Effect of Paraffin Impregnation Modification on Bamboo Properties and Microstructure

Abstract

Phase-change energy-storage paraffin regulates the thermal management of buildings, and the material can regulate room temperature as it absorbs and discharges heat. As a porous adsorbent material, bamboo has high permeability. The aim of this study was to increase the amount of paraffin inside bamboo and the latent heat of the phase change. It was performed using vacuum pressurization (VP) and ultra-high-pressure (UHP) impregnation treatments. The effect of UHP impregnation and properties of bamboo were studied. The weight gain, paraffin loss and dimensional changes were measured and compared. The morphology of UHP-impregnated bamboo were characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The main conclusions are as follows: After UHP impregnation, the highest weight gain was 42%. The loss of paraffin was low, and a high weight percentage gain was maintained. The crystallinity of cellulose decreased to 24% at 100 MPa. The latent heat of the bamboo slices was up to 25.66 J/g at 50 MPa, and the phase change temperature was close to room temperature. At 150 MPa, the hydroxyl content was reduced, and the hydrophilicity decreased. In addition, the content of substances such as hemicellulose in the amorphous zone was reduced under UHP, no new characteristic peaks appeared, and no chemical modifications occurred. The vascular bundles were compressed and dense, and the pores and cell gaps decreased. The thin-walled cells were deformed, and the original cell structure was completely destroyed. The surface of the cells was wrapped or covered with paraffin, confirming that the paraffin could impregnate the bamboo cells under UHP. Therefore, bamboo impregnated with paraffin can regulate temperature and save energy in buildings. It is resistant to biological attacks, and UHP improves the impregnation efficiency.

1. Introduction

Bamboo is an abundant and widely used resource in China. As an alternative to wood, it can greatly alleviate the current pressure on wood resources [1]. Bamboo has a high material utilization rate and a variety of utilization forms. Currently, it is widely used in producing furniture, building structures, garden engineering materials and handicrafts for indoor decoration [2]. With a short cycle, bamboo grows much faster than wood. On average, it will grow into a useful material within three years [3]. It is also inexpensive but is subject to a wide range of defects such as cracking, decay, warping, insect infestation and mold [4,5,6].

Bamboo is a porous material, which is beneficial for liquid penetration. The impregnation modification treatment improves the defects in bamboo and increases the utilization rate [7]. Atmospheric impregnation is slow and usually takes 1 to 2 days [8]. The vacuum-pressurization treatment could speed up the time of liquid penetration to less than 1 h. This has less impact on the properties of the material [9]. The UHP impregnation technology can increase the pressure to 100 MPa in a very short time. This further reduces the duration of impregnation. The bamboo cells are deformed and cracked under pressure, which improves the penetration between cells [10].

The application of phase-change materials (PCMs) enables the material to absorb and release heat according to temperature changes and maintain the temperature within a certain range [11]. It is commonly used in indoor building materials to adjust the temperature of the building [12]. Paraffin is an inert material that does not react with other components during the modification of wood and bamboo [13], but as a PCM it has low thermal conductivity [14,15]. Enhancing the thermal conductivity and increasing the phase change latent heat of the composite material reinforces the phase-change energy-storage performance and plays an important role in temperature regulation and energy saving in indoor environments [16,17]. In addition, it is highly resistant to biological attacks [18,19]. The material is treated with paraffin to form a protective layer on the surface, which decreases water absorption and prevents mold growth.

Paraffin is water-insoluble and hydrophobic [20], and is used more in wood than in bamboo modification. Wood has a high porosity [21], with longitudinally permeable tissues such as vessels and axial parenchyma and tangentially permeable tissues such as wood rays, including tracheids, cell wall pits, and many other cellular pores [22]. Bamboo has only longitudinal vascular bundles, parenchyma cells, and other permeability channels, making bamboo impregnation difficult [23]. The transverse permeability can be improved by vacuum pressurization [24], heat treatment [25], ultrasonic treatment [26] and microwave treatment [27]. Impregnating bamboo slices with paraffin can reduce the moisture content [28] and form a layer of paraffin film on the bamboo surface, which can reduce the contact angle. In general, after paraffin impregnation, the overall moisture content of bamboo decreases, and the paraffin on the surface can prevent external moisture from entering the bamboo cells [29].

In this study, paraffin was impregnated into bamboo slices using VP and UHP impregnation treatments. It was studied in terms of weight percentage gain, paraffin loss, size change, crystallinity, thermal effect, chemical composition and microscopic morphology. The effects of different pressure impregnation methods and UHP impregnation at different pressures on bamboo slices were compared, and the optimal treatment process was selected. Currently, there are few technologies for UHP modification of wood and bamboo, which has an effect on the impregnation penetration of the liquid. UHP modification helps improve the efficiency of the modification process, and it is expected to reinforce the energy storage performance and reduce the internal moisture content of bamboo to strengthen its dimensional stability and extend its service life. The optimal UHP treatment process provides reference conditions for future research on mold resistance.

2. Materials and Methods

2.1. Materials

Bamboo slices: 2~3-year-old moso bamboos from Banqiao Town, Lin’an District, Hangzhou City, Zhejiang Province were selected, and bamboo sections that were more than 3 m above the ground without cracks, discoloration, mold, etc., were intercepted and cut into several bamboo strips in the longitudinal direction. The outer and inner skin were removed, and the bamboo was prepared into 50 × 20 × 5 mm (longitudinal direction × tangential direction × radial direction) bamboo slices without joints. Each group of bamboo slices was sampled from the same location in the same bamboo tube, and each group was composed of 6 pieces to ensure that the material properties were similar.

High-purity paraffin: produced by Guangzhou Zhongjia New Material Technology Co., Ltd. (Guangzhou, China) with a phase transition temperature of 28 °C and a latent heat value of 180 J/g.

2.2. Preparation of Paraffin-Impregnated Bamboo

All the fresh bamboo slices were dried in the oven at 60 °C for 36 h until the quality was stable.

The UHP impregnation (BYBY600-10, Wenzhou Binyi Machinery Co., Ltd., Wenzhou, China) treatment group was heated in a water bath to 70 °C to melt the paraffin. Then, the bamboo slices and paraffin were put into the vacuum plastic bag, which was sealed with the vacuum laminator, and the air was exhausted under the vacuum degree −0.1 MPa. At last, the UHP (50 MPa, 100 MPa, 150 MPa) impregnation took place at 60 °C for 25 min.

In the VP impregnation treatment group, the bamboo slices were impregnated in a vacuum at 60 °C (vacuum 10 min, pressure 0.7 MPa maintained for 1 h).

After impregnation treatment, all the modified bamboo slices were placed in a dry indoor environment until the quality was stable.

2.3. Characterizations

2.3.1. Weight Percent Gain

The weight percent gain was determined by the quality of the bamboo slices before and after impregnation. The calculation equation was as follows:

$$\mathrm{WPG}=\frac{{\mathrm{m}}_1-{\mathrm{m}}_0}{{\mathrm{m}}_0}\times 100\%$$

(1)

where WPG is the weight percent gain (%), m₀ is the initial weight of bamboo (g) and m₁ is the final weight of bamboo (g).

2.3.2. Paraffin Loss

The paraffin loss was measured by the quality of the bamboo slices every 2 h under 60 °C in the oven until the difference between two adjacent tests was 0.002 g. The loss rate (%) was calculated as follows:

$${\mathrm{WPG}}^{\prime }=\frac{{\mathrm{m}}_2-{\mathrm{m}}_0}{{\mathrm{m}}_0}\times 100\%$$

(2)

$$\mathrm{Loss}\mathrm{rate}=\mathrm{WPG}-{\mathrm{WPG}}^{\prime }$$

(3)

where WPG′ and m₂ are the weight percent gain (%) and the weight of bamboo after drying at 60 °C (g).

2.3.3. The Dimensional Change

The dimension of longitudinal direction, radial direction and tangential direction were measured after different impregnation treatment, and the size change (%) was calculated with the equation as follows:

$$\mathrm{Size}\mathrm{change}=\frac{{\mathrm{L}}_1-{\mathrm{L}}_0}{{\mathrm{L}}_0}\times 100\%$$

(4)

where L₀ is the initial size of bamboo (mm); L₁ is the final size of bamboo (mm).

2.3.4. XRD

The bamboo sample powder was irradiated with X-ray diffraction (XRD 6000, Shimadzu, Japan), and the incident angle θ and the corresponding XRD intensity were measured using the standard detection mode. The scanning angle 2θ ranged from 5–60°, the interval was 0.02° and the scanning rate was 3°/min.

2.3.5. DSC

The differential scanning calorimetry instrument (DSC, Q2000, TA, New Castle, DE, USA) was used to detect the latent heat of the phase change of the bamboo slices. Ten mg of the sample was placed in a crucible and filled evenly. The sample volume did not exceed 2/3 of the crucible volume and was placed on the right sample rack after pressing the tablet. Meanwhile, on the left side the same empty crucible was placed as the reference plate; the measuring temperature range was 10–70 °C, and the heating rate was 5 °C/min.

2.3.6. FTIR

The bamboo slices were ground and pressed into transparent flakes, and then measured and analyzed on the Fourier transform infrared spectrometer (FTIR, Nicolet IS10, Thermo Fisher Scientific, Waltham, MA, USA). The spectral range was 500–4000 cm⁻¹, the resolution was 4 cm⁻¹ and each sample was scanned 32 times.

2.3.7. SEM

The microscopic morphology of the bamboo slices was analyzed by a scanning electron microscope (SEM, TM3030, Hitachi, Japan). The filling of paraffin on the cross section and longitudinal section of the bamboo slices was observed to judge the impregnation effect of paraffin.

3. Results

3.1. Analysis of the Weight Percent Gain of the Bamboo Slices

The analysis in

Table 1. The weight percent gain of the bamboo slices.

Pressure/MPa	Weight Percent Gain/%
VP-0.7	12
UHP-50	42
UHP-100	34
UHP-150	35

shows the weight percent gain of VP and UHP impregnation treatment. Compared with the VP impregnation treatment, the weight percent gain of the UHP-impregnated bamboo slices was 272–340% higher.

The weight percent gain was 42% at 50 MPa, and it decreased as the pressure continued to increase. This may have been due to the excessive pressure that caused the ducts and fiber cells of bamboo to compress. The paraffin was compressed before it entered the interstitial space, which may have caused more compression damage to the bamboo while the amount of impregnated paraffin decreased. In addition, the higher the pressure, the faster the relief speed [30], which leads to part of the paraffin inside the bamboo being removed while the pressure is relieved, thereby reducing the weight percent gain of the bamboo slices.

3.2. Analysis of Paraffin Loss

Table 2. Loss rate of paraffin.

Pressure/MPa	VP-0.7	UHP-50	UHP-100	UHP-150
Weight percent gain after drying/%	8	34	24	30
Loss rate/%	4	8	10	4

shows the loss rate of bamboo slices at 60 °C. It was generally not high, within 10%.

Although the loss rate of the UHP-impregnated bamboo slices was higher than that of the VP group, it was determined based on the initial weight percentage gain. The UHP bamboo slices were approximately 20%–30% higher and remained high after the loss experiment, whereas it was only 8% in the VP group. The loss rate did not increase with the pressure. At 150 MPa, the loss rate decreased to 4%. When the pressure is too high, the internal cells of bamboo are quickly destroyed, the amount of paraffin impregnated is limited and the loss rate of bamboo slices decreases. This indicates that bamboo slices can maintain high weight percent gain and a small loss rate after UHP impregnation [31]. With improvements in the permeability of the bamboo, the amount of paraffin impregnation increased. After the paraffin was dissolved, it was impregnated into the bamboo interior. The melting point of paraffin is generally 60–70 °C, and it is frozen at room temperature; therefore, it has a lower loss rate. A protective film was formed on the bamboo surface to prevent water entrance and mildew growth [32].

3.3. Analysis of the Size Change of the Bamboo Slices

Table 3. The size change of the bamboo slices.

Longitudinal Direction/%			Tangential Direction/%			Radial Direction/%
50 MPa	100 MPa	150 MPa	50 MPa	100 MPa	150 MPa	50 MPa	100 MPa	150 MPa
0	0	−1	18	17	16	11	11	11

shows the size change of UHP-impregnated bamboo slices in different directions. The UHP greatly impacted the weight percentage gain and changed the size of the bamboo slices [33]. There was a slight extension in the longitudinal direction, with an average of 0%, which was evident in both the tangential and radial directions. The tangential direction shrank more significantly than the radial direction, reaching 18%. The size change at different pressures was not significantly different; higher pressures did not result in greater shrinkages of the bamboo slices. The shrinkage reached a maximum at 50 MPa.

Based on the cell structure of bamboo, vascular bundles split and thin-walled cells deformed after UHP impregnation, reducing bamboo size. Bamboo has many vertical fiber cell compositions; a single fiber is long and difficult to compress, and the cross-sectional structure is mainly bamboo joints. There are mostly internode-disordered vascular bundles and a small number of pores [34]. In the UHP-dipping tank, the pressure applied to the bamboo slices was evenly distributed, but no proportional shrinkage occurred, which was related to the cell structure of the bamboo slices. The raw material was thin and hollow and had not been flattened. After the outer and inner parts of the bamboo were removed, the radial thickness decreased, which caused the UHP to limit the radial compression on the bamboo slices.

3.4. XRD Spectrum Curve Analysis of the Bamboo Slices

The XRD patterns of the untreated, vacuum-pressurized and UHP-impregnated bamboo slices are shown in Figure 1. Three crystal peaks were observed at approximately 16.04, 21.74, and 43.94°. There were no changes in the position and height of the crystalline peaks compared to those of the untreated bamboo slices. The results showed that the different pressures did not affect the crystal structure of bamboo cellulose.

It can be seen from

Table 4. Crystallinity of the bamboo slices treated with paraffin at different pressure.

Pressure/MPa	CrI/%
0 MPa	39
VP−0.7 MPa	36
UHP−50 MPa	36
UHP−100 MPa	24
UHP−150 MPa	32

that the crystallinity of cellulose is 39%, which changed less after vacuum pressurization and 50 MPa pressure impregnation treatments, that is, 36% and 36%, respectively. The crystallinity decreased to 24% at 100 MPa and increased to 32% at 150 MPa. This is related to the degree of cell destruction within the bamboo slices at different pressures [35]. Low-pressure vacuum impregnation did not affect the cellular structure of the bamboo slices and had less influence on the crystalline properties of cellulose compared with UHP treatment. At 50 MPa, the cellular structure was only slightly compressed; the fiber cells were subjected to less compressive forces, and the crystallinity of the cellulose exhibited no clear change. When the pressure was increased to 100 MPa, the thin-walled cells underwent severe deformation and cracking under UHP, and the adjacent fiber cells were crushed and broken, leading to a reduction in cellulose crystallinity [36]. When the pressure continued to increase, the compression of the bamboo cells under 150 MPa pressure further increased, the fiber cells were more closely aligned and the crystallinity of the cellulose increased [37].

3.5. DSC Analysis of the Bamboo Slices

Figure 2 shows DSC endothermic and exothermic curves of the raw material, paraffin, and two different methods of impregnation with paraffin. Paraffin has high performance of phase-transition heat storage. The phase-change melting temperature is 31.92 °C, and the latent heat of melting is 168.72 J/g; the freezing temperature is 19.72 °C, and the latent heat of freezing is 166.91 J/g.

Table 5. DSC of the bamboo slices with different impregnation treatments.

	Pressure (MPa)	VP-0.7	UHP-50	UHP-100	UHP-150
Melting	Temperature (°C)	27.33	25.66	26.19	25.16
	Latent heat (J/g)	11.58	24.66	16.73	18.89
Freezing	Temperature (°C)	21.17	21.27	23.47	21.70
	Latent heat (J/g)	11.52	24.34	13.16	17.76

shows the DSC test results of VP and UHP impregnation treatment bamboo slices.

This indicates that bamboo does not affect phase-change energy storage. The latent heat of the phase change of the modified bamboo was much lower than that of paraffin, which was mainly related to the amount of impregnated paraffin [38]. The properties of the paraffin changed during the UHP treatment. There were two convex peaks when the latent heat of freezing of the bamboo slices was measured. The smaller peak is the freeze–freeze phase transition peak, the larger peak is the freeze–melt phase transition peak and the actual latent heat of the phase change of the bamboo slices is the sum of the two peak areas [39]. Under different pressure conditions, the latent heat of the phase transition first decreased and then increased, indicating that the amount of impregnated paraffin was affected by the pressure impregnation and relief process. The paraffin content and the latent heat were higher in the UHP impregnation treatment than in the vacuum pressurization treatment. At 50 MPa, the weight gain and the impregnation of paraffin were the highest. Pressure mainly increases the impregnation of paraffin and does not affect the crystallinity of cellulose, and the latent heat of the bamboo slices is the highest at 50MPa. The melting temperature of paraffin is 25.76 °C, and the latent heat of melting is 24.66 J/g; the freezing temperature is 21.69 °C, and the freezing latent heat is 24.34 J/g. At 100 MPa, paraffin impregnation is reduced in the pressure-relief process of the equipment. The fiber cells were compressed and partially fractured, and the destructive effect of pressure on the fiber cells was greater while the amount of impregnated paraffin decreased, resulting in a reduction in both the crystallinity and latent heat of the cellulose of the bamboo slices. At 150 MPa, due to the high pressure, some of the paraffin could enter the bamboo cells, and the impregnation amount increased. The latent heat of the bamboo slices increased, while the compression force on the fiber cells was elevated, the arrangement was tighter and the crystallinity of the cellulose increased [40].

3.6. FTIR Analysis of the Bamboo Slices

Figure 3 shows the infrared spectra of the paraffin, untreated bamboo, and vacuum-pressurized and UHP-impregnated bamboo slices. The peak at approximately 3463 cm⁻¹ is the hydroxyl group absorption peak of hemicellulose, cellulose and lignin [41]. At 150 MPa, the height of this peak decreased significantly because the paraffin entered the interior of the cells, thereby reducing the hydrophilicity of the bamboo slices. The peak at approximately 2928 cm⁻¹ is the absorption peak for the aromatic methyl and methylene stretching vibrations and paraffinic hydrocarbons [42]. The main component of paraffin is a mixture of a range of common alkanes; therefore, an absorption peak appears at this position [43,44]. In addition, the asymmetric deformation vibrations of paraffin methylene absorption peaks [45] are near 1457 cm⁻¹ and 1347 cm⁻¹, and the in-plane wobble vibration peak of paraffin methylene [46] is near 720 cm⁻¹. The absorption peaks at approximately 1745 cm⁻¹ and 1639 cm⁻¹ are the non-conjugated carbonyl group stretching vibration and aromatic skeleton vibration [47], respectively, and the peak at approximately 1053 cm⁻¹ is the ether link stretching vibration absorption peak in the amorphous region of cellulose [48]. The heights of the three peaks decreased, indicating that the hemicellulose content was reduced under the UHP. The modified bamboo slices combined the characteristic absorption peaks of bamboo and paraffin, and the absorption peaks mostly overlapped. No new absorption peaks were observed in the spectrum, further indicating that the modified bamboo slices did not generate new groups. Paraffin and bamboo are primarily formed by intermolecular forces. The intensity of each characteristic peak under different pressures showed little difference, indicating that pressure had little effect on the chemical composition of the bamboo slices.

3.7. SEM Analysis of the Bamboo Slices

Figure 4 shows the SEM microstructures of the cross- and radial sections of bamboo slices after treatment with 150 MPa, the impregnation temperature is 70 °C and the time is 10 min. The internal cell structures of bamboo slices after UHP impregnation with paraffin wax were observed at the levels of 1 mm, 500 μm and 200 μm at 60, 200 and 500 times magnification respectively. Figure 4a–c and Figure 4g–i are the cell structure of untreated bamboo, while Figure 4d–f and Figure 4j–l are the cell structure of UHP-impregnated paraffin-treated bamboo. Figure 4a–f are the cell structure of the cross-section of bamboo, and Figure 4g–l are the cell structure of the radial section of bamboo.

Figure 4d shows the cross-section of the bamboo treated with UHP impregnation of paraffin. The cell interstices, ducts, thin-walled cells and other tissues were completely filled with paraffin. The main pore structures were squeezed and the number of pores, such as vascular bundles and thin-walled cells, was reduced, indicating that macromolecular paraffin can be impregnated into the interior of bamboo cells under UHP. As shown in Figure 4e, the vascular bundles were crushed by compression, the small thin-walled cells inside were completely crushed, the ducts were filled, the xylem was squeezed and compacted, and no extrusion cracking occurred. The basic outer outline of the vascular bundle was still visible, and the surrounding thin-walled cell tissue was compressed. The distance between adjacent vascular bundles decreased, and the density of vascular bundles per unit area increased, promoting the degree of compactness of the bamboo slices. Figure 4f shows that the thin-walled cells were extruded, deformed and crumpled together. The round or oval cell structure was extruded and crumpled after UHP impregnation with paraffin, and there was severe deformation and cracking. However, some starch granules may have been lost owing to compression.

From the analysis of the radial section, the thin-walled cell tissues were intact and arranged (Figure 4g), whereas the cells were damaged during the UHP treatment, producing larger cracks, and the surface was covered by more paraffin (Figure 4j). The fibers around the thin-walled cells were damaged under pressure, whereas most of the long longitudinal fibers, as well as the tissues, such as the ducts, remained intact (Figure 4k). The fibers and ducts were covered with a large amount of paraffin, and the fibers were more closely arranged under pressure (Figure 4k,l)

4. Conclusions

In this study, bamboo was treated with paraffin via VP and UHP impregnation. The weight gain and paraffin loss rates of the bamboo slices were measured. Dimensional changes were measured after UHP impregnation. XRD, DSC and FTIR patterns were analyzed, and cellular changes were observed using SEM. The main conclusions are as follows.

(1)

After UHP impregnation, the weight gain rate of the bamboo was higher than that after vacuum-pressurized impregnation, with a maximum weight gain rate of 42% at 50 MPa. The loss of paraffin was low, and a high rate of weight gain was maintained.

(2)

The UHP greatly impacted the dimensional changes, and the shrinkage rate of the tangential dimension was higher than that of the radial dimension, with a maximum of 18%. The average rate of change of the longitudinal dimension was 0%; that is, the transverse compression of the bamboo slices was higher than that of the longitudinal slices.

(3)

The crystallinity of the cellulose decreased to 24%, owing to the compression and destruction of the fiber cells. When the pressure was greater than 100 MPa, the fiber cells were further compressed and arranged more closely, and the crystallinity increased to 32%.

(4)

The latent heat of the bamboo increased after paraffin impregnation, with the best latent heat performance at 50 MPa and a latent heat temperature close to room temperature.

(5)

The hydrophilicity of the bamboo was reduced. There was a reduction in the amount of material in the amorphous region subjected to UHP. There is only a physical interaction between the paraffin and bamboo.

(6)

Thin-walled cells were squeezed, deformed and cracked, and their surfaces were completely covered with paraffin. The ducts were compressed and became smaller, and the xylem became denser. The fibroblasts adjacent to the thin-walled cells were crushed, and most of the remaining fibroblasts were more closely aligned.

The UHP impregnation treatment improved the impregnation and penetration of paraffin into the bamboo cells. It can be used for temperature regulation and energy saving in buildings. The surface of the bamboo sheet wrapped in paraffin resists mold growth.