Epoxy Thermosets Research Articles

Degradable, intrinsically flame-retardant, low-water-absorbing vanillin-derived epoxy thermoset with a Schiff base structure

Given the issues of non-renewability, poor degradability, and high flammability associated with petroleum-derived bisphenol A-type epoxy (DGEBA) resins, developing degradable and flame-retardant epoxy resins from renewable biomass is of great importance. However, it is still a challenge at present. Herein, a novel Schiff base diepoxy compound (BV-EP) was successfully synthesized from lignin-derived vanillin and commercial amine 9,9-Bis(4-aminophenyl)fluorene (BAPF) through a simple, two-step procedure. After cross-linking with 4,4ʹ-diaminodiphenylmethane (DDM), the vanillin-derived Schiff base epoxy thermoset (BV-EP/DDM) displayed better thermal properties (glass transition temperature Tg: 197 °C) and flame retardancy than the control sample (DGEBA/DDM). The peak heat release rate (PHRR) and the total heat release (THR) of BV-EP/DDM were reduced by 60.1 % and 19.8 %, respectively, compared with those of DGEBA/DDM. Furthermore, the degradable Schiff base and the hydrophobic fluorenyl Cardo structure provide the BV-EP/DDM with good acid-catalytic degradation and low water absorption properties. In this work, the fluorenyl Cardo structure plays the role of “killing three birds with one stone” in enhancing the performance of BV-EP/DDM. This synergistic strategy paves the way for fabricating more practical degradable and intrinsically flame-retardant epoxy resins from renewable biomass resources.

Thermal volume expansion as seen by Temperature-modulated optical refractometry, Oscillating dilatometry and Thermo-mechanical analysis

This paper revisits the experimental assessment of the thermal volume expansion behaviour via the novel technique of Temperature-modulated optical refractometry (TMOR) and compares it to two classical dilatometry techniques: Oscillating dilatometry (OD) and Thermo-mechanical analysis (TMA). This is done at the example of a model liquid (n-tetradecane, C14H30) and a homogeneous and isotropic, cross-linked epoxy thermoset in the viscoelastic state. It is shown that the thermal volume expansion coefficients of C14H30 obtained via TMOR and OD agree exceptionally well with each other and indeed reflect thermodynamic constants, if certain experimental boundary conditions are met. In the case of TMA analyses of the epoxy thermoset, the measured data was found to be biased rather easily, especially due to the tracking force that induces additional shape changes beyond thermal expansion. The controlled combination of TMOR and TMA allows differentiating between a “true” thermal volume expansion behaviour and additional shape changing effects.

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.

High biomass content, anti-flammable and degradable epoxy thermosets by curing a tyramine-derived epoxy monomer with a furan-derived diamine for non-destructively recyclable carbon fiber composite application

High biomass content, anti-flammable and degradable epoxy thermosets were prepared by curing a tyramine-derived epoxy monomer (TVEP) with a furan-derived diamine (DFDA) for non-destructively recyclable carbon fiber composite application.

An efficient and economical degradation strategy for epoxy thermoset based on low cost transesterification catalyst

Epoxy thermoset materials are widely used in the field of electrical engineering, including gas-insulated enclosed switches and drytype transformers. It is a challenge to degrade and recycle traditional epoxy thermoset,...

Synthesis of graphitic carbon nitride-based nanocomposites and their mechanical properties in epoxy compositions.

Thermoset epoxy resins are widely used in research and commercial applications. Zeolite imidazole framework-8 (ZIF-8), graphitic carbon nitride (GCN, g-C3N4), and S-doped graphitic carbon nitride (SCN, S-g-C3N4) composites were synthesized as accelerators and their effects on the physical properties of epoxies were examined. An ultrasound-assisted method was used to prepare ZIF-8/GCN and ZIF-8/SCN nanocomposites while g-C3N4 and S-g-C3N4 were prepared from the calcination of melamine and thiourea, respectively. The surface morphology, and particle size were characterized by scanning electron microscopy, and X-ray diffraction. The properties of synthesized nanocomposites were measured using Fourier-transform infrared spectroscopy. After the accelerator was added to the epoxy composites, their activation energies were calculated using differential scanning calorimetry. The tensile strength and flexural strength were measured using a universal testing machine and impact strength was measured by using an Izod impact strength tester. The impact strength of ZIF-8/SCN nanocomposites was enhanced by 45.2%. The storage stability of the epoxy compositions with different catalysts was evaluated by measuring the variation of viscosity with time at a constant temperature.

Bio-based hyperbranched epoxy resins: synthesis and recycling.

Epoxy resins (EPs), accounting for about 70% of the thermosetting resin market, have been recognized as the most widely used thermosetting resins in the world. Nowadays, 90% of the world's EPs are obtained from the bisphenol A (BPA)-based epoxide prepolymer. However, certain limitations severely impede further applications of this advanced material, such as limited fossil-based resources, skyrocketing oil prices, nondegradability, and a "seesaw" between toughness and strength. In recent years, more and more research has been devoted to the preparation of novel epoxy materials to overcome the compromise between toughness and strength and solve plastic waste problems. Among them, the development of bio-based hyperbranched epoxy resins (HERs) is unique and attractive. Bio-based HERs synthesized from bio-derived monomers can be used as a matrix resin or a toughener resulting in partially or fully bio-based epoxy thermosets. The introduction of a hyperbranched structure can balance the strength and toughness of epoxy thermosets. Here, we especially focused on the recent progress in the development of bio-based HERs, including the monomer design, synthesis approaches, mechanical properties, degradation, and recycling strategies. In addition, we advance the challenges and perspectives to engineering application of bio-based HERs in the future. Overall, this review presents an up-to-date overview of bio-based HERs and guidance for emerging research on the sustainable development of EPs in versatile high-tech fields.

Solvent–base mismatch enables the deconstruction of epoxy polymers and bisphenol A recovery

The use of an alkaline base mismatched with an apolar aromatic solvent provides a new route to disassemble thermoset epoxy resins resulting in the recovery of the polymer building block bisphenol A.

Exchangeable disulfide bond containing highly flexible epoxy vitrimers with shape‐memory, self‐healing, and UV shielding attributes

AbstractWith the rise in momentous resource wastage and severe environmental pollution, a viable path for sustainability is through the use of reprocessable and recyclable materials like vitrimers. Herein, we demonstrated the synthesis of dynamic disulfide‐containing epoxy resin by condensing an amide derivative of 2,2′‐dithiodibenzoic acid with epichlorohydrin. The aforementioned epoxy resin was cured utilizing the two bio‐based curing agents poly(amido amine) and cystamine to produce three distinct epoxy vitrimers. The use of reactive diluent reduced the generation of volatile organic compounds during the curing process. The existence of disulfide bond and aliphatic moiety in the epoxy vitrimers established high elongation at break with good tensile strength. Owing to the dynamic bond, the vitrimers exhibited self‐healing properties with a healing efficiency of 71.48% along with good shape memory abilities. Additionally, the disulfide bond dynamic exchange reaction allowed the polymer chain architecture to topologically reorganize, making them reprocessable and degradable. Moreover, these epoxy vitrimers have the ability to shield ultraviolet radiation as well as inhibits oxidation, and have good chemical resistance properties. The investigated epoxy thermoset thus provides a lot of potential for application as a durable thin film material.

Chemically recyclable and reprogrammable epoxy thermosets derived from renewable resources

Bio-based epoxy networks were synthesized using a vanillin-based epoxy monomer and bio-derived diamines. These networks were recycled using both acidic depolymerization and transimination methods. Reprogramming was achieved using different amines.

“Polymer to polymer” recycling and body-temperature triggered shape memory of a carvone-based epoxy thermoset system achieved using a thiol-Michael covalent adaptable network

Closed-loop chemical recycling and body temperature-triggered shape memory of an epoxy thermoset system were achieved by using reversible thiol-Michael chemistry.

Fluorinated‐polyhedral oligomeric silsesquioxane (F‐POSS) functionalized halloysite nanotubes (HNTs) as an antifouling additive for epoxy resin

AbstractAntifouling epoxy resin serves as a member of advanced nanocomposite coatings and engineered surfaces. Nevertheless, developing an antifouling epoxy composites is a state‐of‐the‐art technology. Herein, a complex nanostructure is tailored to be served as an advanced additive giving the antifouling characteristics to the epoxy resin. F‐POSS@HNT hybrid nanostructure, that is, halloysite nanotubes (HNTs) decorated with fluorinated polyhedral oligomericsil sesquioxane (F‐POSS), was synthesized and characterized. Chemical structure changes, thermal stability and morphology of hydroxylated HNTs (hHNTs) intermediate and F‐POSS@HNT ultimate nanostructures were analyzed by FTIR spectroscopy, TGA and TEM, respectively. The F‐POSS@HNTs catalyzed the epoxy‐amine crosslinking reaction, taking “Good” or “Excellent” crosslinking tags an quantified by the “Cure Index.” The apparent activation energy values were calculated for the epoxy (reference), epoxy/hHNTs, and epoxy/F‐POSS@HNTs systems (26, 51, and 47 kJ mol−1, respectively). Contact angle measurements were performed via dynamic tests demonstrating improved hydro‐ and oleophobicity of the thermoset composites. The advancing contact angle with diiodomethane increased 36% and 30% for nanocomposites containing 5 and 10 wt.% of the developed hybrid nanostructure, respectively, compared with the neat epoxy. Likewise, 54% and 67% reduction in paraffin fouling in the same order confirmed their antifouling ability. Regarding self‐cleaning characteristics, 24% and 33% surface recovery were observed, respectively.Highlights Antifouling surface was achieved in epoxy thermoset nanocomposites for potential industrial applications. Self‐cleaning behavior correlated with low wetting and antifouling properties. F‐POSS@HNT nanofillers were synthesized and successfully added to epoxy matrix. Both hHNT and F‐POSS@HNT showed a beneficial autocatalytic effect on epoxy‐amine reaction yielding satisfactory polymer crosslinking. Activation energy was reduced by incorporating F‐POSS@HNT compared with hHNT, signature of facilitated crosslinking.

Thermal Conductivity and Orientation Structure of Liquid Crystalline Epoxy Thermosets Prepared by Latent Curing Catalyst

Improvements in the performance of electronic devices necessitate the development of polymer materials with heat dissipation properties. Liquid crystalline (LC) epoxies have attracted attention because of the orientation of their polymer network chains and their resultant high thermal conductivity. In this study, a diglycidyl ether of 1-methyl-3-(4-phenylcyclohex-1-enyl)benzene was successfully synthesized as an LC epoxy and the LC temperature range was evaluated via differential scanning calorimeter (DSC). The synthesized LC epoxy was cured with m-phenylenediamine (m-PDA) as an amine-type curing agent and 1-(2-cyanoethyl)-2-undecylimidazole (CEUI) as a latent curing catalyst, respectively. The LC phase structure and domain size of the resultant epoxy thermosets were analyzed through X-ray diffraction (XRD) and polarized optical microscopy (POM). High thermal conductivity was observed in the m-PDA system (0.31 W/(m·K)) compared to the CEUI system (0.27 W/(m·K)). On the other hand, in composites loaded with 55 vol% Al2O3 particles as a thermal conductive filler, the CEUI composites showed a higher thermal conductivity value of 2.47 W/(m·K) than the m-PDA composites (1.70 W/(m·K)). This difference was attributed to the LC orientation of the epoxy matrix, induced by the hydroxyl groups on the alumina surface and the latent curing reaction.

Novel rich aromatic and phosphorus-containing compound cured epoxy resins toward outstanding comprehensive performances

Epoxy resin (EP) is quite flammable, which limits its application in areas such as coatings and electronic and electrical materials. In this study, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) (or diphenylphosphine oxide (DPO)) 3-cyclohexene-1-formaldehyde (CC) and 4,4’diaminodiphenylmethane (DDM) were selected for the synthesis of two rich aromatic and phosphorus-containing compounds (DOPO-CC and DPO-CC). DOPO-CC and DPO-CC were introduced into epoxy pre-polymers to prepare epoxy thermosets with flame retardancy. When 5 wt% DOPO-CC (P content of 0.50 wt%) or DPO-CC (P content of 0.52 wt%) was added, the LOI values of the epoxy thermosets increased from 25.5% (pure EP) to 33.5% (EP added DOPO-CC) and 31.0% (EP added DPO-CC), respectively, and these epoxy thermosets also achieved the UL-94 V-0 rating. Additionally, cone calorimeter tests showed that the peak heat release rate (PHRR) and total heat release (THR) of epoxy thermoset containing 5 wt% DPO-CC reduced by 42.0% and 39.7% compared to those of pure EP, respectively. By reducing the thermal degradation rate (condensed phase) and combining and quenching free radicals (gaseous phase), the introduction of DOPO-CC or DPO-CC can efficiently improve the flame retardancy of epoxy thermosets. More importantly, the epoxy thermosets containing DOPO-CC or DPO-CC exhibited better mechanical properties than pure EP, which could be proven by the storage modulus and tensile strength. This study provided a new class of rich aromatic and phosphorus-containing compounds that can serve as co-curing agents for epoxy thermosets with outstanding comprehensive performances.

Ultrastrong and High-Tough Thermoset Epoxy Resins from Hyperbranched Topological Structure and Subnanoscaled Free Volume.

The strength and toughness of thermoset epoxy resins are generally mutually exclusive, as are the high performance and rapid recyclability. Experimentally determined mechanical strength values are usually much lower than their theoretical values. The preparation of thermoset epoxy resins with high modulus, high toughness, ultrastrong strength, and highly efficient recyclability is still a challenge. Here, novel hyperbranched epoxy resins (Bn, n = 6, 12, 24)with imide structures by a thiol-ene click reaction. Bn shows an excellent comprehensive function in simultaneously improving the strength, modulus, toughness, low-temperature resistance, and degradability of diglycidyl ether of bisphenol-A (DGEBA). All the mechanical properties first increase and then decrease with minimization of the free volume properties. The improvement is attributable to uniform molecular holes or free volume by a molecular mixture of linear and hyperbranched topological structures. The precise measurement and controllability of the molecular free volume properties of epoxy resins is first discovered, as well as the imide structure degradation of crosslinked epoxy resins. The two conflicts are successfully resolved between strength and toughness and between high performance during service and high efficiency during degradation. These findings provide a route for designing ultrastrong, tough, and recyclable thermoset epoxy resins.

Dynamic reversible mechanism of phenol-carbamate bonds and their potential for the development of thermo-healing, recyclable and high-performance epoxy thermosets

In this paper, a series of small monomers were synthesized to explore the dynamic reversible properties of phenol-carbamate bonds (PCBs), and the bimolecular exchange mechanism of PCBs was verified. Based on this mechanism, a novel thermo-healing recyclable epoxy resin with high mechanical performance and shape memory function was successfully developed. Meanwhile, the number of reversible phenol-carbamate bonds in this system was increased by molecular structure design, which increased the chance of reversible cleavage and recombination of the dynamic covalent bonds. Compared with our previous work, the thermo-healing efficiency of the epoxy thermosets was greatly increased, and the target polymer achieved an excellent balance between high thermo-healing efficiency and good mechanical performance. Further post-treatment experiments show that the cured epoxy resin can also be well recycled by solvent degradation or hot-pressing process. What's more, a novel carbon fiber (CF) reinforced polymer composite material was synthesized by taking that prepared epoxy thermoset as a matrix, which greatly improve the mechanical properties of epoxy resin, and the CF cloths can be recycled when the epoxy resin is removed by chemical degradation. In addition, due to the existence of reversible crystallizable switching segments in the epoxy resin system, when the temperature is above its Tg, it can be triggered by thermal energy, so that the film can rapidly recover from the temporary shape to the permanent shape. This study also provides a new direction for the development of novel multifunctional intelligent polymers.

Gallic acid ester-based flexible UV-shielding epoxy thermosets with tunable properties

Epoxy thermosets are usually limited by their inherent brittleness and low-impact resistance. Henceforth, a series of bio-based epoxy resins were synthesized from gallic acid (GA) esters. Varying the chain length of flexible poly(ethylene glycol) (PEG) enabled tailoring the thermomechanical properties of the thermosets. Decreasing the chain length of the epoxy prepolymer, resulted in the increase of tensile strength from 2.89 to 15.18 MPa with a decrease in elongation from 121.7 to 16.91% and an increase in viscosity of the epoxy resins from 5.25 to 228.46 Pa.s. The low viscosity of epoxy resins with longer chain lengths offers good processability and broadens their relevance as reactive diluents for resins of high viscosity. Additionally, the thermosets exhibited decent transparency and UV shielding ability with satisfactory biodegradable attributes. The study revealed a direct correlation between chain length and thermomechanical properties of the epoxy thermosets.

Effect of Fiber Sizing Levels on the Mechanical Properties of Carbon Fiber-Reinforced Thermoset Composites.

Fiber sizing is one of the most important components in manufacturing composites by affecting mechanical properties, including strength and stiffness. The sizing of manmade fibers offers many advantages, such as improving fiber/matrix adhesion and bonding properties, protecting fiber surfaces from damage during the processing and weaving stages, and enhancing the surface wettability of polymer matrices. In this work, the influence of fiber sizing levels on carbon fibers' (CFs) mechanical properties is reported at room temperature using single fiber tensile testing (Favimat+), single fiber pullout testing (SFPO), and interfacial elemental analysis by X-ray photoelectron spectroscopy (XPS). Standard modulus CFs (7 ± 0.2 μm in diameter) were sized using two commercially available Michelman sizing formulations. The average solid content for each sizing formulation was 26.3 ± 0.2% and 34.1 ± 0.2%, respectively. HEXION RIMR 135 with curing agent RIMH 137 was used as a model thermoset epoxy matrix during SFPO measurements. A predictive engineering fiber sizing methodology was also developed. Sizing amounts of 0.5, 1, and 2 wt.% on the fiber surface were achieved for both sizing formulations. For each fiber size level, 50 single-fiber tensile testing experiments and 20 single-fiber pull-out tests were conducted. The ultimate tensile strength (σult) of the carbon fibers and the interfacial shear strength (τapp) of the single fiber composite were analyzed. The sizing levels' effect on interfacial shear stress and the O/C (Oxygen/Carbon) surface composition ratio was investigated. Based on our experimental findings, an increase of 6% in fiber performance was recorded for ultimate tensile and interfacial shear strengths. As a result, generalized fiber sizing and characterization methods were established. These developed methods can be used to characterize the strength and interfacial shear strength of manmade fibers with different sizing formulations and solid contents irrespective of the matrix, i.e., thermoset or thermoplastic.

Improved structural efficiency in composite manufacturing via hammered printing for large-curvature fiber paths

Maximizing the structural efficiency of composite materials is often limited by the minimum turning radius of fibers in composite manufacturing. Neglected defects, such as local fiber buckling, become more pronounced in regions with a large curvature, leading to reduced load-carrying capabilities and a deviation from the target design specifications. This study introduces “hammered printing” as a novel approach for achieving a large curvature of the fiber path in composite manufacturing. By utilizing the transition of the modulus with temperature of a thermoset epoxy pre-polymer, the proposed hammered printing technique generates a unique pressure effect that improves adhesion between the fiber tow and the substrate. This enables the production of accurate and uniform composite structures with a minimum curvature radius of 2 mm, while reducing the occurrence of wrinkling and bending deformations. A continuous fiber path with a small radius (2 mm) for open-hole plates was accurately printed using this printing pattern. Compared with conventional laminates and drilled composites, the obtained printed samples exhibit 75.3% and 28.8% increments in the tensile and bearing strengths, respectively, indicating that this continuous small-radius fiber path can effectively retain the strength and safety of fastened composite parts. Finally, comparisons indicate the great potential of the proposed technique as a large-curvature-path and high-performance manufacturing alternative for the cutout of composite assemblies in the aerospace and related engineering industries.

Comparative analysis of thermoplastic and thermoset adhesives performance and the influence on failure analysis in jointed elium‐based composite structures

AbstractThe preference for thermoplastic composites over thermoset ones is steadily growing, especially in constructing resilient, lightweight, and fuel‐efficient structures in industrial settings due to their recyclability and superior energy absorption. Elium®, an acrylic‐based thermoplastic resin, offers enhanced stiffness and improved toughness at room temperature. Investigation into elium‐based composites with diverse fiber reinforcements has revealed elium's potential as an alternative to traditional thermoset epoxy resins. This study explores recyclable elium resin as an adhesive for creating curable and non‐permanent joints in various elium‐based composites. Various joining techniques, including adhesive bonding, mechanical joints, and hybrid bolted/bonded connections, were assessed, with hybrid bonded/bolted (HBB) joints emerging as a preferred solution for structures requiring higher load‐carrying capacity. The study's results indicate that, when using elium adhesive, elium‐hybrid bolted/bonded (HBB) joints demonstrate superior mechanical strength compared to their epoxy counterparts, resulting in improved toughness and stiffness values for each type of composite material. In addition, the research demonstrates superior load‐bearing strengths in bonded glass/elium composite joints using elium adhesive compared to bolted joints, with HBB joints featuring elium adhesive displaying even higher load‐bearing strengths. This research highlights the potential of elium‐based composites and adhesives in various industrial applications, enabling the production of lightweight, robust structures with exceptional damage tolerance.Highlights Investigates recyclable elium resin for curable and non‐permanent joints. Elium‐based composites offer lightweight, robust, and damage‐tolerant structures. Elium HBB joints demonstrate superior strength. FTIR analysis reveals distinct chemical signatures and bonding characteristics. Elium adhesive enhances bond strength.