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polypropylene composites
Construction and Building Materials 79 (2015) 337–344
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Synergistic effect of synthetic zeolites on flame-retardant wood-flour/polypropylene composites Wen Wang, Wei Zhang, Hui Chen, Shifeng Zhang, Jianzhang Li ⇑ MOE Key Laboratory of Wooden Material Science and Application, Beijing Key Laboratory of Wood Science and Engineering, Beijing Forestry University, Beijing 100083, China
h i g h l i g h t s Different loadings of synthetic 4A or 13X zeolites were used to prepare WPCs. The zeolites had effects on both gas and solid phase flame-retardancy mechanism. WPC/APP/zeolites performed better flame retardancy than WPC/APP. WPC/APP/zeolites presented better mechanical properties than WPC/APP.
a r t i c l e
i n f o
Article history: Received 10 September 2014 Received in revised form 15 December 2014 Accepted 4 January 2015
Keywords: Synthetic zeolite Wood-plastic composite Thermal degradation Synergistic mechanism Mechanical property
a b s t r a c t Flame-retardant wood-plastic composites (WPCs) consisting of ammonium polyphosphate (APP) as flame retardant and four different concentrations (2, 4, 6, and 8 wt%) of synthetic zeolites (type 4A and 13X) as synergistic agent were prepared. The results of thermogravimetry analysis showed that the inclusion of synthetic zeolites catalyzed the thermal degradation of WPCs. Both limiting oxygen index and cone calorimetry tests were used to evaluate the flame performance of WPCs, and the results proved that the addition of 4A or 13X zeolites had synergistic effect on the flame retardancy of WPCs. The residues of burned WPCs after cone calorimetry tests were characterized by scanning electron microscopy, energy dispersive spectroscopy, and Fourier transform infrared spectroscopy. All above data in series were integrated to speculate the synergistic mechanism. Moreover, all the measured mechanical tests proved that the incorporation of zeolites increased flexural properties and impact strength, and decreased tensile properties of WPCs. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Now-a-days, wood-plastic composites (WPCs) have emerged as a new generation of construction engineering materials for different indoor and outdoor usage, such as decking for terrace and balconies, landscaping timbers, fencing, furniture and automobile products etc. [1–3]. They have attracted both researchers and manufacturers with excellent performance, including low cost, high dimensional stability during lifetime, high relative strength and stiffness [4]. However, the high flammability of WPCs limits their application in various fields. Therefore, improvement of the flame retardancy of WPCs is increasingly important and many efforts have been devoted to it. The most expeditious method to impart flame retardancy of WPCs is the incorporation of flame retardants (FRs) during the compounding process [5–10]. Ammonium polyphosphate (APP) is ⇑ Corresponding author. Tel./fax: +86 10 62338083. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.conbuildmat.2015.01.038 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
one of the most effective and widely used environmental-friendly FRs for the flame-retardancy modification of WPCs [6–9]. However, the significant disadvantage of APP is that it enhances the flame retardancy of WPCs with obviously deteriorating the mechanical properties, because of the poor compatibility between APP and WPC matrix. In our previous researches, we have used thermosetting resins to microencapsulate APP to overcome this problem [5]. Although nanoclay [11], carbon nanotube [12], and nano-SiO2 [13] have been reported by other researchers as synergistic agent to modify the flammability, the synergistic mechanism was not clear enough. Therefore, further research needs to be done to improve both the flame-retardant and mechanical performance of WPCs and also investigate the synergistic mechanism. Zeolites are hydrated crystalline aluminosilicates with microporous nanostructures and pore sizes ranging from about 3–15 Å. The structure of zeolites consists of 3-dimensional frameworks made up by SiO4 and AlO4 tetrahedra. They possess permanent negative charges in their structural framework, which has the capability of adsorbing or rejecting molecules, thus they are widely used in
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catalysis, separation, adsorption and ion-exchange [14–18]. Furthermore, zeolites have also been used to reinforce nanocomposites [19–21] or as synergistic agents to improve the flame retardancy or thermal stability of polymers [22–25] due to their high mechanical strength, good thermal and chemical stability, but there is little reports about effect of zeolites on the properties of WPCs. In this research, the goal was to investigate the effect of synthetic zeolites (type 4A and 13X) on WPCs using APP as flame retardant. Thermal degradation behavior of WPC/APP and WPC/ APP/zeolites were investigated by thermogravimetry analysis (TGA). The synergistic effects of synthetic zeolites on the flame retardancy of WPCs were evaluated by limiting oxygen index (LOI) and cone calorimetry tests. To further investigate the synergistic mechanism of synthetic zeolites, the residues of WPCs after burning were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR). Moreover, the effects of the zeolites on the mechanical properties of WPCs have also been highlighted in the present study.
(Thermo Scientific, USA) used in the attenuated total reflection mode. Before tests, all samples were grounded into powders. Each spectrum was collected at a resolution of 4 cm 1 and a scan rate of 32 over the range of 4000–650 cm 1. 2.4. TGA To investigate the effect of synthetic zeolites on the thermal decomposition behavior of WPCs, TGA analysis was conducted. All TGA tests were carried out by a Q50 TGA analyzer (TA Instruments, USA) at a linear heating rate of 10 °C/min under pure nitrogen. The temperature ranged from ambient to 700 °C. Before test, each sample was grounded into powders and kept within 5–8 mg in an open platinum pan. 2.5. Fire performance experiments 2.5.1. LOI LOIs of all the WPCs were measured using a HC-2 oxygen index meter (Jiangning Analysis Instrument Company, China) with the sheets (135 6.5 3 mm3) according to ASTM D2863-77. Fifteen replicates were tested for each group. 2.5.2. Cone calorimetry The combustion experiments were carried out with a cone calorimeter (Stanton Redcroft, UK) in accordance with ISO 5660 procedures. Each specimen (100 100 3 mm3) was wrapped in aluminum foil and exposed horizontally to 50 kW/m2 external heat flux. Three replicates were tested for each group.
2. Experimental methods 2.6. SEM–EDS
2.1. Materials The materials were all made in China, including flour of poplar (Populus tomentasa Carr., particle size between 60–80 mesh sieve), Gaocheng Xingda Wood Flour Company, Hebei. Polypropylene (PP, K8303, melt-flow rate 1.5 g/10 min) and maleic anhydride-grafted PP (MAPP, melt-flow rate 120 g/10 min), Beijing Yanshan Petrochemical Co. Ltd. APP (average polymerization degree n > 1000), Shenzhen Jingcai Chemical Co. Ltd. 4A zeolites (Si/Al = 1, compensation cation: Na) and 13X zeolites (Si/Al = 1.5, compensation cation: Na) (chemical reagents), Shanghai Jiuzhou Chemical Co. Ltd. 2.2. Preparation of WPCs First, the wood-flour was oven-dried at 105 °C until the weight stabilized, which took about 24 h. Then, MAPP, APP, and synthetic zeolites (type 4A or 13X) were added into PP and wood-flour (PP/wood-flour = 6:4) with constant weight percentage (Table 1), each group of the raw materials was mixed in a high-speed mixer at a mixing speed of 2900 rpm for 4 min before being melt-blended in a co-rotating twin-screw extruder (KESUN KS-20, Kunshan, China) to produce composite pellets. The corresponding temperatures in the extruder barrel were 165/ 170/175/180/175 °C from hopper to die zones, and the screw speed was 167 rpm. Then the extrudates were cut into small particles about 5 mm, and then dried again at 105 °C for 3 h before being taken out for hand matting. A hot press (SYSMEN-ll, China Academy of Forestry, Beijing, China) was used to produce the composites by compressing the mat at 180 °C with a pressure of 4 MPa for 6 min. After hot pressing, the formed mat was cooled down at 4 MPa for another 6 min at room temperature in a cold press. The control WPC without APP or zeolites was prepared similarly. The dimension of the composites was 270 270 3 mm3 with a target density of 1.0 g/cm3.
The morphologies of residues for WPCs at the end of cone calorimetry tests and the fracture surfaces of impact tested WPCs were investigated by a Hitachi S-3400 SEM analyzer (Philips, Japan) with an acceleration voltage of 5 kV. EDS spectra showed the quantified surface elemental compositions of the residues. 2.7. Mechanical property experiments 2.7.1. Flexural properties Flexural properties including modulus of rupture (MOR) and modulus of elasticity (MOE) were measured as Chinese standard GB/T 9341-2000, which involves a three-point bending test at a crosshead speed of 1 mm/min. Six replicates (60 25 3 mm3) of each group were tested and standard deviations (SDs) were calculated. 2.7.2. Tensile properties Tensile properties including tensile strength and elongation at break were measured according to Chinese standard GB/T 1040-1992 at a testing speed of 2 mm/min. Six dumbbell replicates of each group were tested and SDs were calculated. 2.7.3. Impact strength Unnotched izod impact tests were performed in accordance with Chinese standard GB/T 1843-1996. Six unnotched replicates (80 10 3 mm3) of each group were tested to calculate the izod impact strength.
3. Results and discussion 3.1. FTIR analysis
2.3. FTIR The surface structure of zeolites, WPCs and the residues for WPCs at the end of cone calorimetry tests were examined by a Nicolet 6700 spectrophotometer
Table 1 Formulations of WPCs. WPC type
Control WPC/APP WPC/APP/4A
WPC/APP/13X
Composition based on weight (%) MAPP
APP
8 8 8 8 8 8 8 8 8 8
25 25 25 25 25 25 25 25 25
4A
13X
2 4 6 8 2 4 6 8
Before FTIR tests, all the synthetic zeolites were dried under vacuum at 80 °C for 24 h. Fig. 1a shows the FTIR spectra of 4A and 13X zeolites. Generally, the absorption bands at 3600–3300 cm 1 and 1600 cm 1 could be attributed to the –OH stretching and deformation of zeolites, respectively [26]. The intensive absorption band at around 960 cm 1 could be assigned to the Si–O and Al–O asymmetric stretching belonging to the SiO4 and AlO4 tetrahedra [27]. The bands observed at about 750 and 665 cm 1 were due to the Si–O symmetric stretching and oscillations of chains of aluminosilicate oxygen tetrahedrals, respectively [26,27]. The FTIR spectra of control WPC, WPC/APP and WPC/APP/zeolites are shown in Fig. 1b. A broad absorption band in the region of 3600–3100 cm 1 could be assigned to the –OH groups in control WPC. The intensity of this band slightly decreased with the addition of 4A or 13X zeolites. This may due to the hydrogen-bonding or ionic interactions between WPC and zeolites [25]. The spectrum of control WPC in the region of 3000–1000 cm 1 could be
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a Residual weight (%)
Transmission
a
100
13X
80
60
40
Control WPC/APP WPC/APP/4A WPC/APP/13X
20
4A 0 100
4000
3500
3000
2500
2000
1500
200
1000
300
400
500
600
700
o
Temperature ( C)
Wavenumber (cm-1)
b
b Derivative weight (%/min)
Control
WPC/APP
Transmission
2.0
WPC/APP/4A
WPC/APP/13X
1.5
1.0
Control WPC/APP WPC/APP/4A WPC/APP/13X
0.5
0.0 100
200
300
400
500
600
700
o
4000
3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm )
Temperature ( C) Fig. 2. TGA (a) and DTG (b) curves of WPCs (the addition of 4A or 13X zeolite was 2%).
Fig. 1. FTIR spectra of (a) zeolite powders (type 4A and 13X) and (b) WPCs.
characterized by absorption peaks at 2950, 2917, and 2838 cm 1 (–CH stretching of CH3 groups), 1454 and 1375 cm 1 (–CH bending of CH3 groups), 1166, 1019, and 997 cm 1 (C–O stretching). Moreover, the peaks at the region of 1000–650 cm 1 in the spectrum of control WPC may be due to the presence of –CH stretching. FTIR spectra of WPC/APP and WPC/APP/zeolites (type 4A or 13X) exhibited the peaks at 1210 and 1045 cm 1 could be assigned to P–O stretching of APP. However, the characteristics peaks of zeolites in the region of 1200–600 cm-1 could not be discriminated since they were superimposed by the WPC/APP spectrum. 3.2. Thermal degradation analysis TGA and derivative thermogravimetry (DTG) curves of WPCs are presented in Fig. 2a and b, respectively. Temperatures of 5% mass loss (T5%), two maximum pyrolysis (T1 and T2), and the residual weight at different temperatures are summarized in Table 2. As can be seen from Fig. 2, the thermal degradation process of all WPCs was composed of two steps. All of them were relatively stable up to the temperature above 200 °C, which was higher than the operational temperature for the co-rotating twin-screw extruder and hot presser. Above 250 °C, WPC/APP and WPC/APP/zeolites decomposed more easily and performed relatively lower T5% than the control WPC. The first step started from about 250–350 °C belonging to the decomposition of part of PP and APP, hemicellu-
lose (225–325 °C) and cellulose (325–375 °C) in wood flour [3,28]. T1 was found to be 331.5 °C for the control WPC and 314.0 °C for WPC/APP (Table 2), indicating that the addition of APP significantly accelerated the decomposition of WPC. The second step occurring at about 400–500 °C was more important and faster, which was the further thermal degradation of the lignin in wood flour (250–500 °C), most of PP and APP. The addition of APP into WPC delayed the T2 from 455.5 °C (control) to 468.0 °C, indicating that WPC/APP was more thermal stable than WPC without FR in the high temperature region. Whereas, the incorporation of 4A or 13X zeolites into WPC performed both slightly lower T1 and T2, and higher residual weight (about 27%) at the end of the tests than WPC/APP (18.9%), which indicated that the inclusion of zeolites may catalyze the thermal degradation of APP or WPC to some extent, leading to form better charred layer to protect WPC from further decomposing in the higher temperature region. 3.3. Fire performance of WPCs 3.3.1. LOI tests In order to compare the flammability of the control WPC, WPC/ APP, and WPC/APP/zeolites (type 4A and 13X), LOI tests were conducted, and the results are shown in Table 3. Clearly, the incorporation of APP significantly increased LOI value from 21.0 (control WPC) to 23.6, and further increase can be seen with the introduction of 4A zeolites into WPC. The LOI values of WPC/APP/2% 4A
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Table 2 TGA data for the two decomposition steps of WPCs. WPC type
Decomposition temperature (°C)
Control WPC/APP WPC/APP/4A WPC/APP/13X
Residual weight (wt%)
T5%
T1
T2
500 °C
600 °C
700 °C
278.1 277.1 276.6 276.7
331.5 314.0 312.9 312.3
455.5 468.0 467.1 466.2
10.0 24.4 29.7 29.9
9.0 21.7 28.3 28.4
8.8 18.9 26.5 26.8
Table 3 LOI tests data. WPC
4A zeolites content (%)
13X zeolites content (%)
Type
Control
WPC/APP
2
4
6
8
2
4
6
8
LOIs (%)
21.0
23.6
24.2
24.2
24.1
24.0
23.8
23.8
23.8
23.8
Heat release rate (kW/m2)
800
zeolites and WPC/APP/4% 4A zeolites were both 24.2. Synergism between 4A zeolites and APP can be easily observed at a relatively low loading. However, LOI value decreased contrarily with more addition of 4A zeolites into WPC. The reason for this phenomenon may be that the addition of 4A zeolites could catalyze the esterification reaction between WPC and APP, leading to form a more stable intumescent carbonaceous residue [22,29], thus effectively improved the flame retardancy of WPC. However, when the inclusion of 4A zeolite was higher than 4%, the esterification reaction rate between WPC and APP increased too much, which could has negative effect on the matching between the viscosity and the rate of expansion foaming during the combustion of WPC matrix. The synergistic effect of 13X zeolites on the flame retardancy of WPC/APP can also be seen in Table 3, which shows that LOI value of both WPC/APP/13X zeolites was 23.8, indicating that the addition amount of 13X zeolites into WPC/APP may not obviously increase the viscosity of the melting matrix.
Control WPC/APP WPC/APP/4A WPC/APP/13X
600
400
200
0 0
100
200
300
400
500
600
700
Time (s) Fig. 3. HRRs versus burning time for different WPCs (the addition of 4A or 13X zeolite was 2%).
3.3.2. Cone calorimetry tests Cone calorimetry can well simulate the behavior of material in a real fire and further interpret the results of LOI tests. The heat release rate (HRR) measured by cone calorimetry is a very important parameter in these tests since it expresses the intensity in a fire. We will use the experimental values to compare the flame retardancy of WPCs with the results of the LOI tests. Representative HRR versus burning time curves for the control WPC, WPC/APP, and WPC/APP/zeolites (2% 4A or 13X zeolites) are displayed in Fig. 3. Table 4 lists the data of cone calorimetry tests. Clearly, the HRR curve of the control WPC gradually increased until a peak value (696 kW/m2) was reached, and sharply decreased owing to the consumption of the material. When WPC was composited with 25% APP, the Pk-HRR decreased markedly and the combustion time was prolonged. A higher IT and lower HRR, MLR, and THR are preferable to reduce the flammability of materials. As expected, both the IT, HRR and MLR of WPC/APP/zeolites (type 4A or 13X) were much lower than those of WPC/APP (Table 4). These results indicate that the incorporation of 4A or 13X zeolites had synergistic effect on the flame retardancy of WPC/APP. Moreover, the THR of WPC/APP/4A or 13X zeolites (69 and 68 MJ/m2, respectively) were slightly lower than that of WPC/APP, indicating that zeolites were capable of playing a positive role in flame-retardant WPC via a gas flame retardancy mechanism. Furthermore, WPC/APP/4A or 13X zeolites left more residual weight after combustion than WPC/APP, illustrating the formation of better charred layer in a real fire, which could effectively protect the underlying materials from further thermal
degradation. All these results indicate that both 4A and 13X zeolites are feasible for improving the flame retardancy of WPC. In general, the lower Yco and higher Yco2 are capable to illustrate the more complete combustion reaction and the lower toxicity of exhaust gas during the process of burning. Clearly, the addition of APP into WPC produced negative effect on the exhaust gas parameters, which dramatically increased Yco and decreased Yco2 (Table 4). However, the incorporation of 4A or 13X zeolites into WPC/APP significantly decreased Yco from 0.136 kg/kg (WPC/APP) to about 0.090 kg/kg, and increased Yco2 from 1.50 kg/kg (WPC/APP) to about 1.75 kg/kg, which illustrated that zeolites could effectively reduce the toxicity of exhaust gases during the combustion of WPC. 3.4. Characterizations of the residues for burned WPCs 3.4.1. SEM–EDS analysis The morphology of the surface residues of different WPCs after cone calorimetry tests are shown in Fig. 4. Meanwhile, the quantified elemental compositions of the residues were quantified via EDS as shown in Table 5. Obviously, almost no successive charred residue was left at the end of the test for the control WPC (Fig. 4a). Because of the thermal decomposition of most of organics, the amount of inorganic substances in wood flour containing elemental Si and Al were comparatively high, especially elemental Si, which reached to 24.12 wt%. In contrast, the residues of the other three WPCs were both covered by an expanded char network (Fig. 4b–
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W. Wang et al. / Construction and Building Materials 79 (2015) 337–344 Table 4 Cone calorimetry data of different WPCs. WPC type
ITa (s)
Av-HRRb (kW/m2)
Pk-HRRc (kW/m2)
Av-MLRd (g/s)
Pk-MLRe (g/s)
THRf (MJ/m2)
Ycog (kg/kg)
Yco2h (kg/kg)
Residual weighti (wt%)
Control WPC/APP WPC/APP/4A WPC/APP/13X
13 ± 1 17 ± 1 18 ± 0 18 ± 1
150 ± 1.7 103 ± 1.9 92 ± 1.3 88 ± 2.1
696 ± 5.1 391 ± 3.5 325 ± 3.2 351 ± 2.8
0.041 ± 0.001 0.034 ± 0.003 0.029 ± 0.001 0.027 ± 0.002
0.218 ± 0.006 0.146 ± 0.011 0.124 ± 0.007 0.132 ± 0.006
89 ± 0.4 71 ± 0.2 69 ± 0.3 68 ± 0.5
0.042 ± 0.001 0.136 ± 0.001 0.090 ± 0.001 0.095 ± 0.001
2.18 ± 0.006 1.50 ± 0.005 1.75 ± 0.006 1.74 ± 0.006
5.8 ± 0.3 19.2 ± 0.1 25.7 ± 0.4 23.9 ± 0.2
Mean ± standard deviation. a Ignition time. b Average heat release rate. c Peak heat release rate. d Average mass lose rate. e Peak mass lose rate. f Total heat release. g Yield of carbon monoxide. h Yield of carbon dioxide. i Residual weight at the end of the test.
Fig. 4. SEM images of the surface char formation for different WPCs. ((a) control WPC; (b) WPC/APP; (c) WPC/APP/2% 4A zeolite; (d) WPC/APP/2% 13X zeolite) Note: The scale bars of the images were 1500.
Table 5 Surface elemental compositions of the char residuals for different WPCs analyzed by EDS. WPC type
C (wt%)
N (wt%)
O (wt%)
P (wt%)
Si (wt%)
Al (wt%)
Control WPC/APP WPC/APP/4A WPC/APP/13X
10.90 30.35 2.89 3.20
7.51 4.38 6.17 6.74
54.51 46.51 57.15 55.57
0 18.63 28.44 28.25
24.12 0.12 3.69 4.23
2.96 0.01 1.66 2.01
d), leading to form the protective structure to improve the flame retardancy of WPC. Compared with the residue of the control WPC, much more little carbonaceous particles joined together to form a barrier on the chars of WPC/APP (Fig. 4b). However, the chars of WPC/APP with a large number of channels or gaps were not closed, and this structure could allow the combustible gases and melting polymer to overflow, resulting in the further combustion. Clearly, the surface of the char for WPC/APP/zeolites (type 4A or 13X) was successive and compact with some bubbles randomly
dispersed (Fig. 4c and d). These bubbles formed during the decomposition process of APP to release volatile gases, such as NH3 and H2O [31], and zeolites to release H2O. Moreover, inclusion of zeolites may capture more elemental O and P during the combustion of WPC (Table 5). Therefore, the burned WPC/APP/zeolites formed much more stable and compact charred layer, which could prevent the transportation of heat and combustible gases to the surface of the burning WPC, thus decrease pyrolysis reactions and improve fire resistance [29,31]. 3.4.2. FTIR analysis Fig. 5 compares the spectra of the residues for different WPCs at the end of cone calorimetry tests. The major absorption peaks of the control WPC were at 1562 cm 1 (C@O stretching), 1433 cm 1 (O–H bending), and the region of 1020–650 cm 1 (C–H bending). The notable absorption peak of WPC/APP at 907 cm 1 could be attributed to P–O stretching of APP. Clearly, the intensity of WPC/APP characteristic peak at 1260 cm 1 (P–O stretching) notably increased with the inclusion of 4A or 13X zeolites. Moreover,
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transportation of heat and combustible gases to the surface of the burning WPC, and the synthetic zeolites could decompose to SiO2 and Al2O3 (Fig. 5) during the esterification reaction between wood flour and PP, leading to the catalysis of the esterification reaction and reinforcement of the charred layer. Moreover, the addition of zeolites may capture more elemental O and P (Table 5), thus improve the integrity of charred layer and flame retardancy [13]. Consequently, the inclusion of zeolites could have synergistic effects on solid-phase flame-retardancy mechanism.
Controll
T Transmission i i
WP PC/A APP WP PC/A APP/44A WP PC/A APP/113X
3.5. Mechanical performance of WPCs
12600
782 10043
4000 0
35000
30000
22500
20000
1500
1 1000
-1
W enum Wave mberr (cm m ) Fig. 5. FTIR spectra of the residuals for different WPCs.
the significant absorption peaks at 1043 and 782 cm 1 in the spectra of the residues for WPC/APP/4A zeolites and WPC/APP/ 13X zeolites could be assigned to Si–O and Al–O asymmetric stretching of SiO2 and Al2O3, which reveal Si and Al based substrates produced during the combustion of WPC. 3.4.3. Synergistic mechanism of synthetic zeolites on flame-retardant WPCs Although zeolites have been used as synergistic agent to improve the flame retardancy of polymers [22–25], their mechanism of synergistic effects on flame retardancy of WPCs has not been reported and explored clearly. After initial speculation, it is found that during the combustion of WPC/APP/zeolites, the flame-retardancy system could generate a series of incombustible gases, including NH3 and H2O from the degradation of APP, and H2O from the decomposition of synthetic zeolites, which could dilute O2 concentration and absorb heat above the surface of burning matrix. Therefore, the incorporation of synthetic zeolites could play an important role in the gas-phase flame-retardancy mechanism. Based on further analysis, it can be concluded that the addition of synthetic zeolites could further decrease the initial thermal degradation temperature of flame-retardancy WPC, and promote char formation of the composite matrix as shown in Fig. 2. During heating, WPC/APP/zeolites formed much more stable and compact charred layer than WPC/APP (Fig. 4b–d), which could prevent the
The mechanical performance, including tensile and flexural properties, and impact strength of the control WPC, WPC/APP, and WPC/APP/zeolites (type 4A and 13X) with different addition amount of zeolites are shown in Table 6. The data presented were the average of six readings. Obviously, both of the mechanical properties of WPC decreased with the addition of APP as previous reported [4,5,9]. With incorporation of 2% 4A or 13X zeolites, MOR, MOE, and impact strength of the WPC obviously increased, especially MOE increased by 9.5% and 16.3% respectively, while tensile strength and elongation at break of the WPC decreased evidently, especially WPC/APP/2% 4A zeolites decreased by 12.8% and 33.1% for these two items respectively. There have researches reported a decrease in tensile strength when combining PP with zeolites [16,30], and further efforts need to be done to deal with this issue. However, both these mechanical property values increased with the increasing 4A zeolites loading up to 6%, beyond that the values decreased contrarily. Whereas the different addition amount of 13X zeolites into WPC showed a disparate mechanical performance trend from WPC/APP/4A zeolites. WPC/APP/2% 13X zeolites performed the optimal mechanical properties, and the mechanical performance decreased with the increasing addition of 13X zeolites into WPC. These results suggest that the preferable loading of 4A and 13X zeolites was 6% and 2%, respectively. The observed increase in mechanical properties indicated that the synthetic zeolites could act as a reinforcing agent that binds the polymer chain inside the WPC matrix and restricts the mobility of the polymer chains [32,33]. However, excess zeolites could form agglomerates inside the WPC matrix and act as microcrack initiator, thus playing a negative role in the mechanical performance. To further investigate the effect of zeolites on mechanical properties of WPCs, morphologies of the fracture surfaces of impact tested WPCs were observed as shown in Fig. 6. As compared with the control WPC (Fig. 6a), the micrograph of fracture surface of WPC/APP (Fig. 6b) showed some randomly scattered APP particles with clean surface. This indicates that APP particles can be pulled out easily from the WPC matrix by breaking the interface due to
Table 6 Mechanical properties of different WPCs. Content of zeolite (%)
Tensile properties
Flexural properties
Unnotched izod impact strength (KJ/m2)
Tensile strength (MPa)
Elongation at break (%)
MOR (MPa)
MOE (GPa)
C 0 4A zeolite
35.51 ± 0.45 35.39 ± 1.45
9.93 ± 0.59 6.97 ± 0.35
69.68 ± 1.58 66.06 ± 1.71
2.20 ± 0.07 2.64 ± 0.15
15.51 ± 2.18 13.42 ± 1.14
2 4 6 8 13X zeolite
30.86 ± 1.43 32.31 ± 0.94 32.54 ± 0.62 32.06 ± 0.38
4.66 ± 0.20 5.66 ± 0.15 5.62 ± 0.19 4.79 ± 0.74
71.51 ± 1.73 72.01 ± 1.48 73.78 ± 1.59 72.22 ± 2.05
2.89 ± 0.11 2.94 ± 0.12 3.13 ± 0.10 2.99 ± 0.12
14.30 ± 0.13 14.45 ± 0.73 16.25 ± 1.51 14.63 ± 1.41
2 4 6 8
33.98 ± 0.61 31.04 ± 1.14 30.80 ± 1.87 27.93 ± 1.60
6.80 ± 0.21 5.13 ± 0.50 4.39 ± 0.74 4.01 ± 0.52
73.87 ± 0.90 73.14 ± 0.23 71.21 ± 1.37 70.46 ± 1.33
3.07 ± 0.13 3.00 ± 0.06 2.88 ± 0.18 2.80 ± 0.14
14.14 ± 0.14 12.23 ± 1.07 11.34 ± 0.07 11.17 ± 1.29
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Fig. 6. SEM micrographs of the fracture surfaces of impact strength tested WPCs. ((a) Control; (b) WPC/APP; (c) WPC/APP/2% 13X zeolites; (d) WPC/APP/6% 13X zeolites) Note: The scale bars of the images were 2500.
the poor adhesion between APP and the WPC matrix, thus decreased the mechanical performance of WPC (Table 6). The image of WPC/APP/2% 13X zeolites (Fig. 6c) shows that in addition to APP particles, some well dispersed zeolite particles were embedded into the matrix by the wetting of zeolites particles with the PP. With the increasing 4A zeolites loading up to 6%, zeolites agglomerated in the fracture surface of WPC (marked by squares in Fig. 6d), acting as microcrack initiator to decrease the mechanical properties.
4. Conclusion In the present study, the effect of different loading (2, 4, 6, and 8 wt%) of synthetic zeolites (type 4A and 13X) on thermal degradation, flammability and mechanical properties of wood-flour/polypropylene composites containing APP as flame retardant were investigated. WPC/APP/zeolites (type 4A or 13X) showed slightly lower T1 and T2, and left much more residues as compared with WCP/APP based on TGA analysis. Both LOI and cone calorimetry tests proved that the incorporation of synthetic zeolites had synergistic effect with APP, and could effectively improve the flame retardancy of WPCs. The synergistic effect was also studied by SEM–EDS and FTIR, and the results showed that inclusion of synthetic zeolites could have synergistic effects on both the gas-phase and solid-phase flameretardancy mechanism. Furthermore, WPC/APP/zeolites (type 4A or 13X) performed better flexural properties and higher impact strength than WPC/APP, especially the WPC/APP/6% 4A zeolites and WPC/APP2% 13X zeolites displayed the optimal mechanical properties.
Acknowledgements The authors are very grateful for financial support from Special Fund Forestry Research in the Public Interest (Project 201204702).
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Synergistic effect of synthetic zeolites on flame-retardant wood-flour/polypropylene composites Wen Wang, Wei Zhang, Hui Chen, Shifeng Zhang, Jianzhang Li ⇑ MOE Key Laboratory of Wooden Material Science and Application, Beijing Key Laboratory of Wood Science and Engineering, Beijing Forestry University, Beijing 100083, China
h i g h l i g h t s Different loadings of synthetic 4A or 13X zeolites were used to prepare WPCs. The zeolites had effects on both gas and solid phase flame-retardancy mechanism. WPC/APP/zeolites performed better flame retardancy than WPC/APP. WPC/APP/zeolites presented better mechanical properties than WPC/APP.
a r t i c l e
i n f o
Article history: Received 10 September 2014 Received in revised form 15 December 2014 Accepted 4 January 2015
Keywords: Synthetic zeolite Wood-plastic composite Thermal degradation Synergistic mechanism Mechanical property
a b s t r a c t Flame-retardant wood-plastic composites (WPCs) consisting of ammonium polyphosphate (APP) as flame retardant and four different concentrations (2, 4, 6, and 8 wt%) of synthetic zeolites (type 4A and 13X) as synergistic agent were prepared. The results of thermogravimetry analysis showed that the inclusion of synthetic zeolites catalyzed the thermal degradation of WPCs. Both limiting oxygen index and cone calorimetry tests were used to evaluate the flame performance of WPCs, and the results proved that the addition of 4A or 13X zeolites had synergistic effect on the flame retardancy of WPCs. The residues of burned WPCs after cone calorimetry tests were characterized by scanning electron microscopy, energy dispersive spectroscopy, and Fourier transform infrared spectroscopy. All above data in series were integrated to speculate the synergistic mechanism. Moreover, all the measured mechanical tests proved that the incorporation of zeolites increased flexural properties and impact strength, and decreased tensile properties of WPCs. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Now-a-days, wood-plastic composites (WPCs) have emerged as a new generation of construction engineering materials for different indoor and outdoor usage, such as decking for terrace and balconies, landscaping timbers, fencing, furniture and automobile products etc. [1–3]. They have attracted both researchers and manufacturers with excellent performance, including low cost, high dimensional stability during lifetime, high relative strength and stiffness [4]. However, the high flammability of WPCs limits their application in various fields. Therefore, improvement of the flame retardancy of WPCs is increasingly important and many efforts have been devoted to it. The most expeditious method to impart flame retardancy of WPCs is the incorporation of flame retardants (FRs) during the compounding process [5–10]. Ammonium polyphosphate (APP) is ⇑ Corresponding author. Tel./fax: +86 10 62338083. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.conbuildmat.2015.01.038 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
one of the most effective and widely used environmental-friendly FRs for the flame-retardancy modification of WPCs [6–9]. However, the significant disadvantage of APP is that it enhances the flame retardancy of WPCs with obviously deteriorating the mechanical properties, because of the poor compatibility between APP and WPC matrix. In our previous researches, we have used thermosetting resins to microencapsulate APP to overcome this problem [5]. Although nanoclay [11], carbon nanotube [12], and nano-SiO2 [13] have been reported by other researchers as synergistic agent to modify the flammability, the synergistic mechanism was not clear enough. Therefore, further research needs to be done to improve both the flame-retardant and mechanical performance of WPCs and also investigate the synergistic mechanism. Zeolites are hydrated crystalline aluminosilicates with microporous nanostructures and pore sizes ranging from about 3–15 Å. The structure of zeolites consists of 3-dimensional frameworks made up by SiO4 and AlO4 tetrahedra. They possess permanent negative charges in their structural framework, which has the capability of adsorbing or rejecting molecules, thus they are widely used in
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catalysis, separation, adsorption and ion-exchange [14–18]. Furthermore, zeolites have also been used to reinforce nanocomposites [19–21] or as synergistic agents to improve the flame retardancy or thermal stability of polymers [22–25] due to their high mechanical strength, good thermal and chemical stability, but there is little reports about effect of zeolites on the properties of WPCs. In this research, the goal was to investigate the effect of synthetic zeolites (type 4A and 13X) on WPCs using APP as flame retardant. Thermal degradation behavior of WPC/APP and WPC/ APP/zeolites were investigated by thermogravimetry analysis (TGA). The synergistic effects of synthetic zeolites on the flame retardancy of WPCs were evaluated by limiting oxygen index (LOI) and cone calorimetry tests. To further investigate the synergistic mechanism of synthetic zeolites, the residues of WPCs after burning were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR). Moreover, the effects of the zeolites on the mechanical properties of WPCs have also been highlighted in the present study.
(Thermo Scientific, USA) used in the attenuated total reflection mode. Before tests, all samples were grounded into powders. Each spectrum was collected at a resolution of 4 cm 1 and a scan rate of 32 over the range of 4000–650 cm 1. 2.4. TGA To investigate the effect of synthetic zeolites on the thermal decomposition behavior of WPCs, TGA analysis was conducted. All TGA tests were carried out by a Q50 TGA analyzer (TA Instruments, USA) at a linear heating rate of 10 °C/min under pure nitrogen. The temperature ranged from ambient to 700 °C. Before test, each sample was grounded into powders and kept within 5–8 mg in an open platinum pan. 2.5. Fire performance experiments 2.5.1. LOI LOIs of all the WPCs were measured using a HC-2 oxygen index meter (Jiangning Analysis Instrument Company, China) with the sheets (135 6.5 3 mm3) according to ASTM D2863-77. Fifteen replicates were tested for each group. 2.5.2. Cone calorimetry The combustion experiments were carried out with a cone calorimeter (Stanton Redcroft, UK) in accordance with ISO 5660 procedures. Each specimen (100 100 3 mm3) was wrapped in aluminum foil and exposed horizontally to 50 kW/m2 external heat flux. Three replicates were tested for each group.
2. Experimental methods 2.6. SEM–EDS
2.1. Materials The materials were all made in China, including flour of poplar (Populus tomentasa Carr., particle size between 60–80 mesh sieve), Gaocheng Xingda Wood Flour Company, Hebei. Polypropylene (PP, K8303, melt-flow rate 1.5 g/10 min) and maleic anhydride-grafted PP (MAPP, melt-flow rate 120 g/10 min), Beijing Yanshan Petrochemical Co. Ltd. APP (average polymerization degree n > 1000), Shenzhen Jingcai Chemical Co. Ltd. 4A zeolites (Si/Al = 1, compensation cation: Na) and 13X zeolites (Si/Al = 1.5, compensation cation: Na) (chemical reagents), Shanghai Jiuzhou Chemical Co. Ltd. 2.2. Preparation of WPCs First, the wood-flour was oven-dried at 105 °C until the weight stabilized, which took about 24 h. Then, MAPP, APP, and synthetic zeolites (type 4A or 13X) were added into PP and wood-flour (PP/wood-flour = 6:4) with constant weight percentage (Table 1), each group of the raw materials was mixed in a high-speed mixer at a mixing speed of 2900 rpm for 4 min before being melt-blended in a co-rotating twin-screw extruder (KESUN KS-20, Kunshan, China) to produce composite pellets. The corresponding temperatures in the extruder barrel were 165/ 170/175/180/175 °C from hopper to die zones, and the screw speed was 167 rpm. Then the extrudates were cut into small particles about 5 mm, and then dried again at 105 °C for 3 h before being taken out for hand matting. A hot press (SYSMEN-ll, China Academy of Forestry, Beijing, China) was used to produce the composites by compressing the mat at 180 °C with a pressure of 4 MPa for 6 min. After hot pressing, the formed mat was cooled down at 4 MPa for another 6 min at room temperature in a cold press. The control WPC without APP or zeolites was prepared similarly. The dimension of the composites was 270 270 3 mm3 with a target density of 1.0 g/cm3.
The morphologies of residues for WPCs at the end of cone calorimetry tests and the fracture surfaces of impact tested WPCs were investigated by a Hitachi S-3400 SEM analyzer (Philips, Japan) with an acceleration voltage of 5 kV. EDS spectra showed the quantified surface elemental compositions of the residues. 2.7. Mechanical property experiments 2.7.1. Flexural properties Flexural properties including modulus of rupture (MOR) and modulus of elasticity (MOE) were measured as Chinese standard GB/T 9341-2000, which involves a three-point bending test at a crosshead speed of 1 mm/min. Six replicates (60 25 3 mm3) of each group were tested and standard deviations (SDs) were calculated. 2.7.2. Tensile properties Tensile properties including tensile strength and elongation at break were measured according to Chinese standard GB/T 1040-1992 at a testing speed of 2 mm/min. Six dumbbell replicates of each group were tested and SDs were calculated. 2.7.3. Impact strength Unnotched izod impact tests were performed in accordance with Chinese standard GB/T 1843-1996. Six unnotched replicates (80 10 3 mm3) of each group were tested to calculate the izod impact strength.
3. Results and discussion 3.1. FTIR analysis
2.3. FTIR The surface structure of zeolites, WPCs and the residues for WPCs at the end of cone calorimetry tests were examined by a Nicolet 6700 spectrophotometer
Table 1 Formulations of WPCs. WPC type
Control WPC/APP WPC/APP/4A
WPC/APP/13X
Composition based on weight (%) MAPP
APP
8 8 8 8 8 8 8 8 8 8
25 25 25 25 25 25 25 25 25
4A
13X
2 4 6 8 2 4 6 8
Before FTIR tests, all the synthetic zeolites were dried under vacuum at 80 °C for 24 h. Fig. 1a shows the FTIR spectra of 4A and 13X zeolites. Generally, the absorption bands at 3600–3300 cm 1 and 1600 cm 1 could be attributed to the –OH stretching and deformation of zeolites, respectively [26]. The intensive absorption band at around 960 cm 1 could be assigned to the Si–O and Al–O asymmetric stretching belonging to the SiO4 and AlO4 tetrahedra [27]. The bands observed at about 750 and 665 cm 1 were due to the Si–O symmetric stretching and oscillations of chains of aluminosilicate oxygen tetrahedrals, respectively [26,27]. The FTIR spectra of control WPC, WPC/APP and WPC/APP/zeolites are shown in Fig. 1b. A broad absorption band in the region of 3600–3100 cm 1 could be assigned to the –OH groups in control WPC. The intensity of this band slightly decreased with the addition of 4A or 13X zeolites. This may due to the hydrogen-bonding or ionic interactions between WPC and zeolites [25]. The spectrum of control WPC in the region of 3000–1000 cm 1 could be
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a Residual weight (%)
Transmission
a
100
13X
80
60
40
Control WPC/APP WPC/APP/4A WPC/APP/13X
20
4A 0 100
4000
3500
3000
2500
2000
1500
200
1000
300
400
500
600
700
o
Temperature ( C)
Wavenumber (cm-1)
b
b Derivative weight (%/min)
Control
WPC/APP
Transmission
2.0
WPC/APP/4A
WPC/APP/13X
1.5
1.0
Control WPC/APP WPC/APP/4A WPC/APP/13X
0.5
0.0 100
200
300
400
500
600
700
o
4000
3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm )
Temperature ( C) Fig. 2. TGA (a) and DTG (b) curves of WPCs (the addition of 4A or 13X zeolite was 2%).
Fig. 1. FTIR spectra of (a) zeolite powders (type 4A and 13X) and (b) WPCs.
characterized by absorption peaks at 2950, 2917, and 2838 cm 1 (–CH stretching of CH3 groups), 1454 and 1375 cm 1 (–CH bending of CH3 groups), 1166, 1019, and 997 cm 1 (C–O stretching). Moreover, the peaks at the region of 1000–650 cm 1 in the spectrum of control WPC may be due to the presence of –CH stretching. FTIR spectra of WPC/APP and WPC/APP/zeolites (type 4A or 13X) exhibited the peaks at 1210 and 1045 cm 1 could be assigned to P–O stretching of APP. However, the characteristics peaks of zeolites in the region of 1200–600 cm-1 could not be discriminated since they were superimposed by the WPC/APP spectrum. 3.2. Thermal degradation analysis TGA and derivative thermogravimetry (DTG) curves of WPCs are presented in Fig. 2a and b, respectively. Temperatures of 5% mass loss (T5%), two maximum pyrolysis (T1 and T2), and the residual weight at different temperatures are summarized in Table 2. As can be seen from Fig. 2, the thermal degradation process of all WPCs was composed of two steps. All of them were relatively stable up to the temperature above 200 °C, which was higher than the operational temperature for the co-rotating twin-screw extruder and hot presser. Above 250 °C, WPC/APP and WPC/APP/zeolites decomposed more easily and performed relatively lower T5% than the control WPC. The first step started from about 250–350 °C belonging to the decomposition of part of PP and APP, hemicellu-
lose (225–325 °C) and cellulose (325–375 °C) in wood flour [3,28]. T1 was found to be 331.5 °C for the control WPC and 314.0 °C for WPC/APP (Table 2), indicating that the addition of APP significantly accelerated the decomposition of WPC. The second step occurring at about 400–500 °C was more important and faster, which was the further thermal degradation of the lignin in wood flour (250–500 °C), most of PP and APP. The addition of APP into WPC delayed the T2 from 455.5 °C (control) to 468.0 °C, indicating that WPC/APP was more thermal stable than WPC without FR in the high temperature region. Whereas, the incorporation of 4A or 13X zeolites into WPC performed both slightly lower T1 and T2, and higher residual weight (about 27%) at the end of the tests than WPC/APP (18.9%), which indicated that the inclusion of zeolites may catalyze the thermal degradation of APP or WPC to some extent, leading to form better charred layer to protect WPC from further decomposing in the higher temperature region. 3.3. Fire performance of WPCs 3.3.1. LOI tests In order to compare the flammability of the control WPC, WPC/ APP, and WPC/APP/zeolites (type 4A and 13X), LOI tests were conducted, and the results are shown in Table 3. Clearly, the incorporation of APP significantly increased LOI value from 21.0 (control WPC) to 23.6, and further increase can be seen with the introduction of 4A zeolites into WPC. The LOI values of WPC/APP/2% 4A
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Table 2 TGA data for the two decomposition steps of WPCs. WPC type
Decomposition temperature (°C)
Control WPC/APP WPC/APP/4A WPC/APP/13X
Residual weight (wt%)
T5%
T1
T2
500 °C
600 °C
700 °C
278.1 277.1 276.6 276.7
331.5 314.0 312.9 312.3
455.5 468.0 467.1 466.2
10.0 24.4 29.7 29.9
9.0 21.7 28.3 28.4
8.8 18.9 26.5 26.8
Table 3 LOI tests data. WPC
4A zeolites content (%)
13X zeolites content (%)
Type
Control
WPC/APP
2
4
6
8
2
4
6
8
LOIs (%)
21.0
23.6
24.2
24.2
24.1
24.0
23.8
23.8
23.8
23.8
Heat release rate (kW/m2)
800
zeolites and WPC/APP/4% 4A zeolites were both 24.2. Synergism between 4A zeolites and APP can be easily observed at a relatively low loading. However, LOI value decreased contrarily with more addition of 4A zeolites into WPC. The reason for this phenomenon may be that the addition of 4A zeolites could catalyze the esterification reaction between WPC and APP, leading to form a more stable intumescent carbonaceous residue [22,29], thus effectively improved the flame retardancy of WPC. However, when the inclusion of 4A zeolite was higher than 4%, the esterification reaction rate between WPC and APP increased too much, which could has negative effect on the matching between the viscosity and the rate of expansion foaming during the combustion of WPC matrix. The synergistic effect of 13X zeolites on the flame retardancy of WPC/APP can also be seen in Table 3, which shows that LOI value of both WPC/APP/13X zeolites was 23.8, indicating that the addition amount of 13X zeolites into WPC/APP may not obviously increase the viscosity of the melting matrix.
Control WPC/APP WPC/APP/4A WPC/APP/13X
600
400
200
0 0
100
200
300
400
500
600
700
Time (s) Fig. 3. HRRs versus burning time for different WPCs (the addition of 4A or 13X zeolite was 2%).
3.3.2. Cone calorimetry tests Cone calorimetry can well simulate the behavior of material in a real fire and further interpret the results of LOI tests. The heat release rate (HRR) measured by cone calorimetry is a very important parameter in these tests since it expresses the intensity in a fire. We will use the experimental values to compare the flame retardancy of WPCs with the results of the LOI tests. Representative HRR versus burning time curves for the control WPC, WPC/APP, and WPC/APP/zeolites (2% 4A or 13X zeolites) are displayed in Fig. 3. Table 4 lists the data of cone calorimetry tests. Clearly, the HRR curve of the control WPC gradually increased until a peak value (696 kW/m2) was reached, and sharply decreased owing to the consumption of the material. When WPC was composited with 25% APP, the Pk-HRR decreased markedly and the combustion time was prolonged. A higher IT and lower HRR, MLR, and THR are preferable to reduce the flammability of materials. As expected, both the IT, HRR and MLR of WPC/APP/zeolites (type 4A or 13X) were much lower than those of WPC/APP (Table 4). These results indicate that the incorporation of 4A or 13X zeolites had synergistic effect on the flame retardancy of WPC/APP. Moreover, the THR of WPC/APP/4A or 13X zeolites (69 and 68 MJ/m2, respectively) were slightly lower than that of WPC/APP, indicating that zeolites were capable of playing a positive role in flame-retardant WPC via a gas flame retardancy mechanism. Furthermore, WPC/APP/4A or 13X zeolites left more residual weight after combustion than WPC/APP, illustrating the formation of better charred layer in a real fire, which could effectively protect the underlying materials from further thermal
degradation. All these results indicate that both 4A and 13X zeolites are feasible for improving the flame retardancy of WPC. In general, the lower Yco and higher Yco2 are capable to illustrate the more complete combustion reaction and the lower toxicity of exhaust gas during the process of burning. Clearly, the addition of APP into WPC produced negative effect on the exhaust gas parameters, which dramatically increased Yco and decreased Yco2 (Table 4). However, the incorporation of 4A or 13X zeolites into WPC/APP significantly decreased Yco from 0.136 kg/kg (WPC/APP) to about 0.090 kg/kg, and increased Yco2 from 1.50 kg/kg (WPC/APP) to about 1.75 kg/kg, which illustrated that zeolites could effectively reduce the toxicity of exhaust gases during the combustion of WPC. 3.4. Characterizations of the residues for burned WPCs 3.4.1. SEM–EDS analysis The morphology of the surface residues of different WPCs after cone calorimetry tests are shown in Fig. 4. Meanwhile, the quantified elemental compositions of the residues were quantified via EDS as shown in Table 5. Obviously, almost no successive charred residue was left at the end of the test for the control WPC (Fig. 4a). Because of the thermal decomposition of most of organics, the amount of inorganic substances in wood flour containing elemental Si and Al were comparatively high, especially elemental Si, which reached to 24.12 wt%. In contrast, the residues of the other three WPCs were both covered by an expanded char network (Fig. 4b–
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W. Wang et al. / Construction and Building Materials 79 (2015) 337–344 Table 4 Cone calorimetry data of different WPCs. WPC type
ITa (s)
Av-HRRb (kW/m2)
Pk-HRRc (kW/m2)
Av-MLRd (g/s)
Pk-MLRe (g/s)
THRf (MJ/m2)
Ycog (kg/kg)
Yco2h (kg/kg)
Residual weighti (wt%)
Control WPC/APP WPC/APP/4A WPC/APP/13X
13 ± 1 17 ± 1 18 ± 0 18 ± 1
150 ± 1.7 103 ± 1.9 92 ± 1.3 88 ± 2.1
696 ± 5.1 391 ± 3.5 325 ± 3.2 351 ± 2.8
0.041 ± 0.001 0.034 ± 0.003 0.029 ± 0.001 0.027 ± 0.002
0.218 ± 0.006 0.146 ± 0.011 0.124 ± 0.007 0.132 ± 0.006
89 ± 0.4 71 ± 0.2 69 ± 0.3 68 ± 0.5
0.042 ± 0.001 0.136 ± 0.001 0.090 ± 0.001 0.095 ± 0.001
2.18 ± 0.006 1.50 ± 0.005 1.75 ± 0.006 1.74 ± 0.006
5.8 ± 0.3 19.2 ± 0.1 25.7 ± 0.4 23.9 ± 0.2
Mean ± standard deviation. a Ignition time. b Average heat release rate. c Peak heat release rate. d Average mass lose rate. e Peak mass lose rate. f Total heat release. g Yield of carbon monoxide. h Yield of carbon dioxide. i Residual weight at the end of the test.
Fig. 4. SEM images of the surface char formation for different WPCs. ((a) control WPC; (b) WPC/APP; (c) WPC/APP/2% 4A zeolite; (d) WPC/APP/2% 13X zeolite) Note: The scale bars of the images were 1500.
Table 5 Surface elemental compositions of the char residuals for different WPCs analyzed by EDS. WPC type
C (wt%)
N (wt%)
O (wt%)
P (wt%)
Si (wt%)
Al (wt%)
Control WPC/APP WPC/APP/4A WPC/APP/13X
10.90 30.35 2.89 3.20
7.51 4.38 6.17 6.74
54.51 46.51 57.15 55.57
0 18.63 28.44 28.25
24.12 0.12 3.69 4.23
2.96 0.01 1.66 2.01
d), leading to form the protective structure to improve the flame retardancy of WPC. Compared with the residue of the control WPC, much more little carbonaceous particles joined together to form a barrier on the chars of WPC/APP (Fig. 4b). However, the chars of WPC/APP with a large number of channels or gaps were not closed, and this structure could allow the combustible gases and melting polymer to overflow, resulting in the further combustion. Clearly, the surface of the char for WPC/APP/zeolites (type 4A or 13X) was successive and compact with some bubbles randomly
dispersed (Fig. 4c and d). These bubbles formed during the decomposition process of APP to release volatile gases, such as NH3 and H2O [31], and zeolites to release H2O. Moreover, inclusion of zeolites may capture more elemental O and P during the combustion of WPC (Table 5). Therefore, the burned WPC/APP/zeolites formed much more stable and compact charred layer, which could prevent the transportation of heat and combustible gases to the surface of the burning WPC, thus decrease pyrolysis reactions and improve fire resistance [29,31]. 3.4.2. FTIR analysis Fig. 5 compares the spectra of the residues for different WPCs at the end of cone calorimetry tests. The major absorption peaks of the control WPC were at 1562 cm 1 (C@O stretching), 1433 cm 1 (O–H bending), and the region of 1020–650 cm 1 (C–H bending). The notable absorption peak of WPC/APP at 907 cm 1 could be attributed to P–O stretching of APP. Clearly, the intensity of WPC/APP characteristic peak at 1260 cm 1 (P–O stretching) notably increased with the inclusion of 4A or 13X zeolites. Moreover,
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transportation of heat and combustible gases to the surface of the burning WPC, and the synthetic zeolites could decompose to SiO2 and Al2O3 (Fig. 5) during the esterification reaction between wood flour and PP, leading to the catalysis of the esterification reaction and reinforcement of the charred layer. Moreover, the addition of zeolites may capture more elemental O and P (Table 5), thus improve the integrity of charred layer and flame retardancy [13]. Consequently, the inclusion of zeolites could have synergistic effects on solid-phase flame-retardancy mechanism.
Controll
T Transmission i i
WP PC/A APP WP PC/A APP/44A WP PC/A APP/113X
3.5. Mechanical performance of WPCs
12600
782 10043
4000 0
35000
30000
22500
20000
1500
1 1000
-1
W enum Wave mberr (cm m ) Fig. 5. FTIR spectra of the residuals for different WPCs.
the significant absorption peaks at 1043 and 782 cm 1 in the spectra of the residues for WPC/APP/4A zeolites and WPC/APP/ 13X zeolites could be assigned to Si–O and Al–O asymmetric stretching of SiO2 and Al2O3, which reveal Si and Al based substrates produced during the combustion of WPC. 3.4.3. Synergistic mechanism of synthetic zeolites on flame-retardant WPCs Although zeolites have been used as synergistic agent to improve the flame retardancy of polymers [22–25], their mechanism of synergistic effects on flame retardancy of WPCs has not been reported and explored clearly. After initial speculation, it is found that during the combustion of WPC/APP/zeolites, the flame-retardancy system could generate a series of incombustible gases, including NH3 and H2O from the degradation of APP, and H2O from the decomposition of synthetic zeolites, which could dilute O2 concentration and absorb heat above the surface of burning matrix. Therefore, the incorporation of synthetic zeolites could play an important role in the gas-phase flame-retardancy mechanism. Based on further analysis, it can be concluded that the addition of synthetic zeolites could further decrease the initial thermal degradation temperature of flame-retardancy WPC, and promote char formation of the composite matrix as shown in Fig. 2. During heating, WPC/APP/zeolites formed much more stable and compact charred layer than WPC/APP (Fig. 4b–d), which could prevent the
The mechanical performance, including tensile and flexural properties, and impact strength of the control WPC, WPC/APP, and WPC/APP/zeolites (type 4A and 13X) with different addition amount of zeolites are shown in Table 6. The data presented were the average of six readings. Obviously, both of the mechanical properties of WPC decreased with the addition of APP as previous reported [4,5,9]. With incorporation of 2% 4A or 13X zeolites, MOR, MOE, and impact strength of the WPC obviously increased, especially MOE increased by 9.5% and 16.3% respectively, while tensile strength and elongation at break of the WPC decreased evidently, especially WPC/APP/2% 4A zeolites decreased by 12.8% and 33.1% for these two items respectively. There have researches reported a decrease in tensile strength when combining PP with zeolites [16,30], and further efforts need to be done to deal with this issue. However, both these mechanical property values increased with the increasing 4A zeolites loading up to 6%, beyond that the values decreased contrarily. Whereas the different addition amount of 13X zeolites into WPC showed a disparate mechanical performance trend from WPC/APP/4A zeolites. WPC/APP/2% 13X zeolites performed the optimal mechanical properties, and the mechanical performance decreased with the increasing addition of 13X zeolites into WPC. These results suggest that the preferable loading of 4A and 13X zeolites was 6% and 2%, respectively. The observed increase in mechanical properties indicated that the synthetic zeolites could act as a reinforcing agent that binds the polymer chain inside the WPC matrix and restricts the mobility of the polymer chains [32,33]. However, excess zeolites could form agglomerates inside the WPC matrix and act as microcrack initiator, thus playing a negative role in the mechanical performance. To further investigate the effect of zeolites on mechanical properties of WPCs, morphologies of the fracture surfaces of impact tested WPCs were observed as shown in Fig. 6. As compared with the control WPC (Fig. 6a), the micrograph of fracture surface of WPC/APP (Fig. 6b) showed some randomly scattered APP particles with clean surface. This indicates that APP particles can be pulled out easily from the WPC matrix by breaking the interface due to
Table 6 Mechanical properties of different WPCs. Content of zeolite (%)
Tensile properties
Flexural properties
Unnotched izod impact strength (KJ/m2)
Tensile strength (MPa)
Elongation at break (%)
MOR (MPa)
MOE (GPa)
C 0 4A zeolite
35.51 ± 0.45 35.39 ± 1.45
9.93 ± 0.59 6.97 ± 0.35
69.68 ± 1.58 66.06 ± 1.71
2.20 ± 0.07 2.64 ± 0.15
15.51 ± 2.18 13.42 ± 1.14
2 4 6 8 13X zeolite
30.86 ± 1.43 32.31 ± 0.94 32.54 ± 0.62 32.06 ± 0.38
4.66 ± 0.20 5.66 ± 0.15 5.62 ± 0.19 4.79 ± 0.74
71.51 ± 1.73 72.01 ± 1.48 73.78 ± 1.59 72.22 ± 2.05
2.89 ± 0.11 2.94 ± 0.12 3.13 ± 0.10 2.99 ± 0.12
14.30 ± 0.13 14.45 ± 0.73 16.25 ± 1.51 14.63 ± 1.41
2 4 6 8
33.98 ± 0.61 31.04 ± 1.14 30.80 ± 1.87 27.93 ± 1.60
6.80 ± 0.21 5.13 ± 0.50 4.39 ± 0.74 4.01 ± 0.52
73.87 ± 0.90 73.14 ± 0.23 71.21 ± 1.37 70.46 ± 1.33
3.07 ± 0.13 3.00 ± 0.06 2.88 ± 0.18 2.80 ± 0.14
14.14 ± 0.14 12.23 ± 1.07 11.34 ± 0.07 11.17 ± 1.29
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Fig. 6. SEM micrographs of the fracture surfaces of impact strength tested WPCs. ((a) Control; (b) WPC/APP; (c) WPC/APP/2% 13X zeolites; (d) WPC/APP/6% 13X zeolites) Note: The scale bars of the images were 2500.
the poor adhesion between APP and the WPC matrix, thus decreased the mechanical performance of WPC (Table 6). The image of WPC/APP/2% 13X zeolites (Fig. 6c) shows that in addition to APP particles, some well dispersed zeolite particles were embedded into the matrix by the wetting of zeolites particles with the PP. With the increasing 4A zeolites loading up to 6%, zeolites agglomerated in the fracture surface of WPC (marked by squares in Fig. 6d), acting as microcrack initiator to decrease the mechanical properties.
4. Conclusion In the present study, the effect of different loading (2, 4, 6, and 8 wt%) of synthetic zeolites (type 4A and 13X) on thermal degradation, flammability and mechanical properties of wood-flour/polypropylene composites containing APP as flame retardant were investigated. WPC/APP/zeolites (type 4A or 13X) showed slightly lower T1 and T2, and left much more residues as compared with WCP/APP based on TGA analysis. Both LOI and cone calorimetry tests proved that the incorporation of synthetic zeolites had synergistic effect with APP, and could effectively improve the flame retardancy of WPCs. The synergistic effect was also studied by SEM–EDS and FTIR, and the results showed that inclusion of synthetic zeolites could have synergistic effects on both the gas-phase and solid-phase flameretardancy mechanism. Furthermore, WPC/APP/zeolites (type 4A or 13X) performed better flexural properties and higher impact strength than WPC/APP, especially the WPC/APP/6% 4A zeolites and WPC/APP2% 13X zeolites displayed the optimal mechanical properties.
Acknowledgements The authors are very grateful for financial support from Special Fund Forestry Research in the Public Interest (Project 201204702).
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