Properties of phenolic adhesives formulated with activated organosolv lignin derived from cornstalk

Highlights

• Organosolv cornstalk lignin is activated via alkalification and methylolation.

• Untraditional lignin based phenolic adhesive development from activated lignin.

• Activated lignin is directly used to replace NPF for adhesive purpose.

• Activated lignin/NPF adhesive condensation is easier than NPF self-condensation.

• Excellent bonding strength of ML/NPF adhesive than NPF for plywood lamination.

Abstract

Organosolv lignin isolated from cornstalk was activated via alkalification with sodium hydroxide or by sodium hydroxide catalyzed methylolation to make aqueous lignin solution (AL) and methylolated lignin solution (ML). Sodium hydroxide solution well dissolved the organosolv lignin even though the viscosity of AL was comparatively higher than that of ML. Different from traditionally replacing phenol with lignin during phenolic formaldehyde (PF) resin synthesis, this study applied the activated lignin as reactive prepolymers that could polymerize directly with neat phenol formaldehyde resin (NPF) under curing for production of wood adhesives. It was found that the lignin/NPF adhesives with or without methylolation cured more easily than the NPF, although the Lignin/NPF adhesives were slightly less thermally stable than the NPF. When applied as a binder to 3-ply plywood, the NPF led to dry and wet bonding strength of 2.33 MPa and 2.09 MPa, respectively. The AL/NPF (25/75, w/w) adhesive resulted in reduced dry and wet bonding strengths of 1.88 MPa and 1.74 MPa, respectively, but AL/NPF adhesive with a higher AL/NPF weight ratio of 50/50 reduced the dry strength to 1.03 MPa, and caused delamination of the 3-ply plywood when heated in boiling water. Lignin methylolation however greatly enhanced the dry/wet bonding strengths of the 3-ply plywood. The ML/NPF (50:50) adhesive produced 3-ply plywood with dry and wet tension shear strengths of 1.45 M P and 1.43 MPa, respectively, both above the ASTM or JIS standard requirements for plywood. In particular, 3-ply plywood samples with ML/NPF (25/75) adhesive afforded superb dry and wet strengths of 2.34 MPa and 2.30 MPa, respectively, even higher than those of the NPF-bonded samples. In addition, the free formaldehyde emission from the bonded 3-ply plywood with the AL/NPF was lower than that with the NPF, although the use of ML/NPF slightly increased the free formaldehyde emission.

Introduction

Lignocellulosic biomass including forestry biomass such as sawdust, bark, logging residues, and agricultural biomass such as cornstalk, wheat straw, rice straw and bagasse has been considered as an alternative source for chemicals and materials. Each year, billions of tons of agricultural biomass are produced globally and most of the crop straw is treated as agricultural waste and combusted (Zhao et al., 2020). Lignocellulosic biomass can be converted into bio-chemicals by direct liquefaction (Shi et al., 2019) and fractionation (Shui et al., 2016). Cornstalk contains 36–43 % cellulose, 28–43 % hemicellulose, and 13–19.00 % lignin (Mei et al., 2016; Sui and Chen, 2015; Boufi and Chaker, 2016) and it is a typical lignocellulosic biomass that is currently under utilized. Cornstalk can be well utilized separately with its polysaccharide derivatives stream and lignin products.

Lignin as the second most abundant biopolymer after cellulose is a renewable and sustainable source for aromatic chemicals on the earth. The global lignin production is expected to reach $913.1 million by 2025. Lignosulphonate (∼88 %) and kraft lignin (∼9%) are the two principal categories of technical lignin (Bajwa et al., 2019). Lignin is a complex polymer with a multifunctional nature, making it challenging to process. Currently, lignin is a vastly under-utilized polymer and mainly used for energy recovery in mills. Less than 2% of the available lignin was sold, primarily in the formulation of dispersants, adhesives and surfactants. However, abundant research on adding value to lignin have been reported. As a main side-product from pulping and cellulosic bio-ethanol industries, lignin displays complex structures depending on the plant species and isolation process.

A new category organosolv lignin (∼2%) is now gaining popularity due to the production of second generation biofuels (bioethanol production). The organosolv lignin segment is expected to experience the highest growth over the coming years, at an estimated growth rate of over 5% from 2016 to 2025 (Bajwa et al., 2019). Organosolv processes are considered more environmentally friendly than sulfite or Kraft pulping. Organosolv pulping employing organic solvents especially organic acids such as formic acid and acetic acid as the reagents for delignification of lignocellulosic biomass have gain significant attention for lignocellulosic biomass fractionation (Zhang et al., 2016; Li et al., 2016). During organosolv process, lignin is recovered from the spent solvent by precipitation, which generally involves adjusting concentration, pH and temperature. Most organosolv lignin is insoluble in water between pH 2 and 7 but dissolves in alkali and many polar organic solvents. In addition, organosolv lignin is generally of high-purity and low molecule weight, it has relatively narrow molecular weight distribution (Glasser et al., 1993). The structure of organosolv lignin is more homogeneous than that of other technical lignin such as kraft lignin or lignosulfonates (Turunen et al., 2003; Norgren and Edlund, 2014).

Lignin applications for polymer materials has attracted growing attention, as it is particularly suitable for producing various types of thermosetting resins such as phenolic resins, epoxy resins, etc (Xu and Ferdosian, 2017). Lignin has demonstrated a great potential for partially or even completely substituting phenol in phenolic resins synthesis as intensively reported over the past decades (Österberg et al., 2020; Pang et al., 2017; Mansouri and Salvado, 2006; Oliet et al., 2006; Kalami et al., 2017). In 2017, industrial production of lignin based phenolic resin in which 50 % phenol was replaced by lignin was reported by Prefere resin company (Prefere Resins, 2017). As biorefineries processing biomass advance, more useful and economically feasible lignin materials will become vastly and commercially available. In addition to replacing phenol during phenolic resin synthesis, lignin is reactive with PF resins and can crosslink with PF resin for various applications. For instance, addition of a small amount of lignin (3.4–9.4 %) during PF resole synthesis promotes the condensation reaction, which has been evidenced by the increased molar masses and high ratios of methylene bridges to the sum of free ortho and para aromatic groups with respect to the reference PF resin, and later lignin addition led to more methylene bridges (Turunen et al., 2003). Blending 20 % lignin into phenolic resin for auto-motive brake pads contributes to competitive advantages relative to the controls prepared with neat phenolic resin (Nehez, 1998).

In our previous studies, cellulose fractionated from cornstalk was successfully applied for sodium carboxymethyl cellulose synthesis (Shui et al., 2016, 2017). This study aims at utilizing cornstalk derived organosolv lignin for preparing bio-phenolic adhesive. The novelty of the manuscript is to develop a lignin based phenolic resin by activating organosolv lignin derived from cornstalk and directly formulating the activated lignin with neat phenol formaldehyde (NPF) resin for preparation of bio-based wood adhesive. Different from the traditional lignin-based phenolic resins development by replacing phenol with lignin during the PF resin synthesis, the new resins demonstrated in this study was produced by activation of organsolv lignin derived from cornstalk, and directly formulating the activated lignins as reactive prepolymers with an NPF to produce wood adhesives. Effects of lignin activation on lignin/PF adhesives in terms of curing properties, thermal stability, bonding strength as well as free formaldehyde emission from the bonded 3-ply plywoods are investigated.