Ultraviolet resonance Raman spectroscopy analysis of carbonyl groups present after different bleaching stages for various bleach sequences
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Different bleaching reagents have different efficiencies of removing chromophore groups from chemical pulps. The objective of this work was to evaluate the effect of different bleaching sequences on the removal of chromophore groups, especially carbonyls, which are suspected to cause brightness reversion. The bleaching sequences analyzed comprise the stages: chlorine dioxide, acid hydrolysis at high temperature, alkaline extraction with hydrogen peroxide, pressurized hydrogen peroxide, and hydrogen peroxide. After bleaching an oxygen-delignified eucalypt kraft pulp with these sequences, the pulps were analyzed for their final brightness, brightness reversion, and pulp viscosity; the bleached pulps were also analyzed using ultraviolet resonance Raman spectroscopy in the infrared region. The infrared analysis indicated that bleaching stages that used hydrogen peroxide, such as pressurized hydrogen peroxide or hydrogen peroxide, in the terminal position reduced the amount of carbonyl groups in the bleached pulp as measured by the absorption band intensity. This study observed that the inclusion of a hot acid hydrolysis stage in the bleach sequence improved the final brightness and brightness stability of the bleached eucalyptus pulp. The replacement of a chlorine dioxide brightening stage by a hydrogen peroxide stage at the end of a bleach sequence yielded higher pulp brightness, and less brightness reversion. The use of pressurized hydrogen peroxide with oxygen resulted in less brightness reversion.
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