Boron-based fire retardancy for natural polymeric materials

The shift from fossil resources to natural polymers as the building blocks of a global bioeconomy is hampered by the intrinsic flammability of these bio-derived materials. In this paper, the recent advances in boron-based fire retardancy of natural materials are reviewed, highlighting the transition from macro-scale salt impregnation to molecular-level engineering of boron chemistry. Boron compounds act as a dual Lewis acid catalyst for dehydration and subsequent char formation, and as a glassy physical barrier to slow down the release of fuel and the diffusion of oxygen. The boron chemistry in the context of the physical constraints dictated by the natural material is analyzed. In solid wood and bamboo, the challenge is to use in situ mineralization and covalent grafting to overcome water solubility and leaching. In engineered wood composites and bio-based adhesives, boron moves from a passive additive to a structural element in the form of borate ester crosslinks. In flexible textiles, boron forms sol-gel architectures and synergistic combinations with phosphorus and nitrogen to achieve wash durability. Boron plays a crucial role in stabilizing high porosity nanocellulose aerogels and foams. The key challenges are identified to fulfil the potential of boron chemistry as a safe and sustainable approach for high performance natural materials.