Doctoral dissertation: Hybrid aspen bark as a raw material for new product applications

The extractives and chemical composition of hybrid aspen bark allow it to be used in several applications in the chemical industry. In the doctoral dissertation of Pasi Korkalo, who is a researcher at the Natural Resources Institute Finland (Luke) and a doctoral student at the University of Oulu's Sustainable Chemistry unit, the chemical composition of the bark biomass of hybrid aspen was studied,  activated carbon was produced from the bark extract, and a pesticide chemical was produced from the bark's hemicellulose. All chemical products were separated from the biomass following cascade processing principles.

The hybrid aspen tree, a species that originated from the crossbreeding of European and American aspen, was introduced to Finland in the 1950s. Its fast growth made it an attractive raw material for the wood industry. However, the hybrid aspen did not reach the same value as other dominant tree species in the market, as the market changed significantly before the cultivated hybrid aspen plantations were ready for harvesting. 

“Currently, hybrid aspen is seen as an opportunity to increase diversity in conifer-dominated forests. Applied chemistry can explore new uses and added value of the trees”, says Pasi Korkalo. 

Enhanced utilization of bark by means of cascade processing 

The extractives and chemical structure of hybrid aspen bark make it suitable for various applications in the chemical industry. The goal of the doctoral research was to produce different fractions from the same bark biomass for further processing, by using the cascade processing  principle.  

First, rigid carbon foam was produced from hybrid aspen bark extracts, which can replace fossil-based foam chemicals in insulation or filler applications that require heat tolerance. Rigid foam also works well as a starting material for activated carbon. 

After separating the extractives from the bark, the extraction residue was further utilized by torrefication, i.e., roasting in oxygen-free conditions. The method produced a distillate that was found to be an effective chemical in controlling certain weeds and fungal diseases of plant crops. The study found that hot water extraction of bark , that was applied to separate raw material for the foams, improved the performance of the chemical produced from the   extraction residue against weeds. 

Finally, after the production of chemicals, torrefied bark biomass still remained, which could be further utilized as a raw material for biochar or in energy production. 

Korkalo concludes: "By using cascade processing of the bark, several possibilities for use and added value can be achieved from a single raw material source. This, in turn, can arouse new interest in the possibilities of using hybrid aspen and the biomass separated from the species." 

The thesis defence will be organised on 3 April in the Science Centre Pilke in Rovaniemi at 12 o'clock. The research was implemented with the support of the Finnish Natural Resources Research Foundation, at the Natural Resources Institute Finland (2019–2021), and the dissertation was conducted at the Sustainable Chemistry Research Unit of the University of Oulu.