Epoxy Vitrimers Research Articles

High-Performance Epoxy Vitrimers with the Joint Action of Dual Dynamic Covalent Bonds

High-Performance Epoxy Vitrimers with the Joint Action of Dual Dynamic Covalent Bonds

Covalent adaptable network offers “sustainable” closed‐loop circularity in epoxy vitrimers

AbstractTraditional Epoxies take hours to cure before their properties can be realized and this restricts its use in applications where quick turn‐around time is imperative. Herein, we developed fast curing Epoxies (which cure within 2 h) using Vitrimer chemistries. A unique dynamic curing agent (bio‐based Schiff motif) consisting of an aldehyde (here, cinnamaldehyde) and an amine (here, mussel inspired dopamine) is used to facilitate imine exchange during the curing process and maintains the network integrity‐ a key requirement to achieve excellent structural properties in epoxy based components. By incorporating this covalent adaptable network, various epoxies were designed which can cure within 2 h. The resulting epoxy vitrimers exhibited high tensile strength (68 MPa), exceptionally high Tg (128°C), fast stress relaxation (a relaxation time of 15.2 s at 180°C) and an activation energy of 105 kJ/mol. This associative covalent adaptable network (imine exchange) also rendered the epoxies re‐processable and re‐usable and offers closed‐loop circularity in plastics. The resulting epoxy also exhibited self‐healing property triggered by the exchangeable bonds. The self‐healing behavior of this epoxy was manifested from the surface conductivity measurements wherein the conducting pathway (facilitated by spraying conducting particles on the vitrimer epoxy substrate) was restored post self‐healing in substrates containing epoxy vitrimers. In addition, the epoxies cured using this Schiff based motif dissolves in a specific solvent and can be recovered and re‐purposed. The excellent mechanical properties achieved by retaining the network integrity together with the self‐healing properties offers myriad applications for the designed epoxy cured using bio‐based Schiff motif.Highlights Developed fast curing Epoxies that cures in 2 h using Vitrimer chemistries. Fully bio‐based hardener was used to realize the network topology. The dynamic exchange reaction results in re‐processability, reusable, recyclable and self‐healable that is not possible in the case of traditional epoxy. It exhibited high tensile strength, exceptional Tg, fast stress relaxation and an activation energy of 105 KJ/mol.

Preparation and characterization of novel epoxy Vitrimer asphalt binders based on reversible covalent network

Although epoxy asphalt possesses excellent mechanical properties, good fatigue resistance, and high durability, it is not recyclable. The aim of this study is to apply the principle of renewable Vitrimer materials, using zinc acetate catalyst and multi-organic carboxylic acids for curing epoxy asphalt. Several epoxy asphalt materials are prepared including dynamic hydroxyl-ester covalent bond epoxy Vitrimer asphalt (DHVE-EVA), dynamic disulfide covalent bond epoxy Vitrimer asphalt (DTS-EVA), and double dynamic covalent bond epoxy Vitrimer asphalt (DTS-DHVE-EVA), and their physical and chemical properties are systematically investigated. Fourier transform infrared characterization studies reveal that zinc ions promote the participation of ester groups in the complexation reaction, thus facilitating the exchange reaction of the ester dynamic covalent bonds. The results of the thermogravimetric analysis show that DHVE-EVA has the best thermal stability, whereas DTS-EVA has inferior thermal stability due to the susceptibility of the S-S bond tends to break when undergoing heating. The thermal stability of DTS-DHVE-EVA is in between DHVE-EVA and DTS-EVA. Investigation into the mechanical properties shows that DHVE-EVA has better usability compared with DTS-EVA and DTS-DHVE-EVA. The tensile strength of DHVE-EVA can still be maintained at 3.0 MPa when its maximum tensile ratio reaches more than 300%. Scanning electron microscope images show that DHVE-EVA exhibits a plastic fracture morphology, while DTS-EVA specimens exhibit a brittle morphology. In addition, thermal regeneration tests show that DHVE-EVA can achieve self-healing at high temperatures and can recover more than 98% of its tensile strength by hot pressing after crushing. These results indicate that DHVE-EVA has good regenerative properties, which demonstrates its potential for engineering applications.

Reprocessed, shape-memory and self-healing robust epoxy resin by hindered urea bond

Given the fossil fuel energy crisis and serious environmental pollution in recent years, one way to alleviate the problem is to introduce dynamic covalent chemistry into thermosetting resins to give the material processable and recyclable properties, creating a new type of eco-friendly material. Hindered urea bonds can undergo reversible exchange reactions at high temperatures to decompose into amine and isocyanate. This reversible addition reaction is currently a prominent research focus in the development of reprocessable thermosets. In this work, we synthesize a novel curing agent with hindered urea bonds without any catalysts. Then characterize the resulting epoxy vitrimers based on hindered urea bond (HUG-EP) using FT-IR and 1H NMR, confirming the successful introduction of hindered urea bonds into the epoxy resin. A variety of tests are carried out, such as those to assess thermal stability, swelling, shape-memory, re-processability and tensile strength. The results demonstrate that HUG-EP can rapidly self-healed within 20 min at 160 °C, with the tensile strength was maintained at 87.15 % of the original during the welding experiments. Furthermore, it can also be reprocessed by hot-pressing several times at 160 °C. Thus, the introduction of hindered urea bonds into the crosslinked network proves to be an effective and practicable approach for producing thermosetting resins with reprocessing, self-healing, and shape memory capabilities.

Well-dispersed silica nanoparticle-reinforced epoxy vitrimer composites prepared by dynamic crosslinking

Well-dispersed silica nanoparticle-reinforced epoxy vitrimer composites prepared by dynamic crosslinking

Additive Manufacturing of Shape Memory Thermoset Composites with Directional Thermal Conductivity

AbstractShape memory epoxy vitrimers and their composites are candidate materials for multiple engineering applications due to the commercial availability of their precursors combined with their functionality, mechanical properties, and recyclability. However, the manufacturing of vitrimer composites through conventional mould‐casting limits the flexibility in the design of complex parts. In this work feedstock inks are formulated based on reduced graphene oxide and hexagonal boron nitride (hBN) to 3D‐print epoxy vitrimer‐based composites by direct ink writing (DIW). The introduction of hBN platelets (up to 22 vol.%) and their alignment during printing enhances the fracture resistance of the material and induces directional thermal transport. The in‐plane thermal conductivities (3 W m−1 K−1) are nearly one order of magnitude higher than the matrix material. The high conductivity results in faster actuation times and can be combined with the printing process to build structures designed to manage heat flow.

Construction of robust and fully bio-based epoxy vitrimers with facile malleability, self-healing ability, and recyclability from peach gum polysaccharide and itaconic acid

Fully bio-based epoxy vitrimers have been explored for sustainable industrial development and environmental conservation. However, the manufacture of fully bio-based epoxy vitrimers with high mechanical properties and thermostability remains a challenge. Here, for the first time, a novel strategy to fabricate natural peach gum-based epoxy vitrimers with the superior mechanical properties and thermostability comparable to commercial epoxy resin is provided by using peach gum and itaconic acid. The strategy includes the synthesis of peach gum polysaccharide with terminal carboxyl (PGP-COOH) and itaconic acid derivative with terminal epoxy (EIA). Fully bio-based epoxy networks with dynamic β-hydroxy ester bonds (PGE) were obtained by a reaction between PGP-COOH and EIA and the structures of PGE were confirmed by Fourier transform infrared (FTIR) analysis. The mechanical properties and thermostability of PGE networks were studied by tensile testing and Thermogravimetric (TG) analysis. Additionally, the self-healing, physical reprocessing and chemical recycling performances of PGE networks were evaluated. Finally, PGE is applied in recycled adhesion for different substrates. This work demonstrates that the PGE networks possess comparable mechanical properties and thermostability to the commercial epoxy resin. This study gives a novel approach to prepare fully bio-based epoxy thermosets with high performances.

Catalyst-free epoxy vitrimers from rosin: Highly mechanical performance, fast self-healing, and facile recycling

Recyclable epoxy vitrimers from biomass are significant for sustainable industrial development. However, the reprocessing of epoxy vitrimers comes at the cost of their mechanical properties and thermal stability and it is a challenge to obtain epoxy vitrimers with facile recycling ability, high mechanical properties and thermal stability, simultaneously. Present work offers a way to prepare the high-performance epoxy vitrimers with dual dynamic β-hydroxy ester bonds and boroxines from rosin derived glycidyl ester epoxy resin and 3-aminobenzeneboronic acid. The presence of tertiary amines and dynamic β-hydroxy ester bonds, as well as dynamic boroxines in the network accounts for the effective malleable behavior without the assistance of a catalyst. The resultant rosin-based epoxy vitrimers possess the Tg around ∼83.2 °C with the Ts above 170 °C, the E′ of 36.1 MPa and the tensile strength around ∼60.9 MPa comparable to commercial BPA epoxies, which attribute to the rigid rosin structure and high crosslink degree. The integrated self-healing of scratches after heating at 120 °C for 30 min is achieved, and the self-healing efficiency in terms of Young’s modulus is 97.8 %. The welding efficiency of tensile strength reaches 94.4 % after welding treatment at 120 °C for 30 min. The reprocessed samples with the first generation obtained from hot press (8 MPa) at 140 °C for 3.5 h exhibit the tensile strength of 54.9 MPa. A close-loop recycling of rosin-based epoxy vitrimers was demonstrated, the retention ratio the recycled sample in terms of tensile strength of 82.6 %. Therefore, the REB networks with unique properties are desired alternatives to commercial epoxy resin and show the potential applications in transportation, aerospace and construction materials.

Imine-Containing Epoxy Vitrimer Cured by Active Ester: Properties and Theoretical Analysis

Imine-Containing Epoxy Vitrimer Cured by Active Ester: Properties and Theoretical Analysis

Extrudable, robust and recyclable bio-based epoxy vitrimer via tailoring the topology of a dual dynamic-covalent-bond network

Vitrimers offer possibilities for sustainable materials due to their reprocessability and recyclability. However, enhancing both their mechanical strength and network dynamics simultaneously remains a challenge. Previous attempts to address this issue have not achieved the desired mechanical strength comparable to traditional cured resins and have also fallen short in dynamic performance during compression molding and extrusion processing. In our study, we have successfully created an epoxy vitrimer, EOCY, by a rational design of the topology of a dual dynamic-covalent-bond (D-DCBs) network based on disulfide and imine bonds. The network of this design incorporates a high density of DCBs through the combined utilization of a rigid monomer containing imine bonds and a flexible crosslinker that carries disulfide bonds. The rigid epoxy monomers in EOCY result in superior tensile strength (85.35 MPa), high Tg (112.01 °C), high Young's modulus (2.01 GPa) and improved solvent resistance. Moreover, the flexibility of the curing agent and plentiful D-DCBs lead to reduced activation energy (Ea = 42.23 kJ/mol) and short relaxation time (τ = 4.84 s at 140 °C), enabling EOCY to withstand multiple cycles of compression molding and continuous extrusion processing. Our vitrimer also demonstrates recyclability through various chemical methods, leveraging the unique properties of disulfide and imine bonds. In summary, this study presents a rational approach to network structure design for vitrimers with exceptional mechanical and dynamic properties.

Molecularly engineered cardanol derived epoxy vitrimers based on dynamic disulfide and dynamic ester exchanges with desirable dynamic response, degradability, and recyclability

Dynamic covalent chemistry provides a solution to the recycling problem of epoxy resins, and the graphene functional composites. Nevertheless, the study on chemical structures of the curing agent, and the type of dynamic covalent bonds associated with the nature of the epoxy resins still face challenge due to unclear mechanism and improper design strategy. Herein, a bio-based epoxy monomer was designed from cardanol, and used to fabricate a double dynamically crosslinked epoxy vitrimer. The mechanical properties, dynamic mechanical properties, thermal stability, relaxation behavior, self-healing efficiency, recovery, and reprocessing efficiency of the resulting resins were investigated, focusing on the effects of the soft and hard structure of the curing agent, and the types of dynamic covalent bonds. Bio-based epoxy vitrimers demonstrated several advantages, including multi-stimulus responsiveness, multi-channel self-healing ability, mechanical and chemical recovery, and biodegradability. Specifically, the epoxy vitrimers containing rigid furan rings exhibited better mechanical properties (6.85 MPa) and stability (Tg = 40.81 °C) compared to those with flexible fatty chains. Additionally, the double dynamically crosslinked vitrimer showed faster stress relaxation time (5.6 min) and lower activation energy (36.58 kJ/mol) compared to systems with a single dynamic bond. Finally, the addition of graphene enabled the production of cardanol-based composites with conductivity. Overall, the ease with bio-based epoxy vitrimer and its composites with excellent degradability can be fabricated, utilized, recycled and re-used, which is a novel direction for sustainable composites.

The Design, Synthesis, and Characterization of Epoxy Vitrimers with Enhanced Glass Transition Temperatures.

A series of epoxy vitrimers (EVs) with enhanced glass transition temperatures (Tgs) were synthesized by curing epoxy resin E51 with different ratios of phthalic anhydride and sebacic acid as curing agents, and 1,5,7-triazabicyclic [4.4.0] dece-5-ene as a transesterification catalyst, and their curing dynamics, rheological properties, mechanical properties, and thermal stability were comprehensively investigated. By adjusting the molar ratio of the anhydride to the carboxylic acid in the curing agent, the Tgs of the EVs increased from 79 to 143 °C with the increase in the anhydride content. In particular, the material EV-5.5 with a high usable Tg of 98 °C could undergo stress relaxation through the transesterification reaction when exposed to high temperatures (160 to 200 °C), and the correlation between the relaxation time and temperature follows the Arrhenius equation. Moreover, EV-5.5 exhibited elastomeric behavior, where brittle fractures occurred before yielding, which demonstrated a tensile strength of 52 MPa. EV-5.5 also exhibited good thermal stability with a decomposition temperature (Td5) of 322 °C. This study introduces new possibilities for practical applications of thermoset epoxy resins under special environmental conditions.

Cure kinetics, glass transition advancement and chemo-rheological modelling of an epoxy vitrimer based on disulphide metathesis

This study develops cure and chemo-rheological models for a vitrimeric system appropriate for use as matrix in continuous fibre composites based on a disulphide-endowed amine/epoxy formulation. The focus is on the processing conditions relevant to liquid composite moulding. Kinetics is investigated using Differential Scanning Calorimetry. An autocatalytic model is developed, simulating the reaction with an average reaction rate error of 5.2 % and degree of cure error of 3 %. The viscosity development is modelled at relatively low degrees of cure, focusing on the liquid moulding infusion window, yielding results with an average relative error of 4.8 %. The glass transition temperature advancement is represented by the Di Benedetto equation, whilst the topological transition temperature of the system is determined using stress relaxation experiments and found to be 149.5 °C. These results set the scene for the development of composites based on the matrix system investigated in this work.

Recoverable Rosin-Based Epoxy Vitrimers with Robust Mechanical Properties and High Thermostability

Recoverable Rosin-Based Epoxy Vitrimers with Robust Mechanical Properties and High Thermostability

Exploring the Limits of High-Tg Epoxy Vitrimers Produced through Resin-Transfer Molding.

Over the past few years, scientists have developed new ways to overcome the recycling issues of conventional thermosets with the introduction of associative covalent adaptable networks (i.e., vitrimers) in polymer materials. Even though various end-use vitrimers have already been reported, just a few of them have targeted high-performance industrial applications. Herein, we develop a promising high-performance epoxy vitrimer based on a commercially available resin widely used in aeronautics with the highest glass transition temperature (Tg) of 233 °C ever reported for a vitrimer. A complete study of its physicochemical properties and cure kinetics was conducted, enabling the construction of the first time-temperature-transformation (TTT) diagram reported in the literature. This diagram allows a full determination of the processing and curing parameters leading to the manufacturing of vitrimer samples by the resin-transfer molding (RTM) process. The reshapability and limits therefrom of this high-Tg vitrimer were evaluated by three successful thermoforming cycles without degradation.

Using epoxy/acid stoichiometry to rationally tune the thermomechanical and reprocessing properties of epoxy vitrimers

AbstractEpoxy vitrimers immerged as an appealing alternative to traditional plastics. However, high loading of catalyst and harsh processing conditions are prerequisites to fully exploit the vitrimer properties. In this work, stoichiometric ratio, an often neglected factor, was utilized to further accelerate the rate of dynamic transesterification. Epoxy vitrimers with four epoxy/acid stoichiometries (60:30, 40:30, 30:30, and 24:30) were prepared to comparatively analyze their network structures, thermomechanical properties, and reprocessing properties. It was found that the off‐stoichiometry epoxy vitrimers have almost the similar excellent thermal stability as epoxy thermosets. Vitrimers with excess epoxy showed increased tensile strength (from 26.3 to 42.7 MPa) and decreased elongation at the break (from 5.5% to 1.7%). Meanwhile, excess epoxy induced abundant of hydroxy groups in the vitrimers, imparting satisfactory repairability, reformability, and recyclability. Specifically, epoxy/acid (60:30) repaired 94% in 10 min at 180°C, and maintained over 93% mechanical properties after reforming and recycling. However, simply increasing the epoxy content will compromise the structural integrity of the network, simultaneously resulting in reduced toughness of epoxy vitrimers. These findings demonstrated that epoxy/acid stoichiometry can be a powerful tool to develop epoxy vitrimers with both excellent thermomechanical and reprocessing properties.

Influence of Catalyst Content and Epoxy/Carboxylate Ratio on Isothermal Creep of Epoxy Vitrimers.

In the present work, a commercial epoxy based on epoxy anhydride and tertiary amine was modified by a metallic catalyst (Zn2+) to induce vitrimeric behavior by promoting the transesterification reaction. The effect of two different epoxy/acid ratios (1 and 0.6) at two different zinc acetate amounts (Zn(Ac)2) on the thermomechanical and viscoelastic performances of the epoxy vitrimers were investigated. Creep experiments showed an increase in molecular mobility above the critical "Vitrimeric" temperature (Tv) of 170 °C proportionally to the amount of Zn(Ac)2. A procedure based on Burger's model was set up to investigate the effect of catalyst content on the vitrimer ability to flow as the effect of the dynamic exchange reaction. The analysis showed that in the case of a balanced epoxy/acid formulation, the amount of catalyst needed for promoting molecular mobility is 5%. This system showed a value of elastic modulus and dynamic viscosity at 170 °C of 9.50 MPa and 2.23 GPas, respectively. The material was easily thermoformed in compression molding, paving the way for the recyclability and weldability of the thermoset system.

Catalyst-free, sustainable epoxy vitrimers from epoxidized soybean oil and natural sugar alcohols

Vitrimers based on dynamic transesterification (DTE) are usually thought to highly rely the use of catalysts, for example, Lewis acids and tertiary amines to promote the bond exchange reaction, which bring additional concerns on human health and the environment. Herein, we report a family of fully bio-based epoxy vitrimers that rearrange their topologies via DTE in the absence of any catalyst. Specifically, epoxidized soybean oil (ESO) was efficiently cured by conjugated carboxylic groups formed in-situ through the ring-opening of maleic anhydride (MA) by sugar alcohols (SAs) in a one-pot and solvent-free manner to produce these vitrimers. Abundant -OH groups from SAs strongly enhanced mechanical properties the without hindering stress relaxation or re-processing of the vitrimers. Meanwhile, welding, crack-healing and shape editing at high temperatures was achieved. The vitrimer samples exhibited various recovery rates of tensile strength upon 3 times of mechanical recycling.

Improving the Recyclability of an Epoxy Resin through the Addition of New Biobased Vitrimer.

In recent decades, the use of thermoset epoxy resins (ER) has spread to countless applications due to their mechanical properties, heat resistance and stability. However, these ERs are neither biodegradable nor recyclable due to their permanent crosslinked networks and usually, they are synthesized from fossil and toxic precursors. Therefore, reducing its consumption is of vital importance to the environment. On the one hand, the solution to the recyclability problems of epoxy resins can be achieved through the use of vitrimers, which have thermoset properties and can be recycled as thermoplastic materials. On the other hand, vitrimers can be made from natural sources, reducing their toxicity. In this work, a sustainable epoxy vitrimer has been efficiently synthesized, VESOV, by curing epoxidized soybean oil (ESO) with a new vanillin-derived Schiff base (VSB) dynamic hardener, aliphatic diamine (1,4-butanediamine, BDA) and using 1,2-dimethylimidazole (DMI) as an accelerator. Likewise, using the same synthesized VSB agent, a commercial epoxy resin has also been cured and characterized as ESO. Finally, different percentages (30, 50 and 70 wt%) of the same ER have been included in the formulation of VESOV, demonstrating that only including 30 wt% of ER in the formulation is able to improve the thermo-mechanical properties, maintaining the VESOV's inherent reprocessability or recyclability. In short, this is the first approach to achieve a new material that can be postulated in the future as a replacement for current commercial epoxy resins, although it still requires a minimum percentage of RE in the formulation, it makes it possible to recycle the material while maintaining good mechanical properties.

High-Strength and Multi-Recyclable Epoxy Vitrimer Containing Dual-Dynamic Covalent Bonds Based on the Disulfide and Imine Bond Metathesis

High-Strength and Multi-Recyclable Epoxy Vitrimer Containing Dual-Dynamic Covalent Bonds Based on the Disulfide and Imine Bond Metathesis