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. 2026 Mar 1;19(5):954. doi: 10.3390/ma19050954

Densification and Mechanical Enhancement of Invasive South African Hardwoods: Prosopis glandulosa and Acacia mearnsii

Matin Naghizadeh 1, Marthie E Niemand 2, Ernst H G Langner 1, Aimin S Sivanda 2, Karel G von Eschwege 1,*
Editor: Antonio Caggiano
PMCID: PMC12985863  PMID: 41828222

Abstract

Wood used in construction varies in density, leading to differences in strength and rigidity. Wood densification has recently emerged as a promising technique to address these limitations and enhance material performance. This study explores the potential of two abundant and low-cost invasive hardwood species in South Africa—Prosopis glandulosa (Honey Mesquite) and Acacia mearnsii (Black Wattle)—as sources for producing densified wood. A range of strengthening methods, including chemical, pressure, and heat treatments, were applied and compared. After partial delignification and hot pressing, sample thicknesses were reduced by 40% for Prosopis and 50% for Acacia, yielding substantial increases in flexural strength of 216% (22.61 MPa) for Prosopis and 334% (24.65 MPa) for Acacia. In addition to anatomical imaging, analyses of lignosulphonate content, and thermogravimetric profiling, the study also evaluated several practical, carpentry-relevant mechanical properties. These included comparative tests for flexural and compressive strength, nailing and sanding performance, as well as assessments of water absorption, electrical resistivity, and flame-holding capacity.

Keywords: invasive species, densified wood, wood strength, low cost, delignify

1. Introduction

Wood, a material utilized by humans for millennia, remains a popular choice in construction. Due to its low density, its versatility extends to transportation, particularly shipbuilding. Nevertheless, this low density is often linked to a lack of strength and rigidity, which restricts its use in high-stress situations [1,2]. Wood densification recently emerged as a promising solution. To help overcome the limitations of untreated wood, such as inferior mechanical properties and dimensional instability, a process of delignification and mechanical heat-compression was recently reported [3,4]. This treatment process leads to a significant increase in the overall density and strength of the wood.

In general, wood qualities can be readily modified through the use of various techniques, such as hot pressing, steam and/or chemical treatment, and hydrothermal alteration. Low-cost wood densification, achieved through mechanical compression, results in a material that exhibits enhanced strength, hardness, and stiffness. Compressed wood is highly versatile and can be used in various applications, including furniture, flooring, and structural components in building and vehicles [5,6,7]. Utilization of densified wood in such diverse sectors exhibits encouraging outcomes, particularly when used in load-bearing structural elements [8].

A recent study compared the mechanical characteristics of natural and densified cedar, oak, and poplar woods [9]. The findings revealed that natural oak exhibits the highest tensile strength at 115 MPa. Following the densification process, oak’s strength significantly increased to 584 MPa. The fracture values for natural oak and its densified counterpart were measured at 1.8 MJ m−3 and 5.3 MJ m−3, respectively. In comparison, poplar wood demonstrated a strength of 56 MPa in its natural state, which is less than half that of oak. However, after densification, poplar’s strength significantly improved to 432 MPa. Cedar wood had the lowest natural strength among the three kinds of wood, measuring 47 MPa. Densification of cedar wood resulted in a notable enhancement in strength, reaching 550 MPa. These findings underscore the considerable improvement in the mechanical properties of wood that can be achieved through the process of delignification (DL).

Wang et al. [10] examined the impact of alkali and maleic acid hydrotropic DL on poplar wood. The study focused on the removal of different amounts of lignin and hemicellulose through the DL process. After DL treatment, wood samples were subjected to hot pressing at 180 °C. The researchers then analyzed the changes in the wood’s microstructure, chemical content, and dimensional stability. The results showed that excessive removal of lignin during the DL process negatively affected the dimensional stability of the wood before heat treatment, i.e., removing too much lignin made the wood less dimensionally stable.

The study conducted by Maturana et al. investigated the effectiveness and consistency of a partial DL procedure on three different types of hardwoods: Carapa guianensis (Andiroba), Brosimum utile (Sande), and Dipteryx oleifera (Choiba) [11]. The DL process eliminated a fraction of the lignin and hemicellulose. While there were only minor alterations in wood density after DL, there was a significant decrease in both the modulus of elasticity (MOE) and modulus of rupture (MOR). Previous research has shown that spruce veneers that have been partially delignified and densified through the use of alkalis and organic solvents can exhibit exceptional tensile properties [12]. This process significantly improves the bending strength and modulus of elasticity, resulting in the production of sturdy plywood. This method enhances the mechanical properties of veneers while maintaining interlaminar shear strength. The preservation of wood strength was significantly improved by partially removing lignin, which increased its mesoporosity without causing irreversible damage, a common issue with complete DL [13].

In [14,15], DL and densification of green and dried poplar involved solutions of sodium hydroxide and sodium sulfite at different temperatures for 7 h. Before densification, the wood underwent a washing process and subsequent analysis, resulting in an 80% reduction in thickness. Different temperatures and durations of pressing were employed throughout the process of densification. The bending strength significantly increased, reaching a maximum value of 450 MPa. This improvement was observed under ideal conditions, which involved eliminating approximately 90% of the lignin content and subjecting the material to a pressing time of 24 h at a temperature of 130 °C.

Prosopis glandulosa and Acacia mearnsii are commonly employed in various applications, such as construction, due to their adaptability and rapid growth rates. Originating from Mexico and the southwestern USA, Prosopis glandulosa finds utility in rustic construction, posts, and furniture within its natural habitat and beyond. Both species are prized for their wood, which serves as a fuel source and timber in arid and semi-arid regions [16,17]. However, in South Africa, both these species are classified as invasive, posing a significant threat to native ecosystems by displacing indigenous flora and diminishing biodiversity.

Limited data has been published on the densification of solid wood and its mechanical properties; studies with a focus on alterations in density, strength, hardness, and dimensional stability that occur as a result of the densification procedure have only investigated a few selected wood species [9]. Reduction in swelling as a result of resilience to moisture uptake was also observed [18,19,20,21,22]. Understanding the performance and durability of wood-based buildings relies heavily on the properties of the wood materials. Current and future investigations should aim to provide experimental data to help answer inquiries from the construction sector regarding the practical use of machining techniques and the suitability of densified wood for various applications.

Prosopis glandulosa, commonly known as Honey Mesquite, was introduced to South Africa in the late 1880s from South, Central, and North America. While the wood of Prosopis glandulosa is known for its hardness [23,24,25], large impenetrable tracts of Prosopis thickets cause loss of valuable pasture in the arid and semi-arid areas in South Africa. The negative impacts of this aggressive invader also include a change in soil chemistry, the soil microbial balance, and soil quality, while eradicating Prosopis glandulosa is extremely costly and difficult [26].

Black Wattle, or Acacia mearnsii, is an invasive tree species that thrives in South Africa’s various habitats. This Australian evergreen tree has been introduced and cultivated worldwide, including in South Africa, where it is used in ecological studies and industry. Acacia mearnsii has extraordinary growth qualities. It grows quickly under suitable conditions. This tree can reach 5–10 m in 5–7 years, presenting a striking shape with a straight and slender trunk. Acacia mearnsii wood is distinctive and flexible. A wood density of 670 kg/m3 to 850 kg/m3 makes this species highly valued in industrial and commercial industries [27,28,29,30,31].

Despite their invasive tendencies, these species’ economic and versatile attributes continue to drive their utilization [27,32]. To gain a deeper understanding of the capabilities of enhanced natural materials, we conducted a comprehensive comparative analysis of wood samples that were treated in different ways. Analyses were performed on all samples, encompassing their flexural and compressive strengths, resistance to nailing and sanding, moisture absorption and flammability, thermal stability, and electrical conductivity/resistance. The research findings presented here offer useful insights into the diverse characteristics and serve as a foundation for future novel uses of cost-effective, improved wood.

2. Experimental Section

2.1. Plant Material Collection and Identification

The wood samples of Prosopis glandulosa (Honey Mesquite) and Acacia mearnsii (Black Wattle) used in this study were collected from invasion sites in South Africa. Prosopis glandulosa wood was supplied by Dr. C. P. Cronje (Zoning & Permissions Advisor) from the Prieska area in the Northern Cape Province (GPS coordinates: −29.808780 S, 22.714670 E). Acacia mearnsii wood was supplied by NTE (https://nte.co.za/, accessed on 13 February 2026) and was collected from Ptermaritzburg area in the KwaZulu-Natal Province (GPS coordinates: −29.0203132 S, 30.7789173 E).

Formal species identification was confirmed by co-author Marthie E. Niemand, who is affiliated with the Centre for Environmental Management at the University of the Free State, Bloemfontein, South Africa.

Both Prosopis glandulosa and Acacia mearnsii are classified as invasive alien plant species in South Africa under the National Environmental Management Biodiversity Act, 2004 (NEMBA). Acacia mearnsii is listed as a Category 2 invasive species, and Prosopis glandulosa is listed as Category 1b (in Eastern Cape, Free State, North-West, and Western Cape) and Category 3 (in Northern Cape) according to the Alien and Invasive Species (AIS) Regulations under NEMBA. Under these regulations, landowners are legally obliged to manage and control the spread of these species. The collection of wood samples for research and commercial purposes complies with the requirements of NEMBA and the Conservation of Agricultural Resources Act, 1983 (CARA), and is consistent with the IUCN Policy Statement on Research Involving Species at Risk of Extinction and the Convention on the Trade in Endangered Species of Wild Fauna and Flora. No specific collection permits were required as the removal of invasive plant material contributes to mandated control efforts.

Due to the invasive nature of these species and the focus of this study on wood material properties rather than botanical taxonomy, voucher specimens were not deposited in a herbarium. Both species are widespread across more than 100,000 square kilometers in South Africa, and extensive documentation of their distribution and characteristics is available in the scientific literature.

2.2. Chemicals and Instruments

The delignification process of wood from two species, Prosopis glandulosa from the Northern Cape Province and Acacia mearnsii from the KwaZulu-Natal Province, involved the use of sodium hydroxide (NaOH, >97%, Merck, Rahway, NJ, USA), sodium sulfite (Na2SO3, >98%, Merck), and distilled water.

A 6-ton mechanical press fitted with a 2500 kg load cell (Adam Equipment, UK, type GK Indicator Platform scale) and a temperature-controlled press chamber (inner dimensions: 100 × 45 × 45 mm), which was made in-house, was employed for wood densification (Figure 1).

Figure 1.

Figure 1

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Temperature-controlled press chamber.

The same equipment was used for measuring the flexural and compressive strengths, as well as the force required to press a nail through the wood samples. Small mechanical devices required for the various tests were manufactured in-house. The sanding test was conducted using a belt sander, with a sanding belt speed of 250 m per minute and P120 grit aluminum oxide sandpaper. A UNI-T, China, UT513A insulation resistance tester (Dongguan, China) was used to measure the electrical resistivity of all wood and reference samples (1 mm thickness), up to 5000 V. A METTLER TOLEDO, Switzerland, TGA/SDTA851 analyzer (Greifensee, Switzerland) was used to conduct thermogravimetric (TGA) analyses under ambient conditions. TGA data were analyzed using METTLER STARe Evaluation software 9.01. A Bernzomatic® UL100 propane torch (Chilton, WI, USA), with a flame temperature of ca 1980 °C in air, was used for the flame tests. A Carl Zeiss Microscopy GmbH, Germany, ZEISS Stereo Discovery V8 optical microscope was used for imaging the microscopic cell structures of the different wood samples. Safranin O (Sigma-Aldrich, St. Louis, MO, USA) was used for staining the lignin for microscopic analysis.

2.3. Sample Treatment

To partially delignify the wood, samples with dimensions of 95 × 25 × 25 mm were submerged in boiling solutions containing NaOH (50 g, 2.5 M) and Na2SO3 (25 g, 0.4 M) for 40 and 80 min periods [33]. Subsequently, the wood blocks were washed (20 min) twice in boiling water (1 L) to remove residual chemicals.

For densification, all samples were subjected to a pressure of 5 MPa at a temperature of 100 °C. An untreated sample of each wood species was also cold-pressed at room temperature for comparison. The compression times were 3 h for cold pressing, 5 h for hot pressing, and 12 h for the samples that underwent the delignification process.

2.4. Tests and Analyses

Flexural strength was assessed through the implementation of the three-point test, wherein a load is exerted at the midpoint of a supported sample until it fractures (Figure 2) [34,35].

Figure 2.

Figure 2

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Setup for three-point flexural strength test.

Flexural strength (σ) was determined using the formula:

σ = 3FL/2wd2

where F represents the maximum force applied in Newtons, L is the distance between two supporting pins, w denotes the width of the sample, and d is the depth/thickness of the sample, all in meters. Processed samples of different thicknesses were subjected to a compressive force, which was applied perpendicular to the wood grain in the transverse direction.

Compressive strength (P, in MPa) was determined using the formula:

P = F/A

where F represents the highest load (or load until failure) applied to the material (in Newtons) and A is the cross-sectional area of the sample that withstands the load. To measure the compressive strength, the dimensions of all wood samples were adjusted to form cubes with a side length of 10 mm. The samples were placed between two flat steel plates and subjected to compression. The maximum force exerted on the samples, just before collapse, was measured. The force was applied in the same direction as the wood grain (longitudinal direction).

Nail resistance tests were conducted similarly to compression testing. A hardened, sharp steel nail measuring 20 mm in length and 2 mm in thickness was affixed to the press. The nail was then driven into the center of each wooden sample (transverse direction), which had dimensions of 10 × 10 mm and 15 mm in length. The maximum force exerted during this process was documented.

Abrasion resistance or sanding tests were performed by using a belt sander on a 1 cm2 pre-weighed sample for a period of 5 s [36]. The weight of the belt sander was 2.94 kg, which exerted a constant pressure of 0.29 MPa on the samples. After the sanding, the samples were weighed again, and the percentage mass loss was calculated and recorded.

For the moisture uptake tests, wood samples measuring 10 × 10 × 5 mm were dried in an oven at 110 °C for 24 h and then weighed at room temperature. The dried specimens were then immersed in water for 24 h at ambient temperature. After immersion, the moist samples were dried using a paper towel and subjected to atmospheric drying for 20 min before being re-weighed. The mass gain was transformed into percentage water absorption [37].

Thermal stabilities of all samples were assessed using TGA. The samples were subjected to controlled heating at a rate of 10 °C per minute until they reached a temperature of 600 °C in the presence of ambient air [38].

Flame-holding tests on wood samples measuring 20 × 10 × 10 mm were subjected to a direct gas flame for 15 s. After the torch was removed, the sample’s ability to sustain a flame was assessed by documenting the duration it took for the flame to die out on its own.

The lignosulfonate content of selected samples was quantitatively determined through the UV–vis spectrophotometry methods of Hon and Dence [39,40]. Specifically, the absorbances of slightly acidic aqueous solutions of the lignin samples were measured in the 210–400 nm wavelength range. Lignin content was then quantified as a mass percentage relative to the solid samples, with the results referring specifically to the lignosulfonate fraction.

An optical microscope was utilized to assess the microscopic anatomical properties of both Prosopis glandulosa and Acacia mearnsii species, both before and after treatment, but not from the same pieces of wood. A WSL sliding microtome was used to prepare thin sections from the transverse plane of each sample to compare the general anatomical structure of both treated and untreated wood. Each thin section was cut to between 12 and 14 µm in thickness, and stained using a 1% Safranin O staining solution, following the standard approach reported by Ruzin [41]. The lignin in the cell walls, which had become stained, exhibited a vivid red to deep pink color, thereby accentuating the observable alteration in cell structure prior to and following treatment.

Electrical resistivity measurements, including breakthrough voltage, were conducted on samples of Prosopis glandulosa and Acacia mearnsii species. The measurements were taken under three different conditions:

  • As is, under ambient conditions;

  • After drying the samples for 2 h at 120 °C;

  • After saturating the samples in furniture oil (raw linseed oil) and wiping them dry.

Measurements were also conducted on reference materials using meter settings of 500, 1000, 2500, and 5000 volts, all within the permissible range.

3. Results and Discussion

Figure 3 depicts the experimental arrangement used in this study. All samples were uniformly resized as specified. The heating chamber’s cavity size, as shown in the photo in Section 2.1, was larger than that of the samples. This allowed for lateral expansion and also provided space for moisture evaporation under compression at a high temperature of 100 °C.

Figure 3.

Figure 3

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Experiment design.

In contrast to previously reported open-compression platforms [42,43,44,45], we constructed a sealed press chamber to ensure rapid, homogeneous heating and temperature regulation. Water vapor was able to exit through small apertures located on the sidewalls. The compression periods for samples that were not boiled or delignified were determined based on the duration required for the applied mechanical pressure to reach a stable state, i.e., where it stops decreasing. Untreated wood achieved pressure stability after around 2 h when pressed at room temperature. However, when pressed at 100 °C, it took up to 4 h for the pressure to stabilize. The extended duration can be attributed to the loss of moisture and chemical reconfiguration at higher temperatures. In both situations, an extra hour was included, resulting in press times of 3 h for cold pressing and 5 h for hot pressing. The wet DL samples were boiled for 40 and 80 min and promptly hot-pressed after being washed twice in clean boiling water. In this case, a prolonged compression time of 12 h was required to achieve fully dried and densified wood. Pressing the wood for a short duration in these situations resulted in samples that were swollen in the center due to the presence of residual water trapped inside the wood fibers. Furthermore, reducing the temperature to below 100 °C considerably decelerated the wood drying process.

In previously reported studies, researchers focused on removing only a portion (40%) of the lignin before compression, with long delignification boiling times of up to 7 h [9]. Our initial testing showed that shorter boiling times are sufficient, which is crucial because it can lead to significant cost savings in terms of energy consumption during large-scale production. Consequently, lignin extraction periods were set to 40 and 80 min. Subsequent investigations and comparisons of different wood species that have potential economic uses were conducted under the same conditions outlined in this paper. The objective was to reduce production time, as well as chemical and energy expenses throughout the entire process, which is necessary for developing the foundation for a sustainable business model. In addition to reducing the drying time of the delignified samples, it was also considered important to report comparisons with other more affordable treatments, such as direct cold pressing of untreated wood, hot pressing, and water-boiling followed by pressing—all without delignification.

Lignin content, as determined by UV/vis spectrophotometry, was associated with mass reductions of 13% and 18% for the 40 min and 80 min Prosopis glandulosa DL procedures, respectively. The lignin content of other Prosopis species, such as Prosopis laevigata, has been reported to range from 29.8% to 31.4% [46,47]. Typically, tropical hardwood species have lignin levels that range from 10 to 40% [48]. The analytical data for the extractions of Acacia mearnsii showed mass reductions of 23% and 25%, respectively. The results are consistent with the lignin contents that have been reported for many Acacia species, which range from 21 to 25% [49].

The microscopic morphologies of the untreated (left) and delignified (right) wood samples are shown in Figure 4. The diffuse porous wood of both Acacia mearnsii and Prosopis glandulosa appears to be mostly intact before and after treatment, except for a slight reduction in lignification in the treated samples. The fibers of both species are thick-walled, adding to the wood strength of these species and therefore, less distortion of the cells was expected. However, after treatment, the fibers of both exotic species exhibited thinner cell walls with larger visible lumina, but without any change in shape, which was again expected.

Figure 4.

Figure 4

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Optical microscopy images of cell structures in untreated (A,C), and 80 min delignified (B,D) Prosopis glandulosa (top) and Acacia mearnsii (bottom) wood samples.

Acacia mearnsii has solitary vessel elements of similar diameters dispersed throughout the ring structure, which showed no noticeable distortion after treatment. Although the ray cells are intact after treatment, Prosopos glandulosa has larger, more pronounced rays than Acacia mearnsii. The rays can be described as 2–3 seriate.

These two species are angiosperms, which are much stronger due to their fibers and vessel elements that form part of the wood structure compared to, for example, pines, which are gymnosperms that do not have fibers or vessel elements in their wood anatomy. Their wood structure consists mainly of tracheids and rays. Thus, less distortion is expected in Acacia mearnsii and Prosopis glandulosa than in pines.

TGA analyses (Figure 5) indicated that the thermal deterioration of both untreated and delignified wood samples can similarly be divided into three distinct stages of mass loss.

Figure 5.

Figure 5

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TGA curves of Prosopis glandulosa (top) and Acacia mearnsii (bottom) wood samples. (─) untreated, (─) 80 min delignified. Mass loss percentages are indicated in brackets.

  • The first phase (I), occurring between room temperature and 150 °C, is attributed to the liberation of approximately 5% water for all samples—this is the only phase that is relevant to the densification process.

  • Phase I is followed by significant mass loss (Phase II), starting at 240 °C, which is the result of decomposition of lignin components in several stages. Thermal decomposition of Prosopis samples concludes at approximately 410 °C, while for Acacia, it concludes at around 350 °C. The mass loss of delignified Prosopis was approximately 50%, nearly identical to the 45% mass loss of the untreated sample, likely due to significant densification. The delignified Acacia sample, with 50% densification, did not exhibit a substantially different mass loss compared to the untreated sample, both around 55% during Phase II.

  • The final phase (III) for Prosopis involves rapid degradation after 420 °C, concluding at around 500 °C. For Acacia, this step begins at a much lower temperature, 350 °C, and is characterized by fast degradation ending at approximately 480 °C. This final step corresponds to the breakdown of the cellulose structure.

Comparing the TGA curves at higher temperatures, the untreated Acacia samples showed an upward shift compared to the 80 min treated samples, while the Prosopis species exhibits almost no upward shift, aligning with its inherently denser nature.

Figure 6 shows the decrease in thickness (from the original 25 mm) of the wood samples subjected to different treatments. The distinction between the dense Prosopis glandulosa (top) and Acacia mearnsii (bottom), which is almost half the weight (see Supporting Information Table S2), is clearly apparent. The 80 min delignified samples consistently reached the smallest thickness dimensions under compression. Specifically, the 80 min delignified Prosopis was compressed to 60% of its original thickness under 5 MPa of pressure, while Acacia was compressed to 50%. This difference in compressibility is attributed to the more permeable and lighter structure of Acacia, as observed in the microscope images (Figure 4C,D). The impact of DL is evident, with the 80 min delignified sample resulting in the most compacted material. The most significant reduction in thickness, from 24 to 19 mm, is observed when transitioning from cold to hot pressing for both species. This represents a 24% reduction in original thickness. Both Prosopis and Acacia exhibited substantial mass loss, as indicated in SI Table S2. The overall mass reduction using hot pressing alone (without prior DL) is likely due to the loss of water and leaching of liquefied organic components during the extended 5 h hot-compression period (see SI Table S1).

Figure 6.

Figure 6

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Thickness comparison of Prosopis glandulosa (top) and Acacia mearnsii (bottom) wood samples that were ■—untreated, ■—cold-pressed, ■—hot-pressed (100 °C), ■—boiled in water then hot-pressed, ■—delignified for 40 min and hot-pressed, and ■—delignified for 80 min and hot-pressed. Reported values are the means of two experiments.

Figure 7 (left) compares the flexural strengths determined based on the points of maximum pressure just before the point of fracture. The flexural strength data is derived from compressive forces applied perpendicular to the wood’s growth direction, in contrast to compressive strengths. The Acacia 80 min delignified sample exhibited a 334% increase in flexural strength, reaching 24.65 MPa. This finding aligns with the research conducted by Song et al. [9], which found that partially delignified materials ended up stronger compared to samples where lignin was completely removed. This is consistent with the measured lignosulphonate content in the 80 min DL sample, as pointed out here earlier. In comparison to the reduction in thickness at a pressure of 5 MPa, the Prosopis sample showed a 216% increase, reaching 22.61 MPa.

Figure 7.

Figure 7

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Flexural (left) and compressive (right) strengths, calculated from maximum pressures measured on Prosopis glandulosa (top) and Acacia mearnsii (bottom) wood samples that were ■—untreated, ■—cold-pressed, ■—hot-pressed (100 °C), ■—boiled in water then hot-pressed, ■—delignified for 40 min and hot-pressed, and ■—delignified for 80 min and hot-pressed. Reported values are the means of two experiments.

During compressive strength measurements, increasing pressure was applied along the growth direction of the wood samples until the point of collapse. Smaller samples of a similar size (10 mm cubes) for subsequent testing were all cut from the two original larger samples that were prepared under each process. The maximum pressures/compressive strengths were recorded and are presented in Figure 7 (right). The compressive strength increased by approximately 150% for Prosopis and 130% for Acacia after 80 min of delignification and hot pressing. These results are consistent with related flexural strength trends. Again, the results for both the flexural and compressive strength measurements showed that the 80 min partial DL with the hot press approach increased the wood strength the most.

Densified and strengthened wood from partially delignified Acacia mearnsii shows greater thickness reduction and strength gains during heat-compression compared to Prosopis glandulosa, primarily due to intrinsic wood properties. Acacia mearnsii typically has a higher initial lignin content (around 25–30% vs. Prosopis glandulosa’s 20–25%), facilitating more effective delignification and cell wall collapse under pressure as lignin acts as a rigid matrix that, when partially removed, allows for denser hydrogen bonding between cellulose microfibrils. Its fiber structure features longer, finer fibers with a more uniform helical arrangement and higher microfibril angle flexibility, promoting superior compressibility and plasticization during hot-pressing, unlike the coarser, more irregular fibers and vessel-rich microstructure of Prosopis glandulosa, which resist deformation and lead to less efficient packing. Additionally, Acacia mearnsii’s lower initial density (0.55–0.85 g/cm3) and higher extractive content (up to 25%) enable greater relative densification (up to 80% thickness reduction) and enhanced mechanical interlocking post-compression, yielding tensile strengths exceeding 400 MPa, while Prosopis glandulosa’s denser starting matrix (often >0.69 g/cm3) and silica inclusions limit shrinkage to ~50–60% and cap strength improvements. These differences underscore Acacia mearnsii’s microstructural advantages for optimized densification outcomes [50,51,52,53,54,55,56].

The authors have chosen to investigate and present data from various less costly densification processes as the business case for viable industrial mass production will be heavily dependent on production time, energy and water consumption, as well as the use of chemicals, together with costs associated with chemical waste disposal, etc. From both the thickness reduction and strength data, it is clear that prior water boiling followed by hot pressing most closely approaches the results obtained from costly chemical delignification processes.

Carpenters raised questions about the suitability of densified wood for everyday woodwork machining. To test and quantify some of these applications, methods were devised to assess the resistance of densified wood to nailing and sanding, as described in the preceding Experimental section. Regarding nail resistance, a 2 mm thick steel nail was pressed into different samples of similar sizes, and the point with highest pressure was recorded (see Figure 8 (left)). The observations closely matched the flexural strength trends seen for both samples. The pressure required to drive the nail into densified Prosopis increased by 120% (6.37 MPa compared to 2.94 MPa for untreated wood). Small/partial to no cracking was observed for the Prosopis samples (see SI Table S3). Despite the small size of the samples (10 × 10 × 15 mm), the appearance of tiny cracks directly opposite each other around the relatively thick nail indicated that there was no significant resistance to driving nails into compressed wood. The observations for Acacia, however, differed—its samples exhibited random cracking, with some showing full cracks while others showed none. This clearly demonstrates that the behavior depends not only on the wood species but also on variations within individual samples. Therefore, it was deemed important to present the data exactly as recorded, without bias, to accurately reflect the conditions likely to be encountered in large-scale industrial applications.

Figure 8.

Figure 8

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Nail and sanding resistances measured for the Prosopis glandulosa (top) and Acacia mearnsii (bottom) wood samples that were ■—untreated, ■—cold-pressed, ■—hot-pressed (100 °C), ■—boiled in water then hot-pressed, ■—delignified for 40 min and hot-pressed, and ■—delignified for 80 min and hot-pressed. Reported values are the means of two experiments.

To simulate practical carpentry working conditions during the abrasion/sanding test, the weight (2.94 kg) of a running belt sander itself was used to apply constant pressure to wooden blocks with an identical surface area. The weight loss of the blocks was measured after sanding for 5 s, see Figure 8 (right). As expected, untreated samples with lower densities exhibited the highest mass loss. The untreated Prosopis sample showed 14% mass loss compared to 2.5% for the densified sample, corresponding to an 82% reduction in material loss, with sanding accounting for only 2.5%. Similarly, the untreated Acacia sample had a mass loss that was 334% greater than the most dense sample.

Clearly, densified wood may still readily be nailed and sanded, but nailing and sanding are systematically more difficult for denser samples. In addition, during sizing and machining of the different samples, careful consideration was given to performance and relative ease of sawing, planing, and drilling of treated samples. Notably, no material property changes were observed that could prevent any of these machining techniques or hamper them significantly.

In general, the densified samples have a smoother, almost plastic-like appearance and are less likely to create splinters. During flexural stress testing, the natural wood samples slowly developed cracks and splinters while the pressure increased. In contrast, the densified samples experienced abrupt and complete breaking without splintering, accompanied by a distinct loud snap sound, resembling the behavior of hard plastics.

Figure 9 illustrates flame-holding capacities (ability to sustain a flame) of the various samples, i.e., how long after igniting the wood samples with a gas torch before the flames die down. In both species, the downward trends followed corresponding increases in wood densities. For several reasons, this follows the expectation that low-density wood.

Figure 9.

Figure 9

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Flame-holding capacity of Prosopis glandulosa (top) and Acacia mearnsii (bottom) wood samples that were ■—untreated, ■—cold-pressed, ■—hot-pressed (100 °C), ■—boiled in water then hot-pressed, ■—delignified for 40 min and hot-pressed, and ■—delignified for 80 min and hot-pressed. Reported values are the means of two experiments.

  • Contains more air spaces, which allow for better oxygen flow;

  • Allows heat to penetrate more easily;

  • Typically has a larger surface area relative to its mass;

All of these may lead to faster ignition and more rapid and sustained burning.

Densified wood is thus seen to sustain fire less readily, at least directly after initial exposure. This observation is not indicative of other fire properties since it is also known that dense wood tends to burn longer and provide more heat energy over time due to its higher mass and energy content.

Water absorption tests were conducted on all samples of comparable size, and the results were recorded as the percentage increase in mass (refer to SI Table S4). Before immersion, the samples were dried and then placed in water at room temperature for 24 h. Densified samples were found to not exhibit reduced water absorption. This is consistent with the microscopy photos (Figure 4), which showed that the vessel elements of Prosopis glandulosa and Acacia mearnsii did not collapse and still allowed a sufficient capacity for water uptake. Throughout this procedure, Acacia samples showed weight increases varying from 56 to 90%, indicating a certain level of expansion. In other words, the wood, which became heavier and more compact, could not absorb much water unless the volume simultaneously expanded. The denser Prosopis samples exhibited lower water absorption capacities ranging from approximately 36% to 51% of its original dry weight. As with otherwise untreated wood, it is crucial to seal densified wood to prevent water uptake, especially under wet environmental conditions.

Lastly, in order to investigate the national electrical utility’s concerns regarding high-voltage electrical equipment placed on “non-conducting” wood, a series of electrical resistivity tests was conducted as part of this study (refer to Table 1). Plates of 1 cm2 and with a thickness of 1 mm were used in these measurements. Resistivities were tested using potential differences of up to 5000 V. In all instances and as anticipated, electrical resistance decreased by orders of magnitude as the voltage increased five-fold. For instance, at 1000 V, the resistance of the 80 min DL sample, which was initially 34 GΩ (giga-ohm), decreased to 1 GΩ at 5000 V.

Table 1.

Electrical resistance through 1 mm thick samples of Prosopis glandulosa (dark gray columns), Acacia mearnsii (light gray columns) as is, pre-dried, and oiled. Resistivities of reference materials were also measured under similar experimental conditions (bottom). Ambient temperature—ca 14 °C. Relative humidity—ca 50%. STD—sample as is. ∞—resistance too high to measure. Arc—resistance collapse.

Samples Voltage
(V)		 Std
(GΩ)		 Dried
(GΩ)		 Oiled
(GΩ)
Untreated 500
1000
2500
5000		 ∞
∞
72
28		 ∞
25
5
1.4		 ∞
∞
∞
388		 ∞
∞
∞
463		 ∞
∞
∞
223		 ∞
∞
∞
301
Cold pressed 500
1000
2500
5000		 ∞
∞
28
Arc		 ∞
33
7
3.5		 ∞
∞
∞
283		 ∞
∞
∞
281		 ∞
∞
∞
255		 ∞
∞
∞
223
Hot pressed 500
1000
2500
5000		 ∞
∞
27
Arc		 ∞
38
7
2		 ∞
∞
∞
273		 ∞
∞
∞
197		 ∞
∞
∞
208		 ∞
∞
∞
173
Boiled water 500
1000
2500
5000		 ∞
∞
11
4		 ∞
34
5
1		 ∞
∞
∞
221		 ∞
∞
∞
180		 ∞
∞
∞
204		 ∞
∞
∞
193
40 min DL 500
1000
2500
5000		 ∞
31
3
0.5		 ∞
31
6
1		 ∞
∞
∞
215		 ∞
∞
∞
171		 ∞
∞
∞
183		 ∞
∞
∞
170
80 min DL 500
1000
2500
5000		 ∞
34
4
1		 ∞
32
4
0.8		 ∞
∞
∞
189		 ∞
∞
∞
197		 ∞
∞
∞
123		 ∞
∞
∞
169
Perspex 2500 ∞
5000 385
PVC 2500 ∞
5000 271
Teflon 2500 ∞
5000 361
Borosilicate glass 2500 ∞
5000 201
Paper 2500 2
5000 Arc
Oiled Paper 2500 3
5000 Arc
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As for increased sample density, in general, no significant impact was seen on electrical resistance in the case of otherwise (Std) untreated samples. Only in the pre-dried and oiled samples were definite and similar trends observed; at 5000 V, resistivities systematically decreased by roughly 50% with increased wood density. It is thus concluded that atmospheric moisture uptake in the Std samples plays a significant role in its resistance to flow of electrical current. Resistivities in all samples where moisture was removed or excluded were so high that readings could only be obtained at 5000 V and with very high values ranging from 170 to 463 GΩ.

For the purpose of comparison, measurements were conducted on several well-known insulating materials under similar (Std) conditions, namely Perspex (polymethylmethacrylate), PVC (polyvinylchloride), Teflon (polytetrafluoroethylene), borosilicate glass, and white paper (primarily composed of cellulose, either dried or oiled). Paper (even when oiled) performed worst in terms of electrical insulating capacity (2–3 GΩ) compared to all the other reference materials, which measured roughly similar (201–385 GΩ) to the various dried and oiled wood samples.

Electrical insulators typically possess strongly bonded electrons and a limited number of mobile charge carriers, resulting in high resistance and low conductivity. Applying a potential difference to an insulator allows for the determination of the electric field based on the voltage gradient across the material. Variations in the form or composition of the insulator lead to distinct voltage gradients. Electrical breakdown occurs when the electric field in a region exceeds the dielectric strength, causing the area to become conductive. This initiates further degradation until a conductive pathway is formed. The voltage at which the initial breakdown takes place is known as the breakdown voltage of the insulator material [57,58,59,60,61]. As shown in Table 1, voltage breakdowns only occurred with two of the Prosopis samples and paper at 5000 V.

4. Conclusions

This study presents comprehensive experimental data on the densification, machining behavior, and physicochemical properties of two invasive South African hardwood species, Prosopis glandulosa and Acacia mearnsii. Partial delignification using NaOH/Na2SO3 removed approximately 13–18% of the lignin in Prosopis and 23–25% in Acacia, which was followed by hot pressing at 5 MPa and 100 °C.

Densification resulted in substantial thickness reductions, with Prosopis compressed to 60% and Acacia to 50% of their original thickness after 80 min of delignification. Correspondingly, mechanical properties improved significantly. Flexural strength increased by 216% for Prosopis (to 22.61 MPa) and 334% for Acacia (to 24.65 MPa), while compressive strength increased by 253% for Prosopis and 231% for Acacia.

Nail resistance increased by 120%, from 2.94 MPa to 6.37 MPa in the most dense samples, indicating enhanced surface durability. Thermal degradation behavior showed three distinct stages, with the major decomposition occurring between 240 and 410 °C for Prosopis and between 240 and 350 °C for Acacia. Electrical resistivity of dried and oiled densified samples ranged from 170 to 463 GΩ at 5000 V, comparable to that of conventional insulating polymers, highlighting their potential as electrical insulating materials.

Despite densification, water absorption remained high (36–51% for Prosopis and 56–90% for Acacia), indicating that surface sealing is required for moisture-exposed applications. Flame-holding capacity decreased with increasing density, suggesting improved resistance to ignition, although dense samples may burn longer due to higher mass.

Overall, densification substantially enhanced mechanical performance, particularly for Acacia, while Prosopis showed more moderate improvements due to its higher initial density. The results demonstrate that invasive woody biomass can be transformed into high-performance engineered materials suitable for structural, furniture, and electrical insulation applications. Species-specific responses highlight the need for tailored processing conditions. Wood species differ in their ability to be densified (compressed) mainly because of differences in their anatomy, cell wall chemistry, moisture behavior, and microstructure. Low-density woods densify better because there is more space to collapse, while high-density woods already have thick walls, limited void space, and resist further compression. This is the reason why species like pine densify much more dramatically than very dense hardwoods, as will be illustrated in our next publications.

Large-scale production faces some hurdles, including high energy consumption during prolonged compression (often 24 h), challenges in chemical recovery of delignification agents, and the need for continuous processing methods to handle varying wood species as current batch processes limit industrial scalability. Long-term durability issues are also to be considered: despite gains in compressive strength and stiffness, densified wood exhibits moisture sensitivity, leading to the “springback” or set-recovery effect under cyclic humidity changes where absorbed water acts as a plasticizer, weakening hydrogen bonds and causing dimensional instability, swelling, or reduced load-bearing capacities. Weather resistance remains a concern for outdoor applications due to vulnerabilities to UV radiation, fungal decay, and termite attacks unless further treated, and thermal instability at elevated temperatures can induce creep or degradation, necessitating advanced post-treatments like lignin regeneration or hydrophobic coatings to ensure reliability in real-world cyclic environments. Overall, ongoing research is essential to optimize these woods for commercial viability without compromising sustainability [9,62,63,64].

Acknowledgments

The authors would like to thank the Central Research Fund of the University of the Free State for financial support, the Ihlati Bush Resort and Flip Zeederberg for supplying wood samples, and Mpact Innovation Centre Stellenbosch for performing the lignin analyses.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma19050954/s1, Table S1: Process times of various wood samples. DL—delignified.; Table S2: Weight changes during processing.; Table S3: Crack comparison during nail tests. pc—partial crack, nc—no crack, fc—full crack.; Table S4: Water absorption.

materials-19-00954-s001.zip (110.9KB, zip)

Author Contributions

M.N. conducted the investigation, data curation, and methodology, and wrote the original manuscript draft. K.G.v.E. conceptualized and supervised the project, acquired funding, provided resources, contributed to the investigation, and reviewed and edited the manuscript. E.H.G.L. contributed to funding acquisition, performed TGA analyses, and assisted in manuscript editing. M.E.N. obtained funding for the WSL microtome and, together with A.S.S., carried out optical microscopy analyses, and contributed to editing the manuscript. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding Statement

Research funding is received from the Central Research Fund at the University of the Free State.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

References

  • 1.Spear M.J., Curling S.F., Dimitriou A., Ormondroyd G.A. Review of Functional Treatments for Modified Wood. Coatings. 2021;11:327. doi: 10.3390/coatings11030327. [DOI] [Google Scholar]
  • 2.Bekhta P., Krystofiak T. Performance and Modification of Wood and Wood-Based Materials. Forests. 2023;14:963. doi: 10.3390/f14050963. [DOI] [Google Scholar]
  • 3.Luan Y., Fang C., Ma Y.-F., Fei B. Wood Mechanical Densification: A Review on Processing. Mater. Manuf. Process. 2022;37:359–371. doi: 10.1080/10426914.2021.2016816. [DOI] [Google Scholar]
  • 4.Sadatnezhad S.H., Khazaeian A., Sandberg D., Tabarsa T. Continuous Surface Densification of Wood: A New Concept for Large-Scale Industrial Processing. BioResources. 2017;12:3122–3132. doi: 10.15376/biores.12.2.3122-3132. [DOI] [Google Scholar]
  • 5.Pagel C.L., Lenner R., Wessels C.B. Investigation into Material Resistance Factors and Properties of Young, Engineered Eucalyptus Grandis Timber. Constr. Build. Mater. 2020;230:117059. doi: 10.1016/j.conbuildmat.2019.117059. [DOI] [Google Scholar]
  • 6.de Jesus Albuquerque N.M.C., de Almeida D.H., de Araujo V.A., Vasconcelos J.S., Christoforo A.L., de Almeida T.H., Lahr F.A.R. Physical and Mechanical Properties of Eucalyptus Saligna Wood for Timber Structures. Ambient. Construído. 2019;19:233–239. [Google Scholar]
  • 7.Namari S., Drosky L., Pudlitz B., Haller P., Sotayo A., Bradley D., Mehra S., O’Ceallaigh C., Harte A.M., El-Houjeyri I., et al. Mechanical Properties of Compressed Wood. Constr. Build. Mater. 2021;301:124269. doi: 10.1016/j.conbuildmat.2021.124269. [DOI] [Google Scholar]
  • 8.Sotayo A., Bradley D., Bather M., Sareh P., Oudjene M., El-Houjeyri I., Harte A.M., Mehra S., O’Ceallaigh C., Haller P., et al. Review of State of the Art of Dowel Laminated Timber Members and Densified Wood Materials as Sustainable Engineered Wood Products for Construction and Building Applications. Dev. Built Environ. 2020;1:100004. doi: 10.1016/j.dibe.2019.100004. [DOI] [Google Scholar]
  • 9.Song J., Chen C., Zhu S., Zhu M., Dai J., Ray U., Li Y., Kuang Y., Li Y., Quispe N., et al. Processing Bulk Natural Wood into a High-Performance Structural Material. Nature. 2018;554:224–228. doi: 10.1038/nature25476. [DOI] [PubMed] [Google Scholar]
  • 10.Wang J., Liu J., Li J., Zhu J.Y. Characterization of Microstructure, Chemical, and Physical Properties of Delignified and Densified Poplar Wood. Materials. 2021;14:5709. doi: 10.3390/ma14195709. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 11.Maturana J.C., Arroyave C., Hurtado A., Echeverría F., Correa E. Effect of Alkaline Delignification Process on the Anatomical Characteristics and Some Mechanical Properties of Tropical Woods of Different Densities. 2022. [(accessed on 15 June 2025)]. Available online: https://www.researchsquare.com/article/rs-2188263/v1.
  • 12.Jakob M., Stemmer G., Czabany I., Müller U., Gindl-Altmutter W. Preparation of High Strength Plywood from Partially Delignified Densified Wood. Polymers. 2020;12:1796. doi: 10.3390/polym12081796. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 13.Grönquist P., Frey M., Keplinger T., Burgert I. Mesoporosity of Delignified Wood Investigated by Water Vapor Sorption. ACS Omega. 2019;4:12425–12431. doi: 10.1021/acsomega.9b00862. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 14.Wehsener J., Bremer M., Haller P., Fischer S. Bending Tests of Delignified and Densified Poplar. Wood Mater. Sci. Eng. 2023;18:42–50. doi: 10.1080/17480272.2022.2134049. [DOI] [Google Scholar]
  • 15.Wang J., Fishwild S.J., Begel M., Zhu J.Y. Properties of Densified Poplar Wood through Partial Delignification with Alkali and Acid Pretreatment. J. Mater. Sci. 2020;55:14664–14676. doi: 10.1007/s10853-020-05034-2. [DOI] [Google Scholar]
  • 16.Pasiecznik N. Prosopis glandulosa (Honey Mesquite) CABI Compend. 2016 doi: 10.1079/cabicompendium.44439. [DOI] [Google Scholar]
  • 17.Fagg C.W., Stewart J.L. The Value of Acacia and Prosopis in Arid and Semi-Arid Environments. J. Arid Environ. 1994;27:3–25. doi: 10.1006/jare.1994.1041. [DOI] [Google Scholar]
  • 18.Cabral J.P., Kafle B., Subhani M., Reiner J., Ashraf M. Densification of Timber: A Review on the Process, Material Properties, and Application. J. Wood Sci. 2022;68:20. doi: 10.1186/s10086-022-02028-3. [DOI] [Google Scholar]
  • 19.Kocaefe D., Huang X., Kocaefe Y. Dimensional Stabilization of Wood. Curr. For. Rep. 2015;1:151–161. doi: 10.1007/s40725-015-0017-5. [DOI] [Google Scholar]
  • 20.Desalegn G., Tadesse W. Eucalyptus Species Management, History, Status and Trends in Ethiopia. Volume 15. Universidad Politécnica de Madrid; Madrid, Spain: 2010. Characteristics and Potential Uses of Eucalyptus Timber Species Grown in Ethiopia; p. 29. [Google Scholar]
  • 21.Marini L.J., Cavalheiro R.S., De Araujo V.A., Cortez-Barbosa J., de Campos C.I., Molina J.C., Silva D.A.L., Lahr F.A.R., Christoforo A.L. Estimation of Mechanical Properties in Eucalyptus Woods towards Physical and Anatomical Parameters. Constr. Build. Mater. 2022;352:128824. doi: 10.1016/j.conbuildmat.2022.128824. [DOI] [Google Scholar]
  • 22.Foelkel C. Papermaking Properties of Eucalyptus Trees, Woods, and Pulp Fibers. Eucalyptus Online Book & Newsletter. 2009. [(accessed on 26 February 2026)]. pp. 1–110. Available online: https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.eucalyptus.com.br/eucaliptos/ENG14.pdf&ved=2ahUKEwjL2tyCnPuSAxWnS2cHHcaYE4MQFnoECBgQAQ&usg=AOvVaw2ZgKFgjGbzZRBV_4PF46Iw.
  • 23.Shackleton R.T., Le Maitre D.C., Richardson D.M. Stakeholder Perceptions and Practices Regarding Prosopis (Mesquite) Invasions and Management in South Africa. Ambio. 2015;44:569–581. doi: 10.1007/s13280-014-0597-5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 24.Steenkamp H.E., Chown S.L. Influence of Dense Stands of an Exotic Tree, Prosopis glandulosa Benson, on a Savanna Dung Beetle (Coleoptera: Scarabaeinae) Assemblage in Southern Africa. Biol. Conserv. 1996;78:305–311. doi: 10.1016/S0006-3207(96)00047-X. [DOI] [Google Scholar]
  • 25.Shackleton R.T., Le Maitre D.C., Richardson D.M. Prosopis Invasions in South Africa: Population Structures and Impacts on Native Tree Population Stability. J. Arid Environ. 2015;114:70–78. doi: 10.1016/j.jaridenv.2014.11.006. [DOI] [Google Scholar]
  • 26.Malongweni S.O., Mrubata K., van Tol J., Abd Elbasit M.A.M., Harebottle D.M. Consequences of Invasive Prosopis (Mesquite) on Vegetation, Soil Health, Biodiversity, and Compliance of Management Practices in South African Rangelands: A Review. Grasses. 2025;4:2. doi: 10.3390/grasses4010002. [DOI] [Google Scholar]
  • 27.Chan J.M., Day P., Feely J., Thompson R., Little K.M., Norris C.H. Acacia mearnsii Industry Overview: Current Status, Key Research and Development Issues. South. For. J. For. Sci. 2015;77:19–30. [Google Scholar]
  • 28.Beck-Pay S.L., Laing M.D., Yobo K.S. Determination of Productivity, Rooting and Nutrient Profile of Acacia mearnsii (Black Wattle) Hedge Plants. Aust. For. 2023;86:58–67. doi: 10.1080/00049158.2023.2225348. [DOI] [Google Scholar]
  • 29.Gwate O., Mantel S.K., Gibson L.A., Munch Z., Gusha B., Palmer A.R. The Effects of Acacia mearnsii (Black Wattle) on Soil Chemistry and Grass Biomass Production in a South African Semi-Arid Rangeland: Implications for Rangeland Rehabilitation. Afr. J. Range Forage Sci. 2021;38:270–280. doi: 10.2989/10220119.2021.1875048. [DOI] [Google Scholar]
  • 30.Bairu M.W., Coetzer W.G., Amelework A.B. Tracing the Genetic Origin of Two Acacia mearnsii Seed Orchards in South Africa. S. Afr. J. Bot. 2019;126:70–77. doi: 10.1016/j.sajb.2019.07.045. [DOI] [Google Scholar]
  • 31.Bairu M.W., Amelework A.B., Coetzer W.G. Genetic Diversity and Population Structure of Six South African Acacia mearnsii Breeding Populations Based on SSR Markers. J. Plant Res. 2021;134:1243–1252. doi: 10.1007/s10265-021-01331-2. [DOI] [PubMed] [Google Scholar]
  • 32.Traynor H., Trevor N.Z., Tshabalala P.C.H. What’ll We Do with Wattle? The Dualistic Nature of Acacia mearnsii as Both a Resource and an Alien Invasive Species, Swaziland. Alternation. 2008;15:180–205. [Google Scholar]
  • 33.Xu B.-H., Wang T.-T., Yu K.-B., Zhao Y.-H., Zhang B. Physical and Mechanical Properties of Densified Poplar by Partial Delignification. Wood Mater. Sci. Eng. 2023;18:1285–1290. doi: 10.1080/17480272.2022.2128696. [DOI] [Google Scholar]
  • 34.Ricardo P., Hein G., Brancheriau L. Comparison between three-point and four-point flexural tests to determine wood strength of eucalyptus specimens. Maderas Cienc. Tecnol. 2018;20:333–342. [Google Scholar]
  • 35.Brancheriau L., Bailleres H., Guitard D. Comparison between Modulus of Elasticity Values Calculated Using 3 and 4 Point Bending Tests on Wooden Samples. Wood Sci. Technol. 2002;36:367–383. doi: 10.1007/s00226-002-0147-3. [DOI] [Google Scholar]
  • 36.Sydor M., Mirski R., Stuper-Szablewska K., Rogoziński T. Efficiency of Machine Sanding of Wood. Appl. Sci. 2021;11:2860. doi: 10.3390/app11062860. [DOI] [Google Scholar]
  • 37.Keskin H., Atar M., Togay A. Impacts of Impregnation with Imersol-Aqua on the Compression Strength of Some Solid Wood Materials. Constr. Build. Mater. 2008;22:1402–1408. doi: 10.1016/j.conbuildmat.2007.04.011. [DOI] [Google Scholar]
  • 38.Haurie L., Giraldo M.P., Lacasta A.M., Montón J., Sonnier R. Influence of Different Parameters in the Fire Behaviour of Seven Hardwood Species. Fire Saf. J. 2019;107:193–201. doi: 10.1016/j.firesaf.2018.08.002. [DOI] [Google Scholar]
  • 39.Hon D.N.S., Shiraishi N. Wood and Cellulosic Chemistry, Revised, and Expanded. CRC Press; Boca Raton, FL, USA: 2000. [Google Scholar]
  • 40.Dence C.W. In: The Determination of Lignin. Lin S.Y., Dence C.W., editors. Springer; Berlin/Heidelberg, Germany: 1992. pp. 33–61. (Springer Series in Wood Science). [Google Scholar]
  • 41.Brandizzi F. Ruzin SE. 1999. Plant Microtechnique and Microscopy. 322 Pp. Oxford, New York: Oxford University Press. £32.50 (Softback) Ann. Bot. 2000;86:708. doi: 10.1006/anbo.2000.1231. [DOI] [Google Scholar]
  • 42.Sandberg D., Kutnar A., Mantanis G. Wood Modification Technologies-a Review. Iforest-Biogeosciences For. 2017;10:895–908. doi: 10.3832/ifor2380-010. [DOI] [Google Scholar]
  • 43.Scholz G., Windeisen E., Liebner F., Bäucker E., Bues C.-T. Wood Anatomical Features and Chemical Composition of Prosopis Kuntzei from the Paraguayan Chaco. IAWA J. 2010;31:39–52. doi: 10.1163/22941932-90000004. [DOI] [Google Scholar]
  • 44.Carrillo A., Mayer I., Koch G., Hapla F. Wood Anatomical Characteristics and Chemical Composition Of Prosopis Laevigata Grown in the Northeast of Mexico. IAWA J. 2008;29:25–34. doi: 10.1163/22941932-90000167. [DOI] [Google Scholar]
  • 45.Harzmann L.J. Kurzer Grundriss Der Allgemeinen Tropenholzkunde. S. Hirzel Verlag Leipzig; Stuttgart, Germany: 1988. [Google Scholar]
  • 46.Neiva D.M., Godinho M.C., Simões R.M.S., Gominho J. Encouraging Invasive Acacia Control Strategies by Repurposing Their Wood Biomass Waste for Pulp and Paper Production. Forests. 2024;15:822. doi: 10.3390/f15050822. [DOI] [Google Scholar]
  • 47.Goets S.A., Kraaij T., Little K.M. Seed Bank and Growth Comparisons of Native (Virgilia Divaricata) and Invasive Alien (Acacia mearnsii and A. Melanoxylon) Plants: Implications for Conservation. PeerJ. 2018;6:e5466. doi: 10.7717/peerj.5466. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 48.Rojas-Sandoval J., Pasiecznik N. Acacia mearnsii (black wattle) CABI Compend. 2022 doi: 10.1079/cabicompendium.2326. [DOI] [Google Scholar]
  • 49.Nair K.P.P. 10—Wattle. In: Nair K.P.P., editor. The Agronomy and Economy of Important Tree Crops of the Developing World. Elsevier; London, UK: 2010. pp. 301–311. [Google Scholar]
  • 50.Pometti C.L., Pizzo B., Brunetti M., Macchioni N., Ewens M., Saidman B.O. Argentinean Native Wood Species: Physical and Mechanical Characterization of Some Prosopis Species and Acacia Aroma (Leguminosae; Mimosoideae) Bioresour. Technol. 2009;100:1999–2004. doi: 10.1016/j.biortech.2008.09.061. [DOI] [PubMed] [Google Scholar]
  • 51.Ngarava S. Invasive Alien Tree Species (Acacia mearnsii) Shaping the Energy Insecurity Complex in Rural South Africa. For. Policy Econ. 2025;178:103578. doi: 10.1016/j.forpol.2025.103578. [DOI] [Google Scholar]
  • 52.Nohegar A. Investigating the Effect of Air Pollution on the Accumulation of Heavy Metals in Plant Species in Arid and Desert Areas (Case Study: Acacia, Prosopis and Ziziphus C, in Kahouristan, Hormozgan Province) Environ. Eros. Res. J. 2022;12:1–18. [Google Scholar]
  • 53.Clulow A.D., Everson C.S., Gush M.B. The Long-Term Impact of Acacia mearnsii Trees on Evaporation, Streamflow and Ground Water Resources. Water Research Commission; Pretoria, South Africa: 2011. [Google Scholar]
  • 54.Denno K. High Voltage Engineering in Power Systems. CRC Press; Boca Raton, FL, USA: 2018. [Google Scholar]
  • 55.Ryan H.M., editor. High Voltage Engineering and Testing. Institution of Engineering and Technology; Stevenage, UK: 2001. [Google Scholar]
  • 56.Ray S. An Introduction to High Voltage Engineering. 2nd ed. PHI Learning Pvt. Ltd.; Delhi, India: 2013. [Google Scholar]
  • 57.Haddad A., Warne D.F. In: Advances in High Voltage Engineering. Haddad A., Warne D., editors. Volume 40 Institution of Engineering and Technology; Stevenage, UK: 2004. [Google Scholar]
  • 58.Große T., Müller U., Jakob M., Feist F., Luxner M.H., Knoebl W. Circularity Days Future Automotive Production Conference FAPC & Werkstoffsymposium. Springer; Berlin/Heidelberg, Germany: 2024. 3d-Shaped High-Strength Parts from Partially Delignified and Densified Wood—Introduction of the Project HolzF3; pp. 165–178. [Google Scholar]
  • 59.Paul B.N., Shukla S.R., Kelkar B.U., Nagraik P. Production Processes, Material Properties and Applications of Densified Wood: An Overview. J. Indian Acad. Wood Sci. 2024;21:235–254. doi: 10.1007/s13196-024-00348-z. [DOI] [Google Scholar]
  • 60.Gondaliya A.M., Hoque M., Raghunath S., Foster E.J. Green and Sustainable Fabrication of DES-Pretreated High-Strength Densified Wood. Wood Sci. Technol. 2024;58:1901–1923. doi: 10.1007/s00226-024-01594-7. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 61.Jalali S., Borges C.D.S.P., Carbas R.J.C., Marques E.A.D.S., Bordado J.C.M., da Silva L.F.M. Characterization of Densified Pine Wood and a Zero-Thickness Bio-Based Adhesive for Eco-Friendly Structural Applications. Materials. 2023;16:7147. doi: 10.3390/ma16227147. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 62.Huang W., Jin Y., Guo Y., Deng J., Yu H., He B. Fabrication of High-Performance Densified Wood via High-Pressure Steam Treatment and Hot-Pressing. Polymers. 2024;16:939. doi: 10.3390/polym16070939. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 63.Huang D., Li J., Li S., Hu J., Cao Z., Guo Y., Ding Y., Zhu M., Chen Y. Self-Densified Super-Strong Wood. J. Bioresour. Bioprod. 2025;10:199–208. doi: 10.1016/j.jobab.2025.03.001. [DOI] [Google Scholar]
  • 64.Li K., Zhao L., Ren J., He B. Interpretation of Strengthening Mechanism of Densified Wood from Supramolecular Structures. Molecules. 2022;27:4167. doi: 10.3390/molecules27134167. [DOI] [PMC free article] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

materials-19-00954-s001.zip (110.9KB, zip)

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Articles from Materials are provided here courtesy of Multidisciplinary Digital Publishing Institute (MDPI)

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