The Characteristics of Citric Acid-Bonded Composite Boards Produced from Alkaline-Modified Fibrovascular Bundle of Salacca Sumatrana Fronds

ABSTRACT

Fibrovascular bundle (FVB) of Salacca sumatrana fronds is a natural fiber derived from palm plants. The fiber typically had long dimensions and the potential to be used as raw material for oriented boards. Therefore, this study aimed to evaluate the properties of oriented boards made from alkaline-modified FVB of Salacca sumatrana frond. FVB was modified with an alkali pre-treatment, comprising 1% NaOH, 1% NaOH + 0.2% Na₂SO₃, and 1% NaOH + 0.4% Na₂SO₃ to improve the dimensional stability. The sample was then manually formed into a three-layer oriented mat using a forming box, which was oriented at 3 different angles of 0°, 45°, and 90°. The results showed that the 1% NaOH + 0.2% Na₂SO₃ treatment gave the lowest thickness swelling and water absorption, as well as the highest modulus of rupture and elasticity values. Based on Fourier Transform Infrared (FTIR) analysis, there was a bond between FVB hydroxyl groups and the carboxyl groups in citric acid, showing an effective bonding mechanism. Furthermore, orientated boards with an orientation of 90° were the best among all chemically modified types. The results showed that the modified natural fiber of FVB formed a bond with citric acid, leading to excellent mechanical properties.

摘要

漆树叶的纤维维管束（FVB）是从棕榈植物中提取的一种天然纤维. 该纤维通常具有长尺寸，并且有潜力用作定向板的原材料. 因此，本研究旨在评估用碱改性苏门答腊黄FVB制成的定向板的性能. 用碱预处理对FVB进行改性，包括1%NaOH、1%NaOH +0.2%Na2SO3和1%NaOH +0.4%Na2SO3，以提高尺寸稳定性. 然后使用成型箱将样品手动成型为三层定向垫，成型箱以0°、45°和90°的3个不同角度定向. 结果表明，1%NaOH +0.2%Na2SO3处理的厚度膨胀率和吸水率最低，断裂模量和弹性值最高. 基于傅立叶变换红外光谱（FTIR）分析，柠檬酸中的FVB羟基和羧基之间存在键合，显示出有效的键合机制. 此外，在所有化学改性类型中，取向为90o的取向板是最好的. 结果表明，改性后的FVB天然纤维与柠檬酸形成结合，具有优异的力学性能.

KEYWORDS:

• Oriented board
• fibrovascular bundle
• salacca sumatrana
• frond
• alkaline modification
• natural fiber

摘要:

• 定向板
• 纤维血管束
• 苏门答腊萨拉卡
• 叶
• 碱性改性
• 天然纤维

Introduction

Fibrovascular bundle (FVB) of Salacca sumatrana frond is a natural fiber derived from palm plants and often used for manufacturing composite boards (Hakim et al. 2022). Previous studies showed that natural fiber, such as bamboo, jute, as well as FVB from Salacca frond typically exhibited various mechanical properties, including high tensile strength and good Young’s modulus (Hakim et al. 2021b; Sridhar, Gobinath, and Kırgız 2023). However, FVB has been reported to have inherent drawbacks, such as high water absorption, high anisotropic characteristics, low compatibility with some matrices and adhesives, and low durability. Despite the challenges, these materials are often considered the most environmentally beneficial alternative to synthetic fibers, considering both economic and sustainability perspectives (Mahesh et al. 2022; Sridhar, Gobinath, and Kırgız 2022). The hydrophilic nature of FVB often decreases the bonding ability, thereby affecting the mechanical properties of the produced composite boards. To overcome this limitation, chemical modification treatment can effectively remove wax content and impurities from FVB surface to improve the roughness and hydrophobicity. Consequently, this augments the bonding ability with adhesives, leading to enhanced mechanical properties of boards. The modified surface properties of FVB play an essential role in facilitating interfacial bonding when used as raw material for composite boards (Gholampour and Ozbakkaloglu 2020). According to previous studies, natural fiber, such as FVB, offers several advantages over synthetic variants, including lower cost, lower density, easier recyclability, and biodegradability, which increases the environmental friendliness of these materials (Karimah et al. 2021).

Several studies showed that composites made from alkaline surface treatment of natural fiber had excellent physical and mechanical properties (Kathiresan and Meenakshisundaram 2022; Mukesh and Godara 2019). A previous study carried out alkaline pre-treatment for Salacca zallaca frond FVB using NaOH + Na₂SO₃ to produce boards with good dimensional stability and mechanical properties (Hakim et al. 2021a). The NaOH+Na₂SO₃ treatment results showed an increased crystallinity index and exposure of more OH groups on the surface of modified FVB, showing the potential as a raw material for composite boards. Based on the results, geometrically long fiber, such as FVB, pre-treated with alkali was more suitable for producing these products (Boumediri et al. 2019). Furthermore, orienting the raw material more compactly has been reported to improve the mechanical properties and dimensional stability. This is because the modified FVB containing a higher number of -OH groups due to NaOH and NaOH+Na₂SO₃ treatment, often forms a stronger bond with the adhesive. In another report, the combination NaOH/NaS can be used as a modifier to improve mechanical properties (compressive and flexural strength) in the transformation of geopolymer slag binder mortar (Biricik et al. 2021; Kirgiz and Biricik 2024). Based on these studies, chemical modification using NaOH/Na₂SO₃ combination is believed to improve the physical and mechanical properties of natural fiber as raw materials for biocomposites.

This current study focused on modification treatment (concentration and time) and fiber orientation regarding modified FVB of Salacca sumatrana frond, which was used for manufacturing oriented boards. The results showed that modified FVB raw materials enhanced the quality of oriented boards compared to unmodified materials. Furthermore, the fiber orientation angle plays a crucial role in improving the quality of the products obtained.

The use of citric acid adhesives serves as an effective alternative in the manufacture of environmentally friendly composite boards (Widyorini et al. 2019). In previous reports, the use as a binder has been proven to increase the physical and mechanical properties of particleboard made from salacca frond particles (Widyorini et al. 2018b) and elephant dung fibers (Widyorini et al. 2018a). Citric acid was used in this current study to add value to the orientation boards produced in terms of environmental friendliness. Based on the results, there are no existing reports on the manufacture of fiber-oriented boards from modified FVB of Salacca sumatrana frond combined with citric acid, as well as the bonding mechanism. Therefore, this study aims to evaluate the quality of oriented boards made from alkaline-modified FVB of Salacca sumatrana frond based on the physical and mechanical properties, and the interface bonding mechanisms between citric acid and the raw materials.

Materials and methods

Materials

FVB from Salacca sumatrana frond was used as a raw material for the oriented board manufactured in this study. Furthermore, FVB length was cut into approximately 25 cm before chemical treatment was performed. The chemical solution was prepared by adjusting the concentrations of sodium hydroxide (NaOH) anhydride and sodium sulfite (Na₂SO₃). Citric acid (anhydrous) applied as a binder for the board was supplied by Rudang Chemical Company (Medan, Indonesia), without requiring any further purification.

Chemical treatments

Modified FVB of Salacca sumatrana frond was chemically modified with NaOH, Na₂SO_3, and the combination before manufacturing the oriented board, as presented in Table 1.

Table 1. The chemical modification of FVB.

Treatment	NaOH + Na2SO3 Combination	Immersion time (min)
A	Untreated, water immersion	30
B	Untreated, water immersion	60
C	1% NaOH	30
D	1% NaOH	60
E	1% NaOH + 0.2% Na2SO3	30
F	1% NaOH + 0.2% Na2SO3	60
G	1% NaOH + 0.4% Na2SO3	30
H	1% NaOH + 0.4% Na2SO3	60

The chemical properties of FVB

To prepare the samples for chemical composition analysis, extractives were removed from 2 g of oven-dried samples with an ethanol-toluene mixture (at a ratio of ratio 1 L:427 mL) through soxhlet extraction for 4 h, following ASTM D 1105–96. The extractive-free samples were then used for the analysis of cellulose (ASTM D 1103–84), hemicellulose (ASTM D 1104–84), lignin Klason (ASTM D 1106–84), and ash content (ASTM D 1102–84), which was performed in triplicate.

The contact angle measurement

Figure 1 shows the contact angle measurement according to Schellbach et al. (2016). The contact angle was observed under a light microscope with two parallel fibers, which were attached to the sample holder with a distance of 1–2 mm. The citric acid solution was dripped by a micropipette between the 2 fibers to obtain a liquid that hung between the 2 FVBs. The liquid of the citric acid droplet image was photographed using a stereo optical camera facility. Furthermore, the image was analyzed using IC-Measure software version 2.0.0.245 to measure the contact angle of FVB. Measurements were made at 4 angles formed between FVB and the liquid in contact with the sample, followed by averaging.

Figure 1. Contact angle measurement based on Schellbach et al. (2016) methods. a: diameter of fibrovascular bundle; b: liquid (water); c: distance between FVB’s; d: water meniscus; and ϴ: contact angle water to fibrovascular bundle.

The manufacturing of the oriented board

The board manufactured had dimensions of 250 mm x 250 mm x 8 mm (length x width x thickness) with a target density of 0.8 g/cm³. The citric acid solution used as the adhesive was dissolved in water to prepare a 60% concentration with a resin content of 30% based on air-dried FVB. This preparation was then sprayed onto the raw materials and oven-dried at 75°C for ±8 h, thereby reducing the moisture content to 4–6%. FVB was manually formed into a three-layer oriented mat using a forming box. For each FVB, 3 different board orientations (0°, 90°, and 45°) were prepared to individually consist of 3 layers in a weight ratio of face:core:back = 30%:40%:30%. Subsequently, the mat was hot-pressed at 180°C under a pressure of 3 MPa for 3 replications in 10 min.

Evaluation of oriented board properties

All samples were maintained under environmental conditions for approximately a week to reach a moisture content of ± 6% before evaluating the oriented board properties based on the Japanese Industrial Standard (JIS) for particleboard (JIS, A 5908 2003). The mechanical properties of the oriented board, including modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB) strength, and screw holding power (SHP), were measured using a universal testing machine (Tensilon RTF 1350, Tokyo, Japan). Furthermore, the physical properties tested were thickness swelling (TS) and water absorption (WA). To calculate static bending (MOR and MOE), samples measuring 200 × 50× 8 mm³ were subjected to the three-point bending method under dry conditions with a span distance of 150 mm and a crosshead speed of 10 mm/min. IB value was then randomly measured from the two surfaces of each specimen with dimensions 50 × 50× 8 mm³. SHP of 75 × 50× 8 mm³ specimens was estimated from 2 positions on each sample board at a speed of 2 mm/min. TS and WA of 50 × 50× 8 mm³ specimens were evaluated through water immersion for 24 h at 20°C.

Fourier Transform Infrared (FTIR) spectroscopy

The oriented board samples were subjected to a series of treatments, including boiling for 2 h to remove unreacted citric acid, conditioning in room temperature water for 1 h, drying at 35 ± 5°C for 12 h, and powdering to a 100 mesh size. FTIR spectroscopy was performed at approximately 25°C using an FTIR-4200 spectrophotometer (8201PC-Shimadzu, Tokyo, Japan) and KBr disc method with a resolution of 12 cm⁻¹. This process was carried out to determine the assignment of absorbance bands to specific functional groups.

Results and discussion

Chemical components of modified FVB

Based on Table 2, the α-cellulose component of modified FVB of Salacca sumatrana frond increased successively from 46.15 ± 0.1% (1% NaOH; 30 min) to 54.44 ± 0.22% (1% NaOH + 0.2% Na₂SO₃; 30 min). This increase did not necessarily indicate an actual elevation in the α-cellulose component, but a reduction in other components due to degradation. NaOH + Na₂SO₃(1:0.2%/60”) treatment caused a decrease in value to 51.28 ± 0.14%, while NaOH + Na₂SO₃(1:0.4/60”) caused a further reduction to 39.78 ± 0.12%, due to the dissolution of the cellulose component. The decrease in α-cellulose content was affected by the concentration of NaOH and Na₂SO₃, which played a role in degrading amorphous cellulose at higher concentrations. The results were consistent with previous studies, which used different treatments on various plant fibers. For instance, the α-cellulose content of modified FVB of Salacca sumatrana frond treated with 3% NaOH + steaming was found to be 54.53% (Darmanto et al. 2019). Meanwhile, Areca frond (Dypsis lutescens) treated with 15% NaOH had an increased α-cellulose content of 63.45% (Shanmugasundaram, Rajendran, and Ramkumar 2018). Compared to this current study, the fiber of Ficus benghalensis root previously exposed to 5% (w/w) NaOH had a 70.4% content (Ganapathy et al. 2019).

Table 2. Chemical components of modified FVB.

Chemical treatment	α-cellulose (wt%)	Hemicellulose (wt%)	Lignin (wt%)	Ash content (wt%)
A	44.39 ± 0.08	31.75 ± 0.37	33.29 ± 0.57	0.88 ± 0.08
B	44.71 ± 0.11	31.50 ± 0.27	33.18 ± 1.00	0.86 ± 0.32
C	46.15 ± 0.10	29.40 ± 1.16	32.15 ± 0.12	0.82 ± 0.09
D	48.50 ± 0.08	29.73 ± 0.88	31.45 ± 0.41	0.82 ± 0.06
E	54.44 ± 0.22	28.31 ± 0.31	27.98 ± 0.79	0.78 ± 0.08
F	51.28 ± 0.14	24.78 ± 0.11	24.96 ± 0.35	0.55 ± 0.05
G	41.10 ± 0.02	22.01 ± 0.34	25.05 ± 0.35	0.58 ± 0.16
H	39.78 ± 0.12	21.91 ± 1.60	20.87 ± 0.66	0.57 ± 0.06

The results showed that the modification treatment decreased the hemicellulose component of modified FVB of Salacca sumatrana frond along with increasing immersion time and concentration of NaOH and Na₂SO₃. The water immersion treatment for 30 min reduced hemicellulose to 31.75 + 0.37%, while the modification with 1% NaOH + 0.4% Na₂SO₃ for 60 min led to a decrease of the content to 21.91 + 1.60%. These values were lower than the 74.09% obtained from a 3% alkaline treatment on modified FVB of Salacca sumatrana frond (Darmanto et al. 2019). A previous study stated that a treatment of 5% NaOH on banyan tree root fiber caused a 10.74% decrease (Ganapathy et al. 2019). Approximately, nothing or 0% value was lost when 15% NaOH treatment was applied to some natural fibers (Shanmugasundaram, Rajendran, and Ramkumar 2018). Hemicellulose was a constituent of lignocellulosic materials, which was easily degraded once treated with alkali because the structure was non-linear and more amorphous compared to the crystalline components.

The hemicellulose reduction pattern was also followed by the lignin component after being subjected to NaOH + Na₂SO₃ modification. The lignin in modified FVB of Salacca sumatrana frond decreased progressively from 33.29 + 0.57% when immersed in water for 30 min to 20.87 + 0.66% after NaOH+Na₂SO₃ treatment (1:0.4/60’). This decline followed a similar pattern observed in previous studies, where alkali served as a modifier. Treatment with alkali and Na₂SO₃ could effectively reduce the lignin content of lignocellulosic materials, including natural fibers (Arnata et al. 2019).

FVB ash content decreased during water immersion from 0.88 ± 0.08% to 0.57 ± 0.66%. Based on the results, only a few previous studies observed the ash component in the alkaline modification process. Shanmugasundaram et al. (2018) reported a significant reduction following the reaction with 5% NaOH, and a complete disappearance after 10% and 15% NaOH treatments. Meanwhile, Ganapathy et al. (2019) showed that the inorganic content increased in Ficus benghalensis root fibers exposed to 5% NaOH. The wax component (wax) found in some natural fibers had also been noted to reduce relatively in chemical modification processes.

Contact angle between FVB modified and citric acid solution

According to Kose et al. (2018), the contact angle was the angle produced between the normal lines of solid and liquid surfaces, which were formed at the junction of the liquid, solid, and gas phases. In this study, the contact angle was calculated to assess the wetness of FVB in relation to the binder (citric acid solution). Figure 2 shows the contact angle image between the modified FVB of Salacca sumatrana frond and citric acid solution.

Figure 2. Contact angle image between modified FVB of the S. sumatrana frond and the citric acid solution.

The treatment of NaOH 1%+Na₂SO₃ 0.4% for 60 min of immersion gave the best contact angle value of 35.75°. During alkali modification, changes in chemical composition and surface roughness caused changes in the contact angle of water (Chen et al. 2018). In comparison to the water immersion treatment, this parameter was reduced by the modification of NaOH and the combination of NaOH+Na₂SO₃. Table 3 shows the value between FVB modified and citric acid solution. According to the contact angle between modified FVB and citric acid solution, the combination of NaOH+Na₂SO₃ could reduce the value. A smaller value often led to a better bonding and adhesion process between raw materials and citric acid, leading to enhanced mechanical properties of the oriented board.

Table 3. Contact angle value between FVB modified and citric acid solution.

Treatment and time of immersion	Contact angle (^o)
A: water for 30 min	65.25 ± 4.79
B: water for 60 min	61.50 ± 10.02
C: NaOH 1% for 30 min	56.00 ± 8.04
D: NaOH 1% for 60 min	55.50 ± 8.27
E: NaOH 1%+Na2SO3 0.2% for 30 min	50.24 ± 12.09
F: NaOH 1%+Na2SO3 0.2%for 60 min	45.50 ± 6.42
G: NaOH 1%+Na2SO3 0.4% for 30 min	41.75 ± 6.70
H: NaOH 1%+Na2SO3 0.4%for 60 min	35.75 ± 2.75

The treatment of NaOH+Na₂SO₃ at low concentrations was very effective in reducing the contact angle. Some of the amorphous hemicellulose components were degraded during NaOH and combination of NaOH+Na₂SO₃ treatment, which caused FVB surface roughness to increase due to the appearance of twisting, microfibril, and the loss of some impurities and wax (Cai et al. 2016). Surface changes due to degradation and accessibility of hydroxyl groups on FVB surface were 2 essential factors affecting the level of wettability.

Board density

In this study, the density values of oriented boards made from modified FVB had a coefficient of variation (CV) of 2.07%. This showed that the sample density values were homogeneous and uniform. The uniformity of the oriented board density could be attributed to the even distribution pattern of citric acid adhesive, consistent heat pressing during board formation, and the relatively uniform dimensions of the raw material (Mesquita et al. 2018). The distribution of board density in this study is presented in Figure 3.

Figure 3. Histogram of board density. A: water (30’), B: water (60’), C: 1% NaOH (30’), D: 1% NaOH (60’), E: 1% NaOH + 0.2% Na₂SO₃ (30”), F: 1% NaOH + 0.2% Na₂SO₃ (60‘), G: 1% NaOH + 0.4% Na₂SO₃ (30’), and H: 1% NaOH + 0.2% Na₂SO₃ (60”).

Water Absorption (WA) and Thickness Swelling (TS)

Figure 4 shows the water absorption and thickness swelling of the oriented board. The highest WA values were obtained in the water immersion treatment (30 min), namely 52.31% (0°), 53.22% (45°), and 54.74% (90°). Meanwhile, the lowest WA values were 43.07% (0°), 45.95% (45°), and 46.21% (90°), as shown in the combined treatment of NaOH+Na₂SO₃ (1:0.2/60’). The highest TS values were 20.44% (0°), 21.40% (45°), and 21.80% (90°), which were achieved in the water immersion treatment (30 min). The lowest TS values of 15.15% (0°), 16.90% (45°), and 16.18% (90°) were observed with the use of NaOH+Na₂SO₃ (1:0.2/60 min).

Figure 4. Histogram of board water absorption and thickness swelling. A: water (30’), B: water (60’), C: 1% NaOH (30’), D: 1% NaOH (60’), E: 1% NaOH + 0.2% Na₂SO₃ (30”), F: 1% NaOH + 0.2% Na₂SO₃ (60‘), G: 1% NaOH + 0.4% Na₂SO₃ (30’), and H: 1% NaOH + 0.4% Na₂SO₃ (60”).

The particleboard manufactured by Widyorini et al. (2018b) using frond and citric acid had a water absorption capacity of 54.2%. This study also reported that the extractive substances contained in the Salacca frond particles did not affect the particleboard WA (Widyorini et al. 2019). Kusumah et al. (2016) found that the use of 30% citric acid effectively reduced composite boards’ WA. In this current study, an alkaline modification performed with NaOH+Na₂SO₃ (1:0.2/60%) successfully decreased WA capacity to 43.0% when a 30% adhesive was used.

Modulus of Rupture (MOR) and Modulus of Elasticity (MOE)

MOR and MOE values of the oriented board were observed in 2 positions, namely the perpendicular (⊥) and parallel position (//). At all 3 orientation angles (0°, 45°, and 90°), the MOR value was found to be higher in the perpendicular test position compared to the parallel one. The results were consistent with previous studies, which also reported similar higher values (Hakim et al. 2021a). Furthermore, MOR values generally increased in boards treated with NaOH+Na₂SO₃ (1:0.2) but decreased during exposure to NaOH+Na₂SO₃ (1:0.4).

The highest MOR of the oriented board made from FVB at an angle of 0° was 26.8 MPa (⊥) and 14.4 MPa (//). At 45°, the highest value was 14.7 MPa (⊥) and 13.4 MPa (//), while at 90°, values of 16.8 Mpa (⊥) and 15.7 Mpa (//) were obtained. Meanwhile, the lowest MOR was obtained from the 30-minute water immersion treatment, namely 12.1 MPa (⊥) and 5.6 MPa (//) at 0°, 9.1 MPa (⊥) and 7.1 MPa (//) at 45°, and 8.8 MPa (⊥) and 8.0 MPa (//) at 90°, as presented in Figures 5.

Figure 5. Histogram of MOR and MOE of 0°, 45°, and 90° orientation, respectively. ꓕ: perpendicular,//: parallel, A: water (30’), B: water (60’), C: 1% NaOH (30’), D: 1% NaOH (60’), E: 1% NaOH + 0.2% Na₂SO₃ (30”), F: 1% NaOH + 0.2% Na₂SO₃ (60‘), G: 1% NaOH + 0.4% Na₂SO₃ (30’), and H: 1% NaOH + 0.4% Na₂SO₃ (60”).

Umemura et al. (2013) reported an MOR value of 10.7 MPa from a wood waste particleboard bonded with citric acid. A previous study showed that pressing particleboard at 180°C and using citric acid as an adhesive during production increased MOR value by 19.6 MPa (Umemura, Sugihara, and Kawai 2014). Liao et al. (2016) added sucrose to citric acid to manufacture low-density particleboards with MOR values > 6.0 MPa. In this study, 30% citric acid and a pressing temperature of 180°C were used to produce oriented boards with values of 28.8 MPa. This could be attributed to both the long fiber form of the raw material and the alkaline modification treatment capable of enhancing the composite boards’ mechanical value (Khakpour et al. 2020). Long natural fibers were used as raw materials to develop composite boards with good mechanical properties (Elanchezhian et al. 2018).

Alkaline modification of long fiber, such as natural fiber and FVB, could augment the accessibility of cellulose, hemicellulose, and lignin to increase the interface bond between FVB and citric acid adhesive (Shanmugasundaram, Rajendran, and Ramkumar 2018), thereby affecting the strength of the composite boards produced (Oushabi et al. 2017).

The highest MOE values were 8.2 GPa (⊥) and 4.8 GPa (//), observed for an orientation angle of 0° in the combined treatment of NaOH+Na₂SO₃ (1:0.2/60 min) and NaOH+Na₂SO₃ (1:0.2/30 min). During NaOH treatment (1/60 min), boards oriented at 45° had MOE of 4.6 GPa (⊥) and 3.9 GPa (//), while those at 90° had 4.9 GPa (⊥) and 4.7 GPa (//). The lowest values were 3.2 GPa (⊥) and 1.3 GPa (//), 2.5 GPa (⊥) and 2.3 GPa (//), as well as 2.8 GPa (⊥) and 2.4 GPa (//), obtained for orientation angles 0°, 45°, and 90°, respectively. Furthermore, these values were obtained during the soaking of samples in water for 30 min.

Santoso et al. (2017) found a MOE value of 0.5 GPa from particleboard manufactured using Nypa frutican as a raw material in combination with citric acid and maltodextrin. Widyorini et al. (2018b) obtained 4.1 GPa by adding maltodextrin and applying a hotpress treatment to the samples. Kusumah et al. (2016) employed sorghum particles and citric acid to yield 5.27 MPa. This current study recorded an MOE of 8.2 MPa, which was higher compared to previously reported values. The mechanical properties of the composite board were influenced by the dimensions of the raw material in long fiber form. The perpendicular bending test identified higher mechanical properties compared to parallel bending due to the increased compatibility generated from the orientation of FVB.

Internal Bond (IB)

The oriented board had the highest IB values of 0.43 MPa (0°) and 0.36 MPa (90°) in the combined treatment of NaOH+Na₂SO₃ (1:0.2/60”), while the lowest was 0.26 MPa (0°) and 0.22 MPa (90°) obtained during exposure to NaOH+Na₂SO₃ (1:0.4/60”). NaOH+Na₂SO₃ and an orientation angle of 0° increased the IB of boards made from bark FVB, as presented in Figure 6.

Figure 6. Histogram of the board’s internal bond. A: water (30’), B: water (60’), C: 1% NaOH (30’), D: 1% NaOH (60’), E: 1% NaOH + 0.2% Na₂SO₃ (30”), F: 1% NaOH + 0.2% Na₂SO₃ (60‘), G: 1% NaOH + 0.4% Na₂SO₃ (30’), and H: 1% NaOH + 0.4% Na₂SO₃ (60”).

Boards oriented at 90° and 45° had lower values compared to 0° because the 90° oriented samples were arranged crosswise, consisting of 3 layers, including face, core, and back. Consequently, the area of adhesion and contact between fibers on the surface of 1 layer with another was reduced. Some IB values shown in the current results, namely 0.37 MPa (Widyorini et al. 2016) and 0.15 MPa (Umemura, Sugihara, and Kawai 2014) were still higher compared to those in previous studies that used bamboo and wood waste particles, respectively, bonded with citric acid. This value was lower than the 0.46 MPa found by Hakim et al. (2021a) in samples produced (at 0°) from modified FVB of Salacca zalacca frond.

Screw holding power

The highest SHP values for the oriented boards were 527 N (0°), 531 N (45°), and 521 N (90°). These values were achieved in the combined treatment of NaOH+Na₂SO₃ (1:0.2/60’), while the lowest was found to be 326 N, 317 N, and 310 N, respectively, in NaOH+Na₂SO₃ treatment (1:0.4/60’), as presented in Figure 7.

Figure 7. Histogram of the board’s screw holding power. A: water (30’), B: water (60’), C: 1% NaOH (30’), D: 1% NaOH (60’), E: 1% NaOH + 0.2% Na₂SO₃ (30”), F: 1% NaOH + 0.2% Na₂SO₃ (60‘), G: 1% NaOH + 0.4% Na₂SO₃ (30’), and H: 1% NaOH + 0.4% Na₂SO₃ (60”).

This study found that the treatment combination to obtain the best modification effect on SHP was 0.2% NaOH + 1% Na₂SO₃. Furthermore, modifying raw materials improved the bonding shared with adhesives. Size and dimensions were other factors affecting SHP, as longer and thicker materials produced higher strength in composite boards. The SHP of oriented boards was greater compared to particleboard produced with a similar adhesive (citric acid) (Widyorini et al. 2019). Finer particles transferred stress less effectively from FVB to FVB, leading to faster screw thread damage in particle-shaped raw materials compared to those with a long-fiber structure. A single-layer board with a 0° orientation had slightly higher strength than three-layered boards created at angles of 45° or 90°.

FTIR Analysis

The effect of NaOH and NaOH+Na₂SO₃ modification treatments on the chemical structure of FVB-oriented board bonded with citric acid is presented in Figure 8. The presence of ester functional groups was detected at a wavelength peak of 1733 cm⁻¹, showing the C=O stretching group and the formation of a carbonyl group (C=O) (Widyorini et al. 2019). This showed that the carboxyl groups in citric acid had reacted or bonded with the hydroxyl groups in modified FVB. E and F treatments (NaOH+Na₂SO₃ 1:0.2% used for 30 and 60 min, respectively) showed peaks containing the highest intensity, while G and H with a higher concentration of Na₂SO₃, exhibited a decrease in intensity. The results provided compelling evidence for the mechanical properties of the oriented board, where E and F treatments yielded the best mechanical value.

Figure 8. FTIR spectrogram of modified FVB. A: water (30’), B: water (60’), C: 1% NaOH (30’), D: 1% NaOH (60’), E: 1% NaOH + 0.2% Na₂SO₃ (30”), F: 1% NaOH + 0.2% Na₂SO₃ (60‘), G: 1% NaOH + 0.4% Na₂SO₃ (30’), and H: 1% NaOH + 0.4% Na₂SO₃ (60”).

Figure 9 shows the FTIR spectrogram of modified FVB (E and F treatments) and the orientation boards produced. The spectrogram of modified material (FVB+E and FVB+F) lacked a peak at 1733 cm⁻¹, while the orientation board (board+E and board+F) displayed a peak at the same wavelength. This peak showed the formation of carbonyl (C=O stretching) and/or C=O ester groups, probably from the interaction between hydroxyl groups in modified FVB and carboxyl in citric acid (Umemura and Kawai 2015). Consequently, the formation of ester on the oriented board made from modified FVB could improve the adhesive bond shared with the raw material.

Figure 9. FTIR spectrogram of modified FVB and their oriented boards. FVB+E: fibrovascular bundle modified with 1% NaOH + 0.2% Na₂SO₃ (30’), board+E: oriented board with raw material FVB+E, FVB+F: fibrovascular bundle modified with 1% NaOH + 0.2% Na₂SO₃ (60’), board+F: oriented board with raw material FVB+F.

Conclusions

In conclusion, the combination of 1% NaOH + 0.2% Na₂SO₃ with a modification time of 30 min (E treatment) succeeded in suppressing WA and the development of oriented board TS. This also positively improved the mechanical properties of the oriented board. MOR and MOE tests showed that an orientation angle of 0° yielded better results for the testing position perpendicular to the boards’ grain. Furthermore, the contact angle between FVB and citric acid solutions showed that FVB with 1% NaOH + 0.2% Na₂SO₃ for 30 min immersion gave better spreads of citric acid solution in FVB surface. The bonding mechanism of FVB modified by 1% NaOH + 0.2% Na₂SO₃ with 30 min immersion (E treatment) and citric acid adhesive showed good bond quality. Based on the results, the FTIR spectrogram showed a peak intensity at 1733 cm⁻¹, indicating the detection of carbonyl C-O stretching and ester groups.

Research highlights

• The alkaline modified FVB of S. sumatrana frond improved surface properties by proving the contact angle value analysis.

• The oriented board made of S. sumatrana FVB modified FVB with a combination of NaOH + Na₂SO₃ can improve physical and mechanical properties.

• The bonding mechanism between fibrovascular bundle and citric acid analyzed by FTIR with evidence of the formation of ester on the board.

Acknowledgments

The authors are grateful for the financial support provided by the post-doctoral grant 2023 from the Directorate of Research Universitas Gadjah Mada (UGM) (No. 3662/UN1.P.II/Dit-Lit/PT.01.03/2023) and UGM Reputation Improvement Team toward World Class University, Yogyakarta, Indonesia.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the the post-doctoral grant 2023 from Universitas Gadjah Mada, Yogyakarta, Indonesia.