Recyclable Epoxy Research Articles

Eugenol-Based Siloxane-Containing Recyclable Epoxy Resins with Tunable Dynamic Properties, Reprocessability, and Degradability

Eugenol-Based Siloxane-Containing Recyclable Epoxy Resins with Tunable Dynamic Properties, Reprocessability, and Degradability

Biobased recyclable epoxy composites reinforced with carbon nanotubes for electromagnetic interference shielding and Joule heating

AbstractCurrent epoxy thermosets are facing various challenges such as resource scarcity, environmental concern, and limited functionalities. Here, we present a novel solvent free approach to fabricate multifunctional carbon nanotubes (CNTs) reinforced biobased and recyclable epoxy thermoset composites through precuring in an internal mixer followed by curing in an oven. The epoxy matrix, a covalent adaptable network (CAN) based on dynamic imine bonds, was synthesized from entirely biobased feedstocks, including glycerol triglycidyl ether, vanillin, and 1,10‐diaminodecane. The shear force during precuring facilitated dispersion of CNTs in CAN matrix, allowing for high CNTs loading (up to 20 wt%). The well‐dispersed CNTs not only reinforced mechanical properties of CAN but also introduced new functionalities like electrical conductivity, electromagnetic interference (EMI) shielding capacity, and Joule heating performance. The composite with 20% CNTs exhibited a tensile strength and Young's modulus of 78.1 MPa and 3.07 GPa, respectively, marking a 34.2% and 63.6% improvement over pristine CAN. It also demonstrated a high electrical conductivity of 35.3 S/m, resulting in a remarkable EMI shielding effectiveness (22.8 dB) and excellent Joule heating performance (reaching 131.7°C at 27 V input). Furthermore, these composites are thermally and chemically recyclable due to dynamic imine bonds, promoting sustainability and circular economy.

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

Novel recyclable epoxy resin with releasable residual stress and synergistically enhanced dielectric properties and healing ability

It has been a challenge to improve the performance of epoxy insulation in the whole life cycle by achieving the reduction of residual stress during manufacturing, the healing of damages during application, and the recycling after decommissioning. In this paper, a novel recyclable epoxy resin (SEP) with releasable residual stress and synergistically enhanced dielectric properties and healing ability was successfully developed by introducing dynamic thiocarbamate bonds (DTBs). A reduction in residual stresses by 45 % was detected after SEP was annealed at 30 °C below the glassy transition temperature (Tg) for 8 h, during which the mechanical properties remained unchanged. Synergistically improved dielectric properties and damage-healing ability were also observed in SEP. A high healing efficiency of 94.4 % for electrical breakdown damage was found in SEP, meanwhile its dielectric properties were superior to commercial epoxy resin. Additionally, SEP featured good recyclability, including reprocessability and degradability. All those excellent properties were ascribed to the dissociation and recombination of DTBs triggered by thermal stimulation. This work provides an effective solution to a series of issues throughout the full lifecycle of epoxy materials.

Recent progress in degradation and recycling of epoxy resin

Epoxy resin is widely used in electrical equipment and electronic devices fields due to its excellent electrical, thermal, and mechanical properties. However, its internal three-dimensional covalent interconnection structure brings barriers to its degradability and recycling because covalent bonds cannot be broken easily. With the replacements of power equipment and electronic devices, there will be more and more epoxy resins and their composites in them to be treated and effective recycling is of great significance for resource conservation and environmental protection. In this review article, recent progress in degradation and recycling of epoxy resin is introduced and the effect of three traditional degradation methods is discussed. The drawbacks of these methods are thought to come from the intrinsic properties of these epoxy resins. So the urgency of developing new kinds of degradable epoxy resins is proposed. Then different types of new degradable epoxy resins are reviewed. Degradation mechanisms of the opened-loop recycling and recycling methods of the closed-loop recycling are summarized in detail. Finally, the challenges and perspectives are discussed based on their current developments. This review comprehensively considers both traditional degradation methods and new methods for developing degradable epoxy resins. It covers not only an overview of the state-of-the-art advances of degradation and recycling of epoxy resin but also the prospects that provide reference for the synthesis of degradable epoxy resin materials.

Comparison of carbon‐reinforced composites manufactured by vacuum assisted resin infusion with traditional and fully recyclable epoxy resins

AbstractThis study presents a comparative analysis of carbon fiber reinforced polymer (CFRP) composites manufactured through vacuum assisted resin infusion (VARI) using a traditional epoxy resin (E), a fully‐recyclable epoxy resin system with (BBR10) and without (BBR) the addition of a reactive diluent (R*Diluent). Various mechanical and thermal tests were conducted to assess their performance. The BBR10 laminate, incorporating 10 wt% R*Diluent, exhibited competitive mechanical performance, comparable to traditional (E) and fully‐recyclable laminates (BBR). Despite a slightly lower ultimate tensile strength (UTS) compared with BBR, BBR10 demonstrated improved flexural strength and modulus. Low‐velocity impact testing confirmed comparable strength between VARI‐produced composites with the recyclable matrix (BBR and BBR10) and the traditional one (E). X‐ray mCT investigations revealed distinct void arrangements in the CFRP laminates. Additionally, a chemical approach was employed for recovering high fractions of fibers from CFRP laminates with a recyclable matrix (BBR and BBR10). Chemical recycling achieved an almost 100% yield for long carbon fibers.Highlights Comparative analysis of CFRP composites manufactured through VARI. Diluent addition allowed to tailor the recyclable epoxy viscosity. Mechanical characterization of traditional and fully recyclable epoxy resins. Investigation by X‐ray mCT of potential flaws and manufacturing defects. Chemical recycling of CFRP laminates with a recyclable matrix.

Effect of Methoxy Groups on the Structure–Properties Relationship of Lignin-Derived Recyclable Epoxy Thermosets

Effect of Methoxy Groups on the Structure–Properties Relationship of Lignin-Derived Recyclable Epoxy Thermosets

Toughened and Recyclable Epoxy Networks Featuring Nanostructures and Dynamic Covalent Bonds

Toughened and Recyclable Epoxy Networks Featuring Nanostructures and Dynamic Covalent Bonds

Towards fire-safe, smoke-suppression and recyclable epoxy composite from bio-derived flame retardant

Epoxy-based vitrimers appear as a promising alternative to common epoxy materials. Nevertheless, the possibility of epoxy vitrimers in practical applications is limited by their high flammability. In this study, a novel bio-based casein phosphopeptide chelating zinc (CPP-Zn) is purposely developed, specifically designed to serve as a flame retardant and smoke suppressant for epoxy vitrimer, operating through a condensed phase mechanism. To further enhance the fire safety of epoxy resins, DOPO derivative (DDP) is synthesized through grafting with bio-based itaconate and subsequently used as a synergistic flame retardant in combination with CPP-Zn. DDP exhibits gas phase flame retardant mechanism through the release of free radical scavangers generated during combustion. As a result of the combination of condensed and gas phase effect, the composite achieves self-extinguishing within 1 second, attains a V-0 rating in UL-94 testing, and boasts an LOI of 32.3 %. Additional, it shows a remarkable 73.7 % reduction in heat release rate (HRR) and 27.2 % in total smoke production (TSP) compared with neat epoxy vitrimer. The experimental results also evidence that the characteristic features of vitrimers, self-healing and recyclability, have been retained in the prepared flame-retarded composites. The remolded 50D/EST-25Cz is still able to maintain 72.2 % of its tensile strength. This opens promising avenues for the development of highly fire-safe and eco-friendly epoxy resins.

Closed-loop recycling of tough epoxy supramolecular thermosets constructed with hyperbranched topological structure

The regulation of topological structure of covalent adaptable networks (CANs) remains a challenge for epoxy CANs. Here, we report a strategy to develop strong and tough epoxy supramolecular thermosets with rapid reprocessability and room-temperature closed-loop recyclability. These thermosets were constructed from vanillin-based hyperbranched epoxy resin (VanEHBP) through the introduction of intermolecular hydrogen bonds and dual dynamic covalent bonds, as well as the formation of intramolecular and intermolecular cavities. The supramolecular structures confer remarkable energy dissipation capability of thermosets, leading to high toughness and strength. Due to the dynamic imine exchange and reversible noncovalent crosslinks, the thermosets can be rapidly and effectively reprocessed at 120 °C within 30 s. Importantly, the thermosets can be efficiently depolymerized at room temperature, and the recovered materials retain the structural integrity and mechanical properties of the original samples. This strategy may be employed to design tough, closed-loop recyclable epoxy thermosets for practical applications.

Electronic effects of substituents on the properties of recyclable epoxy resins containing aromatic acetal groups

Both environmental and economic concerns are pioneering the development of recyclable thermosetting polymers. In this study, three liquid acetal-containing di-epoxide monomers (ADEs) with different para-substituted (-OCH3, -Cl and –NO2) phenyl groups were synthesized. The refractive indices and viscosities of the three di-epoxide liquids increase in the sequence of ADE-3 (-OCH3), ADE-1 (-Cl) and ADE-2 (-NO2). Once cured with methylhexahydrophthalic anhydride, the three cured ADEs exhibit high glass transition temperatures (181–218 °C). Due to the presence of dynamic acetal bonds, all three cured ADEs are reprocessable, repairable, weldable, and degradable. Interestingly, the electronic effect of the substituents on the phenyl group significantly impacts the structural configuration, reprocessability and degradation behaviors of the resultant cured ADEs. As the electron-withdrawing effect of the substituents on the phenyl group increases, the stress relaxation rates of the cured ADEs decrease at the same temperature. Under optimized reprocessing conditions, the reprocessed ADE-1 (-Cl) and ADE-2 (-NO2) resins show the highest (117 %) and lowest (67 %) retention rates for tensile strength, respectively. The degradation rates of the three cured ADEs in the acidic solutions decrease with increasing the electron-withdrawing effect of the substituent on the phenyl group. Finally, a di-epoxide curing system based on ADE-3 was applied to prepare carbon fiber-reinforced composites, and nondestructive recovery of the carbon fibers was attained by degradation of the resin matrix.

Thermal and mechanical properties of recyclable epoxy asphalt vitrimers

Thermal and mechanical properties of recyclable epoxy asphalt vitrimers

Mechanical Characterization of Recyclable and Non-Recyclable Bio-Epoxy Resins for Aerospace Applications

The use of composites in the aerospace industry has been increasing exponentially. However, conventional epoxy resins, derived from petroleum sources, are not sustainable, making them non-degradable and environmentally harmful. In order to foster a sustainable environment, replacing conventional thermoset epoxies with bio-sourced carbon epoxies is imperative. With the enhancement in technology, it is possible to combine vegetable oils or bio-based copolymers with resins to make it recyclable in nature. Hence, it is necessary to study bio-based epoxies and carry out material characterization and see how they behave differently from conventional epoxies. This study examines the mechanical properties of different types of epoxy resins, which includes conventional, recyclable, and non-recyclable bio-epoxies. Tensile, bending, fracture toughness, and compression tests are performed in accordance with ASTM and ISO standards. The results show that the recyclable bio-epoxy exhibits comparable or superior properties when compared with conventional and non-recyclable bio-epoxies, particularly in terms of impact resistance. Recyclable epoxy, examined in the current study, shows a 73% higher strain energy release rate as compared to conventional epoxy. These results suggest that bio-epoxies could serve as a viable alternative to conventional epoxy.

Dual dynamic network structures of recyclable epoxy resins with high strength and toughness via sacrificial hydrogen-bonding clusters and imine bonds: surpassing the strength-toughness trade-off

Thermosetting epoxy resins with high strength, high toughness, and recyclability are strongly sought after, but due to crosslinked structure, balancing strength and toughness and achieving degradation and reprocessing of resins is a challenge. Enhancing strength without compromising toughness during designing recyclable epoxy resin is a challenging task. Herein, a solution is demonstrated where isofluorodioxanone diisocyanate (IPDI), Poly (propylene glycol) bis(2-aminopropyl ether) (D230), and naturally occurring vanillin synthesized a novel imine-functionalized epoxy monomer (IDV-EP), and cured with diamino-diphenylmethane (DDM), 2-(4-aminophenyl)-1H-benzimidazol-5-amine (APBIA) and 4, 4-diamino benzoyl triphenylamine (DABA), respectively, for the purpose of building different contents of the sacrificial hydrogen-bonding clusters (SHBC) in topological network structures. As the hydrogen-bonding (H-bonding) binding energy of the crosslinked network increases, the strength and toughness of the resin were increased concurrently, SHBC was demonstrated to be a feasible method to achieve the strength-toughness trade-off of thermosets, in which IDV-EP-DABA illustrated a tensile strength of 90 MPa, an elongation at break of 16.5%, as well as toughness of 9.73 MJ/m3, which was higher than that of the ordinary bisphenol A-type epoxy thermosetting resin (DGEBA/DDM) by 1.8, 3.7 as well as 6.7 times, respectively. Furthermore, the covalent adaptive networks (CANs) formed by imine bonds provide the resins with effective capabilities for swift degradation (rapid degradation in acid/solvent, room temperature and static conditions) and reprocessing (only a slight decrease in the mechanical properties after hot pressed at 180°C/10 MPa for 15 minutes) , showcasing promising prospects for sustainable utilization.

Biobased and Recyclable Epoxy Composite with High Thermal Conductivity Based on Disulfide Exchange

Biobased and Recyclable Epoxy Composite with High Thermal Conductivity Based on Disulfide Exchange

High performance, self-healable and recyclable epoxy thermosets by integrating dynamic hindered urea bonds

During recent decades, intrinsic self-healable thermosetting materials by integrating dynamic covalent bonds (DCBs) have been widely used. In this study, through the molecular structure design, a flexible polyurethane curing agent based on hindered urea bonds (HUBs) was successfully prepared by the reaction of Hexamethylene diisocyanate trimer (THDI) and sterically hindered secondary amine, which was further used as a curing agent to cure epoxy resin. Therefore, a new type of epoxy thermosetting material (TBEP) with dynamic hindered urea bond was synthesized and the preparation and performance of TBEP were confirmed and characterized by infrared spectroscopy, thermomechanical analysis and thermal stability test. The results show that the HUBs are successfully integrated into the epoxy resin and endow the epoxy thermosets with brilliant self-healable, reprocessable, recyclable and shape memory properties. Simultaneously, the existence of flexible polyurethane chain in the crosslinking network effectively solves the brittleness problem of epoxy resin, promotes the mechanical performance of epoxy thermosets, and further broadens the its application field. This work provides a simple and efficient method to design flexible and self-healable epoxy thermosets with brilliant mechanical performance, which solves the contradiction between outstanding mechanical properties and self-healing ability of epoxy resin to some extent, and is environmentally friendly and cost-effective in acquisition and application.

Recyclable epoxy resins with different silyl ether structures: structure–property relationships and applications in diverse functional composites

In this work, three liquid silyl ether-containing cycloaliphatic epoxides (CESI1, CESI2 and CESI3) with different numbers of pendent phenyl groups were synthesized and cured by methylhexahydrophthalic anhydride. The high glass transition temperatures of the cured epoxy resins range from 150 to 184 °C, which increase with the increasing number of pendent phenyl groups. However, the tensile strengths of the cured epoxy resins decrease as the number of pendent phenyl groups increase. Incorporating dynamic silyl ether bonds enables the cured epoxy resins to be reprocessable and degradable. It is demonstrated that the pendent phenyl groups have a significant impact on the dynamic behaviors and degradation rates of cured epoxy resins. The reprocessing efficiencies and degradation rates of the three epoxy resins monotonously decrease with increasing the number of pendent phenyl groups. Finally, the CESI1 resin with the lowest viscosity was used as the polymer matrix for carbon fiber reinforced composites, electromagnetic wave absorbing coatings and silver-filled conductive adhesives. Notably, non-destructive recovery of carbon fibers, ferromagnetic particles and silver flakes can be achieved through the degradation of the epoxy matrix.

Dynamic reversible mechanism of phenol-carbamate bonds and their potential for the development of thermo-healing, recyclable and high-performance epoxy thermosets

In this paper, a series of small monomers were synthesized to explore the dynamic reversible properties of phenol-carbamate bonds (PCBs), and the bimolecular exchange mechanism of PCBs was verified. Based on this mechanism, a novel thermo-healing recyclable epoxy resin with high mechanical performance and shape memory function was successfully developed. Meanwhile, the number of reversible phenol-carbamate bonds in this system was increased by molecular structure design, which increased the chance of reversible cleavage and recombination of the dynamic covalent bonds. Compared with our previous work, the thermo-healing efficiency of the epoxy thermosets was greatly increased, and the target polymer achieved an excellent balance between high thermo-healing efficiency and good mechanical performance. Further post-treatment experiments show that the cured epoxy resin can also be well recycled by solvent degradation or hot-pressing process. What's more, a novel carbon fiber (CF) reinforced polymer composite material was synthesized by taking that prepared epoxy thermoset as a matrix, which greatly improve the mechanical properties of epoxy resin, and the CF cloths can be recycled when the epoxy resin is removed by chemical degradation. In addition, due to the existence of reversible crystallizable switching segments in the epoxy resin system, when the temperature is above its Tg, it can be triggered by thermal energy, so that the film can rapidly recover from the temporary shape to the permanent shape. This study also provides a new direction for the development of novel multifunctional intelligent polymers.

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.

Sustainable epoxy composites with high thermal conductivity using poly(lipoic acid) modified liquid metal as crosslinkers

With the increasing demand for electronics integration and the increasing accumulation of e-waste, thermal management materials combing high conductivity and sustainability is highly desired. Herein, bio-based recyclable epoxy composites with significantly improved thermal conductivity were prepared by utilizing poly(lipoic acid) modified liquid metal (LM) as the crosslinkers. The strong interactions between poly(lipoic acid) and LM enable the formation of a poly(lipoic acid) shell layer coating on the LM droplet core, which facilitates the dispersion of LM droplets as well as interfacial thermal transport in the composites. As a result, the composite with only 25 vol% LM exhibits a thermal conductivity of 0.82 W/(m·K), which is higher than those of the previous studies with randomly filled LM at the same loading. In addition, attributed to the existence of dual dynamic bonds (disulfide bonds and β-hydroxyl ester bonds), the composites are reprocessable and their thermal conductivities are maintained after reprocessing. Moreover, the composites can be degraded under mild reducing conditions. This work presents a novel approach for the preparation of high-performance thermal management materials featuring recyclability and degradability.