Renewable and flexible thermosetting epoxies based on functionalized biorefinery lignin fractions

Highlights

• Industrial lignins were refined into well-defined and characterized fractions

• Novel thermoset resin materials were fabricated with modified lignin samples

• Refined lignins were introduced with oxirane moieties through epichlorohydrin

• Modified lignin fractions were cross-linked with flexible polyether diamines

• Lignin-based thermoset resins with enhanced flexible and mechanical properties

Abstract

Epoxy resin materials are spread all over the daily life with their excellent physical, mechanical, and insulating properties. However, the biologically toxic bisphenol A used in epoxy resins formulations has brought a long lasing environmental problem. It is therefore urgent to design and develop more biodegradable alternatives to mitigate the plastic menace. Lignin is an abundant biopolymer with great potential to replace petroleum-based chemicals; however, its valorization is commonly limited because of the heterogeneity. In this regard, a green and simple strategy to fabricate renewable and flexible lignin-based thermosetting epoxies with enhanced mechanical strength has been developed. Specifically, industrial biorefinery lignins are firstly fractioned through a green and simple gradient precipitation and then modified with introduced epoxy groups into the lignin macromolecule under a mild reaction condition. The cross-linking treatment facilities improved interfacial bonding forces between modified lignin fractionations and bisphenol A diglycidyl ether (BADGE) with varied contents (5–15 wt%). The fabricated cured epoxy thermosetting plastic exhibits an enhanced tensile strength (29.7%) and elongation (26.8%) as compared to those of the pure commercial BADGE polymer. We envision that the present strategy provides a new possibility for lignin valorization and design of high-performance flexible thermosetting epoxies for remarkable multifunctionality.

Introduction

Lignin is the most abundant and renewable polyphenolic constituent in nature and is widely regarded as a sustainable biopolymer alternative to petroleum [[1], [2], [3]]. Although large quantities of lignin are yearly available, the heterogeneity and structural complexity restrain lignin's utilization for industrial applications [4,5]. The largest source of lignin is discharged as a by-product of the pulp and paper industry or biomass refineries, of which less than 2% of the 150–180 million tons is used for value-added products such as concrete additives, surfactants, and dispersants [[6], [7], [8]]. Lignin is a typically recalcitrant biopolymer that has an intrinsic property and heterogeneous structure. Nevertheless, it is inexpensive and has manifold excellent properties, such as high thermal stability, high carbon content, biodegradability, and favorable stiffness [9]. To fully exploit the potential of lignin, it is urgent to solve the issues of its variability, because industrial processes require replicable batches of lignin to be converted into commodities and functional materials [[10], [11], [12], [13], [14]]. Moreover, even with the same source and procedure, the resulting lignin may still present different characteristics. At present, three lignin fractional purification methods, including solvent fractionation, membrane fractionation, and pH-dependent precipitation, have been developed. Notably, lignin fractionation through multiple organic solvents restrains its industrial utilization because of the limits of time consuming and high cost [15]. The membrane fractionation method shows shortcomings of membrane fouling and expensive cost. Currently, the pH-mediated fractionation process exhibits the merits of green, practical, and economic advantages [10,16].

Recently, the synthesis of bio-based thermosetting materials furnishes a broad application prospect for lignin, and a series of bio-based aromatic polyphenols have been investigated to prepare epoxy resin materials as potential substitutes for bisphenol A (BPA) [[17], [18], [19]]. Bisphenol A (BPA) is produced by the condensation of petrochemically derived phenol and acetone in an acidic medium. It has been pointed out that BPA has a low biological toxicity, and its price is on an upward trend recently [20]. Epoxy resin is a kind of high molecular polymer that is widely used in the production of thermosetting materials in various fields, including composite materials, metal coatings, adhesives, and electronic components [[21], [22], [23]]. Bisphenol A diglycidyl ether (BADGE) is obtained as a typical epoxy resin through the reaction of BPA and epichlorohydrin with different ratios. In particular, epoxy thermosets are extremely inert and hard to be completely degraded in natural environments, thus increasing research concentrations on producing these multifunctional materials from renewable resources [24]. Feghali et al. [25] reported a novel method for preparing biobased epoxy thermoset polymers from lignin hydrogenolysis oils. The depolymerized lignin oils were first obtained from in situ Pd-catalyzed mild hydrogenolysis of native hardwood lignin, secondly reacted with epichlorohydrin to produce biobased epoxy resins, then blended with BADGE, and finally curd with diethylenetriamine. The functional groups of lignin macromolecule, such as phenolic and aliphatic hydroxyl groups as well as carboxylic acid, facilitate its chemical modification. Therefore, the resulting formed oligomer or prepolymer with a specific moiety can promote the chemical crosslinking (curing) properties [26]. From the perspective of plastic replacement, through fractionation and chemical functionalization, the sustainable structural lignin with homogeneity and narrow polydispersity can serve as a reproducible and tenable epoxy resin precursor [27,28]. However, it is still a challenge to make macroscopic epoxy resin with tailor-made biopolymer from industrial lignin due to its complicacy and heterogeneity.

Here, we have developed a green and effective approach for the synthesis of lignin-based thermoset resin materials with enhanced mechanical strength. The structure–property relationships between lignin characteristics and thermomechanical performance of those biodegradable resins were well discussed. This study aims to utilize industrial lignin on a large scale and propose a strategy to support the long-term industrialization of lignin-based thermoset plastics after fractionation and characterization of industrial lignin. Accordingly, we first fractioned the industrial biorefinery lignin through a green and simple gradient precipitation with reduced lignin heterogeneity. We combined the state-of-the-art nuclear magnetic resonance (NMR) technologies [¹³C, ³¹P, and two-dimensional (2D) heteronuclear single quantum coherence (HSQC)], gel permeation chromatography (GPC), Fourier-transform infrared spectroscopy (FT-IR), and thermogravimetry to comprehensively reveal the structural feature of industrial lignin fractions. Subsequently, a mild reaction condition was used to introduce epoxy groups into the lignin macromolecule. Notably, the cured epoxy thermosetting plastics, which were mixed with the epoxidized lignin and commercial BADGE, were then cross-linked with polyether amine showed a more pronounced ductility. From an environmental and economic perspective, it is incredibly valuable to develop and utilize such lignin substitute products in composite materials.