Phase-Change-Material-Impregnated Wood for Potential Energy-Saving Building Materials

Abstract

PCMs (phase change materials) are ideal for thermal management solutions in buildings. This is because they release and store thermal energy during melting and freezing. When this material freezes, it releases a lot of energy in the form of latent heat of fusion or crystallization energy. Conversely, when the material melts, it absorbs the same amount of energy from its surroundings as it changes from a solid to a liquid state. In this study, Oriental spruce (Picea orientalis L.) sapwood was impregnated with three different commercial PCMs. The biological properties and the hygroscopic and thermal performance of the PCM-impregnated wood were studied. The morphology of PCM-impregnated wood was characterized through the use of scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). PCM-impregnated wood demonstrated low performance in terms of storing and releasing heat during phase change processes, as confirmed by DSC. The results show that PCMs possess excellent thermal stability at working temperatures, and the most satisfying sample is PCM1W, with a phase change enthalpy of 40.34 J/g and a phase change temperature of 21.49 °C. This study revealed that PCMs are resistant to wood-destroying fungi. After the 96 h water absorption test, the water absorption of the wood samples decreased by 28%, and the tangential swelling decreased by 75%. In addition, it has been proven on a laboratory scale that the PCM material used is highly resistant to biological attacks. However, large-scale pilot studies are still needed.

1. Introduction

With the increase in the world population, the development of the construction industry in a globalising world has increased. Building energy consumption, which constitutes approximately 40% of the total energy consumption, has increased rapidly [1,2]. Therefore, conserving energy and utilising daylight instead of artificial lighting will be effective in reducing energy consumption in the building sector. The most efficient way to benefit from solar energy is to store the sun’s energy. Heat storage technology also aids in the efficient use of energy and is crucial in terms of energy savings. Phase change materials (PCMs) have been investigated for thermal energy storage applications because they have high thermal storage densities, and the nearly isothermal process can store energy using latent heat [3,4]. PCMs are ideal for thermal management solutions in buildings. This is due to the fact that they both store and release thermal energy during melting and freezing. When this material freezes, it releases a significant amount of energy as latent heat of fusion or crystallisation energy. When a material melts, it absorbs the same amount of energy from its surroundings as it transitions from a solid to a liquid state [5]. Owing to this property, they can be used in a variety of applications, including the following: (1) building insulation, as PCMs can be used to help regulate indoor temperatures and reduce heating and cooling costs in buildings; (2) thermal energy storage: PCMs can be used in thermal energy storage systems to store and release the excess heat generated by renewable energy sources such as solar panels.; (3) refrigeration and air conditioning, PCMs can be used to improve efficiency and reduce energy consumption in refrigeration and air conditioning systems; (4) clothing and textiles: PCMs can be incorporated into clothing and textiles to provide people working in hot or cold environments with comfortable, temperature-regulating clothing; (5) packaging, PCMs can be used in packaging to help temperature-sensitive products, such as food and pharmaceuticals, to maintain their temperature; (6) electronics, PCMs can be used to regulate temperatures and reduce the risk of overheating in electronics such as laptops and smartphones; and (7) These are just a few of the many potential applications of PCMs, and research into their use is ongoing as scientists and engineers seek new and innovative ways to leverage their unique properties [6,7,8,9]. However, the long-term use of PCMs is limited due to the fact that they create a problem of leaching from the environment when they absorb heat from the environment during phase change [2,10]. Researchers have tried various methods to overcome this problem. The most important of these methods are microencapsulation and polymer blending [11,12]. The technique in which a liquid or solid package is covered with another material is called microencapsulation. In this technique, the main part of the PCM matrix, Jams, are used as shells to microencapsulate PCMs. These are melamine–formaldehyde (MF) jams [13,14], poly (urea–urethane) [15], polyurea [16,17], urea–formaldehyde (UF) jams [18], and acrylic jams [19]. The high encapsulation costs and low thermal conductivity of the encapsulated material have limited the use of the encapsulation technique [20,21,22]. Another method to reduce PCM leaching is to produce shape-stabilised PCMs (SPCMs) or form-stable composites. The advantage of the SPCM method is that the material used remains solid even at temperatures higher than its melting point [20,21,22,23,24]. Organic and inorganic materials are used in this technique. When using an organic polymer matrix, the chemical compatibility and thermal stability of polymers should be considered [20]. Various polymer materials are used in this technique, such as low-density polyethylene [25], styrene–maleic anhydride copolymer (SMA) [26], polymethyl methacrylate [27,28], high-density polyethylene [29], polyurethane [30], polypyrrole [31], polyvinyl alcohol [32,33,34], and biodegradable polymers, such as cellulose, chitosan, and agarose [35].

Since ancient times, wood has been used as a building material. The superiority of wood materials is enhanced by their sustainability and renewable nature. The incorporation of PCMs into the wood lignocellulosic matrices to reduce the leaching of the material from the environment represents a thermal energy storage technology that is a flexible and reliable way to store heat [36]. PCMs added to the building construction material melt with the heating of the building envelope during the day, causing less heat entry into the building. PCMs, which freeze at night when the outdoor temperature decreases, contribute to the heating of the building by releasing heat. Thus, with PCMs used in the building material, it is possible to benefit from solar energy passively while a more homogeneous temperature distribution is achieved within the building, and heating and cooling can be realized at lower costs.

There are a few studies using wood and wood flour (WF) as components of PCMs. [36,37,38,39]. In these studies, myristyl alcohol [36], fatty acid [37], a eutectic mixture of lauric acid and myristic and capric-palmitic acid [38,39], paraffin [40,41], a eutectic mixture of capric acid (CA), and stearic acid (SA) [42] were used as PCMs. Cheng and Feng [36] developed FSPCMCs (form-stable phase change composites) used on WF as a by-product of the timber industry via delignification and impregnation with myristyl alcohol (MA). The melting and crystallization enthalpy of delignified WF myristyl alcohol phase change composites were slightly higher than the calculated values, especially at the MA loading content of 65 to 75 wt%. According to the result, the latent heat of 166.5 J/g was measured in the composite material produced in the material containing 75% PCM. Jiang et al. [37] impregnated four kinds of WFs as the support material in the fatty acid (FA)/WF composite material used in the production of a form-stable phase change material (FSPCM). The phase change transition temperature and latent heat of the FA/WF composites were found in the range from 53 °C to 40 °C and 103 J/g to 88 J/g, respectively. In another study, eutectic mixtures of lauric acid and myristic acid were used as an FSPCM [38]. WF samples with porous structures were used as form-stable materials. The latent heat value of the composite material prepared with the eutectic mixture/WF was increased by 15% to 54.2% [38]. In another study, wood plastic composites were fabricated using a co-rotating twin-screw extruder using graphite and high-density polyethylene poplar (Populus tomentasa Carr.) WF. The authors reported a latent heat of 26.8 J/g for the composite material obtained. They also determined that the graphite added to the composite material increased the thermal conductivity of the material and reduced the bending properties [41]. In another study, Scots pine (Pinus sylvestris L.) sapwood was impregnated with a eutectic mixture of capric acid (CA) and stearic acid (SA) as a PCM via a vacuum process. The physical, mechanical, and thermal properties of the wood samples were investigated. According to the results obtained, after 264 h of water removal, low water absorption (WA) and high anti-swelling efficiency (ASE) increased the bending and compression strength of the wood, and the digital scanning calorimetry results showed that a good latent heat value of approximately 94 J/g was obtained at 23.94 °C [42].

Buildings receive a high percentage of solar energy. The impregnation of wood with PCMs will increase the structural applications of wood and wood-based materials, which contain a limited thermal mass, in buildings. Thus, the thermal mass of the buildings can be increased, and a contribution can be made to reduce the environment and climate change by building energy-efficient, thermal comfort, and low-carbon emission buildings. Oriental spruce (Picea orientalis L.) sapwood was used in this study due to being widely available in Turkey, low cost for buildings, and suitable for cross-laminated wood glulam use. Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) were performed to characterize the chemical properties and morphology of the PCM-impregnated wood. The thermal degradation stability of the PCM-impregnated wood was determined by differential scanning calorimetry (DSC) and thermogravimetry (TG/DTA) techniques. In addition, the performance of the spruce wood samples impregnated with PCMs against white rot fungus (Trametes versicolor (L.) Lloyd (Mad-697)) and brown rot fungus (Coniophora puteana (Mad-515)) was also evaluated.

2. Materials and Methods

2.1. Materials

Oriental spruce (Picea orientalis L.) wood samples consisting of sapwood samples with a density of 0.35 g/cm³ were used in this study. The sample sizes used in the study were cut at 5 mm (radial, R) × 15 mm (tangential, T) × 30 mm (longitudinal, L) dimensions for decay testing. The other analyses were performed using samples of these dimensions. PCMs of normal paraffin n-C14 were purchased from Sichuan Aishpeier New Material Technology Co., Ltd. (Çengdu, China). The PCM organic phase change material is based on paraffin. The features of the products are listed in Table 1. The only difference between the phase change materials used is their melting and solidification temperatures; everything else is the same. Paraffin is a common choice for use as a PCM in building construction because it has several desirable properties. One of the most important is its high thermal energy storage capacity. This means that it can absorb and store a large amount of heat energy per unit volume, which makes it very efficient for regulating the temperature of a building. Paraffin is also relatively inexpensive and widely available, which makes it a cost-effective option for use in building construction. Additionally, it is stable and non-toxic, making it safe for use in buildings. Another advantage of using paraffin is that it is a solid at room temperature, which means it will not leak or evaporate over time, making it a long-term storage solution. Finally, paraffin is also a good thermal insulator which helps to keep the heat stored inside the material, making the building more energy efficient. All of these properties make paraffin a suitable choice for use as a PCM in building construction and energy-saving projects.

2.2. Impregnation of Wood Samples with PCM (PCMW)

The wood samples prepared in the decay test sample size were conditioned for 1 month at 25 °C and 65% relative humidity before impregnation. The conditioned wood samples were dried in an oven (103 °C) until they reached a constant weight (2%–3%). The wood samples that reached a constant weight state were impregnated with PCMs. The impregnation process was carried out under a vacuum (650 mmHg) for 45 min and atmospheric pressure for 4 h. The temperature was fixed at 30 °C in a vacuum and atmospheric pressure process. Then, to calculate the weight gain of the samples, they were kept at 80 °C until they reached full dry weight. The PCMs were fabricated inside of the wood after over-drying, and the preparation phase change energy storage wood was named PCMW. The weight percentage gain (WPG) achieved for PCM1, PCM2, and PCM3 was 79.33%, 24.03%, and 67.17%, respectively. After the atmospheric pressure treatment, the wood samples were conditioned for 4 weeks, and the oven dry mass was recorded prior to a leaching test at 20 °C and 65% relative humidity in the air-conditioning cabinet (NÜVE TK 252).

2.3. Characterization of PCMs and PCMW

The thermal stability of the PCM and PCMW was evaluated using a thermogravimetric analyzer (Perkin Elmer Pyris™ 1 TGA) at a scanning rate of 10 °C min⁻¹ in the temperature range of 20–500 °C under a sustained stream of nitrogen atmosphere. The graphics of the TG and DTG were drawn with the Origin 2019b program. The chemical structure of PCM, PCMW and spruce wood samples were recorded using a Fourier transformation infrared spectroscope (Bruker, Karlsruhe, Germany). The spectra ranged between 400 and 4000 cm⁻¹ wavelength and with a 4 cm⁻¹ resolution using a Bruker ATR-FTIR spectrometer. The radial surface of the wood was used to ATR-FTIR. The measurements were made with solid samples. The spectra were baseline corrected and smoothed using the OPUS software (Bruker Optics GmbH, Ettlingen, Germany). The graphics of the results obtained were drawn with the Origin 2019b program. The thermal properties of the PCM and PCMW were measured using a differential scanning calorimeter (DSC, Perkin Elmer Pyris™ 1 DSC, temperature accuracy: ±0.2 °C, enthalpy accuracy: ±5%) under a sustained stream of argon, with the cooling and heating rate of 5 °C/min. The test temperature range was set from 0 to 40 °C. The graphics of the melting and solidifying temperature were drawn with the Origin 2019b program.

2.4. Leaching and Decay Test

The leaching test was carried out according to the conditions specified by the EN84 [43] standard for 14 days prior to the decay test. The purpose of leaching is to evaluate any loss in the effectiveness of decay resistance. The decay test was carried out according to the principles of EN 113-2 [44]. Some changes were made, such as the sample dimensions, the use of Kolle flasks, and the total test period in the standard EN 113. A brown rot fungus, namely Coniophora puteana (Mad-515) and a white rot fungus, namely Trametes versicolor (L.) Lloyd (Mad-697) were inoculated to a sterile malt extract agar medium in Petri dishes. The wood samples were placed on the growing mycelium in each Petri dish and then incubated at 20 °C and 70% relative humidity for 2 months. The weight loss was calculated on the basis of the oven-dried weight before and after the decay test.

2.5. Water Absorption Test

In the study, wood samples with a dimension of 20 mm × 20 mm × 10 mm were tested for their water absorption properties. The test was carried out by immersing the oven-dried samples in a beaker filled with distilled water. The relative water absorption and tangential swelling were calculated based on the initial oven dry weight and tangential direction of the samples. Ten replicates were used for each group.

3. Results and Discussion

3.1. Thermal Stability Analysis of PCMs-Impregnated Woods

The TG and DTG curves of the PCMs tested are given in Figure 1 and Figure 2, while the TG and DTG curves of wood and PCM-impregnated wood samples (PCMW) are shown in Figure 3 and Figure 4.

It appears that the PCMs in Figure 1 and Figure 2 and Table 2 decomposed at low temperatures, specifically at 200 °C, and lost all of their weight at this temperature. This suggests that the materials were not stable at this temperature and may not be suitable for certain applications. The wood samples impregnated with PCMs were found to deteriorate at higher temperatures, in contrast to the PCMs alone, which decomposed at lower temperatures. This suggests that the addition of PCMs to the wood samples may have an impact on their thermal stability and may affect their suitability for certain applications. It would be interesting to know the exact temperature range at which the deterioration of wood samples occurs, as well as the type of PCM used for impregnating the wood. Water and volatile compounds are removed at around 100 degrees in the wood samples. Hemicellulose, cellulose, and lignin, which are the main components of wood, degrade at 100–365 °C, 270–400 °C, and 400–600 °C temperature ranges, respectively. Lignin is one of the main components of wood and is responsible for providing the material with its strength and rigidity. The temperatures mentioned, between 410 and 620 °C, are in the range of temperatures at which lignin begins to break down and degrade. At these temperatures, cellulose and hemicellulose are also broken down, but lignin is the first component to degrade [45]. Wood impregnated with PCM1 showed the slowest degradation in this range of temperatures, followed by samples impregnated with PCM2 and PCM3. Untreated wood showed the fastest degradation rate, but it revealed the least degraded sample, cumulatively. The impregnation of PCMs helps to stabilise the composite sample at 324 °C, 326 °C, and 320 °C for PCM1, PCM2, and PCM3, respectively. While the highest weight loss occurred at 366 °C (PCM1W), 368 °C (PCM2W, and 362 °C (PCM3W) in wood samples impregnated with PCMs, the highest weight loss was achieved in the spruce wood sample at 359 °C with the rate 72.20%, 68.59%, 70.82%, and 70.88%, respectively. The PCM1-, PCM2-, and PCM3-impregnated wood left a higher residue of 17.31%, 19.40%, and 15.24% at 600 °C, respectively, compared to the wood with 17.40%. This result indicates that PCMs-impregnated wood had thermal stability at extreme temperatures compared to the control wood. In the literature study, it was reported that the residual rate of the Scot’s pine wood samples impregnated with capric acid was the same as the control samples, but the thermal stability increased [46].

3.2. FT-IR Analysis of PCMs-Impregnated Wood Samples

The FT-IR spectra of the wood, PCM, and impregnated wood are shown in Figure 5 and Figure 6. The FTIR spectrum of spruce wood is shown in Figure 5. The 3342 cm⁻¹ peaks show strong main OH stretching, and 2800–3000 cm⁻¹ shows C–H stretching in the methyl and methylene groups. The other characteristics peak at 1735 cm⁻¹, acetyl groups in xylan, and other non-conjugated carbonyls), C=O Stretch in xylan (hemicelluloses), the peak at 1508 cm⁻¹ shows the aromatic skeletal vibration of lignin, 1427 cm⁻¹ Aromatic skeletal vibration in lignin with C-H deformation 1369 and 897 cm⁻¹ are related to carbohydrates. The peak at 1265 cm⁻¹ indicates a C-O stretching lignin ring in the G units and a 1026 cm⁻¹ C-O vibration in cellulose and hemicellulose.

The FT-IR spectra of paraffin-based PCMs and PCM-impregnated wood (PCMW) are shown in Figure 6. Since the PCMs are paraffin-based, they showed paraffin properties. In the infrared (IR) spectrum of a PCM, the peak at 2912–2921 cm⁻¹ and 2852–2847 cm⁻¹ are typically associated with the symmetric stretching vibrations of the -CH₃ and -CH₂ groups, respectively. These peaks are specific to organic compounds and are a result of the bond between the carbon and hydrogen atoms in these groups. The IR spectrum of a PCM can provide valuable information about its chemical structure and properties, which can be useful for identifying and characterizing different types of PCMs. The peak at 2953–2957 cm⁻¹ represents the hydrogen-bonding vibration of methyl (CH₃-). The asymmetric deformation vibration peaks of the paraffin methyl group (CH₃-) are around 1466–1470 cm⁻¹, and the symmetrical deformation vibration of the paraffin methyl group (CH3-) are around 1369–1377 cm⁻¹.

Some changes in the chemical structure of spruce wood occurred upon impregnation with PCMs. Impregnation is the process of introducing a liquid into a solid; in this case, the PCM is introduced into the spruce wood samples. Depending on the type of PCM and the impregnation method used, chemical reactions may occur between the PCM and the wood, which can lead to changes in the chemical composition of the wood. Additionally, the presence of the PCM within the wood may affect the physical properties of the wood, such as its dimensional stability. In general, the chemical structure of PCMs is also seen in wood samples. In particular, peaks of 2852 cm⁻¹, 2922 cm⁻¹, and 2958 cm⁻¹ were also observed in wood samples. In addition, significant increases in the 1465 and 1261 cm⁻¹ peaks in the PCM3W samples indicate asymmetric deformation vibration of the paraffin methyl group (CH3-) and C-O stretch lignin ring in guaiacyl units, respectively. In the PCM2W samples, an increase was observed at the 1259 cm⁻¹ peak, which means the CO and OH groups in the hemicellulose, cellulose, and lignin; in PCM1W samples, on the other hand, a peak occurred at 1465 cm⁻¹, which means C-H deformation of lignin and carbohydrates. In addition, the 720 cm⁻¹ peak, which is one of the PCMs characteristic peaks, appeared at 721 cm⁻¹, 798 cm⁻¹, and 804 cm⁻¹ in wood samples (PCM1W, PCM2W, PCM3W), respectively.

3.3. Thermal Properties Analysis of PCMs-Impregnated Woods

The thermal properties of PCMs and PCMW, including onset temperature, peak temperature, and latent heat during melting and solidifying processes, were analyzed using DSC, and the specific experimental data are listed in Table 3. Figure 7 describe the DSC curve of PCM1-3 and Figure 8 describe the DSC curve of PCM1-3W. The melting temperatures of PCM1, PCM2, and PCM3 were 22.07 °C, 21.77 °C, and 28.30 °C, respectively, while they were determined as 21.49 °C for PCM1W, 26.63 °C for PCM3W. The results obtained might be attributed to the following: (1) confinement of PCMs in the pores of the wood after the impregnation process, (2) the hydrogen bond between the paraffin and the lignin, cellulose, and hemicellulose components of the wood, (3) and the effect of capillary forces between the PCM molecules and cell walls of the wood. The melting latent heats of the PCMs were 168.71 J/g, 158.97 J/g, and 211 J/g, respectively, while PCMs impregnated with wood were lower than PCMs, which were 40.34 J/g for PCM1W and 30.33 J/g for PCM3W. No results were obtained for the PCM2W samples (Table 3). From the DSC data in Table 3, it can be seen that the solidifying temperatures of the PCMs are 19.27 °C, 18.43 °C, and 24.07 °C, respectively, wherein PCM1W is higher than PCM1 and PCM3W is lower than PCM3. In the literature study, it was found that the impregnation ratio of the CA-PA mixture in the DW reached 61.2%. At this impregnation rate, 94.4 J/g of latent heat was obtained [39]. Amini et al. [46] reported that a retention value of 267 kg/m³ was obtained for the Scotch pine wood samples impregnated with an 80% concentration of capric acid. A latent heat of 70.5 J/g was obtained at this retention level. In a study by Mathis et al. [9], red oak (Quercus rubra L.) and sugar maple (Acer saccharum Marsh.) were impregnated in a reactor with a microencapsulated phase change material. A latent heat storage of 7.6 J/g was measured for the impregnated red oak samples, and 4.5 J/g was measured for the impregnated sugar maple samples.

The energy storage efficiency can be calculated by the following formula:

E = [(ΔHPCMW_m + ΔHPCMW_s)/(ΔHPCM_m + ΔHPCM_s)] × 100(1)

where ΔHPCMW_m and ΔHPCMW_s are the latent heat of the PCM-impregnated wood during the melting and solidifying process, ΔHPCM_m and ΔHPCM_s are the latent heat of the PCM during the melting and solidifying process, and E refers to the energy storage efficiency of PCMW. According to this Equation (1), the energy storage efficiency of the PCM1W is 23.56%, and PCM3W is 11.75%.

3.4. Mass Loss of PCMs-Impregnated Woods

When the untreated control sample was exposed to white rot fungus Trametes versicolor (L.) Lloyd (Mad-697) and brown rot fungus Coniophora puteana (Mad-515) for 8 weeks of incubation, the highest mass loss (22.80% and 22.56%, respectively) was recorded. As the decay test standard (EN 113-2, 2020) stated, the minimum mass loss of the control samples should not exceed 20% mass loss. Therefore, these values corresponded to the “nondurable” class of durability ratings.

Conversely, minimum weight losses of <10% were noticed in the specimens impregnated with PCM1. The modified samples were usually more extremely attacked by the white rot fungus T. versicolor (from 2% to 4.7%) than by the brown rot fungus C. puteana (from 3% to 8.2%).

In studies on paraffin-based materials, it has been reported that wood attacked by wood-rotting fungi might degrade more slowly when impregnated with waxes. Wood-rotting fungi are a common source of decay and deterioration in wooden structures, and they can significantly reduce the strength and durability of wood. According to research, impregnating wood with waxes can slow the rate of degradation caused by wood-rotting fungi. This is because the waxes create a barrier that prevents the fungi from penetrating the wood and breaking down the cellulose and lignin. Additionally, some waxes also contain fungicides that can kill or inhibit the growth of fungi. This technique is known as “wax impregnation”; it is one of the methods used to protect the wood from decay and extend the service life of wooden structures. Paraffin wax, when applied to wood, forms a barrier that prevents the penetration of both fungal enzymes and fungal mycelium into the cell walls and wood structural components. Filling the wood cell walls and lumens with paraffin wax creates a physical barrier that makes it difficult for fungal enzymes to break down the cellulose and lignin in the wood. Furthermore, the wax barrier prevents fungal mycelium, the vegetative growth of the fungus, from penetrating the wood, preventing the fungus from spreading and colonising the wood. This helps to protect the wood and extend its service life by slowing fungal degradation. The wax penetration can be conducted by using a pressure method, by melting the wax and immersing the wood in the wax, or by using a vacuum method. Specific changes in the molecular structure of wood that cause an increase in its resistance to decay occur as a result of parallel PCM impregnation. The inhibition of fungal growth is also a result of polysaccharide dihydroxylation, resulting in a lower moisture content of the wood [47,48,49]. In this study, while an 80% final humidity was obtained in the control samples, the final humidity was obtained between 40% and 50% in the test samples. According to Liu et al. [50], the highest mass loss (64%) occurred when the untreated post-decay sample was exposed to white rot fungus for 12 weeks of incubation, indicating that Trametes versicolor was active. The Chinese standard states that a mass loss higher than 45% demonstrates the effectiveness of the fungal tests. The mass losses of the three paraffin wax emulsion-treated groups were reduced to 62%, 61%, and 58%. It has been reported that the paraffin wax emulsion process used at low concentrations only slows and does not completely prevent fungal attacks. According to Humar et al. [51] paraffin wax inhibits the growth of fungi by reducing the moisture content. The durability of the wood improved when treated with paraffin wax. Other reports found that the paraffin wax treatment can slow the growth of basidiomycetes due to the hydrophobicity of the paraffin wax [48,51]. However, in this study, it seems that the paraffin wax material included in the capsule provided a high level of protection.

3.5. Hygroscopic Properties of PCMs-Impregnated Woods

Figure 9 shows the water absorption and tangential swelling of spruce wood impregnated with the three PCMs from 20 min to 96 h soaking time.

In this study, all of the PCM-impregnated samples exhibited significantly lower water absorption and tangential swelling than that of the control samples. The 96 h water absorption test results show that the control samples had a higher water content (73.4%) compared to the test samples (57.69%, 56.8%, and 57.24%). Similarly, while the tangential swelling of the control samples was 7.98%, the tangential swelling rates of the test samples were 2.7%, 3.0% and 2.45%, respectively. For water absorption, the effects were less prominent during the first 2 h of soaking, but the absorption inhibition effects of the PCM-treated wood started to show significant differences as the soaking period progressed. As for tangential swelling, PCM impregnation bestowed significantly better dimensional stability to the wood, and the effect can be observed immediately even after merely 20 min soaking period. The PCMs used in this study are not water-soluble and do not leach easily. When applied to wood, they are deposited into the wood cells and lumen, which helps to limit moisture absorption and protect the wood from decay. The interaction between the wood and paraffin can also prevent hydrogen bonds between the wood and water molecules from forming. Because paraffin is a hydrocarbon, its molecules can interact with the wood cells and form a barrier, making it more difficult for water molecules to penetrate and be absorbed [52]. There was no difference between the PCMs used. The wood samples impregnated with all three PCMs absorbed the same amount of water. Longer soaking periods can result in higher water absorption in wood. When the wood is soaked in water for a longer period, it allows more time for moisture to penetrate into the wood. As the water is absorbed, the wood cells expand, causing the wood to increase in weight and volume. Water absorption refers to this increase in weight and volume, and the longer the wood is soaked, the higher the water absorption value. Water absorption can cause the wood to swell and change dimensions, which can affect the physical and mechanical properties of the wood [46].

4. Conclusions

In this study, important properties of spruce wood samples impregnated with PCMs were revealed. Wood samples with three different PCMs were impregnated using the vacuum pressure process. Wood samples impregnated with PCMs showed positive properties for use as thermal-regulating components in buildings.

• The DSC results revealed melting enthalpy values of up to 41.63 J/g to PCM1, which are beneficial for latent heat energy storage.

• The fungal test results showed that wood samples impregnated with PCMs showed resistance to fungi, specifying that it is safe to treat wood with PCMs.

• Spruce wood samples impregnated with PCMs have been shown to have potential as thermal regulation building materials. PCMs are capable of absorbing and releasing heat energy as they change phase (i.e., solid to liquid and vice versa), which can help to minimize temperature fluctuations in a building, thereby reducing energy consumption for heating and cooling. This is particularly useful in reducing temperature changes during the day and night.

• Potential future plans for PCMW include the following:

• The integration of PCMW into building materials, such as wall insulation and roof tiles, for passive thermal energy storage.

• Investigation of the potential for using PCMW in combination with thermal management systems, such as heat pumps and refrigeration systems, to improve overall energy efficiency.

Author Contributions

Writing—review and editing, A.C., S.H.L., and P.A.; Writing—original draft, A.C.; Visualization, A.C.; Validation, P.A.; Resources, A.C.; Methodology, A.C.; Investigation, A.C.; Formal analysis, A.C. and S.H.L.; Data curation, A.C.; Conceptualization, P.A. and M.A.A.G.; Funding acquisition, M.A.A.G. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

No data were used for the research described in the article.

Acknowledgments

The authors would like to thank Bursa Technical University, Faculty of Forestry Dean’s Office for the use of the FTIR device.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. TGA curves of the PCM1-3.

Figure 2. DTG curves of the PCM1-3.

Figure 3. TGA curves of the PCMW1-3.

Figure 4. DTG curves of the PCMW1-3.

Figure 5. FTIR-ATR spectra of spruce wood (Control).

Figure 6. FTIR-ATR spectra of the different PCMs and PCMs impregnated wood (PCMW).

Figure 7. Melting and solidifying DSC curves of the PCMs.

Figure 8. Melting and solidifying DSC curves of the PCM-impregnated wood (PCMW).

Figure 9. Water absorption (%) and tangential swelling (%) of wood impregnated with PCMs.

Table 1

Technical specifications of the PCMs used in this work.

Product Name	Normal Paraffin n-C14
Product properties	Liquid
Appearance	Transparent
Enthalpy	180–200 J/kg
Specific heat	3.22 kj/kgK
Coefficient of thermal conductivity	0.21 Wm/K−1
Color	Colorless
Melting point (PCM1, PCM2, PCM3)	20 °C, 25 °C, 30 °C
Density	0.77 g/cm3
Solubility	Water Insoluble (0.09 ug/L, 25 °C)Soluble in: ether, alcohol, acetone
Chemical stability	Generally stable

Table 2

TGA data for PCMs, spruce wood and PCM-impregnated wood samples.

	Degradation Interval (°C)	% Mass Loss
PCM1	95–224	100
PCM2	95–203	100
PCM3	128–201	100
Spruce wood	180–400	72.20
PCM1W	200–390	68.59
PCM2W	200–390	70.82
PCM3W	200–390	70.88

Table 3

DSC data of the PCM and PCM impregnated wood.

Samples	Melting			Solidifying
	Onset Temperature (°C)	Peak Temperature (°C)	Latent Heat (J/g)	Onset Temperature (°C)	Peak Temperature (°C)	Latent Heat (J/g)
PCM1	20.63	22.07	168.71	20.27	19.27	179.09
PCM1W	19.19	21.49	40.34	19.65	18.41	41.63
PCM2	19.72	21.77	158.97	19.92	18.43	170.16
PCM2W	-	-	-	-	-	-
PCM3	25.75	28.30	211.26	25.09	24.07	210.15
PCM3W	24.77	26.63	30.33	25.04	24.18	19.19

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