Changes in light absorption coefficient spectra as a result of TCF bleaching

Light absorption coefficient, k, spectra in the wavelength range from 200 t o 700 nm have been determined for a number of modified kraft and alkaline s ulfite pulps after O, OAZ and OAZQP* bleaching (O=oxygen, A=acid treatment, Z=ozone, Q=complexing agent, P*=peroxide stage with Mg addition). The diff erent pulp types have earlier shown differences in bleachability and select ivity when bleached in an OAZQP* sequence and also in other sequences. Howe ver, the reason for the differences in bleachability are not fully understo od. In this paper, the changes in k spectra after the different bleaching s tages for the different pulp types are shown, and the differences are discu ssed, The accuracy in the determination of k is also discussed and why it i s preferable to measure k rather than brightness. There were no significant differences between the shape of the k spectra fo r the different pulp types that could explain the known differences in blea chability between the pulp types. However, the levels of the k spectra afte r the oxygen stage could be correlated to the known differences in bleachab ility between the pulps, The k spectra for the ASAM pulp, the pulp that has shown best bleachability, was significantly lower compared to the other in vestigated pulps. Ozone and peroxide bleached pulps showed k spectra of app roximately the same shape as the oxygen-delignified pulps. In this study, k values as high as about 10 m(2)/kg were correctly measured. At higher k va lues, the light scattering coefficient (s) began to deviate due to too high a light absorption. Dilution, i.e. mixing the pulp with a brighter pulp of lower and known k value so that the k value of the mixture does not exceed 10 m2/kg is a way of circumventing the problem. However, there are also so me limitations with the pulp mixing method, which are discussed.