High-density binderless bamboo brush handles via high-consistency mechano-enzymatic pretreatment: Micro-filler effect and machinability

Abstract

The resurgence of traditional culture has driven market demand for high-quality writing brushes. Natural bamboo frequently suffers from hygroscopic cracking, whereas polymer substitutes face the dual challenges of aesthetic deficiency and formaldehyde emission. While flat-molded binderless technology offers environmental advantages, it remains inadequate for addressing the high density and durability requirements of cylindrical artifacts. This study uses high-consistency mechano-enzymatic (HCME) pretreatment and cylindrical molding. It transforms bamboo processing residues into high-density binderless brush handles. Under HCME treatment, fibers fibrillate and parenchyma cells fragment. These changes induce microstructural reorganization. This reorganization constitutes a critical micro-filler effect. Porosity decreased to 3.27%, enabling a high density of 1.27 g/cm³. A thickness swelling (TS) below 5.1% effectively mitigated hygroscopic defects, while the material exhibited robust ink resistance. Attributable to micro-brittleness, chips broke cleanly during lathe turning; this eliminated fiber tearing and yielded a mirror-like surface finish. This formaldehyde-free approach achieved mechanical performance comparable to the tactile sensation of precious hardwoods but provided a potential pathway for extending binderless technology to the manufacturing of high-value cultural artifacts.

Full Article

High-density Binderless Bamboo Brush Handles via High-consistency Mechano-enzymatic Pretreatment: Micro-filler Effect and Machinability

Zheng Xue  ,^a and Chen Hang  ,^b,*

The resurgence of traditional culture has driven market demand for high-quality writing brushes. Natural bamboo frequently suffers from hygroscopic cracking, whereas polymer substitutes face the dual challenges of aesthetic deficiency and formaldehyde emission. While flat-molded binderless technology offers environmental advantages, it remains inadequate for addressing the high density and durability requirements of cylindrical artifacts. This study uses high-consistency mechano-enzymatic (HCME) pretreatment and cylindrical molding. It transforms bamboo processing residues into high-density binderless brush handles. Under HCME treatment, fibers fibrillate and parenchyma cells fragment. These changes induce microstructural reorganization. This reorganization constitutes a critical micro-filler effect. Porosity decreased to 3.27%, enabling a high density of 1.27 g/cm³. A thickness swelling (TS) below 5.1% effectively mitigated hygroscopic defects, while the material exhibited robust ink resistance. Attributable to micro-brittleness, chips broke cleanly during lathe turning; this eliminated fiber tearing and yielded a mirror-like surface finish. This formaldehyde-free approach achieved mechanical performance comparable to the tactile sensation of precious hardwoods but provided a potential pathway for extending binderless technology to the manufacturing of high-value cultural artifacts.

DOI: 10.15376/biores.21.3.5729-5748

Keywords: Bamboo processing residues; HCME pretreatment; Binderless technology; Chinese brush handles; High density; Hydrophobicity; Machinability

Contact information: a: Hezhou University, 18 West Ring Road, Hezhou, Guangxi 542899, China; b: Suzhou Industrial Park Vocational and Technical College, 1 Ruoshui Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, China; *Corresponding author: elena7@ivt.edu.cn

Graphical Abstract

INTRODUCTION

Chinese brushes serve as carriers of intangible cultural heritage, possessing significance that transcends their function as simple writing tools. The resurgence of the “Guochao” (national trend) has driven market demand for stationery that combines traditional aesthetics with modern durability (Qian et al. 2024). However, traditional brush manufacturing is constrained by the inherent thin walls and density variation of bamboo culms. This material is susceptible to humidity, which induces hygroscopic cracking, resulting in poor consistency and difficulties in long-term preservation (Li et al. 2022). Early attempts at substitution involved wood-plastic composites (WPC) or phenolic resin-impregnated bamboo. While these solutions addressed dimensional stability, the artificial plastic texture remains inferior to natural tactile qualities (Ziaei Tabari et al. 2017; Xu et al. 2023). Furthermore, the potential risk of formaldehyde emission (Lu et al. 2012) contradicts the eco-friendly philosophy intrinsic to calligraphy (Lao and Chang 2023). Binderless technology offers a sustainable alternative. Biomass components, specifically lignin and hemicellulose, undergo in-situ activation and reorganization under hot-pressing without exogenous adhesives. The application of such eco-friendly flat fiberboards in furniture validates their potential to replace traditional resin-based composites (Cheng et al. 2024).

Binderless composites have advanced significantly in flat building materials. However, translating them into cylindrical brush handles remains challenging. Brush handles require exceptional density, tactile quality, and water resistance. Existing research on binderless bamboo materials has been largely confined to two-dimensional flat board preparation (Pintiaux et al. 2015), while three-dimensional special-shaped profile molding has remained as an unresolved challenge. During cylindrical molding, the combined effects of radial pressure gradients and heat transfer hysteresis hinder effective densification in the geometric core layer. Consequently, a void remains in the market for high-value cultural artifacts that are both genuinely green and high-performing. The density of conventional binderless bamboo boards rarely exceeds 1.0 g/cm³. In contrast, Pterocarpus santalinus provides a unique tactile sensation due to its high density of 1.1 to 1.2 g/cm³, a quality unattainable by low-density materials (Arunkumar and Joshi 2014). Current densification theories rely heavily on the physical entanglement of long fibers (Shi et al. 2023). Mechanisms that achieve high density via microstructural filling must be explored. Such mechanisms are needed to ensure a low center of gravity. A low center of gravity is necessary for writing stability. Furthermore, writing brush applications inherently involve water and colloid-rich ink. Although enzymatic pretreatment enhances bonding, the hydrophilicity of the material persists (Sun et al. 2021; Wang et al. 2022), creating a high risk of hygroscopic swelling. Conventional low-consistency enzymatic hydrolysis often induces excessive cellulose degradation. The resulting material brittleness leads to chipping during lathe turning and fine carving, which compromises precision machining requirements (Ding et al. 2023).

This study realizes the high-value utilization of bamboo processing residues through a high-consistency mechano-enzymatic (HCME) strategy. Traditional low-consistency enzymatic hydrolysis typically limits solid content to below 10%. This causes substantial loss of natural binders (e.g., lignin) into the waste liquid. In contrast, HCME technology not only preserves these key components but also significantly reduces water consumption and subsequent drying energy. Mechanical shear force and cellulase are applied synergistically at 35% solid content. Bamboo fibers undergo in-situ fine defibrillation. Meanwhile, lignin surface activation is triggered. High temperature and pressure within custom cylindrical molds induce material self-bonding, ultimately forming high-performance composite brush handles.

This study’s objectives were the following: application of binderless technology to Chinese traditional brush handle manufacturing; elucidation of the micro-filler effect; and demonstration of the hydrophobic and machining mechanisms. This study develops a prototype for a cultural artifact that is completely formaldehyde-free, retains a natural bamboo scent, and exhibits excellent performance. The study reveals how parenchyma cell fragments generated during HCME fill the voids in the fiber skeleton. This mechanism constitutes the basis for overcoming density bottlenecks, and thereby addressing the theoretical challenge of achieving a texture comparable to precious hardwoods in binderless materials.

LITERATURE REVIEW

Evolution of Bamboo-based Composites

As an alternative to traditional wood, fast-growing bamboo offers significant potential (Adier et al. 2023). The technical iteration of bamboo-based composite preparation has evolved from “full-gluing” to “binderless” processes. Although scrimber and laminated bamboo lumber exhibit excellent strength, their heavy reliance on phenolic or urea-formaldehyde resins during manufacturing poses health risks associated with formaldehyde emission (Lao and Chang 2023).

Binderless technology has emerged as a green solution. Steam explosion, heat treatment, or mechanical activation induces biomass to achieve self-bonding (Chen et al. 2023). While mechanical reinforcement of two-dimensional flat materials dominates current research (Cheng et al. 2024), converting these into 3D special-shaped profiles, such as cylinders, presents severe challenges regarding material fluidity and densification uniformity. Research on molding processes and applications for fine crafts remains scarce.

Self-bonding Mechanism and Microstructural Reorganization

High densification and uniform molding of 3D profiles depend on dual breakthroughs at the microscopic level: the multi-scale interlocking network formed by long fiber skeletons and micron-scale fillers (Wang et al. 2022), and the in-situ activation and redistribution of lignin during hot-pressing (Cheng et al. 2024; Pintiaux et al. 2015).

Studies indicate that for binderless biomass materials to exceed the density of high-grade hardwoods (> 1.1 g/cm³), internal microscopic voids must be effectively eliminated. Constructing a multi-scale interlocking network constitutes one of the most effective pathways to achieve ultra-high density comparable to high-grade hardwoods (> 1.1 g/cm³). (Wang et al. 2022). Traditional mechanical separation removes parenchyma cells, which impairs packing efficiency. Moderately fragmented parenchyma cells can act as micro-fillers, embedding within the gaps of the long fiber skeleton to achieve three-dimensional interlocking (Lao and Chang 2023). However, systematic research on the precise regulation of fragmentation degree and its contribution to extreme densification remains absent.

The core mechanism endowing binderless materials with water resistance lies in the glass transition and redistribution of lignin (Cheng et al. 2024; Pintiaux et al. 2015). Thermoplastic lignin softens during hot-pressing and migrates to the surface layer, where a natural hydrophobic binding layer forms. However, the tight internal bonding forces within natural bamboo constitute significant resistance to migration. Biological enzyme pretreatment selectively severs the hydrogen bond network; the exposed active sites of lignin accelerate activation and flow (Cheng et al. 2024; Wang et al. 2022; Chen et al. 2023). Existing literature rarely addresses the directional migration mechanism of lignin to the material surface under the synergistic action of high-intensity mechanical force and enzymatic hydrolysis, nor its application in complex profiles.

Figure 1 depicts the proposed mechanistic pathway transforming bamboo processing residues into high-performance brush handles. Fibers and parenchyma cells in the raw material undergo structural reorganization and chemical modification via the HCME process. Fiber fibrillation coupled with cell fragmentation and lignin exposure ultimately leads to three key performance indicators: high density, hydrophobicity, and machinability.

Fig. 1. Conceptual framework diagram

EXPERIMENTAL

Preparation and Screening of Raw Materials

Bamboo processing residues from 5-year-old Moso bamboo harvested in Zhejiang Province, China, formed the material basis of this study, with sawdust and shavings constituting the primary raw materials. Established standardized procedures regulated the collection and pretreatment steps (Cheng et al. 2024). To ensure the fineness of the finished brush handles, the raw materials were crushed and screened. Bamboo powder (BP) with a particle size distribution between 40 and 60 mesh was selected as the substrate and dried at 60 °C to constant weight to eliminate moisture influence. Chemical composition determination followed the standard analytical procedures of the National Renewable Energy Laboratory (NREL) (Sluiter et al. 2008). The enzyme preparation used was commercial-grade cellulase (CTec3) produced by Novozymes, with a nominal enzyme activity of 9.5 FPU/mL.

Synergistic HCME Pretreatment

A modified HCME strategy realized efficient defibrillation and in-situ activation while maintaining high solid content. Cheng et al. (2024) provided the reference frame for core parameter settings. An acetate buffer (pH 4.8) preheated to 50 °C dissolved the cellulase and was sprayed uniformly onto the dry BP. A solids content of 35% (w/w) strictly simulated an industrial high-consistency environment. The enzyme dosage was set at 0.007 g (CTec3) per gram of dry substrate. A twin-screw kneader equipped with a Sigma mixer (NH-1, Rugao Guanchen Machinery Factory, China) supported the reaction at 50 °C. To investigate the influence of treatment extent on handle performance, the experiment established five time gradients: 0 h, 0.5 h, 2 h, 4 h, and 8 h (labeled as BP-0 to BP-8, respectively). Upon completion, samples were immediately placed in a 100 °C environment for 10 min to deactivate the enzymes, then dried at 60 °C for subsequent use.

Directional Molding of Brush Handle Blanks

The specific geometric characteristics of brush handles dictated the use of a dedicated stainless steel mold. An inner diameter of 12 mm combined with a length of 200 mm defined the molding space. This experiment referenced the molding strategy in Shi et al. (2023). This study adopted a layered filling and axial compression process using a closed mold. This approach overcomes pressure transmission attenuation during cylindrical molding. It also improves density uniformity under high pressure. Quantitatively weighed HCME-pretreated BP was manually filled and pre-compacted. A flat vulcanizer performed the hot-pressing task. A core process parameter matrix of 180 °C, 20 MPa pressure, and 10 min holding time was established. After the hot-pressing cycle, the mold was allowed to cool naturally to room temperature under pressure; subsequent demolding yielded high-density binderless self-bonding bamboo composite (BSBC) cylindrical rods.

Precision Machining and Surface Finishing

Machinability assessment and final product fabrication drove the post-processing steps that simulated traditional brush-making crafts. Ding et al. (2023) guided the optimization of cutting parameters to avoid surface defects. A small computer numerical control (CNC) lathe turned the rods into standard forms with a slightly thicker center and tapered ends. A rotational speed of 1500 rpm and a feed rate of 0.5 mm/rev determined the cutting conditions. High-end stationery requires strict tactile standards. Sanding the handles with 400-, 800-, and 1200-mesh sandpaper was performed sequentially. Wool wheel polishing finally resulted in a mirror-like luster.

Characterization and Application Simulation

Multi-scale characterization methods achieved a comprehensive assessment of the microstructural and macro-performance of the BSBC. The BP-0 to BP-8 sample series was used to investigate the chemical structural evolution during HCME pretreatment. A Nicolet iS10 Fourier transform infrared (FTIR) spectrometer equipped with an attenuated total reflectance (ATR) accessory was used. The spectra were collected over a range of 4000 to 400 cm⁻¹ with a resolution of 4 cm⁻¹. Cellulose crystallinity index (CrI) was determined using an X-ray diffractometer (XRD, Bruker D8 Advance, Germany) with a Cu Kα radiation source (λ = 1.5406 Å), a scanning range (2θ) of 5° to 40°, and a scanning speed of 2°/min. The CrI values were calculated using the Segal method (Segal et al. 1959) based on the intensity of the (200) crystalline peak (around 20 ≈ 22°) relative to the amorphous background intensity (around 20≈18°). Thermal stability was evaluated via thermo-gravimetric analysis (TGA, TGA 550, TA Instruments, USA), heating samples from room temperature to 600 °C at a rate of 10 °C/min under a nitrogen atmosphere. The NREL standard procedures governed the quantitative analysis of cellulose, hemicellulose, and lignin.

A GeminiSEM 360 scanning electron microscope was employed to comparatively observe the defibrillation state of BP before and after HCME treatment. In-depth analysis of the microscopic bonding characteristics of BSBC fracture surfaces revealed the reinforcement mechanism. Gold sputtering for 60 s endowed the samples with the conductivity and clarity required for testing. An accelerating voltage of 3 to 5 kV set the scanning baseline. A SkyScan 2214 micro-computed tomography (Micro-CT) system performed quantitative characterization of material densification. The internal pore structure underwent non-destructive scanning. An operating voltage of 50 kV, a current of 80 µA, and a resolution of 4.00 µm defined the imaging precision. Three-dimensional model reconstruction and porosity calculations were executed by Avizo software following the methodological logic of Wang et al. (2022).

The GB/T 17657 (2022) standard established the baseline for physical and mechanical performance determination. Data for density, 24 h thickness swelling, three-point flexural strength, and internal bonding strength were obtained using these standards. Although originating from board standards, their applicability for evaluating high-density biomass composites has been frequently validated in relevant research (Li et al. 2022). Mechanical tests were conducted on Instron 5848 (Instron, UK) and WDW-E100D (Jinan Shijin Group, China) universal testing machines. A contact angle meter (OCA20, Dataphysics Instrument, Germany) recorded dynamic water contact angle (WCA) changes (0 to 60 s) on the material surface to evaluate wettability. Statistical significance among groups was evaluated using one-way ANOVA, followed by Duncan’s multiple range test (p < 0.05). The detailed pairwise comparisons are reported in Supporting Information (Table S1). One-way ANOVA was used to determine whether statistically significant differences existed among multiple treatment groups, and Duncan’s test was subsequently applied to identify specific group differences.

This study referenced the processing quality evaluation standards proposed by Ding et al. (2023). Machinability was assessed by observing microscopic surface smoothness and edge integrity after turning. Ink resistance tests validated handle durability. Finished handles underwent full immersion in standard calligraphy ink for 24 h. Capturing phenomena of surface ink penetration or swelling were used to quantify the material’s resistance to colloidal ink.

Fig. 2. The methodological workflow of the study

Figure 2 illustrates the four key preparation stages from bamboo processing residues to finished brush handles. Raw material screening and preparation occurred first. The HCME pretreatment established the material foundation. Directional hot-pressing molding within a cylindrical mold realized structural reorganization. The final form of the binderless brush handles was sculpted via precision lathe turning and polishing processes.

RESULTS

Microstructural Reorganization Driven by HCME

Scanning electron microscopy (SEM) images intuitively revealed the noticeable reshaping effect of HCME pretreatment on the micromorphology of bamboo powder (BP). Figure 3 presents the complete evolution. The original vascular bundles and parenchyma cell structure of Moso bamboo in Fig. 3a served as a reference. BP-0 (Fig. 3b) maintained a smooth and dense fiber bundle appearance, with blocky parenchyma cells remaining structurally intact. The quantitative proportion of bamboo fibers and parenchyma cells in the raw BP is provided in Supporting Information (Fig. S1), which confirms the coexistence of fiber bundles (60.1%) and a considerable fraction of parenchyma tissue (39.9%) as the structural basis for the subsequent micro-filler effect.

Prolonging the mechano-enzymatic synergistic treatment time drove the progressive evolution of fiber morphology. BP-0.5 (Fig. 3c) and BP-4 (Fig. 3d) show increased fiber surface roughness, signs of fiber bundle loosening and initial defibrillation, and the shedding and fragmentation of parenchyma cells. The 8-h treatment (BP-8, Fig. 3e) catalyzed the most marked qualitative change. Bamboo fibers presented strong “fibrillation” (broom-like) characteristics, where dense fiber bundles were peeled into microfibers, leading to a sharp expansion in specific surface area.

The combined effect of strong shear force and enzymatic hydrolysis pulverized the originally aggregated parenchyma cells into numerous micron-scale fragments. A multi-scale physical interlocking network was jointly constructed by these refined fiber branches and cell fragments; this established the foundation for the densified structure formed during the hot-pressing process.