Improving the Reactivity of Sugarcane Bagasse Kraft Lignin by a Combination of Fractionation and Phenolation for Phenol–Formaldehyde Adhesive Applications

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.

A Comparison among Lignin Modification Methods on the Properties of Lignin–Phenol–Formaldehyde Resin as Wood Adhesive

“…Other reasons explained later also contribute to such an acceleration effect. Several different research groups have indicated that modified LPF resin cures at lower temperatures and faster than a PF resin [ 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ].…”

Lignin and Lignin-Derived Compounds for Wood Applications—A Review

“…The formaldehyde emission of lignin-containing resins has to be examined with care in both PF and UF resins. Several publications reported higher formaldehyde emissions, due to the lower reactivity of the lignin compared to other monomers [ 44 , 45 , 46 ], while others reported similar or even lower formaldehyde emissions [ 47 , 48 , 49 , 50 ] upon replacing part of the phenol or urea monomers with lignin. This indicates that the formaldehyde emission is highly dependent on the ratio of monomers to formaldehyde, and on the reaction conditions.…”

“…However, plywood produced with an LPF resin with 40% phenolated Kraft lignin fractionated by solubilization in different solvents was inferior to plywood produced with PF resin for all fractions. Despite this, the properties of the plywood met the Chinese national standards GB/T14732 [ 44 ]. Lignin can function as a fire retardant and photo stabilizer in epoxy resins [ 78 , 79 ].…”

“…Technical lignins obtained from different plant species have different properties, and different ratios, for example, in black liquor, can lead to varying quality in the final application [ 22 , 23 ]. An answer to that challenge might be lignin fractionation or cleavage [ 15 , 44 ]. In the field of wood modification the size of the lignin molecules is problematic, as only small molecules can enter the cell wall [ 34 , 35 , 36 ].…”

Synthesis of High-Water-Resistance Lignin-Phenol Resin Adhesive with Furfural as a Crosslinking Agent

“…Several strategies have also been directed to improve the water resistance of LPF resin adhesive. On the one hand, scholars have tried to develop multiple methods to realize the efficient utilization of lignin, such as sulfonation, phenolation, demethylation, hydrolysis, hydrogenation, and oxidative cracking [11][12][13][14][15]. On the other hand, reagents were introduced into the adhesive.…”