Molecular Understanding of the Distinction between Adhesive Failure and Cohesive Failure in Adhesive Bonds with Epoxy Resin Adhesives

In the development of adhesives, an understanding of the fracture behavior of the bonded joints is inevitable. Two typical failure modes are known: adhesive failure and cohesive failure. However, a molecular understanding of the cohesive failure process is not as advanced as that of the adhesive failure process. In this study, research was developed to establish a molecular understanding of cohesive failure using the example of a system in which epoxy resin is bonded to a hydroxyl-terminated self-assembled monolayer (SAM) surface. Adhesive failure was modeled as a process in which an epoxy molecule is pulled away from the SAM surface. Cohesive failure, on the other hand, was modeled as the process of an epoxy molecule separating from another epoxy molecule on the SAM surface or breaking of a covalent bond within the epoxy resin. The results of the simulations based on the models described above showed that the results of the calculations using the model of cohesive failure based on the breakdown of intermolecular interactions agreed well with the experimental results in the literature. Therefore, it was suggested that the cohesive failure of epoxy resin adhesives is most likely due to the breakdown of intermolecular interactions between adhesive molecules. We further analyzed the interactions at the adhesive failure and cohesive failure interfaces and found that the interactions at the cohesive failure interface are mainly accounted for by dispersion forces, whereas the interactions at the adhesive failure interface involve not only dispersion forces but also various chemical interactions, including hydrogen bonds. The selectivity between adhesive failure and cohesive failure was explained by the fact that varying the functional group density affected the chemical interactions but not the dispersion forces.

Performance Evaluation for Ultra-Lightweight Epoxy-Based Bipolar Plate Production with Cycle Time Reduction of Reactive Molding Process

The commercial viability of fuel cells for vehicle application has been examined in the context of lightweight material options, as well as in combination with improvements in fuel cell powertrain. Investigation into ultra-lightweight bipolar plates (BPs), the main component in terms of the weight effect, is of great importance to enhance energy efficiency. This research aims to fabricate a layered carbon fiber/epoxy composite structure for BPs. Two types of carbon fillers (COOH-MWCNT and COOH-GNP) reinforced with woven carbon fiber sheets (WCFS) have been utilized. The conceptual idea is to reduce molding cycle time by improving the structural, electrical, and mechanical properties of BPs. Reducing the reactive molding cycle time is required for commercial production possibility. The desired crosslink density of 97%, observed at reactive molding time, was reduced by 83% at 140 °C processing temperature. The as-fabricated BPs demonstrate excellent electrical conductivity and mechanical strength that achieved the DOE standard. Under actual fuel cell operation, the as-fabricated BPs show superior performance to commercial furan-based composite BPs in terms of the cell potential and maximum power. This research demonstrates the practical and straightforward way to produce high-performance and reliable BPs with a rapid production rate for actual PEMFC utilization.

An Update on Graphene Oxide: Applications and Toxicity

Graphene oxide (GO) has attracted much attention in the past few years because of its interesting and promising electrical, thermal, mechanical, and structural properties. These properties can be altered, as GO can be readily functionalized. Brodie synthesized the GO in 1859 by reacting graphite with KClO3 in the presence of fuming HNO3; the reaction took 3-4 days to complete at 333 K. Since then, various schemes have been developed to reduce the reaction time, increase the yield, and minimize the release of toxic byproducts (NO2 and N2O4). The modified Hummers method has been widely accepted to produce GO in bulk. Due to its versatile characteristics, GO has a wide range of applications in different fields like tissue engineering, photocatalysis, catalysis, and biomedical applications. Its porous structure is considered appropriate for tissue and organ regeneration. Various branches of tissue engineering are being extensively explored, such as bone, neural, dentistry, cartilage, and skin tissue engineering. The band gap of GO can be easily tuned, and therefore it has a wide range of photocatalytic applications as well: the degradation of organic contaminants, hydrogen generation, and CO2 reduction, etc. GO could be a potential nanocarrier in drug delivery systems, gene delivery, biological sensing, and antibacterial nanocomposites due to its large surface area and high density, as it is highly functionalized with oxygen-containing functional groups. GO or its composites are found to be toxic to various biological species and as also discussed in this review. It has been observed that superoxide dismutase (SOD) and reactive oxygen species (ROS) levels gradually increase over a period after GO is introduced in the biological systems. Hence, GO at specific concentrations is toxic for various species like earthworms, Chironomus riparius, Zebrafish, etc.

Synthesis and derivatization of epoxy-functional sterically-stabilized diblock copolymer spheres in non-polar media: does the spatial location of the epoxy groups matter?

Epoxy-functional sterically-stabilized diblock copolymer nanoparticles are prepared via PISA in mineral oil and then derivatized using various reagents and reaction conditions.

The Use of the ATD Technique to Measure the Gelation Time of Epoxy Resins

In this paper, we investigated the thermodynamics of the resin curing process, when it was a part of composition with graphite powder and cut carbon fibers, to precisely determine the time and temperature of gelation. The material for the research is a set of commercial epoxy resins with a gelation time not exceeding 100 min. The curing process was characterized for the neat resins and for resins with 10% by weight of flake graphite and cut carbon fibers. The results recorded in the analysis of temperature derivative (ATD) method unequivocally showed that the largest first derivative registered during the test is the gel point of the resin. The innovative approach to measuring the gelation time of resins facilitates the measurements while ensuring the stability of the curing process compared to the normative tests that introduce mechanical interaction. In addition, it was found during the research that the introduction of 10% by weight of carbon particles in the form of graphite and cut carbon fibers rather shortens the gelation time and lowers the temperature peak due to the effective absorption and storage of heat from the cross-linking system. The inhibiting (or accelerating) action of fillers is probably dependent on chemical activity of the cross-linking system.

The correlated effects of polyetheramine-functionalized graphene oxide loading on the curing reaction and the mechanical properties of epoxy composites

In this study, the effects of polyetheramine (D230) functionalized graphene oxide loading on the curing reaction, thermal and mechanical properties of epoxy composites were studied and the correlation between structure and property of epoxy composite was established. In the functionalization of graphene oxide (GO), the effect of the mass ratio of D230 to GO on chemical properties of the functionalized GO was investigated. Results showed that D230 were successfully covalently grafted onto surface of two-dimensional functionalized GO sheet. The functionalized GO sheets prepared under optimal condition of D230/GO ratio of 1:1 dispersed evenly in epoxy composites, indicating the possibility of the epoxy composite fabrication by the solvent-free technique. The analysis of qualitative Cure Index suggested that epoxy composites were subjected to excellent curing. The quantitative evaluation of curing kinetics demonstrated that the functionalized GO exhibited a chemical facilitation on the curing reaction. However, the functionalized GO simultaneously physically restricted the curing reactivity, especially at high loading. These contributed to the improved interfacial properties and high toughness of the epoxy composites. Compared to neat epoxy, the epoxy composites showed effective tensile strength improvement of ∼10.0% (77.0 MPa), tensile modulus enhancement of ∼7.7% (3.34 GPa), flexural modulus increment of ∼12.1% (3.43 GPa), and flexural strength increment of ∼10.6% (124.3 MPa). This study demonstrated an effective and environment-friendly strategy to design GO reinforced epoxy composites with favorable dispersion and interfacial bonding, and it further clarified the relationship between the crosslinking network/interfacial structure and the mechanical properties of epoxy composites.

Assessing the Flexural Properties of Epoxy Composites with Extremely Low Addition of Cellulose Nanofiber Content

Epoxy resins are a widely used common polymer due to their excellent mechanical properties. On the other hand, cellulose nanofiber (CNF) is one of the new generation of fibers, and recent test results show that CNF reinforced polymers have high mechanical properties. It has also been reported that an extremely low CNF addition increases the mechanical properties of the matrix resin. In this study, we prepared extremely-low CNF (~1 wt.%) reinforced epoxy resin matrix (epoxy-CNF) composites, and tried to understand the strengthening mechanism of the epoxy-CNF composite through the three-point flexural test, finite element analysis (FEA), and discussion based on organic chemistry. The flexural modulus and strength were significantly increased by the extremely low CNF addition (less than 0.2 wt.%), although the theories for short-fiber-reinforced composites cannot explain the strengthening mechanism of the epoxy-CNF composite. Hence, we propose the possibility that CNF behaves as an auxiliary agent to enhance the structure of the epoxy molecule, and not as a reinforcing fiber in the epoxy resin matrix.

Enhanced covalent p-phenylenediamine crosslinked graphene oxide membranes: Towards superior contaminant removal from wastewaters and improved membrane reusability

The increasing depletion of freshwater necessitates the re-use and purification of wastewaters. Among the existing separation membrane materials, graphene oxide (GO) is a promising candidate, owing to its tunable physicochemical properties. However, the widening of GO membranes pore gap in aqueous environments is a major limitation. Crosslinking agents can be incorporated to alleviate this problem. This study describes a comparative analysis of uncrosslinked and p-Phenylenediamine (PPD) crosslinked GO membranes' water purification performance. Dip-coating and dip-assisted layer-by-layer methods were used to fabricate the uncrosslinked and crosslinked membranes respectively. The covalent interaction between GO and PPD was confirmed by Fourier Transform Infra-Red and X-ray Photoelectron Spectroscopy. The excellent membrane topographical continuity and intactness was assessed by means of Scanning Electron Microscopy, while water contact angle measurements were undertaken to evaluate and confirm membrane hydrophilicity. The improvement impact of the crosslinker was manifested on the enhancement of the stability and performance of the membranes during nanofiltration tests of aqueous solutions of methylene blue in a homemade nanofiltration cell operated at 1 bar.

Graphene Oxide and Oxidized Carbon Black as Catalyst for Crosslinking of Phenolic Resins

Influence of different graphite-based nanofillers on crosslinking reaction of resorcinol, as induced by hexa(methoxymethyl)melamine, is studied. Curing reactions leading from low molecular mass compounds to crosslinked insoluble networks are studied by indirect methods based on Differential Scanning Calorimetry. Reported results show a catalytic activity of graphene oxide (eGO) on this reaction, comparable to that one already described in the literature for curing of benzoxazine. For instance, for an eGO content of 2 wt %, the exothermic crosslinking DSC peak (upon heating at 10 °C/min) shifted 6 °C. More relevantly, oxidized carbon black (oCB) is much more effective as catalyst of the considered curing reaction. In fact, for an oCB content of 2 wt %, the crosslinking DSC peak can be shifted more than 30 °C and a nearly complete crosslinking is already achieved by thermal treatment at 120 °C. The possible origin of the higher catalytic activity of oCB with respect to eGO is discussed.

Catalytic Activity of Oxidized Carbon Waste Ashes for the Crosslinking of Epoxy Resins

In this study, two different fillers were prepared from carbon-based ashes, produced from the wooden biomass of a pyro-gasification plant, and starting from lignocellulosic waste. The first type was obtained by dry ball-milling (DBA), while the second one was prepared by oxidation in H₂O₂ of the dry ball-milled ashes (oDBA). The characterization of the fillers included wide-angle x-ray diffraction (WAXD), thermogravimetric, and Fourier-transform infrared spectroscopy (FTIR) analysis. The DBA and oDBA fillers were then tested as possible catalysts for the crosslinking reaction of a diglycidyl ether of bisphenol A (DGEBA) with a diamine. The cure reaction was studied by means of rheometry and differential scanning calorimetry (DSC). The oDBA filler exhibits both a higher catalytic activity on the epoxide–amine reaction than the DBA sample and improved mechanical properties and glass transition temperature. The results obtained indicate, hence, the potential improvement brought by the addition of carbon-based waste ashes, which allow both increasing the flexural properties and the glass transition temperature of the epoxy resin and reducing the curing time, acting as a catalyst for the crosslinking reaction of the epoxy resin.

Glycidyl Methacrylate-Emulsion Styrene Butadiene Rubber (GMA-ESBR)/Silica Wet Masterbatch Compound

In the tire industry, solution styrene butadiene rubber (SSBR), which can introduce a functional group with good reactivity to silica at chain ends, is used to increase rolling resistance performance by considering fuel economy. However, this is not environmentally friendly because SSBR uses an organic solvent for polymerization, and it is difficult to increase its molecular weight. Functionalized emulsion SBR (ESBR) can solve the problems of SSBR. The molecular weight of ESBR molecules can be easily increased in an eco-friendly solvent, i.e., water. A functionalized ESBR introduces a functional group with good reactivity to silica by introducing a third monomer during polymerization. In this field, glycidyl methacrylate (GMA) has been reported to show the best properties as a third monomer. However, for GMA-ESBR, the viscosity is high and processability is disadvantageous. Therefore, we polymerized GMA-ESBR and manufactured silica compounds to clarify the causes of these problems. In addition, wet masterbatch (WMB) technology, which is a new compound manufacturing method, was applied to manufacture the silica compound, and the physical properties are compared with those of a dry masterbatch. The results clarified the problem of GMA-ESBR, which could be solved by using WMB technology.

Rheological Properties and Thermal Conductivity of Epoxy Resins Filled with a Mixture of Alumina and Boron Nitride

Electronic packaging materials with high thermal conductivity and suitable viscosity are necessary in the manufacturing of highly integrated electronic devices for efficient heat dissipation during operation. This study looked at the effect of boron nitride (BN) platelets on the rheology and thermal conductivity of composites based on alumina (Al₂O₃) and epoxy resin (EP) for the potential application as electronic packaging. The viscosity and thermal conductivity of the composite were increased upon increasing filler content. Furthermore, thermal conductivity of the BN/Al₂O₃/EP was much higher than that of Al₂O₃/EP at almost the same filler loadings. These unique properties resulted from the high thermal conductivity of the BN and the synergistic effect of the spherical and plate shapes of these two fillers. The orientation of BN platelets can be controlled by adjusting their loading to facilitate the formation of higher thermally conductive pathways. The optimal content of the BN in the Al₂O₃/EP composites was confirmed to be 5.3 vol %, along with the maximum thermal conductivity of 4.4 W/(m·K).

Breathable Microgel Colloidosome: Gas-Switchable Microcapsules with O2 and CO2 Tunable Shell Permeability for Hierarchical Size-Selective Control Release

Microcapsules enabling precise delivery and controlled release are highly desirable. However, it is still challenging to control the release profile by regulating the microcapsule shell permeability. In this work, gas-switchable microgel colloidosome (MGC) with oxygen (O₂) and carbon dioxide (CO₂) dual gas-tunable shell permeability has been developed and tested for control release of water-soluble cargo molecules, based on the size exclusion mechanism. The O₂ and CO₂ dual gas-switchable poly(2-(diethylamino)ethyl methacrylate-co-2,3,4,5,6-pentafluorostyrene), P(DEA-co-FS), microgels having surface modified with amino group (-NH₂) were synthesized and used to stabilize oil-in-water (O/W) Pickering emulsions. The oil-soluble poly(propylene glycol) diglycidyl ether (PPGDGE) was added as an intermicrogel cross-linker. The cross-linking between adjacent microgel particles at the water-oil interface was achieved through the amine-epoxy reaction of PPGDGE with the amine groups at the particle surface. Fluorescent-labeled dextran model cargo molecules of 10 kDa (D₁) and 2000 kDa (D₂) were uploaded under CO₂ treatment and locked inside the MGC with N₂ treatment. The O₂ and CO₂ dual-gas switchable properties offered the MGC with tunable shell permeability, which allowed the hierarchical release of D₁ and D₂ based on size exclusive mechanism. This work provides a robust method for preparation of gas-switchable microcapsules with tunable permeability and size-exclusive hierarchical release profile, promising for multiple ingredient controllable release, separation, and reaction.

GO/PEDOT:PSS nanocomposites: effect of different dispersing agents on rheological, thermal, wettability and electrochemical properties

In this work glucose (G), α-cyclodextrin (α-CD) and sodium salt of carboxymethyl cellulose (CMCNa) are used as dispersing agents for graphene oxide (GO), exploring the influence of both saccharide units and geometric/steric hindrance on the rheological, thermal, wettability and electrochemical properties of a GO/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) nanocomposite. By acting on the saccharide-based additives, we can modulate the rheological, thermal, and wettability properties of the GO/PEDOT:PSS nanocomposite. Firstly, the influence of all the additives on the rheological behaviour of GO and PEDOT:PSS was investigated separately in order to understand the effect of the dispersing agent on both the components of the ternary nanocomposite, individually. Subsequently, steady shear and dynamic frequency tests were conducted on all the nanocomposite solutions, characterized by thermal, wettability and morphological analysis. Finally, the electrochemical properties of the GO/PEDOT composites with different dispersing agents for supercapacitors were investigated using cyclic voltammetry (CV). The CV results revealed that GO/PEDOT with glucose exhibited the highest specific capacitance among the systems investigated.

Epoxy Resins Toughened with Surface Modified Epoxidized Natural Rubber Fibers by One-Step Electrospinning

Epoxidized natural rubber fibers (ERFs) are developed through one-step electrospinning and directly deposited into epoxy resins without collecting and distributing of fibers. The shape of ERFs shows rough surface due to different evaporation rate of solvent mixture consisting of chloroform and dichloromethane and the average diameter of ERFs is 6.2 µm. The increase of ERFs loading from 0 to 20 wt % into the epoxy resin increases the fracture strain significantly from 1.2% to 13% and toughness from 0.3 MPa to 1.9 MPa by a factor of 7. However, the tensile strength and Young’s modulus decrease about 34% from 58 MPa to 34 MPa and from 1.4 GPa to 0.9 GPa, respectively. Due to the crosslinking reactions between oxirane groups of ERFs and amine groups in the resin, surface roughness and the high aspect ratio of ERFs, ERFs result in more effective toughening effect with the minimum loss of tensile properties in epoxy resins.

Catalytic Activity of Oxidized Carbon Black and Graphene Oxide for the Crosslinking of Epoxy Resins

This article compares the catalytic activities of oxidized carbon black (oCB) and graphene oxide (eGO) samples on the kinetics of a reaction of diglycidyl ether of bisphenol A (DGEBA) with a diamine, leading to crosslinked insoluble networks. The study is mainly conducted by rheometry and Differential Scanning Calorimetry (DSC). Following the same oxidation procedure, CB samples are more efficiently oxidized than graphite samples. For instance, CB and graphite samples with high specific surface areas (151 and 308 m²/g), as oxidized by the Hummers' method, exhibit O/C wt/wt ratios of 0.91 and 0.62, respectively. Due to the higher oxidation levels, these oCB samples exhibit a higher catalytic activity toward the curing of epoxy resins than fully exfoliated graphene oxide.

Recent advances in organic reactions catalyzed by graphene oxide and sulfonated graphene as heterogeneous nanocatalysts: a review

Over the past decade, the application of carbocatalyst systems has been preferred over that of homogeneous catalytic systems because of their advantages such as physical and thermal stability of the catalysts in successive reaction runs and reusability.