Tailored Thermosetting Wood Adhesive Based on Well-Defined Hardwood Lignin Fractions

Luyao Wang,Sebastian Von Schoultz,Teija Tirri,Yongchao Zhang,Lari Vähäsalo,Chunlin Xu,Xiaoju Wang,Patrik C Eklund,Irina Sulaeva,Thomas Rosenau,Lucas Lagerquist,Andrey Pranovich,Stefan Willför,Rajesh Koppolu

doi:10.1021/acssuschemeng.0c05408

Abstract

By aiming at tailoring the bonding strength of a thermosetting lignin-containing phenol-formaldehyde (LPF) wood adhesive, different fractions of an industrial hardwood alkaline lignin have been prepared through sequential solvent fractionation (i-PrOH, EtOH, and MeOH). Those fractions were comprehensively characterized by GPC, GC, Py/GC–MS, and NMR techniques. Lignin fractions with low molar mass and narrow dispersity, including the i-PrOH-soluble and EtOH-soluble ones, were of high purity and had more reactive sites for LPF adhesive synthesis and better accessibility due to lower degree of condensation than the high molar mass ones. Some recalcitrance of integrating high molar mass fractions covalently into the PF adhesive was observed, which was also true in the case of lignin phenolation. The tailored bonding strength of the LPF adhesive, tested by gluing wood pieces, provided strong evidence for molecular structure–performance correlation; the i-PrOH-PF had the lowest activation energy, the highest curing enthalpy, and the strongest bonding strength of 2.16 MPa. This study demonstrates a clear structure–property-application relationship of technical hardwood lignin in the LPF adhesive field, which might pave the way for a more effective bulk valorization.

Highlights

Lignin is by far the most abundant aromatic bioresource composed of phenyl propane units as the main building block.[1,2] Because of the structural similarities, lignin represents a natural, promising sustainable alternative to the petroleumbased phenol in phenol−formaldehyde (PF) wood adhesive.[3,4]native lignin is not the lignin that is available for applications, with its structure being sometimes drastically different from the lignins produced by the pulping industry or upon cellulosic ethanol production, which are denoted as “technical lignins”.5 Technical lignin is a bulk feedstock with a global annual output of 70 million tons.[6]
To tackle with the issues associated with the inherent heterogeneity of technical lignin, a variety of lignin fractionation methodologies have been proposed to offer relatively homogeneous lignin fractions with well-defined characteristics, i.e., defined range of molar mass, narrow ĐM, as well as selective interunit linkages and enriched functional groups that are desired in the respective applications
The wet bonding strength of the glued plywood can be tailored according to the molar-massdependent differences of the integrated birch alkaline lignin (AL) fractions, e.g., amount of reactive sites, degree of condensation, and impurity content

Summary

■ INTRODUCTION

Lignin is by far the most abundant aromatic bioresource composed of phenyl propane units as the main building block.[1,2] Because of the structural similarities, lignin represents a natural, promising sustainable alternative to the petroleumbased phenol in phenol−formaldehyde (PF) wood adhesive.[3,4]. Birch AL fractions with low MW and narrow ĐM, e.g., i-PrOH-s and EtOH-s fractions, contain less impurity (carbohydrate and ash) contents, more noncondensed phenolic−OH groups, and lower degree of condensation than MeOH-s fraction or residue lignin These molar-massdependent differences of birch AL fractions turned out to be of high relevance for the bonding strength performance of the corresponding LPF wood adhesives. It is worth noting that the resoles containing low MW lignin fractions, e.g., i-PrOH-PF and EtOH-PF, have a lower activation energy (the corresponding DSC thermograms are shown in Figure S5 and the detailed calculation procedure is listed in Table S6) and higher curing exothermic heat than those containing high MW lignin fractions (Table 2) This indicates once more that pubs.acs.org/journal/ascecg birch AL fractions of low MW and DC% allow for higher chemical accessibility and/or lower steric hindrance in the corresponding resole polymerization/curing reaction. The water resistance capacity of the cured LPF wood adhesive would deteriorate for the residue-PFP, owing to the high hemicellulose content of 8.4 wt % in the residue lignin

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES

Tailoring the Molecular and Thermo-Mechanical Properties of