Sustainable alternative for bisphenol A epoxy resin high-performance and recyclable lignin-based epoxy vitrimers
Graphical abstract
Introduction
Lignin is the most abundant resource of renewable aromatic structures in the world, and has been wildly studied because its low-cost and highly branched structure, which is considered as the most promising renewable natural polymer to replace petroleum-based chemicals (Chatel and Rogers, 2016; Sun et al., 2018). As a type of lignin, enzymatic hydrolysis lignin (EL) has received increasing attention with the rapid development of research on biomass chemicals and bioethanol fuels. EL is obtained by extracting residues from plant stalks and corn cob fermentation of fuel ethanol. Compared with traditional kraft lignin or ligninsulfonate, EL retains the chemical structure of lignin and exhibits greater chemical activity since it has not been treated with high temperature, high pressure, strong acid and strong base (Jin et al., 2010; Yelle et al., 2012). Meanwhile, EL has been used to prepare a large number of polymer materials, such as phenol-formaldehyde resins (Jing et al., 2015), epoxy resins (Wang et al., 2018), adhesives (Jin et al., 2010) and polyurethane (Xue et al., 2019).
Bisphenol A (BPA) epoxy resin is currently the most widely used epoxy resin for their high crosslink density, great rigidity, good excellent electrical insulation and chemical resistance (Auvergne et al., 2014). There is a growing demand in replacing bisphenol A with renewable resources, for the reason that BPA act an endocrine disruptor would cause health and environmental problems (Michałowicz, 2014). Owing to the presence of phenolic groups in the structure of lignin, the replacement BPA with lignin in the synthesis of epoxy resin has attracted increasing attention over the past decades (Guo et al., 2019; Ono et al., 2019; Sun et al., 2018). However, the high molecular mass and low reactivity of raw lignin usually limits its utilization. Thus, chemical modification of lignin is necessary for its applications. Phenolation (Zhang et al., 2019), esterification (Guo et al., 2019) and solvent extraction (Nagatani et al., 2019) have been reported as the effective lignin modification method, and the results showed that aromatic structure of lignin could significantly improve the rigidity and thermal stability of the epoxy resin. But there is still a problem that cannot be ignored, due to the permanent crosslinking network structure inside the epoxy resin, which make it a great challenge to reprocess or remold after their initial formation.
Recently, vitrimers become an important topic because the existence of dynamic covalent bonds in crosslinked network structures, which can solve the insoluble and infusible problems of traditional epoxy resins after curing (Aaontarnal et al., 2011; Legrand and Soulié-Ziakovic, 2016). Vitrimers are permanent covalent crosslinking network which rearrange the topology via thermally stimulated exchange reactions under guaranteed the cross-link density(Bowman and Kloxin, 2012; Denissen et al., 2015), this specific feature enables vitrimers to be processed like vitreous glass without losing network integrity(Zhao et al., 2019). Among recognized dynamic bonds, transesterification is the most studied vitrimer chemistry on account of it is conveniently applied to epoxy/acid and epoxy/anhydride curing systems(Altuna et al., 2013; Tran et al., 2018; Zhao and Abu-Omar, 2019). In cross-linked polymer network, the hydroxyl groups resulting from the ring-open reaction of epoxies can induce the dynamic transesterification with the ester linkages at high temperatures, which can impart epoxy resin be reprocessed and repaired.
There are several lignin and its derivatives-based vitrimers have been reported based on above mechanism, since they possessing abundant hydroxyl groups which can be modified to epoxy groups, such as, Liu et al. (Liu et al., 2017) synthesized vitrimeric materials by eugenol-derived epoxy with succinic anhydride in the presence of zinc-containing catalysts. Zhao et al. (Zhao and Abu-Omar, 2019) developed a recyclable and malleable vanillin-based epoxy thermoset by imine exchange reactions, and Zhang et al. (Zhang et al., 2018). prepared a lignin-based vitrimer material through transesterification reactions. The synthesis of the aforementioned vitrimers confirms the possibility of preparing biobased vitrimers from lignin and its derivatives. However, the irregular and complex chemical structure of lignin leads to poor dispersion and reactivity of lignin-based epoxy vitrimers (LEVs), making the mechanical properties of LEVs much lower than those of its derivatives (vanillin and eugenol)-based vitrimers. which makes it difficult to achieve high lignin replacement rate, excellent mechanical property and reprocessability in a facile method at the same time.
In this work, lignin-based epoxy vitrimers with excellent mechanical properties were developed through glycidyl etherified enzymatic hydrolysis lignin (GEL)/diglycidyl ether bisphenol A (DGEBA) with dodecanedioic acid as curing agent in the presence of zinc catalyst. Our research strategy was to gradually replace DGEBA with GEL in similar benzene ring structure for synthesizing the LEVs. The mechanical properties, self-healing reprocessing and shape memory property of the LEVs can be tuned by different ratios of GEL and DGEBA. The method for preparing LEVs shows one simple and effective approach to prepare lignin based high value-added and functional materials.
Section snippets
Materials
EL was purchased from the Longlive Biological Technology Co.Ltd. (Shandong, China). The chemical characteristics of the lignin has been reported in our previous paper (Xue et al., 2020). In brief, The Mn of lignin is 1530 g/mol, Mw is 2560 g/mol, polydispersity index is 1.67, total hydroxyl content is 7.6 mmol/g, phenolic hydroxyl content is 2.73 mmol/g, and carboxyl content is 0.85 mmol/g. DGEBA, dodecanedioic acid (DEA), 1,2,3-propanetricarboxylic acid (PTA), tetrabutylammonium bromide and
Synthesis and characterizations of GEL and LEVs
The synthetic routes to the LEVs are shown in Scheme 1a, at first, GEL was produced by EL reacted with epichlorohydrin, excess epichlorohydrin was used as solvent to reduce the viscosity and hydrolyzable chlorine content in epoxy prepolymers in the modified progress (Vargiu et al., 1979). The EEW of GEL was measured to be 0.34 mol/100 g by the titration method. And then, GEL and DGEBA were mixed homogeneously through a simple machine. Finally, the mixture of DGEBA and GEL were cured with DEA,
Conclusion
In summary, a high mechanical strength and lignin replacement rate of lignin-based vitrimers were successfully synthesized by gradually replacing DGEBA with glycidyl ether of lignin. The LEVs systems possess three distinct characteristics. Firstly, the tensile strength of vitrimers can reach 46.8 MPa, which the highest strength of lignin-based vitrimers that have been reported, and the mechanical property of vitrimers can be adjusted by changing the ratio of GEL to DGEBA. Secondly, the highest
CRediT authorship contribution statement
Bailiang Xue: Conceptualization, Methodology, Writing - review & editing, Funding acquisition, Project administration. Rui Tang: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing, Visualization. Danwei Xue: Formal analysis, Validation, Visualization. Ying Guan: . Yongchang Sun: Resources, Validation. Wei Zhao: Data curation. Jiaojun Tan: Data curation. Xinping Li: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors wish to express their gratitude for the grants from the National Key Research and Development Program of China (2017YFB0307903), Natural Science Foundation of China (21706154), and the Foundation of Key Laboratory of Pulp and Paper Science and Technology of Ministry of Education of China (KF201916).
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