Wood Consolidant Solution Based on Decorated MWCNTs Tested on Real Wood Samples from Banloc Castle

Abstract

Historical buildings are highly prone to degradation because they are continuously exposed to the external environment, which represents an extremely aggressive factor. Globally, there are so many historical buildings that need urgent restoration. This paper focuses on finding a new consolidant for real oak old wood and presents a new recipe based on multi-walled carbon nanotubes (MWCNTs) decorated with zinc oxide (ZnO) nanoparticles dispersed in PHBHV solution, aimed at improving old wood properties. The research was conducted on Banloc Castle oak wood, which is predominant throughout the castle. The obtained treatment was applied by brushing onto the wood surface, while the retention and uniform application of the consolidation were confirmed by optical microscopy. One major advantage of the treatment is that the natural color of the wood is not affected, with the total color difference being very small. Another advantage gained after consolidation was the enhanced hydrophobic behavior of the old wood confirmed through water absorption, humidity and contact angle tests. In contrast, untreated wood exhibited hydrophilic behavior and high water and moisture absorption capacity, making aged wood extremely vulnerable to environmental degradation over time. Mechanical tests confirmed that the consolidant solution significantly improved the properties of the wooden material, due to the effective impregnation of the treatment into the wood structure. Furthermore, the MWCNT-based consolidant inhibited the growth of the Aspergillus niger strain, providing antifungal protection and preventing the colonization of microorganisms within the wood structure and its subsequent degradation. Through the methods investigated in this work, it was proven that the treatment is suitable for the consolidation of aged and degraded oak wood materials.

1. Introduction

Globally, the preservation of history is an important subject. Historic buildings are the most prone to degradation because they come into direct contact with the external environment, which is an extremely aggressive factor. So, their conservation and restoration is essential. Of all the old materials, wood is the most sensitive to the external environment, mostly because of wood-destroying organisms, and their conservation and restoration has been priority [1,2,3]. Wood exposed to the outdoor environment struggles with three major factors: high humidity, extreme temperatures and biological attack (insects, fungi and microorganisms) [4]. Over the years, several consolidants were proposed, but most have various disadvantages. These solutions can be applied on wood by various techniques, such as spraying, brushing, dipping, infusion, impregnation, injection, and so on [5]. Both the treatment and application method of the solution are important. Treatment can achieve the prevention of further damage and restoration of the degraded wood, by improving its strength, durability, dimensional stability, and other wood properties if the ideal treatment is selected and the right application method is employed.

Wood is a material highly exposed to external environmental factors, being found in many buildings, especially older ones. Over time, several materials have been proposed to protect and strengthen the wood. For example, acryloids and epoxies have been proposed in order to strengthen the wood structure; however, it was found that the used materials do not significantly increase the resistance of wood against fungi and may even be themselves used as a substrate [6]. In another study, it was found that these consolidants increase the mechanical resistance of wood, but no tests have been conducted on their antifungal or absorption properties [7]. Harandi d. and co-workers investigated the performance of Poly (vinyl butyral-co-vinyl alcohol-co-vinyl acetate) solution on fir wood. It was demonstrated that the treatment created a more hydrophobic surface, compared to untreated wood. Also, the applied treatment increased wood hardness locally, being enhanced with solution concentration, but the antifungal character was not investigated [8]. In another study, Paraloid B72 and Regalrez 1126 were used individually and in combination to investigate their antifungal effect on wood. It was found that when applied together, they slowed the growth of both fungal species (Fomitopsis palustris and Trametes versicolor) compared to the results obtained when they were individually applied [9]. Munteanu M. and co-workers used acrylic polymers (Paraloid B72 and Paraloid B67) in order to restore old lime trees and old oak trees. It was reported that, in both cases, the treatments significantly changed the natural color of the wood, and none of the wood samples returned to their original color [10]. Biological durability tests were carried out on treated birch wood with methyltrimethoxysilane and 3-mercaptopropyltrimethoxysilane with 25 wt% in ethanol against Basidiomycete fungi. An efficiency of approximately 90% (very durable) was reported for the wood treated with methyltrimethoxysilane, while an efficiency of approximately 65% (moderately durable) was observed and reported for the wood treated with 3-mercaptopropyltrimethoxysilane [11].

One real solution is offered by nanotechnology, as nanomaterials have played a key role in the development of new wood preservatives [1,12,13]. Serafini I. and Ciccola A. reported that ZnO nanoparticles could represent a good choice for wood preservation and restoration due to their chemical stability and the possibility of preventing dust accumulation and UV-induced aging. However, the disadvantage reported by this group was that ZnO nanoparticles were able to prevent fungal growth for only three fungal species (Alternaria alternata, Penicillium chrysogenum, and Penicillium pinophilum), whereas in the case of Aspergillus niger the nanoparticles stimulated microbial growth [14]. In another study, the antibacterial activity of the coating based on ZnO-carboxymethyl-chitosan was investigated on S. aureus and E. coli. The results showed that all coatings produced larger inhibition zones than the untreated samples, and the resistance of E. coli was higher compared to S. aureus. [15]. Another important well studied property of ZnO nanoparticles is their hydrophobic character. In a study, ZnO nanoparticles were dispersed in ethanol and the colloidal suspension was spin-coated on a silicone substrate several times in order to evaluate its hydrophobicity. It was reported that the water contact angle of the untreated substrate was below 5°, whereas after coating the water contact angle increased to 150°, turning the material into a hydrophobic one [16].

In recent years, researchers have attempted to overcome the limitations of ZnO nanoparticles by incorporating them into a polymeric matrix. For example, Abbasi J. and co-workers obtained a consolidant based on polyvinyl butyral and ZnO nanoparticles and tested the product on old dried wood samples in order to evaluate its effect on wood penetration and wettability. It was found that the obtained consolidant led to a reduction in water penetration and wettability of the samples, compared to the control. Also, the degradation rate under accelerated aging conditions of samples treated with the obtained consolidant was lower in comparison with the control [17].

Previous studies on wood consolidants have shown that reducing water uptake and moisture can significantly improve durability by limiting moisture-induced degradation mechanisms, such as fungal degradation [18]. However, excessive pore blocking may also affect vapor transport and moisture exchange within hygroscopic substrates. Belt T. and co-workers reported that the improved decay resistance of modified wood is associated with reduced moisture uptake and limited diffusion processes within the cell wall [19].

In conservation science, maintaining a balance between hydrophobic protection and vapor permeability is considered essential for long-term compatibility. In this context, the proposed MWCNTs_ZnO + PHBHV solution represents a multifunctional treatment approach combining mechanical reinforcement, reduced water absorption, hydrophobic behavior, and antifungal activity. Unlike conventional single-component treatments, the hybrid nanocomposite structure may provide synergistic effects through the interaction between the polymer matrix, carbon nanotubes, and ZnO nanoparticles.

In recent years, there has been a tendency in wood conservation to investigate new efficient materials and improve the existing materials that have interesting features such as eco-friendliness, reversibility, and highly compatibility with wood [1,20,21]. Harandi, D. and M. Moradienayat obtained a consolidant based on nanocrystalline cellulose/ZnO nanofibers dispersed in Polyvinyl Butyral solution and applied the solution on the wood. It was reported that the obtained coating provided protection against moisture and UV light [22]. In another study, lignin-ZnO was dispersed in poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and the obtained solution was applied on oak wood specimens by brushing. It was reported that, compared to control, the treated samples presented an improvement in hydrophobicity and fungal resistance [23]. Poly(3-hydroxybutyrate) seems a good choice due to its intrinsic hydrophobic character, low acidity and biodegradability, but its application is quite limited due to its mechanical properties and processability. In order to enhance its properties, David M.E. and co-workers obtained a solution based on MWCNTs decorated with ZnO nanoparticles dispersed in poly(3-hydroxybutyrate) (PHBHV). The authors combined the hydrophobic character and biodegradability of PHBHV with the ideal mechanical properties of MWCNTs and the antimicrobial, antifungal and UV resistance of ZnO nanoparticles in order to obtain an ideal solution for wood protection and preservation [21].

Unlike traditional polymeric consolidants or inorganic nanoparticle coatings, the proposed PHBHV-based nanocomposite combines biodegradability with multifunctional surface protection, offering not only hydrophobic improvement but also structural consolidation and greater compatibility with sustainable conservation strategies.

The reported data suggested that the treated wood did not change its natural color (in the case of 0.1% MWCNTs_ZnO + PHBHV and 0.2% MWCNTs_ZnO + PHBHV) and properties such as water absorption, humidity and mechanical tests significantly improved after treatment (in the case of 0.2% MWCNTs_ZnO + PHBHV and 0.4% MWCNTs_ZnO + PHBHV). Also, greater fungal inhibition was observed for samples treated with 0.2% MWCNTs_ZnO + PHBHV, ensuring good protection against mold and fungi [4]. Due to the promising data obtained in our previous article, the present study represents a deepening of the research. Thus, the solution based on 0.2% MWCNTs decorated with ZnO nanoparticles dispersed in PHBHV was chosen and applied on real old and degraded wood by brushing.

In order to deeply investigate the consolidation capacity of the obtained solution, old wood from Banloc Castle was selected.

The novelty of the study lies in demonstrating the consolidant’s ability to protect and improve the properties of the historic wood material, which is in a highly degraded state. Due to the fact that old historic wood is very sensitive to environmental conditions, acting as a hygroscopic material that absorbs and releases moisture, in large quantities, in response to the surrounding environment, it is difficult to find a suitable consolidant. Furthermore, historic wood is constantly and for many years subject to movements—such as swelling, shrinkage and deformation—when exposed to temperature and humidity fluctuations, which leads to severe degradation over time. Borda M. and Hill C.A.S. reported that historical wood will not reach the same moisture equilibrium levels compared to young wood under the same environmental conditions [24], which means that it is more than necessary to study the behavior of old wood and wood in a state of severe degradation in order to state with certainty whether the treatment is appropriate for both young and old wood.

Banloc Castle is included in the category of historical buildings and is positioned in the center of Banloc commune, being the most important monument in the area. The property was finished in 1759 and has a “U”-shaped plan, with brick thick walls [25,26]. The castle is an imposing building and consists of: a basement, a ground floor, an intermediate floor and a massive wooden roof. The walls of the castle are made of bricks and layers of lime mortar. The roof is solid and robust, with a wooden supporting structure that reflects the construction technique of the Habsburg period. The joinery is made of oak wood and is simply executed. The windows and doors have artistic brass ironwork from the 19th century. Oak is the main wood found throughout the castle, such as the built-in wardrobes, the main staircase, including the small wardrobe under the staircase, the stairs to the cellar and the corridors [27,28,29]. Oak wood has historically represented one of the principal materials used in European heritage architecture and wooden artifacts due to its high mechanical strength and natural durability. Historical oak elements are commonly encountered in doors, staircases, beams, structural frames, furniture, and decorative architectural components [4,30]. Therefore, the wood material taken and chosen for future studies was part of an interior door from Banloc Castle.

2. Materials and Methods

This study was carried out in order to demonstrate the efficiency of the previously obtained treatment [4] on aged and degraded oak wood. Based on the previous study, the best consolidant was composed from 0.2% MWCNTs_ZnO NPs dispersed in a solution of 2% PHBHV (molecular weight of 67,000 g/mole containing 2% hydroxyvalerate—Good Fellow). In this study, the consolidant was applied by brushing—in three layers on each side of the wooden material.

2.1. Preparation of Wooden Material

A piece of oak wood with heritage value from a window frame of an interior door belonging to Banloc Castle was chosen in order to study the efficiency of the selected consolidant (Figure 1). The wooden piece was carefully cleaned with a brush, after which it was sized into pieces of approximately 2.5–3 cm in length, 2–2.2 cm in width and 0.86 cm in thickness.

Due to the intrinsic heterogeneity of historical wood, special attention was paid to sample selection and conditioning. All specimens were conditioned under identical laboratory conditions prior to testing (the sized wooden pieces were conditioned for one month at a temperature T = 20 ± 5 °C and a relative atmospheric humidity φ = 55 ± 5%).

After consolidant application, the wooden pieces were allowed to dry at room temperature and were reconditioned for another month. After that, the treated samples and control samples were used in order to investigate the treatment applicability.

Although natural variability cannot be completely eliminated in severely aged wood, these measures were intended to improve experimental reproducibility and comparability between untreated and treated samples.

2.2. Consolidant Retention (CR)

CR was investigated by a gravimetric method using (Equation (1) [31]), where the data were recorded before and after treatment. The degree of wooden impregnation was monitored in order to evaluate the effectiveness of the applied treatment. The determinations were performed in triplicate for each sample (control vs. treated), and the arithmetic mean was calculated.

$$\mathit{CR}={\displaystyle \frac{(mf-mi)}{mi}}\times 100$$

(1)

where CR is the mass percentage increase in [%]; m_f is the final mass of the treated and conditioned sample in [g]; and m_i is the initial mass of the conditioned sample in [g].

2.3. Optical Microscopy (OM)

In order to investigate the uniformity of the applied treatment on the wood surface optical microscopy was used. A NovexMicroscope BBS, which offers the possibility to investigate the samples in transmitted light with magnification between 4 and 100×, was used. The equipment has a digital video camera (EUROMEX, Arnhem, The Netherlands) attached, which, through the microscope software (ZenPro 2.3), allowed the acquisition of data in real time.

2.4. Fourier Transform Infrared Spectroscopy (FTIR)

A GX-type spectrometer (Perkin Elmer, Waltham, MA, USA), which allows measurements in the range of 4000–600 cm⁻¹, was used in order to record the IR spectra. The recording of the spectrum collection was performed in total attenuation reflection mode at a resolution of 4 cm⁻¹ for the accumulation and mediation of 32 spectra.

2.5. Wavelength Dispersive X-Ray Fluorescence (WDXRF)

In order to determine the qualitative and quantitative elementary composition of the wood WDXRF was performed. The WDXRF instrument is equipped with 3 crystal analyzers (with automated exchange): LiF (200) (Rigaku, Tokyo, Japan) for heavy elements (Ti-U), PET and RX 25 for light elements (O-Mg and Al-Sc) at 200 W power (50 kV tens, 4 mA int). Detection limit: 1 ppm–10 ppb; accuracy < 0.1–0.5%; the elements ranged from 8 O to 92 U.

2.6. Colorimetric Tests

A very important aspect of a consolidant is that once it is applied on the material it should not influence the natural color of the material. For this reason, colorimetric tests were carried out. A CR-410 colorimeter (Konica Minolta, Tokyo, Japan) set in the CIE L*a*b* system (CIE 1986) was used. For each sample, three determinations were performed before and after the application of the treatment, and the arithmetic mean was calculated for each sample. The total color difference (ΔE_xfinal) was calculated according to [32] Equation (2):

$$\Delta Ex final={({\Delta L}_X^2+{\Delta a}_x^2+{\Delta b}_x^2)}^{1/2}$$

(2)

where ΔL is the difference in lightness, calculated with the formula: ΔL = L_{treated sample} − L_{untreated sample}; Δa is the chromatic deviation of the coordinates of a* coordinates, calculated with the formula: Δa = a_{treated sample} − a_{untreated sample}; and Δb is the chromatic deviation of the b* coordinates, calculated with the formula: Δb = b_{treated sample} − b_{untreated sample}.

2.7. Water Absorption Test (WA)

The wood samples were dried in an oven at 103 °C and then were left to cool at room temperature and weighed (W₁). Subsequently, the wood pieces were immersed for 24 h in distilled water at room temperature, and then they were removed from the water, wiped with a towel and weighed (W₂). Their mass was determined with an accuracy of 0.01 g. WA was calculated according to [33] Equation (3). The determinations were performed in triplicate for each sample (control vs. treated), and the arithmetic mean was calculated.

$$WA={\displaystyle \frac{W2-W1}{W1}}\times 100$$

(3)

2.8. Humidity Test (H)

Wood samples were immersed in distilled water for 30 min and then weighed (W_i). Subsequently, wood samples were dried for 1 h at 100 °C in the oven and left for 24 h at room temperature, according to ISO 22157-1: 2004 [34]. Finally, the samples were weighed (Wf), and H was calculated according to [35] Equation (4). Their mass was determined with an accuracy of 0.01 g. The determinations were performed in triplicate for each sample (control vs. treated), and the arithmetic mean was calculated.

$$H={\displaystyle \frac{Wi-Wf}{Wf}}\times 100$$

(4)

2.9. Contact Angle

Hydrophobicity is another characteristic that an ideal consolidant solution should offer to the materials. Contact angle measurements were made in order to examine the hydrophobicity of the wood surface before and after treatment. The wood sample was positioned on a straight surface near a light source, and 6 μL of distillated water was dropped at a single point on the wood surface. After the drop touched the surface, photos were taken at a distance of 10 cm from the wood sample, every 15 sec for 1 min, in order to calculate the contact angle at different times. The measurements were recorded at a temperature of 23 ± 1 °C. DropAnalysis plugin LB-ADSA from ImageJ 1.52v was used to calculate the samples’ contact angle. For each sample, two determinations were performed (control vs. treated), and their arithmetic mean was calculated.

2.10. Mechanical Tests

Wooden mechanical strength, expressed by determining the rebound number, was investigated by using a Silver Schmidt Proceq hammer, type L, with impact energy of 0.735 Nm [36]. This procedure was chosen due to the limited availability and heritage significance of the Banloc Castle wood samples in order to avoid destructive mechanical testing methods.

For each sample, 10 measurements were performed, with a minimum edge distance of 5 mm. The hammer was positioned at 90° on the sample. The compressive strength, expressed in MPa, was calculated according to Equation (5) using the arithmetic values of the rebound number, determined in duplicate:

Compressive strength = 2.77 ∗ e^0.048∗Q

(5)

where 2.77 is the device constant, and Q is the rebound number.

2.11. Artificial Aging Test by Exposure to Humidity Variations

Wood is a hygroscopic material which absorbs and desorbs humidity from the environment due to the hydroxyl groups in the cell walls. The dimensional stability of wood and durability under real conditions were investigated by hysteresis, according to ISO 12571:2013 [37], using a KK 115 Smart PRO climatic chamber, POL-EKO Aparatura, Wodzisław Śląski, Poland, with the fan used at 20% capacity and light at 50%. The wood samples were placed in the climatic chamber with controlled temperature and humidity. By maintaining the temperature at 25 °C, and varying the humidity from 35% to 85%, the samples reached a constant mass after approximately one hour. The determinations were performed in triplicate for each sample (control vs. treated), and the arithmetic mean was calculated.

The samples were weighed after each cycle, with an accuracy of 0.01 g and the moisture content was calculated with Equation (6):

$${AC}_x={\displaystyle \frac{{SH}_x-{SI}_x}{{SI}_x}}\ast 100$$

(6)

where AC_x—moisture content, in [%]; SH_x—sample mass at humidity x, in [g], and SI_x—sample mass at initial environmental conditions, in [g].

Sorption/desorption kinetics for each wood sample per unit area depending on the specific humidity value was expressed in the sorption/desorption curve.

2.12. Antifungal Activity

Each sample was embedded, individually, in a specific culture medium for the growth and isolation of Aspergillus niger. As a culture medium for growing and isolating fungi, solid Sabouraud medium was used. A suspension of sterile physiological water with a concentration of 1–3 × 106 spores/mL from a fresh fungal culture grown for 4 days on solid PDA medium (Scharlau, peptone—4; glucose—20; agar—15 (g/L)) was used as inoculum. Petri dishes with sterile Sabouraud medium were seeded in cloth with a sterile swab. The sample was placed in the middle of the Petri dish, and the Petri dishes were incubated at 28 °C for up to 5 days. During this time, the plates were observed and photographed to visually assess the absence or presence of growth of Aspergillus niger on the surfaces of the wood samples.

Fungal growth was semi-quantitatively analyzed using ImageJ software. The fungal-colonized area was determined by threshold-based image segmentation and expressed as a percentage of the total Petri dish surface.

2.13. Statistical Analysis

Statistical analyses were conducted in Microsoft Excel using two-tailed independent Student’s t-tests with the assumption of unequal variances (Welch’s t-test). Mean values between untreated and treated wood samples were compared for mechanical strength, water absorption capacity, and average size of the wood voids. Data are presented as mean ± standard deviation (SD). Statistical significance was set at p < 0.05.

3. Results and Discussion

Based on the latest published research of our group [4], the obtained solutions applied to oak wood samples revealed that the brushing application led to a much more uniform layer of the solution on the wood surface, and the retention of the consolidant on the wood surface was superior compared to solutions applied by spraying or immersion. Furthermore, investigation of the water behavior showed that the consolidants applied by brushing and spraying contributed to a high hydrophobicity, respectively leading to an increase in the contact angle. Regarding the mechanical tests, the brushing application method proved to be the most effective, confirming the formation of a uniform layer that penetrated the wood surface, making the wood more compact and resistant.

The concentration of the consolidating solutions also played an important role. Colorimetric tests established that the highest total color difference was observed for solutions containing 0.4% nanocomposite, indicating a slight change in the natural color of the wood. In addition, water absorption and humidity tests showed that the best results were obtained for samples treated with solutions containing 0.2% and 0.4% nanocomposites. These findings confirm that the use of lower concentrations (e.g., 0.1% nanocomposite) does not provide sufficient protection for the wood material.

Regarding the mechanical resistance, it was observed that the best results were obtained for wood pieces treated with the solutions containing decorated carbon, which confirms that the presence of nanoparticles makes a contribution in this regard. Also, the presence of nanoparticles on the surface of the nanotubes brought an advantage in terms of antifungal activity, where it was confirmed that wood pieces treated with solutions based on MWCNTs_ZnO + PHBHV and MWCNTs_Ag + PHBHV were able to slow down the growth of the Aspergillus niger strain and the Penicillium sp. mycelium. Therefore, of all the compositions applied to wooden pieces without heritage value, we selected the consolidation solution based on MWCNTs_ZnO + PHBHV for application to the wooden pieces from Banloc Castle, with a nanocomposite concentration of 0.2%, applied by brushing.

3.1. Investigation of the Pieces from Banloc Castle

3.1.1. Consolidant Retention

The percentage mass calculated for the wood pieces with heritage value increased after treatment; using Equation (1), it was 0.5752% (Figure 2). Compared to the values obtained for the oak wood pieces without heritage value (young wood) [4], it is observed that the consolidation retention in this case is optimal, slightly exceeding the value obtained in the case of the same treatment (due to the stage of degradation of the wood material). This difference is due to the fact that the old wood, from the castle, has a higher capacity to absorb the treatment (being drier—over 200 years old), thus allowing the consolidation solution to penetrate deeper into the wood material.

3.1.2. Optical Microscopy

Signs of severe aging, such as significant voids and cracks, are present in the untreated wood material (

Table 1. Optical microscopy images of the wood samples from Banloc Castle.

Sample	Central Zone (4×)	Particle Size Average on the Surface of Wooden Pieces (µm)
Control		-
MWCNTs_ZnO + PHBHV

). In the case of wood treated with the consolidating solution, it is observed that the applied solution was uniformly distributed on the sample surface, penetrating into the pores of the wood. A slightly whitish layer on the surface of the wood was observed due to the presence of the polymer. Using ImageJ software, the particles on the wood surface were identified and their average diameter was calculated. It can be seen that most of the particles had small diameters (below 0.5 μm), which means that they did not agglomerate on the surface of the wood and were able to enter the voids of the wood material to improve its properties. The treatment penetrates the porous wood structure and forms a continuous polymeric network within lumina, microvoids, and cell wall defects.

Based on the microscopy images, the average voids presented in the wood material were calculated by using ImageJ software [38]. The results showed that the average size of the voids on the wood surface is bigger before treatment, which suggests that after applying the treatment, a significant part of the voids and cracks present on the wood surface is covered, leading to a possible increase in resistance to external factors (

Table 2. Voids present on the wood surface before and after treatment. The results are presented as the mean ± S.D. of three replicates.

Sample	Voids on the Central Zone (4×)	Average Size of the Voids (µm)	Standard Deviation	p-Value
Control		22.31	5.84	<0.001
MWCNTs_ZnO + PHBHV		16.44	3.73

).

To evaluate the effect of the treatment on the number of voids identified on the wood surface, a comparison was made between two independent groups: untreated wood and treated wood. The data were analyzed using the t-test for two independent samples with unequal variances (Welch’s t-test). The mean number of voids for the untreated wood surface was significantly higher (22.31 ± 2.34) compared to the treated wood surface (16.44 ± 1.63). Statistical analysis revealed a significant difference between the two groups (t(4) = 10.76, p < 0.001). The two-sided p-value (p = 0.00042) indicates that the treatment applied to the wood significantly reduces the presence of surface voids. Thus, the null hypothesis regarding the absence of differences between the groups was rejected.

Fourier Transform Infrared Spectroscopy (FTIR)

IR spectra recorded for both untreated and treated wood is presented in Figure 3. Characteristic bands include OH stretching vibration at 3300 cm⁻¹, C−H stretching vibration at 2914 cm⁻¹, C−O deformation vibrations at 1596, 1501, 1459, and 1424 cm⁻¹, C−H grouping at 1321 cm⁻¹ and C= C stretching vibrations in the aromatic rings at 1224 cm⁻¹, which are characteristic of lignin and holocellulose in the wood structure [39]. At 1723 cm⁻¹ the characteristic band of hemicellulose (C=O stretching vibration) [40] is present, at 1369 cm⁻¹ the band is assigned to the bending vibrations (deformations) of CH₂, and at 1030 cm⁻¹ the band corresponds to the C−O−C stretching vibration of the primary alcohol in cellulose and hemicellulose. Regarding the treated wood a much sharper band can be observed at 1719 cm⁻¹, characteristic of the C=O group in the polymer structure. The appearance of the Zn-C group at 974 cm⁻¹ and Zn-O at 517 cm⁻¹ is also observed.

Wavelength Dispersive X-Ray Fluorescence (WDXRF)

The composition of untreated and treated samples was investigated using WDXRF analysis to confirm the presence of the top components in the structure of the old wood, which is over 200 years old (

Table 3. Composition of untreated and treated samples investigated by WDXRF.

Component	Wood (%)	Wood + 0.2% MWCNTs_ZnO + PHBHV (%)
SiO2	14.6348	21.0604
SO3	5.0322	8.2864
Cl	3.2932	4.5325
K2O	20.1770	6.7275
CaO	27.8629	29.9315
Fe2O3	12.4192	8.6095
P2O5	4.2106	9.1521
Al2O3	4.9567	7.1266
MnO	1.5218	0.5090
MgO	1.2877	1.8021
PbO	4.6039	0.9369
ZnO	-	1.3255

). The major components in the structure of the untreated wood are silicon, potassium, calcium, iron and sulfur. It is observed that after treating the wood with the consolidating solution based on MWCNTs_ZnO + PHBHV, this trend does not change, but instead a new component (ZnO) appears, in low proportions, thus confirming the successful deposition of the nanocomposite system onto the wood substrate. Besides the appearance of ZnO after treatment, variations in the relative proportions of other oxides were also observed. The WDXRF data are expressed as normalized relative percentages, meaning that the introduction of additional Zn-containing species can influence the calculated proportions of the remaining components. Furthermore, historical wood represents a highly heterogeneous substrate, characterized by non-uniform mineral distribution, degradation products, and possible environmental contamination accumulated during long-term aging. Consequently, local variations in elemental composition and measurement position may also contribute to the observed differences in oxide contents.

3.1.3. Colorimetric Tests

An adequate consolidant does not have to change the natural color of the wood, and for this reason colorimetric tests were performed. The wood pieces chosen from Banloc Castle are over 200 years old, which means that the wood has been subject to severe degradation for a long time, without benefiting from protective treatment. Another indication of the major degradation of the piece of wood refers to the darkening of its natural color. Colorimetric tests revealed that the brightness (L_x) has significantly diminished (compared to the young oak wood without heritage value [4] that was not subject to degradation over time and severe climatic conditions, where L_x = 76.67), which leads to a major blackening of the oak wood color over time (

Table 4. Chromatic parameters obtained for untreated and treated wood belonging to Banloc Castle. The results are presented as the mean ± S.D. of three replicates.

Sample/Parameter	Control—Arithmetic Mean	Standard Deviation	Wood + MWCNTs_ZnO + PHBHV—Arithmetic Mean	Standard Deviation
Lx	59.79	0.56	59.79	0.68
ax	4.81	0.35	4.62	0.56
bx	7.47	0.43	9.22	0.73
∆Lx calculated relative to control	-	-	0	0.09
∆ax calculated relative to control	-	-	−0.19	0.14
∆bx calculated relative to control	-	-	1.74	0.34
ΔEx calculated relative to control	-	-	1.75	0.42

). After treatment, a very low value of the total color difference (ΔE_{x final}, calculated with Equation (2)) was observed, which confirms that the used treatment is imperceptible on the wood surface. This supports the fact that the selected treatment can be successfully used both in the case of degraded wood and young wood, without modifying their natural color. An important advantage of the treatment based on 0.2% MWCNTs_ZnO + PHBHV is represented by its slightly dark color, which is lost on the surface of a degraded wood material over time (a darker shade of the surface color being characteristic of these types of wood), which makes the treatment imperceptible to the naked eye. Also, the low values obtained compared to the control, in the case of the other parameters (such as ∆L_x, ∆a_x, ∆b_x), confirm that the treatment does not modify the natural color of the old wood.

3.1.4. Water Absorption and Humidity Test

These tests were carried out in order to investigate whether the selected treatment is able to prevent water absorption in old and damaged wood. An old wood has the disadvantage of being dry, which means that it easily absorbs humidity from the external environment, thus leading to a much faster degradation [41]. This characteristic plays an important role in the long-term performance of wood materials, influencing dimensional stability, mechanical behavior, and biological degradation processes [18]. The obtained value for the Banloc untreated wood (

Table 5. Water absorption and humidity for untreated and treated wood belonging to Banloc Castle. The results are presented as the mean ± S.D. of three replicates.

Sample	Water Absorption (%)	Standard Deviation	p-Value	Humidity (%)	Standard Deviation
Control	64.7391	9.18	<0.001	15.0980	4.15
MWCNTs_ZnO + PHBHV	47.1569	6.26		8.3540	2.78

) in the water absorption test confirms that the untreated wood shows a much higher absorption. Water absorption measurements indicated a significant decrease in moisture uptake after treatment. Untreated wood samples exhibited an average water absorption of 64.73 ± 0.23%, whereas treated samples showed significantly lower values of 47.16 ± 0.20% (Welch’s t-test, p < 0.001). After treatment, the water absorption in the wood significantly decreased, which means that the consolidation solution entered and covered the gaps in the wood, thus creating a uniform protective layer that prevented water from entering in the wood. The treatment reduced water absorption by approximately 27%, suggesting decreased porosity accessibility and improved resistance to water penetration. The reduction in water absorption for the treated samples is attributed to partial pore blocking, decreased capillary transport, and reduced accessibility of hydrophilic hydroxyl groups within the wood cell wall. This is also applied in the case of humidity absorption from the external environment; the untreated wood absorbed almost double the amount of humidity compared to the treated sample.

The enhanced reduction in water and moisture absorption observed for the MWCNTs_ZnO + PHBHV consolidant can be comparatively discussed against previously reported nano-ZnO-based wood protection systems, which primarily demonstrated improvements in UV resistance, water repellency, and dimensional stability [42], thereby highlighting the added multifunctional value of the proposed nanocomposite treatment.

3.1.5. Contact Angle

This property was investigated by measuring the contact angle of samples belonging to Banloc Castle before and after treatment. The hydrophobicity of the surface is a very important aspect in order to obtain adequate consolidation. The higher the value of the contact angle is, the greater the hydrophobicity of the surface is, thus preventing the penetration of water into the wooden material. In

Table 6. Contact angle values for untreated and treated wood from Banloc Castle. The results are presented as the mean ± S.D. of two replicates.

Sample		Image	Contact Angle (°)	Standard Deviation
Control	t = 0		87.010	1.18
	t = 15		55.146	1.24
	t = 30		39.951	0.55
MWCNTs_ZnO + PHBHV	t = 0		95.780	0.87
	t = 15		91.991	1.11
	t = 30		87.091	0.65
	t = 45		86.574	0.87
	t = 60		85.445	0.73

, both the images obtained at different times and calculated contact angle values are presented, from the moment the droplet reached the surface of the wood. Initially, for the untreated sample a hydrophilic contact angle (θ < 90°) was obtained, which decreased rapidly in the following seconds. As can be seen, the untreated wood completely absorbed the water after 30 s. After applying the consolidation treatment, by brushing, it is observed that the hydrophobicity of the surface improved considerably, due to the combined effect of reduced surface energy and increased micro/nanoscale surface roughness induced by ZnO nanoparticles. At t = 0, we have a hydrophobic contact angle (90° < θ), which decreases slightly over time. It is observed that after 60 s, the value of the contact angle has slightly decreased below 90°. The gradual decrease in the contact angle over time is attributed to the partial penetration of water into the porous wood structure and residual capillary absorption phenomena. Nevertheless, the treated samples maintained high contact angle values throughout the measurement period, indicating persistent hydrophobic behavior and reduced surface wettability compared with untreated wood. Similar decreases in contact angle over 60 s have been reported for hydrophobically modified wood surfaces, even when highly water-repellent treatments were applied [43]. Also, Wang, K. and co-workers reported that highly water-repellent wood surfaces exhibit time-dependent wettability changes due to progressive liquid penetration into the porous and the hygroscopic wood structure [44].

3.1.6. Mechanical Tests

The mechanical tests were carried out in order to investigate whether the chosen treatment has the capacity to improve the mechanical properties of the wood material in an advanced state of degradation. From the obtained results (

Table 7. Mechanical properties of untreated and treated wood belonging to Banloc Castle. The results are presented as the mean ± S.D. of two replicates.

Sample	Rebound Number (Q)	Standard Deviation	Compressive Strength (MPa)	Standard Deviation	p-Value
Control	30.3	0.50	11.8609	0.12	0.037
MWCNTs_ZnO + PHBHV	32.3	0.46	13.0559	0.14

), it can be concluded that the presence of the applied consolidation solution improves the mechanical resistance of the wood material. The increase in rebound-derived mechanical resistance is mainly due to the impregnation of the treatment applied in the wood that strengthen the walls of the wood cells and filled the voids of the wood material structure [45]. Also, the mechanical integrity of the wood improved, due to the presence of carbon nanotubes in the consolidant composition that act as nanoscale reinforcing elements capable of stress transfer and crack-bridging. The independent two-tailed t-test showed statistically significant differences between the treated and untreated samples (p = 0.037). The treated samples presented high values of mechanical properties compared to the untreated samples.

3.1.7. Artificial Aging Test by Exposure to Humidity Variations

A major disadvantage of wood refers to its increased capacity to absorb/desorb moisture from/to the external environment, which leads to stresses and deformations in the structure of the wood material. In Figure 4, the sorption/desorption curves of the untreated and treated wood pieces are represented, and by comparing the two sets of samples it is observed that the untreated wood absorbs the most moisture from the external environment and desorption is slower. The treatment used to protect the wood material prevented moisture from penetrating the wood structure. Also, desorption is fast, and the percentage of moisture retained in the wood material is lower, compared to the control sample. The results obtained are consistent with the literature, which shows that treating the wood surface reduces the porosity and hygroscopicity of the material, limiting water and moisture absorption by reducing the accessibility of hydroxyl groups and improving the hydrophobic character of the surface, which leads to superior dimensional stability and increased durability [17,46].

3.1.8. Antifungal Activity Testing

Figure 5 and Figure 6 show the fungal growth on the wooden pieces from Banloc Castle at 48 and 72 h. In the case of the untreated wooden piece, the growth of the Aspergillus niger strain is observed on the wood surface at 48 h. In the case of the sample treated with the selected solution, a delay in the sporulation of the Aspergillus niger strain is observed, suggesting a considerable slowdown in the growth of the strain on the surface of the wood.

Fungal filaments have grown on the surface of the samples, and the sporulation of the Aspergillus niger strain continues rapidly at 72 h (Figure 6). In the case of the treated sample, a very slow growth is observed, the sporulation of the Aspergillus niger strain is easily observed in the left corner at 72 h.

The treated samples exhibited a substantially lower fungal surface coverage compared with the untreated samples, indicating significant inhibition of fungal colonization (

Table 8. Inhibition area at 48 h and 72 h. The results are presented as the mean ± S.D. of two replicates.

Sample	Inhibition Area (%) at 48 h	Standard Deviation	Inhibition Area (%) at 72 h	Standard Deviation
Control	19.96	4.72	53.49	11.23
MWCNTs_ZnO + PHBHV	0.56	0.22	1.09	0.67

).

The improved performance of the treated wood can be explained by the synergistic action of the PHBHV polymer matrix, carbon nanotubes, and ZnO nanoparticles. The PHBHV matrix likely acted as a pore-filling consolidant, reducing capillary water uptake and improving structural cohesion within the wood structure [47]. Simultaneously, CNTs contributed as nanoscale reinforcing agents, enhancing stress transfer and limiting crack propagation under mechanical loading [48], demonstrated by mechanical tests. The presence of ZnO nanoparticles plays a dual role by contributing to hydrophobicity through increased surface roughness and by providing antifungal activity via reactive oxygen species generation and membrane disruption mechanisms [49]. Taken together in a solution, the MWCNTs_ZnO + PHBHV produced a multifunctional treatment capable of improving the physical, mechanical, and biological resistance of the wood material.

From a conservation perspective, additional aspects such as reversibility and vapor permeability must also be considered. Although the MWCNTs_ZnO + PHBHV treatment significantly reduced water absorption, this does not necessarily imply complete suppression of vapor diffusion through the wood structure. Maintaining adequate vapor permeability is essential for hygroscopic materials in order to prevent internal moisture accumulation and associated mechanical stresses [50]. Furthermore, as with many polymer-based consolidants, complete reversibility cannot be fully guaranteed due to partial penetration of the nanocomposite into the porous wood network. Nevertheless, the treatment was applied at a low concentration in order to minimize excessive pore occlusion and preserve substrate compatibility. Future studies should specifically investigate vapor transport behavior and retreatability in order to further assess the suitability of the proposed treatment for conservation applications.

Although the present work focused on historical wood samples originating from Banloc Castle, the observed treatment effects may extend beyond this specific substrate. Previous experiments performed on younger wood samples using the same treatment showed comparable improvements in hydrophobicity, mechanical properties and fungal behavior, suggesting broader applicability of the treatment system. Historical wood often presents heterogeneous pore structures and altered cell wall chemistry, which can influence consolidant penetration and distribution, but, in principle, the treatment should be able to be successfully applied to all types of wood.

4. Conclusions

Oak wood pieces over 200 years old, coming from Banloc Castle, were chosen in order to investigate the consolidation capacity of the selected solution (0.2% MWCNTs_ZnO + PHBHV). The treatment was applied by brushing on the wood surface and the successful retention of a significant amount of consolidation in the pores of the wood material was demonstrated. Also, the retention and uniform application of the consolidation on the wood surface was confirmed by optical microscopy. Colorimetric tests have confirmed that the treatment does not alter the natural color of the wood, with the total color difference being very small.

Through water absorption, humidity and contact angle tests, the water absorption rate, droplet size and droplet residence time were studied for untreated wood samples and consolidated products, respectively. As a result of the tests, it was demonstrated that untreated wood has a high water and moisture absorption capacity, a property that makes aged wood extremely vulnerable to the external environment and time. Also, the high absorption capacity of the wood was demonstrated by investigating the contact angle, where the water droplet was completely absorbed by the wood after 30 s. After treating the wood pieces with the consolidation solution, a significant decrease in the water and moisture absorption capacity was obtained, which confirms the creation of a protective layer on the wood surface, which prevents water from penetrating the material. Also, the surface of the wood material changed after applying the treatment, presenting a hydrophobic character. The treatment also produced a moderate increase in rebound-derived mechanical resistance, thus suggesting a potential consolidating effect on the degraded wood material. The study of the behavior during accelerated aging by exposure to humidity variations reconfirmed the fact that untreated wood has an increased capacity to absorb moisture from the external environment. Also, water desorption is slower when the humidity in the external environment drops to 35%. In the case of the treated wood pieces, it is observed that the used treatment protected the wood material, preventing moisture from penetrating the wood structure. In this case, rapid desorption was observed at low humidity and a low percentage of moisture was retained in the wood material. The performed antifungal tests showed that old oak materials treated with 0.2% MWCNTs_ZnO + PHBHV inhibited the growth of the Aspergillus niger strain. Thus, it can be concluded that the applied treatment to the surface of old wood pieces provides antifungal protection, preventing the colonization of microorganisms in the structure of the wood material, and, respectively, its disintegration. Through the methods investigated by our group, it was demonstrated that the selected treatment is a good choice for the consolidation of aged and degraded oak wood materials. However, in real conservation applications, wooden materials are continuously exposed to humidity fluctuations, temperature variations, biological attack, and wetting–drying cycles, which may influence the long-term stability and performance of the treatment system. In particular, aging phenomena such as polymer degradation, nanoparticle redistribution, changes in vapor permeability, or loss of hydrophobicity may occur over extended periods of exposure. Future studies should be conducted to investigate the wood vapor transport behavior, long-term durability under fluctuating environmental conditions and retreatability in order to further assess the suitability of the proposed treatment for conservation applications.