Influence of heartwood and the age of tree on the properties of particleboards from pine (Pinus sylvestris) .2. Physical-technical properties and formaldehyde release of particleboards made from sap- and heartwood of pine

Chips from sap- and heartwood of 20- and 126-year-old pine of the same site were chemically characterized. One-layer boards of 20 mm thickness were prepared from the chips using melamine-urea-phenol-formaldehyde resin (MUPF-resin), phenolformaldehyde resin (PF-resin), tannin-formaldehyde resin (TF-resin) and adhesives based on polymeric diisocyanate (PMDI). 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. The results reveal: Sap- and heartwood chips differ in their chemical properties and bonding behaviour. The age of the tree has also a dominant influence on the bonding characteristics of the chips. The addition of heartwood water extractives to PF- and MUPF-resin decreases the pH value and increases the viscosity, the attained viscosity of the resins remained unchanged for 6 h. In addition, water extractives of heartwood increase significantly the gelation time of TF-resin. Thickness swelling, water absorption and equilibrium moisture content of heartwood boards were always lower irrespective of the binder used than those of the corresponding sapwood boards. Moreover, bending and internal bonding strength of heartwood boards from 40-year-old trees were much higher than those produced from corresponding sapwood. The strength properties of the heartwood boards deteriorate, however, with increasing age of the tree. In addition, MUPF- and TF-bonded boards from heartwood were in general of lower formaldehyde release compared with boards from sapwood. The chemical properties of heart- and sapwood boards depend on the binder and the age of tree.