Structural Epoxy Research Articles

Debondable Epoxy-Acrylate Adhesives using β-Amino Ester Chemistry.

The reuse of multilayered materials, which are held together by structural epoxy adhesives, is a major challenge since the bonded substrates cannot be easily separated for recycling. In this research, we explore a one-pot strategy based on β-amino ester chemistry for the development of modified epoxy adhesives with on-demand debonding potential. For this, a formulation of commercially available acrylate, epoxy and amine compounds is used. The research starts with a systematic study, demonstrating the influence of the different compounds on the thermal and adhesive properties of the materials. Subsequently, the potential for debonding is demonstrated using rheological measurements and tensile tests. The fast, catalyst-free Aza-Michael reaction enables the straightforward preparation of such epoxy-based adhesives, while the reverse reaction allows for debonding at 120 °C. In general, a chemical design is demonstrated for producing an industrially attractive generation of debondable epoxy-based adhesives.

Analyses and control of interphase structures and adhesion properties of epoxy resin/epoxy resin for development of CFRP adhesion systems

In the adhesion of carbon fiber reinforced plastics (CFRP), the boundary regions were composed of only epoxy resins of CFRP matrix and adhesives, and their similar epoxy structures pose significant difficulty in studying the adhesion mechanism. Herein, we focused on structure and properties of laminates of epoxy resin substrates and adhesives with different curing conditions of substrates. We prepared laminates using deuterated epoxy adhesives or fluorinated epoxy adhesives, and their boundary regions were identified through confocal Raman scattering measurements. The regions were distributed into “penetration” and “interphase”. In particular, the penetration phase is a key of the adhesion properties. The penetration behaviors strongly depended on the crosslinking densities of the adherends, suggesting that the penetration regions would be directly impacted by the curing conditions of the substrates and molecular size of epoxy adhesive precursors. Our findings provide insights into novel designs of the reliable adhesion-based manufacturing systems of CFRP.

Design of self-healing and anticorrosion epoxy coating with active multiple hydrogen bonds based on grafted polyetheramine

The healing of epoxy coatings in damaged areas significantly influences their service life and protection quality. Intrinsic self-healing coatings, which enable multiple healing cycles without the need for additional functional carriers, have emerged as a promising coating technology. Nevertheless, the cross-linked structure of thermoset epoxy coating system restricts the large-scale migration of molecular chains, posing a significant challenge to achieve intrinsic healing of resin without compromising its mechanical properties. In this study, we report a design scheme for epoxy coating with intrinsic self-healing properties based on active multiple hydrogen bonds. Grafting aminobenzothiazole at the end of polyetheramine D230 chain, the grafted polyetheramine (GD230) was synthesized and served as the auxiliary curing agent, whose accurate molecular structure has been confirmed by different characterizations. The self-healing coating with reversible dynamics network based on hydrogen bonds was prepared by crosslinking through epoxy resin and mixed curing agent of polyetheramine D230 and GD230. Various measurements have been adopted to evaluate the self healing and anticorrosion performance. Results showed that the functional coating with a ratio of 8:2 for D230 to GD230 has satisfied both performances on the steel substrate. After immersion in 3 wt% NaCl solution for 120 days, the impedance value of functional coating still remains about 1010 Ω cm2. After the coating was scratched and immersion for 96 h, the impedance value of functional coating increased by about two orders of magnitude compared to that in blank coating. Mechanical testing and theoretical calculations were used to support dynamic crosslinking model to reveal the self-healing mechanism.

Standard specimen geometries do not always lead to consistent fatigue results for epoxy adhesives

This paper questions the recommendation regarding the use of standard specimen geometries, (Type I, Type II, and Type IV), for estimating the tensile quasi-static and fatigue properties of structural epoxy adhesives. The work presents results from an experimental program investigating the performance of structural epoxy adhesives indicating a significant effect of the specimen geometry, especially when referring to fatigue loading. Simple finite element models are also developed to facilitate the comparison of the stress distribution along the three specimen geometries. The fatigue experimental results allowed the derivation of probabilistic S-N curves, showing higher fatigue sensitivity of Type I specimens compared to Type II and IV. Furthermore, probability distribution function (PDF) curves of the equivalent static strength estimated by using Sendeckyj’s wear-out model attributed lower mean strength and higher variance for Type I specimens validating the fatigue data.

Thermal conductivity of epoxy/multilayered graphene composites prepared with different curing agents

Epoxy/multilayer graphene (ML-graphene) composites were prepared using different curing agents to control the graphene dispersion by changing the curing reactivity. With increasing initial reactivity, the aggregation size of the ML-graphene decreased and their thermal conductivity increased. In particular, the thermal conductivity of the composite prepared with p-phenylenediamine showed a maximum value of 1.46 W/(m·K) at 25 wt% ML-graphene loading because of the highest initial curing reactivity. The application of a magnetic field led to graphene alignment along the applied field, resulting in two times higher thermal conductivity than that of the corresponding system without magnetic field. The relationship between the interfacial affinity for epoxy/graphene and thermal conductivity was also investigated. As a result, resulting in a biphenyl epoxy composite showed higher thermal conductivity (6.17 W/(m·K)) than that of the bisphenol-A epoxy composite. This is derived that the π-conjugated and planar structure of biphenyl epoxy can easily interact with the surface of graphene.

Biomass-derived epoxy resin and its application for high-performance natural fiber composites

The development of bio-based materials with superior comprehensive properties from biomass resources remains a significant challenge, providing an alternative to conventional petroleum-based materials. This study explores the lignin-derived vanillyl alcohol epoxy (VAE) resin-containing mono and di (m&d) epoxy structure as an adhesive to develop environment-friendly natural fiber-reinforced composites to reduce the carbon footprint. The synthesized m&dVAE containing mono and di-epoxidized (m&d) aromatic rings, when cured with 4, 4´-diaminodiphenyl methane (DDM) hardener, exhibits higher record tensile strength ∼124.0 ± 8.43 MPa and tensile modulus ∼2.88 ± 0.35 GPa compared to a commercial petroleum-based epoxy, diglycidyl ether bisphenol A (DGEBA) thermoset. Additionally, it demonstrated higher adhesion shear strength (∼19.16 ± 0.58 MPa) with cellulose nanofibers film than DGEBA. Moreover, fabricated green composite possesses excellent flexural strength of ∼203.72 ± 2.08 MPa and stiffness of ∼11.58 ± 0.38 GPa than the petroleum-based thermoset composite. The composite’s fracture surface morphology shows that the resin and fibers have good interfacial adhesion. Notably, the sustainable composite showed good hydrophobicity and an excellent heat-resistant index of 144.4°C. The m&dVAE resin can be an alternative to petroleum-based resins as an adhesive, and its environment-friendly sustainable composite could be a promising candidate to replace synthetic materials for high-performance structural applications.

Review of Recyclable Bio-based Epoxy Resins with Dynamic Chemical Bonds

Epoxy thermosetting resins are usually reliant on fossil fuel-based resources, commonly diglycidyl ether bisphenol A (DGEBA) type epoxy monomers. Most raw materials of these thermoset resin are toxic to the health of human, and their eternal cross-links make them difficult to reuse and recycle. To alleviate concerns about the environment and human health, it is an effective way to design new bio-based epoxy thermosetting materials to replace petroleum based thermosetting materials. The introduction of cleavable and dynamic bonds for bio-based thermosetting materials can also realize the recycling of bio-based epoxy resin. In this way, the damaged thermosetting materials can be recovered to prolong their service lifetime and reduce the thermosetting waste. This review article aims to outline the latest improvements in intrinsically recyclable bio-based epoxy thermosetting materials. This review first describes the synthesis method of recyclable bio-based epoxy, then reviews the structure, recyclable and other properties of bio-based epoxy containing different dynamic bonds, and finally summarizes the challenges and opportunities for the recyclable bio-based epoxy.

Synthesis and cryogenic mechanical properties of degradable epoxy resins containing hexahydro-s-triazine structure

Epoxy resins are widely used in cryogenic applications due to their distinguished mechanical and electrical insulation properties. To avoid generating enormous amounts of waste that affect the environment and cause economic losses, covalent adaptable networks can be introduced into the epoxy structure to make the thermosetting network reversible. In this study, ester bond and hexahydro-s-triazine structure were adopted in epoxy structures. Two new degradable epoxy resins (HPBE and HPHE) with variances in the content of benzene rings were synthesized which were confirmed by FTIR and NMR. After curing, the thermal, mechanical, thermal conducting properties of the resulting materials were examined. The products exhibited tensile strength and flexural strength all above 120 MPa at 77 K, with glass transition temperature of 89.1 °C and 86.9 °C, respectively. Meanwhile, HPBE and HPHE demonstrated favorable thermal stability, with thermal conductivity comparable to that of the commonly used epoxy resin DGEBA. These resins could be degraded under mild conditions and demonstrated high hydrolysis efficiency (approximately 90 %). This investigation presented a viable method for the application of degradable thermosetting materials in cryogenic environments and the possibility for repairing of high-value equipment.11HPBE: The first degradable epoxy resin synthesized in this paper. HPHE: The second degradable epoxy resin synthesized in the paper.EP-4000: A kind of polyether modified bisphenol A epoxy resins.

The influence of combination of surface roughness and nanostructure of adhesive on the strength of adhesively bonded joints

Nowadays, adhesively bonded joints are frequently employed in engineering designs as they enable the manufacture of joints with great strength while remaining light. These joints used in aviation sectors, where lightness and reliability are important, are generally designed to join metal-metal, metal-composite and composite-composite adherends. The surface roughness of the adherends with its degree and the way the adhesive interacts with the surface are the most crucial parameters in these applications. In the current work, surface roughness and normal stress strength of nano-composite adhesive in butt joint subjected to tensile loading were investigated. DP460 liquid structural epoxy from 3 M was used as adhesive, AA2024-T3 aluminum alloy as adherend, Graphene-COOH and Carbon Nanotube-COOH as nanostructures while sandpaper and sandblasting methods were used for introducing roughness to the adherend surfaces. A two dimensional axisymmetric FE model was developed to get an insight about the joint failure. When the failure load obtained from experiments was examined, an increasing trend in maximum force values was noticed with an increase in surface roughness, but this increase in joint strength decreased when the surface roughness increased to very high values. In addition, minimum surface roughness reduces the adhesive's adhesion to the surface, thus reducing the normal stress strength of the joint. When 1 % Graphene-COOH nanostructure was added to the adhesive, the ultimate normal stress of the joints increased from 9 % to 21 %. However, when the same percentage of Carbon Nanotube-COOH was added, the increase was from 27 % to 62 %. Different normal stress values of the joints were obtained using by sanding and sandblasting methods even though the roughness resulting from the two treatments was similar.

Effect of hybridization and composite architecture symmetry in the bonded joint performance of jute/carbon fibre-reinforced composites

This study investigates the effect of hybridization and composite architecture symmetry in the bonded joint performance of jute/carbon fibre-reinforced composites. Jute and carbon fibre bidirectional fabrics were used as reinforcement fibres for interlaminar hybrid epoxy composite adherends. The effect of the number of carbon fibre layers (1 and 2) as well as layup symmetry on the mechanical performance of the bonded joints was studied by testing single lap joints (SLJs) bonded with two adhesive types (a structural ductile bi-component epoxy adhesive and a brittle one). It was found that the hybridization of jute fibre reinforced composites with carbon fibres significantly improved the bonded joint performance for all tested specimens, when compared to the neat jute fibre reinforced composites (maximum of approx. 69 %). However, when compared to the neat carbon fibre (CFRP) adherend joints, the hybrid jute/carbon adherend joint configurations studied here achieved up to approximately 94 % (ductile adhesive system) and 97 % (brittle adhesive system) of maximum joint load efficiency of the CFRP synthetic joint. The effect of composite architecture symmetry was more pronounced for the brittle adhesive joints, whereas the failure loads of the ductile adhesive joints exhibited a plateau-like trend (ductile adhesives accommodate better the bending moments during SLJ loading, while brittle adhesives are more sensitive to adherend stiffness). A jute/carbon fibre hybrid composite bonded structure presents a potentially cost-effective and efficient alternative for reducing the structural carbon footprint, without a significant compromise in mechanical properties.

Investigation of substrate materials laminated CPW-Fed patchantennas: Opportunities and challenges

This article investigates the design of antennas fed by coplanar waveguide (CPW) utilizing different substrate materials and techniques. The article illustrates the impact of several materials such as FR-4, Rogers, Arlon, etc on electrical properties such as impedance bandwidth (IBW), reflection coefficient (|S11|), and gain. The article also examines the effects of design approaches such as including stubs, slots, arrays, etc on antenna |S11| parameters. The resonance frequencies (fr) of the suggested antennas may be modified by manipulating the electrical dimension, the dielectric constant of the substrate materials, and their respective thicknesses. The structure of the Flame-Retardant fiberglass epoxy (FR-4) proposed antenna covers anoperating frequency range of 3.3 to 10.1 GHz. It can operate at six distinct frequency bands, making it appropriate for applications including C-band (4-8GHz), X-band (8-12GHz), 5G (3.5-6.0GHz), and 6G (7-20GHz). The antenna designed and simulated using computer simulation software (CST) electromagnetic simulator.

Insight into the relationship between interfacial structure and thermal conductivity in epoxy resin/Al2O3 composite

AbstractThe relationship between interfacial structure and thermal conductivity in epoxy resin/Al2O3 composite is important but is still elusive. Here, we illuminate this issue by investigating the role of four silane coupling agents in thermal conductivity for epoxy resin/Al2O3 composites. The results show that 3‐glycidoxypropyltrimethoxysilane (KH560) is the best silane coupling agent to modify Al2O3, resulting in the lowest viscosity and the highest thermal conductivity of the epoxy resin/Al2O3‐KH560 composite compared with alternative systems. This is attributed to the strong interfacial interaction between Al2O3‐KH560 and epoxy resin, as confirmed by the Fowkes model and broadband dielectric spectroscopy. The interfacial thermal resistance between the epoxy resin and Al2O3 is also quantitatively measured via a photothermal radiometry technique. The epoxy resin/Al2O3‐KH560 also has the lowest interfacial thermal resistance (1.50 ± 0.02 × 10−6 m2 K W−1), compared with the other three systems. This study advances the understanding of the relationship between interfacial structure and thermal conductivity in polymer composites.Highlights The relationship between interfacial structure and thermal conductivity in epoxy resin/Al2O3 composite is understood. The interfacial structure is characterized by broadband dielectric spectroscopy. The interfacial thermal resistance is quantitatively measured via a photothermal radiometry technique.

Adjusting the dynamic and mechanochromic properties of aromatic disulfide epoxy vitrimers by unsaturation of epoxy monomers

AbstractDisulfide‐contained vitrimers that possess unprecedented functions, such as reprocessing and self‐healing, are considered to be one of the most promising materials since they also exhibit mechanochromic behaviors. A few reports have given the phenomenon of mechanochromism, but the relationship between their structure and mechanochromic behavior needed to further research. Herein, three aromatic disulfide‐contained epoxy vitrimers with different degree of unsaturation are prepared by changing the epoxy monomer. The effect of network structure on the dynamic and mechanochromic properties are investigated. With increasing the degree of unsaturation from the aliphatic to aromatic epoxy vitrimers, the Tg‐value increases and the chain mobility decreases, so the aliphatic epoxy vitrimers shows the lowest activation energy, the lowest temperature of malleability Tv, and the shortest relaxation times. In addition, these epoxy vitrimers all showes reversible mechanochromic behaviors due to the generation and coupling of sulfenyl radicals, which can be accelerated by the high temperature and solvent. Meanwhile, the epoxy structure has a great influence on the mechanochromic behavior, including the absorption wavelength and fading kinetics of grinding powders. These results can be utilized to design desired vitrimer materials for various applications that requires both the intrinsically self‐report damage and the dynamic properties such as recycling.

Structure and Properties of Epoxy Resin/Graphene Oxide Composites Prepared from Silicon Dioxide-Modified Graphene Oxide.

In this study, graphene oxide (GO) was modified via electrostatic interactions and chemical grafting by silica (SiO2), and two SiO2@GO hybrids (GO-A and GO-B, respectively) with different structures were obtained and carefully characterized. Results confirmed the successful grafting of SiO2 onto the GO surface using both strategies. The distribution of SiO2 particles on the surface of GO-A was denser and more agglomerated, while it was more uniform on the surface of GO-B. Then, epoxy resin (EP)/GO composites were prepared. The curing mechanism of EP/GO composites was studied by differential scanning calorimetry and in situ infrared spectra spectroscopy. Results of tensile tests, hardness tests, dynamic mechanical analysis, and dielectric measurement revealed that EP/GO-B exhibited the highest tensile properties, with a tensile strength of 79 MPa, a 43% increase compared to raw EP. Furthermore, the addition of fillers improved the hardness of EP, and EP/GO-B showed the highest energy storage modulus of 1900 MPa. The inclusion of SiO2@GO hybrid fillers enhanced the dielectric constant, volume resistivity, and breakdown voltage of EP/GO composites. Among these, EP/GO-B displayed the lowest dielectric loss, relatively good insulation, and relatively high volume resistivity and breakdown voltage. A related mechanism was proposed.

A derivative of trihydroxynaphthalenone and a pyrone metabolite from the endophytic fungus Talaromyces purpurpgenus

A newly discovered trihydroxynaphthalenone derivative, epoxynaphthalenone (1) involving the condensation of ortho-hydroxyl groups into an epoxy structure, and a novel pyrone metabolite characterized as pyroneaceacid (2), were extracted from Talaromyces purpurpgenus, an endophytic fungus residing in Rhododendron molle. The structures of these compounds were elucidated through a comprehensive analysis of their NMR and HRESIMS data. The determination of absolute configurations was accomplished using electronic circular dichroism (ECD) calculations and CD spectra. Notably, these recently identified metabolites exhibited a moderate inhibitory activity against xanthine oxidase (XOD).

Damage-tolerant behavior of structural epoxies with thin PEI and PVDF interlayers

This work examines the crack-arresting capability of PEI and PVDF thermoplastic interlayers in short-glass fiber modified and core–shell rubber toughened epoxy materials under mode-I fracture loading. Experimental results show that the initial crack can be effectively arrested by the thermoplastic-epoxy interface and reinitiation of this crack at the thermoplastic layer requires up to 2.5 times higher load than the crack initiation load of the pristine epoxies. The interlayered designs also exhibit a significant increase in energy absorption by up to 43 times more than their pristine counterparts. Competing damage mechanisms and failure events are captured by microscopy images, digital image correlation and high-speed photography observations. Additionally, elasto-plastic fracture mechanics models based on configurational material forces theory are developed for a preliminary crack-arresting material selection and to elucidate the material inhomogeneity effects.

Mode-I and II fracture behavior of metal-to-composite bonded interfaces with a graphene nanoplatelet-reinforced structural epoxy adhesive

Epoxy adhesive joints are increasingly being used to bond structural components in automotive, marine, and aircraft bodies. However, poor fracture toughness energy and low resistance to crack growth in structural epoxy adhesives are important drawbacks of these adhesives. To overcome this challenge, this paper aims to investigate the influence of dispersing graphene nanoplatelets (GNPs) into Axson-Sika Adekit A 140-1 structural epoxy adhesive on the fracture behavior of aluminum-to-glass fiber-reinforced polymer composite joints under mode-I and mode-II fracture tests. Double cantilever beam (DCB) and end-notched flexure specimens were fabricated to perform mode-I and mode-II fracture tests. For each specimen, three specimen groups including neat and graphene nanoplatelet reinforced with 0.05 wt.%, 0.1 wt.%, and 0.2 wt.% were considered. Results revealed that, for both of mode-I and mode-II tests, the optimum fracture toughness energy was obtained by inserting 0.1 wt.% of nanoparticles. By addition of 0.05 wt.% and 0.1 wt.% of GNPs, mode-I fracture toughness energy was improved by 90% and 115% compared with neat DCB specimens, while mode-II energy release rate increased by 55% and 83%, respectively. Finally, debonded surfaces were examined using optical microscope and scanning electron microscopy in order to find failure patterns and microstructural reinforcing mechanisms.

Investigation of the Bending Properties of Ex situ Functional Metal Foams

In each industry, compromises have to be made when choosing materials. Lower-density materials have a significant advantage in the automotive sector, especially due to rising fuel prices, since weight reduction can lower overall fuel consumption. The advantageous properties of metal foams, such as low density, high specific strength, and excellent energy absorption, should be researched and exploited in as many areas and ways as possible. This research aims to perform and evaluate the bending tests of ex situ functional metal foams: aluminum alloy matrix (AlSi7Mg) was used, which was filled with Ø2.5–3.0 mm lightweight expanded clay aggregate particles and surrounded by thin-walled aluminum tubes (AlMgSi0.5) with a wall thickness of 2 mm and an outer diameter of Ø32 mm. Empty tubes, foam-filled tubes (with and without structural epoxy adhesive) and metal matrix syntactic foams were compared based on their flexural strength and energy absorption capacity. Quasi-static three-point bend tests were carried out up to 25 mm deflection. The foam-filled tubes with epoxy adhesive showed an average of 6% increase in flexural strength compared to the foam-filled tubes without adhesive and a 145% increase compared to the metal matrix syntactic foams.

Mechanism of stress generation at the epoxy-epoxy interface

Epoxy resins are widely used in power equipment, and due to the material as well as process requirements, multi-layer epoxy structures inevitably exist. The interlayer interface between epoxy layers, formed via secondary curing, causes a potential weak link in the material's properties. This work experimentally measures the temperature and strain at the epoxy-epoxy interface under various conditions, including the presence or absence of epoxy and differing chemical doses of curing agent. Subsequently, an epoxy-epoxy interface strain resin simulation model is established to examine the mechanism of interface stress formation by exploring the curing kinetics of epoxy resins using the Straink method. Experimental measurements and simulation calculations reveal the presence of an interaction between interlayer interfaces during the secondary curing. Changing the dose of curing agent in the post-pouring layer can change the interaction between the layers and regulate the interlayer interface performance.

Gamma radiation impact on dielectrics and RH sensitivity of nylon–carbon–GNP–epoxy composites

AbstractComposites of carbon–graphene nanoplatelets (GNP) epoxy with nylon‐66 interleaving were fabricated and investigated to understand the impact of γ‐irradiation on their dielectrics and relative humidity (RH) sensitivity by capacitance and water absorption measurements. The moisture sensitivity was investigated by measuring memory effect (hysteresis) and response time of the capacitance. Composites were γ‐irradiated at doses of 1000 and 3000 kGy and studied by the same set of measurements. X‐ray diffraction analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy were used to understand the impact of γ‐irradiation on the morphology and surface roughness of the composites. Results of these measurements suggest that 3000‐kGy dose γ‐irradiation produces good chain scission and cross‐linking, resulting in reduced water absorption and a 360% increase in capacitance values. The sensitivity of capacitance for 3000‐kGy γ‐irradiated composite was found to be highest at 40 pF/%RH, and hysteresis was 1.8% RH and not observed in non‐irradiated composites. Based on these observations, it seems that 3000‐kGy γ‐irradiated composite could be a potential candidate for moisture content diagnostics during condition monitoring of the carbon–epoxy structures in several industrial applications and needs to be further investigated.Highlights Composites of carbon–GNP epoxy with nylon‐66 interleaving were γ‐irradiated. 3000‐kGy dose γ‐irradiated composites exhibit 360% increase in capacitance. Sensitivity and hysteresis were higher than those of 1000‐kGy and non‐irradiated samples. Good chain scission and cross‐linking resulted in reduced water absorption.