lable at ScienceDirect

Polymer Degradation and Stability 94 (2009) 291–296

Contents lists avai

Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Combustion properties and thermal degradation behavior of polylactide with aneffective intumescent flame retardant

Jing Zhan a,b, Lei Song a, Shibin Nie a, Yuan Hu a,*

a State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, PR Chinab Department of Polymer Science and Engineering, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, PR China

a r t i c l e i n f o

Article history:Received 15 November 2008Received in revised form14 December 2008Accepted 18 December 2008Available online 31 December 2008

Keywords:PolylactideIntumescent flame retardantCombustion propertyThermal stability

* Corresponding author. Tel./fax: þ86 551 3601664E-mail address: yuanhu@ustc.edu.cn (Y. Hu).

0141-3910/$ – see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.polymdegradstab.2008.12.015

a b s t r a c t

An intumescent flame retardant spirocyclic pentaerythritol bisphosphorate disphosphoryl melamine(SPDPM) has been synthesized and its structure was characterized by Fourier transformed infraredspectrometry (FTIR), 1H and 31P nuclear magnetic resonances (NMR). A series of polylactide (PLA)-basedflame retardant composites containing SPDPM were prepared by melt blending method. The combustionproperties of PLA/SPDPM composites were evaluated through UL-94, limiting oxygen index (LOI) testsand microscale combustion calorimetry (MCC) experiments. It is found that SPDPM integrating acid, charand gas sources significantly improved the flame retardancy and anti-dripping performance of PLA.When 25 wt% flame retardant was added, the composites achieved UL-94 V0, and the LOI value wasincreased to 38. Thermogravimetric analysis (TGA) showed that the weight loss rate of PLA wasdecreased by introduction of SPDPM. In addition, the thermal degradation process and possible flameretardant mechanism of PLA composites with SPDPM were analyzed by in situ FTIR.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Nowadays, biodegradable polymeric material has attractedmore and more attention due to the deterioration of environmentalpollution and the quick decrease of the petroleum energy sources[1]. Polylactide (PLA) is one of the biodegradable polymers that canbe used as promising alternatives to the normal petroleum basedcommodity resins, because they can be derived from renewableresources, such as corn, potato and other agricultural products [2].Moreover, it has good biocompatibility, mechanical properties,thermal plasticity, high degree of transparency and gas perme-ability, therefore PLA holds tremendous promise for various end-use applications such as biomedical fields, household, engineering,packing industries and so on [3–7].

Just like other plastics, the poor fire-resistance of PLA restrictsits application and development in some important fields, but onlya little study has been done about the flame retardant polylactideup to now. Reti et al. [8] studied the efficiency of different intu-mescent formulations to flame retardant PLA, and the quantity ofthe additives has been optimized to decrease the quantity of APP inthe formulation. Bourbigot and Solarski have studied flammabilityof PLA/clay nanocomposites, and found a significant decrease of

.

ll rights reserved.

peak of heat release rate, but failed to flammability tests (LOI,UL-94) [9,10].

It is well known that intumescent flame retardants (IFR) areefficient in some polymers and are widely used as halogen-freeadditives due to their advantages of little smoke and low toxicity[11]. The IFR system usually experiences an intense expansion andforms protective charred layers, thus well protecting the under-lying material from the action of the heat flux or flame duringcombustion [12,13]. Melamine salt of pentaerythritol phosphate(MPP) is a very effective IFR that chemically combines acid source,carbonization agent, and blowing agent into one molecule whichwill play the best synergism between the three components [14,15].However the addition of MPP as a salt would easily induce thedegradation of the polymer during the process especially for thebiodegradable polymeric material. Therefore a phosphamide withvery similar chemical structure to MPP is synthesized and expectedto avoid this problem.

In this paper, an intumescent flame retardant SPDPM has beensynthesized from spirocyclic pentaerythritol bisphosphorate dis-phosphoryl chloride (SPDPC) and melamine. Then flame retardantPLA composites with SPDPM were prepared and their combustionproperties were evaluated through UL-94, LOI tests and microscalecombustion calorimetry (MCC) experiments. In addition, TGA andin situ FTIR were used to investigate the thermal degradationprocess of the composites.

J. Zhan et al. / Polymer Degradation and Stability 94 (2009) 291–296292

2. Experimental

2.1. Materials

PLA was supplied by Cargill Dow. Melamine, N,N-dime-thylformamide (DMF) and pyridine were provided by ShanghaiChemicals Co. SPDPC was synthesized by the reaction of phos-phorus oxychloride with pentaerythritol as reported [16]. Allsolvents were purified before used.

2.2. Synthesis of SPDPM

A 1000 ml four-necked round bottom flask was equipped witha mechanical stirrer, reflux condenser connected with a calciumchloride tube, thermometer, and a pressure funnel. The flask wascharged with 600 ml DMF, 50 ml pyridine and 25.2 g (0.2 mol)melamine. The mixture was strongly stirred, gradually heated untilreflux. Then 100 ml DMF with 29.7 g (0.1 mol) SPDPC was addedwithin about 60 min. The reaction was kept about 6 h under reflux,and then the solution was filtered before cooled. The white productwas washed with DMF and water and dried at 80 �C under vacuumto a constant weight (70.2% yield). The synthesis route is illustratedin Scheme 1.

2.3. Preparation of PLA/SPDPM composites

PLA pellets were dried under vacuum at 80 �C for overnightbefore used. All the samples were prepared on a two-roll mixingmill (XK-160, Jiangsu, China) at 165 �C, and the roll speed wasmaintained at 35 rpm. PLA was firstly added to the mill at thebeginning of the blending procedure. After PLA was molten, SPDPMwas then added to the matrix and processed about 10 min untila visually good dispersion was achieved. The resulting sampleswere compressed and molded into sheets (3 mm thickness).

2.4. Characterization

The 1H and 31P NMR spectra were recorded with an AVANCE300 Bruker spectrometer using tetramethylsilane as an internalreference and DMSO-d6 as a solvent. The Fourier transforminfrared (FTIR) spectra were recorded with a MAGNA-IR 750spectrometer (Nicolet Instrument Company, USA). The in situ FTIRspectra were recorded in the range of 250–400 �C with a heatingrate of 10 �C/min.

UL-94 vertical burning tests were performed with plasticsamples of dimensions 130�13� 3 mm, suspended verticallyabove a cotton patch. The classifications are defined according tothe American National Standard UL-94. LOI tests were measuredaccording to ASTM D2863. The apparatus used was an HC-2 oxygen

Scheme 1. Synthesis of SPDPM.

index meter (Jiangning Analysis Instrument Company, China). Thespecimens used for the test were of dimensions 100� 6.5� 3 mm.

A Govmark MCC-2 microscale combustion calorimetry (MCC)was used to determine the flammability characteristics of PLAcomposites according to ASTM D 7309-07. About 5 mg specimenswere thermally decomposed in an oxygenated environment ata constant heating rate of 1 K/s.

TGA experiments were performed using a Q5000 IR thermoa-nalyzer instrument under air flows of 25 ml/min. The specimens(about 10 mg) were heated from room temperature to 600 �C ata linear heating rate 20 �C/min.

3. Results and discussion

3.1. Characterization of SPDPM

Fig. 1 is the 1H NMR spectrum of SPDPM. It can be found that the1H of –OCH2– from SPDPC resonates in the d¼ 4.0 ppm and 3.9 ppmwith equal proton signals in intensity. The proton peak of N–H fromthe amide bond shifts to 7.5 ppm due to the deshielding effect ofP]O, and the 1H of –NH2 induced the broad peaks between2.6 ppm and 3.8 ppm as a result of the hydrogen bond. 31P NMRspectrum of SPDPM is showed in Fig. 2, and the only one peak at40.1 ppm further validates the purity and structure of thecompound.

The FTIR spectrum of SPDPM is showed in Fig. 3. The strong andwide absorption peaks between 3500 and 3000 cm�1 can beascribed to the asymmetrical and symmetrical stretching vibrationsof N–H bond; meanwhile the bending vibration of N–H appears at1693 cm�1. The peak at 2896 cm�1 belongs to the stretchingvibration of C–H, and three peaks at 1561 cm�1, 1510 cm�1 and1395 cm�1 correspond to the stretching vibration of the triazinering. Compared with the peak 1300 cm�1 of P]O in SPDPC, thestretching vibration of P]O bond falls to 1241 cm�1 witha shoulder peak at 1218 cm�1 due to the resonant effect betweenP]O and N. Moreover, several peaks between 1020 cm�1 and1150 cm�1 are associated with stretching vibration of P–O–C. Allthe analysis above verifies the successful synthesis of SPDPM.

3.2. Combustion properties of PLA/SPDPM composites

LOI measurements and UL-94 vertical burning tests are widelyused to evaluate the flame retardant properties of materials. Table 1

Fig. 1. 1H NMR spectrum of SPDPM.

Fig. 2. 31P NMR spectrum of SPDPM.

Table 1Composition of the samples and the flame retardancy of the composites.

Samples PLA SPDPM (wt%) LOI (wt%) UL-94 (%) Dripping rating

PLA 100 0 21 NR HeavyPLA/5 SPDPM 95 5 28 V2 HeavyPLA/15 SPDPM 85 15 33.5 V2 SlightlyPLA/25 SPDPM 75 25 38 V0 No dripping

J. Zhan et al. / Polymer Degradation and Stability 94 (2009) 291–296 293

shows the LOI values, UL-94 rating and dripping behaviour of all thespecimens. It is found when only 5 wt% SPDPM is added the LOI ofPLA composite is increased 7 units compared to that of pure PLA.The sample with 25 wt% SPDPM obtains the maximum of LOI 38,and UL-94 V0 is achieved. Meanwhile, the anti-dripping effect isimproved due to the incorporation of SPDPM.

The photographs of the specimens after LOI tests are shown inFig. 4. It can be seen that during PLA composites burning processobvious intumescent char forms with the SPDPM contentincreasing. The intumescent char layer covers the surface ofmaterial and provides resistances of both mass and heat transfer,protects the inner materials and postpones its degradation to forma combustible substance [17].

Microscale combustion calorimetry (MCC) is evaluated asa screening test for efficacy of flame retardant in polymers from justa few milligrams of specimen. The test method is a thermal analysismethod that improves upon previous methods by directlymeasuring the heat of combustion of the gases evolved duringcontrolled heating of the samples [18].

Fig. 3. FTIR spectrum of SPDPM.

Fig. 5 is the heat release rate (HRR) curves of PLA compositeswith different SPDPM contents and the corresponding combustiondata are presented in Table 2. It is found that the peak HRR (pHRR)of PLA composites decreases with increasing SPDPM content.Compared to pure PLA, the pHRR of the sample containing 15 wt%SPDPM decreases from 475 W/g to 292 W/g. However there is nofurther decrease when the flame retardant additive is increased to25 wt%. It is possible because that the pHRR would not decrease anymore when the flame retardant composite has expanded to someextent during burning, as seen from Fig. 4 the PLA/15 SPDPMcomposites gain a good expanding and the degree of intumescencedoes not change much with 25 wt% SPDPM. Meanwhile, the totalheat release hc decreases as the flame retardant additives increase,and the reduction is 33.5% when 25 wt% SPDPM is incorporated.The heat release capacity hc is a relatively good predictor of the heatrelease rate in flaming combustion and the propensity for ignition,and low values of hc are indication of low flammability and low fullscale fire hazard [19]. From Table 2 it can be seen hc shows thesimilar change trend as pHRR. In addition, the temperature atmaximum pyrolysis rate (Tmax) decreases with the increasingSPDPM content because of the low initial thermal stability ofSPDPM. All the results above show that SPDPM can effectivelyimprove the flame retardancy of PLA.

3.3. Thermogravimetric analysis

The TGA and DTG curves of PLA/SPDPM composites are shownin Fig. 6, and the related data are listed in Table 3. It can be seenfrom the TGA curves that SPDPM possesses less thermal stabilitythan pure PLA at the beginning of decomposition, due to theincorporation of melamine segment. Therefore the 5% losstemperature (T�5 wt%) and the maximum weight loss temperature

Fig. 4. Pictures of the samples after LOI tests: (a) PLA/5 SPDPM; (b) PLA/15 SPDPM;(c) PLA/25 SPDPM.

Fig. 5. The HRR curves of PLA and its composites at 1 K/s heating rate.

Table 2Part data recorded in MCC experiments.

Samples hc (J/g K) pHRR (w/g) hc (kJ/g) Tmax (�C)

PLA 492 475 16.4 390PLA/5 SPDPM 400 398 14.4 389PLA/15 SPDPM 291 292 12.2 377PLA/25 SPDPM 291 291 10.9 369

J. Zhan et al. / Polymer Degradation and Stability 94 (2009) 291–296294

(Tmax) of the PLA composites with SPDPM are all lower than purePLA. On the contrary, SPDPM possesses high thermal stability charat high temperatures with 40 wt% char residue at 600 �C. Thereforethe char residue of flame retardant PLA composites at finaltemperature increases with the increasing SPDPM content, forexample the PLA/25 SPDPM sample yields 12 wt% char residue(calculated theoretically 10 wt% char residue). This is because at theelevated temperature, the intumescent char forms and wouldprevent the inner matter of the composites from further degrada-tion. In addition, as is clearly shown in DTG curves, the weight lossrate of flame retardant PLA composites descends as the SPDPMadd-on increases.

3.4. Thermal decomposition process

The thermal degradation process of pure PLA and PLA/25 SPDPMsample was characterized by in situ FTIR which provided

Fig. 6. TGA and DTG curves of PLA, SP

information about the mass transfer and the decomposed sample. Itcan be found from Fig. 7 at 330 �C, almost all of the ester bonds ofthe pure PLA have been pyrolyzed according to the decrease of thepeaks at 1750 cm�1 corresponding to C]O, and the wide peaksfrom 1300 cm�1 to 1000 cm�1 ascribed to the stretching vibrationof C–O–C. Meanwhile the stretching and bending vibrations of C–Hat 2940 cm�1, 2990 cm�1, 1380 cm�1, and 1460 cm�1 also disap-pear. The new peaks at 1370 cm�1, 1410 cm�1 and 1580 cm�1 can beassigned to some complexes containing C]C bond, such asaromatic species that will produce char residue at highertemperature.

Fig. 8 shows the degradation process of PLA/25 SPDPM. It can befound at 310 �C the wide peaks between 3500 and 3000 cm�1 andthe peak at 1693 cm�1 corresponding to NH2 group almost disap-pears, and the new wide peak at 1400 cm�1 can be assigned to thetriazine ring stretch [20]. These results indicate the condensation ofmelamine segment with the release of NH3 which is consistent tothe TGA results. On the other hand some peaks corresponding tovibration of the bonds from PLA also have a large decrease at 310 �Ccompared to 250 �C. With further increase of the temperature, theintensity of the peaks from 1750 cm�1 to 1710 cm�1 ascribed toC]O reduces but disappears until 400 �C. Meanwhile the decreaseof wavenumber for C]O is probably due to the formation ofO]C–O–P]O and unsaturated ester bond. As a result of the breakof P–N bond and the formation of pyrophosphate the peak of P]Obond shifts from 1260 cm�1 to 1290 cm�1. The wide peak from1000 cm�1 to 1150 cm�1 becomes clearer as a result of the degra-dation of P–O–C and C–O–C and the formation of some new peaks.The peaks at 1140 cm�1 and 1020 cm�1 with increasing intensitycan be ascribed to the stretching vibration of PO2/PO3 in phosphatecarbon complexes. The peaks at 1090 cm�1 and 880 cm�1 belong tothe stretching vibrations of P–O–P bond, and this indicates thatsome phosphate groups link to each other by sharing one oxygenatom, leading to the formation of poly(phosphoricacid), such asP2O5 and P4O10 [21,22]. Some unsaturated compounds, such asaromatic species are formed according to the new wide peaksappear at about 1580 cm�1 corresponding to the stretching vibra-tion of C]C bond.

In order to give a more complete picture of the flame retardancemechanism of SPDPM acting on PLA, Fig. 9 shows the FTIR differ-ential spectra between the contiguous temperatures to comparethe pyrolyzed compound of PLA and its composite. As is shown inFig. 9, for the pure PLA most peaks are ascribed to ester bond before330 �C, and this reveals that the degradation process is mainly thebreak and pyrolysis of ester bond; whereas almost none of thesepeaks exists with the temperature increase. The new positive peaksat 1610 cm�1, 1580 cm�1 and negative peak at 1410 cm�1 at higher

DPM and their composites in air.

Table 3Thermal properties of PLA and PLA/SPDPM composites.

Samples T�5 wt% (�C) Tmax (�C) Char residue at 600 �C (wt%)

PLA 325 365 0SPDPM 268 – 40PLA/5 SPDPM 324 365 2.1PLA/15 SPDPM 311 362 7.2PLA/25 SPDPM 303 356 12.0

Fig. 7. In situ FTIR spectra for the degradation process of the pure PLA at differenttemperatures.

Fig. 9. The FTIR differential spectra between the contiguous temperature.

J. Zhan et al. / Polymer Degradation and Stability 94 (2009) 291–296 295

temperature range imply the continuous aromatization andcarbonization of the C]C bond formed from the pyrolysis of esterbond. However for the composites containing SPDPM, the twocharacteristic absorption bands of ester bond (1000–1300 cm�1 forC–O–C and 1710–1750 cm�1 for C]O) still exist till 400 �C. Onereason for this phenomenon is the formation of intumescent

Fig. 8. In situ FTIR spectra for the degradation process of the PLA/25 SPDPMcomposites at different temperatures.

protective charred layers that will inhibit the degradation of esterbond. Another possible reason is the change of the degradationprocess of PLA as a result of the incorporation of SPDPM. Thepyrolysis process of the composites with the flame retardant ismainly the ester exchange reaction due to the catalysis of phos-phoric acid or polyphosphoric acid derived from the degradation ofSPDPM.

4. Conclusions

A novel compound SPDPM was successfully synthesized andused to flame retardant PLA. It is found from the combustionexperiments’ results that when 25 wt% SPDPM was added, the LOIof PLA composites was increased to 38, and UL-94 V0 was achieved,meanwhile the dripping behavior was effectively improved. MCCresults showed SPDPM could significantly decrease the heat releasecapacity hc and pHRR of PLA composites. The flame retardantmechanism of SPDPM in PLA was not only due to the intumescentprotective charred layer resulting from combining acid, carbon andgas sources in SPDPM but also for the change of the degradationprocess of PLA according to the analysis of the in situ FTIR. The TGAand in situ FTIR results showed the low thermal stability of mela-mine segment results in the decrease of the onset decompositiontemperature.

Acknowledgements

The work was financially supported by the Program for NewCentury Excellent Talents in University, and National 11th 5-yearProgram (Nos. 2006BAK01B03, 2006BAK06B06, and2006BAK06B07).

References

[1] Yu L, Dean K, Li L. Polymer blends and composites from renewable resources.Prog Polym Sci 2006;31(6):576–602.

[2] Ajioka M, Enomoto K, Suzuki K, Yamaguchi A. The basic properties of poly(-lactic acid) produced by the direct condensation polymerization of lactic-acid.J Environ Polym Degrad 1995;3(4):225–34.

[3] Dorgan JR, Lehermeier HJ, Palade LI, Cicero J. Polylactides: properties andprospects of an environmentally benign plastic from renewable resources.Macromol Symp 2001;175:55–66.

J. Zhan et al. / Polymer Degradation and Stability 94 (2009) 291–296296

[4] Nagata F, Miyajima T, Yokogawa Y. A method to fabricate hydroxyapatite/poly(lactic acid) microspheres intended for biomedical application. J Eur CeramSoc 2006;26(4–5):533–5.

[5] Tokiwa Y, Calabia BP. Biodegradability and biodegradation of poly(lactide).Appl Microbiol Biotechnol 2006;72(2):244–51.

[6] Sinclair RG. The case for polylactic acid as a commodity packaging plastic.J Macromol Sci Pure Appl Chem 1996;A33(5):585–97.

[7] Meinander K, Niemi M, Hakola JS, Selin J-F. Polylactides-degradable polymersfor fibres and films. Macromol Symp 1997;123:147–53.

[8] Reti C, Casetta M, Duquesne S, Bourbigot S, Delobel R. Flammability propertiesof intumescent PLA including starch and lignin. Polym Adv Technol2008;19(6):628–35.

[9] Solarski S, Mahjoubi F, Ferreira M, Devaux E, Bachelet P, Bourbigot S, et al.(Plasticized) polylactide/clay nanocomposite textile: thermal, mechanical,shrinkage and fire properties. J Mater Sci 2007;42(13):5105–17.

[10] Bourbigot S, Fontaine G, Duquesne S, Delobel R. PLA nanocomposites: quan-tification of clay nanodispersion and reaction to fire. Int J Nanotechnol2008;5(6–8):683–92.

[11] Horacek H, Pieh S. The importance of intumescent systems for fire protectionof plastic materials. Polym Int 2000;49(10):1106–14.

[12] Nishihara H, Tanji S, Kanatani R. Interactions between phosphorus- andnitrogen-containing flame retardants. Polym J 1998;30(3):163–7.

[13] Riva A, Camino G, Fomperie L, Amigouet P. Fire retardant mechanism in intumes-cent ethylene vinyl acetate compositions. Polym Degrad Stab 2003;82(2):341–6.

[14] Chen YH, Wang Q. Reaction of melamine phosphate with pentaerythritol andits products for flame retardation of polypropylene. Polym Adv Technol2007;18(8):587–600.

[15] Wang DY, Liu Y, Wang YZ, Artiles CP, Hull TR, Price D. Fire retardancy ofa reactively extruded intumescent flame retardant polyethylene systemenhanced by metal chelates. Polym Degrad Stab 2007;92(8):1592–8.

[16] Chen DQ, Wang YZ, Hu XP, Wang DY, Qu MH, Bing Y. Flame-retardant andanti-dripping effects of a novel char-forming flame retardant for the treatmentof poly(ethylene terephthalate) fabrics. Polym Degrad Stab 2005;88(2):349–56.

[17] Bourbigot S, Le Bras M, Duquesne S, Rochery M. Recent advances for intu-mescent polymers. Macromol Mater Eng 2004;289(6):499–511.

[18] Lyon RE, Walters RN, Stoliarov SI. Screening flame retardants for plastics usingmicroscale combustion calorimetry. Polym Eng Sci 2007;47(10):1501–10.

[19] Lyon RE. Seventh international symposium on fire safety science. Worcester,MA: Worcester Polytechnic Institute; 2002.

[20] Costa L, Camino G, Dicortemiglia MPL. Mechanism of thermal-degradation offire-retardant melamine salts. ACS Symp Ser 1990;425:211–38.

[21] Bugajny M, Bourbigot S. The origin and nature of flame retardance inethylene–vinyl acetate copolymers containing hostaflam AP 750. Polym Int1999;48:264–70.

[22] Liu W, Chen DQ, Wang YZ, Wang DY, Qu MH. Char-forming mechanism ofa novel polymeric flame retardant with char agent. Polym Degrad Stab2007;92(6):1046–52.