Characterization of reactive systems for lignin-based materials

Abstract

Lignin is the second most abundant biopolymer on earth but is rarely used as polymeric aromatic source because of its high structural variety. Two different approaches are studied in this work for the valorization of lignin, both of them give a perspective to add a sustainable resource for material synthesis. Oxypropylation, e.g. the modification of lignin with propylene oxide, yields liquid polyol mixtures suitable for the synthesis of polyurethane foams. Poly(propylene glycol) is formed as a side product, reducing the overall lignin content in the final foam. The extent of this side reaction was found to be highly sensitive to the type of catalyst. Potassium tert-butoxide as catalyst gives 10% less side product than the usually used potassium hydroxide. Formation of side products is even lower using the amidine 1,8- Diazabicyclo[5.4.0]undec-7-ene as catalyst. Combined use of propylene oxide and cyclic propylene carbonate as modifiers lead to presence of carbonate linkages in the polymer chains grafted to lignin. The properties of lignin-based polyols can thus be tailored for a desired application. Mechanically stable organic aerogels are prepared from an acid-catalyzed polycondensation of a mixture of 40 wt% resorcinol and 60 wt% lignin and an appropriate amount of formaldehyde. Gels form within a few minutes. Dimethyl sulfoxide/H2O is the best solvent system for the preparation of stable gels with low shrinkage and without the need for supercritical drying. Scale-Up allows for production of monolithic gels with dimensions of 15x15x3 cm or below. The obtained detailed understanding in form of a structure-property relationship makes it possible to tailor the amount of acid and formaldehyde appropriate for a certain application.