# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 3 Bio-Based Preservatives and Impregnators
Among the tested tannin molecules, valonia and chestnut tannins resulted in the lowest mass loss after fungi-induced degradation. Despite the encouraging findings for tannin molecules, one of the main limitations for their exploitation is their elevated water solubility, which makes them easily leach out. Therefore, expedients are to be taken into consideration to foresee the future use of tannins as wood preservatives. Tannin copolymer formulations, prepared with hexamine, formaldehyde, furfural, glyoxal, furfuryl alcohol, and maleic anhydride, revealed improved leaching and fire resistance, as well as increased hardness [98]. However, the properties of these new formulations were negatively affected by artificial weathering. Overall, these studies reveal the great potential of tannins as wood preservatives but, at the same time, they highlight the requirement for further research and investigations. Monotergenes are extracted from several plants and plant oils. A systematic study identified the monoterene carvacrol as a possible agent with anti-fungal activity. In fact, this compound was found to be toxic to the wood white-rot fungi _Trametes hirsuta_, _Schizphylhls commune_, and _P. sanguineus_, with low values of IC\({}_{50}\)[99]. Extracts of propolisis were also investigated for anti-fungal characteristics [100]. When used as a wood impregnant, the extracts derived from propolisis could limit the fungal decay induced by the brown-rot fungus _Conophora puteana_. Additionally, phenolic compounds were identified within the propolisis extract, which were able to provide antimicrobial activity. [PERSON] et al. reported that a coconut shell pyrolytic oil distillate (CSPOD) was effective in protecting wood from both brown-rot and white-rot fungi. This anti-fungal activity was mainly attributed to the presence of phenolic compounds in the distillate [101]. Caffeine is an alkaloid that can be easily extracted from plants. This widespread natural compound has been found to improve the resistance of spruce wood against termites and brown-rot fungi. The main drawback is that caffeine is prone to leaching out from wood in the presence of water, making it not perfectly suitable for the treatment of outdoor wood [102]. Since ancient times, essential oils have been widely applied for different purposes. Considering their long history of use, also in the traditions of ancient local civilizations, they represent a valid and promising alternative as wood preservatives. In fact, many essential oils are reported to have anti-fungal properties. For example, essential oils extracted from _Lippiia organoides_ displayed elevated anti-fungal properties, and the effect was attributed to the presence of thymol [103]. Overall, several plant essential oils have been proven to exhibit anti-fungal properties, such as _Origanum vulgare_, _Cymbopogon citratus_, _Thymus vulgaris_, _Pelargonium graveolens_, _Cinnamomum zyelanicum_, and _Eugenia caryophyllata_, and they and/or their main constituents can be employed to impregnate wood surfaces in order to ensure their preservation [104]. Eventually, [PERSON] et al. conducted a comprehensive study to investigate the effect of several natural products on the inhibition of fungi growth on southern yellow pine. The best inhibitory effect was reported for salicylic acid, tea tree oil, and cinnamon bark oil, with highly encouraging outcomes after a test in the mould chamber [105]. Finally, one of the most environmentally sustainable lines of research involves the recovery of active substances from waste products. This offers the dual advantages of having a recyclable raw material, instead of searching for new sources, and reusing a waste product, thus reducing the issues related to waste disposal. Coffee silverskin is the perfect example of an extremely diffused industrial waste derived from the process of coffee roasting. Its potential for wood preservation was first explored by [PERSON] et al.