Due to increasing concerns associated with environmental problems such as greenhouse gas emissions, bio-polymers have attracted great attention as a substitution for petroleum-based polymers [
1,
2]. Natural and renewable resources such as lignin [
3], starch [
4], poly lactic acid [
5], and cellulose [
6] are potential candidates for the synthesis of bio-based industrial polymers. Epoxy resins provide a wide range of applications including protective coatings, constructions, components for the electrical and electronic industry, composites, and adhesives due to having a combination of excellent chemical and mechanical properties [
7,
8]. In the presence of curing agents, epoxy resins form cross-linked networks in which the reactions of the curing process are irreversible. Several kinds of chemical compounds such as aliphatic and aromatic amines, anhydrides, mercaptans, and phenols can be used to cure epoxy resins [
9,
10,
11]. Each curing agent has a specific curing profile including the time and temperature, leading to the formation of epoxy networks possessing various physical and mechanical features. For instance, aliphatic amines can cure epoxy resin at room temperature, while the curing of epoxy resins using anhydrides requires higher temperatures [
12]. As petroleum-based aliphatic and aromatic amines are generally hazardous materials, they can potentially cause some health and environmental problems [
13,
14]. Therefore, it is necessary to synthesize and develop curing agents from renewable materials to replace petroleum-derived materials with bio-based compounds. Although several bio-based compounds that are able to cure epoxy resins have been found, their numbers are limited, and most importantly, their thermo-mechanical properties are not comparable with petroleum-based curing agents. For instance, furanic amines synthesized from polysaccharides and sugars suffer from low reaction yields [
15,
16,
17]. Moreover, most bio-based amines, which can be employed as curing agents are amino acids, which are synthesized by either enzymatic or fermentation methods [
18]. Lysine [
19,
20] and tryptophan [
21,
22] amino acids as curing agents in curing DGEBA have been reported, with studies suggesting that their glass transition temperatures (
Tg) and thermal degradation temperatures are significantly lower than those of petroleum-based curing agents. Furthermore, bio-based anhydrides, as curing agents for epoxy resin, can also be synthesized from various bio-sources such as terpene [
23] and rosin [
24]. Qin et al. [
25] used rosin-based anhydride to cure epoxy resin and the resulting epoxy system had both excellent thermal and mechanical properties. Tannic acid was used as a phenolic curing agent whose results showed that the tensile strength and
Tg of the cured epoxy system using this curing agent were lower than those of commercial systems [
26]. Aminated grape seed oil was utilized as a polyamine curing agent which formed a cured epoxy network possessing a very low
Tg (−38 °C), categorized as an amorphous thermoset polymer [
27]. Phenalkamine was extracted from cardanol by the Mannich condensation reaction in the presence of formaldehydes and amines, and it was applied as an amine curing agent to cure epoxy resins [
28,
29]. Although this type of curing agent is set to be deployed as a commercial bio-based curing agent, formaldehyde compounds are hazardous and carcinogenic reactants. Darroman et al. [
30] synthesized a new aromatic bio-based amine from cardanol without using formaldehyde, but the thermal and mechanical properties of its cured epoxy network were not satisfactory.
Wang et al. [
31] synthesized aminated lignin by the Mannich reaction and it was applied to decolorize anionic azo-dyes. Adding amine groups to lignin increases both lignin’s solubility in water and its reactivity with nucleophiles. Moreover, lignin-based amines synthesized via the Mannich reaction were used in waterborne polyurethanes [
32]. It was reported that various kinds of aminated lignin improved the mechanical properties and aging resistance of polyurethane systems. In another study, lignin, after being treated with epichlorhydrin and amination by the Mannich reaction, was used to adsorb heavy metals from aqueous solutions [
33]. By grafting alkaline lignin with methyl amine and formaldehyde, a Mannich base biosorbent was prepared [
34]. In addition, the Mannich reaction was applied to the amination of lignin for different purposes [
35,
36,
37,
38].
Lignin is the second most plentiful renewable polymer in the earth after cellulose and it can be extracted from wood and annual plants using various extraction methods [
39]. Lignin is classified as a phenolic polymer and its chemical structure includes phenylpropane units derived from three aromatic alcohol precursors (monolignols) consisting of
p-coumaryl, coniferyl, and sinapyl alcohols [
40,
41]. The chemical structure of lignin is shown in
Figure 1. In several studies, epoxy resins have been synthesized by the epoxidation of lignin [
42,
43,
44]. In addition, it has been used to synthesize phenolic resins, adhesives, polyolefins, and other miscellaneous applications [
45]. Pan et al. [
46] made a lignin-based epoxy resin from the reaction of epichlorohydrine with lignin and then epoxied lignin was reacted with propane diamine to create a cured epoxy system. The prepared lignin curing agent in the mentioned study possesses a low number of amine groups, requiring a co-curing agent for the full-curing of epoxy resin. Moreover, the lignin-based epoxy resins usually have a low
Tg and thermal stability because the low number of epoxy rings hinders the formation of a high crosslink density in the cured epoxy system. Therefore, the synthesis of lignin-based curing agents may be preferable to prepare high performance epoxy systems. In this regard, few studies have been done, revealing that the prepared lignin curing agent possesses a low amount of amine groups, requiring a commercial co-curing agent for the full-curing of epoxy resin [
47]. In this study, primary amine groups were directly introduced into the lignin structure using a synthesized nanocatalyst of cobalt/copper supported on nanoalumina to enhance the hydrogen amine equivalent of a lignin-based curing agent. Then, this synthesized lignin-based curing agent was used to cure the Diglycidyl Ether of Bisphenol A (DGEBA) epoxy resin, and its thermal and mechanical properties were investigated.