# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 4 Bio-Based Fillers
### Reinforcing Bio-Based Fillers  
Similar to the necessity for weather endurance, wood coatings must also exhibit favourable mechanical attributes, encompassing traits such as hardness and resistance to abrasion. Once again, an array of bio-derived additives can be harnessed to enhance the capabilities of protective layers, infusing multifaceted functionality into the polymer matrix of the coating. For instance, a recent study aimed to unveil the combined effects of two natural additives on the longevity and protective attributes of a bio-based wood coating [41]. This research highlighted the intriguing synergy between spirulina and wax, which afforded vivid colouring and specific aesthetic qualities to the paint. Additionally, they enhanced the surface's water-repellent characteristics and increased the abrasion resistance of the composite layer. Likewise, consistent with strategies aimed at reinforcing the coating's ability to withstand environmental influences, the primary focus lies on utilizing lignocellulosic material to elevate the mechanical attributes of the coating. For example, [PERSON] et al. presented new strategies for lignin applications in durable particulate coatings [115]. They outlined the benefits of these strategies in comparison with both conventional synthetic and bio-based coatings presently in use. In particular, bio-based surface coatings with multiple protective features have been formulated by utilizing water-dispersible colloidal lignin particles (CLPs) alongside an epoxy compound known as glycerol diglycidyl ether (GDE). By employing GDE/CLP ratios of 0.65 and 0.52 g/g, the resulting cured films of CLP-GDE exhibited remarkable resistance against abrasion and elevated temperatures. The unique spherical morphology and chemical composition of CLPs enabled them to serve as both a hardening agent and a particulate element within the coating. This multifunctional role negated the necessity for an underlying binding polymer matrix. Likewise, a recent investigation showcased the enhanced application of lignin and proposed innovative avenues for crafting versatile waterborne wood coatings [116]. The organosolv lignin derived from biorefineries underwent a transformation into colloidal lignin micro-nanospheres (LMNS), reinforcing and fortifying the polymer matrix of the coating. This intervention led to a notable 55% surge in tensile strength and a 40% increase in elongation. Furthermore, the waterborne coating displayed substantial enhancements in wear resistance, adhesion, and various other attributes upon application to wood surfaces. Nevertheless, cellulose currently stands as the material with the most significant capacity for enhancing the protective qualities of wood coatings. Indeed, both the industrial sector and academia are profoundly focused on exploring the protective attributes offered by cellulose derivatives. On a broader scale, an investigation aimed at unveiling the impact of a substantial concentration of cellulose fibres on the durability and protective characteristics of a bio-based wood coating [37]. The study focused on the advantages and disadvantages associated with the extensive incorporation of cellulose fibres as fillers in wood paint. It cautioned against an overabundance of these fibres, indicating the necessity for a defined threshold to prevent significant alterations in the coating's composition and subsequent weakening of its protective attributes. However, the optimal capabilities of cellulose can be harnessed most effectively on a nanoscale level. A recent research investigation showcased that the integration of nanocellulose derived from blueberry pruning residues, along with titanium dioxide and silica dioxide nanoparticles, into waterborne varnishes yielded advancements in the mechanical strength and durability of wooden surfaces [16]. Notably, enhancements were observed in adhesion and resistance to abrasion, all while maintaining stability in other aspects, such as optical clarity, colour, and coating gloss.