Assessment of covalent bond formation between coupling agents and wood by FTIR spectroscopy and pull strength tests

Abstract In the focus was the question whether metal alkoxide coupling agents – titanium, silane, and zirconium – form covalent bonds to wood and how they improve coating adhesion. In a previous work, a downshift of the lignin infrared (IR) band ∼1600 cm -1 was shown to be consistent with the formation of ether linkages between lignin and titanium coupling agent. In the present work, changes were found in the attenuated total reflectance-Fourier transform IR (ATR-FTIR) spectra of lignin and wood mixed with silane, and titanium coupling agents, and to a lesser extent for a zirconium coupling agent. This was seen as evidence for covalent bonds between lignin phenolics and the coupling agents. No spectral changes were observed when the coupling agents were mixed with the wood constituents cellulose and hemicellulose. For verification of the results, a modified EN 311 wet adhesion pull strength test was performed with softwood panels painted with a solvent-borne alkyd/acrylic coating. The results revealed an improved adhesion for all tested coupling agents compared to the untreated reference. The spectroscopic and pull test results underline that the presence of the lignin moiety in wood is of central importance for improved wood coating adhesion.