Formaldehyde-Free Method for Incorporating Lignin into Epoxy Thermosets

Abstract

A series of liquid and curable lignin-containing epoxy prepolymers were prepared for making renewable epoxy thermosets. First, lignin was modified to phenolated lignin (PL) in a solvent-free reaction. PL was subsequently co-oligomerized with salicyl alcohol (SA) in water without the use of formaldehyde to obtain fully bio-based polyphenols (PL-SA). Glycidylation of lignin-based polyphenols yielded exclusively liquid epoxy prepolymers without production of solid co-products. The liquid epoxy prepolymers (with lignin content up to 21 wt %) were curable with amine hardener (diethylenetriamine) to generate homogeneous thermosets, which required no epoxy co-prepolymer. The structural evolution from starting monomers to epoxy thermosets was followed by nuclear magnetic resonance and Fourier transform infrared spectroscopy. Compared to common syntheses in which lignin is glycidylated prior to being blended with epoxy co-prepolymers, the herein reported methodology conferred networks with increased α-relaxation temperature (106–114 vs 96 °C), storage modulus (1843–2151 vs 1828 MPa), cross-link density (8.2–16.0 vs 5.4 mmol/cm3) and tensile properties (stress of 66.9–68.1 vs 28.7 MPa, and strain of 3.3–3.7 vs 1.4%). Moreover, bio-based thermosets exhibited comparable or superior thermomechanical properties to conventional bisphenol A (BPA)-based counterpart. By producing liquid-phase lignin-containing epoxy prepolymers, this study provides a formaldehyde-free method for incorporating lignin into epoxy thermosets without the need for additional co-prepolymers.

New Reactive Isoeugenol Based Phosphate Flame Retardant: Toward Green Epoxy Resins

“…Thus, in the view of decreasing the environmental impact, saving petroleum resources and replacing toxic BPA for the synthesis of epoxy, bio-based thermosets are widely studied. During the last decades many research works have therefore been dedicated to the specific domain of bio-based epoxy thermosets pointing out the potential of rosin, 4 cardanol, 5 iso-eugenol, 6,7 eugenol, [8][9][10][11][12] , vegetable oil, 13 modified lignin, [14][15][16] vanillin, [17][18][19] terpene derivatives, 20 daidzein, 21 ... to replace DGEBA epoxy monomer. Among the most promising feedstocks to replace BPA, building blocks derived from lignin, have been particularly pointed out.…”

New Eco-Friendly Synthesized Thermosets from Isoeugenol-Based Epoxy Resins

“…This way induces a variability of the resulting materials properties since the content of lignin in different types of biomass resources varies strongly. An alternative approach consists of lignin fragmentation [10][11][12] leading to a wide range of building blocks, such as vanillin [13][14][15], rosin [16], eugenol [17][18][19][20] and isoeugenol [21,22], which are then epoxidized.Within this context, our research team has recently described both the synthesis of an epoxy monomer derived from isoeugenol, named as GlycidylEther Epoxy isoeugenol (GEEpiE) [21], and its curing with camphoric anhydride [22]. Behaviours of the resulting thermoset are consistent with its use as a bio-based epoxy matrix for structural composites.…”

Recent Advances in the Application of Functionalized Lignin in Value-Added Polymeric Materials

“…In the preparation of lignin-incorporated novolac polyphenols, Zhao et al liquified the solid organosolv lignin by successive demethylation, phenolation, phenol-formaldehyde reaction and glycidylation, yet the lignin content was limited to 12 wt% due to the poor compatibility [ 37 ]. To solve this problem, the organosolv lignin was first phenolated with catechol to break down the lignin backbone and increase the number of hydroxyl groups, and subsequently, the catechol ortho site of the phenolated lignin was condensed with salicyl alcohol in water without using formaldehyde as the coupling agent ( Figure 6 ) [ 38 ]. In the cured epoxy thermosets, the final lignin content was up to 19 wt% and total biomass content including catechol and salicyl alcohol derived from renewable sources reached 65–69 wt%.…”