Zum Verhalten des Splint- und Kernholzes der Lärche (Larix decidua) bei der Herstellung von feuchtebeständigen Spanplatten unter Einsatz verschiedener Bindemittel

Chips from sap- and heartwood of 20, 40 and 102-year-old larch (Larix decidua) were prepared and chemically characterized. From the chips one-layer boards of 20 mm thickness were prepared using melamine-urea-phenol-formaldehyde resin (MUPF-resin), phenolformaldehyde resin (PF-resin), polymeric diisocyanate (PMDI), and tannin-formaldehyde resin (TF-resin). The physical and chemical properties of the boards were determined. Moreover, the influence of hot water extractives on the pH-value, viscosity and gelation time of the resins was assessed. Sap- and heartwood chips are different in their chemical properties as well as in bonding behaviour. The age of the tree has also an influence on the bonding characteristics of the chips. Thickness swelling, water absorption and equilibrium moisture content of heartwood boards were always lower than those of sapwood boards. Moreover, bending and internal bond strength of heartwood boards made from 20- and 40-year-old trees are much higher than those produced from sapwood. The strength properties of the boards deteriorate, however, with increasing age of the tree. This applies for both heartwood and sapwood boards. The water extractives of heartwood boards are of lower pH-value and higher buffering capacity than the corresponding boards from sapwood. Moreover, the emanation of acetic acid from heartwood boards is much higher than that of formic acid. In addition, boards from heartwood are in general of lower formaldehyde release compared with boards from sapwood. The addition of heartwood extractives to MUPF-resin increases the viscosity and gelation time of the resin and decreases its pH-value, whereas in case of PF-resin no such increase in the gelation time was registered. The extracts of heartwood increase the viscosity and gelation time of TF-resin.