Highly Flame-Retardant\nBiovitrimer Utilizing P–O–C\nDynamic Bonds for Closed-Loop Recyclable Thermally Conductive Pads

Biovitrimer pads\nwith high flame retardancy and high thermal conductivity\nwere developed using tannic acid (TA), phosphoric acid (PA), and poly(vinyl\nalcohol) (PVA). For the first time, it was discovered that this type\nof thermal pad is formed through dynamic bonding of P–O–C\ncovalent bonds. The vitrimer transition temperature (<i>T</i><sub>v</sub>), which indicates dynamic covalent bonding, was controlled\nwithin the 74–91 °C range by adjusting the TA, PA, and\nPVA composition ratio. Below <i>T</i><sub>v</sub>, the vitrimer\nexhibited excellent elastic recovery properties, with a recovery rate\nof over 90% when subjected to 100% strain. The material demonstrated\na tensile strength of 8 MPa and 430% elongation at break. Interestingly,\nsamples deformed at 100 °C (above <i>T</i><sub>v</sub>) and then rapidly cooled showed a shape memory effect, returning\nto their original form when reheated above <i>T</i><sub>v</sub>. Thanks to the phosphoric acid in the vitrimer, these materials\nachieved high limiting oxygen index values exceeding 55% and the highest\nV-0 rating in the UL-94 test, demonstrating excellent flame retardancy\nnot seen in existing vitrimers. Combining biovitrimer and hexagonal\nboron nitride (<i>h</i>-BN) produced a composite film with\na thermal conductivity of up to 3.8 W/m·K, proving its effectiveness\nas a heat dissipation pad. Additionally, the vitrimer’s properties\nallowed for the complete recovery of both <i>h</i>-BN and\nbiovitrimer through recycling. A closed-loop recycling process demonstrated\nthat a composite film with the same heat dissipation performance could\nbe remanufactured using reclaimed biovitrimer and <i>h</i>-BN.