Anhydride-cured Epoxy Research Articles

Mechanoluminescence driven by oxidation reactions in epoxy resins

The luminescence of two imidazole or anhydride cured epoxy resins has been investigated under the combined effect of mechanical stress and temperature using a special experimental setup developed for this purpose. Rupture and cyclic mechanical tests have been conducted in air and in nitrogen between room temperature and 110 °C. Luminescence was acquired through both temporal and spectral acquisitions. Light emission is observed only in air, and fits a Zhurkov model of bond scission. Spectral measurements show that adding a mechanical component to a thermal stress does not excite new molecular groups, indicating that degradation mechanisms under thermal stress or both thermal and mechanical stresses could be similar. A simple two-step model is proposed to describe luminescence as a combination of bond scission and light decay.

Towards a zero-waste chemcycling of thermoset polymer composites: Catalyst assisted mild solvolysis for clean carbon fiber liberation and circular coating development

Thermoset materials and their reinforced composites are widely employed in the aircraft, wind energy and construction sectors. Their 3D-crosslinked network and their chemical and physical heterogeneity make them particularly difficult to be recycled. Nowadays, the management of composite scraps and end-of-life waste is still based on landfilling or incineration practices, which are clearly non-compliant with the principles of the circular economy. In this work, a catalysed solvolysis process in mild conditions (T = 180 °C, t = 1–3 h, catalyst 1–7 wt%) was applied for the chemical recycling (chemcycling) of anhydride-cured epoxy resins and their carbon fiber reinforced composites. The selection of the hydroxylated solvents followed thermodynamic considerations (Hansen solubility parameters) and green chemistry principles. The quality of the liberated fibers was studied through thermogravimetric analysis, scanning electron microscopy and single-fiber micromechanical testing, highlighting high surface purity and 100% retention of their pristine mechanical properties (Young's modulus, elongation at break and ultimate strength). The organic recyclates were characterized through gel permeation chromatography, Fourier-transform infrared spectroscopy and chemical titration, and directly reused as hydroxylated binders for the formulation and application of bicomponent polyurethane protective coatings. The resulting coatings were characterized by high chemical resistance (> 100 double rubs at methyl-ethyl ketone test), high surface scratch hardness (3H to 5H), good substrate adhesion (1.5–4 MPa), and excellent optical clarity and surface gloss. These results demonstrate the potential zero-waste reusability of all fractions derived from the chemical recycling of carbon fiber reinforced composites, in line with the principles of the circular economy.

An efficient and economical degradation strategy for epoxy thermosets based on a low-cost transesterification catalyst

Epoxy thermoset materials are widely used in electrical engineering, but it is a challenge to degrade and recycle traditional epoxy thermoset wastes. In this work, an effective method was proposed to recycle anhydride-cured epoxy thermoset wastes.

Performance Evaluation of Insulating Material Prepared From Recycled Anhydride-Cured Epoxy Resin by Alcoholysis

Performance Evaluation of Insulating Material Prepared From Recycled Anhydride-Cured Epoxy Resin by Alcoholysis

Influence of the stoichiometric ratio on the material properties and the aging behavior of anhydride-cured epoxy systems

Abstract When partial discharge and dielectric heating effects can be excluded, comparatively little is known about the aging behavior of polymeric insulation materials under combined environmental and electric field stress. Since it was found in an earlier study that hygroelectrical stress leads to a strong reduction in the residual breakdown strength of anhydride-cured epoxy samples, the goal of this contribution is to determine if a material modification could enhance the insulation performance under hygroelectrical stress. For this purpose, epoxy samples of different stoichiometric ratios (SRs) were manufactured and hygroelectrically stressed. The material properties were evaluated by AC breakdown strength measurements as well as by Fourier-transform infrared (FTIR) spectroscopy before and after each aging sequence. Gravimetric analysis quantified the water diffusion behavior and the (steady-state) water absorption during exposure to different relative humidity levels at room temperature. The breakdown strength decreases for both increasing and decreasing SR compared to S R = 95 % . Moreover, FTIR measurements revealed that an increase of the SR leads to a higher amount of unreacted anhydride which correlates with an increasing amount of absorbed water. Thus, the mutual presence of water in the atmosphere and of unreacted anhydride in the epoxy material is identified as the main risk factor for epoxy degradation under the influence of electrical stress, since unreacted anhydride favors stronger water absorption and the presence of water inside the material is attributed to a lowering of the potential barrier for molecular bond breaking. Remarkably, the breakdown strength of non-aged samples is lower at S R = 80 % compared to S R = 95 % , but the long-term performance with respect to insulation aging is enhanced, which correlates with the non-/less-existing unreacted anhydride in the material.

Retarding thermal-oxidative aging of anhydride-cured epoxy asphalt binder and mixture by incorporating antioxidant Irganox 1010

ABSTRACT This research focused on the stabilisation of anhydride-cured epoxy asphalt (EA) over thermal-oxidative aging. EA binders with graduated Irganox 1010 antioxidant contents at 0, 0.3, 0.6, 0.9, and 1.2 wt% were prepared and then subjected to accelerated oven-conditioning aging at 85 and 100°C. The thermal stability, viscosity, mechanical parameters, and chemical structure of EA containing Irganox 1010 were evaluated during aging. The results showed that Irganox 1010 contributed to enhanced thermal stability, higher initial viscosity, and longer allowable construction time for EA. The 0.3–0.9 wt% Irganox 1010 had good compatibility with EA without weakening performance. During aging, Irganox 1010 effectively restrained degradation of the EA binder’s mechanical properties and the EA mixture’s low-temperature deformation. The variations of functional groups indicate that Irganox 1010 prominently restrained the oxidation of the asphalt phase and lightly affected the post-curing of the resin phase. There was a strong correlation between decreasing rate of bending strain of the mixture and increasing rate of sulfoxide in the binder. The mutual synergistic effect of resin and antioxidant improves the oxidation resistance of asphalt. Considering the aging improvement and economic cost, it is recommended to use 0.6 wt% Irganox 1010 as an antioxidant for anhydride-cured EA. Highlights Epoxy asphalt (EA) binder and mixtures incorporating antioxidant Irganox 1010 were analysed. Irganox 1010 plays a crucial role in aging protection effect of EA binder and mixture. Irganox 1010 antioxidant prominently restrains the oxidation of the asphalt phase and lightly affects the post-curing of the resin phase. There was a strong correlation between the bending strain of mixture and sulfoxide of binder. Addition of 0.6 wt% Irganox 1010 is recommended as an antioxidant for EA materials.

Mixed-frequency medium-voltage aging analysis of epoxy in the absence of partial discharges and dielectric heating

Premature failures of polymeric insulation under inverter-type electrical stress are predominantly associated with partial discharge (PD) erosion or dielectric heating. In the present contribution, an approach for aging analysis in the absence of the aforementioned mechanisms is proposed and applied to anhydride-cured epoxy samples, which are designed with a recessed shape to achieve PD-free aging. Dielectric heating was found to be negligible under all applied experimental conditions. Aging of samples was performed with a specialized setup for the generation of mixed-frequency medium-voltage (MF-MV) waveforms under controlled temperature and humidity conditions. The health state of samples was evaluated before and after different aging sequences by analysis of potential aging markers, namely the short-term AC breakdown strength, the complex dielectric permittivity (real and imaginary part), the volume resistivity, the glass transition temperature and the characteristic absorbance peaks obtained by Fourier-transform infrared spectroscopy (FTIR). Of these, only the breakdown strength exhibited significant aging effects under hygroelectric stress, which is hypothesized to be attributed to localized microcracking caused by electromechanical stress. Pure electrical MF-MV stress (i.e. at room temperature and dry conditions) was not found to be critical under the applied experimental conditions. By means of FTIR, hydrolysis was excluded as a possible aging mechanisms. In summary, the proposed aging analysis approach was found to be suitable to reveal aging effects empirically as well as to give indications about the underlying aging mechanisms without the need for excessively long or accelerated lifetime testing.

Reconstruction of new high-performance epoxy thermoset based on the full utilization of the degradation products of waste epoxy thermoset

Chemcycling of waste plastics seeks high-efficiency degradation and high utilization of degradation products. However, it is difficult to achieve such a goal for epoxy thermosets due to their dense network structure and lack of active groups in degradation products. In most cases, only a small fraction of the degradation products can be used to reconstruct new epoxy thermosets after waste epoxy thermosets are degraded which accounts for a very low proportion in the new epoxy thermoset. Herein, we report a chemical upcycling of waste anhydride-cured epoxy thermosets to new epoxy thermosets based on the degradation-modification-reconstruction strategy. The epoxy thermoset is degraded using a simple and efficient alkali/alcohol system with the yields of about 90 % of two degradation products. The two degradation products are further modified into new epoxy monomer and curing agent respectively. All the modified products are directly used to prepare new epoxy thermosets with high performance without additional epoxy monomer or curing agent added. This work realizes the full utilization of the degradation products of waste epoxy thermoset, and the degradation products account for more than 80 % of the new epoxy thermoset. Carbon footprint and economic calculations show that the strategy is environmental-friendly and cost efficient.

Facile recycling of anhydride-cured epoxy thermoset under mild conditions with multifunctional hydrazine hydrate

Environmental economics is accelerating the urgency to develop recycling technologies for the ever-growing quantity of discarded thermoset polymers. Herein, we developed a mild and energy-saving process for high-efficiency degradation and reuse of anhydride-cured epoxy thermoset with the aid of hydrazine hydrate. The degradation degree of the epoxy resin reached 99.6% at 120 °C within a short time of 60 min. During the reaction, the ester bonds in the cross-linked network were selectively cleaved by the amination of hydrazine hydrate, and the epoxy resin was fully converted to new monomers that contained hydrazide and hydroxyl groups, respectively. Moreover, the degradation mechanism of the epoxy resin in hydrazine hydrate was studied and a nucleation model was utilized to predict the actual degradation behavior of the system. Finally, the degradation products can be directly mixed with epoxy precursor to prepare a new waterborne epoxy coating with good comprehensive properties. This work not only demonstrates a new way to realize the efficient degradation of epoxy resins, but also provides a facile and efficient recycling protocol for thermosets.

Bona fide upcycling strategy of anhydride cured epoxy and reutilization of decomposed dual monomers into multipurpose applications

Waste epoxy materials become an enormous problem to society and the environment. The advantages of epoxy resins derive from their rigid and chemically stable networks, but these qualities also make them difficult to dispose of or recycle. Herein, we demonstrated an efficient degradation of anhydride cured epoxy resin by aminolysis in aminoethanol without using any catalysts. The epoxy resin was fully decomposed at 160 °C in 4 h, resulting in two distinct high purity monomers (HHPA-OH and BPA-OH). To fully realize this upcycling approach, The BPA-OH was used to synthesize a polyurethane coating with an excellent glass transition temperature (88.9 °C), scratch hardness (8H), gouge hardness (6H), adhesive strength (5B), and strong solvent resistance. The HHPA-OH with two hydroxyl groups was reacted with methacrylic anhydride to form a dimethacrylate monomer which was then used as a viable crosslinker for photo-curable 3D printing thermosetting polymer with tensile strength as high as 64 MPa and impact strength of 4.86 kJ/m2. This work demonstrates a feasible pathway to convert anhydride cured epoxy waste to new monomeric recyclates for superior polymer products.

Insights into anhydride-cured epoxy resin system using dynamic chemo-rheological modeling

Insights into anhydride-cured epoxy resin system using dynamic chemo-rheological modeling

Aging degradation of anhydride-cured epoxy asphalt binder subjected to ultraviolet exposure

ABSTRACT This research investigated the aging degradation of anhydride-cured epoxy asphalt (EA) subjected to ultraviolet (UV) exposure. A UV accelerated aging test was applied to age EA binders with graduated asphalt contents at 25%, 35%, 45%, 55%, and 65%. The effects of aging on the mechanical, chemical properties, and microstructure of EA were investigated. Results show that the more asphalt is introduced, the higher the mass loss rate of the EA binders, and the more prominent the decrease in elongation and fracture energy during aging. The storage modulus and hardness of EA binders increased after aging. The tensile strength and glass transition temperature of EA binder with 25%-45% asphalt content decreased, while those of EA binder with 55%-65% asphalt content increased. Besides, several carbonyl compounds formation, hydroperoxides formation, and loss of alkyl compounds were observed. Also, when the asphalt content was lower than 45%, holes and microcracks appeared on the surface, while above 45% asphalt content wrinkles. Degradation of the resin network in EA binders was evident by the loss of ester compounds and the microcrack of the resin phase. Overall, the resin network can reduce UV damage to asphalt and improve the photo-oxidative aging resistance of EA materials.

Curing Kinetics and Dielectric Properties of Anhydride Cured Epoxy Resin With Different Accelerator Contents

The addition of accelerator reduces the curing reaction temperature, changes the curing reaction process, and affects the morphology of the crosslinking structure, which would lead to the difference in the dielectric properties of the epoxy resin. In this study, the curing behaviors and dielectric properties of diglycidyl ether of bisphenol A (DGEBA)/methyl hexahydrophthalic anhydride (MHHPA) were investigated with respect to the contents of the curing accelerator, Tris-(dimethylaminomethyl) phenol (DMP-30). The curing behavior of epoxy resin was studied by differential scanning calorimetry (DSC), and it was found that the system conforms to the Sestak–Berggren kinetic model [SB ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${m},{n}{)}$ </tex-math></inline-formula> ]. The accelerator content has a saturation effect on the dielectric properties of the epoxy resin curing sample, and excessive content can lead to the increase of dielectric constant and loss and decrease of breakdown strength. The cured sample with 0.5% accelerator addition has the best dielectric properties and the highest breakdown strength. The analysis concludes that the optimum accelerator content allows for a more rational curing crosslinking network.

Curing Regime-Modulating Insulation Performance of Anhydride-Cured Epoxy Resin: A Review.

Anhydride-cured bisphenol-A epoxy resin is widely used in the support, insulation and sealing key components of electrical and electronic equipment due to their excellent comprehensive performance. However, overheating and breakdown faults of epoxy resin-based insulation occur frequently under conditions of large current carrying and multiple voltage waveforms, which seriously threaten the safe and stable operation of the system. The curing regime, including mixture ratio and combination of curing time and temperature, is an important factor to determine the microstructure of epoxy resin, and also directly affects its macro performances. In this paper, the evolution of curing kinetic models of anhydride-cured epoxy resin was introduced to determine the primary curing regime. The influences of curing regime on the insulation performance were reviewed considering various mixture ratios and combinations of curing time and temperature. The curing regime-dependent microstructure was discussed and attributed to the mechanisms of insulation performance.

Toughening of epoxy thermosets by self-assembled nanostructures of amphiphilic comb-like random copolymers.

Amphiphilic comb-like random copolymers synthesized from poly(ethylene glycol) methyl ether methacrylate (PEGMMA) and stearyl methacrylate (SMA) with PEGMMA contents ranging between 30 wt% and 25 wt% were demonstrated to self-assemble into various well-defined nanostructures, including spherical micelles, wormlike micelles, and vesicle-like nanodomains, in anhydride-cured epoxy thermosets. In addition, the polymer blends of the comb-like random copolymer and poly(stearyl methacrylate) were prepared and incorporated into epoxy thermosets to form irregularly shaped nanodomains. Our research findings indicate that both the comb-like random copolymers and polymer blends are suitable as toughening modifiers for epoxy. When added at a concentration of 5 wt%, both types of modifiers lead to substantial improvements in the tensile toughness (>289%) and fracture toughness of epoxy thermosets, with minor reductions in their elastic modulus (<16%) and glass transition temperature (<6.1 °C). The fracture toughness evaluated in terms of the critical stress intensity factor (KIC) and the strain energy release rate (GIC) increased by more than 67% and 131% for the modified epoxy thermosets containing comb-like random copolymers.

Mechanical Behavior and Thermal Oxidative Aging of Anhydride-Cured Epoxy Asphalt with Different Asphalt Contents

Mechanical Behavior and Thermal Oxidative Aging of Anhydride-Cured Epoxy Asphalt with Different Asphalt Contents

Influences of material and processing conditions on the depolymerization speed of anhydride-cured epoxy during the solvent-assisted recycling

Thermosetting polymers with permanent crosslinking networks have been intensively used in high-performance applications requiring thermal stability, chemical resistance, and structural integrity. However, they are extremely hard to be reprocessed and recycled using conventional techniques. The recently developed solvent-assisted recycling technique is shown to depolymerize and recycle thermoset wastes using low-toxic solvents and mild processing conditions. The macromolecular polymer chains can be cleaved at target covalent bonds on the chain backbone through bond exchange reactions with solvent molecules. In this study, the influences of various material and processing conditions on the network depolymerization speed are studied from both experimental and theoretical points of view. A diffusion-reaction chemomechanics modeling framework is defined to link the physical properties of organic solvent and microscale reaction kinetics to the overall depolymerization speed of thermosets at various material and processing parameters. The study reveals how the temperature, network crosslinking density, and physical properties of the solvent, in particular, the solvent diffusivity and solvent/polymer interactions would affect the depolymerization speed. It enhances our understanding of the underlying fundamentals of thermoset recycling using organic solvents, which provides valuable insights for industrials to optimize the recycling conditions and develop appropriate strategies for waste management. • Influencing mechanisms of various material and processing parameters are revealed. • A theory is defined to link solvent properties to the depolymerization speed. • There is an optical content of good solvent to maximize the depolymerization speed. • There are threshold values for solvent hydroxyl-hydrocarbon ratio and interaction parameters.

Fast and sustainable recycling of epoxy and composites using mixed solvents

Chemical recycling of thermosets and composites can reclaim polymer matrix and high-value reinforcement to protect the environment and save resources. However, efficient and sustainable chemical recycling of epoxy thermosets is still a significant challenge, specifically, improving the decomposition rate, easily separating and reusing the decomposed components under mild condition. Herein, we report the fast and sustainable recycling of epoxy and composites by small-molecule assisted bond exchange reaction (BER) in a mixed solvent. The anhydride-cured epoxy resin with embedded catalyst was rapidly decomposed by the mixed solvent containing a high boiling point alcohol and inert good solvent via transesterification below 150°C at normal pressure. The epoxy decomposition rate was optimized by altering the good solvent types and solvents mixing ratios. We found the epoxy decomposition rate was the fastest in dimethylformamide/ethylene glycol (50/50) due to balanced solvent diffusion/swelling and reaction rate. After epoxy decomposition, the depolymerized epoxy oligomer (DEO) and residual solvents were readily separated by reduced pressure distillation below 80°C. The DEO as an additive (up to 20%) was repolymerizated with fresh epoxy resin to make epoxy materials, which showed similar mechanical properties to the original epoxy with slight deterioration even for several recycling cycles. The reclaimed solvent was used for the next round of recycling. With the optimized condition, carbon fiber reinforced epoxy composite was also recycled to reclaim the high-value carbon fiber with unchanged mechanical properties. This work presents a sustainable and fast recycling paradigm for industrial-grade epoxy and composite with potential for upscale engineering applications.

Bio-based phosphorus-containing benzoxazine towards high fire safety, heat resistance and mechanical properties of anhydride-cured epoxy resin

Endowing anhydride-cured epoxy resin (EP) with excellent flame retardancy while maintaining good comprehensive properties has become a great challenge. In this context, a bio-based phosphorus-containing benzoxazine monomer (SFD-BOZ) composed of DOPO, furfurylamine, salicylaldehyde and paraformaldehyde was successfully synthesized via a two-step reaction, and then blended into EP/methyltetrahydrophthalic anhydride (MeTHPA) system. Though SFD-BOZ exhibited no significant effect on the curing activity of EP/MeTHPA system, the introduction of SFD-BOZ made FREP show better heat resistance. Besides, the flame retardancy of EP was markedly improved due to the incorporation of SFD-BOZ.The modified EP with 20.7 wt% SFD-BOZ achieved V-0 rating of UL-94 and obtained limited oxygen index (LOI) value of 35.2%, which was 76% higher EP. Also, its peak heat release rate (PHRR), total heat release (THR) and total smoke production (TSP) dramatically declined by 53%, 36% and 45% as compared to that of EP, respectively. Furthermore, the flexural and tensile strengths of modified EP showed the maximum increments of 9% and 49%, respectively. The incorporation of SFD-BOZ manifested the potential for developing high-performance epoxy resin.

Efficient catalytic degradation of anhydride-cured epoxy resin by amphiphilic molecule catalysts

A novel strategy was proposed for the complete degradation of ACEP by an amphiphilic catalyst in water under mild conditions.