Bisphenol A-based epoxy thermosets
involve both environmental and
health risks. By reacting a vegetal oil-based epoxide with an aromatic
diacid containing S–S bonds a thermoset is produced. Herein,
reprocessable thermosets were synthesized, the recyclability being
designed through a dual mechanism: that of disulfide metathesis and
of transesterifications. To assess the feasibility of the reprocessing,
a series of 10 initiators was tested to probe their effect not only
on the cross-linking reaction but also on the recyclability. This
study introduces for the first time the key role of the initiator
on the material performance and on their reprocessing. A very good
reprocessability was obtained for thermosets prepared using as initiator
the imidazole. Moreover, the thermosets exhibit complete chemical
recyclability in 1 N NaOH at 80 °C, after 3 days, without needing
additional chemicals. The reprocessed materials have similar performance
with the virgin ones, even after 10 cycles of reprocessing.
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.
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