# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 4 Bio-Based Fillers
### Green Flame Retardant and Intumescent Systems  
Since the 1970s, flame retardants have been utilized in a wide array of consumer and industrial goods with the aim of diminishing a material's susceptibility to ignition or serving as a barrier (absorbing heat) to combustion. Notably, substantial emphasis has been placed on synthetic fire retardants throughout this time period. A considerable portion of commercially accessible flame retardants comprises organic compounds derived from oil sources (such as organo-halogenated, organo-phosphorous, and organo-nitrogen compounds). However, owing to their reliance on petroleum, these compounds face concurrent challenges, including depleting petroleum resources, geopolitical complexities, and contributions to global warming. Furthermore, certain compounds, particularly those with halogen components, have come under scrutiny owing to potential links to specific health and environmental issues. Thus, during recent times, there has been a notable surge in the scholarly literature focusing on the creation of flame retardants derived from sustainable sources [127; 128]. The prohibition of certain halogenated substances and the pursuit of substitute options have played a pivotal role in motivating the exploration and formulation of these novel bio-based additives. Given this perspective, recent developments have highlighted the effectiveness of vanillin as a promising bio-based option. For instance, [PERSON] et al. [86] successfully synthesized an innovative epoxy reactive flame retardant with phosphate content. This was achieved through a single-step reaction involving vanillin and benzene phosphorous oxydichloride (BPOD). Comprehensive analyses demonstrated marked enhancements in both the thermal stability and flame retardancy of the environmentally friendly coatings. As a result, this study introduced a sustainable and eco-friendly approach to crafting flame-retardant wood coatings, showcasing substantial potential for applications in the realm of wood-related endeavours. Similarly, [PERSON] et al. [129] synthesized a polybasic carboxylic acid (HCPVC) by combining vanillin and hexachlorocyclotriphosphate. This compound was utilized as a curing agent in a wood epoxy coating. Consequently, the resultant coating displayed an elevated char yield alongside notable flame retardant properties. Additionally, the study unveiled that the formation of a compact and swelling char layer effectively curtailed further combustion and pyrolysis within the condensed phase. Furthermore, the outcomes of thermogravimetric analysis coupled with infrared spectrometry (TGA-FTIR) confirmed that the environmentally friendly coating released more inert gases during the heating process, while concurrently impeding extensive decomposition within the gaseous phase. In a recent development, [PERSON] et al. [130] conducted an assessment of the viability of suberin, a bio-polyester found in cork, for the creation of bio-based flame-retardant substances. Meanwhile, [PERSON] et al. [131] accomplished the synthesis of a bio-based co-curing agent that imparted flame-retardant capabilities to both epoxy and wood. This innovation was aimed at constructing functional thermosetting coatings with enhanced flame resistance. Continuing the exploration of environmentally friendly additives, [PERSON] et al. [132] successfully produced a bio-derived flame-retardant curing agent for ammonium hydrogen phytate (AHP). This was achieved through the precise control of the molar ratio between phytic acid and urea in the reaction process. When compared with a control sample of wood coating prepared with a commercial curing agent containing ammonium chloride, the resulting wood coating (referred to as MP) not only showcased significantly enhanced thermal stability and flame retardancy, but also demonstrated comparable characteristics in terms of Tg, hardness, adhesion, and water resistance. Notably, the preparation process for MP was straightforward, amenable to scaling up, and employed an environmentally friendly water-based solvent.