Recycling Of Thermosets Research Articles

Dynamic covalent epoxy network of hyperbranched-synergistic-supramolecular: Catalyst-free reprocessing, and application in carbon fiber composites recycling

Plastic recycling, especially the recycling of thermosets, is a crucial step towards improving waste management and achieving economic recycling. Here, a method utilizing non-covalent supramolecular interactions and synergistic hyperbranched structures is reported to endow transesterification-based thermosetting materials with recyclability in the absence of a catalyst. A hyperbranched epoxy resin curing agent (HPCA) containing amide bonds, terminated by amine and ester, was designed and synthesized, and further cured with bisphenol A epoxy resin. The hyperbranched topological structure and its abundant amide bonds contribute to the formation of a dense hydrogen bonding network, enhancing the reprocessability, thermal, and mechanical properties of the material. As a result, the resin can be reprocessed by hot pressing at 190 °C and 10 MPa for 40 min without catalyst. Moreover, amide and ester bonds endow resin materials with excellent degradation performance in alkaline solutions, laying the foundation for the recycling and utilization of carbon fibers in composite materials.

Dual-Factor-Controlled Dynamic Precursors Enable On-Demand Thermoset Degradation and Recycling.

Thermosets are well known for their advantages such as high stability and chemical resistance. However, developing sustainable thermosets with degradability and recyclability faces several principal challenges, including reconciling the desired characteristics during service with the recycling and reprocessing properties required at the end of life, establishing efficient methods for large-scale synthesis, and aligning with current manufacturing process. Here a general strategy is presented for the on-demand degradation and recycling of thermosets under mild conditions utilizing dynamic precursors with dual-factor-controlled reversibility. Specifically, dynamic triazine crosslinkers are introduced through dynamic nucleophilic aromatic substitution (SNAr) into the precursor polyols used in polyurethane (PU) synthesis. Upon removal of the catalyst and alcohol, the reversibility of SNAr is deactivated, allowing for the use of standard PU polymerization techniques such as injection molding, casting, and foaming. The resulting cyanurate-crosslinked PUs maintain high stability and diverse mechanical properties of traditional crosslinked PUs, yet offer the advantage of easy on-demand depolymerization for recycling by activating the reversibility of SNAr under specific but mild conditions-a combination of base, alcohol, and mild heat. It is envisioned that this approach, involving the pre-installation of dual-factor-controlled dynamic crosslinkers, can be broadly applied to current thermosetting plastic manufacturing processes, introducing enhancedsustainability.

From biomass to vitrimers: Latest developments in the research of lignocellulose, vegetable oil, and naturally-occurring carboxylic acids

Thermosets are widely used for their excellent properties, but their permanent cross-linking makes them difficult to recycle. Covalent adaptive networks can address the recycling of thermosets due to their unique reversible transformations, which led to the concept of vitrimers. Vitrimer materials are expected to replace conventional plastics because they combine the removable properties of thermoplastic materials with the advantages of the bulk network structure of thermoset materials. This review details bio-based vitrimers based on cellulose, lignin, vanillin, eugenol, vegetable oils, and natural carboxylic acids, and provides an outlook on the future of vitrimers. With the increasing shortage of fossil resources, it is important to find a low-cost, resource-rich, and easily available biomass resource to develop fully bio-based vitrimer materials for material recycling and sustainable development.

Direct and Catalyst-Free Ester Metathesis Reaction for Covalent Adaptable Networks.

Thermosetting polymers possess excellent environmental resistance and mechanical properties but cannot be reprocessed due to their covalently cross-linked structures. Recycling of thermosets via the implantation of dynamic covalent bonds offers a promising solution. Here, we report the direct and catalyst-free ester metathesis of N-acyloxyphthalimide (NAPI) at about 100 °C without the requirement of hydroxyl groups and its utilization for the fabrication of covalent adaptable networks (CANs). NAPI metathesis has interesting sigmoid kinetics with a fast exchange rate, which proceeds via a free radical chain mechanism, guaranteeing a fast associative exchange under a rather low dissociation. The bifunctional molecule of NAPI as both the radical precursor and substrate is the key to the dissociatively initiated associative (DAssociative) mechanism and kinetic behavior. Based on the efficient NAPI metathesis, polyester networks, poly(N-acyloxyphthalimides) (PNAPIs), show excellent malleability. Notably, PNAPIs exhibit exceptional solvent resistance and mechanical stability at elevated temperatures owing to the unique DAssociative mechanism, suggesting exciting opportunities for designing recyclable thermosetting polymers.

Reversible Crosslinking of Commodity Polymers via Photocontrolled Metal-Ligand Coordination for High-Performance and Recyclable Thermoset Plastics.

Thermoset plastics, highly desired for their stability, durability, and chemical resistance, are currently consumed in over 60 million tons annually across the globe, but they are difficult to recycle due to their crosslinked structures. The development of recyclable thermoset plastics is an important but challenging task. In this work, recyclable thermoset plastics are prepared by crosslinking a commodity polymer, polyacrylonitrile (PAN), with a small percentage of a Ru complex via nitrile-Ru coordination. PAN is obtained from industry and the Ru complex is synthesized in one step, which enables the production of recyclable thermoset plastics in an efficient way. In addition, the thermoset plastics exhibit impressive mechanical performance, boasting a Young's modulus of 6.3GPa and a tensile strength of 109.8MPa. Moreover, they can be de-crosslinked when exposed to both light and a solvent and can then be re-crosslinked upon heating. This reversible crosslinking mechanism enables the recycling of thermosets from a mixture of plastic waste. The preparation of recyclable thermosets from other commodity polymers such as poly(styrene-coacrylonitrile) (SAN) resins and polymer composites through reversible crosslinking is also demonstrated. This study shows that reversible crosslinking via metal-ligand coordination is a new strategy for designing recyclable thermosets using commodity polymers.

Mechanochemistry recycling of polyurethane foam using urethane exchange reaction

Although the recycling and reuse of polyurethane foam (PUF) is a promising route to deal with this type of waste commodity polymer, highly efficient and green processing protocols are a great challenge due to their stable crosslink structure. In this study, a strategy of urethane exchange reactions was proposed to transform commercial rigid PUF into PUF sheets. The process was composed of the pulverization of the foam, the pretreatment method and the hot pressing. The effects of the pretreatment method on the structure and properties of the recycled commercial PUF were studied. The results show that the ball milling pretreatment method can effectively pulverize the commercial PUF into powder of 74.5 µm and a specific surface area as high as 2991.0 m2/kg, which can promote the urethane exchange reaction in close contact with the catalyst. The recycle method of ball milling pretreatment makes the crosslink density and tensile strength of the PUF sheets reach 2236.8 mol/m3 and 32.6 MPa, which means an increase of 9.9 times and 3.3 times, respectively, compared to the method of other pretreatment. Moreover, PUF sheets can still maintain 71.5% of the tensile strength after two cycles of recycling, indicating the potential of this recycling method for PUF. This simple and efficient method enhances the sustainability of rigid PUF utilization and shows the enormous potential for value-added recycling of thermosets.

Recycling of Thermoset Materials and Thermoset-Based Composites: Challenge and Opportunity.

Thermoset materials and their composites are characterized by a long life cycle with their main applications in aircrafts, wind turbines and constructions as insulating materials. Considering the importance of recovery and valorization of these materials at their end-of-life, avoiding landfilling, the interest concerning their recycling grows continuously. The thermoset materials and their composites, to be successfully recovered and valorized, must degrade their three-dimensional structures and recover the mono-oligomers and/or fillers. The thermoset materials could successfully degrade through thermal treatment at different temperatures (for example, above 1000 °C for incineration, ca. 500 °C for oxidation/combustion of organic constituents, etc.), chemical degradation by catalyst, irradiation with or without the presence of water, alcohol, etc., and mechanical recycling, obtaining fine particles that are useful as filler and/or reinforcement additives. Among these recycling methods, this mini-review focuses on the formulation and recovery method of innovative thermoset with in-build recyclability, i.e., materials having chemical links that could be degraded on-demand or containing dynamic covalent bonds to have re-processable and/or recyclable thermoset. This issue could be considered the future perspective in developing novel thermoset materials. The aim of this review is to get an overview of the state of the art in thermoset recycling and of the most commonly used thermoset composites, recovering valuable reinforcing fibers. Additionally, in this work, we also report not only known recycling routes for thermoset and thermoset-based composites, but also new and novel formulating strategies for producing thermosets with built-in recyclability, i.e., containing chemical-triggered on-demand links. This mini-review is also a valuable guide for educational purposes for students and specialized technicians in polymer production and recycling.

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.

A REVIEW ON RECYCLING OF THERMOSETS

Thermosets are a type of plastics that pose a major problem when it comes to easy reusability and recycling because they become irreversibly rigid when subjected to progressive increase in temperatures. Epoxy resins, Phenol Formaldehyde, and Polyurethanes are a few of the many thermosets that are widely employed in several different fields. The rigid, brittle, opaque thermosets, in general, possess qualities like good mechanical strength at elevated temperatures, good chemical resistance in addition to even being self-extinguishing at times (mostly with the help of additives), with low smoke emissions, as highlighted by the British Plastics Federation. However, these properties which serve as an advantage to the end product also affect their recyclability, which is difficult and limited, because of their ability to undergo cross-linking on heating, which ultimately results in the formation of strong covalent bonds that cannot be broken easily. Several commercial techniques are now available for recycling thermosets, which include - Mechanical Processing, Thermal Processing and Chemical Processing. These methods demand higher amount of energy for offering feasible results. There are several studies that have focused on degradation (which is the essential first step that progressively leads to recycling) of thermosets. This review highlights the importance of such studies, techniques and methodologies on aspects including feasibility, cost, sustainability and technological innovation.

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.

Closed-loop chemical recycling of thermosetting polymers and their applications: a review

This review provides an overview of the closed-loop recycling of thermosets via hydrolysis and dynamic exchange reactions.

Recyclable High-Performance Epoxy-Anhydride Resins with DMP-30 as the Catalyst of Transesterification Reactions.

Epoxy-anhydride resins are widely used in engineering fields due to their excellent performance. However, the insolubility and infusibility make the recycling of epoxy resins challenging. The development of degradable epoxy resins with stable covalent networks provides an efficient solution to the recycling of thermosets. In this paper, 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) is incorporated into the epoxy-glutaric anhydride (GA) system to prepare high-performance epoxy resins that can be recycled below 200 °C at ordinary pressure via ethylene glycol (EG) participated transesterification. The tertiary amine groups in DMP-30 can catalyze the curing reaction of epoxy and anhydride, as well as the transesterification between ester bonds and alcoholic hydroxyl groups. Compared with early recyclable anhydride-cured epoxy resins, the preparation and recycling of diglycidyl ether of bisphenol A (DGEBA)/GA/DMP-30 systems do not need any special catalysts such as TBD, Zn(Ac)2, etc., which are usually expensive, toxic, and have poor compatibility with other compounds. The resulting resins have glass transition temperatures and strengths similar to those of conventional epoxy resins. The influences of GA content, DMP-30 content, and temperature on the dissolution rate were studied. The decomposed epoxy oligomer (DEO) is further used as a reaction ingredient to prepare new resins. It is found that the DEO can improve the toughness of epoxy resins significantly. This work provides a simple method to prepare readily recyclable epoxy resins, which is of low-cost and easy to implement.

Vitrimerization: Converting Thermoset Polymers into Vitrimers.

Thermoset polymers with permanently cross-linked networks have outstanding mechanical properties and solvent resistance, but they cannot be reprocessed or recycled. On the other hand, vitrimers with covalent adaptable networks can be recycled. Here we provide a simple and practical method coined as "vitrimerization" to convert the permanent cross-linked thermosets into vitrimer polymers without depolymerization. The vitrimerized thermosets exhibit comparable mechanical properties and solvent resistance with the original ones. This method allows recycling and reusing the unrecyclable thermoset polymers with minimum loss in mechanical properties and enables closed-loop recycling of thermosets with the least environmental impact.

A multiscale chemomechanics theory for the solvent – Assisted recycling of covalent adaptable network polymers

Thermosetting polymers are hard to be recycled using conventional methods due to their permanently crosslinked networks. It was recently reported that covalent adaptable network (CAN) polymers could be fully decomposed in organic solvents utilizing bond-exchange reactions (BERs). Re-polymerization occurs by heating the decomposed solution to evaporate the solvent. This prominent feature of CANs provides exciting opportunities to enable the green and sustainable recycling of thermosets. In this paper, we develop a multiscale chemomechanics theory to study the re-polymerization process of CANs, where the soluble chain segments connect at the tails via BERs. At the macromolecular level, the chemical reaction rates are formulated by considering the distance and diffusivity of reactive species, which determine the average chain segment length and network degree of curing (DoC). The evolution rule of DoC is then fed into the continuum-level multi-branched model to capture the thermomechanical properties of CANs at different curing states. The established theory can predict the conversion ratio of functional groups, solution viscosity, volume shrinkage, and glass transition behaviors of re-polymerizing CANs. It also reveals the influencing mechanisms of various material and processing conditions, which paves the road for the immediate engineering applications of the green and sustainable recycling approach for thermosets and their composites.

Unraveling the Mechanistic Origins of Epoxy Degradation in Acids.

Water diffusion into polymers like thermosetting epoxies is well-studied; however, comparably little has been reported thus far on the related but very different mechanism of acid diffusion and the corresponding influence on material degradation. The diffusion of hydrochloric acid into an amine-cured epoxy system was studied in this work using gravimetric analysis and dielectric monitoring concurrently, and the mass uptake behavior was observed to differ significantly compared with water diffusion, faster by an order of magnitude. A unique 3-stage diffusion of acid into epoxy was observed due to the influence of Coulombic interactions between oppositely charged ionic species diffusing at different rates. Material characterization studies have revealed that the dominant degradation mechanism is physical in nature, with the formation of surface cracks driven by the swelling stresses due to the core–shell swelling behavior in highly concentrated hydrochloric acid, leading to an erosion-type degradation phenomenon. The insights gained from understanding acid electrolyte diffusion could serve to design a more effective and efficient process to enable thermoset recycling by facilitating rapid material breakdown or the design of acid-resistant materials for various applications in chemical storage tanks, batteries, and protective coatings in a corrosive environment.

Reprocessable and Recyclable Crosslinked Polyethylene with Triple Shape Memory Effect

AbstractThermosetting polymers, such as epoxy resins, have found widespread applications because of their excellent thermomechanical and solvent resistance properties. However, the recycling of thermosets remains a challenge, due to the irreversible covalent crosslinking bonds in the network. The exchangeable covalent bonds in vitrimers open opportunities in the way that materials can be dynamically repaired. However, in the case of polyolefins, the strategy of exchangeable vitrimers is still underexplored. Herein, a simple way is reported to prepare a high‐density polyethylene (HDPE) vitrimer with a triple shape memory effect, where exchangeable esters are introduced into the network of covalently crosslinked HDPE. With the incorporation of polyethers or polyesters into the crosslinked network, the two obtained vitrimers show outstanding recyclability and a good triple shape memory effect. This new strategy of preparing an HDPE vitrimer might allow the high‐value utilization of crosslinked polyolefins.

Recycling of Epoxy Thermoset and Composites via Good Solvent Assisted and Small Molecules Participated Exchange Reactions

Thermosetting polymers and composites are a class of high-performance materials with significant industrial applications. However, recycling of thermosets and their filling matters are significantl...

Thermoset composite recycling – Driving forces, development, and evolution of new opportunities

Thermoset composites represent a substantial challenge for recycling, even as composite products increase in market interest. The concept of putting all future thermoset composite products into landfills over the next decades is unlikely to continue. This paper examines the three eras in the history of thermoset product recycling, the drivers for increased recycling, and possible future trends. Technology for managing thermoset composite products at end-of-life first focused on retrieving fiber and to a lesser extent resin. Then in a second era, research focused on better utilization of recovered fiber and finally the third era is now keeping more of the original resin–fiber structure to reuse these composites. Drivers are emerging to stimulate thermoset recycling, including States with success in recycling other challenging products (tires, carpets, automobile parts, etc.) setting policy and fees to encourage recycling. The evolution of heat recovery as a thermoset recycling option in Europe is another driver. Additionally, efforts at certification of recycled fiber quality may stimulate greater reuse.

PRODUÇÃO DE RESINA TROCADORA DE ÍONS A PARTIR DE POLIETILENO RETICULADO

Entre os termofixos existentes no mercado, encontra-se o polietileno reticulado (PEX), que é utilizado no encapsulamento de fios de alta tensão. Nesse processo, uma grande quantidade de resíduos é gerada e descartada, onerando a produção e prejudicando o meio ambiente. Desse modo, este trabalho propôs a modificação química do PEX por meio de reações de sulfonação, que consistem na introdução de grupos SO3H em sua estrutura. Estudou-se a influência da pré-irradiação em micro-ondas, do tempo de reação e da concentração de SO3 livre no ácido sulfúrico fumegante por FTIR-ATR, DRX e ICP. Pelos resultados obtidos verificou-se que PEX-SO3H pode ser classificado como uma resina fortemente catiônica. Os melhores resultados foram obtidos com 30 minutos de reação e 65% de SO3 livre sem pré-irradiação em micro-ondas. Apesar da avaliação econômica não ter sido satisfatória, esse estudo abre novas possibilidades para a reciclagem de termofixos.Entre os termofixos existentes no mercado, encontra-se o polietileno reticulado (PEX), que é utilizado no encapsulamento de fios de alta tensão. Nesse processo, uma grande quantidade de resíduos é gerada e descartada, onerando a produção e prejudicando o meio ambiente. Desse modo, este trabalho propôs a modificação química do PEX por meio de reações de sulfonação, que consistem na introdução de grupos SO3H em sua estrutura. Estudou-se a influência da pré-irradiação em micro-ondas, do tempo de reação e da concentração de SO3 livre no ácido sulfúrico fumegante por FTIR-ATR, DRX e ICP. Pelos resultados obtidos verificou-se que PEX-SO3H pode ser classificado como uma resina fortemente catiônica. Os melhores resultados foram obtidos com 30 minutos de reação e 65% de SO3 livre sem pré-irradiação em micro-ondas. Apesar da avaliação econômica não ter sido satisfatória, esse estudo abre novas possibilidades para a reciclagem de termofixos

Sandwich Injection Moulding of Thermosetting Materials. Part I: Initial Experiments

Thermoset co-injection moulding is a novel technology that might be an alternative for thermoset recycling. This paper describes some experiments on the sandwich moulding of two polyesters, a powder coating and a BMC using a new special design thermoset co-injection machine and manifold system. The first thermoset sandwich moulding has been made and a number of initial moulding problems are presented and discussed.