Epoxy Thermosets Research Articles

Chemical and Solvent-Based Recycling of DGEBA-Based Epoxy Thermoset and Carbon-Fiber Reinforced Epoxy Composite Utilizing Imine-Containing Secondary Amine Hardener.

Epoxy systems are essential in numerous industrial applications due to their exceptional mechanical properties, thermal stability, and chemical resistance. Yet, recycling epoxy networks and reinforcing materials in epoxy composites remains challenging, raising environmental concerns. The critical challenge is the recovery of well-defined molecules upon depolymerization. To address these issues, an innovative strategy is developed utilizing imine-containing secondary amine hardener (M1). The reaction of M1 with DGEBA produced high-performance epoxy thermoset P1, which exhibits Young's modulus of 2.18 GPa and tensile strength of 63.4 MPa, and excellent stability in neutral aqueous conditions. Upon carbon-fiber reinforcement, Young's modulus and tensile strength are significantly elevated to 10.99GPa and 328.3MPa, respectively. The reactive secondary amine functionalities enabled the tailored network to display a well-defined growth pattern, yielding only well-defined molecules and intact carbon fibers upon acidic depolymerization. Consequently, the recycled polymers retained properties identical to those of P1. Notably, it is discovered that despite the cross-linked nature of the epoxy networks, complete dissolution in dichloromethane facilitated straightforward solvent-based recycling, allowing the recovery of undamaged carbon fibers and an epoxy thermoset with properties matching the virgin material. Presented novel monomer design and approach showcased two important and efficient recycling options for epoxy systems.

Chemical Recycling of Epoxy Thermosets: From Sources to Wastes

As one of the most widely used thermosets due to its excellent performances, epoxy resin (EP) is widely used in various fields and often employed as a component of composite actuator devices, strengthening their mechanical properties. However, the expanding production of EP inevitably leads to the accumulation of waste end-of-life equipment and the corresponding increasingly serious environmental problems. This review summarizes the recycling strategies of EP, divided into two perspectives: recycling from wastes and sources. Chemical recycling is expected to be the future of waste EP treatment, and we discuss the chemical recycling methods of existing waste EP based on different mechanisms, including the selective cleavage of ester bonds, C–N bonds, and C–O bonds. On the other hand, epoxy vitrimer networks based on various dynamic covalent linkages are also outlined, which can respond to multiple external stimuli and provide materials with recyclability from the origin. Therefore, the use of epoxy vitrimer actuators can prevent waste generation throughout the whole lifecycle. We present some issues of concern in both waste-based and source-based recycling strategies and emphasize the significance of scaling-up. Finally, we summarized the current situation and present some future perspectives with the aim of making practical contributions to environmental issues.

Enhanced Thermal and Flexural Properties of Alkalized Date Palm/Kenaf Fiber-Reinforced Epoxy Hybrid Composites: A Comparative Study of Untreated and Treated Fibers

ABSTRACT This study evaluates the thermal and mechanical behavior of hybrid composites made from date palm (DP) and kenaf fibers (KF) in an epoxy thermoset matrix. Both DP and KF fibers are abundant and renewable, reducing environmental impact. KF offers high tensile strength and stiffness, improving composite strength and rigidity, while DP fibers offer toughness and impact resistance, enhancing durability. The composites were developed in 30%, 50%, and 70% DP-KF compositions using layup and hot-pressing methods. Fiber treatment through alkalization was analyzed for its impact on mechanical and thermal properties to find the optimal fiber ratio. The hybrid composites were subjected to DMA, TGA, and flexural tests. The treated composites exhibited fewer impurities and increased cellulose content, resulting in enhanced flexural performance. This improvement was evident through higher storage modulus, lower loss modulus, and reduced tan δ when exposed to thermal stress. The 30% DP and 70% KF composites exhibited best performance. The treated 30DP-70KF exhibited improvement in the flexural strength and modulus by 13.8% and 2.7%, respectively. TGA demonstrated that treated 30DP-70KF composites had a higher glass transition temperature (Tg) and better thermal degradation resistance. These findings highlight the potential of natural fiber hybrids in creating sustainable, high-performance composite materials for construction and industrial applications.

Excellent flame retardant and tough epoxy thermosets with imidazolium tributyl phosphate ionic liquid

The brittleness and flammability of epoxy thermosets (EP) have largely limited their applications. In this work a phosphorus-containing ionic liquid 1-methylimidazolium tributyl phosphate ([Mim]TBP) is designed and synthesized to develop high-performance and flame-retardant epoxy thermosets. [Mim]TBP significantly accelerates the curing and improves the toughness of epoxy thermosets. The tensile toughness, tensile strength, impact strength and flexural strength of samples with 10phr [Mim]TBP increase by 419.7 %, 100.2 %, 71.7 % and 58.3 %, compared with pure EP, respectively. The increased toughness is due to the significant energy dissipation caused by the ionic bonds breaking between Mim and TBP. When the [Mim]TBP content exceeds 10 phr, the limiting oxygen index (LOI) can reach over 30 % and UL-94 test pass the V-0 rating. The peak heat release rate (pHRR), total heat release (THR), total smoke release (TSR) and peak CO production (PCOP) values effectively decrease by 31.5 %, 30.6 %, 48.3 % and 6.3 %, respectively, thanks to the gas and condensed phase mechanisms of the [Mim]TBP. A new strategy was provided for the development of toughness and fire safety performance epoxy thermosets for high-performance industrial applications.

A facile biomass-based coating for flax fabric toward plant fiber/biobased epoxy composite with enhanced fire safety

The natural abundance, biodegradability, and low density of plant fibers, together with biobased epoxy thermoset resin, have driven the increasing popularity of plant fiber/polymer composites (PFRPs) to wider applications in various industries. However, the striving for biomass-based flame retardants (FRs) treatment for PFRPs remained a bottleneck due to polymers’ inherent vulnerability against fire and the increasing environmental awareness. In this work, a facile two-step aqueous solution coating process was proposed for fabric surface treatment of flax fabric using fully biobased phytic acid and chitosan from polysaccharides. The treated flax fabric demonstrated self-extinguishing behavior when ignited and showed a decrease in peak heat release rate (PHRR) by 58% under combustion. The laminate produced by this treated flax fabric and biobased epoxy resin showed a decrease of PHRR by 36% and an increase of more than 200% for the time of torch fire burn-through, demonstrating intriguing flame retardance brought by only FRs treatment on flax fabric reinforcements. Various measurements were done to elaborate on the role of treated flax fabric in the flame retardancy of polymer composites.

Synergetic Hybridization Strategy to Enhance the Dynamicity of Poorly Dynamic CO2‐derived Vitrimers achieved by a Simple Copolymerization Approach

AbstractCopolymerization allows tuning polymer's properties and a synergetic effect may be achieved for the resulting hybrid, i.e., outperforming the properties of its parents as often observed in natural materials. This synergetic concept is herein applied to enhance both dynamicity and properties of vitrimeric materials using poorly dynamic hydroxyurethane and non‐dynamic epoxy thermosets. The latter generates activated hydroxyl, promoting exchange reactions 15 times faster than pure polyhydroxyurethanes. This strategy allows obtaining catalyst‐free high‐performance vitrimers from conventional epoxy‐amine formulations and an easily scalable (bio‐)CO2‐based yet poorly efficient dynamic network. The resulting hybrid network exhibits modulus retention superior to 95% with fast relaxation (<10 min). The hydroxyurethane moieties actively participate in the network to enhance the properties of the hybrid. The material can be manufactured as any conventional epoxy formulation. This new strategy to design dynamic networks opens the door to large‐scale circular high‐performance structural carbon fiber composites (CFRP). The CFRP can be easily reshaped and welded from flat plates to complex geometries. The network is degradable under mild conditions, facilitating the recovery and re‐use of high‐added‐value fibers. This accessible and cost‐effective approach provides a versatile range of tunable dynamic epoxides, applicable across various industries with minimal adjustments to existing marketed products.

Ultratough, Robustness, and Reprocessable Thermoset Epoxy Resins Synergistically Enhanced by Hierarchical Coordination Interactions and Dynamic Covalent Networks

AbstractEpoxy resins, ubiquitous and indispensable thermosetting material in various industries, suffer from the resistance to crack initiation, poor ductility, and irreparable permanent cross‐linked network, which hinder their utilization in high‐performance applications and are detrimental to the development of a sustainable economy. However, achieving the trade‐off between toughness, robustness, and reprocessability of epoxy resins remains a formidable challenge. Herein, an epoxy resin that combines ultratoughness, robustness, and reprocessability by incorporating a hierarchical crosslink structure embedded in a transesterification network is reported. The construction of hierarchical coordination structures facilitates formation of dense nano‐hard domains, enabling enhancement of both strength and toughness at multilength scales. As a result, the epoxy resin, integrating covalent adaptable networks with sacrificial non‐covalent networks, exhibits exceptional elongation at break (280%), modulus (1.8 GPa), stress at break (56.3 MPa), and tensile toughness (142 MJ m−3), showcasing its excellent endurance and ability to undergo multiple reprocessability. The application of this epoxy resin in carbon fiber‐reinforced polymer composites further underscores its potential as the resulting HCEV‐CFRPs exhibit unprecedented tensile properties and facilitate multiple non‐destructive recycling of high‐value carbon fiber under mild conditions. This research provides novel design principles for recyclable and high‐performance materials that combine strength and toughness.

Bio-derived epoxy thermosets incorporating imine linkages: Towards sustainable and advanced polymer materials

In the search for sustainable and eco-friendly alternatives, Schiff-based epoxy thermosets are emerging as a promising strategy due to the inherent dynamic and reversible nature of their imine functional groups, which enables thermal reprocessability and chemical recyclability. This article explores the synthesis of Schiff-based epoxy thermosets using sustainable approaches. Starting from vanillin or syringaldehyde, the synthesis involves the incorporation of imine linkages into the epoxy network. A new strategy has been employed by self-polymerization without the use of additional curing agents. This approach conducts to novel thermosets with superior material properties that exceed those reported in previous studies. Thermo-mechanical investigations showed that the designed thermosets have high mechanical properties, with storage moduli at room temperature ranging between 1.5–2.2 GPa, and glass transition values between 138 and 249 °C. Thermal analyses allowed to evaluate the Limit Oxygen Index (LOI) which ranged from 33 % to 36 %, demonstrating excellent flame-retardant properties without the addition of supplementary additives. Their reduced density (0.74–1.09 g/cm3) makes them suitable for applications where weight considerations are critical. These results obtained results place the designed Schiff-based materials as promising candidates for a wide range of high-end applications, particularly in industries that prioritize eco-friendly processes and materials.

Biobased recyclable epoxy composites reinforced with carbon nanotubes for electromagnetic interference shielding and Joule heating

AbstractCurrent epoxy thermosets are facing various challenges such as resource scarcity, environmental concern, and limited functionalities. Here, we present a novel solvent free approach to fabricate multifunctional carbon nanotubes (CNTs) reinforced biobased and recyclable epoxy thermoset composites through precuring in an internal mixer followed by curing in an oven. The epoxy matrix, a covalent adaptable network (CAN) based on dynamic imine bonds, was synthesized from entirely biobased feedstocks, including glycerol triglycidyl ether, vanillin, and 1,10‐diaminodecane. The shear force during precuring facilitated dispersion of CNTs in CAN matrix, allowing for high CNTs loading (up to 20 wt%). The well‐dispersed CNTs not only reinforced mechanical properties of CAN but also introduced new functionalities like electrical conductivity, electromagnetic interference (EMI) shielding capacity, and Joule heating performance. The composite with 20% CNTs exhibited a tensile strength and Young's modulus of 78.1 MPa and 3.07 GPa, respectively, marking a 34.2% and 63.6% improvement over pristine CAN. It also demonstrated a high electrical conductivity of 35.3 S/m, resulting in a remarkable EMI shielding effectiveness (22.8 dB) and excellent Joule heating performance (reaching 131.7°C at 27 V input). Furthermore, these composites are thermally and chemically recyclable due to dynamic imine bonds, promoting sustainability and circular economy.

Constructing piperazine pyrophosphate@LDH@rGO with hierarchical core-shell structure for improving thermal conductivity, flame retardancy and smoke suppression of epoxy resin thermosets

The integration and miniaturization development of electronic devices placed the great demand for epoxy resin (EP) thermosets with excellent thermal management, flame retardancy and electrical insulation. Herein, a multifunctional additive piperazine pyrophosphate@layered double hydroxide@reduced graphene oxide (PPAP@LDH@rGO) with hierarchical core-shell structure was constructed by solvothermal and electrostatic self-assembly methods to meet the above requirements. Profiting from the distinct structure of PPAP@LDH@rGO, the thermal conductivity of EP/PPAP@LDH@rGO reached 0.951 W m−1 K−1 at 7 wt% addition (3 wt% rGO containing), which is 320.8 % increment compared to pristine EP. Meanwhile, when 4 wt% PPAP@LDH@rGO was added, the EP thermoset reached UL-94 V-0 rating during vertical burning tests with the limiting oxygen index of 29.8 % due to the synergistic effect of PPAP, LDH and rGO. The release of hazard products including smoke and carbon monoxide for EP/PPAP@LDH@rGO visibly declined during combustion. Besides, the EP thermosets also well-maintained the electrical insulation and mechanical properties. This work provided an alternative approach for preparing high performance EP thermosets which was suitable to be applied in electronics and electrical fields.

High-homogeneous recyclable self-cured epoxy resins based on imine

The production of high-performance epoxy resins currently depends on combining rigid epoxy oligomers and non-flexible hardeners. However, the high melting points and incompatibilities of raw materials, along with the inhomogeneous crosslinked networks of thermoset resins, challenge the development of high-performance epoxy resins simultaneously enhancing strength and toughness with excellent processing properties. To address this issue, we propose a self-curing strategy to obtain a novel thermosetting epoxy resin by simply melting the epoxy oligomers containing the Schiff base structure. The mechanism of self-cured was also revealed by density functional theory. Remarkably, this method eliminates the need for curing agents, catalysts, or active hydrogen structures. The resulting self-cured epoxy resin (VEP-SC) simplifies processing steps. In particular, VEP-SC forms a more homogeneous cross-linked network and exhibits superior mechanical properties due to the unique preparation method. Compared to traditional thermosetting epoxy cured with a curing agent, the tensile strength, elongation at break, and flexural strength of VEP-SC were 133.4 %, 168.4 %, and 276.2 % higher than those of VEP-MeHHPA, respectively. Furthermore, the Schiff base structure imparts the vitrimeric behavior of self-cured epoxy resin, achieving recyclability. This approach provides innovative solutions for developing versatile and high-performance epoxy resins.

Recyclable fully biobased high-performance epoxy thermosets

Thermosetting polymeric materials play a pivotal role in numerous sectors due to their versatility in tailoring physicochemical and mechanical properties and their exceptional performances. The aim of this study is to design 100 % bio-based high performant and recyclable thermosets. To achieve this goal, a natural and renewable-based epoxy monomer, triglycidyl ether of phloroglucinol (TGPh), was crosslinked with biobased anhydrides to create novel polymeric resins. Through detailed investigations, it was found that the designed materials have storage moduli values at room temperature ranging from 2.1 to 3.4 GPa, with damping factors spanning from 86 °C to 163 °C depending on the nature of the anhydride. Tensile tests corroborated the performant mechanical properties of the designed bio-resins, showing Young's modulus values ranging from 1.2 to 1.5 GPa. Furthermore, the water absorption values are very low, ranging from 0.25 % to 0.35 %. Besides, the designed bio-resins exhibited rapid dissolution in TBD/EG solution, as well as thermo-mechanical reprocessability.

Design of self-healing and anticorrosion epoxy coating with active multiple hydrogen bonds based on grafted polyetheramine

The healing of epoxy coatings in damaged areas significantly influences their service life and protection quality. Intrinsic self-healing coatings, which enable multiple healing cycles without the need for additional functional carriers, have emerged as a promising coating technology. Nevertheless, the cross-linked structure of thermoset epoxy coating system restricts the large-scale migration of molecular chains, posing a significant challenge to achieve intrinsic healing of resin without compromising its mechanical properties. In this study, we report a design scheme for epoxy coating with intrinsic self-healing properties based on active multiple hydrogen bonds. Grafting aminobenzothiazole at the end of polyetheramine D230 chain, the grafted polyetheramine (GD230) was synthesized and served as the auxiliary curing agent, whose accurate molecular structure has been confirmed by different characterizations. The self-healing coating with reversible dynamics network based on hydrogen bonds was prepared by crosslinking through epoxy resin and mixed curing agent of polyetheramine D230 and GD230. Various measurements have been adopted to evaluate the self healing and anticorrosion performance. Results showed that the functional coating with a ratio of 8:2 for D230 to GD230 has satisfied both performances on the steel substrate. After immersion in 3 wt% NaCl solution for 120 days, the impedance value of functional coating still remains about 1010 Ω cm2. After the coating was scratched and immersion for 96 h, the impedance value of functional coating increased by about two orders of magnitude compared to that in blank coating. Mechanical testing and theoretical calculations were used to support dynamic crosslinking model to reveal the self-healing mechanism.

Sustainably sourced and DOPO-derived triols as hardeners for epoxy thermosets: A promising solution to synchronously improve flame retardancy, mechanical strength and toughness

The flammability and brittleness of the diglycidyl ether of bisphenol A (DGEBA)-type epoxy resin greatly restrict its application in electronic appliances and structural materials. However, the synchronous improvement in the flame retardancy, mechanical strength, and toughness of DGEBA-type epoxy thermosets remains a significant challenge. To overcome this challenge, in this study, a sustainably sourced and DOPO-derived triol (DOPO-GL-DCDA) was synthesized from a bio-based material, glycerol 1, 3-diglycerolate diacrylate (GL-DCDA). A series of epoxy thermosets with different phosphorus contents were prepared by introducing DOPO-GL-DCDA into the DGEBA matrix. EP/DOPO-GL-DCDA demonstrated excellent flame retardancy and mechanical properties without compromising transparency. Specifically, EP/DOPO-GL-DCDA-1.0P exhibited a limit oxygen index (LOI) of 34.0 % and a UL-94 V-0 rating, and the total heat release (THR) and peak heat release rate (PHRR) of EP/DOPO-GL-DCDA-1.5P decreased by 36.6 % and 46.8 %, respectively, compared with those of pure EP. In addition, compared with those of pure EP, the tensile strength, impact strength, fracture toughness (KIC), and fracture energy (GIC) of EP/DOPO-GL-DCDA-1.5P increased by 62.8 %, 69.9 %, 62.9 %, and 99.0 %, respectively. Moreover, EP/DOPO-GL-DCDA had better dielectric properties than pure EP. Under the premise of maintaining transparency, this work provided a promising solution to synchronously improve the flame retardancy, mechanical strength, and toughness of epoxy thermosets.

Epoxy-based multifunctional re-bondable polymer with self-healing, shape memory and superb bonding properties

Thermoset epoxy resin-based materials are widely used, but their permanent cross-linked network limits their processability and reusability, which can lead to environmental burdens. In this work, by exploiting the weak reactivity of aniline to design appropriate reaction ratios, we achieved a linear link between the epoxy resin and the curing agent. This linear link, along with the crosslinking points provided by the flexibly branched polyurethanes, avoids the inherent brittleness associated with the highly crosslinked network of conventional epoxy resins. As a result, the adhesive exhibits extraordinary improvements in extensibility and toughness. The lap shear strength, tensile strength and elongation at break reach 11.9 MPa, 14.4 MPa and 607 %, respectively. The fracture toughness is as high as 109.6 kJ/m2, far beyond the existing epoxy adhesives. The synergistic effect of disulfide bonds and hydrogen bonds confers the adhesive with self-healing and repeatable bonding characteristics. The multi-level hydrogen bonding and appropriate phase separation structure are key to optimizing toughness, resulting in excellent comprehensive performance. The introduction of polyurethane not only improves toughness but also enhances the interfacial bonding force between the adhesive and the substrate, broadening the scope of applications. The prepared high-performance polymers provide new insights into reusable epoxy adhesives.

Comparison study of Izod impact properties on 3D printed thermoplastic and thermoset carbon fiber composite at different infill density

PurposeThis study aims to evaluate the low energy impact characteristics of 3D printed carbon fiber thermoplastic and thermoset polymer composite using the Izod impact test. The effects of infill density are examined on the Izod impact properties of 3D printed thermoset polymer and thermoplastic composite specimens. Furthermore, a thorough investigation is conducted into the effect of heat treatment using a hot-air oven on both types of 3D printed composite specimens. To characterize the impact characteristics of each specimen, the fracture surfaces caused by impact load are inspected, and the fracture mechanism is studied using scanning electron micrographs.Design/methodology/approachIzod Impact specimens of thermoset (epoxy resin) and thermoplastic carbon fiber of different infill density (70, 75, 80, 85, 90 and 100%) are fabricated using the different fiber impregnation 3D printing process. To carry out the heat treatment process, printing of composites is done for each infill design from both thermoset and thermoplastic composites and the impact characteristics of specimens are evaluated on a pendulum test-rig using the ASTM D-256 standard. Using a scanning electron microscope, each fracture zone underwent four separate scanning processes, ranging in size from 2 µm to 100 µm.FindingsThe impact resistance of the 3D printed thermoset and thermoplastic composite material is significantly influenced by the type of fiber placement and infill density in the matrix substrate. Because of the weak interfacial strength between the layers of fiber and polyamide 6, the specimen printed with continuous fiber implanted at the part exhibited reduced impact resistance. At 75% infill density, the impact specimen printed with coextruded fiber showed the highest impact resistance with a 367.02% greater magnitude than the continuous fiber specimen with the same infill density.Originality/valueThis work presents a novel approach to analyze the low energy impact characteristics and three-dimensional printing of carbon fiber reinforced thermoplastic and carbon fiber reinforced thermoset and thermoplastic composite material.

Comparison of the in situ compressive behavior of carbon fibers woven ply laminates exposed to one‐sided heat flux: Thermoplastic versus thermosetting composites

AbstractThis study investigates the influence of a combined thermal heat flux (imposed by a cone calorimeter) and a compressive loading on the deformation and damage mechanisms within quasi‐isotropic carbon fibers reinforced thermoplastic (polyetherether ketone [PEEK] and polyphenylene sulfide [PPS]) and thermosetting (epoxy) laminates. First, thermogravimetric Analyses conducted under nitrogen provide valuable information on the thermal decomposition of these materials depending on the matrix nature (580–510°C and 350°C, respectively) which is a key factor to explain the mechanical behavior under fire conditions. Second, under the same testing conditions (creep loading under a 50 kW/m2 heat flux), the time to failure is about 17 times as high (C/PEEK), twice as high (C/PPS) with respect to C/epoxy laminates. At the ply scale, the exposure of thermoplastic matrices (PEEK and PPS) to a 50 kW/m2 heat flux causes softening and thermal decomposition, facilitating the micro‐buckling of fiber bundles in matrix‐rich areas. At the laminates scale, this mechanism ultimately leads to the formation and propagation of plastic kink bands in the transverse direction. In epoxy‐based laminates, the onset of matrix pyrolysis occurs at lower temperatures (350°C) during thermal aggression, and thermal decomposition has started in all the plies of the laminates. Combined with the low ductility of the epoxy matrix, the compressive response is elastic‐brittle and results from the buckling of 0° plies outside the thermally‐damaged area.Highlights Study on the compression of polymer composites exposed to a 50 kW/m2 heat flux. The influence of the matrix nature (thermoplastic vs thermoset) is discussed. Time to failure is about 17 times higher in C/PEEK compared to C/Epoxy. Plastic kink bands propagate in thermoplastic composites at T > Tg. C/Epoxy laminates undergo buckling outside the thermally damaged area.

Novel phosphorous-containing epoxy thermosets with improved anti-flammability, smoke suppression, and dielectric properties

As one of the most commonly used thermosetting resins, the flammability of epoxy resins (EPs) has become a key factor limiting the expansion of their application range. Herein, a novel phosphorous-containing epoxy monomer (EDPPO) was synthesized using diallyl amine and diphenyl phosphoryl chloride as the starting materials. Subsequently, EDPPO was thermally cured with two diamine hardeners, 4, 4′-diaminodiphenyl sulfone (DDS) and 4,4′-dithiodianiline (DTDA). Additionally, commercial diglycidyl ether of bisphenol A (DGEBA)-type epoxy pre-polymers were cured with DDS and DTDA as comparative samples. The cured EDPPO-based thermosets showed excellent flame-retardant properties coupled with smoke suppression. Specifically, the limiting oxygen index (LOI) for the EDPPO/DDS (P content 6.4 wt.%) reached 32.0 %, and the UL-94 vertical burning test reached a V-0 rating. In addition, the PHRR, THR, and TSP values of EDPPO/DDS were 61.6 %, 49.1 %, and 58.5 % lower than those of DGEBA/DDS, respectively. The flame retardant mechanism analysis indicated that the excellent flame retardancy for cured EDPPO-based thermosets was attributed to the combined modes of action in condensed (promoting the charring capacity) and gaseous phases (quenching the active radicals). More importantly, EDPPO/DDS and EDPPO/DTDA exhibited lower dielectric constant and dielectric loss than DGEBA/DDS and DGEBA/DTDA. This work provided novel phosphorous-containing epoxy thermosets with improved anti-flammability, smoke suppression, and dielectric properties.

SEPS functionalized PEG copolymer for improved toughness without obvious plasticization in epoxy resin

AbstractThe effects of grafting level of polyethylene glycol (PEG) grafted styrene ethylene propylene styrene (SEPS) on the intrinsic brittleness of epoxy thermosets were studied. The SEPS‐g‐PEG block copolymer (BCP) was synthesized by grafting various grafting degree of PEG (i.e., 10%, 20%, 30%, and 40%) on polystyrene (PS) blocks of SEPS following Williamson ether synthesis method. A total of 10 wt% of the BCP modifiers were dispersed in epoxy thermosets. PEG side chains were miscible in the epoxy which formed a uniform BCP dispersion within the epoxy thermoset matrix. As the degree of PEG grafting was increased, the BCP phase was absorbed in the epoxy matrix. This absorption occurred because the increase in PEG grafting provided more miscible PEG chains to bind the BCP and epoxy nanostructures together. The differential scanning calorimeter and dynamic mechanical thermal analysis analyses indicated that BCP toughened epoxy exhibited an improved glass transition temperature (Tg). At a 40% PEG grafting degree in the BCP, the critical stress intensity factor (KIC) and critical strain energy release rate (GIC) increased up to 80% and 180%, respectively. This trend suggested that the energy associated with fractures could be absorbed through the deformation of the softer phases when a load was applied to them.

Enhancing thixotropic properties of epoxy resin and mechanical properties of epoxy resin thermosets by polyethyleneimine functionalized carbon nanotubes

Epoxy resin (ER) thermosets are widely applied in various fields due to their high mechanical strength, thermal stability and chemical corrosion resistance. However, the brittleness of ER thermosets and low thixotropic property of ER matrix largely limits their further development. Here, this article develops branched polyethyleneimine (PEI) functionalized carbon nanotube (PEI@CNT) as high-performance nanofiller for simultaneously strengthening and toughening ER thermosets. Benefiting from good dispersity and rich amine groups of PEI@CNT, effective interfacial bonding between PEI@CNT and ER is achieved. The ER thermoset with 0.5 wt% PEI@CNT shows increase in tensile strength by 24.8 % and fracture elongation by 30.0 % compared with pristine ER thermoset. The enhanced mechanical properties of PEI@CNT modified ER thermosets can be ascribed to the chemical interaction between PEI@CNT and ER matrix as well as the formed crack propagation, which can dissipate a lot of fracture energy, further improving the strength and toughness of the ER composites. More importantly, the thixotropic property of ER matrix is largely improved after adding the PEI@CNT filler. This work reports a high-performance nanofiller for enhancing mechanical properties of ER thermosets and processability of ER matrix.