Studies on the Color of Lignin Part.X

Abstract

Lignin is a phenolic compound and contains a small amount of catechol group. Especially, the catechol group exists more abundantly in thiolignin because of the demethylation during sulfate cooking. Therefore, the color of lignin and pulp is influenced by the formation of lignin-metal chelate. This investigation was undertaken to clarify the effects of iron and copper on the color of lignin and pulp.The results are as follows : (1) The residual lignin in mangrove sulfate pulp forms the lignin-iron chelate which affects the color of pulp. The amount of chelate site in pulp, however, is a little. The all chelate sites in sulfate pulp are completely blocked through the lignin-iron chelate with the iron in wood and the iron introduced during pulping process. Even if the iron would be added into sulfate pulp, the iron could not form a chelate and would be merely absorbed in pulp. Consequently, the color of sulfate pulp is slightly affected by the addition of iron (Fig. 1 and Table 1).(2) 7080 ppm of iron is introduced in lignin from the stainless steel autoclave during the soda or sulfate cooking. Moreover, a considerable amount of heavy metal is introduced into pulp from mill water and pulping apparatus (Table 2).(3) The molar extinction coefficients of thiolignin at each wavelength increase linearly with increasing the amount of iron added. This, however, does not vary with more than 7, 000 ppm of iron. Based on 7, 000 ppm of iron, 0.025 per phenyl propane unit is estimated as the chelate site in softwood thiolignin which affects the coloration (Fig. 3 and Table 3).(4) The darkening ratio at each wavelength was calculated as follows : D= (e-e0) /e×100where D=darkening ratioe=molar extinction coefficient of original lignine0=molar extinction coefficient of lignin which does not contain the ironThe darkening ratio in the range of 400700 mμ is less than 10% in softwood thiolignin and is 4050% in softwood lignosulfonate. The darkening ratio of MWL is almost 100% in the range of longer wavelength than 570 mμ. The color of sulfomethylated thiolignin increases with increasing the molecular weight. This, however, can not be explained on the basis of the amount of α-carbonyl group and heavy metal in lignin (Fig. 5, 6 and Table 5).(5) Even if lignin is methylated with diazomethane and dimethyl sulfate, the amount of heavy metal in lignins does not vary, but decreases considerably by sodium borohydride or sodium hydrosulfite reduction. By the reduction treatment, the lignin-iron chelate is destroyed, and iron is released from lignin. Consequently, it is conceivable that the brihgtening of lignin by the reduction with sodium borohydride or sodium hydrosulfite is due to the destruction of lignin-iron chelate together with the reduction of quinoid structure (Fig. 79 and Table 4).