New Insights into Chromophore Chemistry of Eucalypt Pulps Assessed by Uv-resonance Raman Micro-spectroscopy

UV-Resonance Raman (UV-RR) spectroscopy with laser beam excitation source of 325 nm was applied to a solid-state study of chromophores in Eucalyptus globulus kraft pulps. The amount of chromophore structures in pre-bleached/fully bleached pulps absorbing at 325 nm linearly correlated with signal at ca 1600 cm in UV-RR spectra. The characteristic vibrations from particular pulp chromophore structures have been assigned from experiments with model compounds thus allowing establishing an UV-RR database. Among the components of bleached eucalypt kraft pulp, xylan-lignin complex was suggested to be an important source of chromophores. The monitoring of pulp bleaching in ECF sequences by UV-RR allowed a proposition about formation of new polysaccharide-derived chromophores that hindered the brightness development in the final D stages. These chromophore systems are co-responsible for the brightness reversion of fully bleached pulps during thermal accelerated ageing and are vulnerable to alkali extraction stages in standard ECF sequences. This study revealed UV-RR spectroscopy as a promising tool for quantitative monitoring of chromophores during pulp bleaching and its relationship to thermal ageing.