Using a wood-based panel for interior construction, researchers from the Fraunhofer WKI (institute for wood research) demonstrated the application possibilities for a newly developed, formaldehyde-free bio-adhesive at Ligna 2023.

The researchers, based in Braunschweig (Germany), presented a 100% bio-based condensation resin for the wood-based materials industry that can be manufactured and processed using conventional process and equipment technology.

Compared to the phenol-formaldehyde resins often used until now, the new lignin-HMF resin is harmless to health and free from petrochemical raw materials.

With the highlight ‘Multi-material model Wall I Ceiling I Roof’, Fraunhofer WKI presented eight innovations for the construction industry. The ceiling-system module is the floor of the model.

Fraunhofer WKI collaborated with the Argentinian Instituto de Materiales de Misiones (IMAM) on the development of the adhesive. As regionally available production residues are recycled in the manufacturing process, the research team created a sustainable and economically attractive solution for the production of wood-based materials.

Research process

Wood-based materials such as particle board, OSB, plywood or fibre-based materials (MDF, HDF) are used in large quantities as sustainable building materials in the construction industry and furniture production. They are also utilised in vehicle construction and could play a significantly greater role in the future.

Amongst other substances, phenol-formaldehyde resins have been used as binders (adhesives) in their manufacture to date. These resins are, firstly, critical to health and, secondly, are produced from fossil raw materials.

“We wanted to develop an alternative to conventional phenol-formaldehyde resins that is not harmful to health and, furthermore, is made from renewable, regionally available production residues,” explained Dr. Steven Eschig, Project Manager at the Fraunhofer WKI.

“To ensure that the new adhesive could be produced and processed using standard process and equipment technologies, we made it our goal to develop a comparable alternative. In other words, a bio-condensation resin.”

The researchers sought a way to replace the health-critical components phenol and formaldehyde. As a replacement for phenol, the research team successfully tested lignin, the most abundant biopolymer in plant biomass after cellulose.

Lignin is produced in large quantities, for example during paper production, and has so far been used primarily for energy through incineration. On account of its phenolic substructures, lignin is a promising raw material for the substitution of petrochemical phenol.

As a substitute for formaldehyde, the researchers tested hydroxyl methyl furfural (HMF), a sugar derivative. Currently, HMF is produced on an industrial scale from fructose.

In addition, the Argentine partners investigated the production of HMF from natural sugar polymers such as starch or cellulose. For this purpose, starch and cellulose are enzymatically broken down into their individual sugar components – glucose.

The glucose is then isomerised to fructose, which is subsequently converted to HMF in a thermal process in which water is eliminated. At the Fraunhofer WKI, researchers converted lignin and HMF to aqueous condensation resins.

“An important aspect of the resin development was the precise adjustment of the ratio of lignin and HMF. Among other things, the composition and origin of the lignin is hereby crucial. In our tests to date, kraft lignin provided the best results in terms of curing behaviour and tensile strengths,” said Dr. Eschig.

“In pressing and tensile tests on the automated bonding evaluation system, shear tensile strengths of more than 5 mega-Pascals were achieved. The pressing times in our investigations to date lie within the range of 30 to 90 seconds. The press temperatures are preferentially in the range of 130°C to 150°C,” he said, summarising the results to date.

More advantages

With the newly developed lignin-HMF resin, the advantages of sustainable wood-based materials are expanded even further. Thanks to the completely bio-based alternative to petrochemical condensation resins, the wood-based materials industry will become less dependent on fossil raw materials.

Furthermore, the avoidance of formaldehyde in resin production means that wood-based materials now contain only extremely low levels of formaldehyde, which occurs naturally in wood.

For the transition of adhesives and wood-based materials production to the new bio-condensation resins, high investment costs are not incurred. The wood-based materials industry is therefore provided with an economically attractive possibility for fulfilling legal requirements and increasingly stringent customer demands in terms of sustainability and health protection.

Courtesy: Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut WKI in Riedenkamp, Braunschweig, Germany.

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