Lignin phosphorylation to enhance mechanical and physical properties and reduce formaldehyde emissions in plywood panels

Lignin is gaining more attention for its potential use in adhesives for wood-based composites owing to its accessibility, molecular structure, barrier properties, and potential for chemical modification. In order to assess the physical, thermal, and mechanical performances of alkaline lignin (AL) and its ability to reduce formaldehyde emissions in synthetic phenol formaldehyde (PF) adhesives used to glue plywood panels, AL was surface-modified by a phosphorylation treatment to produce phosphorylated alkali lignin. In this study, lignin was isolated from alkali-treated powdered argan shells and subsequently phosphorylated to obtain phosphorylated lignin (P-AL). The isolated lignin was characterized by two-dimensional (2D-¹H and ¹³C NMR) heteronuclear single quantum coherence (HSQC) spectra to confirm its successful isolation from argan shells. Both AL and P-AL were then subjected to a series of analytical techniques, including Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) with elemental mapping, X-ray fluorescence (XRF) spectrometry, phosphorus nuclear magnetic resonance (³¹P NMR) spectroscopy, conductimetric titration (to determine the phosphorylation rate), thermogravimetric and derivative thermogravimetric analysis (TGA/DTG) and X-ray diffraction (XRD) analysis. Afterwards, P-AL was formulated with PF resin and applied to plywood panels, following characterizations to evaluate mechanical properties, such as bond strength (BS), modulus of rupture (MOR), modulus of elasticity (MOE), shear strength (SS), and formaldehyde emission (FE). Results showed that lignin phosphorylation had a significantly positive effect on plywood adhesion, especially in terms of FE, which decreased from 2.5 to 1.89 mg^-1 m^-2 h^-1, while maintaining higher mechanical properties, including an MOE of 4144 MPa and an MOR of 66 MPa.