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Dec 20, 2010 - Res. , 2011, 50 (2), pp 713–720 ... Intumescent flame-retardant poly(lactic acid)/starch (PLA/starch) biocomposites were prepared by ...
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Ind. Eng. Chem. Res. 2011, 50, 713–720

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Flame Retardancy and Thermal Degradation of Intumescent Flame Retardant Poly(lactic acid)/Starch Biocomposites Xin Wang,†,‡ Yuan Hu,†,‡,* Lei Song,† Shanyong Xuan,† Weiyi Xing,† Zhiman Bai,† and Hongdian Lu†,§ State Key Laboratory of Fire Science, UniVersity of Science and Technology of China, Anhui 230026, P. R. China, Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for AdVanced Study, UniVersity of Science and Technology of China, Suzhou, Jiangsu 215123, P.R. China, and Department of Chemical and Material Engineering, Hefei UniVersity, Hefei, Anhui 230022, P. R. China

Intumescent flame-retardant poly(lactic acid)/starch (PLA/starch) biocomposites were prepared by means of melt blending. Microencapsulated ammonium polyphosphate (MCAPP) was added to the PLA/starch biocomposites not only to improve its flame retardancy but also to restrain the reaction between ammonium polyphosphate and starch during processing. The flame-retardant properties of PLA/starch biocomposites were evaluated by limiting oxygen index, UL-94 test, and microscale combustion calorimetry (MCC) test. The results of MCC showed that the peak of heat release rate and total heat release of PLA/starch biocomposites decreased dramatically compared with those of pure PLA. The thermal degradation and gas products of PLA/ starch/MCAPP systems were monitored by thermogravimetric analysis and thermogravimetric analysis-infrared spectrometry. Scanning electron microscopy and X-ray photoelectron spectroscopy were utilized to explore the surface morphology and chemical components of the char residues. Introduction With the growing environmental awareness and shortage of natural resource, there is a strong demand for biodegradable polymers as means of solving the disposal problem due to nondegradable petroleum-based plastics.1-3 Among biodegradable polymers, polylactic acid (PLA) has received an increasing amount of attention because of its biodegradability, abundant renewable source, and excellent mechanical properties. It has various applications such as in automotive components, electrical industry, building materials, and the aerospace industry due to ecological and economical advantages.4-7 These fields require a remarkable flame retardant grade, but the flammability is a major drawback of PLA which limits its applications; thus, modification for flame retardancy of PLA is necessary.8 For environmental concerns, halogen-free flame retardation has aroused great attention in recent years, because halogencontaining flame retardant materials produce a lot of smoke and toxic gases during burning. Intumescent flame retardant (IFR) is considered as a promising halogen-free flame retardant additive due to its advantages of low smoke, low toxicity, low corrosion, and no molten dropping during a fire.9-12 Typically, an IFR system includes three basic components: an acid source, a carbonization agent, and a blowing agent. Its proposed mechanism is based on the charred layer acting as a physical barrier, which isolates the transfer of mass and heat between gas and condensed phases.13-15 The aim of this work is to enhance the flame retardant properties of PLA biocomposites. So, IFR system was selected for improving the flame retardancy of PLA/starch biocomposites. In the system, microencapsulated ammonium polyphosphate (MCAPP) is used as the acid source, melamine (MA) acts as * To whom correspondence should be addressed. Fax: +86-5513601664. E-mail: [email protected]. † State Key Laboratory of Fire Science, University of Science and Technology of China. ‡ Suzhou Key Laboratory of Urban Public Safety, University of Science and Technology of China. § Hefei University.

the blowing agent, while starch functions as the carbonization agent. Starch is an inexpensive, biodegradable, biocompatible and renewable polyol,16 which could be used as a natural carbon source. It 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.17,18 Furthermore, APP is microencapsulated by polyurethane not only to improve its compatibility with PLA but also to retard the reaction between acid and carbonization agent during processing. In our previous work,19 biodegradable poly(vinyl alcohol) (PVA) composites filled with starch and MCAPP were prepared. The starch together with MCAPP conferred excellent flame retardant effect on PVA. This intumescent flame retardant system, including starch and MCAPP, is expected to be incorporated into other polymers to obtain the composites with enhanced flame resistance. Therefore, in this study, the intumescent flame retardant system was blended with PLA to obtain flame retardant and environment friendly biocomposites. The flame retardant and thermal properties of the biocomposites were evaluated by limiting oxygen index (LOI), UL-94, microscale combustion calorimetry, and thermogravimetric analysis. The themal degradation process was investigated by real time Fourier transform infrared spectrometry (RTFTIR) and thermogravimetric analysis-infrared spectrometry (TG-IR). The chemical components and surface morphology of the char residues were explored by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Experimental Section Materials. PLA resin (NatureWorks 4032D) in granular form was supplied by Cargill Dow Inc. Native potato starch was obtained from Shangdong Jincheng Co., Ltd. (Zhaoyuan, China). Ammonium polyphosphate microencapsulated (MCAPP, average size 10 µm) by polyurethane was provided by Hefei KeYan Co. Melamine (MA) was purchased from Shanghai Chemical Reagent Corp. The intumescent flame retardant (IFR) was composed of MCAPP and MA (the weight ratio was 2:1).

10.1021/ie1017157  2011 American Chemical Society Published on Web 12/20/2010

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Table 1. Formulations and Flame Retardancy of IFR-PLA Systems composition (wt %)

flame retardancy

sample code

PLA

IFR

starch

LOI (%)

PLA-1 PLA-2 PLA-3 PLA-4 PLA-5 PLA-6 PLA-7 PLA-8 PLA-9 PLA-10

100 80 80 80 80 95 90 70 70 70

0 20 17.5 15 10 0 0 30 25 20

0 0 2.5 5 10 5 10 0 5 10

20.0 27.0 28.5 30.0 31.5 22.0 23.0 33.0 38.0 41.0

UL-94 rating burning burning V1 V1 V1 burning burning V1 V0 V0

and dripping and dripping

and dripping and dripping

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 Ltd), using Al Ka excitation radiation (hν ) 1253.6 eV). Scanning electron microscopy (SEM) was performed on the surfaces and cross sections of the char residues using a Hitachi X650 scanning electron microscope. The specimens were previously coated with a conductive layer of gold. Results and Discussion

Preparation of Samples. PLA, MCAPP, MA, and starch were dried in a vacuum oven at 80 °C overnight before use. Then PLA, MCAPP, MA, and starch were melt-mixed in a twinroller mill for 15 min; the temperature of the mill was maintained at 175 °C, and the roller speed was 30 rpm. The composition of samples is listed in Table 1. After mixing, the samples were hot-pressed at about 170 °C under 10 MPa for 10 min into sheets with the thickness of 3.0 ( 0.1 mm for UL94 and LOI. Measurements. Wide-angle X-ray diffraction patterns of the samples were recorded on an X-ray diffractometer (Rigaku Dmax/rA), using Cu KR radiation (λ ) 0.154 18 nm) at 40 kV and 20 mA. LOI was measured according to ASTM D2863. The apparatus used was an HC-2 oxygen index meter (Jiangning Analysis Instrument Co.). The specimens used for the test were of dimensions 100 × 6.5 × 3 mm3. The vertical test was carried out on a CFZ-2 type instrument (Jiangning Analysis Instrument Co.) according to the UL 94 test standard. The specimens used were of dimensions 130 × 13 × 3 mm3. The combustion properties were evaluated using a microscale combustion calorimeter (GOVMARK MCC-2). The samples were tested according to ASTM D 7309-07. Approximately 4 ( 1 mg of each sample was raised into the heated tube of a pyrolysis-combustion flow calorimeter (PCFC) that was purged with nitrogen. The sample was gradually heated to 900 °C at a heating rate of 1 °C/s. The gaseous pyrolysis products mix in the gas stream with oxygen prior to entering the combustion zone of the PCFC, where they are completely oxidized. Oxygen and nitrogen flow rates were set at 20 and 80 cm3/min, respectively. Heat release rate (HRR) in watts per gram of sample (W/g) was calculated from the oxygen depletion measurements. The total heat release (THR) in kJ/g was obtained by integrating the HRR curve.20 Reported data herein are the average of three tests of each sample. Thermogravimetric analysis (TGA) was carried out using a Q5000IR (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. The samples were run in triplicate; the temperature reproducibility of the instrument is (1 °C, while the mass reproducibility is (0.2%. The real time Fourier transform infrared spectra (RTFTIR) were recorded using a Nicolet MAGNA-IR 750 spectrophotometer equipped with a heating device and a temperature controller. Powders of samples were mixed with KBr powders, and the mixture was pressed into a tablet, which was then placed in a ventilated oven. The temperature of the oven was raised at a heating rate of 10 °C/min. Thermogravimetric analysis-infrared spectrometry (TG-IR) was performed using the TGA Q5000 IR thermogravimetric

XRD Analysis. Figure 1 shows the XRD patterns of native starch and the PLA/starch biocomposite. As shown in Figure 1, starch exhibits mainly two strong intensity peaks at 17.5° and 23.0° (2θ). After mixing starch with PLA by melt blending, obvious changes can be found. The above-mentioned peaks of starch coincides with the amorphous area of the PLA matrix. The intra- and intermolecular hydrogen bonds are responsible for the ordered crystalline structure in starch.21 During the blending process, the formation of intramolecular hydrogen bonds is reduced, which results in the destruction of the ordered crystalline structure. Flame Retardancy of PLA and PLA/Starch Composites. The effect of starch on the LOI values and UL-94 results of the intumescent flame retardant PLA composites is shown in Figure 2. Pure PLA is highly combustible and is not classified in the UL-94 rating. PLA/10 wt % starch and PLA/15 wt % starch are also combustible, and neither can reach a UL-94 rating. The LOI value of PLA/15 wt % starch is just 23.0%. When MCAPP and MA are combined with PLA/starch composites, their flame retardant properties are greatly improved. It can be seen that the LOI values of the PLA composites increase with the increase of starch content. The LOI value of the sample PLA-10 is as high as 41.0%. With 15 wt % MCAPP and 7.5 wt % MA, both the sample PLA-9 and PLA-10 can pass the V0 rating in the UL-94 test, while the sample PLA-8 reaches V1 rating. From the above result, it can be concluded that starch is a effective carbonization agent for PLA, which can promote the formation of the protective char layer, so the flame retardant properties of PLA biocomposites improve. The residual chars of PLA and PLA/starch composites (containing 15 wt % MCAPP and 7.5 wt % MA) at the end of

Figure 1. XRD patterns of native starch and the PLA/starch biocomposite.

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Figure 2. Effect of starch on the LOI values and UL-94 results of the IFR-PLA composites.

Figure 3. Photos of char residues of samples after combustion: (a) PLA, (b) PLA-8, (c) PLA-9, and (d) PLA-10.

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 pure PLA, whereas the surface of PLA/starch composite residue is covered with an expanded char network. The results demonstrate that IFR system (MCAPP/MA/starch) possesses an excellent intumescent effect. As the content of starch increases, the intumescent properties are observed obviously. Microscale Combustion Calorimeter. Cone calorimetry is a commonly used approach to study flame retardancy and quantitatively measure HRR. However, it requires large quantities (25-100 g) of materials for accurate and reproducible determinations. Lyon and Walters at Federal Aviation Administration (FAA) developed the microscale combustion calorimeter (MCC), which measures flammability of materials on milligram quantities.20 MCC is a new, rapid laboratory scale test that uses thermal analysis methods to measure chemical properties related to fire. Using the MCC, the HRR curves of PLA and IFR-PLA systems are given in Figure 4. It can be found that the HRR plots of IFR-PLA systems (PLA-8, -9, and -10) contain two peaks, as shown in Figure 4. The first peak occurs between 270 and 300 °C, which corresponds to degradation of the intumescent flame retardant. The second peak occurs between 320 and 400 °C and represents combustion of PLA matrix. The PHRR values, obtained from the peak of the HRR curves, are summarized in Table 2. Neat PLA exhibits the highest PHRR of 398 W/g and the highest maximum HRR temperature of 375

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Figure 4. Heat release rate curves of PLA and IFR-PLA systems. Table 2. MCC Results of PLA and IFR-PLA Systems samples

PHRR (W/g)

THR (kJ/g)

max HRR temp (°C)

PLA-1 PLA-8 PLA-9 PLA-10

398 145 133 97

13.9 9.4 7.8 6.8

375 351 358 365

°C. The sample PLA-8 exhibits PHRR value of 145 W/g. Addition of MCAPP and MA leads to about 64% reduction in PHRR. The sample PLA-10, which contains the MCAPP, APP, and starch simultaneously, exhibits the lowest PHRR value (97 W/g). THR, calculated from the total area under the HRR peaks, is another important parameter used to evaluate fire hazard. PLA-8 and PLA-9 exhibit THR values of 13.9 and 9.4 kJ/g, respectively. PLA-10 exhibits the lowest THR values of 6.8 kJ/g, corresponding to about 51% reduction in the THR of neat PLA. As shown in Table 2, it can be founded that the incorporation of starch into intumescent flame retardant PLA systems can reduce the peak heat release rate and total heat release obviously. Thermal Stability of PLA and PLA/Starch Composites. The typical TG and DTG traces for PLA and PLA/starch composites under air atmosphere are given in Figure 5. The initial decomposition temperature (Td) can be considered as the temperature at which the weight loss was 5%. The relative thermal stability of the samples was compared by the temperature of 5% and 75% (Td,-75%) weight loss, the temperature of maximum rate of weight loss (Tmax), and the percent char yield at 600 °C. These data are listed in Table 3. The pure PLA starts to decompose at 328 °C and the thermal degradation process only has one stage. The stage is in the temperature range of 300-400 °C, corresponding to a strong DTG peak at 368 °C (Tmax) and the weight loss is about 97%. The neat PLA at 600 °C does not produce any char residue. As for IFR-PLA systems, the thermal degradation process of PLA-8, -9, and -10 has two stages in the temperature range of 250-300 and 300-380 °C. The IFR-PLA systems begin to lose weight at about 260 °C, which is earlier compared with that of pure PLA. This can be explained by the fact that decomposition of starch occurs between 250 and 300 °C. The first stage is only about 15 wt % weight loss derived from the degradation of the starch and MCAPP. The major weight loss (approximately 65% decrease) occurs at the second stage from 300 to 380 °C and then the weight loss shows no change, which reflects the thermal stability of the char layer formed during

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Figure 6. FTIR spectra of PLA-10 at different degradation temperatures. Table 4. Assignment of Dynamic FTIR Spectra of PLA-10

Figure 5. (a) TG and (b) DTG traces for PLA and PLA/starch composites under air atmosphere. Table 3. TG Data of PLA and IFR-PLA Systems in Air samples Td (°C) Td,-75% (°C) Tmax (°C) char residue (%) at 600 °C PLA PLA-8 PLA-9 PLA-10

328 263 260 259

373 368 539 580

368 278, 349 267, 329 268, 331

0.2 16.3 19.6 22.2

degradation. At higher temperature (beyond 500 °C), the weight continues to decrease at a increased rate from around 500-600 °C. The increased rate of mass loss illustrates the continued degradation of the char layer. The residue left at 600 °C increases significantly, as starch is combined with MCAPP and MA. These results indicate that the incorporation of starch into intumescent flame retardant PLA systems can improve the thermal stability of the IFR-PLA composites at higher temperature and promote the formation of char layer. Thermal Degradation of PLA and PLA/Starch Composites. Dynamic FTIR is employed to evaluate the thermal degradation process of PLA-10. The FTIR spectra of PLA-10 at different degradation temperatures are shown in Figure 6. It can be seen that peaks at 3472, 3425, 2992, 2940, 1765, 1657, 1550, 1460, 1262, and 1091 cm-1 are the characteristic absorptions of PLA-10. The assignment of dynamic FTIR spectra of PLA-10 is presented in Table 4. The relative intensities of the characteristic peaks do not show obvious change below 250 °C. At 250 °C, the bands at 3472, 3425, and 1657 cm-1 corresponding to the NH2 group decrease

Wavenumber (cm-1)

assignment

3472, 3425 2992, 2940 1765 1657 1550 1262 1091 1140, 1020 880

N-H or O-H stretching vibration -CH3 and -CH stretching vibration CdO stretching vibration N-H deformation vibration stretching vibration of triazine ring PdO stretching vibration stretching vibration of C-O-C stretching vibration of PO2/PO3 stretching vibrations of P-O-P

dramatically, and this can be caused by the release of ammonia from MA. Meanwhile, almost all of the ester bonds of the PLA have been pyrolyzed according to the decrease of the peaks at 1765 cm-1 corresponding to CdO, and the wide peaks around 1091 cm-1 ascribed to the stretching vibration of C-O-C. The relative intensities of C-H stretching vibration at 2992 and 2940 cm-1 and deformation vibration at 1360 cm-1 also decrease. While the temperature rises up to 280 °C, it is found that the intensities of the absorption peaks at 2992, 2940, 1765, 1657, 1550, 1460, 1262, and 1091 cm-1 nearly disappear, indicating that the main decomposition happens at this stage. This is consistent with the TGA results. With further increase of the temperature, two new peaks at 1140 and 1020 cm-1 with increasing intensity can be ascribed to the stretching vibration of P-O-C and PO2/PO3 in phosphate-carbon complexes.22 The peaks at 1090 and 880 cm-1 belong to the stretching vibrations of the P-O-P bond, and this indicates the formation of poly(phosphoric acid), such as P2O5 and P4O10.23 The presence of poly(phosphoric acid) catalyzes the formation of char. The residual char can prevent materials from further degrading during combustion. Volatilized Products of PLA Biocomposites Analyzed by TG-FTIR. TG-FTIR is used to analyze the gas products during the thermal degradation. The 3D TG-FTIR spectra of gas phase in the thermal degradation of PLA and PLA-10 are shown in Figures 7 and 8. In Figure 7, peaks in the regions of around 3400-3600, 2750-3150, 2250-2400, 1300-1450, and 1100-1200 cm-1 are noted. Some of the gaseous decomposition products of the PLA are unambiguously identified by characteristic, strong FTIR signals, such as methane (3015 cm-1) and water (3575 cm-1).24 The main share of the bands of the decomposition products is attributed to the functional groups with characteristic, unambiguous band

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

Figure 10. FTIR spectrum of pyrolysis products for PLA-10 at the maximum decomposition rate.

Figure 8. 3D TG-FTIR spectrum of gas phase in the thermal degradation of PLA-10.

Figure 9. FTIR spectrum of pyrolysis products for PLA at the maximum decomposition rate.

positions: compounds containing carbonyl group (1760 cm-1) and methyl-substituted compounds (2980 cm-1).19,24,25 FTIR spectra of pyrolysis products of PLA at maximum decomposition rates are shown in Figure 9. The main products of the thermal decomposition of PLA are compounds containing -OH (such as H2O, 3400-3600 cm-1), CO2 (2360 cm-1), aliphatic ethers (1120 cm-1), hydrocarbons (C-H stretching at 1373 cm-1), compounds containing carbonyl group (1760 cm-1), etc.19,24,25 It is well-known that depolymerization is the main process associated with the thermal degradation of polymers. In the process of depolymerization, the main decomposition products are CO2, H2O, hydrocarbons, etc.

As shown in Figure 10, the evolved gas analysis for PLA-10 at maximum decomposition rates exhibits characteristic bands of H2O (3575 cm-1), CO2 (2360 cm-1), hydrocarbons (-CH3 and -CH2- groups, 2980-2850 and 1200-1300 cm-1), and compounds containing carbonyl group (1760 cm-1),19,24,25 which is similar to that of pure PLA. Additionally, the appearance of the new absorption band at 3340 cm-1 is attributed to the release of NH3; the new absorption band at 1510 cm-1 appears which is due to the structures containing aromatic rings. Moreover, it is probably that other new absorption bands, such as 1260 cm-1 (PdO) and 1118 cm-1 (-P-O-P-O), almost coincide with the characteristic peaks of the PLA matrix. It can be concluded that the polyphosphate structures are formed by decomposition of IFR. These polyphosphate structures can react with other pyrolysis products containing a hydroxyl group to catalyze the formation of intumescent carbonaceous chars. The absorbance of pyrolysis products for PLA and PLA-10 vs time is revealed in Figure 11. It can be seen that the pyrolysis products for PLA-10 begin to release at about 24.7 min, whereas those for PLA begin to release at about 27.7 min. It can be interpreted that IFR can catalyze the thermal decomposition of PLA. However, the absorbance intensity of pyrolysis products for PLA-10 (except CO2) is much lower than that for PLA, especially hydrocarbons. Consequently, the addition of IFR can reduce the release of combustible gas and the weight loss. Meanwhile, the release of nonflammable gases (such as CO2 and NH3) originated from the thermal decomposition of IFR can dilute the combustible gas, thus retarding the combustion. The results of the pyrolysis products release correspond well to MCC. Chemical Structure and Morphology of the Residual Char. The chemical components of the residual char for PLA10 (heated in muffle furnace for 10 min at 600 °C) are investigated by XPS, as shown in Figure 12. The results of the XPS spectra are presented in Table 5. The peak at 284.6 eV is attributed to C-H and C-C in aliphatic and aromatic species. The peak at 286.0 eV is characteristic of C-O (ether and/or hydroxyl group).26 Moreover, the peaks at 287.4 and 288.5 eV can be assigned to CdO and/or CdN, respectively.27 Three bands at 530.9, 532.5, and 533.5 eV are observed from the O1s spectra. The peak at 530.9 eV can be attributed to the dO in phosphate or carbonyl groups, and the peak at around 532.5 eV is assigned to -O- in C-O-C, C-O-P, and/or C-OH groups.28 The peak at 533.5 eV corresponds to chemisorbed oxygen and/or adsorbed water. For N1s spectra, two peaks at

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Figure 11. Absorbance of pyrolysis products for PLA and PLA-10 vs time: (a) total, (b) H2O, (c) hydrocarbons, (d) CO2, (e) carbonyl compounds, and (f) aromatic compounds. Table 5. XPS Results of the Residual Char of PLA-10 system

binding energy (eV)

area (%)

C1s O1s N1s P2p

284.6, 286.0, 287.4, 288.5 530.9, 532.5, 533.5 399.1, 400.6 133.8

62.8 22.0 8.2 7.0

around 399.1 and 400.6 eV are observed, which may be assigned to the nitrogen functionality in melamine structures (399.1 eV) and pyrrolic (400.6 eV) group.28 As for the spectra of P2p, the peak at around 133.8 eV can be attributed to the pyrophosphate and/or polyphosphate.19 The samples PLA and PLA-10 were heated in muffle furnace for 5 min at 600 °C. The residual chars for PLA-10 were obtained, whereas the pure PLA does not produce any char

residue. Figure 13 presents the SEM photographs of the residual chars for PLA-10. The char residue presents the cellular structures, which results from the released gases of the blowing agent. Moreover, the surface char is dense and compact. Conclusion In our research, starch together with microencapsulated ammonium polyphosphate (MCAPP) was utilized to develop a biobased polylactide (PLA) composite with improved flame retardancy. Pure PLA was highly combustible, while PLA composites containing 30% IFR could reach UL-94 V0 with a high LOI value of 41.0. The incorporation of IFR into PLA obviously decreased the peak heat release rate (PHRR) and total heat release (THR) of the composites in the MCC test.

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Figure 12. (a) C1s, (b) O1s, (c) N1s, and (d) P2p XPS spectra of the char residue of PLA-10.

Figure 13. SEM micrographs of the char residues for PLA-10: (a) low amplification and (b) high amplification.

The TGA results indicated that the addition of IFR into PLA could dramatically improve the char yields and the thermal stability of the char at high temperature compared with neat PLA. These results were consistent with the data of dynamic FTIR. The data of TG-FTIR showed that the main products of the thermal decomposition of PLA are H2O, CO2, compounds containing carbonyl group, hydrocarbons, etc.

The presence of IFR catalyzed the degradation of PLA, and less flammable gas products were released at the thermal degradation process, so the PHRR and THR were reduced. The residual char which was mainly composed of carbon/ pyrophosphate and or polyphosphate compounds and some holes were presented on the surface of the char. This intumescent char layer can inhibit heat transmission and

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retard inner polymer degradation; thus, the fire resistance of polylactide was improved. Acknowledgment The work was 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), the Opening Project of State Key Laboratory of Environmental Adaptability for Industrial Product, and the Program for the Graduate Innovation Fund in University of Science and Technology of China. Literature Cited (1) Li, S. M.; Ren, J.; Yuan, H.; Yu, T.; Yuan, W. Z. Influence of ammonium polyphosphate on the flame retardancy and mechanical properties of ramie fiber-reinforced poly(lactic acid) biocomposites. Polym. Int. 2010, 59, 242–248. (2) Yu, T.; Ren, J.; Li, S. M.; Yuan, H.; Li, Y. Effect of fiber surfacetreatments on the properties of poly(lactic acid)/ramie composites. Compos. Part A 2010, 41, 499–505. (3) Rasal, R. M.; Janorkar, A. V.; Hirt, D. E. Poly(lactic acid) modifications. Prog. Polym. Sci. 2010, 35, 338–356. (4) Zhang, J. F.; Sun, X. Z. Mechanical properties of poly(lactic acid)/ starch composites compatibilized by maleic anhydride. Biomacromolecules 2004, 5, 1446–1451. (5) Li, B. H.; Yang, M. C. Improvement of thermal and mechanical properties of poly(L-lactic acid) with 4,4-methylene diphenyl diisocyanate. Polym. AdV. Technol. 2006, 17, 439–443. (6) Garlotta, D. A literature review of poly(lactic acid). J. Polym. EnViron. 2001, 9, 63–84. (7) Carrasco, F.; Pagesb, P.; Gamez-Perezc, J.; Santanac, O. O.; Maspoch, M. L. Processing of poly(lactic acid): Characterization of chemical structure, thermal stability and mechanical properties. Polym. Degrad. Stab. 2010, 95, 116–125. (8) Li, S. M.; Yuan, H.; Yu, T.; Yuan, W. Z.; Ren, J. Flame-retardancy and anti-dripping effects of intumescent flame retardant incorporating montmorillonite on poly(lactic acid). Polym. AdV. Technol. 2009, 20, 1114– 1120. (9) Gao, M.; Wu, W.; Yan, Y. Thermal degradation and flame retardancy of epoxy resins containing intumescent flame retardant. J. Therm. Anal. Calorim. 2009, 95, 605–608. (10) Gao, M.; Yang, S. S. A novel intumescent flame-retardant epoxy resins system. J. Appl. Polym. Sci. 2010, 115, 2346–2351. (11) Nie, S. B.; Song, L.; Guo, Y. Q.; Wu, K.; Xing, W. Y.; Lu, H. D.; Hu, Y. Intumescent flame retardation of starch containing polypropylene semibiocomposites: Flame retardancy and thermal degradation. Ind. Eng. Chem. Res. 2009, 48, 10751–10758. (12) Li, B.; Xu, M. J. Effect of a novel charring-foaming agent on flame retardancy and thermal degradation of intumescent flame retardant polypropylene. Polym. Degrad. Stab. 2006, 91, 1380–1386. (13) Bourbigot, S.; Le Bras, M.; Duquesne, S.; Rochery, M. Recent advances for intumescent polymers. Macromol. Mater. Eng. 2004, 289, 499– 511.

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ReceiVed for reView August 13, 2010 ReVised manuscript receiVed November 2, 2010 Accepted December 1, 2010 IE1017157