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Integrated surface densification and furfurylation for enhancing mechanical performance and dimensional stability of fast-grown Chinese fir

  • Bingbin Kuai , Yunhui Liang , Yan Liu , Fei Guo , Xuexia Zhang , Rilong Yang and Yan Yu EMAIL logo
Published/Copyright: December 29, 2025
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Holzforschung
From the journal Holzforschung Volume 80 Issue 2

Abstract

Surface densification combined with bio-based polymer impregnation offers a sustainable approach for enhancing the performance of fast-growing softwoods. In this study, a one-step and a catalyst-assisted two-step furfuryl alcohol (FA) modification strategy were applied to Chinese fir (Cunninghamia lanceolata) prior to thermal surface densification. The effects of FA concentration (10 %, 30 %, and 50 %) and treatment route on density distribution, weight percentage gain, colorimetric change, mechanical properties, dimensional stability, and chemical structure were systematically evaluated. The one-step treatment with 50 % FA produced the high MOR (123 MPa), compressive strength (65 MPa) accompanied by improved surface darkening and reduced thickness swelling (4.3 % and 22.8 %) under 65 % and 98 % humid conditions. Scanning electron microscopy revealed progressive cell wall deformation and densification, while FTIR analysis (1738 and 1715 cm−1) confirmed in situ FA polymerization and crosslinking. Compared to untreated and water-treated controls, FA-modified samples exhibited superior performance with reduced set recovery (28.2 %) and enhanced structural rigidity (MOE:13.6 GPa). The results demonstrate that FA-assisted surface densification is a tunable and resource-efficient method for upgrading low-density plantation wood for structural and decorative applications.

Keywords: surface densification; furfuryl alcohol modification; mechanical properties; dimensional stability

1 Introduction

Chinese fir (Cunninghamia lanceolata), a major plantation species in southern China, has gained attention for its rapid growth and commercial availability. However, its practical applications are severely limited by several inherent defects. Compared to other fast-grown species such as poplar and radiata pine, Chinese fir exhibits significantly lower basic density, inferior mechanical properties, high anisotropy, and poor dimensional stability under moisture cycling. These deficiencies hinder its use in structural applications and moisture-sensitive environments. Consequently, strategies aimed at improving both mechanical performance and dimensional stability are crucial for increasing the added value and application scope of Chinese fir in modern wood-based industries (Luan et al. 2021; Luo et al. 2022, Qiu et al. 2018). To enhance the utilization of these abundant but underperforming resources, densification by either mechanical or chemical means has become a widely adopted approach. By reducing porosity and compressing cell walls, densification significantly increases surface hardness, bending strength, and dimensional stability, making softwoods comparable to higher-grade hardwoods in selected applications (Cabral et al. 2022; Sandberg et al. 2017).

Conventional densification techniques often rely on high densification ratio, applying heat and pressure to reduce thickness uniformly throughout the wood volume. While such methods can achieve notable improvements in stiffness and strength, they are typically accompanied by challenges such as spring-back behavior under humidity, limited structural permanence, and high energy or chemical input (Antikainen et al. 2013; Hosseinihashemi et al. 2022). Moreover, overdensification may lead to collapse of cell walls and internal defects, especially in species with fragile anatomical structures like Chinese fir (Wang et al. 2014).

To address these limitations, surface densification has emerged as a targeted alternative. By concentrating deformation and reinforcement in the outermost wood layers, where mechanical wear and moisture exchange are most significant, surface densification offers a more material-efficient route to property enhancement (Rautkari et al. 2011; Wu et al. 2020). However, without chemical stabilization, the compressed surface often fails to retain its densified state under cyclic moisture conditions, resulting in swelling, shape recovery, and diminished performance. One strategy to enhance the permanence of surface densification involves coupling it with chemical impregnation and polymerization treatments, which fix the deformed structure and introduce moisture resistant networks (Kuai et al. 2024). Traditional chemical modifiers such as urea-formaldehyde or phenol-formaldehyde resins have demonstrated success in improving dimensional stability and durability, but concerns over formaldehyde emissions, poor biodegradability, and high processing temperatures have driven the search for greener alternatives (Gabrielli and Kamke 2009; Kupfernagel et al. 2022; Sun et al. 2013).

Furfuryl alcohol (FA), derived from hemicellulose-rich biomass, has gained increasing attention as a bio-based, thermally activated monomer capable of in situ polymerization within wood tissues. Its small molecular size facilitates penetration, and its furan ring structure contributes to water resistance and thermal stability after polymerization (Tenorio et al. 2022). FA-modified wood exhibits enhanced resistance to biological degradation and moisture, with improved mechanical performance (Li et al. 2020; Li et al. 2021; Shen et al. 2021). Although furfurylation combined with thermal compression has been proven effective in improving wood properties, several scientific challenges remain unresolved. Conventional one-step furfurylation approaches often lead to excessive and uneven resin distribution, particularly toward the core, resulting in inefficient chemical usage and insufficient surface fixation (Zhang et al. 2022). Furthermore, many existing studies focus on bulk impregnation without optimizing localized polymer localization, which is critical for dimensionally unstable softwoods. Additionally, the relationship between polymer distribution, density profile, and functional performance is often poorly understood or insufficiently quantified (Liu et al. 2023).

To address the challenges associated with the low density and poor dimensional stability of fast-growing Chinese fir, this study proposes a surface-oriented furfurylation strategy in which the catalyst and monomer are introduced separately, followed by controlled hot pressing to induce both surface densification and in situ polymerization. This two-step approach aims to minimize chemical input while enhancing structural performance and moisture resistance. By systematically comparing one-step and two-step FA infusion routes, the study investigates how monomer catalyst distribution influences surface polymerization behavior, structural efficiency, and dimensional stability. The overarching goal is to develop a process-efficient and environmentally responsible method to upgrade plantation-grown softwoods for high-value structural and decorative applications.

2 Materials and methods

2.1 Wood materials

Chinese fir (C. lanceolata) was harvested from plantation forests in Sanming, Fujian Province, China, with tree ages ranging from 26 to 30 years. Logs were processed into boards with dimensions of 60 mm × 30 mm × 2000 mm (R × T × L). Mature sapwood portions with stable grain and no visible defects were selected and further cut into specimens of 8, 11, and 16 mm (radial thickness) × 50 mm (T) × 120 mm (L). Prior to chemical treatment, all wood specimens were oven-dried at 103 ± 2 °C until reaching a constant weight, corresponding to a final moisture content below 3 %, as determined by weight loss method. The average oven-dry density of the material was 330 kg/m3. Furfuryl alcohol (FA), citric acid, ammonium chloride, borax, and itaconic anhydride were analytical grade and used without further purification. Detailed reagent specifications are listed in Table 1.

Table 1:

Mass fraction of modified solution ratio of furfuryl alcohol.

Number Mass fraction (%)
Citric acid Itaconic anhydride Ammonium chloride Borax Furfuryl alcohol Water
1 0.5 0.5 0.75 0.9 10 87.35
2 0.5 0.5 0.75 0.9 30 67.35
3 0.5 0.5 0.75 0.9 50 47.35

2.2 Surface densification treatments

The overall workflow of the surface-oriented wood modification strategies, including one-step and two-step furfurylation, and as well as water-treated, is illustrated in Figure 1. The one-step method (OS-SDW) was used as a reference to represent conventional furfurylation processes, where the furfuryl alcohol solution and catalyst are applied simultaneously. In contrast, the two-step method (TS-SDW) separates the application of catalyst and monomer to promote controlled in situ polymerization closer to the wood surface, aiming to improve chemical efficiency and surface fixation. Comparing these two configurations enables a systematic investigation of how treatment sequence affects polymer localization, surface densification, and resultant properties. For each treatment condition, a minimum of six replicate specimens (n = 6) were prepared and tested. All reported data are presented as mean values with corresponding standard deviations.

Figure 1: Schematic diagram of two different methods for preparing surfaced densified wood: control = untreated Chinese fir; PW-SDW = one-step surface densified wood with pure water pretreatment; OS-SDW-10/30/50 % = one-step surface densified wood treated with 10 %, 30 %, and 50 % furfuryl alcohol (FA); TS-SDW = two-step surface densified wood treated with pure FA.
Figure 1:

Schematic diagram of two different methods for preparing surfaced densified wood: control = untreated Chinese fir; PW-SDW = one-step surface densified wood with pure water pretreatment; OS-SDW-10/30/50 % = one-step surface densified wood treated with 10 %, 30 %, and 50 % furfuryl alcohol (FA); TS-SDW = two-step surface densified wood treated with pure FA.

2.2.1 One-step FA surface modification (OS-SDW)

Wood samples (11 mm × 50 mm × 120 mm) were sealed at both end-grain surfaces using paraffin wax and submerged at ambient temperature and pressure into FA solutions of varying concentrations (Table 1: 10 %, 30 %, and 50 % v/v). The liquid-to-wood volume ratio was maintained at 4:1. Samples were removed periodically, surface-dried, and weighed until an FA uptake of approximately 12 % was achieved.

2.2.2 Two-step FA surface modification (TS-SDW)

In the first step, a catalyst solution was prepared by dissolving 0.5 % citric acid, 0.75 % ammonium chloride, 0.9 % borax, and 0.5 % itaconic anhydride in distilled water. Samples were vacuum-treated (25 min) to evacuate air, then pressure-impregnated at 0.8 MPa with the catalyst solution for 2 h. Following impregnation, samples were dried at 40 °C for 3 h, 60 °C for 2 h, 80 °C for 2 h, and finally oven-dried at 103 °C to constant weight. In the second step, paraffin-coated dry samples were immersed in pure FA at room temperature (volume ratio 4:1), with intermittent weighing to achieve 12 % FA uptake.

2.2.3 Thermal surface densification

FA-loaded specimens were hot-pressed to a target thickness of 8 mm (densification ratio = 27 %) using a platen heated to 120 °C. Densification involved a 2.5 min preheating stage followed by 3 min of active pressing and 60 min of sustained pressure. Cooling was rapidly performed to 40 °C under pressure before releasing the sample.

2.3 Physical properties test

Weight percent gain (WPG): determined based on the oven-dry mass before (m 0) and after (m 1) treatment were described by Eq. (1):

(1) WPG = m 1 − m 0 m 0 × 100   %

Swelling rate (SW): swelling from dry to saturated states was measured under 65 % and 98 % RH at 23 ± 2 °C. Swelling rate was calculated according to Eq. (2):

(2) SW = X 1 − X 0 X 0 × 100   %

where X represents the measured dimension (thickness, width, or volume) before (X 0) and after (X 1) completely saturated conditions under controlled relative humidity. Radial thickness was selected as the primary indicator for densification behavior due to its sensitivity to surface compression. Tangential width and total volume were also recorded for supplementary analysis.

Set-recovery (SR): After three wet-dry cycles and immersion in 95 °C water, radial dimension recovery was calculated as Eq. (3) using initial, compressed, and recovered thicknesses.

(3) SR = R 2 − R 0 R 1 − R 0 × 100   %

where R 0 is the radial thickness of the densified sample after drying; R 1 is the radial thickness of the natural wood after drying; and R 2 is the radial thickness of the densified sample after soaking and drying.

Vertical density profiles (VDP): measured along the radial direction (DENSE-LAB, EWS, Germany) at 0.1 mm intervals using an X-ray densitometer (5 mm × 50 mm × 50 mm specimens).

2.4 Mechanical properties test

Modulus of rupture (MOR) & modulus of elasticity (MOE): following ISO 13061-3 and ISO 13061-4, 5 mm × 5 mm × 100 mm specimens were tested via 3-point bending with a span of 60 mm and a loading rate of 5 mm/min. Compressive strength (CS): determined along the grain on 5 mm × 5 mm × 8 mm specimens per ISO 13061-17, at a loading rate of 3 mm/min. Each test group consisted of at least 10 replicates.

2.5 Optical properties test

Color parameters (L*, a*, b*) were measured using a CR-400 Chroma Meter (Konica Minolta, Japan). Measurements were taken on the radial face of each specimen both before and after treatment. Six measurements were recorded on the same surface per sample and averaged to obtain representative values.

2.6 FTIR spectroscopy analysis

Wood powder was collected from the outer 0–2 mm layer of the radial surface of each specimen. Samples were prepared for four groups: untreated control, water-treated (PW-SDW), one-step furfurylated wood with 50 % furfuryl alcohol concentration (OS-SDW-50 %), and two-step furfurylated wood (TS-SDW). The powder was sieved through a 200-mesh screen and dried at 103 °C for 4 h. Each pellet was made by mixing 1 mg of sample with 100 mg of KBr, followed by grinding and pressing. FTIR spectra were collected using a PerkinElmer FT-Spectrometer (Nexus670) over the range of 400–4,000 cm−1, with 64 scans and a resolution of 4 cm−1. Spectra were normalized to the maximum absorbance peak within each spectrum to facilitate comparison. All measurements were performed in triplicate.

2.7 Microscopy techniques

Scanning electron microscopy (SEM): specimens (5 mm × 5 mm × 10 mm) were cut, microtomed, mounted on conductive tape, and sputter-coated with platinum. Microstructure was observed using a Nova Nano SEM 230 (FEI, Czech Republic) at variable magnifications.

Confocal laser scanning microscopy (CLSM): to investigate the spatial distribution of furfuryl alcohol (FA) resin within the wood matrix, transverse sections (20 μm thick) were prepared and thoroughly rinsed in deionized water. To suppress lignin autofluorescence, the samples were stained in 1 % toluidine blue solution for 10 min and rinsed again until clear. Sections were mounted on glass slides with a drop of distilled water and sealed with coverslips using nail polish. CLSM observation was conducted using a STELLARIS 5 system (Leica Microsystems, Germany) with an excitation wavelength of 488 nm (argon laser) and detection at 500–550 nm.

2.8 Dynamic vapor sorption (DVS) analysis

Moisture sorption behavior of the wood samples was characterized using a Dynamic Vapor Sorption analyzer (DVS Advantage, Surface Measurement Systems Ltd., UK). Approximately 20–30 mg of oven dried wood powder was used for each test. The temperature was maintained at 25 °C throughout the experiment. Relative humidity (RH) was first increased from 0 % to 90 % in 10 % increments, followed by smaller steps of 5 % from 90 % to 95 %, and then decreased back to 0 % following the same RH steps. At each RH step, the sample was allowed to reach equilibrium, defined as a mass change rate of less than 0.02 % per minute. The equilibrium moisture content (EMC) at each humidity level was recorded to generate sorption-desorption isotherms.

2.9 Contact angle measurement

The surface wettability of the wood samples was evaluated by measuring the static water contact angle using a contact angle goniometer (SDC 350 KS). A 10 μL droplet of deionized water was gently deposited onto the sample surface using a microsyringe. The contact angle was recorded at intervals of 10 s for up to 60 s using high-resolution imaging software. For each sample group, measurements were performed at three different locations on the same surface and averaged to obtain representative results. All tests were conducted at ambient conditions (23 ± 2 °C, 50 % RH).

2.10 Statistical analysis

All experimental results were expressed as mean ± standard deviation (SD), based on at least three replicates. One-way analysis of variance (ANOVA) was conducted using SPSS software (version 25, IBM Corp., USA) to evaluate statistical significance among different treatment groups. Duncan’s multiple range test was applied to determine significant differences at a confidence level of p < 0.05. Different lowercase letters in the figures indicate statistically significant differences. Data visualization and plotting were performed using Origin software (version 2018, OriginLab Corporation, USA).

3 Results and discussion

3.1 Surface densification performance and distribution

As illustrated in Figure 2b–d, Surface densification following FA treatment resulted in significant changes in WPG and VDPs in Chinese fir. To isolate the effects of FA impregnation, a control sample of untreated wood was subjected to the same hot pressing conditions. This yielded a peak surface density of 484 kg/m3, compared to 310 kg/m3 in natural, uncompressed wood, demonstrating that mechanical compression alone produces limited densification. In contrast, the one-step FA-modified samples (OS-SDW) exhibited much higher surface densities (Figure 2a). Interestingly, the VDP peak decreased slightly with increasing FA concentration: OS-SDW-10 % reached 1,189 kg/m3, OS-SDW-30 % reached 1,087 kg/m3, and OS-SDW-50 % reached 1,008 kg/m3. This trend may result from surface polymer formation at higher FA concentrations, which restricts inward diffusion and reduces compressibility. The two-step FA modification (TS-SDW) produced the highest vertical density peak at 1,217 kg/m3 and displayed a distinctive double-peak VDP profile. This feature suggests localized polymer fixation at both surface zones, likely due to catalyst predistribution and thermal gradients during pressing. Such a distribution may promote polymerization near the outer regions, enhancing surface-specific reinforcement. Overall, these results confirm that FA-assisted densification significantly enhances surface consolidation beyond what is achievable through compression alone. Additionally, the treatment pathway has a strong influence on polymer distribution and final density gradients, which are critical for mechanical performance and dimensional behavior (Huang et al. 2024).

Figure 2: Surface densification performance of wood: (a) density of samples after furfuryl alcohol (FA) modification and densification; (b) weight percentage gain (WPG) after FA treatment; (c, d) vertical density profiles (VDP) of treated samples; (e, f) SEM cross-sections showing lumen collapse and fiber compaction in PW-SDW and OS-SDW-30 %; (g, h) confocal laser scanning microscopy images of (g) untreated and (h) FA-treated wood, indicating polymer distribution (control = untreated Chinese fir; PW-SDW = one-step surface densified wood with pure water pretreatment; OS-SDW-10/30/50 % = one-step surface densified wood treated with 10 %, 30 %, and 50 % FA; TS-SDW = two-step surface densified wood treated with pure FA).
Figure 2:

Surface densification performance of wood: (a) density of samples after furfuryl alcohol (FA) modification and densification; (b) weight percentage gain (WPG) after FA treatment; (c, d) vertical density profiles (VDP) of treated samples; (e, f) SEM cross-sections showing lumen collapse and fiber compaction in PW-SDW and OS-SDW-30 %; (g, h) confocal laser scanning microscopy images of (g) untreated and (h) FA-treated wood, indicating polymer distribution (control = untreated Chinese fir; PW-SDW = one-step surface densified wood with pure water pretreatment; OS-SDW-10/30/50 % = one-step surface densified wood treated with 10 %, 30 %, and 50 % FA; TS-SDW = two-step surface densified wood treated with pure FA).

In Figure 2b, the weight percentage gain (WPG) data is almost consistent with the density trend. Interestingly, the pure water treated (PW-SDW) samples exhibited a non-negligible weight gain of approximately 4.8 %, which can be attributed to water absorption and potential leaching-reabsorption dynamics during the soaking process. This mass increase likely results from the hygroscopic nature of the cell wall, which can temporarily retain bound water even after oven-drying. Although PW-treated wood showed only a slight change in WPG, most of FA-modified samples exceeded 8.5 %, indicating that FA polymers were deposited inside the wood. The two-step FA treatment achieved similar WPG levels, but with greater vertical density, likely due to initial catalyst fixation limiting FA penetration depth. This modified distribution contributed to its distinctive bimodal density profile and may favor surface-related property enhancement without overloading the wood interior (Liu et al. 2024).

The combination of densification and FA polymerization generated compressive closure of cell lumens and resin anchoring within tracheid walls, as supported by SEM observations (Figure 2e and f). This led to both structural stiffening and chemical stabilization of the densified layer. Notably, OS-SDW-30 % achieved the most favorable balance between surface densification and polymer infiltration, producing a smooth and continuous high-density surface layer, which would significantly influence mechanical behavior and moisture resilience in subsequent analyses (Yang et al. 2023). Confocal laser scanning microscopy (CLSM) revealed distinct fluorescence localized within the cell wall regions of FA-treated Chinese fir, confirming that furfuryl alcohol successfully penetrated and polymerized in the cell wall matrix (Figure 2g and h). This provides direct visual evidence supporting effective in situ polymerization at the microstructural level, contributing to improved dimensional stability and mechanical reinforcement.

3.2 Optical properties and surface appearance

The visual and chromatic responses of Chinese fir following FA treatment and surface densification are shown in Figure 3a and b. All modified samples exhibited perceptible darkening compared to the untreated control, with the extent of color change positively correlated with FA concentration and treatment severity. The lightness parameter (L*) decreased significantly from 81.0 in the control to 24.7 in the TS-SDW sample, accompanied by marked reductions in both a* and b*, indicating a shift toward darker and less saturated tones. The OS-SDW-50 % group achieved the most balanced and uniform surface coloration, while OS-SDW-30 % and TS-SDW samples displayed more intense browning, occasionally accompanied by local scorching. These visual alterations are attributed to in situ FA polymerization during hot pressing, which promotes furan ring formation and carbonization, particularly at elevated concentrations or in catalyst-assisted systems (Iroegbu and Ray 2023). The visual surface morphology (Figure 3c and d) corroborates these results, revealing smoother and more consolidated surfaces in the FA-treated groups compared to the control, which retained its original fibrous texture.

Figure 3: Color changes of surface densified Chinese fir under different treatment conditions: (a) CIE lab color parameters (L*, a*, b*) of OS-SDW samples treated with 10 %, 30 %, and 50 % furfuryl alcohol (FA), compared with control and PW-SDW; (b) color comparison among control, PW-SDW, and TS-SDW; (c, d) surface photographs after hot pressing, showing progressive darkening with increasing FA concentration (control = untreated Chinese fir; PW-SDW = one-step surface densified wood with pure water pretreatment; OS-SDW-10/30/50 % = one-step surface densified wood treated with 10 %, 30 %, and 50 % FA; TS-SDW = two-step surface densified wood treated with pure FA).
Figure 3:

Color changes of surface densified Chinese fir under different treatment conditions: (a) CIE lab color parameters (L*, a*, b*) of OS-SDW samples treated with 10 %, 30 %, and 50 % furfuryl alcohol (FA), compared with control and PW-SDW; (b) color comparison among control, PW-SDW, and TS-SDW; (c, d) surface photographs after hot pressing, showing progressive darkening with increasing FA concentration (control = untreated Chinese fir; PW-SDW = one-step surface densified wood with pure water pretreatment; OS-SDW-10/30/50 % = one-step surface densified wood treated with 10 %, 30 %, and 50 % FA; TS-SDW = two-step surface densified wood treated with pure FA).

Notably, the TS-SDW samples showed more pronounced surface melting and resin concentration near the outermost layers. This is likely due to the two-step approach, wherein catalytic fixation of FA precursors in the initial stage restricts deep penetration, enhancing polymer formation at or near the surface during subsequent heating. While this results in lower L* values and higher color uniformity, it may also introduce risk of thermal degradation if temperature control is not optimal (Hadi et al. 2022).

In summary, FA-based surface densification imparts both functional and visual changes to Chinese fir. Moderate FA concentration (30 %) provides uniform surface aesthetics without over-darkening, while two-step modification achieves maximum chromatic transformation at the cost of potential surface overheating. These observations offer insight into the tunability of surface appearance via chemical formulation, with relevance for decorative wood applications requiring controlled color and texture.

3.3 Mechanical properties

The mechanical properties of surface-densified Chinese fir following various furfuryl alcohol (FA) treatments are presented in Figure 4. All modified samples exhibited substantial improvements in bending strength (MOR), modulus of elasticity (MOE), and compressive strength (CS) compared to the untreated control. The enhancement was most pronounced in the OS-SDW-30 % treatment, where MOR and MOE increased by approximately 81.3 % and 115.4 %, respectively, relative to the native wood (Figure 4a). In contrast, the two-step FA treatment (TS-SDW) resulted in moderate mechanical gains, and with greater variability, possibly due to localized resin accumulation and inhomogeneous surface deformation.

Figure 4: Mechanical performance of surface densified Chinese fir: (a) modulus of rupture (MOR) and modulus of elasticity (MOE); (b) compressive strength; (c) X-ray diffraction patterns and calculated relative crystallinity of samples under different furfuryl alcohol (FA) concentrations and treatment methods (control = untreated Chinese fir; PW-SDW = one-step surface densified wood with pure water pretreatment; OS-SDW-10/30/50 % = one-step surface densified wood treated with 10 %, 30 %, and 50 % FA; TS-SDW = two-step surface densified wood treated with pure FA).
Figure 4:

Mechanical performance of surface densified Chinese fir: (a) modulus of rupture (MOR) and modulus of elasticity (MOE); (b) compressive strength; (c) X-ray diffraction patterns and calculated relative crystallinity of samples under different furfuryl alcohol (FA) concentrations and treatment methods (control = untreated Chinese fir; PW-SDW = one-step surface densified wood with pure water pretreatment; OS-SDW-10/30/50 % = one-step surface densified wood treated with 10 %, 30 %, and 50 % FA; TS-SDW = two-step surface densified wood treated with pure FA).

Compressive strength followed a similar trend to flexural properties, with OS-SDW-50 % showing the highest value (65 MPa). The increased resistance to deformation is linked to the densified microstructure and resin-induced stiffening. The lower performance in the TS-SDW group, despite its higher WPG and vertical density, implies that mechanical integrity is governed not merely by mass addition but by the homogeneity of polymer integration and the balance between stiffness and flexibility within the modified zone.

The observed of mechanical properties improvements are attributed to the synergistic effects of surface densification and in situ FA polymerization. Thermo-densification induced lumen collapse and cell wall buckling in the surface region, which were stabilized by crosslinked furan resin infiltration. This was further confirmed by SEM, which revealed densely compacted tracheid walls and reduced intercellular voids in the FA-treated groups. In the OS-SDW-30 % samples, the cell wall distortion was moderate yet continuous, forming a cohesive surface layer that resisted bending and axial densification (Zhao et al. 2024). In contrast, the TS-SDW groups might exhibited over-densification and localized microcracking, suggesting that excessive polymer content may embrittle the cell structure or impair internal stress relaxation during pressing. A positive trend between surface density and mechanical performance was qualitatively observed, as shown in Figures 2a and 4b, supporting the contribution of densification to mechanical reinforcement. Among the OS-SDW samples, the 50 % FA treatment showed the highest compressive strength and surface density, indicating a more effective reinforcement of the surface region. This treatment level also maintained moderate dimensional stability, offering a favorable performance balance across key metrics within the tested concentration range. Hence, furfuryl alcohol-assisted surface densification significantly enhances the mechanical performance of Chinese fir through structural reinforcement and polymer stabilization of compressed wood cells (Chen et al. 2025; Zheng et al. 2022). The OS-SDW-50 % one-step treatment emerged as the most effective formulation, offering high mechanical efficiency without inducing surface brittleness or compromising toughness.

X-ray diffraction (XRD) analysis revealed that the crystallinity of the treated wood varied with different modification strategies (Figure 4c and s1). Compared with the untreated control (54.4 %), relative crystallinity increased notably in PW-SDW (68.2 %) and OS-SDW-30 % (69.3 %), which also corresponded to significantly enhanced bending and compressive strengths. This trend supports the hypothesis that moderate FA treatment and thermal compression facilitate the rearrangement and alignment of cellulose microfibrils, promoting a denser and stiffer wood matrix. The TS-SDW group also showed a high crystallinity of 66.4 %, aligning with its improved strength characteristics. Interestingly, although OS-SDW-50 % showed the highest compressive strength (65 MPa), its crystallinity index (59.3 %) was lower than that of OS-SDW-30 %, suggesting that excessive FA content may hinder crystalline ordering due to resin oversaturation or restricted molecular rearrangement. Notably, this sample also exhibited a marked decline in bending strength (123 MPa), in contrast to the peak observed at OS-SDW-30 % (139 MPa). This simultaneous reduction in crystallinity and bending strength indicates a potential correlation, where lower crystalline integrity may compromise the material’s resistance to flexural deformation, despite improvements in compressive capacity.

3.4 Dimensional stability analysis

The dimensional stability of surface-densified Chinese fir was evaluated under both moderate (65 % RH) and high-humidity (98 % RH) environments, as well as after three wet-dry cycles. As shown in Figure 5a and b, all FA-treated samples exhibited reduced thickness swelling rate compared to the control, with the OS-SDW-50 % group and TS-SDW achieving the lower swelling values under both humidity levels. This improvement reflects the synergistic effect of physical densification and chemical stabilization (Yang et al. 2019). However, when subjected to accelerated aging through three wet-dry cycles in 95 °C hot water (Figure 5c), the TS-SDW group exhibited significantly lower set recovery than the OS-SDW groups. This enhanced dimensional stability may be attributed to the two-step modification’s more effective surface-localized polymer crosslinking, which provided greater resistance to stress relaxation and spring-back under thermal moisture cycling. The findings suggest that while FA concentration influences short-term swelling behavior, the treatment route has a stronger impact on long-term shape retention and resistance to irreversible deformation. In contrast, the control and PW-SDW groups retained high recovery capacity due to elastic cell wall rebound.

Figure 5: Dimensional stability and chemical structure analysis of surface-densified Chinese fir: (a, b) swelling rates in radial, tangential, and volumetric directions under 65 % and 98 % relative humidity; (c) set recovery (%) after three wet-dry cycles; (d) FTIR absorbance spectra of untreated and treated samples; (e) contact angle over 60 s for untreated and OS-SDW-50 % samples; (f) equilibrium moisture content (EMC) under varying RH measured by DVS (control = untreated Chinese fir; PW-SDW = one-step surface densified wood with pure water pretreatment; OS-SDW-10/30/50 % = one-step surface densified wood treated with 10 %, 30 %, and 50 % furfuryl alcohol (FA); TS-SDW = two-step surface densified wood treated with pure FA).
Figure 5:

Dimensional stability and chemical structure analysis of surface-densified Chinese fir: (a, b) swelling rates in radial, tangential, and volumetric directions under 65 % and 98 % relative humidity; (c) set recovery (%) after three wet-dry cycles; (d) FTIR absorbance spectra of untreated and treated samples; (e) contact angle over 60 s for untreated and OS-SDW-50 % samples; (f) equilibrium moisture content (EMC) under varying RH measured by DVS (control = untreated Chinese fir; PW-SDW = one-step surface densified wood with pure water pretreatment; OS-SDW-10/30/50 % = one-step surface densified wood treated with 10 %, 30 %, and 50 % furfuryl alcohol (FA); TS-SDW = two-step surface densified wood treated with pure FA).

FTIR analysis provided molecular-level evidence supporting these observations (Figure 5d). The peak at 1,738 cm−1 corresponds to non-conjugated C=O stretching, indicating the presence of oxidized furfuryl groups. The 1,715 cm−1 peak is commonly associated with conjugated carbonyl functionalities, suggesting the formation of stable polymeric furan structures. These changes, along with the observed reduction in O–H stretching around 3,400 cm−1, confirm that furfuryl alcohol underwent in situ polymerization and interacted with wood cell wall components (Huang et al. 2008; Yang et al. 2019). In addition, the increased band at 1,507 cm−1 corresponds to aromatic C=C stretching, suggesting enhanced rigidity due to the incorporation of furanic rings (Mubarok et al. 2023). This shift in chemical composition is consistent with the reduced hygroscopicity and improved dimensional stability observed in FA-modified samples. Moreover, notable decreases in the intensity of bands at 1,458 cm−1 and 1,268 cm−1, corresponding to C–H deformation in lignin and C–O stretching in aromatic ethers, suggest partial lignin degradation or rearrangement during FA treatment (Lems et al. 2019; Nordstierna et al. 2008). This modification likely enhances polymer interpenetration and interfacial bonding, contributing to the improved mechanical strength observed in OS-SDW-30 % samples (see section 4.3). Taken together, the FTIR data reveal that furfuryl alcohol functions not only as a bulking agent but also as a chemical crosslinker, forming a hydrophobic and structurally stiff surface layer that anchors compressed cellular structures, reduces swelling, and improves resistance to shape recovery.

Contact angle measurements were conducted to further evaluate the hydrophobicity of the modified wood surfaces. As shown in Figure 5e, the untreated control sample exhibited a sharp decline in contact angle from 85.7° at 0 s to 0° at 40 s, indicating complete water absorption and high surface wettability. In contrast, the OS-SDW-50 % sample maintained a stable contact angle of 90.7° throughout 60 s, suggesting a significant improvement in surface hydrophobicity due to FA polymerization and surface densification. This persistent non-wettability is likely attributed to the formation of a crosslinked FA network within the cell wall region and the microstructural smoothing from compression, effectively reducing moisture uptake at the surface level. As shown by the DVS results, the equilibrium moisture content (EMC) decreased progressively from the control to PW-SDW and OS-SDW groups, with OS-SDW-50 % showing the lowest EMC under high humidity conditions. Although TS-SDW exhibited a slight increase in EMC compared to these group, it remained significantly lower than that of the untreated and water-treated samples. Together, these findings provide clear evidence of enhanced dimensional stability resulting from furfuryl alcohol-assisted surface densification.

4 Conclusions

This study presents an effective and sustainable strategy to improve the surface properties of Chinese fir through furfuryl alcohol-assisted thermal densification. One-step FA modification at 50 % concentration resulted in a surface density increase to 640 kg/m3 and improved compressive strength up to 65 MPa, alongside enhanced dimensional stability. Microstructural observations revealed densified zones with compressed lumens and resin infiltration, while FTIR spectra confirmed chemical fixation through FA polymerization. The catalyst-assisted two-step process further altered swelling behavior but showed trade-offs in mechanical performance. Comparative analysis of the one-step and two-step processes revealed that the two-step approach achieved more localized surface polymerization, as indicated by double-peaked density profiles and improved dimensional after wet-dry cycling. While WPG was similar, the structural efficiency and functional performance were enhanced in the two-step method, supporting its effectiveness for targeted wood surface modification. Overall, FA-modified densified wood exhibited a favorable balance of stiffness, shape retention, and moisture resistance, with minimal chemical usage and surface-localized modification. These findings indicate that combining FA modification with surface densification provides an efficient method to upgrade fast-growing plantation wood, achieving structural enhancement with minimal chemical use and targeted surface treatment.

Corresponding author: Yan Yu, College of Material Engineering, Fujian Agriculture and Forestry University, Fuzhou 350108, China; and National Forestry and Grassland Administration Key Laboratory of Plant Fiber Functional Materials, Fuzhou 350002, China, E-mail:
Bingbin Kuai and Yunhui Liang contributed equally to this work.

Funding source: High-level Talent Team Project of Fujian Agriculture and Forestry University

Award Identifier / Grant number: 118360020

Funding source: National Natural Science Foundation of China

Award Identifier / Grant number: 32271790

Acknowledgments

The authors are thankful for financial support from the National Natural Science Foundation of China (32271790) and the High-level Talent Team Project of Fujian Agriculture and Forestry University (118360020).

  1. Research ethics: Not applicable.

  2. Informed consent: Not applicable.

  3. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  4. Use of Large Language Models, AI and Machine Learning Tools: None declared.

  5. Conflict of interest: The authors state no conflict of interest.

  6. Research funding: National Natural Science Foundation of China (32271790) and the High-level Talent Team Project of Fujian Agriculture and Forestry University (118360020).

  7. Data availability: Data will be made available on request.

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Supplementary Material

This article contains supplementary material (https://doi.org/10.1515/hf-2025-0108).

Received: 2025-09-10
Accepted: 2025-12-12
Published Online: 2025-12-29
Published in Print: 2026-02-24

© 2025 the author(s), published by De Gruyter, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

  1. Frontmatter
  2. Wood Chemistry
  3. Solubility and structures of β-O-4 type lignin model compounds with different molar masses in aqueous 1,4-dioxane
  4. Wood Physics/Mechanical Properties
  5. Influence of frequency and recovery interval on fatigue behavior of wood in compression
  6. Anisotropic thermal conductivity as a function of wood density: experimental data comparison across eight wood species
  7. Wood Technology/Products
  8. Integrated surface densification and furfurylation for enhancing mechanical performance and dimensional stability of fast-grown Chinese fir
  9. Lignin recovery from Norway spruce using steam pretreatment and hydrotropic extraction
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https://doi.org/10.1515/hf-2025-0108
Keywords for this article
surface densification; furfuryl alcohol modification; mechanical properties; dimensional stability
Creative Commons
BY 4.0

Articles in the same Issue

  1. Frontmatter
  2. Wood Chemistry
  3. Solubility and structures of β-O-4 type lignin model compounds with different molar masses in aqueous 1,4-dioxane
  4. Wood Physics/Mechanical Properties
  5. Influence of frequency and recovery interval on fatigue behavior of wood in compression
  6. Anisotropic thermal conductivity as a function of wood density: experimental data comparison across eight wood species
  7. Wood Technology/Products
  8. Integrated surface densification and furfurylation for enhancing mechanical performance and dimensional stability of fast-grown Chinese fir
  9. Lignin recovery from Norway spruce using steam pretreatment and hydrotropic extraction
  10. Fractionation of beech wood pyrolysis liquids with supercritical and gaseous CO2
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