Orthogonal imine and disulfide exchange in a biobased covalent adaptable network: toward healable and recyclable thermosets

Recent advances in recyclable thermosets and thermoset composites based on covalent adaptable networks

A Strong and Rigid Coordination Adaptable Network that Can Be Reprocessed and Recycled at Mild Conditions

Traditional epoxy thermosets cannot be reprocessed or recycled due to their permanent covalent cross-linking network. Covalent adaptable networks (CANs) emerge as a solution, endowing epoxy thermosets with recyclability, reprocessability and self-healing ability to tackle the recycling issue. Nevertheless, the existing covalent adaptable epoxy network exhibits low mechanical robustness, glass transition temperature and thermal stability. Herein, we have developed a covalent adaptable epoxy network based on dynamic amine terminated hyperbranched polyamide (AHPA) to fabricate catalyst-free and high-performance epoxy vitrimers. The incorporation of thermoactivated rearrangement of AHPA enables the obtained epoxy vitrimers to possess remarkable reprocessability, along with good thermal stability, high glass transition temperature and excellent creep resistance. The epoxy vitrimers can be easily reprocessed without compromising thermal and mechanical properties even after multiple cycles, presenting a promising design of dynamic hyperbranched polymers for constructing adaptive and recyclable epoxy thermosets for sustainable engineering applications.

Covalent adaptable networks (CANs) are polymer materials that are covalently cross-linked via dynamic covalent bonds. The cross-linked polymer network is generally expected to be insoluble, as is seen for traditional thermosets. However, in recent years, it has become apparent that-under certain conditions-both dissociative and associative CANs can be dissolved in a good solvent. For some applications (e.g., those that require long-term (chemical) stability), the solubility of CANs can be problematic. However, many forget that (selective) solubility of CANs can also be applied advantageously, for example, in recycling or modification of the materials. In this work, we provide results and insights related to the tunable solubility of imine-based CANs. We observed that selected CANs could be fully dissolved in a good solvent without observing dissociation of imines. Only in an acidic environment (partial) dissociation of imines was observed, which could be reverted to the associated state by addition of a base. By adjusting the network composition, we were able to either facilitate or hamper solubility as well as control the size of the dissolved particles. DLS showed that the size of dissolved polymer particles decreased at lower concentrations. Similarly, decreasing cross-linking density resulted in smaller particles. Last, we showed that we could use the solubility of the CANs as a means for chemical recycling and postpolymerization modification. The combination of our studies with existing literature provides a better understanding of the solubility of CANs and their applications as recyclable thermosets.

Sustainable Green Polymerizations and End-of-Life Treatment of Polymers.

Chapter 15 - Recyclable thermoset polymers: beyond self-healing

Covalent adaptable networks (CANs) are cross-linked polymers that have mechanical properties similar to thermosets at operating conditions yet can be reprocessed by cross-link exchange reactions that are activated by a stimulus. Although CAN exchange dynamics have been studied for many polymer compositions, the tensile properties of these demonstration systems are often inferior compared to those of commercial thermosets. In this study, we explore toughening CANs capable of forming covalent bonds with a reactive filler to characterize the trade-off between improved toughness and longer reprocessing times. Polycarbonate (PC) and polyurethane (PU) CANs were toughened by incorporating cellulose modified with cyclic carbonate groups as a reactive filler with loadings from 1.3 to 6.6 wt %. The addition of 6.6 wt % of the cellulose derivative resulted in a 3.2-fold increase in average toughness for the PC CANs, yet it only increased the characteristic relaxation time of stress relaxation (τ*) via disulfide exchange at 180 °C from 63 to 365 s. The cellulose-containing samples also showed >80% recovery in crosslinking density and mechanical properties after reprocessing. The addition of 3.2 wt % of the functionalized cellulose into a polyethylene glycol-based PU CAN led to a 2.3-fold increase in toughness while increasing τ* at 140 °C from 106 to 157 s. These findings demonstrate the promise of functionalized cellulose as an inexpensive, renewable, and sustainable filler that toughens CANs containing hydroxyl groups.

Covalent adaptable networks (CANs) are cross-linked polymers that have mechanical properties similar to thermosets at operating conditions, yet can be reprocessed by cross-link exchange reactions that are activated by a stimulus. Although CAN exchange dynamics have been studied for many polymer compositions, the tensile properties of these demonstration systems are often inferior compared to commercial thermosets. In this study, we explore toughening CANs capable of forming covalent bonds with a reactive filler to characterize the trade-off between improved toughness and longer reprocessing times. Polycarbonate (PC) and polyurethane (PU) CANs were toughened by incorporating cellulose modified with cyclic carbonate groups as a reactive filler with loadings from 1.3-6.6 wt%. The addition of 6.6 wt% of the cellulose derivative resulted in a 3.2-fold increase in average toughness for the PC CANs, yet only increased the characteristic relaxation time of stress relaxation (*) via disulfide exchange at 180 °C from 63 s to 365 s. The cellulose-containing samples also showed >80% recovery in crosslinking density and mechanical properties after reprocessing. The addition of 3.2 wt% of the functionalized cellulose into a PEG-based PU CAN led to a 2.3-fold increase in toughness, while increasing * at 140 °C from 106 s to 157 s. These findings demonstrate the promise of functionalized cellulose as an inexpensive, renewable, and sustainable filler that toughens CANs containing hydroxyl groups.

Recycling of thermoset polymers is of significant importance, which could be solved by utilizing a new class of thermosetting polymer called “vitrimer.” Vitrimers are cross-linked polymers with dynamic covalent networks permitting the rearrangement of networks at elevated temperature. This readily enables it to remold, reprocess, and recycle similar to thermoplastics. Vitrimers from biomass-derived building blocks are expanding due to raising awareness of bio-based products and ingenious way towards a greener future. The polymer network has exchangeable bonds like transesterification, vinylogous urethane, and metathesis have been adopted in designing the vitrimers. However, the research on bio-based vitrimers is limited to the academics and lab scale and requires significant attention. This book chapter aims to provide a detailed summary of topical efforts directed towards understanding and controlling the cross-linking reactions to modulate the bio-based vitrimers properties. First, the basic mechanism of the covalent adaptable network is enlightened, along with thermally activated associative covalent bond exchange reactions for different vitrimers. A recent progress on vitrimers made from renewable raw materials (vegetable oil, lignin, sugar, rosin, and its derivatives) is highlighted. The concept of vitrimer exchangeable reaction, synthesis routes, and their properties are discussed. Also, a detailed survey was carried out to understand the effect of reinforcement as covalently adaptable network on vitrimer properties. Besides, the fascinating features of vitrimers such as thermal reparability and self-healing characteristics are discussed. Finally, the challenges in preparing this exchangeable polymer network and the outlook of utilizing the vitrimer for practical applications is also part of the discussion. This chapter will greatly help the researchers to expand their strategies towards the development of renewable vitrimers for securing a sustainable future.

Traditional Epoxies take hours to cure before their properties can be realized and this restricts its use in applications where quick turn‐around time is imperative. Herein, we developed fast curing Epoxies (which cure within 2 h) using Vitrimer chemistries. A unique dynamic curing agent (bio‐based Schiff motif) consisting of an aldehyde (here, cinnamaldehyde) and an amine (here, mussel inspired dopamine) is used to facilitate imine exchange during the curing process and maintains the network integrity‐ a key requirement to achieve excellent structural properties in epoxy based components. By incorporating this covalent adaptable network, various epoxies were designed which can cure within 2 h. The resulting epoxy vitrimers exhibited high tensile strength (68 MPa), exceptionally high Tg (128°C), fast stress relaxation (a relaxation time of 15.2 s at 180°C) and an activation energy of 105 kJ/mol. This associative covalent adaptable network (imine exchange) also rendered the epoxies re‐processable and re‐usable and offers closed‐loop circularity in plastics. The resulting epoxy also exhibited self‐healing property triggered by the exchangeable bonds. The self‐healing behavior of this epoxy was manifested from the surface conductivity measurements wherein the conducting pathway (facilitated by spraying conducting particles on the vitrimer epoxy substrate) was restored post self‐healing in substrates containing epoxy vitrimers. In addition, the epoxies cured using this Schiff based motif dissolves in a specific solvent and can be recovered and re‐purposed. The excellent mechanical properties achieved by retaining the network integrity together with the self‐healing properties offers myriad applications for the designed epoxy cured using bio‐based Schiff motif.Highlights Developed fast curing Epoxies that cures in 2 h using Vitrimer chemistries. Fully bio‐based hardener was used to realize the network topology. The dynamic exchange reaction results in re‐processability, reusable, recyclable and self‐healable that is not possible in the case of traditional epoxy. It exhibited high tensile strength, exceptional Tg, fast stress relaxation and an activation energy of 105 KJ/mol.

Thermoset polymers have become integral to our daily lives due to their exceptional durability, making them feasible for a myriad of applications; however, this ubiquity also raises serious environmental concerns. Covalent adaptable networks (CANs) with dynamic covalent linkages that impart efficient reprocessability and recyclability to thermosets have garnered increasing attention. While various dynamic exchange reactions have been explored in CANs, many rely on the stimuli of active nucleophilic groups and/or catalysts, introducing performance instability and escalating the initial investment. Herein, we propose a new direct and catalyst-free C═C/C═N metathesis reaction between α-cyanocinnamate and aldimine as a novel dynamic covalent motif for constructing recyclable thermosets. This chemistry offers mild reaction conditions (room temperature and catalyst-free), ensuring high yields and simple isolation procedures. By incorporating dynamic C═C/C═N linkages into covalently cross-linked polymer networks, we obtained dynamic thermosets that exhibit both malleability and reconfigurability. The resulting tunable dynamic properties, coupled with the high thermal stability and recyclability of the C═C/C═N linkage-based networks, enrich the toolbox of dynamic covalent chemistry.

Covalent adaptable networks (CANs) represent a novel class of polymeric materials crosslinked by dynamic covalent bonds. Since their first discovery, CANs have attracted great attention due to their high mechanical strength and stability like conventional thermosets under service conditions and easy reprocessability like thermoplastics under certain external stimuli. Here, we report the first example of ionic covalent adaptable networks (ICANs), a type of crosslinked ionomers, consisting of negatively charged backbone structures. More specifically, two ICANs with different backbone compositions were prepared through spiroborate chemistry. Given the dynamic nature of the spiroborate linkages, the resulting ionomer thermosets display rapid reprocessability and closed-loop recyclability under mild conditions. The materials mechanically broken into smaller pieces can be reprocessed into coherent solids at 120 °C within only 1 min with nearly 100% recovery of the mechanical properties. Upon treating the ICANs with dilute hydrochloric acid at room temperature, the valuable monomers can be easily chemically recycled in almost quantitative yield. This work demonstrates the great potential of spiroborate bonds as a novel dynamic ionic linkage for development of new reprocessable and recyclable ionomer thermosets.

The regulation of topological structure of covalent adaptable networks (CANs) remains a challenge for epoxy CANs. Here, we report a strategy to develop strong and tough epoxy supramolecular thermosets with rapid reprocessability and room-temperature closed-loop recyclability. These thermosets were constructed from vanillin-based hyperbranched epoxy resin (VanEHBP) through the introduction of intermolecular hydrogen bonds and dual dynamic covalent bonds, as well as the formation of intramolecular and intermolecular cavities. The supramolecular structures confer remarkable energy dissipation capability of thermosets, leading to high toughness and strength. Due to the dynamic imine exchange and reversible noncovalent crosslinks, the thermosets can be rapidly and effectively reprocessed at 120 °C within 30 s. Importantly, the thermosets can be efficiently depolymerized at room temperature, and the recovered materials retain the structural integrity and mechanical properties of the original samples. This strategy may be employed to design tough, closed-loop recyclable epoxy thermosets for practical applications.

Thermosets are polymeric materials that contain permanent networks and thus are difficult to recycle. They are not reprocessable once cured and often do not degrade under mild conditions. Over the past decades, the use of polymeric materials in fire safety applications has increased, and so is the need for them to be more sustainable. From this standpoint, recently two major challenges in designing next-generation thermosets have attracted much attention in the scientific community: embedded fire safety and reprocessability/recyclability. In this review, a detailed report on research progress in design of fire-safe and thermomechanical reprocessable/recyclable thermosets is presented. Such thermosets are designed not only to enable the reuse and recycling of the polymer material but also recover valuable components (carbon fibers or rare additives) that are encapsulated in the matrix. The flame retardant recyclable thermoset materials are categorized based on the chemistry of the labile bonds (covalent adaptable networks): i.e. (i) esters (carboxylic and phosphate esters), (ii) sulfur-containing linkages, (iii) nitrogen-containing structures, and (iv) other phosphorus-containing structures. In addition, the use of bio-based raw materials in constructing these thermosets is also highlighted. The synthetic route, fire performance, recycling method, degradation mechanism, and progress in various approaches being developed by researchers towards recyclable and fire-safe thermosets are summarized in detail in this review.

Thermosetting polymers have played an important and irreplaceable role, such as adhesives, coatings, composites, etc., while they are difficult to be recycled due to their permanent crosslinked networks. The last few years have witnessed the rapid development of thermosetting polymers based on dynamic covalent bonds, also known as covalent adaptable networks (CANs), which could be easily recycled (reprocessable, weldable or even degradable). The need to develop easily recyclable thermosetting polymers based on dynamic covalent bonds, as well as a summary of the research progress is highlighted in this chapter. The easily recyclable thermosets are divided into three categories based on: (1) the dynamic associative exchange reaction, (2) the dynamic dissociative exchange reaction or (3) two mechanisms together exchange reaction. The recycling methods and mechanisms, and research progress of each category of recyclable thermosets are described in this chapter, as well as the applications of the recyclable thermosets are also summarized. Finally, the conclusions and perspectives are highlighted.