The Hygroscopicity and Strength Properties of Thermally Modified Gigantochloa scortechinii Bamboo from Peninsular Malaysia

Abstract

Bamboo is a lignocellulosic material characterized by its high hygroscopicity, which refers to the ability of material to absorb and retain moisture from the surrounding environment. This attribute could adversely affect its dimensional stability and resistance against deterioration agents. Thus, a study was conducted to investigate the effect of thermal modification on the hygroscopic, mechanical, and chemical properties of three-year-old Gigantochloa scortechinii, a native and highly exploited bamboo species in Malaysia. Overall, heat treatment effectively reduced the equilibrium moisture content and improved the dimensional stability of bamboo, with samples treated at 210 °C exhibited the most significant moisture resistance of up to 95.6% anti-swelling efficiency (ASE). The heat-treated bamboo exhibited an improvement in modulus of elasticity (MOE) at intermediate temperatures (170–190 °C) whereas modulus of rupture (MOR) declined at 210 °C. Chemical analysis indicated that a significant reduction in hemicellulose content and a relative increase in α-cellulose and lignin contributed to the improved moisture resistance of heat-treated bamboo. The results demonstrate the viability of heat treatment in producing quality thermally modified bamboo as an alternative raw material for construction materials and furniture manufacturing, thereby contributing to the development of Malaysia’s bamboo industry.

1. Introduction

Bamboo is one of the most important non-wood resources, belonging to the family Poaceae and subfamily Bambusoideae [1]. Bamboo plantations span approximately 36.8 million hectares worldwide [2]. Malaysia has around 0.6 million hectares of bamboo stands, with an average stand density of 100 to 200 clumps per hectare [3]. It is estimated that 31% of bamboo plantations are in Peninsular Malaysia, 45% in Sarawak, and 24% in Sabah [4]. About 70 bamboo species are available in Malaysia, with Peninsular Malaysia having the highest number of species (59), followed by Sabah and Sarawak with 30 and 20 species, respectively [5].

Bamboo is an ideal engineered structural material for the construction industry due to its exceptional mechanical properties, high strength-to-weight ratio, and abrasion resistance [6]. Study [7] reported that the modulus of rupture (MOR) and modulus of elasticity (MOE) of 4-year-old Schizostachyum zollingeri bamboo are 142 Nmm⁻² and 10,005 Nmm⁻², respectively, which are comparable to the MOR and MOE of selected plantation timber species such as Acacia mangium and Hopea odorata [8,9].

Due to its fast-growing characteristic, satisfactory strength properties and flexibility for diverse products, bamboo has been accepted as a renewable resource that can offer great potential to contribute to Malaysia’s economic growth. The export value of bamboo products in Malaysia increased by 66.1% in 2023 as compared to 2022 [10]. Asia has become the largest bamboo exporter, covering about 67% of the world exports with a value of USD 71 million [11].

Bamboo is a highly hygroscopic material, absorbing and releasing moisture in response to changing environmental conditions [12]. This high moisture absorption could affect its dimensional stability, leading to swelling and shrinkage and causing structural problems such as cracks and splits [13,14,15]. Additionally, bamboo’s moisture sensitivity could compromise the performance of bamboo-based products, such as weakening bonding strength in laminated bamboo and increasing the susceptibility of warping in the flooring deck. Thus, bamboo should be well-treated before use to reduce the moisture-related problem.

Heat treatment is well-known as an effective and chemically-free treatment method for bamboo. Heat treatment refers to a process of exposing bamboo to temperatures above the boiling point of water for a specific duration [15]. This treatment process varies with heat transfer media, treatment temperature, and exposure duration (

Table 1. Heat treatment method for bamboo.

Heat Transfer Media	Temperature (°C)	Duration (min)	References
Air	50–230	60–180	[14,17,18,19,20,21]
Steam	140–200	10–60	[22,23]
Oil	100–220	30–180	[13,16]
Inert Gas	180–220	120	[12]

). Heat treatment could effectively reduce the hygroscopicity of bamboo by altering its cellular composition. Study [12] found that Phyllostachys pubescens (Moso Bamboo) heat-treated at 180 °C and 220 °C for 2 h exhibited higher anti-water absorption efficiency and anti-swelling efficiency, along with lower equilibrium moisture content, compared to untreated samples. Similarly, study [16] treated Gigantochloa scortechinii in crude palm oil at 140–220 °C for 30–60 min, and found reduction in equilibrium moisture content and volumetric shrinkage by 4%–27% and 17%–53%, respectively. These results confirm that heat treatment reduces bamboo’s hygroscopicity, especially at higher temperatures and longer durations. In addition to improved dimensional stability, heat treatment also affects the appearance of bamboo. Study [17] reported that Dendrocalamus asper and Bambusa blumeana turned bright brown after treatment at 160 °C, enhancing their aesthetic appeal due to fiber caramelization caused by carbohydrate modification.

Strength enhancement is generally observed up to a specific temperature limit. However, these improvements are limited to a particular temperature range, after which further increases in temperature cause a reduction in these properties. For example, study [23] reported that the modulus of rupture (MOR) and modulus of elasticity (MOE) of Phyllostachys heterocycla bamboo increased by 11% and 17% when treated at 140 °C, but decreased by 47% and 20%, respectively, when the temperature was increased to 180 °C. Temperatures exceeding 200 °C have been shown to adversely affect the mechanical properties of bamboo. For example, study [22] observed a significant reduction in MOR and MOE when the temperature was increased to 200 °C. Similarly, study [12] reported a 33% reduction in MOR when the bamboo was treated at 220 °C. Depending on the species, the mechanical properties of bamboo could be enhanced when treated under optimal heat treatment conditions such as at 140 °C for 10 to 30 min [21]. In some studies, heat treatment caused insignificant effect on the mechanical properties of bamboo [15].

The influence of heat treatment on the physical and mechanical properties of bamboo is directly associated with the modification of its chemical constituents such as cellulose, hemicellulose and lignin composition during the heat treatment process. Bamboo treated at a temperature above 160 °C was found to have a significant degradation of hemicellulose and cellulose, accompanied by an increase in lignin content due to condensation reactions [23]. Due to its branched and amorphous structure, hemicellulose exhibits low thermal stability, making it more susceptible to decomposition, which also affects cellulose stability through the degradation of acetyl groups [19]. FTIR analysis confirms these changes through reduced C-O-C and O-H absorption bands, indicative of hemicellulose degradation, and increased C=C stretching, reflecting lignin condensation [16,18]. The resulting increase in lignin and extractive contents, alongside a degradation of hemicellulose, may reduce the strength of bamboo but enhance its resistance against moisture absorption under different environmental conditions.

The drying process could also be further optimized by integrating heat treatment techniques to develop an effective processing method for bamboo. Research on the thermal modification of Semantan bamboo (Gigantochloa scortechinii), an important bamboo species in Malaysia remains limited. Previous studies focused on oil heat treatment, particularly using palm oil as the heating medium [16,24,25]. However, alternative thermal methods such as air heat treatment have not been explored for this species within Malaysia. Thus, a study was conducted to enhance the processing techniques of Semantan bamboo (Gigantochloa scortechinii) by implementing solar drying and air heat treatment methods. The resulting improvements in the quality of bamboo material could support the extensive utilization of bamboo as a sustainable alternative resource and subsequently contribute to the development of the bamboo-based industry in Malaysia.

2. Experimental

2.1. Material Preparation

The material used in this study was the 3-year-old Semantan bamboo (Gigantochloa scortechinii) obtained from Sungai Lui, Selangor, Malaysia. Bamboo culms with average dimensions of 9000 mm (length) × 97 mm (diameter) × 6 mm (wall thickness) were cut at about 300 mm above the ground and further cut into three equal lengths of 3000 mm each. The bamboo culms were dried to an approximate moisture content of 12% using an industrial-scale solar-assisted kiln dryer with a capacity of 34 m³. The kiln’s temperature and relative humidity settings were recorded regularly. The drying process was monitored by weighing randomly selected bamboo culms until they reached a constant weight. The dried culms were then split and trimmed to obtain bamboo strips with dimensions 500 mm (L) × 20 mm (W) × culm wall thickness (T).

2.2. Heat Treatment

The bamboo strips were heat-treated in a forced-air convection oven (Memmert ULE 600, Memmert GmbH + Co.KG, Schwabach, Germany) with air as a heat transfer medium. The oven was equipped with a microprocessor-based PID temperature controller and a built-in timer to ensure precise monitoring and regulation of temperature. The treatment temperature was set to 130 °C, 150 °C, 170 °C, 190 °C, and 210 °C for the various treatments. The bamboo strips were exposed at each treatment temperature for a duration of 2 h. The airflow within the oven was maintained at 1.5 m s⁻¹. A total of 150 specimens were subjected to heat treatment, with 30 replicates for each treatment temperature. The solar-dried bamboo strips without heat treatment were used as the control group.

2.3. Physical Properties Test

2.3.1. Weight Loss

The weight loss (W_L) and weight loss rate (WLR) of the specimens after heat treatment were calculated using Formulae (1) and (2), respectively:

$${\mathrm{W}}_{\mathrm{L}} \left(\mathrm{g}\right)={\mathrm{W}}_{\mathrm{i}}-{\mathrm{W}}_{\mathrm{f}}$$

(1)

$$\mathrm{W}\mathrm{L}\mathrm{R} (\%)=\left({\displaystyle \frac{{\mathrm{W}}_{\mathrm{i}}-{\mathrm{W}}_{\mathrm{f}}}{{\mathrm{W}}_{\mathrm{i}}}}\right)\times 100$$

(2)

where ${\mathrm{W}}_{\mathrm{i}}$ and ${\mathrm{W}}_{\mathrm{f}}$ are the weights of the specimen before and after heat treatment, respectively.

2.3.2. Moisture Content

The moisture content (MC) of the specimen before and after heat treatment and the moisture loss rate (MLR) were calculated using Formulae (3) and (4), respectively:

$$\mathrm{M}\mathrm{C} (\%)=\left({\displaystyle \frac{{\mathrm{W}}_{\mathrm{g}}-{\mathrm{W}}_{\mathrm{o}}}{{\mathrm{W}}_{\mathrm{o}}}}\right)\times 100$$

(3)

$$\mathrm{M}\mathrm{L}\mathrm{R} (\%)=\left({\displaystyle \frac{{\mathrm{M}\mathrm{C}}_{\mathrm{i}}-{\mathrm{M}\mathrm{C}}_{\mathrm{f}}}{{\mathrm{M}\mathrm{C}}_{\mathrm{i}}}}\right)\times 100$$

(4)

where ${\mathrm{W}}_{\mathrm{g}}$ is the weight before or after treatment, ${\mathrm{W}}_{\mathrm{o}}$ is the oven dry weight, and ${\mathrm{M}\mathrm{C}}_{\mathrm{i}}$ and ${\mathrm{M}\mathrm{C}}_{\mathrm{f}}$ are the moisture content of the specimen before and after heat treatment, respectively.

2.3.3. Density

The density of each specimen before and after heat treatment was determined using the water-displacement method and the loss rate in the density (DLR) was calculated using Formula (5):

$$\mathrm{D}\mathrm{L}\mathrm{R} (\%)=\left({\displaystyle \frac{{\mathsf{\rho }}_{\mathrm{i}}-{\mathsf{\rho }}_{\mathrm{f}}}{{\mathsf{\rho }}_{\mathrm{i}}}}\right)\times 100$$

(5)

where ${\mathsf{\rho }}_{\mathrm{i}}$ and ${\mathsf{\rho }}_{\mathrm{f}}$ are the densities of the specimen before and after heat treatment, respectively.

2.3.4. Equilibrium Moisture Content Test

The untreated and heat-treated bamboo were then conditioned in a temperature-humidity chamber (Tabai Espec Corporation, Osaka, Japan) set at three different equilibrium moisture content (EMC) levels (

Table 2. Temperature and relative humidity conditions for each EMC level.

EMC Level (%)	Temperature (°C)	Relative Humidity (%)
18	20	85
12	20	64
8	20	40

). During each conditioning phase, the weight of each sample was recorded periodically until a constant weight was attained. After completing all conditioning treatments, a 25 mm wide MC specimen was cut from the edge of each strip and oven-dried at 103 ± 2 °C for 24 h. The EMC values achieved under the respective conditions, as well as the moisture exclusion efficiency (MEE) were calculated using Formulae (6) and (7), respectively:

$$\mathrm{E}\mathrm{M}\mathrm{C} \left(\%\right)=\left({\displaystyle \frac{{\mathrm{W}}_{\mathrm{a}}-{\mathrm{W}}_{\mathrm{o}}}{{\mathrm{W}}_{\mathrm{o}}}}\right)\times 100$$

(6)

$$\mathrm{M}\mathrm{E}\mathrm{E} \left(\%\right)=({\displaystyle \frac{{\mathrm{M}\mathrm{C}}_{\mathrm{U}\mathrm{T}}-{\mathrm{M}\mathrm{C}}_{\mathrm{H}\mathrm{T}}}{{\mathrm{M}\mathrm{C}}_{\mathrm{U}\mathrm{T}}}})\times 100$$

(7)

where ${\mathrm{W}}_{\mathrm{a}}$ is the weight attained at the respective condition, and ${\mathrm{M}\mathrm{C}}_{\mathrm{U}\mathrm{T}}$ and ${\mathrm{M}\mathrm{C}}_{\mathrm{H}\mathrm{T}}$ are the moisture content of the untreated and heat-treated specimens, respectively.

2.4. Water Absorption Test

The water absorption test was conducted using in-house method based on ASTM D4446M-13 [26]. Both untreated and heat-treated bamboo strips were conditioned to 12% EMC before being fully immersed in distilled water for 2 h. The strips were then removed, weighed and measured for their dimension. Subsequently, the samples were re-immersed for an additional 22 h, followed by another weighing and dimensional measurement. After completing the immersion process, a 25 mm-wide specimen was cut from the edge of each strip and oven dried at 103 ± 2 °C for 24 h. The water absorption rate (WAR), volume swelling coefficient (VSC), anti-water absorption efficiency (AWAE) and anti-swelling efficiency (ASE) of both untreated and heat-treated bamboo strips were calculated using Formulae (8)–(11), respectively.

$$\mathrm{W}\mathrm{A}\mathrm{R} \left(\%\right)=\left({\displaystyle \frac{{\mathrm{W}}_{\mathrm{i}}-{\mathrm{W}}_{\mathrm{f}}}{{\mathrm{W}}_{\mathrm{i}}}}\right)\times 100$$

(8)

$$\mathrm{V}\mathrm{S}\mathrm{C} \left(\%\right)=\left({\displaystyle \frac{{\mathrm{V}}_{\mathrm{i}}-{\mathrm{V}}_{\mathrm{f}}}{{\mathrm{V}}_{\mathrm{i}}}}\right)\times 100$$

(9)

where ${\mathrm{W}}_{\mathrm{i}}$ and ${\mathrm{V}}_{\mathrm{i}}$ are the weight and volume of the specimen before water immersion, and ${\mathrm{W}}_{\mathrm{f}}$ and ${\mathrm{V}}_{\mathrm{f}}$ are the weight and volume of the specimen after immersion.

$$\mathrm{A}\mathrm{W}\mathrm{A}\mathrm{E} \left(\%\right)=\left({\displaystyle \frac{{\mathrm{W}\mathrm{A}\mathrm{R}}_{\mathrm{C}}-{\mathrm{W}\mathrm{A}\mathrm{R}}_{\mathrm{H}\mathrm{T}}}{{\mathrm{W}\mathrm{A}\mathrm{R}}_{\mathrm{C}}}}\right)\times 100$$

(10)

$$\mathrm{A}\mathrm{S}\mathrm{E} \left(\%\right)=\left({\displaystyle \frac{{\mathrm{V}\mathrm{S}\mathrm{C}}_{\mathrm{C}}-{\mathrm{V}\mathrm{S}\mathrm{C}}_{\mathrm{H}\mathrm{T}}}{{\mathrm{V}\mathrm{S}\mathrm{C}}_{\mathrm{C}}}}\right)\times 100$$

(11)

where ${\mathrm{W}\mathrm{A}\mathrm{R}}_{\mathrm{C}}$ and ${\mathrm{V}\mathrm{S}\mathrm{C}}_{\mathrm{C}}$ are the WAR and VSC of the untreated sample, and ${\mathrm{W}\mathrm{A}\mathrm{R}}_{\mathrm{H}\mathrm{T}}$ and ${\mathrm{V}\mathrm{S}\mathrm{C}}_{\mathrm{H}\mathrm{T}}$ are the WAR and VSC after heat treatment.

2.5. Mechanical Properties Test

The mechanical properties of heat-treated strips were evaluated in accordance with the methodologies outlined by the Indian Standard BIS [27]. Specimens measuring 6 mm × 20 mm × 160 mm were prepared for the 3-point static bending test, with the central internode selected for testing. The specimens were thoroughly inspected before testing to ensure they were free from defects such as cracks and crookedness. A total of 120 specimens were subjected to bending test, with 20 replicates for each treatment temperature including untreated control group.

Testing was conducted using a 10 kN Universal Testing Machine (Instron 3364, Illinois Tool Works Inc., Norwood, MA, USA). The specimens were positioned horizontally on two parallel rollers, each approximately 2 cm in diameter, with a span length of 140 mm. The skin surface of the specimens was oriented downward. A load was applied continuously at the center of the specimen, with the movable head set to a constant speed of 6.5 mm min⁻¹ [28]. The modulus of rupture (MOR) and modulus of elasticity (MOE) for each specimen were calculated using Formulae (12) and (13), respectively:

$$\mathrm{M}\mathrm{O}\mathrm{R} (\mathrm{M}\mathrm{P}\mathrm{a})={\displaystyle \frac{3{\mathrm{P}}^{\prime }\mathrm{l}}{{2\mathrm{b}\mathrm{h}}^2}}$$

(12)

$$\mathrm{M}\mathrm{O}\mathrm{E} (\mathrm{M}\mathrm{P}\mathrm{a})={\displaystyle \frac{{\mathrm{P}\mathrm{l}}^3}{{4\mathrm{b}\mathrm{h}}^3\mathrm{d}}}$$

(13)

where ${\mathrm{P}}^{\prime }$ is the maximum load, $\mathrm{l}$ is the span length, $\mathrm{b}$ is the width of the specimen, $\mathrm{h}$ is the depth of the specimen, $\mathrm{P}$ is the load at proportional limit, and $\mathrm{d}$ is the deflection at proportional limit.

2.6. Chemical Composition Test

Holocellulose consists of both alpha-cellulose and hemicellulose. Therefore, the hemicellulose content can be determined by deducting the alpha-cellulose content from the holocellulose content. The holocellulose content of untreated and heat-treated bamboo was determined according to study [29]. Approximately 5 g of the finely ground sample was mixed with 100 mL distilled water and heated up to 80 °C in a water bath. Then, 1.5 g of sodium chlorite and 10 drops of 10% acetic acid were added, and the mixture was stirred gently until the solution turned white. The holocellulose residue was then filtered, washed, and dried in an oven at 105 °C for 24 h until a constant weight was achieved. The holocellulose content was calculated using Formula (14).

$$\mathrm{Holocellulose} \mathrm{content} \left(\%\right)=\left({\displaystyle \frac{{\mathrm{W}}_{\mathrm{o}}}{{\mathrm{W}}_{\mathrm{g}}}}\right)\times 100$$

(14)

where $W_o$ is the weight of dried holocellulose after drying and $W_g$ is the initial weight of the sample before chemical treatment.

The alpha-cellulose content of heat-treated bamboo was determined according to study [30]. Approximately 5 g of the finely ground sample was dissolved in 100 mL of 17.5% sodium hydroxide. The mixture was stirred gently and allowed to stand at room temperature for 30 min. The residue was then filtered, washed, and treated with a 10% acetic acid solution. After another washing step, the sample was dried in an oven at 105 °C for 24 h until a constant weight was achieved. The alpha-cellulose content was calculated using Formula (15):

$$\mathrm{Alpha}-\mathrm{cellulose} \mathrm{content} \left(\%\right)=\left({\displaystyle \frac{{\mathrm{W}}_{\mathrm{o}}}{{\mathrm{W}}_{\mathrm{g}}}}\right)\times 100$$

(15)

where $W_o$ is the weight of dried alpha-cellulose after drying and $W_g$ is the initial weight of the sample before chemical treatment.

The lignin content of heat-treated bamboo was determined according to study [31]. Approximately 1.0 g of finely ground sample was mixed with 15 mL of 72% sulfuric acid and stirred continuously at room temperature for 2 h. The mixture was then diluted with 560 mL distilled water and heated at 100 °C for 4 h. The residue was filtered, washed, and dried in an oven at 105 °C for 24 h until a constant weight was achieved. The dried residue was then placed in a muffle furnace at 600 °C for 3 h, then weighed. The lignin content was calculated using Formula (16):

$$\mathrm{Lignin} \mathrm{content} \left(\%\right)=\left({\displaystyle \frac{{\mathrm{W}}_{\mathrm{o}}-{\mathrm{W}}_{\mathrm{a}}}{{\mathrm{W}}_{\mathrm{g}}}}\right)\times 100$$

(16)

where $W_o$ is the weight of dried lignin residue, $W_a$ is the weight of ash, and $W_g$ is the initial weight of the sample.

2.7. Statistical Analysis

Statistical analysis was conducted using IBM SPSS Statistic software version 26. A one-way analysis of variance (ANOVA) was performed to determine whether the mean differences between treatment temperature were significant or not. If the differences were significant, Tukey’s HSD test was conducted to determine which of the means were significantly different from one another.

3. Results and Discussion

3.1. Physical Properties

3.1.1. Weight Loss Rate (WLR), Moisture Loss Rate (MLR) and Density Loss Rate (DLR)

Table 3. WLR, MLR, and DLR of heat-treated bamboo.

Temperature (°C)	WLR (%)	MLR (%)	DLR (%)
130	9.79 a	76.66 a	8.42 a
	(0.59)	(0.72)	(1.45)
150	11.16 b	80.62 b	11.15 b
	(0.14)	(0.19)	(1.67)
170	12.18 c	93.90 c	15.86 c
	(0.21)	(0.24)	(2.90)
190	15.45 d	98.09 d	18.34 d
	(0.17)	(0.18)	(1.27)
210	18.20 e	100.06 e	22.40 e
	(0.7)	(0.54)	(3.22)

WLR = weight loss rate, MLR = moisture loss rate, DLR = density loss rate. Values in parentheses are standard deviation. Mean values in the same column with different superscript letter are significantly different at p < 0.05.

shows the WLR and MLR of heat-treated bamboo strips under various treatment temperatures. At the lowest treatment temperature of 130 °C, WLR and ML were recorded at 8.38% and 9.47%, respectively, indicating a relatively minor reduction in mass. As the temperature increases to 150 °C, both WLR and MLR exhibited noticeable increments, reaching 11.16% and 12.67%, respectively. This trend continued at higher treatment temperatures of 170 °C and 190 °C, with the most significant reductions was observed at 210 °C, reaching 18.2% for WLR and 22.25% for MLR, respectively. DLR indicates that the density of heat-treated bamboo decreases as the heat treatment temperature increases. Study [32] similarly reported that the density of heat-treated bamboo was lower than untreated bamboo when subjected to tung oil heat treatment at temperatures above 180 °C.

The results demonstrated a strong positive linear correlation between heat treatment temperature and WLR/MLR, as shown in Figure 1. High coefficient of determination (R² > 0.9) confirms that temperature significantly influenced the extent of weight and moisture loss during heat treatment. The observed mass reduction could be primarily attributed to the thermal decomposition process, including the volatilisation of organic compounds and the degradation of lignocellulosic structures [33]. Meanwhile, moisture loss occurs due to the removal of both free and bound water within the bamboo cell due to heat-induced desorption and structural modifications [34].

These findings are consistent with previous studies on the thermal treatment of bamboo. Study [35] reported that the mass loss of 5-year-old Phyllostachys pubescens reached 29% when subjected to air heat treatment at temperatures above 160 °C for 4 h, demonstrating that elevated temperatures significantly reduce bamboo’s weight and moisture content. The weight loss caused by the increased temperature leads to more pronounced moisture evaporation due to the breakdown of organic constituents, ultimately contributing to considerable structural alterations [36].

3.1.2. Equilibrium Moisture Content

Figure 2 presents the EMC of untreated and heat-treated bamboo under three environmental conditions. The results demonstrated a decreasing trend in EMC with increasing heat treatment temperature, indicating that heat-treated bamboo absorbed less moisture than untreated bamboo. At 20 °C/40% RH, EMC was highest for untreated bamboo (8%), suggesting significant moisture uptake from the environment. In contrast, bamboo treated at 210 °C shows a substantially lower EMC of 2.67%, indicating improved moisture resistance. A similar trend was observed at 20 °C/85% RH, where the EMC of untreated bamboo at 14.01% decreased to 9.51% following heat treatment at 210 °C. Likewise, at 20 °C/64% RH, the EMC of untreated bamboo at 10.50% reduced to 6.98% after treatment at 210 °C. These findings indicate that heat treatment significantly reduced the EMC of bamboo by approximately 30%–60% compared to untreated samples, with EMC reduction tended to increase at higher treatment temperatures.

These findings align with previous studies on the effect of heat treatment on bamboo. Study [23] reported that saturated steam heat treatment at 160 °C for 30 min reduced the EMC of 4-year-old Moso bamboo (Phyllostachys heterocycle) by 33.5% compared to untreated sample. Similarly, study [14] found that bamboo treated at 180 °C and 200 °C for 8 h exhibited lower EMC than untreated samples when exposed to relative humidity conditions of 11%, 45% and 75%.

EMC refers to the moisture level at which bamboo stabilises under specific temperature and relative humidity (RH) conditions. A lower EMC indicates reduced water absorption, which enhances dimensional stability and resistance to moisture-related issues such as swelling or decay. This reduction in EMC enhances dimensional stability by minimizing the likelihood of swelling or shrinkage due to fluctuations in humidity [12]. Furthermore, decreased moisture absorption enhances bamboo’s resistance to fungal decay, as fungi require high moisture content to thrive [24].

The results were further supported by the increase in MEE with increasing heat treatment temperatures (Figure 2), indicating that heat-treated bamboo exhibits greater moisture absorption resistance than untreated bamboo. At 20 °C/40% RH, MEE gradually increased with temperature, with bamboo treated at 130 °C achieved 37.48%, while treatment at 210 °C achieved the highest MEE value of 67.01%. A similar trend was observed at condition 20 °C/85% RH and 20 °C/64% RH, where the MEE of bamboo treated at 130 °C reached 2.57% and 11.55% respectively, when compared to untreated. Meanwhile, treatment at 210 °C further improved moisture resistance to 32.11% and 33.48%, respectively. This trend indicates that higher heat treatment temperatures significantly enhance bamboo’s ability to resist moisture from the surrounding environment.

MEE quantifies bamboo’s resistance to moisture absorption, with higher values indicate greater reduction in water uptake, enhanced dimensional stability, and improved resistance to humidity changes [37]. Heat treatment modifies the bamboo cell wall structure, increasing its hydrophobicity and reducing its susceptibility to dimensional changes [22].

3.1.3. Dimensional Stability

Figure 3 and Figure 4 present the dimensional stability of heat-treated bamboo following water absorption tests conducted over two different immersion durations: 2 h and 24 h. For the 2 h immersion, WAR exhibited an increasing trend from the untreated sample to the sample treated at 130 °C to 170 °C. The hypothesis suggests that the cellular structure of bamboo remains largely intact during short-term water immersion and water can still penetrate freely due to the insufficient degradation at low temperature. Further investigation should be conducted to confirm this hypothesis. Later on, a decreasing trend from 170 °C to 210 °C was observed, suggesting reduced water uptake at higher temperatures. VS was highest in untreated bamboo at 4.22%, whereas the lowest value was recorded at 210 °C (0.13%), demonstrating improvement in dimensional stability with increasing treatment temperatures. Similarly, the AWAE was most pronounced at 210 °C (58.61%), whereas the lowest value was observed at 130 °C (6.17%). The ASE exhibited a comparable trend, with values increased significantly from 12.25% in the sample treated at 130 °C to 95.59% in the sample treated at 210 °C, thereby further highlighting the superior resistance of heat-treated bamboo to dimensional changes resulting from exposure to elevated heat treatment temperatures.

For the 24 h immersion, moisture absorption increased significantly compared to the 2 h immersion. WAR reached a maximum of 29.82% in the untreated sample, while the lowest value of 9.84% was observed in the 210 °C sample. AWAE varied significantly, with the 210 °C sample achieved the highest efficiency at 66.99%, while the lowest value of 14.58% was observed for the 130 °C sample. VS remained highest in the untreated sample (11.34%), whereas the lowest value of 4.00% was recorded for the 210 °C sample, reinforcing the effectiveness of heat treatment in minimizing swelling. In addition, the sample subjected to treatment at 210 °C exhibited the highest ASE of 64.72%, whereas the lowest ASE value of 0.58% was recorded in the sample treated at 130 °C. These findings further support that higher treatment temperatures improves the dimensional stability of bamboo.

A comparison between the 2 h and 24 h immersion durations reveals a pronounced increase in moisture absorption and dimensional swelling as a result of prolonged water exposure. The untreated sample exhibited the highest increase in WAR with 23.56% increment, whereas the thermally modified sample treated at 210 °C showed the lowest increment at 7.25%.

This substantial reduction in WAR demonstrating the effectiveness of thermal modification in enhancing bamboo’s resistance to water absorption over extended immersion period. AWAE increased with immersion duration, with the lowest increment was observed for sample treated at 210 °C (8.38%) and the highest increase of 12.84% recorded at 130 °C, further confirming that higher treatment temperatures reduce bamboo’s water absorption capacity.

In both untreated and heat-treated samples, VS increased with prolonged immersion time. The untreated sample exhibited a 7.12% increase after 24 h immersion compared to 2 h, whereas the 210 °C sample showed the lowest swelling increment at 2.84%. In contrast, ASE decreased with extended immersion duration; the 150 °C sample exhibited slight ASE reduction (3.81%), whereas the 190 °C sample showed the highest decline (45.82%). These findings indicate that prolonged water exposure leads to increased moisture absorption and greater dimensional changes, while heat-treated bamboo samples exhibit improved resistance to both water absorption and associated dimensional changes compared to untreated bamboo.

Overall, heat treatment markedly reduced both WAR and VS of bamboo. Higher treatment temperatures further reduce the moisture uptake and swelling, demonstrating improved dimensional stability. Samples treated at 210 °C demonstrated the lowest WAR and VS and the highest AWAE and ASE values, suggesting superior water resistance and reduced susceptibility to deformation.

These observations align with a previous study by study [38]. They reported that heat treatment significantly reduced the water absorption and water absorption thickness expansion rate of Moso bamboo when subjected to heat treatment at 140–220 °C for 2 h. Similarly, study [23] found that heat treatment effectively reduced the moisture absorption of 4-year-old Moso bamboo (Phyllostachys heterocycle) by 33.4% to 46.7% compared to untreated bamboo after undergoing saturated steam heat treatment at 160–180 °C for 30 min. In addition, findings from study [16] demonstrated that oil heat treatment at temperatures of 140–220 °C for 30 to 60 min successfully enhanced the dimensional stability of bamboo. Their findings also indicated that bamboo became less hygroscopic when subjected to higher temperatures and longer heat treatment durations.

3.2. Mechanical Properties

Table 4. Strength properties of heat-treated bamboo.

Temperature (°C)	MOR (MPa)	MOE (MPa)
UT	129.18 a	16,165 ab
	(24.76)	(2737)
130	120.41 a	14,928 ab
	(32.66)	(2890)
150	122.68 a	14,125 a
	(30.70)	(2475)
170	110.43 ab	15,164 ab
	(28.35)	(2754)
190	123.66 a	16,858 b
	(14.80)	(2058)
210	97.83 b	15,762 ab
	(25.60)	(1640)

Values in parentheses are standard deviation. Mean values in the same column with different superscript letters are significantly different at p < 0.05.

shows the strength properties of heat-treated bamboo subjected to different treatment temperatures. Overall, the effect of treatment temperature on the MOR and MOE values was found to be variable. ANOVA test indicates that treatment temperature significantly influenced the MOR and MOE values at a 95% confidence level across all treatment temperatures (p < 0.05).

MOR value of bamboo exhibited a decreasing trend after heat treatment compared to the untreated sample. Post hoc test (PHT) results indicate that MOR values at specific treatment temperatures did not show significant differences among any pairwise comparison groups, except for the bamboo treated at 210 °C, which showed a significant reduction (p < 0.05) compared to the untreated samples.

The MOE values of bamboo treated at 130 °C and 150 °C exhibited a decreasing trend compared to the untreated sample. Meanwhile, an increase in MOE was observed at 170 °C to 190 °C before declining at 210 °C. PHT analysis revealed that the MOE values across all treatment temperatures did not show significant differences among any pairwise comparison groups. The PHT analysis results also indicate that the increase in MOE at 190 °C is significantly different compared to the sample treated at 150 °C (the sample with the lowest MOE value). Despite that, the result did not show a significant difference compared to the untreated sample.

Previous studies have reported varying effects of heat treatment on the mechanical properties of bamboo. Study [15] found that the bending properties of Phyllostachys viridiglaucescens bamboo were not significantly influenced by thermal treatment when heat-treated using an LPG-gas torch method. In contrast, study [22] observed that the bending properties of Bambusa blumeana Schultes f. bamboo culms subjected to intense heat treatment at 200 °C for 60 °C minutes decreased significantly, with MOE and MOR reductions of up to 28.1% and 51.9%, respectively, compared to the untreated sample. However, samples treated at 150 °C to 175 °C exhibited a more gradual decline, with values varying insignificantly across this temperature range.

Study [21] found that the mechanical properties of bamboo vary depending on treatment temperature, with the optimal values was observed at 170 °C before declining as the temperature increases to 200 °C. Study [13] reported that Phyllostachys bamboo culm from France achieved optimal MOE when heat-treated at 180 °C for 1 to 2 h. Similarly, study [39] found that bamboo’s mechanical properties decreased slightly at 150 °C, but declined significantly at 200 °C. These results are consistent with the findings of the present study, where bamboo samples treated at 150 °C exhibited the lowest MOE, while the highest MOE was recorded at 190 °C, followed by a decline at 210 °C.

3.3. Chemical Composition

Holocellulose, α-cellulose and hemicellulose content in bamboo after heat treatment at 130 °C, 150 °C, 170 °C, 190 °C, and 210 °C are presented in

Table 5. Chemical content of heat-treated bamboo.

Temperature	Chemical Content (%)
	Holocellulose	A-Cellulose	Hemicellulose	Lignin
UT	84.25 a	36.09 a	48.16 a	21.86 ab
	(2.65)	(0.27)	(2.65)	(0.43)
130	82.25 ab	46.09 b	36.16 bc	23.12 abc
	(3.22)	(1.4	(3.22)	(0.51)
150	86.42 a	46.22 b	40.2 b	23.79 bc
	(0.58)	(0.21)	(0.58)	(0.31)
170	80.41 ab	47.21 b	33.2 c	21.42 a
	(1.39)	(1.65)	(1.39)	(0.43)
190	75.49 bc	42.69 ab	32.8 c	23.18 abc
	(1.29)	(3.73)	(1.29)	(1.45)
210	68.94 c	46.62 b	22.32 d	24.21 c
	(1.59)	(1.63)	(1.59)	(0.58)

Values in parentheses are standard deviation. Mean values in the same column with different superscript letters are significantly different at p < 0.05.

. The holocellulose content, comprising α-cellulose and hemicellulose, ranged from 68.94% to 86.42% in both untreated and heat-treated bamboo.

The increase in treatment temperature significantly affected the holocellulose content of bamboo (p < 0.05). A significant decrease was observed in holocellulose content when treatment temperature exceeds 150 °C, compared to the untreated and the 130 °C treatment (Figure 5). This reduction is attributed to chemical reactions occurring in wood and wood-based materials at temperatures exceeding 150 °C, which leads to substantial alterations in their chemical composition upon exposure to elevated thermal conditions [40].

The α-cellulose content of heat-treated bamboo exhibited a significant increase with increasing treatment temperatures compared to the untreated sample (Figure 5). Cellulose consists of crystalline α-cellulose and amorphous cellulose. The increase in α-cellulose content after heat treatment is attributed to the degradation of hemicellulose and the amorphous regions of cellulose, leaving behind a higher proportion of crystalline α-cellulose [41]. Study [42] also reported that the α-cellulose content increased from 45.64% in the untreated sample to 49.48–52.79% in the heat-treated sample.

Heat-treated bamboo exhibited a significant decrease in hemicellulose content with increasing temperature compared to the untreated sample. The reduction at 130 °C and 150 °C was minimal, but a significant decline was observed as the temperature increased from 170 °C to 210 °C (Figure 5). Study [23] similarly reported that hemicellulose decomposition in Phyllostachys heterocycla bamboo begins to decompose as treatment temperature reaches 160 °C. The difference in hemicellulose content between 130 °C and 150 °C was 4.0%, while decreases of 7.0% and 0.4% were observed as the temperature increased from 150 °C to 170 °C and from 170 °C to 190 °C, respectively. The total reduction in hemicellulose composition from 190 °C to 210 °C was approximately 10.5%.

The decline in hemicellulose content in heat-treated bamboo could be attributed to thermal degradation during the heat treatment process. Hemicellulose, which consists of branched and amorphous polysaccharides, possesses low thermal stability, making it more susceptible to decomposition at elevated temperatures compared to other chemical components [38]. The degradation of hemicellulose strengthens the interaction of the hydroxyl groups through the formation of stronger hydrogen bonds. This structural rearrangement reduces the availability of free hydroxyl groups, thereby decreasing the hygroscopic capacity of bamboo fibers [43]. As a result, heat-treated bamboo exhibits significantly lower moisture absorption compared to untreated bamboo, ultimately reducing its equilibrium moisture content and enhancing dimensional stability [19].

These chemical decompositions also explain the observed increase in MEE at higher treatment temperatures, which is closely associated with the degradation of hemicellulose and the reduction of extractives [19]. Moreover, the overall mass loss resulting from the decomposition of these components contributes to a decrease in density [17]. The structural performance of bamboo may be diminished at higher temperature exceeding 200 °C due to severe thermal degradation of hemicellulose. Therefore, an optimal treatment temperature should be determined to produce quality heat-treated bamboo material with enhanced mechanical properties.

Lignin content increased with rising treatment temperatures, although these increases were not statistically significant compared to the untreated sample. At 130 °C and 150 °C, lignin content increased by 1.26% and 1.93%, respectively, compared to the untreated sample. A slight decrease of 0.44% was observed when the treatment temperature was increased to 170 °C. Further increases of 1.32% and 2.35% were observed when the treatment temperature was increased to 190 °C and 210 °C, respectively. Previous research has also shown an increasing trend in lignin content when bamboo is treated at 160–180 °C [44] and 180–220 °C [45]. This phenomenon is attributed to the pyrolysis of hemicellulose and condensation reactions in lignin during heat treatment [46].

4. Conclusions

1.

This study investigated the effect of heat treatment on the physical, mechanical and chemical properties of bamboo. The findings show that thermal modification significantly reduces equilibrium moisture content (EMC) and swelling behavior of Gigantochloa scortechinii bamboo. Treatment at 210 °C resulted in the lowest moisture uptake and the highest anti-swelling, as well as moisture exclusion efficiencies, indicating substantial improvements in dimensional stability.

2.

These improvements are closely related to the thermal degradation of hemicellulose, which contributes to bamboo’s hydrophilic nature. An increase in α-cellulose and a decrease in holocellulose content indicate a modification in the chemical structure that reduces hygroscopicity.

3.

The modulus of rupture (MOR) and modulus of elasticity (MOE) of heat-treated bamboo showed varying effects at different treatment temperature. A slight enhancement in MOE was observed at intermediate temperatures (170–190 °C), while both MOR and MOE declined at 210 °C, implying a loss in structural integrity due to thermal degradation.

4.

This study demonstrates that thermal treatment at 190 °C for two hours significantly reduces the hygroscopicity of G. scortechinii bamboo without diminishing its strength properties, thereby enhancing its suitability for value-added applications.

5.

Through proper optimisation of heat treatment parameters, this method offers a sustainable, chemically-free approach to improving bamboo quality, highlighting the potential of G. scortechinii as a viable alternative to timber and contributing to the advancement of Malaysia’s bamboo-based industry.