Review of reversible dynamic bonds containing intrinsically flame retardant biomass thermosets

The Covalent Linking of Organophosphorus Heterocycles to Date Palm Wood-Derived Lignin: Hunting for New Materials with Flame-Retardant Potential

Environmentally acceptable and renewably sourced flame retardants are in demand. Recent studies have shown that the incorporation of the biopolymer lignin into a polymer can improve its ability to form a char layer upon heating to a high temperature. Char layer formation is a central component of flame-retardant activity. The covalent modification of lignin is an established technique that is being applied to the development of potential flame retardants. In this study, four novel modified lignins were prepared, and their char-forming abilities were assessed using thermogravimetric analysis. The lignin was obtained from date palm wood using a butanosolv pretreatment. The removal of the majority of the ester groups from this heavily acylated lignin was achieved via alkaline hydrolysis. The subsequent modification of the lignin involved the incorporation of an azide functional group and copper-catalysed azide-alkyne cycloaddition reactions. These reactions enabled novel organophosphorus heterocycles to be linked to the lignin. Our preliminary results suggest that the modified lignins had improved char-forming activity compared to the controls. 31P and HSQC NMR and small-molecule X-ray crystallography were used to analyse the prepared compounds and lignins.

Synthesis of novel vanillin-amine hardeners fully derived from renewable bio feedstocks and their curing with epoxy resins to produce recyclable reprocessable vitrimers

Biobased epoxy vitrimers have reached intense interest in recent decades. The triggerable reverse bonds can be introduced into these crosslinked epoxy vitrimers through epoxy resins or hardeners. This study synthesized two imine hardeners, such as vanillin-butanediamine (V-BDA) and vanillin-hexanediamine (V-HDA), using biobased vanillin, butanediamine, and hexanediamine and their chemical structures were ensured by FTIR, ¹HNMR, ¹³CNMR, and TOF-MS. The two novel hardeners were used to cure epoxy resins, rendering vitrimers with good reprocessability, self-healing, recyclability, and solvent resistance due to the reversible imine bonds. The flexural strengths and modulus of these cured resins were consistent with those of epoxy resins that were hardened with traditional amine-based hardeners. The cured resins maintained 100% of their T_g and flexural properties after being reprocessed up to three times. It was revealed that the cured epoxy vitrimers could be degraded entirely in a particular acidic solution capable of bond-exchanging reactions within 12 h at 50 ᵒC, allowing the thermoset matrix to be chemically recycled and the monomers regenerated. This versatile recyclability, combined with the use of fully biobased feedstocks to prepare the hardeners, provides an attractive approach to help achieve a sustainable circular composite economy.

Metal Coordination in Polyimine Covalent Adaptable Networks for Tunable Material Properties and Enhanced Creep Resistance

Covalent adaptable networks (CANs) can replace classical thermosets, as their unique dynamic covalent bonds enable recyclable crosslinked polymers. Their creep susceptibility, however, hampers their application. Herein, an efficient strategy to enhance creep resistance of CANs via metal coordination to dynamic covalent imines is demonstrated. Crucially, the coordination bonds not only form additional crosslinks, but also affect the imine exchange. This dual effect results in enhanced glass transition temperature (T_g ), elasticmodulus (G') and creep resistance. The robustness of metal coordination is demonstrated by varying metal ion, counter anion, and coordinating imine ligand. All variations in metal or anion significantly enhance the material properties. The T_g and G' of the CANs are correlated to the coordination bond strength, offering a tunable handle by which choice of metal can steer material properties. Additionally, large differences in T_g and G' are observed for materials with different anions, which are mostly linked to the anion size. This serves as a reminder that for coordination chemistry in the bulk, not only the metal ion is to be considered, but also the accompanying anion. Finally, the reinforcing effect of metal coordination is proved insensitive to the metal-ligand ratio, emphasizing the robustness of the applied method.

A Critical Review of Sustainable Vanillin-modified Vitrimers: Synthesis, Challenge and Prospects

Nearly 90% of thermosets are produced from petroleum resources, they have remarkable mechanical characteristics, are chemically durable, and dimensionally stable. However, they can contribute to global warming, depletion of petroleum reserves, and environmental contamination during manufacture, use, and disposal. Using renewable resources to form thermosetting materials is one of the most crucial aspects of addressing the aforementioned issues. Vanillin-based raw materials have been used in the industrial manufacturing of polymer materials because they are simple to modify structurally. Conversely, traditional thermosetting materials as a broad class of high-molecular-weight molecules are challenging to heal, decompose and recover owing to their permanent 3-D crosslinking network. Once the products are damaged, recycling issues could arise, causing resource loss and environmental impact. It could be solved by inserting dynamic covalent adaptable networks (DCANs) into the polymer chains, increasing product longevity, and minimizing waste. It also improves the attractiveness of these products in the prospective field. Moreover, it is essential to underline that increasing product lifespan and reducing waste is equivalent to reducing the expense of consuming resources. The detailed synthesis, reprocessing, thermal, and mechanical characteristics of partly and entirely biomass thermosetting polymers made from vanillin-modified monomers are covered in the current work. Finally, the review highlights the benefits, difficulties, and application of these emerging vanillin-modified vitrimers as a potential replacement for conventional non-recyclable thermosets.