Laminated wood material reinforced with bacterial cellulose (Kombucha SCOBY) Part 2: Selected mechanical properties

Abstract

Kombucha pellicles (Kombucha microbial culture) were evaluated as a source of bacterial cellulose to use in the production of laminated wood material. The goal was to improve the mechanical properties. The Kombucha pellicles were produced at room temperature over an eight-week fermentation period, after which bacterial cellulose sheets approximately 1 mm thick were obtained by drying. These sheets were used in the production of laminated wood material together with Scots pine (Pinus sylvestris L.), Oriental beech (Fagus orientalis L.), and Anatolian chestnut (Castanea sativa Mill.) wood species, utilizing urea formaldehyde, polyurethane, and polyvinyl acetate adhesives. To evaluate the mechanical properties, the following tests were applied: compressive strength parallel to the grain, adhesive strength, bending strength, and modulus of elasticity in bending. The findings revealed that although bacterial cellulose displayed a mechanically strong structure, overall mechanical performance did not reach the desired level due to insufficient adhesion between lamina layers. The results indicated that further research is needed to improve structural compatibility and adhesion mechanisms for the use of this material in advanced mechanical applications.

Full Article

Laminated Wood Material Reinforced with Bacterial Cellulose (Kombucha SCOBY) Part 2: Selected Mechanical Properties

Mehmet Budakçı  ,^a Diğdem Şağban  ,^a Mustafa Korkmaz  ,^a and İzham Kılınç  ^b,*

Kombucha pellicles (Kombucha microbial culture) were evaluated as a source of bacterial cellulose to use in the production of laminated wood material. The goal was to improve the mechanical properties. The Kombucha pellicles were produced at room temperature over an eight-week fermentation period, after which bacterial cellulose sheets approximately 1 mm thick were obtained by drying. These sheets were used in the production of laminated wood material together with Scots pine (Pinus sylvestris L.), Oriental beech (Fagus orientalis L.), and Anatolian chestnut (Castanea sativa Mill.) wood species, utilizing urea formaldehyde, polyurethane, and polyvinyl acetate adhesives. To evaluate the mechanical properties, the following tests were applied: compressive strength parallel to the grain, adhesive strength, bending strength, and modulus of elasticity in bending. The findings revealed that although bacterial cellulose displayed a mechanically strong structure, overall mechanical performance did not reach the desired level due to insufficient adhesion between lamina layers. The results indicated that further research is needed to improve structural compatibility and adhesion mechanisms for the use of this material in advanced mechanical applications.

DOI: 10.15376/biores.21.4.9382-9406

Keywords: Bacterial cellulose; Mechanical properties; Kombucha pellicle; Lamination; Sustainable materials

Contact information: a: Department of Wood Products Industrial Engineering, Faculty of Forestry, Düzce University; Düzce, Türkiye; b: Department of Design, Interior Design, Vocational School of Technical Sciences, Batman University, Batman, Türkiye; *Corresponding author: [email protected]

Graphical Abstract

INTRODUCTION

Wood is a renewable biomaterial that is abundant in nature, energy-efficient to process, and aesthetically appealing; furthermore, when combined with appropriate engineering approaches, it is capable of providing high mechanical performance (Budakçı et al. 2016; Korkmaz et al. 2025). Due to these qualities, wood has been utilized for centuries, particularly in structural applications such as load-bearing systems, flooring, roofing, and façade cladding (Kılınç et al. 2023; Küçüktüvek et al. 2023). However, as wood is a natural polymer composite, it inevitably has certain structural heterogeneities and defects. Factors, such as growth rings, variations in fiber orientation (anisotropy), knots, resin pockets, and reaction wood, complicate its mechanical behavior, rendering its performance under tension, compression, and bending loads relatively unpredictable (Fu et al. 2021; Grottesi and Coelho 2023). For instance, knots induce stress concentrations that diminish material strength, while the anisotropic nature results in significant discrepancies between strength and elasticity values measured parallel and perpendicular to the grain. Consequently, overcoming these limitations to fully realize the potential of wood as a structural material depends on enhancing the reliability of its mechanical properties, which remains a critical area of research.

One of the most significant engineering solutions developed to mitigate the impact of these natural defects and optimize mechanical performance is laminated wood technology (Keskin and Togay 2003; Çiftçi et al. 2024). This technology relies on bonding relatively thin wood layers—such as lamellae or veneers—under pressure using high-performance adhesives, with their grain orientations typically arranged parallel to one another. The lamination process enhances the overall homogeneity and mechanical resistance of the material by distributing the influence of natural defects inherent in solid wood, such as knots, across different layers and reinforcing weak zones (Gáborík et al. 2016; Ngadianto et al. 2023).

Furthermore, by engineering the quality and arrangement of the lamellae, it is possible to manufacture structural elements with optimized mechanical properties—such as modulus of rupture (MOR), tensile strength, compressive strength, and modulus of elasticity (MOE) tailored for specific loading conditions (Wu et al. 2023). To further enhance the mechanical capacity of laminated wood, reinforcement with high-strength synthetic composites, including glass fiber reinforced polymer (GFRP), carbon fiber reinforced polymer (CFRP), and aramid fiber reinforced polymer (AFRP), is frequently employed in critical stress zones (Borri et al. 2005; Yahyaei-Moayyed and Taheri 2011; Wei et al. 2013). While these hybrid systems offer superior mechanical performance, the high energy consumption associated with synthetic polymer production, coupled with recyclability challenges and environmental sustainability concerns, is driving researchers to seek more ecological and bio-based reinforcement alternatives (Dicker et al. 2014; Başeğmez 2025).

Biopolymers derived from renewable sources, which promise superior mechanical properties, have gained significant attention in recent years as eco-friendly reinforcement elements for composite materials (Ladero et al. 2017). Among these biopolymers, bacterial cellulose (BC) stands out due to its unique morphology and exceptional mechanical characteristics. Bacterial cellulose is synthesized extracellularly by certain bacterial species—such as those belonging to the genus Gluconacetobacter—through the fermentation of simple sugars such as glucose. It has a complex three-dimensional network structure. This network consists of ultra-fine cellulose fibrils with nanometer-scale diameters (20 to 100 nm) and a high aspect ratio (Tabarsa et al. 2017; Hervy et al. 2018). Such a fibrous architecture provides BC with a remarkable combination of properties, including a very high degree of crystallinity, a significantly higher Young’s modulus compared to plant-based cellulose, high tensile strength, low density, and complete biodegradability (Chen et al. 2021). Furthermore, BC sheets, which can be produced through kombucha culture fermentation, represent a promising resource for potential applications of this material. However, it should be noted that the economic feasibility and sustainability of BC production depend on process optimization and the use of alternative low-cost substrates, and therefore require further investigation. In particular, the long time period for development of the cultured fiber mats appears to pose a barrier to industrial scale-up.

Previous studies have demonstrated that the integration of bacterial cellulose into various polymer matrices—both synthetic and bio-based—can result in composites with significantly enhanced mechanical properties (Mbituyimana et al. 2021; Li et al. 2022). For instance, even low concentrations of BC added to matrices, such as epoxy or polylactic acid, can increase tensile strength, flexural strength, and toughness (Chiciudean 2011). Furthermore, it has been reported that BC layers applied to the surface of wood veneers lead to substantial increases in both tensile strength and modulus of elasticity (Ananto et al. 2021). These findings suggest that BC possesses significant potential within laminated wood structures, either as an intermediate layer in the bond line or by being integrated directly onto the lamella surfaces. Such integration can improve stress transfer and prevent crack propagation, thereby increasing the load-bearing capacity and overall mechanical resistance of the final composite, facilitated by the ability of BC’s nano-fibrillar structure to form a robust interface with wood cell walls.

Considering this information, the present study investigated the feasibility of utilizing bacterial cellulose layers derived from Kombucha pellicles in the production of laminated wood materials. To evaluate the mechanical performance of the manufactured specimens, density, compressive strength parallel to the grain (ISO 13061-17 2017), adhesion strength (TS EN 205 2017), modulus of rupture (TS 2474 1976), and modulus of elasticity in bending (TS 2478 1976) tests were conducted.

EXPERIMENTAL

This study is a continuation of the previously published work entitled “Laminated Wood Material Reinforced with Bacterial Cellulose Sheets Derived from Kombucha Pellicles: Part 1 – Some Physical Properties.” Similar material preparation methods and experimental procedures were employed to ensure experimental consistency. However, the present study focuses on mechanical properties, and the related descriptions have been revised and restructured accordingly to ensure originality.

Materials

Preparation of wood specimens

In this study, wood from Scots pine (Pinus sylvestris L.), Oriental beech (Fagus orientalis L.), and Anatolian chestnut (Castanea sativa Mill.), all commonly utilized in the furniture industry, was employed. In accordance with TS 2470 (1976), first-grade, defect-free blanks with dimensions of 400 mm × 100 mm × 10 mm were prepared in the radial direction. The blanks were conditioned at a temperature of 20 ± 2 °C and a relative humidity of 65 ± 3% until reaching a constant weight, following the TS 2471 (1976) standard. The wood materials were selected as first-grade, defect-free specimens in accordance with TS 2470 (1976), ensuring comparable density characteristics within each species. Under these controlled conditions, all specimens were assumed to reach equilibrium moisture content prior to lamination. The samples were reconditioned under the same environmental conditions. The moisture content of the BC sheets was not directly measured; however, under the specified conditioning conditions, it is expected to reach equilibrium with the ambient environment. Detailed physical and structural characteristics of the BC material have been reported in Part 1 of this study (Budakcı et al. 2025). After conditioning, the specimens were machined to final dimensions of 400 × 100 × 6.6 mm³ for the control group and 400 × 100 × 6 mm³ for the BC-reinforced group. All specimens were subsequently re-conditioned under the same environmental parameters prior to lamination.

Adhesive

Polyvinyl acetate (PVAc), polyurethane (PU), and urea-formaldehyde (UF) adhesives were utilized in the production of the laminated wood. The adhesives were applied at a spread rate of 170 g/m², monitored with a precision of 0.01 g, in accordance with the manufacturer’s recommendations and procedures used for laminated wood bonding (ASTM D5751-99 2019). A constant adhesive spread amount per unit area was used for all adhesive types to ensure direct comparability among the experimental groups and to isolate the effect of BC reinforcement. The fundamental properties of the adhesives and the corresponding pressing parameters are summarized in Table 1.

Table 1. Adhesive Properties and Pressing Parameters Used in The Study

Kombucha Pellicle Production

Bacterial cellulose (BC), utilized as a reinforcing layer in this study, was obtained through the static fermentation of a kombucha-based culture medium. The medium was prepared by dissolving sucrose (90 g/L) and black tea (5 g/L) in distilled water. After the solution cooled to room temperature, previously fermented kombucha liquid (200 mL per L) and approximately 24 g of wet SCOBY were introduced as inoculum (Reiss 1994). All materials used during preparation were treated with hot water (~ 90 °C) to reduce contamination risk. Fermentation was conducted under static conditions for approximately 28 days at a temperature of 22 ± 4 °C and a relative humidity of 80 ± 5%.

Fig. 1. Kombucha-derived BC pellicles: (a) air-drying and conditioning process; (b) stacked dried pellicles. Reproduced from Part 1 (Budakcı et al. 2025) to ensure experimental consistency

During fermentation, the acidity of the medium was monitored using a digital pH meter, and pH values were maintained within the range of 3.0 to 4.0. This condition may lead to pH-related incompatibilities between BC and adhesive systems, and its potential effects on bonding performance are discussed in the relevant section. At the end of the fermentation period, cellulose pellicles with an approximate thickness of 10 mm were formed. The pellicles were then rinsed repeatedly with distilled water to remove residual compounds from the fermentation medium. Drying was performed at room conditions by placing the pellicles on parchment paper and allowing moisture to evaporate naturally. The drying process continued for approximately 30 days, and the samples were periodically turned at two-day intervals to ensure homogeneous drying. Thickness measurements were taken from eight different locations (corners and edge midpoints) using a digital caliper, and average values were calculated. The final thickness of the dried sheets was found to be 1.0 mm with a standard deviation of 0.15 mm. Detailed physical, structural, and chemical characterization of the BC sheets (including density, surface morphology, crystallinity, and chemical composition) was not conducted in the present study. These properties have been comprehensively reported in Part 1 of this study (Budakcı et al. 2025). The current work focuses specifically on evaluating the mechanical performance of laminated wood composites incorporating BC layers. Prior to lamination, the samples were conditioned at 20 ± 2 °C and 65 ± 3% relative humidity.

Preparation of Test Samples

The conditioned wood layers were manufactured into three-layer laminated wood composites with dimensions of 400 × 100 × 20 mm³ using a hydraulic press. The lamellae were arranged in a parallel (unidirectional) configuration, with the grain direction of all layers aligned parallel to the longitudinal axis of the specimens, and no cross-lamination was applied. This layering arrangement is illustrated in Fig. 2.

Fig. 2. Generalized schematic of the laminated wood structure, showing BC-reinforced and control configurations used in the study

The PVAc and UF adhesives were applied to the surfaces using a roller, whereas the PU adhesive was applied with a notched trowel. In the BC-reinforced laminated materials (Fig. 3), the dried BC sheets were integrated between the wood lamellae. In this study, the adhesive was applied only to the wood surfaces (single glue spread, SGS). The BC sheets were not directly coated with adhesive due to their dense and low permeability structure, which limits adhesive penetration. Additionally, the use of a double glue spread (DGS) method was not adopted in order to maintain process consistency and to avoid excessive adhesive accumulation at the interface.

Fig. 3. Production stages of BC-reinforced laminated wood materials: (a) adhesive application, (b) layering, (c) pressing, and (d) conditioning. Reproduced from Part 1 (Budakcı et al. 2025) to ensure experimental consistency

The laminated specimens, initially produced at 400 × 100 × 20 mm³, were subsequently dimensioned for mechanical testing in accordance with the relevant standards. For the compression strength parallel to the grain test, specimens were prepared with dimensions of 30 mm (length) × 20 mm (width) × 20 mm (thickness). For the bonding strength test, the dimensions were 150 × 20 × 10 mm³. For the modulus of rupture (MOR) and modulus of elasticity (MOE) tests, specimens were sized to 320 × 20 × 20 mm³. To eliminate moisture content variations arising from the gluing and machining processes, all specimens were conditioned at 20 ± 2 °C and 65 ± 3% relative humidity until reaching a constant weight, following the TS 13061-17 (2017) standard. For each experimental combination (wood species × adhesive type × configuration), 10 replicate specimens were prepared and tested for each mechanical property (compression strength, adhesion strength, MOR, and MOE).

Methods

Compressive strength parallel to grain test

The compressive strength parallel to the grain (σ_c) was determined according to the TS 13061-17 (2017) standard. The widths (a) and thicknesses (b) at the midpoints of the air-dry specimens were measured using a digital caliper with a precision of ± 0.01 mm. The loading rate of the universal testing machine was adjusted to ensure specimen failure within 1.5 to 2 min from the onset of loading, and the process continued until fracture occurred (Fig. 4). The compressive strength parallel to the grain was calculated using Eq. 1,

 (1)

where σ_c is the compressive strength parallel to grain (N/mm²), F_max is the maximum force at failure (N), a is the width of the specimen (mm), and b is the thickness of the specimen (mm).

Fig. 4. Compressive strength test parallel to the fibers

Adhesion strength test

The adhesion strength (σ_a) was determined according to the TS EN 205 standard. Air-dry specimens, prepared in accordance with the standard, were subjected to a tensile force in a universal testing machine at a crosshead speed that induced failure at the adhesive line within 15 to 60 s (Fig. 5). The adhesion strength was calculated using Eq. 2:

 (2)

where F_max is the maximum force at failure (N) and A is the bonding surface area (mm²), calculated as A = a × l, where a is the width (mm) and l is the length of the bonding surface (mm).