# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 2 Bio-Based Coating's Matrix
### Vegetable Oils and Their Derivatives  
Vegetable oils represent a \"green\" and eco-friendly solution to be used as wood coatings and preservatives. In fact, many essential oils, and some of their derivatives, have been known since ancient times for their antimicrobial or antifungal effects [44]. Another advantage of vegetable oils is the fact that they induce a smooth feel and provide the coated surface with a small coefficient of friction [45]. Natural oils can be directly applied to the wood surface, but they may also serve as the starting point for the synthesis of the bio-based coating matrix. Oils can be classified as drying and non-drying. The former become harder when exposed to the atmospheric environment. On the other hand, non-drying oils remain in the liquid state and are, in general, not useful for the production of coatings. Indeed, coatings should be able to provide protection to the wood substrate, remaining adhered and intact, even when the wood object is moved, touched, or simply exposed to aggressive environmental conditions. The hardening process occurs because drying oils possess functional groups that are able to react and be oxidized by atmospheric oxygen [46]. Linseed oil and tung oil represent the most known and explored drying oils in the field of coatings. Linseed oil, alternatively referred to as flaxseed oil or flax oil when used for consumption, is a clear to slightly yellow oil derived from the matured, dried seeds of the flax plant (scientifically known as _Linum__sitiatissimum_). The oil can be acquired through a process involving pressing and, occasionally, solvent extraction. Similarly, tung oil, also known as China wood oil, is a drying oil derived from pressing the seed found within the nut of the tung tree, scientifically known as _Vernicia__fordii_. When exposed to air, tung oil undergoes a process of polymerization, causing it to harden. This results in a transparent finish with a rich, almost glossy appearance, akin to a wet surface. It is primarily employed for the purpose of enhancing and safeguarding wood. Both tung and linseed oils provide hydrophobicity to the wood's surface, but some differences in their performances have been reported [47]. On one hand, tung oil provides a hydrophobic effect that can be appreciated soon after its application; on the other hand, linseed oil requires a longer time to complete film formation. Nevertheless, after that time, a higher contact angle is measured for linseed oil than that for tung oil [47]. In addition to the hydrophobic properties, another important requirement for a wood coating is the ability to curb the colonization of wood-decay fungi. Both tung oil and linseed oil have been found to be protective against brown-rot and white-rot fungi, with better protective performance associated with tung oil [48]. [PERSON] et al. reported that the treatment of wood with tung oil could improve the dimensional stability of the material and, most importantly, reduce moisture absorption [49]. The SEM micrographs in Figure 2 show how the pits visible in the untreated wood (left) were occluded after the application of tung oil (right), thus preventing the absorption of moisture. In addition to the protective effect provided by the oil, additives and/or pigments can be added to increase the performance of the coating. The presence of pigments and additives will certainly have an influence on the degree of wear and decay upon weathering [36]. For example, the addition of 5% bemp-derived biochar (BC) to tung oil resulted in increased hydrophobicity of the coated surface. Although the degree of hydrophobicity did not directly correlate with the amount of introduced BC, a higher BC content was associated with improved protection against the weathering effects [50]. Pigments extracted from wood-decay fungi solubilized in raw linseed oil could extend the service life of the coating [40], while the addition of nanofibrilated cellulose (NFC) has been reported to improve the wear resistance of linseed oil-based coatings [51].