Preparation of Intumescent Flame Retardant Poly (butylene succinate

Jul 21, 2010 - supplied by Wuhai city Tianyuan Chemical Industry Co., Ltd. (China). All chemicals were dried in an oven overnight at 70. °C before us...
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Ind. Eng. Chem. Res. 2010, 49, 8200–8208

Preparation of Intumescent Flame Retardant Poly(butylene succinate) Using Fumed Silica as Synergistic Agent Yangjuan Chen,†,‡ Jing Zhan,† Ping Zhang,† Shibin Nie,† Hongdian Lu,† Lei Song,† and Yuan Hu*,† State Key Laboratory of Fire Science and Department of Polymer Science and Engineering, UniVersity of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, PR China

A novel intumescent flame retardant (IFR) poly(butylene succinate) (PBS) with an antidripping property using fumed silica as a synergistic agent was prepared. Ammonium polyphosphate, melamine, and fumed silica were added in PBS via melt blending. It was revealed that the flame retardant PBS exhibited both excellent flame retardance and antidripping properties when the three components of IFR coexisted at an appropriate proportion. The lowest total loading of flame retardant could be reduced to 17 wt % with the synergism of fumed silica, for the goal of vertical flammability (UL-94) V-0 rate. Scanning electron microscopy and X-ray photoelectron spectroscopy were employed to characterize the morphology and composition of residual char, respectively. The combustion properties and thermal degradation behavior of the IFR-PBS composites were fully evaluated, and the possible flame retardant mechanism was proposed. 1. Introduction Recently, poly(butylene succinate) (PBS) has emerged as an excellent biodegradable material to overcome the worldwide environmental problem of white pollution caused by traditional nondegradable plastics.1-3 It is prepared via condensation polymerization of butanediol and succinic acid, and each of them can be facilely obtained from renewable bioresources. This white semicrystalline thermoplastic polymer holds tremendous promise for various end-use applications (such as biomedical fields, agricultural films, one-off daily necessities, packing materials, and foaming products) due to its combined advantages of good processing capabilities, thermal properties, and mechanical properties.4 With further development of PBS serial polyester, the application fields will be increasingly broadened. However, the potential applications of PBS have been restricted mainly owing to its poor fire-resistance, and the formation of melting drip during burning usually leads to the spread of fire. What’s more, there is limited information available about flame retardant PBS up to now, excepting Kuan et al.5 who did some research on it in 2006. They prepared PBS/ ammonium polyphosphate (PBS/APP) composites with improved flame retardancy and antidripping properties via a unique water-cross-linking technique. Nevertheless, the water-crosslinking reaction in hot water may lead to the formation of voids due to the hydrolysis of PBS. These voids would decrease some of the mechanical properties of PBS, such as tensile strength and elongation at break. Meanwhile, the water-cross-linking reaction could limit the crystallization rate of the composites, and decrease Tg of the PBS/APP composites from 32.7 to 23.2 °C (after a 4-h water-cross-linking reaction). Thus, development of novel high efficient methods for flame retardant PBS is quite crucial. It is well-known that intumescent flame retardants (IFR) are efficient in some polymers and are widely used as halogen-free additives owing to their advantages of little smoke and low toxicity.6 The IFR system usually experiences an intense * To whom correspondence should be addressed. Tel./Fax: +86 551 3601664. E-mail: [email protected]. † State Key Laboratory of Fire Science. ‡ Department of Polymer Science and Engineering.

expansion and forms protective charred layers, thus well protecting the underlying material from the action of the heat flux or flame during combustion.7,8 An IFR system is composed of three components: an acid source (precursor for catalytic acidic species), a carbonization agent (or char forming agent), and a blowing agent.9,10 It is worth noting that the technique of marrying IFR to PBS should consist of an efficient approach of preparing a novel flame retardant and biodegradable material, which has not been reported to the best of our knowledge. Moreover, there are only a few articles dealing with the flammability of polymer nanocomposites with silica. For example in 2002, Kashiwagi et al. reported flame retardancy of PMMA nanocomposite with silica.11 The flame retardant mechanism of silica has also been discussed in some studies.12,13 In the current study, intumescent flame retardant PBS (IFRPBS) with significant flame retardancy and antidripping properties was prepared by melt blending with APP and melamine (MA), containing fumed silica as a synergistic agent. The acceptable lowest loading of additives and the optimum ratio of the flame retardant ingredients were investigated. Combustion properties of the IFR-PBS composites were evaluated by limiting oxygen index (LOI), vertical flammability (UL-94) tests, and microscale combustion calorimetry (MCC) experiments. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the morphology and composition, respectively, of residual char. Moreover, thermogravimetric analysis (TG), in situ Fourier transform infrared spectroscopy (in situ FTIR), and thermogravimetric analysis/infrared spectrometry (TG-IR) were used to investigate the influence of flame retardant additives on the thermal degradation process of PBS in condensed and gas phases. The possible flame retardant mechanism was proposed. 2. Experimental Section 2.1. Materials. PBS (Mw ) 190 kDa, relative density (25 °C) ) 1.26, melt flow index (190 °C/2.16 kg) ) 11 g/10 min, hydroxyl end-capped) was purchased from Anqing Hexing Chemicals Co., Ltd. (Anhui, China). APP-II (soluble fraction in water 1000) in powder was obtained from Shandong Shi’an Chemical

10.1021/ie100989j  2010 American Chemical Society Published on Web 07/21/2010

Ind. Eng. Chem. Res., Vol. 49, No. 17, 2010 Table 1. The Composition and Naming of the IFR-PBS Composites Formulation name

PBS (wt %)

APP+MA loading (wt %)

PBS PA PAM6 PAM5 PAM3 PAM1 PAM PAMS1 PAMS2 PAMS3 17PAMS 16PAMS

100 75 75 75 75 75 80 80 80 80 83 84

0 25a 25 25 25 25 20 19 18 17 15b 14b

a

Without the addition of MA. controlled at 15:3:2.

b

SiO2 loading (wt %) 0 0 0 0 0 0 0 1 2 3 2b 2b

APP:MA (ratio of mass)

6:1 5:1 3:1 1:1 5:1 5:1 5:1 5:1 5:1 5:1

The ratio of APP/MA/SiO2 was

Co., Ltd. (China). Melamine (g99%) was provided by Country medicine group chemical agent Co., Ltd. (Shanghai, China). Fumed silica nanoparticles with an average size of 20 nm were supplied by Wuhai city Tianyuan Chemical Industry Co., Ltd. (China). All chemicals were dried in an oven overnight at 70 °C before used. 2.2. Preparation of IFR-PBS Composites. All the samples were prepared on a two-roll mixing mill (XK-160, Jiangsu, China) at 110 °C, and the roll speed was maintained at 30 rpm. PBS was first added to the mill at the beginning of the blending procedure. After the PBS had melted, APP and MA (and fumed silica) were then added to the matrix, and the mixture was processed for about 10 min until a visually good dispersion was achieved. The resulting samples were compressed and molded into sheets (3 mm thickness). The composition and naming of the IFR-PBS composites are listed in Table 1. 2.3. Characterizations. Limiting oxygen index (LOI) tests were measured according to ASTM D2863. The apparatus used was an HC-2 oxygen index meter (Jiangning Analysis Instrument Company, China). The specimens used for the test were of dimensions 100 × 6.5 × 3 mm3. UL-94 vertical burning tests were performed with plastic samples of dimensions 130 × 13 × 3 mm3, suspended vertically above a cotton patch. The classifications are defined according to the American National Standard UL-94. The MCC tests were carried out using a Govmak MCC-2 microscale combustion calorimeter; 4-6 mg samples were heated to 650 °C at a heating rate of 1 °C/s in a stream of nitrogen flowing at 80 cm3/min. The volatile anaerobic thermal degradation products in the nitrogen gas stream were mixed with a 20 cm3/min stream of pure oxygen prior to entering a 900 °C combustion furnace. The MCC data obtained were reproducible to about (3%. TG experiments were performed using a Q5000 IR thermoanalyzer instrument under air flows of 60 mL/min. The specimens (about 5-10 mg) were heated from room temperature to 700 °C at a linear heating rate of 20 °C/min. SEM observations were conducted by high-resolution JEOL JSM-6700 field-emission scanning electron microscopy (FESEM) and environmental scanning electronic microscopy (ESEM) AMRAY1000B. The studied surfaces were first sputter-coated with a thin layer of gold before the measurement. The XPS spectra were recorded with a VG Escalab mark II spectrometer (VG Scientific Ltd., UK), using Al Ka excitation radiation (1486.6 eV) and calibrated by assuming the binding energy of carbonaceous carbon to be 284.6 eV.

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The in situ FTIR spectra were recorded in the range of room temperature (rt) to 550 at 10 °C/min (air atmosphere) on a MAGNA-IR 750 spectrometer (Nicolet Instrument Company, USA). Thermogravimetric analysis/infrared spectrometry (TG-IR) was performed using the TGA Q5000 IR thermogravimetric analyzer that was interfaced to the Nicolet 6700 FT-IR spectrophotometer. About 5.0 mg of the sample was put in an alumina crucible and heated from 30 to 700 °C. The heating rate was set as 20 °C/min (nitrogen atmosphere, flow rate of 45 mL/min). 3. Result and Discussion 3.1. Nanoscale Dispersion of Fumed Silica in PBS Matrix. A homogeneous dispersion of fumed silica nanoparticles, together with strong interfacial interactions between the polymer matrix and fumed silica nanoparticles, can effectively improve the thermal, mechanical, and rheological performances of the polymer matrix.14 To reveal the dispersion of fumed silica in the PBS matrix, the fracture surfaces of the samples PAM and PAMS2 were investigated in detail by SEM. The samples were frozen well in liquid nitrogen and quickly broken off to obtain random brittle fractured surfaces. A layer of gold was sputtercoated uniformly over all of the fractured surfaces before SEM observations. From Figure 1, it could be seen that the fumed silica nanoparticles were dispersed uniformly in the PBS matrix, as a result of the sufficient shear force imposed during the melt compounding process. A few aggregates which did not exceed 200 nm in diameter were observed in higher magnification images, resulting from the interactions between the fumed silica nanoparticles. In contrast, the fracture surface of the sample without fumed silica nanoparticles was smooth and featureless. 3.2. Combustion Characteristics: LOI, UL-94, and MCC. LOI measurements and UL-94 vertical burning tests are widely used to evaluate the flame retardant properties of materials. From Table 2, it can be found that the LOI value of the sample PAM5 obtained the maximum of 39, which was increased by 15 units in comparison with that of pure PBS, and a UL-94 V-0 rate was achieved. Meanwhile, the antidripping effect was obtained. Nevertheless, the LOI and UL-94 results of the samples with other ratios of APP/MA were unsatisfactory as shown in Table 2. So the optimum ratio of APP/MA was fixed at 5:1. However, when the IFR loading was decreased to 20 wt %, just UL-94 V-1 rate was obtained with slight dripping. But after the addition of fumed silica (the total addition of flame retardant was still controlled at 20 wt %), a UL-94 V-0 rate could be reached. The LOI value was increased from 34 (PAM) to 36 (PAMS2). Because of the synergistic effect of fumed silica, the acceptable lowest loading of intumescent flame retardant could be reduced to 17 wt % for the goal of UL-94 V-0 rate. The digital photos of the specimens after LOI tests are shown in Figure 2. In accordance with LOI and UL-94 results, samples (PAM5 and PAMS2) with the ratio of APP/MA 5:1 formed notable intumescent char during burning process. The intumescent charred layer provided resistances of both mass and heat transfer, and postponed the degradation process which would form combustible substance.10 MCC is one of the most effective bench scale methods for investigating the combustion properties of polymer materials. It uses an oxygen consumption calorimeter to measure the rate and amount of heat which is produced by complete combustion of the fuel gases generated during controlled heating of a milligram-sized sample.15 The heat release rate (HRR) curves

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Figure 1. SEM microphotographs of the fractured surfaces: (a) and (b) PAMS2; (c) PAM. Table 2. LOI and UL-94 Results of Pure PBS and Flame Retardant PBS Composites sample

LOI (vol %)

UL-94 rate

dripping

ignite the absorbent cotton

PBS PA PAM6 PAM5 PAM3 PAM1 PAM PAMS1 PAMS2 PAMS3 17PAMS 16PAMS

24 31 35 39 36 34 34 35 36 35 33 31

NR NR V-2 V-0 V-0 V-2 V-1 V-0 V-0 V-0 V-0 V-1

yes yes no/yesa no no no/yesa no/yes no no no no no/yes

yes yes no no no no no no no no no no

a

No/yes corresponds to the first/second flame application.

Figure 3. The HRR curves of IFR-PBS composites at 1 °C/s heating rate. Table 3. Part Data Recorded in MCC Experiments

Figure 2. Pictures of the samples after LOI tests: (a) PAM6; (b) PAM5; (c) PAM3; (d) PBS; (e) PAM; (f) PAMS2.

of IFR-PBS composites are shown in Figure 3, and the corresponding combustion data are presented in Table 3. The peak of heat release rate (PHRR) was found to be one of the most important parameters in the evaluation of fire safety, and low values of PHRR were an indication of low flammability and low full-scale fire hazard.16,17 It could be observed that a PBS/IFR system with 2 wt % fumed silica showed a best synergistic effect. The PHRR of PAMS2 was decreased by 18%,

samples

PHRR (w/g)

THR (kJ/g)

TPHRR (°C)

PAM PAMS1 PAMS2 PAMS3

886 872 724 861

15.5 14.9 13.8 14.3

382 381 384 383

comparing with that of PAM. In the meanwhile, the total heat release (THR) of PAMS2 decreased to 13.8 from 15.5 kJ/g for PAM. It implied that the presence of fumed silica catalyzed the formation of a more effective intumescent char which could act as a superior thermal insulator and mass-transport barrier to protect the underlying material from further burning and reduce its heat release. The MCC results demonstrated that the thermal decomposition of IFR-PBS was suppressed by the synergistic effect of fumed silica at suitable loading. 3.3. Thermogravimetric Analysis. The TG and DTG (differential thermogravimetry analysis) curves of IFR-PBS with different loadings of fumed silica were shown in Figure 4, and the related data were listed in Table 4. After the addtion of fumed silica (the total addition of flame retardant were still controlled at 20 wt %), the char residues at 700 °C were increased obviously comparing with that of PAM, especially when the loading was 2 wt %. It indicated that the synergistic

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Figure 4. TG and DTG curves of flame retardant PBS with different loadings of silica in air. Table 4. Thermal Properties of Flame Retardant PBS with Different Loadings of Silica

samples

T-5 wt % (°C)

Tmax (°C)

char residue at 700 °C (wt %)

PAM PAMS1 PAMS2 PAMS3

320 315 321 314

346 345 353 349

7.5 10.4 11.7 12.3

effect between IFR and fumed silica could catalyze the char formation of PBS at high temperature. Moreover, the onset and maximum degradation temperature of PAMS2 were both increased in comparison with that of PAM. Regarding the analyses above, a conclusion could be drawn that fumed silica could be used as a good flame-retardant synergistic agent for an IFR-PBS system, and the optimum ratio of N/P/Si is crucial to fully exert flame retardance on PBS. 3.4. Morphological Characterization of the Char Residue. SEM photographs of Figure 5 showed the microstructures of outer and inner char of PAM and PAMS2 after being heated in a muffle furnace for 10 min at 600 °C. As shown in Figure 5a,b, it could be observed that there were some cracks and spots on the outer char of PAM, and its inner char was fragmental. Therefore, they could not provide a good flame shield for the material beneath. In contrast, as shown in Figure 5c,d, both the outer and inner char of PAMS2 were more compact than that of PAM, and had a lot of bubbles. The intumescent charred layer could slow down heat and mass transfer between gas and condensed phases, and prevent the underlying polymeric substrate from further attack by heat flux in a flame.

Figure 5. SEM micrographs of the char formed after combustion (a) and (b) outer and inner char of sample PAM; (c and d) outer and inner char of sample PAMS2.

Table 5. The Relative Content before and after Normalization of Elements Analyzed by XPS sample PAM outer char

PAM inner char

PAMS2 outer char

PAMS2 inner char

element and line

atom %

C 1s O 1s N 1s P 2p C 1s O 1s N 1s P 2p C 1s O 1s N 1s P 2p Si 2p C 1s O 1s N 1s P 2p Si 2p

43.91 45.6 2.87 7.61 43.1 45.29 3.79 7.82 38.51 50.03 2.35 7.73 1.38 45.63 43.4 3.03 7.36 0.58

normalized atom %

27.39 72.61 32.64 67.36 20.51 67.45 12.04 27.62 67.09 5.29

3.5. X-ray Photoelectron Spectroscopy Analysis. The chemical components of the residual char after conbustion were investigated by XPS. The surface of the char layer was liable to be contaminated by carbon and oxygen during XPS analysis.18,19 So we should normalize the XPS data to deduct the deviation in quantitative analysis. From Table 5, it could be seen that, for both of the samples with and without fumed silica, the relative phosphorus content of the outer char was more than that of the inner char, but the change of nitrogen content was the opposite. This common phenomenon was due to the formation of various kinds of phosphorus-containing compounds in the outer char. These phosphorus-containing compounds were cross-linked into a network structure and suppressed the degradation of the material inside. The balance between density and surface area of the additive and polymer melt determined whether the additive accumulated near the sample surface or sank through the polymer melt layer.12 Fumed silica had a large surface area, low density, and low superficial free energy. Consequently, it accumulated on the sample surface without sinking through the PBS melt to act as a thermal insulation layer and also to reduce the PBS concentration near the surface. The conjecture was proven by the evident increase of silicon content of PAMS2 from inner char to outer char as seen from Table 5 and Figure 6. Moreover, the ratios of P/N in the outer and inner char of PAMS2 were both more than that of PAM, indicating release of more small molecules containing nitrogen. A physically

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Figure 6. The Si2p spectra of XPS for inner and outer char of PAMS2.

Figure 7. In situ FTIR spectra for the degradation process of pure PBS at different temperatures.

strong charred layer with lots of bubbles was formed, which was proven in SEM micrographs. The more stable intumescent charred layer could not only prevent the melt from dripping effectively but also hinder the propagation of oxygen and heat into the interior substrate. 3.6. In Situ FTIR Analysis. The chemical structure changes during the thermal degradation of pure PBS, PAM, and PAMS2 were monitored by in situ FTIR. The peaks at 2962 cm-1 and 1725 cm-1 were assigned to the C-H and CdO bonds stretching vibrations, respectively.20 With the increase of temperature, the peak at 1725 cm-1 shifted to 1742 cm-1 because of part of the hydrogen-bond-associated CdO bonds tranformed into dissociated ones. The disappearances of the peaks at 2962 and 1742 cm-1 were accelerated after the addition of flame retardant, which illustrated that APP and MA could catalyze the thermal degradation of PBS. From Figure 7 and Figure 8, it could be seen that there was a peak at 3453 cm-1, which was attributed to the absorption of terminal hydroxyl group of pure PBS. However, there were two peaks at 3420 and 3470 cm-1 for flame retardant PBS. It could be explained by the additional absorptions of the N-H stretching vibrations of -NH2 and NH4+ groups in MA and APP besides that of hydroxyl-terminated group, which could cause the coupling/ splitting of infrared absorption peaks.21 Assignment of the peak around 1400 cm-1 had caused a great controversy.22,23 According to our study, we attributed the band

Figure 8. In situ FTIR spectra for the degradation process of PAM at different temperatures.

Figure 9. In situ FTIR spectra for the degradation processes of PAM and PAMS2 at 320-400 °C.

Figure 10. FTIR spectra of total pyrolysis products of PBS, PAM, and PAMS2.

at about 1430 cm-1 to the combined absorptions of ammonium ions and phosphorus oxynitrides of APP as well as the amine groups of MA. When the temperature was increased to 320 °C, the band was attributed to the absorptions of the phosphorus oxynitrides and melon, and it shifted to 1400 cm-1. This was

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Figure 11. FTIR spectra of pyrolysis products at the maximum decomposition rate of PBS, PAM, and PAMS2.

because the scission of the P-O-N bond and the elimination of NH3 were proceeding continuously in APP during pyrolysis. On the other hand, in addition to the sublimation, melamine underwent progressive endothermic condensation during heating. For pure PBS, the peaks at 1256 and 1157 cm-1 could be ascribed to the stretching vibration of the C-O bond.24 However, for flame retardant PBS, the peak at 1256 cm-1 could be assigned to the absorptions of the C-O bond and PdO bond. When the pyrolysis temperature was increased from room temperature to about 350 °C, it shifted to 1290 cm-1, which was due to the weakening of the hydrogen-bonding effect around the PdO bond as a result of the formation of pyrophosphate.25 PBS exhibited two kinds of crystal forms designated as Rand β-forms, and these crystal structures were investigated by

-1

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Ichikawa et al. The absorptions at 1047 and 958 cm might be associated with the crystallization of PBS. The peak splitting was induced by crystal field, and they merged into one at 1019 cm-1 above 100 °C. But for flame retardant PBS, they started to merge at about 200 °C and became weaker. It indicated that the IFR made an important influence on the crystallization behavior of PBS. And the absorption still existed even at 550 °C, indicating the formation of P-O-C bond.27 The peaks at 1086 and 870 cm-1 corresponded to to the asymmetric and symmetric stretching vibrations of a P-O bond.28 It could be seen from Figure 8 that the two peaks became wider and stronger as the thermal degradation temperature increased, which gave positive evidence that various kinds of P-O-P structures such as P2O5 and P4O10 were formed in the intumescent charred layers.29,30 In the meanwhile, the P-O-C bond degraded into the more stable P-O-P bond. Because of the addition of a low amount of fumed silica, the absorption of Si-O bond31 was not obvious or might be overlapped by other peaks. The three absorptions in flame retardant PBS at 1290, 1086, and 870 cm-1 still existed and became clearer in the spectra at 550 °C, indicating that the residues were mainly composed of stable structures, such as PdO and P-O-P groups. It could be observed in Figure 9 that the intensities of the absorptions that belonged to PBS (such as 2962 and 1742 cm-1) of PAMS2 were stronger than that of PAM, while the difference in the intensities of the absorptions belonging to the flame retardant (such as 1400, 1290, 1086, and 870 cm-1) between PAMS2 and PAM was the opposite. It indicated that a more effective intumescent charred layer was formed with the synergistic effect of fumed silica, which could substitute and inhibit further degradation of PBS during combustion. This layer acted not only as thermal insulation to protect the materials underneath

Figure 12. The absorbance of pyrolysis products for pure PBS, PAM, and PAMS2 Vs time.

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Figure 13. Possible flame retardant mechanism of IFR-PBS composite with fumed silica.

but also as a barrier against the migration to the surface of the thermal degraded products. 3.7. Volatilized Products of Pure PBS and IFR-PBS Analyzed by TG-IR. The TG-IR technique that directly gives identification of the evolved products can significantly contribute to an understanding of thermal degradation mechanisms.32 Therefore, it was selected to characterize the evolved products formed during the thermal degradation of pure and flame retardant PBS. The intensities of absorbance were all normalized to the samples’s content. It could be found from the FTIR spectra of total pyrolysis products as shown in Figure 10 that the maximum decomposition of PBS, PAM, and PAMS2 were at 20.2, 16.5, and 17.7 min, respectively. The total release of gas products decreased significantly after the addition of fumed silica. The gaseous products at the maximum decomposition temperature of PBS, PAM, and PAMS2 are shown in Figure 11. They are easily identified by their characteristic absorbance: water (H2O) (3668 cm-1), unsaturated alkane (3080, 908 cm-1), saturated hydrocarbons (2880-2980 cm-1), carbon dioxide (CO2) (2360 and 2315 cm-1, 667 cm-1), methanol (CH3OH) (1050-1070, 669 cm-1), and ammonia (NH3) (927 cm-1).33,34 The peak at 669 cm-1 was due to the mutual absorptions of C-O bending vibration of CO2 and O-H deformation vibration of CH3OH. It is worth mentioning that the intensity of the peak at 908 cm-1 was increased significantly after the addition of flame retardant. This phenomenon could be clarified in that the absorption of C-H deformation vibration of gaseous alkene was overlapped by the additional absorption of NH3 degraded from APP and MA. The band attributed to the absorption of the C-O

bond was increased from 1051 cm-1 in pure PBS to 1070 cm-1 in PAM, but then shifted to 1065 cm-1 in PAMS2. It might be interpreted that the hydrogen-bonding effect around the C-O bond was weakened during pyrolysis, but then strengthened as a result of the hydrogen-bonding effect between PBS and fumed silica. Furthermore, there were some aromatic compounds (characteristic peak at 1598 cm-1) formed during the degradation process of flame retardant PBS. It was noteworthy that the peak at 1762 cm-1 attributed to carboxylic acid moved to 1810 cm-1 attributed to acid anhydride. This phenomenon could be interpreted that acid anhydride could be formed at a high temperature in result of the reactions among carboxylic acid molecules. From the comparison of intensities, it could be found the amount of acid anhydride was more than the amount of carboxylic acid, which was the main reason for the weakening of the hydrogen-bonding effect. The changes in the peaks at 2360, 1810, and 1762 cm-1 implied that more CO2 could be formed from the decomposition of the compounds containing carbonyl during pyrolysis, which was catalyzed by acidic species degraded from APP. Nevertheless, it was restrained by fumed silica, indicating that there might be a synergistic effect between silicon, nitrogen, and phosphorus. To clearly understand the changes of the formed products, the relationship between the intensity of the characteristic peaks and temperature for some of the volatilized products were plotted in Figure 12. With the combination of Figure 10 and Figure 11, the shifts in decomposition temperature illustrated that the degradation of PBS was catalyzed by acidic species degraded from APP on the whole, but postponed by fumed silica to some extent. This could be explained by the fact that fumed silica

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improved the thermal stability of PBS, and the interactions between fumed silica and PBS led to the formation of an intumescent char with higher quality which could protect the polymer from decomposing further. From the aforementioned analyses, it was concluded that the volatilized products of the thermal degradation of flame retardant PBS were H2O, saturated hydrocarbons, unsaturated alkane, CO2, carboxylic acid, acid anhydride, aromatic compounds, NH3, and CH3OH. These small volatile gas products were very important for the formation of an intumescent charred layer, which could cause the char to expand and dilute the oxygen and flammable gas during burning. 4. Possible Flame Retardant Mechanism It was speculated that the fumed silica was prone to esterification and transesterification reactions with PBS (or its degradation products) and polyol phosphate compounds degraded from APP during the combustion process, which was catalyzed by the decomposed acidic species. In addition to the cross-linking reactions, there was a strong hydrogen-bonding effect between fumed silica and PBS. These factors have led to the formation of stable three-dimensional interpenetrating networks in the carbon layer. The complicated networks led to an increase in the molecular weight, which could heighten the viscosity of the carbon melt during pyrolysis and combustion. In the temperature range of intumescence, the apparent viscosity of the melt increased with time. The viscosity was high enough to manacle gaseous products resulting from the degradation of the material and also to accommodate stress induced by solid particles and the presumed high pressure of the trapped gases. Most importantly, it can support melted PBS so as to prevent the melted component from dripping during combustion, This has been proven in the vertical flammability test. In the meanwhile, fumed silica was easy to accumulate on the sample surface owing to its large surface area, low density, and low superficial free energy in heating. Thus flame retardant PBS with fumed silica could form a physically strong charred surface layer during combustion. The more stable intumescent charred layer could not only effectively prevent the melt from dripping but also hinder the propagation of oxygen and heat into the interior substrate. Furthermore, the inert volatile products are very important for diluting the oxygen and flammable gas during burning in the gas phase. The possible flame retardant mechanism was depicted in Figure 13. 5. Conclusion We have prepared a novel intumescent flame retardant PBS using a combination of APP and MA as intumescent ingredients and fumed silica as synergistic agent. The flame retardant PBS exhibited both excellent flame retardance and antidripping properties when the three components of flame retardant coexisted at an appropriate proportion. The lowest total loading of flame retardant could be reduced to 17 wt % for the goal of a UL-94 V-0 rating with the synergistic effect of fumed silica. MCC results showed that fumed silica could significantly decrease PHRR and THR of flame retardant PBS. The XPS data indicated that silicon was easy to migrate and gather on the material surface in the heating process. It was speculated that three-dimensional interpenetrating networks were formed in the carbon layer of flame-retardant PBS with fumed silica during combustion, leading to the formation of a physically strong charred surface layer. The more stable intumescent charred layer could not only effectively prevent the melt from

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dripping but also hinder the propagation of oxygen and heat into the interior substrate. Acknowledgment The work was financially supported by the Program for Specialized Research Fund for the National Natural Science Foundation of China (No. 50903080), the Doctoral Program of Higher Education (No. 200803580008) and China Postdoctoral Science Foundation (No. 20080430101). Literature Cited (1) Mochizuki, M.; Hirami, M. Structural effects on the biodegradation of aliphatic polyesters. Polym. AdV. Technol. 1997, 8, 203–209. (2) Ray, S. S.; Okamoto, K.; Okamoto, M. Structure-property relationship in biodegradable poly(butylene succinate)/layered silicate nanocomposites. Macromolecules. 2003, 36, 2355–2367. (3) Shibata, M.; Makino, R.; Yosomiya, R.; Takeishi, H. Poly(butylene succinate) composites reinforced with short sisal fibres. Polym. Polym. Compos. 2001, 9, 333–338. (4) Fujimaki, T. Processability and properties of aliphatic polyesters, ‘BIONOLLE’, synthesized by polycondensation reaction. Polym. Degrad. Stab. 1998, 59, 209–214. (5) Kuan, C. F.; Kuan, H. C.; Ma, C. C. M.; Chen, C. H. Flame retardancy and nondripping properties of ammonium polyphosphate/ poly(butylene succinate) composites enhanced by water crosslinking. J. Appl. Polym. Sci. 2006, 102, 2935–2945. (6) Liepins, R.; Pearce, E. M. Chemistry and toxicity of flame retardants for plastics. EnViron. Health. Persp. 1976, 17, 55–63. (7) Nishihara, H.; Tanji, S.; Kanatani, R. Interactions between phosphorus- and nitrogen-containing flame retardants. Polym. J. 1998, 30, 163– 167. (8) Riva, A.; Camino, G.; Fomperie, L.; Amigouet, P. Fire retardant mechanism in intumescent ethylene vinyl acetate compositions. Polym. Degrad. Stab. 2003, 82, 341–346. (9) Bourbigot, S.; Duquesne, S. Fire retardant polymers: Recent developments and opportunities. J. Mater. Chem. 2007, 17, 2283–2300. (10) Bourbigot, S.; Bras, M. L.; Duquesne, S.; Rochery, M. Recent advances for intumescent polymers. Macromol. Mater. Eng. 2004, 289, 499– 511. (11) Kashiwagi, T.; Morgan, A. B.; Antonucci, J. M.; VanLandingham, M. R.; Harris, R. H.; Awad, W. H.; Shields, J. R. Thermal and flammability properties of a silica-poly(methylmethacrylate) nanocomposite. J. Appl. Polym. Sci. 2003, 89, 2072–2078. (12) Kashiwagi, T.; Gilman, J. W.; Butler, K. M.; Harris, R. H.; Shields, J. R.; Asano, A. Flame retardant mechanism of silica gel/silica. Fire Mater. 2000, 24, 277–289. (13) Kashiwagi, T.; Shields, J. R.; Harris, R. H.; Davis, R. D. Flameretardant mechanism of silica: Effects of resin molecular weight. J. Appl. Polym. Sci. 2003, 87, 1541–1553. (14) Bian, J. J.; Han, L. J.; Wang, X. M.; Wen, X.; Han, C. Y.; Wang, S. S.; Dong, L. S. Nonisothermal crystallization behavior and mechanical properties of poly(butylene succinate)/silica nanocomposites. J. Appl. Polym. Sci. 2010, 116, 902–912. (15) Lyon, R. E.; Walters, R. N.; Stoliarov, S. I. Screening flame retardants for plastics using microscale combustion calorimetry. Polym. Eng. Sci. 2007, 47, 1501–1510. (16) Gilman, J. W. Flammability and thermal stability studies of polymer layered-silicate (clay) nanocomposites. Appl. Clay Sci. 1999, 15, 31–49. (17) Zanetti, M.; Kashiwagi, T.; Falqui, L.; Camino, G. Cone Calorimeter Combustion and Gasification Studies of Polymer Layered Silicate Nanocomposites. Chem. Mater. 2002, 14, 881–887. (18) Reich, T.; Nefedov, V. I. Quantitative XPS surface analysis: Correction for contamination layer. J. Electron. Spectrosc. 1991, 56, 33– 49. (19) Smith, G. C. Evaluation of a simple correction for the hydrocarbon contamination layer in quantitative surface analysis by XPS. J. Electron. Spectrosc. 2005, 148, 21–28. (20) Liang, H.; Shi, W. F. Thermal behaviour and degradation mechanism of phosphate di/triacrylate used for UV curable flame-retardant coatings. Polym. Degrad. Stab. 2004, 84, 525–532. (21) Wang, Z. Z.; Zhou, S.; Hu, Y. Intumescent flame retardation and silane crosslinking of PP/EPDM elastomer. Polym. AdV. Technol. 2009, 20, 393–403.

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ReceiVed for reView April 30, 2010 ReVised manuscript receiVed June 21, 2010 Accepted July 5, 2010 IE100989J