Solid wood utilization of Litchi: Study on physicochemical properties and drying technology

doi:10.21203/rs.3.rs-4976979/v1

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Solid wood utilization of Litchi: Study on physicochemical properties and drying technology

https://doi.org/10.21203/rs.3.rs-4976979/v1

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Litchi is an important economic crop with a wide planting area and large yield worldwide. To explore the solid wood utilization value of litchi, this work systematically tested the physical and chemical properties of litchi wood, summarized the drying characteristics using the 100℃-Test Method, and formulated the drying schedule of litchi wood to evaluate the drying quality. The results showed that the drying time of litchi wood from an initial moisture content of 41.38–11.28% was 435 hours, and the drying rate was 0.069%/h. According to GB/T 6491 − 2012 "Sawn Timber Drying Quality Standard," the comprehensive quality grade of litchi wood was grade 2. Dried sawn timber can meet the quality requirements of solid wood furniture and handicrafts. In addition, the porous structure of litchi wood mainly consisted of micro and medium pores, with a porosity of 31.93% and an average pore size of 23.07 nm. The small pore cavities on the cell wall resulted in difficult water transfer, leading to a slow drying speed. However, the porous structure of litchi wood has a positive effect on dimensional stability.In addition, the shrinkage grade of litchi wood was medium, comparable to white oak, ash, and rubber wood under the same environmental conditions. Moreover, the linear and volumetric swelling rates of litchi wood were only higher than rubber wood but lower than white oak. Litchi wood exhibited a larger hygroscopic hysteresis, significantly higher than that of control wood, indicating better dimensional stability. In conclusion, litchi wood is an excellent renewable biological resource and has excellent drying, processing, physical, and mechanical properties, making it a potential candidate for solid wood utilization.

Litchi wood

Solid Wood utilization

Wood drying

Dimensional stability

Dynamic vapour adsorption

Litchi (Litchi. Chinensis Sonn.) is a subtropical hard-wood species with sweet and delicious fruits(Qin et al., 2024). The Data show that the planted area of litchi is about 800,000 hm² all over the world in 2023, which is cultivated in about 32 countries in the world, and China has the largest area of litchi cultivation in the world(Chen et al., 2023; Wang et al., 2020), accounting for about 65% of the total planted area of the world(Chen et al., 2023). Litchi is one of the most important economic crops in China. In addition, litchi wood is an excellent hardwood species. Generally speaking, the litchi wood with a good growth condition is dark in color with beautiful texture, as well has the advantages about solid material, corrosion resistance, decay resistance and other, known as “Chinese rose wood” (Liang, 2015; Chen, 2013). Depending on the age of the tree, the diameter at breast height of the litchi wood is generally 15–30 cm, with a maximum of 156 cm (Liang et al., 2015), which is can be highly utilizable. Therefore, the solid wood furniture and crafts made of litchi wood are also popular with people. Nowadays, due to climate warming, litchi has flowering defects(Huang et al., 2024), the fruiting rate is greatly reduced, and the effective life of most litchi is only 20 ~ 30 years. These trees will be felled with no longer bear fruit by fruit farmers. In addition, in order to ensure the yield of litchi, farmers have to trim the branches of litchi every year, which leads to a large amount of litchi wood resources surplus(Chen et al., 2023).

Prior to this study, several scholars have investigated the applications of litchi wood. Li et al. utilized litchi wood as the primary raw material for particleboard production (Li et al., 2024, 2023). Chen et al. processed litchi wood into wood-plastic composite materials (Chen et al., 2023). Huan et al. modified litchi wood with Fe or Mn elements after converting it into biochar, applying it to remediate land contaminated by heavy metals (Huan et al., 2020). Additionally, Yang et al. conducted a chemical analysis of litchi wood vinegar, discovering that it possesses significant antioxidant and antibacterial properties, making it a potential substitute for conventional antioxidants and antibiotics (Yang et al., 2016). However, the majority of these studies have either limited applications or are associated with high costs.

The utilization of solid wood is a crucial method for enhancing the added value of wood, owing to its unique color, aroma, and texture. Solid wood utilization preserves the natural properties of wood, resulting in furniture that is not only aesthetically pleasing and durable but also contributes positively to energy conservation and emission reduction (Geng et al., 2019). In recent years, the supply of solid wood materials has become increasingly scarce due to rising demand in both domestic and international furniture markets. Consequently, the exploration and development of superior solid wood substitute materials have become a significant focus for researchers (Wang and Xu, 2020; Ye, 1999). Litchi wood, an agricultural residue with a broad cultivation area and excellent performance characteristics, presents a promising alternative for solid wood applications. The key factor in the machinability of solid wood utilization is wood drying, which significantly influences the cost and efficiency of wood processing (Bond and Espinoza, 2016; Cheng et al., 2023; Yin and Liu, 2021). However, systematic studies on the drying characteristics and wood properties of litchi wood are currently lacking, particularly in terms of dimensional stability, mechanical properties, and machinability.

To optimize the utilization of solid litchi wood resources, this study evaluated its drying characteristics and proposed an appropriate drying process. The dimensional stability and mechanical properties of litchi wood were assessed in comparison with commonly used commercial woods. The pore structure of litchi wood was characterized using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP), while its moisture absorption properties were analyzed through dynamic vapor sorption (DVS). We hope that this research will draw attention to litchi wood, address its current underutilization in the solid wood market, and facilitate its industrial production, thereby enhancing the value-added utilization of the litchi crop industry chain.

1.1 Materials

The primary material used in this study was litchi wood, while the control materials included white oak (Quercus alba L.), red oak (Quercus rubra), ash (Fraxinus excelsior L.), and rubber wood (Hevea brasiliensis). Their air-dry densities are presented in Table 1. The average moisture content of litchi wood was 46.66%. After drying at 70°C, the defect-free portions were ground into wood powder using a cutting mill. The litchi wood powder with a particle size of less than 60 mesh was separated using a screen, then baked to absolute dryness at 103°C, and stored in a sealed bag. This portion of the sample was tested for chemical composition. A string-cutting plate without natural defects and without a pith core was selected and cut into 10 pieces of 200 mm (straight grain) × 100 mm (tangential) × 20 mm (radial) sawn timber using a precision push table saw for the evaluation of drying characteristics. A total of 25 pieces of litchi sawn timber of 1000 mm × natural width × 30 mm were selected for the drying process test. Twelve random samples were selected from these for the testing of drying quality, and the remaining samples were used for other performance tests.

Table 1

Air-dry density of litchi wood and control wood
	Litchi wood	White Oak	Red Oak	Ash	Rubber wood
Density(cm/g³)	0.98	0.61	0.70	0.68	0.65

1.2 Drying test method

2.2.1 Evaluation of drying characteristics

The drying characteristics were tested by using the100℃ test method proposed by Professor Masa Terasawa (He, 1998) and related studies (Lu et al., 2021; Wang, 2002; Luo et al., 2019; Cai et al., 2002) discussed.

2.2.2 Drying schedule

According to the 100℃ test method, the comprehensive grades of initial cracking, internal cracking, cross-sectional deformation, and drying speed of litchi wood are all 4 and above, indicating that the drying defects are serious and that it belongs to a difficult-to-dry wood species. Therefore, to ensure drying quality, it is necessary to apply a more moderate drying schedule along with timely intermediate and final humidification treatments. Referring to the 100℃ test method for drying defect grades, and based on relevant research (Cai et al., 2002; He, 1998) and pre-tests, the drying schedule for 30mm thick litchi wood sawn timber was determined as shown in Table 2.

Table 2

Moisture content drying schedule for litchi lumber with 30mm thickness
Stage		Moisture content (%)	Dry bulb temperature (℃)	Wet bulb Temperature (℃)
	Thermal pretreatment		40	39
1		> 40	40	39
2		40 ~ 35	42	39
3		35 ~ 30	45	41
	Intermediate treatment		45	41
4		30 ~ 25	48	43
5		20 ~ 25	55	47
	Intermediate treatment		55	54
6		20 ~ 15	60	49
7		< 15	65	51
	Final moisture treatment		65	62

2.2.3 Drying technology test

In this paper, according to LY/T 1068–2012 " Technology rules of kiln drying swan timber ", two chord-cut plates without joints and cracking defects were selected as water content and stress test plates. Before drying, samples of 10mm were intercepted at 50mm of the end of the moisture content test plate and weighed. After drying with 103 ± 2℃ oven, the mass was weighed and the initial moisture content was detected. The two ends of the test plate are sealed with sealant. During the drying process, the moisture content test plate is removed from the drying kiln every 24h, its quality is measured, and the moisture content of the sample during the drying process is calculated according to Eq. (1).

$$\:Mi=\frac{{m}_{1}-{m}_{0}}{{m}_{0}}\:\:$$

Where: M_i is the moisture content of the specimen (%); m₁ is the specimen mass (g); m₀ is the specimen's absolute dry mass (g).

The stress test specimens were prepared by slicing method. The specimen of 10mm is cut along the chord, and the specimen is cut into 5 equal parts according to the thickness. After the test piece is placed in a cool and dry place for 24h, its length l and deflection f are measured, and the stress index Y of the test piece during the drying process is calculated according to Eq. (2).

$$\:{Y}_{n}=\frac{f}{l}\times\:100\%$$

Where: Y_n is the stress index of layer n test piece; f is the deflection of the test piece after deformation (mm); l is the length of the test piece (mm).

1.3 Dimensional stability test

Since there is no uniform standard for dimensional stability testing, the dimensional stability was characterized by the dry shrinkage and dry and wet expansion of different woods. The dry shrinkage and wet swelling of litchi wood and control wood were determined according to GB/T 1927–2021 "Test method for physical and mechanical properties of small clear wood specimens - Part 6: Determination of shrinkage" and "Test method for physical and mechanical properties of small clear wood specimens - Part 8: Determination of swelling".

1.4 Physical and mechanical properties

The moisture content and density of litchi wood were determined according to the Part 4 and Part 5 of GB/T 1927–2021 “Test methods for physical and mechanical properties of small clear wood specimens”. According to GB/T 1927–2021 Part 9, Part 10, Part 11 and Part 19, the flexural strength, flexural elastic modulus, compressive strength parallel to grain and hardness were tested respectively.

2.4 Main chemical composition content

The content of cellulose and hemi-cellulose in litchi wood was quantitatively determined by the method of National Renewable Energy Laboratory (NREL). According to GB/T 2677.6–1994 " Fibrous raw material-Determination of solvent extractives" and GB/T 2677.8 1994 " Fibrous raw material-Determination of acid-insoluble lignin" standard determination of benzene/ethanol extract, ash and lignin content in litchi wood.

2.5 Mercury intrusion porosimetry (MIP)

The mercury injection instrument (Auto-Pore IV 9500, America) was used to tested the porosity and pore size distribution of litchi wood. The cylindrical litchi wood with diameter of 25mm and thickness of 2.5mm after drying was placed in a mercury injection chamber, liquid mercury was added to the chamber, and then the indoor pressure was increased from 0.1psia to 60000psia. The balance time between each pressure is set to 10s. The aperture distribution is determined according to Washburn Eq. (3):

$$\:\tau\:=\frac{2\gamma\:\cdot\:\text{cosθ}}{p}$$

Where: τ is the pore radius, mm; p is pressure, psia; γ = 0.48N /m, is the surface tension of mercury; θ = 140° is the wetting Angle of mercury.

2.6 Dynamic water absorption (DVS)

The dynamic water absorption analyzer (DVS Adventure, United Kingdom) was used to measure the water absorption properties of Litchi wood, White Oak (WO), Red Oak (RO), Ash, and Rubber wood (RW). The moisture absorption/desorption isotherm was also drawn. The test sample size was 2 mm (tangential) × 2 mm (radial) × 2 mm (along the grain), with an average mass of 30 mg. During the hygroscopic process, humidity was increased in 10% increments. The relative humidity was increased from 0–95%. After the hygroscopic process was completed, the relative humidity was decreased from 95–0%. The internal temperature of the instrument was maintained at a constant 25℃, and the sample quality and internal relative humidity of the instrument were automatically recorded every 60 seconds. When the mass change of the sample at each humidity stage was less than 0.002%·min-1, it was considered that adsorption at this stage had reached equilibrium, and the process automatically proceeded to the next adsorption stage. The test was repeated twice for each tree species to ensure data accuracy.

2.7 Scanning electron microscope (SEM)

Scanning electron microscopy (SEM, S-4800 FE-SEM, Japan) was used to observe the microstructure of the chord section and diameter section of litchi wood. The wood rays, ducts, pores and pitted pores in litchi were observed. The photos were taken at different magnifications and analyzed.

3.1 Drying characteristics

Based on the 100℃ test method, the drying characteristics of litchi wood were statistically analyzed. As shown in Table 1, the comprehensive drying rate was classified as grade 5, indicating that the drying rate of litchi wood is very slow. The overall grade for initial cracking was 4, with most cracking defects being short and fine, and no wide cracks observed. The comprehensive grade for section deformation was also 4, with most specimens exhibiting a narrow middle and thick sides on the cross-section, suggesting a tendency for shrinkage defects during the drying process. Additionally, most litchi wood samples displayed internal cracks during drying, with the comprehensive grade for internal crack defects being 4. These findings indicate that litchi wood is prone to internal cracking during the drying process. This serves as a preliminary basis for formulating an appropriate drying schedule.

Table 3

The drying defects and grades of Litchi wood
Number	Initial check	Internal check	Cross-section deformation	Drying rate
1	No.3	No.3	No.2	No.4
2	No.2	No.4	No.4	No.5
3	No.2	No.1	No.3	No.4
4	No.4	No.3	No.3	No.4
5	No.3	No.4	No.4	No.5
6	No.3	No.3	No.3	No.4
7	No.2	No.4	No.4	No.5
8	No.3	No.2	No.4	No.4
9	No.2	No.2	No.3	No.4
10	No.2	No.4	No.4	No.4
Comprehensive grade	No.4	No.4	No.4	No.5

3.2 Drying technology analysis

3.2.1 Moisture content, temperature and drying speed rate

During the entire drying process, to maintain an appropriate drying speed, the temperature and relative humidity of the drying medium should be controlled according to the actual moisture content of the wood (Tu et al., 2004; Cai et al., 2002). Based on the drying schedule (Table 2), the drying curve and drying rate curve are illustrated in Fig. 2a and 2b, respectively. It is observed that the initial moisture content of litchi sawn wood with a thickness of 30 mm was, on average, 41.38%, and the final moisture content after drying was 11.28%. The total drying time was 18.125 days, resulting in a drying rate of 0.069%/h. As shown in Fig. 2b, the drying rate of litchi wood exhibited an overall trend of initially increasing and then decreasing. The drying rate during the first stage (MC > 40%) was 0.086%/h, which is lower than that of the second (40% < MC < 35%) and third stages (35% < MC < 30%). This is because, during the first stage, the moisture content of the wood is higher than 40%. To prevent surface and end cracks, a low temperature and a small dry-wet bulb temperature difference of 3℃ were employed. When the moisture content dropped below 40%, the dry bulb temperature increased and the temperature difference between the dry and wet bulbs widened, resulting in an increased drying rate. During the fourth stage (30% < MC < 25%) and subsequent stages, the drying rate slowed progressively as the moisture content decreased. This is because the moisture content inside the wood gradually fell below the fiber saturation point, leading to the evaporation of absorbed water and an increase in the required drying power. Additionally, the high density of litchi wood makes it difficult for vapor and liquid water to migrate to the wood surface, further slowing the drying rate. Therefore, it is necessary to increase the drying temperature and the dry-wet bulb temperature difference during the later stages of drying litchi wood sawn timber.

3.2.2 Layer moisture content and stress change in drying progress

The layer moisture content and drying residual stress of litchi wood are illustrated in Fig. 3. As depicted in Fig. 3a, during the drying process from an initial moisture content of 41.38–11.28%, the moisture content of the surface and core layers decreased at different rates over time. Notably, in the first six days, the surface layer exhibited the fastest decrease. By the 6th day, the moisture content of layers 1 and 5 and layers 2 and 4 were 17.71%, 19.67%, 28.67%, and 31.02%, respectively, all of which were below the fiber saturation point (FSP). In contrast, the moisture content of layer 3 was 37.29%, exceeding the FSP, indicating the highest moisture content gradient and the fastest drying rate within the wood. As the surface layer's moisture content gradually decreased, the core layer maintained a consistent drying rate until the 8th day, when the moisture content of layer 3 reached 30.29%. Subsequently, the drying rate of litchi sawn timber slowed down as the moisture content gradient began to diminish. By the 12th day, the moisture content of layers 1, 5, 2, and 4 had reached the equilibrium moisture content (EMC), measuring 14.39%, 15.19%, 17.01%, and 17.59%, respectively, while the moisture content of layer 3 was 23.26%. At this stage, the moisture content of both the surface and core layers was below the FSP, and the moisture content gradient had decreased. By the 18th day, the moisture content gradient had further reduced to 2.56%, meeting the moisture content deviation requirements specified in GB/T 6491 − 2012 "Drying Quality Standard for Sawn Timber," thus concluding the drying process.

Correspondingly, changes in wood moisture content were accompanied by variations in drying stress (Fig. 3b). At the onset of the drying process, the moisture content within the wood was uniform, and no strain was generated. By the end of the 3rd day, the moisture in layers 1 and 5 had evaporated due to heat, reducing the moisture content below the FSP and initiating free shrinkage. The shrinkage of layers 1 and 5 was constrained by the undried inner layers, generating tensile stress, while the compressive stress in the inner layers balanced the surface layer. By the 6th day, due to the maximum moisture content gradient, the tensile stress in layers 1 and 5 peaked, and the corresponding maximum compressive stress in the inner layers was the highest. After 8 days of drying, the moisture content in the wood core layer also fell below the FSP, leading to free shrinkage and a reduction in tensile stress in layers 1 and 5. By the 12th day, layers 1, 5, 2, and 4 had gradually reached the EMC, ceasing to produce free shrinkage. At this point, the moisture content of layer 3 was 23.26%, and its free shrinkage was constrained by layers 1, 5, 2, and 4, resulting in tensile stress. Consequently, layers 1 and 5 transitioned from tensile stress to compressive stress. At the end of the drying process, the moisture content across all wood layers was relatively uniform, with residual stress causing minor strain; layers 1, 5, 2, and 4 exhibited compressive stress, while layer 3 exhibited tensile stress. The stress variation observed during the drying process of litchi sawn timber aligns with the findings of Tu et al. (2009). During the drying process, surface water evaporates first, and water from the core layer migrates to the surface, creating a moisture content gradient with higher levels inside and lower levels outside. The greater the moisture content gradient, the faster the drying rate and the higher the drying stress, consistent with the observations of Cai et al. (2003).

3.2.3 Drying quality

According to the requirements of GB/T 6491 − 2012 " Drying quality of sawn timber", the drying quality and visible drying defects of litchi wood were statistically analyzed, as shown in Tables 2 and 3. The statistical results showed that the average moisture content of dried litchi was 11.28%, and the single quality grade was grade 2. The drying evenness and thickness deviation of drying were 1.04% and 2.56%, and the individual quality grades were all grade 1. The residual stress value is 0.22%, and the single quality grade was 2. In addition, the average bending degree and distortion degree of litchi wood were less than 1%, and the single quality grade was 1. The average crook grade is 0.85%, and the individual quality grade was 2. The average cross-section deformation is 0.79mm, and the individual quality grade was 2. All the dried litchi sawn timber had no internal crack or longitudinal crack with a width greater than 2mm, and the single quality grade was grade 1. The above results showed that the comprehensive quality grade of visible defects of litchi sawn timber was grade 2 by using the drying schedule summarized in this study. It shows that the drying parameters set in this paper are reasonable, and the quality of drying samples can meet the requirements of national standards, and can meet the manufacturing quality requirements of furniture, building wood, solid wood flooring and other solid wood products.

Table 3

The statistic of drying quality of litchi wood
	Moisture Content/%	Drying evenness/%	Thickness moisture content deviation/%	Residual stress/%
Average value	11.28	1.04	2.56	0.22
Grade	No.2	No.1	No.1	No.2

Table 4

The statistic of visible drying defects litchi wood
	Bow (%)	Crook (%)	Twist (%)	Slit (%)	Internal check(%)	Collapse depth (mm)
Average value	0.28	0.85	0.65	0	0	0.79
Grade	No.1	No.2	No.1	No.1	No.1	No.2

3.3 Main chemical composition content

Cellulose, hemicellulose, and lignin are the primary components of the wood cell wall, significantly influencing the mechanical strength and dimensional stability of wood (Yu et al., 2019). The main chemical composition of litchi wood is illustrated in Fig. 4. The analysis revealed that the contents of cellulose, hemicellulose, and lignin in litchi wood were 39.24%, 13.80%, and 38.50%, respectively, with ethanol extractives accounting for 5.98%. In comparison, the cellulose, hemicellulose, lignin, and extractives content in typical hardwoods are 45%, 30%, 20%, and 3.5%, respectively (Liu & Zhao, 2012). Notably, the hemicellulose content in litchi wood is significantly lower, whereas the lignin and extractives contents are higher than those in general hardwoods. The high lignin content contributes to the dark color of litchi wood and affects its dimensional stability and mechanical strength.

3.4 Microstructure and pore characteristics

To investigate the impact of the pore structure of litchi wood on fluid permeability, we quantitatively characterized the pore structure using Mercury Intrusion Porosimetry (MIP) and visually observed it through Scanning Electron Microscopy (SEM). The results of the mercury intrusion are presented in Fig. 5(a) and (b). As illustrated in Fig. 5(a), liquid mercury begins to penetrate the internal pores of the litchi wood sample when the pressure reaches 74 psia. With increasing pressure, the mercury suction rate stabilizes because the larger pores remain unfilled. At a pressure of 2300 psia, liquid mercury starts to infiltrate the smaller pores, resulting in a rapid increase in the mercury suction rate, which reaches its peak. When the pressure reaches 35000 psia, the medium-sized pores in the litchi wood sample are completely filled, and the liquid mercury gradually enters the micropores until all pores are invaded, causing the suction rate to decrease gradually and eventually reach zero. This preliminary finding indicates that medium-sized pores predominantly exist in litchi wood (Xiang et al., 2021), mainly within the cell walls, their pores, and microfilament gaps.

The mercury withdrawal curve of litchi wood reveals that it does not retrace the mercury intrusion curve as pressure decreases, exhibiting significant hysteresis. This behavior suggests the presence of ink-bottle pores in litchi wood (Ding et al., 2008; Plötze and Niemz, 2011; Vitas et al., 2019). Pores of varying sizes are connected in series to form bottleneck pore channels, which facilitate fluid accumulation within the material but hinder fluid discharge (He et al., 2024). Consequently, the presence of ink-bottle pores in the pore structure of litchi wood leads to challenging water migration and slow drying, contributing to the prolonged drying period of litchi wood.

Figure 5(b) illustrates the relationship between pore size and integral pore volume, obtained through the logarithmic integration of the mercury intrusion curve of litchi wood, representing the pore volume change rate corresponding to each pore size. For litchi wood, the highest increase rate in pore volume corresponds to a pore size of 17.12 nm, indicating the highest number of micropores at this size. Additionally, there are relatively many pores around 4.52 nm and 349.3 nm, suggesting the presence of some medium-sized pores and a few large pores. It is evident that the cell wall of litchi wood contains more small pores, primarily micropores and medium-sized pores, with almost no large pores. The porosity of litchi wood is 31.93%, and the average pore size is 23.07 nm (Table 4).

Table 4

Porosity and average pore size of Litchi wood
Sample	Porosity (%)	Average pore size(nm)
Litchi wood	31.93	23.07

Radial and tangential sections of litchi wood were examined using scanning electron microscopy (SEM), with the results presented in Fig. 6(a)-(f). The cross-sectional analysis revealed that litchi wood is a diffuse-porous wood with a sparse distribution of conduits, most of which are arranged in a solitary vessel pattern. Occasionally, multiple vessel elements were observed, containing white deposits. The cell lumina are small, and the cell walls are thick. In the radial section, the wood rays of litchi wood appeared slender and densely packed, exhibiting a non-overlapping structure. These rays are homocellular, predominantly single-rowed, occasionally double-rowed, with ray cell heights ranging from 6 to 10 cells. These observations are consistent with those reported by Chen et al. (2013). The vessel walls of litchi wood exhibited few attachments and numerous perforations, primarily simple perforation plates, with some scalariform perforation plates of similar length and width, which were unblocked. The average sizes of the pit cavity and pit aperture in litchi wood were measured to be 5.33 µm and 0.47 µm, respectively, using Digimizer.

Wood is a natural porous material characterized by a multi-layered, hierarchical pore structure. The large pore structures, such as vessels and tracheids, facilitate water transport and provide structural support, while the rich pore network enhances connectivity and improves water transport efficiency. The specific features of ducts, rays, and the number and size of pits enable litchi wood to exhibit efficient water migration (Liu and Zhang, 2021). However, the dense pore structure impedes water migration during the drying process, potentially leading to the formation of high-pressure water vapor within the cells, resulting in deformation or cracking. Additionally, during the drying process, surface tension effects cause the wood pores to shrink, reducing porosity and affecting water transport efficiency. The small diameter of the cell lumina and the thick cell walls of litchi wood contribute to a low water diffusion rate and prolonged drying time, which are primary factors for the low timber yield of litchi wood.

3.5 Dimensional stability analysis

Wood is a sustainable biomass material; however, its inherent properties, such as hygroscopicity and dimensional stability, limit its applications (He et al., 2023). The shrinkage and swelling of litchi wood, white oak (WO), red oak (RO), ash, and rubber wood (RW) were analyzed, and the results are presented in Fig. 6a and 6b. The tangential shrinkage rates of litchi wood, WO, RO, ash, and RW were found to be 6.49%, 7.21%, 7.23%, 7.46%, and 3.87%, respectively, while the radial shrinkage rates were 4.31%, 3.55%, 4.93%, 4.88%, and 2.68%, respectively, under the same environmental conditions. Litchi wood exhibited minimal size changes. The shrinkage ratio (R/T) differences between litchi wood and the control woods were 1.51, 2.03, 1.47, 1.53, and 1.44, respectively. The R/T difference grades between litchi wood and WO, ash, and RW were medium, whereas for RO, it was large, consistent with previous findings (Shen et al., 2020). From a volumetric perspective, the volume shrinkage rates of litchi wood and the control woods were 10.53%, 11.81%, 10.5%, 11.98%, and 6.45%, respectively. Except for RW, the other species maintained a volume shrinkage rate of approximately 11%, indicating that the volumetric change of litchi wood from fully saturated to fully dry was comparable to that of common market species. Gong et al. suggested that the tangential and radial shrinkage of wood decreases with increasing temperature. Therefore, in practical applications, the linear drying shrinkage rate of litchi wood will remain low, effectively reducing defects such as internal cracking and wrinkling, consistent with the findings in section 3.2.3.

As shown in Fig. 6b, the tangential swelling rates of litchi wood and the control woods were 8.22%, 10.05%, 8.53%, 11.79%, and 6.36%, while the radial swelling rates were 5.4%, 4.46%, 6.25%, 7.43%, and 3.69%, respectively. Under identical environmental conditions, the wet expansion rate of wood was larger than the dry shrinkage rate, indicating hygroscopic hysteresis in the studied species. In terms of volume, the wet expansion rates of all species were 14.07%, 15.32%, 14.95%, 20.11%, and 9.7%, respectively. No significant difference was observed in the wet expansion volume among litchi wood, white oak, and red oak, suggesting that litchi wood exhibited more stable size changes compared to white oak and red oak. In summary, the shrinkage and swelling of litchi wood are more stable than those of the control woods, making it suitable for furniture and outdoor building materials.

The primary reason for the observed differences in dry shrinkage and swelling is that wood with higher density exhibits lower internal porosity and smaller pore size, which affects its multi-layer water adsorption capacity. Consequently, it is posited that the shrinkage and swelling of the thick cell walls in high-density tree species are limited, necessitating the absorption of more water (Fredriksson et al., 2023; Yin et al., 2023). Additionally, the hygroscopicity of wood significantly influences its dimensional changes. Figure 7c illustrates the water adsorption isotherms of litchi wood, RO, WO, ash, and RW. It is evident that the water adsorption of wood follows an "S" type pattern, classified as a Class II isotherm, characterized by multi-molecular layer absorption. Based on varying relative humidity (RH), Fig. 7c is divided into three regions. Under identical humidity conditions, the equilibrium moisture content (EMC) of litchi wood in low humidity (Region I) is markedly lower than that of other woods. When RH exceeds 20% (Region II), the EMC of litchi wood begins to rise, indicating an increase in hygroscopicity. When RH surpasses 40% (Region III), the hygroscopicity of litchi wood significantly increases and continues to do so. The hygroscopic properties of litchi wood may be attributed to its chemical composition and cell wall structure (Fredriksson et al., 2023). At low moisture content, the hemicellulose content in litchi wood is significantly lower than in other woods, resulting in fewer hydroxyl groups available to bind with water, leading to a lower EMC under the same humidity conditions. As humidity increases, the orientation of microfibrils in the S1 and S3 layers of the cell wall plays a crucial role (Fredriksson et al., 2023; Yin et al., 2023), resulting in an increased EMC. In summary, the moisture content of wood cells is influenced by multiple factors, including chemical composition and cell wall structure, with no single factor being solely decisive. The unique chemical composition of litchi wood contributes to its distinctive moisture absorption characteristics.

Furthermore, as shown in Fig. 7d, under the same humidity conditions, the absorption hysteresis of litchi wood is significantly greater than that of the control wood. When RH is 20%, 40%, 60%, and 80%, the lag values are 2.79%, 3.76%, 4.02%, and 3.52%, respectively, due to the higher lignin content in litchi wood compared to other tree species (Hill et al., 2009; Yang et al., 2018; Zhou et al., 2016). Except for litchi wood, the absorption hysteresis of control wood begins to decrease significantly when RH reaches 70%, consistent with the findings of Chen et al. (Chen et al., 2020), indicating that within this humidity range, the softening of hemicellulose primarily affects hygroscopic hysteresis (Olsson and Salmen, 2004). However, the absorption hysteresis of litchi wood begins to decrease significantly at 60% RH, earlier than other woods, suggesting that in addition to hemicellulose softening, partial lignin degradation may have occurred (Hill et al., 2009). Studies have shown (Garcia Esteban et al., 2005) that the drying shrinkage coefficient of wood is inversely proportional to the hysteresis rate; a higher hysteresis rate corresponds to a lower drying shrinkage coefficient and better dimensional stability. Ishimaru et al. (Arai Mizutani et al., 2001) pointed out that moisture absorption hysteresis is an adjustment process for wood to achieve true equilibrium, requiring a prolonged time interval to recover. This also implies that a higher moisture absorption hysteresis value extends the time interval for wood moisture content to change and reach equilibrium due to moisture absorption, resulting in smaller dimensional changes.

For wood building materials, the significant consequence of hygroscopicity is its impact on the dimensional changes of wood, namely dry shrinkage and wet expansion (Fredriksson et al., 2023; Ouyang et al., 2022; Yang et al., 2018). Studying the moisture absorption properties can ensure the quality and longevity of wood during its application (Patera et al., 2016). By examining the moisture adsorption isotherm of litchi wood, the EMC can be accurately predicted based on environmental temperature and humidity, which is crucial for establishing drying standards for litchi wood and controlling dimensional stability during the use of wood products.

3.6 Mechanical property

The mechanical properties of litchi wood were evaluated in accordance with GB/T 1927–2022, "Test Method for Physical and Mechanical Properties of Small Sample Wood without Defects," and the results are presented in Fig. 7. The flexural strength and modulus of litchi wood were determined to be 87.97 MPa and 5.94 GPa, respectively, which are lower than those of red oak, white oak, ash, and rubber wood. However, the compressive strength of litchi wood was found to be 62.45 MPa, surpassing that of other hardwoods. Additionally, litchi wood exhibited a surface hardness of 12288 N, which is higher than that of most broad-leaved woods, indicating superior processing performance. These findings suggest that litchi wood is suitable for use in construction, decorative materials, or solid wood furniture production, offering strong processing capabilities albeit with limited bending strength.

In summary, this paper made a comprehensive study on the physical and chemical properties and drying characteristics of litchi wood, and the important conclusions are as follows:

The comprehensive grade of initial cracking, internal cracking, cross section deformation and drying rate of litchi wood is 4 or above, which is a difficult to dry wood. Using the drying process in this paper, the drying time from the initial moisture content of 41.38–11.28% was 11.825d, and the comprehensive drying quality grade was 2, which met the requirements of solid wood floors and solid wood furniture supplies, indicating that the drying standard in this paper was reasonable and effective, and litchi wood sawn wood could be artificially dried. The differential shrinkage of litchi was 1.51, and the differential shrinkage grade was medium, which was similar to that of white oak, ash and rubber wood. The volume wet expansion rate was 14.07%, lower than that of white oak, red oak and ash wood, and the dimensional stability was good. The flexural strength and flexural elastic modulus of Litchi wood are 87.98MPa and 5942MPa respectively, the hardness is 12288N, and the longitudinal compressive strength is 62.45MPa. The above physical and mechanical characteristics of Litchi wood show that Litchi wood sawn wood meets the standard of solid wood furniture and can be used for wood products such as solid wood furniture and solid wood flooring.

The pores in litchi were mainly medium pores and micropores, with a porosity of 31.93% and an average pore size of 23.07nm; There are a large number of pitted holes on the pipe wall, mainly interrow pitted holes, a few ladder pitted holes, small pitted cavities, and the drying speed of litchi sawn wood is slow.

The content of cellulose, hemicellulose and lignin in the chemical composition of litchi wood was 39.24%, 13.80% and 38.50%, respectively, and the extract accounted for 3.14%. Affected by the content of chemical components, the water adsorption curve of litchi showed that the adsorption capacity was weak in the low humidity area, and increased with the increase of relative humidity, and showed a higher hygroscopic lag than that of the control wood.

Through this study, the drying process of litchi wood sawn timber with industrial feasibility was given, and its potential for solid wood utilization was confirmed by evaluating its physical and chemical properties. It was also found that the chemical composition and mechanical properties of Litchi wood were significantly different from those of other woods, but the mechanism of the effect of chemical composition on the properties of Litchi wood was still unclear, and further study was needed.

CRediT authorship contribution statement

Dongdong Liang: Investigation, Collectively wrote the paper. Xiuyi Lin: Revised the manuscript. Shihuan Chen: Investigation, data analyze. Xianju Wang: Revised the manuscript. Qiaofang Zhou: Investigation, Supervision. Hong Yun: Resources, Supervision. Dengyun Tu: Investigation, Supervision, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by Guangdong Forestry Science and Innovation Project (grant numbers: 2022KJCX016), Maoming Laboratory Independent Research Project (grant numbers: 2022ZD002) and Guangdong Provincial Department of Science and Technology, China (grant numbers: 2022A1515010502).

Data Availability

Data will be made available on request.

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Solid wood utilization of Litchi: Study on physicochemical properties and drying technology

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