Abstract
Structural, thermal, and physicochemical performance were studied for sodium hydroxide (NaOH) (12 wt%)/itaconic acid polyamidoamine-epichlorohydrin (IA-PAE) (0 to 30 wt%) cross-linked tissue-equivalent composites made from defatted soy flour, soy protein concentrate, and soy protein isolate-bonded Rhizophora spp. (particle size: 149 µm), to enhance interfacial bonding, stability, and overall material performance. The Fourier transform infrared spectra confirmed effective crosslinking and a reduction in hydroxyl groups, with the formation of ester/ether linkages. Thermal analysis showed improved stability and a higher char yield. Microstructural analysis revealed denser, more homogeneous matrices with enhanced interfacial bonding. Average densities ranged (0.94 ± 0.11 to 1.04 ± 0.20 g/cm3), matched those of water and other commercial tissue substitute biomaterials, while moisture content (6.51 ± 0.19 to 8.06 ± 0.50%) was consistent with JIS A-5908 (2015) limits. Mechanical properties exceeded standards (internal bonding strength (IBS) up to 0.73 ± 0.01 MPa; modulus of rupture (MOR): 19.21 ± 0.50 MPa; modulus of elasticity (MOE): 7.70 ± 0.33 GPa), and dimensional stability improved significantly (water absorption: ≥ 20%, thickness swelling: ≥ 10%). Optimal performance was achieved at 20% IA-PAE. NaOH/IA-PAE crosslinking effectively enhances structural, thermal, and physicomechanical properties, producing sustainable composite particleboards suitable for tissue-equivalent and radiation-related applications.
Download PDF
Full Article
Sodium Hydroxide/Itaconic Acid Polyamidoamine-Epichlorohydrin Cross-linked Soy Protein Bonded Rhizophora spp. Composites
Damilola Oluwafemi Samson,a,b Siti Hajar Zuber 
,a,* Saadiatul Nur Aqilah Mohd Zakaria,a Muhammad Safwan Ahmad Fadzil,a Abel Blessing Olorunsola,b Mohd Zahri Abdul Aziz,c Seán B. Leen,d and Pouyan Ghabezi d
Structural, thermal, and physicochemical performance were studied for sodium hydroxide (NaOH) (12 wt%)/itaconic acid polyamidoamine-epichlorohydrin (IA-PAE) (0 to 30 wt%) cross-linked tissue-equivalent composites made from defatted soy flour, soy protein concentrate, and soy protein isolate-bonded Rhizophora spp. (particle size: 149 µm), to enhance interfacial bonding, stability, and overall material performance. The Fourier transform infrared spectra confirmed effective crosslinking and a reduction in hydroxyl groups, with the formation of ester/ether linkages. Thermal analysis showed improved stability and a higher char yield. Microstructural analysis revealed denser, more homogeneous matrices with enhanced interfacial bonding. Average densities ranged (0.94 ± 0.11 to 1.04 ± 0.20 g/cm3), matched those of water and other commercial tissue substitute biomaterials, while moisture content (6.51 ± 0.19 to 8.06 ± 0.50%) was consistent with JIS A-5908 (2015) limits. Mechanical properties exceeded standards (internal bonding strength (IBS) up to 0.73 ± 0.01 MPa; modulus of rupture (MOR): 19.21 ± 0.50 MPa; modulus of elasticity (MOE): 7.70 ± 0.33 GPa), and dimensional stability improved significantly (water absorption: ≥ 20%, thickness swelling: ≥ 10%). Optimal performance was achieved at 20% IA-PAE. NaOH/IA-PAE crosslinking effectively enhances structural, thermal, and physicomechanical properties, producing sustainable composite particleboards suitable for tissue-equivalent and radiation-related applications.
DOI: 10.15376/biores.21.3.7769-7789
Keywords: Soy protein-based adhesives; Rhizophora spp.; NaOH/IA-PAE crosslinking; Composite particleboards; Sustainable biomaterials; Renewable resources
Contact information: a: Department of Diagnostic Imaging and Radiotherapy, Centre of Diagnostic, Therapeutic and Investigate Studies, Faculty of Health Sciences, Universiti Kebangsaan Malaysia, 50300, Kuala Lumpur, Malaysia; b: Department of Physics, Faculty of Sciences, University of Abuja, 90021, Abuja, Nigeria; c: Biomedical Imaging Department, Pusat Kanser Tun Abdullah Ahmad Badawi, Universiti Sains Malaysia, Bertam, 13200, Kepala Batas, Pulau Pinang, Malaysia; d: School of Engineering, University of Galway, H91 TK33, Galway, Ireland;
* Corresponding author: hajarzuber@ukm.edu.my
Graphical Abstract
INTRODUCTION
Mangrove wood from the genus Rhizophora spp. (Rh. spp.) has attracted increasing interest as a sustainable raw material for engineered wood products, including particleboard, fiberboard, and plywood. Its fast growth, high density, and favorable mechanical strength make it particularly suitable for composite applications (Shakhreet et al. 2013; Samson et al. 2020; Zuber et al. 2022). Beyond structural benefits, the unique anatomical features of Rh. spp., such as its stilt-like root system, enhance nutrient transport and environmental resilience, further supporting its use in advanced material systems (Ng et al. 1999). In addition to traditional applications, Rh. spp.-based materials have been investigated for radiation-related uses, including photon and electron radiotherapy, as well as X-ray attenuation, because of their promising shielding properties and tissue substitute performance (Banjade et al. 2001; Watanabe and Constantinou 2006; Abuarra et al. 2014; Hamid et al. 2018; Anugrah et al. 2020; Zuber et al. 2022; Samson et al. 2023; Tayo et al. 2025).
Despite these benefits, untreated Rh. spp. wood is susceptible to dimensional changes, biological decay, and mechanical wear over time. To address these problems, recent advances in wood-based composites have integrated wood adhesive systems to improve bonding performance and long-term stability (Abuarra et al. 2014; Samson et al. 2023; Zuber et al. 2025). Currently, approximately 95% of wood adhesive systems used worldwide to develop wood composites are based on non-renewable resources (synthetic adhesives) (Karliati et al. 2024). Conventional fossil-based adhesives, such as urea-formaldehyde, melamine-urea-formaldehyde, and phenol-formaldehyde resins, are widely used, with urea-formaldehyde particularly prevalent in the manufacturing of composite particleboards. While non-renewable resources offer strong bonding, improved efficiency, and performance, their environmental and health risks, such as formaldehyde emissions, coupled with their unsustainable nature, have fueled the search for cleaner alternative adhesives (Samson et al. 2020; Karliati et al. 2024; Xu et al. 2024; Tayo et al. 2025). Thermoplastic alternatives are more flexible but lack permanent cross-links, leading to reduced thermal and structural stability (Rowell et al. 2013; Marta et al. 2026).
In this context, bio-based adhesives made from renewable resources have become sustainable options because their by-products have resulted in optimized, economically feasible, and environmentally friendly value chains (Gui et al. 2013; Frihart and Satori 2013; Samson et al. 2025; Tayo et al. 2025; Marta et al. 2026; Younesi-Kordkheili and Pizzi 2026). Through reshaping linear industrial value chains to minimize pollution and waste generation, this shift supports a trajectory marked by greater sustainability and inclusivity, effectively addressing climate change and reducing reliance on fossil-based raw materials. Many renewable resources, including soy protein, tannins, corn starch, lignin, and animal glue, have so far been explored as alternatives for developing bio-adhesives (Abuarra et al. 2014; Xu et al. 2024; Samson et al. 2025; Tayo et al. 2025; Baskaran et al. 2026; Marta et al. 2026).
Among these biopolymers, soy protein-based adhesives (such as DSF-defatted soy flour, SPC-soy protein concentrate, and SPI-soy protein isolate) have been the most abundant oilseed-derived materials and have gained significant attention due to their biodegradability, availability, and excellent film-forming properties, as well as their use in tissue regeneration (Chien and Shah 2012; Xu et al. 2024; Samson et al. 2025). Soy proteins have complex hierarchical structures (α-helices and β-sheets) that influence their physicochemical behavior and adhesion capabilities. However, inherent limitations, including hydrophilicity, low water resistance, and moderate mechanical strength, restrict their use in high-performance composites (Hemmilä et al. 2017). To overcome these challenges, chemical modification and cross-linking are employed (Gui et al. 2013; Samson et al. 2020). Alkaline treatment with sodium hydroxide (NaOH) promotes protein unfolding and reactivity, while cross-linking agents, such as itaconic acid polyamidoamine-epichlorohydrin (IA-PAE), improve network formation, leading to enhanced mechanical strength, water resistance, and interfacial bonding (Gui et al. 2013; Gao et al. 2019; Samson et al. 2020). These modifications are also expected to affect radiation attenuation by altering the composite’s density and elemental composition.
This study focused on developing and characterizing Rh. spp.-based particleboards bonded with soy protein adhesives (DSF, SPC, and SPI) modified with NaOH (12 wt%) and varying levels of IA-PAE (0 to 30 wt%). The physicochemical, mechanical, and dimensional stability properties were evaluated, along with elemental atomic compositions. The performance of the composites was assessed in accordance with relevant standards, including the Japanese Industrial Standard (JIS A 5908 2015), the American Society for Testing and Materials (ASTM D1037 1999), and European standards (EN 319/EN 310 1993).
EXPERIMENTAL
Preparation and Particle Size Analysis of Rhizophora spp. Wood
Due to the natural variability in Rh. spp. wood from the local mangrove forest reserve in Perak, Malaysia, the middle trunk sections were selected. The debarked logs were cleaned, oven-dried, and cut into smaller pieces with a band saw, then planed into chips (~8370 μm). The chips were pre-ground (~1500 μm) with a mechanical grinder (3 krpm) and further milled to ≤ 500 μm using an SK-100 cutting mill (Retsch, Germany).
Fig. 1. Flowchart of the production process of bio-based Rh. spp. composites
The resulting particles (moisture content: 5 to 8%) were sieved (≤ 149 µm mesh) for 60 min using an automatic horizontal vibrating sieve (Tai-Yi, Taiwan), then conditioned at room temperature for 1 h and stored in sealed containers at ambient conditions for one month. Rh. spp. particles (≤ 149 µm) were used, as this size was previously identified as optimal for improving the physical and mechanical properties of the composites (Samson et al. 2020, 2023). Figure 1 illustrates the overall flowchart of the present study.
Preparation of NaOH/IA-PAE-modified Bio-Adhesives
The DSF, SPC, and SPI-based adhesives were purchased from Wachsen Industry and Shandong Wonderful Biotech (Qingdao, China) and supplied as mechanically pulverized, dehydrated powders (DSF: ≥ 50% w/w gross protein on a dry basis, ≤ 40% carbohydrates, ≤ 5.3% moisture, ≤ 6.5% ash, ≤ 1% crude fat; SPC: ≥ 65% w/w gross protein on a dry basis, ≤ 35% carbohydrates, ≤ 5.7% moisture, ≤ 8% ash, ≤ 0% crude fat; SPI: ≥ 90% w/w gross protein on a dry basis, ≤ 10% carbohydrates, ≤ 5% moisture, ≤ 6% ash, ≤ 0% crude fat). NaOH (≥ 97% purity) was obtained from Sigma-Aldrich (USA). The IA-PAE cross-linker was prepared according to established procedures reported in the literature (Samson et al. 2020).
DSF-, SPC-, and SPI-based/NaOH/IA-PAE adhesives were prepared following the method reported by Samson et al. (2020) with some modifications. For DSF-SPC systems, 35 g (dry basis) was dispersed in distilled water (65, 43, 33, and 23 g, corresponding to different solid contents) and mechanically stirred at 600 rpm for 30 min at 25 ℃. IA-PAE resin (0, 10, 20, and 30 wt%, based on protein weight) was then added, and mechanical stirring was continued for an additional 30 min. For SPI formulations, 18 g of SPI was dissolved in distilled water (82, 60, 50, and 40 g), and mechanically stirred at 600 rpm for 30 min at 25 ℃. IA-PAE (0 to 30 wt%, based on SPI weight) was subsequently added dropwise, followed by further mechanical stirring for 30 min. In all cases, the adhesive slurries were adjusted to pH 11.0 using 2 M NaOH (12 wt%), via dropwise addition over 30 min, and further mechanically mixed for 30 min to ensure homogeneity. The resulting formulations were designated as DSF-SPC-SPI/NaOH/IA-PAE adhesives. The composite variants were defined solely by protein type (DSF, SPC, or SPI) and IA-PAE content (0, 10, 20, and 30 wt%). A total of twelve adhesive formulations were prepared and evaluated.
Preparation of NaOH/IA-PAE-modified Composite Particleboards
Rh. spp. particles were uniformly blended with the respective adhesive formulations (DSF-, SPC-, or SPI-based + NaOH + IA-PAE) using a rotary mixer (DFY-1000, China) at 25 rpm for 40 min before composite fabrication. The mixtures were then formed into composite mats using a square mold with dimensions (27.0 x 27.0 x 0.5 cm3) (length x width x thickness) and pre-pressed (0.49 MPa, 10 min) in a laboratory environment maintained at 25 ℃ and 55% relative humidity (RH). Thermal consolidation was performed using a hydraulic hot press (185 ℃, 20 MPa, and 18 min). The fabricated particleboards were cooled and conditioned at ambient conditions for one week before testing, as shown in Fig. 2. All composites were manufactured as single-layer particleboards. For each adhesive formulation, five replicate particleboards were fabricated and evaluated to ensure the reliability and reproducibility of the results according to JIS A-5908 (2015), ASTM D1037 (1999), and EN 319 (1993)/EN 310 standards (1993). The masses of air, Rh. spp. particles, adhesive mixtures, and distilled water were measured using Eq. 1,
Fig. 2. Manufactured composite particleboards
The prepared composite particleboards were sectioned into labeled test specimens and are presented in Fig. 3.
Fig. 3. Schematic representation of specimen preparation; test specimens were collected from multiple locations across each particleboard to account for composite heterogeneity.
Specimen 1 was set aside for other tests, while specimens 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 were used for internal bonding strength (IBS) testing. Specimens 3, 5, 7, 9, 11, 13, 15, 17, and 19 were designated for water absorption (WA) and thickness swelling (TS) measurements. Specimen 21 was used to evaluate flexural properties (MOR-modulus of rupture; MOE-modulus of elasticity). All measured parameters are presented as mean ± standard deviation (SD), as indicated in the figures and tables. In addition, statistical analyses were conducted to evaluate differences between the unmodified and modified adhesive variants.
Fourier Transform Infrared (FTIR) Spectrometry Analysis
The FTIR spectra were collected on an IRPrestige-21 spectrophotometer (Shimadzu, Japan) in diffuse reflectance mode. Prior to analysis, the cured adhesive and composite samples were oven-dried, finely ground, and sieved to obtain a homogeneous powder. Approximately 1 mg of sample was thoroughly mixed with KBr (100 mg) and pressed into disc pellets (thickness: 0.5 mm, diameter: 12.71 mm) at 32 MPa for 3 min, as shown in Fig. 4. Spectra were recorded over the range of 4000 to 400 cm-1 at a resolution of 4 cm-1 with 32 scans.
Fig. 4. FTIR spectroscopy: (a) Metal moulds used for making the pellets, (b) Sample pellets, and (c) Disc pellets of crimped samples
Thermal Properties
Thermogravimetric (TG) and derivative thermogravimetric (DTG) analyses were performed on an STA 6000 (Perkin-Elmer, USA) under a nitrogen atmosphere. An empty aluminum pan served as the reference. A 5 mg ground sample was sealed in an aluminum pan. The sample was heated from 30 to 900 ℃ at 10 ℃/min (20 mL/min N2) with a 5 min hold at 900 ℃. Mass loss was recorded as a function of temperature, and the peak temperature in the DTG curve was used as an indicator of thermal stability.
Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray (EDX) Analysis
The morphology of the composites was examined using an FESEM (FEI Quanta FEG-650, Netherlands). Sample cross-sections (0.5 x 0.5 x 0.5 cm3) were oven-cured at 120 ℃, mounted on metal straps, and sputter-coated with approximately 0.5 µm of gold (~60%) for 30 s at 45 mA (Polaron SC515, UK). Micrographs were acquired at 5 to 20 kV, with a 30° tilt and 500x magnification. Chemical compositions were determined and compared with water and other commercial tissue substitute biomaterials using the EDX system integrated with the FESEM.
Density
Specimens (5.0 x 5.0 x 0.5 cm3) were prepared and labeled accordingly, and ρ with its associated uncertainties was calculated using Eq. 2,
where m, l, w, and h denote the respective mass (g), length (cm), width (cm), and thickness (cm), while dm, dl, dw, and dh represent the respective uncertainties.
Moisture Content (MC)
Each sample was weighed in a petri dish, oven-dried at 105 ℃ for 24 h, then cooled in a desiccator for 10 min. The final weight was recorded, and MC (%) was evaluated using Eq. 3:
(3)
Solid Content (SC)
Approximately 3 g of each sample was placed in an aluminum pan and oven-dried at 103 ± 2 ℃ for 24 h. The weight was recorded, and the average SC (%) was calculated using Eq. 4:
(4)
Internal Bonding Strength (IBS)
Specimens (5.0 x 5.0 x 0.5 cm3) were prepared as shown in Figs. 3 and 5.
Fig. 5. Sample setup for IBS testing and specimen configuration: (a) Composite particleboard test samples, (b) IBS mould blocks, and (c) UTM-5582 with integrated test samples
The IBS was measured using an INSTRON universal testing machine (UTM-5582, USA) at a crosshead speed of 2 mm/min. Samples were bonded between preheated metal blocks (150 ℃) with adhesive applied evenly to both surfaces and then conditioned at room temperature for 24 h. The bonded samples were mounted in tensile fixtures, and load was applied perpendicular to the particleboard surface until failure. The maximum load at fracture was recorded, and IBS was assessed using Eq. 5,
(5)
where l is the span (cm) between two supporting points, and w is the width (cm) of the testing sample.
Flexural Properties (MOR and MOE)
For the bending test, specimens measuring 27 x 5 x 0.5 cm3 were prepared as shown in Figs. 3. and 6. The 3-point bending test was conducted with a span-to-depth ratio of 52:1 and a crosshead speed of 10 mm/min. The support span was set to 15 cm, and the load was applied centrally until failure. The collected load-deflection data were used to calculate MOR and MOE (Eqs. 6 and 7),
(6)
(7)
Where Fmax, l, h, t, and sd represent breaking force (N), support point spacing (cm), thickness (cm), time (s), and downward displacement speed (cm/s). The mechanical parameters were then assessed through various tests. The IBS and MOR were evaluated according to JIS A-5908 (2015) Types 8, 13, and 18, EN 319 (1993), and EN 310 (1993), while the MOE was determined in accordance with ASTM D1037 (1999) M-2 and EN 310 (1993).
Fig. 6. Sample orientation for MOR and MOE testing
Water Absorption and Thickness Swelling Tests
Rectangular specimens (5.0 x 5.0 x 0.5 cm3), as shown in Fig. 3, were prepared and oven-dried at 105 ℃ for 24 h. The initial weight W0 and thickness (h0) were measured with an accuracy of 1 x 10-3 g and 1 x 10-2 mm, respectively. The specimens were then immersed horizontally in distilled water at 23 ± 2 ℃ for 24 h. After immersion, the weight gain (W24) and thickness (h24) were immediately measured. Water absorption (WA24) and thickness swelling (TS24) were calculated as percentage changes using Eqs. 7 and 8:
(7) 
(8)
RESULTS AND DISCUSSION
FTIR Analysis
The FTIR spectra of DSF-SPC-SPI/Rh. spp. and NaOH/IA-PAE-modified composites exhibited consistent chemical transformations that were indicative of cross-linking and interfacial enhancement.
Fig. 7. FTIR spectra of composite adhesives, revealing the functional groups and structural changes after alkaline treatment and cross-linking: (a) DSF/Rh spp., (b) DSF/NaOH/IA-PAE/Rh. spp., (c) SPC/Rh. spp., (d) SPC/NaOH/IA-PAE/Rh. spp., (d) SPI/Rh. spp., and (e) SPI/NaOH/IA-PAE/Rh. spp.
Unmodified adhesive variants as shown in Figs. 7a, 7c, and 7e displayed broad O–H/N–H stretching (3240 to 3500 cm-1), amide I (1641 to 1653 cm-1), amide II (1562 to 1572 cm-1), and C–O bands (1062 to 1091 cm-1), indicating strong hydrogen bonding between DSF/SPC/SPI and lignocellulosic components, consistent with typical protein-fiber interactions (Chen et al. 2013; Anil and Trina 2019). After NaOH/IA-PAE treatment, notable spectral changes occurred and are presented in Figs. 7b, 7d, and 7f. The O–H/N–H band (3416 to 3443 cm-1) generally decreased, suggesting hydroxyl consumption and stronger intermolecular interactions. Amide I band shifted to higher wavenumbers (1649 to 1653 cm-1) with increased intensity, while amide II bands (1570 to 1575 cm-1) became more prominent, confirming protein structural changes and covalent cross-linking with IA-PAE (Samson et al. 2020). Increased C–N and bands in the 1319 to 1462 cm-1 region support the formation of new ester and ether linkages (Chen et al. 2025). The presence of ester groups could facilitate adhesion in bonding wood-based composites (Karliati et al. 2024). Carbohydrate peaks shifted from 1060 cm-1 to 1091 cm-1, indicating NaOH-induced disruption of the lignocellulosic structure and improved access for cross-linking. The reduction in C–H stretching (2965 cm-1) and lower C–O/C–O–C band intensity (1120 cm-1) suggests restricted chain mobility and hydroxyl participation in bonding.
Thermal Analysis
Figure 8a shows the multi-stage degradation of TG analysis, with clear improvement after NaOH/IA-PAE modification. Initial weight loss below 100 ℃ (1 to 2%) resulted from moisture evaporation; the smaller loss in NaOH/IA-PAE-modified composites indicates reduced hygroscopicity due to effective cross-linking (Zou et al. 2024). In the 100 to 250 ℃ range, modified samples retained more mass (92 to 93%), reflecting restricted chain mobility and strengthened intermolecular interactions (Xu et al. 2024; Zou et al. 2024). During the main degradation stage (250 to 350 ℃), which involves cellulose depolymerization and soy protein backbone cleavage, DSF-SPC-SPI/Rh. spp. composites demonstrate slightly higher weight retention (74 to 76% at 294 ℃), whereas DSF-SPC-SPI/NaOH/IA-PAE/Rh. spp. composites display broader mass-loss curves, indicating a more uniform network that allows gradual energy release.
At higher temperatures (400 to 800 ℃), treated composites consistently retained more residue (DSF/NaOH/IA-PAE: 33.2% at 550 ℃; 26.1% at 722 ℃; SPC/NaOH/IA-PAE: 31.1%), confirming improved carbon retention and reduced volatilization (Xu et al. 2024). Final char yields at 898 ℃ (DSF/NaOH/IA-PAE: 17.6%, SPC/NaOH/IA-PAE: 15.1%, SPI/NaOH/IA-PAE: 12.8%) further indicate enhanced structural compactness and thermal stability. The DTG results showed faster moisture release (30 to 70 ℃) but lower degradation rates at 70 to 150 ℃, indicating better intermediate stability, as depicted in Fig. 8b. In the 200 to 400 ℃ range, sharper peaks at slightly lower temperatures (−3.03% min-1) suggested controlled, synchronized breakdown of cross-linked networks. At 700 to 900 ℃, smoother degradation confirmed stabilized char formation, showing that modification redefined degradation pathways while enhancing thermal resilience.
Fig. 8. (a) TG and (b) DTG profiles of soy protein particleboards; The plots compare unmodified (DSF, SPI, SPC) and NaOH/IA-PAE cross-linked composites, highlighting multi-stage thermal degradation
FESEM and EDX Analysis
Figure 9 illustrates improvements in microstructure and interfacial bonding across the adhesive composites. The DSF/Rh. spp.-bonded composite showed a heterogeneous, porous structure with uneven adhesive spread, noticeable voids, and noticeable particle pull-out, suggesting poor wetting and weak adhesion.
These findings are presented in Fig. 9a. SPC/Rh. spp. revealed slight improvement but remained porous, with visible gaps at the interface and predominant debonding failure, as shown in Fig. 9b. In contrast, SPI/Rh. spp. formed a more uniform matrix that effectively coated and connected particles, reducing voids and improving stress transfer, as evidenced by partial cohesive failure in Fig. 9c. Notable densification was observed after NaOH treatment, and IA-PAE crosslinking is presented in Fig. 9d, 9e and 9f. These modified adhesive variants had denser, more uniform structures with better binder continuity, reduced porosity, and fewer microcracks. Enhanced interfacial adhesion is reflected in smoother transitions and less debonding in wood composites (Xian-Qing et al. 2020). Smoother adhesive films often lead to greater adhesive-wood surface interaction by improving molecular interactions between the adhesive and Rh. spp. wood (Wibowo and Park 2021). Among all composite samples, SPI/NaOH/IA-PAE/Rh. spp. demonstrated the most integrated structure, featuring a dense interpenetrating network and ductile fracture characteristics (Samson et al. 2020; Tayo et al. 2025).