# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 4 Bio-Based Fillers
### Weathering Resistance Bio-Based Fillers  
Certainly, ensuring the ability to withstand weathering is a crucial factor for a wood coating, given that wood is especially vulnerable to the effects of outdoor elements. Bearing this in mind, different varieties of bio-derived additives have recently exhibited impressive performance in enhancing the coating's longevity. As a result, they contribute to prolonging the lifespan of outdoor wood components. For instance, heme-based biocarbon (BC) particles have been incorporated into tung oil to function as UV absorbers [50]. The introduction of these particles led to an enhancement in the water-repellent characteristics of the coating and resulted in improved colour preservation throughout the on-site weathering examination. Likewise, [PERSON] et al. [109] utilized olive leaf extract as a supplement in polyacrylate coatings along with TiO\({}_{2}\) or ZnO nanoparticles and a UV-absorber of the 2-(2-hydroxyphenyl)-benzotriazole (BTZ) type. This combination was investigated for its ability to withstand natural or accelerated weathering. The study emphasized a decrease in the occurrence of wood discolouration due to the inclusion of olive leaf extract, which enhanced resistance against UV radiation. Adopting a distinct strategy, [PERSON] et al. [110] developed a wood coating through the curing of levulinic acid (LLA),Ich served at an innovative bio-derived solvent for the mild solvolysis liquefaction of wood. The resulting cured coatings exhibited robust pull-off adhesion to the beech wood surface, demonstrated water resistance, and displayed minimal colour change upon exposure to UV light. Lignin itself stands as another interesting functional asset. Indeed, [PERSON] et al. [111] extracted lignin from wood waste, utilizing a non-solvent technique to produce lignin nanoparticles (LNPs). Consequently, wooden specimens that were dip-coated with these LNPs exhibited encouraging surface alterations reminiscent of a cohesive film composed of merged LNPs. These treated samples demonstrated notably superior performance in simulated weathering trials compared with untreated control samples. Much like lignin, tannins, which are also derived from wood, offer an opportunity to enhance the protective qualities of coatings. In alignment with this objective, [PERSON] et al. [112] endeavoured to enhance the weathering durability of Scots pine wood by applying water-based wood coatings infused with valex, mimosa, and pine tannins. The investigation demonstrated that, among the tannn varieties, valex and pine tannins stood out as the most effective agents in transparent coatings, offering substantial resistance against surface deterioration. In a similar vein, condensed tannins derived from tree bark have been evaluated as effective  
Figure 3: Illustration of the functionality of bio-based fillers in wood protective systems. additives to impart a protective function to acrylic-based coating resins [113]. Beyond their potent antioxidant capabilities, the UV-absorbing properties of both original and chemically altered tannins have been observed to be influenced by factors such as pH and the extent of esterification or etherification. Native and modified tannins containing maleate or methylcarboxylate groups, while maintaining robust antioxidant activity, exhibited notably enhanced coating durability and performance when compared with the utilization of synthetic photostabilizers. Furthermore, condensed tannins subjected to esterification with a high degree of substitution also outperformed synthetic additives, underscoring the intrinsic UV-absorbing potential of these materials in enhancing the effectiveness of acrylic and styrene-acrylic coating systems. Lastly, carnauba wax has been combined harmoniously with zinc oxide nanoparticles within multi-layer coatings to thwart the deterioration and extend the lifespan of timber [114]. The resultant coating exhibited a distinct blend of superhydrophobicity, exceptional moisture buffering capacity, and partial UV shielding.