19 / 2023-06-14 22:58:08

Enabling reprocessability and flame-retardancy of epoxy thermosets via reactive phosphonated moieties and their application in FRPCs

phosphonate,covalent adaptable network,repairable and reprocessable polymer,fiber reinforced polymer composites,fire safe coating,flame-retardant mechanism,Phosphonated epoxy thermosets,Phosphite,polyphosphite

Phosphorus Material Chemistry and Applied Science > 1) Phosphorus chemistry in material science

The increasing amount of fossil-based plastic waste ending up in our environment is one of the most pressing issues of the oncoming decades. Thus, development of re-usable polymers with a prolonged useful lifetime heralds the switch for a transition towards a more circular economy. Various recyclable materials have been synthesized via incorporation of wide variety of covalent exchangeable bond. Some of these materials which contain sufficient carboxylic ester, disulfide, siloxane, imine, diketoenamine, Diels−Alder adduct, dioxaborolane bonds etc., can "flow" again like thermoplastics through network topological rearrangement by thermally triggered catalytical bond exchanges. Such network structures are fixed at product operating temperatures when the exchange reaction kinetics are frozen. Among the various dynamic covalent bonds, phosphorous ester based transesterification reaction may provide a multifaceted solution. Phosphorous ester can also offer excellent fire protection, and broad industrial relevance due to ready availability of monomers and straightforward synthesis procedure. Phosphate triester based thermosets brought new inspirations into multifunctional vitrimer material. If we could replace the phosphate with phosphonate, the P-C bond will theoretically bring even better material stability and flame retardancy, as it is a chemically and thermally stable analog of a P–O bond. 

A novel dynamic phosphonate ester bonds rich thermoset has been designed and synthesized in our group via a simple one-pot, two-step synthetic pathway, which demonstrated exceptional reparability, recyclability and flame retardancy. The chemical structures and compositions of the bisphosphonate precursor and final thermosets are confirmed by 1H, 13C, and 31P NMR spectroscopy, elemental analysis, and Fourier transform infrared spectroscopy. The covalent incorporation of phosphonate moieties in the thermoset matrix introduced sufficient dynamic P-O ester bonds, and promoted the exchange of network strands under moderate heating condition, resulting in scratch reparability and recyclability. Due to the presence of phosphorus in the structure, the thermoset also exhibited excellent flame retardancy in varied fire tests.