Curing Process Of Epoxy Resin Research Articles

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.

Preparation and construction mechanism of thermal conductive epoxy‐based composite via magnetic field induced orientation

AbstractBN was modified with Fe3O4 to confer it with paramagnetic responsivity. The scanning electron microscopy and energy dispersive spectrometer (EDS) results demonstrated that with the assistance of an external magnetic field, the paramagnetic BN particles within an epoxy matrix are effectively aligned along the direction of the magnetic field during the curing process of epoxy resin, hence forming continuous thermal conduction pathways. Therefore, the thermal conductivity of the epoxy‐based composite filled with 30 wt% of BN and externally applied with a 50 mT magnetic flux density was 0.7417 (W/m·K), an improvement of 207.89% relative to the pure epoxy resin. The establishment of continuous thermal pathways facilitates effective phonon conduction, thereby further enhancing the thermal conductivity of the material. Meanwhile, this study investigates the chain formation mechanism of Fe3O4‐modified BN under the influence of a magnetic field. When subjected to an applied magnetic field, the magnetic BN embedded in an epoxy resin matrix undergoes magnetization, rotation, and contact. Subsequently, multiple particles initially form short chains, then aggregate into longer chains aligned with the direction of the magnetic field. The findings indicate that the magnetic field induced particle alignment method holds significant potential in the fabrication of high thermal conductivity polymer composites with low filler loading.

Improving the mechanical performance of phase change microcapsule/epoxy composites via alternately enhancing interface bonding and strength of microcapsules

Microcapsules containing phase change materials (MPCMs) can be integrated with polymeric matrices to develop intelligent thermoregulating composites for applications in temperature management and thermal energy storage. However, the mechanical properties of such composites can be severely compromised due to the weak interfacial adhesion and the poor microcapsule rupture strength. This paper aims to investigate the failure mode of microcapsule/epoxy composites and present corresponding implementable methods to improve their strength. The MPCMs/epoxy composite primarily fails due to poor microcapsule-matrix interface debonding resulting from the poor compatibility between MPCMs and epoxy resin. A silane coupling agent (EPTMS) was employed to modify the surface of MPCMs, forming E-MPCMs, and created a crosslinked interface network during the epoxy curing process. The tensile strength of composite containing 20 wt% E-MPCMs has been improved by 23.55 % while the failure mode switching to microcapsule rupture. Furthermore, the microcapsule shell enhanced by TiO2 nano-reinforcements achieves a 105.20 % improvement in microcapsule rupture strength. The incorporation of EPTMS modified TiO2-MF hybrid microcapsules (ET-MPCMs) resulted in a significant increase of 33.79 % in the tensile strength of ET-MPCMs/epoxy composite with the failure mode changing to microcapsule-matrix interface debonding again. The strategy of alternately enhancing subset properties derived from the competition failure mode of the composite opens new perspectives for the preparation of high-performance multifunctional composites.

Anisotropic dispersion promoting and passivation protecting of CA@JNs@Zn2+ Janus nanosheets on epoxy anticorrosive coatings

Defects in the curing process of epoxy coatings tend to deteriorate the anticorrosion performance of the coatings. An important way to enhance the anticorrosion performance of epoxy coatings is to add flaky fillers. In this paper, anisotropic Janus nanosheets (JNs) were prepared and modified by cardanol (CA) and Zn2+. Then, the obtained CA@JNs@Zn2+ Janus composite nanosheets were introduced into epoxy coatings. When the content of CA@JNs@Zn2+ is 0.2 %, the mechanical properties and corrosion resistance of the coating were effectively improved. The tensile strength of 0.2 %-CA@JNs@Zn2+/EP coating is 79 % higher than that of pure epoxy coating. The EIS analysis showed that the low-frequency impedance of the coating remained as high as 16.45 Gohm·cm2 after 35 days of immersion, and remained above 10 Gohm·cm2 during the whole immersion period, which demonstrating the optimal corrosion resistance. This paper provides a theoretical basis for the preliminary application of Janus material in the field of anticorrosion, and the prepared composite coatings are expected to be used for anticorrosion in harsh environment.

Investigation of the effect of polyesteramides based on secondary PET on the curing process and properties of epoxy filling compositions

Filling compositions based on epoxy-diane resin have been developed, the curing process using polyesteramides has been investigated, and the activation energies of the curing reaction have been calculated. The effect of increasing the amount of PET on the thermal characteristics and physico-mechanical properties of filling compositions is shown. The test results show that the filling compositions obtained using the developed polyesteramides are superior in properties to compositions based on industrial amine hardeners.

Understanding cure and interphase effects in functionalized graphene‐epoxy nanocomposites

AbstractAgglomerations effects of graphene‐based nanofillers are often reported in the literature to be the main reason on the deterioration of the mechanical properties, especially at high filler loadings. In our study, we focused on the correlated effects of plasma‐treated graphene nanofillers on the curing reaction and mechanical properties of an epoxy matrix. Specifically, we explored the effect of dispersion state, planar size, filler content, surface functionalization and stoichiometric ratio on the epoxy curing process. The surface of the treated graphene nanofillers were studied in detail by X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy and X‐ray diffraction (XRD). The results indicated greater presence of oxygen containing groups with the crystallinity to be unaffected after the plasma process. Dynamic Mechanical Analysis (DMA) was used to assess the changes in both the Tg and the mechanical properties of graphene‐epoxy nanocomposites. Rheological and microscopic data showed that a well‐dispersed material was achieved at high filler loadings with the use of calendaring and plasma functionalization. Although, a well‐dispersed material was achieved on the bulk composite, no further mechanical reinforcement was observed at high filler loadings. The adsorption of epoxy groups onto the graphene nanofillers' surface, leading to a stoichiometric imbalance between the epoxy chains and hardener molecules, was proposed to explain the results.

Development of Methods for Determining Molecular Weights of Epoxy Resin

Molecular weight, as one of the characteristic indexes of the intrinsic structure of epoxy resin, has a decisive impact on the curing process of epoxy resin, the structure of cured products and even the properties of composites. It is an important indicator for the quality evaluation of epoxy resin. This paper introduces various instrumental analysis techniques for molecular weight testing, including gel permeation chromatography, nuclear magnetic resonance spectroscopy, vapor pressure osmometry and mass spectrometry, and also compares and reviews the principles, characteristics, applications, and problems of each method, and finally looks into the research prospects.

In situ monitoring of epoxy resin curing process: Using glass transition as a bridge

Dielectric analysis (DEA) is an effective method for monitoring the curing process of epoxy resin (EP) in situ, but the influence of curing temperature on the measurement results limits the application of DEA in curing process monitoring with variable temperature. In this study, the theoretical basis of using the glass transition as a bridge and monitoring the curing degree by ionic conductivity was analyzed. DEA experiments and differential scanning calorimeter (DSC) experiments were carried out and the mathematical model of ionic conductivity, glass transition temperature, and curing degree of the EP established. The parameters in the model were only related to the composition of EP system in the model as verified for implementing curing-degree monitoring of the EP system cured at variable temperature. The FT-IR spectra measurement was carried out and the activation energy of curing reaction was calculated by results of DEA and DSC, which verified the reliability and scientificity of the method. The method of using glass transition as a bridge can monitor the curing degree in the curing process with variable temperature and extends DEA to the monitoring of this process under variable temperatures.

Spectroscopically Detecting Molecular-Level Bonding Formation between an Epoxy Formula and Steel.

The formation of the interfacial adhesion between an epoxy adhesive and a substrate was normally accompanied by the epoxy curing process on the substrate. Although the debate on the formation mechanism of the interfacial adhesion is still ongoing, this issue can causally be resolved by studying the interfacial structural formation between the epoxy adhesive and the substrate. Herein, to reveal the interfacial structural formation of a representative formula composed of epoxy (digylcidyl ether of biphenyl A, DGEBA) and amine hardener (2,2'-(ethylenedioxy) diethylamine, EDDA) with the steel substrate upon curing and postcuring treatments, sum-frequency generation (SFG) vibrational spectroscopy with a sandwiched transparent window/epoxy adhesive/steel setup was applied to detect and track the buried molecular-level structures at the epoxy adhesive/steel interface. An X-ray photoelectron spectroscopic (XPS) experiment was performed to probe the intentionally exposed interface to disclose the occurring interfacial chemical reaction. The reaction between the epoxy groups and the steel-surface OH groups and the molecular reconstruction of interfacial epoxy methyl groups upon curing and postcuring steps were confirmed. The latter also indirectly indicated the formation of the additional hydrogen bonding and the former bonding reaction at the interface. The above two spectroscopic experimental results matched up with the further examination of the adhesion strength. Therefore, this work elucidates the formation of the interfacial bonding between the epoxy formula and the steel substrate upon curing and postcuring treatments at the molecular level, thus providing an in-depth insight into the origin of the interfacial adhesion.

Effects of Silicon Dioxide/Graphene Oxide Hybrid Modification on Curing Kinetics of Epoxy Resin.

In this study, SiO2-grafted graphene oxide (GO-SiO2) was prepared using the oxygen-containing group on the GO surface as the active site of the reaction. The chemical structure, morphology, and particle size of GO and GO-SiO2 were carefully investigated by Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, thermogravimetry, transmission electron microscopy, scanning electron microscopy, and atomic force microscopy, and the results proved that the grafting modification was successful. Furthermore, epoxy (EP)/GO composites were prepared, and the effects of unmodified GO and GO-SiO2 on the curing kinetics of EP were comparatively studied by differential scanning calorimetry (DSC). The results showed that, compared with neat EP and EP/GO, GO-SiO2 significantly reduces the curing temperature of the composites, indicating that GO-SiO2 has a more significant catalytic effect on the curing process of EP. The calculation results of the Kissinger method showed that the curing activation energy of EP/GO-SiO2 is obviously lower than that of EP/GO and neat EP. Results of the Ozawa method showed that the introduction of GO-SiO2 reduces the curing activation energy during the whole curing process, and in the middle and late stages of curing (α = 0.5–1) can significantly reduce the curing activation energy. The related mechanism has been proposed.

Fluoro-substituted polyaniline deeply incorporation with inert h-BN for interface improvement in anti-corrosion

Inert inorganic anti-corrosion fillers enjoy the advantages of stable performance, but most of them face the problems of poor compatibility and disability. Therefore, strategies about addressing the above problems need to be developed. Herein, based on interface enhancement strategy, one typical inert inorganic filler h-BN was armed with fluoro-substituted polyaniline (PFAN). To this end, diazotization reaction was utilized to introduce aniline group to h-BN, and emulsion polymerization was employed to grow the micro-size copolymer of aniline and 2-fluoroaniline. Benefitting from the above interface enhancement attempt, the compatibility of h-BN with resin was enhanced, which was explained by the chemical armed PFAN participated in the curing process of epoxy. Moreover, the disability of h-BN was also improved owed to this interface armor. Therefore, combining the fully developed barrier properties of h-BN with passivate ability of PFAN, the coating containing a small addition amount (0.9 wt%) of the composite fillers to epoxy coatings significantly improved their long-term corrosion resistance, which was confirmed by the EIS result that its low frequency impedance value was maintained above 10 10 and was about 10 3 times larger than that of pure EP coating after being immersed in 3.5 wt% NaCl solution for 120 days.

Fiber bragg grating sensors to monitor strain response of epoxy resin during curing and cryogenic temperature

The epoxy resin is one of the critical components of large-scale superconducting magnets which find wide applications in magnetic confinement fusion, high energy accelerator and magnetic resonance imaging. In general, magnetic confinement fusion, high energy accelerator and magnetic resonance will be used at cryogenic temperatures. An approach with respect to real-time monitor of epoxy curing and cryogenic process is practically significant for magnetic reliability, maintainability as well as future mass production. Fiber Bragg Grating (FBG) sensors are excellent candidates of this measurement for their anti-electromagnetic interference characteristics and high sensitivity. The aim of this work is to use the embedded Fiber Bragg Gratings (FBG) to obtain real-time strain response of curing epoxy, and the strain change of the epoxy at cryogenic temperature from 4.2K to 300K. An FBG pre-tensioning device was designed to keep the FBG embedded in the epoxy in a stretched state during the epoxy curing process. Moreover, strain gauge sensors were also embedded to monitor the curing process at the same time. There are subtle differences in strain values obtained by the two sensors, which illustrate higher sensitivity of FBGs. This work verifies the reliability of FBG monitoring the strain response of epoxy during curing process as well as the cryogenic service.

Electroactive shaping and shape memory of sequential dual-cured off-stoichiometric epoxy/CNT composites

Sequential dual-cured epoxy composites, based on off-stoichiometric thiol–epoxy mixtures catalysed by 1-methylimidazole, have been developed by adding carbon nanotubes (CNT). The epoxy curing process initially consists in two thermally activated curing stages: a first thiol–epoxy reaction and later homopolymerization at a higher temperature. This system presents easy shaping/conforming and shape memory properties through thermo-mechanical treatments. Addition of the electrical CNT network into the epoxy matrix allows electrical switching, which increases its performance and in-situ applicability.The obtained results confirmed that CNTs catalyse the homopolymerization epoxy reaction, hindering the sequential curing process, due to the π–π anchoring of imidazole catalyser over the CNTs surface, enhanced by donor–acceptor interaction.Non-doped off-stoichiometric resins present relatively high thermal strength, with a glass transition temperature in the range of 73–109 °C, and high stiffness, with a storage modulus close to 2–3 GPa. They can be easily conformed at low temperature, 60 °C, before their second curing stage, showing a high shaping efficiency (around 90%) and full fixing efficiency (>98%).Nanocomposites with 0.2% CNT present efficient Joule heating, triggering the shape memory at low voltage, <80 V, with fixing and recovery efficiencies of 60–85%. In addition to its high in-situ applicability, the electrical resistive heating is faster and more efficient than conventional heating in an oven.

Wetting mechanism and interfacial bonding performance of bamboo fiber reinforced epoxy resin composites

A polyacrylate-based wetting agent, BYK-358N, was used to improve the interfacial bonding between the bamboo fibers and epoxy matrix. Fourier transform infrared spectrometer and differential scanning calorimetry were used to study the curing process of epoxy resin. The wettability of the treated bamboo fiber, which by the untreated and wetting agent treated epoxy, was studied using the surface tension of epoxy and bamboo fiber. The analysis of dynamic contact angle and interfacial shear stress confirmed that the composite interface was improved. Finally, Vacuum-assisted resin transfer molding was used to prepare bamboo fiber reinforced epoxy resin (BF/EP) composites, and the tensile properties were measured. The results showed that the addition of the wetting agent could effectively improve the interface compatibility of BF/EP composites. The interfacial shear stress of BF/EP composites treated with 1% wetting agent was 24.36 MPa, tensile strength was 111.54 MPa, and Young's modulus was 3.90 GPa, which was 165.7%, 99.7%, and 66.7% higher than that of untreated BF/EP composites.

Heterogeneous dynamics in the curing process of epoxy resins

Epoxy resin is indispensable for modern industry because of its excellent mechanical properties, chemical resistance, and excellent moldability. To date, various methods have been used to investigate the physical properties of the cured product and the kinetics of the curing process, but its microscopic dynamics have been insufficiently studied. In this study, the microscopic dynamics in the curing process of a catalytic epoxy resin were investigated under different temperature conditions utilizing X-ray photon correlation spectroscopy. Our results revealed that the temperature conditions greatly affected the dynamical heterogeneity and cross-linking density of the cured materials. An overview of the microscopic mechanism of the curing process was clearly presented through comparison with the measurement results of other methods, such as 1H-pulse nuclear magnetic resonance spectroscopy. The quantification of such heterogeneous dynamics is particularly useful for optimizing the curing conditions of various materials to improve their physical properties.

Molecular Size Effect on Curing Process for Epoxy and Amine Mixture

The curing process of epoxy and amine mixtures was investigated from the viewpoint of molecular size of reactants via all-atom molecular dynamics simulation. Of four mixtures composed of either smaller or larger epoxy and amine molecules, the system of both smaller epoxy and amine reached the highest conversion at a given time, whereas the conversion for the combination of both larger epoxy and amine was the lowest. These can be explained in terms of their diffusion coefficients. In addition, the size effect on the reaction kinetics was more striking for epoxy than amine in the curing process. This was because epoxy reacted only once but amine did twice. When the primary amine reacted with epoxy, the resultant secondary amine was incorporated into the three-dimensional network. Hence, even if the primary amine had a higher mobility, the movement of the part after the reaction slowed down, making the further reaction to be the tertiary amine slow. Finally, it was shown that although the smaller epoxy reacted faster than the larger one in the mixed system of larger/smaller epoxy and amine, there was no difference in the reaction progress depending on the molecular size of amine.

Kinetic Analysis of the Curing Process of Biobased Epoxy Resin from Epoxidized Linseed Oil by Dynamic Differential Scanning Calorimetry.

The curing process of epoxy resin based on epoxidized linseed oil (ELO) is studied using dynamic differential scanning calorimetry (DSC) in order to determine the kinetic triplet (Ea, and A) at different heating rates. The apparent activation energy, Ea, has been calculated by several differential and integral isoconversional methods, namely Kissinger, Friedman, Flynn–Wall–Ozawa (FWO), Kissinger–Akahira–Sunose (KAS) and Starink. All methods provide similar values of Ea (between 66 and 69 kJ/mol), and this shows independence versus the heating rate used. The epoxy resins crosslinking is characterized by a multi-step process. However, for the sake of the simplicity and to facilitate the understanding of the influence of the oxirane location on the curing kinetic, this can be assimilated to a single-step process. The reaction model has a high proportion of autocatalytic process, fulfilling that αM is between 0 and αp and αM < . Using as reference the model proposed by Šesták–Berggren, by obtaining two parameters (n and m) it is possible to obtain, on the one hand, the kinetic parameters and, on the other hand, a graphical comparison of the degree of conversion, α, versus temperature (T) at different heating rates with the average n and m values of this model. The good accuracy of the proposed model with regard to the actual values obtained by DSC gives consistency to the obtained parameters, thus suggesting the crosslinking of the ELO-based epoxy has apparent activation energies similar to other petroleum-derived epoxy resins.

Continuously thermal conductive pathway of bidisperse boron nitride fillers in epoxy composite for highly efficient heat dissipation

This study presents a combined strategy of bidisperse boron nitride (BN) and surface modification to improve the thermal conductivity (κ) of the BN-epoxy composite. Surface modification of hexagonal BN was carried out with benzoic acid (BA). The optimal weight ratio of large and small BN (BNl and BNs) was 8:2 for their connectivity in the epoxy matrix. For 50 wt% filler content, the BNl8s2-BA composite has a much higher κ of 2.71 W/m K (out-plane direction), which is 12.3, 1.32, and 1.50 times higher than that of the pure epoxy, and its composite with BNl10s0-BA and BNl8s2, respectively. The chemical-sensitive analyses indicate that carboxyl-carboxylate interaction occurred in partially deprotonated benzoic acid on BN during the epoxy curing process, resulting in in-situ formation of a continuously thermal conductive pathway, which is responsible for the improved κ.

Mechanical properties of carbon nanotubes/epoxy nanocomposites: Pre-curing, curing temperature, and cooling rate

The effects of composite fabrication, such as pre-curing, curing temperature, and cooling rate, were studied. In this work, the pre-curing was defined as heat treatment of Multi-Walled Carbon Nanotubes (MWNCTs) with Diglycidyl Ether of Bisphenol A (DGEBA) epoxy resin. Acid purified MWCNTs were characterized by Raman spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR). The pre-curing facilitated bonding between MWCNTs and epoxy via the oxirane ring of DGEBA, which accelerated the curing process of epoxy and increased mechanical properties. The elevated curing temperature on the pre-cured sample further improved the composite’s mechanical properties by increasing interfacial bonding due to cross-linking. The rapid cooling using liquid nitrogen during pre-curing treatment prevented re-agglomeration of MWCNTs, showing smaller agglomerates and improving the mechanical properties. Agglomeration was characterized by scanning electron microscopy, while the bonding between MWCNTs and epoxy was examined by the length of fibre pull-out on the fracture surface. Tensile testing was deployed for mechanical properties characterization. The degree of cure was determined by FTIR and Differential Thermal Analysis (DTA).

Monitoring bond strength of adhesively planted rebars in concrete using electro-mechanical impedance (EMI)

Electro-mechanical impedance (EMI) is among the many types of gauges that can be used for structural health monitoring (SHM) of concrete structures. The bond strength between concrete and planted rebar plays an important role in the integrity of concrete structures strengthened by post-installed anchors. This paper presents a method to monitor the development of epoxy bond strength between concrete and a planted rebar by the EMI technique. Three concrete cubes each with a planted rebar were fabricated and used for a set of monitoring experiments. To conduct experiments, a lead zirconate titanate (PZT) patch was attached to the polished side surface of the planted rebar. During the monitoring test, conductance spectra signals from three PZT patches were acquired at the 0th, 20th, 40th, 60th, 80th, 100th, and 120th min of the experiment. Furthermore, a pull-out test was conducted on two concrete beams with three planted rebars each. The root mean square deviation (RMSD) and mean absolute percentage deviation (MAPD) were applied to compare instantaneous conductance signatures with the initial, baseline conductance signature across different frequency ranges to ascertain the growth of bond strength during the curing process of epoxy. The results of the monitoring experiments and the pull-out test demonstrate the effectiveness of EMI technique in monitoring bond strength development between concrete and planted rebars. The results also show potential application prospects of the EMI technique in the SHM of reinforced concrete structures.