Recyclable Epoxy Research Articles

On the Response to Hygrothermal Ageing of Fully Recyclable Flax and Glass Fibre Reinforced Polymer Composites.

The present work studies the response to hygrothermal ageing of natural fibre composites (NFCs) against synthetic fibre composites when using three different types of polymers as matrices. For ageing, coupons were fully immersed in distilled water at 23, 40, and 60 °C for a total ageing period of 56 days. Flax fibre-reinforced composites, using two recyclable polymer systems: (i) a bio-based recyclable epoxy and (ii) an acrylic-based liquid thermoplastic resin, were tested against conventional glass fibre-reinforced composites employing a synthetic (petroleum-based) epoxy. Different fibre/polymer matrix material combinations were tested to evaluate the effects of hygrothermal ageing degradation on the reinforcement, matrix, and fibre/matrix interface. The hygrothermal ageing response of unaged and aged composite coupons was assessed in terms of flexural and viscoelastic performance, physicochemical properties, and microscopy (SEM-Scanning Electron Microscopy).

Thermo-mechanical and insulating robust epoxy vitrimer for fully recyclable fiber reinforced composites relied on salen agent

Environmental concerns raise the urgent demand of recyclable materials for electrical and electronic applications. Epoxy and its glass fiber reinforced composites are the most widely used materials in these areas, but are generally unrecyclable. Herein, we reported a recyclable epoxy vitrimer and its glass fiber reinforced composite by using salen agent and DGEBA-type epoxy to ensure its molecular design flexibility and industrial adaptability. Comparing with the conventional amine-cured epoxy, the obtained epoxy vitrimer shows a higher glass transition temperature of 140 °C, a superior tensile strength of 86.7 MPa and an enhanced breakdown strength (180 MV m−1). Furthermore, based on the reversible imine-imine and amine−imine exchange reactions, the epoxy vitrimer could be recycled by both mechanical and chemical methods. An admirable recycling efficiency was achieved based on electrical properties. Moreover, its corresponding glass fiber reinforced composites could be recycled in a closed-loop way while the reclaimed fabric was comparable to its origin. This study demonstrates the potential application of salen agent to develop fully recyclable electrical insulating component and opens a window to further improve the material properties of epoxy vitrimer based on the great design flexibility of salen agent.

Epoxy Thermosets Designed for Chemical Recycling.

Epoxy thermosets constitute a significant portion of high-performance plastics, as they possess excellent thermal and mechanical properties that are applicable in a wide range of industries. Nevertheless, traditional epoxy networks show strict limitations regarding chemical recycling due to their covalently crosslinked structures. Although existing methods provide partial solutions for recycling of epoxy networks, it is urgent to develop more effective, sustainable, and permanent strategies that will solve the problem at hand. For this purpose, developing smart monomers with functional groups that enable the synthesis and development of fully recyclable polymers is of great importance. This review highlights recent advancements in chemically recyclable epoxy systems and their potential to support a circular plastic economy. Moreover, we evaluate the practicality of polymer syntheses and recycling techniques, and assess the applicability of these networks in industry.

A facile way to fabricate renewable P/N/S-containing ferulic acid-derived epoxy thermosets with excellent flame retardancy

The development of sustainable epoxy thermosets combining high performance, acceptable thermal stability, and outstanding flame retardancy has remained a major challenge. Herein, a facile way were developed to fabricate flame-retardant recyclable epoxy thermosets by the ferulic acid-based epoxy resin (FAE) as matrix and a furfural-based 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-Oxide (DOPO) derivative (DOF) as reactive flame retardant. The obtained bio-based epoxy thermosets showed outstanding fire-resistance and higher glass transition temperature (Tg) compared to bisphenol A epoxy thermoset. With 1.26% loading of phosphorus, it possessed a Tg of 132 °C, a limiting oxygen index (LOI) value of 33.2% and a residual char of 42.1% at 700 °C acquired from TGA analyses. It not only exhibited excellent self-extinguishing effect and reached the UL-94 V-0 rating, but showed remarkable reduction on the heat release and smoke production. SEM observation and Raman spectrum manifested that the epoxy thermosets showed an outstanding ability on intumescent and compact char formation, which could suppress the exchange of heat, oxygen and flammable volatiles.

Reprocessable, chemically recyclable, and flame-retardant biobased epoxy vitrimers

Conventional epoxy thermosets exhibit outstanding mechanical performances, excellent thermal and dimensional stabilities, and prominent resistances to chemical/solvent corrosions but are limited because they are dependent on fossil resources, cannot be reprocessed, and are not fire safe. Herein, we reported biobased, flame-retardant, reprocessable, and chemically recyclable epoxy thermosets synthesized with bio-based glycerol triglycidyl ether (GTE) as an epoxy monomer, vanillin derived imine compound as a hardener, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) as a reactive fire-retardant agent. The curing kinetics investigation manifested that the curing reaction was activated with increasing DOPO content. The relationship between structure and properties of the vitrimers were studied in detail. The BEVs showed excellent physical properties with a tensile strength and glass transition temperature higher than 80 MPa and 100 °C, respectively. They possessed outstanding reprocessability and could be chemically recycled easily owning to the presence of the exchangeable imine bonds. The BEV with 6 wt% DOPO showed excellent fire resistance with a UL-94 V-0 rating and a LOI at 26.4% as well as much lower peak heat release rate and total heat release than the control vitrimer without DOPO. The BEVs with combination of excellent mechanical properties, reprocessability, chemical recyclability, and flame retardancy showed a bright future as an alternative to conventional epoxy thermosets for better sustainability and fire-safety.

Performance of Concrete with Recycled Epoxy Plate Waste as aPartial Replacement for Fine Aggregates

The waste recycling of epoxy resin-based fiber reinforced plastics is an important topic of current environmental protection. This work investigated the potential engineering benefits of applying simply crushed recycled epoxy resin-based fiber reinforced plastic in structural concrete. In this paper, the mixing and mechanical properties of concrete products with crushed and recycled waste epoxy plates instead of some fine aggregate were studied experimentally. Three kinds of recycled crushing materials with different particle sizes below 4mm, 4mm to 9mm, and 9mm were used to prepare 10 groups of concrete samples with different proportions by replacing sand with 5, 10, and 15% volumes, respectively. The results showed that the density of the recycled plastic concrete was reduced and the slump of the concrete was increased, with the increase of the recycled crushing materials, and the mechanical properties of concrete were improved within the 5%-10% admixture, and the best results of comprehensive mechanical properties were achieved when the admixture was 10% and the particle size was 4-9mm. These results indicated that it is feasible to replace concrete fine aggregates with recycled crushing materials of waste epoxy plate, and this study not only explores a new way to recycle waste epoxy resin-based fiber reinforced plastic, but also reduces the over-exploitation of sand natural resources, which have a high social and economic value.

Preparation and properties of lignin-based vitrimer system containing dynamic covalent bonds for reusable and recyclable epoxy asphalt

Conventional epoxy asphalt is a thermosetting material that is difficult to reprocess and cannot be easily recycled after disposal. In this study, a lignin-based epoxy asphalt (ELEA) with thermoplastic and thermosetting properties was prepared by introducing a lignin-based vitrimer system, which has a dynamic, covalently bonded, cross-linked ester network, into the asphalt. The mechanical properties (tensile strength, 1.605 MPa; elongation at break, 382.38 %) also meet the requirements of epoxy asphalt for steel bridge deck pavement. The viscosity, thermal properties and high-temperature reversibility of ELEA show that ELEA has good operability, thermal stability and potential for recycling, in line with the trend towards the development of a circular economy. The use of biomass-based derivatives for the epoxy resin and curing agent in ELEA also reduces dependence on non-renewable petroleum resources and provides a useful reference for the high-value and efficient development and utilization of industrial lignin.

Synthesis Strategy and Basic Validation of Recyclable Epoxy Asphalt

The inefficient regeneration of epoxy asphalt is an increasingly serious problem that has greatly restricted its application and development. To alleviate this severe difficulty, recyclable epoxy asphalt has been designed on the basis of the reversibility of dynamic covalent bonds, taking into consideration its nonrenewable characteristics. The synthesis strategy and basic validation of recyclable epoxy asphalt were therefore preliminarily implemented in this study. According to the screening of reversible covalent bond-forming reactions (Diels–Alder reaction), the design scheme of recyclable epoxy asphalt was first proposed. Then the preparation and regeneration processes of recyclable epoxy asphalt were designed and adjusted to give a series of optimal solutions. Finally, samples of recyclable epoxy asphalt and conventional epoxy asphalt were analyzed and tested microscopically before and after regeneration. In comparison with conventional epoxy asphalt, recyclable epoxy asphalt exhibited outstanding sustainability and damping characteristics. Moreover, the damping characteristics of recyclable epoxy asphalt after regeneration were still better than those of conventional epoxy asphalt, which indicated that recyclable epoxy asphalt displayed promising performance. According to the regeneration efficiency, it can be concluded that recyclable epoxy asphalt was successfully prepared. However, mechanical properties of recyclable epoxy asphalt still need to be optimized from the perspective of the proportions of materials.

High-Strength, Degradable and Recyclable Epoxy Resin Based on Imine Bonds for Its Carbon-Fiber-Reinforced Composites

Carbon fiber (CF) is widely used in the preparation of carbon-fiber-reinforced polymer composites (CFRP) in which it is combined with epoxy resin due to its good mechanical properties. Thermosetting bisphenol A epoxy resin, as one of the most common polymer materials, is a non-renewable resource, leading to a heavy environmental burden and resource waste. To solve the above problems and achieve high mechanical and thermal properties comparable to those of bisphenol A, herein, a high-performance, degradable and recyclable bio-based epoxy resin was developed by reacting the lignin derivative vanillin with 4-amino cyclohexanol via Schiff base. This bio-based epoxy resin showed a Young's modulus of 2.68 GPa and tensile strength of 44 MPa, 36.8% and 15.8% higher than those of bisphenol A epoxy, respectively. Based on the reversible exchange reaction of the imine bond, the resin exhibited good degradation in an acidic environment and was recoverable by heat treatment. Moreover, the prepared epoxy resin could be used to prepare carbon fiber (CF)-reinforced composites. By washing off the epoxy resin, the carbon fiber could be completely recycled. The recovered carbon fiber was well preserved and could be used again for the preparation of composite materials to realize the complete recovery and utilization of carbon fiber. This study opens a way for the preparation of high-performance epoxy resin and the effective recycling of carbon fiber.

High-performance and fully recyclable epoxy resins cured by imine-containing hardeners derived from vanillin and syringaldehyde

Increasing concerns about environmental protection have driven the rapid development of recyclable and sustainable thermosetting polymers. Herein, two biobased epoxy hardeners containing imine bonds were prepared from lignin-derived aromatic aldehydes (vanillin and syringaldehyde). The epoxy resins cured by these two hardeners showed comparable mechanical performances in comparison to traditional epoxy resins. Owing to the dynamic feature of incorporated imine bonds, both cured epoxy resins could be repeatedly reprocessed via hot pressing like thermoplastics, and were degradable in the amine solution. It is noteworthy that a closed-cycle chemical recycling process could be realized by utilizing the degradation products to prepare fresh epoxy resins. Notably, the flexural strength and modulus of the chemically recycled and thermally reprocessed epoxy resins were nearly 100% recovered. In addition, both cured epoxy resins showed acceptable glass-transition temperature, thermal stability and chemical resistance. Taken together, our work offered a simple method to develop biobased epoxy resins with versatile recyclability and high performance recovery rates.

Surface modification of aramid fiber meshes – the key to chemically recyclable epoxy composites

Fiber surface functionalization can play a dual role in the development of fiber reinforced polymer composites; improving the overall performance and enabling recovery of high-quality fibers.

Bio-Based Vitrimers from 2,5-Furandicarboxylic Acid as Repairable, Reusable, and Recyclable Epoxy Systems.

In this work, a series of bio-based epoxy vitrimers were developed by reacting diglycidyl ether of bisphenol A (DGEBA) and bio-based 2,5-furandicarboxylic acid (FDCA) at different molar ratios. Triazabicyclodecene was used as a transesterification catalyst to promote thermally induced exchange reactions. Differential scanning calorimetry, gel content measurements, and Fourier transform infrared spectroscopy were used to study the FDCA-DGEBA crosslinking reaction. The transesterification exchange reaction kinetics of such crosslinked systems was characterized via stress relaxation tests, evidencing an Arrhenius-type dependence of the relaxation time on temperature, and an activation energy of the dynamic rearrangement depending on the molar composition. In addition, self-healing, thermoformability, and mechanical recycling were demonstrated for the composition showing the faster topology rearrangement, namely, the FDCA/DGEBA molar ratio equal to 0.6. This work provides the first example of bio-based epoxy vitrimers incorporating FDCA, making these systems of primary importance in the field of reversible, high-performance epoxy materials for future circular economy scenarios.

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.

Thermo-healing and recyclable epoxy thermosets based on dynamic phenol-carbamate bonds

In recent years, the reversible addition reaction of isocyanates and active hydrogen compounds has aroused wide concern. The synthesis of a novel dynamic covalent system based on phenol-carbamate bonds (PCBs) and its application in the preparation of thermo-healing and recycled epoxy resins have become a research hotspot. Herein, a compound with a phenolic hydroxyl group was designed and synthesized, which was used to react with isocyanate to obtain the dynamic covalent system of PCBs. Based on PCBs, a cross-linked polyurethane curing agent and the epoxy thermosetting resin with recyclability and thermo-healing ability were prepared. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), stress relaxation analysis and tensile test were carried out on the synthesized epoxy resins based on PCBs (PCBs-EP). The results showed that the dynamic PCBs endow epoxy thermosets with outstanding reprocessability, thermo-healing, welding, recycling and shape memory properties. Based on the reversible dynamic phenol-carbamate bonds, PCBs-EP can be thermally healed within 2 h under 100 °C, and the thermo-healing effect is obvious. More surprisingly, the tensile strength of PCBs-EP reaches 95.89 (±1.06) MPa, and its tensile strength still maintains 74.13% of the original after healed and 70.6% of the original after welded. Therefore, the strategies for incorporating dynamic covalent PCBs into thermosetting epoxy resins provide a feasible strategy for preparing cross-linked epoxy thermosets with high thermo-healing efficiency and high mechanical strength in a cost-effective manner.

Influence of the Epoxy/Acid Stoichiometry on the Cure Behavior and Mechanical Properties of Epoxy Vitrimers.

Bisphenol A epoxy resin cured with a mixture of dimerized and trimerized fatty acids is the first epoxy vitrimer and has been extensively studied. However, the cure behavior and thermal and mechanical properties of this epoxy vitrimer depend on the epoxy/acid stoichiometry. To address these issues, epoxy vitrimers with three epoxy/acid stoichiometries (9:11, 1:1 and 11:9) were prepared and recycled four times. Differential scanning calorimetry (DSC) was used to study the cure behavior of the original epoxy vitrimers. The dynamic mechanical properties and mechanical performance of the original and recycled epoxy vitrimers were investigated by using dynamic mechanical analysis (DMA) and a universal testing machine. Furthermore, the reaction mechanism of epoxy vitrimer with different epoxy/acid stoichiometry was interpreted. With an increase in the epoxy/acid ratio, the reaction rate, swelling ratio, glass transition temperature and mechanical properties of the original epoxy vitrimers decreased, whereas the gel content increased. The recycling decreased the swelling ratio and elongation at break of the original epoxy vitrimers. Moreover, the elongation at break of the recycled epoxy vitrimers decreased with the epoxy/acid ratio at the same recycling time. However, the gel content, tensile strength and toughness of the original epoxy vitrimers increased after the recycling. The mechanical properties of epoxy vitrimers can be tuned with the variation in the epoxy/acid stoichiometry.

Two birds with one stone: The route from waste printed circuit board electronic trash to multifunctional biomimetic slippery liquid-infused coating

Finding appropriate usage of waste printed circuit board (WPCB), a kind of electronic trash, is an important task from background of environment protection and waste management. Due to the intrinsic high hindrance barrier capability, recycled epoxy resin in rich source from WPCB gives opportunity for corrosion protection of metals based on Two birds with one stone ideology. One stone in this ideology is to apply waste as resource, and two birds respectively indicate using PCB waste as valuable material and inhibiting metal corrosion. In this paper, by combining biomimetics strategy, we adopt a three-step method including anodic oxidation, hydrophobic treatment and infusion of composite containing epoxy resin from WPCB and dimethyl silicone oil to finally form WPCB based slippery liquid-infused porous surface (W-LIS). Morphology, composition, wettability and electrochemical properties are characterized by sophisticated techniques. Adding recycled hydrophobic/oleophilic epoxy resin from WPCB into oil phase can effectively increase the barrier effect and raise anticorrosion ability. Moreover, W-LIS delivers anti-freezing adhesion and biofouling inhibition property, which is obviously superior to LIS and lotus inspired superhydrophobic (SHS) coating. The various excellent performances of W-LIS indicate epoxy resin from waste printed circuit board can act a versatile role in field of metal corrosion prevention.

Room‐Temperature Green Recyclable Epoxy Composites with Enhanced Mechanical and Thermal Properties Cross‐Linked via B−O−C Bonds

AbstractWithout any catalyst, a novel way for the preparation of epoxy (EDSPs) and their composites by cross‐link via dynamic B−O−C bonds was presented. The EDSPs with a glass transition temperature (Tg) of 194 °C were resistant to various solvents and can be green degraded in ethanol at room temperature. The dynamic properties of EDSPs also give the composites excellent recyclability, as the expensive carbon and glass fibers can be recovered intact from the composites under mild conditions at room temperature. Mechanical performance tests showed that the carbon fiber composites (CF/EDSPs) can achieve a flexural strength of 849 MPa and an interlaminar shear strength of 58 MPa. Even after recycling and re‐preparation, 99 % of the original flexural strength and 93 % of the original interlaminar shear strength are still maintained. Scratch repair experiments shown that scratch self‐healing can be achieved in only 10 s at 200 °C.

Cost-efficient and recyclable epoxy vitrimer composite with low initial viscosity based on exchangeable disulfide crosslinks

Epoxy (EP) vitrimer with excellent processing and recyclability is of great potential for industrial application. In this study, we prepared an EP vitrimer based on the low viscosity homogenous mixture of diglycidyl ether of bisphenol A (DGEBA) and a liquid hardener dimethyl 3,3′-dithiodipropionate (DTDP) catalyzed by 2,4,6-tris (dimethylaminomethyl) phenol (DMP30). The optimal curing condition was determined to be 110 °C × 12 h for the original EP vitrimer. Owing to the exchangeable disulfide crosslinks, rapid stress relaxation was found for the vitrimer with a relaxation time of 1.9 s at 160 °C. Moreover, the prepared EP vitrimer possessed excellent self-healing ability and recyclability, with recovery ratio of at least 94.1% during 3 times recycling. The self-healing ability, recyclability and processing performance remain excellent for the vitrimer even after incorporating 30 wt% boron nitride (BN), which can be recycled using DMF as a solvent.

Benefits and Opportunities of Reusing Waste Rotor Blades Materials from a Life Cycle Perspective

The present Life Cycle Analysis (LCA) intends to investigate the environmental benefit of using natural fibres and/or recyclable epoxy resins for future manufacturing of small/medium wind turbine blades to handle thermoset polymer composites that are designed to be recyclable at the end of life”). LCA comparison of a modelled blade based on flax fibres reinforced recyclable epoxy resin and a traditional blade made of glass fibres and non recyclable epoxy resin is presented. In the production phase the environmental impacts of the flax fibre reinforced recyclable epoxy resin composite blade are higher than the blade based on glass fibre epoxy composite mainly due to the higher amount of epoxy resin necessary to satisfy the design criteria of the blade e.g. fatigue and deflection. The end of life is significative as the environmental impacts are reduced by the recycling and recovery of the fibres and the resin, being the resin more significative.

Flame retardancy and chemical degradation of epoxy containing phenylphosphonate group under mild conditions

The development of methods for recycling of thermosetting resin is difficult due to their structure. To regulate the flame retardancy and chemical degradation behavior of epoxy (EP), a phenylphosphonate based EP monomer (DGEP) was prepared and cured using 4,4-diamino diphenylmethane (DDM). For comparison, bisphenol A EP (DGEBA/DDM) was also prepared. The modes of flame retardancy and chemical degradation have been investigated. The DGEP/DDM displays a UL-94 V0 rating while the DGEBA/DDM may not be classified in the UL-94 test. Moreover, DGEP/DDM is stable in most solvents and is degraded after 40 min in a solution of methanol/sodium hydroxide (NaOH)/water (H2O) at 50 °C, while DGEBA/DDM is stable under the same conditions. Degradation occurs by hydrolysis and alcoholics of phenylphosphonate in the main chain of EP under alkaline conditions. In addition, DGEP/DDM has a similar glass transition temperature and tensile properties as those for DGEBA/DDM. The introduction of phenylphosphonate into EP may represent a useful method for the preparation of flame retardant and recyclable EP.