Epoxy Thermosets Research Articles

Multicolor Mechanochromic Epoxy Thermosets That Recognize the Intensity, Type, and Duration of Mechanical Stimulation.

Mechanochromic polymers that exhibit multiple color changes under external mechanical stimulation show great potential for sensor applications. Herein, an epoxy thermoset that can reveal the intensity, type, and duration of mechanical stimulation via a combination of disulfide (DS) and rhodamine (Rh) mechanochromophores is reported. A unique multicolor transition occurs upon ball mill or manual grinding because of the different activation energies of DS and Rh. The epoxy changes color depending on the ball mill grinding duration. Simultaneous activation occurs with a mechanochromic time lag between DS and Rh, and the collision energy strongly affects the relative intensity. A more dramatic multicolor response is observed using a mortar and pestle, as sequential activation occurs upon gentle and strong grinding. Various types of mechanical stimulation can cause different aggregates of the activated Rh moiety and vary the relative mechanosensitivities of Rh and DS, which lead to a different color response.

Curing and thermal comparative study of rosin epoxy thermosets with bio-based imidoamines and commercial curing agents

This paper includes the curing of previously synthesized multifunctional bio-based epoxy resins (PEMPAE/TMPAE) with rosin-based imidoamine curing agents (IAMDK/IASDK/IAEDK) which are nucleophilic addition reaction products of diamino diphenyl ether (DDE) and dimaleopimaryl ketone (DMPK), a dehydrodecarboxylated derivative of MPA. Curing dynamics of rosin-based epoxy resins (PEMPAE/TMPAE) with rosin-based imidoamine curing agents (IAMDK/IASDK/IAEDK) were evaluated using differential scanning calorimetry. For comparison, the bio-based epoxy resins were also cured with rosin derivative dimaleopimaryl ketone (DMPK) and commercial-based diamino diphenyl ether (DDE). The universal testing machine and thermogravimetric analyzer) were used to measure the mechanical properties and thermal stability of the cured epoxy samples. The samples’ chemical resistance was assessed by calculating the weight loss in percent after they were submerged in solutions of NaOH, HCl, and NaCl. We also used scanning electron microscopy (SEM) to examine the morphological changes. Rosin-based epoxy cured with imidoamine curing agents outperformed the commercial epoxy in terms of performance.

Nitrogen‐Containing Polymers Derived from Terpenes: Possibilities and Limitations

AbstractNitrogen‐containing polymers are widely applied materials, including polyamides, polyurethanes, and polyureas as well as epoxy thermosets. Their indispensability and the urgent need to replace fossil‐based polymers considering more sustainable alternatives lead to significant efforts in exploring renewable feedstock as potential new building blocks. Being obtained from non‐edible plant parts and often occurring as waste products, terpenes represent a promising group of renewable compounds that are of possible interest in the synthesis of nitrogen‐containing polymers. The structural diversity of terpenes in combination with the mechanical properties that nitrogen‐containing moieties bring into polymers can give rise to novel biobased materials that outstand mechanical and thermal properties of conventional fossil‐based counterparts. In this review, an overview of the use of terpene compounds in the synthesis of nitrogen‐containing polymers is given, covering different approaches to make use of terpenes as a valuable pool of renewable monomers.

Development of Sustainable High Performance Epoxy Thermosets for Aerospace and Space Applications.

There is an imperative need to find sustainable ways to produce bisphenol A free, high performance thermosets for specific applications such as the space or aerospace areas. In this study, an aromatic tris epoxide, the tris(4-hydroxyphenyl)methane triglycidyl ether (THPMTGE), was selected to generate high crosslinked networks by its copolymerization with anhydrides. Indeed, the prepared thermosets show a gel content (GC) ~99.9% and glass transition values ranged between 167-196 °C. The thermo-mechanical properties examined by DMA analyses reveal the development of very hard materials with E' ~3-3.5 GPa. The thermosets' rigidity was confirmed by Young's moduli values which ranged between 1.25-1.31 GPa, an elongation at break of about 4-5%, and a tensile stress of ~35-45 MPa. The TGA analyses highlight a very good thermal stability, superior to 340 °C. The Limit Oxygen Index (LOI) parameter was also evaluated, showing the development of new materials with good flame retardancy properties.

A furan-derived epoxy thermoset with inherent anti-flammability, degradability, and raw material recycling

The production of multifunctional thermosets with flammability, degradability and raw material recycling from epoxy thermosets made from renewable resources is one of the hottest topics in the context of sustainable development. In this work, we fabricated a fully bio-based epoxy thermoset by curing an as-synthesized furan-derived epoxy monomer (HMF-DDDS-EP) with a furan-based hardener (DFA). Owing to its unique structure containing a Schiff base and disulfide bonds, the cured HMF-DDDS-EP/DFA thermoset integrates a high glass transition temperature, high tensile strength, inherent anti-flammability, degradability, and recyclability. Specifically, a glass transition temperature as high as 171 °C, tensile strength of 62.9 MPa, a storage modulus of 2,356 MPa and outstanding anti-flammability (UL-94 V-0 rating and high LOI of 36.0%) were observed for this fully bio-based epoxy thermoset. Additionally, it was capable of degrading under mildly acidic conditions because of the cleavage of the Schiff base into the original aldehyde monomer. This fully bio-based epoxy thermoset can be considered a representative for fostering the synthesis of advanced thermosetting materials derived from renewable resources.

High‐performance epoxy composites improved by uniformly dispersed and partly thermal reduced graphene oxide sheets

AbstractHigh‐performance graphene oxide (GO) and graphene reinforced epoxy composites have made great development in various properties (such as, mechanical and thermal property) compared with the neat epoxy thermosets. However, under curing conditions, the GOs are thermal reduced and give rise to aggregation, deteriorating the properties of the epoxy/GO composites. Here, GOs and thermal reduced GOs (rGOs) could be uniformly dispersed in epoxy resin under the assistant of the dispersant, pyrene group functionalized polyethylene glycol (Py‐PEG‐Py). The thermal reduced behavior of GOs had been proved by the tests of X‐ray photoelectron spectroscopy (XPS). And the dispersion of individual GO sheets within epoxy matrix was confirmed by the measurements of dynamic laser scattering (DLS). Accordingly, the remarkable enhancement in performances of GOs reinforced epoxy was achieved. Typically, incorporation of as less as 0.03 wt% GOs and 3.0 wt% Py‐PEG‐Py, flexural strength (113.2 MPa) increases by 56.5%, fracture toughness (KIC) (1.94 MPa m0.5) increases by 55%, respectively. This work confirms GO can be thermal reduced under curing conditions and also highlights the importance of the microscopic uniformity of GO or rGO within epoxy matrix on the mechanical performances of composites.

Dynamic crush response of compression molded planar isotropic carbon fiber reinforced composites

AbstractVehicle mass reduction can be achieved through the utilization of carbon fiber reinforced composites within the vehicle body structure. In this study, planar isotropic compression molded carbon fiber reinforced composites are studied for their potential utilization as automotive primary energy absorbing structures. The energy absorption behavior of a discontinuous randomly oriented 2‐D planar isotropic carbon fiber reinforced nylon 6 thermoplastic composite and a continuous 2 × 2 twill quasi‐isotropic carbon fiber reinforced epoxy thermoset composite are studied by dynamic flat plaque crush testing. Post‐mortem failure modes of both materials are related back to their load‐displacement and energy‐displacement curves under different test conditions. It is shown that crush fixture support length dictates the failure mode and specific energy absorption (SEA) of all fiber reinforced composites tested. In addition, elevated temperature testing of the nylon 6 carbon fiber reinforced composites reduces the SEA in all test configurations. Finally, an automotive carbon fiber composite front end energy absorbing structure is considered as the replacement over a legacy metallic energy absorbing structure. Mass reduction benefits beyond the improved SEA of the composite as compared to the metallic structure it replaces are discussed by considering the increased crush length efficiency of the composite structure which can reduce the vehicle front end length, and therefore the mass of the vehicle. These results are expected to direct the future development of fiber reinforced composites structures in energy absorbing automotive structures.

Machine-Learning-Assisted Design of Highly Tough Thermosetting Polymers.

Despite advances in machine learning for accurately predicting material properties, forecasting the performance of thermosetting polymers remains a challenge due to the sparsity of historical experimental data and their complicated crosslinked structures. We proposed a machine-learning-assisted materials genome approach (MGA) for rapidly designing novel epoxy thermosets with excellent mechanical properties (high tensile moduli, high tensile strength, and high toughness) through high-throughput screening in a vast chemical space. Machine-learning models were established by combining attention- and gate-augmented graph convolutional networks, multilayer perceptrons, classical gel theory, and transfer learning from small molecules to polymers. Proof-of-concept experiments were carried out, and the structures designed by the MGA were verified. Gene substructures affecting the modulus, strength, and toughness were also extracted, revealing the mechanisms of polymers with high mechanical properties. The developed strategy can be employed to design other thermosetting polymers efficiently.

Facile fabrication of high‐performance epoxy systems with superior mechanical properties, flame retardancy, and smoke suppression

AbstractThe realization of high‐performance thermosetting polymers with superior mechanical properties, flame retardancy and smoke suppression is very necessary but not yet implemented. Herein, a flame‐retardant curing agent (BICPD) combining cyclotriphosphazene and phosphaphenanthrene groups was synthesized, and used to prepare high‐performance, flame‐retardant epoxy (EP/BICPD) systems. Our results showed that the EP/BICPD systems began to cure at 110°C and solidify rapidly under heating. In comparison to the common epoxy system, the EP/BICPD thermosets exhibited improved glass transition temperature and mechanical properties. Most impressively, the EP/BICPD thermosets possessed outstanding flame retardancy and smoke suppression. The EP/BICPD‐20 thermoset achieved the LOI value of 37.7% and passed the UL94 V‐0 rating. Additionally, compared with the common epoxy thermoset, the peak of heat release rate (pHRR) and total smoke production (TSP) of EP/BICPD‐20 thermoset were reduced by ~70.0% and ~43.4%, respectively. The significant enhancements in flame retardancy and smoke suppression was mainly due to the collaboration of cyclotriphosphazene and phosphaphenanthrene in condensed and gaseous phases. Therefore, the high‐performance EP/BICPD systems with high glass transition temperature, satisfied mechanical properties, superior flame retardancy and smoke suppression showed profound scientific and industrial significance.

Multiscale analysis of the effect of interfacial thermal conductance between fillers and epoxy resin on the effective thermal conductivity of their composites

The heat dissipation performance of the epoxy molding compound(EMC) is very important for high power application. In this paper, the thermal transport properties of thermoset epoxy(EP) based EMC containing silica particles(SiO2Ps) and diamond particles(CPs) were calculated using two different scale approaches. Firstly, the representative volume element(RVE) of EMC was modeled using molecular dynamics(MD) to calculate the effective thermal conductivity(ETC). The crosslinking of EP between Diglycidyl Ether of Bisphenol A(DGEBA) resin and JEFFAMINE® D-230 hardener was modeled. The results indicate that the CPs can increase the ETC of composite by 71.4 % than that of SiO2Ps. The interfacial thermal conductance(ITC) of Silica/EP interface was 1.33 times higher than that of Diamond/EP interface. The discontinuous temperature gradients on interface was observed in the RVE filled with dispersed fillers. In addition, a macroscale approach was used to determine the ETC of the EMC using finite element method(FEM) and the Maxwell theoretical model. The effects of fillers' shape and the ITC of fillers/EP interface on the ETC of EMC were analyzed. With the increase of the ITC, the ETC of the composite increases gradually. When the ITC exceeded a critical value, the ETC did not change. The chain fillers were more conducive to heat transport. The results of the two multiscale modeling techniques were well consistent.

From the Design of Novel Tri- and Tetra-Epoxidized Ionic Liquid Monomers to the End-of-Life of Multifunctional Degradable Epoxy Thermosets

From the Design of Novel Tri- and Tetra-Epoxidized Ionic Liquid Monomers to the End-of-Life of Multifunctional Degradable Epoxy Thermosets

Hydraulic-based testing and material modelling to investigate uniaxial compression of thermoset and thermoplastic polymers in quasistatic-to-dynamic regime

The strain rate dependence of thermoplastics and thermosets in the range 10-3–102 s−1 is still unclear due to experimental difficulties. This interval cannot be fully treated using a quasistatic-based electromechanical machine nor a high-speed test relying on the Kolsky bar method. This paper presents a uniaxial compression test set-up to cover the aforementioned range providing robust stress–strain measurements up to 100 s−1. This set-up is assessed at room temperature with a semi-crystalline polymer (SCP) polyether-ether-ketone (PEEK) KT880-NT and a toughened thermoset epoxy PR520. High-speed 3D stereo digital image correlation (DIC) and an infra-red camera are used for data acquisition. The macroscopical true stress–strain curves demonstrate the importance of the thermal softening effect taking place in this range, accompanied with a temperature rise. In terms of characterization, an advanced unified SCP (USCP) model is employed to quantify the rate- and temperature-dependence for both polymers. A straightforward FEM-based parameter identification procedure is proposed requiring only two compressive stress–strain curves. The USCP model is further applied to PEEK 450G from literature to highlight the features of SCPs. The comparison of PEEK and PR520 is discussed pointing out the characterization challenges due to the transitional quasistatic-to-dynamic testing regime.

Nanocomposites of epoxy with one-dimensional fibrous poly(ε-caprolactam) nanocrystals via crystallization-driven self-assembly

In this contribution, we reported the preparation of nanocomposites of epoxy with one-dimensional (1D) poly(ε-caprolactam) (PA6) nanocrystals via crystallization-driven self-assembly. First, a novel diblock copolymer composed of poly(ethylene oxide) and poly(ε-caprolactam) subchains (PEO-b-PA6) was synthesized via the anionic ring-opening polymerization (ROP) of ε-caprolactam. It was found that the PEO-b-PA6 diblocks displayed crystallization-driven self-assembly (CDSA) behavior in the selective solvents (e.g., water); the 1D fibrous nanocrystals of PA6 were obtained via CDSA approach. Such a CDSA behavior was utilized to generate 1D fibrous PA6 nanocrystals into epoxy thermosets. In this case, the epoxy precursors were used as the solvent selective for PEO subchain of the diblock. Notably, the 1D fibrous PA6 nanocrystals were generated in the epoxy precursors via the CDSA approach. Upon curing, the nanocomposites (i.e., the nanostructured thermosets containing PA6 nanocrystals) were successfully obtained. It was found that epoxy thermoset was significantly nanoreinforced by the PA6 nanocrystals. In the meantime, the fracture toughness of the materials was significantly improved with the incorporation of 1D fibrous PA6 nanocrystals.

Full Recycling and Re-Use of Bio-Based Epoxy Thermosets: Chemical and Thermomechanical Characterization of the Recycled Matrices.

High performances of thermosets deriving from their covalent intermolecular cross-link bonds result in their low recyclability hindering the full exploitation of a truly circular approach for cured thermosets. In this experimental work, the recyclability of a bio-based fully recyclable epoxy resin using a mild chemical recycling process was demonstrated. The recycled polymer obtained was fully characterized to ascertain its structure and properties. MALDI (Matrix-Assisted Laser Desorption/Ionization), GPC (Gel Permeation Chromatography) and NMR (Nuclear Magnetic Resonance) spectroscopy to determine the chemical structure of the recycled polymer were used. The thermomechanical properties of the cured virgin network and of the recycled product obtained were measured by DSC (Differential Scanning Calorimetry) and DMA (Dynamic Mechanical Analysis). Thermogravimetric analysis of the recycled polymer was also performed. The recycled polymer was transformed into a polyurethane by reacting it with an isocyanate. The synthetized polyurethane obtained therefrom was thoroughly characterized by thermogravimetric analysis. This approach proved the possibility to up-scale the recycled product making it available for novel applications exploiting its re-use.

Mechanically robust, hydrothermal aging resistant, imine-containing epoxy thermoset for recyclable carbon fiber reinforced composites

Imine compounds have been proved to possess great potential in the design and preparation of sustainable epoxy thermosets. However, it is still a great challenge to develop imine-containing hardener with definite structure due to the randomness of condensation reaction between aldehydes and primary amines. Herein, a novel amine hardener containing imine bond is synthesized successfully by selecting 4-imidazole formaldehyde and 3-aminobenzylamine as raw materials. The asymmetric structure of raw material can avoid uncontrollable reaction and ensure that this hardener possesses definite chemical structure. Due to high activity of hardener, the resultant epoxy thermosets with high cross-linked density exhibit excellent thermostability and fine hydrothermal aging resistance. Furthermore, such thermosets show good mechanical properties (tensile strength of ∼76 MPa), which are comparable to that of commercial Diglycidyl Ether of Bisphenol A epoxy resin cured by 4,4-Diaminodiphenyl methane. More interestingly, increasing the equivalent ratio of active hydrogen to epoxy can introduce more dynamic covalent C=N linkage into the cross-linked network, thus the epoxy thermoset further achieves the decomposition within 3 h and the corresponding composite laminates also are recycled efficiently under mild condition. This work demonstrates the feasibility and potential of preparing high-performance degradable epoxy thermoset and corresponding recyclable composites based on well-designed imine-containing hardener.

Multifunctional Materials from Aerobic Degradation of Epoxy Thermoset via Phosphotungstic Acid-Involved Activation and Bonding.

The growing scale of production of wind turbines represents a big challenge for chemical recycling of amine-cured epoxy resin (EP) to achieve high-efficiency degradation and high-value utilization of degradation products. Here, H2 O2 /phosphotungstic acid (HPW) catalytic oxidation system is demonstrated to completely degrade EP thermoset with the solid recovery rate of 96% at a reaction temperature of 80 °C for 4 h. Owing to protonation and bonding effect of HPW to the amine groups, the degradation products had a weight-average molecular weight of 4285 with narrow molecular weight distribution. They were used as dye adsorption blend films and supramolecular adhesives based on hydrogen bonding and coordination bonding respectively. The work demonstrates a feasible and promising method to recover the EP thermoset into high-performance materials.

Synthesis of a bio-based and intrinsically anti-flammable epoxy thermoset and the application of its carbonized foam as an efficient CO2 capture adsorbent

The production of multifunctional carbon foams from biomass has attracted great interest due to the increasing awareness of green and sustainable development. Here, we report a bio-based epoxy thermoset prepared by curing a furan-derived di-functional epoxy monomer (5-hydroxymethyl-2-furaldehyde (HMF)-3, 5-diamino-1,2,4-triazole (GU)-EP) with a furan-derived curing agent, 5,5′-methylenedifurfurylamine (DFA). For comparison, the cured commercial diglycidyl ether of bisphenol-A was used as a thermoset epoxy resin. Due to its unique structure rich in aromatic and nitrogenous structures, the cured HMF-GU-EP/DFA thermoset possessed exceptional charring ability. This fully bio-based epoxy thermoset had a very high char yield (55.8 wt%) at 800 °C under nitrogen, while the cured diglycidyl ether of bisphenol-A/4, 4′-diaminodiphenylmethane was only 13.6 wt%. Due to this excellent carbonization ability, the cured HMF-GU-EP/DFA was carbonized at 700 °C to obtain a carbon foam as a CO2 absorbent. The Brunauer–Emmett–Teller surface area and total pore volume were measured to be 472 m2/g and 0.22 cm3/g, respectively. The amounts of CO2 adsorbed with this carbon foam were 4.72 and 2.15 mmol/g at 273 and 298 K, respectively. In conclusion, this study provides a bio-based, strong and rigid, inherently flammable, and highly carbonizable epoxy thermoset and the potential application of its carbonized foam as an efficient adsorbent for CO2 capture.

Cryo-Assisted Extrusion 3D Printing of Shape Memory Polymer-Graphene Composites

Abstract A cryo-system assisted extrusion 3D printing technique has been developed to fabricate Shape Memory Polymer Epoxy (SMPE) thermoset into a near-net shape. With a unique cryogenic sprayer assisted printing system, it is possible to 3D print "pure" thermoset polymer, otherwise which was limited to thermoplastic or highly functionalized co-polymeric thermoset systems. The temperature was maintained below 10oC during 3D printing, which enabled dimensionally accurate structures. Graphene Nanoplatelet (GNP) reinforced SMPE ink was synthesized to be compatible with cryo-assisted extrusion 3D printing of shape memory composites. The addition of a mere 0.1 wt.% GNP exhibited 15% faster shape recovery due to improved thermal conductivity. Also, GNP addition imparted a 30% improvement in the ultimate tensile strength and a 17% increase in 3D printed composites' elastic modulus. Dynamic mechanical testing showed that the addition of 0.1 wt.% GNP in SMPE increased the composite's storage modulus by 365% and loss tangent by 66% compared to pure 3D printed SMPE, indicating enhanced damping characteristics. This study is the first to demonstrate a non-existent 3D printing technology for “pure thermoset” polymers with and without conductive graphene, that has the potential to develop an energy-efficient actuation system, passive thermal sensors, and self-deployable communication system structures.

Thermal Properties and Flammability Characteristics of a Series of DGEBA-Based Thermosets Loaded with a Novel Bisphenol Containing DOPO and Phenylphosphonate Units.

Despite a recent sustained preoccupation for developing biobased epoxies with enhanced applicability, such products have not been widely accepted for industry because of their inferior characteristics compared to classic petroleum-based epoxy thermosets. Therefore, significant effort is being made to improve the flame retardance of the most commonly used epoxies, such as diglycidyl ether-based bisphenol A (DGEBA), bisphenol F (DGEBF), novalac epoxy, and others, while continuously avoiding the use of hazardous halogen-containing flame retardants. Herein, a phosphorus-containing bisphenol, bis(4-(((4-hydroxyphenyl)amino)(6-oxido-6H-dibenzo[c,e][1,2]oxaphosphinin-6-yl)methyl)phenyl) phenylphosphonate (BPH), was synthesized by reacting bis(4-formylphenyl)phenylphosphonate with 4-hydroxybenzaldehyde followed by the addition of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) to the resulting azomethine groups. Environmentally friendly epoxy-based polymer thermosets were prepared by using epoxy resin as polymer matrix and a mixture of BPH and 4,4'-diaminodiphenylsulfone (DDS) as hardeners. A hyperbranched phthalocyanine polymer (HPc) and BaTiO3 nanoparticles were incorporated into epoxy resin to improve the characteristics of the final products. The structure and morphology of epoxy thermosets were evaluated by infrared spectroscopy and scanning electron microscopy (SEM), while the flammability characteristics were evaluated by microscale combustion calorimetry. Thermal properties were determined by thermogravimetric analysis and differential scanning calorimetry. The surface morphology of the char residues obtained by pyrolysis was studied by SEM analysis.

High-performance and fire-resistant epoxy thermosets derived from plant-derived ferulic acid

Development of epoxy thermosets derived from biomasses with intrinsic flame retardancy and high mechanical performances is of increasing demand. Here we synthesize a novel epoxy monomer (DGEFA) from ferulic acid that can be directly obtained from many plants and offers intrinsic flame retardancy due to its conjugation structure. By using 4,4’-diaminodiphenylmethane (DDM) as a curing agent, the cured epoxy resin (DGEFA-DDM) possesses a high glass transition temperature (178.5 °C). The DGEFA-DDM also exhibits high mechanical performances, i.e., tensile strength of 84.7 MPa and flexural strength of 145.1 MPa. Besides, the DGEFA-DDM presents a char yield of 40.6 % under N2, a V-1 rating of UL-94 and a limiting oxygen index of 32.8 %. The cone calorimeter test shows that the DGEFA-DDM has low heat and smoke release performances with a total heat release of 49.2 MJ/m2 and a total smoke production of 16.0 m2, indicating a high intrinsic flame retardancy. This work provides a simple way for the preparation of epoxy resins from bio-based feedstocks possessing physical properties and fire safeties.