THE KINETICS OF LIGNIN REACTIONS DURING OXYGEN BLEACHING .4. THE REACTIVITIES OF DIFFERENT LIGNIN MODEL COMPOUNDS AND THE INFLUENCE OF METAL-IONS ON THE RATE OF DEGRADATION

The reactivities of different lignin model compounds with oxygen in al kaline media have been compared. Generally, phenolic structures with a conjugated side chain like stilbene and enol ethers react very rapidl y during oxygen bleaching, whereas structures like propylguaiacol and beta-aryl ethers are more resistent. Some of the reaction mechanisms i nvolved are probably of a radical type because new crosslinked structu res can be formed with even lower reactivity. The influence of metal i ons on the reactivity has also been studied. We have found that Mg2+ a nd transition metal ions like Mn2+, Cu2+ and Fe2+ influence the rate o f degradation as well as the rate of dimerization. Most of the metal i ons gave a lower rate of degradation as measured at pH 11. The additio n of a small amount of hydrogen peroxide to the oxygen alkali system i ncreases the rate of degradation.