| Abstract: | CONCLUSION. Coniferaldehyde,structures are the major leucochromophore present in spruce lignin. These end groups are also present to a large extent in unbleached mechanical pulps and are responsible for a major portion of the absorption of UV-radiation above 350 nm. Mild sulfite treatment leads to a considerable reduction in the content of coniferaldehyde structures. After bleaching of mechanical and chemimechanical pulps with alkaline hydrogen peroxide, coniferaldehyde structures are to a large extent eliminated and they cannot be considered to be major contributor to the photo-yellowing of such pulps. They may, however, still be responsible for a part of the remaining colour in these pulps. Stilbenes seem to be present in native spruce lignin in trace amounts only. During wood grinding or chip refining, part of the diarylpropane structures present in the lignin are converted into diguaiacyl stilbenes. Alkaline hydrogen peroxide bleaching leads to a further significant formation of stilbenes and these structures seem to be responsible for most of the absorption of UV-light around 350 nm in bleached high yield pulps. Diguaiacyl stilbene, derived from diarylpropane structures, was found to be the most important leucochromophoric structure responsible for the fast initial discoloration of bleached high yield pulps through conversion into coloured products. Coniferyl alcohol and phenylcoumaran structures are rather stable, and neither phenylcoumorone nor stilbene structures seem to be formed from phenylcoumarans during high yield pulping and bleaching. Pyruvaldehyde enol ether structures may be formed in the pulp lignin from detached glyceraldehyde side chain structures by mild acid treatment at elevated temperatures. Coniferyl alcohol, coniferaldehyde, pyruvaldehyde enol ether and vanillin structures all absorb UV-radiation above 300 nm. These structures may therefore also contribute to part of the yellowing of high yield pulps. |
|---|
