Polymer–Solvent Phase Behavior of Lignin with Hot Aqueous Solvent Systems

Lignin is one of the world’s most abundant biopolymers, comprising almost 30% of all woody biomass; only cellulose is more common. Furthermore, it is unique among renewable biopolymers in having significant aromatic content, giving it the potential for polymeric systems applications ranging from electrodeposited coatings to polyurethane foams to an inexpensive precursor for carbon fibers. We have discovered that lignins exhibit some of the classic features of polymer­­–solvent phase behavior when combined with selected aqueous organic solvent systems at elevated temperatures; additionally, this liquid-liquid phase behavior has been found to be useful for solvating, fractionating, and even cleaning lignins. For example, when raw alkali lignins are combined with hot acetic acid–water mixtures, two equilibrium liquid phases are formed, one solvent-rich and one polymer (lignin)-rich. The metal salts in the raw lignin (typically 10,000 ppm) are extracted into the solvent phase, resulting in a purified “ultrapure” lignin as the polymer phase. Furthermore, the lignin also partitions between the two phases according to mol wt, with the distribution of the lignin being controlled by the organic/water ratio in the solvent mixture. Finally, the lignin polymer phase is highly solvated, rendering it easily processable.

Determining the temperature at which the lignin–solvent mixture undergoes the phase transition from solid–liquid equilibrium to liquid–liquid equilibrium can be difficult, as visual observation is impeded by the opalescence of the system. Thus, a detection method based on electrochemical impedance spectroscopy (EIS) has been developed and was used to locate SLE to LLE phase boundaries for two lignin polymer–solvent systems over a range of organic/water solvent ratios. We expect that this EIS method can be applied to other polymer–solvent systems when standard techniques such as differential scanning calorimetry (DSC) fail.

Liquid–liquid equilibrium (LLE) phase behavior has been measured for the acetic acid–water–lignin system at 70 and 95 °C, with the results being presented as ternary phase diagrams depicting all regions of both fluid and solid phase behaviour. Measurements for at least one other lignin–solvent–water system will also be presented. A number of important thermodynamic properties relevant to the processing of lignin can be obtained from these diagrams, including the degree of solvation in the lignin (polymer)-rich phase as a function of solvent composition, the size of the LLE region available for polymer processing, and the distribution of lignin between the polymer-rich and solvent-rich phases. Our ability to both control and model the size of the LLE region will also be discussed, along with the implications of the observed polymer–solvent phase behavior for the development of a viable purification and fractionation process for raw, unpurified lignins.