# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 2 Bio-Based Coating's Matrix
### Natural Biopolymers for Wood Protection  
Natural biopolymers, including the most famous chitosan and cellulose, are promising solutions to be used for wood protection. The drying behaviour of three natural biopolymers was investigated in the context of the preservation of archaeological wooden objects, namely chitosan, alginate, and cellulose [72]. The study concluded that chitosan and alginate improved the thermal stability of wood and could thus be considered as potential wood consolidants. However, cellulose nanocrystals did not display good performance in terms of wood protection, being, therefore, less promising in this field of application [72]. Chitosan is a natural biopolymer that can be isolated from the outer shells of shrimps and crustaceans. The potential of chitosan for the development of bio-based wood coatings has been widely explored. [PERSON] et al. used chitosan to limit the leaching of caffeine out from the wood substrate. Indeed, despite having anti-fungal activity, caffeine is extremely prone to leaching in the presence of water, considerably limiting its exploitability, especially considering that outdoor wood objects are frequently exposed to conditions of potentially high humidity. While the wood treated just with caffeine had reduced anti-fungal activity and a mass loss of about 21%, the chitosan-caffeine preparation extended the resistance of wood owing to the limited caffeine leaching provided by the chitosan [73]. A modified chitosan was ionically cross-linked with nitrilotris(methylenphosphonic acid) (NTMP), and the obtained complexed formulation reduced the flammability of wood specimens, decreasing both the total heat release rate and the peak heat release rate [74]. One of the main damaging factors for coated wood surfaces is exposure to UV radiation. [PERSON] and colleagues attempted to deal with this issue by creating layer-by-layer coatings with chitosan and CeO\({}_{2}\) nanoparticles as UV-protective molecules [19]. Owing to the addition of the CeO\({}_{2}\) nanoparticles, the colour variation induced by exposure to UV rays was considerably reduced, especially the yellowing of the wood surface, which is a characteristic outcome when the wood substrate is exposed to solar radiation. Cellulose nanofibril (CNF) is one of the most abundant and common biopolymer nanofibrils, and it possesses good qualities for it to be employed as a bio-carrier for coating formulation. [PERSON] et al. combined CNF with graphitic carbon nitride nanosheets (gCNNS) as a UV absorber to shield the wood material from UV radiation [75]. After 15 days of accelerated weathering, the gCNNS/CNF coating significantly improved the colour stability of the wood substrate, suggesting the excellent performance of this complex formulation as a barrier to UV radiation. As one of the most common biopolymers, several studies have explored the characteristics of lignin for the development of wood coatings. For example, lignin-based polyurethane coatings have been produced, with a high content of lignin. When compared with the uncrosslinked lignin precursor, the new polyurethane coating had higher thermal stability and hydrophobicity, suggesting that this may represent a new direction to generate novel bio-based coatings [76]. [PERSON] et al. produced a bio-based polyurethane starting from lignin and isocyanate, testing the effectiveness of using different proportions between the two components [77]. From one side, the lignin-based coating resulted in a hydrophobic surface, but from the other side, superficial cracks were formed when the coating was formulated with a high lignin concentration. This represents an issue for its applicability to wooden surfaces, as cracks increase the permeability of the coating, and water uptake is known to have a negative impact on wood decay. Therefore, some expedients must be adopted to counter this problem. For example, additives might be introduced with low surface tension to reduce the rate of crack formation.