Reduced kraft lignin

Softwood kraft lignin, 2) NaBH -reduced softwood kraft ligniny (3) Bjorkman spruce milled wood lignin... 

The acidity of kraft lignin is decreased by NaBH4 reduction. Less sodium hydroxide (0.07 mole NaOH per C6-C3 less) was needed to bring the solution of the reduced lignin to the same alkalinity (pH 10.9) than was needed for untreated kraft lignin. 

In the latter preparation, about 0.05 a-CO groups per C6-C3 were found in phenolic units (26). It is known that the phenolic hydroxyl in acetovanillone or in similar phenolic ketones is quite acidic (21) (pK = 8) reducing these ketones would decrease the acidity to the usual level of phenols (pK = 10, or still higher in sterically hindered phenols). This observation is a further indication that our present concept concerning the presence of a-CO groups in kraft lignin (24) is right. 

The exponential factors of Kuhn-Mark-Houwink-Sakurada(KMHS) equation for kraft lignin (KL) are 0.11, 0.13, and 0.23 in dimethylformamide (DMF) at 45.2°C, in DMF at 77.7°C, and in 0.5N sodium hydroxide at 30.2°C, respectively [16]. The fact that KMHS exponential factors of KL in DMF are small indicates that the molecular mass of lignin scarcely affects the reduced viscosity. This suggests that the lignin molecules in DMF have a compact spherical structure and approach the limit of an Einstein sphere, a constant-density sphere. The above results had also been reported by the research group of Goring [4]. 

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