紫外線による紙の劣化機構

抄録

In the previous papers, we have studied the influence of UV treatments on physical and optical properties of paper, and several characteristic results for UV treatments were obtained. In this paper, therefore, the mechanisms of the changes on physical properties, especially such as double folds, Young's modulus and water absorption values, by UV treatments of paper were examined in terms of molecular-order analyses of pulp and rosin chemicals.
Cellulose in amorphous regions and hemicellulose are depolymerized through oxidative and/or radical reactions by UV treatments. This depolymerization leads to the formation of stable intermolecular hydrogen bond by molecular rearrangement, and thus the decrease in hydroxyl groups, which can interact with moisture or water, causes the decrease in hydrophilic properties of paper. This new hydrogen bond formation brings about the increase in Young's modulus, and even the slight decrease in moisture contents of paper probably causes the drastic decrease in double folds. On the other hand, porous structures of paper were found to be unchanged by UV treatments, and cross-linking reactions between cellulose chanis were not likely to form during the treatments.
Rosin chemicals are degraded to form hydrophilic groups such as carboxylic acid from their double bonds by UV treatments. This causes the decrease in sizing degrees of rosin-sized paper, while the UV treatments bring about the increase in sizing degrees for rosin-free paper such as neutral paper owing to the formation of the stable hydrogen bond, as described above.