Properties of phenolic adhesives formulated with activated organosolv lignin derived from cornstalk

Abstract

Organosolv lignin isolated from cornstalk was activated via alkalification with sodium hydroxide or by sodium hydroxide catalyzed methylolation to make aqueous lignin solution (AL) and methylolated lignin solution (ML). Sodium hydroxide solution well dissolved the organosolv lignin even though the viscosity of AL was comparatively higher than that of ML. Different from traditionally replacing phenol with lignin during phenolic formaldehyde (PF) resin synthesis, this study applied the activated lignin as reactive prepolymers that could polymerize directly with neat phenol formaldehyde resin (NPF) under curing for production of wood adhesives. It was found that the lignin/NPF adhesives with or without methylolation cured more easily than the NPF, although the Lignin/NPF adhesives were slightly less thermally stable than the NPF. When applied as a binder to 3-ply plywood, the NPF led to dry and wet bonding strength of 2.33 MPa and 2.09 MPa, respectively. The AL/NPF (25/75, w/w) adhesive resulted in reduced dry and wet bonding strengths of 1.88 MPa and 1.74 MPa, respectively, but AL/NPF adhesive with a higher AL/NPF weight ratio of 50/50 reduced the dry strength to 1.03 MPa, and caused delamination of the 3-ply plywood when heated in boiling water. Lignin methylolation however greatly enhanced the dry/wet bonding strengths of the 3-ply plywood. The ML/NPF (50:50) adhesive produced 3-ply plywood with dry and wet tension shear strengths of 1.45 M P and 1.43 MPa, respectively, both above the ASTM or JIS standard requirements for plywood. In particular, 3-ply plywood samples with ML/NPF (25/75) adhesive afforded superb dry and wet strengths of 2.34 MPa and 2.30 MPa, respectively, even higher than those of the NPF-bonded samples. In addition, the free formaldehyde emission from the bonded 3-ply plywood with the AL/NPF was lower than that with the NPF, although the use of ML/NPF slightly increased the free formaldehyde emission.