Methanol fractionation of softwood Kraft lignin: Impact on the lignin properties

The development of technologies to tune lignin properties for high-performance lignin-based materials is crucial for the utilization of lignin in various applications. Here, the effect of methanol (MeOH) fractionation on the molecular weight, molecular weight distribution, glass transition temperature (T_g), thermal decomposition, and chemical structure of lignin were investigated. Repeated MeOH fractionation of softwood Kraft lignin successfully removed the low-molecular-weight fraction. The separated high-molecular-weight lignin showed a T_g of 211°C and a char yield of 47%, much higher than those of asreceived lignin (T_g 153°C, char yield 41%). The MeOH-soluble fraction of lignin showed an increased low-molecular-weight fraction and a lower T_g (117°C) and char yield (32%). The amount of low-molecular-weight fraction showed a quantitative correlation with both 1/T_g and char yield in a linear regression. This study demonstrated the efficient purification or fractionation technology for lignin; it also established a theoretical and empirical correlation between the physical characteristics of fractionated lignins.