Development and structural comparison of alkaline and organosolv coconut husks lignin as an eco-friendly lignin-phenol-glyoxal (LPG) wood adhesives

The development of eco-friendly wood adhesives have gained more interest among adhesives industries due to the concerns about using carcinogenic formaldehyde and petroleum-based phenol in commercially available adhesives. Therefore, many studies have been done by using lignin to partially replace phenol and completely substitute formaldehyde with non-toxic glyoxal in a wood adhesive formulation. This study focused on using different percentages of lignin substitution (10 %, 30 % and 50 wt%) of alkaline and organosolv coconut husk lignin into soda lignin-phenol-glyoxal (SLPG), Kraft lignin-phenol-glyoxal (KLPG) and organosolv lignin-phenol-glyoxal (OLPG) adhesives. The adhesives were further characterized using various analyses and it showed that 50 % lignin substitution was the optimum rate percentage with 50 % SLPG adhesive giving the highest solid content, shorter gel time and more viscosity compared to control (PF and PG), KLPG and OLPG adhesives. Mechanical properties revealed that 50 % SLPG adhesive showed an improvement performance of tensile strength (TS: 68.98 ± 0.19 MPa), internal bonding (IB: 17.01 ± 1.07 Nmm-2), and cross-linking density panels (775.51 ± 8.15 kg m-3) due to the higher amount of molecular weight (Mw) as well as higher phenolic-OH that improved the cross-linking reaction between phenol-glyoxal with G-type unit in lignin structure.