Epoxy Vitrimers Research Articles

Self-healable epoxy/graphene oxide vitrimers and their recyclable glass fiber reinforced composites

Glass fiber-reinforced polymers (GFRPs) made from thermoset polymers have been widely used across various industries. However, these composites have significant drawbacks, including poor interfacial adhesion between the fiber and matrix, as well as a substantial contribution to waste and environmental pollution due to the challenges in recycling, reprocessing, and reuse. Herein, firstly graphene oxide (GO) was incorporated in vitrimer matrix to fabricate recyclable vitrimer nanocomposites with improved interfacial interaction, high thermal stability, low dielectric constant, fast stress relaxation as well as rapid self-healing ability. The nanocomposite with 0.2 wt% GO is optimized to achieve excellent remoulding (89% of flexural strength) and self-healing (60 min for a 48 μm scratch) properties due to dynamic disulfide bond exchange mechanism. Furthermore, epoxy/GO matrix was used to develop glass fiber reinforced composites via vacuum assisted resin infusion molding process (VARIM). The developed composites demonstrate tremendous mechanical strength (250 MPa), shape-memory, weldability, and degradable properties. Due to the rapid chemical degradation of epoxy vitrimer under mild conditions in the thiol (2-mercaptoethanol and 1-otanethiol), facilitated by thiol disulfide exchange reaction, glass fibers (GFs) can be effectively recycled. The performance of recycled glass fibers closely matches that of original fibers, exhibiting nearly identical woven structure and mechanical properties. The single yarn of recycled glass fibers can achieve tensile strength of 85% (1-octanethiol) and 72% (2-mercaptoethanol) of the original glass fibers, thus, the recycling of glass fibers would be highly advantageous in terms of achieving the sustainability goals.

Self-healing amine- and carboxy-cured bio-based epoxy vitrimers driven by the disulfide metathesis reaction

Self-healing amine- and carboxy-cured bio-based epoxy vitrimers driven by the disulfide metathesis reaction

High biomass content in epoxy vitrimers: a study on bio-based and reprocessable thermosets

High biomass content in epoxy vitrimers: a study on bio-based and reprocessable thermosets

Advanced Vitrimer with Spiropyran Beads: A Multi-Responsive, Shape Memory, Self-Healing, and Reprocessable Smart Material.

Thermosetting materials have limitations in terms of reshaping and recycling due to their irreversible bond structures, leading to significant plastic waste issues. Recently, epoxy vitrimers based on dynamic covalent bond exchange have been introduced as promising alternatives to traditional thermosets. Particularly, they demonstrate significant potential applications in the field of multi-responsive materials. In this research, a self-healable and mechano-responsive vitrimer (EB-V) is successfully prepared, incorporating epoxide spiropyran beads (ESP beads) derived from citric acid and epoxy derivatives. To enable self-reporting of cracks through color changes, ESP beads are covalently bonded to the vitrimer via an epoxy-carboxylic acid reaction. The photochromic properties of EB-V are demonstrated by color and fluorescence changes, and its tensile strength increased from 2.0 to 6.8MPa compared to the control sample. Dynamic mechanical analysis confirmed the covalent exchange reaction of the vitrimer, revealing its reconfigurable behavior and stress relaxation at elevated temperatures. Furthermore, EB-V exhibited exceptional properties, including self-healing and reprocessability. As a smart material, it holds great promise for a wide range of applications, such as sensors, actuators, 4D printing, and industrial safety diagnostics.

Catalyst-free sustainable epoxy vitrimers with fire safety offered by phosphorus-containing compounds

In this study, we designed and incorporated a phosphorus-rich compound, DPT, which possesses inherent catalytic properties, into a typically sustainable epoxy vitrimer. This innovative strategy enables the fabrication of flame retarded epoxy vitrimers that require no additional catalysts. DPT was synthesized through a reaction between triethanolamine (TEA) and dibenzyl chlorophosphonate (DPCP). The successful synthesis of DPT is attested to by FTIR and NMR analyses. DPT effectively accelerated the curing process of the epoxy vitrimers, yielding materials with exceptional mechanical strength and inherent self-healing abilities. Notably, the pHRR of epoxy vitrimer incorporated with DPT is reduced to 296.3 kW/m2, showing a remarkable 54% reduction compared to that of the epoxy vitrimer catalyzed by conventional catalysts. The THR is also decreased from 84.3 to 42.4 MJ/m2. Comprehensive analyses using cone calorimetry and TG-FTIR revealed that DPT effectively inhibits the emission of toxic gases. Crucially, the presence of DPT in the epoxy matrix facilitated the formation of a dense and stable phosphorus-laden carbon layer, which not only enhanced flame retardancy but also imparted exceptional smoke suppression capabilities. These findings underscore the potential of DPT for developing high-performance, catalyst-free, flame-retardant epoxy vitrimers.

Self-Repairable Carbon Fiber-Reinforced Epoxy Vitrimer Actuator with Multistimulus Responses and Programmable Morphing.

Smart shape-changing structures in aerospace applications are vulnerable to damage in harsh environments. Balancing high mechanical performance with self-repair capabilities poses a challenge due to inherent trade-offs between strength and flexibility. To address this challenge, an asymmetric bilayer-structured actuator was fabricated using commercially available continuous carbon fiber tows (CFs) as the passive layer and a dynamic cross-linked epoxy vitrimer as the active layer. The construction of the vitrimer-CF actuator involves a simple and scalable hot-pressing process, resulting in a tensile strength of 234 MPa and an interfacial bonding strength of 405 N·m-1. This actuator exhibits remarkable deformation capability (210°/7 s) and an efficient self-repair ability under various stimuli, including thermal (60-160 °C), light (0.4-1.0 W·cm-2), electric (2-4 V), and solvent (acetone). By adjustment of the orientation angle of CFs, complex left-handed and right-handed curling structures can be achieved. Leveraging the insights from photothermal/electrothermal actuation mechanisms, a quadruped crawling robot is developed capable of crawling 4 cm with a single light illumination. The actuator can lift objects 45 times its weight when subjected to light stimuli. Additionally, a flap actuator is constructed to achieve an angle change of 63° within 10 s under an electric stimulus, enabling remote control over the aircraft flight angle. These results demonstrate the potential of the vitrimer-CF actuator for advanced applications in intelligent aerospace structures.

Epoxy-imine vitrimer having low dielectric constant and high wave transmission efficiency for mobile communication applications

The rapid growth of telecommunication technology calls for express evaluation and development for low dielectric constant (Dk) materials with higher performance requirements. At mean time, the extensive use of these dielectric materials generates increasing concerns over their crosslinking structures that cannot be readily recycled without causing environment pollution. Herein, a curing agent comprising dynamically exchangeable imine bonds was synthesized via the condensation of terephthalaldehyde (TPA) and isophorondiamine (IPDA). Epoxy vitrimers cured by this curing agent exhibited low Dk (2.75–2.79) and excellent wave transmission efficiency (>90.5 %) at high frequency range of 8.2–12.4 GHz. The cured resins can also be recycled through chemical degradation, producing reusable monomers. The cured resins demonstrate vitreous properties and its dielectric, thermal properties and wave transmission efficiency maintained unchanged after reprocessing. This work offers a strategy to prepare low Dk epoxy vitrimer used in communication field.

Bio-Based Self-Healing Epoxy Vitrimers with Dynamic Imine and Disulfide Bonds Derived from Vanillin, Cystamine, and Dimer Diamine.

The condensation reactions of 4,4'-(ethane-1,2-diylbis (oxy)) bis(3-methoxybenzaldehyde) (VV) with cystamine, 1,6-hexamenthylene diamine, and a dimer diamine (PriamineTM 1075) produced three types of vanillin-derived imine-and disulfide-containing diamines (VC, VH, and VD, respectively). Thermal curing reactions of polyglycerol polyglycidyl ether with VD and mixtures of VC/VD and VH/VD produced bio-based epoxy vitrimers (BEV-VD, BEV-VC/VD, and BEV-VH/VD, respectively). The degree of swelling and gel fraction tests revealed the formation of a network structure, and the crosslinking density increased with a decreasing VD fraction. The glass transition temperature, tensile strength, and tensile modulus of the cured films increased as the VD fraction decreased. In contrast, the thermal degradation temperature of the cured films increased as the VD fraction increased. All the cured films were healed by hot pressing at 120 °C for 2 h under 1 MPa at least three times. The healing efficiencies, based on tensile strength after the first healing treatment, were 75-78%, which gradually decreased as the healing cycle was repeated. When imine-and disulfide-containing BEV-VC/VD and imine-containing BEV-VH/VD with the same VC/VD and VH/VD ratios were used, the former exhibited a slightly higher healing efficiency.

Flame retardant epoxy vitrimers employing a Meldrum's acid-functionalized and phosphorus-containing bisphenol compound as the cross-linking agent for conventional epoxy

In addition to recycling and reprocessing features, designs of vitrimers possessing critical properties required for conventional cross-linked polymers are of interest. In this work, a newly designed Meldrum's acid (MA)-functionalized and phosphorus-containing bisphenol compound (P-MADV) compound is synthesized. Employing P-MADV as a curing agent, simultaneous formation of permanent and dynamic covalent linkages through one-pot reaction has been achieved in formation of flame retardant epoxy resin vitrimers. The newly synthesized P-MADV compound is an effective cross-linking agent for commercially-available epoxy resins for synthesis of flame-retardant epoxy vitrimers through the conventional cross-linking conditions and processes. The cross-linked epoxy vitrimer passes the flame test under UL 94V-0 specification contributed from the phosphorus-containing groups. The flame retardant vitrimer also exhibits a storage modulus of 1.2 GPa at 50 °C and a Tg (tanδ peak) of 185 °C, which are comparable to the values found with conventional phenolic novolac-cross-linked epoxy resin (storage modulus: 1.6 GPa; Tg: 155 °C). Without addition of catalysts for dynamic bond exchanging reaction, the cross-linked epoxy vitrimer shows stress relaxation time of 397 s at 230 °C and an activation energy of stress relaxation of 97.8 kJ mol−1. The dynamic features warrant the epoxy vitrimer being recyclable under thermally pressing at 230 °C and 16 MPa for 1 h. The recycled samples exhibit thermal and mechanical properties being comparable to the pristine resin. This work has demonstrated an attractive approach for preparation of flame retardant epoxy resin vitrimers in viewpoints of both manufacturing processes and resins properties.

Influence of castor oil‐derived bio‐resin on mechanical and visco‐elastic properties of dynamic covalent imine bond based epoxy and its composite

AbstractThe widespread use of thermoset composites has raised significant environmental and economic concerns due to their non‐recyclable nature. This study addresses these issues by developing vitrimers and their composites featuring dynamic imine bonds synthesized from vanillin, 3,3′‐dimethyl‐4,4′‐diaminodicyclohexylmethane, and epoxy. To enhance toughness and bio‐content in the vitrimer and its composites, a green reactive monomer, epoxy methyl ricinoleate (EMR), was synthesized via the transesterification of epoxidized castor oil (ECO) and incorporated as the reactive diluent. The effect of EMR on bio‐resin mechanical, thermo‐mechanical, and relaxation times (τ) of the vitrimer was examined. The inclusion of EMR reduced the relaxation time, indicating improved dynamic bond exchange efficiency. EMR incorporation decreased the relaxation time, suggesting enhanced efficiency in the interchange of dynamic bonds. In addition, the incorporation of 20% EMR increased the impact strength of the epoxy vitrimer by 37.8%. The study also details the manufacturing and characterization of greener composites made with these vitrimers and flax fabric, which were evaluated for various mechanical and thermo‐mechanical properties. Morphological analysis using a scanning electron microscope revealed good interfacial adhesion between the fiber and matrix. Furthermore, the flax fiber vitrimer‐based composite demonstrated effective chemical recyclability. The results indicate that these composites possess mechanical and thermal‐mechanical properties well‐suited for lightweight engineering applications.Highlights Recyclable vitrimers were developed from vanillin and epoxy. Toughness and bio‐content were enhanced by adding EMR. Relaxation time was reduced by EMR, improving bond exchange efficiency. Epoxy vitrimer impact strength was increased by 37.8% with 20% EMR. Green composites with vitrimers and flax fabric were manufactured.

Investigation on Polyether Sulfone Toughening Epoxy Vitrimer: Curing and Dynamic Properties.

Diglycidyl ether of bisphenol A crosslinking with glutaric anhydride is used to form the conventional "covalent adaptive network", polyether sulfone (PES) by coiling and aggregating on the adaptive network is used to significantly increase the uncured resin viscosity for improving the processability of epoxy resin, but inevitably affecting the curing reaction and dynamic transesterification reaction. This study investigates the crucial roles of PES in curing dynamics and stress relaxation behavior. The results indicate that although PES does not directly participate in the crosslinking reaction of polyester-based epoxy vitrimers. Moreover, the isothermal curing studies reveal that the addition of PES can greatly bring forward the reaction rate peak from conversion α = 0.6 to α = 0.2, meaning that the curing mechanism transfers from chemical control to diffusion control. Dynamic property analysis shows that the addition of PES significantly accelerates stress relaxation, especially at lower temperatures, leading to low viscous flow activation energy Eτ and relatively insensitive stress relaxation behavior to temperature. Introducing PES into vitrimer resin greatly improves crosslinking density (2.31×10⁴molm- 3), enhancing glass transition temperature (82.68°C), tensile strength (68.66MPa), and fracture toughness (6.25%). Additionally, the modified vitrimer resin exhibits satisfying shape memory performance and reprocessing capability.

Recyclable Epoxy Vitrimer with High Strength and Rapid Stress Relaxation under Mild Temperatures

Recyclable Epoxy Vitrimer with High Strength and Rapid Stress Relaxation under Mild Temperatures

Protocatechualdehyde-based epoxy vitrimer with low dielectric, excellent flame retardancy, and recyclability

Conventional bisphenol A epoxy resins cannot meet the requirements of the new generation of electronic/electrical industries in terms of dielectric properties and flame retardancy and are challenging to reprocess after molding, resulting in waste of resources and environmental hazards. Therefore, a tetrafunctional active ester curing agent (TIE) was successfully synthesized by a one-pot method using biomass protocatechualdehyde as raw materials, followed by curing DGEBA to prepare an epoxy vitrimer (TIE/DGEBA). MHHPA/DGEBA was chosen for comparison since the –OH was also absent after curing. The results showed that TIE/DGEBA had good combinatorial performance, thanks to the unique structure of TIE, including active ester units, siloxane chains, and aromatic Schiff base groups. TIE/DGEBA possessed a lower dielectric constant (2.9 vs. 4.0) compared to MHHPA/DGEBA, which was attributed to the synergistic effect of the less-polar siloxane chain segments, bulky benzene structure, and lower cross-linking density (1971 vs. 733 mol/cm3) of TIE/DGEBA. The aromatic Schiff base not only imparted good thermal stability (T30% = 418.0 vs. 422.2 °C) to TIE/DGEBA but ensured that it allowed for recycling and reprocessing. After solvent recycling, the resin can be remolded without significant change in tensile strength (53.0 vs. 46.3 MPa). TIE/DGEBA displayed better flame retardancy with higher residual weight (28.2 % vs. 5.5 %), lower total heat release (29.6 vs. 43.5 kJ/g), and good resistance to thermal oxidization. The structural and performance characteristics of TIE/DGEBA offer a strategy for designing multifunctional epoxy-based materials in electrical/electronic applications.

The effect of graphene properties on the extrusion of a shape memory epoxy vitrimer

Thermoset polymers exhibit very appealing mechanical and functional properties. Direct ink writing (DIW) could open new possibilities in the design and fabrication of intricate thermoset parts, but it often requires the use of additives such as fumed silica or nanoclays to modify the rheology of uncured epoxies. However, relatively large concentrations are usually needed what can be detrimental to properties. Graphene-derived additives are an appealing alternative, but we need to understand the key physicochemical characteristics that define an optimum graphene rheology modifier. Here we compare the effect of three different carbon fillers on the viscoelastic response of a reprocessable epoxy vitrimer with shape memory capabilities, graphene oxide (GO), reduced graphene oxide (rGO), and graphene powder (GP), and assess the effect of their chemistry and morphology. The analysis shows that large (∼20 μm in size) rGO flakes enable the formation of strong, printable gels, through Van der Waals interactions and physical entanglement. The vitrimer could be successfully printed by incorporating 5 wt% of rGO. The printed parts exhibit tensile strengths (30–60 MPa), moduli (2–3 GPa), strength recovery after reprocessing (∼80 %), shape-memory properties comparable to the pure epoxy, and improved water resistance due to the introduction of hydrophobic rGO.

Creep and recovery of vitrimers under thermo-chemo-mechanical coupling effects

Reprocessable, repairable, and recyclable (“3R”) vitrimers have experienced rapid development over the past decade and demonstrate significant potential for diverse applications. The creep performance of vitrimers is crucial for their dimensional stability under load-bearing conditions at relatively low temperatures and serves as a key metric for evaluating their reprocessability at high temperatures. However, the impact of dynamic covalent polymer networks (DCPNs) on vitrimer mechanics, particularly creep properties, remains debated. Systematic experimental studies on the creep of vitrimers across a wide temperature range are lacking. We use the classic epoxy vitrimer as a model system to investigate the impact of DCPNs on vitrimer mechanics, especially focusing on the creep and recovery behavior across a wide temperature range, spanning from room temperature to the glass transition temperature (Tg) and further to the topology freezing transition temperature (Tv). We systematically examined the effects of temperature, stress, and catalysts on vitrimer creep, revealing the influence of DCPNs. Our findings demonstrate significant thermo-chemo-mechanical coupling effects in the creep mechanics of vitrimers, a facet not comprehensively acknowledged in existing studies. In addition, at temperatures below Tg, vitrimers exhibit superior creep resistance compared to pure epoxy resin due to metal coordination, ensuring excellent dimensional stability under load-bearing conditions. Conversely, at high temperatures, active bond exchange reactions in vitrimers accelerate creep and result in greater residual deformation, highlighting exceptional reprocessability. This study provides new insights into the materials and mechanics of vitrimers.

Fabricating epoxy vitrimer with excellent mechanical and dynamic exchange properties via network topology design

Epoxy vitrimers are a novel type of environmentally friendly materials that possess exceptional properties, including self-healing, recyclability, and reprocessability. They offer a solution to the issue of traditional thermosetting epoxy resins, which are not recyclable. However, the incorporation of dynamic bonds typically results in a decrease in mechanical properties. Therefore, achieving a balance between the mechanical properties and dynamic exchange capability is crucial. In this work, a series of epoxy resin systems with various topological network characteristics were prepared by adjusting the proportions of bisphenol A epoxy resin (E51) and polyethylene glycol diglycidyl ether (EGDGE) epoxy resins in combination with a curing agent containing vinylogous urethane (VU) dynamic bonds. Through further study, we found that with the increase of E51 content, the cross-linking density of the network gradually increased, and the flexibility of the macromolecular chains decreased (favorable for enhancing the material's mechanical performance). Simultaneously, the free volume of the network gradually increased, and the tightness of the macromolecular chains decreased (favorable for improving the material's dynamic exchange ability). These two effects combined to make E51-40 exhibit the lowest activation energy for dynamic covalent bond exchange and the lowest topological transition temperature. Compared to E51-0, the tensile strength and glass transition temperature of E51-40 were improved by 22.23 % and 25.30 %, respectively. After remoulding experiment at 140 °C for 1h, the material exhibited a high tensile strength recovery of 91.34 %. Therefore, starting from the design of molecular structure, adjusting the network topology is an effective means to balance the mechanical and dynamic exchange properties of epoxy vitrimers. This provides experimental guidance and theoretical insights for designing high-performance epoxy vitrimer materials.

Catalyst-free gallic acid-based epoxy vitrimers with reprocessability and high glass transition temperature

General approaches for preparing epoxy vitrimers required lots of catalysts to accelerate transesterification to form the cross-linked network and the residual catalyst not only resulted in toxicity but also poor compatibility with other components. Herein, a green and facile strategy for preparing gallic acid epoxy resin (GA-EP) was developed under catalyst-free condition using renewable gallic acid, xylitol and hexahydro 4-methylphthalic anhydride (MHHPA) as feedstocks. The results indicated that the bio-based epoxy vitrimer had higher glass transition temperature (122 °C), satisfactory mechanical, thermal and degradation properties compared to neat Bisphenol A-based epoxy vitrimer. Furthermore, recycled GA-EP was ground into powder and added to the fresh resin to investigate its reusability. Compared to the original, the recycled epoxy vitrimer system demonstrated comparable performance, except for minor reductions in glass transition temperature (Tg) and modulus, likely stemming from the inherent inhomogeneity of the reprocessed material. This work provides a new approach to the production of high Tg bio-based epoxy vitrimers without adding any catalyst.

Curing kinetics and mechanical properties of the epoxy vitrimer based on transesterification and its composite laminates

AbstractThe resin is a fundamental factor in determining the performance and usage conditions of its composites. The epoxy vitrimer, which combines both thermoplastic and thermosetting properties, shows significant potential in extending the service life of material. The curing process significantly affects the macroscopic properties of the vitrimer and the resulting carbon fiber reinforced plastics (CFRP). Effective control of curing parameters constitutes the critical factor for the assurance of product quality. In the study, a differential scanning calorimetry (DSC) was utilized to establish a curing kinetics model for a modified, self‐healable epoxy vitrimer based on transesterification. Then the molecular structure, thermodynamic, and mechanical properties of the cured vitrimer were analyzed. Next vitrimeric CFRP (vCFRP) laminates were fabricated, and the tensile and bending properties of the vCFRP laminates were investigated. The results showed the curing process was obtained: 95°C/1 h + 150°C/2 h + 170°C/2 h. The thermodynamic and mechanical properties of cured vitrimer could meet practical requirements. The minimal differences observed in the stress–strain curves of the specimens indicated that the curing process was appropriately configured. The prepared laminates exhibited performance comparable to that of commercial thermosetting composites.

Influence of monomer structure and catalyst concentration on topological transition and dynamic properties of dicarboxylic acid‐epoxy vitrimers

AbstractThis study delineates the dependence of thermophysical behavior of acid‐epoxy vitrimers on their formulation. The stress relaxation due to the bond exchange reaction and the glass transition temperature of acid epoxy vitrimers are investigated, with respect to the influence of catalyst content and acid chain length. This is carried out for a range of dicarboxylic acids and catalyst concentrations formulated and characterized using calorimetry and dynamic mechanical analysis. The influence of acid chain length on the bond exchange rate, topological transition, and glass transition temperatures of the vitrimers is found to be significant. The activation energy of the exchange reaction varies over a wide range from 73 to 104 kJ/mol and the topology freezing temperature from 66 to 136°C with the behavior governed by the interplay between crosslinking density, network flexibility and density and distance of functional groups, with an increase of catalyst concentration leading to lower topological transition temperature and the dependence on chain length showing non‐monotonic behavior. The glass transition decreases by about 30°C as the carbon chain length increases from 6 to 14 carbons due to enhanced monomer flexibility and is not affected by the concentration of catalyst.

Bio-Based Epoxy Vitrimers with Excellent Properties of Self-Healing, Recyclability, and Welding.

The development of more recyclable materials is a key requirement for a transition towards a more circular economy. Thanks to exchange reactions, vitrimer, an attractive alternative for recyclable materials, is an innovative class of polymers that is able to change its topology without decreasing its connectivity. In this work, a bisphenol compound (VP) was prepared from saturated cardanol, i.e., 3-pentadecylphenol and vanillyl alcohol. Then, VP was epoxidized to obtain epoxide (VPGE). Finally, VPGE and citric acid (CA) were polymerized in the presence of catalyst TBD to prepare a fully bio-based vitrimer based on transesterification. The results from differential scanning calorimetry (DSC) showed that the VPGE/CA system could be crosslinked at around 163 °C. The cardanol-derived vitrimers had good network rearrangement properties. Meanwhile, because of the dynamic structural elements in the network, the material was endowed with excellent self-healing, welding, and recyclability.