Lignin-based foams as insulation materials: a review

The bulk use of renewable polymers is currently largely limited to cellulose and, less significantly, hemicelluloses. Technical lignins are only applied in novel materials to a rather limited extent, although bulk lignin utilization is a worldwide research object. Native lignins, which belong to the second or third most abundant biopoly-mers of terrestrial plants, are mostly used in the form of technical lignins from wood pulping processes; they are employed in low-performance sectors or simply burnt for the generation of energy. Technical lignins are available in huge quantities and have a large application potential, mainly in areas where their aromatic nature is of relevance. This review presents the state of the art of foamed lignin-based polymers (lignofoams) as high-performance insulation materials. In the focus of this presentation are the fundamental foaming principles and influential agents that have an improvement potential concerning the matrix interactions between technical lignins (including lignosulfonates) and a copolymer in foam composites. The different approaches for foam preparation are critically compared. In general, the reviewed papers disclose that the lignin part in foams should be less than 37%. There are significant difficulties to improve the properties of lignofoams, and thus intensive research is needed to find better formulations and new technologies.