# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 4 Bio-Based Fillers
### Green Flame Retardant and Intumescent Systems
This study presents a sustainable and ecologically mindful approach to developing high-performance, flame-retardant wood coatings, showing substantial promise within the wood and furniture industries. Employing an alternative approach, [PERSON] et al. [133] successfully developed an eco-friendly and highly effective flame-retardant coating utilizing tea saponin (TS) derived from natural sources. Through their findings, the researchers demonstrated that, when integrated with other elements within flame-retardant coatings, TS exerts a direct and noteworthy influence on the microstructure of the char layer, as well as flame retardancy and pyrolysis performance. These effects contribute to TS serving a dual role: functioning as a gas-phase flame-retardant mechanism by acting as a blowing agent, and simultaneously acting as a carbon agent within the condensed-phase flame-retardant mechanism. The significant synergistic effects of natural-based TS, as revealed in this study, establish a solid theoretical foundation for the development of emerging environmentally friendly,bio-based flame-retardant materials. Moreover, this research uncovers a novel avenue for harnessing the potential of TS, thus exploring a fresh pathway for its utilization. In the context of intumescent coatings, [PERSON] et al. [134] developed wood coatings containing varying proportions of industrial fillers, namely titanium dioxide (TiO\({}_{2}\)) and aluminium trihydroxic (Al(OH)\({}_{3}\)), in conjunction with bio-fillers like eggshell and rice husk ash. The inclusion of these bio-based fillers within water-based intumescent formulations led to a significant enhancement in the fire resistance of wooden substrates. For instance, at an incident heat flux of 30 kW/m\({}^{2}\), the effective heat of combustion witnessed a reduction of over 40%. Similarly, the average peak to heat release rate decreased from 193.2 to 150.3 kW/m\({}^{2}\) for the wood sample protected by the formulation containing both industrial and bio-fillers. Additionally, the application of these investigated coatings yielded a notable decrease in the back surface temperature of the wooden specimens. Likewise, in a recent investigation, montmorillonite (MMT) was harnessed as an inorganic synergistic agent to formulate a water-based intumescent flame retardant (IFR) decorative coating designed for plywood [135]. The findings highlight that the IFR coating, which was modified with 7 wt.% MMT, showcased the most robust fire resistance among the samples subjected to testing, exhibiting a fire duration surpassing 20 min. This enhanced fire performance was evident in a substantial reduction of the specific extinction area by 44.12 m\({}^{2}\) kg\({}^{-1}\) as compared with the coating lacking MMT, as determined via cone calorimeter analysis. Ultimately, bio-derived flame retardants emerge as environmentally friendly, cost-effective, and readily accessible substances. Incorporating these bio-based flame retardants into polymeric materials contributes to reducing their flammability characteristics, thereby enhancing the safeguarding of wood substrates. Furthermore, they function as a heat barricade, constraining the spread of fuel or flames, thus providing protection to the composite surface against heat and air exposure. Even in this specific capacity, recent studies have underscored the remarkable efficacy of bio-based additives, which present applications with considerable assurances, to the extent that they could replace outdated systems that are less attuned to environmental preservation. Table 3 provides a brief encapsulation of the noteworthy findings linked to bio-based fillers and additives in wood coatings.