Implication of chemical deterioration on mechanical performance of wood
Sammanfattning: The oak wood of the 17th century warship Vasa has shown significant changes in the chemical and mechanical properties as compared to recent oak. The most important factors contributing to these changes are the incorporation of iron compounds during waterlogging and the uptake of polyethylene glycol (PEG) in the course of the preservation treatment. To investigate the effect of iron-dependent oxidative degradation reactions, samples of recent birch, pine and oak were impregnated with aqueous iron(II) chloride solution and thereafter exposed to air or pure oxygen at controlled relative humidity in long-term experiments, followed by tensile strength (TS) measurements. Iron-impregnated samples exposed to oxygen displayed significant effects already after one week and with a reduction in TS of ~50% after one year. Samples treated with additional PEG displayed less reduction in TS. The extent of oxidative degradation was monitored quantitatively following the O₂ consumption in closed vials with different relative humidity (RH), as a function of time. For oak, the initial O₂ consumption was high and declined with time. After 200 d, the accumulated O₂ consumption was 0.3-0.4 mmol∙(g wood)-1. The formation of carbon dioxide and oxalic acid was positively correlated with RH. Samples kept for 1500 d at RH55% had accumulated 0.044 mmol oxalic acid∙(g wood)-1, which equals the average oxalic acid content in the interior of Vasa oak. FT-IR analysis on iron(II) impregnated samples and on Vasa wood showed an increased signal intensity at 1710 cm-1, assigned to carbonyl functions, in samples with reduced TS. The signal at 1710 cm-1 is not primarily related to oxalic acid. The results show that chemical changes and mechanical performance of iron contaminated wood are strongly correlated and that the state of degradation is observable using IR spectroscopy. Overall, the results confirm that iron compounds have a detrimental effect in wood and indicate that PEG might act as an antioxidant in oxidative degradation processes. Concerning the Vasa, it may be concluded that most degradation related to iron compounds and oxidative processes has taken place during the first period of conservation when the wood was exposed to oxygen in a still very humid state. Thus, the current rate of oxidative degradation under the present relatively dry museum conditions should be relatively low.
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