Waste sawdust as a raw material: Synthesis, modification and characterization of novel bio-based phenol-formaldehyde wood adhesives

Abstract

Lignocellulosic biomass is a sustainable alternative to petroleum-derived chemicals for the development of bio-based wood adhesives. However, limited by the complexity and reactivity of biomass, bio-based adhesives are not competitive in terms of physicochemical properties. In this study, a novel bio-based phenol-formaldehyde resin was prepared directly using phenol to liquefy waste sawdust without removing the residue, and modified with a small amount of melamine. The results demonstrated that the melamine-modified bio-based phenol-formaldehyde resin (MBPF) exhibited better physicochemical qualities. Particularly, the free phenol content was 0.36 %, 57.6 % less than in common phenol-formaldehyde resin. Various characterizations of resins showed that liquefied sawdust could successfully substitute phenol in the resin system. Ortho hydroxymethylphenol was produced by an addition reaction between the liquefied sawdust or phenol and formaldehyde during the resin production process. A reticulated cross-linking structure was then formed between the hydroxymethylate liquefied sawdust and polyhydroxymethylphenol through the dehydrated condensation reaction. The degraded sawdust chain segments were successfully embedded in the resin, increased the molecular weight of the resin prepolymer and improved the initial thermal stability. After a tiny amount of melamine was added, the resin produced a denser cross-linked network, which improved resin condensation activity, decreased the curing time, and successfully increased the bonding force between the liquefied sawdust and resin molecules. The wet shear strength of MBPF resin could reach 1.43 MPa, which was equivalent to PF resin.