Cross-linked Epoxy Research Articles

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.

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.

Understanding creep in vitrimers: Insights from molecular dynamics simulations

Vitrimers offer malleability and recyclability due to covalent adaptable networks, however, they are prone to low-temperature creep. In this work, we investigate the molecular mechanisms of vitrimer creep using molecular dynamics simulations. We model the interplay between dynamic bonding with mechanical loading using a topology-based reaction scheme. The creep behavior is compared against cross-linked epoxies with dynamic reactions to understand the unique aspects related to dynamic bonding. It is found that the free volume that arises from tensile loads is reduced in vitrimers through dynamic bond rearrangement. An important feature that distinguishes the secondary creep behavior between epoxies and vitrimers is the orientation of the dynamic bonds during loading. In vitrimers, the dynamic bonds preferentially align orthogonal to the loading axis, decreasing the axial stiffness during secondary creep, resulting in larger creep strain compared to epoxies. Over longer timescales, such increased strain leads to void growth, resulting in tertiary creep.

Asynchronous Ring Opening of Cyclic Carbonate and Glycidyl Ether Induced Phase Evolution Towards Heat-Free and Rapid-Bonding Superior Epoxy Adhesive.

Structural adhesives that do not require heating are in high demand in the automotive and electronics industries. However, it remains a challenge to develop robust adhesives that rapidly achieve super adhesion near ambient temperature. Herein, a room-temperature curable, fast-bonding, and super strong epoxy-based structural adhesive was designed from the perspective of cross-scale structure, which lies in threefold pivotal aspects: (i) high branching topology of glycerol carbonate-capped polyurethane (PUGC) increases the kinetics of the ring-opening reaction, contributing to fast crosslinking and the formation of abundant urethane and hydroxyl moieties; (ii) asynchronous crosslinking of epoxy and PUGC synergistically induces phase separation of PUGC within the epoxy resin and the resulting PUGC domains surrounded by interpenetrated shell serves to efficiently toughen the matrix; (iii) abundant dynamic hydrogen bonds including urethane and hydroxyl moieties, along with the elastomeric PUGC domains, dissipate energy of shearing force. As a result, the adhesive strength rapidly grows to 16 MPa within 4 hours, leveling off to 21 MPa after 7 hours, substantially outperforming commercial room-temperature curable epoxy adhesives. The results of this study could advance the field of high-performance adhesives and provide valuable insights into designing materials for efficient curing at room temperature.

Asynchronous Ring Opening of Cyclic Carbonate and Glycidyl Ether Induced Phase Evolution Towards Heat‐Free and Rapid‐Bonding Superior Epoxy Adhesive

AbstractStructural adhesives that do not require heating are in high demand in the automotive and electronics industries. However, it remains a challenge to develop robust adhesives that rapidly achieve super adhesion near ambient temperature. Herein, a room‐temperature curable, fast‐bonding, and super strong epoxy‐based structural adhesive was designed from the perspective of cross‐scale structure, which lies in threefold pivotal aspects: (i) high branching topology of glycerol carbonate‐capped polyurethane (PUGC) increases the kinetics of the ring‐opening reaction, contributing to fast crosslinking and the formation of abundant urethane and hydroxyl moieties; (ii) asynchronous crosslinking of epoxy and PUGC synergistically induces phase separation of PUGC within the epoxy resin and the resulting PUGC domains surrounded by interpenetrated shell serves to efficiently toughen the matrix; (iii) abundant dynamic hydrogen bonds including urethane and hydroxyl moieties, along with the elastomeric PUGC domains, dissipate energy of shearing force. As a result, the adhesive strength rapidly grows to 16 MPa within 4 hours, leveling off to 21 MPa after 7 hours, substantially outperforming commercial room‐temperature curable epoxy adhesives. The results of this study could advance the field of high‐performance adhesives and provide valuable insights into designing materials for efficient curing at room temperature.

Tailoring the Structure and Physico-Chemical Features of Cellulose-Based Hydrogels Using Multi-Epoxy Crosslinking Agents.

Hydrogel features can be designed and optimized using different crosslinking agents to meet specific requirements. In this regard, the present work investigates the physico-chemical features of cellulose-based hydrogels, designed by using different epoxy crosslinkers from the same glycidyl family, namely epichlorohydrin (ECH), 1,4-butanediol diglycidyl ether (BDDE), and trimethylolpropane triglycidyl ether (TMPTGE). The effect of the crosslinker's structure (from simple to branched) and functionality (mono-, bi- and tri-epoxy groups) on the hydrogels' features was studied. The performances of the hydrogels were investigated through the gel fraction, as well as by ATR-FTIR, DVS, SEM, DSC, and TG analyses. Also, the swelling and rheological behaviors of the hydrogels were examined. The advantages and limitations of each approach were discussed and a strong correlation between the crosslinker structure and the hydrogel properties was established. The formation of new ether bonds was evidenced by ATR-FTIR spectroscopy. It was emphasized that the pore size is directly influenced by the crosslinker type, namely, it decreases with the increasing number of epoxy groups from the crosslinker molecule, i.e., from 46 ± 11.1 µm (hydrogel CE, with ECH) to 12.3 ± 2.5 µm (hydrogel CB, with BDDE) and 6.7 ± 1.5 µm (hydrogel CT, with TMPTGE). The rheological behavior is consistent with the swelling data and hydrogel morphology, such as CE with the highest Qmax and the largest pore size being relatively more elastic than CB and CT. Instead, the denser matrices obtained by using crosslinkers with more complex structures have better thermal stability. The experimental results highlight the possibility of using a specific crosslinking agent, with a defined structure and functionality, in order to establish the main characteristics of hydrogels and, implicitly, to design them for a certain field of application.

Universal Copolymerization of Crosslinked Polyether Electrolytes for All-Solid-State Lithium-Metal Batteries.

Solid polymer electrolytes (SPEs) are pivotal in advancing the practical implementation of all-solid-state batteries. Poly(1,3-dioxane) (PDOL)-based electrolytes have attracted significant attention due to the pseudo-high conductivity achieved through sophisticated in situ polymerization methods; however, such PDOL-based electrolytes present challenges of crystallization over time and monomers residual during processing. In this study, integrating LiTFSI and LiDFOB as a universal copolymerization strategy for developing high-performance PDOL electrolytes with a wide range of epoxy crosslinkers is proposed. It is discovered that this approach leverages the protective effects of TFSI anions on the boron active center and catalyzes polymer chain growth via crosslinking. The homogenously crosslinked (benzene-centered) PDOL electrolyte exhibits remarkable thermo-mechanical stability (up to 100°C), high ion migration number (tLi+ = 0.42), a wide electrochemical window (≈5.0V vs Li+/Li), and high ionic conductivity (4.5×10-4 S cm-1). Notably, the crosslinked PDOL electrolyte is in the all-solid-state with minimal monomer/oligomer residual, exhibiting no crystallization during relaxation, delivering a robust performance in all-solid-state lithium metal batteries.

Fracture of Epoxy Networks Using Atomistic Simulations.

Predicting fracture properties through all-atomistic simulations poses challenges due to classical force field limitations in breaking covalent bonds and the computational demands of reactive force fields like ReaxFF. In addressing this, we propose a scale-bridging method for forecasting the fracture behavior of highly cross-linked epoxy combining classical force fields, the LAMMPS package REACTER, and for bond breaking a parameter based on experimental distance criterion. In our analysis, we anticipate the macroscopic fracture energy GC of the epoxy network through the application of a continuum fracture mechanics model developed for fibrils. In addition, we extract the value of the stress intensity factor KI. This modeling approach is specifically implemented for a frequently used epoxy system that consists of bisphenol F and DETDA hardener. Notably, our results demonstrate a robust correlation with existing literature and experimental studies. Moreover, our approach boasts a substantial computational time advantage, facilitating calculations that are significantly faster compared to those performed using reactive force fields.

Preparation and evaluation of Urea‐F amine crosslinkers

AbstractThis study presents the preparation of a long‐chain amine solution, employing the concept of urea‐formaldehyde synthesis. We also investigate the performance shifts in the cured product post‐urea addition. The Urea‐F curing agent is prepared by reacting EDA/DETA/TETA/TEPA, formaldehyde, and urea, and crosslinked with bisphenol A epoxy resin (E51). The crosslinking agent solves the problem of urea not being suitable for curing epoxy resin, and it improves the problem of low mechanical strength of traditional amine crosslinked epoxy resin cured materials. The cured product underwent evaluation through acid and alkali resistance solution measurement and mechanical and thermal tests. The results show enhanced heat resistance in the cured product, with its maximum thermal decomposition rate exceeding 350°C. The cured products have better mechanical properties than the amine monomers, with enhanced bending strength and superior acid–base solution resistance. The scanning electron microscopy micrograph indicates that the cured product fractures have a tight internal structure.

Size Effect of Graphite Nanosheet-Induced Anti-Corrosion of Hydrophobic Epoxy Coatings

In order to broaden the selectivity of graphite nanosheet additives on epoxy resin-based coatings and verify the size effect, this work aims to dope graphite nanosheets of different sizes into the three-dimensional structure produced by cross-linking and curing epoxy resin and polyamide resin. In addition, a micro-nano level secondary structure and a surface with special roughness are constructed to obtain the composite epoxy hydrophobic coating. The influence of the size effect of graphite nanosheets on the hydrophobic performance and corrosion resistance of the coating is summarized as well. Among them, the optimized doping size (2.2 μm) of graphite nanosheets in the epoxy coating showed the largest impedance arc of 2.58 × 108 Ω cm2, which could form an excellent nano-network covering the micropores to impede the diffusion of corrosive medium. Through simulation calculation analysis, we also found that the edge site of graphene is more effective in capturing H2O and O2; therefore, a smaller size of graphene with a large edge can be more favorable. This work will be used as a reference for the industrial application of graphite anti-corrosive coating.

Mechanical properties analysis of cross-linked epoxy resin

Abstract For this study, bisphenol A epoxy resin (DGEBA) was used as the resin matrix, and 3,3’-diaminodiphenyl sulfone (33DDS) was used as the curing agent. The effects of different cross-linking densities on the mechanical properties of epoxy resins were studied by molecular dynamics (MD) simulation, and the changes in the mechanical properties of epoxy resins under five different cross-linking density levels were predicted [1]. The results showed that as the cross-linking density of the epoxy resin increased, the mechanical parameters (such as elastic modulus, shear modulus, and bulk modulus) of the epoxy resin system also increased. This indicates the importance of improving the curing process of epoxy resin to enhance its mechanical properties.

Chitosan toughened epoxy resin by chemical cross-linking: Enabling excellent mechanical properties and corrosion resistance

There is a growing demand for the development of epoxy resin modified with biomaterials, aiming to achieve high toughness. Herein, chitosan crosslinked epoxy resin (CE) was synthesized by diisocyanate as a bridge. With 4,4′-diamino-diphenylmethane (DDM) as the curing agent, thanks to the unique cross-linking structure of the CE resin and the presence of carbamate groups, the cured CE/DDM exhibited superior properties compared to commercially available epoxy resin (E51). The tensile strength of the cured CE-3/DDM reached 90.17 MPa, the elongation at break was 11.2 %, and the critical stress intensity factor (KIC) measured 1.78 MPa m1/2. These values were 21.4 %, 151.6 %, and 81.6 % higher than those of the cured E51/DDM, respectively. It is worth noting that the addition of biomass material chitosan did not reduce the thermal stability of the resin. Additionally, the CE coatings on the metal substrate exhibited exceptional corrosion resistance, as evidenced by higher impedance values in electrochemical impedance spectroscopy (EIS) and polarization voltages in the Tafel curve compared to those of the E51 coating. This study opens up a novel approach to modifying epoxy resin with biomass materials with high toughness and corrosion resistance, without sacrificing other performance.

A novel flame retardant epoxy thermoset based on renewable honokiol and furfuryl alcohol

In this study, honokiol epoxy resin was directly functionalized by bio-based material honokiol, which was combined with furfuryl alcohol-derived flame retardant by Diels-Alder reaction to prepare bio-based epoxy cross-linking system. After curing with 4,4′-diaminodiphenyl methane, HE1/FP0.5/DDM (121.37 °C) offered higher glass transition temperature than EP/DDM (91.75 °C). Meanwhile, the HE1/FP0.5/DDM exhibited excellent thermal stability and flame retardancy compared to EP/DDM. The char residue of HE1/FP0.5/DDM was obviously increased from 12.8% (EP/DDM) to 35.6% at 700 °C. Notably, the HE1/FP0.5/DDM achieved a limiting oxygen index value of 37.4% and UL-94 V-0 rating. Cone calorimeter demonstrated that the peak heat release rate, total heat release and fire growth rate of HE1/FP0.5/DDM were 400.29 kW/m2, 33.45 MJ/m2 and 4.40 kW/m2·s, reduced by 57.47%, 63.39% and 41.1% compared with EP/DDM, respectively. The satisfactory flame retardancy and smoke suppression were attributed to the flame-retardant function of FP in the condensed and gas phases. Thus, this work not only provided a simple but also eco-friendly approach to achieve flame retardant thermosets from honokiol to replace bisphenol A epoxy.

Design of Soft/Hard Interface with High Adhesion Energy and Low Interfacial Thermal Resistance via Regulation of Interfacial Hydrogen Bonding Interaction.

Adhesion ability and interfacial thermal transfer capacity at soft/hard interfaces are of critical importance to a wide variety of applications, ranging from electronic packaging and soft electronics to batteries. However, these two properties are difficult to obtain simultaneously due to their conflicting nature at soft/hard interfaces. Herein, we report a polyurethane/silicon interface with both high adhesion energy (13535 J m-2) and low thermal interfacial resistance (0.89 × 10-6 m2 K W-1) by regulating hydrogen interactions at the interface. This is achieved by introducing a soybean-oil-based epoxy cross-linker, which can destroy the hydrogen bonds in polyurethane networks and meanwhile can promote the formation of hydrogen bonds at the polyurethane/silicon interface. This study provides a comprehensive understanding of enhancing adhesion energy and reducing interfacial thermal resistance at soft/hard interfaces, which offers a promising perspective to tailor interfacial properties in various material systems.

Effect of processing conditions on the properties of vitrimerized polybutylene terephthalate prepared by reactive extrusion

This study examined the effects of continuous reactive extrusion on the properties of vitrimerized thermoplastics derived from polybutylene terephthalate (PBT). PBT formulations were prepared using epoxy (crosslinker) and zinc acetylacetonate (transesterification catalyst) to form dynamic covalent networks. Reactive extrusion was performed at different screw rotation speeds (50, 100, 200 rpm) with controlled residence times to compare the effects of the shear stress during the formation of crosslinks on the vitrimer properties. The rheological behavior of the extruded vitrimerized PBT suggests that competition between chain scission and branching occurs during the dynamic crosslinking of thermoplastics. Increasing the shear stress and/or time in the extruder was found to favor chain scission over crosslinking which makes gelation unattainable. In the case of injection molded specimens, tensile and X-ray diffraction tests of vitrimerized PBT extruded at different shear stress indicated a significant relationship between the degree of crosslinking, the orientation of crystallites, and the ability of the polymer network to withstand deformation at elevated temperatures.

Molecular Dynamics Simulations on Epoxy Resin Composite via Grafting Acryloyl-chloride to Inhibit Electron Transport and Improve Thermal-mechanical Properties

Electrical, thermal, and mechanical properties of cross-linked epoxy resin (EP) modified by the chemical grafting of acryloyl chloride (AC) were studied to explore the trapping mechanism of charge transport inhibition. The bound state traps deriving from grafted molecules were analyzed by first-principles calculations combined with electron transmission spectra to study the underlying mechanism of the electrical properties. In contrast to pure EP, the EP-graft-AC (EP-g-AC) represents significantly depressed conductivity due to the electron scattering from polar-groups of the grafted AC molecule. The substantial deep traps are generated in EP-g-AC molecules by the polar group of grafted AC and accordingly decrease charge mobility and raise the charge injection barrier, consequently suppressing space charge accumulation and charge carrier transport. EP-g-AC polymer acquires a significant amelioration in thermal and mechanical properties, as indicated by the higher cohesive energy density, glass transition temperature, and decomposition temperature in consistence with the lower thermal vibrations compared with pure EP polymer, except that the resulting higher fractional free volume is not preferable, which is attributed to the mixing incompatibility of the grafted AC molecules with EP molecular-chains.

Aging of Adhesives of External Reinforcement Systems of Building Structures. Part 2. Structural Studies

The structural changes of cross-linked epoxy polymer adhesive for external reinforcement systems of building structures after exposure to artificial climatic factors have been studied by the method of IR Fourier spectroscopy. The aim of the study was to determine the significance of influencing factors (UV radiation, temperature, water and alkaline environment) for the development of a method for accelerated assessment of the durability of selected systems. The objectives of the study were to identify the mechanism of aging and obtain differential spectra of aged and initial samples. The processes of destruction of samples of cross-linked epoxy polymer of amine curing under the influence of artificial climatic factors of aging have been studied by IR spectroscopy. It has been established that the most significant factors influencing structural changes in the polymer are UV radiation and an alkaline solution of NaOH and KOH.

Ultra-high cross-linked active ester-cured epoxy resins: Side group cross-linking for performance enhancement

Active ester hardeners are a unique class of epoxy hardeners that undergo a curing reaction without generating secondary hydroxyl groups. This characteristic enables the production of low dielectric and low moisture absorption epoxy resins. In this study, two active ester hardeners, bisphenol A diisobutyrate (DIB) and bisphenol A dimethacrylate (DMA), were used to prepare epoxy resins by reacting with a resveratrol-based epoxy monomer (REP). By cross-linking the double bonds in the resin (using dicumyl peroxide, DCP), the properties of the resin can be dramatically changed. Among the resins prepared, REP/DMA/DCP exhibited the highest cross-linking density, approximately 48 times that of REP/DIB. Its storage modulus at 300 °C remained at 600 MPa, indicating an equivalent glass transition temperature (Tg) higher than 300 °C. Additionally, the cross-linking of the ester side groups increased the initial degradation temperature by 12–13 °C, reaching 395–396 °C. Regarding the coefficient of thermal expansion (CTE), the cross-linking of ester side groups had no significant effect on the CTE in the glassy state. However, it drastically reduced the CTE above the glass transition temperature. The average CTE (30–300 °C) of REP/DMA/DCP was only 106 ppm/°C. Furthermore, the cross-linking of the ester side groups appeared to reduce the dielectric constant but increased the dielectric loss and the water absorption rate. Nevertheless, REP/DMA/DCP's dielectric constant at 10 MHz of 3.07, dielectric loss of 0.01, and saturated water absorption of 0.77 wt% remain in the low range.

Correction to “Study of the impact of epoxidized plant oil phospholipids and hydroxyl radicals on crosslinked epoxy resins”

Correction to “Study of the impact of epoxidized plant oil phospholipids and hydroxyl radicals on crosslinked epoxy resins”

A review on improved physical and thermal properties of oxide nanoparticles reinforced epoxy composites

Epoxy resins are well-known because of their desirable thermal and mechanical characteristics in a variety of fields, including the automotive, construction, and aerospace sectors. However, the inherent brittle nature of highly cross-linked epoxy resins generally leads to weakness in resisting the formation of cracks and their movement. The brittleness of the epoxy resins is one of the major obstacles inhibiting its use at a wider scale. Therefore, many researchers focused on reinforcement of epoxy resins by different types of nanostructures including carbon nanotubes (CNTs), organic/inorganic nanofillers to provide higher strength, without diminishing other essential thermo-physical characteristics of the nanocomposites. Most of the review articles focused on the CNT-reinforced epoxy composites and very limited review articles are available that focus on the oxide nanofiller reinforced epoxy composites. In this review article, epoxy nanocomposites reinforced with alumina (Al2O3), titania (TiO2), silica (SiO2), and zirconia (ZrO2) nanoparticles have been investigated. The influence of the oxide nanoparticles in modifying the physical and thermal properties of the epoxy nanocomposites has been presented, compared, and critically analysed to optimize the performance of epoxy nanocomposites.