Bond quality of poplar plywood reinforced with hemp fibers and lignin-phenolic adhesives
Ramesh Karri, Reijo Lappalainen, Laura Tomppo, Ranjana Yadav
Abstract
This study investigated the potential of plywood reinforced with short hemp fibers and bonded with lignin-phenol-formaldehyde adhesives for structural applications. Furthermore, bond quality was examined by the evaluation of the physical and mechanical properties of the fabricated plywood as per standard methods of tests. The plywood bonded with phenol-formaldehyde resin was used as a control group. Four different types of plywoods were produced by the hot-pressing method. Subsequently, moisture content, density, modulus of elasticity, modulus of rupture, tensile strength, and matrix shear strength were tested and results were evaluated. Field emission scanning electron microscopy was used to analyze the fractured surface of tested plywood. The results indicated that lignin-phenol-formaldehyde resin with a replacement of phenol up to 50% weight with lignin (sodium lignosulfonate) can be utilized for bonding plywood. The modulus of elasticity of plywood bonded with lignin-phenol-formaldehyde resin at 7.65 GPa is higher than the phenol-formaldehyde resin at 5.28 GPa. The other properties of both lignin-phenol-formaldehyde and phenol-formaldehyde bonded plywood were found to be similar. In addition, the 3-point bending strength of plywood reinforced with short hemp fibers increased the strength of the composite by 95.3 MPa compared with the control group of 78.5 MPa. Furthermore, the moisture content, modulus of rupture, tensile strength, and shear strength test results of fiber-reinforced plywood confirm the specific requirements of structural plywood.