Background of Tannin-Based Adhesives
Tannin-based adhesives have been used in industrial wood-based panel manufacturing since the 1970s and are suitable for load-bearing exterior applications. Furthermore, it is possible to use these tannin adhesives not only for wood-based panels, but also for higher-quality wood-based composites, such as cross-laminated timber.
Industrial tannin-based adhesives can achieve mechanical properties and water resistance comparable to those of conventional structural adhesives. However, their use has traditionally been limited by formaldehyde emissions, as curing typically occurs through the condensation of tannins with formaldehyde. To address this issue, alternative crosslinkers—such as furfural, glyoxal, hexamine, tris(hydroxymethyl)nitromethane, and larger aldehydes—have been successfully explored in laboratory-scale tannin-based adhesive formulations.
Non-Isocyanate Polyurethanes from Polyphenols
Among the various approaches to synthesizing non-isocyanate polyurethanes (NIPUs), the condensation of polyphenol carbonates with multifunctional amines has shown significant promise. Condensed tannins, in particular, exhibit higher reactivity and superior wood-bonding performance compared to lignin.
One studied route involves the transesterification of tannins with dimethyl carbonate (DMC), followed by polycondensation with hexamethylenediamine (DA). From a sustainability perspective, cradle-to-grave life cycle analyses have demonstrated that NIPUs derived from tannin carbonates offer environmental advantages over other tannin-based polymeric adhesives and their synthetic aminoplastic counterparts. An example of this process is illustrated in Figure 1.
Figure 1. Synthesis route for the transesterification of tannin QSF with dimethyl carbonate (DMC) to tannin carbonate (QSF-Cb) in stage A and the urethane formation of QSF-Cb with hexamethylenediamine (DA) to TANIPU-DA resin and additional nucleophilic crosslinking of TANIPU-DA with HEXAMINE (TD; TANIPU-DA-TD) in stage B. Source
Developing High-Performance Bio-Adhesives for Wood-Based Boards
For bio-based adhesives to become a viable alternative to synthetic ones, they must deliver comparable bonding performance. In addition, practical factors such as application methods and curing times are crucial in industrial production, as they directly affect the production rate and overall cost of wood-based boards.
SuperBark aims to develop a formaldehyde-free adhesive that is at least 95% bio-based. These new adhesive systems, derived from polyphenols found in softwood bark extracts, must meet the bonding performance requirements defined in European Standard tests EN 302 Parts 1 and 2, and EN 314 Parts 1 and 2, which are essential for plywood classification. For other wood-based boards under development, such as medium-density fibreboard (MDF) and particleboard (PB), the relevant standards—EN 622-5 and EN 312—are followed for classification.
Softwood bark extracts contain a mixture of polyphenolic compounds rather than a single component. They also include co-extractives, which reduce the concentration of reactive polyphenols useful for polycondensation, polyaddition, or polymerization reactions involving electrophilic aromatic substitution.
Because purification is both time-consuming and expensive, finding simple and cost-effective methods to incorporate these co-extractives into the final adhesive is essential. The project explores ways to integrate identified co-extracts into the adhesive network formation and to develop fast-curing, bio-based polyphenolic resins. During curing, these resins form durable polymer networks on and within wood materials such as birch, spruce, and pine. The resulting adhesive systems must not only fit into existing industrial manufacturing processes but also meet the classification requirements of the relevant European standards.
Collaboration and Industrial Relevance
To achieve these objectives, research institutions VTT, Fraunhofer ICT, and TECNALIA are collaborating closely with industry partners Metsä Wood and Kastamonu Entegre within WP3. Multiple synthesis routes are being explored to develop adhesives tailored for plywood, particleboard, and MDF production—aligned with market demand and the interests of the industrial partners. At the current stage of the project, several polyphenol-based adhesive formulations are being optimized and evaluated, along with the pressing and curing conditions for different types of wood-based boards.
Detlef Schmiedl, PhD, WP3
Fraunhofer ICT, located in Pfinztal, Germany, is a leading research institute with over 100 laboratories, multiple pilot plants, and three test centers specializing in chemical processes, polymer engineering, explosives technology, and energy systems. Within SuperBark, Fraunhofer ICT leads WP3, focusing on the analytical characterization and chemical modification of polyphenolic extractives from softwood bark to develop reactive intermediates for bio-based adhesives.
To learn more about the work in WP3, contact WP3 leader Detlef Schmiedl at detlef.schmiedl@ict.fraunhofer.de.