowed and unyellowed TMPs. Taken together, these results provide strong support for a previously suggested yellowing hypothesis in which a hydroquinone/p-quinone couple was seen as an important leucochromophore/chromophore system in mechanical pulps.