Bio-based Adhesive From Yeast Bacteria

Adhesives based on polyurethane can be manufactured easily and are extremely versatile. Around 60% of the mass of such adhesives consists of so-called polyols, a group of organic compounds that are usually produced petrochemically.

“Since polyols make up a large proportion of the adhesive, they are a good lever to make the product more sustainable. We chose sophorolipids as substitutes for the polyols - a special type of biosurfactants,”explains project manager Prof. Dr. Ulrich Schörken from the Faculty of Applied Sciences at TH Cologne.

Starmerella bombicola serves as the basis

To obtain sophorolipids, the scientists fed the yeast of the Starmerella bombicola strain with sugar and oil. As a result, these sophorolipids are excreted as metabolites. “Product variations result from adapting the feeding of the bacteria. For example, a lactonic form of the sophorolipids, which is characterized by a closed, ring-like shape; or an acidic, open-chain form. With our experiments, we have also synthesized a new class of sophorolipids that can serve as an emulsifier in the future,”Schörken describes the biotechnological process.

In a solvent-free crystallization process and a final freeze drying, the lactonic form of the sophorolipids could be produced with a purity of almost 98%. In contrast, the acidic form was 71%.

The research team therefore decided to continue working with the lactonic form. “If the sophorolipid is used as a whole, it can be used as a cross-linking polyol, for example to increase the mechanical strength and chemical resistance of the adhesives. However, since these only make up a very small proportion of the adhesive, sustainability hardly increases,”says Prof. Dr. Marc Leimenstoll from the Faculty of Applied Sciences at TH Cologne.

60% of the adhesive from sustainable sources

Therefore, the team focused on the fatty acid residue attached to each sophorolipid molecule. To do this, the team split off the so-called hydroxy fatty acid and used it to produce a bio-based polyester polyol. This is comparable to petrochemical polyester polyols and could replace them. "This would make 60 percent of the adhesive from sustainable sources," says Leimenstoll.

However, the properties of the adhesive can change due to the new polyols. For example, it would be significantly more water-repellent, which could be an advantage when gluing oily surfaces. In addition to adhesives, foams or lacquers could also be areas of application for the new polyols.

Industrial manufacturing still far away

"But there is still a long way to go in industrial production," emphasizes Leimenstoll. Further intensive research work is required, particularly in the purification and chemical manufacturing processes. "Our project has shown the potential of sophorolipids for more sustainable production in the adhesive industry," continues Leimenstoll.

The PureGlue research project was funded over three years by the Renewable Resources program of the Federal Ministry of Food and Agriculture. Two cooperative doctorates were also carried out within his framework.