Studies on the Color of Lignin (II)

Abstract

The brightness and the brightness reversion of high yield pulp are related to the color of lignin in pulp. Brown or dark brown color partially prevents lignin from utilizations. The exact nature of chromophoric systems in lignin, however, has not been known yet.The purpose of this work was to discuss chromophoric groups and systems of softwood thiolignin responsible for their absorption spectra in visible region and reflectance spectra. From present knowledges of chemical structure of lignin the following chromophoric groups anal systems are predicted as to the color of lignin.Chromophoric groups : alcoholic hydroxyl group, phenolic hydroxyl group, double bond, carbonyl group, carboxyl group etc.Chromophoric systems : quinone, quinone methide, biphenyl etc.The thiolignin used here was the commercial softwood thiolignin (free type) purified with dioxaneether. In order to distinguish the effect of chromophoric groups and systems the following samples were prepared from purified thiolignin (TL).DA-1 and DA-3 : TL was methylated once with diazomethane and was methylated repeatedly three times, respectively.DA+NaOH : DA-3 was treated with 1N sodium hydroxyde solution at 80°C in nitrogen gas for 1 hr.DA+R-300 : DA-1 was reduced with sodium borohydride for 300 hr.DS-1 and DS-3 : TL was methylated once with dimethyl sulfate and was methylated repeatedly three times, respectively.DS+R-300 : DS-3 was reduced with sodium borohydride for 300 hr.R-20 and R-500 : TL was reduced with sodium borohydride for 20 hr. and 500 hr., respectively.The above thiolignins were purified with dioxane-ether to remove the effect of variation of apparent density. The reflectance spectra (Fig. 3) and the absorption spectra in ultraviolet and visible regions (Fig. 4) were observed by the same method adopted in the previous paper (Japan Tappi, 20, 369 (1966)). The visual efficiency, the dominent wave length and the percent saturation based on the XYZ system of C.I.E. were calculated from the reflectance spectra and the transmission spectra (Table 1).Moreover, various thiolignins of apparent density were prepared to discuss the relationship between apparent density and color of thiolignin powder.The results are as follows : (1) The visual efficiency of thiolignin powder is increased linearly with decrement of apparent density (Fig. 1).(2) The straight line relationships are observed between increase of methoxyl content and increase of the visual efficiency, and between increase of methoxyl content and decrease of the percent saturation (Fig. 5 and 6).(3) Alcoholic hydroxyl group affects scarcely on the color of thiolignin. Phenolic hydroxyl group considerably affects on the color as much as carbonyl group (Fig. 7). Unit effect of carbonyl group, however, affects much more than that of phenolic hydroxyl group (Fig. 8).(4) The methylated thiolignin reduced with sodium borohydride is still light-colored. Therefore, it can be predicted that stable free radical structure and carbonium ion structure in thiolignin may contribute markedly to the color of thiolignin. Moreover, molecular compound of electron donor-acceptor type is also expected to contribute to the color of lignin.