Flame Retardancy and Thermal Degradation - American Chemical

Ind. Eng. Chem. Res. 2009, 48, 10751–10758

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APPLIED CHEMISTRY Intumescent Flame Retardation of Starch Containing Polypropylene Semibiocomposites: Flame Retardancy and Thermal Degradation Shibin Nie,†,‡ Lei Song,† Yuqiang Guo,† Kun Wu,† Weiyi Xing,† Hongdian Lu,§ and Yuan Hu*,‡ State Key Laboratory of Fire Science and Department of Polymer Science and Engineering, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P.R. China, and Department of Chemical and Material Engineering, Hefei UniVersity, Hefei, Anhui, 230022, P.R. China

Starch containing polypropylene (SCP) semibiocomposites were prepared by melted blend method. Microencapsulated ammonium polyphosphate (MCAPP) was added to the SCP not only to improve its flame retardant properties but also to restrain the reaction between ammonium polyphosphate (APP) and starch during processing. The flame retardant properties of SCP have been investigated by limited oxygen index (LOI), UL-94 test, and cone calorimeter test. The results of cone calorimeter show that the peak of heat release rate and total heat release of SCP decreases substantially compared with that of pure PP. The thermal degradation and gas products of PP/starch/MCAPP systems were monitored by thermogravimetric analysis (TGA) and thermogravimetric analysis-infrared spectrometry (TG-IR). Scanning electron micrograph (SEM) and X-ray photoelectron spectroscopy (XPS) measurements were utilized to investigate the chemical structure, as well as the surface morphology of the residual char. Introduction Polypropylene (PP) is one of the most widely used polyolefins and has broad applications in cars, cables, electronics, architectural materials, and so on. However, because of the continuously increasing plastic wastes and environmental pollution, the use of PP often brings considerable problems with regard to reusing or recycling after the end of the lifetime. A simple disposal such as landfill disposal is more and more unpopular with regard to the increasing environmental awareness. Therefore, environmentally compatible biocomposites are looked for and become more and more hot in recent research. One approach is the embedding of natural biodegradable polymer into thermal plastic polymer resulting in composites which are not completely biodegradable, but are compatible with the environment.1 Starch is one of the most important biodegradable polymers and is widely used due to its low cost and potential thermoplastic properties. Starch can be blended with various biodegradable polymers, such as poly(lactic acid), polycaprolactone, and poly(propylene carbonate) to meet the necessary requirements for various applications.2-4 It is also reported that starch can be blended with thermoplastic polymer (e.g., polystyrene and polyethylene) to provide biodegradation properties to the composites.5,6 However, there are some problems limiting the development of starch containing semibiocomposites. Starch is a highly hydrophilic macromolecule, whereas some polymers such as PP and PE are nonpolar and hydrophobic, so the compatibility is very weak. Moreover, the thermal stability of starch is not very good, especially since the hydroxyl groups in the starch are sensitive to the temperature of compounding processes. Beside the above problems, easy combustibility and * To whom correspondence should be addressed. Fax: 86-5513601664. E-mail address: [email protected]. † State Key Laboratory of Fire Science, University of Science and Technology of China. ‡ Department of Polymer Science and Engineering, University of Science and Technology of China. § Hefei University.

melt dripping limit its applications, so it is necessary to make these kind materials flame retardant. Until now, many researches have done a lot of work to improve the compatibility of starch with hydrophobic polymer, such as the modification of hydrophobic polymers, the modification of starch, and the introduction of compatibilizers into the blends of starch and hydrophobic polymer.7-12 However, other problems especially flammability lack investigation. Intumescent flame retardants (IFR) are remarkable because they are environmental friendly, halogen-free, and very efficient. A typical and widely studied IFR system is the combination of an acid source (ammonium polyphosphate (APP)), a carbon source (pentaerythritol), and a gas source (melamine).13,14 In this paper, starch was blended with PP to form a semibiodegradable composite, and PP grafted maleic anhydride (PP-g-MA) was added to the PP-starch system to improve the compatibility between PP and starch. Microencapsulated APP (MCAPP) was selected as the flame retardant to act as an acid and blowing agent, but also to retard the reaction between acid and hydroxyl groups during compounding process due to the presence of the shell. The flame retardant and thermal properties of semibiodegradable composites were investigated by different methods. The surface morphology of the residual char was investigated to explain the differences of the flame retardancy among different systems. Experimental Details 2.1. Materials. PP (homopolymer, melt-flow rate ) 2.5 g/10 min) was supplied as pellets by Yangzi Petrochemical Co., Ltd. (China). PP graft maleic anhydride (PP-g-MA, 0.6-1.0 MA%) is bought from Ningbo Nengzhiguang New Materials Technology Co., Ltd. Ammonium polyphosphate is bought from Shandong Shian Chemical Co., Ltd. Microencapsulated ammonium polyphosphate (MCAPP) which was microencapsulated by melamine-formaldehyde resin was kindly provided by KeYan Co, and melamine formaldehyde:APP is 1:10 by weight.

10.1021/ie9012198 CCC: $40.75  2009 American Chemical Society Published on Web 11/02/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 24, 2009

Table 1. Formulations of the Starch Containing Polypropylene Semibiocomposites sample SCP0 SCP1 SCP2 SCP3 SCP4 SCP5 SCP6 SCP7 SCP8 SCP9

composition PP PP PP PP PP PP PP PP PP PP

75 55 35 75 55 45 35 25 35

wt wt wt wt wt wt wt wt wt

% % % % % % % % %

+ + + + + + + + +

5 5 5 5 5 5 5 5 5


% % % % % % % % %

MAPP MAPP MAPP MAPP MAPP MAPP MAPP MAPP MAPP

+ + + + + + + + +

20 40 60 20 20 20 20 20 20


Native potato starch was obtained from Shangdong Jincheng CO., Ltd. (Zhaoyuan, China). 2.2. Preparation of Samples. PP, MCAPP, and starch were dried in a vacuum oven at 80 °C overnight before use. Then PP, PP-g-MA, MCAPP, and starch were melt-mixed in a twinroller mill for 10 min; the temperature of the mill was maintained at 170 °C, and the roller speed was 30 rpm. The samples are listed in Table 1. The resulting semibiocomposites were hot-pressed into sheets with suitable thickness and size for UL-94, limited oxygen index (LOI), and cone calorimeter tests. 2.3. Characterization. X-ray diffraction (XRD) pattern was performed on the 1 mm thick films with a Japan Rigaku D/MaxRa rotating anode X-ray diffractometer equipped with a Cu KR tube and Ni filter (λ ) 0.1542 nm). LOI was measured according to ASTM D2863, and the apparatus used was an HC-2 oxygen index meter (Jiangning Analysis Instrument Company, China). The specimens dimensions used for test were 100 × 6.5 × 3 mm. The vertical burning test was carried out on a CFZ-2-type instrument (Jiangning Analysis Instrument Company, China), and the specimens used for the test is 100 × 13 × 3 mm. The combustion properties were evaluated using a cone calorimeter. All samples (100 × 100 × 3 mm) were exposed to a Stanton Redcroft cone calorimeter according to ISO-5660 standard procedures. Thermogravimetric analyses (TGAs) were carried out using a TGA Q 5000IR (TA Instruments) thermo-analyzer instrument at a linear heating rate of 20 °C/min under an air flow. Samples were measured in an alumina crucible with a mass of about 5.0 mg. Thermogravimetric analysis-infrared spectrometry (TG-IR) was performed using the TGA Q5000 IR thermogravimetric analyzer that was linked to the Nicolet 6700 FTIR spectrophotometer. About 5.0 mg of the sample was put in an alumina crucible and heated from 30 to 600 °C. The heating rate was 20 °C/min (nitrogen atmosphere, flow rate of 60 mL/min). X-ray photoelectron spectroscopy (XPS) was carried out with a VG Escalab Mark II spectrometer (Thermo-VG Scientific Ldt, UK), using Al Ka excitation radiation (hV ) 1486.6 eV).

% % % % % % % % %

UL-94 starch starch starch MCAPP MCAPP + 20 wt % starch MCAPP + 30 wt % starch MCAPP + 40 wt % starch MCAPP + 50 wt % starch APP + 40 wt % starch

burning burning burning burning burning burning V2 V0 V0 V0

and and and and and and

dripping dripping dripping dripping dripping dripping

polypropylene (SCP) biocompomsites. PP/40 wt % starch and PP/60 wt % starch are combustible, and neither can reach a UL-94 V0 rating. The LOI value of PP/60% starch is just 20.5%. When APP and MCAPP are combined with SCP composites, their flame retardant properties are greatly improved. With APP or MCAPP (20 wt %), the sample PP/40 wt % starch/20 wt % APP or PP/40 wt % starch/20 wt % MCAPP can pass the V0 rating in a UL-94 test. The LOI value of PP/40 wt % starch/20 wt % MCAPP is as high as 28.2% (see Figure 2). The flame retardant properties of SCP semibiocomposites are also influenced by the weight ratio of PP and starch. Keeping 20 wt % MCAPP unchanged, when the weight ratio of starch and PP is higher than 1.1:1, all the samples can pass the UL-94 V0 rating. When the weight ratio of starch and PP is 2:1, the LOI value can reach 31.2%. From the above result, it can be concluded

Figure 1. XRD patterns of PP, native starch, and PP/starch semibiocomposite.

3. Results and Discussion 3.1. XRD Analysis. Figure 1 shows the XRD patterns of PP, native starch, and the PP-starch semibiocomposite. As shown in Figure 1, starch exhibits mainly three strong intensity peaks at 15.2°, 17.2°, and 23.5°. After mixing starch with PP by melted blend, obvious changes can be found. The abovementioned peaks of starch almost disappear. It is aroused by the destruction of crystalline structure of starch during the blending process with PP. 3.2. LOI and UL-94 Analysis of Flame Retarded Starch Containing Polypropylene Semibiocomposites. Table. 1 shows UL-94 results for the various starch containing

Figure 2. LOI results of the PP/starch/20 wt % MCAPP systems.


Figure 3. HRR curves of the PP/starch/MCAPP systems.

the higher the content of starch, the better the flame retardant property obtained. MCAPP and starch can form an intumescent system to protect the matrix from further burning, so the flame retardant properties of SCP semibiocomposites improve. 3.3. Cone Calorimeter Test. Cone calorimetry is a smallscale test, but it has good correlation with real fire disaster and provides a wealth of information on the combustion behavior. The heat release rate (HRR) is recognized to be the most important parameter to evaluate the development, spreading, and intensity of fires.15 Figure 3 shows the HRR curves of flame retarded SCP semibiocomposites. The peak HRR (PHRR) of the SCP composite with 60 wt % starch is 698.5 kW/m2, with a 33.8% decrease compared with that of pure PP (1140.0 kW/ m2). When PP and starch are mixed in a certain weight ratio, with the addition of 20 wt % MCAPP, the HRR peak value of the SCP semibiocomposite is greatly decreased. When the weight ratio of starch and PP is 1.1:1, the SCP composite has the lowest PHHR value (231.0 kW/m2), with a 79.8% decrease compared with that of pure PP. With the decrease of the weight ratio of starch and PP, the PHRR of the SCP system shows a small change, increasing from 231.0 to 255.5 kW/m2. Compared with PP/60 wt % starch and PP/20 wt % MCAPP, the HRR peaks of flame retarded SCP composites are obviously decreased, and the burning process is also prolonged. The burning time of PP/40 wt % starch/20 wt % MCAPP is about 525 s, much longer than that of PP/60 wt % starch (185 s) and PP/20 wt % MCAPP (305 s). It can be explained that starch is a hydroxyl-containing compound and can act as a carbon source. With adding MCAPP, starch with MCAPP can form an intumescent flame retardant system. With the presence of the efficient intumescent char layer, the amount of heat transferred to the polymer matrix and combustible gas escaping from the matrix decrease greatly and further degradation of PP is prevented, so the inner matrix can be protected for a longer time. It also can be observed that the ignition time of PP/40 wt % starch/20 wt % MCAPP (20 s) is shorter than that of PP/60 wt % starch (35 s) and PP/20 wt % MCAPP (30 s), and this demonstrates that MCAPP will react with starch at low temperature and release some small molecules which are easy to ignite. For PP/60 wt % starch and PP/20 wt % MCAPP, the total heat release (THR) is almost the same, but the PP/20 wt % MCAPP has a much slower heat release rate. It can be seen from Figure 4 that the THR drops from 96.0 MJ/m2 for PP to 80.0 MJ/m2 for PP/60 wt % starch and finally to 60.3 MJ/m2

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Figure 4. THR curves of the PP/starch/MCAPP systems.

Figure 5. SEM photographs of the outer surface of the chars after the LOI test: (a) PP/20 wt % MCAPP, (b) PP/60 wt % starch, (c) PP/40 wt % starch/ 20 wt % MCAPP, (d) PP/50 wt % starch/20 wt % MCAPP.

for PP/40 wt % starch/20 wt % MCAPP. Clearly there is some synergistic effect that occurs between starch and MCAPP. 3.4. Analysis of the Morphology and Chemical Structure of the Residual Char. The morphologies of the chars from the sample at the end of LOI and cone calorimeter tests were investigated by scanning electron microscopy (SEM) and a digital camera, as shown in Figures 5 and 6. Under magnification of 300×, it can be observed from Figure 5a and b that the surface char layer of PP/20 wt % MCAPP or PP/60 wt % starch is very compact and smooth, but neither of them can form an intumescent char layer during the combustion to protect the matrix. Some small bubbles can be observed in the PP/40 wt % starch/20 wt % MCAPP system (Figure 5c), and this shows that an intumescent system is formed. However, the outer surface of this char is not compact. The outer surface of residual char of PP/50 wt % starch/20 wt % MCAPP (Figure 5d) seems more compact, so it can provide a much better barrier to the transfer of the heat, combustible gases, and free radicals during a fire. So PP/50 wt % starch/20 wt % MCAPP has the best flame retardant properties among all the SCP systems. Figure

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Figure 6. Digital photos of the chars formed during the cone calorimeter test: (a) PP, (b) PP/60 wt % starch, (c) PP/20 wt % MCAPP, (d) PP/20 wt % starch/20 wt % MCAPP, (e) PP/40 wt % starch/20 wt % MCAPP. Table 2. XPS Data of PP/40 wt % Starch/20 wt % MCAPP after the LOI Test

C1s C1s C1s C1s O1s O1s P2p N1s

energy (ev)

atom %

284.8 286.2 287.5 289.0v 531.6 533.2 133.9 401.7

70.3 6.7 2.2 1.1 6.2 9.6 2.3 1.6

6 shows the digital photos of residual chars at the end of cone calorimeter test. As we can see from these pictures, pure PP has nothing left (Figure 6a) and PP/60 wt % starch (Figure 6b) and PP/ 20 wt % MCAPP (Figure 6c) only have a little char residue left after burning. The char residue increases greatly with the addition of MCAPP to the SCP semibiocomposites, and this is direct evidence that MCAPP has some synergistic effect with starch. It can be observed from Figure 6 that the content of starch has a great influence on the morphology of the char layer. PP/40 wt % starch/20 wt % MCAPP (Figure 6e) has a thick and compact char layer compared with that of PP/20 wt % starch/20 wt % MCAPP (Figure 6d), and this is consistent with the SEM result. The changes of the morphology of char residue are useful to explain the differences of the flame retardancy among different systems, and strongly support the results discussed in this paper. The chemical components of the residual char of PP/40 wt % starch/20 wt % MCAPP after the LOI test is investigated by XPS. The XPS analysis and spectra are shown in Table 2 and

Figure 7. XPS spectra of PP/40 wt % starch/20 wt % MCAPP after the LOI test: (a) C1S spectrum, (b) O1s spectrum.

Figure 7. Figure 7a presents the C1S spectrum. The peaks at 284.8 and 286.2 eV are assigned to CsH and CsC in aliphatic and aromatic species and CsO in ether, hydroxyl groups, CsOsP in hydrocarbonated phosphate, and/or CsN in heterocyclic compounds, respectively. The peaks at the peaks at 287.5 and 289.0 eV are attributed to carbonyl and carboxyl groups, respectively.16,17 For the O1s spectrum (Figure 7b), two peaks are found at around 531.6 and 533.2 eV. It is impossible to distinguish inorganic and organic oxygen because the O1s band is structureless. The peak at around 531.6 and 533.2 eV can be assigned to dO in carbonyl or phophate groups and sOs in CsOsC, CsOsP, or CsOH groups, respectively.18 3.5. Analysis the Difference between MCAPP and APP. The TGA curves of native starch, MCAPP, starch/APP (2:1 by weight), and starch/MCAPP (2:1 by weight) are shown in Figure 8. It can be seen that the decomposition of starch is divided into two temperature region after the temperature of 100 °C. Before the temperature of 100 °C, weight loss of the starch is due to the desorption of water. The first step decomposition of starch occurs between 254 and 344 °C and probably corresponds to the loss of water. The second step between 344 and 640 °C can be ascribed to decomposition of the char. MCAPP shows two main decomposition processes. Its volatile products at the first step (260-500 °C) are mainly ammonia and water, and cross-linked polyphosphoric acids are formed simultaneously. The second step is the main decomposition process of MCAPP occurring at above 500 °C. The thermal stability of starch/MCAPP (2:1, weight ratio) and starch/APP systems (2:1, weight ratio) is analyzed by TGA to evaluate the


Figure 8. TGA curves of native starch, MCAPP, starch/APP (2:1 by weight), and starch/MCAPP (2:1 by weight) under an air atmosphere.

occurrence of reaction between starch and APP (or MCAPP) during processing. It can be seen that the starch/MCAPP system produces 88.0% residue at 260 °C, 50.5% at 500 °C, and 27.5% at 640 °C. But, the calculated residue of the starch/MCAPP system, obtained by adding the data of MCAPP and starch, should be 95.8% at 260 °C, 30.5% at 500 °C, and 22.6% at 640 °C. The starch/MCAPP system has poorer thermal stability compared to that of the calculated one before 297 °C. When the temperature is higher than 297 °C, the starch/MCAPP system shows improved thermal stability over that of the calculated one. The above result directly demonstrates that there are chemical interactions between starch and MCAPP due to the acceleration of the phosphorylation reaction, and the esterification between starch and polyphosphoric acid. Due to the release of small molecules from these reactions, the starch/APP system shows a poorer thermal stability compared to that of the calculated one at initial decomposition. The formation of bridges or cross-links between starch and MCAPP can inhibit the further decomposition at high temperature. Starch/APP begins to decompose at about 200 °C which is near the processing temperature. If APP is used in the SCP system, APP would react with starch at a lower temperature under high shear force during processing. The initial decomposition temperature of starch/MCAPP is about 240 °C which is higher than the processing temperature, so it can overcome this problem. The color of PP/40 wt % starch/20 wt % APP turns into black after the processing, while the color of PP/40 wt % starch/20 wt % MCAPP is just slightly yellow. All of the above results indicate that microencapsulation can restrain the reaction between APP and starch during processing. 3.6. Thermal Properties of SCP Semibiocomposites. The TGA results of the PP/starch/MCAPP systems at a heating rate of 20 °C/min are shown in Figure 9. The 10% weight loss temperature (T-10wt%), the maximum decomposition temperature (Tmax), and the char residue at 600 °C are listed in Table 3. The curve for pure PP shows one step degradation of total weight loss in the range of 230-350 °C and has no char residue beyond the temperature of 500 °C. Compared to pure PP, the PP/starch/ MCAPP systems have poorer thermal stability before 250 °C. From the above discussion, it comes to a conclusion that a charring layer forms due to the reaction between MCAPP and starch. Above 250 °C, PP/strach/MCAPP systems have much better thermal stability compared with pure PP. The ratio of PP and starch also affects the thermal behavior of the flame retarded

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Figure 9. TGA curves of the PP/starch/MCAPP systems under air atmosphere. Table 3. TGA Results of the PP/Starch/MCAPP Systems samples

T-10 wt % (°C)

Tmax (°C)

char residue at 600 °C (wt %)

PP starch SCP5 SCP8

253.2 287.2 264.8 256.5

288.6 299.0 299.0 299.0

0.5 6.6 17.0 28.4

SCP semibiocomposites. PP/50 wt % starch/20 wt % MCAPP has lower thermal stability than PP/20 wt % starch/20 wt % MCAPP below 305 °C, but shows higher thermal stability above this temperature. The reason for this can be explained by the chemical reaction between MCAPP and starch. With the content of starch increasing, more starch can take part in the reaction with MCAPP, and more small molecules can be released, so the initial thermal decomposition is faster than that of the low content of starch. The char residue of PP/50 wt % starch/20 wt % MCAPP at 600 °C is about 28.4 wt %, much more than that (17.0 wt %) of PP/20 wt % starch/20 wt % MCAPP. 3.7. Volatilized Products of SCP Semibiocomposites Analyzed by TG-IR. TG-IR was used to analyze the gas products during the thermal degradation. From the TGA cure (Figure 10) of PP/40 wt % starch/20 wt % MCAPP under nitrogen, the sample shows two degradation steps. The first step occurs at about 270 °C, and the second occurs at 400 °C. The 3D TG-IR spectra of the gas phase in the thermal degradation of PP/40 wt % starch/20 wt % MCAPP are shown in Figure

Figure 10. TGA curve of the PP/40 wt % starch/20 wt % MCAPP semibiocomposite under nitrogen atmosphere.

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Figure 11. 3D TG-IR spectra of the gas phase in the thermal degradation of PP/40 wt % starch/20 wt % MCAPP.

Figure 12. Characteristic spectra obtained at 15.1, 20.4, 22.9, 23.8, 26.8, and 30.0 min from the TG-IR test.

11. In this figure, FTIR spectra of all the volatile pyrolysis products evolved at different times are shown. And, the characteristic spectra obtained at 15.1, 20.4, 22.9, 23.8, 26.8, and 30.0 min are shown in Figure 12. Some small molecular gaseous species, CO2, H2O, CO, are easily identified by their characteristic absorbance: saturated hydrocarbons (-CH3 and -CH2- groups: 2950-2850 and 1515-1370 cm-1), unsaturated alkane (3076 and 890 cm-1), H2O (3400-4000 cm-1), NH3 (927 cm-1), and CO2 (2381 cm-1).19 The peak at 2381 cm-1 appears at about 15.1 min indicating the appearance of CO2. The peaks at 2950-2850 cm-1 appear at about 20.4 min indicating the appearance of saturated hydrocarbons. When the time increases to 22.9 min, the appearance of the absorptions at 3076 cm-1 could prove to the formation of unsaturated alkane. The

appearance of saturated hydrocarbons and unsaturated alkane groups is mainly attributed to the decomposition of the PP. In order to clearly understand the change of the formed products, the relationship between intensity of the characteristic peak and temperature for volatilized saturated hydrocarbons, unsaturated alkane, H2O, NH3, and CO2 is plotted in Figure 13. It can be found that the IR result is in delay compared with the TGA result. This can be explained by two reasons. The first is that pyrolysis products of thermal degradation may condense in the junction, and the second is that pyrolysis products should take some time to reach the IR equipment. The characteristic peaks for NH3 and H2O appears at 13.3 min, disappears at 16.1 min, and then appears at 22.4 min again. The reason for the initial production of NH3, H2O, and CO2 at low temperature is


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Figure 13. Relationship between intensity of characteristic peak and temperature for volatilized saturated hydrocarbons, unsaturated alkane, H2O, NH3, and CO2.

that APP can release NH3 around 280 °C to form poly(phosphorus acid) and poly(phosphorus acid) can react with the hydroxyl group of the starch to release H2O. Furthermore, the presence of poly(phosphorus acid) can catalyze the decomposition of starch and release some small molecules such as CO2. These small volatile gas products are very important for the formation of an intumescent char layer, which can cause the char to expand and dilute the oxygen and flammable gas during burning. Associated with the analysis of the TGA result, the first decomposition step of PP/40 wt % starch/20 wt % MCAPP may be due to the release of small molecules. At high temperature, the intumescent char layer decomposes and loses its protective effect, so the SCP composites begin to decompose and release some small molecules. The characteristic peaks for hydrocarbons initially show a peak value at 23.8 min and rapidly decrease. It can be found that the volatilized products of the thermal degradation of PP/40 wt % starch/20 wt % MCAPP at the maximum decomposition are saturated hydrocarbons, unsaturated alkane, H2O, and NH3, etc.

The TG-IR result indicates that the initial weight loss for SCP semibiocomposites is due to the release of NH3, H2O, and CO2 at low temperature. The reason for this can be explained by the fact that APP can release NH3 around 280 °C to form poly(phosphorus acid) and poly(phosphorus acid) can react with the hydroxyl group in the starch to release H2O. Furthermore, the presence of poly(phosphorus acid) can catalyze the decomposition of starch to release some small molecules such as CO2. The volatilized products produced during the thermal degradation of PP/40 wt % starch/20 wt % MCAPP at the maximum decomposition are saturated hydrocarbons, unsaturated alkane, H2O, NH3, etc. Acknowledgment The work was financially supported by the Program for Specialized Research Fund for the Doctoral Program of Higher Education (200803580008), the Program for Science and Technology of SuZhou (SG-0841), and China Postdoctoral Science Foundation (20080430101).

Conclusion In this work, starch containing polypropylene (SCP) semibiocomposites were prepared by the melted blend method. Microencapsulated ammonium polyphosphate (MCAPP) was added to the semibiocomposites not only to improve their flame retardant properties but also to restrain the reaction between ammonium polyphosphate (APP) and starch during processing. With 20 wt % MCAPP, the sample PP/40 wt % starch can pass a UL-94 V0 rating. The LOI value of PP/40 wt % starch/ 20 wt % MCAPP is as high as 28.0%. The results of cone calorimeter testing show that the peak of heat release rate (HRR) and total heat release (THR) of SCP semibiocomposites decrease substantially compared with those of pure PP: the HRR value decreases from 1140.0 to 231.0 kW/m2 and the THR value decreases from 96.0 to 60.3 MJ/m2. SEM results demonstrate that the quality of char layers of PP/MCAPP/starch systems is superior to those of PP/MCAPP and PP/starch systems. From TGA results, it can be found that PP/50 wt % starch/20 wt % MCAPP has weaker thermal stability than PP/20 wt % starch/20 wt % MCAPP below 305 °C, but it shows better thermal stability above that temperature. The char residue of PP/50 wt % starch/20 wt % MCAPP at 600 °C is about 28.4 wt %, much more than that (17.0 wt %) of PP/20 wt % starch/20 wt % MCAPP.

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ReceiVed for reView August 1, 2009 ReVised manuscript receiVed October 8, 2009 Accepted October 17, 2009 IE9012198

Flame Retardancy and Thermal Degradation - American Chemical

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