Abstract
In this study, lignin, an aromatic compound from the forestry industry, was used as a renewable material to synthesize a new aromatic amine curing agent for epoxy resin. Firstly, lignin was separated from black liquor and hydroxyl groups were converted to tosyl groups as leaving groups. Then, primary amination was conducted using an ammonia solution at high pressure and temperature, in the presence of a nano-alumina-based catalyst. The structure of the nanocatalyst was confirmed by FT-IR, ICP, SEM, and XPS analyses. According to the FT-IR spectra, a demethylation reaction, the substitution of hydroxyl groups with tosyl groups, and then an amination reaction were successfully performed on lignin, which was further confirmed by the 13C NMR and CHNS analyses. The active hydrogen equivalent of aminated lignin was determined and three samples with 9.9 wt %, 12.9 wt %, and 15.9 wt % of aminated lignin, as curing agents, were prepared for curing the diglycidyl ether of bisphenol A (DGEBA). The thermal characteristics of the curing process of these epoxy samples were determined by DSC and TGA analyses. Moreover, the mechanical performance of the cured epoxy systems, e.g., the tensile strength and Izod impact strength, were measured, showing that in the presence of 12.9 wt % aminated lignin, the mechanical properties of the aminated lignin-epoxy system exhibited the best performance, which was competitive, compared to the epoxy systems cured by commercial aromatic curing agents.
Highlights
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]
The synthesized cobalt and copper nanoparticles were trapped in the porous spaces, catalyzing the amination reaction without the dispersion of the reaction in the environment of [51]
Lignin functional groups were firstly converted to hydroxyl groups through a demethylation process, before being converted to a tosyle-lignin intermediate
Summary
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]. 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]. As petroleum-based aliphatic and aromatic amines are generally hazardous materials, they can potentially cause some health and environmental problems [13,14]. It is necessary to synthesize and develop curing agents from renewable materials to replace petroleum-derived materials with bio-based compounds. 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. 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 )
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