Cure kinetics of several epoxy–amine systems at ambient and high temperatures

Epoxy-crosslinker curing reactions and the extent of the reactions are critical parameters that influence the performance of each epoxy system. The curing of an epoxy prepolymer with an amine functional group may be accompanied by side reactions such as etherification. Commercial epoxy prepolymers were cured with different commercial amines at ambient as well as at elevated temperatures. Singularly, only epoxy–amine reactions were observed with diglycidyl ether of bisphenol-A (DGEBA)-based epoxides in our research even upon post-curing at 200°C. Etherification side reaction was found to occur at a cure temperature of 200°C in epoxides possessing a tertiary amine moiety. A combined goal of our research was to understand the effect of tougheners on the cure of epoxy–amine blend. To discern the effect of tougheners on the cure, core–shell rubber (CSR) particles were incorporated into the epoxy–amine blend. It was observed that CSR particles did not restrict the system from proceeding to complete reaction of epoxy moieties. Besides, CSR particles were found to accelerate the epoxy-amine reaction at a lower level of epoxy conversion. The lower activation energy of epoxy–amine reaction of CSR incorporated system compared to control supported the catalytic effect of CSR particles on the epoxy-amine reaction of epoxy prepolymer and amine blends.     

Effects of nanosilica and titanium oxide on the performance of epoxy–amine nanocoatings

Different types of composite coatings were prepared by the blending of colloidal nanosilica (SiO₂) and titanium dioxide (TiO₂) in epoxy resin to investigate their coating performances. A fixed amount of silica nanoparticles (20 wt %) and different amounts (5, 10, and 15 wt %) of microsized TiO₂ particles were used in the coatings. The functional groups of the formulated coatings were confirmed by Fourier transform infrared spectroscopy. These results indicate that the SiO₂–TiO₂ particles interacted well with epoxy. Scanning electron microscopy images of the composite coatings revealed a good dispersion of TiO₂ particles at a lower amount of loading; this improved the adhesiveness, glass-transition temperature, thermal stability, and chemical resistance properties. At higher loadings, the performances decreased. The composite coatings were also characterized by their UV radiation-absorption properties with an ultraviolet–visible spectrophotometer. Interestingly, this property was found to be enhanced at higher loadings. An impressive result was noticed in the nanocomposites in terms of oxygen transmission rate performance compared to that of the neat epoxy. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47901.     

Novel formulations of high-performance epoxy–amine networks based on the use of nanoscale phase-separated antiplasticizers

The addition of an antiplasticizing agent in epoxy–amine resin formulations was revisited. The choice, as the antiplasticizer, of a chemical species which is fully miscible with the mixture of monomers but gives rise to nanoscale phase separation along the network construction was shown to be greatly favorable. Small domains, enriched in additive molecules, segregate in the polymer matrix, which is itself plasticized by residual additive molecules. Use of the additives did not change markedly the cure cycle and the total extent of reaction after full cure. The main effects of this special morphology on network properties were the depression of the glass transition temperature, T_g, of the matrix and the appearance of some damping (so-called μ-relaxation) in the temperature range intermediate between the secondary relaxation and the main mechanical relaxation. Networks prepared in this way were shown (1) to save a sufficiently high value of T_g in the view of the applications, (2) to present a higher modulus and higher toughness around room temperature than those of the conventional systems without an additive, and (3) to exhibit a lower water uptake at equilibrium than that of their regular homolog. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 759–774, 2001     

Evaluation of adhesion of continuous fiber–epoxy composite/aluminum laminates

Continuous fiber composite/metal laminates (FMLs) offer significant improvements over currently available composite materials for aircraft structures due to their excellent fatigue endurance and low density. Glass fiber–epoxy composite laminae and aluminum foil (GLARE) are commonly used to obtain these hybrid laminates. In this work, FMLs were produced by treating the aluminum foil to promote adhesion bonding by two methods: sulphuric chromic acid etching (SCAE) and chromic acid anodization (CAA). The surface treatments were evaluated by contact angle, roughness and scanning electron microscopy techniques. In order to compare different families of fiber composite/metal laminates, carbon fiber and glass fiber fabrics were used as reinforcements for the hybrid laminates. The adhesion of the hybrid laminates was evaluated by scanning electron microscopy (SEM) and three-point bending test. CAA resulted in better wetting properties. The interlaminar shear strength results for both carbon fiber-epoxy/metal and glass fiber-epoxy metal, were close to the interlaminar shear strength results found in the literature (approx. 40.0 MPa).     

Construction of improved isothermal TTT cure diagram based on an epoxy–amine thermoset

Cure degree plays a pivotal role in determining the final properties of thermosetting resin, while the parameter cannot be visually presented by the classic isothermal time–temperature-transformation (TTT) diagram. An improved isothermal TTT cure diagram is built for an epoxy–amine thermoset with the visual relationship between temperature, time, and cure degree during the whole curing. As for the improved isothermal TTT cure diagram, the curing surface and the gelation plane were developed using Vyazovkin method and rheological analysis in turn, and the variation between glass transition temperature (T _g) and curing degree was described by Dibenedetto's equation. The obtained improved isothermal TTT diagram of epoxy–amine thermoset was constructed by the combination of calorimetric and rheological analysis. The fitting results of vitrification surface and gelation plane obtained via improved isothermal TTT diagram were in good agreement with experimental results. In addition, the experimental gelation curve of epoxy–amine thermoset is directly linked to the steepest location of curing surface. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47279.     

Investigation of the curing kinetics of alkyd–melamine–epoxy resin system

Properties of coatings based on alkyd resin can be improved via blending with other suitable resins. Recent studies assessed that many properties could be improved by blending with epoxy resins as well as with melamine resins. The aim of this work was to investigate the effect of epoxy resin content on the curing process in alkyd–melamine–epoxy three component blends. The coatings with two mixing ratios of alkyd/melamine (70:30 and 80:20) were formulated. They were made into baking enamels by blending with 3 and 5 wt% of epoxy resin on total resin solid. Curring kinetics was investigated by differential scanning calorimetry (DSC) and application of Ozawa isoconversional method. Fourier transform infrared spectroscopy (FTIR) was used to follow major curing reactions. The absorbance of –OH and –N–CH₂R, showed significant reduction and confirmed that the epoxy resin reacts and inserts in enamel structure. It was found that resin system with alkyd/melamine ratio of 70:30 and 3 wt% of epoxy resin has the lowest apparent activation energy of 141.5 kJ mol⁻¹ and needs the shortest time of 34.2 min to reach final apparent degree of cure. Isothermal DSC experiments have confirmed these findings. The samples with 30 wt% of melamine resin had higher hardness of baked enamels then samples with 20 wt%. They also showed an increase of hardness with the increase of epoxy resin content.     

Characterisation and modelling of CNT–epoxy and CNT–fibre–epoxy composites

Abstract

This research presents an experimental and theoretical investigation on the effects of carbon nanotube (CNT) integration within neat epoxy resin (nanocomposites) and a carbon fabric–epoxy composite (multiscale composites). An approach is presented for the prediction of mechanical properties of multiscale composites. This approach combines woven fibre micromechanics (MESOTEX) with the Mori-Tanaka model which was used for the prediction of mechanical properties of nanocomposites in this research. Nanocomposite and multiscale composite samples were manufactured using cast moulding, resin infusion, and hand lay-up process. The CNT concentrations in the composite samples were from 0 to 5 wt-%. The samples were characterised using tensile, shear and flexural tests. The discrepancy between the theoretical predictions and the experimental observations was hypothesised to be due to dispersion and bonding issues and SEM images are presented in support of the hypothesis.     

Influence of the amine structure on the combustion and explosion of vapor–air mixtures

The enthalpy of formation and combustion is calculated for 15 aliphatic amines by various methods. These characteristics depend on the number of carbon atoms and the number of amino groups in the amine molecule. The combustion parameters are calculated when P = 1 atm and V = const, for mixtures of aliphatic amines with air. The adiabatic combustion temperature of mixtures of limiting composition is practically independent of the amine structure. The calculated values of the maximum pressure exceed 1000 kPa.     

Effect of surface pre-treatment on the adhesive strength of epoxy–aluminium joints

A detailed study of the effect of pre-treatment applied on the surface characteristics of aluminium substrates and on the adhesive strength of epoxy–aluminium joints is reported. The variation of the density, composition and aspect of the adherends were analysed as a function of the applied pre-treatment. In order to determine the influence of alloying elements, two different aluminium alloys were used, A1050 and A2024. The adhesive strength was measured by the lap shear test, using several epoxy resins to analyse the influence of the adhesive nature.A chromate-free treatment based on the sulphuric acid-ferric sulphate etch provided an improved joint strength compared to dichromate-sulphuric acid etching, alkaline etching or mechanical abrasion. This increase is associated to the porous oxide layer formed, but it depends on the adhesive nature used. The joints with Al–Cu–Mg alloy substrates generally presented higher adhesive strength values than those with pure aluminium adherends, due to the selective etching of some allowing elements and intermetallic compounds, which have different electrochemical potential.     

Influence of surface morphology on the adhesion strength of epoxy–aluminum interfaces

Adhesively bonded aluminum joints have been increasingly used in the automotive industry because of their structural and functional advantages. Interfacial debonding in these joints has become a major concern limiting their performance. The present work is focused on experimental investigation of the influence of surface morphology on the interfacial fracture behavior of the epoxy- aluminum interface. The specimens used in this experimental study were made of an epoxy- aluminum bimaterial strip in the form of a layered double cantilever beam (LDCB). The LDCB specimens were debonded by peeling off the epoxy layer from the aluminum substrate using a steel wedge. Interfacial fracture energy was extracted from the debonding length using a solution for the specimen geometry based on a model of a beam on an elastic foundation. This model was validated by direct finite element analysis. The experimental results establish a direct correlation between the surface roughness of aluminum substrate and the fracture resistance of the epoxy-aluminum interface. The results emphasize the importance of choosing surface features at an appropriate length scale in studying their effects on interfacial fracture resistance.     

Warpage of carbon–epoxy composite channels

Abstract

Finite element models have been developed of the warpage occurring during the cure of unidirectional carbon fibre-epoxy resin channels. These were based on equivalent experimental channels that were formed on a male mould, with the distortions determined separately after cure and post-cure. To quantify the warpage, the decrease in enclosed angle, or spring forward, of the two corners of the U-shaped cross-sections were calculated; values were determined using displacements from both the finite element predictions and measurements of the experimental channels. The experimental channels were fabricated so that several different factors affecting the distortions could be investigated. These included: fibre orientation; cured or post-cured state; conditions of post-cure; fillet radius of the channel corners; and channel thickness, width and depth. Results across the different channels showed predictions of 1° spring forward where the fibres followed the cross-section profile (0° channels), which were fairly accurate, at 75-85% of the experimental values. However, for the channels that had the fibres aligned parallel to the channel length (90° channels), negligible values were predicted, which were considerably lower in magnitude than the experimental values of 0° to-5° (spring back). Subsequent inhomogeneous models and optical microscopy work indicated that the unpredicted spring back in the more flexible 90° channels was caused by a thin (<0·1 mm) resin layer on the outer surface of all the channels. The small underprediction of spring forward in the stiffer 0° channels was attributed to unmodelled cure shrinkage, which was moderated by some reduced spring back due to the presence of a resin layer.     

Study on interfacial interaction energy of Cu–SAM–epoxy systems in a hot and humid environment

Due to poor adhesion, the interfacial delamination is one of the typical failure modes in electronic packages. In this paper, two kinds of self-assembly monolayers (SAMs), SAMA and SAME, are added to Cu–epoxy interface and the effects of temperature, moisture, and cross-link conversion on the modified interfaces are investigated with molecular dynamics (MD) simulation. The results show that the interfacial interaction energy of the systems with SAMA increases with the increasing temperature, decreasing moisture content, and cross-link conversion. However, the interfacial interaction energy of the systems with SAME decreases with the increasing temperature and moisture content, while it is reluctant to the cross-link conversion. In addition, the simulation reveals that the covalent bonds between SAMA and epoxy enhance the interfacial adhesion of Cu–epoxy. However, the nonbond interactions of SAME and epoxy resin weaken the interfacial adhesion. This paper provides a new method for research and valuation the effects of SAM or other adhesive on interfacial adhesion. MD simulation is an efficient tool in predicting the performances of materials.     

A rapid quantitative protocol for measuring carbon nanotube degree of dispersion in a waterborne epoxy–amine matrix material

The available literature makes it very clear that accurate measurements of carbon nanotube dispersion quality are very complicated and the typical characterization is neither simple nor reliable. Most methods to quantify carbon nanotube dispersion reported in the literature require investigator-chosen assumptions or software interpretations that are impractical at best and misleading at worst for facile application. Herein, we report on the use of visible light absorption-based method(s) and validate that these were quantitative for discerning dispersibility differences for MWCNTs with three distinct surface chemistry modifications and concentration levels blended with polymeric materials. Ultimately, the dispersion quality was quantified via the trendline slope of the thickness-normalized absorbance values as a function of MWCNT concentration. Extremely poor dispersions were represented by statistically insignificant slope trendlines. Our data revealed that hydroxyl surface modification increased MWCNT dispersibility by a factor of ~2.8 and ~2.6 compared to the as-received MWCNT formulations via the absorption and the blackness methods, respectively. These results support and quantifiably validate that simple optical blackness values directly measured the degree of dispersion for MWCNTs in coatings applied to substrates, and our data support that this is a simple and effective quality control metric.     

Experimental and analytical investigations of creep of epoxy adhesive at the concrete–FRP interfaces

This paper presents the results of experimental and analytical investigations on the long-term behavior of epoxy at the interface between the concrete and the fiber-reinforced-polymer (FRP). Double shear experiments under sustained service load were performed on nine specimens composed of two concrete blocks connected by FRP sheets bonded to concrete using epoxy. The primary investigation parameters included the ratio of shear stress to ultimate shear strength, the epoxy thickness and the epoxy time-before-loading. Loading was sustained for periods up to nine months. We show that the magnitude of shear stress to ultimate shear strength and the epoxy time-before-loading could be the most critical parameters affecting creep of epoxy at the concrete–FRP interfaces. It was also found that the creep of epoxy can result in failure at the interfaces due to the combined effect of relatively high shear stress to ultimate shear strength and thick epoxy adhesive. This can have an adverse effect on the designed performance of reinforced concrete (RC) structures strengthened with FRP. Based on the experimental observations, rheological models were developed to simulate the long-term behavior of epoxy at the concrete–FRP interfaces. It is shown that the long-term behavior of epoxy at the interfaces can be properly modeled by analytically for both loading and unloading stages.     

A study of the NOx storage catalyst of Ba–Fe–O complex oxide

The complex oxide Ba–Fe–O catalysts were prepared by sol–gel method. The XRD, DTA, NO-TPD, XPS and NSC measurements were used to characterize the structures, NO_x storage property and sulfur resistance ability. It is concluded that when coadsorption of NO and O₂ at 400 °C, the sample calcined at 750 °C possesses high NO_x storage capacity and sulfur resistance. The perovskite type BaFeO₃ and BaFeO_3−x phases are the active centers in the catalyst for NO_x storage.     

Effect of metal-substitution on the redox behaviors of mono-transition metal-substituted Wells–Dawson tungstoarsenates

Mono-transition metal (Mo, V, and Nb)-substituted Wells–Dawson tungstoarsenates were investigated to elucidate the effect of metal-substitution on their redox behaviors. In the electrochemical analysis, an additional redox transition was observed for molybdenum- and vanadium-substituted tungstoarsenates, while a negatively shifted redox transition was observed for niobium-substituted tungstoarsenate. First electron reduction potential of the catalysts showed the consistent trend with UV–visible absorption edge energy of the catalysts. Oxidative dehydrogenation of benzylamine to dibenzylimine was carried out as a model reaction to probe oxidation catalysis. Yield for dibenzylimine increased with increasing first electron reduction potential and with decreasing UV–visible absorption edge energy.     

Evaluation of the performance of a constructal mixer with the iodide–iodate reaction system

A novel fluidic mixer, which takes advantage of impingement mixing and is designed according to the constructal approach, is evaluated by the Villermaux/Dushman method. The effects of different configurations (including the structure of the fluid collector and the diameter of the nozzles on the fluid injector) on the mixing performance (i.e. the segregation index and the energy dissipation rate) are determined. The segregation index is smaller, or in other words the degree of mixing is better, when a branch type fluid collector or smaller nozzles are used, however, at the cost of higher energy dissipation rate. When the flow rate is sufficiently high, mixing caused by the impingement of streams is almost complete, rendering the mixing in the branched channels unnecessary. As a result, if very high degree of mixing is pursued, the collector with a simple empty space should be used in the mixer to reduce energy consumption.     

Experimental determination of the Hamaker constants for solid–water–oil systems

The van der Waals interaction (vdW) is a fundamental interaction in colloid and interface science. Regardless of the methods used in deriving the vdW interaction between two bodies as a function of their separation distance, the Hamaker constant is always an essential parameter involved. In this paper, a simple experimental method is presented to determine the Hamaker constant. As an example, the Hamaker constant of a solid-water-oil system is related to its surface and interfacial energies, which can be measured accurately. Based on the proposed method, the effects of two typical solid surfaces and three kinds of aqueous solutions on the Hamaker constant and wettability of the solid-water-oil system are studied. It is found that hydrophilic and hydrophobic solid surfaces will lead to rather different Hamaker constants and wettability behaviour. The detailed experimental results also show that the ionic surfactant solutions have a strong influence, whereas the pH value of the aqueous phase has a limited effect on the Hamaker constant. In addition, the electrolyte solutions do not strongly affect the Hamaker constant for the oil phase interacting with the solid surface across an electrolyte solution. Such determined Hamaker constants are in reasonable agreement with the reported Hamaker constants for oils (dodecane and hexadecane), mica, and metals (Ag, Au, and Cu) interacting across a pure water phase.     

Evaluation of the performance of heat pump-assisted distillation of an ethanol–water mixture

The process of separation of an ethanol–water mixture was simulated. A heat pump system was then implemented on the distillation column with the highest energy requirements in the process. Complete elimination of hot and cold utilities of the pretreatment column was obtained through an improved system by application of a heat pump at a compressor work of 406.6 kW. Installation of the heat pump system on the pretreatment column of the system results in a reduction of 10.7% of annual operating costs and 6.6% of the total annual costs (TAC). However, the capital costs of the process will enhance by 22.6%.     

Numerical simulation of the mechanical properties of a carbon-fiber-reinforced hollow glass microsphere–epoxy syntactic foam

The addition of carbon fibers has a great influence on the mechanical properties of hollow glass microsphere (HGM)–epoxy syntactic foam. Thus, to elucidate the reinforcement mechanism, the numerical simulation of HGM- and carbon-fiber-filled epoxy matrixes was carried out. The effect of the orientation of carbon fibers on the elastic modulus and stress distribution was studied. The effect of the elastic modulus of the matrix on the change of force was also studied. We noted that the orientation of carbon fibers affected the elastic modulus of the matrix, and when the carbon fibers were distributed in the direction of force, the elastic modulus of the matrix reached its maximum. The maximum stress of HGMs decreased with increasing matrix elastic modulus, and the mechanical properties of the syntactic foam increased with increasing elastic modulus of the matrix. When the carbon fibers were distributed in the direction of the force, the enhancement effect was the best. Because the carbon fibers had a higher elastic modulus than the matrix, the degree of compressive deformation of the carbon fibers was smaller than that of the matrix. During compression, carbon fibers were pulled out and consumed a lot of energy. Thus, the mechanical properties of the syntactic foam were improved. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47083.