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Zhang, S., Wu, Y., Guo, X., Shen, H., Zhang, M., and Wang, M. (2019). "Synergistic effect of hierarchically porous 4A zeolite on improving the fire retardancy and reducing the fire toxicity of ammonium polyphosphate treated wood composites," BioRes. 14(1), 2013-2028.


The synergistic effect of hierarchically porous zeolites on the fire retardancy and fire toxicity of ammonium polyphosphate (APP) treated wood composites was studied in this work. Hierarchically porous 4A (H4A) zeolite and 4A zeolite were hydrothermally synthesized and characterized. Several groups of wood composites containing APP as fire retardant and H4A/4A zeolites as synergists were fabricated. Thermogravimetric (TG) results showed that H4A zeolite increased the char formation and enhanced the thermal stability of APP treated wood at higher temperature. Cone calorimetry test results suggested that H4A zeolite reduced the heat release rate (HRR) and decreased CO produce rate (COPR) of APP treated wood composites. Moreover, H4A also exhibited better smoke suppression than 4A zeolite when the APP content was small. These results demonstrated that H4A zeolites may be a promising synergistic component for improving the fire retardancy and reducing the fire toxicity of APP treated wood composites.

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Synergistic Effect of Hierarchically Porous 4A Zeolite on Improving the Fire Retardancy and Reducing the Fire Toxicity of Ammonium Polyphosphate Treated Wood Composites

Shaodi Zhang,a,c Yuhui Wu,a,c Xi Guo,a,c Haiying Shen,a,c Mingchang Zhang,a,c and Mingzhi Wang a,b,c,*

The synergistic effect of hierarchically porous zeolites on the fire retardancy and fire toxicity of ammonium polyphosphate (APP) treated wood composites was studied in this work. Hierarchically porous 4A (H4A) zeolite and 4A zeolite were hydrothermally synthesized and characterized. Several groups of wood composites containing APP as fire retardant and H4A/4A zeolites as synergists were fabricated. Thermogravimetric (TG) results showed that H4A zeolite increased the char formation and enhanced the thermal stability of APP treated wood at higher temperature. Cone calorimetry test results suggested that H4A zeolite reduced the heat release rate (HRR) and decreased CO produce rate (COPR) of APP treated wood composites. Moreover, H4A also exhibited better smoke suppression than 4A zeolite when the APP content was small. These results demonstrated that H4A zeolites may be a promising synergistic component for improving the fire retardancy and reducing the fire toxicity of APP treated wood composites.

Keywords: Wood composites; Hierarchically porous 4A zeolite; Hydrothermal synthesis; Fire retardancy; Fire toxicity

Contact information: a: MOE Key Laboratory of Wooden Material Science and Application, Beijing, 100083, China; b: Beijing Key Laboratory of Wood Science and Engineering, Beijing, 100083, China; c: College of Material Science and Technology, Beijing Forestry University, Beijing, 100083, China;

* Corresponding author:


Wood composites have become a prevalent construction engineering material due to their low density and good mechanical properties (Wang et al. 2014a). However, the cellulose, hemicellulose, and lignin content of wood composites are flammable; the danger of heavy economic loss and casualties restricts composite application in dwellings and other buildings (Lowden and Hull 2013).

Wood composites can be chemically modified with fire retardants. Ammonium polyphosphate (APP) and ammonium dihydrogen phosphate (ADP) were typically used as fire retardants of wood owing to their eco-friendly properties compared with halogen fire retardants (LeVan 1984; Diniz 2010; Jiang et al. 2010; Lowden and Hull 2013; Jiang et al. 2015). APP can decompose to polyphosphoric acid when heated, and the acid catalyzes the dehydration of cellulose and increase the production of char residue of wood. The residue acts as a protective layer to prevent the emission of combustible gases and the impregnation of oxygen. Thus, the propagation and intensity of fire can be restrained. However, toxic gases including CO are emitted due to the incomplete combustion of wood (Wang et al. 2014b; Yan et al. 2017), which leads to heavy casualties or long-term damage in humans (Stec and Hull 2010). Furthermore, smoke production increases dramatically when a high concentration of APP solution (>15%) is used. Because smoke reduces visibility and impedes escape from fire, the inhibition of smoke is just as vital as CO (Naeher et al. 2007). Therefore, the modification of the APP fire-retardant system is urgently required to improve the fire-retarding efficiency and to reduce toxicity during fire. Inorganics such as zeolites have been applied as synergists of fire retardants to solve these problems, with positive results (Demir et al. 2005; Lin et al. 2011).

Zeolites are crystalline aluminum silicates, comprised of aluminum and silicate oxygen tetrahedral through corner sharing, giving rise to a three-dimensional framework with intracrystalline microporous structures (Breck 1974). As nanoscale porous materials, zeolites exhibit superior catalytic property, size or shape selectivity, and absorbability; they have become increasingly important in many industrial applications, e.g., in automotive catalysis, sewage purification, and petroleum processing (Zorpas et al. 2000; Vermeiren and Gilson 2009; Wang et al. 2011; Deka et al. 2013). There are many types of zeolites, and they are named LTA, FAU, ZSM-5, etc., based on the framework structure and channel type. LTA type zeolites have a three-dimensional, 8-ring channel system. The typical pore size of synthesized LTA zeolites is 4 Å (0.4 nm), so it is known also as 4A zeolite. Previous researchers have also clarified the efficiency of microporous 4A zeolite by improving the fire retardancy of APP treated materials. Fire toxicity was inhibited in multiple studies. Bourbigot et al. (1996) added 4A and other zeolites into the intumescent (APP)-pentaerythritol (PER) system treated polymers, resulting in a great improvement in the fire retardant properties; the presence of zeolites increased the thermal stability of materials at high temperatures. Wang et al. (2015) found that 4A zeolite catalyzes the thermal degradation of APP treated wood-plastic composites, leading to the formation of a more stable and compact char layer, and gas/solid-phase fire retardancy mechanisms of zeolites were also speculated. Wang et al. (2014b) treated plywood with APP and 4A zeolite and investigated the combustion behavior of obtained plywood by cone calorimetry. The heat release and emission of CO were decreased by the APP/4A system. In a recent study, different zeolites were used to modify the fire retardant coating of wood, and the results indicated that the FAU type 13X zeolite with larger pore size exhibited better smoke suppression properties than the 4A zeolite (Wu et al. 2017). In summary, zeolite catalyzes char formation and enhances the strength of a char layer to prevent wood from oxygen, and the special porous structure may act as a physical barrier to limit the transfer of heat and reduce the production of toxic gases through absorption and catalytic property.

The small aperture size (ranging from 3 to 15 Å) and relatively long transport path of microporous zeolites may cause mass transport limitations, leading to the weakening of its catalytic and absorptive performance. These drawbacks have also restrained the application of microporous zeolites in other fields like the conversion of biomass. To overcome these shortages, research efforts have focused on introducing mesopores into zeolites, including the attempts of mesoporous zeolites and the synthesis of hierarchically porous zeolites (Choi et al. 2006; Pérez-Ramírez 2012; Ates et al. 2015; Gamliel et al. 2016). In particular, hierarchically porous zeolites contain both micro- and mesoporosity and have shown better mass transport performance and improved catalytic efficiency than microporous zeolites (Gueudré et al. 2014; Fu et al. 2015; Besser et al. 2016). However, the topic of synergistic effect of hierarchically porous zeolites on fire retardancy of fire retardant-treated wooden materials has not been reported yet, and this may be of considerable significance for improving the fire retardancy of wood or other lignocellulosic materials and reducing fire casualties.

In this work, hierarchically porous 4A (H4A) and 4A zeolite were hydrothermally synthesized. Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DTPAC) was used as a template to introduce intracrystalline mesoporous into the framework of 4A zeolite to obtain H4A zeolite. Several groups of wood composites containing APP as fire retardant and H4A/4A zeolites as synergists were fabricated. The fire retardancy and fire toxicity of APP treated wood composites were assessed by thermogravimetric (TG) analysis and cone calorimetry.



Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DTPAC, 60 wt% in methanol) was purchased from Beijing Bailingwei Technology Co. Ltd. (Beijing, China). Analytical grade reagents for the synthesis of zeolites including sodium hydroxide (NaOH), sodium aluminate (NaAlO2), and sodium metasilicate nonahydrate (NaSiO2·9H2O) were provided by local companies in China.

Ammonium polyphosphate (APP, average polymerization degree: n > 1000) was purchased from Ji’nan Taixing Fine Chemical Co., Ltd. (Jinan, China). Wood flour of poplar (W, Populus tomentosa, particle size between 20 to 60 mesh sieve) was provided by Gaocheng Xingda Wood Flour Company (Hebei, China), and dried in an oven at 103 °C for 8 h prior to usage. The phenol-formaldehyde (PF) resin adhesive was synthesized by batch copolymerization using phenol and formaldehyde with a molar ratio of 1:2.2, following the reported method (Li et al.2017). The solid content of the resin was 45%, the viscosity was 100 to 150 mPa s, with a pH of 11 to 12. The distilled water used in this work was self-made in the lab.

Synthesis of H4A/4A Zeolite

H4A zeolite was prepared according to published procedures (Choi et al. 2006). First, 1.2 g of NaOH and 30.0 g of distilled water were added to a 500 mL polypropylene flask and stirred until a clear solution was prepared, and 8.2 g of NaAlO2 was dissolved in the NaOH solution (solution A). Subsequently, 28.4 g of NaSiO2·9H2O was dissolved in 69.1 g of distilled water in another 500 mL polypropylene flask, and 3.3 g of DTPAC was added into the NaSiO2·9H2O solution while stirring to obtain solution B. Solution B was added to solution A, leading to the formation of a gel having the mole composition of 1Al2O3:3.3Na2O:2SiO2:128H2O:0.08 DTPAC. The gel was stirred by a blender for 2 h until a homogeneous mixture was obtained. The mixture was heated in a water bath with electromagnetic stirring for 4 h at 95 °C; the crystallization process of target zeolite occurs in this procedure. After it was vacuum filtered with distilled water for 4 times, the crystallized zeolite was oven-dried at 110 °C for 3 h and calcined in an air atmosphere at 550 °C for 6 h in a muffle furnace to remove DTPAC. The mesoporosity was introduced into the zeolite particle through the calcination process.

For the synthesis of 4A zeolite, solutions A and B were prepared following the same procedure mentioned above, except that DTPAC was not added in solution B, and the mixture of solution A and B was stirred in a polypropylene flask for 2 h at room temperature, then heated at 100 °C for 4 h in an oven. The product was also filtered and dried like H4A zeolite but did not undergo the calcination.

The synthesized zeolites were milled to powder to break up aggregates.

Characterization of H4A/4A Zeolite

Powder X-ray diffraction (XRD) patterns were acquired with Cu Kα radiation at 40 kV and 40 mA (Bruker, D8 ADVANCE, Billerica, MA, USA). The data were recorded in the 2θ range of 5 to 50° with an angular step size of 0.02° and a counting time of 8 s per step. The morphology of zeolites was obtained by field emission scanning electron microscope (FE-SEM; SU8010, HITACHI, Chiyoda, Tokyo, Japan). The N2 adsorption/desorption isotherms were determined using an automated physisorption analyzer (BEL max 00097, BEL, Bengaluru, India) at -196 °C. The samples were activated by removing the water of hydration at 350 °C under 10-4 Pa before the adsorption measurement. The pore size distributions were calculated by the Barrett-Joyner-Halenda (BJH) method using the adsorption branch for the mesopores analysis.

Thermogravimetric Analysis

Poplar wood flour (60 mesh) was blended with APP and/or zeolites in different ratios, as shown in Table 1. TG analysis was carried out in a thermogravimetric analyzer (TGA Q50 V20.10 Build 36, TA instruments, New Castle, DE, USA) using a heating rate of 10 °C ·min-1 up to 800 °C under air atmosphere (79% of nitrogen and 21% of oxygen).

Preparation of Wood Composites

Six groups of wood composites were fabricated. The wood flour with particle sizes of 20 to 60 meshes were blended with APP and/or zeolites by different combination ratios, as shown in Table 1. PF resin adhesive (10 % of wood flour weight) was manually blended in the mixture, which was then compressed in a mould. The semi-finished slabs were hot-pressed under 1.8 to 2.5 mPa pressure at 150 °C for 10 min. The fabricated wood composites panels were cut into specimens of 100 mm × 100 mm × 10 mm dimensions for cone calorimetry.

Table 1. Combinations and Ratios of Different Samples for TG Analysis and Cone Calorimetry

Cone Calorimetry

Cone calorimetry is a standard method for burning materials with specified dimensions under a constant heat flux to observe the combustion behavior. It can simulate the fire environment precisely and has been widely used in the field of wood fire retardants (Steen-Hansen and Kristoffersen 2007; Kim et al. 2012). A cone calorimeter (FTT00007, East Grinstead, UK) was used with an external heat flux of 50 kW·m-2, according to the ISO 5660 standard (2015). The data gathered to investigate the combustion properties of wood composites included heat release rate (HRR), total heat release (THR), smoke produce rate (SPR), total smoke production (TSP), and CO produce rate (COPR).


Characterization of Zeolites

The FE-SEM images of 4A and H4A zeolites are presented in Fig. 1. The 4A zeolite exhibited regular cubic shape with glazed surfaces. The H4A zeolite showed a distinct change in morphology; homogeneous and globular particles in the range of 2 to 3 μm with rugged surfaces were observed.

Fig. 1. FE-SEM images of 4A and H4A zeolites. (a), (b) are FE-SEM images of 4A zeolite with magnification of 4 k and (c), (d) are for H4A zeolite with magnification of 50 k. The inset of (c) and (d) showed the morphology of singular particles and the region that has been magnified was marked by a rectangle.

Fig. 2. XRD patterns of H4A and 4A zeolites synthesized in this study

Figure 2 reveals the XRD patterns of samples. Both 4A and H4A zeolites showed characteristic peaks of LTA type zeolites (Treacy and Higgins 2007), suggesting that crystalline LTA type zeolites were successfully synthesized. The only difference was a slight decrease in the intensity of the diffraction peak, indicating that the size of the crystalline domain of H4A zeolite was decreased because of the DTPAC addition (Cho et al. 2009).

Fig. 3. N2 adsorption/desorption isotherms of H4A and 4A zeolite; the H4A zeolite show a typical type-IV isotherm.

The N2 adsorption/desorption isotherm of H4A zeolite presents a typical type-IV isotherm, according to Fig. 3. The pore size analysis by the BJH algorithm gave a sharp distribution of mesopores centered at around 10.0 nm (Fig.4), indicating that the mesoporosity was introduced into H4A zeolite successfully.


Fig. 4. The pore size distribution of H4A and 4A zeolites

TG Analysis

Figures 5 and 6 show the TG and derivative thermogravimetry (DTG) curves of different samples. The results are summarized in Tables 2 and 3. The thermal degradation of wood can be divided into three stages, as shown in TG and DTG curves. Wood (W) starts to absorb heat at first. The water evaporates under 135 °C, and about 3.8% of the mass was lost during this process (Table 2). The second stage at 135 to 375 °C corresponds to the decomposition of wood, including the dehydration, rearrangement, and pyrolysis of cellulose, hemicellulose, and lignin. Approximately 67.9% of mass was lost, and the mass loss rate reached the maximum value (R1max: 0.0087%·°C-1) at T1max of 320 °C (Table 3). Some stable char remained during this process (Yunchu et al. 2000). With the temperature increasing, the third stage occurred at around 380 to 500 °C, when the char was oxidized and approximately 4.7% of residue was retained at 800 °C.


Fig. 5. TG curves


Fig. 6. DTG curves, (a) Enlarged detail of the 500 °C to 800 °C region

The residues of both W + 4A (8.9%) and W + H4A (6.3%) were higher than that of W. Metal cations have a strong effect on pyrolysis of cellulose and could increase the formation of char, e.g., the Na+ within the framework of zeolites (Soares et al. 1998). More residue demonstrates that the sample is more thermal-stable, which suggests that the fire retardancy had been improved (Rowell 1984). Another plausible explanation is that the solid-acidity of zeolites exhibits similar catalytic performance to phosphoric acid (Moreno-Recio et al. 2016).

Table 2. TG Analysis Data of Different Samples under Air Atmosphere

Table 3. DTG Data of Different Samples under Air Atmosphere

Less residue remained from W + APP + H4A/4A than that of W + APP due to the reduction of APP content, as shown in Table 2. APP is the main driver in improving fire retardancy. However, the combination of APP and zeolites showed excellent synergistic effects in other aspects. In the second stage, the R1max of W + APP + H4A and W + APP + 4A was much smaller than that of W + APP, and the T1max of W + APP + H4A was 298 °C, 16 °C higher than that of W + APP. Thus, the unique porous structure of H4A zeolite could enhance the thermal stability of wood. In the third stage, the initial decomposing temperature of W + APP was 356 °C, which increased to 395 °C for W + APP + H4A/4A (Table 2). The DTG curves of W + APP + H4A/4A were lower than that of W + APP after 500 °C (inset of Fig. 6), especially for W + APP + H4A, which means that the remaining char was more thermally stable at a higher temperature.

These results indicated that H4A zeolite increased the char residue at a low content of APP and also improved the thermal stability of the wood and the char by catalyzing the formation of an intercalated network, leaving Si-O-P and P-O-C bonds in the char (Wang et al. 2015; Yan et al. 2017).

Cone Calorimetry Test

Heat release

Heat release rate is a critical parameter for evaluating fire intensity and the velocity of fire propagation. The HRR curves of samples are illustrated in Fig. 7, and the data are listed in Table 4. Wood showed two heat release peaks during the combustion process. The first heat release peak corresponded to the burning of volatile gases produced by the pyrolysis of surface wood. After the pyrolysis of surface wood and the combustion of volatile gases, a protective char layer was generated, such that the heat transfer was slow and pyrolysis was inhibited. Subsequently, the char layer cracked, and the under-layer substrates began pyrolysis. More combustible volatiles escaped, leading to the appearance of the second peak (Spearpoint and Quintiere 2000; Marney and Russell 2008). The HRR declined sharply and reach a plateau, due to the formation of carbonaceous residue and the gradual extinguishment of the fire.


Fig. 7. The heat release rate (HRR) curves of different treated samples

Both pHRR1 and pHRR2 of W + APP were lower than those of W (26% less for pHRR1 and 47% less for pHRR2). The first peak represents the combustion of flammable gases produced by the degradation of wood. APP inhibited wood combustion effectively by releasing NH3 to dilute the combustible gases and by promoting the char formation to restrain the emission of heat, as discussed previously. The second peak of W + APP appeared earlier (485 s) than that of W (550 s) but was deferred with the further addition of H4A. This result suggests that the thermal stability of the char layer of W + APP + H4A is enhanced due to the formation of Si-O-P and P-O-C bonds within the structure.

The first heat release peaks of zeolite treated samples (W + 4A/H4A, W + APP + 4A/H4A) were all decreased, as shown in Fig. 7 and Table 4. Zeolites can release H2O to dilute the flammable gases, and its framework will act as a barrier to restrain the transmission of heat and O2 to inhibit wood combustion (Wang et al. 2015). The second peak (pHRR2) of W + 4A was increased slightly and occurred 15 s earlier than that of W. In contrast, for W + H4A, the pHRR2 of W + H4A declined to 132.73 kW·m-2 and was delayed from 550 s for W to 615 s. This result clarified that the unique framework of H4A zeolite makes the structure denser and more difficult to crack. Accordingly, H4A zeolite exhibited better performance in reducing the heat release compared with 4A zeolite with 10 wt% of fire-retardant.

Table 4. Results of Cone Calorimetry Test of Different Samples

Smoke production

Because smoke reduces visibility and impedes escape from fire, it is a principal cause of fire casualties (Naeher et al. 2007). Smoke release rate (SPR) and total smoke production (TSP) are thus crucial parameters. Figure 8 shows the SPR curves of different samples, and TSP values are listed in Table 4. Smoke was released rapidly at around 450 s to 700 s, which was attributed to the glowing combustion and further burning of under-layer wood. Accordingly, the smoke production was mainly affected by the coherence and thermal stability of char layer. The SPR curves of treated samples were all lower than W, as shown in Fig. 8. This result suggested that treated wood produces more coherent and thermal-stable char layers. Although the TSP of W + APP was only 16.64 m2·s-1, 68% less than W when the ratio of APP was only 10%, it would have increased dramatically if APP was more than 15%, as shown in earlier research (Wang et al. 2014b). H4A and 4A zeolites exhibited obvious differences in the reduction of smoke. The TSP were 31.45 m2·s-1 and 38.15 m2·s-1 for W + 4A and W + APP + 4A, while the TSP of W + H4A and W + APP + H4A were 23.15 m2·s-1 and 22.25 m2·s-1, respectively, as shown in Table 4. The SPR curves of H4A treated samples with or without APP were also much lower than those of 4A treated ones. Thus, H4A zeolite may strengthen the char layer by making it compact. Therefore, the smoke could not pass through, and it might also absorb the solid particles and droplets of smoke due to the presence of mesopores (as indicated in the N2 adsorption/desorption isotherms and pore size distribution, Fig. 3 and Fig. 4).


Fig. 8. The smoke production rate (SPR) curves of different treated samples

CO production

The toxicity of gases is a cause of casualties during fire (Stec and Hull 2010). Carbon monoxide (CO) is emitted when wood burns incompletely. The CO production rate (COPR) is shown in Fig. 9, and the COY of different samples is given in Table 4. The COPR of W exhibited two peaks, corresponding to the flame combustion of surface wood and glowing combustion of the char layer, respectively, as shown in Fig. 9. The second peak of COPR of W was delayed from 570 s to around 620 s and was lowered when H4A was added (Fig. 9, W + H4A).

For W + APP, the second peak was shifted to 200 s to 400 s, and the peak value were two times larger than that of W. This result can be explained by the formation of the charcoal barrier on the wood surface, which prevents the under-layer wood from contacting O2, leading to the incomplete combustion of wood. The increase of COPR could cause a serious threat to people’s lives. When H4A was added with APP, the large COPR peak disappeared (Fig. 9). This suggested that H4A zeolite can inhibit the emission of CO, which is of great importance for saving a human life in a fire disaster. The second COPR peak of W + APP + H4A was smaller than that of W + APP + H4A. This result suggests that H4A zeolite was more effective than 4A zeolite in reducing the COPR when the APP content is low. The COY of W was increased from 0.045 kg·kg-1 to 0.054 kg·kg-1 for W + APP, while it was reduced by about 50% with the addition of H4A zeolite (COY of W + H4A: 0.023 kg·kg-1), and for W + APP + H4A, the COY was close to that of W + H4A. Therefore, a small amount of H4A zeolite (3.33 wt% of wood flour) is enough to eliminate the CO yield produced by twice as much APP as zeolite (6.67 wt% of wood flour).