Formation of Chromophores and Leucochromophores during Manufacturing of Mechanical Pulp

The formation of chromophores and leucochromophores during the manufacturing of thermomechanical pulp (TMP) has been investigated. As analytical tools, advanced UV-VIS and NMR spectroscopy have been used. With the exception of some pilot plant scale refining experiments, that were carried out at Valmet Fibertech (Sundsvall), the work has been conducted at the Royal Institute of Technology (Stockholm). When wood is converted into TMP, both chromophoric and leucochromophoric structures are formed. The greatest changes in the light absorption coefficient spectrum of the material takes place in the first refining stage, with a predominant increase in the region below 400 nm. Addition of a mixture of DTPA and sodium sulphite to the wood, before refining, results in a decreased formation of leucochromophoric structures, however. The increase in light absorbance, caused by the refining of wood, does not appear to be correlated to any conversion of β-1 structures into stilbenes, despite an observed reduction in the amount of a known dienone structure, which act as a precursor for β-1 structures in lignin. Rather, a certain amount of oxygen oxidation, resulting in the formation of aryl α-carbonyl structures, seem to take place during refining. In addition, metal ion complexes appear to play an important role. Coloured metal ion complexes, absorbing light throughout the entire visible region, appear to be formed during refining. By treating the pulp lignin with EDTA, most of this colour can be removed, however.