http://manuscript.elsevier.com/S0079670022000132/pdf/S0079670022000132.pdf

Biobased vitrimers: towards sustainable and adaptable performing polymer materials

Turning thermosets into fully sustainable materials requires utilization of biobased raw materials and design for easy recyclability. Here, dynamic covalent chemistry for fabrication of covalent adaptable networks (CANs) could be an enabling tool. CAN thermosets ideally combine the positive material properties of thermosets with thermal recyclability of linear thermoplastics. Among the dynamic covalent bonds, imine bond, also called Schiff base, can participate in both dissociative and associative pathways. This induces potential for chemical recyclability, thermal reprocessability and self-healing. This review presents an overview of the current research front of biobased thermosets fabricated by Schiff base chemistry. The discussed materials are categorized on the basis of the employed biobased components. The chemical approaches for the synthesis and curing of the resins, as well as the resulting properties and recyclability of the obtained thermosets are described and discussed. Finally, challenges and future perspectives are briefly summarized. This article is protected by copyright. All rights reserved.

Designed from Biobased for Recycling: Imine-Based Covalent Adaptable Networks.

Nowadays, Fiber Reinforced Polymer composites (FRPCs) are extensively utilized due to their remarkable properties such as high stiffness, excellent strength to weight ratio, resistance to wear, corrosion etc. Earlier, the FRPCs are prepared through synthetic fibers in order to attain high strength in conjunction with high elastic modulus. However, with the increasing economic and environmental factors regarding the accumulation of plastic waste, the development of natural and hybrid (combination of any two) fibers were started. The mechanical properties of FRPCs are largely determined by the way loads are transferred between the matrix and fibers, or by the strength of the bond between the fiber-matrix interfaces. Additionally, these factors play a significant role in determining the overall performance of FRPCs. Therefore, this review discusses the recent advancements in enhancing the interaction between fiber and matrix by means of chemical treatment and the inclusion of nanoparticles. The resulting mechanical performance of the end composites and their intended applications are also presented. Few targeted application areas of FRPCs such as aerospace, automobile, mechanical and biomedical implants were discussed in detail. 

Critical Review on Advancements on the Fiber-Reinforced Composites: Role of Fiber/Matrix Modification on the Performance of the Fibrous Composites

Epoxy resins possess poor flame retardancy and have difficulty in both recycling and reprocess ability which largely limit their application. Herein, two vanillin-based epoxy monomers, shorted as VAD-EP and VDP-EP, containing both imine bonds and inherently fire retarding phosphorus elements, were synthesized via in situ condensation reaction with DDM and addition reaction with DOPO, followed by the epoxidation by epichlorohydrin. Epoxy vitrimer materials were then cured by D230 diamine hardener by adjusting the proportion of VAD-EP and VDP-EP monomer. When the mass ratio of VAD-EP and VDP-EP was 8:2, A8P2-D230 reached the fire retarding UL-94 V0 rating and LOI value of 27.0% with only 0.66% of phosphorus content, this behavior was ascribed to condensed phase and free radical scavenging mechanisms. When the mass ratio was 7:3, the tensile strength and elastic modulus of the A7P3-D230 epoxy were 75.5 MPa and 2.5 GPa, respectively, i.e. 48.6% and 18.8% higher than A10P0-D230 sample. Meanwhile, the material showed high stress relaxation rate, due to the presence of dynamic imine bonds in the topological crosslinking network. As a consequence, the epoxy vitrimer exhibited excellent self-healing capability, reprocessability and degradation behaviors. The biobased epoxy vitrimer was then used to prepare the carbon fibers (CFs) reinforced epoxy composites, results showed that CFs can be completely recycled. Interestingly, the recycled CFs maintain unchanged chemical structure, mechanical properties and morphology as the original CFs. 

/pdf/design-of-inherent-fire-retarding-and-degradable-bio-based-3zk13mw7.pdf

Design of inherent fire retarding and degradable bio-based epoxy vitrimer with excellent self-healing and mechanical reprocessability

Researchers have attempted to use Schiff base vitrimer as a matrix resin for preparation of intrinsically recyclable CFRP materials. Because most of the reported Schiff base vitrimer resin systems have high viscosity or are in solid state, solvent is often used during product fabrication to reduce the viscosity and slow down reactions during handling. In this work, a low viscosity Schiff base vitrimer resin is prepared from commercially available diamine, dialdehyde, and bisphenol A epoxy in a solvent-free one-pot synthesis. The obtained crosslinked polymer presents a tensile strength up to 59.6 MPa and an elongation at break up to 7.1%. Moreover, compared with those highly acid-sensitive Schiff base vitrimers in the literature, the vitrimer in this work has excellent chemical resistance toward various solvents. Due to the imine bond exchange reaction, the material exhibits fast stress relaxation at elevated temperatures (>150 °C). By the same thermally induced imine bond exchange reaction, the CFRP prepared from this vitrimer matrix shows unique welding, shape changing, and chemical recycling properties. The CFRP strips can be thermally transformed to a new permanent shape at 180 °C under force, while the welded CFPR strips present a lap shear strength of 8.33 MPa and elongation at break of 2.99%. Hydrolysis of imine linkages is achieved in mild acidic solutions (0.2 M HCl), and the recovered carbon fibers show little damage and are reused to prepare new CFRP. The findings from the study will help to bring Schiff base vitrimer resins from research stage to practical composite applications. This work has demonstrated a highly engineerable Schiff base vitrimer resin and their application in fiber composites with shape adaptability and recyclability. • Low viscosity Schiff base vitrimer resins were developed from commercially available feedstocks in solvent-free conditions. • The mechanical properties of the Schiff base vitrimers can be manipulated with different ratios of aldehyde and epoxy in the system. • The Schiff base vitrimers are stable in organic solvents and acidic solutions but degradable in acidic solution at high temperature. • Carbon fiber reinforced composites were successfully fabricated with remarkable welding, shape changing, and chemical recycling properties. 

Highly engineerable Schiff base polymer matrix with facile fiber composite manufacturability and hydrothermal recyclability

Traditional epoxy resin materials are widely used in coatings, composite materials, electronic packaging materials, etc. They are usually made of unsustainable fossil resources and cannot be recycled under mild conditions. Degradable thermosetting resins with dynamic covalent structure provide a potential solution to this conflict. In this paper, using biomass energy vanillin, m-xylylenediamine and 1, 6-hexanediamine as raw materials, two dynamic imine bond curing agents were synthesized, and then cured with DGEBA to prepare two bio-based imine epoxy vitrimers. The thermal and mechanical properties of two imine epoxy vitrimers were studied and compared in detail. Results showed that the two types of polymers exhibit excellent thermal stability and solvent resistance. At the same time, the tensile strength, modulus and elongation at break were comparable to or even better than those of conventional bisphenol A epoxy resin. In addition, due to the hydrolysis of the dynamic imine bonds, vitrimers had degradable characteristic, and its degradation also exhibited temperature, solvent and acidity dependence. More importantly, the recyclable carbon fiber reinforced polymer composites made of these two vitrimers could be completely degraded under weak acid conditions, and the nondestructive recycling of carbon fiber composites could be realized. We envision that this vitrimers with simple process, excellent comprehensive properties and degradability will make it a potential candidate for applications in sustainable structural materials.

Degradable bio-based epoxy vitrimers based on imine chemistry and their application in recyclable carbon fiber composites

Self-Healing, Reprocessable, Degradable, Thermadapt Shape Memory Multifunctional Polymers Based on Dynamic Imine Bonds and Their Application in Nondestructively Recyclable Carbon Fiber Composites

Epoxy resins are widely used for electronic packaging materials due to their good performance. A common way to cure epoxy is to add anhydride as a hardener, to this, we added a less used curing method, catalytic curing, to study their effect on the curing system and curd samples. Polymers containing naphthalene rings usually have low moisture absorption, good thermal stability, dimensional stability, and dielectric properties. Here, we used two naphthalene epoxy monomers (1,6- naphthalene diglycidyl ether (1,6-NDE) and 2,7-NDE) and cured them with two methods (anhydride (methyl nadic anhydride, MNA) / accelerator (Tris(dimethylaminomethyl)phenol, DMP-30) curing system and catalyst (DMP-30) curing system). We obtained four types of epoxy resins and compared the differences in their structure and performance. All epoxy resins had excellent tensile properties at around 100 MPa tensile strength. Catalyst cured resins had better thermal stability and higher glass transition temperature that close to 180 ˚C (T g -DMA), compared with anhydride cured resins. Meanwhile, 1,6-naphthalene epoxy resins had better performance than 2,7-naphthalene epoxy resins in high glass transition temperature and high crosslink density. Overall, all naphthalene epoxy resins had good performances, and catalyst-cured resins performed better and provided an additional method. Herein, these types of naphthalene epoxy resins have potential applications in the field of electronic packaging. • Molecular arrangement inferred from mode layer spacing and crosslink density. • Spatial resistance affects the cross-linking density and the performance of the resins. • Better thermal stability and solvent resistance of catalyst cured resins. 

High-performance naphthalene epoxy resins cured by catalyst for packaging materials

In this study, an aerospace thermosetting composite was co-curing joined by Polyether-ether-ketone (PEEK) and Polyethylenimine (PEI) films, with an aim of developing advanced composite joints. The semi-crystalline PEEK films were surface activated upon a UV-irradiation technique to obtain a strong film–composite interface, while the amorphous PEI films could be directly used. The fracture behaviour of the composite joints was evaluated and compared with benchmark aerospace adhesive joints. The experimental results proved remarkable mode-I and mode-II fracture resistance of the PEEK co-cured joints at 22 °C and 130 °C, while the PEI co-cured joints exhibited excellent mode-I fracture resistance at 22 °C and mode-II fracture resistance in both testing temperature cases. Extensive elongation, tearing and fracture of the PEEK/PEI plastics were proved to be the main mechanisms for toughness enhancement. Overall, this work had successfully demonstrated the effectiveness of developing advanced composite joints via a co-curing process using high-performance thermoplastic films. 

On the application of strong thermoplastic–thermoset interactions for developing advanced aerospace-composite joints

Given the fossil fuel energy crisis and serious environmental pollution in recent years, one way to alleviate the problem is to introduce dynamic covalent chemistry into thermosetting resins to give the material processable and recyclable properties, creating a new type of eco-friendly material. Hindered urea bonds can undergo reversible exchange reactions at high temperatures to decompose into amine and isocyanate. This reversible addition reaction is currently a prominent research focus in the development of reprocessable thermosets. In this work, we synthesize a novel curing agent with hindered urea bonds without any catalysts. Then characterize the resulting epoxy vitrimers based on hindered urea bond (HUG-EP) using FT-IR and 1H NMR, confirming the successful introduction of hindered urea bonds into the epoxy resin. A variety of tests are carried out, such as those to assess thermal stability, swelling, shape-memory, re-processability and tensile strength. The results demonstrate that HUG-EP can rapidly self-healed within 20 min at 160 °C, with the tensile strength was maintained at 87.15 % of the original during the welding experiments. Furthermore, it can also be reprocessed by hot-pressing several times at 160 °C. Thus, the introduction of hindered urea bonds into the crosslinked network proves to be an effective and practicable approach for producing thermosetting resins with reprocessing, self-healing, and shape memory capabilities. 

Reprocessed, shape-memory and self-healing robust epoxy resin by hindered urea bond

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Degradable bio-based epoxy vitrimers based on imine chemistry and their application in recyclable carbon fiber composites

Self-Healing, Reprocessable, Degradable, Thermadapt Shape Memory Multifunctional Polymers Based on Dynamic Imine Bonds and Their Application in Nondestructively Recyclable Carbon Fiber Composites

High-performance naphthalene epoxy resins cured by catalyst for packaging materials

On the application of strong thermoplastic–thermoset interactions for developing advanced aerospace-composite joints

Reprocessed, shape-memory and self-healing robust epoxy resin by hindered urea bond