Transverse Compression Strength and Fracture of Spruce Wood Modified by Melamine-Formaldehyde Impregnation of Cell Walls

In comparison with mechanical properties in longitudinal direction, the transverse mechanical properties of wood are far from satisfying. In an attempt to modify wood for improved transverse compression strength and stiffness, spruce wood samples were impregnated with an aqueous solution of melamine-formaldehyde resin. After polymerization, the tangential compression strength of treated samples was 82% higher compared to the untreated reference, and radial compression strength had increased by 290%. The average resin concentration in the cell walls of treated samples amounted to 0.125 g g-1, as measured with the UV-microscope. In contrast, less than 2% of the cell cavities (lumina), were found to be filled with resin. Thus the improvement of strength and stiffness obtained is attributed to modification of the cell wall and not to filing of tracheid lumina. Yielding-behavior under excessive compression load changed from plastic buckling in the reference samples to brittle fracture of cell walls in the treated samples. Even though a certain increase of brittleness has to be tolerated, it is demonstrated that melamine-formaldehyde reinforcement of cell walls has a distinctly improving effect on the properties of spruce in transverse compression