Abstract
The research aim of this work is to determine the influence of lignin modification methods on lignin–phenol–formaldehyde (LPF) adhesive properties. Thus, glyoxal (G), phenol (P), ionic liquid (IL), and maleic anhydride (MA) were used to modify lignin. The modified lignins were used for phenol substitution (50 wt%) in phenol–formaldehyde adhesives. The prepared resins were then used for the preparation of wood particleboard. These LPF resins were characterized physicochemically, namely by using standard methods to determine gel time, solids content, density, and viscosity, thus the physicochemical properties of the LPF resins synthesized. The panels dimensional stability, formaldehyde emission, bending modulus, bending strength, and internal bond (IB) strength were also measured. MA-modified lignin showed by differential scanning calorimetry (DSC) the lowest temperature of curing than the resins with non-modified lignin and modified with IL, phenolared lignin, and glyoxal. LPF resins with lignin treated with maleic anhydride presented a shorter gel time, higher viscosity, and solids content than the resins with other lignin modifications. Equally, the particleboard panels prepared with LPF resins with maleic anhydride or with ionic liquid had the lowest formaldehyde emission and the highest mechanical strength among all the synthesized resins. The dimensional stability of all panels bonded with modified lignin LPF resins presented no difference of any significance.
Highlights
Phenol–formaldehyde (PF) resins are used to bind moisture and water-resistant woodbased panels such as particleboard, plywood, and oriented strandboard (OSB), giving stability to temperature and preventing panels from delaminating [1,2]
This shows that the modified LPF adhesive had higher solids content, higher viscosity, density, and a shorter gel time than the resins made from unmodified lignin and modified by the other three treatments
Based on the physicochemical test analysis results, the resins modified by maleic anhydride and ionic liquid treated lignin had higher solids of all the resins synthesized
Summary
Phenol–formaldehyde (PF) resins are used to bind moisture and water-resistant woodbased panels such as particleboard, plywood, and oriented strandboard (OSB), giving stability to temperature and preventing panels from delaminating [1,2]. Research aimed at using lower-cost biomaterials as phenol substitutes in PF resins is increasing. In this regard, previous work indicated that lignin, due to its chemical similarity with phenol, showed the best results among all-natural materials [4]. The main obstacle to increasing lignin application to phenolic adhesives is due to its slower reaction than phenol with formaldehyde. For this reason, it is proven that lignin must be modified before any use of it in the PF resin for wood adhesive. This work aims to compare the best proposed modification methods of lignin for use in a PF resin
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