State of the Art

Bark Valorization

Attempts at valorizing bark and especially tannins extracted from bark are not new. For example, hydrolyzable tannins from european hardwood species such as oak and chestnut are commercially produced and used for leather tanning. Other niche markets exist for bark extractives, in particular in the pharmaceutical sector. In addition, tannins have long been considered for polymer formulations such as adhesives. Also, within the framework of the ERA-NET project PROBARK, significant progress is being made in extracting bark tannins from European softwood species as well as utilizing them as adhesives and matrices in biocomposites.

Tannin Foams

A recent innovation by the research group of Prof. Pizzi (Université de Lorraine, LERMAB, Epinal, France) for valorising tannin by the production of tannin-based rigid insulation foams. These foams in their natural state have been shown to have excellent mechanical properties and be excellent thermal and acoustic insulators (they do not burn at all). In fact, tannin-based foams are excellent candidates to replace the petroleum-derived polyurethane or phenolic rigid foams that are currently used in building insulation. All these foams have considerable lower cost of the most diffuse commercial foams today, namely polyurethane foams, with the advantage of being resistant to fire and of being 98% of natural origin.

Tannin Foam Chemistry

Since the first use of tropical condensed tannins from Mimosa (Acacia mearnsii, formerly Mollissima, De Wildt) for foam production much progress has been made on fine-tuning tannin foam chemistry and on understanding structure/properties relationships. The chemistry of these tannin foams is based on the co-reaction of tannins with furfuryl alcohol and formaldehyde to form a cross-linked network. The exothermic self polymerization of furfuryl alcohol produces sufficient heat to induce foaming by the boiling of low boiling point solvents, such as diethyl ether or others. The two processes cross-linking and foaming have to be well harmonized with each other in order to control the foam properties. Unfortunately, today such high-performance foams can only be produced from condensed tannins from Mimosa and Quebracho which are extracted from tropical species and do not grow well or even at all to any great extent in Europe. Furthermore the tonnage of tannin produced in the world from these species is rather limited. While tannins from common European species have also shown some potential in blend formulations, the bark from each tree species exhibits a unique chemistry and reactivity, thus requiring a unique recipe and technology for its use in foams. For example, Pine (Pinus radiata) tannins have a very high reactivity with formaldehyde in comparison to tannins from Mimosa.