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Polymer Degradation and Stability 94 (2009) 1230–1237

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Polymer Degradation and Stability

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

Polyhedral oligomeric silsesquioxane as flame retardantfor thermoplastic polyurethane

Serge Bourbigot*, Thomas Turf, Severine Bellayer, Sophie DuquesneProcedes d’Elaboration des Revetements Fonctionnels (PERF), LSPES – UMR/CNRS 8008, Ecole Nationale Superieure de Chimie de Lille (ENSCL),Avenue Dimitri Mendeleıev – Bat. C7a, BP 90108, 59652 Villeneuve d’Ascq Cedex, France

a r t i c l e i n f o

Article history:Received 10 March 2009Received in revised form7 April 2009Accepted 12 April 2009Available online 3 May 2009

Keywords:Reaction to fireFlame retardantPOSSIntumescenceSolid state NMRPolyurethane

* Corresponding author. Tel.: þ33 0 3 20 43 48 88;E-mail address: serge.bourbigot@ensc-lille.fr (S. Bo

0141-3910/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2009.04.016

a b s t r a c t

In this work, the reaction to fire of thermoplastic polyurethane (TPU) containing polyhedral oligomericsilsesquioxanes (or POSSs) was investigated by mass loss calorimetry. This composite exhibits a largereduction of peak of heat release rate (PHRR) compared to virgin TPU. The protection occurs via anintumescent mechanism. Mechanism of protection is examined in chemical and physical ways. Solidstate NMR of carbon and silicon on heat-treated materials reveals that there is no significant chemicalinteraction between TPU and POSS. Nevertheless the intumescent char is characterized as ceramifiedchar made of silicon network in a polyaromatic structure. The expansion occurs because of the partialvolatilization of the organic part of POSS and because of the evolving degrading products of TPU.The formation of this intumescent structure makes an efficient insulating material at the surface of thesubstrate limiting heat and mass transfer and then decreasing heat release rate.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Structure of polyhedral oligomeric silsesquioxanes (or POSSs)has been first reported in 1946 [1], but it is only recently that POSS-based hybrid polymers have received increasing attention becauseof the unique structure of the POSS macromer, which is a well-defined cluster with an inorganic silica-like core (Si8O12) sur-rounded by eight organic corner groups [2]. The nanoscopic size ofPOSS enables POSS segments to effectively reinforce polymer chainsegments and control chain motion at the molecular level throughmaximizing the surface area and chemical interactions of thenanoreinforcement with the polymer [3,4]. POSS is thus a candidateto design polymer nanocomposite. POSS reinforcement of polymerchains on a molecular level is analogous to the macroscopicreinforcement that fibres provide in composite structures.

Increased environmental awareness and fire safety concernshave pushed the plastic industry to look for environmentallyfriendly alternatives [5]. Replacing traditional halogenated fireretardants with nonhalogenated alternatives is a pressing concernand also, important for the image of companies (‘‘eco-label’’). Ofparticular interest is the developed nanocomposite technologyconsisting of a polymer and nanoparticles because those materials

fax: þ33 0 3 20 43 65 84.urbigot).

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often exhibit remarkably improved mechanical and various otherproperties including flame retardancy as compared with those ofvirgin polymer [6–8]. Lichtenhan et al. have first shown the effi-ciency of using POSS in commodity and engineering polymers fordesigning FR materials [9]. As an example, peak of heat release rate(PHRR) (cone calorimetry experiment at an external heat flux of35 kW/m2) of polyether block amides polymer (PEBAX) isdecreased by 77% when using POSS compared to virgin polymer.More recently, Fina et al. [10,11] used dimeric and oligomericAl- and Zn-isobutyl silsesquioxane (POSS) in polypropylene (PP)and they showed that the incorporation of Al-POSS in PP leads toa decrease in combustion rate with respect to PP, resulting ina decrease of heat release rate (�43% at 10 wt.% POSS loading) aswell as reduction in CO and CO2 production rates, while Zn-POSSdoes not significantly affect the PP combustion behaviour.

Thermoplastic polyurethanes (TPUs) are well known for theirhigh performance (excellent abrasion resistance, high tensile,compressive and tear strength, good flexibility over a wide range oftemperatures, good hydrolytic stability, selection of a wide range ofhardness) but they exhibit as many thermoplastics poor flameretardancy. In a previous work, we have shown that TPU/POSScomposite used as coating on woven PET fabrics permits 50%reduction in PHRR. The suggested mechanism is char formation atthe surface of the material which can act as an insulating barrier[12]. It is then our goal in this paper to investigate the reaction to

S. Bourbigot et al. / Polymer Degradation and Stability 94 (2009) 1230–1237 1231

fire of TPU as bulk polymer using POSS as potential flame retardant(FR). The morphology of the material will be investigated bytransmission electron microscopy (TEM) and the reaction of firewill be then evaluated by mass loss calorimetry. The mode of actionof POSS will be studied using chemical and physical approaches:(i) heat gradient and expansion of the material will be measuredusing a novel method and (ii) chemical characterization of the heat-treated materials will be done by solid state NMR of carbon andsilicon. Finally, a mechanism of action will be proposed.

2. Experimental

2.1. Materials and processing

TPU was polyester polyurethane supplied by BASF (ElastollanC85A) as pellets and used as received. Poly(vinylsilsesquioxane)(POSS) was supplied by Hybrid Plastics (USA) under the brandname Fire Quench (Fig. 1). It was used as received.

TPU was melt-mixed with POSS using a Brabender mixerrunning in nitrogen flow at 50 rpm for 10 min and at 180 �C. Thetotal loading is 10 wt.%.

2.2. Transmission electron microscopy

All samples were ultramicrotomed with a diamond knife ona Leica ultracut UCT microtome, at cryo temperature (�120 �C) togive sections with a nominal thickness of 70 nm. Sections weretransferred to Cu grids of 400 meshes. Bright-field TEM images ofnanocomposites were obtained at 300 kV under low dose condi-tions with a Philips CM30 electron microscope, using a Gatan CCDcamera. Low magnification images were taken at 17 000� andhigh-magnification images were taken at 100 000�. The materialswere sampled by taking several images of various magnificationsover 2–3 sections per grid to ensure that analysis was based ona representative region of the sample.

2.3. Fire testing

LOI (Minimum Oxygen Concentration to Support Candle-likeCombustion of Plastics) was measured using a Fire Testing Tech-nology instrument on sheets (100 � 10 � 3 mm3) according to thestandard ‘oxygen index’ test (ISO4589). It measures the minimum

Fig. 1. Approximate structure of poly(vinylsilsesquioxane).

concentration of oxygen in a nitrogen/oxygen mixture requiredto just support combustion of a test sample under specified testconditions in a vertical position (the top of the test sample is ignitedwith a burner).

UL-94 classification was obtained on sheets (127 � 12.7 �3.2 mm3) according to the conditions of the standard test (ASTM D3801) i.e. in a vertical position (the bottom of the sample is ignitedwith a burner). This test provides only a qualitative classification ofthe samples (V0, V1 and V2 labelled samples).

FTT (Fire Testing Technology) Mass Loss Calorimeter was usedto carry out measurements on samples following the proceduredefined in ASTM E 906. The equipment is identical to that used inoxygen consumption cone calorimetry (ASTM E-1354-90), except thata thermopile in the chimney is used to obtain heat release rate (HRR)rather than employing the oxygen consumption principle. Ourprocedure involved exposing specimens measuring 100 mm� 100mm � 3 mm in horizontal orientation. External heat flux of35 kW/m2 was used for running the experiments. This flux corre-sponds to common heat flux in mild fire scenario. The mass losscalorimeter was used to determine heat release rate (HRR). Whenmeasured at 35 kW/m2, HRR is reproducible to within�10%. The datareported in this paper are the average of three replicated experiments.

2.4. Thermal analysis

Thermogravimetric analyses were carried out at heating rate of10 �C/min in synthetic air flow or in nitrogen using a Setaram TG 92thermobalance at a flow rate of 60 mL/min. In each case, samples(10 mg) were positioned in open vitreous silica pans. The precisionon the temperature measurements is�1.5 �C in the range 50–850 �C.

In order to determine whether a potential increase or decreasein the thermal stability happens between two additives mixedtogether, the weight difference curves between experimental andcalculated TG curves were computed as follows:

C MPOSS(T): values of weight given by the TG curve of POSS,C MTPU(T): values of weight given by the TG curve of TPU,C Mexp(T): values of weight given by the TG curve of the

composite TPU–POSSC Mth(T): theoretical TG curve computed by linear combination

between the values of weight given by the TG curve of bothcomponent: Mth(T) ¼ xMPOSS(T) þ (1 � x)MTPU(T) where x isthe POSS content in the composite.

C D(T): weight difference curve: D(T) ¼ Mexp(T) � Mth(T).

D(T) curves allow us to show a potential increase or decrease inthe thermal stability of the system when two additives are mixedtogether [13].

2.5. Solid state NMR

In order to investigate the degradation of samples (POSS andTPU–POSS), heat treatments were carried out in synthetic air atdifferent characteristic temperatures for 3 h in a tubular furnace.The residues were then analyzed by solid state NMR.

Measurements were conducted using a Bruker Avance 400spectrometer operating at 9.4 T for 13C. 13C spectra were performedusing a 4-mm probe at 100.6 MHz. Ground samples were packed in4-mm fused zirconia rotors and sealed with Kel-F caps. Samplespinning rate at the magic angle (MAS) was approximately 15 kHz.High-power 1H decoupling and 1H–13C cross-polarisation (CP) wereused. All spectra were acquired with contact times of 1 ms. Arepetition time of 5 s was used for all the samples. Typically, 4096scans were necessary to obtain spectra with good signal to noiseratio. Tetramethylsilane was used as reference.

Fig. 2. Experimental set-up for measuring the swelling during a mass loss experimentusing infrared camera.

Time (s)0 200 400 600 800

Sw

ellin

g (%

)

0

5

10

15

20

25

30

Fig. 4. Typical relative expansion as a function of time of an intumescent coating ona steel plate during a mass loss calorimeter experiment (external heat flux¼ 35 kW/m2).

S. Bourbigot et al. / Polymer Degradation and Stability 94 (2009) 1230–12371232

Solid state NMR of 29Si was performed at 20 MHz on BrukerAvance 100 spectrometer operating at 2.35 T with MAS at 10 kHzand with CP (contact time ¼ 8 ms). Bruker probe heads equippedwith 7-mm rotor assembly were used with the same type of rotorsdescribed above. A repetition time of 10 s was used for all samples.Tetramethylsilane was used as reference.

2.6. Swelling and heat gradient

An experimental set-up was designed to shoot a mass losscalorimeter experiment using an infrared camera (Fig. 2).

The advantage of infrared camera is to get clear images to makeimage analysis. Typical infrared images at the beginning of theexperiment and at the maximum of expansion of the intumescentcoating are shown in Fig. 3. Note here that an intumescent coatingwas applied on steel plate but it is not expansion involving TPU-based materials.

Using image analysis in dynamic conditions (from a movie),swelling of the intumescent can be measured and quantified(see the arrow in Fig. 3). In this approach, it is assumed duringcalculation on images that the expansion is homogeneous andoccurs in one dimension. Typical curve exhibits a sigmoidal shapeshowing first a rapid development of intumescence and seconda pseudo-steady state at longer times (Fig. 4). The benefit of thisapproach is to get a quantitative phenomenological model whichmight be included in further modelling.

In addition to those measurements, thermocouples areembedded in the materials and located inside the plaque, at theinterface air/material and at 5 mm beyond the surface as shown inFig. 5. It measures heat gradients in the expanded char and so, toquantify the provided protection. The thermocouple located beyondthe surface of the coating gives measurements of temperature in

Fig. 3. IR images of an intumescent coating on steel plate upon h

intumescent layers close to the surface. In this experimental set-up,it is necessary to assume that additional conductive effects due tothe thermocouples are negligible.

3. Results and discussion

3.1. Morphology of TPU–POSS

TEM is used to study the composites morphology at the finersize-scale level. At lower magnification (not shown) POSS particlesare found to make macroaggregates evenly dispersed in TPU. Athigher magnification (Fig. 6), the photographs reveal that POSSparticles tend to form micron-sized aggregates probably formedduring the melt state of the processing. The high-magnificationphotograph (Fig. 6b) also reveals that the sizes of POSS particles haveellipsoidal shape exhibiting a size lying between 200 and 400 nm.TPU–POSS is then not a nanocomposite but a microcomposite.

3.2. Reaction to fire

The incorporation of 10 wt.% POSS in TPU decreases by 80% thepeak of heat release rate (PHRR) compared to virgin TPU (80 kW/m2

vs. 430 kW/m2) during a mass loss calorimetry experiment (Fig. 7).Nevertheless, time to ignition of the composite (60 s) is twiceshorter than that of virgin TPU (120 s). Note also there is nosignificant enhancement of LOI (22 for the virgin TPU vs. 23 vol.%for TPU–POSS) and UL-94 (V-2 at 3.2 mm in the two cases) whenincorporating POSS in TPU.

eating at t ¼ 0 s (a) and at the maximum of expansion (b).

Fig. 5. Experimental set-up for measuring heat gradient in an intumescent coatingduring a mass loss calorimeter experiment at the beginning of the experiment (a) andat the steady state (b).

S. Bourbigot et al. / Polymer Degradation and Stability 94 (2009) 1230–1237 1233

The interesting feature is the formation of a large intumescentchar when burning TPU–POSS (Fig. 8). It suggests that this char canprovide the protection of interest limiting heat and mass transfer asit makes in the case of conventional intumescent system. Evidenceof this must be given, that is why we have measured the expansionof the material and heat gradients inside the char.

According to our experimental protocol described in theExperimental section, Fig. 9 shows the expansion of TPU-basedmaterials as a function of time including pictures taken at differentcharacteristic times. The expansion of TPU–POSS starts rapidly at25 s and before the ignition of the composite (60 s). At 60 s, theswelling of the sample due to the formation of an expanded char isat about 250% but it is not effective enough to avoid the ignition.The swelling reaches a plateau at about 300% between 100 s and200 s corresponding to the highest HRR values. When the expan-sion increases again (up to 370%), HRR decreases suggesting thatthis additional expansion provides further protection to stop thecombustion of the sample. Concurrently, virgin TPU exhibits theformation of a char (TPU is a char former polymer) with relativelylow expansion (up to 200%) which is developed after the ignition ofthe sample, i.e. during the combustion of the material.

Fig. 10 shows temperatures as a function of time in TPU andTPU–POSS. Temperatures increase rapidly in the two materials

Fig. 6. TEM images of TPU–POSS comp

because of the external heat flux. When TPU–POSS ignites (t¼ 60 s),temperatures of the two thermocouples (located in TPU–POSS)jump and then they reach a pseudo-plateau (100 s < t < 250 s). Atthose times, the thermocouple located at 5 mm beyond the surfaceis in the char and we can measure a heat gradient of about 125 �C ina 5-mm thick piece of char. It gives then evidence that the forma-tion of this intumescent char limits heat transfer and providesprotection. In comparison with virgin TPU, temperature jumpsrapidly after its ignition (t > 100 s) and reaches 600 �C at 280 swhen HRR values are the highest.

3.3. Thermal analysis

The TGA plots of the pyrolysis and of the thermo-oxidativedegradation of POSS are shown in Fig. 11. Pyrolytic degradation ofPOSS (Fig. 11a) occurs in one apparent main step starting at 210 �Cand yielding a residue of 53 wt.% at 400 �C. This residue is slightlydegraded between 450 and 600 �C (residue at 800 �C ¼ 50 wt.%).The thermo-oxidative degradation of POSS occurs in two apparentmain steps (Fig. 11b). The first step starts at 210 �C up to 300 �C (thisstep can be superimposed to that of the pyrolytic degradation) andthe second exhibits a smooth decreasing slope from 300 �C to800 �C (final residue ¼ 62 wt.%). Lichtenhan et al. [14] investigatedthe thermolysis of some POSS (POSS macromers and POSS copol-ymers) and they assigned the pyrolytic decomposition to the partialloss of the organic substituents of POSS followed by formation ofSiOxCy networks. Fina et al. [15] investigated the thermo-oxidativedegradation of POSS similar to our POSS (poly(vinylsilsesquioxane))and evidenced an acceleration of its degradation in the presence ofoxygen. Our TGA trace shows the contrary probably because ourPOSS was not washed and purified like Fina et al. did. Neverthelessand according to the works of Lichtenhan et al. [14] and Fina et al.[15], we can suggest that the thermo-oxidative degradation of POSSfollows a similar degradation pathway as that in nitrogen in a firststep. Oxygen reacts then with the degraded products stabilizingthem and slowing down the degradation. The complete mechanismof degradation will be discussed in the next section.

Degradation of TPU starts at 280 �C and occurs in one apparentsingle step in nitrogen while it occurs in two steps in air (Fig. 11). Inair and nitrogen, TG curves are similar up to 400 �C and it isassigned to the depolycondensation of TPU [16]. In air, the forma-tion of relatively stable transient char is observed between 400and 500 �C. This char is decomposed by oxidation at higher

osite at different magnifications.

0 100 200 300 400 500 600 700 800 900

HR

R (kW

/m

2)

0

50

100

150

200

250

300

350

400

450

Virgin TPUTPU-POSS

Fig. 7. Heat release rate (HRR) as a function of time of virgin TPU compared toTPU–POSS composite (external heat flux ¼ 35 kW/m2).

Time (s)

0 100 200 300 400 500 600

Sw

elin

g (%

)

0

100

200

300

400

Virgin TPUTPU-POSS

TPU-POSS@500s

TPU-POSS@130s

TPU-POSS@40sVirgin TPU@300s

Fig. 9. Swelling as a function of time of virgin TPU compared to TPU–POSS compositeduring a mass loss calorimetry experiment with infrared pictures showing the swellingat different characteristic times (external heat flux ¼ 35 kW/m2).

S. Bourbigot et al. / Polymer Degradation and Stability 94 (2009) 1230–12371234

temperatures yielding a 1.5 wt.% residue. The decomposition of TPUin nitrogen does not exhibit any transient char but yields a higherchar yield at 800 �C (6 wt.%).

The incorporation of POSS in TPU does not modify the apparentdegradation pathway in nitrogen except the higher char yield(6 wt.% vs. 11 wt.%) due to POSS (Fig. 11a). Curve of weight lossdifference confirms this assumption (Fig. 12) and it shows nosignificant interactions between TPU and POSS during degradation.On the contrary, POSS seems to destabilize TPU in air. The degra-dation of TPU–POSS starts at lower temperatures than virgin TPU(Fig. 11b) and its remaining mass is lower than TPU up to 400 �C.TPU–POSS exhibits a higher transient char yield (27 wt.% vs.20 wt.%) and higher final char yield (3.5 wt.% vs. 1.5 wt.%) because ofthe incorporation of POSS. The curve of weight loss differencereveals interactions between TPU and POSS from 200 �C up to800 �C (Fig. 12). It is always negative evidencing that POSS desta-bilizes TPU during thermo-oxidative degradation.

600

3.4. Solid state NMR

In the previous section, we have evidenced interactions betweenPOSS and TPU when degrading in thermo-oxidative conditions. Thecurve of weight loss difference defines 240, 380, 480 and 650 �C ascharacteristic temperatures at which heat treatment will be done in

Fig. 8. Residue of TPU–POSS composite after a mass loss calorimetry experiment(external heat flux ¼ 35 kW/m2).

air. The resulting residues will be analyzed by solid state NMRof carbon and silicon in order to elucidate the mechanism ofdegradation.

29Si NMR spectroscopy is a powerful tool in acquiring furtherinformation about the structure of oligomeric species. It is then ofgreat interest when investigating the degradation of materialscontaining POSS. The chemical shift of silicon is determined by thechemical nature of its neighbours, namely, the number of siloxanebridges attached to a silicon atom. M, D, T and Q structures form thecommonly used notation corresponding to one, two, three and fourSi–O– bridges, respectively. We will label the silicon sites with theconventional Mn, Dn, Tn and Qn, notation (Mn¼ R3–Si–OR0, Dn¼ R2–Si–(OR0)2, Tn¼ R–Si–(OR0)3 and Qn¼ Si(OR0)4), where n denotes thenumber of bridging oxygens surrounding the silicon atom [17]. Interms of chemical shifts, the assignments are as follows: M (þ6 to�12 ppm), D (�20 to�55 ppm), T (�65 to�85 ppm), and Q (�85 to�112 ppm), relative to the peak for tetramethylsilane (TMS) [18].Fig. 13 shows 29Si NMR spectra of POSS heat treated at differenttemperatures. At room temperature POSS exhibits two peaks at�81 and �83 ppm assigned to silicon T3 resonances. When heatingPOSS up to 480 �C, silicon T2 (�66 ppm) and Q3 (�101 ppm)resonances appear while T3 broadens and decreases in intensity.

Time (s)

0 100 200 300 400 500

Tem

peratu

re (°C

)

0

100

200

300

400

500

Virgin TPU (thermocouple at interface)TPU-POSS (thermocouple at interface)TPU-POSS (thermocouple at 5 mm up)

Fig. 10. Temperature as a function of time of virgin TPU compared to TPU–POSScomposite during a mass loss calorimetry experiment (external heat flux ¼ 35 kW/m2).

Temperature (°C)

0 200 400 600 800

Rem

ain

in

g m

ass (w

t.-%

)

0

20

40

60

80

100

POSS (N2)TPU (N2)TPU-POSS (N2)

Temperature (°C)

0 200 400 600 800

Rem

ain

in

g m

ass (w

t.-%

)

0

20

40

60

80

100

POSS (air)TPU (air)TPU-POSS (air)

a b

Fig. 11. TG curves of POSS, virgin TPU and TPU–POSS in pyrolysis (a) and thermo-oxidative conditions (b) (heating rate ¼ 10 �C/min).

-125-120-115-110-105-100-95-90-85-80-75-70-65-60-55-50-45-40

Chemical shift(ppm)

RT

240°C

320°C

480°C

650°C

Fig. 13. 29Si CP/MAS NMR of POSS heat treated at different characteristic temperatures.

S. Bourbigot et al. / Polymer Degradation and Stability 94 (2009) 1230–1237 1235

This result suggests a redistribution of the silicon environments asshown by Belot et al. [19] and by Lichtenhan et al. [14]. At 650 �C,only one broad band is observed centred at �106 ppm. Deconvo-lution of this latter band (not shown) shows it is an overlap of twobands at �104 and �111 ppm. They can be assigned to silicon Q3

and Q4 resonances indicating the complete decomposition of POSS.13C NMR spectra of heat-treated POSS are shown in Fig. 14. At

room temperature, POSS exhibits two groups of two peaks locatedat 138.5 and 137.5 ppm, and 130 and 128.5 ppm. Those peaks can beassigned to carbons in the vinyl group of POSS (see Fig. 1) [20]. Thedownfield peaks at 138.5 and 137.5 ppm are assigned to carbonlinked to silicon (CH group) in the cage and in the polymeric chainsrespectively. The high field peaks are assigned to CH2 of the vinylgroup linked to the cage and in the polymeric chains respectively. At240 �C, an additional broad band appears centred at 24 ppm whichcan be assigned to Si–R bonds where R is a methylene chain [21].Only two relatively broad bands (at 138 and 128.5 ppm) can also bedistinguished where the peaks of undegraded POSS were located. Itis consistent with the analysis by 29Si NMR which indicated thepartial degradation of POSS yielding a T2 and Q3 structure. At 320 �C,we note the increasing intensity of the broad band centred at24 ppm and the broadening of the two bands located at 138 and128.5 ppm. A low intensity band can be distinguished at � 5 ppmwhich can be assigned to methyl group linked to silicon in T2 unit

Temperature (°C)

0 200 400 600 800

Rem

ain

in

g m

ass (w

t.-%

)

-15

-10

-5

0

5

AirN2

Fig. 12. Curves of weight difference in pyrolysis and thermo-oxidative conditions(heating rate ¼ 10 �C/min).

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Chemical shift(ppm)

RT

240°C

320°C

480°C

650°C

Fig. 14. 13C CP-DD/MAS NMR of POSS heat treated at different characteristictemperatures.

-140-130-120-110-100-90-80-70-60-50-40-30-20-100

Chemical shift(ppm)

RT

240°C

320°C

480°C

650°C

Fig. 15. 29Si CP/MAS NMR of TPU–POSS heat treated at different characteristictemperatures.

S. Bourbigot et al. / Polymer Degradation and Stability 94 (2009) 1230–12371236

[21]. At higher temperatures, the two peaks broaden and overlapwhile the broad band centred at 24 ppm exhibits a decreasingintensity (it disappears at 650 �C) and the band at�5 ppm increasesin intensity. Note that an additional peak at 4 ppm appears which isassigned to methylene carbons in (Si–O3)–CH2–CH2–(Si–O3) [21]suggesting the formation of vinyl cross-links and the double peak inthe 120 ppm region becomes a broad band suggesting the formationof phenyl group linked to silicon. At 650 �C, only one broad band isobserved centred at 128 ppm assigned to polyaromatic char [22]. Itis justified because of the formation of cross-linked silicon networkevidenced by 29Si NMR. Note that the formation of char starts onlyat 650 �C evidenced by visual observation showing only blackresidues after heat treatment at 650 �C.

NMR study of the degradation of POSS suggests the followingmechanism. When heating up POSS (up to 320 �C), its cage starts todegrade and exhibits redistribution reactions leading to theformation of T2 and Q3 units in addition to the original T3 unit of

0102030405060708090110130150170190210

Chemical shift(ppm)

RT

240°C

320°C

480°C

650°C

1200

RT

24

320

480

650

a b

Fig. 16. 13C CP-DD/MAS NMR of TPU (a) and TPU–POSS (b)

POSS. Concurrently, there is the formation of polyethylene chainslinked on Si–O– suggesting partial polymerization of vinyl groups.At higher temperatures, the POSS cage is destroyed evidenced bythe main presence of T2 units in the residue as well as by thepolyethylene chains. The detection of cross-linked (Si–O3)–CH2–CH2–(Si–O3) and the appearance of phenyl groups linked to Si–Osuggests the formation of a so-called ceramified char which reachesa higher degree of cross-linking at 650 �C (char in silicon network).

The incorporation of POSS in TPU does not significantly modifythe degradation of POSS (Fig. 15). In Fig. 15, 29Si CP/MAS NMRspectra reveal the same modifications and the same degradationpathway of POSS as in the case of the virgin POSS. The same speciesare observed and they appear (or disappear) at the same temper-atures (except at 240 �C). The only noticeable difference is that thedegradation of POSS seems to be slowed down as no T2 and Q3 unitsare observed on the spectrum at 240 �C.

Fig. 16 shows 13C CP/DD MAS NMR of TPU and TPU–POSS heattreated at different temperatures. According to those spectra, thedegradation of TPU and TPU–POSS looks similar. At room temper-ature, characteristic bands of polyurethane can be distinguished.Bands at 25 and 33 ppm are assigned to carbons in methylenechains probably in a soft segment of TPU and that at 40 ppm istypical of methylene group bridging two aryl groups [23]. The bandat 65 ppm is characteristic of carbon adjacent to urethane group[24]. In the 100–140 ppm region, 4 bands can be clearly distin-guished which are assigned to protonated carbon in phenyl ring(bands at 130, 123 and 119 ppm) and to quaternary carbon in phenylring (band at 137 ppm) [25,26]. Carbonyl groups in urethane and inester groups are observed at 154 and 173 ppm respectively [24].Note we can detect additional bands on the spectra at 480 �C (broadband centred at 15 ppm and two bands at 5 and �5 ppm) and at650 �C (broad band centred at�5 ppm). Those bands correspond tothe degraded POSS and they can be assigned as above.

When heating TPU and TPU–POSS to 320 �C, the same bands asthose observed at room temperature appear on the spectra but theybecome broader. It indicates the beginning of the degradation ofTPU as shown on TG curves (Fig. 11b). From 480 �C, only one broadband centred at 125 ppm assigned to aromatic carbon in a charredmaterial is observed. At those temperatures, TPU and TPU–POSS aretherefore degraded yielding char.

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Chemical shift(ppm)

0°C

°C

°C

°C

heat treated at different characteristic temperatures.

S. Bourbigot et al. / Polymer Degradation and Stability 94 (2009) 1230–1237 1237

3.5. Mechanism of action

Incorporation of POSS in TPU provides flame retardancy propertiesvia an intumescent mechanism. This observation done by mass losscalorimetry suggests a mechanism in the condensed phase. It isevidenced by TGA showing the formation of transient char andinteractions between TPU and POSS during the thermo-oxidativedegradation of TPU–POSS. NMR analysis on heat-treated materialsshows that the degradation of POSS yields a charred ceramic. WhenPOSS is incorporated in TPU, the same species are formed and thepresence of TPU does not play a specific role in terms of chemicalinteractions (formation of new species). Nevertheless it is noteworthythat the degradation of POSS is slowed down in TPU. It suggests thatPOSS would catalyze the degradation of TPU explaining the negativeinteractions between TPU and POSS during the thermo-oxidativedegradation of TPU–POSS. To summarize our results, the degradationof TPU–POSS yields a ceramified char made of silicon network ina polyaromatic structure. It constitutes the intumescent material butthe question we should answer is how does it work? When TPU–POSSundergoes an external flux, it starts to degrade and yields a viscouscharred paste. This paste can be expanded because of the partialvolatilization of the organic part of POSS and because of the evolvingdegrading products of TPU. From Fig. 9, we can estimate the swellingrate to 8%/s. The formation of this intumescent structure limits HRRmaking an efficient insulating material (large thermal gradient wasmeasured in such structure) at the surface of the substrate. It isnoteworthy that the rapid expansion of TPU–POSS occurs before theignition of the material. The combination of the expansion gettingcloser of the heat flux and of the high viscous material limiting heattransfer and accumulating heat at the surface leads to relatively shortignition time. On the contrary, virgin TPU melts and can dissipate heatbecause vigorous bubbling leading to relatively long ignition time.

4. Conclusions

In this work, we have investigated the reaction to fire of TPU–POSS composite. It exhibits a large reduction of PHRR compared tovirgin TPU. The protection occurs via an intumescent mechanism.The intumescent material is composed of ceramified char made ofsilicon network in a polyaromatic structure. This material formsa thermal barrier at the surface of the substrate limiting heat andmass transfer leading to a limited HRR.

Acknowledgement

The authors are indebted to Mr. Bertrand Revel for skilfultechnical assistance in NMR experiments and for helpful discussion.

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