# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 2 Bio-Based Coating's Matrix
### Natural Biopolymers for Wood Protection
These results show that, although some substances of natural origin have excellent protective qualities, further studies and improvements may be required in the view of industrial application to enhance their final performance. The concept of flame retardancy is extremely vital, as a coating for wood should first reduce the risk of flammability in case of fire. Therefore, bio-based compounds have also been investigated for their anti-flammable performances. Some bio-based natural substrates are already known to display fire-retardant characteristics. A study conducted on several natural substrates, including starch, chitosan, rice bran, and fish gelatine, reported fish gelatine as a promising fire-proofing agent [78]. A waterborne flame-retardant itaconate-based unsaturated polyester was synthetized with the flame retardants FRC-6, N-(hydroxymethyl) acrylamide (NMA), and \(\gamma\)-methacryloxypropyltrimethoxysilane (MPS) [79]. The coating provided the wood with flame protection owing to the formation of a stable char layer, acting as a barrier. With the aim of generating a bio-based coating able to offer protection against flame propagation, [PERSON] et al. produced an epoxy flame retardant, PPDEG-EP [80]. After mixing with the curing agent diamindiphenylmethane (DDM), the eugenol-derived coating proved to be effective in providing wood with anti-flammability properties. In a different study, researchers investigated the fire protection offered by an epoxy coating from the natural substance luteolin [81], which was mixed with a furan-derived hardener. Compared with the reference control, the new wood coating resulted in a decrease in the peak heat release rate of 55.8%, a reduced total heat release of 12.4%, and a decreased smoke production of 11.5%. Interestingly, the produced new coating was safe and nontoxic to human fibroblast cells, providing additional value for its use for wood protection. Another study proved eugenol to be a potential natural source for wood coatings synthesis [82], supporting future industrial investments and efforts for studies on this natural compound. Indeed, [PERSON] and colleagues generated an epoxy monomer from eugenol, to obtain a final wood coating with high char yields and thermomechanical properties. Tannins are plant-derived polyphenols, largely exploited as natural polymers with active protective properties. The potential of tannins as wood protective agents has been explored, in conjunction with the biopolymer lignin [83]. [PERSON] et al. generated polyphenic resins based on these two natural compounds and compared their protective properties for wood substrates with respect to the best commercially available fireproofing coatings. The novel resins based on tannins and lignin displayed reduced heat release during combustion and delayed propagation of flames, showing the potential of compounds of natural origin for the design of wood coatings with anti-flammable performance. Good flame-retardant performance has also been associated with the natural compound benzoxazine (BZ) [84]. BZ can be mixed with polyurethane to create an interpenetrating network system, with excellent hydrophobicity values and improved resistance both to acids and to basic substances, compared with the reference polyurethane. Most importantly, the investigated network could provide the wood-coated sample with fire protection. A promising biopolymer for the production of \"green\" wood coatings is poly(lactic acid) (PLA), produced from the bacterial fermentation of sugar beet or cornstarch. A polyester methacrylate resin, obtained from PLA and \(\varepsilon\)-caprolactone (CL), was mixed with cellulose nanocrystals (CNCs), and the coating was applied to pine wood [85]. This formulation reduced the rate of water absorption by 65%, and the presence of CNCs was found to further positively contribute to the slow water absorption. Vanillin is a phenolic aldehyde, which is mainly extracted from vanilla bean.