3.1.2. Green Wood Protectors' Active Compounds

Wood is a frequently used natural, renewable, relatively inexpensive and readily available building material used in the construction of structural beams, facilities, structures and wood objects (e.g., furniture and home decors). The use of wood in construction brings several benefits, e.g., wood is resistant to high temperatures, stretching (tensile strength) and electrical currents, it can absorb unwanted sounds (especially desirable in the construction of concert halls) and is highly machinable. Generally, wood can be divided into two types: hardwood (e.g., maple, oak, mahogany, beech and teak) and softwood (e.g., birch, pine and ash). Depending on the type, they differ in physical properties, such as density, strength, moisture content, etc. [107,108]. However, all of the types are exposed to factors such as weather conditions (moisture), fungi and insects, which contribute to its degradation [109].

Biological corrosion of wood causes significant changes in its structure, as well as in its chemical and psychical properties and can lead to complete material deconstruction (wood decay). To prevent wood and wood-based materials from these damages, various chemical wood preservatives are used. However, most of the traditional biocides used for wood protection are often highly toxic (Table 4) and can leach out from the preservative-treated wood, posing a serious risk to the environment, human and animal health [108,110]. For example, conventional synthetic wood preservative—CCA (Copper Chromium Arsenate)—contains arsenic and chromium (VI), which are easily leached from the wood surface and contaminate the surrounding soil. Arsenic is also known to be carcinogenic and, therefore, the use of CCA for wood conservation has been restricted since 2003 by the U.S. Environmental Protection Agency (EPA) [107,111]. To protect the environment and society, new alternative wood preservatives based on non-toxic and biodegradable natural substances should be developed. AP contains huge quantities of unused active substances, especially phenolic compounds, which are known to be potential antifungal and antibacterial agents [66,112].

Some types of wood (e.g., Alaska cedar, redwood) show natural resistance to insects, microorganisms and decay, due to the presence of extractives in hardwood [113–115]. Benzoic and cinnamic acids as well as their phenolic derivatives were found to be one of the extractive components responsible for the natural resistance of wood [116]. These compounds are plant secondary metabolites responsible for plant protection against biotic (insects, bacteria and fungi) and abiotic (drought, cold, heat and UV light) environmental stress [117,118]. Numerous studies have investigated the use of benzoic, cinnamic acids and their phenolic derivatives recovered from plant sources as potential natural and non-toxic wood protection agents (Table 5) [119–124]. The influence of benzoic, salicyli, syringic and vanillic acids on oil palm diseases caused by *Ganoderma boninense* was investigated in the study of Surendran et al. [119]. *G. boninense* is the major pathogen for basal stem rot (BSR) disease. Among all studied compounds, benzoic acid turned out to be the best *G. boninense* inhibitor. During all days of the measurement, benzoic acid at a concentration of 5 mM inhibited the growth of the tested pathogen. On the 120th day, the following

weight loss was observed in the woodblocks treated successively with salicylic (≈34%), syringic (≈40%) and vanillic acids (≈75%) (C = 5 mM). For comparison, the mass loss of the untreated control woodblocks was 71.8% [119]. Sekine et al. [120], investigated the bioactivity of latifolin and its derivatives (Table 4) against termites and white- and brown-rot fungi. The results showed that latifolin exhibited significantly higher antifungal and anti-termite activity than the other tested compounds. For example, the value of inhibition rate of *T. versicolor* for latifolin was 79.1%, while for its derivatives, this was in the range of 13.2% to 21.8% [120]. In the studies of Little et al. [121], three flavonoids (quercetin, morin and catechin) and tannic acid were investigated as potential termite repellers. The results showed that wood treated with 3% tannic acid and 4% catechin caused high termite mortality—75% and 50%, respectively [121]. The anti-termite activity of flavonoids (apigenin, quercetin, biochanin A, genistein and taxifolin) was also reported in other works [122,123]. Efhamisisi et al. [124] impregnated 3-ply beech plywood with a mixture of 20% tannin solution and 1% boric acid (to enhance the crosslinking properties and prevent tannin loss). The results showed that such treatment significantly increased the resistance of panels against termites (*R. flavipes*) and fungal (*T. versicolor*) attack [124].

**Table 4.** Examples of toxic substances used in wood preservatives [110].


\* LD50—the amount of toxic substance (mg) per kg of body weight, which causes the death of 50% of a group of the tested animals.

**Table 5.** Phenolic compounds as natural wood preservatives.

