A Highly Hydrophobic Siloxane-Nanolignin Coating for the Protection of Wood

Abstract

Wood, a vital material for both modern and heritage objects, is particularly susceptible to degradation caused by water due to its hydrophilic nature and porous structure. Therefore, developing sustainable strategies to protect wood is of significant importance. This study aims to produce a highly hydrophobic coating for the protection of wood following a straightforward procedure and using materials that are compatible with wood. First, nano/sub-microlignin (NL) is isolated and produced from beech wood through a one-step tailored organosolv process. Next, NL is incorporated into Sivo 121, a water-borne and solvent-free silane system recommended by the manufacturer for protecting wood surfaces. Composite coatings containing various concentrations of NL and Sivo 121 are applied to chestnut (Castanea spp.) and oak (Quercus spp.). The impact of NL concentration on the contact angles of water drops (CAs) and colour changes (ΔE) of the treated wood specimens is investigated. The coating with 4% w/w NL demonstrates enhanced hydrophobicity (CA = 145°) and has a negligible effect on the colour of pristine oak (ΔE < 3). The wetting properties of coated oak are not affected after 100 tape peeling cycles. However, the coating exhibits poorer performance on chestnut, i.e., CA = 135°, which declines after 80 peeling cycles, and ΔE > 5. The drop pH does not have any noticeable effect on CA. The latter remains stable even after prolonged exposure of coated oak and chestnut samples to artificial UV radiation and outdoor environmental conditions. Finally, the composite coating offers good and comparable protection for both wood species in the biological durability soil burial test

1. Introduction

Wood has been a vital resource since antiquity, serving diverse purposes such as generating energy, producing paper, and acting as a construction material for furniture, small objects, and large-scale buildings. As numerous objects, including artefacts of cultural heritage and archaeological relics, are crafted from wood, developing effective strategies for the protection and preservation of wood is of great importance. Wood is particularly vulnerable to degradation effects induced by water due to its hydrophilic character and porous structure. These inherent properties of wood enable water molecules to penetrate its pore system, leading to hygroexpansion, accumulation of pollutants, and spread of microorganisms. Therefore, various methods have been developed in recent years to treat and coat wooden surfaces, aiming to modify their wettability from hydrophilicity to hydrophobicity or even superhydrophobicity, as outlined in several review articles, e.g., [1,2,3,4,5,6,7]. Among the various suggested methods, a highly versatile approach involves embedding nanoparticles (NPs) into polymer coatings. NPs act as fillers, increasing the surface roughness of the composite coating and thereby promoting hydrophobicity [8].

Among the various NPs, silicon dioxide (SiO₂) NPs have been commonly selected and used as additives to silane-based materials [8,9,10,11], perfluoroalkyl methacrylic copolymers [12], polyvinyl alcohol [13], epoxy resins [14,15], and acrylic polymers [16] to produce superhydrophobic composite coatings on wood. In another approach, SiO₂ NPs were introduced in a NaOH etching process, which was applied to polydopamine-coated wood samples [17]. In some of these methods, the use of low surface energy agents, such as octadecyltrichlorosilane (OTS) [13,17] and 1H,1H,2H,2H-perfluorooctyltrimethoxysilane [14,15], was included in the coating’s formulation. In an alternative method, SiO₂ NPs were functionalised with OTS [18] or 1H,1H,2H,2H-perfluoroalkyltriethoxysilanes [19] and deposited on wood which obtained superhydrophobic properties [18,19]. Titanium dioxide (TiO₂) NPs were also used and mixed with various polymers, including, for instance, perfluorooctyltriethoxysilane (PFOTS) [20] and polydimethysiloxane (PDMS) [21], to produce superhydrophobic and highly hydrophobic coatings on wood, respectively. Superhydrophobic wood was obtained by Yang et al. using aluminum oxide (Al₂O₃) NPs, which were embedded in a multilayer composite coating consisting of polydopamine, 3-mercaptopropyltriethoxysilane, and OTS [22]. Aluminum hydroxide (Al(OH₃)) NPs modified with stearic acid were mixed with PDMS, and the coating was deposited on lignocellulose composite, which obtained superhydrophobicity as well as flame retardancy [23]. Duan et al. induced superhydrophobic and antibacterial properties in wood veneer using a coating that was prepared by self-polymerization of dopamine, deposition of copper (Cu) NPs, and fluorosilane [24]. Notably, several methods have been developed to achieve superhydrophobicity on wood without directly using engineered nanoparticles as additives. With this approach, NPs were formed in situ through hydrothermal treatments [25,26,27], hydrolysis [28], chemical plating [29], and reduction [30].

The use of metal-based NPs for treating wood has a significant drawback due to their chemical incompatibility with wood’s natural composition, which includes cellulose, hemicellulose, and lignin. Moreover, inorganic NPs are non-degradable and derived from non-renewable resources. To overcome these challenges, efforts have been directed toward utilizing lignin-based NPs as additives in coating formulations to enhance the hydrophobic properties of wood while maintaining better chemical compatibility, as described next. Li et al. prepared a mixture containing lignin NPs, cellulose nanocrystals (CNCs), and polyvinyl alcohol (PVA), which was sprayed onto wood surfaces and then modified by 1H,1H,2H,2H-perfluorooctyltrichlorosilane to obtain superhydrophobicity [31]. Liu et al. prepared a lignin-based superhydrophobic powder via modifying kraft lignin through 1H,1H,2H,2H-perfluorodecyl-triethoxysilane (PFDTES) substitution reaction and constructed superhydrophobic coatings by depositing the suspended PFDTES–lignin powder on wood and other substrates [32]. A fluorine-free superhydrophobic coating on wood was produced by Ren et al. using lignin nanospheres, methyltrimethoxysilane (MTMS), and hexadecyltrimethoxysilane (HDTMS) as modifiers; CNC as a reinforcer; and PDMS as adhesive material [33]. Lignin/Fe₃O₄ NPs were prepared by Huang et al. who, furthermore, used CNCs and TiO₂ as reinforcers and PDMS as adhesive material to prepare a superhydrophobic coating on wood by a one-step spray/dip process [34]. In a simpler approach, Ma et al. produced lignin micro/nano-spheres, which were mixed with 1H,1H,2H,2H-perfluorooctyltrimethoxysilane and an epoxy resin to fabricate a functional superhydrophobic coating on wood [35]. A similar approach was followed in a later report by Ma et al., who used the same silane and epoxy resin, but the added NPs were rod-like ZnO/lignin nanosphere composites [36]. In another study, phosphorylated lignin was attached to the surfaces of CNC@SiO₂ rods, which were modified with MTMS and HDTMS [37]. PDMS and epoxy resin were used as adhesives to produce the final superhydrophobic coating on wood [37]. Kassaun and Fatehi showed that a silsesquioxane-grafted kraft lignin and aluminum phosphate binder can produce a fluorine-free superhydrophobic coating on wood [38]. Finally, Zhang et al. used enzymolysis lignin as the main material to construct a rough surface structure, which was enhanced with both calcium carbonate (CaCO₃) and chitosan, whereas MTMS and HDTMS were used as the hydrophobic modifiers and PDMS as the adhesive [39].

Lignin’s overall potential has been advanced by tailoring its structure and particle size towards nano-scale sizes [40,41,42]. Due to its compatibility with wood, nanolignin (NL) has recently attracted the interest of heritage conservation science, and it has been tested as a consolidant for waterlogged archaeological wood [43] and as a protective coating for wooden [44] and other cellulosic artefacts [45]. Currently, the most commonly used methods of reducing the size of lignin particles include solvent/antisolvent and acid precipitation, ultrasonication, homogenization, and ball milling [41,42,46,47,48,49]. All of the above methods primarily utilize technical lignins extracted from biomass via various biorefinery processes (i.e., organosolv and enzymatic hydrolysis) or recovered as byproducts in the pulp and paper industry, i.e., kraft and lignosulfonates. These methods are indeed very successful in reducing lignin’s particle size to the nanoscale (<100 nm); however, they are time- and energy-consuming and, in most cases, provide low yields (ca. < 10%). An alternative is the direct (in situ) isolation of nano/sub-micro-sized lignin via a tailored organosolv pretreatment/fractionation of biomass [50]. In this work, the isolation of nano/sub-microlignin (570 nm) from beech wood via a one-step tailored organosolv process was achieved without any additional processing steps.

As previously described, lignin-based materials developed to enhance wood hydrophobicity have yielded significant results, but their production involved complex and/or multi-step processes [31,32,33,34,35,36,37,38,39]. This study aims to produce a coating for the protection of wood following a straightforward procedure and using materials that are compatible with wood. The primary focus is on reducing wood wettability and imparting enhanced hydrophobicity, while other properties of the treatment (e.g., effect on the colour of pristine wood, biological durability, and others) are also investigated. Specifically, organosolv nano/sub-microlignin (NL), derived from beechwood biomass through tailored organosolv pretreatment [50], was incorporated into a solvent-free silane system (Dynasylan Sivo 121) which is recommended for wood protection. The resulting composite coating is deposited and evaluated for the protection of chestnut and oak. These two wood species were selected and included in the study as they are among the most valuable wood species, extensively used in Europe since antiquity [51,52]. Both species are ring-porous hardwood species characterized by good mechanical strength-to-weight ratio, grain design of high aesthetics, and high biological durability [53], even though their sapwood part, as with most wood species’ sapwood, is less durable to microorganism action than heartwood, making the application of protective treatment, before being exposed outdoors, a necessity. Given that Sivo 121 is used for the impregnation and protection of wood surfaces, this research can have strong practical implications.

2. Experiments

2.1. Materials

The tree trunks utilized for the purposes of the current work were harvested from Chalkidiki area (North Greece), and they were selected to fulfil specific criteria, such as to be as straight as possible, sturdy, with a relatively large diameter, and without any apparent defects. The diameters of chestnut (Castanea sativa) and oak (Quercus pubescens) trunks were measured to be 19.1 cm and 23 cm, respectively. After debarking, the tree age and the growth ring width were assessed. The age of the chestnut trunk was approximately 32 years old, while the trunks of oak wood were between 35 and 38 years old, without bearing any apparent defect. The mean value of growth ring width for chestnut and oak wood used in the current work was measured to be 3.8 mm and 3.0 mm, respectively. Mainly sapwood part was included in the tests, as it is considered more susceptible to dimensional instability and biological deterioration. According to ISO3061-1 [54], all wood specimens were placed for approximately 2 weeks in a chamber under stable conditions (20 ± 2 °C, 65 ± 1% relative humidity) until they reached constant weights. The latter was measured in accuracy to the nearest 0.001 g (wet mass). Specimens were then oven dried at 103 ± 2 °C for 48 h, placed in desiccators towards cooling process, avoiding moisture absorption, and were weighed again (dry mass). The mean equilibrium moisture content (EMC) of wood specimens was found to be 8.6 ± 0.13% for chestnut and 9.5 ± 0.21% for oak.

Dynasylan Sivo 121 is a water-borne and solvent-free silane system which is commercially available, and it is provided by the manufacturer (Evonik, Essen, Germany) for the protection of wood surfaces. Sivo was used as received.

Organosolv nano/sub-microlignin (NL) was produced according to the following procedure [50]. Beechwood sawdust (Lignocel HBS 150–500) was mixed with ethanol/H₂O (60:40, v/v) solution in an autoclave batch reactor in the presence of 1.9% wt H₂SO₄ as acid catalyst. The organosolv pretreatment was performed at 175 °C for 60 min. Lignin was precipitated with the addition of tailored amounts of deionized H₂O and was then separated from the liquid with filtration. The recovered NL was thoroughly dried under vacuum prior to use. Lignin’s particle size was measured using a dynamic light scattering (DLS) instrument (Litesizer 500, Anton Parr - GmbH, Graz, Austria).

2.2. Coating Preparation and Characterisation

NL was initially dispersed in 5 mL of acetone in varying amounts. These dispersions were then combined with Sivo 121 to produce mixtures containing NL at concentrations of 0.5%, 1%, 2%, 3%, and 4% w/w. These concentrations were commonly used in the past to incorporate engineered nanoparticles into siloxane materials, enhancing the hydrophobic properties of the resulting composite coating [8,9,16]. The resulting dispersions were agitated for 30 s using a vortex mixer, and subsequently, 0.25 g were applied dropwise onto chestnut and oak specimens (length 5 cm × width 5 cm × height 2 cm). Coated wood specimens were left at room conditions for 24 h for curing. The above procedure was repeated using pure Sivo without NL.

Contact angles (CAs) of water drops (3 μL) were measured in triplicate using the ImageJ (version 1.53k) software. CAs were measured on coated wood specimens. Moreover, solutions of pure Sivo and NL in acetone were spin-coated on smooth glass slides and CAs were measured to find out the intrinsic wetting properties of the two materials. The surface structures of uncoated and coated wood samples were studied using a JSM-6390LV Scanning electron microscope (JEOL, Tokyo, Japan). To investigate the chemical composition of Sivo, SEM was coupled with an Oxford INCA Energy Dispersive X-ray microanalytical system (Oxford, UK). For the preparation of the samples, a JEOL JEE4X carbon evaporator system(JEOL, Tokyo, Japan) was employed. Roughness measurements of pristine woods were carried out using a Mitutoyo Surftest SJ-301 profilometer (Mitutoyo, Kanagawa, Japan). A PCE-CSM 1 spectrophotometer (PCE instruments, Hamble-le-Rice, UK) was used for colourimetry, and the results were evaluated using the L*, a*, b* coordinates of the CIE 1976 scale. The total colour changes ($\Delta \mathrm{E}$) of the coated chestnut and oak samples with respect to the colour of the pristine (uncoated) woods were calculated using the following equation:

(1)$\Delta \mathrm{E}=\sqrt{({\Delta \mathrm{L}}^{\mathrm{*}2}+{\Delta \mathrm{a}}^{\mathrm{*}2}+{\Delta \mathrm{b}}^{\mathrm{*}2}}$

where ${\Delta \mathrm{L}}^{\mathrm{*}}$, ${\Delta \mathrm{a}}^{\mathrm{*}}$, and $\Delta {\mathrm{b}}^{\mathrm{*}}$ are the changes in the lightness, the red–green component, and the yellow–blue component of the CIE 1976 scale, respectively. Measurements were conducted in triplicate, and mean values and standard deviations were calculated.

Several tests were conducted to evaluate the performance and stability of the selected composite coating, composed of Sivo and 4% w/w NL. For comparison, wood samples coated solely with Sivo and, where applicable, uncoated woods were included in the following studies. Unless otherwise specified, all tests were performed in triplicate.

The capillary water absorption test was conducted according to the procedure described in EN 15801 (CEN, 2009) [55]. In particular, coated and uncoated wood samples were placed with the treated surface onto a filter paper pad (Whatman paper, No. 4) and were partially immersed up to about 0.2 cm in distilled water. The lateral faces of the specimens were sealed using a waterproof Teflon tape. Samples were weighted periodically (every 10 min) for 2 h in total.

The biological durability soil burial test was based on the standard of ISO 846:2024 [56] and conducted in typical oak forest soil located at a large oak forested area in the campus of AUTh in Finikas region of Thessaloniki city (North Greece). According to the standard, the level of microorganisms’ action was tested prior to the placement of wood samples in the soil by initially burring in the soil untreated strips of cotton fabric (of known dry weight) and left there for 7 days, in order to evaluate the level of its biodeterioration. The strips retained less than 25% of the fabric’s original tensile strength after the period of 7 d burial; therefore, it was considered that the microorganisms action was acceptable. Furthermore, the visual assessment of the fabric and its recorded small mass loss also suggested that the microorganisms were adequately fresh and active, which was the green light for the burial process of the wood samples to follow. Wood samples were dried in an oven at 75 °C for 24 h, weighed to record the dry initial weight, and were subsequently buried in the soil outdoors at a depth of 10 cm, as shown in Figure 1. The samples remained buried for 15 consecutive days, after which they were retrieved, cleaned with a brush, dried (75 °C for 24 h), and weighed again in order to calculate the mass loss of the samples attributed to microorganisms’ catastrophic action. This process was repeated after 30 and 60 days of burial. For each of the two wood species, 15 samples were buried. Five samples were coated with Sivo, five were treated with the selected composite material (Sivo + 4% w/w NL), and five remained uncoated as controls.

Drops of aqueous solutions, which were prepared using HCl or NaOH and corresponded to a pH range from 1 to 12.5, were placed on coated woods, and CAs were measured. The tape peeling test was conducted using Scotch Tape 600 (3M) in accordance with the ASTM D3359-97 standard (Method A) [57]. The coated surfaces of the wood blocks were subjected to repeated attachment–detachment cycles. The test was concluded either after 100 cycles or when the tested surface obtained hydrophilic properties, i.e., CA < 90°.

To evaluate the stability of the coatings under UV radiation, a custom-built chamber equipped with a UV source (Osram Dulux S Blue, 9 W/78 V, UVA 300–400 nm) was used. The chamber temperature was maintained at 27.7 °C, with a radiation intensity of 1.064 W/m². The distance between the sample surfaces and the UV lamp was set at 32 cm. The coated wood samples were exposed to UV radiation for a total duration of 2 months. Moreover, coated and uncoated wood samples were placed outdoors for about the same duration, from mid-August to mid-October 2024, to test their behaviour under natural environmental conditions, according to the standard of ISO 16053:2022 [58].

3. Results and Discussion

3.1. Material Characterisation

Preliminary studies were conducted to determine key properties of the wood specimens, Sivo 121 and organosolv nano/sub-microlignin NL), which are essential for interpreting the results presented in the subsequent sections. Wood densities were determined by measuring the masses and volumes of chestnut and oak specimens at the moisture contents specified in the Experiments section, i.e., 8.6% for chestnut and 9.5% for oak. The resulting densities were 0.562 ± 0.015 g/cm³ and 0.785 ± 0.02 g/cm³ for chestnut and oak, respectively. The roughnesses of the surfaces of the two wood species were 7.19 ± 2.22 μm for chestnut and 8.04 ± 3.56 μm for oak.

As the wetting properties of a surface are strongly influenced by its chemistry, SEM-EDS was employed in the chemical composition of Sivo. The material was applied to chestnut and oak samples, and the results are included in the Supplementary File (Figures S1 and S2). For comparison, the SEM-EDX results for uncoated chestnut and oak are provided in Figures S3 and S4, respectively. According to the results of Figures S1–S4, silicon (Si) and fluorine (F) were detected in the composition of Sivo. The detection of Si confirms the silane-based structure of Sivo, as described by the manufacturer, whereas the detection of F suggests that the silanes were functionalized with fluorine moieties to lower the surface energy. The contact angle (CA) of water drops on Sivo, which was spin-coated on smooth glass slides, was notably high (CA = 111.3 ± 1.2°), confirming the low surface energy of Sivo.

The size and the inherent hydrophobic/hydrophilic character of fillers, added to organic coatings, are two key properties which affect the wettability of the resulting composite coatings [59,60]. For this reason, dynamic light scattering (DLS) was employed to measure the size of NL and found to be 570 nm, as shown in Figure S5. The CA of NL spin-coated on glass was 88.6 ± 1.3°, highlighting the notable hydrophobic nature of the NL used in this study compared to other lignins [61,62].

3.2. Effects of NL Concentration on Wetting Properties and Colour

Figure 2 shows CA measurements as a function of NL concentration for Sivo-NL composite coatings applied to chestnut and oak. CAs for samples coated with pure Sivo (i.e., without NL) are included in the figure and are discussed first. The CAs for samples coated with pure Sivo are 130° and 135° for treated chestnut and oak, respectively. The higher CA (= 135°) observed for coated oak compared to coated chestnut (= 130°) is attributed to the higher roughness of oak. Previous studies have demonstrated that the surface morphology of a polymer coating mirrors the texture of its substrate, with rough substrates resulting in rougher polymer coatings [63]. The differences in the surface roughnesses of the two wood species can be attributed to their different anatomical characteristics. More specifically, oak wood presents a lower width of annual growth rings compared to chestnut, which makes the whole tissue denser with narrow transition zones between earlywood and latewood. Additionally, the higher roughness of oak wood could be attributed to the higher number of rays being cut tangentially (exclusively multiseriate) than in chestnut (exclusively uniseriate) [64,65].

Attention now turns to the effect of the NL concentration on CA. According to the results of Figure 2, the CAs on both coated woods increase gradually with NL concentration, up to 3% w/w. Further increase in NL to 4% w/w did not have any effect on CA. Overall, for coated chestnut, CA increased from 130° (0 NL) to 136° (4% w/w NL), whereas for coated oak, the corresponding increase was larger, from 135° (0 NL) to 145° (4% w/w NL). Although the threshold for superhydrophobicity (CA = 150°) was not achieved, it is noteworthy that the extreme non-wetting state was closely approached, particularly in the case of oak. Previous studies have demonstrated that the CA increases with the concentration of NPs used as fillers in polymer matrices, as NPs enhance surface roughness [8,9,16,59,60,63]. A similar effect was observed with the addition of NL to Sivo, as evidenced by the SEM images in Figure 3a,b. The surface of the composite coating (Figure 3b) is noticeably rougher than that of pure Sivo (Figure 3a). However, it is reported that extreme water repellency was not observed. As shown in the photograph of Figure 3c, a water drop remained pinned on the tilted surface of the chestnut, which was coated with a composite coating of Sivo and 4% w/w NL. This behaviour resembles the rose-petal effect, which is characterized by a high CA, indicating enhanced hydrophobicity and pinned drops [66]. Video S1 shows pinned drops on oak coated with the same composite coating, i.e., Sivo + 4% w/w NL. Furthermore, the video shows that the drops roll off the coating surface when the coated wood sample is agitated. This indicates that some energy is required to initiate the roll-off motion of the drops, suggesting that the composite coating exhibits a certain degree of water repellency.

To improve the water-repellent properties of the composite coating, a post-treatment step can be introduced using a low-surface-energy agent. In Video S2, the standard lotus effect is demonstrated on the surface of a composite coating (Sivo + 4% w/w NL) applied to oak and subsequently treated with 1H,1H,2H,2H-perfluorooctyltriethoxysilane. As illustrated in Video S2, a water drop rolls effortlessly off the surface. This demonstrates that the method developed here can be easily adapted by adding an additional post-treatment step to create lotus-like coatings on wood. However, the aim of this study is to explore the simple and one-step method involving the deposition of Sivo-NL dispersions on wood without any post-treatment modification.

Based on the wetting properties discussed so far, either the NL concentration of 3% w/w or 4% w/w could be selected for further consideration, as they provide the highest degree of hydrophobicity (Figure 2). However, the colour change induced by a conservation treatment is a critical consideration. Therefore, the effect of NL concentration on the colour of chestnut and oak was assessed before finalising the NL concentration to be used. The results of the colourimetric measurements are summarised in Figure 4 and show that the colour changes induced to oak specimens by all treatments were $\Delta \mathrm{E}$ < 3, and therefore, they were imperceptible to the naked eye. However, for the chestnut specimens, $\Delta \mathrm{E}$ ranged between 4.50 and 6.00. Notably, the acceptable threshold for conservation purposes of cultural heritage objects is $\Delta \mathrm{E}<5$ [67], which falls within the range of the reported $\Delta \mathrm{E}$. Consequently, the $\Delta \mathrm{E}$ results raise concerns about the suitability of the treatment for chestnut heritage objects. It is stressed, however, that NL had virtually no impact on the measured $\Delta \mathrm{E}$. The colour changes induced by the composite coatings are comparable with those induced by pure Sivo, a product recommended by the supplier for wood protection. The colour coordinates, ${\Delta \mathrm{L}}^{\mathrm{*}}$, ${\Delta \mathrm{a}}^{\mathrm{*}}$, and $\Delta {\mathrm{b}}^{\mathrm{*}}$, which were used to calculate $\Delta \mathrm{E}$ are provided in Table S1 of the Supplementary File. For the chestnut specimens, the addition of NL to the coating formulation increased the ${\mathrm{b}}^{\mathrm{*}}$ component, although the concentration of NL did not influence this effect. However, the addition of NL to the coating formulation did not have any noticeable effect on $\Delta \mathrm{E}$ when the coating was applied on oak.

Based on the wetting properties of the coatings and the colour changes they induced in the pristine woods, the composite coatings comprising Sivo and either 3% w/w NL or 4% w/w NL could be selected for further testing. These two coatings exhibited the highest hydrophobicity while showing no significant inferiority to the other coatings in the colourimetric measurements. Selecting the 4% w/w NL concentration is safer to ensure maximum hydrophobicity because NL concentrations slightly lower than 3% w/w (e.g., 2% w/w) result in lower CA (Figure 2). Therefore, the composite coating comprising Sivo and 4% w/w NL is selected for the following studies. For comparison, wood samples coated with pure Sivo (without NL) and, where applicable, uncoated wood samples are also included.

3.3. Water Absorption by Capillarity

The curves of water absorption by capillarity are shown in Figure 5 for uncoated wood and wood samples coated with Sivo and the selected composite, comprising Sivo and 4% w/w NL. The results show that uncoated oak absorbs nearly twice as much water per unit area as uncoated chestnut. This could be attributed to the slightly higher hemicellulose content of oak (compared to chestnut), which constitutes a highly amorphous and hygroscopic component bearing a high number of hydroxyl groups compared to cellulose or lignin [68]. Notably, as described in the Experimental, EMC results of the raw, uncoated wood materials also revealed a slight tendency of oak to absorb and retain higher moisture content compared to chestnut wood (8.6% for chestnut and 9.5% for oak).

The results in Figure 5 for the coated wood samples indicate that adding NL to the protective layer had a positive, though not substantial, impact on reducing capillary water absorption. For both chestnut (Figure 5a) and oak (Figure 5b), the curves for samples coated with Sivo and the selected NL-based composite are very similar. However, the composite coating provided slightly better protection overall compared to Sivo alone. Interestingly, the coated oak and chestnut absorbed nearly the same amount of water. This suggests that the protective coatings had a more pronounced effect on oak than on chestnut, as uncoated oak was significantly more prone to water absorption than uncoated chestnut.

3.4. Durability

The results of the biological durability soil burial test are provided in Figure 6. Focusing on the uncoated woods, it is seen that chestnut (Figure 6a) is more susceptible to degradation than oak (Figure 6b), as expected, and could be mainly attributed to the anatomical characteristics of chestnut, which is characterized by lower density than oak and therefore, its higher porosity facilitates the microorganisms penetration and movement inside the wood mass. Density could be roughly considered as an index of biological durability, as well as mechanical durability of wood [64]. Additionally, the higher extractive content of oak wood, which is largely toxic to microorganic substances (terpenes, resin acids, phenolic substances, alkaloids), retards microorganisms’ action and acts as a protective factor to the substrate [68]. According to the results of Figure 6a,b, the application of either Sivo or composite coating reduced the relative mass loss <1%. Consistent with the findings for capillary water absorption, the composite coating provided slightly better protection against soil-induced degradation compared to Sivo, though the improvement was not significant. Finally, the two photographs in Figure 6c,d reveal the optical effect of the soil burial test.

The chemical and mechanical stabilities of the selected composite coating were evaluated, with the results presented in Figure 7 and Figure 8, respectively. For comparison, wood samples coated with pure Sivo were also included in the studies. As shown in Figure 7, drops with a wide range of pH values were applied to the Sivo and composite coatings on woods, and the corresponding CAs were measured. The results indicate that the CAs remained stable and were practically unaffected by the pH of the drops, regardless of whether the coatings were applied to chestnut or oak. The results of the tape peeling test, shown in Figure 8, demonstrate that the enhanced hydrophobicity of the selected composite coating on oak remained unaffected for up to 100 attachment–detachment cycles. However, the CA of the composite coating on chestnut began to decline after 80 cycles and fell into the hydrophilic range (<90°) after 100 cycles. The Sivo coating appeared to be more susceptible to the tape peeling test. The CAs of Sivo on oak and chestnut began to decrease after 80 and 70 attachment–detachment cycles, respectively. Moreover, the Sivo coating on chestnut became hydrophilic after 80 cycles. Overall, the results from Figure 7 and Figure 8 demonstrate the effective chemical and mechanical durability of the composite coating, consisting of Sivo with 4% w/w NL.

Two additional tests were performed to assess the stability of the coatings. Figure 9 shows the variations in CAs of coated wood samples over time under two conditions: outdoor exposure (Figure 9a,b) and UV light exposure in an artificially accelerated ageing chamber (Figure 9c,d). The environmental conditions during the outdoor exposure are described in Table S2. The results of Figure 9 demonstrate the stability of the composite and the pure Sivo coatings against environmental conditions and UV radiation. The wettabilities of both coatings remained unaffected after 3 months of outdoor exposure (Figure 9a,b) and 2 months of accelerated ageing in the UV chamber (Figure 9c,d). The message delivered from Figure 9 is that the selected composite coating maintained its enhanced hydrophobic properties throughout the entire exposure period of the tests.

4. Conclusions

A highly hydrophobic coating with chemical, biological, and mechanical durability was developed for wood protection, using environmentally friendly materials that are either compatible with or recommended for wood preservation. First, nano/sub-microlignin (NL), with a mean size of 570 nm, was isolated and produced successfully from beech wood via a one-step tailored organosolv process. Contact angles of water drops (CAs) on chestnut and oak specimens coated with composite coatings, consisting of Sivo 121 (recommended for wood protection) and NL, increased with the NL concentration. Enhanced hydrophobicity was achieved on coated oak, corresponding to a maximum CA = 145°, whereas the maximum CA reported for treated chestnut was 135°. These results were obtained for the composite coating, which was prepared using 4% w/w NL. Notably, the surface of the coating exhibited the rose-petal effect, whereas enhanced water repellency could be achieved only after a post-treatment step with a low surface energy agent.

The colour changes induced in both wood species by the deposition of pure Sivo and Sivo-NL composite coatings with various NL concentrations were comparable, indicating that the presence of NL had no significant impact on the colour change. The total colour changes ($\Delta \mathrm{E}$) of oak and chestnut treated samples ranged between 2.5 and 3.0 as well as 4.5 and 6.0, respectively.

Based on the above results, a concentration of 4% w/w NL was selected for further investigations. The Sivo + 4% w/w NL coating demonstrated better protection for oak than chestnut in both the capillary water absorption test and the tape peeling test. The composite coating provided good and comparable protection for both wood species in the biological durability soil burial test. Finally, the coating showed good chemical stability, as CA was not affected by the drop in pH and durability against UV radiation and outside environmental conditions.

Author Contributions

Conceptualization, I.K. and K.S.T.; methodology, I.K., K.S.T. and V.K.; validation, P.N.M. and V.K.; investigation, M.M.M.R., C.P.P., E.P., V.T. and M.P.; resources, P.K.S.; data curation, M.M.M.R., V.K. and I.K.; writing—original draft, I.K., V.K., M.M.M.R. and C.P.P.; writing—review and editing, I.K., V.K. and K.S.T.; supervision, I.K. and K.S.T. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures

Figure 1. Photographs illustrating the procedure to conduct the biological durability soil burial test. The photographs show (a) the samples placed at a depth of 10 cm and (b) the burial site after covering the samples with soil.

Figure 2. Contact angle of water drops (CA) on coated chestnut and oak vs. the NL concentration.

Figure 3. SEM images showing the surface structures of chestnut coated with (a) pure Sivo and (b) Sivo + 4% w/w NL. (c) Pinned drop on chestnut tilted to a perpendicular position and coated with Sivo + 4% w/w NL.

Figure 4. Colour change ([Forumla omitted. See PDF.]) in the coated wood samples vs. the NL concentration. The colours of the wood samples changed due to the application of the coatings.

Figure 5. Results of the test of water absorption by capillarity: amount of water absorbed per unit area vs. treatment time for uncoated woods, woods coated with Sivo, and woods coated with the selected composite (Sivo + 4.0% w/w NL). Results for (a) chestnut and (b) oak samples are shown.

Figure 6. (a,b) Results of the biological durability soil burial test: mass loss (%) vs. the time wood samples remained buried in the soil. Results for uncoated woods, woods coated with Sivo, and woods coated with the selected composite (Sivo + 4.0% w/w NL) are shown. The two photographs show oak specimens (c) before and (d) after the test.

Figure 7. CA vs. the pH of drops on woods coated with Sivo and woods coated with the selected composite (Sivo + 4.0% w/w NL). Results for (a) chestnut and (b) oak samples are shown. Photographs of drops on chestnut coated with the composite material are included in (a).

Figure 8. Results of the tape peeling test: CA vs. peeling cycles on woods coated with Sivo and woods coated with the selected composite (Sivo + 4.0% w/w NL). Results for (a) chestnut and (b) oak samples are shown. The figure includes photographs of water drops on wood surfaces coated with the composite material, captured before testing (0 cycles) and after 100 peeling cycles.

Figure 9. CA vs. exposure time for wood samples kept (a,b) outdoors and (c,d) within the UV chamber. Results for wood samples coated with Sivo and wood samples coated with the selected composite (Sivo + 4.0% w/w NL) are shown.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings15030293/s1, Figure S1: SEM-EDX mapping results for chestnut sample which was coated with Sivo; Figure S2: SEM-EDX mapping results for oak sample which was coated with Sivo; Figure S3: SEM-EDX mapping results for uncoated chestnut sample; Figure S4: SEM-EDX mapping results for uncoated oak sample; Figure S5: Particle size distribution of organosolv nanolignin (NL), as measured by DLS; Table S1: Mean values of ${\Delta \mathrm{L}}^{\mathrm{*}}$, ${\Delta \mathrm{a}}^{\mathrm{*}}$ and $\Delta {\mathrm{b}}^{\mathrm{*}}$ which were used to calculate the colour changes ($\Delta \mathrm{E}$) as reported in Figure 4; Table S2: Environmental conditions measured by the Meteorological Station of Finikas, Thessaloniki for August-October, 2024; Video S1: pinned drops on oak coated with Sivo + 4% w/w NL; Video S2: a water drop rolls off an oak surface coated with Sivo + 4% w/w NL, which was post-treated with 1H,1H,2H,2H-perfluorooctyltriethoxysilane.

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