Epoxy Thermosets Research Articles

Review on the Synthesis, Recyclability, Degradability, Self-Healability and Potential Applications of Reversible Imine Bond Containing Biobased Epoxy Thermosets

Epoxy thermosets need to be designed for simple recycling and biomass resource utilization in order to be fully sustainable building materials. The development of covalent adaptive networks (CANs) using adaptive covalent chemistry (ACC) may be helpful in this regard. Several reversible covalent bonds are incorporated into the epoxy polymer to overcome the challenge of reprocessability or recyclability, degradability and self-healability. The imine bond, also referred to as the Schiff base, is one of the reversible covalent bonds that can participate in both associative and dissociative reactions. This opens up possibilities for mechanical and chemical recycling as well as self-healing. This review summarises the progress related to the synthesis and mechanical and thermal properties of epoxy thermosets based on reversible imine bonds derived from different sustainable resources over the past few decades. The feedstocks, physical and thermal properties, recycling conditions, degradability and self-healability of the biomass epoxy thermosets are addressed along with the main obstacles, prospective improvements and potential applications.

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.

High toughness plant oil-based epoxy resins simultaneous with low dielectric properties

Developing novel multifunctional polymeric materials from biomass to replace traditional electric encapsulation materials such as epoxy resins and polydimethylsiloxane is of great significance for energy consumption and sustainable development. Here, a series of bio-based epoxy thermosets were prepared using a solvent-free ring-opening polymerization-initiated process from different epoxidized plant oils. Key factors such as reactive sites and optimized curing conditions for this system were studied systematically. Moreover, the relative properties of the resulting products, including thermal, mechanical, and dielectric performance were investigated and discussed deeply in correlation with their structure and reactivity. The findings in this work demonstrated that the adjustable chain stiffness and cross-link density of cured networks, achieved through the use of different types of vegetable oils, allows for tailoring the comprehensive properties of these networks. Among the prepared samples, soybean oil-based thermoset exhibited superior tensile strength (7.5 ± 0.28 MPa), elongation at break (87.56 ± 4.80%), glass transition temperature (Tg) of approximately 23 °C, storage modulus of 5.84 MPa, and a dielectric constant of approximately 2.95 at 10 MHz. These properties endow soybean oil-based thermoset as a suitable candidate substrate for encapsulation materials, in comparison with commercial polydimethylsiloxane. The findings suggest that the functional materials designed in this study hold promise as alternatives to petroleum-based polymers and offer paramount potential for applications in diverse electronic device applications.

Molecular design and properties of intrinsic flame‐retardant P‐N synergistic epoxy resin

AbstractIn recent years, the poor weather resistance and aging resistance of additive flame retardants have caused researchers to pay attention to reactive flame retardants. A novel P‐N coacting epoxy curing agent with intrinsic flame retardancy was designed and synthesized. The mechanical properties, crosslinking curing properties and flame‐retardant properties of intrinsic flame‐retardant epoxy resin were characterized. The results show that the cross‐linking curing performance of hexa (3,5‐diamino‐1,2,4 triazolyl)‐cyclotriphosphonitrile) (VCP) is lower than that of DDM. This is due to the decrease in cross‐linking density caused by the VCP ring molecular structure. Therefore, the mechanical properties of the epoxy resin cured with VCP decreased, but the flame‐retardant properties of the material significantly improved. The limiting oxygen index of the VCP/EP flame retardant epoxy thermosets was 27.3%, reaching the UL 94 V‐1 level. The peak heat release rate and total heat release rate of the VCP/EP flame retardant epoxy thermosets were significantly reduced. The flame retardancy mechanism was studied by Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, and x‐ray photoelectron spectroscopy. The results show that the intrinsic flame‐retardant P‐N coacting epoxy resin has excellent curing and flame‐retardant properties.

Hyperbranched Dynamic Crosslinking Networks Enable Degradable, Reconfigurable, and Multifunctional Epoxy Vitrimer.

Degradation and reprocessing of thermoset polymers have long been intractable challenges to meet a sustainable future. Star strategies via dynamic cross-linking hydrogen bonds and/or covalent bonds can afford reprocessable thermosets, but often at the cost of properties or even their functions. Herein, a simple strategy coined as hyperbranched dynamic crosslinking networks (HDCNs) toward in-practice engineering a petroleum-based epoxy thermoset into degradable, reconfigurable, and multifunctional vitrimer is provided. The special characteristics of HDCNs involve spatially topological crosslinks for solvent adaption and multi-dynamic linkages for reversible behaviors. The resulting vitrimer displays mild room-temperature degradation to dimethylacetamide and can realize the cycling of carbon fiber and epoxy powder from composite. Besides, they have supra toughness and high flexural modulus, high transparency as well as fire-retardancy surpassing their original thermoset. Notably, it is noted in a chance-following that ethanol molecule can induce the reconstruction of vitrimer network by ester-exchange, converting a stiff vitrimer into elastomeric feature, and such material records an ultrahigh modulus (5.45GPa) at -150°C for their ultralow-temperature condition uses. This is shaping up to be a potentially sustainable advanced material to address the post-consumer thermoset waste, and also provide a newly crosslinked mode for the designs of high-performance polymer.

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.

Optically transparent and high-strength glass-fabric reinforced composite

Fiber reinforced polymer composites (FRPs) are widely utilized in various industrial applications due to their significant advantages such as low cost and superior mechanical properties. However, owing to the trade-off between high mechanical strength and high optical transparency with typical FRPs, it is technically challenging to achieve high mechanical performance while meeting the optical requirements for transparent electronics and automotive applications. We herein report the synthesis of a transparent fiber reinforced polymer (tGFRP) by incorporating reinforced E-glass fiber into refractive-index-tunable thermosetting epoxy resin and the consequential advantageous opto-mechanical properties. By doping organic molecules, the optical property of the epoxy-based resin system has been efficiently engineered, achieving the match of the chromatic dispersions of the E-glass fiber and the epoxy resin. By the means of a novel technique derived from infusion treatment and in-situ polymerization combined with a liquid composite molding (LCM) method, both surface and bulk defects have been efficiently mitigated. With the refractive-index of the epoxy matrix matched with that of the embedded fiber fabrics, high transparency up to 88% has been realized with 10v.% fiber loading (500 μm thick tGFRP). An outstanding transparency and superior mechanical properties were achieved on 2 mm thick samples, maintaining up to 85% transmittance even when using 25 layers of E-glass fabric, corresponding to 50 v.% fiber. This work has shed light on the development of transparent composite materials for applications such as in construction, transportation, and protective equipment.

Cure characterisation and prediction of thermosetting epoxy for wind turbine blade manufacturing

In the wind industry, the increasing length of turbine blades and the use of thick composite sections make it vital to understand the thermoset and the underlying curing process. In this study, the curing of a specific thermoset epoxy for application in wind turbine blades is characterised by differential scanning calorimetry. The thermoset is analysed under dynamic and isothermal conditions to determine the complex cure behaviour under these various conditions. The data is fitted to determine firstly; the cure kinetics of the material through a kinetic model. Secondly; the phenomenon of the glass transition temperature Tg is studied to predict the development of Tg as a function of the degree of cure. Thirdly, a Diffusion-Kinetic model was utilised to make more accurate cure predictions based on the interaction between kinetic chemical reactions and diffusion control affecting the reaction rate through the incorporation of the glass transition temperature. A simple experimental framework for validating the diffusion-kinetic model was used to make a final validation of the model predictions on a larger scale. A set of final parameters is presented for application in a full diffusion-kinetic model for cure predictions and simulations beyond this study.

Sustainable Bio-Based Epoxy Resins with Tunable Thermal and Mechanic Properties and Superior Anti-Corrosion Performance.

Bio-based epoxy thermoset resins have been developed from epoxidized soybean oil (ESO) cured with tannic acid (TA). These two substances of vegetable origin have been gathering attention due to their accessibility, favorable economic conditions, and convenient chemical functionalization. TA's suitable high phenolic functionalization has been used to crosslink ESO by adjusting the -OH (from TA):epoxy (from ESO) molar ratio from 0.5:1 to 2.5:1. By means of Fourier-transform infrared spectroscopy, resulting in thermosets that evidenced optimal curing properties under moderate conditions (150-160 °C). The thermogravimetric analysis of the cured resins showed thermal stability up to 261 °C, with modulable mechanical and thermal properties determined by differential scanning calorimetry, dynamical mechanical thermal analysis, and tensile testing. Water contact angle measurements (83-87°) and water absorption tests (0.6-4.5 initial weight% intake) were performed to assess the suitability of the resins as waterproof coatings. Electrochemical impedance spectroscopy measurements were performed to characterize the anti-corrosive capability of these coatings on carbon steel substrates. Excellent barrier properties have been demonstrated due to the high electrical isolation and water impermeability of these oil-based coatings, without signs of deterioration over 6 months of immersion in a 3.5 wt.% NaCl solution. These results demonstrate the suitability of the developed materials as anti-corrosion coatings for specific applications.

Synergistic effects in the co-pyrolysis of thermoset epoxy resin and biomass on kinetics and product distribution

With the rapid growth of wind power, batteries, and aerospace industries, sustainable disposal of thermoset plastics has become a long-term concern. However, pyrolysis, a highly promising solution for this issue, has seen limited studied on thermoset plastics. Drawing inspiration from the synergistic interactions observed between thermoplastics and biomass, this study explored the co-pyrolysis of a thermoset epoxy resin, diglycidyl ether of bisphenol A, cured with 4,4′-Diaminodiphenylmethane, with ash-free biomass pseudo-components (cellulose, lignin, and xylan) using thermogravimetry and Py-GC/MS. The results revealed distinctive synergistic effects in Epoxy-Cellulose and Epoxy-Lignin mixtures, delaying the onset and hastening the conclusion of weight loss. This was accompanied by increased activation energies, assessed through the Friedman and Kissinger-Akahira-Sunose methods, as well as higher char yields. Conversely, in Epoxy-Xylan, synergistic effects led to reduced activation energy across most α values, resulting in a lower onset temperature and a heightened maximum rate of mixture degradation. Regarding product distribution, all three pseudo-components, particularly lignin, enhanced the selectivity of bisphenol A. Additionally, cellulose and xylan altered the kinetics of N–C and C(N)–C(OH) bond cleavage, reducing the amount and altering the distribution of 2-B-N compounds (compounds featuring two benzene rings and nitrogen). Conversely, the presence of resin reduced the volatiles from cellulose and xylan, promoting phenol-type products and –H and alkylene side chains while diminishing guaiacol-type products and alkyl side chains. Three mechanisms were proposed to elucidate these observed synergistic effects: catalytic effects of –OH groups from biomass pseudo-components, hydrogen exchange or hydrogen bonding between resin-derived nitrogen and biomass-derived oxygen-containing functional groups, and radical reactions. These insights into the co-pyrolysis of biomass and cured epoxy resin can inform the selection of suitable feedstocks for various applications.

Progressive approximation method toward uniformly-dispersed black phosphorene/epoxy composites

Black phosphorus (BP) nanosheets are commonly used flame retardants in polymers, but the challenge of poor dispersion in the polymer matrix makes their addition relatively large, which undoubtedly increases the cost of BP-based flame-retardant materials due to the expensive price of BP. Here, progressive approximation is adopted as a macroscopic methodological guidance for BP dispersion, where the dispersions and aggregates in partially dispersed solutions are continuously separated by multiple experiments to finally approach a uniform dispersion state. Additionally, a bio-based interfacial layer is also constructed on the BP surface (i.e., BNTP) to reduce black phosphorus self-aggregation and further promote dispersion as well as achieve synergistic flame retardancy through microscopic interfacial engineering. The rationality of this method is verified by BNTP modified epoxy thermosets, which achieve a UL-94 V0 level and a LOI of 29.4 % with an ultralow BNTP loading (0.3 wt%). This work provides a facile concept for achieving uniform dispersion of 2D nanosheets in organic matrices from a mathematical methodological guidance, and simultaneously contributes to a significant reduction in the BP content in polymer matrices, solving the cost problem of BP-based flame-retardant materials and promoting their industrialization.

Experimental study on interfacial shear strength between date palm fibers and thermoset resins

Date palm fibers (DPFs) can be potentially used in novel applications including composites. In this study, the Microbond technique was used to measure the interfacial shear strength (IFSS) between DPFs extracted from the mesh (ME), the midrib (MI), and the Spadix stem (SS) and three types of thermoset resins, i.e., unsaturated polyester (P), epoxy (E), and phenolic Resol (R). The fibers’ tensile strength was measured and the aspect ratio of the fibers (critical length/diameter) for different fiber-resin systems was determined. It was shown that there is a clear relationship between resin droplet contact angle and IFSS. SS fibers provided the highest and ME fibers the lowest IFSS values among the fibers. Among the resins, E provided the highest and R the lowest adhesion to all fibers. The IFSS values of the E with MI, SS, and ME fibers were 7.62, 8.89, and 5.88 MPa, for the R matrix were 2.94, 3.02, and 2.33 MPa, and for the P matrix were 5.59, 5.97 and 5.17 MPa, respectively. It was shown that there is a clear relationship between IFSS values and contact angles of different fiber/matrix systems.

A Self-Healing Thermoset Epoxy Modulated by Dynamic Boronic Ester for Powder Coating.

Thermoset powder coatings exhibit distinctive characteristics such as remarkable hardness and exceptional resistance to corrosion. In contrast to conventional paints, powder coatings are environmentally friendly due to the absence of volatile organic compounds (VOCs). However, their irreversible cross-linking structures limit their chain segment mobility, preventing polymers from autonomously repairing cracks. Dynamic cross-linking networks have garnered attention for their remarkable self-healing capabilities, facilitated by rapid internal bond exchange. Herein, we introduce an innovative method for synthesizing thermoset epoxy containing boronic ester moieties which could prolong the life of the powder coating. The epoxy resin system relies on the incorporation of two curing agents: one featuring small-molecule diamines with boronic bonds and the other a modified polyurethane prepolymer. A state of equilibrium in mechanical properties was achieved via precise manipulation of the proportions of these agents, with the epoxy composite exhibiting a fracture stress of 67.95 MPa while maintaining a stable glass transition temperature (Tg) of 51.39 °C. This imparts remarkable self-healing ability to the coating surface, capable of returning to its original state even after undergoing 1000 cycles of rubbing (using 1200-grit abrasive paper). Furthermore, the introduction of carbon nanotube nanoparticles enabled non-contact sequential self-healing. Subsequently, we introduce this method into powder coatings of different materials. Therefore, this work provides a strategy to develop functional interior decoration and ensure its potential for broad-ranging applications, such as aerospace, transportation, and other fields.

Molecular design and properties of a P‐N synergistic flame retardant epoxy resin curing agent

AbstractThe utilization of epoxy resin (EP) is widespread across various industries. Nevertheless, its combustibility restricts its broader utilization. Additive flame retardants have proven effective in enhancing EP flame retardancy in recent decades. However, poor weather and aging resistance have always been the defects of added flame retardants. A reactive flame retardant (VPD) was synthesized from 3, 5‐diamino‐1, 2, 4‐triazole and phenylphosphonic dichloride, and was used to prepare VPD/EP flame retardant epoxy thermosets. The mechanical properties of the VPD/EP flame retardant epoxy thermosets are lower than those of pure epoxy, because the distance between molecules of VPD is increased compared with that of DDM, the cross‐linking density of the polymer after curing is decreased, and the intermolecular force is reduced. The VPD/EP flame retardant epoxy thermosets have a 40% reduction in total heat release compared to pure EP, an increase in limiting oxygen index value from 24.7% (pure EP) to 29.4%, and a V‐1 rating in UL‐94 testing. The obtained EP has not only good flame retardancy but also better weather resistance and aging resistance than additive flame retardants.Highlights An intrinsic flame‐retardant P and N coacting EP was designed and synthesized. The intrinsic flame‐retardant EP has excellent weather and aging resistance. The presence of the P, the residual char generated by combustion is denser.

Flame retardant, strong and tough, low dielectric epoxy thermoset enabled by a novel bi-DOPO type epoxy with aggregation-induced emission

Multifunctional epoxy resins with excellent flame retardancy, high strength, high toughness, low dielectric loss and strong fluorescence show great potentials in electrical/electronic applications; however, simultaneously achieving those goals are difficult. To address those challenges, an epoxy resin with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) pendant group and flexible aliphatic spacer (DOPOEP) was developed. V-0 rating in UL-94 test and LOI of 33.1% are achieved at only 0.45 wt% phosphorus loading, benefitting from the flame retardant mechanism in gas and condensed phase. Compared with neat DGEBA, the cured epoxy with 10 wt% DOPOEP shows 13.4% and 37.6% improvements in tensile strength and impact strength, respectively; in addition, the tensile strength of the cured epoxy with 20 wt% DOPOEP is as high as 102.2 MPa, a 35.3% increase, without compromising toughness. The strengthening effects of DOPOEP derive from π-π stacking and hydrogen bonding, and the improved toughness is due to the flexible aliphatic spacer and reduced crosslinking density. Meanwhile, free volume reduction was observed after introducing DOPOEP, which is responsible for the improved modulus. DOPOEP addition also lowers the hydroxyl group concentration, leading to lower dielectric constant and dielectric loss tangent. Moreover, the aggregation-induced emission behavior of DOPOEP was reported, and its application in anti-counterfeiting field was explored. In air, the fluorescence of DOPOEP-containing thermoset steadily diminishes with increasing temperature, however the fluorescence changing trend is complex in vacuum. Besides, the fluorescence of DOPOEP is effective in the visual detection of defects and flaws in cured products. This work provides a new direction for developing high-performance epoxy with multifunction.

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.

Revolutionizing epoxy performance: A new flame retardant with phosphorus and maleimide for enhanced cure behavior, thermal stability, flame retardancy, and mechanical properties

AbstractA novel phosphorus‐based maleimide, tris[4‐(2,5‐dioxopyrrol‐1‐yl)phenyl] phosphite (TDPPI), was employed as a flame retardant in epoxy with moderate transparency, enhanced thermal stability, and improved mechanical strength. SEM studies ascertained the uniform dispersion of TDPPI and its compatibility with the epoxy. Differential scanning calorimetry showed that the epoxy compounded with 2.5 wt% of TDPPI achieved 100% curing with the glass transition temperature of 43.8°C. The thermogravimetric study revealed that the presence of 2.5 wt% of TDPPI in epoxy increased the residual mass found at 600°C by 23%. The epoxy thermoset compounded with 2.5 wt% of TDPPI passed the Underwriter's Laboratory (UL‐94) test with V‐1 classification and achieved the limiting oxygen index of 30.7% with a reduced burning rate of about 58.7%. At 1.5 wt% of the TDPPI, the resulting epoxy thermoset exhibited an enhancement in the tensile strength (23%), Young's modulus (37%), flexural strength (9%), and Izod impact strength (37%) relative to the neat epoxy. In conclusion, 2.5 wt% of TDPPI is adequate for imparting flame retardancy to epoxy composites with improved mechanical strength because of the maleimide‐phosphorus synergistic effect.Highlights TDPPI is a phosphorus and maleimide containing flame retardant. TDPPI improves thermal stability, flame retardancy and mechanical strengths. Only 2.5 wt% of TDPPI is required for epoxy to achieve high flame retardancy. Maleimide presence lowers phosphorus requirement for epoxy flame retardancy. TDPPI is a promising new flame retardant material for epoxy.

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.

Long-chain amino acids and hydrogen bonding enhanced epoxy thermosets: High latency, thermal and mechanical properties, and its application in recyclable carbon fiber composites

Thermoset carbon fiber reinforced polymer (CFRP) composites are widely used and in increasing demand in composites industries. Among them, CFRP with one-component epoxy matrix can optimize the preparation process. However, high-value carbon fiber (CF) is difficult to recover and no cases of latent CFRP with CF recyclability have been reported. In this study, long aliphatic chains were innovatively introduced into amino acid type latent curing agents to improve the latent performance and the solubility in organic solvents. The latent performance of curing agents improved with the increase of chain length due to the difficulty in proton transfer of ammonium ions along to the long chain. The latent curing agent derived from 1,10-diaminodecane exhibited good solubility in ethanol and good compatibility with epoxy monomers, making it had the potential for industrial use in the preparation of CFRP. Moreover, amide bonds in the synthetic amino acid were prone to form hydrogen bonds to improve the thermal and mechanical properties of cured resin, while ester bonds in the amino acids imparted the resin with degradability in alkaline environments, thereby endowing CFRP with CF recyclability without damaging surface morphology and chemical structure. This work will open a door for the recycling research of latent CFRP.

Sustainable biobased epoxy thermosets with covalent dynamic imine bonds for green composite development

The widespread use of thermoset composites has raised great environmental and economic awareness, due to the lack of recyclability of the resins and fibres. This work is focused on the development of a sustainable, recyclable and reprocessable epoxy vitrimer by using an epoxidized linseed oil resin and a vanillin derived Schiff base as dynamic hardener. Different epoxy:dynamic hardener ratios have been employed, analyzing its influence in the final physico-chemical properties of the thermoset resin. The reprocessability and the recyclability of the vitrimers have been demonstrated. This work also describes the manufacturing and characterization of greener composites obtained with these vitrimers and flax yarn fibres. A good adhesion between the fibres and the vitrimer is observed. Moreover, the characterization of developed composites indicated that their thermal and mechanical properties allow their use in lightweight engineering applications. Finally, chemical recyclability and compostability of the flax fibre vitrimer-based composite have been demonstrated.