Highly fibrillated and intrinsically flame-retardant nanofibrillated cellulose for transparent mineral filler-free fire-protective coatings

• After post-boronation, Q-NFC films show excellent flame retardancy and transparency. • Both boron and lignin enhance flame retardancy of Q-NFC in a synergistic manner. • The presence of lignin imparts UV-shielding function to flame-retardant Q-NFC. • Boron-modified Q-NFCs act as fire-extinguishing coatings of papers and PET films. Developing functional nanocellulose-based materials while well maintaining their inherent excellent attributes is crucial to achieve the value-added applications. However, their inherent flammability largely limits their potential applications in many fields with fire-safety concerns. In this work, an intrinsically flame-retardant nanofibrillated cellulose (NFC) was successfully prepared from kraft and unbleached bamboo pulps, respectively, via a three-step strategy involving quaternized pre-treatment, mechanical disintegration and subsequent borylation. TEM images show that the well-defibrillated cellulose nanofibrils with an average diameter of 5–9 nm were successfully obtained. The morphological, physicochemical, mechanical, thermal and flame-retardant characteristics of NFC suspensions, and their resulting films were systematically characterized. The borylated NFC film from the kraft pulp (B/Q-NFC) exhibits remarkably improved char yield while retaining high degree of fibrillation, optical transparency and mechanical properties of the Q-NFC one. Despite relatively lower optical transmittance and tensile strength, the lignin-containing NFC (L-Q-NFC) from unbleached pulp exhibits better thermal stability and outstanding UV-shielding function. Flame-retardant properties of borylated NFC samples were comprehensively investigated in terms of microscale combustion calorimetry (MCC), limiting oxygen index (LOI), vertical burning, and cone calorimetry tests. A synergistic effect was noted between B and lignin species in promoting flame resistance of Q-NFC matrix (L-B/Q-NFC). When being used as flame-retardant coatings on the filter paper and PET film, both of borylated NFC films display excellent flame-retardancy and smoke-suppression effects. In combination with of surface and bulk chemical composition analysis, TGA results as well as oxygen-permeability and thermal diffusivity measurements, the flame-retardant mechanism of borylated NFC films was elucidated. These favorite features enable them to act as sustainable fire-protective coatings for many fields such as thermal insulation, electronics, separators for lithium-ion battery, etc.