Preparation of ultra-flame retardant wood materials with mechanical reinforcement and water resistance through biomimetic mineralization of carbonated apatite

The preparation of wood products with mechanical reinforcement, flame retardancy, and water resistance is still a problem to be solved. In this work, carbonated apatite (CAp) was bio-mineralized in wood vessel by the method of gradient impregnation. The CAp@wood75G sample with the weight gain of 75 wt% showed the greatest bending strength and compressive strength due to the formation of organic–inorganic composites. With the high bending strength and compressive strength, the CAp@wood75G sample exhibited elevated limiting oxygen index (LOI) of 60.4% and self-extinguished immediately in the vertical burning test. In cone calorimetry test, the peak of heat release rate and total heat release for CAp@wood75G sample were lessened by 68.2 and 53.1% respectively, compared with that of natural wood sample. Furthermore, the LOI of CAp@wood75G with self-extinguishment was still as high as 38.4% even after immersed in water for 24 h. Furthermore, the flame retardant mechanism was explored and generalized. The CAp was decomposed to produced CO2 and H2O to dilute the combustible gases in the air, and catalyzed wood to form the barrier of heat and oxygen during burning. Moreover, CAp produced inorganic salts which played the protective layer role. To conclude, this efficient processing method by bioinspired mineralization was eco-friendly and effective.


Introduction
Wood, as a natural material with superior mechanical properties and environmental friendliness, has been widely used in housing construction and furniture for thousands of years (Wimmers 2017;Jiang et al. 2022). Wood is composed of C, H, and O with a porous structure (Jiang et al. 2022;Fang et al. 2022), which is flammable, limiting its widespread utilization, particularly in fire-sensitive applications. Therefore, improving the flame retardancy of wood is crucial.
According to the structure and composition of wood, efforts in three aspects were made to prepare the flame-retardant wood initially: one is to impregnate particles including flame-retardant elements into the pipe structure of wood with improved mechanical properties (Lu et al. 2021;Samanta et al. 2022); the second one is to process the wood by spraying (Wang et al. 2021a, b;Zhang et al. 2022), brushing (Zhao et al. 2021), polymerizing , precipitation , and densifying (Gan et al. 2020) to acquire the flame retardant coating; the third one is to use the elements originally contained in wood to carry out the mineral growth of inorganic flame retardant (Guo et al. 2019). Although these methods could reduce the fire hazards, the water resistance for the treated wood had been rarely tested.
In order to satisfy the water resistance in the actual application, different strategies have been designed and developed for the water resistance of flame retardant wood. For example, phenolic resins (Hoyos-Martínez et al. 2021), furfural Zhang et al. 2021a, b, c) and other organic substances Zhang et al. 2021a, b, c) were used to enhance the connection between flame retardant and wood conduit. However, the materials released many hazardous volatile organic compounds (such as acetone, furfuryl alcohol, and formaldehyde), which did not meet the requirements of environmental development. From the perspective of environmental protection and the durability of flame retardants, a new flame retardant treatment method has been developed. Biomimetic mineralization could produce organic-inorganic hybrid materials, which not only obtained flame retardant wood, but also improved the water resistance and mechanical properties (Merk et al. 2016). Pondelak et al. mineralized wood with CaCO 3 , and the flame retardant performance was improved (Pondelak et al 2021). Inspired by nature's matrix-mediated biomineralization, struvite was produced inside the wood through mineralization by Guo et al. (2019), and both the flame retardancy and mechanical properties were improved. Furthermore, the main characteristics of wood products are substantial improvements in fire performance with maintained eco-efficiency, durability, and other properties of the original wood product (Östman et al. 2006).
Carbonated apatite (CAp) (CAp, Ca 10−x (PO 4 ) 6−x (CO 3 ) x (OH) 2 , with 0 ≦ x ≦ 2) as an environmentally friendly inorganic clay, shows excellent biocompatibility and heat resistance (Nifant'ev et al. 2021;Ulian and Valdrè 2020). Because of its unique molecular formula and structure, it could be a potential material that can be mineralized in the interior of wood.
In this work, CAp was synthetized and mineralized with high quantity in the wood by gradient impregnation. The effects of different mineralization on the mechanical properties of wood were explored, and both the fire behavior and water resistance were tested. The flame retardant mechanism was also discussed. This work provided a facile method to prepare wood with improved mechanical properties, flame retardancy, and water resistance.

Materials
Wood samples with the size of 100 × 100 × 80 mm 3 came from the Chinese firs (an arbor of Taxodiaceae and Cunninghamia, Guizhou, China). Ammonium dihydrogen phosphate and urea were produced by Shanghai Macklin Biochemical Technology Co., Ltd. Calcium chloride anhydrous (CaCl 2 ) was purchased from Tianjin Jinke Institute of Fine Chemicals. All chemicals used were analytically pure.

Preparation of salt solution
The clear salt solution A were prepared by mixing CaCl 2 and NH 4 H 2 PO 4 with a molar ratio of 5:3 with the concentration of 0.8, 1.6, 2.4 and 3.2 mol/L, respectively. Urea (60 g) was dissolved in 500 mL of DI water in the beaker to form the solution B.

Impregnation and mineralization treatment of wood samples
The impregnation process through concentration gradient was as following. Firstly, the wood samples with the moisture content of 8% and the size required for each test (six parallel samples were prepared for each test) were put in the vacuum dryer under the − 0.1 MPa for 2 h at 25 °C to remove the gases in the vessel. Secondly, the wood was impregnated in the mixed solution A with the concentration of 0.8, 1.6, 2.4 and 3.2 mol/L one by one under − 0.1 MPa for different time at 25 °C, and the soaking time of each concentration was 2 h. The obtained wood was dried in the oven at 50 °C to get the constant mass. Then the obtained wood was immersed in the vapor environment of solution B for 2 h. After that, the wood sample was got out and dried at room temperature. The obtained woods by gradient impregnation samples were signed as CAp@wood nG (n representing the weight gain of each sample). In order to highlight the advantages of gradient impregnation, we prepared a sample directly immersed in solution A with 3.2 mol/L, which was named as CAp@wood nI , and the immersion time was the same as the time corresponding to the maximum weight gain of CAp@ wood nG . The weight gain of the treated wood after dried to constant weight was calculated as the following formula: where W t represented the weight after immersed in the vapor environment of solution B for t h, W 0 represented the initial weight.
The schematic diagram of preparation process was shown in Fig. 1.

Characterization
Fourier transform infrared (FTIR) spectra were tested using the Nicolet Nexus 670 at the wavenumbers range of 4000-500 cm −1 with averaging 32 scans. Powder samples were blended with KBr before testing.
X-ray diffraction (XRD) was recorded by a D/max-2500 X-ray diffraction instrument to characterize the successful immersion of CAp into the wood, and the CAp@wood 75G was sampled in the position at a distance of 3 cm from the wood surface.
Scanning electron microscope (SEM, HITACHI S4800) was applied to observe the morphology of the sample before and after combustion.
The X-ray photoelectron spectroscopy (XPS) were got by an ESCALAB250 (Thermo VG) spectrometer.

Weight gain
Before fitting, C 1s spectra was shifted to the position of 284.5 eV. Thermogravimetry analysis (TGA) was conducted by a TA Instrument (Q50 thermal analyzer, USA). The temperature range was from 30 to 800 °C in the N 2 atmosphere, the heating rate was 10 °C/min with the sample weight of 3.5 mg.
The limiting oxygen index (LOI) was obtained by a JF-3 oxygen index apparatus. The test was conducted according to ASTM D 2863-17 with the sample size of 80 × 10 × 5 mm 3 .
The vertical burning behavior was measured to compare their burning performance carrying out on a CZF-3 instrument according to ASTM D 3801. The sample size was 100 × 10 × 4 mm 3 .
The combustion behavior of wood samples was simulated by a cone calorimeter according to ASTM E1354-17 with the sample size of 100 × 100 × 10 mm 3 .
The bending strength and compressive strength were tested in accordance with the Chinese national standard GB/T 1936-2009and GB/T 1935-2009, respectively. The specimens for bending strength testing were 20 × 20 × 300 mm 3 , and those for compressive strength were 20 × 20 × 30 mm 3 . Six parallel samples were tested under the specified experimental conditions.

Results and discussion
The bending strength The bending strength of woods obtained by gradient impregnation and direct impregnation respectively was shown in Fig. 2. One can see that with the increase of weight gain, the bending strength and compressive strength of each sample were also increased. Moreover, after the weight gain reached 45 wt%, the change range of bending strength and compressive strength of the sample were not obvious. The sample with the highest bending strength and compressive strength with the weight gain of 75wt% labeled as CAp@wood 75G was discussed in the following work. Furthermore, the weight gain of the sample prepared by direct immersion method with the same immersion time as that of CAp@wood 75G sample was only 15wt%, and the bending strength and compressive strength for CAp@wood 15I was only 76 and 58 MPa respectively due to the low adding of CAp. Fig. 2 The stress strain curves (a, c), average bending strength (b), average compressive strength (d), the digital photos (e) and SEM images (f) for different wood samples ◂ One can see that the irregular degree of the fracture surface was increased with the increase of the weight gain of the CAp in the digital photos (Fig. 2e). From the SEM images (Fig. 2f), when the weight gain was between 0 and 30 wt%, the section of wood fracture was flat, and the position of CAp in the wood hole was visible. When the weight gain was greater than 45 wt%, the fracture surface of the wood was jagged, and the holes of the wood with CAp in the fracture surface cannot be clearly distinguished.

Characterization of the CAp and CAp@wood sample
The FTIR spectrum and SEM image of the CAp were shown in Fig. 3a, b. From Fig. 3a, one can see that the peaks at 1031 and 563 cm −1 were due to PO 4 3− (Xu et al. 2021), and the peaks at 1467 and 1658 cm −1 assigned to CO 3 2− appeared (Opitz et al. 2021). It was proved that urea promoted the substitution of CO 3 2− in the PO 4 3− position. The peaks at 3600-3000 cm −1 belonged to -OH were also exhibited ). The morphology of CAp was also observed by SEM, and it was found that the CAp had an irregular shape, and there were many ravines on the surface of CAp (Fig. 3b). As shown in Fig. 3c, there were typical characteristic peaks of cellulose crystal at 2θ = 16.0° and 22.5° for the natural wood (Peter 2021), CAp showed two characteristic diffraction peaks at 2θ = 25.9° and 31.6°, which were attributed to crystal plane (002) and (211), respectively (Moore et al. 2016). The other weak diffractions at 23.2°, 26.8°, and 29.6° in 2θ were detected in the CAp pattern. For the CAp@wood 75G sample, the two peaks of natural wood were weakened as the crystal structure of cellulose was broken partly and the characteristic peaks at 2θ = 25.9°, 31.6 o and 29.6° corresponding to CAp were appeared. Compared to Fig. 3dd 1 (natural wood), the Fig. 3d 2 indicated that the particles of CAp exhibited in the internal wood. It was also proved that the CAp was impregnated into the wood successfully.

Thermal stability
The thermal stability for natural wood and CAp@ wood 75G samples was tested by the TGA in N 2 , and the TGA and DTG curves were shown in Fig. 4. The natural wood was pyrolyzed with two stages (Yang , the first stage was mainly the evaporation of absorbed water, and the second stage was mainly the pyrolysis of cellulose and hemicellulose. The decomposition of lignin was accompanied by each stage of degradation (Fig. 4a,  b). After the biomineralization of CAp in the wood, the two decomposition stages of wood were still existed. However, the decomposition peak of water was disappeared below 100 °C, the decomposition peak of CAp was appeared between 150 and 300 °C, and the decomposition peak of cellulose and hemicellulose between 350 and 420 °C was reduced, which indicated that the CAp hindered the degradation of natural wood. The initial decomposition temperature (T 5% ) for CAp@wood 75G sample was increased to 190.8 °C because of the immersion of CAp, and the maximum decomposition rate (R max ) for CAp@ wood 75G samples was decreased to 0.4%/°C (see Table 1). Furthermore, the char residue at 800 °C was increased to 38.5%, but that of natural wood was only 18.4%. It was indicated that the treated wood showed the higher stability and more char residue. When the temperature was above to 400 °C, the weight of CAp@wood 75G sample did not change, which indicated the high temperature stability.

LOI and vertical burning test
The fire behavior for different wood samples was characterized by the LOI and vertical burning test firstly, and its results were exhibited in Fig. 5 and Table 2. The natural wood burnt fiercely and burnt out in just 75 s in the vertical burning test, and its LOI was 23.1%. The LOI value of CAp@wood 75G sample was achieved 60.4%, and CAp@wood 75G sample extinguished immediately in the vertical burning test, which showed the high flame resistance of CAp (Fig. 5a).
The water resistance was characterized by immersing samples in water for 24 h under ambient temperature. After immersion in water for 24 h, the CAp@ wood 75G sample was still extinguished immediately in the vertical burning test with the LOI of 38.4%,  which exhibited the great water resistance for CAp@ wood 75G (Fig. 5b).

Cone calorimetry test
The curves for heat release rate (HRR), total heat release (THR) and mass of wood samples after cone calorimeter were shown in Fig. 6 and some key data was summarized in Table 3. Figure 6a indicated that there were two peaks for HRR of the natural wood sample: the first peak of HRR (pk 1 -HRR) was 188 kW/m 2 which was caused by the decomposition of hemicellulose, cellulose, and partially lignin; the second peak of HRR (pk 2 -HRR) was 198 kW/ m 2 which was due to the degradation of lignin. For CAp@wood 75G sample, the pk 1 -HRR and pk 2 -HRR was decreased by 38.8 and 68.2% respectively. Furthermore, the time to pk 2 -HRR was delayed from 202 to 351 s. The THR value of CAp@wood 75G sample was decreased by 53.1% compared with that of natural wood sample (Fig. 6b). Additionally, the char residue mass for the CAp@wood 75G sample was improved to 42.1% from 10.0% of natural wood sample (Fig. 6c). The time to ignition (TTI) was also delayed from 10 to 25 s, which improved the difficulty of fire occurrence. Generally, fire performance index (FPI) was the value of TTI/pHRR, and higher FPI values indicated a lower fire hazard of the materials (Zhou et al. 2021). The FPI value of CAp@wood 75G sample was calculated as 0.11 m 2 ·s/kW, which was higher than that of natural wood (0.09 m 2 ·s/kW), demonstrating that the fire hazard of CAp@wood 75G sample was also reduced.  The exploration for flame retardant mechanism The SEM image of char residue To understand the flame retardant effects in the condensed phase, the morphology of char residues was analyzed. The digital photos of char residue for natural wood and CAp@wood 75G were shown in Fig. 7a, b respectively. It was illustrated that the residue of natural wood was fragile and not intact (Fig. 7a). In contrast, the residue of CAp@wood 75G sample was integrated. The white ashes on the surface was the amorphous Ca salt (Koda et al. 1993) (Fig. 7b). The micro morphology of char residue was observed by SEM and shown in Fig. 7c, d. It was exhibited that the residue of natural wood sample exhibited broken structures of vessels, and the wood was damaged completely (Fig. 7c). In contrast, the SEM image of CAp@wood 75G sample char was intact, and complete structures of vessels were well preserved. Granular particles were also observed in the vessels of char (Fig. 7d). It was indicated that the CAp promoted the   Fig. 8 The FTIR (a) and XPS spectra (b-e) for char residues of the natural wood and CAp@wood 75G after cone calorimeter test formation of char, and the particles produced from the degradation of CAp forming a protective layer with good heat resistance in wood (the char residue was still 61.7% until 800 °C in Fig. 7e, f.

FTIR and XPS analysis
The FTIR and XPS were used to analyze the char residue of natural wood and CAp@wood 75G after the cone calorimeter, and the results were shown in Fig. 8. In Fig. 8a, the absorption peaks at 955 and 1031 cm −1 were attributed to the existence of CAp (Zhang et al. 2021a, b, c). The characteristic peaks at 980 and 1020 cm −1 were ascribed to P-O-C (benzene ring C and aliphatic C respectively) (Wang et al. 2021a, b). FTIR results showed that CAp was decomposed to produce phosphoric acid which promoted the formation of high-quality char. The XPS survey spectra were shown in Fig. 8b-e. Besides the peaks ascribed to C 1s and O 1s in two samples, special peak of P 2p was found in the char for CAp@wood 75G (Fig. 8b). The existence of P in the char of CAp@wood 75G sample indicated that the CAp was decomposed to produce phosphoric acid which participated in the formation of char . Furthermore, the bonding status of C 1s was different in the char of natural wood and CAp@wood 75G sample as shown in Fig. 8c, d. The C 1s of the natural wood in Fig. 8c was fitted to O-C=O (287.9 eV), C-O (286.3 eV), and C-C (284.5 eV). Whilst for the C 1s of CAp@wood 75G sample, there were also three peaks, but the relative peak area was changed. The increased mount of oxygen element mean that the burning of wood was incomplete. It was stated that the particles remained in the wood surface, preventing wood from being exposed to oxygen after combustion. In Fig. 8e, the peak at 134.6 eV and 136.8 eV in CAp@wood 75G was assigned to C-O-P and pyrophosphate (Zheng et al. 2022), which also illustrated that the existence of CAp produced crosslinked char.

The flame retardant mechanism
The flame retardant mechanism for CAp@wood 75G sample was proposed according to the above analysis and shown in Fig. 9. The CAp@wood 75G sample showed the great flame retardancy during the burning process in three aspects. Firstly, the CAp was decomposed by heat to produced CO 2 and H 2 O (Zhang et al. 2016), which diluted the combustible gases in the air. Secondly, the CAp was decomposed to produce phosphoric acid, which promoted the formation of compact and crosslinked char. Thirdly, particles produced from the degradation of CAp could form a protective layer with good heat resistance in wood. Therefore, the CAp@wood 75G sample exhibited outstanding flame resistance.

Conclusion
Carbonated apatite (CAp) was bio-mineralized in wood vessel by the method of gradient impregnation. The obtained wood showed excellent bending strength compressive strength, flame retardancy (LOI: 60.4%; self-extinguishment) and water resistance. Furthermore, the peak of heat release rate and total heat release for CAp@wood 75G sample was reduced by 68.2 and 53.1% respectively in comparison with that of natural wood. The flame retardant mechanism analysis illustrated that CAp released invert gases to dilute combustible gases. Furthermore, CAp produced inorganic salts and catalyzed the formation of protective layer during the burning process. This work provided an efficient fire-retardant treatment and expanded the use of wood.