Light-induced brightness reversion is currently a topic of considerable interest for the pulp and paper industry. The objective of this study was to describe the effect of using different irradiation sources on the accelerated photoyellowing properties of untreated and chemically modified bleached high-yield pulps. The wavelength distribution of the irradiation source strongly influenced the photochemistry of both untreated and acetylated pulps. UV/VIS absorption difference spectra of aspen chemithermomechanical pulp (CTMP), irradiated with broadband UV-radiation, exhibited an apparent absorption maximum at 370 nm with a shoulder at 415 nm, whereas the aspen CTMP aged with the UV/VIS-fluorescent lamps exhibited an absorption peak at 360 nm and substantially less absorption in the entire visible region (lambda>400 nm). Two major absorption peaks, at 330-345 nm and at 425 nm, were observed in the absorption difference spectra of irradiated spruce CTMP regardless of the irradiation source used. However, in addition to the photodiscoloration, photobleaching with lambda(max)similar to 390 nm was observed when the spruce CTMP was irradiated with the UV/VIS-fluorescent lamps. Acetylation was found to slow down the UV-light induced reactions, but could also promote photobleaching reactions when the pulps were subjected to an irradiation source emitting light in the visible range. Generally no further discoloration was observed when the acetylated CTMPs (acetyl content, 8-10%) were irradiated with the UV/VIS-fluorescent lamps under the aging conditions used in this work. This investigation has shown the importance of choosing a light source that resembles the actual reversion situation as closely as possible (e.g., contains both an ultraviolet and a visible component) if realistic accelerated light-induced reversion conditions are to be obtained.
