Enhancement of Thermal–Acoustic Properties of Pinus radiata by Impregnation of Bio-Phase-Change Materials

Abstract

Using fatty acids has generated significant interest in the building sector for improving energy storage in the form of latent heat. In this work, using vacuum impregnation, we analyzed the properties of a capric acid and myristic acid eutectic (83-17%) as a bio-based phase change material in Pinus radiata. The delignification of Pinus radiata samples facilitated the impregnation process, which was carried out using the Kraft pulping method. Morphological, chemical, mechanical, thermal, and acoustic impedance analyses were performed. The results revealed that impregnating PCM in Pinus radiata samples increases the thermal inertia of the impregnated samples, which is comparable to that of delignified samples. Additionally, the analyses showed no significant difference between natural and delignified samples after treatment with PCM.

1. Introduction

Wood has a good strength-to-weight ratio, is recyclable, and is relatively fast to process, all at a low cost [1]. However, its main limitation is its low thermal inertia. Its use induces large thermal fluctuations during the day and seasons, leading to discomfort and high energy consumption [2]. To optimize the energy performance of this porous material, phase-change materials (PCMs) have attracted particular interest for improving the energy efficiency of buildings, and these materials can be incorporated, for example, into wood [2,3]. They can store energy in the form of latent heat by exploiting their thermal properties. During the phase change of the material due to temperature variation, it stores (melts) or releases (crystallizes) energy in the form of latent heat [4,5,6]. The thermal properties of a PCM are related to its nature, i.e., its temperature and enthalpy of phase change, as well as its conductivity [6,7,8,9,10,11,12,13]. It is therefore of interest to develop a wood/PCM material with high thermal inertia, which implies that it would require a significant amount of energy to change its initial thermal condition [14], making it possible to improve the thermal efficiency of living spaces. Wood is not particularly effective for sound isolation; increasing its density by impregnating it with a PCM could enhance its acoustic insulating properties [15]. The effect of impregnation with PCMs has been studied in potential biomaterials for construction based on coffee waste [16], with an improvement observed in the acoustic absorption capacity, especially in the medium-frequency band, which, according to the authors, is similar to ambient noise.

PCMs are divided into three categories: organic, inorganic, and eutectic. The latter combines the first two families but is commonly regarded as a distinct group. In developing more environmentally friendly materials, particular interest is paid to organic bio-PCMs, such as fatty acids (FA), to replace paraffin derived from hydrocarbons [9,13,17]. These materials have several advantages, including their renewable nature, a remarkable enthalpy of phase change, improved thermal conductivity, and an attractive cost, making them suitable for thermal insulation in buildings. Among the most studied are capric acid (CA), lauric acid (LA), myristic acid (MA), palmitic acid (PA), and stearic acid (SA), which exhibit a temperature range of increasing state change, between 31.4 and 69 °C [13]. To obtain an adequate phase-change temperature, mixtures of these acids were studied at 20–25 °C. Eutectic compositions have enabled PCMs to achieve the desired thermal properties and have shown particular promise in applications in building materials. The study conducted by Duquesne et al. highlighted the remarkable properties of CA/MA and CA/PA eutectics [4,13]. In particular, the eutectic CA/MA with melting and crystallization temperatures of 24.14 and 21.37 °C, respectively, an enthalpy of fusion of 137.3 J/g, and a conductivity of 0.27 W/m/K is a promising PCM.

Several studies have already attempted to address this challenge by integrating this material into the wood, maintaining its thermal properties without impacting the mechanical properties of the matrix. The most common method is the vacuum impregnation of the PCM in the wood [18]. Many studies have been conducted using organic PCMs [14,19,20,21], particularly FA [21,22,23,24,25,26]. Some authors have studied delignification to increase the porosity of the wood and favor impregnation with PCM [14,26,27,28]. Lignin is the third most abundant family of compounds in wood, after cellulose and hemicellulose, and corresponds to a hydrophobic biopolymer comprising mainly p-hydroxyphenyl, guaiacyl, and syringyl [29]. It can account for up to 30% of the wood’s mass.

This work involved delignification and impregnation with a capric/myristic acid mixture in Pinus radiata samples. The delignification of Pinus radiata was studied to enhance the impregnation of the PCM, which was achieved through vacuum impregnation. Scanning electron microscopy, differential scanning calorimetry, thermal conductivity, and acoustic impedance were carried out to analyze the effect of the PCM on the wood samples.

2. Materials and Methods

2.1. Materials

The specimens used in this study were made from Pinus radiata, sourced from the southern region of Chile, with planed edges and a uniform cross-section. The specimens used had dimensions of 7 × 22 × 100 mm. Pinus radiata was chosen for its excellent availability, versatility, and profitability, which means that it is widely used for structural applications involving bending loads. Industrial applications such as paper pulp, plywood, sawn timber, and chipboard production have also established it as one of the most adaptable and versatile tree species worldwide [30]. The main direction of orientation of the samples was parallel to the fibers in the longitudinal direction. Capric acid (CA), myristic acid (MA), ammonium persulfate, and sodium sulfite (Na₂SO₃) were ordered from Sigma-Aldrich. Sodium hydroxide and hydrogen peroxide (H₂O₂) were sourced from Merck.

2.2. Delignification of Pinus Radiata

The wood samples were dried in an oven at 103 °C for 24 h. Delignification was performed in an alkaline medium [26,27], using a solution of 2.5 M of NaOH (Merck) and 0.4 M of sodium sulfite (Sigma-Aldrich, St. Louis, MO, USA), which was boiled in a reflux reactor from 1 h to 6 h, to solubilize the lignin by the method of “kraft pulping” (a dominant alkaline pulping method due to its high-quality pulp production) [31]. Later, the samples were washed in distilled water and then boiled in 2.5 M H₂O₂ (Merck) to remove the last traces of lignin, until the samples turned white, indicating the process was complete. The samples were then rinsed with hot distilled water and immersed in three successive baths of acetone [32]. Finally, they were dried in the open air overnight and then in the oven at 103 °C for 24 h.

2.3. Preparation of a Capric/Myristic Acid Mixture as PCM

The preparation of the eutectic point of the capric acid (CA) and myristic acid (MA) mixture followed a precise procedure to ensure an accurate composition. According to the literature [13], the eutectic point of the capric acid (CA) and myristic acid (MA) mixture was 83 CA-17 MA wt.%, with a melting temperature of approximately 24.3 °C, as verified by DSC analysis using a DSC 4000 System (PerkinElmer, Waltham, MA, USA). With an analytical balance with a precision of ±0.1 mg, the acids were individually weighed and transferred into separate beakers. Each acid was then liquefied on a hot plate at a temperature above its melting point under an extractor hood. The liquid mixtures were combined in a single beaker and thoroughly homogenized to obtain a uniform mixture.

2.4. Impregnation of the Capric/Myristic Acid Mixture as a PCM in Pinus Radiata

The impregnation method was based on the Bethell process [33], which involves immersing the wood samples in a PCM mixture using a vessel placed in a vacuum oven (Memmert VO29) at 60 °C, which maintains the wood dilated and the PCM in a liquid state. During the impregnation process, a pressure of 15 kPa was applied for 20 min using a vacuum pump to extract air from the porous matrix of the wood. Later, the vacuum pressure was released, and the sample was maintained at atmospheric pressure for 45 min. Then, a vacuum step of 60 kPa was applied for 10 min to prevent the impregnant from recoiling and percolating through the wood surface, as shown in Figure 1.

Figure 1. Impregnation program [33].

Figure 1. Impregnation program [33].

It is worth mentioning that the Bethell (full-cell) process begins with an initial vacuum of at least −85 kPa for 15 min or more, depending on the permeability and cross-sectional dimensions of the treated wood. The preservative solution is then introduced into the impregnation vessel and penetrates the wood’s pore system. Pressure is applied (usually between 800 and 1400 kPa, above atmospheric pressure) for 1–5 h, but occasionally many hours, depending on the permeability and cross-sectional dimensions. Pressure is maintained until the wood is fully impregnated or the rate of preservative uptake by the wood becomes negligible. The pressure is then released while the preservative solution drains from the vessel. The final step is to use the final vacuum (usually about −50 kPa for 5–10 min) to prevent the back effect and the bleeding of preservatives from the surface of the treated wood; this approach is commonly used for water-based preservatives.

Once impregnation was complete, the wood samples were removed and dried with paper towels, keeping the temperature of the wood samples below the melting point. The retention property of the PCM by the wood sample wDas calculated using the following formula:

$$PC{M}_{retention}={\displaystyle \frac{m_{impregnated Pinus sample}-m_{dry Pinus sample}}{V_{dry Pinus sample}}}$$

where the m_{impregnated Pinus sample} represents the mass of the impregnated Pinus radiata sample, the m_{dry Pinus sample} is the mass of the unimpregnated sample, and the V_{dry Pinus sample} is the volume of the unimpregnated Pinus radiata sample. The dimensions of the samples are measured in each direction of space with a caliper at three points.

2.5. Attenuated Total Reflection Fourier Transform Infrared Characterization (ATR-FTIR)

The capric/myristic acid mixtures were chemically analyzed by attenuated total reflectance spectroscopy (Shimadzu IR Spirit FTIR) at 24 °C with 40% relative humidity, averaging at 10 scans with a resolution of 1 cm⁻¹, which allows the achievement of high-resolution scans to obtain the unique absorption bands of functional groups in the sample [33].

2.6. Differential Scanning Calorimetry

The influence of impregnating the capric/myristic acid mixture as a PCM in Pinus radiata samples was studied using differential scanning calorimetry (DSC 4000 System, PerkinElmer), allowing us to estimate the eutectic composition of the acid mixture. DSC was conducted in a N₂ atmosphere of 20 mL/min, starting at 10 °C, which was maintained for 5 min. Then, the samples were heated at 40 °C for 5 min, increasing 1 °C/min. Later, the samples were cooled to 10 °C at 1 °C/min and then maintained at 10 °C for 5 min.

2.7. Mechanical Properties: Excitation Vibration Pulse

The mechanical properties of the Pinus radiata specimens were measured using the Sonelastic^® device, which uses a mechanical wave to calculate the modulus of elasticity of wood according to ASTM E1876-15 [34]. Pinus radiata samples were excited by a pulse generating an acoustic wave. Compression and tension responses were produced when the sample was subjected to bending excitation. The analysis of this wave and its characteristic frequencies enabled us to determine the longitudinal elastic, flexural, and shear (G) moduli [35]. Each measurement was repeated 5 times. The mechanical properties were measured by exciting the material in the direction of the fibers. The pulse was, therefore, generated at the first end of the material, and the response was recorded at the opposite end.

2.8. Morphological Characterization

Surface analysis of Pinus radiata specimens was conducted using a field-emission scanning electron microscope (FE-SEM, Zeiss GeminiSEM 360) operated at an accelerating voltage of 5 keV. This enabled relative chemical analysis using energy-dispersive X-ray spectroscopy (EDX, Oxford Max Ultra40).

Micro-computed tomography (micro-CT) analysis of Pinus radiata samples was also conducted using a high-resolution Bruker SkyScan 1272 microtomography system (Kontich, Belgium) with a rotation of 0.4° at 80 kV and 125 mA. The system was equipped with an aluminum filter of 0.25 mm thickness and a voxel size of 12 µm resolution. Three-dimensional scanned images were obtained using CTvox software (Version 3.3.1). The images were reoriented in space using Data Viewer software v1.5.1.9 to standardize sample positioning. Quantitative assessments were performed using micro-CT and software analysis in a volume of interest (VOI). Images of each sample were obtained using visualization software. Image processing and analysis were performed with CTAn software version 1.14.4.1. Images obtained from micro-CT were corrected by removing the background.

2.9. Thermal Analysis

Non-destructive analysis was performed using a Hot Disk TPS 1500 to determine the thermal properties of each specimen, according to ISO 22007-2 [36]. The tested specimens were cylindrical samples, previously dry at room temperature (20 °C), with a diameter of 40 mm and a thickness of 20 mm. For thermogravimetric analysis (TGA) using a PerkinElmer TGA 4000, cubic samples with a thickness of 1 mm were used. The Pinus radiata samples were placed inside a crucible and stabilized for 20 min. Thermogravimetric analysis was performed under a N₂ atmosphere, using a flow rate of 20 mL/min with a two-step program: a 1 min holding time at 20 °C and a temperature scan from 20 °C to 800 °C, heated at a rate of 10 °C/min.

2.10. Acoustic Measurements

The sound absorption measurement tests of the Pinus radiata samples were conducted for cylindrical samples of 40 mm and 20 mm in thickness, using an impedance tube with an internal diameter of 40 mm, equipped with 2 microphones according to ISO 10534-2 requirements and ASTM E1050 standards [37,38]. Using a sonometer positioned 5 cm from the entrance of the tube, a white noise signal was applied and adjusted to 90 Db. Then, the sound spectrum was adjusted for 1/8 frequency bands between 100 Hz and 4000 Hz. The sound absorption test had to establish the wave number and consider the sample thickness. Then, the software calculated the surface impedance and sound absorption coefficient [39]. An initial test was conducted using an acoustic isolation foam sample with the exact dimensions of the samples studied. Samples were analyzed once the sound absorption measurements were similar to those reported in references.

3. Results and Discussion

3.1. Thermo-Chemical Analysis of Capric/Myristic Acid Mixtures

DSC analysis of the capric/myristic acid mixtures allowed us to determine the eutectic point between them, analyzing the melting temperature and the enthalpy of fusion of the mixture.

Table 1. Thermal properties of some fatty acids and their eutectics.

Acid Mixture	Peak Temperature (°C)	Melting Temperature (°C)	Melting Enthalpy (J/g)
MA	55.57	53.86	151
CA	32.95	31.55	133
82%MA-18%CA	25.04	22.36	139
83%MA-17%CA	25.18	22.09	146
84%MA-16%CA	25.27	22.39	113

shows that the mixture of 83 wt% CA and 17 wt% MA is the eutectic mixture [4]. The lowest melting temperature is approximately 22 °C, with a melting enthalpy of around 146 J/g, corresponding to the energy the mixture can store as latent heat.

Figure 2 shows the attenuated total reflectance–Fourier transform infrared (ATR-FTIR) spectra of the eutectic capric/myristic acid mixture as a PCM, revealing peaks at 2800–3000 cm⁻¹, which correspond to the stretching vibration of O-H groups [25], and the peak associated with the carbonyl group C=O present in the PCM molecules, observed at 1700–1715 cm⁻¹ [40]. Additionally, the ATR-FTIR spectra exhibit 940 and 1460 cm⁻¹ peaks, corresponding to stretching and bending vibrations of C-O and C=O groups, respectively [22]. The peaks at 1245 and 670 cm⁻¹ are associated with O-H vibrations.

3.2. Delignification and Impregnation of Pinus Radiata Using PCM

Figure 3 illustrates the gradual development of a brown color during the delignification of Pinus radiata samples, a characteristic of lignosulfonate extraction. This indicates that the lignin concentration and the intensity of the brown color increase with the delignification time.

Table 2. Variation in physical parameters of Pinus radiata samples as a function of the delignification time.

Delignification Time (h)	Mass Variation (%)	Volume Variation (%)	Density Variation (%)
1	−10.98 ± 0.3	−9.01 ± 2.1	−2.3 ± 2.5
2	−13.98 ± 0.7	−15.03 ± 1.5	+1.35 ± 1.9
3	−14.62 ± 1.4	−16.95 ± 1.8	+2.83 ± 2.8
6	−26.21 ± 0.5	−17.53 ± 1.5	−10.51 ± 1.6

shows the mass, volume, and density variations in Pinus radiata samples after delignification, indicating a reduction in these properties. This can be associated with the hydrophobic molecules that compose the lignin, which protect the wood against moisture. To avoid the deformation of the wood structure, a post-treatment with acetone was applied before drying [32].

Figure 4 shows cross-sectional digital images of Pinus radiata samples, delignified and impregnated with the eutectic capric/myristic acid mixture as a PCM, revealing the increase in porosity and cracks after delignification. These cracks increased with exposure time but decreased after the impregnation process. Increasing porosity could facilitate a higher PCM loading within the sample matrix than the non-delignified Pinus radiata.

3.3. Morphological Analysis

Figure 5 shows SEM images of Pinus radiata samples, revealing the effect of delignification and impregnation with the eutectic capric/myristic acid mixture as a PCM. As can be seen, the naturally impregnated samples display partial surface coverage with the PCM. In contrast, the delignified samples exhibit a surface completely covered with the PCM [14,26]. This suggests that the delignification process enhances PCM impregnation, likely due to increased sample porosity [41,42].

Additionally, the Pinus radiata samples were analyzed using micro-computed tomography (see Figure 6), revealing that the delignification process enhanced the sample porosity, as shown in Figure 4C.

The micro-CT analysis also revealed that impregnation with the eutectic CA/MA mixture as a PCM reduced the porosity of both the natural and delignified samples. The reduction in total porosity would affect the thermal and acoustic properties of the samples, as previously mentioned, because the air in the sample bulk would be replaced with a capric and myristic acid mixture [41,42,43], as shown in

Table 3. Variation in porosity of Pinus radiata samples.

Sample	Total Porosity (%)
Natural	62.37 ± 2.24
Delignified	71.75 ± 0.43
Natural + PCM	48.67 ± 5.31
Delignified + PCM	65.53 ± 2.37

.

3.4. Mechanical Tests

Figure 7A shows the effect of the delignification and impregnation of the eutectic capric/myristic acid mixture as a PCM on the longitudinal elastic, flexural, and shear moduli of Pinus radiata samples, revealing a drastic decrease after 1 h of delignification, revealing that the delignification process did not significantly influence the mechanical moduli after a longer delignification time. However, adding a capric/myristic acid mixture as a PCM reduced the longitudinal elastic moduli. Additionally, the effect of the PCM on the mechanical properties of the samples was evaluated. Figure 7B reveals that impregnation with the capric and myristic PCM does not affect the flexural and shear moduli.

3.5. Thermal Properties

Thermal conductivity plays a crucial role in evaluating the heat transfer performance of the developed samples. This study analyzed the impact of delignification and impregnation with a capric/myristic acid mixture on the thermal conductivity of cylindrical Pinus radiata samples along the longitudinal axis at 20 °C, as shown in Table 4. The natural Pinus radiata samples exhibited a thermal conductivity (k) of 0.166 W/(m·K), which increased to 0.347 W/(m·K) after PCM impregnation—a 121.68% improvement. This enhancement is attributed to the infiltration of PCM into the porous structure of Pinus radiata. Naturally, the air trapped within the wood pores acts as an insulator, reducing thermal conductivity. However, upon PCM impregnation, the air in these cavities is replaced by the PCM, which has a higher thermal conductivity than air, as reported by Temiz et al. [44]. Also, Nazari et al. [45] found an increase in thermal conductivity in wood-based samples infused with a biobased PCM, highlighting the reduction in air pockets and the formation of continuous heat transfer pathways. Additionally, the thermal conductivity of delignified PCM-impregnated samples increased by 150.68% compared to the natural sample. This improvement is likely due to the delignification process, which enhances the matrix porosity (as confirmed by micro-CT analysis), allowing for better PCM incorporation. This, in turn, facilitates more continuous phonon pathways, improving heat conduction [46]. Table 4 shows the thermal conductivity of two common building materials, fiber cement and plaster, comparable to natural wood but nearly three times lower than that of PCM-infused composites.

Incorporating PCMs into the Pinus radiata samples enhances thermal conductivity and significantly increases thermal inertia, which is determined by a material’s mass and heat capacity. As shown in Table 4, the specific heat capacity of the PCM-infused composites increases, reflecting an increase in thermal mass. The natural PCM sample and the delignified PCM sample exhibit rises by 80.31% and 122.24%, respectively. This improvement is attributed to the PCM’s ability to absorb and release thermal energy during phase transitions, leveraging its latent heat storage capacity. Consequently, the overall heat capacity of PCM composites increases, approaching that of the reference materials, plasterboard and fiber cement board.

The influence of the PCM on the thermal inertia of Pinus radiata samples aligns with findings by Nazari et al. [47], who reported that a higher thermal mass results in greater thermal inertia, allowing for more efficient thermal management during temperature fluctuations. This enhanced thermal inertia makes the developed PCM composites suitable for energy-efficient building applications, helping maintain stable indoor temperatures [48]. Vasco et al. [21] also demonstrated a rapid increase in thermal conductivity and heat capacity after impregnating Pinus radiata with octadecane. Similarly, Feng Qiu et al. [49] investigated the thermophysical properties of a novel shape-stable phase-change material (SSPCM) based on fly ash modified with lauric acid, reporting an enhancement in thermal properties due to the incorporation of the PCM.

Table 4. Variation in the thermal properties of Pinus radiata samples.

Pinus radiata	κ (W/(mK))	C (J/m3 K)	I (J/m2 s1/2 K)
Natural	1.7 × 10−1 ± 7 × 10−3	1.5 × 105 ± 9 × 104	1.5 × 102 ± 6 × 101
Natural + PCM	3.5 × 10−1 ± 7 × 10−3	3.4 × 105 ± 6 × 104	3.4 × 102 ± 5 × 101
Delignified + PCM	4.2 × 10−1 ± 2 × 10−2	3.5 × 105 ± 7 × 104	3.9 × 102 ± 4 × 101
Fiber cement board [50]	1.7 × 10−1 ± 2 × 10−3	6.9 × 105 ± 1 × 103
Plasterboard [50]	1.4 × 10−1 ± 1 × 10−3	7.7 × 105 ± 1 × 103

Table 4. Variation in the thermal properties of Pinus radiata samples.

Pinus radiata	κ (W/(mK))	C (J/m3 K)	I (J/m2 s1/2 K)
Natural	1.7 × 10−1 ± 7 × 10−3	1.5 × 105 ± 9 × 104	1.5 × 102 ± 6 × 101
Natural + PCM	3.5 × 10−1 ± 7 × 10−3	3.4 × 105 ± 6 × 104	3.4 × 102 ± 5 × 101
Delignified + PCM	4.2 × 10−1 ± 2 × 10−2	3.5 × 105 ± 7 × 104	3.9 × 102 ± 4 × 101
Fiber cement board [50]	1.7 × 10−1 ± 2 × 10−3	6.9 × 105 ± 1 × 103
Plasterboard [50]	1.4 × 10−1 ± 1 × 10−3	7.7 × 105 ± 1 × 103

The TGA of Pinus radiata samples was conducted to assess the effect of the eutectic capric/myristic acid mixture as a PCM, as shown in Figure 8. The natural sample (red curve) exhibited an initial weight loss below 100 °C due to moisture evaporation, followed by a significant mass reduction between 200 °C and 400 °C, corresponding to the degradation of hemicellulose, cellulose, and lignin, leaving behind a notable char residue [49]. The pure PCM (black curve) began degrading between 175 °C and 250 °C, indicating the volatilization and breakdown of fatty acids, consistent with the typical degradation behavior of fatty acid-based PCMs [51]. The delignified wood (purple curve) exhibited a two-stage, hemicellulose breakdown between 200 °C and 315 °C, followed by cellulose decomposition between 315 °C and 400 °C. Notably, the delignified sample exhibited a lower degradation temperature and a higher char residue. This reduction in thermal stability is attributed to the extraction steps involved in the delignification process [52]. However, despite this degradation, studies suggest that delignified wood forms a more stable carbonaceous residue due to crosslinking or dehydration reactions in cellulose, increasing char yield [53]. The PCM-impregnated Pinus radiata samples (green and blue curves) exhibited degradation onset between 175 °C and 250 °C, likely due to the decomposition of the PCM. Notably, the weight loss in the impregnated natural sample closely matched that in the untreated sample, suggesting that the PCM remains stable within the wood matrix and does not degrade it as in a pure PCM. Hanif et al. [51] observed that encapsulating a PCM within a porous wood structure improved its thermal stability. However, the delignified PCM-impregnated sample exhibited lower char formation than the natural impregnated sample, likely due to its increased porosity, facilitating oxygen penetration and accelerating degradation. Overall, the higher residual weight of the impregnated samples compared to a pure PCM suggests reduced volatilization and enhanced PCM retention within the wood structure. This stabilization of the PCM within the natural matrix highlights its potential for energy-efficient construction and insulation applications.

3.6. Acoustic Measurements

Figure 9 shows the sound absorption coefficient (SAC) variation of Pinus radiata samples resulting from delignification and impregnation with the eutectic capric/myristic acid mixture, as determined using an acoustic impedance analyzer. As can be seen, the sound absorption coefficient of the natural Pinus radiata samples varied with the frequency range, reaching a maximum SAC value at 2000 Hz, especially for the delignified sample impregnated with the PCM, which is higher compared to isolator materials used in construction applications, such as plywood panels.

It is worth mentioning that at every frequency, the delignified impregnated sample exhibits higher SAC values than the natural and natural impregnated samples, due to the higher porosity present in the delignified and impregnated sample, as shown in

Table 5. Variation in the density of Pinus radiata samples.

Pinus radiata Sample	Sound Absorption Coefficient at 2000 Hz	Density (kg/m3)
Natural	0.205	3.81 × 10+2 ± 1 × 10+1
Natural impregnated	0.215	7.19 × 10+2 ± 5 × 100
Delignified and impregnated	0.275	9.18 × 10+2 ± 1 × 10+1
B. pendula (30 mm thick) [59]	0.16	~2.34 × 10+2
P. sylvestris (30 mm thick) [59]	<0.15	~2.63 × 10+2
Hard maple [60]	0.259	4.52 × 10+2
Silver poplar [60]	0.432	4.76 × 10+2
Plywood panel [54]	0.1	-
Fiberglass board [54]	0.95	-
Painted concrete [61]	0.020	-
Styrofoam 100 kg/m2 [61]	0.22	-

. This response can be attributed to the porosity absorbing the acoustic waves, transforming them into heat [54]. As a consequence, the sound is dispersed [39,55,56,57], which can enhance the sound absorption efficiency. Lashgari et al. [58] determined that the absorption coefficient increased at lower frequencies by enhancing the bulk density of wood chip samples. Beyond a certain density level, the porosity of the samples diminished, and as a consequence, the SAC decreased, explaining the acoustic behavior of the delignified sample. Table 5 also illustrates the impact of impregnation and delignification on the density of the Pinus radiata sample.

The SAC values for the three samples analyzed at 2000 Hz varied between 0.205 and 0.275, similar to those reported by Khrystoslavenko et al. [59] for different types of wood. The authors noted the average SAC values of common wood types, birch (B. pendula), pine (P. sylvestris), and oak (Q. robur), using thicknesses of 10 mm, 18 mm, 25 mm, and 30 mm. The SAC values were close to 0.16 at 2000 Hz for 30 mm birch, 0.13 at 2000 Hz for 18 mm birch, 0.15 at 4000 Hz for 25 mm pine, and 0.12 at 5000 Hz for 30 mm birch. It is worth mentioning that the SAC values not only depend on the dimensions of samples and density but also on the structural properties, such as cellulose crystallinity, as demonstrated by Kolya et al. [60].

It has been reported that incorporating a PCM can influence the sound absorption coefficient, depending on the analyzed frequency range and concentration. For instance, Choi et al. [16] analyzed the influence of an MPCM on the sound absorption coefficient of DCWB samples using an acoustic impedance tube test at low, medium, and high frequencies. At low frequencies, the sound absorption coefficient decreased with the addition of the MPCM. At the mid-frequency, the DCWB 3, 5, and 10 wt% samples exhibited a higher sound absorption coefficient than DCWB 0%. The authors reported that samples without the MPCM exhibited better sound absorption coefficients at high frequencies. Furthermore, the authors proposed that the MPCM favored a low sound absorption coefficient as the density increased.

4. Conclusions

This work involved a delignification process and impregnation with a capric/myristic acid mixture on Pinus radiata samples. The delignification of Pinus radiata was studied to improve the impregnation of the PCM, which was achieved by vacuum impregnation. Micro-CT analysis highlights the role of the delignification process before the impregnation due to the increase in porosity. Furthermore, the mechanical properties were not significantly modified by impregnation using capric acid/myristic acid as a phase-change material, regardless of whether the samples were delignified. Thermal analysis revealed that impregnating Pinus radiata with the capric acid/myristic acid phase-change material significantly enhanced the thermal inertia. Therefore, Pinus radiata can store more thermal energy using a mixture of capric acid and myristic acid as a phase-change material. The sound absorption coefficient of Pinus radiata increases when using capric acid and myristic acid as a phase-change material, as the density increases and the porosity diminishes after impregnation.