Tung Oil-Derived Epoxy Vitrimers with High Mechanical Strength, Toughness, and Excellent Recyclability

Abstract

The application of diglycidyl ether of bisphenol A (DGEBA) is severely limited due to the poor toughness of its thermosets, and the growing concern for sustainability calls for materials with recyclability. Thus, two tung oil-based compounds (DA-MA and TA) with different contents of carboxylic acid and anhydride were prepared and used to cure DGEBA to fabricate vitrimers, a kind of thermoset cross-linked through reversible covalent bonds. Test results show that vitrimers derived from tung oil (EP-DA-MA and EP-TA) exhibit excellent mechanical strength and toughness. In particular, the tensile strength, elongation at break, bending strength, bending toughness, and impact strength of EP-DA-MA are 63.17 MPa, 5.34%, 92.96 MPa, 1.09 kJ·m–2, and 7.73 kJ·m–2, respectively, while these parameters of the control, DGEBA cured with commercial methyl nadic anhydride, are 60.86 MPa, 1.77%, 78.94 MPa, 0.14 kJ·m–2, and 4.42 kJ·m–2. Meanwhile, high content of reversible ester bonds and secondary hydroxyl groups may facilitate the transesterification in the recycling process; thus, the crushed vitrimers of EP-DA-MA and EP-TA can be remolded without additional catalysts, and the tensile strength of the recycled samples can reach almost 100% of the original sample size, suggesting excellent malleability and recyclability. Therefore, the tung oil-derived epoxy vitrimers are promising candidates for utilization as adhesives, coatings, and matrix resins.

Readily Recyclable, Auto-Catalyzed Tung Oil-Derived Vitrimers and Carbon Fiber-Reinforced Composites

“…In contrast, for TOMA vitrimers, there is no tertiary amine catalyst system, and the curing process is only assigned to a direct reaction of the carboxylic acid with epoxy. Furthermore, the curing enthalpy (Δ H ) of TOMA vitrimers is significantly lower than that of TTMA vitrimers under a tertiary amine catalyst system …”

“…Due to the conjugated double bond structure of TTA and TO, both TTMA and TOMA were easily capped with dienophile by reacting with maleic acid to form TO-based acid curing agents and used to cure DGEBA by a ring-opening reaction . The structure change before and after the curing reaction of vitrimers was investigated using FTIR (Figure S4a,b).…”

“…Due to the conjugated double bond structure of TTA and TO, both TTMA and TOMA were easily capped with dienophile by reacting with maleic acid to form TO-based acid curing agents and used to cure DGEBA by a ring-opening reaction. 28 The structure change before and after the curing reaction of vitrimers was investigated using FTIR (Figure S4a,b). Obviously, the characteristic peaks of 910 (epoxy group) and 1704 cm −1 (carboxyl group) disappeared, and a broad peak appeared at 3450 cm −1 , indicating the two groups successfully reacted to form β-hydroxy ester structures.…”

Epoxy Thermosets Designed for Chemical Recycling

“…[51] Typically, such ester-based epoxy thermosets are obtained by the reaction between an epoxy precursor and an anhydride [52] or a carboxylic acid (Scheme 2). [53] Secondary and tertiary ester linkages containing epoxy thermosets can be degraded upon heat treatment even without any catalyst; however, their thermal performance (i. e., thermal decomposition temperature (T d5 % )) is limited. [54,55] In contrast, primary ester-containing networks are more thermally stable and generally require chemical treatment to be disconnected.…”

Readily recyclable, high-performance catalyst-free tung oil-derived vitrimer and carbon fiber reinforced composites

“…Due to the conjugated double bond structure of TTA and TO, both TTMA and TOMA were easily capped with dienophile by reacting with maleic acid to form TO-based acid curing agents, and used to cure DGEBA by a ring-opening reaction [28]. The structure change before and after curing reaction of vitrimers was investigated using FTIR (Fig S3a-b).…”