Hybrid Wood Composites with Improved Mechanical Strength and Fire Retardance Due to a Delignification–Mineralization–Densification Strategy

Abstract

The shortage of wood resources and the policy of logging restrictions have hindered the development of the wood industry. The development of fast-growing wood can effectively solve the problem of wood shortages and the discrepancy between supply and demand; however, the softness and poor strength of fast-growing wood make it difficult to use directly and restrict its applications. Meanwhile, the inflammability of wood is also a crucial hindrance to its application. In this work, hybrid wood composites with high strength and excellent fire retardance were developed by using a combined strategy of “delignification–mineralization–densification”. Delignification promoted the deposition of minerals inside the wood, and the mineralization process was able to significantly increase the fire retardance performance of the hybrid wood. The densification treatment made the wood and minerals closely packed, which was conducive to the improvement of the strength and fire retardance performance of the hybrid wood. The resulting hybrid wood composites showed enhanced mechanical strength (the tensile strength, flexural strength, and compressive strength were 180.6 MPa, 159.8 MPa, and 86.5 MPa, respectively) and outstanding fire retardance, and this strategy provided a feasible pathway towards the high-value application of fast-growing wood.

1. Introduction

Wood is a natural biomass material with the advantages of being light weight, strong, easy to process, and environmentally friendly; it is widely used in various fields, such as construction, transportation, furniture, and decoration [1,2,3,4,5]. However, with the fast development of the wood industry and the policy of restricting the exploitation of forest resources, wood resources have fallen into a shortage situation [6,7]. The development of artificial forests can effectively alleviate the contradiction between supply and demand. However, the softness, low density, low strength, susceptibility to decay, and deformation of fast-growing wood make it difficult to use directly and restrict the development of the wood industry [8,9]. Therefore, improving the optimal utilization of fast-growing wood has important practical significance for alleviating supply–demand discrepancies and protecting the natural ecological environment.

Wood modification usually uses physical, chemical, or combined physical–chemical methods to improve its properties, including dimensional stability, mechanical strength, decay resistance, flame retardancy, acid/alkali resistance, etc., thus achieving high-value utilization [10,11,12]. Modifications of fast-growing wood mainly include heat treatment, compression, resin impregnation, chemical grafting, wood mineralization, nanoparticle modification, etc., and each modification method has its own advantages. Among them, wood mineralization and compression, as modification technologies for wood reinforcement, have the advantages of high efficiency and environmental friendliness [13,14,15,16]. Wood mineralization and compression densification modification technology are important measures for improving the strength of fast-growing timber; this is of great practical importance and yields social benefits, giving full play to the advantages of fast-growing wood resources.

Biomineralization refers to the process of organisms generating inorganic minerals through the regulation of biomolecules, which have excellent mechanical properties due to their orderly organic–inorganic arrangement. Biomineralization is the organic unification of macroscopic properties and microstructures, which provides a new way of thinking about and approaching wood mineralization. Wood mineralization is usually carried out by calcification and silicification, of which calcification is the most commonly used method. The principle of mineralization is to impregnate the wood with a metal salt solution, which combines with the internal components of the wood (such as cellulose, hemicellulose, and lignin) to form a new composite material with water-, fire-, and corrosion-resistance properties. The advantages of mineralization treatments include stable material properties, environmental sustainability, and a long service life. At present, mineralization treatment is one of the most important technologies in the wood processing industry and is widely used in construction, furniture, flooring, ships, and other fields. However, many wood mineralization processes involve a two-step impregnation method, such as the preparation of calcium carbonate mineralized wood [17,18]. This involves using a solution of calcium ions and carbonate ions to impregnate the wood in order to perform in situ deposition mineralization. However, the minerals prepared using the two-step method are unevenly distributed inside and outside the wood, which affects the overall structure and mechanical strength of the mineralized wood. If the mineralized liquid is permeated into the pore structure of the wood in one step, then it is mineralized in situ using a heat treatment, an acid–alkali treatment, or another method, and the distribution of the minerals in the wood can be made more uniform [15]. In addition, the deposition depth and uniformity of the minerals in wood play an important role in improving its flame retardancy [19].

The mechanical strength of wood is positively correlated with its density, and the strength of wood can be improved by increasing the density of fast-growing wood. There are two main modification methods in common use: one densifies wood by extruding the cell lumens or cell gaps of wood that has been softened and pretreated; the other densifies wood by impregnating the cell lumens or cell gaps of the wood with organic or inorganic components. In a dry state, even a small force on the wood can lead to the destruction of the wood cell walls, so the wood needs to be pre-treated to soften and increase its plasticity before it is compacted by hot pressing. The most common pretreatment methods include boiling, high-temperature steam, microwave heating, impregnation, chemical delignification, etc. Each pretreatment method involves different processes, and their impacts on the performance of the wood after compression and densification are somewhat different [20,21,22]. The compressed dense wood prepared with a chemical delignification pretreatment has a high compression ratio and exhibits significant performance improvement. Hu’s group developed a wood compression method, which first partially removes the lignin from the wood and then compresses it. The results showed that the cell walls of the wood were completely collapsed and highly compacted. At the micro-level, the cellulose nanofibers of the wood were tightly arranged in an orderly configuration, greatly increasing the density of hydrogen bonds between the cellulose nanofibers. Consequently, it produced lightweight, ultra-strong, and super-tough wood, and all of the other mechanical properties, including compressive strength, toughness, stiffness, hardness, and impact resistance, were more than ten times that of natural wood [23]. Unfortunately, this super-strong wood still faced the problem of poor flame retardancy.

In this work, mineralized compressed wood with high strength and fire retardance properties was prepared using a strategy of “partial delignification–mineralization–densification”. The delignified wood skeleton was used as a biomimetic template to induce the mineralization assembly of the inorganic substance (magnesium ammonium phosphate, MAP), forming a hierarchically ordered organic–inorganic composite system. After compression densification, a highly dense mineralized wood was constructed, endowing the modified wood with high strength and excellent flame retardancy performance. The tensile, flexural, and compressive strengths of the mineralized compressed wood were 180.6 MPa, 159.8 MPa, and 86.5 MPa, respectively. The microstructure morphology, chemical composition, thermal properties, mechanical properties, and flame retardancy performance of the mineralized compressed wood were characterized and analyzed. This modification strategy of “partial delignification–mineralization–densification” can observably improve the strength and flame retardancy of fast-growing wood, which is of great significance for optimizing the utilization of fast-growing wood and broadening its application field.

2. Materials and Methods

2.1. Materials

Fast-growing poplar wood (Populus spp.) was obtained from Beijing Wood Works, China, and the sapwood was processed to a size of 100 mm (longitudinal) × 30 mm (tangential) × 10 mm (radial). Sodium hydroxide (NaOH, 95.0%), sodium sulfite (Na₂SO₃, 99.0%), magnesium sulfate (MgSO₄, 99.5%), potassium dihydrogen phosphate (KH₂PO₄, 99.0%), and aqueous ammonia (NH₃·H₂O, 25.0%) were procured from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The above reagents were used without further purification.

2.2. Partial Delignification of the Wood Samples

First, the wood samples were dried to absolute dryness in an oven at 100 °C and were then weighed. Then, the wood samples were submerged in a mixture of 2.5 M NaOH and 0.4 M Na₂SO₃ and boiled for 2 h. The wood was then washed several times with hot deionized water to remove residual reagents.

2.3. Wood Mineralization

MgSO₄ (199 g) and KH₂PO₄ (225 g) were added to deionized water (1.5 L) and stirred vigorously until the solution became clear. The mineralization solution was sealed for later use. The wood samples were placed in a vacuum-pressurized impregnation tank and vacuumed for 30 min, and then the mineralized liquid was sucked into the impregnation tank to immerse the wood samples; it was finally pressurized for 2 h. After that, the wood was removed and the surface solution was wiped off. The wood samples were placed in a sealed dryer, 150 mL NH₃·H₂O was measured into the beaker, and this beaker was placed in the sealed dryer. The whole system was sealed and kept for 24 h at 25 °C, and then the wood was dried to absolute dryness in an oven at 40 °C and weighed.

2.4. Mineralized Wood Compression

The mineralized wood samples were placed in a sealed desiccator containing deionized water for humidity adjustments. After 24 h, wood samples were put into a mold for compression. First, they were cold pressed at 20 MPa for 3 h, then hot pressed at 100 °C and 20 MPa for 2 h to obtain the MAP mineralized compressed wood.

2.5. Characterization

The microstructures of the natural wood, delignified wood (DW), mineralized delignified wood (MDW), and compressed mineralized delignified wood (CMDW) were observed using a scanning electron microscope (SEM, SU8010, Hitachi, Japan). Before characterizing the wood samples by SEM, the cross-section of the wood was repaired and smoothed with a rotary microtome; then, the smoothed wood surface was sprayed with gold, glued to the sample stage with a conductive adhesive, and observed by SEM at an accelerating voltage of 15 kV. The microstructures of the CMDW samples (the distributions of minerals in the CMDW) were analyzed using microcomputed tomography (Micro-CT, SkyScan1276, Bruker, Belgium). This technique uses X-rays to scan the sample and then converts the signal into a tomographic image that can clearly show the differences in the internal structure of the material. The corresponding spatial resolution of the X-ray radiography was about 6.0 μm/pixel. The chemical compositions of the wood and the DW samples were characterized using Fourier transform infrared (FTIR, iS50, Nicolet, Gibraltar, WI, USA) spectrometry in the attenuated total reflection (ATR) mode using a smart iTX™ diamond accessory. The scanning range was between 4000 and 450 cm⁻¹ with a resolution of 4 cm⁻¹. The X-ray diffraction (XRD, Model ULTIMA IV, Rigaku, Japan) patterns of the wood, DW, and MDW samples were tested on the Rigaku Ultima IV diffractometer using Cu Ka radiation in the 2θ range 10–80° at a 5° min⁻¹ scan rate. The thermal degradation behavior of the wood, DW, and MDW samples was assessed using thermogravimetric analysis (TGA, TGA55, and TA Instruments). The wood sample was tested under a nitrogen atmosphere in the range of 30–600 °C at a heating rate of 10 °C min⁻¹. The surface hardness of the wood, DW, MDW, and CMDW samples was tested using a TH210 durometer and expressed as Shore D hardness. Each sample was measured at 20 points and the average value was taken as the final hardness value. The tensile, flexural, and compressive strengths of the wood, DW, MDW, and CMDW samples were assessed using a universal testing machine (SHIMADZU AGS-X) at ambient conditions (25 °C and 50% RH) with a 10 kN load cell. The size of the sample for the tensile test was 100 mm (longitudinal) × 10 mm (tangential) × 3 mm (radial). The tensile test was carried out in the longitudinal direction of the wood, and the tensile speed was set to 5 mm min⁻¹. The size of the sample for the flexural test was 100 mm × 10 mm × 3 mm, the feed speed of the upper indenter was set to 5 mm min⁻¹, and the span was 6 mm. The size of the sample for the compressive test was 20 mm (longitudinal) × 5 mm (tangential) × 3 mm (radial). The compressive test was conducted in the longitudinal direction of the wood, and the compressive speed was set to 5 mm min⁻¹. Six samples from each group were tested for mechanical properties. A cone calorimeter (CONE) was used to assess the combustion performance of the CDW and CMDW samples at a heat flux of 35 kW·m⁻² according to ISO5660. Prior to testing, the wood samples were baked to an absolutely dry state. The CONE test was performed on three samples from each group.

3. Result Analysis

3.1. Microstructures and Chemical Components

The structure of a material determines its performance. Wood is a naturally porous polymeric material consisting of cellulose, hemicellulose, and lignin. Figure 1 shows the microstructure morphology of the cross-section of the natural wood, DW, MDW, and CMDW samples. The cross-section of the natural wood exhibited a honeycomb-like cell structure consisting of cell walls and cell lumens, and the cell walls slightly shrank after partial delignification treatment. After in situ mineralization with MAP, the wood cell lumens were filled with a large number of mineral components. After densification, the wood components and mineral components were squeezed against each other. Closely stacked wood and mineral components were expected to improve the density and surface hardness of the CMDW, as well as the mechanical properties and fire-retardant properties of the wood.

In order to further investigate the distribution of minerals in the structure of the CMDW sample, a micro-CT characterization was conducted for CMDW (with a size of about 10 mm × 8 mm × 4 mm), with a scanning resolution of 6.0 μm. Figure 2a,b show the scans of the three-section position schematic and the actual position coordinates of the three exported scanned sections, and the three exported scanned sections are shown in Figure 2c,d. The dark gray area represented the wood components, and the bright white areas represented inorganic mineral components, which can be clearly distinguished. This indicates that the wood inside the structure is filled with a large number of inorganic mineral components, demonstrating that the mineralization solution can penetrate into the interior of the wood and successfully mineralize in situ. In fact, the mineralized precursor solution was first impregnated into the interior of the wood, and then the mineral was induced to grow in situ in the wood via ammonia fumigation. This prevented the problem of the uneven distribution of minerals inside and outside the wood that is caused by the traditional two-step solution impregnation mineralization method, and ammonia fumigation helped the mineralization to occur deeper in the wood. This was beneficial for improving the mechanical performance and flame retardancy of the CMDW.

Cellulose, hemicellulose, and lignin are the three major elements that make up the chemical composition of wood. Hu et al. demonstrated that the compressed dense wood prepared using a chemical delignification pretreatment had high levels of compressibility and exhibited significant performance enhancement [23]. Therefore, the wood prepared in this experiment was also delignified to partially remove the hemicellulose and lignin components. In addition, the pore structure of the delignified wood was improved, which facilitated the dispersion and penetration of the subsequent mineralization solution in the wood.

In the FTIR analysis, the absorption peaks at 1738 cm⁻¹ and 1235 cm⁻¹ in the natural wood were assigned to the carboxyl groups of lignin and hemicellulose or the uronic acid groups of hemicellulose [24,25], which disappeared or decreased in the DW after delignification. However, the characteristic peaks at 1593, 1506, and 1453 cm⁻¹ were ascribed to the aromatic vibrations of lignin [26,27], which exhibited no obvious change. These results demonstrate that more hemicellulose components in wood were removed, while fewer lignin components were removed after two hours of delignification.

The crystalline structures of the natural wood, DW, and MDW samples were characterized using XRD, as shown in Figure 3b,c. For the natural wood and DW samples, the diffraction peaks at 15.9°, 22.7°, and 34.5° were attributed to the crystal planes (101), (002), and (040) of cellulose, respectively [28,29]. Compared to the natural wood, the peak intensity at 15.9° and 22.7° became stronger in the DW, which was mainly due to the increase in the relative content of cellulose with the partial removal of lignin and hemicellulose. For the XRD spectrum of the MDW sample, the numerous peaks demonstrated the presence of MAP minerals [30].

The characterization of the above microstructure and chemical composition demonstrated the more uniform deposition of MAP minerals inside the partially delignified wood. Meanwhile, this was a prerequisite for the high strength and flame retardancy of the CMDW sample.

3.2. Mechanical Performance

The strength of wood is, to some extent, closely related to its density, and the variation in the wood density in each group also reflects the degree of inorganic mineralization and densification. The density results for natural wood, DW, MDW, and CMDW are shown in Figure 4a. The density of the natural wood was 0.497 g/cm³, which decreased to 0.438 g/cm³ after a partial delignification treatment for two hours, indicating that a small amount of hemicellulose and lignin components were removed. After mineralization, the density of the MDW was increased to 0.628 g/cm³, which further increased to 1.357 g/cm³ after the densification treatment. The high density represents the dense accumulation between the wood composition and MAP minerals of the CMDW sample, which could play a positive role in its mechanical strength. The thickness of the CMDW after compression was approximately one-third the thickness of the MDW.

The surface hardness of different wood samples is shown in Figure 4b; there is also a correlation between the hardness of wood and its density. The surface hardness of the natural wood, DW, and MDW was 58.7, 55.6, and 58.2, respectively (the unit is Shore D), values that differed only a little. After mineralization and compression, the value of the surface hardness of the CMDW sample was significantly increased to 88.1, which was 50.1% higher than that of natural wood.

Improving the mechanical properties of fast-growing wood so that it can be used as a structural load-bearing material is important for its high-value utilization; it is also a prerequisite for replacing scarce high-quality wood resources. The tensile strength, flexural strength, and compressive strength, and the corresponding elasticity modulus of the natural wood, DW, MDW, and CMDW samples were tested, and the results are shown in Figure 5. The tensile, flexural, and compressive strengths of the natural wood were approximately 83.6, 85.8, and 59.9 MPa, respectively, and the mechanical performance of the DW samples was slightly reduced after delignification because of the partial removal of the hemicellulose and lignin components. After the mineralization treatment, the mechanical performance of MDW was slightly improved. After compression densification, the mechanical properties of the CMDW samples were significantly improved, with tensile, flexural, and compressive strengths of 180.6 MPa, 159.8 MPa, and 96.5 MPa, which were approximately 2.2, 1.9, and 1.6 times those of natural wood, respectively. Meanwhile, the corresponding moduli of the CMDW sample were 3.9, 3.4, and 1.9 times those of natural wood, respectively. The results indicate that the strength and modulus of the modified wood were significantly enhanced after mineralization and compression. Therefore, the delignification–mineralization–densification strategy can significantly improve the density, surface hardness, and mechanical strength of fast-growing wood.

3.3. Thermal Degradation Properties

The thermal degradation properties of wood, DW, and MDW were assessed via thermogravimetric analysis. For natural wood, there are three main components (cellulose, hemicellulose, and lignin) of degradation with the increase in temperature. Hemicellulose decomposition was observed from 250 °C to 310 °C [31]. The cellulose degradation temperature was approximately 310 °C−390 °C, with a maximum peak of about 360 °C−370 °C [32]. The lignin exhibited high levels of structural diversity, and its degradation was slow and generally occurred from 200 °C−900 °C, with low levels of weight loss [33]. Compared to the natural wood, the peak degradation temperature of the cellulose of the DW sample decreased from 362 °C to 345 °C, and the residual weight was slightly increased; this was caused by the partial removal of the hemicellulose and lignin components. However, for the MDW samples, a new degradation peak near 200 °C occurred, resulting from the conversion of mineralized MAP to magnesium hydrogen phosphate (MHP). The MAP decomposed at a lower temperature than wood, absorbing heat and releasing non-combustible gases and amorphous MgHPO₄ from the degradation, promoting the formation of insulating carbon [34]. Meanwhile, the cellulose degradation temperature was further reduced to about 283 °C. Notably, the residual weight of MDW at 600 °C remained at 44%, much higher than the values of 16% for DW and 13% for natural wood. This may be due to the enhanced char formation caused by the interaction of the inorganic mineral components of MDW with the organic wood components, acting as an insulating fire barrier to prevent internal wood burning (Figure 6).

3.4. Fire Retardance Properties

Cone calorimetry (CONE) is one of the most suitable test instruments for describing materials’ combustion behaviors. Its test environment is close to the burning environment of a real fire, and the obtained test data can be used to assess the burning behavior of materials in a fire [35]. To evaluate the fire retardant performance of the CMDW sample, a CONE measurement was carried out, and the flame retardancy of the CDW sample (direct compression after partial delignification) was also tested for comparison. The CDW sample exhibited an HRR curve typical of thermally thick charring materials, showing a peak HRR immediately after ignition [36,37] (Figure 7b). The second major HRR peak occurred after 155 s, most likely due to the cracking of the char layer [38]. The CMDW sample also showed two HRR peaks, with the second one being significantly lower and delayed. The THR of the CMDW was only 76% of that of natural wood, 38.69 MJ/m² vs. 50.41 MJ/m² (Figure 7c). In a fire disaster, smoke release and carbon monoxide (CO) production are also key parameters. The SPR of the CDW was consistent with the HRR, reaching a maximum SPR of 0.0123 m²/s, a TSP of 0.94 m², and a TSR of 103.1 m²/m². The CMDW sample exhibited significantly decreased SPR peaks, and the TSP and TSR were highly suppressed (with a TSP of 0.17 m² and a TSR of 14.8 m²/m²) compared to CDW (Figure 7d,f,g). The residual structures of CDW and CMDW after CONE testing are shown in Figure 7a,e, respectively. The CDW disintegrated into small fragments, whereas the CMDW had a compact and robust char residue. The residual yield of the CDW was 0.84%, and that of the CMDW was 31.03%. The CDW without MAP did not show any char formation and completely decomposed into ash, whereas the formation of the char layer was due to the presence of MAP minerals in the wood. The char formed an insulating layer, which was responsible for delaying oxygen diversion, insulation, and reducing the volatilization of combustible gases. Meanwhile, the CMDW also exhibited excellent mechanical strength, effectively preventing the collapse of or damage to the wood structures, providing valuable rescue time in the event of a fire.

Therefore, in addition to high strength and high surface hardness, CMDW also has excellent flame-retardant properties, which significantly improves the defects of fast-growing wood in terms of its softness, low strength, and easy combustion; it is significant for broadening the applications of wood and realizing its high-value applications.

4. Conclusions

In summary, a mineralized compressed wood with improved mechanical strength and fire retardance properties was fabricated via partial delignification, mineralization, and densification processes. The partial delignification promoted the penetration and deposition of mineralized liquids in the wood, while the ammonia fumigation initiated MAP growth without bringing the wood into direct contact with alkaline solutions. The formation of minerals within the wood structure made bulk treatments possible, which resulted in a good depth of penetration of the mineral particles, providing fire-retardant properties. The compression treatment caused the inorganic minerals and organic wood components to be closely stacked, which was conducive to improving the strength and flame-retardant performance of the modified wood. The resulting hybrid wood exhibited improved mechanical strength; compared to the natural wood, the tensile, flexural, and compressive strengths of CMDW increased by 2.2, 1.9, and 1.6 times, respectively. The increased char layer of the hybrid wood was highly beneficial for wood applications as it acted as an insulating barrier, delaying the burning process in the event of a fire. This wood modification strategy of “partial delignification, mineralization, and densification” provides a feasible scheme for the fabrication of high-strength and high-performance wood composite materials; additionally, it provides an innovative scheme for the application of fast-growing wood.