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Polymer Degradation and Stability 97 (2012) 2619e2626

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Polymer Degradation and Stability

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

Ignition of polypropylene/montmorillonite nanocomposites

Alberto Fina a,b,*, Fabio Cuttica a,b, Giovanni Camino a,b

aDipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, V.le Teresa Michel 5, 15121, Alessandria, ItalybMember of INSTM: Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali - Via G. Giusti, 9 - 50121 Firenze, Italy

a r t i c l e i n f o

Article history:Received 9 May 2012Received in revised form2 July 2012Accepted 2 July 2012Available online 13 July 2012

Keywords:Time to ignitionIgnition timeIgnition temperatureNanocompositesFlame retardancy

* Corresponding author. Dipartimento di Scienzatecnico di Torino, V.le Teresa Michel 5, 15121, Alessan

E-mail address: alberto.fina@polito.it (A. Fina).

0141-3910/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymdegradstab.2012.07.0

a b s t r a c t

This paper addresses the flaming ignition of polypropylene and polypropylene-based nanoclaycomposites, under irradiation in a cone calorimeter. The surface temperature as a function of time wasmeasured using thermocouples on the surface and the chemical changes occurring in the polymerleading to evolution of volatiles were studied by means of spectroscopic analyses of the surface. Ther-mogravimetric analyses were also carried out to study thermal and thermoxidative volatilization atvariable heating rates, which correlate to the production of the critical concentration of combustiblevolatiles for ignition.

The effect of both microdispersed and intercalated nanoclays on ignition time and ignition tempera-ture is addressed and analysed in details.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Ignition of polymers under external irradiation depends on theheat-induced chemical changes and physical evolution of volatiles,eventually leading to the production of a critical flammable vola-tiles flux, sufficient to reach the flammability limit of the volatilesabove the surface. Ignitionwould then occur by either self-ignition,if temperature of the gas mixture is above its auto-ignitiontemperature, or by piloted ignition (e.g. spark or flame) whichbrings a portion of the fuel/air mixture above ignition temperature.Ignition of polymers has been extensively studied since the ’60s[1e9], but fundamental aspects concerning their chemical changesand surface temperature prior and during the combustion processare not yet understood in details. Indeed, especially in the case ofcomplex polymer-based formulations containing fire retardantsand/or nanoparticles, the thermophysical evolution of thecondensed phase before ignition in a fire test has been rarelystudied, also owing to experimental difficulties in such propertiesmeasurement. On the other hand, polymers containing additives ornanoparticles often exhibit unpredictable time to ignition if blind

Applicata e Tecnologia, Poli-dria, Italy.

All rights reserved.17

reference is made only to the material thermal behaviour such asthat shown in thermogravimetry. As an example, variable trends fortime to ignition have been reported for polymer/layered silicatenanocomposites: a reduction of ignition time was often observed[10e12] compared to the reference polymers, but the oppositeeffect is also reported in many other cases [13e16].

Despite different hypotheses being made to interpret modifi-cation of ignition time in nanocomposites, including thermalinstability of layered clays organic modifiers [17e19], polymerdegradation triggering by catalytic effects [20,21] and moltenpolymer viscosity [22,23], little work has been devoted to the studyof chemical and physical changes in the material during fire tests.The use of temperature measurements of the specimens surfacehas been recently shown [24] to help in understanding phenomenaoccurring during ignition in polymer nanocomposites, providinginsights in the melting, which controls the time to reach thedecomposition temperature in semicrystalline polymers, anddecomposition, which controls the production of volatile fuel.Furthermore, the analysis of residues quenched before or soon afterignition was shown to provide information on the thickness ofmaterial contributing to volatiles production as well as on materialchemical evolution.

In this paper, investigation on ignition of polypropylene andpolypropylene/montmorillonite nanocomposites by the measure-ment of ignition temperatures and the analysis of chemical changeson the material surface evolution is reported.

A. Fina et al. / Polymer Degradation and Stability 97 (2012) 2619e26262620

2. Experimental

2.1. Materials

Polypropylene (PP) was Moplen HP 500N by LyondellBasell (I).Maleic anhydride grafted polypropylene (PPgMA, Polybond 3200by Crompton - US) was used as a compatibiliser between PP andmontmorillonite which was supplied by Southern clays (US), eitherunmodified (Sodium Cloisite e CNaþ) or organomodified withdimethyl, dehydrogenated tallow quaternary ammonium (Cloisite20A e C20A).

2.2. Processing

PP/PP-g-MA/montmorillonite were melt compounded in a Leis-tritz ZSE 18 HP 40D twin screw extruder equipped with gravimetricfeeders, operating at 300 rpm, 4.5 kg/h throughput and the screwprofile as described elsewhere [25]. PP and PPgMA pellets wereloaded in the main feed whereas montmorillonite was added to themelt by a side feeder at 14D. The composition of prepared formu-lations, is reported in Table 1.

100 � 100 � 6 mm3 slabs were prepared by injection moulding,to be used in the cone calorimeter (see below) as well as a source ofspecimens for characterisations, which were obtained by cuttingthe slabs to shapes suitable for each type of test, to ensure that allspecimens had the same morphology.

2.3. Characterisation

X-ray diffraction (XRD) was performed on 30 � 30 � 6 mm3

specimens using a Philips X-Pert diffractometer in qeq reflectionconfiguration, with Cu Ka radiation (l ¼ 1.5406 Å) at 2�/min scanrate.

Differential scanning calorimetry (DSC) was carried out on8e9mg samples using a TAQ20 instrument in hermetic aluminiumpans, under nitrogen flow (50 mL/min) at 10 �C/min heating ratefrom 0 to 200 �C. Crystallization degree (Xc) was calculated from theenthalpy peak area normalized to the actual polymer weight frac-tion, according to:

Xc ¼ DHDH0 �Wpolymer

where DH is the measured melting enthalpy, DH0 is the PP meltingenthalpy for 100% crystalline degree (189 J/g [26]) and Wpolymer isthe polymer weight fraction.

Thermogravimetry was performed on 10 � 0.5 mg samplesusing a TA Q500 instrument. Tests were carried out in alumina panseither in oxidizing (air) or inert (nitrogen) atmosphere, at 10 �C/minor 50 �C/min heating rates, from 50 to 800 �C. TONSET is defined asthe temperature corresponding to 5% mass loss and TMAX is thetemperature at maximum mass loss rate.

Isothermal gravimetric tests were carried out at 320 �C in air for20 min.

Rheological tests were performed on a strain-controlled rheom-eter ARES (TA Instruments) equipped with a forced air convectionoven and parallel plate geometry. Specimens were obtained by

Table 1Compounds composition.

Formulation PP MoplenHP 500N

PPgMAPolybond 3200

Cloisite Naþ Cloisite 20A

PP 95 5 e e

PP/CNaþ 90 5 5 e

PP/C20A 90 5 e 5

compression moulding of pellets at 200 �C. All measurements wereconducted under nitrogen atmosphere at 190 �C.

2.4. Cone calorimeter testing

A Fire Testing Technology (FTT) oxygen consumption calorim-eter (“Cone Calorimeter”) apparatus was used. Specimens, wrappedin aluminium foil, leaving an upper edge of 3 mm to avoid overflowof molten polymer, were located in the cone calorimeter sampleholder, resting on top of two ceramic pads 10 mm thick each. Theresulting distance between sample surface and cone heater loweredge was 25 mm (Fig. 1).

Surface temperature measurements were performed duringcone calorimeter tests, using K-type 0.5 mm stainless steelsheathed thermocouples by Tersid (I). Thermocouples were care-fully placed and supported to keep contact with the upper surfaceof the sample throughout the experiment as described and re-ported in a previous paper [24]. Temperature of the sample bottomlayer was measured by inserting a K-type 1 mm stainless steelsheathed thermocouple parallel to the specimens surface betweenthe polymer specimen and the aluminium foil (Fig. 1).

Temperature signals were recorded using a TC-08 thermocoupledata logger by Pico Technology (UK) and the original data plots arereported without smoothing.

Tests were carried out at 25, 35 and 50 kW/m2, using sparkpiloted ignition. All tests were performed at least three times inorder to check reproducibility. Average temperature-time curvesare reported.

Averaged ignition times (TTI) and temperatures are reportedwith their experimental error estimated as (maximum val-ue�minimum value)/2.

Interrupted combustion tests were performed by quenching theflame just after ignition, directing a stream of nitrogen at roomtemperature on the material surface on which infrared spectrumwas recorded by Attenuated Total Reflectance (ATR) using a dia-mond crystal on a Perkin Elmer Frontier instrument. Cross sectionsof these residues were obtained by fragile fracture after quenchingin liquid nitrogen.

3. Results and discussion

3.1. Montmorillonite dispersion

Montmorillonite dispersion in the polymerwas characterised byX-Ray diffraction (XRD) and rheology. The 2q angle for the mont-morillonite either pristine (CNaþ) or organophilic (C20A), 001diffraction peak and ensuing calculated interlayer distance beforeand after their dispersion in PP, are reported in Table 2. In the caseof composite based on CNaþ, a negligible increase in the clayinterlayer distance was observed (from 11.9 to 12.4 Å), due toorganophobic character of the clay, which is well known to hinderintercalation of non polar polymers such as PP. In the case oforganically modified C20A, intercalation is observed, the averageinterlayer distance increasing from 23.9 Å to 27.6 Å.

It is worth noting that XRD analysis reflects the nanocompositestructure in the surface layer of the specimen, which can be highlyoriented as a consequence of injection moulding. This is of directrelevancy to the ignition of composites,whichprimarily involves thesurface layer of the specimens; however, the analysis of the bulkdispersion of nanoparticles is clearly complementary to assess thestructure of the composites. Rheological characterisation wastherefore used to properly assess the structure of the composites inthe bulk. The complex viscosity curves for PP, PP/CNaþ and PP/C20Aare reported in Fig. 2. The unfilled polymer exhibits a Newtonianplateau at about 1300 Pa s in the frequency range below 1 rad/s and

Fig. 1. Experimental setup for cone calorimeter tests and temperature measurements. Inset shows detail of positioning of the bottom thermocouple, laying parallel to thespecimen surface.

1000

10000 PP PP/C20A PP/CNa+

[Pa*

s]

A. Fina et al. / Polymer Degradation and Stability 97 (2012) 2619e2626 2621

clear non Newtonian shear thinning behaviour above that defor-mation frequency. Different trends were observed with the two PP/montmorillonites composites. PP/CNaþ exhibits a slightly lowerviscosity compared to PP over the whole frequency range with nosignificant differences in the shear thinning behaviour. On the otherhand, PP/C20A shows both clear shear thinning and viscosity higherthan PP over all the frequency range explored, suggesting theformation of a percolating network of nanoclay platelets [25].

Based on the experimental evidences from XRD and rheology,the structure of PP/CNaþ and PP/C20A can be described as micro-composite and intercalated nanocomposite, respectively.

3.2. Thermal analysis

Differential scanning calorimetry was carried out to evaluate thethermal transitions of the different materials, which are relevant totheir behaviour when exposed to an irradiation source, prior toignition. DSC results are summarized in Table 3: very similarmelting temperatures (Tm) and melting enthalpies (DHm) weremeasured, evidencing minor effect of the clay on the crystallisationbehaviour of PP (Xc). Thus, specimens of the different formulationwill require similar melting heat, with no significant effect on themelting time, which will be shown below to contribute to theignition time.

Thermogravimetric analyses were carried out to assess PP andcomposites thermal and thermal oxidative decomposition behav-iour which is relevant to their ignition and combustion. Indeed,polymer materials ignition upon exposure to a heating source isdirectly related to polymer thermal volatilisation kinetics. Reachingthe concentration of combustible volatiles, required to obtain anignitable mixture above the material surface, depends on thevolatiles composition and on polymer volatilisation rate. Thedecomposition scenario for polymer materials under irradiance inair is quite complex because the availability of oxygen to interactwith the polymer, affecting its decomposition mechanism, dependson oxygen diffusion into thematerial which is a function of time, i.e.of the heating rate. Thus, both thermal and thermal oxidativedecomposition should be taken into account when discussingpolymers thermal volatilisation under irradiation. Furthermore,polymer heating rate under constant irradiation power is not

Table 2(001) diffraction peak for montmorillonite pristine or dispersed in composites.

2q angle [�] Interlayer distance [Å]

CNaþ 7.4 11.9PP/CNaþ 7.1 12.4C20A 3.7 23.9PP/C20A 3.2 27.6

constant but it depends on its heat capacity, which in turn dependson temperature, as well as on the occurrence of thermal transitionssuch as melting and decomposition. Thus, to simulate polymerthermal volatilisation in pre-ignition and ignition conditions, boththe effect of testing atmosphere (nitrogen or air) and of heating rate(10 �C/min and 50 �C/min) were explored. TGA plots are shown inFig. 3 and the measurements of TONSET, TMAX and mass residue at800 �C obtained at different heating rates are reported in Table 4.

Polypropylene decomposition is well known to lead to completevolatilisation in both inert and oxidative atmosphere and the samewas previously reported for maleic anhydride grafted poly-propylene [27]. Thus, the blend of polypropylene and maleicanhydride-grafted polypropylene, referred to as PP, also decom-poses nearly completely in a single step, both in nitrogen and in air.By increasing the rate of heating from 10 to 50 �C/min, the PPweight loss curve shifts to higher temperatures, as expected indynamic thermogravimetry, due to heating rate dependence ofweight loss rate.

The presence of oxygen in the decomposition atmospheresignificantly accelerates the volatilization process because oxygeninitiates, by hydrogen abstraction, the PP radical degradation chainreaction (oxygen initiated PP pyrolysis), at a much lower temper-ature than thermal initiation by CeC bond scission taking place innitrogen atmosphere [28]. For example, on heating at 10 �C/min theonset of weight loss in air is observed at 271 �C and the maximumrate of volatilization is at 342 �C, whereas corresponding values innitrogen are 416 and 466 �C, respectively. A similar trend is alsoobserved on heating at 50 �C/min.

The presence of microdispersed Cloisite Naþ induces 19�C/12 �Cincrease of PP TONSET in nitrogen atmosphere upon heating at 10 �C/min and 50 �C/min, respectively, whereas TMAX is very slightlyaffected (þ6�C/þ4 �C respectively). This is likely due to an inter-action between volatile decomposition products and nanoclay,

0,1 1 10 100100

η*

ω [rad/s]

Fig. 2. Complex viscosity plots for PP and composites.

Table 3Melting temperatures, melting enthalpies and crystallization degree.

Material Tm [�C] DHm [J/g] Xc [%]

PP 170 65 34PP/CNaþ 168 62 33PP/C20A 168 64 34

Table 4Thermal degradation of PP and PP composites.

Heatingrate[�C/min]

Atmosphere Material TONSET[�C]

TMAX

[�C]Rate_decMAX

[%/�C]Residueat 800 �C [%]

10 Nitrogen PP 416 466 2.45 1.3PP/CNaþ 435 472 2.82 7.3PP/C20A 430 451 6.79 4.5

Air PP 271 342 1.30 1.5PP/CNaþ 269 379 1.55 6.2PP/C20A 296 436 2.66 5.1

50 Nitrogen PP 487 534 2.37 1.2PP/CNaþ 499 538 2.65 5.5PP/C20A 498 498 4.72 4.1

Air PP 352 459 1.26 1.3PP/CNaþ 360 465 1.22 6.2PP/C20A 395 508 2.42 4.1

A. Fina et al. / Polymer Degradation and Stability 97 (2012) 2619e26262622

either by adsorption or diffusion barrier effect, resulting in slowerrelease of polymer fragments to the gas phase, although somewhatlimited by the low degree of dispersion reached with Cloisite Naþ.Indeed, pristine polymer chains are unable to diffuse between claynanolayers, owing to their organophobicity and high viscosity.Partial clay intercalation/exfoliation becomes only possible afterscission of polymer molecules to more mobile chain fragments,occurring during the thermal degradation process [29] giving raiseto partial volatiles evolution delay. A residue is left at the end of theheating run, corresponding to the amount of the added clay, whichis not volatile. On the other hand, when heating at 10 �C/min in air,CNaþ delays TMAX by 37 �C while keeping TONSET approximatelyunmodified. This effect is explained by a clay barrier effect towardsoxygen diffusion from the atmosphere to the polymer, occurringsoon after the weight loss onset, owing to accumulation of clay onthe surface as a consequence of volatilization of the surface poly-mer layer. Here again, the barrier effect is due to the increase of claydispersion in the PP matrix occurring on heating, which is largerthan in nitrogen because, besides chain shortening due to chainbonds scission, in air also some polymer oxidation takes place,making the polymer chain fragments somewhat polar and hencemore compatible with clay layers [29]. Furthermore, as claydispersion increases, migration of clay layers towards thecomposite surface increases [30], contributing to the creation of theinsulating clay surface. Similar results are obtained on heating at50 �C/min, with however a lower volatilisation delay compared to

Fig. 3. PP and composites TGA curves in nitrogen (a,

the lower heating rate, owing to the competition between rate ofoxygen initiated PP pyrolysis and rate of the oxygen screeningsurface clay layer build up.

More complex features are observed in the decompositionpathway of PP in the intercalated nanocomposite PP-organomodified Cloisite (PP/C20A). Indeed, while the thermaldecomposition onset in nitrogen is slightly delayed in temperatureas compared to PP, the weight loss rate rapidly increases soon afterthe onset, to reach a maximum decomposition rate more thandouble compared to PP, although the temperature is 15 �C lower. Asa consequence, the volatilization is completed at a lower temper-ature (460 �C) than in the case of PP and PP/CNaþ (500 �C). This islikely due to the catalytic action of the clay on thermal degradationof PP, in agreement with literature on thermal decomposition of PPcontaining sepiolite clays [31,32], which becomes active whenthere is extended PP-clay molecular contact as in the case of thedegrading intercalated PP-organomodified montmorillonite

c) and in air (b, d) obtained at 10 and 50 �C/min.

Table 5Time to ignition (TTI) for PP and its composites, at different heat fluxes.

Heat flux [kW/m2] PP [s] PP/CNaþ [s] PP/C20A [s]

25 117 � 17 116 � 2 125 � 535 70 � 3 49 � 2 67 � 450 31 � 2 23 � 2 31 � 4

A. Fina et al. / Polymer Degradation and Stability 97 (2012) 2619e2626 2623

nanocomposite. Thus, on heating PP-montmorillonite intercalatednanocomposites at 10 �C/min in nitrogen, the weight loss ratedecrease due to barrier effect of clay layers to diffusion of volatilecompounds, found in PP/CNaþ microcomposite, is overwhelmed bytheir PP decomposition catalytic effect in the PP/C20A nano-composite. When heating in air, both TONSET and TMAX are notablyincreased as compared to PP: this is explained by a more efficientbarrier to oxygen exerted by the nanodispersed clay as compared toPP/CNaþ microcomposite, leading to a depleted oxygen concen-tration in the molten polymer and, consequently, to a highertemperature needed for its volatilization.

From the results discussed above, a complex scenario arises:indeed, the presence of nanodispersed clay layers in the PP matrixmay accelerate or slow down the volatilization of PP depending onthe concentration of oxygen available in the material, which in pre-ignition fire scenarios is a function of several material-independentparameters, including the specimen thickness and geometry, theimposed heat flux and the ventilation.

3.3. Combustion behaviour

3.3.1. PPThe typical well known reduction of heat release rate (HRR)

induced by the presence of dispersed clay [10] is seen in Fig. 4 for PPat 50 kW/m2 and was observed as well at 25 and 35 kW/m2.

Time to ignition (TTI) measured for the different compositionsand heat fluxes are reported in Table 5. For each formulation,reduction of time to ignition is generally observed with increasingheat flux. This is due to the higher thermal power transmitted tothe specimen which increases its rate of heating, thus shorteningthe time required to reach ignition temperature. Specific trends areobserved in the presence of clays: CNaþ generally decreased theignition time compared to PP, except at the lowest heat flux, whereno significant difference is noticed. On the other hand, C20A doesnot show significant differences in ignition time as compared toreference PP.

Aiming at understanding the phenomena explaining thesetrends, the temperature of the surface layer of the specimensirradiated under the cone calorimeter was measured as a functionof irradiation time.

Repeatability of surface temperature is shown in Fig. 5 for PP at35 kW/m2, taken as an example, and is considered satisfactory forthe purpose of this work.

The averaged surface temperature of PP measured during conecalorimeter tests at different heat fluxes are reported in Fig. 6.

At 25 kW/m2 the surface temperature increases at about 10 �C/suntil PP melting temperature is reached (165 �C, 10 s); then thetemperature increases at a much lower rate (<1 �C/s), owing to thelatent melting heat. After complete melting of the specimen surface

0 100 200 300 400 500 600

0

200

400

600

800

1000

1200

1400

Heat flux = 50 kW/m2

PP PP/CNa+

PP/C20A

HR

R [k

W/m

2 ]

Time [s]

Fig. 4. HRR plots for PP and its composites, at 50 kW/m2 irradiance.

layers (Fig. 6, 50 s), the surface temperature increases at a faster rateagain (4 �C/s), but lower than before melting, owing to higherspecific heat of the liquid polymer as compared to the original solidpolymer. Above 260 �C (75 s), temperature at which the endo-thermic polymer thermal volatilisation begins in air (Fig. 3),a second quasi-stationary low temperature increase stage(about 1 �C/s) is reached, during which volatile degradation prod-ucts are evolved. It is worth noting that both melting and decom-position are expected to result in a constant temperature plateauduring the heating of a specimen in thermal equilibrium; however,in the present conditions it is not surprising that the materialthermal equilibrium is not fully reached, owing to the low effi-ciency of thermal exchange within the polymer specimen.

Ignition occurs when the lower flammability limit for thevolatiles/air mixture is reached (i.e. the critical mass flux of volatilefuel, referred to as critical ignition flux, CIF) after 117 � 17 s PPexposure at 25 kW/m2, when the measured surface temperature is316 � 1 �C, after 42 s at volatilisation temperature (>260 �C). Arapid temperature increase (ca.10 �C/s) follows, owing to additionalheat flux from the flame.

The measured temperature at ignition evidences that the fuelwhich leads to ignition at 25 kW/m2 is produced in thermaloxidative conditions, provided that in the range of temperatures260e316 �C no significant volatilization of PP was observed by TGAunder nitrogen (Fig. 3), evenwhen tested at the lowest heating rate(0.16 �C/s). Experimental evidence for the occurrence of extensiveoxidation of the polymer in the top layer prior to ignition is given bythe specimen surface ATR-FTIR absorption spectrum at ignitiontime (Fig. 7) showing PP oxidation by C¼O absorption peak at1720 cm�1 and the OH absorption band centred at about3350 cm�1.

Fig. 8 shows that the PP specimen bottom temperature duringirradiation continuously increases in the early stage at a lower ratecompared to the surface (about 0.5 �C/s, when heating at 25 kW/m2). Such a relatively low heating rate is indeed expected, takinginto account that heat transfer through the specimen is mainly dueto conduction through solid or liquid PP and convection in themolten PP, whereas in-depth radiation absorption representsa minor contribution, owing to the significant specimen thickness(6 mm).

0 10 20 30 40 50 60 70 800

100

200

300

400

500

600 Test 1 Test 2 Test 3

Tem

pera

ture

[°C

]

Time [s]

Fig. 5. Repeatability of surface temperature measurements for PP at 35 kW/m2.

0 100 200 300 4000

100

200

300

400

500

TTI=70 s

TTI=117 s

Tem

pera

ture

[°C

]

Time [s]

25 kW/m2

35 kW/m2

50 kW/m2

TTI=31 s End of melting

Beginning of decomposition

Fig. 8. Temperature curves of the bottom sample layer during Cone experiment for PP:arrows indicate when ignition occurs, figures on arrows indicate time to ignition andcorresponding bottom temperature.

0 25 50 75 100 1250

100

200

300

400

500387 ± 7°C

387 ± 5°C

316 ± 1 °C

Tem

pera

ture

[°C

]

Time [s]

25 kW/m2

35 kW/m2

50 kW/m2

Fig. 6. Surface temperature curves for PP at different heat fluxes. Arrows indicate timeto ignition, figure on arrows show surface temperature at ignition.

A. Fina et al. / Polymer Degradation and Stability 97 (2012) 2619e26262624

On the other hand, poor heat transfer through PP specimens isconfirmed by overlapping of heating and melting processes in thebulk as shown by absence of a sharp melting temperature plateaushown on the surface (Fig. 6). Specimen melting completion isshown by sudden increase of bottom temperature-time curve slopedue to end of endothermic melting process since melting latentheat contribution to the heat balance disappears and all the heattransmitted through the specimen results into a rise of polymertemperature. Specimen complete melting is observed at timeslonger than ignition time, evidencing that the specimen is notcompletely molten before ignition, which gives a rough estimationof the average gradient temperature established in the specimenthickness, in the range of at least 30 �C/mm. This phenomenon isclearly different from the full melting before ignition previouslyobserved for PET [24]. The explanation of the different behaviours ismost likely in the more than one order of magnitude difference inviscosity for the two polymers above melting. Indeed, while PETexhibits a newtonian viscosity plateau at low frequency about40 Pa s (at 260 �C), compared with the value of about 1300 Pa s forPP (Fig. 2) thus evidencing a completely different flow behaviour.

Comparing Figs. 6 and 8 it is seen that at 25 kW/m2, PP specimenbottom decomposition, shown by the temperature-time curveslope reduction (240 s) due to heat absorbed by the thermal vola-tilisation process involving then the whole specimen, takes place

4000 3500 3000 2500 2000 1500

C=O stretching1720 cm-1

PP@TTI, 50 kW/m2

PP@TTI, 35 kW/m2

PP@TTI, 25 kW/m2

Abso

rban

ce

Wavenumber [cm-1]

PP

O-H stretching3350 cm-1

=0,05

Fig. 7. ATR-FTIR spectra for pristine PP and top surface of PP specimens after inter-rupted cone tests at time to ignition, for different imposed heat fluxes.

165 s after surface decomposition begins (75 s) as a result of the lowrate of heat transfer from sample surface to bottom.

By increasing heat flux to 35 and 50 kW/m2, surface tempera-ture plots show less time resolved melting and decompositionstages, progressively losing the corresponding quasi-stationarysteps observed at 25 kW/m2. Despite the dynamic response of thethermocouples may also play a role, this behaviour is indeed ex-pected, due to increasing of the thermal power to the material,shifting the surface temperature control from thermodynamics tokinetics. Furthermore, the temperature measured at ignition at 35and 50 kW/m2 is about 387 �C, which is significantly higher thanthe 316 �C measured at 25 kW/m2. This could be partly due to thehigher heating rates observed at higher heat flux, increasing thetemperature for polymer volatilisation as shown above by TGA.However, the same ignition temperature was found at 35 and50 kW/m2, despite significantly different heating rates wereobserved in Fig. 6. Another possible explanation is proposed, basedon oxidation kinetics. The oxygen-polymer reaction rate is rela-tively low, being a reaction between PP in condensed phase anda gas whose concentration in a function of time, i.e. the heatingrate. Thus, above 25 kW/m2, the heterophasic bimolecular oxygen-PP reaction might not have time to occur significantly beforetemperature is high enough to start monomolecular thermallyinitiated CeC chain bonds scission, contributing to the productionof volatiles critical flux required for ignition (CIF).

Specimens bottom temperature on exposure to 35 and 50 kW/m2 follows the same trend observed at 25 kW/m2 with a shift of thecurves at shorter times due to increased heat flux.

3.3.2. PP compositesIn the presence of clays, general features of temperature trend

are mostly retained (Fig. 9), including the shape of both surface andbottom temperature-time plots, as well as the fact that bottomlayer of polymer is not yet molten at the ignition time. However,some differences are clearly visible, in terms of different ignitiontemperatures, reported in Fig. 10 as a function of heat flux, for thedifferent formulations.

In particular, the nanocomposite shows a trend similar to that ofPP, with increase of ignition temperature towards pyrolysiscontrolled PP volatilisation as heat flux increases to 50 kW/m2. Themicrocomposite ignites instead at the same temperature (316-7 �C)independently of the irradiation flux, which is close to thetemperature (322 �C) at which PP ignites at the lowest irradiation(25 kW/m2) and is within the range of oxygen initiated PP pyrolysis.This behaviour is the result of the interplay of several physical andchemical processes such as: polymer viscosity control of heatconduction and convection, filler effect on heat absorption in thetop layer and/or reflection/reradiation, oxygen diffusion, polymer

0 25 50 75 100 125 1500

100

200

300

400

500

25 kW/m2

35 kW/m2

50 kW/m2

316 ± 3 °C

317 ± 13 °C

322 ± 3 °CTe

mpe

ratu

re [°

C]

Time [s]0 25 50 75 100 125 150

0

100

200

300

400

500

25 kW/m2

35 kW/m2

50 kW/m2

381 ± 9°C

337 ± 7°C

315 ± 8 °C

Tem

pera

ture

[°C

]

Time [s]

0 100 200 300 4000

100

200

300

400 25 kW/m2

35 kW/m2

50 kW/m2

TTI=23 s

TTI=49 s

TTI=116 sTem

pera

ture

[°C

]

Time [s]

End of melting

0 100 200 300 4000

100

200

300

400

TTI=67 s

25 kW/m2

35 kW/m2

50 kW/m2

TTI=31 s

TTI=125 sTe

mpe

ratu

re [°

C]

Time [s]

End of melting

a b

c d

Fig. 9. Temperature vs. time plots for PP/CNaþ surface/bottom temperature (a/c) and PP/C20A surface/bottom temperature (b/d) at 25, 35 and 50 kW/m2, compared with corre-sponding plots for PP (dashed lines). Arrows indicate time to ignition.

at TTI,50 kW/m2

Abso

rban

ce

PP PP/CNa+ PP/C20A

A. Fina et al. / Polymer Degradation and Stability 97 (2012) 2619e2626 2625

thermal oxidation, polymer pyrolysis, chain fragments volatilisa-tion etc.

In this scenario, the properties of the surface layer prior toignition, are of paramount importance in the control of polymerand composites ignition. Thus the specimens surface was analysedby means of FTIR spectroscopy to gain insight on the chemical andphysical evolution. FTIR spectra for the different residues obtainedfrom interrupted combustion during tests at 50 kW/m2, are re-ported in Fig. 11, this flux corresponding to the shortest timeavailable for clay accumulation (i.e. the “worst case”). Additionallyto the partial oxidation of PP (CO stretching at 1720 cm�1), both PP/CNaþ and PP/C20A showed accumulated clay on the surface of thespecimen at the time to ignition, as evidenced by the increasedafter the interrupted cone test of the SieOeSi absorption bandcentred at about 1030 cm�1. The accumulation of clay on thesurface may occur both as a consequence of polymer ablation,leaving unvolatile particles on the surface of the surface or by

25 30 35 40 45 50250

300

350

400

PP PP/CNa+ PP/C20A

381 ± 9 °C

315 ± 8 °C

316 ± 3 °C317 ± 13 °C

316 ± 1 °C

387 ± 7°C387 ± 5°C

322 ± 3 °C

Igni

tion

tem

pera

ture

[°C

]

Heat Flux [kW/m2]

337 ± 7 °C

Fig. 10. Surface ignition temperatures vs. heat flux for PP and clays composites.

migration of the particles in the polymer melt, as discussed else-where [30,33].

Similar results were also obtained at lower heat flux. Unfortu-nately, no clear trend in the extent and continuity of the accumu-lated clay can be drawn as a function of neither clay type norirradiance, possibly owing to the inhomogeneity of the surface.

1800 1600 1400 1200 1000 800

Wavenumbers [cm-1]1800 1600 1400 1200 1000 800

before testing

Abso

rban

ce

Fig. 11. FTIR spectra for PP and relative composites, before testing and after inter-rupted cone calorimeter test at TTI.

A. Fina et al. / Polymer Degradation and Stability 97 (2012) 2619e26262626

Based on these observations, the accumulation of clays on thespecimen surface is most likely playing an important role, bychanging the diffusivity of oxygen and by changing the radiationabsorption.

4. Conclusions

Ignition of PP and PP composites containing clays, eithermicrodispersed or intercalated, was studied by means of temper-ature measurements on the top layer of polymer specimens duringtests in cone calorimeter at variable imposed heat flux and byresidues characterisation prior to ignition.

The results evidenced a dependency of the ignition temperatureon the imposed heat flux. Indeed, PP showed an increase in ignitiontemperature with increasing the imposed heat flux, i.e. increasingthe heating rate, possibly explained by a reduction in oxygenavailability for the polymer decomposition, shifting the decompo-sition mechanism from thermoxidative decomposition towardspurely thermal scission.

Significant differences in the PP evolution were observedcompared to PET previously studied [24]. In particular, PP ignitionappears to be obtained as a consequence of decomposition ofa relatively thin surface layer, sufficient to produce the critical massflux of volatile fuel for ignition, whereas almost the whole spec-imen thickness appeared to take part to volatiles production in thecase of PET, at least at low imposed heat flux. This behaviour isexplained by the higher viscosity of PP compared to PET, thussignificantly reducing convective flows in the specimen thickness,i.e. reducing the heat exchange by convection. Therefore, thesurface layer properties are even more important controlling theignition behaviour of PP, compared with PET.

In the presence of clays, differences in time to ignition wereobserved, depending on both the degree of clay dispersion and theimposed heat flux. Investigation of surface temperatures and bothsurface and bulk material evolution allowed to gain insights on thephenomena controlling ignition. In particular, accumulation of theclaybymigrationand/orpolymerablationappears toplaya role in thediffusion of oxygen, controlling ignition at low imposed heat flux. Onthe other hand, increasing the imposed heat flux, effects on oxygenbarrier becomes irrelevant to polymer decomposition, and theprocess becomes controlled byan interplay of other factors, includingsurface radiative absorption/emissions, convective heat exchange inthe bulk and possible catalytic effects in the thermal scission of PPchains and in the oxidation of volatile gases on the specimen surface.

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