No abstract

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 new approach for reprocessing of existing thermoset waste is presented. This work demonstrates that unrecyclable thermoset materials can be reprocessed using the concept of associative dynamic bonding, vitrimers. The developed recycling methodology relies on swelling the thermoset network into a solution of a catalyst, which enables transesterification reactions allowing dynamic bond exchange between ester and hydroxyl groups within the thermoset network. Thermal and mechanical properties for recycled polyurethane and epoxy networks are studied and a strategy to maintain the properties of recycled materials is discussed. The developed methodology promises recycling and even upcycling and reprocessing of previously thought intractable materials. Moreover, processability of vitrimerized thermosets with common thermoplastic manufacturing methods opens up the possibility of tuning recycled networks by adding nanoparticles. This flexibility keeps the application window of recycled thermosets very broad.

This work describes the preparation and characterization of biobased fiber reinforced resins using bacterial cellulose (BC) as the matrix reinforcing phase and diglycidyl ether diphenolate ethyl ester (DGEDP-ethyl) as the biobased epoxy resin. BC mats were prepared by static cultivation of strain Gluconacetobacter xylinus ATCC 700178 in Hestrin−Schramm medium augmented with mannitol in sterile containers. After freeze-drying, the surface of the BC matrix fibers was modified to introduce trimethylsilyl moieties (BC TMS ). Reinforced by BC TMS nanofiber networks were fabricated by impregnation of BC TMS matrixes with the resin mixture followed by hot-pressing and curing. Resulting DGEDP-ethyl/BC TMS composites with 5, 10, 20, and 30%-by-vol BC TMS network loading were formed. The BC network proved effective in reinforcing the epoxy resin matrix. The composite Young's modulus (E T ) increased from 1.22 ± 0.41 GPA for the neat DGEDP-ethyl thermoset to 8.8 ± 0.98 for the composite with 30%by-vol BC TMS . Furthermore, the storage modulus (E′) increased for DGEDP-ethyl/30%BC TMS relative to the neat DGEDPethyl resin below T g (30 °C) by 3-fold (2.27 to 7.7 GPA) and above T g (180 °C) by 100-fold. This work highlights the potential to use prefabricated BC matrixes produced by microbial fermentation along with a biobased epoxy resin to provide highperformance biobased composites.

The curing kinetics of a family of biobased epoxies derived from n-alkyl diphenolate esters differing in ester side chain length were compared with diglycidyl ether of bisphenol A (DGEBA). Isothermal isoconversional analysis and Kamal−Sourour model fitting by differential scanning calorimetry (DSC) were utilized to obtain reaction constants. The biobased epoxides and DGEBA have reaction orders that are comparable while the autocatalytic rate constant of DGEBA was larger than those of the biobased epoxies. As the n-alkyl side chain length of diphenolate esters increased, the autocatalytic rate constant decreased. Furthermore, the non-autocatalytic rate constant for DGEBA is smaller than that of the biobased epoxies. The cause for the difference in rate constants is discussed, and applications are assigned to the epoxies based on curing kinetics.