In Situ Polymerization of Furfuryl Alcohol with ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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In Situ Polymerization of Furfuryl Alcohol with Ammonium Dihydrogen Phosphate in Poplar Wood for Improved Dimensional Stability and Flame Retardancy Lizhuo Kong,†,‡ Hao Guan,†,‡ and Xiaoqing Wang*,†,‡ †

Research Institute of Forestry New Technology, Chinese Academy of Forestry, Beijing 100091, People’s Republic of China Department of Wood Modification, Research Institute of Wood Industry, Chinese Academy of Forestry, Xiangshan Road, Haidian District, Beijing 100091, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Fast-growing plantation wood normally possesses some undesirable intrinsic properties, such as dimensional instability, inferior mechanical strength, and flammability, limiting its usage as an engineering material. Herein, we report a green and facile approach for upgrading the lowquality poplar wood via a combined treatment with biomassderived furfuryl alcohol (FA) and ammonium dihydrogen phosphate (ADP) acting as a flame-retardant additive. Wood/ PFA/ADP composites were prepared by impregnation of the FA precursor solutions into the wood matrix, followed by in situ polymerization upon heating to form a hydrophobic FA resin/ADP network within the wood scaffold. In-depth scanning electron microscopy coupled with enregy-dispersive X-ray spectroscopy (SEM-EDX) and confocal laser scanning microscopy (CLSM) analyses reveal the wide distribution of the FA resin/ADP complexes inside the cell walls and also in the cell lumens. The incorporation of hydrophobic FA resin into wood results in reduced water uptake and remarkably enhanced dimensional stability, as well as generally improved mechanical properties. The addition of a small amount of ADP greatly enhances the flame retardancy of the modified wood and also effectively suppresses smoke generation during its combustion by reducing the heat-release rate and promoting char formation, as proven by cone calorimetry. The FA resin/ADP complexes increase phosphorus fixation in wood and reduces its leaching into water, suggesting a long-term fire protection of wood in service. Such modified poplar wood with overall enhanced properties could be potentially utilized as a reliable engineering material for structural applications. KEYWORDS: Poplar wood, Furfuryl alcohol (FA), Polymerization, Dimensional stability, Flame retardancy

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INTRODUCTION The depletion of fossil resources and growing environmental concerns in recent years call for more sustainable development in the search for renewable resources to supply energy and materials for our modern society. 1,2 In this context, lignocellulosic biomass is highly desirable in view of its renewability and sustainability, holding a promising potential as an alternative to fossil resources for wide applications. Among the available natural materials, wood is one of the key candidates attracting growing interest from academia and industries, because of its unique intrinsic features.3 Given its light weight, high mechanical strength, easy processability, and local availability, wood has long been used as an engineering material for building and construction. Owing to overexploitation of natural forests and associated logging restrictions in many countries, fast-growing forest plantations will play an increasingly important role in the wood supply in © XXXX American Chemical Society

the future. However, despite its stock abundance, planted forest trees (e.g., poplar) produce wood that normally possesses some undesirable intrinsic properties, such as dimensional instability, inferior mechanical strength, and flammability, limiting its usage as an engineering material.4 Therefore, modifying the lowquality plantation wood to improve its physical and mechanical properties for value-added applications is highly imperative but remains a challenge. In order to improve the dimensional stability, mechanical properties, durability, and fire resistance of wood, many chemical modification strategies have been proposed, including acetylation,5,6 resin impregnation,7,8 silanization,9,10 in situ grafting polymerization of monomers,11,12 and mineralization of Received: October 1, 2017 Revised: December 18, 2017

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DOI: 10.1021/acssuschemeng.7b03518 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering

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wood tissues.13,14 Although chemical modification with polymers or minerals yields wood materials with improved properties, most of the treatments involve harmful chemicals or solvents, which may present serious environmental and health concerns during the processing and use of the modified wood. For instance, wood impregnated with low-molecular-weight phenol−formaldehyde (PF) or melamine−formaldehyde (MF) resins is known to release hazardous volatile organic compounds, such as formaldehyde and phenols, limiting their indoor applications. Grafting polymerization of vinyl monomers such as styrene (ST) and methyl methacrylate (MMA) often involve the use of toxic organic solvents to carry chemicals inside the wood microstructure.15,16 Driven by ecological and health concerns, the development of nontoxic and green chemical approaches for wood modification is highly pursued. In this context, biosourced chemicals derived from renewable resources (e.g., plant oils,17 glucose18 and rosin19) represent an environmentally benign and sustainable alternative to the conventional fossil-based chemicals for wood protection. As a biomass-derived chemical, furfuryl alcohol (FA) is a promising wood modification agent, because of its strong polarity (high affinity for cell wall components) and good solubility in water (water-based system). FA can be readily obtained from furfural via hydrogenation.20,21 Furfural is normally derived from pentose-rich agricultural residues such as rice hulls, bagasse, and corncobs. Because of its environmental friendliness, furfurylation of wood has gained widespread interest from academia and industries over the last 10 years.22−26 This process is based on the impregnation of wood with FA monomers, followed by in situ polymerization of FA to form a hydrophobic dark brown polymeric gel within wood in the present of a catalyst at elevated temperatures. The resulting furfurylated wood exhibited improved dimensional stability,27,28 enhanced wood density and hardness,29 and good resistance toward microbial decay and insect attack.30 More importantly, no negative environmental impact has been identified, with respect to the usage and disposal of furfurylated wood.31 In view of these virtues, furfurylation appears to be an attractive way to upgrade the low-quality plantation wood for valueadded applications. Despite improved physical and mechanical properties of wood by furfurylation, very little is known about the fire resistance of furfurylated wood. Fire safety is a major concern for wood materials used in structural applications. Although classical flame retardants (e.g., ammonium phosphate, halogen, boron, silica-based systems) are known to provide effective protection of wood from fire, possible leaching of these inorganic salts from treated wood in contact with water undermines their protective effects. In this study, in view of the low-quality poplar wood that fails to meet the requirements of engineering materials, a combined treatment using biosourced FA and ammonium dihydrogen phosphate (ADP) was proposed to modify the overall properties of wood, in terms of dimensional stability, mechanical properties, and flame retardancy. Upon in situ polymerization within wood, the formed hydrophobic poly(furfuryl alcohol) (PFA) with crosslinked structures is expected to encapsulate ADP and fix the flame retardant within wood, thus reducing the leaching of ADP from the treated wood. The distribution and deposition of PFA and ADP within the cellular structure of the wood were examined, and the physical and mechanical properties, especially the thermal stability and flame retardancy of the resulting wood, were investigated.

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EXPERIMENTAL SECTION

Materials. Italian poplar (Populus euramevicana cv., “I-214”) was obtained from a fast-growing plantation located in Hebei Province, China. Clear wood samples were cut from the sapwood region of the tree trunk and air-dried under natural atmospheric conditions for 4 months before use. Furfuryl alcohol (FA), maleic anhydride (MA), ethyl alcohol (EtOH), and ammonium dihydrogen phosphate (ADP) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used as received without further purification. Deionized (DI) water was used as the solvent for preparation of the impregnation solutions. Preparation of Impregnation Solutions. The water-based impregnation solutions consisted of FA, EtOH, MA, and ADP. MA was used as a catalyst and EtOH as a solvent mediator. Typically, FA (30 wt %), EtOH (5 wt %), and MA (2 wt %) were added into DI water. The mixture was stirred continuously at room temperature until the solution became transparent. ADP was used as a flame-retardant additive in the impregnation solutions. Different amount of ADP (1, 3, 5, and 7 wt %) was added to explore its impact on the overall properties of the modified wood. Preparation of Wood/PFA/ADP Composites. Before impregnation, all samples were oven-dried at 103 °C for 10 h, and the ovendry weight and dimensions of the samples were measured. The asprepared solutions were impregnated into the wood samples in a custom-built chamber using a full cell process, with an initial vacuum treatment for 30 min, followed by a pressure treatment (1 MPa) for 3 h. After impregnation, the samples were wiped with tissue paper to remove excessive solutions on the surface. For avoiding solution evaporation during the curing stage, the samples were wrapped in aluminum foil, and cured in an oven at 103 °C for 3 h, allowing full polymerization of FA within the wood matrix. Afterward, the aluminum foil was removed. The samples were first dried at 60 and 80 °C for 2 h, respectively, and finally at 105 °C until a dry state was achieved. Overall, wood/PFA/ADP composites with differing ADP contents were prepared. Unless specified otherwise, wood/PFA/ADP refers to the modified wood with the precursor solution containing 5 wt % ADP throughout the paper. For comparison, wood samples treated with FA alone were also prepared, denoted as wood/PFA. Characterization. The morphology of the furfurylated wood was characterized by field-emission scanning electron microscopy (FESEM) (Model SU8010, Hitachi) equipped with an energy-dispersive X-ray (EDX) detector for mapping. The microscopic distribution of polymerized FA resin within the furfurylated wood was examined using confocal laser scanning microscopy (CLSM) (Model C2-SIM, Nikon, Japan) with both 40× and 60× (water immersion) objective lens. The excitation laser wavelength was 488 nm, and the detector ranges were 500−550 nm. Cross-sectional samples of ∼20 μm in thickness were prepared for acquiring images, which were color-coded according to fluorescence emission intensity. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 spectrometer (Thermo Scientific, USA) over the wavenumber range of 400−4000 cm−1 at a resolution of 4 cm−1, using the KBr pellet method. Solid-state 13C NMR spectra were measured at room temperature on a Bruker Avance III 400 MHz NMR spectrometer, using a magic angle spinning (MAS) speed of 2.5 MHz and a rotation frequency of 5000 MHz. Thermogravimetric (TG) analysis was performed on a thermal analyzer (SDT Q600 V8.3 Build 101) from room temperature up to 800 °C at a heating rate of 10 °C/min in a nitrogen atmosphere. Physical and Mechanical Properties. The physical and mechanical properties of wood samples were tested in accordance with “China National Standard Testing Methods for Wood Physical and Mechanical Properties” (GB/T 1929-2009). Measurements of weight percent gain (WPG), bulking effect, antiswelling efficiency (ASE), and water uptake (WA) was carried out with 10 replicates of cubic samples with a size of 20 mm × 20 mm × 20 mm (R × T × L). WPG and bulking effect due to impregnation were determined by comparison of the dry weights and volumes of the control and treated samples, respectively. For the determination of ASE, both the treated B


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Figure 1. Schematic illustration of the fabrication process of the wood/PFA/ADP composite derived from natural wood. and control samples at the oven-dry state were soaked in water for 5 days. The volume of the sample before and after water immersion was measured to determine the volumetric swelling coefficient. ASE was thus calculated by comparison of the volumetric swelling coefficients of the treated and control samples. For a realistic determination of the ASE values of the modified wood, multiple water soaking cycles were performed on the samples. The ASE values for the second and third soaking cycles were calculated according to the reported method.12 Water uptake (WA) of the samples was measured as an index of water repellency. Oven-dry wood samples were soaked in water for 39 days in total. After a certain time interval, the samples were taken out of the beaker and wiped with a piece of tissue paper to remove the excess water before weight measurement. WA of the sample was calculated based on the initial dry and final wet weight after water soaking. Modulus of elasticity (MOE), modulus of rupture (MOR), and compressive strength parallel to grain (CS) were measured for evaluating the mechanical properties of wood samples. End-matched samples with dimensions of 20 mm × 20 mm × 300 mm (R × T × L) were used for MOE and MOR tests and 20 mm × 20 mm × 30 mm (R × T × L) for CS test. MOE and MOR were measured under static three-point bending, using a universal testing machine (Model AG2000A, Shimadzu Corp., Japan). Ten samples were used for each of the above mechanical tests. The typical stress−strain (load-deflection) curves for the tested samples are presented in Figure S1 in the Supporting Information. Flammability Test. The flammability properties of wood samples was measured using a cone calorimeter (Fire Testing Technology Ltd., East Grinstead, U.K.) under a heat flux of 50 kW/m2 in the horizontal position, according to the ISO 5660-1 standard. Wood samples of 100 mm × 100 mm × 10 mm (R × T × L) were conditioned at 20 °C and 65% relative humidity (RH) prior to the combustion test. All sides of the samples were wrapped in aluminum foil, except for the upper face, which was exposed to the heat flux. The fire response parameters measured include peak heat release rate (PHRR), total heat release (THR), and total smoke production (TSP). Leaching Test. Leaching tests were conducted in accordance with Japanese Industrial Standard JIS A 9201 (Japanese Standards Association 1991) with some modifications. Wood/PFA/ADP samples were submerged in deionized water under magnetic stirring (500 rpm) at 25 °C for 12 h, and then dried in an oven at 60 °C for 24 h. The leaching and volatilization procedures were conducted for seven cycles. After each cycle, leachate was sampled to analyze the phosphorus content via inductively coupled plasma−mass spectrometry (ICP-MS) (Model 7700, Agilent, USA). For comparison, wood samples treated with a pure ADP aqueous solution (5 wt %) were also prepared for the leaching test.

structure with many open channels (cell lumens) along the growth direction. The cell walls are natural fiber composites consisting of stiff cellulose microfibrils embedded in the soft matrix of hemicelluloses and lignin. Because of the hygroscopic nature of amorphous cellulose and hemicelluloses, wood cell walls swell or shrink upon variation of moisture contents, leading to dimensional changes in the wood. Such moisturerelated dimensional instability is particularly prominent for the fast-growth poplar wood that contains a large proportion of juvenile wood. On the other side, with its naturally developed porous structure comprising many cell lumens as well as numerous nanopores present in the cell walls, the wood scaffold possesses inherent room for the incorporation of additional materials for property enhancement. After being introduced into wood under vacuum/pressure conditions, the monomer precursors of FA and ADP could readily penetrate into the porous structure of wood, because of their hydrophilic nature. Upon heating in the presence of a catalyst (MA), the monomer FA is expected to polymerize in situ into a hydrophobic dark brown polymeric resin with a cross-linked structure, anchoring the flame-retardant ADP within the resin network. Morphology and Structure. The natural cellular structure of wood with irregular cell shapes can be viewed from the crosssection of the control sample (Figure 2a). After furfurylation treatment, the honeycomb structure of wood is well preserved, and a few scattered cells with filled lumens (indicated by the

RESULTS AND DISCUSSION The fabrication process of wood/PFA/ADP composite derived from the natural wood scaffold is schematically illustrated in Figure 1. The natural wood possesses a unique cellular

Figure 2. (a−c) SEM images of control wood (a) wood/PFA (b), and wood/PFA/ADP (c); arrows indicate cells with filled lumen. (d−f) Confocal laser scanning microscopy (CLSM) images of the fluorescence from control wood (d), wood/PFA (e), and wood/ PFA/ADP (f).

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C


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tion of FA resin within the wood cell, as shown by CLSM imaging, it is not likely that ADP is evenly distributed across the entire cell walls, which is probably due to the limited lateral resolution of EDX analysis. The chemical reactions involved in wood furfurylation are rather complex. To further investigate the possible chemical bonds formed between FA and the wood cell wall polymers, FTIR spectra were recorded on the corresponding samples (Figure 4a). Compared with the control wood, the wood/PFA and wood/PFA/ADP composites show three notable bands at 1711, 1561, and 790 cm−1, which are assigned to the CO stretch of γ-diketones formed from hydrolytic ring opening of the furan rings, conjugated CC species and skeletal vibration of 2,5-disubstituted furan rings, respectively.32,33 This indicates that the resinification process of FA was successfully completed within wood. In addition, the intensity of the small peak at 897 cm−1 assigned to the β-glycosidic linkages between the sugar units decreases in the spectra of the modified wood,34 suggesting the occurrence of acid hydrolysis of hemicelluloses during wood furfurylation. Noticeably, a new peak at 490 cm−1 due to NH3+ oscillation can be observed in the wood/PFA/ ADP composite spectrum,35 indicating the incorporation of ADP into the resin network, which is in accordance with the EDX mapping result. The 13C NMR spectra of the corresponding samples are shown in Figure 4b. For the control wood, the signals in the region between 50 ppm and 105 ppm are mainly assigned to different carbons in the glucose unit of cellulose.36 The appearance of new peaks in the spectra of the treated wood at 151.1, 108.0, and 27.8 ppm, corresponding to C-2, C-5, C-3, C4 of the furan ring and the −CH2− bridge between the furan rings, respectively,37 indicates that FA resin has been successfully introduced in wood during the furfurylation process. It has been proposed that grafting reactions occur between FA and lignin within the cell wall,31 and chemical bonds formed between FA units and lignin model compounds were verified by NMR analysis.38 However, our NMR results (also FTIR data) provide no evidence for chemical reactions between FA and the wood cell wall polymers. In addition, it is apparent from the spectra that some degradation of carbohydrates has occurred during wood furfurylation. The relative intensity of the signals at 62.9 and 84.0 ppm, assigned to the C-6 and C-4 of amorphous cellulose, respectively,36 is diminished after the treatment. A decrease in the intensity of 21.6 ppm band corresponding to CH3 of acetyl groups attached to hemicelluloses can also be observed. The degradation of amorphous cellulose and hemicelluloses is probably due to acid hydrolysis of the wood during the treatment. Physical and Mechanical Properties. Since the hydrophobic FA resin was incorporation into wood, improvements in the physical and mechanical properties of the modified wood are expected. As shown in Figure 5a, the modified wood exhibits a volume gain (bulking) of ∼10%, independent of the loading amount of ADP. This cell wall bulking indicates that FA resin was incorporated into the cell wall, which is consistent with the CLSM observation. Water uptake was measured to evaluate the water repellency of the modified wood upon water soaking (Figure 5b). The control wood shows a weight gain of ∼170% upon water soaking for 39 days, while the water uptake of the furfurylated wood is remarkably reduced by almost half of that of the control wood, indicating improved water repellency due to FA resin incorporation. The addition of ADP shows little effect on the water uptake capacity of the

arrows) can be observed for both wood/PFA and wood/PFA/ ADP samples (Figure 2b and 2c). This indicates FA polymerization partially occurred in the cell lumens. To better detect the distribution of the polymerized FA resin in the wood cavities, fluorescence imaging using CLSM was conducted on the wood samples. As shown in Figure 2d, when exciting at a wavelength of 488 nm, rather weak fluorescence can be seen from the untreated wood, which is mainly due to autofluorescence emitted from the lignin-rich middle lamella and cell corners. In contrast, a strong fluorescence emission was observed from the furfurylated wood, where both the cell walls and part of cell lumens fluoresce (Figures 2e and 2f). It is known that the fluorophores of Poly(FA) emit primarily between 500 nm and 650 nm.23 This suggests that FA resin was not only deposited in cell lumens but also entered into cell walls. It is noticeable that the FA resin formed within the cell lumens fluoresce more intensely than that in the cell walls. The cell wall polymers likely constitute a restricted environment for FA polymerization, resulting in hindered chain elongation and thus shorter conjugation lengths for the formed FA resin within the cell wall.23 Apart from a few fibers with filled lumens, most of the cell lumens of fibers are coated with a thin layer of FA resin, sealing the lumen surface, which cannot be detected by SEM observation. Furthermore, a closer view of the cell wall reveals more fluorescence emitted from the middle lamella and cell corners than from the cell wall (Figure 2f). It can be assumed that the lignin-rich regions favor the polymerization of FA, resulting in preferred deposition of FA resin within these regions. EDX mapping was further conducted to visualize the elemental composition of the cell wall for the wood/PFA/ ADP sample (Figure 3). It is evident from the EDX maps that

Figure 3. (a) SEM images of wood/PFA/ADP and the corresponding EDX mapping showing the elemental distribution of (b) carbon, (c) oxygen, and (d) phosphorus.

the distribution of elemental carbon and oxygen elements follows the profile of the cell walls, which could be derived from the cell wall polymers and also the incorporated materials. Other than C and O, the signal of phosphorus (P) can also be detected at a discernible level, which appears to be evenly distributed through the entire cell walls (Figure 3d). The elemental phosphorus is derived from ADP, which is likely encapsulated and entangled within the polymerized FA resin network. However, considering the inhomogeneous distribuD


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Figure 4. (a) FTIR and (b) 13C NMR spectra of control wood, wood/PFA, wood/PFA/ADP, and pure FA resin (PFA).

Figure 5. (a) Bulking effect of wood/PFA/ADP composites with different loading amounts of ADP. (b) Water uptake of control and modified wood as a function of soaking time. (c) ASE of modified wood as a function of water soaking cycle.

Table 1. Physical and Mechanical Properties of the Control and Modified Wooda sample control wood/PFA wood/PFA/ ADP a

modulus of rupture, MOR (MPa)

modulus of elasticity, MOE (GPa)

compressive strength, CSb (MPa)

wood density (g/cm3)

weight percent gain, WPG (%)

88.42 (5.40) 78.77 (5.82) 83.26 (4.08)

11.70 (0.78) 14.10 (0.68) 15.52 (0.91)

49.84 (2.12) 78.14 (3.57) 71.26 (1.89)

0.37 (0.01) 0.63 (0.01) 0.58 (0.01)

67.75 (0.03) 68.34 (0.05)

Standard deviations are shown in parentheses. bCompression strength parallel to the grain.

of the control wood with that of treated wood. A large ASE value is indicative of high dimensional stability. The modified wood shows an ASE value as high as ∼50% after three water soaking cycles, implying highly restricted swelling of wood due to impregnation. Considering their small molecules and polarity, the FA monomers can enter the cell wall and polymerize there after curing. The formed hydrophobic FA

treated samples. The incorporated hydrophobic FA resin located in cell cavities and cell wall micropores is expected to occupy the space available for water, thus decreasing the water absorption of wood. The stabilizing effect of FA resin incorporation is also reflected in the dimensional changes of the modified wood, as measured by ASE (Figure 5c). The ASE compares the swelling E


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Figure 6. (a) TG and (b) DTG curves of control wood, wood/PFA and wood/PFA/ADP. The data of pure FA resin (PFA) and PFA/ADP are also displayed.

resin is expected to bulk the cell wall and thus reduce the ability of the cell wall to swell upon water immersion, resulting in improved dimensional stability of the treated wood. In fact, only the part of the FA resin entering the cell walls contributes to the dimensional stability of wood, whereas those deposited in the cell lumen shows little effect. The achieved ASE value (∼50%) of the furfurylated wood is comparable and even higher than the results of the modified wood based on grafting polymerization of styrene into cell walls.12,15 Furthermore, it is noted that the addition of a small amount of ADP has negligible influence on the ASE values, especially after the first soaking cycle, indicating the irrelevance of ADP for the dimensional stability. The wood density and mechanical properties of the modified wood are shown in Table 1. The furfurylated wood (68% WPG) exhibits a significant increase (70%) in wood density, from 0.37 g/cm3 to 0.63 g/cm3, relative to the control wood. In parallel, the parallel-to-grain compression strength (CS) is remarkably improved by 57% after the treatment. It is wellknown that the mechanical properties of wood are positively correlated with wood density.39 However, only a slight improvement of MOE by 20% and a minor reduction in MOR by 11% is observed for the furfurylated wood, despite its high loading of FA resin. The different influences of furfurylation on the compression and flexural strength of wood are in agreement with previous reports.24,28 It can be assumed that the FA resin filled in cell cavities help to reinforce the wood structure, thus increasing its resistance to collapse in compression. In contrast, for the flexural properties, the reinforcing effect of FA resin is probably counteracted by the inevitable acidic degradation of hemicelluloses during the treatment, thus weakening the integrity of the cell wall structure. The flexural properties seem to be more sensitive to the degradation of cell wall components. As a consequence, furfurylation results in only slightly improved MOE and even reduced MOR for the treated wood. Again, the addition of ADP has little effect on the mechanical properties of wood, indicating a dominant role of FA resin in reinforcing the wood structure. Thermal Stability. The thermal stability of the modified wood was evaluated by thermogravimetric (TG) analysis in nitrogen. The TG and DTG curves are shown in Figure 6, and the relevant data are summarized in Table 2. For the natural control wood, the initial small weight loss between room temperature and 200 °C is mainly due to the evaporation of absorbed water in cell walls. After that, the wood components

Table 2. Thermogravimetric Data of the Control and Modified Wood sample

T10%a (°C)

Tmaxb (°C)

residue at 800 °C (%)

control wood/PFA wood/PFA/ADP

274.54 261.24 209.04

372.53 369.96 267.26

10.00 21.41 40.00

a Temperature corresponding to 10% weight loss. corresponding to the maximum rate of weight loss.

b

Temperature

undergo pyrolysis, where the thermally unstable hemicelluloses are the first component to decompose at ∼300 °C and a slight shoulder can be seen in the DTG curve. The abrupt weight loss at ∼370 °C, corresponding to the prominent peak in the DTG curve, can be ascribed to the main degradation of cellulose. The decomposition of lignin occurs over a wider range, from 290 °C to 500 °C.40 The complete pyrolysis of the wood components up to 800 °C results in a 90% weight loss, yielding final residues composed of a thermally stable char. For the furfurylated wood, the incorporation of FA resin has some effects on the thermal behavior of wood. The higher weight loss observed between 200 °C and 300 °C is related to the thermal degradation of the FA resin, involving scission of the weaker chemical bonds.41 The major degradation of the FA resin occurring at 320−400 °C overlaps with cellulose and lignin decomposition. The furfurylated wood yields higher final residues (21%), compared with the control wood, indicating increased char formation and improved thermal stability by the incorporated FA resin. For the wood/PFA/ADP sample, it is interesting to note that the addition of a small amount of ADP has a strong effect on the thermal behavior and final residues of the modified wood. It can be seen that the prominent peak in the DTG curve shifts to a lower temperature of ∼270 °C, while the final mass residue is increased up to 40%, indicating the dominant influence of ADP. The thermal decomposition of ADP is known to produce metaphosphoric acid (HPO3), which can react with the wood components, causing their dehydration and the formation of thermally stable char (carbonization), thus retarding glow.42 The incorporation of ADP into wood increases the char formation significantly, from 21% to 40%, when comparing the wood/PFA and wood/PFA/ADP samples. Flammability Properties. The combustion behavior of the modified wood was qualitatively evaluated by being exposed directly to an open flame using a butane torch. As shown in Figure 7, the control wood ignited within 2 s, leading to a dramatic flame spread due to easy flammability of the natural F


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seen. The initial peak arises from the oxidation of volatile pyrolysis products.43 As burning continues, an insulating char layer is produced afterward, which lowers the HRR, as demonstrated by a valley in the HRR curve. When the protective char layer breaks down, a sudden increase in HRR to its maximum occurs, followed by a rapid decrease and gradual leveling off. For the furfurylated wood, both HRR peaks are apparently increased, indicating an enhanced fire growth rate relative to the control wood, which is consistent with the above burning test (Video S1). In parallel, the furfurylated wood exhibits a higher total heat release (THR) value than the control wood (Figure 8b). This indicates that the incorporated FA resin appears to promote fire spread of the treated wood during combustion. However, the furfurylated wood produced higher amounts of char residues at the end of the test, as opposed to the lesser residues consisting of fragile and gray ashes from the control wood (Figure 8c). By contrast, for the wood/PFA/ADP composite, both HRR peaks are substantially suppressed and the second peak is shifted to a longer exposure time, indicating a greatly lowered fire growth rate and increased char formation. The THR value is also remarkably reduced, and the final black residues appear more compact and less fragile (Figure 8). In general, the fireretardant performance of the modified wood is comparable to that of wood impregnated with clay14 and CaCO3,43 as reported in the literature, in which the HRR peak values are reduced to a similar extent (by ∼50%−60%). The excellent flame retardancy of the wood/PFA/ADP composite emphasizes the crucial role of ADP in inhibiting fire growth and promoting char formation, despite its very low loading in the modified wood. ADP is known to thermally decompose into HPO3, which can react with the wood components, causing their dehydration to form thermally stable char, meanwhile reducing the release of flammable combustion products from

Figure 7. Flammability tests for (a) control wood, (b) wood/PFA, and (c) wood/PFA/ADP using a butane torch. The flame exposure time during the test is displayed.

wood. The incorporation of FA resin results in slightly delayed ignition of the furfurylated wood, while the flame spread is still dramatic. By contrast, with the addition of a small amount of ADP, the wood/PFA/ADP composite can withstand continuous exposure to the flame without being ignited at all, exhibiting excellent flame retardancy of the modified wood, which is also highlighted in Video S1 in the Supporting Information. Cone calorimetry tests were further performed on wood samples to quantitatively study their combustion behavior in a developing fire. In the flammability test, the samples were subjected to a constant heat flux of 50 kW/m2 from a conical heater. Once sufficient combustible volatiles are released from wood, the burning process is initiated by a spark igniter. Figure 8a shows the heat release rate (HRR) curves of the modified and control wood samples, where two principal peaks of the heat-release rate corresponding to flaming combustion can be

Figure 8. (a) Average HRR and (b) THR curves of control wood, wood/PFA, and wood/PFA/ADP and (c) the corresponding photographs of char residues after the combustion. G


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ACS Sustainable Chemistry & Engineering Table 3. Cone Calorimetry Data of the Control and Modified Wooda sample control wood/PFA wood/PFA/ ADP a

first HRR (heat release rate) peak, PHRR1 (kW/m2)

second HRR peak, PHRR2 (kW/m2)

total heat release, THR (MJ/m2)

total smoke production, TSP (m2)

167.86 (4.06) 233.21 (34.94) 156.61 (4.94)

216.49 (16.06) 280.34 (15.54) 93.86 (13.85)

68.69 (2.29) 83.36 (2.65) 45.10 (0.16)

2.18 (0.28) 5.37 (0.37) 0.22 (0.04)

Standard deviations are shown in parentheses.

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wood.42 In addition, incombustible gases such as NH3 and H2O are also produced during the thermal decomposition of ADP. All these factors contribute to the fire-retardant effects of ADP. Apart from the heat release, the production of toxic gases (e.g., CO) or soot during combustion is also important for evaluating the fire hazard. As shown in Table 3, the furfurylated wood shows the highest smoke production, while the wood/PFA/ ADP composite releases the least smoke in correlation with its lowest HRR and THR values, which further proves the fireretardant effects exerted by ADP. Leachability of ADP. To achieve a long-lasting fireretardant effect in wood, the water-soluble ADP should be able to be immobilized in the wood structure, reducing or avoiding its leaching from the treated wood in contact with water. Leaching tests were thus performed to examine the leachability of ADP in the modified wood. For comparison, wood samples treated with ADP alone were also tested. As shown in Table 4, after three leaching cycles, most of the

CONCLUSIONS In summary, we have demonstrated a green and facile method for upgrading the low-quality poplar wood in terms of dimensional stability, mechanical properties, and flame retardancy by using biomass-derived FA in combination with ADP (as a flame-retardant additive). Upon polymerization of FA within the wood matrix, the in situ formed hydrophobic FA resin/ADP complexes are widely distributed inside the cell walls and also in the lumens. The derived wood/PFA/ADP composite possesses remarkably improved dimensional stability over the nature control, because of efficient cell wall bulking. The incorporated FA resin also helps to reinforce the wood scaffold, resulting in generally improved mechanical properties, except only a minor reduction in MOR. More strikingly, the addition of a small amount of ADP greatly enhances the flame retardancy of the modified wood, meanwhile effectively suppressing smoke production during its combustion. The hydrophobic FA resin/ADP complexes allows good fixation of phosphorus in wood, suggesting a long-term fire protection of wood in service. Such modified poplar wood with overall enhanced performances could expand its application in building and construction, replacing the traditional high-quality solid wood derived from natural forests.

Table 4. Phosphorus Concentration in the Leachates of Wood/PFA/ADP and Only ADP Treated Wood Subjected to Cycled Water Leaching Phosphorus Concentrationa (ppm) leaching cycle 1 2 3 4 5 6 7

wood/ADP 1390.89 429.99 145.40 43.34 21.27 12.88 7.78

(10.49%) (3.24%) (1.10%) (0.33%) (0.16%) (0.10%) (0.06%)

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wood/PFA/ADP 360.54 116.84 87.70 83.43 71.65 67.74 52.82

(2.87%) (0.93%) (0.70%) (0.66%) (0.57%) (0.54%) (0.42%)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03518. Typical stress−strain curves for the samples tested in compression and load−deflection curves for the threepoint bending test (PDF) Video showing the flame retardancy of the modified wood (AVI)

a

Values shown in parentheses are the ratio of phosphorus concentration to that in the impregnation solution.

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phosphorus was removed by water from the samples treated with ADP alone. The highest phosphorus release occurs in the first leaching cycle (10.49%). By contrast, for the wood/PFA/ ADP sample, the presence of FA resin results in much less phosphorus release into water. The stabilizing effect of FA resin has also been reported in wood treated with FA/boric acid, in terms of slowing the boron leaching rate.27 For the FA/ADP combined treatment, the use of FA is to improve the dimensional stability of wood and to reinforce its structure; it also functions as a fixing agent capable of forming a hydrophobic FA resin network to anchor ADP in wood, thus reducing its leaching into water, as illustrated in Figure 1. The retarded phosphorus release suggests a long-term fire protection of the treated wood in service, thus extending the service life of wood.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaoqing Wang: 0000-0002-7460-0004 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Nonprofit Institute Research Grant of CAF (No. CAFYBB2016SY025) and the National Natural Science Foundation of China (NSFC Grant No. 31570554). H


Research Article


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