Co-modification of corn straw lignin and its enhancement on glue-free fiberboard based on freezing activated wood fibers

Glue-free fiberboard, as a substitute for traditional fiberboard, does not contain a large amount of synthetic resins, which can avoid harm to human health and environment. However, glue-free fiberboard still has some disadvantages of insufficient mechanical strength and excessive thickness swelling of water absorption. Here, corn straw lignin was co-modified by demethylation/hydrothermal degradation to obtain high hydroxyl content increased by 210.85% compared with raw lignin. The co-modified lignin was then introduced into freezing activated wood fibers to prepare the environment-friendly medium density fiberboard (MDF). The effect of co-modified lignin content on the properties of MDF was investigated. MDF with 3 wt% co-modified lignin exhibited the best comprehensive performance with the modulus of elasticity (MOE) of 5809 MPa, modulus of rupture (MOR) as high as 70 MPa, internal bonding strength (IB) of 2.29 MPa and thickness swelling (TS) as low as 16.65%, completely matching the requirements of the Chinese national standard GB/T 11718-2009 and EU national standard BS EN 622–5–2009. The interaction mechanism of co-modified lignin on the wood fibers was further explored by Fourier infrared spectroscopy (FTIR), differential scanning calorimeter (DSC) and scanning electron microscope (SEM) analysis. The phenolic structures of co-modified lignin could be chemically crosslinked with wood fibers during the hot-pressing process and the large amount of hydroxyl groups provided strong hydrogen bond interactions for the glue-free fiberboard. The glue-free fiberboard enhanced by co-modified lignin with excellent comprehensive performance exhibits a broad application prospect.¹1MDF: medium density fiberboard; MOR: modulus of rupture; MOE: modulus of elasticity; IB: internal bonding strength; TS: thickness expansion; DSC: differential scanning calorimetry