Insights on the physico-chemical properties of alkali lignins from different agro-industrial residues and their use in phenol-formaldehyde wood adhesive formulation

The current study investigates for the first time the physico-chemical performances of lignins from cactus waste seeds (CWS) and spent coffee (SC) in comparison to previously isolated lignins from sugar byproducts (bagasse (SCB) and beet pulp (SBP)). In this work, lignin-phenol formaldehyde (LPF) resins were formulated using various lignin loadings (5-30 wt%), characterized and applied in the manufacturing of plywood panels. Several characterization techniques were applied to identify the chemical and morphological properties, thermal stability, and phenolic content of the extracted lignins, as well as the bonding strength and wood failure of the formulated resins. Results showed that the CWS and SC could be considered as an important source for lignin recovery with a considerable yield of 15.46 % and 27.08 % and an important hydroxyl phenolic content of 1.26 mmol/g and 1.36 mmol/g for CWS and SC, respectively. Interestingly, 20 wt% of extracted lignins in PF adhesives were the optimal formulation showing an improved modulus of elasticity (MOE) of about 3505, 3536 and 3515 N/mm2, and a higher modulus of rupture (MOR) of about 55, 55 and 56 N/mm2 for panels containing CWS, SC and SCB-lignins, respectively, over the reference panels (MOE = 3198 N/mm2 and MOR = 48 N/mm2). Additionally, formaldehyde emission from plywood remarkably decreases by up to 20 % when lignin was incorporated into the PF matrix. Herein, the treatment of the CWS and SC for the extraction of alkali lignin and its application showed a new route to produce high added-value products from underused residues.