Structure-property relationships in flexible and rigid lignin-derived polyurethane foams: A review

Lignin is an essential component of the effort to transition from petroleum-based to biorenewable resources. Although millions of tons of lignin are produced each year, only a small percentage of this material is used for high-value applications. As a source of hydroxyl groups, lignin is a natural candidate to replace traditional polyols in polyurethane synthesis opening a biobased route to the world's sixth most produced plastic. However, the natural heterogeneity, limited solubility, and crosslinked nature of lignin has impeded its use in commercial materials. This review presents a critical look of the research surrounding both flexible and rigid lignin-derived polyurethane foams and the methods of creating lignin-based polyols for their synthesis. A particular emphasis is placed on the interaction of lignin within the traditional polyurethane system and the effect to important properties such as density, compression strength, and thermal conductivity. Lignin has significant effects to natural cellular morphology and reactivity of polyurethane systems, yet new and innovative methods have been studied to overcome these challenges. The benefits of each method towards lignin incorporation are evaluated in terms of their ability to increase the biocontent of the final materials while minimizing the energy and complexity of processing techniques. Important insights are gained from comparing the physical properties of foams arising from each method to the amount of lignin that is actually incorporated in the final material. Several trends are observed regarding how lignin can best be transformed into a useful biomaterial for polyurethane foam synthesis. By creating a landscape to compare each method, the path forward to sustainable polyurethanes from lignin can come into view.