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

An environmental and economic study on the chemical recycling of plastic composites using an engineering constitutive model

Carbon fiber-reinforced polymer composites are widely used in engineering applications due to their exceptional thermal and mechanical properties. Recently developed chemical recycling techniques offer a sustainable solution for recycling these materials, which use organic solvents to depolymerize the composite matrix under mild conditions. However, existing analyses of the environmental impacts and economic viability of composite recycling often rely on rough estimations for input and output data, which significantly limit their predictive accuracy. This study introduces a novel approach by integrating an engineering constitutive model with lifecycle and cost analyses to provide more precise assessments under varying processing conditions. By comparing landfill disposal, mechanical recycling, pyrolysis, solvolysis, and chemical recycling, the study highlights the distinct environmental and economic characteristics of each method. The findings indicate that solvent-assisted chemical recycling offers superior net environmental savings while maintaining low energy consumption, capital investment, and recycling costs. The adoption of the engineering constitutive model also enables parametric analysis and optimization of processing conditions. It shows that by reducing catalyst content and lowering temperature, chemical recycling can reduce costs by over 58 % and improve the benefit-cost ratio by 24.4 % without compromising environmental benefits. Overall, this study provides valuable recommendations for refining recycling processes with a focus on sustainability, while also supporting more informed decision-making aligned with sustainable development principles.

Probing the Solubility of Imine-Based Covalent Adaptable Networks

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.

3D Printing of continuous fiber composites using two-stage UV curable resin

3D printing allows for moldless fabrication of continuous fiber composites with high design freedom and low manufacturing cost per part, which makes it particularly well-suited for rapid prototyping and composite product development. Compared to thermal-curable resins, UV-curable resins enable the 3D printing of composites with high fiber content and faster manufacturing speeds. However, the printed composites exhibit low mechanical strength and weak interfacial bonding for high-performance engineering applications. In addition, they are typically not reprocessable or repairable; if they could be, it would dramatically benefit the rapid prototyping of composite products with improved durability, reliability, cost savings, and streamlined workflow. In this study, we demonstrate that the recently emerged two-stage UV-curable resin is an ideal material candidate to tackle these grand challenges in 3D printing of thermoset composites with continuous carbon fiber. The resin consists primarily of acrylate monomers and crosslinkers with exchangeable covalent bonds. During the printing process, composite filaments containing up to 30.9% carbon fiber can be rapidly deposited and solidified through UV irradiation. After printing, the printed composites are subjected to post-heating. Their mechanical stiffness, strength, and inter-filament bonding are significantly enhanced due to the bond exchange reactions within the thermoset matrix. Furthermore, the utilization of the two-stage curable resin enables the repair, reshaping, and recycling of 3D printed thermosetting composites. This study represents the first detailed study to explore the benefits of using two-stage UV curable resins for composite printing. The fundamental understanding could potentially be extended to other types of two-stage curable resins with different molecular mechanisms.