| Abstrakt: | CONCLUSION. The mechanism of light-induced yellowing or color reversion of unbleached and peroxide-bleached thermomechanical pulps was studied by investigating the phenomenon of photocycling performed alternately using two different radiation wavelengths (370 and 440 or 470 nm). Photoyellowing and photobleaching were demonstrated for both pulps during many cycles. The amplitude, expressed as the change in the reflectance, of the cycles was found to decrease continuously with longer irradiation times. For the unbleached pulp, reversing the sequence showed that photobleaching was more detrimental than photoyellowing when carried out first. The number of photocycles observed for the bleached pulp was calculated to be from 6 to 12, depending on the length of time used for the cycle period. Two possible mechanisms were proposed for photocycling, but further experimental studies are required before any conclusions can be made on which, if either of these applies. |
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