Chemical Reactivity and the Influence of Initiators on the Epoxidized Vegetable Oil/Dicarboxylic Acid System

Abstract

In a sustainable development context, epoxidized vegetable oils (EVO) have unlimited and promising future prospects as renewable and environmentally friendly feedstock. The only drawback to their use is their low and non-selective reactivity compared to the aromatic epoxides. Properly, a small optimized amount of “true” initiators can overcome this issue and also beneficially serve in properties such as glass transition, modulus, strength, elongation at break, and chemical resistance. This paper presents efforts to understand and identify the initiator’s effect to more accurately predict how to select a good initiator on EVO/dicarboxylic acid systems. A new bio-based reprocessable epoxy resin was prepared from epoxidized linseed oil (ELO) and 2,2′-dithiodibenzoic acid (DTBA). The evolution of the chemical structures and the reactions’ mechanisms have been systematically studied by in situ Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopies and differential scanning calorimetry (DSC). A screening of 10 initiators was performed for the ELO/DTBA cross-linking reaction. The influence of the initiator’s structure, basicity, and nucleophilicity was assessed and ranked in terms of the kinetic response including the epoxy–acid reaction rate and the percentage of functional group consumption. An excellent effect achieved by imidazole as an initiator was demonstrated. An attempt has been proposed to corroborate the experimental values with the results of quantum chemistry calculations.

Monitoring the structure–reactivity relationship in epoxidized perilla and safflower oil thermosetting resins

“…Previously, we demonstrated that the use of imidazole as initiator provides efficient epoxy/acid crosslinking, generating thermosets with tunable thermomechanical properties. 45 In the present study, we selected 3 different EVO monomers with three levels of epoxide content varying from 3.7 to 8.0 equiva-lents per mole. The curing of these 3 EVOs with 2 dicarboxylic acids (DCAs), namely 2,2′-dithiodibenzoic acid (DTBA) and diphenic acid (DPA), was investigated by in situ FT-IR analysis.…”

Building thermally and chemically reversible covalent bonds in vegetable oil based epoxy thermosets. Influence of epoxy–hardener ratio in promoting recyclability

“…The initiator, imidazole, was used at an optimal content of 1 wt%, as previously determined. 13 To prepare the reactive mixtures, EVO was heated at 80 1C to decrease its viscosity and thereafter mixed with the initiator and the hardener. The obtained mixtures were placed into silicone moulds and cured in an oven.…”

“…There is scarce information regarding EVO curing with dicarboxylic acids. [7][8][9][10][11][12][13] However, an important aspect of the epoxy/acid reaction is the formation of b-hydroxyesters that are responsible for transesterification reactions, which allow the recyclability of the polymeric networks. 14,15 Another reaction favouring network chain reorganization is the metathesis of disulfide covalent bonds by a dynamic exchange in the main or side chains, usually triggered by external stimuli, such as pressure or temperature, known as the disulfide exchange reaction.…”

“…2,2 0 -Dithiodibenzoic acid (DTBA) was used as a dynamic hardener. Our team already studied how the initiators influence the copolymerization of ELO/DTBA, 13,21 but, to the best of our knowledge, this study reports for the first time the effect of the ratio of the thermoset systems on the chemical recyclability and the thermal reprocessing properties. The epoxy/acid reactions were monitored via differential scanning calorimetry (DSC) and infrared spectroscopy (FT-IR) analyses.…”

Influence of the Presence of Disulphide Bonds in Aromatic or Aliphatic Dicarboxylic Acid Hardeners Used to Produce Reprocessable Epoxidized Thermosets

“…The polymer networks were prepared by reacting the EVOs with diacid hardeners in different R e/a functional ratios epoxy groups/acid groups: R e/a = 2 (e/a = 1/0.5), R e/a = 1.25 (e/a = 1/0.8) and R e/a = 1 (e/a = 1/1). In all the formulations, IM was used as the initiator as we recently showed its good selectivity on epoxy/acid copolymerization and also the positive effect on the thermoset’s recyclability [ 20 , 21 , 22 , 23 , 24 ]. According to previous results [ 21 ], the initiator was used in 1 wt.% of the total amount of the reactive mixture, as the optimal amount for curing.…”