Iroko wood (Milicia excelsa C.C. berg), a good candidate for high-speed rotation-induced wood dowel welding: An assessment of its welding potential and the water resistance of its welded joints

The objective of this study was to determine the explanatory factors for the cold water resistance of welded joints of Iroko wood (Milicia excelsa C. C. Berg). The welding parameters considered and measured were grain direction, welding time, pre-drilled-hole depth, and rotation speed on welding temperature. The pull-out resistance of joints immersed in distilled water from 10 °C to 20 °C for five periods (7, 14, 21, 28, and 35 days) was also tested. The results show that the maximum welding temperatures (125 °C and 231 °C) were reached in less than 5s for all the dowelled specimens and obtained with the specimens inserted in the tangential direction, at 2100 rpm and 25 mm hole depth. The maximum value of the pull-out strength (3.28 MPa) was obtained with specimens inserted in the radial direction, at 1050 rpm and 20 mm hole depth. However, no significant difference was found between this result and those obtained with specimens inserted in the tangential direction, at 2100 rpm and 25 mm hole depth and bonded with PVAc. Despite the drastic decrease in pull-out strength of the specimens immersed in cold water within each group compared to the G1 reference group, some welded joints showed good maximum pull-out strength, greater than 2 MPa, and no failure after 5 weeks in cold water. MALDI-TOF analysis reveals the presence of inhibitory oligomers of the stilbene class at (379 Da = no Na ⁺, and at 402 Da with Na ⁺) chlorophorin (C ₂₄H ₂₈O ₄), and geranyl-trihydroxy-stilbene (364 Da) which remain insensitive to water. In addition, the presence and slow leaching at (381 Da no Na ⁺) of geranyl tetrahydroxy-stilbene, at 380 Da of cudraxanthone I (C ₂₃H ₂₄O ₆), and at (377 Da) of neocyclomorusin (C ₂₅H ₂₄O ₇) is an additional reason to justify the water stability of the Iroko welded joints. In conclusion, Iroko wood is a good candidate for the wood welding technique.