Flame-retardant wood by enzyme-induced precipitation of calcium carbonate and method for its production

Abstract

The application relates to the technical field of wood functional modification, in particular to flame-retardant wood prepared by enzyme-induced calcium carbonate precipitation and a preparation method thereof.The method comprises the following steps: S1, performing partial delignification treatment on wood; S2, immersing the wood in a first solution containing urease under vacuum conditions to enable the urease to penetrate into the interior of the wood; S3, immersing the wood treated in S2 in a second solution containing urea and calcium ions, and performing immersion under vacuum conditions, so that the urease catalyzes the hydrolysis of urea to generate carbonate ions, and the carbonate ions react with calcium ions to generate calcium carbonate precipitation; and S4, performing water bath reaction at 25-30 DEG C and pH 7.8-8.2. S1-S4 is a cycle, and after being repeated for 2-4 times, the flame-retardant wood is obtained through maintenance and drying. Through the synergy of partial delignification and step-by-step immersion plus water bath circulation, deep and uniform deposition of calcium carbonate is realized, and the obtained wood has an extreme oxygen index of 59%, and the flame-retardant performance is obviously improved.

Description

Technical Field

[0001] This application relates to the field of wood functional modification technology, specifically to a flame-retardant wood with enzyme-induced calcium carbonate precipitation and its preparation method. Background Technology

[0002] Wood, as a natural and renewable biomass material, is widely used in building structures, furniture manufacturing, and interior decoration due to its low density, high specific strength, good processing properties, and excellent environmental adaptability. However, wood is inherently a combustible polymer material. Its main components, cellulose, hemicellulose, and lignin, are all composed of carbon, hydrogen, and oxygen elements. When heated, they are prone to dehydration, chain scission, and cracking reactions, producing a large amount of combustible volatiles, leading to rapid combustion and continuous heat release. Therefore, improving the flame-retardant properties of wood is a key issue in expanding its applications in high-safety-level fields.

[0003] Currently, methods to improve the flame retardant properties of wood mainly include flame retardant impregnation, surface coating, and structural modification. While traditional halogen-based flame retardants are highly efficient, they may release corrosive gases and toxic substances during combustion. Halogen-free flame retardant systems, such as phosphorus-based, nitrogen-based, boron-based, and silicon-based flame retardants, can improve combustion behavior through mechanisms such as promoting char formation, free radical capture, and physical insulation, but they still have drawbacks such as high addition amounts and easy migration and loss of flame retardants. In recent years, the construction of green flame retardant systems using biomass resources has gradually gained attention. Among these, biomimetic mineralization technology based on the in-situ deposition of inorganic minerals within wood is considered a promising method for structural regulation.

[0004] Enzyme-induced calcium carbonate precipitation (EICP) is a green mineralization technology based on the urease-catalyzed hydrolysis of urea. Under the action of urease, urea is hydrolyzed to generate carbonate and ammonium ions. The carbonate ions further combine with calcium ions to form calcium carbonate precipitate in situ. Compared with traditional chemical precipitation methods, EICP has advantages such as mild reaction conditions, no need for a strong alkaline environment, and theoretically controllable mineral formation location and rate, thus attracting attention in the field of wood flame retardant modification.

[0005] However, existing EICP mineralization studies on wood typically involve co-immersion of urease, urea, and calcium salts (i.e., the co-bath method) or multiple cycles of treatment. While such methods can introduce calcium carbonate into the wood, because the catalytic and precipitation reactions occur simultaneously and rapidly in the same space, calcium carbonate tends to preferentially nucleate and deposit in large quantities on the wood surface and at vessel openings, forming a relatively dense surface deposit layer. This surface deposit layer blocks the pores and mass transfer channels of the wood, hindering the penetration of reactants into the deeper layers of the wood. Consequently, mineralization is mainly concentrated in the surface and shallow areas, making it difficult to effectively fill the internal cell cavities and vessels. This uneven deposition structure limits the extent to which flame-retardant properties can be improved. For example, in existing reports of biomimetic enzyme-induced mineralization methods, after multiple cycles of treatment, the limiting oxygen index (LOI) of the wood only increased from approximately 19.8% to 24.4%, still within the combustible material category, failing to meet the performance requirements for flame-retardant materials.

[0006] Therefore, how to effectively control the deposition location of calcium carbonate during the EICP process, avoid rapid nucleation and pore blockage on the surface, and enable calcium carbonate to achieve deep and uniform deposition inside the wood, thereby significantly improving the flame retardant properties of the wood, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] To address the technical problem in the prior art where EICP mineralization results in limited improvement in flame retardant performance due to calcium carbonate deposition only on the surface of wood, this application provides a method for preparing flame-retardant wood by enzyme-induced calcium carbonate precipitation, comprising the following steps: S1. Partially delignify the wood to obtain delignified wood; S2. The delignified wood is immersed in a first solution containing urease under vacuum conditions to allow the urease to penetrate into the interior of the wood. S3. The wood treated in S2 is immersed in a second solution containing urea and calcium ions and impregnated under vacuum conditions. Urease catalyzes the hydrolysis of urea to generate carbonate ions. The carbonate ions react with calcium ions to generate calcium carbonate precipitate in situ inside the wood. S4. The wood from S3 is reacted with the solution in a water bath at 25-30°C and pH 7.8-8.2. S1-S4 constitute one treatment cycle. After repeating the cycle 2-4 times, the wood is cured and dried to obtain flame-retardant wood.

[0008] In some embodiments, the partial delignification treatment in S1 includes: immersing the wood in an alkaline solution to partially remove lignin and hemicellulose, then washing it to neutral and drying it.

[0009] In some embodiments, the alkaline solution is a sodium hydroxide solution with a concentration of 0.5–2 wt% and a treatment time of 0.5–4 hours.

[0010] In some embodiments, the urease in S2 is a plant-derived urease.

[0011] In some embodiments, the plant-derived urease is a urease solution extracted from soybean flour.

[0012] In some embodiments, the urea concentration in the second solution in S3 is 0.5–1.0 mol / L, and the calcium ion concentration is 0.5–1.5 mol / L.

[0013] In some embodiments, the vacuum impregnation conditions in S2 and / or S3 are as follows: maintaining a negative pressure of -0.06 MPa relative to atmospheric pressure for 25 minutes, and then maintaining the negative pressure for 2 hours.

[0014] In some embodiments, the water bath reaction time in S4 is 0.5 to 4 hours.

[0015] In some embodiments, the first solution in S2 is the plant-derived urease solution itself, or a solution obtained by diluting or concentrating the plant-derived urease solution.

[0016] This application also provides a flame-retardant wood, prepared by the method described in any of the above-mentioned methods.

[0017] Based on the above technical solution, the method for preparing flame-retardant wood by enzyme-induced calcium carbonate precipitation provided in this application, compared with the prior art, uses a multi-cycle process of partial delignification treatment and stepwise impregnation followed by water bath reaction to allow calcium carbonate to be deposited deeply, uniformly and fully inside the wood cell cavity, while avoiding pore blockage caused by rapid nucleation on the surface, thereby constructing a denser and continuous inorganic flame-retardant network and significantly improving the flame-retardant performance of the wood. Attached Figure Description

[0018] The accompanying drawings, which are provided to further illustrate this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application.

[0019] Figure 1 This is a process flow diagram for preparing flame-retardant wood according to the embodiments and comparative examples of this application.

[0020] Figure 2The images show a comparison of the SEM morphology and EDS elemental distribution of untreated wood (RW) and flame-retardant wood (TDW) from Example 1. (a) is a cross-sectional SEM image of RW, (b) is a cross-sectional SEM image of TDW, (c) is a longitudinal SEM image of RW, (d) is a longitudinal SEM image of TDW, (e) is a longitudinal SEM image of DW, (f) is an SEM image showing the pores of the DW wood grain, (g) is an SEM image showing the pores of the DW wood grain, (h) is an EDS scan of RW, and (i) is an EDS scan of TDW.

[0021] Figure 3 The FTIR spectrum (a) and XRD diffraction pattern (b) of the wood from each treatment group are shown.

[0022] Figure 4 The TG and DTG curves of wood in each treatment group under nitrogen and air atmospheres are shown, where (a) is the TG curve under air atmosphere, (b) is the DTG curve under air atmosphere, (c) is the TG curve under nitrogen atmosphere, and (d) is the DTG curve under nitrogen atmosphere.

[0023] Figure 5 The curves for heat release rate (a), total heat release (b), smoke release rate (c), and total smoke release (d) of the wood cone calorimetry test for each treatment group are shown.

[0024] Figure 6 The images show the vertical burning test results of some treated groups of wood according to UL-94 standard, where (a) is RW, (b) is DW, (c) is CRW, (d) is CDW, (e) is TRW, and (f) is TDW. Detailed Implementation

[0025] The present application will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings.

[0026] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0027] Source of materials Poplar (Populus spp.) was purchased from Shijiazhuang, Hebei, China. Sections free of knots, cracks, decay, and fiber tilt defects were selected and processed into specimens of the required size. Sections free of knots, cracks, decay, and fiber tilt defects were specifically chosen to minimize the impact of natural structural variations on the test results.

[0028] The soybean flour was purchased from Mudanjiang City, Heilongjiang Province, China, and was used as a raw material for the extraction of plant-derived urease.

[0029] Urea (CH4N2O), anhydrous calcium chloride (CaCl2), and sodium hydroxide (NaOH) were all of analytical grade and purchased from Nanjing Guangxin Biotechnology Co., Ltd. All chemical reagents were used directly without further purification.

[0030] Wood drying process After the sample is processed, it is first placed in an 80℃ forced-air drying oven for 24 hours for pre-drying, and then the temperature is raised to 103℃ for another 24 hours until the mass is constant. After drying, the sample is cooled to room temperature and its oven-dry mass m0 is recorded for later use.

[0031] Preparation of plant-derived urease solution Soybean flour that has passed through a 100-mesh sieve was mixed with deionized water at a mass-to-volume ratio of 1:10 (1 L of deionized water for every 100 g of soybean flour). After thorough stirring, the mixture was allowed to stand at a low temperature for 12 hours. Subsequently, it was centrifuged at 25°C and 3000 r / min for 15 minutes, and the supernatant was collected. This supernatant was the plant-derived urease solution with a concentration of 100 g / L and was used for subsequent mineralization experiments within 5 hours to avoid a decrease in enzyme activity due to long-term storage.

[0032] General methods for preparing partially delignified wood The dried wood samples were completely immersed in a 1 wt% NaOH solution and treated at room temperature for 2 hours. After treatment, they were thoroughly rinsed and soaked with deionized water, with the water changed every 2 hours until the pH of the washing solution stabilized at 7.0 ± 0.2. Subsequently, the wood was dried in an oven at 103°C to constant weight to obtain partially lignin-treated wood.

[0033] As one of the characterization indicators of the partial delignification treatment effect, the leaching rate of wood before and after treatment was measured. The leaching rate was calculated according to the following formula: Leaching rate = (m0 - m1) / m0 × 100% Where m0 and m1 are the oven-dry masses of the wood before and after soaking in sodium hydroxide solution, respectively.

[0034] For mineralized timber, the percentage increase in weight (WPG) is calculated using the following formula: WPG = (m2 - m0) / m0 × 100% Where m2 is the oven-dry mass after impregnation with mineralization solution and drying, and m0 is the oven-dry mass before treatment.

[0035] Example 1 (TDW) This embodiment provides a method for preparing flame-retardant wood based on a two-step partial delignification treatment combined with a water bath reaction. The specific steps are as follows: S1. Select poplar wood samples after drying and pretreat them according to the general preparation method of some delignified wood to obtain delignified wood.

[0036] S2. Use the above-mentioned plant-derived urease solution as the first solution. Immerse the delignified wood obtained in S1 into the first solution and perform impregnation treatment under vacuum conditions: first, evacuate to -0.06 MPa relative to atmospheric pressure and maintain for 25 minutes, then maintain this negative pressure and continue impregnation for 2 hours to allow the urease to fully penetrate and anchor in the internal pore structure of the wood.

[0037] S3. After removing the wood treated in S2 and removing the residual liquid on the surface, directly immerse it in a bonding solution (i.e., the second solution) containing 1.0 mol / L urea and 1.0 mol / L CaCl2. Apply the same vacuum impregnation process as in S2 (vacuuming at -0.06 MPa for 25 minutes and maintaining negative pressure for 2 hours). This allows the urease that penetrates into the wood to catalyze the hydrolysis of urea to generate carbonate ions. The carbonate ions then react with calcium ions to form calcium carbonate precipitate in situ inside the wood matrix.

[0038] S4. Place the container containing the wood sample and the second solution used for impregnation in S3 in a constant temperature water bath, control the solution temperature at 25-30℃ and the pH value at 8.0, and allow it to react for 2 hours.

[0039] The above S1 to S4 constitute a complete treatment cycle, which is repeated 3 times. After all cycles are completed, the wood sample is taken out and placed in a standard curing chamber for 7 days of wet curing at room temperature to allow calcium carbonate to fully precipitate and stabilize; then it is dried in an oven at 103℃ to constant weight to obtain flame-retardant wood, denoted as TDW.

[0040] As a control indicator of mineralization effect, the weight gain percentage (WPG) of the TDW sample was measured to be 67.24%, which was significantly higher than that of the TRW group without delignification (36.19%), indicating that partial delignification treatment can effectively improve the wood’s adsorption and capacity for mineralized components.

[0041] Comparative Example 1 (RW) The poplar wood was not treated in any way, but only underwent routine drying for later use, and is designated as RW.

[0042] Comparative Example 2 (DW) The poplar wood underwent partial delignification treatment using the same method as the general preparation method for partially delignified wood described in S1 of Example 1, but without subsequent mineralization treatment; this sample was designated DW. The leaching rate of the DW sample was measured to be 3.74%.

[0043] Comparative Example 3 (TRW) The same S2-S4 operation and cyclic maintenance process as in Example 1 were adopted. The difference was that the wood was not partially deligninized. The untreated poplar wood after drying was used directly for the subsequent steps. Other conditions were the same as in Example 1. This was referred to as TRW.

[0044] Comparative Example 4 (CRW) Untreated poplar was mineralized using a single-bath method: a plant-derived urease solution was mixed with a cementing solution containing 1.0 mol / L urea and 1.0 mol / L CaCl2 at a volume ratio of 1:1. The untreated poplar was then immersed in the mixed solution, and the same vacuum impregnation process as in S2 of Example 1 was applied (vacuuming at -0.06 MPa for 25 minutes, maintaining negative pressure for 4 hours, which constitutes one impregnation cycle, and a total of 3 impregnation cycles were repeated). After each impregnation cycle, the sample was removed and the residual solution on the surface was removed. The sample was then wet-cured for 7 days and dried at 103°C to constant weight, denoted as CRW.

[0045] Comparative Example 5 (CDW) Partially delignified poplar was mineralized using a co-bath method: the wood was first partially delignified according to S1 in Example 1, and then mineralized using the same co-bath method as Comparative Example 4, denoted as CDW. The leaching rate of the CDW sample was measured to be 3.85%.

[0046] Comparative Example 6 (PRW) Untreated poplar was mineralized using a pre-hydrolysis method: A plant-derived urease solution was first mixed with a 1.0 mol / L urea solution at a volume ratio of 1:1 and allowed to react for 4 hours to generate carbonate ions, yielding a pre-treated solution. Untreated poplar was then immersed in this pre-treated solution under the same vacuum impregnation process as in Example 1, S2 (-0.06 MPa vacuum for 25 minutes, maintaining negative pressure for 2 hours). After removing the sample and removing residual liquid from the surface, it was then immersed in a 1.0 mol / L CaCl2 solution under the same vacuum-assisted impregnation conditions. This process constituted one cycle and was repeated three times. The sample was then wet-cured for 7 days and dried at 103°C to constant weight, denoted as PRW.

[0047] Comparative Example 7 (PDW) Partially delignified poplar was mineralized using a pre-hydrolysis method: the wood was first partially delignified according to S1 in Example 1, and then mineralized using the same pre-hydrolysis method as Comparative Example 6, denoted as PDW. The leaching rate of the PDW sample was measured to be 3.92%.

[0048] Performance Testing and Results Discussion The following is a systematic performance characterization and comparative analysis of the samples prepared in Example 1 and Comparative Examples 1-7.

[0049] 1. Process Route Description Figure 1 This is a process flow diagram for preparing flame-retardant wood according to this application. (Refer to...) Figure 1 The core process route of this application is: "partial delignification pretreatment → first step vacuum impregnation (urease) → second step vacuum impregnation (urea + calcium salt) → constant temperature water bath reaction", and this process is repeated multiple times, and finally curing and drying are carried out.

[0050] This process involves first introducing urease separately and allowing it to penetrate and anchor within the wood pores, followed by the introduction of a cementing solution containing urea and calcium ions. This allows the urease-catalyzed reaction and the calcium carbonate precipitation reaction to be separated in time and space. Carbonate ions, generated in situ within the wood pores, immediately react with calcium ions to precipitate. A water bath reaction step, under suitable temperature and pH conditions, further promotes the thorough in-situ precipitation. The cyclic operation further increases the amount of mineral deposition and strengthens the flame-retardant network. Partial delignification pretreatment creates unobstructed mass transfer channels for the deep penetration of reactants. Compared to traditional co-bath or pre-hydrolysis methods, this process effectively avoids the problem of rapid surface deposition and pore blockage of minerals, ensuring deep and uniform mineralization of calcium carbonate within the wood.

[0051] 1-1. Analysis of Leaching Rate and Percentage Weight Gain Table 1 lists the leaching rates of wood before and after partial delignification treatment and the percentage increase in weight after mineralization treatment. After mild alkali treatment, the leaching rates of DW, PDW, CDW, and TDW were 3.74%, 3.92%, 3.85%, and 2.94%, respectively, indicating that partial delignification treatment removed only a small amount of wood components, the treatment conditions were mild, and there was no significant damage to the overall structure of the wood.

[0052] After mineralization treatment, the WPG of the non-delignified groups (PRW, CRW, TRW) were 30.77%, 20.19%, and 36.19%, respectively, while the WPG of the delignified groups (PDW, CDW, TDW) increased to 35.29%, 21.15%, and 67.24%, respectively. Among them, the WPG of TDW reached 67.24%, the highest among all groups, and was about 86% higher than that of the TRW group, which followed the same pathway but was not delignified. This indicates that partial delignification treatment significantly increased the wood's capacity to accommodate mineralized components, laying the structural foundation for deep and large-scale deposition of calcium carbonate.

[0053] Table 1. Weight changes of wood before and after modification sample <![CDATA[m0 / g]]> <![CDATA[m1 / g]]> Leaching rate / % <![CDATA[m2 / g]]> WPG / % RW 10.4 / / / / DW 10.7 10.3 3.74 / / PRW 10.4 / / 13.6 30.77 PDW 10.2 9.8 3.92 13.8 35.29 CRW 10.8 / / 12.5 20.19 CDW 10.4 10.0 3.85 12.6 21.15 TRW 10.5 / / 14.3 36.19 TDW 10.4 10.1 2.94 17.46 67.24 (Note: In the table, m0 and m1 are the oven-dry weights before and after partial delignification treatment, respectively, and m2 is the oven-dry weight after mineralization treatment and drying; " / " indicates that the group did not undergo the corresponding treatment or measurement.) 2. Microscopic morphology and elemental distribution analysis (SEM-EDS) The microstructure of the examples and comparative samples was observed using a scanning electron microscope (SEM, Zeiss Sigma 300), and elemental distribution analysis was performed using energy-dispersive X-ray spectroscopy (EDS). Before testing, all samples were dried, fixed onto conductive adhesive, and sputtered with gold to improve conductivity.

[0054] The results are as follows Figure 2 As shown in the SEM image, the untreated RW wood exhibits clear cell cavities, vessels, and pit structures, with smooth cell wall surfaces, exhibiting a typical poplar porosity structure. The SEM image of Example 1 (TDW) shows a large amount of granular deposits adhering to the cell wall surface and filling the cell cavities and some vessels, making the originally smooth cell wall surface rough, indicating that the in-situ reaction successfully generated solid inorganic matter. EDS surface scanning results further verified the introduction of mineral elements. The EDS surface scan of RW shows that RW is mainly composed of C and O elements; the EDS surface scan of TDW shows that after processing with the method of this application, the TDW sample exhibits a significant and relatively uniform Ca element signal in the wood tissue region, and has a high correlation with the spatial distribution of O elements. This indicates that calcium-containing inorganic matter has been successfully introduced into the interior of the wood, rather than merely accumulating on the surface, laying a crucial structural foundation for achieving a significant flame-retardant effect.

[0055] 3. Chemical structure and crystal phase analysis (FTIR, XRD) Fourier transform infrared (FTIR, Tenfor II) spectra of the examples and comparative samples were performed in the range of 400–4000 cm⁻¹. -1 Within range and X-ray diffraction (XRD, Rigaku SmartLab-9kw, scan rate 5° / min, scan range 5°~70°) analysis.

[0056] Figure 3 (a) The FTIR spectra show that all samples exhibit typical characteristic peaks of wood (approximately 3300–3500 cm⁻¹). -1 The -OH stretching vibration at approximately 2900 cm⁻¹ -1 CH stretching vibration at 1000–1100 cm -1 (CO / COC vibration at the location). Compared to RW and DW, Example 1 (TDW) and other mineralized treatment groups showed improvement at approximately 1420 cm⁻¹. -1 870 cm -1 and 710 cm-1 The appearance of new or significantly enhanced absorption peaks nearby are attributed to CO3. 2- The presence of carbonate minerals was confirmed by asymmetric stretching vibration, out-of-plane bending vibration, and in-plane bending vibration. Among them, the carbonate characteristic peaks of the two-step treatment group (TRW and TDW) were more obvious, indicating that their calcium carbonate deposition degree was higher.

[0057] Figure 3 The XRD patterns in (b) further clarified the crystal form of the mineralized products. RW and DW exhibited characteristic diffraction peaks of cellulose type I only near 2θ≈16° and 22°. After EICP treatment, each mineralization group showed a strong diffraction peak at 2θ≈29.4°, corresponding to the (104) crystal plane of calcite-type CaCO3. Meanwhile, diffraction peaks near 39.4°, 43.1°, 47.5°, 48.5°, 57.5°, and 60.7° corresponded to other crystal planes of calcite. This indicates that the minerals generated by the EICP process in this application are stable calcite-type calcium carbonate. The diffraction peak intensity of Example 1 (TDW) was outstanding among all groups, consistent with its cyclic process and the expectation that the water bath reaction would facilitate sufficient mineral growth.

[0058] 4. Thermal stability analysis (TGA / DTG) Thermogravimetric analysis (TGA, DSC3+DHR2, Switzerland) was performed on the samples under nitrogen and air atmospheres, respectively. The test temperature range was 30℃ to 800℃, and the heating rate was 10℃ / min. The results are as follows: Figure 4 As shown.

[0059] Regardless of the atmosphere, untreated wood (RW) exhibited typical lignocellulose pyrolysis behavior, with significant weight loss in the main pyrolysis range of 300–400°C and extremely low residual weight at 800°C. Example 1 (TDW) treated with the method of this application showed a significant change in pyrolysis behavior: the initial decomposition temperature shifted towards higher temperatures, the maximum rate of thermal weight loss decreased substantially, and the residual weight at 800°C was much higher than all comparative examples (including DW with only delignification and TRW, CRW, PRW, etc., which employed other mineralization pathways). Under air atmosphere, the improvement in thermal oxidative stability of TDW was even more pronounced, exhibiting the highest final residual weight retention. This is attributed to the deep, uniformly deposited calcium carbonate inorganic network acting as an inert filler to dilute combustible components, while simultaneously acting as a physical barrier to hinder heat transfer and volatile matter escape, thereby comprehensively enhancing the thermal stability of the wood.

[0060] 5. Combustion behavior and smoke release analysis (cone calorimetry) The dynamic combustion behavior of the samples was evaluated using a cone calorimeter (CCT, Kunshan Modisco Instruments Co., Ltd.) according to ISO 5660-1:2002 standard, with an external heat flux of 50 kW / m². 2 The sample size was 100×100×6 mm³. Key combustion parameters are summarized in Table 1, and the corresponding heat release rate (HRR), total heat release (THR), smoke release rate (SPR), and total smoke release (TSP) curves are shown below. Figure 5 As shown.

[0061] The key data for cone calorimetry and flame retardancy tests of the wood from each treatment group are shown in Table 2 below: Table 2 Summary of key data from cone calorimetry and flame retardancy tests on wood from each treatment group sample LOI / % <![CDATA[pHRR / (kW / m 2 )]]> <![CDATA[THR / (MJ / m 2 )]]> <![CDATA[pSPR / (m 2 / s)]]> <![CDATA[TSR / (m 2 / m 2 )]]> <![CDATA[FGR / (kW / m 2 ·s)]]> UL-94 RW (Comparative Example 1) 20 462.11 34.35 0.0389 651.8 4.27 No grade DW (Comparative Example 2) 23 413.50 28.15 0.0290 258.3 3.72 No grade PRW (Comparative Example 6) 40 177.68 21.08 0.0043 20.9 4.67 V-0 PDW (Comparative Example 7) 48 155.50 21.72 0.0063 41.7 3.17 V-0 CRW (Comparative Example 4) 30 202.16 22.40 0.0307 914.2 1.50 V-0 CDW (Comparative Example 5) 33.5 167.25 19.31 0.0098 77.7 3.55 V-0 TRW (Comparative Example 3) 56 148.52 14.56 0.0093 31.6 3.03 V-0 TDW (Example 1) 59 120.69 9.56 0.0075 37.1 2.56 V-0 Depend on Figure 5 As shown in (a) and Table 2, the peak heat release rate (pHRR) of RW is as high as 462.11 kW / m². 2 It exhibits a sharp combustion peak, indicating that it rapidly and violently releases heat after ignition. The pHRR of Example 1 (TDW) decreased to 120.69 kW / m³. 2 The reduction was 73.88%, and the HRR curve remained at an extremely low level throughout the combustion process. Figure 5 (b) The THR curve shows that the total heat released by RW at the end of combustion is 34.35 MJ / m³. 2 TDW is only 9.56 MJ / m 2 The decrease was 72.17%.

[0062] In contrast, the pHRR and THR of the control group that only underwent delignification (DW) or used the same bath method or pre-hydrolysis method decreased to varying degrees, but were significantly less than the improvement achieved by TDW. Figure 5 (c) and (d) show the smoke release rate (SPR) and total smoke output (TSP) curves, respectively. The RW's TSP is as high as 651.8 m³. 2 / m 2 TDW's TSP dropped to 37.1 m. 2 / m 2 It exhibits good smoke suppression performance. Notably, Comparative Example 6 (PRW) has the lowest TSP (20.9 m³ / s). 2 / m 2 However, its total heat release and LOI performance were inferior to TDW, indicating that different mineralization pathways have different mechanisms for inhibiting heat and smoke release. Regarding the fire growth index (FGR), RW's FGR was 4.27 kW / m³. 2 ·s, TDW decreased to 2.56 kW / m 2The ·s indicates that the method described in this application can effectively slow down the rate of fire development.

[0063] 6. Limiting Oxygen Index and Vertical Burning Rating (LOI, UL-94) The limiting oxygen index (LOI) was determined using an oxygen index meter (JF-3, Nanjing Jiangning District Analytical Instrument Factory) according to ISO 4589-1996 standard. All samples were measured three times, and the average value was taken. Vertical burning tests were conducted according to UL-94 standard using a UL94-X instrument (Motis Fire Technology Co., Ltd., China), following ASTM D3801-10 standard for the samples (125×13×5 mm), with five repeated tests. The results are shown in Table 2 and... Figure 6 As shown.

[0064] The LOI of RW is only 20%, and that of DW is 23%, both of which are classified as flammable materials. TDW's LOI reaches 59%, an increase of 195% compared to RW, the highest among all groups, and has entered the category of flame-retardant materials.

[0065] Figure 6 The vertical burning test records visually demonstrate the differences in flame retardant performance. Comparative Example 1 (RW) and Comparative Example 2 (DW) continued to burn after the initial 10-second ignition, with the flame spreading rapidly and eventually burning completely, exhibiting no self-extinguishing capability. Figure 6 (a) and (b)). Comparative Examples 4 (CRW) and 5 (CDW) exhibited very short afterflame times (approximately 1-2 seconds) after flame removal, but showed significant instability during combustion. Figure 6 (c) and (d)). Example 1 (TDW) and Comparative Example 3 (TRW) ignited for 10 seconds in two separate instances, and immediately self-extinguished upon removal of the flame source (afterflame time approximately 1 second in both cases). Furthermore, no molten droplets ignited the absorbent cotton during the entire process, achieving the highest flame retardant rating V-0 of the UL-94 standard. Figure 6 (e) and (f)). This result, together with the cone calorimetry data, demonstrates that the two-step mineralization method of the present application, which includes a water bath circulation process, can impart good self-extinguishing properties and extremely low combustion intensity to wood.

[0066] 7. Residual Carbon Raman Spectroscopy Analysis Raman spectroscopy was used to analyze the char residue after cone calorimetry using a Lab RAM Odyssey (Horiba Co., Ltd., Japan, laser wavelength 532 nm). The char was analyzed at approximately 1350 cm⁻¹. -1 and 1580 cm -1D-bands corresponding to carbon structural defect regions and G-bands of graphitized sp² carbon structures were observed at the locations. The degree of graphitization of the char residue was evaluated by calculating the integral area ratio (ID / IG) of the D-band to the G-band. The ID / IG values ​​were 2.50 for RW, 2.61 for DW, 2.31 for CRW, and 2.64 for TRW. The higher ID / IG value of TRW compared to RW indicates that its flame-retardant performance advantage is not primarily due to the increased graphitization of the char residue, but rather to the physical barrier effect of the dense composite protective layer formed by calcium carbonate and the char layer. This result is consistent with the mechanism of constructing a stable mineral-char layer composite barrier through deep mineralization in this application.

[0067] 8. Flame Retardant Mechanism Analysis This application organically combines "partial delignification," "two-step impregnation," and "water bath reaction cycle." The partial delignification pretreatment moderately removes some lignin and hemicellulose, increases cell wall porosity, and exposes more active hydroxyl sites, creating conditions for deep penetration of reactants and nucleation and anchoring of minerals.

[0068] The first impregnation step (S2) introduces urease alone, allowing it to penetrate and anchor within the wood pores. The second impregnation step (S3) introduces a binder containing urea and calcium ions. Urease catalyzes the in-situ generation of carbonate ions from urea within the wood pores, which then precipitate with calcium ions. The subsequent water bath reaction step (S4) further drives the precipitation reaction to completion under constant temperature (25–30°C) and weakly alkaline conditions (pH 7.8–8.2), promoting crystal growth and densification. Multiple cycles of this process allow calcium carbonate deposits to gradually accumulate, forming a continuous and thick inorganic flame-retardant network. Under fire conditions, this network achieves synergistic flame retardancy through the following methods: calcium carbonate acts as a non-combustible inert filler to dilute the proportion of combustible components; the decomposition endothermic reaction (CaCO3→CaO+CO2↑) consumes heat and dilutes oxygen; uniformly dispersed mineral particles are tightly bonded to the pyrolytic carbon layer to form a dense "mineral-carbon composite barrier," which effectively blocks the inward transfer of heat and oxygen and inhibits the outward escape of combustible volatiles and smoke particles, thereby significantly improving flame retardant performance in multiple aspects such as reducing heat release, inhibiting smoke generation, and delaying flame spread.

[0069] Results Analysis Based on the above test data, the key technical effects and comparative patterns involved in this application are analyzed in detail as follows.

[0070] 1. The Influence of Mineralization Pathways on Calcium Carbonate Deposition Behavior SEM-EDS results visually reveal the differences in microstructure caused by different mineralization pathways. In the TDW sample obtained by the two-step method plus water bath circulation in this application, calcium carbonate particles penetrate deep into the cell lumen and are uniformly attached to the cell wall, and the distribution of Ca element in the wood cross section is extensive and uniform.

[0071] In contrast, while the co-bath method (Comparative Examples 4 and 5) also generates precipitates within the cell lumen, the simultaneous presence of urease, urea, and calcium ions leads to rapid and synchronous catalytic and precipitation reactions within the same space. Reactants rapidly nucleate on the surface and at the vascular inlets upon contact, often forming a dense surface deposit that hinders the inward penetration of the reaction solution, resulting in a denser calcium signal on the surface and a sparser signal internally. Although the pre-hydrolysis method (Comparative Examples 6 and 7) generates some carbonate ions in advance, precipitation still primarily occurs in the bulk solution and on the wood surface after subsequent mixing with calcium ions. FTIR and XRD results further corroborate this, showing that the two-step method produced stronger and sharper calcite characteristic peaks, indicating superior mineralization and crystal integrity. This demonstrates that introducing urease and urea-calcium salt stepwise (S2, S3) and strengthening with a water bath (S4) can significantly improve the mass transfer and deposition uniformity of EICP mineralization in porous wood media.

[0072] 2. Synergistic enhancing effect of partial delignification on mineralization Comparing the performance indicators of TRW (two-step process + water bath but without delignin) and TDW (two-step process + water bath and delignin) clearly shows the crucial role of partial delignin removal. Although TRW's flame retardant effect is significantly better than other comparative examples, TDW shows lower LOI (59% vs 56%) and pHRR (120.69 vs 148.52 kW / m²). 2 ), THR (9.56 vs 14.56 MJ / m 2The WPG (67.24% vs 36.19%) and residual weight percentage were further improved. This is because mild alkali treatment (S1) dissolved some lignin and hemicellulose, increased the nanopores of the cell wall, making it easier for urease molecules, urea, and calcium ions to diffuse into deeper layers, and exposed more cellulose hydroxyl groups as nucleation sites for calcium carbonate, promoting in-situ anchoring growth of minerals on the cell wall. Unlignified wood has relatively closed pores. Although the two-step method can alleviate surface blockage, the promotion of deep mineralization is still limited by mass transfer resistance, resulting in slightly lower total mineral deposition and uniformity. Therefore, partial delignification (S1) and the two-step method plus water bath circulation (S2-S4) form a synergistic effect, which is a necessary prerequisite for the significant flame retardant effect achieved in this application. It is worth noting that although the limiting oxygen index (LOI) of TRW has reached 56%, close to that of TDW (59%), its weight gain percentage (WPG, 36.19%) is much lower than that of TDW (67.24%). This phenomenon indicates that for undelignified TRW (transfer-wheat wood), the two-step process still allows a considerable amount of calcium carbonate to preferentially deposit on the surface and near-surface areas of the wood, forming a dense mineral coating. This coating effectively isolates the wood in the LOI (Location of Intake) test (primarily characterizing surface burning behavior), thus significantly improving the LOI. However, due to the limited total amount of deposition and uneven distribution, its performance in the cone calorimetry test (characterizing overall burning behavior) (pHRR of 148.52 kW / m², THR of 14.56 MJ / m²) is significantly inferior to TDW (pHRR of 120.69 kW / m², THR of 9.56 MJ / m²). This precisely demonstrates that, through partial delignification treatment, this application not only increases the total amount of mineral deposition but, more importantly, achieves a deep and uniform distribution of minerals within the wood, thereby comprehensively reducing the thermal hazard of fire, rather than merely improving surface burning behavior. In summary, partial delignification (S1) and the two-step water bath circulation (S2-S4) form a synergistic effect, which is a necessary prerequisite for the application to achieve significant flame retardant effect.

[0073] 3. Structure-property relationship between flame retardancy and smoke suppression properties Cone calorimetry and LOI data show that the sample in this application achieved a significant reduction in both heat release and flue gas release simultaneously. TDW showed a 73.88% reduction in pHRR, a 72.17% reduction in THR, and a 94.3% reduction in TSR (from 651.8 to 37.1), all reaching or near-optimal levels. This effect stems from the multiple functions of the deep, uniform calcium carbonate network during combustion: as a non-combustible component, it dilutes the proportion of combustible material in the wood, reducing the calorific value and total volatile matter from the source; the endothermic decomposition reaction consumes some of the combustion heat and releases CO2 to dilute oxygen; the in-situ deposited calcium carbonate tightly binds with the charcoal at high temperatures, forming a hard mineral-charcoal composite barrier, greatly delaying further pyrolysis of the internal matrix. It is noteworthy that although PRW had the lowest TSP, its pHRR and THR were higher than TDW, and its FGR was also higher, indicating that the pre-hydrolysis method tends to form a denser surface cover to suppress flue gas, but there is still a relatively rapid heat release in the early stages of combustion, making its overall fire safety less balanced than the two-step method with water bath circulation. Therefore, the control of heat release and smoke release needs to be weighed according to the application scenario, and the solution in this application achieves a better balance in terms of comprehensive fire safety parameters.

[0074] 4. Relationship between thermal stability and residual carbon structure TGA results showed that the high-temperature residual weight of TDW under nitrogen and air atmospheres was significantly higher than that of the comparative proportions, and the DTG main peak was significantly reduced and delayed. This is consistent with the char formation behavior in the combustion test. Raman spectroscopy analysis revealed that the degree of graphitization of the carbon layer (ID / IG) was not the main explanation for the flame retardant performance of TDW, as TRW had a higher ID / IG value. In fact, the participation of CaCO3 changed the physical structure of the char layer: uniformly distributed CaCO3 particles can act as a skeletal support, making the char layer denser and more continuous, reducing cracks and pores, and thus more effectively insulating heat and mass transfer. This indicates that the "physical integrity" of the char residue is more crucial to flame retardancy than pure "chemical graphitization".

[0075] In summary, the experimental data systematically demonstrate that the core improvement of this application—enzyme-induced mineralization through partial delignification synergistic stepwise impregnation and water bath circulation—effectively solves the problem of mineralization being limited to the surface layer in existing EICP technologies, achieving multi-scale deep deposition of calcium carbonate within the wood, thereby endowing the wood with excellent flame retardant and smoke-suppressing properties.

[0076] The purpose, design concept, mechanism of action, and beneficial effects of this application 1. Purpose This application aims to provide a green and efficient method for preparing flame-retardant wood, which solves the problem that the existing enzyme-induced calcium carbonate precipitation technology has limited flame-retardant performance improvement due to the mineralization being limited to the surface of the wood. It realizes the deep and uniform construction of an inorganic flame-retardant network of calcium carbonate inside the wood, transforming the wood from a combustible material into a flame-retardant material, and significantly improving its fire safety.

[0077] 2. Design concept and mechanism of action This application proposes a two-step EICP enzyme-induced calcium carbonate precipitation modification method for poplar wood, combining partial delignification pretreatment. First, partial delignification is achieved using dilute alkali, widening the internal pore channels and exposing more hydroxyl active sites. Then, a two-step EICP process is employed: urease is first infiltrated into the wood and anchored to the cell wall surface; subsequently, a urea-calcium chloride cementitious solution is introduced. Urease catalyzes the hydrolysis of urea in the wood's cell cavities, vessels, and pits, generating carbonate ions, which react with calcium ions to form calcite-type calcium carbonate in situ. This avoids rapid surface nucleation and pore blockage, achieving a uniform distribution of calcium carbonate minerals within the wood matrix. The flame-retardant enhancement mechanism is as follows: (a) After partial delignification treatment with sodium hydroxide: Remove some of the lignin in the middle layer of the wood to open up the mass transfer channels inside the wood. The cell wall surface exposes a large number of polar hydroxyl groups (-OH), which enhances the interfacial affinity between cellulose and subsequent mineralization components; The overall liquid permeability of the substrate is significantly improved, creating a prerequisite for subsequent mineralization solution penetration.

[0078] (II) EICP in-situ calcium carbonate mineralization and forming process: Urease and urea-calcium chloride cementitious solution are penetrated into all the pores of the wood in two steps, causing in-situ mineral precipitation reaction: The first step involves urease penetrating into the wood and adhering to the inner walls of the pores; The second step involves the urea-calcium chloride binder penetrating the wood. Under the catalysis of urease, the urea hydrolyzes to generate carbonate ions. The carbonate ions react with calcium ions to precipitate calcite-type calcium carbonate solid particles in situ.

[0079] Thanks to the open channels after lignin removal, calcium carbonate particles can fill pores, deposit on cell wall surfaces, and deposit within pits and micropores, while inorganic calcite particles are evenly distributed in the multi-level pores of wood.

[0080] (III) The flame retardant and smoke suppression mechanism relies on the calcium carbonate-char residue composite barrier to achieve flame retardant and smoke suppression, which consists of three layers: physical barrier, high-temperature chemical reaction, and combustion parameter regulation. Multiple barrier layers: During combustion, the wood matrix pyrolyzes to generate a char layer. The calcium carbonate particles evenly distributed in the pores and the char layer together form a composite barrier layer. The barrier isolates external heat conduction, prevents oxygen from penetrating into the interior of the substrate, and restricts the release of internal combustible volatiles, thus inhibiting the release of smoke from the source.

[0081] High-temperature endothermic decomposition of calcium carbonate: When the ambient temperature is above 700℃, calcium carbonate may decompose: CaCO3→CaO+CO2↑; This decomposition reaction absorbs a large amount of combustion heat; at the same time, it releases non-flammable carbon dioxide gas, dilutes the concentration of flammable volatile gases in the pores, and slows down the combustion chain reaction.

[0082] Combustion performance indicators have been optimized: Limiting oxygen index (LOI) has increased, peak heat release rate (pHRR) has decreased, total heat release (THR) and total smoke release (TSR) have decreased, achieving a self-extinguishing flame-retardant effect.

[0083] (iv) Mechanisms for enhancing the mechanical properties of wood: In-situ generated calcite-calcium carbonate particles fill the pores and internal microcracks of the wood; filling the defects reduces the stress concentration points inside the material; the calcium carbonate particles and cell wall polysaccharides are tightly bound together through hydrogen bonds and physical entanglement, forming an organic-inorganic integrated interface structure; the regular porous filling structure optimizes the transmission path of bending loads inside the wood.

[0084] (V) Summary of the mechanism: This application is based on a progressive design concept of "prioritizing structural regulation, separating and reinforcing the reaction process in time and space, and constructing a dense network through cyclic accumulation," as detailed below: (1) Prioritizing structural regulation: Inspired by the porous hierarchical structure of natural wood, a mild partial delignification treatment (S1) was first adopted. By selectively removing some lignin and hemicellulose, the nanopores of the cell wall were moderately enlarged and more active hydroxyl sites of cellulose were exposed while maintaining the integrity of the overall wood skeleton. This structural regulation provided a smooth mass transfer channel for the subsequent deep penetration of urease, urea and calcium ions, while increasing the nucleation sites of calcium carbonate, which is conducive to the in-situ anchoring and growth of minerals on the cell wall.

[0085] (2) Spatiotemporal Separation and Enhancement of the Reaction Process: Abandoning the traditional EICP practice of bathing or premixing urease, urea, and calcium salts together, the mineralization process is decomposed into two independent vacuum impregnation stages: a urease infiltration step (S2) and a mineralization precipitation step (S3), combined with a water bath to enhance the reaction (S4). In S2, a urease solution is introduced separately, using vacuum to allow the urease to fully penetrate deep into the wood pores and anchor to the cell walls. In S3, a cementing solution containing urea and calcium ions is introduced, allowing carbonate ions generated from the hydrolysis of urea by urease to precipitate in situ with calcium ions within the wood pores. This spatiotemporal separation strategy fundamentally avoids the blockage of surface pores caused by the rapid nucleation of urease, urea, and calcium ions upon contact in the bulk solution, ensuring that urea and calcium ions can be transported over long distances to the deep layers of the wood, allowing the precipitation reaction to occur primarily within the wood. The subsequent constant-temperature water bath reaction step (S4), under optimized conditions of 25–30℃ and weak alkalinity (pH 7.8–8.2), further drives the precipitation reaction towards completeness, promoting the continued growth and increased crystallinity of the initially formed calcium carbonate particles, thereby obtaining a more stable and denser calcite-type calcium carbonate deposit. Under these weakly alkaline conditions, the nucleation and growth rate of calcium carbonate are effectively controlled, which is conducive to the formation of a firmly attached mineral layer with good compatibility with the wood matrix.

[0086] (3) Circular accumulation to build a dense network: S1 to S4 are treated as a complete unit and repeated multiple times, so that the amount of calcium carbonate deposited inside the wood gradually accumulates. Each cycle continues to deposit on the basis of the minerals formed in the previous cycle. The mineral particles stack and connect with each other, eventually forming a continuous, thick inorganic flame-retardant network that is tightly integrated with the wood cell wall. This network is not a simple physical filling, but forms an organic-inorganic integrated composite structure with the wood matrix through interfacial forces such as hydrogen bonds.

[0087] (4) Mechanism of flame retardancy: The above-mentioned deep and uniformly deposited calcium carbonate inorganic network achieves high-efficiency flame retardancy under fire conditions through the following multiple synergistic mechanisms: First, calcium carbonate, as a non-combustible inert filler, dilutes the proportion of combustible organic components in the wood, reducing the calorific value and the total amount of combustible volatiles from the source; Second, calcium carbonate undergoes an endothermic decomposition reaction at high temperatures (CaCO3→CaO+CO2↑), which consumes a large amount of heat, and the released CO2 can dilute the concentration of gaseous combustibles and oxygen, thereby inhibiting the combustion chain reaction; Third, the uniformly dispersed calcium carbonate particles are tightly combined with the char layer formed by the pyrolysis of wood during combustion, playing a skeletal support role and forming a dense "mineral-char composite barrier". This barrier can effectively block the transfer of external heat and oxygen to the interior of the wood, inhibit the escape of combustible volatiles and smoke particles generated by internal pyrolysis, thereby significantly improving the flame retardant performance of the material in multiple aspects such as reducing heat release, inhibiting smoke generation and delaying flame spread.

[0088] 3. Beneficial effects Compared with the prior art, this application has at least the following beneficial effects: (1) Significantly improved flame retardant performance: The limiting oxygen index (LOI) increased from 20% of untreated wood to 59%, reaching the standard of flame-retardant materials; the peak heat release rate (pHRR) and total heat release (THR) decreased by 73.88% and 72.17% respectively, and the fire thermal hazard was significantly reduced.

[0089] (2) Good self-extinguishing property: The UL-94 vertical flammability rating reaches V-0, and it extinguishes immediately upon removal of the flame without dripping, which can effectively suppress the spread of flame.

[0090] (3) Good smoke suppression effect: Total smoke emission (TSP) can be reduced by more than 94%, and the harm of smoke is greatly reduced.

[0091] (4) Uniform and efficient mineralization: This method solves the problem that minerals are only deposited on the surface in traditional methods. Calcium carbonate is deeply filled into the pores of each layer inside the wood, resulting in uniform and reliable overall material performance. The weight increase percentage of the TDW group reached 67.24%, which is the highest among all groups, proving that the effective capacity of minerals inside the wood is significantly higher than that of the non-delignified group and the traditional co-bath method and pre-hydrolysis method.

[0092] (5) Green and mild process: Plant-derived urease and biocompatible calcium salts and urea are used as raw materials. The reaction conditions are mild (normal temperature and pressure) and there is no emission of toxic and harmful reagents, which meets the requirements of green chemistry and sustainable development.

[0093] (6) High applicability: This method is applicable in principle to a variety of broad-leaved and coniferous woods, providing a common technical platform for the development of high-performance flame-retardant wood composite materials.

[0094] It should be noted that: The characterization and testing methods used in the embodiments and comparative examples of this application are as follows: Microscopic morphology and elemental distribution analysis (SEM-EDS) was performed using scanning electron microscopy (SEM, Zeiss Sigma 300) to observe the changes in microscopic morphology of wood treated with different methods, combined with energy-dispersive X-ray spectroscopy (EDS) to analyze elemental composition and spatial distribution. Before testing, all samples were dried, fixed onto conductive adhesive, and sputter-coated with gold to improve conductivity. SEM was primarily used to observe mineral deposition in the cell cavities, vessels, and pores of wood after EICP treatment; EDS was used to detect the distribution characteristics of Ca within the wood to verify whether CaCO3 minerals were successfully generated in situ within the wood structure.

[0095] Fourier transform infrared spectroscopy (FTIR) was performed using a Fourier transform infrared spectrometer (Tenfor II) to analyze the chemical structure of the samples. The testing range was 400–4000 cm⁻¹. -1 This study analyzed the effects of different treatment methods on the main chemical components and functional group structure of wood. FTIR was used to analyze the changes in characteristic absorption peaks related to cellulose, hemicellulose, and lignin. Simultaneously, the appearance of CaCO3-related absorption peaks was used to determine mineral deposition, and the possible interfacial interactions between CaCO3 and the wood matrix were explored.

[0096] X-ray diffraction (XRD) analysis was performed using an X-ray diffractometer (Rigaku SmartLab-9kW) to analyze the crystal structure and mineral composition of the samples. The scan rate was 5° / min, and the scan range was 5°–70°. By comparing the changes in XRD diffraction peaks of different treatment groups, the changes in crystal structure after EICP treatment were analyzed, and the crystal form of CaCO3 was determined. Based on the positions of characteristic diffraction peaks, the phases of the mineralization products were identified to confirm the CaCO3 crystal type.

[0097] X-ray photoelectron spectroscopy (XPS) was performed using a Fisher Scientific K-Alpha spectrometer to analyze the elemental composition and chemical state of the sample surface. An argon ion source was used, with five scans and an energy step of 0.05 eV. Full-spectrum scanning determined the major elemental composition of the sample surface, and high-resolution scanning analysis was performed on C 1s, O 1s, and Ca-related peaks. XPS was used to further analyze the elemental changes on the wood surface before and after EICP treatment, revealing the changes in the elemental chemical environment during CaCO3 deposition and combustion char formation.

[0098] Thermal stability analysis (TGA / DTG) was performed using a thermogravimetric analyzer (TGA, DSC3+DHR2, Switzerland) to evaluate the thermal decomposition behavior and thermal stability of wood treated with different methods. Tests were conducted under nitrogen and air atmospheres, with a temperature range of 30°C to 800°C and a heating rate of 10°C / min. -1 The mass loss process, maximum thermal decomposition temperature, and high-temperature residual mass change of the samples were analyzed using thermogravimetric (TG) and differential thermogravimetric (DTG) curves. The influence of CaCO3 mineral deposition on the thermal stability and char formation behavior of wood was evaluated based on the residual mass change.

[0099] Limiting oxygen index (LOI) and vertical burning performance tests (UL-94) were conducted using an oxygen index meter (JF-3, Nanjing Jiangning District Analytical Instrument Factory) according to ISO 4589-1996 standard to determine the limiting oxygen index (LOI) of different wood samples. The LOI test is used to evaluate the minimum oxygen concentration required for a material to sustain combustion, thus characterizing the overall flame retardant performance of the wood. Simultaneously, according to the UL-94 vertical burning test method, the flammability rating of the samples was evaluated using a UL94-X instrument (Motis Fire Technology Co., Ltd., China) according to ASTM D3801-10 standard. The flame retardant rating of EICP-mineralized wood was further evaluated by indicators such as burning duration, dripping occurrence, and self-extinguishing performance.

[0100] Cone calorimeter testing was conducted using a cone calorimeter (CCT, Kunshan Modisco Instruments Co., Ltd.) to test the combustion behavior of samples under real fire conditions. The tests were performed according to ISO 5660-1:2002 standard, with an external heat flux of 50 kW / m². 2 The following key combustion parameters were obtained through cone calorimetry: peak heat release rate (pHRR); total heat release (THR); total smoke production (TSP); and smoke production rate (SPR). These parameters were used to comprehensively evaluate the impact of EICP mineralization treatment on the wood combustion process, heat release behavior, and smoke generation behavior.

[0101] Raman spectroscopy analysis of char residue was performed to further reveal the structural changes in the char layer after the combustion of mineralized wood. A Raman spectrometer (Lab RAM Odyssey, Horiba Co., Ltd., Japan, laser wavelength 532 nm) was used to analyze the char residue after cone calorimetry testing. Analysis was conducted at depths of 1000–1800 cm⁻¹. -1 The characteristic peak variations within the range were analyzed to determine the positions of the D peak (defect structure) and the G peak (graphitized structure), and the ID / IG ratio was calculated. The ID / IG value was used to evaluate the degree of order in the residual carbon structure and the degree of graphitization, thereby analyzing the promoting effect of CaCO3 mineralization on the formation of stable carbon layers.

[0102] In summary, the specific parameters or some commonly used reagents or raw materials in the above embodiments are specific or preferred embodiments under the concept of this application, and not limitations thereof; those skilled in the art can make adaptive adjustments within the concept and protection scope of this application.

[0103] In addition, unless otherwise specified, the raw materials used may be commercially available products in the field or prepared by conventional methods in the field; that is, the reagents and instruments used in this embodiment do not specify the manufacturer or other information, and are all conventional products that can be purchased from the market.

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

[0105] The specific embodiments of this application have been described above, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing flame-retardant wood by enzyme-induced calcium carbonate precipitation, characterized in that, Includes the following steps: S1. Partially delignify the wood to obtain delignified wood; S2. The delignified wood is immersed in a first solution containing urease under vacuum conditions to allow the urease to penetrate into the interior of the wood. S3. The wood treated in S2 is immersed in a second solution containing urea and calcium ions and impregnated under vacuum conditions. Urease catalyzes the hydrolysis of urea to generate carbonate ions. The carbonate ions react with calcium ions to generate calcium carbonate precipitate in situ inside the wood. S4. The wood from S3 is reacted with the solution in a water bath at 25-30°C and pH 7.8-8.

2. S1-S4 constitute one treatment cycle. After repeating the cycle 2-4 times, the wood is cured and dried to obtain flame-retardant wood.

2. The preparation method according to claim 1, characterized in that, In S1, the partial delignin treatment includes: immersing the wood in an alkaline solution to partially remove lignin and hemicellulose, then washing it until neutral and drying it.

3. The preparation method according to claim 2, characterized in that, The alkaline solution is a sodium hydroxide solution with a concentration of 0.5–2 wt%, and the treatment time is 0.5–4 hours.

4. The preparation method according to claim 1, characterized in that, In S2, the urease is a plant-derived urease.

5. The preparation method according to claim 4, characterized in that, The plant-derived urease is a urease solution extracted from soybean flour.

6. The preparation method according to claim 1, characterized in that, In step S3, the urea concentration in the second solution is 0.5–1.0 mol / L, and the calcium ion concentration is 0.5–1.5 mol / L.

7. The preparation method according to claim 1, characterized in that, The vacuum impregnation conditions in S2 and / or S3 are as follows: maintain a negative pressure of -0.06 MPa relative to atmospheric pressure for 25 minutes, and then maintain this negative pressure for 2 hours.

8. The preparation method according to claim 1, characterized in that, In step S4, the water bath reaction time is 0.5 to 4 hours.

9. The preparation method according to claim 5, characterized in that, In step S2, the first solution is the plant-derived urease solution itself, or a solution obtained by diluting or concentrating the plant-derived urease solution.

10. A flame-retardant wood produced by enzyme-induced calcium carbonate precipitation, characterized in that, Prepared by the method according to any one of claims 1 to 9.

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