# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 2 Bio-Based Coating's Matrix
### Vegetable Oils and Their Derivatives
A more comprehensive discussion of the effects of the use of pigments and additives on the coating resistance and performance is provided in Sections 4 and 5. The synergistic effect of tung oil with natural beeswax has also been investigated. As the coating properties of a surface also depend on the surface exterior, micronized sodium chloride (NaCl) particles were added to impart a surface texture after the dip-coating of wood samples [52]. A superhydrophobic coating was obtained, and the presence of tung oil within the coating mixture was found to enhance the temperature stability compared with the one related to the coatings containing wax only. In addition to directly treating the wood surfaces with oil as it is, another solution is to use oils of natural origin as a starting base for the synthesis of resins and coatings. For example, a bio-based epoxide amine nanocoating was synthesized starting from tung oil. The novel resin coating was found to enhance the density of the wood material, decrease water absorption, and improve the mechanical properties of the wood's surface [53]. Acrylated vegetable oils have been used for the production of wood coatings, either in combination with propoxylated glycerol triacrylate [54] or together with a photomitiator for the induction of the curing process [55]. A few years ago, [PERSON] et al. exploited castor oil as a starting material to synthesize a multifunctional castor oil-based bio-mercaptan, with the addition of an organic phosphorus flame-retardant [56; 57]. This wood coating promoted the degradation of films at lower temperatures, but, at the same time, it could reduce the maximum degradation rate and could delay the process of decomposition as well. In a different study, a transparent castor oil-based coating was successfully produced by combining siloxane oligomer and castor oil through epoxidation [58]. The coating object of the study contained grafted polydimethylsiloxane molecules that were associated with the provided roughness and surface morphology, and responsible for a reduced friction coefficient with oily contaminants. In a different study, epoxidized soybean oil was mixed with castor oil maleic anhydride adduct (COMA), which is \"green\" and environmentally sustainable because it is obtained from renewable materials, and methyl nadic anhydride (MNA), with the aim of generating a wood coating with a high content of renewable materials [15]. Castor oil is much exploited in the context of bio-based coatings, since it  
Figure 2: SEM micrographs of untreated wood (**left**) and of the wood after treatment with tung oil (**right**) [49]. From Industrial Crops and Products, 140, [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], Effects of Tung oil treatment on wood hygroscopicity, dimensional stability and thermostability, 111647, Copyright (2019), with permission from Elsevier. offers varying applications and the possibility to be modified to improve its properties, both in terms of protection and in terms of durability. For example, [PERSON] and [PERSON] started from a castor oil-based fatty amide to synthesize a hyperbranched alkyld resin, which was then used as a starting material for the generation of polyurethane. These bio-based generated coatings displayed better performance and properties than the reference linear alkyld polyurethane [59]. One of the possible solutions to improve the performance of coatings is the addition of additives or nanoparticles, which are able to provide improved properties. Based on this, SiO\({}_{2}\) nanoparticles were added to a coating based on castor oil thiolated oligomers to improve its hydrophobicity, and the control of the wetting behaviour was achieved by varying the loading of SiO\({}_{2}\) nanoparticles. When SiO\({}_{2}\) nanoparticles were added at a concentration of 40 wt.%, the highest water contact angle was reported. In addition to hydrophobicity, the coating also showed self-cleaning properties, adding an additional benefit other than the protection for the wood substrate [60].