ARTICLE pubs.acs.org/IECR
Poly(vinyl alcohol)/Ammonium Polyphosphate Systems Improved Simultaneously Both Fire Retardancy and Mechanical Properties by Montmorillonite Jian-Sheng Lin, Ya Liu,* De-Yi Wang, Qing Qin, and Yu-Zhong Wang Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China ABSTRACT: The effect of montmorillonite (MMT) used to improve the fire retardancy of poly(vinyl alcohol) (PVA)/ammonium polyphosphate (APP) was studied. The limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimeter, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM) were used to evaluate the effect of the presence of MMT. Compared to that for the PVA/APP composite, the LOI of PVA/APP/ MMT composite was increased from 27.9 to 30.8 and UL-94 could reach V-0 when the total loading of flame retardant was 15 wt %. The results from cone calorimeter parameters such as the heat release rate (HRR), the total heat release (THR), and mass loss (ML) showed that the addition of MMT improves the flame retardancy of PVA/APP systems significantly. The TGA data showed that MMT improved the thermal stability of the PVA/APP systems at high temperature. Importantly, mechanical properties such as tensile strength and elongation at break for the PVA/APP composite could be enhanced by the presence of MMT.
1. INTRODUCTION Poly(vinyl alcohol) (PVA) has been used in many applications such as coatings, adhesives, fibers, films, hydrogels, biomaterials, etc., because of its excellent mechanical properties, good chemical resistance, easy processability, biodegradablility, and water solublility.1 11 However, the flammability of PVA restricts its applications in some important fields. Therefore, the fire retardance of PVA is very necessary and important. To date a few reports on flame-retardant PVA have appeared in the literature, and most of them are related to halogenated compounds. Generally, traditional halogen flame retardants show good flame retardancy in PVA.12,13 However, they have some serious disadvantages, such as the release of toxins during their decomposition. Therefore, it is necessary to find halogen-free fire retardants for PVA.14 In recent years, layered nanofillers, such as layered double hydroxide (LDH)15 17 and layered silicate,18,19 have been extensively studied because of their wide application in several areas. In our previous study,20 a series of LDHs were synthesized and applied in PVA matrix as synergistic agents used to improve the flame retardancy of APP, showing that it could exhibit an obvious synergistic effect with APP. Montmorillonite (MMT) is a typical layered silicate, which is made up of two tetrahedral silicate sheets with an aluminum oxide octahedral sheet sandwiched in between. In previous work,21,22 MMT was used to further enhance the fire retardancy of a variety of polymer composites. The mechanism of this fire-retardant action is linked to the char formation during combustion, which impedes heat transfer and oxygen contact, thus slowing down polymer pyrolysis.23 However, to the best of our knowledge, there are few reports about the effect of layered silicate on the flammability of poly(vinyl alcohol) flame retarded using APP. The preparation of flame-retardant PVA/APP/MMT nanocomposites is reported here. A study of synergistic effects between APP and MMT on the flame-retardant, thermal, and r 2011 American Chemical Society
mechanical properties has been investigated by the limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimeter, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and mechanical property tests. The main purpose of this study is to study the effect of MMT on the flame retardancy and mechanical properties of PVA/APP systems.
2. EXPERIMENTAL SECTION 2.1. Materials. The clay Closite Na+ was purchased from
Southern Clay Products, Inc., Gonzales, TX, USA. Ammonium polyphosphate (APP) was obtained from Changfeng Chemical Corp. (Shifang Chemical). Poly(vinyl alcohol) (average molecular weight 83 368 g/mol) was provided by Sichuan Weinilun Industry Corporation (Chongqing, China). All the above chemicals were commercial products and were used as received. The water used in this experiment was distilled. 2.2. Preparation of the PVA/APP/MMT Composites. First, the 5% aqueous solution of PVA/APP was prepared by using a three-necked round-bottomed flask with vigorous stirring at 90 °C for 4 h. Distilled water was used to form a suspension of MMT. The suspension was stirred for 24 h at room temperature and sonicated for 30 min. Then the MMT suspension was introduced into a stirred aqueous solution of PVA/APP at a concentration of 0.1 3.0 wt % at 90 °C for 8 h. After ultrasonicating for 1 h at 70 °C with vigorous stirring, the transparent or translucent solution obtained was cast onto a glass plate to form a sheet of suitable thickness and size for the LOI, UL-94, Received: March 19, 2010 Accepted: July 17, 2011 Revised: July 7, 2011 Published: July 17, 2011 9998
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Industrial & Engineering Chemistry Research and cone calorimeter tests. The total content of APP and MMT in PVA was kept at 15 wt %. 2.3. Characterization. 2.3.1. XRD Analysis. The dispersion of montmorillonite in the PVA was evaluated using a Philips X-ray diffractometer (XRD) in steps of 0.03° using Cu KR radiation at 40 kV and 150 mA between 2 and 25° (2θ). 2.3.2. TEM Analysis. The morphologies of the composites containing MMT were observed by using a transmission electron microscope (TEM; H-800-1, Hitachi, Japan). The sample was ultramicrotomed with a diamond knife at room temperature to give 70 90 nm thick sections. 2.3.3. LOI and UL-94 Testing. The LOI values were measured on an HC-2C oxygen index meter (Jiangning, China) with sheet dimensions of 130 6.5 0.5 mm3 according to GB/T 2406-93. The vertical burning tests were carried out on a CZF-2-type instrument (Jiangning, China) with sheet dimensions of 130 13 0.5 mm3 according to ASTM D3801. 2.3.4. Cone Calorimeter. The signals from the cone calorimeter (FTT standard cone calorimeter, Fire Testing Technology Ltd., U.K.) were recorded and analyzed by a computer system. All samples (100 100 3 mm3) were exposed horizontally to an external heat flux of 35 kW/m2 irradiance according to ISO 5660 standard procedures. 2.3.5. Thermogravimetric Analysis (TGA). Thermogravimetric analysis (TGA) on PVA/APP/MMT composites (approximately 7.0 mg) was performed on a TG 209 F1 (Netzsch, Germany) instrument in flowing air (60 mL/min) at 10 °C/min from 40 to 700 °C. 2.3.6. Scanning Electron Microscopy (SEM). The morphologies of the residues of PVA/APP and PVA/APP/MMT composites after the cone calorimeter test were investigated by scanning electron microscopy (SEM) using a JEOL JSM-5009LV instrument operating at an accelerating voltage of 10 kV after gold coating surface treatment. 2.3.7. Tensile Tests. Tensile tests were conducted on an Instron universal material testing system (Model 4320). The samples were cut into strips according to GB/T 1039-92 and tested at room temperature with a gauge length of 20 mm and crosshead speed of 20 mm/min. All the specimens were previously conditioned for more than 1 week under a relative humidity of 50%, and the values were averaged over five measurements. 2.3.8. Fourier Transform Infrared (FTIR) Spectroscopy. The FTIR spectra of residues of PVA/APP with or without MMT at a series of temperatures in a muffle furnace were characterized with KBr powder by use of a Nicolet FTIR 170SX infrared spectrophotometer.
3. RESULTS AND DISCUSSION 3.1. Dispersibility of PVA/APP/MMT Composites. Figure 1 shows the XRD patterns of the MMT, PVA/MMT (MMT 1 wt %) composite, and flame-retardant PVA composite (PVA/APP/ MMT, MMT 1 wt %). From Figure 1, MMT gives a very intense peak at 2θ = 6.6°, corresponding to 1.33 nm of basal spacing. After blending with PVA, the interlayer spacing increased significantly to 2.15 nm (2θ = 4.1°), indicating that some PVA chains are intercalated into the MMT galleries forming an intercalated exfoliated structure.24 In Figure 1, it also can be found that there is an intense peak appearing about 2θ = 19.6°, corresponding to a d-spacing of 0.45 nm for PVA.25 The intensity of this peak is decreased in the presence of MMT with or without
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Figure 1. XRD patterns of (a) MMT, (b) PVA/APP/MMT, (c) PVA/ MMT, and (d) PVA.
Figure 2. TEM of PVA/APP/MMT (MMT 1 wt %) nanocomposite.
Table 1. Effect of Mass Ratio of APP to MMT on the Flame Retardancy of PVA Composites with 15 wt % Total Loading flame retardancy
component (wt %)
a
sample
PVA
APP
MMT
1
100
0
2
85
15.0
3 4
85 85
14.9 14.7
0.1 0.3
5
85
14.5
6
85
14.3
7
85
8
85
APP:MMT
LOI
UL-94
0
19.7
NR
0
27.9
V-1
149:1 49:1
28.9 29.6
V-0 V-0
0.5
29:1
30.5
V-0
0.7
20:1
30.8
V-0
14.0
1.0
14:1
29.1
V-0
12.0
3.0
4:1
25.0
NRa
Sample does not extinguish after the first ignition.
APP particles, as shown in Figure 1b,c. The introduction of clay or APP into polymer matrix may interfere with the crystallinity. The TEM for PVA/APP/MMT (Figure 2) confirms that a mixed morphology has formed in the PVA matrix. Individual silicate layers are exfoliated in the PVA matrix, and some larger intercalated tactoids are visible. Therefore, both XRD and TEM consistently indicate that these samples are in a hybrid structure where both intercalated and exfoliated silicate layers coexist. 3.2. Flame Testing. Table 1 presents the limiting oxygen index (LOI) values and vertical burning test (UL-94) results of pure PVA and PVA systems with different additives. The LOI value of pure PVA is 19.7; the presence of 15 wt % APP in PVA 9999
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Figure 3. Effect of MMT on LOI values of composites.
Figure 4. Heat release rate curves of PVA, PVA/APP, and PVA/APP/ MMT systems vs time.
enhances the LOI value to 27.9, although it cannot reach a V-0 rating. However, the fire retardancy of PVA/APP composite was improved effectively with a small loading of MMT. Figure 3 presents the changes in the LOI values vs the MMT loading level for the PVA/APP/MMT samples with 15 wt % as the total amount of additives. With the increase of the proportion of MMT, the LOI value first increases and then drops. Compared with the PVA/APP composite, the LOI values of PVA/APP/ MMT nanocomposites increase from 27.9 to 30.8 as shown in Table 1. However, the LOI value decreases when the mass ratio of APP to MMT is lower than 20:1, indicating that there is a synergistic effect between APP and MMT and their optimal ratio is 20:1. Most of the PVA/APP/MMT samples could achieve a V-0 rating and do not generate dripping during the UL-94 test. Therefore, the mass ratio of APP to MMT would affect the flame retardancy of PVA/APP/MMT. The cone calorimeter is one of the most effective bench scale methods for investigating the combustion properties of polymer materials,26,27 and the parameters obtained include the time to ignition (TTI), heat release rate (HRR), peak heat release rate (PHRR), total heat release (THR), and mass loss (ML). In order to investigate the effects of APP and MMT in various samples on the fire behavior of the composites, a cone calorimeter has been used.
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Figure 5. Total heat release curves of PVA, PVA/APP, and PVA/APP/ MMT systems vs time.
Figure 6. Mass loss of PVA, PVA/APP, and PVA/APP/MMT systems vs time.
The heat release rate (HRR), particularly the peak heat release rate (PHRR), has been found to be one of the most important parameters to evaluate fire safety.28,29 The HRR curves of PVA, PVA/APP, and PVA/APP/MMT (containing 0.7 wt % MMT) at a heat flux of 35 kW/m2 are illustrated in Figure 4. Compared with neat PVA, the PVA/APP and PVA/APP/MMT composites burn slowly and the peak HRR decreases drastically from 576 to 194 and 157 kW/m2, respectively, with only 15 wt % loading level. Moreover, the combustion process is prolonged. The results suggest that the addition of APP with or without MMT can improve the flame retardancy of PVA remarkably. Figure 5 shows the total heat release (THR) curves of the samples. Obviously, the THR values of PVA/APP and PVA/ APP/MMT composites are lower than that of pure PVA during testing. At the end of burning, pure PVA releases a total heat of 99 MJ/m2, while PVA/APP releases 48 MJ/m2 and PVA/ APP/MMT releases only 43 MJ/m2, which is in accordance with the results of the HRR. The significant decrease in the THR of flame-retardant PVA indicates that a part of the polymer is not completely burnt. This may be because during the burning process a char is formed on the surface of the matrix, which serves as a thermal insulation layer to inhibit polymer pyrolysis and prevent the evolution of combustible 10000
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Table 2. Data of Cone Calorimeter for the Pure and Flame-Retardant PVA Blends sample PVA
a
TTI (s)
PHRR (kW/m2)
FPIa (m2 s/kW)
THR (MJ/m2)
Av-HRR (kW/m2)
char (%)
Av-MLR (g/s)
33
576
0.057
99
243
1.4
0.10
PVA/APP
116
194
0.598
48
118
15.8
0.07
PVA/APP/MMT
129
157
0.822
43
98
26.9
0.06
Calculated from the ratio of TTI to PHRR.
gases to feed the flame, and also separate oxygen from burning materials. In Figures 4 and 5, it is obvious that HRR values of the PVA/ APP/MMT nanocomposite are lower than that of the PVA/APP composite during testing, but THR values of PVA/APP/MMT are almost the same as that of PVA/APP during the first 370 s, and then the THR values of PVA/APP/MMT are clearly lower than that of PVA/APP. The reason for the lower HRR and THR values of the PVA/APP/MMT nanocomposite is that the char of the PVA/APP/MMT nanocomposite may be more compact than that of the PVA/APP composite due to the synergistic effect between APP and MMT, which can protect underlying materials more effectively. This result shows that introducing MMT and APP to PVA can improve the flame retardancy of PVA. The dynamic curves of mass versus time for the above three samples at a heat flux of 35 kW/m2 are shown in Figure 6. It is seen that pure PVA loses its mass faster than PVA/APP and PVA/APP/MMT composites. After combustion, the residues of pure PVA, PVA/APP, and PVA/APP/MMT samples are 1.4, 15.8, and 26.9%, respectively. The PVA/APP/MMT nanocomposite shows the lowest mass loss, which further indicates that the char formed is more compact and stronger to prevent mass and heat transfer. This trend agrees with the results from TGA discussed later. The combustion parameters obtained from the cone calorimeter are summarized in Table 2. The fire performance index (FPI) is calculated from the ratio of TTI to PHRR, with higher numbers indicating better flame resistance performance.30 The FPI of the PVA/APP/MMT is higher than those of PVA/APP and pure PVA as shown in Table 2, indicating a reduced flammability of PVA treated with APP/MMT. The order of time to ignition (TTI) values of samples is PVA (33 s) < PVA/APP (116 s) < PVA/APP/MMT (129 s), which shows that the addition of APP with or without MMT can delay TTI. The addition of APP with or without MMT improves the time to ignition significantly. The reason may be formation of a thermooxidative surface layer (cross-linking phosphate) avoiding homogeneous ignition,20,31 or the release of less combustible gases. Thermal analysis shows that the flame retardant decomposes before polymer decomposition. Thus the delayed ignition time is attributed to the fuel dilution effect of the additive.32 In addition, the average HRR (Av-HRR) of PVA/APP/MMT (98 kW/m2) is lower than that of PVA/APP (118 kW/m2), whereas the AvHRR of neat PVA is 243 kW/m2, indicating that there exists a synergistic effect between APP and MMT for flame-retardant PVA. The mass loss rates (MLR) are 0.10, 0.07, and 0.06 g/s, respectively, for pure PVA, PVA/APP, and PVA/APP/MMT samples. The rates show that APP with or without MMT could effectively prohibit the decomposition of the composites and form an amount of char on the surface, especially with the addition of MMT. These results suggest that the flame retardancy of PVA/APP composite was improved remarkably by MMT, which is in good agreement with the conclusion of LOI measurements.
Figure 7. TGA curves of APP, PVA, PVA/APP, and PVA/APP/MMT systems in air.
Table 3. Thermogravimetric Analysis Data of Various Samples in Air char residue (%) Tinitiala (°C) Tmaxb (°C) 500 °C 600 °C 700 °C
sample PVA APP
255 240
294 560
11.9 75.8
0.5 40.4
0.5 30.1
PVA/APP
233
291
35.9
30.2
18.9
PVA/APP/MMT
215
274
39.5
34.9
24.0
a
Tinitial is the initial degradation temperature (temperature at 5% weight loss). b Tmax is the maximum-rate degradation temperature.
All the results above demonstrate that MMT plays an important role in enhancing the flame retardancy of PVA/APP composites. 3.3. Thermal Decomposition Behaviors. The thermal stability of the composites was studied by TGA under air atmosphere. Figure 7 and Table 3 give thermogravimetric analysis curves and data of PVA, PVA/APP, and PVA/APP/MMT (containing 0.7 wt % MMT). Differential thermogravimetric (DTG) curves for all samples under air are illustrated in Figure 8. Comparison of TGA results for PVA and flame-retarded PVA presented in Figure 7 suggests that the thermal stability of flame-retarded PVA is diminished below 350 °C but enhanced at higher temperatures. It can be seen that the thermal decomposition of PVA composites mainly occurs in the range 220 470 °C with two distinguishable weight loss zones, except pure PVA. At the first step, APP could decompose under heat to produce polyphosphoric acids, water, and ammonia; the polyphosphoric acids could react with the hydroxyl of PVA to yield cross-linked phosphate ester. The resulting residues would decompose further to yield 10001
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Figure 8. DTG curves of APP, PVA, PVA/APP, and PVA/APP/MMT systems in air.
voids and the compact char coating surface of the polymer matrix, which can protect the material effectively at the latter step. The degradation rate of pure PVA becomes very quick after 255 °C, which is related to the first stage of degradation with elimination of volatile products (the main product is water).33,34 Degradation of PVA starts above 270 °C later, and nearly no char residue remains over 550 °C. It is noticed that the Tinitial of the PVA/APP composite is shifted to lower temperatures than that of pure PVA. Meanwhile, the PVA/APP composite has a higher amount of residue at temperatures ranging from 480 to 700 °C. When MMT is added as a synergistic agent in PVA/APP, the thermal degradation behavior of PVA/APP is changed; that is, MMT lowers the Tinitial of PVA/ APP. The PVA/APP/MMT nanocomposite shows higher char residues than that of PVA/APP at 500, 600, and 700 °C, also as shown in Table 3. This fact indicates that MMT can catalyze water elimination, cross-linking, and decomposition reactions between PVA and APP and plays an important role in the hindered diffusion of volatile decomposition products due to the barrier effect of the clay layer structure.35 Therefore, the flame retardancy of PVA is improved, and this is the possible reason why the flame-retardant performance of PVA/APP/MMT is better than those of PVA/ APP and pure PVA. Figure 8 shows DTG curves of the samples which reflect the height of the peak in DTG curves corresponding to the major weight loss stage. From Figure 8, we can see the height of the peak of the PVA/APP sample, which is higher than that of pure PVA but lower when MMT is used in combination with APP. This result may be caused by the less volatile material in the gas phase and more substance remaining in the condensed phase to form a protective compact char. The fact that the LOI value of phosphorus-containing flame-retardant PVA increased with the increase of the char amount and thermal stability suggests that the action mode of phosphorus might be in the condensed phase.36,37 In order to further evaluate the synergistic effect between APP and MMT, theoretical mass loss curves are calculated from a linear combination of their TGA curves for the individual components and compared to the experimental data. Figure 9 shows the TGA curves of pure PVA, PVA/APP/ MMT, and calculated curves of ternary composites, respectively.
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Figure 9. TG curves of pure PVA, PVA/APP/MMT, and calculated PVA/APP/MMT systems.
Figure 10. FTIR spectra of residues from PVA/APP treated for 5 min at (a) 400, (b) 500, (c) 600, and (d) 700 °C.
Calculated residues = residues of PVA 85% + residues of APP 14.3% + residues of MMT 0.7%. Through the comparison of the experimental and calculated TGA curves, it can be seen that there is a decrease in the weight loss onset and an increase in the solid residue of the ternary composites above 370 °C in the experimental curve compared with the calculated curve. It is suggested that there exists an obvious synergistic effect to improve the thermal stability and enhance the char formation, which occurs between MMT and APP. The exact mechanism of interaction between APP and MMT in PVA is not known; however, it is anticipated that these additives slow or prevent depolymerization of the PVA chains. Water from deintercalation and dehydroxylation process catalyzes the formation of polyphosphoric acids, which promotes the formation of a char network leading to the delay of the thermooxidative degradation rate, probably.17 The TGA and cone calorimeter results reveal that there is a synergistic effect between the MMT and APP, contributing to the improved thermal stability and flammability properties. 3.4. FTIR Spectra of Char Residues. In order to analyze how the MMT affects the formation of charred layer, FTIR spectroscopy was used to analyze the charred layer composition of PVA/ APP and PVA/APP/MMT (0.7 wt % MMT). The chars were collected after heating at 400, 500, 600, and 700 °C for 5 min in a 10002
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Figure 11. FTIR spectra of residues from PVA/APP/MMT treated for 5 min at (a) 400, (b) 500, (c) 600, and (d) 700 °C.
muffle furnace. Figures 10 and 11 show the FTIR spectra of PVA/APP and PVA/APP/MMT at a series of temperatures. In Figure 10a, the peak at 3162 cm 1 is attributed to the stretching vibration of N H in NH4+, which does not appear in the spectrum of the PVA/APP/MMT sample in Figure 11a. This indicates that MMT can promote the decomposition of APP. The peak at 3427 cm 1 is attributed to the stretching vibration of OH. The peaks at 989 cm 1 are assigned to P O C absorption, and the peak at 1234 cm 1 is attributed to PdO absorption.18,20 The characteristic peaks of PdO and P O C appear, giving positive evidence of the phosphorus component in the condensed phases. The peaks at 2920 and 2850 cm 1 are assigned to vibrations of CH3 and CH2 . In Figure 10c,d, the intensities of the CH3 and CH2 absorption peaks decrease a lot. However, the intensities of those peaks increase a lot with the loading of MMT in the PVA/APP composite in Figure 11c,d. These results indicate that MMT can enhance char formation at higher temperatures. 3.5. Morphology of Residues from PVA/APP and PVA/ APP/MMT Composites. To further investigate the synergistic effect between MMT and APP on the char formation during combustion, SEM was employed to investigate the microstructure of the char residue of the PVA matrix after the cone calorimeter test, which is shown in Figure 12. Parts a and b of Figure 12 are outer surface and inner surface residues of the PVA/APP composite, and parts c and d of Figure 12 are outer surface and inner surface chars from residues of the PVA/APP/ MMT nanocomposite, respectively. It is observed that there is much residual char for PVA/APP, with or without MMT. The char can form protective shields on the surfaces of materials to protect the matrix effectively by limiting the heat transfer from the heat source to the substrate and flammable compound transfer from the substrate. It can be seen in Figure 12a that the outer surface of the char residue has cracks, and a “foam” char structure with some flaws and big holes is observed in Figure 12b, which may be the primary reason for poor flame retardancy. A continuous and compact char structure is observed in Figure 12c and a char layer with some small holes in Figure 12d is formed, indicating a strong charred layer formed during combustion. This could be explained by MMT catalyzing the cross-linking of the polyphosphate or the phosphate with PVA during combustion to form efficient char both inner and outer for PVA/APP/MMT nanocomposites. This char can prevent the heat transfer between the
Figure 12. SEM morphology of the char samples obtained from (a) PVA/APP outer surface, (b) PVA/APP inner surface, (c) PVA/APP/ MMT outer surface, and (d) PVA/APP/MMT inner surface.
Table 4. Mechanical Properties of PVA Composite and Its Blends with APP or APP/MMT PVA
APP
MMT
tensile strength
elongation at
sample
(wt %)
(wt %)
(wt %)
(MPa)
break (%)
45.5 ( 2.4
228.7 ( 10.7
25.6 ( 1.2
209.0 ( 6.0
0.1 0.3
30.7 ( 0.7 31.1 ( 3.4
269.1 ( 5.2 317.1 ( 4.1
14.5
0.5
34.9 ( 0.3
320.2 ( 1.5
14.3
0.7
40.2 ( 1.7
335.3 ( 10.2
85
14.0
1.0
41.1 ( 0.6
348.8 ( 6.8
85
12.0
3.0
37.2 ( 0.9
313.6 ( 4.1
1
100
2
85
15
3 4
85 85
14.9 14.7
5
85
6
85
7 8
flame zone and the substrate, and thus protect the underlying materials from further burning and pyrolysis. As a result, PVA/ APP/MMT nanocomposites exhibit better flame retardancy than that of the PVA/APP composite, which agrees with the results of LOI, UL-94, TGA, and cone calorimeter testing. 3.6. Mechanical Properties. The results of mechanical properties for PVA, PVA/APP, and PVA/APP/MMT (MMT from 0.1 to 3 wt %) composites are presented in Table 4. From the experimental data shown for PVA and PVA/APP/MMT samples, the addition of APP/MMT reduces tensile strength and increases the elongation at break. However, both the tensile strength and the elongation at break of PVA/APP/MMT nanocomposites are significantly higher than those of the PVA/APP composite. The tensile strength and the elongation at break increase stepwise with the increase of MMT content. When the MMT content is 1 wt %, the tensile strength and the elongation at break reach a maximum of 41.1 MPa and 348.8%, respectively, probably 60.5% and 66.9% increasing, compared with the corresponding values of 25.6 MPa and 209% of the PVA/APP composite. The obvious improvement of the mechanical properties may be attributed to a physical cross-linking network which is 10003
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Industrial & Engineering Chemistry Research produced among layered particles, APP molecules, and polymer chains. This physical cross-linking effect can enhance the mechanical properties. However, with higher MMT content (3 wt %), both the tensile strength and the elongation at break are reduced; the reason may be the possible aggregation of MMT particles.
4. CONCLUSIONS Flame-retarded PVA nanocomposites containing a layered silicate (MMT) and APP are prepared via solution intercalation blending, and their morphologies are evaluated using XRD and TEM. The flame retardancy of PVA could be improved by adding APP and MMT rather than APP alone, with an increase in the LOI values and the improvement of UL-94 rating, while total loading of APP and MMT is kept at 15 wt %. The highest LOI value of all samples is 30.8 and UL-94 can reach a V-0 rating, while the mass ratio of APP to MMT is 20:1. Meanwhile most of the cone calorimeter parameters are improved distinctly. HRR, THR, and ML of PVA/APP and PVA/APP/MMT indicate that MMT can enhance the flame retardancy of PVA/APP. The TG results of PVA, APP, PVA/APP, and PVA/APP/MMT show that MMT has a catalytic effect in the initial decomposition stage and promotes the formation of char at high temperatures. From the analysis of FTIR spectroscopy a conclusion can be drawn that MMT can promote the formation of phosphoric acid and enhance char formation at higher temperatures. From the SEM analysis of the residue of PVA/APP/MMT, a continuous and dense char layer is observed, which could inhibit the transmission of heat during contacting flame. The mechanical property results show that the introduction of MMT into PVA/APP could enhance the tensile strength and elongation at break. All results indicate that MMT has a significant effect on the flame retardancy and mechanical properties of PVA/APP systems at a low content. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86-28-85410755. Fax: +86-28-85410284. E-mail: yya_liu@ 163.com.
’ ACKNOWLEDGMENT The authors gratefully acknowledge the National Science Foundation of China (Grant 50933005) and Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT1026) for financial support. ’ REFERENCES (1) Israel, C.; Joseph, J. G.; David, V. Polymeric alloys of polyphosphonates and acetyl cellulose: I. Sorption and diffusion of benzene and cyclohexane. J. Appl. Polym. Sci. 1974, 18, 2117. (2) Sahmetlioglu, E.; Yuruk, H.; Toppare, L.; Cianga, L.; Yagci, Y. Synthesis and characterization of conducting copolymers of poly(vinyl alcohol) with thiophene side-groups and pyrrole. Polym. Int. 2004, 53, 2138. (3) Ruckenstein, E.; Sun, F. Anomalous sorption and pervaporation of aqueous organic mixtures by poly (vinyl acetal) membranes. J. Membr. Sci. 1994, 95, 207. (4) Fei, J. Q.; Gu, L. X. PVA/PAA thermo-crosslinking hydrogel fiber: preparation and pH-sensitive properties in electrolyte solution. Eur. Polym. J. 2002, 38, 1653.
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