Photoyellowing of groundwood pulps

The photoyellowing of spruce groundwood pulps (GWP) has been studied by use of UV/VIS diffuse reflectance- and FTIR-transmission spectroscopy. Brightness reversion was induced by exposing unbleached, hydrogen peroxide-bleached and sodium borohydride-reduced spruce GWP to simulate daylight for various exposure times. Spectra were recorded and principal component anal. was used to evaluate the spectral data. The information obtained indicates the presence of two alternative photo-induced processes proceeding in parallel during the irradn. The first process, more pronounced during the initial phase of the photoyellowing was ascribed to redox reactions involving structures with absorption bands at 340 or 380 nm and 440 nm in the UV/VIS-spectra. These absorption bands could results from photo-induced transformations involving quinonoid redox couples. The second process, pronounced during both the initial and final phase of the photoyellowing, was interpreted as non-specific oxidns. of lignin causing major chromophore formation.