Flame Retardancy and Thermal Degradation of Intumescent Flame

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Flame Retardancy and Thermal Degradation of Intumescent Flame Retardant Starch-Based Biodegradable Composites Kun Wu,† Yuan Hu,*,† Lei Song,† Hongdian Lu,†,‡ and Zhengzhou Wang*,†,§ State Key Laboratory of Fire Science, UniVersity of Science and Technology of China, Anhui 230026, P. R. China; Department of Chemical and Material Engineering, Hefei UniVersity, Hefei, Anhui 230022, P. R. China; and School of Materials Science and Engineering, Tongji UniVersity, Shanghai 200092, P. R. China

Biodegradable PVA/glycerol-plasticized thermoplastic starch (TPS) and its intumescent flame retardant composites are prepared. Microencapsulated ammonium polyphosphate (MCAPP) was used not only to utilize the charring capacity of the polyhydric compounds but also to restrain the reaction between APP and starch during processing. The flame retardancy and thermal stability of TPS and TPS/MCAPP were characterized by LOI, UL 94, TG, and microscale combustion calorimeter (MCC). TPS/MCAPP composites with only 2 wt % MCAPP can pass V-0 in UL 94 test. However, neat TPS cannot pass any rating. The presence of MCAPP can reduce the total heat release of TPS sharply in MCC test. The thermal degradation and gas products of TPS and TPS/MCAPP were monitored by TG-FTIR and dynamic FTIR. XPS and SEM measurements were utilized to investigate the chemical structure, as well as the surface morphology of the residual char. Introduction Recently, because of the continuously increasing plastic wastes and environmental pollution, the use of biodegradable and renewable materials to replace conventional petroleum plastics is becoming popular.1 Starch-based material is one of the most popular biodegradable plastics and is widely used due to the low cost and potential thermoplastic properties of starch.2 It is reported that starch can be blended with thermoplastic polymer (e.g., polystyrene and polyethylene) to provide biodegradation properties to the composites.3,4 Starch is not truly thermoplastic, but under certain shearing and temperature conditions, water or glycerol can act as a good plasticizer agent to disintegrate granules and overcome the strong interaction of starch molecules.5 Starch can be also added to various biodegradable polymers, e.g., poly(lactic acid), poly(propylene carbonate, polycaprolactone, and poly(vinyl alcohol), to achieve the necessary performance properties for various applications.6-8 Starch-based material can be used where long-term durability is not required. For example, biodegradable composites of poly(vinyl alcohol) (PVA) filled with starch have been studied by many researchers and are widely used in packaging and agricultural sector.9,10 Numerous studies are reported to overcome the starch-based materials’ drawbacks, such as weak mechanical strength and moisture sensitivity.1-11 However, starch-based biodegradable materials are flammable, and their applications are also limited.12,13 For environmental concerns, halogen-free flame retardation (HFFR) has aroused great attention in recent years, because halogen-containing flame-retardant materials on burning produce a lot of smoke and toxic gases. Intumescent flame retardation (IFR) is a promising method of HFFR. Its proposed mechanism is based on the charred layer acting as a physical barrier, which slows down heat and mass transfer between gas and condensed phases.14 The conventional IFR system is composed of three * Corresponding author. Tel./Fax: +86-551-3601664. E-mail: [email protected]. † University of Science and Technology of China. ‡ Hefei University. § Tongji University.

parts: an acid source, a carbonization agent, and a blowing agent. Bourbigot and his co-workers have done extensive studies on the APP/IFR systems in polyolefins and reviewed the recent developments of the IFR systems in great detail.14-17 The aim of this work is to enhance the flame retardant properties of starch-based biodegradable materials. It is known that starch, PVA, and glycerol are all polyhydric compounds. So, IFR is a good choice for the flame retardancy of starch/ PVA/glycerol composites. In order to utilize the charring capacity of above polyhydric compounds, the acid source can be used. But it should be noted that polyhydroxy groups will easily react with an acid source during processing, leading to instability.18 In our previous work, APP was microencapsulated to improve its water resistance and flame retardance in polymer.19 In this paper, microencapsulated APP (MCAPP) was selected as flame retardant. In one aspect, MCAPP can be used as acid and blowing agent. In another aspect, the presence of the shell can retard the reaction between acid and carbonization agent during processing. It can be anticipated that MCAPP would be an effective flame retardant in the biodegradable PVA/glycerolplasticized thermoplastic starch (TPS) composites. Experimental Section In this work, the biodegradable PVA/glycerol-plasticized thermoplastic starch (TPS) composites are prepared in a Brabender-like apparatus. The use of MCAPP as a flame retardant in TPS is evaluated by limiting oxygen index (LOI), UL-94, thermogravimetry (TG), and microscale combustion calorimeter (MCC). The thermal degradation properties of the TPS and TPS/MCAPP are evaluated by TG, dynamic Fourier transform infrared spectra (FTIR), and TG-FTIR. The char residue is characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Materials. Native potato starch was obtained from Shangdong Jincheng CO., Ltd. (Zhaoyuan, Shandong, China). APP with average degree of polymerization n > 1000 was purchased from Hangzhou JLS Flame Retardants Chemical Corp. PVA (polymerization degree ) 1750, degree of alcoholysis ) 98-99%)

10.1021/ie801230h CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

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was kindly supplied by Shanghai DongCang International Trading Co., Ltd. (China). Glycerol, melamine, and formaldehyde were purchased from Shanghai Chemical Reagent Corp. Preparation of Microencapsulated APP. Prepolymer synthesis: melamine and 37% formaldehyde solution (with a mole ratio of 3:1) were put into a three-necked bottle with a stir. The mixture was adjusted to pH 8-9 with 10% Na2CO3 solution, heated to about 80 °C, and kept at that temperature for 1 h. The MF prepolymer solution was prepared and ready for use of the microencapsulation. Preparation of microencapsulated APP: APP (40 g) was first dispersed in 150 mL of ethanol. Then 13 mL of melamine-formaldehyde prepolymer solution was added into the mixture, and the pH of the mixture was adjusted to 4-5 with sulfuric acid. The resulting mixture was heated at 80 °C for 2 h. After that, the mixture was cooled to room temperature, filtered, washed with distilled water, and dried at 105 °C. The MCAPP powder was finally obtained.19 Preparation of TPS and TPS/MCAPP Composites. The starch, PVA and glycerol were first mixed as dry solids. The mixture was placed into a Brabender-like apparatus rotating at a speed of 35 rpm. Both TPS and TPS/MCAPP composites were prepared in a Brabender-like apparatus at a temperature of about 150 °C for 15 min. The ratio of starch, PVA, and glycerol (wt/ wt) was 60:7.5:32.5. After mixing, the samples were hot-pressed at about 160 °C under 10 MPa for 10 min into sheets of suitable thickness for analysis. Measurements. X-ray Diffraction. Wide-angle X-ray diffraction patterns of the samples were recorded on an X-ray diffractometer (Rigaku Dmax/rA, Japan), using Cu KR radiation (λ ) 0.154 18 nm) at 40 kV and 20 mA. Limiting Oxygen Index. LOI was measured according to ASTM D2863. The apparatus used was an HC-2 oxygen index meter (Jiangning Analysis Instrument Co., China). The specimens used for the test were of dimensions 100 × 6.5 × 3 mm. UL 94 Testing. The vertical test was carried out on a CFZ2-type instrument (Jiangning Analysis Instrument Co., China) according to the UL 94 test standard. The specimens used were of dimensions 130 × 13 × 3 mm. Microscale Combustion Calorimeter. The heat release rate (HRR) and the total heat release (THR) were measured in a microscale combustion calorimeter (GOVMARK MCC-2) for studying the molecular-level fire behaviors of materials. About 5 mg of samples were heated at a heating rate of 1 K/s in a nitrogen stream flowing at 80 cm3/min. The volatile, anerobic 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. Thermogravimetry. The samples were examined on a TGAQ5000 apparatus (TA Co., USA). The weight of all samples was kept within 3-5 mg in an open Al pan. Scanning Electron Microscopy. The samples were sputtercoated with a conductive layer. Then the SEM micrographs of the char residue were obtained with a scanning electron microscope AMRAY1000B. Thermogravimetry-Fourier Transform Infrared Spectra. The TG-FTIR instrument consists of analyzer (TGA-Q5000, TA Co., USA) coupled with Fourier transform spectrometer (Nicolet 6700) and the transfer line. The investigations were carried out under nitrogen atmosphere at a flow rate of 35.0 mL/min for TG, with heating rate of 20 °C/min. In order to reduce the possibility of gases condensing along the transfer line, the temperature in the gas cell and transfer line were set to 230 °C.

Figure 1. XRD profiles of native starch and TPS.

Real Time Fourier Transform Infrared Spectra. Real time FTIR spectra were recorded using Nicolet MAGNA-IR 750 spectrophotometer equipped with a ventilated oven having a heating device. The temperature of the oven was raised at a heating rate of about 10 °C/ min. Samples were mixed with KBr powders, and the mixture was pressed into a tablet. Dynamic FTIR spectra were obtained in situ during the thermal degradation of the samples. X-ray Photoelectron Spectroscopy Spectra. The XPS spectra of the char residue were recorded with a VG Escalab mark II spectrometer (VG Scientific Ltd., UK), using Al KR excitation radiation (1253.6 eV). Results and Discussion XRD Analysis. Figure 1 shows the XRD profiles of native starch and thermoplastic starch (TPS). As shown in Figure 1, starch exhibits mainly three weak intensity diffraction peaks at 11.3°, 20.1°, and 26.5°, and three strong intensity peaks at 15.2°, 17.2°, and 23.5°. After the PVA and glycerol treatment, obvious changes can be found in the diffraction pattern of TPS. The above-mentioned peaks of starch almost disappear, while new peaks at 19.9° and 13.0° appear. It is aroused by the destruction of crystalline structure of starch during the blending process with PVA and glycerol. Flame Retardation of TPS and TPS/MCAPP Composites. The effect of MCAPP on the LOI values and UL 94 results of the intumescent flame retardant (IFR) TPS composites is shown in Figure 2. Neat TPS is highly combustible, and its LOI value is only 23.0%. It can be seen that the LOI values of the IFR composites increase with the increase of MCAPP content. The LOI value of the TPS/MCAPP composite containing 2% MCAPP is as high as 31.0% There are no ratings for neat TPS, however, when the mass percentage of MCAPP is 2% UL 94 results of the TPS/MCAPP composites can reach V-0. The residual chars of TPS and TPS/MCAPP (containing 2% MCAPP) at the end of LOI test are shown in Figure 3. It is clear that there is almost no residue left at the end of the LOI test for neat TPS. On the other hand, the surface of TPS/MCAPP residue is covered with an expanded char network. The results indicate that TPS/MCAPP can form an effective char which can prevent the heat transfer and flame spread during combustion. Microscale Combustion Calorimeter. Microscale combustion calorimeter (MCC) is a new, rapid, laboratory scale test that uses thermal analysis methods to measure chemical

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Figure 2. Effect of MCAPP on the LOI and UL 94 results of TPS composites.

Figure 4. HRR curves of TPS and TPS/MCAPP (2 wt %) composites.

Figure 5. TG curves of (a) starch, (b) APP, (c) MCAPP, (d) starch/APP (1/1 by weight), and (e) starch/MCAPP (1/1 by weight). Run under an air flow (60.0 mL/min) at a heating rate of 10 oC/min.

Figure 3. Residual char of TPS and TPS/MCAPP (2 wt %) at the end of LOI test.

properties related to fire. It can quickly and easily measures the key fire parameters of plastics, wood textiles, and composites. From just a few milligrams of specimen a wealth of information on material combustibility and fire hazard is obtained in minutes. The curves of the heat release rate (HRR) of TPS and TPS/ MCAPP composites are shown in Figure 4. The shape of the HRR curves for the neat TPS is a sharp peak followed by a low, broad peak, indicating two steps in the decomposition. Associated data for the TPS are peak HRR ) 23.7, 121.2 w/g, total heat release (THR) ) 9.1 kJ/g. In the case of TPS/MCAPP composite, incorporation of 2% MCAPPP into TPS shifts the two peaks to lower temperatures. Associated data for the TPS/MCAPP are: peak HRR )7.2, 123.9 w/g, THR ) 4.4 KJ/g. The movement of the peaks was due to the dehydration between MCAPP and TSP and formation of an expanded protective shield. With the increase the temperature, further decomposition of the char precursor to a thermally stable structure occurs. Though the addition of MCAPP changes the peak HRR little, the THR reduces sharply. It is because that

the presence of MCAPP catalyzes the formation of a protective char, the char can protect the underlying materials from further burning and reduce its heat release. Thermal Analysis of Starch, Its Mixtures, TPS, and TPS/MCAPP. The TG curves of native starch, APP, MCAPP, starch/APP (1/1 by weight) and starch/MCAPP (1/1 by weight) are shown in Figure 5. It can be seen that the decomposition of starch is composed of two steps after the temperature of 100 °C. Before the temperature of 100 °C, weight loss of the starch is caused by the evaporation of water. Starch begins to decompose at the temperature of 252 °C. The first step is the main decomposition process of starch, and the products at this step are mainly water. At above 363 °C is the second step which is supposed to be the decomposition of the char. APP has two main decomposition processes. The first step begins from about 270 to 500 °C. Its evolution products at this step are mainly ammonia and water, and cross-linked polyphosphoric acids (PPA) are formed simultaneously.20 The second step is the main decomposition process of APP occurring at above 500 °C. For MCAPP, it initial decomposition temperature is similar to that of APP, but MCAPP decomposes faster than APP at the beginning owing to the lower thermal stability of melamine-formaldehyde resin in MCAPP. Above about 600 °C, MCAPP is more thermally stable compared with APP.

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Figure 8. 3D TG/FTIR spectrum of gas phase in the thermal degradation of TPS/MCAPP (2 wt %).

Figure 9. FTIR spectrum of pyrolysis products for TPS at the maximum decomposition rate (23.4 min). Figure 6. TG (a) and DTG (b) curves of TPS and TPS/MCAPP (2 wt %). Run under a nitrogen flow (35.0 mL/min) at a heating rate of 20 oC/min.

Figure 7. 3D TG-FTIR spectrum of gas phase in the thermal degradation of TPS.

Moreover, MCAPP after the decomposition at 700 °C left about 10.5% residue, whereas the residue for APP at this temperature is 2.2%. APP or MCAPP was blended with starch at room temperature with a mass ratio 1:1, and the mixture was characterized by TG to evaluate the occurrence of reaction between starch and APP (or MCAPP) during processing. It is clear that starch/APP begins to decompose at about 195 °C which is a little higher than the processing temperature. If APP is used as the flame retardant, it can be anticipated that APP would react with starch at lower temperature under high shear force during processing. The reaction may have deleterious effect on the properties of IFR TPS composites. For starch/MCAPP, its initial decomposition temperature is about 230 °C which is much higher than

Figure 10. FTIR spectrum of pyrolysis products for TPS/MCAPP (2 wt %) at the maximum decomposition rate (19.6 min).

the processing temperature. The above results indicate that microencapsulation can restrain the reaction between APP and starch during processing. It also should be noted that between the temperature of 261 and 487 °C, starch/APP is more stable than starch/MCAPP. This is because above 261 °C MCAPP begins to decompose and reacts with starch. However, at a temperature higher than 487 °C, the mixture of starch/MCAPP is more thermally stable than that of starch/APP. The residue left at 700 °C for starch/MCAPP is 10.0% which is much higher than 5.4% for starch/APP. The

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Figure 11. Absorbance of pyrolysis products for TPS and TPS/MCAPP (2 wt %) vs time: (a) total; (b) H2O; (c) hydrocarbon; (d) CO2; (e) carbonyl compounds; and (f) CH3OH.

increase of amount of residue for starch/MCAPP may be due to the formation of more thermally stable carbonaceous char. Figure 6 shows the TG and DTG curves of the TPS and its IFR composite filled with 2 wt % MCAPP. The decline on the weight loss of the TPS and the TPS/MCAPP at the beginning is due to evaporation of the water and glycerol (Figure 6a). It has been found that the starch starts to degrade at around 251 °C. The thermal degradation of TPS is composed with two steps beyond 205 °C. The residue weight for TPS at the temperature of 700 °C is about 10.8%. From Figure 6, it can be seen that MCAPP has remarkable influence on the thermal degradation of TPS. Due to the

esterification between MCAPP and TPS, TPS/MCAPP decomposes much faster compared with TPS at lower temperature. However, beyond the temperature of 324 °C, TPS/MCAPP is more stable than TPS. The thermal degradation of TPS/MCAPP occurs in two steps beyond 205 °C. The temperatures of maximum mass loss rate (Tmax) for TPS/MCAPP in the two steps are 261 and 339 °C, respectively, while the values for TPS are 218 and 320 °C, respectively. Moreover, the residue left at 700 °C for TPS/MCAPP is 21.8% which is much higher than that of TPS. TG-FTIR Analysis of TPS and TPS/MCAPP. In this work, TG-FTIR was used to analyze the gas product during the thermal

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Figure 12. FTIR spectra of TPS at different pyrolysis temperatures.

Figure 13. FTIR spectra of TPS/MCAPP (2 wt %) at different pyrolysis temperature.

Figure 14. C1s spectra of the char residue of TPS/MCAPP (2 wt %).

Figure 15. O1s spectra of the char residue of TPS/MCAPP (2 wt %).

Table 1. XPS Results of the Residual Char of TPS/MCAPP (2 wt %) system

binding energy (eV)

atom %

C1s O1s N1s P2p

284.6, 286.0, 287.2, 288.5 530.8, 532.5, 535.86 398.3, 399.9, 401.6 133.8

70.48 22.72 4.95 1.84

degradation. The 3D TG-FTIR spectra of gas phase in the thermal degradation of TPS and TPS/MCAPP are shown in Figures 7 and 8. In Figure 7, peaks in the regions of around 3400-4000 cm-1, around 2700-3000 cm-1, around 2250-2400 cm-1, around 1600-1900 cm-1, around 1250-1500 cm-1, and around 1000-1100 cm-1 were noted. The spectra fit well to the reported FTIR features of gas products such as H2O (3400-4000 cm-1 and 1200-2200 cm-1), hydrocarbons (2800-3000 cm-1), CH2O (C-H stretching, 2782 cm-1; C-O stretching, 1746 cm-1), CO (2177 cm-1), CO2 (2250-2400 cm-1), and CH3OH (C-O stretching, 1068 cm-1; O-H deformation, 660 cm-1).21-23 Although it is known that glycerol is one of the products evolved from TPS degradation, it was not detected by FTIR in this study, as we know that the boiling point of glycerol is about 290 °C. The reason can be explained that vapors of glycerol may condense in the connection between the TG and FTIR equipment when the temperature of the tube is about 230 °C.

Figure 16. N1s spectra of the char residue of TPS/MCAPP (2 wt %).

FTIR spectra of pyrolysis products of TPS at maximum decomposition rates are shown in Figure 9. The main products of the thermal decomposition of TPS are H2O (3621 cm-1), CO2 (2360 cm-1), CO (2180 cm-1), and compounds containing CH3OH (660 cm-1), CH2O (1746 cm-1), and hydrocarbons (2928 cm-1), etc. It is well-known that dehydration and depolymerization are the two main processes associated with the thermal degradation of polysaccharides.24 In the process of dehydration, the sample

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Figure 17. P2p spectra of the char residue of TPS/MCAPP (2 wt %).

may release water, and in the process of depolymerization, the main decomposition products are CO2, CO, CH3OH, and hydrocarbons, etc. Associated with the analysis of Figure 11, it can be concluded that the pyrolysis product for TPS at the beginning (from about 15 min) is mainly composed of H2O, CH3OH, and hydrocarbons, etc. With the increase of temperature (above about 20 min), CO2, CO, and CH2O, etc. are released. Figure 8 shows the 3D TG-FTIR spectrum of gas phase in the thermal degradation of TPS/MCAPP (2 wt % MCAPP). As shown in Figure 10, the evolved gas analysis for TPS/MCAPP exhibited characteristic bands of H2O (3621 cm-1), CO2 (2360 cm-1), CO (2180 cm-1), CH3OH (660 cm-1), CH2O (1746 cm-1), and hydrocarbons (-CH3 and -CH2- groups: 2950-2850 and 1515-1370 cm-1) which is similar to that of TPS. As Figure 6 reveals, the presence of MCAPP increases the weight loss of TPS beyond 240 °C, indicating that MCAPP catalyze the thermal degradation of TPS. This view can be estimated that the release of H2O was increased for TPS/MCAPP compared with TPS at earlier stage (Figure 9). Associated with the analysis of Figure 11, it can be drawn that the pyrolysis products for TPS/MCAPP at the beginning (from about 7.5 min) are mainly composed of H2O and hydrocarbons, etc. With the increase of temperature (above about 17.5 min), CO2, CO, CH3OH, and CH2O, etc. are released. It is interesting that there are two peaks in the absorbance of CO2 and CH3OH for TPS/MCAPP, while there is only one for TPS. Moreover, in Figure 11, TPS/MCAPP release more H2O, hydrocarbons, and CH3OH compared with TPS. It can be interpreted that MCAPP can catalyze the thermal

decomposition of TPS at the beginning and the second peak was aroused by the further decomposition of the char. Thermal Degradation of TPS and TPS/MCAPP. Dynamic FTIR was used to evaluate the thermal degradation mechanism of TPS and TSP/MCAPP (2 wt % MCAPP). Figure 12 shows the FTIR spectra of TPS at different degradation temperatures. A broad band observed at 3392 cm-1 is attributed to free and bound O-H groups of TPS. The absorption band at 2930 cm-1 refers to the C-H stretching present in the TPS. In the region 2000-400 cm-1, the main absorption bands related to bound water (1645 cm-1), C-H bending (1460 cm-1), and bands associated with C-O, C-C, and C-O-H in the region 1200 -900 cm-1 25. Some modifications occurred at the 3392 and 1645 cm-1 band at temperatures of 300 °C, and this behavior can be explained by the release of water and dehydration of TPS. Above 300 °C, it is the depolymerization of TPS. It can be testified by the disappearance of bands in the region 2000-400 cm-1. At this process, a reduction in the degree of polymerization, elimination of water, formation of carbonyl, carboxyl, and hydroperoxide groups (in the presence of air), and evolution of carbon monoand dioxide occurred.25 The FTIR spectra of the TPS/MCAPP at different degradation temperatures are shown in Figure 13. For TPS/MCAPP at room temperature, it has similar profile with that of TPS. In this figure, the characteristic peaks of MCAPP are not obvious due to the content of MCAPP being too low. It is interesting that at 250 °C the intensities of most peaks decrease sharply, which means that the presence of MCAPP catalyzes the thermal degradation of TSP and formation of char. The residual char can prevent the materials from further degradation during combustion. Chemical Components of the Residual Char. It is obvious that the intumescent char influences the flame retardancy and thermal degradation of the composites greatly. The chemical components of the residual char for TPS/MCAPP (heated in muffle furnace for 10 min at 600 °C) were investigated by XPS. As shown in Table 1 and Figure 14, the peak at 284.6 eV can be assigned to C-H and C-C in aliphatic and aromatic species. The peak at 286.0 eV is attributed to C-O (ether and/or hydroxyl group).26 Moreover, the peaks at 287.2 and 288.5 eV are characteristic of CdO and/or CdN, respectively.27 Three bands at 530.8, 532.5, and 533.0 eV are observed from O1s spectra as shown in Figure 15. It is reported that it is impossible to distinguish inorganic and organic oxygen because the O1s band is structureless.26 The peak at 530.8 eV can be attributed to the )O in phosphate or carbonyl groups and the peak centered at 532.5 is assigned to -O- in C-O-C,

Figure 18. SEM micrographs of the char residues: (a) TPS and (b) TPS/MCAPP (2 wt %).

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C-O-P, and/or C-OH groups. The peak at 535.9 eV corresponds to chemisorbed oxygen and/or adsorbed water.29 Figure 16 shows the N1s spectra. The peak at 398.4 eV can be assigned the nitrogen in melon groups formed by the condensation of melamine with the evolution of NH3 at higher temperature. The peak around 399.9 eV is due to the nitrogen in the melamine structures. The peak at 401.6 eV corresponds to quaternary nitrogen and to formation of some oxidized nitrogen compounds.28 As for the spectra of P2p, the peak at 133.8 eV can be attributed to the pyrophosphate and/or polyphosphate.28 SEM Micrograph of the Residual Char. The sample TPS and TPS/MCAPP were heated in muffle furnace for 10 min at 600 °C, and the residual chars were obtained. Figure 18 presents the SEM photographs of above residual chars. For TPS, it can be observed that the char is porous. In another aspect, the char of TPS/MCAPP is coherent. Consequently, the char can provide better flame shield for the underlying material during combustion. Conclusion The effects of MCAPP on the flame retardancy and thermal properties of TPS were remarkable. UL 94 results of the TPS/ MCAPP composites can reach V-0 when the mass percentage of MCAPP is just 2%. However, neat TPS cannot pass any rating. It can be found that addition of MCAPP changes the peak HRR little, but the value of THR reduces sharply in the test of MCC. The data of TG-FTIR show that the main products of the thermal decomposition of TPS are H2O, CO2, CO, CH3OH, CH2O, and hydrocarbons, etc. The presence of MCAPP catalyzes the degradation of TPS, and more gas products were released at the earlier thermal degradation stage. The results are consistent with the data of dynamic FTIR. The residual char of TPS/MCAPP was mainly composed of pyrophosphate and/ or polyphosphate. From above results, it can be concluded that MCAPP can catalyze the formation of a protective char which can protect the underlying materials from further burning. Acknowledgment The financial support from the National Natural Science Foundation of China (No. 20776136) and the program for New Century Excellent Talents in University and National 11th Five-year Program (2006BAK01B03, 2006BAK06B06, 2006BAK06B07) is acknowledged. Literature Cited (1) Acioli-Moura, R.; Sun, X. S. Thermal Degradation and Physical Aging of Poly(lactic acid) and its Blends With Starch. Polym. Eng. Sci. 2008, 48, 829–836. (2) Ohkita, T.; Lee, S. H. Thermal Degradation and Biodegradability of Poly (lactic acid)/Corn Starch Biocomposites. J. Appl. Polym. Sci. 2006, 10, 3009–3017. (3) Pimentel, T. A. P. F.; Duraes, J A.; Drummond, A. L. Preparation and characterization of blends of recycled polystyrene with cassava starch. J. Mater. Sci. 2007, 42, 7530–7536. (4) Santonja-Blasco, L.; Contat-Rodrigo, L.; Moriana-Torro, R.; RibesGreus, A. Thermal characterization of polyethylene blends with a biodegradable masterbatch subjected to thermo-oxidative treatment and subsequent soil burial test. J. Appl. Polym. Sci. 2007, 106, 2218–2230. (5) Coffin, D. R.; Fishman, M. L.; Cooke, P. H. Mechanical and microstructural properties of pectin/starch films. J. Appl. Polym. Sci. 1995, 57, 663–670. (6) Lu, X. L.; Du, F. G.; Ge, X. C.; Xiao, M.; Meng, Y. Z. Biodegradability and thermal stability of poly(propylene carbonate)/starch composites. J. Biomed. Mater. Res. 2006, 77A, 653–658.

(7) Zhang, J. F.; Sun, X. Z. Mechanical Properties of Poly(lactic acid)/ Starch Composites Compatibilized by Maleic Anhydride. Biomacromolecules 2004, 5, 1446–1451. (8) Kweon, D. K.; Kawasaki, N.; Nakayama, A.; Aiba, S. Preparation and Characterization of Starch/Polycaprolactone Blend. J. Appl. Polym. Sci. 2004, 92, 1716–1723. (9) Yoon, S. D.; Chough, S. H.; Park, H. R. Preparation of Resistant Starch/Poly(vinyl alcohol) Blend Films with Added Plasticizer and Crosslinking Agents. J. Appl. Polym. Sci. 2007, 106, 2485–2493. (10) Zou, G. X.; Qu, J. P.; Zou, X. L. Optimization of water absorption of starch/PVA composites. Polym. Compos. 2007, 28, 674–679. (11) Thunwall, M.; Boldizar, A.; Rigdahl, M. Compression Molding and Tensile Properties of Thermoplastic Potato Starch Materials. Biomacromolecules. 2006, 7, 981–986. (12) Matko, S.; Toldy, A.; Keszei, S.; Anna, P.; Bertalan, G.; Marosi, G. Flame retardancy of biodegradable polymers and biocomposites. Polym. Degrad. Stabil. 2005, 88, 138–145. (13) Wittek, T.; Tanimoto, T. Mechanical properties and fire retardancy of bidirectional reinforced composite based on biodegradable starch resin and basalt fibres. Express Polym. Lett. 2008, 2, 810–822. (14) Bourbigot, S.; Le Bras, M.; Duquesne, S.; Rochery, M. Recent Advances for Intumescent Polymers. Macromol. Mater. Eng. 2004, 289, 499–511. (15) Le Bras, M.; Bourbigot, S.; Delporate, C.; Siat, C.; Le Tallec, Y. New intumescent formulations of fire-retardant polypropylenessDiscussion of the Free Radical Mechanism of the Formation of Carbonaceous Protective Material During the Thermo-oxidative Treatment of the Additives. Fire Mater. 1996, 20, 191–203. (16) Almeras, X.; Le Bras, M.; Hornsby, P.; Bourbigot, S.; Marosi, G.; Keszei, S.; Poutch, F. Effect of fillers on the fire retardancy of intumescent polypropylene compounds. Polym. Degrad. Stab. 2003, 82, 325–331. (17) Almeras, X.; Le Bras, M.; Poutch, F.; Bourbigot, S.; Marosi, G.; Anna, P. Effect of fillers on fire retardancy of intumescent polypropylene blends. Macromol. Symp. 2003, 198, 435–447. (18) Liu, M. F.; Huang, X.; Liu, Y.; Wang, Q. Flame Retardant Polyethylene with Intumescent System Containing Macromoleculeencapsulated Low Molecular Weight Charring Agent. Polym. Polym. Compos. 2007, 15, 591–596. (19) Wu, K.; Wang, Z. Z.; Liang, H. J. Microencapsulation of Ammonium Polyphosphate: Preparation, Characterization and its flame retardance in Polypropylene. Polym. Compos. 2008, 29, 854–860. (20) Camino, G.; Grassie, N.; McNeill, I. C. Influence of the fire retardant, ammonium polyphosphate, on the thermal degradation of poly(methyl methacrylate). J. Polym. Sci., Polym. Chem. Ed. 1978, 16, 95– 106. (21) Braun, U.; Schartel, B.; Fichera, M. A.; Jager, C. Flame retardancy mechanisms of aluminium phosphinate in combination with melamine polyphosphate and zinc borate in glass-fibre reinforced polyamide 6,6. Polym. Degrad. Stab. 2007, 92, 1528–1545. (22) Baker, R. R.; Coburn, S.; Liu, C.; Tetteh, J. Pyrolysis of saccharide tobacco ingredients: a TGA-FTIR investigation. J. Anal. Appl. Pyrolysis 2005, 74, 171–180. (23) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction To Infrared and Raman Spectroscopy; Academic Press: Boston, MA, 1990. (24) Rudnik, E. Thermal properties of biocomposites. J. Therm. Anal. Calorim. 2007, 88, 495–498. (25) Marques, P. T.; Lima, A. M. F.; Bianco, G.; Laurindo, J. B.; Borsali, R.; Le Meins, J. F.; Soldi, V. Thermal properties and stability of cassava starch films cross-linked with tetraethylene glycol diacry. Polym. Degrad. Stab. 2006, 91, 726–732. (26) Zhu, S. W.; Shi, W. F. Thermal degradation of a new flame retardant phosphate methacrylate polymer. Polym. Degrad. Stab. 2003, 80, 217–222. (27) Nakayama, Y.; Soeda, F.; Ishitani, A. XPS study of the carbon fiber matrix interface. Carbon 1990, 28, 21–26. (28) Bourbigot, S.; Le Bras, M.; Delobel, R.; Gengembre, L. XPS study of an intumescent coating II. Application to the ammonium polyphosphate/ Pentaerythritol/ ethylenic terpolymer fire retardant system with and without synergistic agent. Appl. Surf. Sci. 1997, 120, 15–29. (29) Gardner, S.D.; Singamsetty, C. S. K.; Booth, G. L. Surface Characterization of Carbon-fibers using angle-resolved XPS and ISS. Carbon 1995, 33, 587–595.

ReceiVed for reView August 11, 2008 ReVised manuscript receiVed December 25, 2008 Accepted December 28, 2008 IE801230H