Study of the solubility and composition of welded wood material at progressive welding times

The solubility in water and carbohydrates/lignin composition after hydrolysis of wood welded by linear friction welding was studied. Welded beech (WB) and welded spruce (WS) were analyzed at welding times (Wt) of 1, 1.5, 2, 2.5, 3 s, and 2, 4, 6, 8, 10, 12 s, respectively. Scratched welded material was first extracted in water, dried, cryo-milled, and then hydrolyzed with sulfuric acid. Maximal percentages of solids dissolved in water were 17.7 and 10 for WB and WS and 2.2 and 1.4 for un-welded beech and spruce, respectively. Among water extracts, lignin, mono-oligosaccharides, acetic acid, vanillin, furfural, 5 hydroxymethylfurfural (5HMF), and syringaldehyde were quantified. Maximal percentages of water soluble lignin in WB and WS were 5 and 3.6, respectively; molecular weight and polydispersity were also determined. Regarding carbohydrates in the water extracts, a maximum of 1.4 % oligosaccharides in WB and 1 % monosaccharides in WS were detected. After hydrolysis, an increase in the amount of Klason lignin and a progressive diminution of some sugars take place at consecutive Wt of WB and WS. This study allows explaining to a good extent the behavior of welded joints face to liquid water. Moreover, the degradation of certain wood components is clearly presented.