| Abstrakt: | CONCLUSION. The present investigation of the action spectra of different chemimechanical pulps confirms our previous results concerning the most efficient wavelengths causing photoyellowing and photobleach.ng. The strongest effect of photoyellowing occurs on irradiation with light of wavelength 310-320 nm, while the extent of photobleaching and the wavelengths that cause this change depend on whether the pulp has been pre irradiated and, if so, also on the amount of radiation the sample has received Although untreated chemimechanical and chemithermomechanical pulps displayed no nhoto-bleachmg effect regardless of radiation wavelength, a minimum in the curve for PC value versus radiation wavelength (corresponding to a maximum in the reflectance) in the vicinity of the normal photobleaching region (ca. 470 nm) was still observed The bleached pulp displayed no such minimum or maximum in the corresponding region For a strongly pre-yellowed pulp, photobleaching is a major reaction in simulated sunlight and only a narrow portion of the spectrum at around 350 nm is able to cause further discoloration of the pulp as measured at 457 nm. In this case, photobleaching starts at 370 nm and continues throughout the whole visible region, being most efficient at 430-450 nm. When measured at 557 nm, this active region is wider, extending from 320 to 410 nm. Action spectra are used to throw more light upon the nature of the cnromophores responsible for the photochemical changes; in certain situations they may even make it possible to identify the chromophores by comparison with their absorption spectra. This is a challenge in the case of wood pulps, too. However, this will require an extensive series of model studies with suitable samples. This seems to be particularly important since lignin model compounds have been found to display very different reflectance spectra in different pulps. Therefore, the direct use of the solution absorption spectra does not appear to be appropriate. Before more detailed conclusions can be made concerning the active chromophores based on the action spectra presented in this study, an investigation of the behavior of lignin model compounds and their intermediates in different pulps is needed. An investigation to this end is already in progress |
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