Using lignin as the precursor to synthesize Fe3O4@lignin composite for preparing electromagnetic wave absorbing lignin-phenol-formaldehyde adhesive

In this study, lignin was used as a precursor to prepare Fe3O4@lignin composites to synthesize an electromagnetic wave (EMW) absorbing adhesive, which is applicable to plywood preparation. The morphology and structural characteristics of the synthesized Fe3O4@lignin composites are investigated with different lignin additions. Results revealed that core Fe3O4 nanoparticles are successfully coated with the lignin through the Mannich reaction. When Fe3O4@lignin nanoparticles are used to substitute 20 % of the phenol to prepare magnetic lignin-phenol-formaldehyde (LPF) adhesives, the resulting performance characteristics (bonding strength of 0.73 - 0.76 MPa and formaldehyde emission of 0.22 - 0.36 mg/L for the bonded poplar plywood) satisfied the Chinese National Standards (GB/T14732). Fe3O4@lignin (1:0.5)-phenol-formaldehyde adhesive demonstrated the best EMW absorbing properties with a reflection loss of - 29.5 dB (15.1 GHz) with a matching thickness of 4 mm, which is significantly higher than those of LPF adhesive ( -1.95 dB at 16.2 GHz) and Fe3O4 phenol-formaldehyde adhesive ( -5.75 dB M 12.1 GHz). The measurement of complex permittivity and complex permeability revealed that the excellent EMW absorption abilities of magnetic LPF adhesives are owing to the enhancement in interfacial polarization, natural resonance, exchange resonance, dielectric polarization originating from the introduced lignin with Fe3O4 in adhesives, which can balance the impedance matching and attenuation ability an the excellent EMW absorbing performance.