Abstract
The low reactivity of lignin hinders its application as a phenol substitute in phenol–formaldehyde (PF) resin. Therefore, the combination of fractionation and phenolation was adopted to enhance the reactivity of lignin for preparing a phenol–formaldehyde resin adhesive. Sugarcane bagasse kraft lignin and its fractions were employed to replace 40 wt% of phenol to prepare a PF adhesive. The fractionation increased the reactivity of lignin, however the as-prepared lignin-based PF (LPF) hardly met its application requirements as an adhesive. Therefore, the phenolation of lignin under an acidic condition was adopted to further improve its reactivity. The phenolated lignin was characterized by FTIR, gel permeation chromatography, and NMR, indicating its active sites increased while its molecular weight decreased. The phenolated lignin was used to replace 40 wt% of phenol to prepare a PF adhesive (PLPF) which was further employed to prepare plywood. The results indicated that the combination of fractionation and phenolation effectively enhanced the reactivity of lignin, and eventually improved the properties of the PLPF and its corresponding plywood. The free formaldehyde content of PLPF decreased to 0.16%. The wet bonding strength of the as-prepared plywood increased to 1.36 MPa, while the emission of formaldehyde decreased to 0.31 mL/L.
Highlights
Wood adhesive is one of the key materials in the manufacture of particleboard, fiberboard and plywood
The results indicated that the reactivity of lignin, the properties of the PF adhesive, and the performances of the plywood were significantly enhanced by the combination of fractionation and phenolation
The bonding strength of plywood prepared with lignin-based PF (LPF) exceeded the plywood (Class I) national requirements of China
Summary
Wood adhesive is one of the key materials in the manufacture of particleboard, fiberboard and plywood. Phenol–formaldehyde (PF) resin adhesive has been widely used in the production of wood-based panels due to its excellent performance and mechanical properties, stability, chemical durability and flame retardancy [1]. A traditional PF adhesive is prepared from non-renewable petroleum products. A large number of industrial lignin as a by-product has been produced by pulp and papermaking mill. Since lignin contains a structural unit which is similar to phenol, it can be used to replace or partially replace phenol to produce lignin-based phenol–formaldehyde (LPF) resin [2]. Industrial lignin has a complicated composition, heterogeneous features and a low reactivity with formaldehyde, resulting in a decreased number of reactive functional groups in the resin, thereby inhibiting curing and lowering the crosslinking density [3]. The fractionation methodologies, including solvent extraction [4,5], selective
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