Extractives are substances which can be dissolved out of plant tissues by means of extraction. Some are specifically extracted as industrial raw materials, for example starch from potatoes or sugar from beets. Substances can, however, also be dissolved through natural processes and subsequently migrate to the surface of the wood. Once there, they can have a lasting adverse effect on the bonding of the wood. Potentially problematic extractives are sugars, resins, fats, acids, quinones and other substances.
In the project, we characterize and evaluate the types and proportions of extractives from four tree species (spruce, Douglas fir, beech, and oak). From each tree species, we take samples at four different growth locations, which differ as greatly as possible as regards the following parameters: water balance/soil moisture, nutrient supply, elevation, and geographic location. In the event that the extractives differ significantly depending on the location, this is represented cartographically.
Tree trunks generally have three extraction zones:
- Lower: noble zone (e.g. furniture, construction timber)
- Middle: industrial wood zone (e.g. sleepers, additive manufacturing)
- Upper: biomass zone (e.g. firewood, wood chips)
For each tree trunk, we extract samples from the three zones in order to be able to provide a statement regarding the extractives content in dependence on the end product, particularly for construction purposes.
In order to determine the influence of the extractives on the bonding, we conduct tests with four adhesives which are also regularly utilized in the production of wood-wood bonds:
- Conventional PF system
- Bio-based PF system
- MUF system
- 1C PUR system
We investigate:
- Proportions and quantities of extractives
- Determination of the pH value of the wood
- Analysis of the gel time and the viscosity of the adhesive with addition of extractives
- Fracture-mechanical tests on bonded wood samples, for example test of longitudinal tensile shear strength in accordance with DIN EN 302-1
In the fracture-mechanical tests, we may, if necessary, additionally apply extractives to the surface of the bonded part in order to make the effects more clearly visible. In an ideal case, a correlation between extractives, locations and quality of the bond can be established. Adhesive compositions could consequently be adapted to the species and origin of the wood.
Fraunhofer Institute for Wood Research 
