Durability, haptic, and optical appearance of wood are improved by coatings, which are nowadays prepared using environmentally compatible and less health-damaging materials. However, contemporary wood coatings have frequently a shortened service life compared to their traditional predecessors. New coating formulations would have a better performance if the adhesion to wood could be improved. In the present work, the chemical interaction between a titanium-based coupling agent, isopropyl triisostearoyl titanate (titanium agent, TA) and lignin has been studied by means of attenuated total reflectance-fourier transform infrared spectroscopy in combination with first principle predictions based on the density functional theory (DFT). In the infrared spectra, a new band at 1586 cm-1 was identified and the DFT predictions confirmed that the new band is because of the covalent bonds in the form of ether linkages between TA and lignin.

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.