On a Biobased Epoxy Vitrimer from a Cardanol Derivative Prepared by a Simple Thiol-Epoxy "Click" Reaction

Abstract

The development of this work lies in the relevant interest in epoxy resins, which, despite their wide use, do not meet the requirements for sustainable materials. Therefore, the proposed approach considers the need to develop environmentally friendly systems, in terms of both the starting material and the synthetic method applied as well as in terms of end-of-life. The above issues were taken into account by (i) using a monomer from renewable sources, (ii) promoting the formation of dynamic covalent bonds, allowing for material reprocessing, and (iii) evaluating the degradability of the material. Indeed, an epoxy derived from cardanol was used, which, for the first time, was applied in the development of a vitrimer system. The exploitation of a diboronic ester dithiol ([2,2'-(1,4-phenylene)-bis[4-mercaptan-1,3,2-dioxaborolane], DBEDT) as a cross-linker allowed the cross-linking reaction to be carried out without the use of solvents and catalysts through a thiol-epoxy "click" mechanism. The dynamicity of the network was demonstrated by gel fraction experiments and rheological and DMA measurements. In particular, the formation of a vitrimer was highlighted, characterized by low relaxation times (around 4 s at 70 °C) and an activation energy of ca. 48 kJ/mol. Moreover, the developed material, which is easily biodegradable in seawater, was found to show promising flame reaction behavior. Preliminary experiments demonstrated that, unlike an epoxy resin prepared from the same monomer and using a classical cross-linker, our boron-containing material exhibited no dripping under combustion conditions, a phenomenon that will allow this novel biobased system to be widely used.

Figure 1

Scheme of the cardanol-based epoxy (EC) cross-linking reaction.

Figure 2

(A) FT-IR spectra of (a) neat mixture EC–DBEDT and (b) thermally treated EC–DBEDT mixture (EC–DBEDT_T). (B) Scheme of the “click reaction” between EC and DBEDT. (C) Photos of left neat EC–DBEDT_T and right swollen EC–DBEDT_T in anhydrous toluene.

Figure 3

(a) DSC traces of blue EC and orange EC–DBEDT_T. (b) TGA curve of blue EC and orange EC–DBEDT_T.

Figure 4

DMTA results (storage modulus, tan δ, and length as a function of temperature) for EC–DBEDT_T.

Figure 5

(a) Elastic (G′) and viscous (G″) moduli and (b) complex viscosity as a function of frequency measured by dynamic frequency sweep tests performed at different temperatures.

Figure 6

(a) Stress relaxation curves; (b) variation of the stress relaxation time vs inverse temperature; and (c) boronic ester metathesis exchange reaction.

Figure 7

(a) Photo of the broken sample and recycled film. (b) TGA curves of the black neat EC–DBEDT_T sample and orange EC–DBEDT_T after the recycle step. (c) FT-IR curves of the black neat EC–DBEDT_T sample and orange EC–DBEDT_T after the recycle step. (d) BOD of EC (blue), EC–DBEDT_T (orange), and microcrystalline cellulose (biodegradable reference) (gray).

Figure 8

Photos of the combustion of (a) vitrimer EC–DBEDT_T and (b) Pripol-based resin.