The degradation of wood surfaces by dilute acids

IRG/WP 3326

P D Evans, W B Banks

Thin radial/longitudinal sections(~100 µ) of Corsican Pine (Pinus nigra) and Lime (Tilia vulgaris), were exposed to Sulphuric, Sulphurous, nitric, acetic, and formic acid at 40°C in the pH range 2-6. After about 3 months exposure to Sulphuric, nitric, acetic and formic acid at pH 2.0 Pine lost some 20-25% of its tensile strength. Losses in tensile strength due to sulphurous acid were greater being about 60-70%. Further prolonged exposure to sulphurous acid for 12 months led to some 90% tensile strength loss in Pine and 95% in Lime. Tensile strength losses also occurred at pH 2.5 and 3.0. For Pine strength losses of some 40% and 20% occurred at pH 2.5 and 3.0 after 12 months exposure. For Lime the losses were somewhat greater being about 70% at pH 2.5 and 50% at pH 3.0. For both species even greater losses in toughness occurred after this period of exposure in the pH range 2-3. Signficant but more erratic losses in toughness also occurred at pH 3.5. The strength losses noted above are over and above those resulting due to exposure of the controls to deionized water alone. During a similar time period exposure to deionized water led to losses in tensile strength for both species of some 15-20%. Losses in toughness were even greater being in the range 20-40%. Lime specimens tested wet showed a tendency to fail by inter-fibre shear in the middle lamella region. Scanning electron micrography also showed degradation of the middle lamella in Pine. It is believed that the degradational phenomena reported here are due to physico-chemical processes and that they may contribute to the degradation of paints and other finishes from wood surfaces.


Conference: 85-05-12/17 Guarujá, Brazil

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