Rapid loss of lignin at wood surfaces during natural weathering
P D Evans, K J Schmalzl, A J Michell
Thin veneers of radiata pine (Pinus radiata D. Don) were exposed to natural weathering over a period of 30 days. Incident UV radiation was monitored at the test site. FTIR internal reflectance spectroscopy was used to study the chemical changes taking place at the veneer surface during weathering. Spectra obtained show that after only 3 days exposure there was substantial delignification at veneer surfaces. Large losses in the acid insoluble lignin content of veneers also occurred during weathering. Losses in the wet finite span tensile strength of veneers were more rapid than losses in dry zero span tensile strength. Since delignification reduces wet finite span tensile strength, but has little effect on dry zero span tensile strength, the results here again indicate that lignin loss at wood surfaces during exterior exposure is surprisingly rapid. Changes in the morphology of transverse surfaces exposed to the weather were also largely confined to the lignin rich middle lamella layers. A stepwise loss in weight of exposed veneers after rainfall supports suggestions made in the literature that lignin degradation products are leached from weathered wood by rain.