Citric acid was used to improve resistance to water of the welded joint of spruce along the end-grain-to-end-grain faces. The use of 10-20 citric acid as treatment during a very short time has been shown to markedly improve both the mechanical performance and the water resistance of welded spruce. Based on the results of the water resistance tests it appears that the application of citric acid as a natural waterproofing additive in wood welding is possible. The use of citric acid thus allows the preparation of welded wood joints satisfying the requirement of frequent short-term and long-term exposure to water. C-13 NMR analysis indicated that lignin modification of the welded material is related to the improvement of water resistance and shear properties of the welded joints.
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