Highly Flame-Retardant Biovitrimer Utilizing P–O–C Dynamic Bonds for Closed-Loop Recyclable Thermally Conductive Pads

Biovitrimer pads with high flame retardancy and high thermal conductivity were developed using tannic acid (TA), phosphoric acid (PA), and poly(vinyl alcohol) (PVA). For the first time, it was discovered that this type of thermal pad is formed through dynamic bonding of P–O–C covalent bonds. The vitrimer transition temperature (Tv), which indicates dynamic covalent bonding, was controlled within the 74–91 °C range by adjusting the TA, PA, and PVA composition ratio. Below Tv, the vitrimer exhibited excellent elastic recovery properties, with a recovery rate of over 90% when subjected to 100% strain. The material demonstrated a tensile strength of 8 MPa and 430% elongation at break. Interestingly, samples deformed at 100 °C (above Tv) and then rapidly cooled showed a shape memory effect, returning to their original form when reheated above Tv. Thanks to the phosphoric acid in the vitrimer, these materials achieved high limiting oxygen index values exceeding 55% and the highest V-0 rating in the UL-94 test, demonstrating excellent flame retardancy not seen in existing vitrimers. Combining biovitrimer and hexagonal boron nitride (h-BN) produced a composite film with a thermal conductivity of up to 3.8 W/m·K, proving its effectiveness as a heat dissipation pad. Additionally, the vitrimer's properties allowed for the complete recovery of both h-BN and biovitrimer through recycling. A closed-loop recycling process demonstrated that a composite film with the same heat dissipation performance could be remanufactured using reclaimed biovitrimer and h-BN.