Control of reactions and network structures of epoxy thermosets

Impact of cross-linking on the time-temperature superposition of creep rupture in epoxy resins

Epoxy resins are an important class of thermosetting resins, and their network structure, formed by the curing reaction of epoxy and amine compounds, plays a crucial role in determining material properties, including creep behavior. We here applied the time-temperature superposition (TTS) principle to analyze the creep behavior of epoxy resins with well-defined network structures that were systematically varied based on the length of the n-alkyl diamine used. The superposition of isothermal creep curves under small stress was achieved through horizontal and vertical shifting, regardless of the length of the n-alkyl diamine. The temperature dependence of the horizontal shift factor was well described by the Williams-Landel-Ferry equation. Creep rupture measurements under large stress conditions revealed specimen rupture, and the time to rupture was plotted against the imposed stress. These plots, acquired at various temperatures, could be superimposed through horizontal shifting. As the diamine length decreased-namely, the distance between cross-linking points-the temperature dependence of the horizontal shift factors deviated from the WLF equation and exhibited Arrhenius-type behavior. The deviation was associated with differences in the fracture process involving chain scission, which became more pronounced as the diamine length decreased. The insights gained in this study should be valuable for controlling creep response and predicting the long-term durability of epoxy resins.

Stereochemistry-Tuned Hydrogen-Bonding Synergistic Covalent Adaptable Networks: Towards Recycled Elastomers with Excellent Creep-Resistant Performance

Covalent adaptable networks (CANs) offer innovative solutions for the reprocessing and recycling of thermoset polymers. However, achieving a balance between easy reprocessing and creep resistance remains a challenge. This study focuses on designing and synthesizing polyurethane (PU) materials with tailored properties by manipulating the stereochemistry of diamine chain extenders. By employing cis- and trans-configurations of diamine extenders, we developed a series of PU materials with varying mechanical properties and creep resistance. The trans-configured materials (R,R-DAC-PU or S,S-DAC-PU) exhibited superior creep resistance and mechanical strength due to dense hydrogen bonding networks. The cis-configured materials (Cis-DAC-PU) exhibited enhanced processability and elasticity. Under 0.1 MPa stress, R,R-DAC-PU showed a mere 3.5 % strain change at 170 °C over 60 minutes, highlighting its superior creep resistance. Both configurations can be recycled via urea bond exchange reactions using hot pressing or solvothermal methods. Density Functional Theory (DFT) calculations indicate that both the (R,R-DCA-UB-U)2 and (S,S-DCA-UB-U)2 segments form six hydrogen bonds with shorter bond lengths, leading to stronger hydrogen-bonding interactions. Conversely, the (Cis-DCA-UB-U)2 segment forms four hydrogen bonds with longer bond lengths, resulting in weaker interactions. This work highlights the critical role of stereochemistry in designing high-performance, recyclable polymer materials with tailored properties.

Enhanced Electro-Optical Properties and Peel Strength of Epoxy-Based Polymer-Stabilized Liquid Crystal Films Enabled by Rapid Cationic Polymerization and Polymer-Network Morphology Regulation

Polymer-stabilized liquid crystal (PSLC) dimming film has attracted widespread attention due to its normally transparent state, energy-saving capability, and excellent electro-optical performance, which has promising applications in smart cars and building windows. However, achieving good electro-optical performance and high peel strength simultaneously still remains challenging. In this study, a PSLC film based on monoepoxy and diepoxy monomers was prepared through rapid cationic polymerization, showing low driving voltages and high peel strength simultaneously. The influence of the content and composition of two epoxy monomers on the microstructures, mechanical properties, and electro-optical performance of the PSLC films were systematically studied. The polymer morphology of PSLC could be effectively modulated by doping monoepoxy monomers. The PSLC film with total monomer content of ≤15 wt% showed enhanced electro-optical properties and peel strength when doping monoepoxy monomers due to the lateral polymer in the networks and denser polymer on the substrate. When the ratio of E6M to E6PM was 12:3, compared with pure E6M, the threshold voltage decreased from 18.2 V to 12.6 V, and the peel strength increased from 62.53 kPa to 136.37 kPa. These PSLC films can adapt to the requirements of different application scenarios by changing the content and proportion of two epoxy monomers, and the strategy has good prospects in the actual production and application of PSLC films.

Degradable Semi-Cycloaliphatic Epoxy Resin for Recyclable Carbon Fiber-Reinforced Composite Materials

The development of an energy-saving method to recycle expensive carbon fibers (CFs) from end-of-life thermosetting resin-based CF-reinforced composites (CFRCs) is strongly desired because of the environmental and economic issues. The replacement of traditional thermosetting matrixes with controllably degradable epoxy resins provides a promising solution to this challenging task. In this work, a liquid acetal-containing semi-cycloaliphatic epoxy resin (H-ER) is designed and synthesized. After curing, H-ER shows simultaneously increased thermal stability, shearing strength, flexural strength, strain at break, and critical stress intensity factors by 126%, 26.5%, 17.0%, and 29.5%, respectively, in comparison with ERL-4221. Particularly, the cured H-ER is sufficiently resistant to organic solvents, bases, and weak acids but degrades rapidly in a modestly strong acidic aqueous solution, and the rate of degradation is controlled by modulating the acidity. GC-MS and FTIR spectra demonstrate that the degradation is indeed due to the cleavage of acetal linkages in the network, and the degradation-generated benzaldehyde may be reused as a raw material for the synthesis of the H-ER resin. More importantly, for the CFRCs using H-ER as a matrix, the CFs are readily recovered without detectable damage and are able to be recycled for CFRC fabrication.

Design and Evaluation of a Reprocessable Bismaleimide Thermoset: Enhancing Functionality and Sustainability Compatibility

Bismaleimide (BMI) resins are high-performance thermosets that are primarily used in aerospace because of their exceptional heat resistance and physical properties. However, their growing demand has led to significant environmentally unfriendly waste. To address this, our research proposes a reprocessable BMI system using a newly synthesized BMI vitrimer (BMIV) with functional groups that form covalent adaptable networks (CANs). To enhance the properties, a symmetrical BMI with two ester groups introduced into the rigid rod molecule was designed as a CAN component. After confirming the structure using various spectroscopic techniques, BMIV was coupled with aromatic diamines via an additional aza-Michael reaction to obtain the cured resins. Subsequently, the mechanical properties and reprocessing behavior of the thermally stable and optimized thermosetting material with the best performance were evaluated, and the evidence, mechanism, and activation energy of the topology rearrangement are reported in detail.

Cross-Linking Reaction of Bio-Based Epoxy Systems: An Investigation into Cure Kinetics

The cure kinetics of various epoxy resin mixtures, comprising a bisphenol epoxy, two epoxy modifiers, and two hardening agents derived from cardanol technology, were investigated through differential scanning calorimetry (DSC). The development of these mixtures aimed to achieve epoxy materials with a substantial bio-content up to 50% for potential automotive applications, aligning with the 2019 European Regulation on climate neutrality and CO2 emission. The Friedman isoconversional method was employed to determine key kinetic parameters, such as activation energy and pre-exponential factor, providing insights into the cross-linking process and the Kamal-Sourour model was used to describe and predict the kinetics of the chemical reactions. This empirical approach was implemented to forecast the curing process for the specific oven curing cycle utilised. Additionally, tensile tests revealed promising results showcasing materials' viability against conventional counterparts. Overall, this investigation offers a comprehensive understanding of the cure kinetics, mechanical behaviour, and thermal properties of the novel epoxy-novolac blends, contributing to the development of high-performance materials for sustainable automotive applications.

Thermo-rheological and kinetic characterization and modeling of an epoxy vitrimer based on polyimine exchange

The present study describes the development of cure kinetics and chemo-rheological models for an epoxy vitrimer based on polyimine exchange to elucidate the potential in terms of processing and accurate process selection. Reaction kinetics is investigated using differential scanning calorimetry. A good agreement between the model and data can be demonstrated for different stoichiometries by selecting a parallel reaction approach consisting of an nth-order and an autocatalytic approach. The suggested chemo-rheological model captures the intrinsically high viscosity of the resin over a broad temperature and curing range, even after the gelation point. The Di-Benedetto equation represents the glass transition temperature advancement with cure while combining the rheological Winter-Chambon criterion and the kinetic model determines the degree of cure at the gelation. These results give important advice for improved process modeling of vitrimeric resins, facilitate accurate process selection, and pave the way towards the development of composites based on the matrix system investigated in this work.

Effects of Curing Agents on the Adhesion of Epoxy Resin to Copper: A Density Functional Theory Study

Epoxy resins are widely used adhesives in industrial fields. To use epoxy resin as an adhesive, it is necessary to mix the epoxy resin with a hardener. Hardeners have various functional groups and skeletons, and the properties of epoxy resins vary depending on the hardener. Although the adhesion of epoxy resins has been extensively studied using density functional theory (DFT) calculations, few studies have evaluated the effect of hardener molecules. Therefore, in this study, DFT calculations of adhesion energies and bonding structures on Cu (111) and Cu2O (111) surfaces are performed for model molecules of adducts of epoxy resin with hardeners having various functional groups and skeletons to evaluate the influence of the hardeners on the adhesion of epoxy resin to the metal surface. The adhesion energy to the Cu (111) surface is governed by the energy due to dispersion forces. Hardeners of the thiol type, which contain relatively heavy sulfur atoms, and hardeners with aromatic rings, displaying high planarity, enable the entire molecule to approach the metal surface, resulting in a relatively high adhesion strength. The calculations for the Cu2O (111) surface show the adhesion strength is more strongly influenced by interactions such as hydrogen bonds between the surface and adhesive molecules than by dispersion forces. Therefore, in adhesion to Cu2O (111), the benzylamine-epoxy adduct with hydrogen bonding and OH-π interactions with the surface, in addition to having a relatively flexible framework, shows a high adhesion strength.

High-Performance One-Component Epoxy Adhesive Based on the Synergistic Effect of Lignin-Derived Triaryl-Imidazole and Phytic Acid

Latent curing agents are essential in the formulation of one-component epoxy resins, yet they are seldom derived from fully biobased chemicals. In the present work, a fully biobased latent curing agent for epoxy resins (BIMPA) was produced by synthesizing an ionic complex of lignin-derived triaryl-imidazole (BIM) and phytic acid (PA). Benefiting from the synergistic effect of BIM and PA, the one-component epoxy resin, composed of BIMPA and commercially available E51, exhibits a storage stability of over 90 days. Upon heating, the ionic complex undergoes decomposition, liberating the active imidazole to cure the precursor. The resulting epoxy resins exhibited a flexural modulus of 3.09 GPa, a flexural strength of 107.47 MPa, a notched izod impact strength of 2.47 kJ/m3, and a shear strength of 41.02 MPa. The outcome can provide an effective supplement for the development of biobased epoxy resins.

Optimizing Epoxy Molding Compound Processing: A Multi-Sensor Approach to Enhance Material Characterization and Process Reliability

The in-line control of curing during the molding process significantly improves product quality and ensures the reliability of packaging materials with the required thermo-mechanical and adhesion properties. The choice of the morphological and thermo-mechanical properties of the molded material, and the accuracy of their determination through carefully selected thermo-analytical methods, play a crucial role in the qualitative prediction of trends in packaging product properties as process parameters are varied. This work aimed to verify the quality of the models and their validation using a highly filled molding resin with an identical chemical composition but 10 wt% difference in silica particles (SPs). Morphological and mechanical material properties were determined by dielectric analysis (DEA), differential scanning calorimetry (DSC), warpage analysis and dynamic mechanical analysis (DMA). The effects of temperature and injection speed on the morphological properties were analyzed through the design of experiments (DoE) and illustrated by response surface plots. A comprehensive approach to monitor the evolution of ionic viscosity (IV), residual enthalpy (dHrest), glass transition temperature (Tg), and storage modulus (E) as a function of the transfer-mold process parameters and post-mold-cure (PMC) conditions of the material was established. The reliability of Tg estimation was tested using two methods: warpage analysis and DMA. The noticeable deterioration in the quality of the analytical signal for highly filled materials at high cure rates is discussed. Controlling the temperature by increasing the injection speed leads to the formation of a polymer network with a lower Tg and an increased storage modulus, indicating a lower density and a more heterogeneous structure due to the high heating rate and shear heating effect.

A Numerical Model to Predict the Relaxation Phenomena in Thermoset Polymers and Their Effects on Residual Stress during Curing, Part II: Numerical Evaluation of Residual Stress

This article proposes a numerical routine to predict the residual stresses developing in an epoxy component during its curing. The scaling of viscoelastic properties with the temperature and the degree of conversion is modeled, adopting a mathematical formulation that considers the concurrent effects of curing and structural relaxation on the epoxy's viscoelastic relaxation time. The procedure comprises two moduli: at first, the thermal-kinetical problem is solved using the thermal module of Ansys and a homemade routine written in APDL, then the results in terms of temperature and the degree of conversion profiles are used to evaluate the viscoelastic functions, and the structural problem is solved in the mechanical module of Ansys, allowing the residual stresses calculation. The results show that the residual stresses mainly arise during cooling and scale with the logarithm of the Biot number.

A Numerical Model to Predict the Relaxation Phenomena in Thermoset Polymers and Their Effects on Residual Stress during Curing-Part I: A Theoretical Formulation and Numerical Evaluation of Relaxation Phenomena

This paper analyzes the effect of crosslinking reactions on a thermoset polymer's viscoelastic properties. In particular, a numerical model to predict the evolution of epoxy's mechanical properties during the curing process is proposed and implemented in an Ansys APDL environment. A linear viscoelastic behavior is assumed, and the scaling of viscoelastic properties in terms of the temperature and degree of conversion is modeled using a modified version of the TNM (Tool-Narayanaswamy-Mohynian) model. The effects of the degree of conversion and structural relaxation on epoxy's relaxation times are simultaneously examined for the first time. This formulation is based on the thermo-rheological and chemo-rheological simplicities hypothesis and can predict the evolution of epoxy's relaxation phenomena. The thermal-kinetic reactions of curing are implemented in a homemade routine written in APDL language, and the structural module of Ansys is used to predict the polymer's creep and stress relaxation curves at different temperatures and degrees of conversion.

Transparent and Efficient Wood-Based Triboelectric Nanogenerators for Energy Harvesting and Self-Powered Sensing

Wood possesses several advantageous qualities including innocuity, low cost, aesthetic appeal, and excellent biocompatibility, and its naturally abundant functional groups and diverse structural forms facilitate functionalization modification. As the most sustainable bio-based material, the combination of wood with triboelectric nanogenerators (TENGs) stands poised to significantly advance the cause of green sustainable production while mitigating the escalating challenges of energy consumption. However, the inherent weak polarizability of natural wood limits its development for TENGs. Herein, we present the pioneering development of a flexible transparent wood-based triboelectric nanogenerator (TW-TENG) combining excellent triboelectrical properties, optical properties, and wood aesthetics through sodium chlorite delignification and epoxy resin impregnation. Thanks to the strong electron-donating groups in the epoxy resin, the TW-TENG obtained an open-circuit voltage of up to ~127 V, marking a remarkable 530% enhancement compared to the original wood. Furthermore, durability and stability were substantiated through 10,000 working cycles. In addition, the introduction of epoxy resin and lignin removal endowed the TW-TENG with excellent optical characteristics, with optical transmittance of up to 88.8%, while preserving the unique texture and aesthetics of the wood completely. Finally, we show the application prospects of TW-TENGs in the fields of self-power supply, motion sensing, and smart home through the demonstration of a TW-TENG in the charging and discharging of capacitors and the output of electrical signals in different scenarios.

pH-Responsive, Thermoset Polymer Coatings for Active Protection against Aluminum Corrosion

This paper describes the synthesis and use of multifunctional methacrylic monomers, which contain basic (amine) functional groups, including an example in which an acid-labile tert-butylcarbamate-protected glycine is used to form a novel methacrylic monomer. The "protected" amino acid-derived functional monomer (BOC-Gly-MA) is copolymerized with an epoxide functional methacrylic monomer (GMA), to deliver novel multifunctional polymers, which are processed into powder coatings and used to study filiform corrosion at the surface of an aluminum substrate. The BOC-Gly-MA-containing copolymers were shown to improve a coating's anticorrosion performance, presenting the lowest average filiform corrosion (FFC) track length, total FFC number, and total corroded surface area (CSA) of the coatings investigated. Further to this, a mode of action for the role of BOC-Gly functional polymers in corrosion protection is proposed, supported by both solution and polymer-aluminum interface studies, delivering new insights into the mode of action of pH-responsive polymer coatings.

Effect of inorganic material surface chemistry on structures and fracture behaviours of epoxy resin

The mechanisms underlying the influence of the surface chemistry of inorganic materials on polymer structures and fracture behaviours near adhesive interfaces are not fully understood. This study demonstrates the first clear and direct evidence that molecular surface segregation and cross-linking of epoxy resin are driven by intermolecular forces at the inorganic surfaces alone, which can be linked directly to adhesive failure mechanisms. We prepare adhesive interfaces between epoxy resin and silicon substrates with varying surface chemistries (OH and H terminations) with a smoothness below 1 nm, which have different adhesive strengths by ~13 %. The epoxy resins within sub-nanometre distance from the surfaces with different chemistries exhibit distinct amine-to-epoxy ratios, cross-linked network structures, and adhesion energies. The OH- and H-terminated interfaces exhibit cohesive failure and interfacial delamination, respectively. The substrate surface chemistry impacts the cross-linked structures of the epoxy resins within several nanometres of the interfaces and the adsorption structures of molecules at the interfaces, which result in different fracture behaviours and adhesive strengths.

Recyclable Curcumin-Based Bioepoxy Resin with On-Demand Chemical Cleavability

We synthesized a novel curcumin-based bioepoxy resin by introducing epichlorohydrin (ECH) into the hydroxyl groups of curcumin and analyzed it using Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). The epoxy equivalent weight (EEW) was determined based on a reaction with sodium hydroxide (NaOH) through titration, and the actual curing process was conducted after exploring the optimal conditions using an amine-based curing agent through dynamic scanning in differential scanning calorimetry (DSC) and isotherm analysis. The cured epoxy resin had a tensile strength, Young's modulus, and glass transition temperature (Tg) of 33 MPa, 1.4 GPa, and 86 °C, respectively. Interestingly, the diunsaturated ketone contained in the epoxy resin showed on-demand chemical cleavability, in that it had been decomposed into an aldehyde and ketone only after having been converted to a hydroxyl ketone through an oxidation reaction. The results of this study can significantly contribute to improving the eco-friendliness and recyclability of epoxy resins used in fields requiring long-term stability and chemical resistance.

Using thermokinetic methods to enhance properties of epoxy resins with amino acids as biobased curing agents by achieving full crosslinking

Fibre-reinforced polymers (FRPs) are used in numerous industrial sectors and contribute to reducing CO2 emissions due to their outstanding properties in lightweight design. However, sustainable alternatives must be developed since the matrix polymers utilised contain substances hazardous to health and the environment. In widely used epoxy resins, the curing agents are mainly critical. Using biomolecules instead of synthetic curing agents can significantly reduce composites' toxicity and petrol-based carbon content. This study considerably exceeds the thermo-mechanical properties of epoxies cured with amino acids described in the literature until now. It demonstrates competitive or even better properties than state-of-the-art epoxies cured with petrol-based amine curing agents. For instance, the tensile strength of arginine-cured epoxy is more than twice as high as reported before and 13.5% higher compared to the petrol-based reference. At the same time, a high elongation at break of over 6% was accomplished, making these polymers suitable as matrix materials in FRPs. Furthermore, the glass transition onset of up to 130 °C is sufficiently high for many applications. The key to success is the development of individual curing profiles based on thermokinetic analysis. The work provides the development and analysis of several biomolecule-cured epoxies with promising property spectra.

An Accurate and Transferable Coarse-Graining Method for the Investigation of Microscopic Fracture Behaviors of Epoxy Thermosets

Coarse-grained modeling shows potential in exploring the thermo-mechanical behaviors of polymers applied in harsh conditions such as cryogenic environment, but its accuracy in simulating fracture behaviors of highly cross-linked epoxy thermosets is largely limited due to the complex molecular structures of the cross-linked networks. We address this fundamental problem by developing a CG modeling method where the backbones and electrostatic interaction (EI) contributions in the cross-linked networks are retained, and thus the potentials of the CG model can be directly extracted, or parametrized on the basis of, existing all-atomistic (AA) force fields. A multilevel parametrization procedure was adopted, where the bond potentials were parametrized relying on the results of density functional theory (DFT) simulation, whereas the nonbond potentials were parametrized by renormalizing the cohesive interaction strength. Remarkably, the CG model can reproduce stress-strain responses highly consistent with the AA simulation results at multiple stages, including elastic deformation, yielding, plastic flow, strain hardening, etc., and the straightforward parametrization procedure can be easily transferred to different materials and thermodynamic conditions. The CG modeling method was then used to build a large-scale representative volume element (RVE) to investigate the microscopic fracture behavior of an epoxy thermoset. It has been discovered that EI contributions play a significant role in generating correct mechanical responses and fracture morphologies. The influences of temperature (i.e., from room to cryogenic temperatures) and strain rates were discussed, and the fracture morphology in the RVE was unveiled and analyzed in a quantitative manner.

Epoxy (Meth)acrylate-Based Thermally and UV Initiated Curable Coating Systems

Recently, photocurable coatings are being used frequently. However, it is worth mentioning that the use of photopolymerization has its drawbacks, especially in the case of curing coatings on three-dimensional surfaces and in places that are difficult to access for UV radiation. However, it is possible to develop a system in which UV technology and thermal methods for curing coatings can be combined. Moreover, the obtained resins are derived from low-viscosity epoxy resins or diglycidyl ethers, making them an ideal building material for photopolymerization-based three-dimensional printing techniques. Due to the need to improve this method, a series of epoxy (meth)acrylates containing both epoxy and (meth)acrylate groups were obtained via the addition of acrylic or methacrylic acid to epoxy resin, diglycydylether of bisphenol A epoxy resin (DGEBA), cyclohexane dimethanol diglycidyl ether (CHDMDE) and neopentyl glycol diglycidyl ether (NPDE). The structures of the synthesized copolymers were confirmed through spectroscopic analysis (FTIR) and studied regarding their nonvolatile matter content (NV) and acid values (PAVs), as well as their epoxy equivalent values (EEs). Due to the presence of both epoxy and double carbon-carbon pendant groups, two distinct mechanisms can be applied: cationic and radical. Hence, the obtained resins can be cured using UV radiation with thermally appropriate conditions and initiators. This type of method can be used as a solution to many problems currently encountered in using UV technology, such as failure to cure coatings in underexposed areas as well as deformation of coatings. Synthesized epoxy (meth)acrylate prepolymers were employed to formulate photocurable coating compositions. Furthermore, the curing process and properties of cured coatings were investigated regarding some structural factors and parameters. Among the synthesized materials, the most promising are those based on epoxy resin, characterized by their high glass transition temperature values and satisfactory functional properties.

Modification of Epoxides with Metallic Fillers-Mechanical Properties after Ageing in Aqueous Environments

The aim of this research was a comparative analysis of selected mechanical properties of epoxy compounds that were modified with metallic fillers and aged in aqueous environments. The tested epoxy compounds consisted of three components: styrene modified epoxy resin based on Bisphenol A, triethylenetetramine curing agent (resin/curing agent ratio of 100:10) and two types of metallic fillers in the form of particles: aluminum alloy (EN AW-2024-AlCu4Mg1) and tin-phosphor bronze (CuSn10P). Samples were subjected to ageing in 4 water environments: low-, medium- and high-mineralized natural water and in a sugar-containing solution for 1, 2 and 3 months. The epoxy samples were subjected to compressive strength tests in accordance with the ISO 604:2002 standard. It was observed that, among others, the compositions seasoned in low-mineralized water usually achieved the highest average compressive strength. As for filler type, using the bronze filler (CuSn10P) usually achieved the highest average compressive strength results.

Block Chemistry for Accurate Modeling of Epoxy Resins

Accurate molecular modeling of the physical and chemical behavior of highly cross-linked epoxy resins at the atomistic scale is important for the design of new property-optimized materials. However, a systematic approach to parametrizing and characterizing these systems in molecular dynamics is missing. We therefore present a unified scheme to derive atomic charges for amine-based epoxy resins, in agreement with the AMBER force field, based on defining reactive fragments─blocks─building the network. The approach is applicable to all stages of curing from pure liquid to gelation to fully cured glass. We utilize this approach to study DGEBA/DDS epoxy systems, incorporating dynamic topology changes into atomistic molecular dynamics simulations of the curing reaction with 127,000 atoms. We study size effects in our simulations and predict the gel point utilizing a rigorous percolation theory to recover accurately the experimental data. Furthermore, we observe excellent agreement between the estimated and the experimentally determined glass transition temperatures as a function of curing rate. Finally, we demonstrate the quality of our model by the prediction of the elastic modulus based on uniaxial tensile tests. The presented scheme paves the way for a broadly consistent approach for modeling and characterizing all amine-based epoxy resins.

Influence of Block-Copolymers' Composition as Compatibilizers for Epoxy/Silicone Blends

The objective of this study was to prepare crosslinked epoxy networks containing liquid silicone particles in order to improve their mechanical properties and obtain less brittle materials. Different copolymers were used as compatibilizers. These copolymers vary in their chemical composition and structure. All of the copolymers contain hydrophobic (PDMS sequences) and hydrophilic groups. The effect of their chemical structure and architecture on the morphology of the dispersed phase, and on the final physico-chemical and flexural characteristics of epoxy/silicone blends, was explored. The morphology of crosslinked formulations was studied by scanning electron microscopy (SEM), and the thermal characteristics (glass transition temperature, Tg, and curing exothermic peak) were determined by differential scanning calorimetry (DSC). The experimental results have shown that the average diameter and particle size distribution of silicone particles depend on the chemical structure and architecture of the compatibilizers. One copolymer has been identified as the best compatibilizer, allowing a lower mean diameter and particle size distribution in addition to the best mechanical properties of the final network (less brittle character). This study has consequently evidenced the possibility of creating in situ silicone capsules inside an epoxy network by adding tailored compatibilizers to epoxy/silicone formulations.

Water-solvent regulation on complete hydrolysis of thermosetting polyester and complete separation of degradation products

As one of the largest productions of thermosetting plastics, unsaturated polyester resin (UPR) is difficult to be effectively chemcycled after it is discarded due to its dense network structure. Herein, we demonstrate a mild method for efficient alkaline hydrolysis of UPR into useful feedstocks in mixed solvents of polar aprotic solvent and a small amount of H₂O by utilizing the fragmentation effect of the solvent on the UPR and the swelling effect of H₂O on the subsequent partially hydrolyzed UPR respectively. The mixed solvents also play a key role in the aggregation and solubility of the degradation products. It is worth noting that the tetrahydrofuran (THF)-H₂O system achieved 100 % separation of degradation products in an energy-efficient way taking advantage of the insolubility of the carboxylate-containing products in THF and the low boiling point of THF. The participation of non-reactive mixed solvents greatly promotes both the degradation and the separation process of thermosetting polymers.

Effect of Synthetic Low-Odor Thiol-Based Hardeners Containing Hydroxyl and Methyl Groups on the Curing Behavior, Thermal, and Mechanical Properties of Epoxy Resins

A novel thiol-functionalized polysilsesqioxane containing hydroxyl and methyl groups was synthesized using a simple acid-catalyzed sol-gel method to develop an epoxy hardener with low odor, low volatile organic compound (VOC) emissions, and fast curing at low temperatures. The synthesized thiol-based hardeners were characterized using Fourier transform infrared spectroscopy, nuclear magnetic resonance, thermogravimetric analysis (TGA), and gel permeation chromatography and compared with commercially available hardeners in terms of odor intensity and VOC emissions using the air dilution olfaction method and VOC analysis. The curing behavior and thermal and mechanical properties of the epoxy compounds prepared with the synthesized thiol-based hardeners were also evaluated. The results showed that synthetic thiol-based hardeners containing methyl and hydroxyl groups initiated the curing reaction of epoxy compounds at 53 °C and 45 °C, respectively. In contrast, commercial thiol-based hardeners initiated the curing reaction at 67 °C. Additionally, epoxy compounds with methyl-containing synthetic thiol-based hardeners exhibited higher TGA at a 5% weight loss temperature (>50 °C) and lap shear strength (20%) than those of the epoxy compounds with commercial thiol-based hardeners.

Simultaneous rheology and cure kinetics dictate thermal post-curing of thermoset composite resins for material extrusion

Thermoset composites are excellent candidates for material extrusion because they shear thin during extrusion but retain their shape once deposited via a yield stress. However, thermal post-curing is often required to solidify these materials, which can destabilize printed parts. Elevated temperatures can decrease the rheological properties responsible for stabilizing the printed structure before crosslinking solidifies the material. These properties, namely the storage modulus and yield stress, must therefore be characterized as a function of temperature and extent of reaction for various filler loadings. This work utilizes rheo-Raman spectroscopy to measure the storage modulus and dynamic yield stress as a function of temperature and conversion in epoxy-amine resins with fumed silica mass fractions up to 10 %. Both rheological properties are sensitive to conversion and particle loading, but only the dynamic yield stress is reduced by elevated temperatures early in the cure. Notably, the dynamic yield stress increases with conversion well before the chemical gel point. These findings motivate a two-step cure protocol that starts at a low temperature to mitigate the drop in dynamic yield stress, then ramps up to a high temperature when the dynamic yield stress is no longer at risk of decreasing to rapidly drive conversion to near completion. The results suggests that structural stability can be improved without resorting to increasing filler content, which limits control over the final properties, laying the groundwork for future studies to evaluate the stability improvements provided by the multi-step curing schedules.

Benchmark Study of Epoxy Coatings with Selection of Bio-Based Phenalkamine versus Fossil-Based Amine Crosslinkers

The phenalkamines (PK) derived from cardanol oil can be used as a bio-based crosslinker for epoxy coatings as an alternative for traditional fossil amines (FA). First, the reaction kinetics of an epoxy resin with four PK and FA crosslinkers are compared by differential scanning calorimetry, illustrating a fast reaction rate and higher conversion of PK at room temperature in parallel with a moderate exothermal reaction. Second, the performance of coatings with various concentrations of PK and PK/FA ratios indicates good mixing compatibility between crosslinkers resulting in higher hardness, scratch resistance, hydrophobicity, and abrasive wear resistance of coatings with PK. The superior performance is confirmed over a broad range of resin/crosslinker ratios, facilitating the processing with viscosity profiles depending on the PK type. Although fossil- and bio-based crosslinkers have different chemical structures, the unique linear relationships between intrinsic mechanical properties (i.e., ductility and impact resistance) and coating performance indicate that the degree of crosslinking is a primary parameter controlling coating performance, where PK simultaneously provides high hardness and ductility. In conclusion, the optimization of the processing range for bio-based PK as a crosslinker for epoxy coatings delivers suitable processing conditions and superior mechanical performance compared to traditional amine crosslinkers.

High-contrast en bloc staining of mouse whole-brain and human brain samples for EM-based connectomics

Connectomes of human cortical gray matter require high-contrast homogeneously stained samples sized at least 2 mm on a side, and a mouse whole-brain connectome requires samples sized at least 5-10 mm on a side. Here we report en bloc staining and embedding protocols for these and other applications, removing a key obstacle for connectomic analyses at the mammalian whole-brain level.

Corrosion Behavior of Aluminium-Coated Cans

Hundreds of billions of aluminium-based cans are manufactured and used every year worldwide including those containing soft drinks. This study investigates and evaluates the performance and quality of two well-known energy and soft drinks brands, Green Cola and Red Bull. Recent health hazards and concerns have been associated with aluminium leakage and bisphenol A (BPA) dissociation from the can's internal protective coating. The cans were examined under four conditions, including coated and uncoated samples, the soft drink's main solution, and 0.1 M acetic acid solution. Electrochemical measurements such as potentiodynamic polarization and impedance spectroscopy (EIS), element analyses using inductively coupled plasma optical emission spectrometry (ICP-OES), and energy dispersive X-ray spectroscopy (EDS) were performed. In addition, sample characterization by scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD) were employed to comprehensively study and analyze the effect of corrosion on the samples. Even though the internal coating provided superior corrosion protection concerning main or acetic acid solutions, it failed to prevent aluminium from dissolving in the electrolyte. Green Cola's primary solution appears to be extremely corrosive, as the corrosion rate increased by approximately 333% relative to the acetic acid solution. Uncoated samples resulted in increases in the percentage of oxygen, the appearance of more corrosion spots, and decreases in crystallinity. The ICP-OES test detected dangerous levels of aluminium in the Green Cola solution, which increased significantly after increasing the conductivity of the solution.

Curing Kinetics of Bioderived Furan-Based Epoxy Resins: Study on the Effect of the Epoxy Monomer/Hardener Ratio

The potential of furan-based epoxy thermosets as a greener alternative to diglycidyl ether of Bisphenol A (DGEBA)-based resins has been demonstrated in recent literature. Therefore, a deep investigation of the curing behaviour of these systems may allow their use for industrial applications. In this work, the curing mechanism of 2,5-bis[(oxiran-2-ylmethoxy)methyl]furan (BOMF) with methyl nadic anhydride (MNA) in the presence of 2-methylimidazole as a catalyst is analyzed. In particular, three systems characterized by different epoxy/anhydride molar ratios are investigated. The curing kinetics are studied through differential scanning calorimetry, both in isothermal and non-isothermal modes. The total heat of reaction of the epoxy resin as well as its activation energy are estimated by the non-isothermal measurements, while the fitting of isothermal data with Kamal's autocatalytic model provides the kinetic parameters. The results are discussed as a function of the resin composition. The global activation energy for the curing process of BOMF/MNA resins is in the range 72-79 kJ/mol, depending on both the model used and the sample composition; higher values are experienced by the system with balanced stoichiometry. By the fitting of the isothermal analysis, it emerged that the order of reaction is not only dependent on the temperature, but also on the composition, even though the values range between 0.31 and 1.24.

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.

Spatial Distribution of the Network Structure in Epoxy Resin via the MAXS-CT Method

We have succeeded in visualizing the spatial heterogeneity of the reaction ratio in epoxy resins by combining medium-angle X-ray scattering (MAXS) and computed tomography (CT). The reaction ratio is proportional to the degree of cross-linking between epoxy and amine in epoxy resins. The reaction ratio and its spatial inhomogeneity affect the toughness of epoxy resins. However, there has been no non-destructive method to measure the spatial inhomogeneity of the reaction ratio, although we can measure only the spatially averaged reaction ratio by Fourier-transform infrared spectroscopy (FT-IR). We found that the scattering peak reflected the cross-linking structures in the q region of MAXS and that the peak intensity is proportional to the reaction ratio. By reconstructing CT images from this peak intensity, we visualized the spatial heterogeneity of the reaction ratio. The application of this method may not be limited to epoxy resins but may extend to studying the heterogeneity of cross-linked structures in other materials.

Network Formation and Physical Properties of Epoxy Resins for Future Practical Applications

Epoxy resins are used in various fields in a wide range of applications such as coatings, adhesives, modeling compounds, impregnation materials, high-performance composites, insulating materials, and encapsulating and packaging materials for electronic devices. To achieve the desired properties, it is necessary to obtain a better understanding of how the network formation and physical state change involved in the curing reaction affect the resultant network architecture and physical properties. However, this is not necessarily easy because of their infusibility at higher temperatures and insolubility in organic solvents. In this paper, we summarize the knowledge related to these issues which has been gathered using various experimental techniques in conjunction with molecular dynamics simulations. This should provide useful ideas for researchers who aim to design and construct various thermosetting polymer systems including currently popular materials such as vitrimers over epoxy resins.

Fused Filament Fabrication of a Dynamically Crosslinked Network Derived from Commodity Thermoplastics

A massive carbon footprint is associated with the ubiquitous use of plastics and their afterlife. Greenhouse gas (GHG) emissions from plastics are rising and increasingly consuming the global "carbon budget". It is, hence, paramount to implement an effective strategy to reclaim postconsumer plastic as feedstock for technologically innovative materials. Credible opportunity is offered by advances in materials chemistry and catalysis. Here, we demonstrate that by dynamically crosslinking thermoplastic polyolefins, commodity plastics can be upcycled into technically superior and economically competitive materials. A broadly applicable crosslinking strategy has been applied to polymers containing solely carbon-carbon and carbon-hydrogen bonds, initially by maleic anhydride functionalization, followed by epoxy-anhydride curing. These dynamic networks show a distinct rubber modulus above the melting transition. We demonstrate that sustainability and performance do not have to be mutually exclusive. The dynamic network can be extruded into a continuous filament to be in three-dimensional (3D) printing of complex objects, which retain the mechanical integrity of vitrimers. Being covalently crosslinked, these networks show a thermally triggered shape-memory response, with 90% recovery of a programmed shape. This study opens up the possibility of reclaiming recycled thermoplastics by imparting performance, sustainability, and technological advances to the reprocessed plastic.

Research progress of low dielectric constant polymer materials

The advent of high frequency communication era presents new challenges for further development of dielectric polymer materials. In the field of communication, efficient signal transmission is critical. The lower the dielectric constant of the dielectric material used, the lower the signal delay and the higher the signal fidelity. The preparation of polymer materials with low dielectric constant or reduce the dielectric constant of polymer materials becomes a key research topic. Summarizing past progress and providing perspective, this paper primarily discusses the intrinsic low dielectric polymers, fluorine doped low dielectric polymers, and microporous low dielectric polymers, while predicting the research trend of low dielectric materials.

Glycidate as a High-Strength Epoxy Adhesive Curable with Amine under Ambient Conditions

This paper reports that glycidates bearing epoxy moieties with adjacent ester can be cured with diethylenetriamine (DETA) under mild conditions and exhibit high adhesiveness. Curing of bifunctional glycidates with DETA gave cross-linked products. The curing started at a lower temperature (7 °C) than the analogous glycidyl ether (27 °C), while the rate of the curing was slower due to the lower activation energy (Ea = 57 kJ/g) and exothermicity (ΔH = 58 J/g) as confirmed by DSC analysis. The curing system of neopentyl glycol diglycidate and DETA effectively adhered aluminum plates by curing at 25 °C, and the strength was more than five times higher than the curing with analogous glycidyl ether. The higher adhesive strength under curing of ambient conditions and facile preparation of monomers are the significant advantages of this curing.

Multicycling of Epoxy Thermoset Through a Two-Step Strategy of Alcoholysis and Hydrolysis using a Self-Separating Catalysis System

Plastic has now become a contradiction between civilization and pollution that human society has to resolve. The recycling of thermosetting plastics in waste plastics is a huge challenge since they are difficult to remold like thermoplastic plastics due to their high crosslinking density. Here, a new strategy was developed to achieve multicycling of anhydride-cured epoxy thermosets. The process consisted of mild and high-efficiency alcoholysis catalyzed by potassium phosphate/low-boiling alcohol system, and subsequent fast hydrolysis to obtain degradation products rich of carboxyl groups. The degradation products were reused as curing agent to prepare new anhydride-cured epoxy thermosets without sacrifice of high strength and stability. Moreover, the new epoxy thermosets could still be repeatedly recycled using the same protocol. The insolubility of potassium phosphate in ethanol at room temperature made the separation and reuse of the catalyst more convenient. The use of low-boiling alcohol not only allowed high-efficiency degradation but also enabled easy separation from the degradation products. The excellent degradation performance was attributed to the improved swelling of the thermoset and the increased solubility of potassium phosphate induced by small amounts of water in the alcohol. This research provides a recycling method that can reintegrate thermoset waste plastics into remodeling ones under the background of circular economy.