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Polymer Degradation and Stability 93 (2008) 2166–2171

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Polymer Degradation and Stability

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

Synergism between flame retardant and modified layered silicate on thermalstability and fire behaviour of polyurethane nanocomposite foams

M. Modesti*, A. Lorenzetti, S. Besco, D. Hrelja, S. Semenzato, R. Bertani, R.A. MichelinPadova University, Department of Chemical Process Engineering, v. Marzolo 9, 35131 Padova, Italy

a r t i c l e i n f o

Article history:Received 22 May 2008Received in revised form 7 August 2008Accepted 18 August 2008Available online 26 August 2008

Keywords:PolyurethaneNanocompositeSynergyPhosphorusFire

* Corresponding author. Tel.: þ39 49 8275541; fax:E-mail address: michele.modesti@unipd.it (M. Mo

0141-3910/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2008.08.005

a b s t r a c t

Synergy in flame retardancy of polyurethane foams between phosphorus-based flame retardant(aluminium phosphinate) and layered silicates has been investigated. We used pristine montmorilloniteas well as ammonium modified clay (commercially available) and diphosphonium modified clay, whichwere synthesised by the intercalation of the quaternary diphosphonium salt according to a procedurereported here. The morphology of the foams was characterised through X-ray diffraction (XRD), whilethermal properties were characterised by oxygen index test, cone calorimeter and thermogravimetricanalysis (TGA). The morphological characterisation showed that pristine and diphosphonium modifiedclays are almost slightly intercalated, while ammonium modified one is very well dispersed. The resultsof thermal characterisation showed that in the presence of phosphinate enhancements of oxygen index,fire behaviour, measured by cone calorimeter, and thermal stability have been achieved. Phosphinate istherefore an efficient flame retardant for polyurethane foams and its flame retardancy action takes placein both condensed and gas phases. Pristine and ammonium modified layered silicate bring someenhancements of thermal stability while having no important effect in decreasing peak heat release rate(PHRR) and total heat evolved (THE) when used in conjunction with phosphinate; their main advantageis related to the enhancement of compactness of the char layer formed. Diphosphonium clay is insteadeffective in further improving the fire behaviour of the foams because of the flame retardancy action ofphosphonium: both PHRR and THE were decreased. The analysis of cone calorimeter data showed thatclays act through physical effect constituting a barrier at the surface which is effective in preventing orslowing the diffusion of volatiles and oxygen, while phosphinate and phosphonium are more effectiveowing to their combined action in both condensed and gas phases.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

It is now well known that organic–inorganic nanocompositesexhibit superior characteristics in terms of mechanical properties,barrier effect, thermal resistance, fire behaviour, etc., in comparisonwith pure polymer constituents or conventionally filled polymers.Dealing with polyurethane, since the first examples of elastomericpolyurethane/clay nanocomposites reported by Wang and Pinna-vaia [1], Chen et al. [2] and Mulhaupt et al. [3], numerous worksreporting improvement on mechanical and thermal properties ofpolyurethane have been published. Several of them studied thethermal stability and fire behaviour of polyurethane nano-composite, although they were mainly dealing with thermoplasticand elastomeric polyurethanes [4–6]. It is reported that the use ofnanoclays generally leads to an increase in thermal stabilitythrough a barrier effect, bringing a delay of thermal degradation

þ39 49 8275555.desti).

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products’ release [5]. It was generally reported that the tempera-ture of the onset of decomposition slightly increases or remainsunchanged [4,5], although Song et al. stated that the onsetdecomposition temperature for polyurethane nanocompositecontaining onium modified clay is lower than neat polymerbecause the acid sites on the clay surface, formed by Hoffmanreaction [9], have a catalytic effect on polymer degradation [6].These acid sites are probably also responsible for the enhancing ofcharring reactions [10]; the higher char residue lowers the peak ofheat release rate in cone calorimeter tests [5–8] and eliminates fire-induced dripping of the nanocomposite sample during UL-94 test[5].

However, most authors agree that in order to comply with theactual standards on fire reaction, nanocomposite technology mustbe combined with conventional flame retardant technology.Synergistic effects between layered silicates and fire retardants areactually the aim of numerous works and have been alreadyreported for styrenic polymers [11], vinyl ester polymers [12] andelastomeric and thermoplastic polyurethanes [6,13]; synergy

M. Modesti et al. / Polymer Degradation and Stability 93 (2008) 2166–2171 2167

between clays and metal hydroxides for ethylene–vinyl acetate hasalso been shown [14,15].

To our best knowledge, in spite of other polymers, no synergisticeffects were reported for flame retardants and layered silicates inpolyurethane nanocomposite foams. Some papers on PU nano-composite foams have been already reported [16–24] and only fewdeal with fire behaviour [20–24]. Seo et al. [20] reported that thethermal stability and fire resistance properties of polyurethanefoam/clay nanocomposite are enhanced when a higher degree ofdispersion of filler is achieved (i.e. using ultrasound treatmentduring montmorillonite modification with isocyanate). Mahfuzet al. [21] reported the enhancement of thermal stability andflexural properties of PU foams through the use of SiC and TiO2

particles. They reported that thermal stability increases whenincreasing TiO2 amount, owing probably also to its catalytic effecton the cross-linking of the polyurethane foams, while for highestSiC content (3 wt%) the fillers weaken the van der Waals interactionbetween polymer chain, thus lowering thermal stability. Pethrickreported [22] that for PU flexible foams, the use of nanoplateletcomposite structures within cell walls inhibits volatile diffusionand enhances the viscoelasticity of the melt phase, preventingdripping. Such foams are able to pass Crib 5 test. Gilman [23] andZammarano et al. [24] reported that the use of carbon nanofibres(CNF) is effective in lowering peak of heat release rate (PHRR) byabout 35% in polyurethane flexible foams. The nanoadditives (e.g.CNF) properly dispersed in the polymeric matrix formed a perco-lated jammed structure, due to particle–particle interactions, sothat the melt behaves rheologically like a gel [25], with inhibition ofdripping and a heat shield effect of the network-structuredprotective layer.

The focus of this work was to study potential synergistic effectsbetween phosphorus-based flame retardants and layered silicatesin polyurethane foams. In particular fairly new phosphorus-basedfire retardants (aluminium phosphinate) were used as well asphosphonium modified montmorillonite; for comparison,commercial unmodified and organically modified layered silicateswere also used. We synthesised and used phosphonium modifiedclay because it was known that it has a greater thermal stabilitythan ammonium modified clay [26]; moreover mono- andbisphosphonium salts are already used as flame retardants fortextiles and paper as well as heat stabilizers for nylon. Thus, the useof phosphonium salts as organic modifiers to layered silicates mayfurther enhance the thermal stability and flammability propertiesof polyurethane nanocomposites.

2. Experimental

2.1. Materials

For phosphonium modified clay preparation (referred in thefollowing as DP), purified Na-montmorillonite (Dellite HPS) havinga cation exchange capacity (CEC) of 128 meq/100 g was supplied byLaviosa Chimica Mineraria (Italy) and used as-received while 1,2-bis(diphenylphosphino)ethane and methyl bromoacetate weresupplied by Sigma–Aldrich. Also unmodified Dellite HPS (referredin the following as MMT) and Cloisite 30B (referred in the followingas OMLS), that is bis(2-hydroxyethyl)methyl tallow ammoniummodified montmorillonite, supplied by Southern Clay Products,were used. The raw materials employed in polyurethane foamsynthesis were polymeric MDI (methane diphenyl diisocyanate),Voranate M600 (Dow Chemicals) characterised by NCO%¼ 30.5;average functionality¼ 2.8, viscosity to 25 �C¼ 600 mPa s; poly-ester polyols: Isoexter 4530 (Coim, Italy): hydroxyl value¼ 510 mgKOH/g, viscosity to 25 �C¼ 11,000 mPa s; Isoexter 4537 (Coim,Italy) hydroxyl value¼ 350 mg KOH/g, viscosity to25 �C¼ 4900 mPa s; polyether polyol: Voranol RN 490 (Dow

Chemicals) hydroxyl value¼ 490 mg KOH/g, viscosity to25 �C¼ 6000 mPa s; catalyst: polycat 8, i.e. N,N-dimethyl cyclo-hexylamine (DMCHA), and polycat 5, i.e. pentamethyl-diethylenetriamine (PMDETA), both supplied by Air Products; surface-activeagent: polysiloxane–polyether copolymer: Tegostab B8471 (Gold-schmidt, Italy); blowing agent: blend of 1,1,1,3,3-pentafluorobutaneand 1,1,1,2,3,3,3-heptafluoropropane (Solkane 365/227, SolvayFluor) and water that reacting with isocyanate leads to theformation of CO2:

The amount of blowing agent used was calculated in order toobtain foams with a density of 33�1 kg/m3. The flame retardantused is aluminium phosphinate, Phoslite IPA, supplied by ItalmatchChemicals (Italy).

2.2. Preparation of rigid polyurethane–clay nanocomposite foams

The detailed synthetic procedure for diphosphonium derivative[CH3OOCCH2(Ph)2PCH2CH2P(Ph)2CH2COOCH3]Br2 was reportedelsewhere [27]. DP clay was prepared from Na-MMT (Dellite HPS)by ion exchange reaction using [CH3OOCCH2(Ph)2PCH2CH2P(Ph)2

CH2COOCH3]Br2 in water. The resulting DP suspension was centri-fuged at 10,000 rpm for 30 min and the solid separated wasrepeatedly washed.

Polyurethane foams were prepared using a two-step procedure.First, a fixed amount of clay (5 wt% on total foam mass), which waspreviously dehydrated overnight in an oven at 110 �C, wasdispersed in the polyol mixture. In order to promote clay dispersiona microwave treatment was used [28]; microwave processing wasapplied through 15 s step, followed by intermediate cooling to50 �C using a water/ice/acetone bath. After filler dispersion, thecatalysts, surface-active agent, flame retardant (IPA, 10 wt% on totalfoam mass) and blowing agents were added to the polyol mixtureand stirred for 1 min with a high-speed mechanical stirrer. Theisocyanate was added to the polyols so formulated; the twocomponents were mixed for 15 s with a high-speed stirrer and thenpoured into an open mould for free rise polymerization. Afterpreparation, the foams were put in an oven at 70 �C for 24 h, inorder to complete the polymerization reaction. After conditioning,several samples were cut in order to carry out characterisation.

2.3. Characterisation

The thermal stability was studied by thermogravimetric analy-ser Q 5000 (TA Instruments), under air atmosphere with heatingrate of 20 �C/min from 25 to 900 �C using alumina pans. Firebehaviour was characterised through limiting oxygen index (LOI)evaluation, according to ISO 4589, and cone calorimeter testaccording to ISO 5660. The samples were irradiated at 50 kW/m2

and the data were collected for the first 600 s. The exhaust gas flowrate was 24 L/s. The heat released was calculated from theconsumption of oxygen due to combustion. The parameters whichwere measured are the heat release rate (HRR, kW/m2), its peakvalue (PHRR, kW/m2) and the time at which peak takes place(TPHRR, s), the total heat evolved (THE, kJ/m2), the total smokereleased (TSR), evaluated through smoke absorbance, the totalmass loss (TML, g) and the CO production (g/g sample).

Morphological study was performed by X-ray diffraction usinga Philips powder diffractometer X’Pert equipped with a Cu Ka(l¼ 1.54 Å) radiation source; XRD experiments were carried onboth clay powders and polyurethane nanocomposite foams.

2 4 6 8 10 12 140

500

1000

1500

2000

2500

in

ten

sity

2 theta

DPOMLSMMTREFIPA-DPIPA-OMLSIPA-MMT

Fig. 1. XRD curves for clays (OMLS, MMT and DP), unfilled PU foam (Ref) and PUnanocomposite foams (IPA–OMLS, IPA–MMT and IPA–DP).

0

10

20

30

40

50

60

70

80

90

100

110

Weig

ht (%

)

50 150 250 350 450 550 650 750

Temperature (°C)

IPA-OMLS–––––––IPA-MMT–––––––IPA-DP–––––––IPA–––––––Ref–––––––

Fig. 2. TGA curves under air atmosphere for unfilled PU foam (Ref), IPA filled PU foam(IPA) and PU nanocomposite foams (IPA–OMLS, IPA–MMT and IPA–DP).

0.0

0.2

0.4

0.6

0.8

Deriv. W

eig

ht (%

/°C

)

RefIPAIPA-DPIPA-OMLSIPA-MMT

M. Modesti et al. / Polymer Degradation and Stability 93 (2008) 2166–21712168

3. Results and discussion

3.1. Morphology

The detailed characterisation of DP is reported elsewhere [27].The XRD patterns of DP after drying at 30 �C showed that the basalspacing increased from 1.26 (corresponding to unmodified MMT) to1.90 nm; the basal spacing value is consistent with a bilayerarrangement of intercalated chains. The 31P NMR spectrum of DP–MMT shows two broad signals centred at 36.9 and 28.6 ppm, whichcan be attributed to phosphine oxide and phosphonium systems,respectively. 31P NMR spectra in the solid state were also used toevaluate the amount of diphosphonium intercalated within MMT. Itwas calculated that the P amount is about 7.5% weight (by weight)in DP–MMT, which may be compared with the value of 10%calculated on the basis of the stoichiometry of the reactions. It isnoteworthy that also on the basis of X-ray fluorescence energydispersive spectroscopy, the P amount on different samples of DP–MMT was evaluated to be 6–7% by weight.

X-ray diffraction (XRD) curves for the different nanocompositesprepared are reported in Fig. 1; the interlayer distance calculatedusing Bragg’s law is reported in Table 1.

For MMT and DP filled foams only a moderate intercalation maybe observed: the interlayer distance increases in both cases of about2–3 Å. Better results were obtained considering OMLS: the inter-layer distance is more than twice than the initial one. This may bedue not only to the greater compatibility between clay and polymerbut also to the tethering reaction that takes place during thedispersion [28] which seems to be responsible for the very goodintercalation achieved.

3.2. Thermal stability

TGA curves in air are reported in Fig. 2 while derivative DTGcurves are reported in Fig. 3. As can be seen, the thermo-oxidativedegradation path is similar for both unfilled and filled foams and

Table 1XRD results and calculated interlamellar distances for clays (OMLS, MMT and DP)and PU nanocomposite foams (IPA–OMLS, IPA–MMT and IPA–DP)

OMLS IPA–OMLS MMT IPA–MMT DP IPA–DP

2q 4.83 2.07 7.09 6.25 4.64 4.75d (Å) 18.29 42.68 12.61 14.15 19.05 18.61

shows two stages: in the first urethane bonds as well as polyolchain segments decompose to form volatile products such asaldehydes, ketones, carbon dioxide and water; in the second stepthe oxidative decomposition of isocyanates and aromaticcompounds takes place. Nevertheless, the change in weight asso-ciated with each step and the residue at high temperature are verydifferent for the foams analysed. The onset of decomposition,which is assumed to be the temperature at which 5 wt% weight losshas taken place (Table 2), is similar for unfilled and IPA foams whileit is delayed by about 10 �C for foams containing clays owing to thebarrier effect, which slows down both the escape of volatiledegradation products from the polymer and the counter diffusionof oxygen, thus reducing polymer degradation. Above 350 �C, theweight retained for IPA filled foam is higher than that of referenceone and even further greater for IPA–nanoclays filled foams.Considering DTG curves, it can be observed that in the presence ofIPA, the rate of weight loss in the first step is higher than that forunfilled PU foam due to the decomposition of phosphinate in this

-0.20 200 400 600 800 1000

Temperature (°C)

Fig. 3. Derivative TGA curves under air atmosphere for unfilled PU foam (Ref), IPAfilled PU foam (IPA) and PU nanocomposite foams (IPA–OMLS, IPA–MMT and IPA–DP).

Table 2TGA and DTG results under air atmosphere for unfilled PU foam (Ref), IPA filled PUfoam (IPA) and PU nanocomposite foams (IPA–OMLS, IPA–MMT and IPA–DP)

T at 5 wt%loss [�C]

Tmax Residue at800 �C [%]

1st step [�C] 2nd step [�C]

Ref 255 301 562 0.4IPA 259 303 558 6.7IPA–MMT 268 313 550 17.4IPA–OMLS 269 313 550 17.2IPA–DP 272 318 562 13.6

Table 3Limiting oxygen index for unfilled PU foam (Ref), IPA filled PU foam (IPA) and PUnanocomposite foams (IPA–OMLS, IPA–MMT and IPA–DP)

Sample LOI [%]

Ref 20.6� 0.5IPA 25.6� 0.5IPA–MMT 25.7� 0.5IPA–OMLS 26.5� 0.5IPA–DP 26.7� 0.5

M. Modesti et al. / Polymer Degradation and Stability 93 (2008) 2166–2171 2169

temperature range; however, when nanoclays are also used inconjunction with IPA, the rate of weight loss decreases andbecomes similar to the reference one, the temperature of themaximum rate of weight loss increases and the weight loss in thisstep is decreased. Considering the second step of weight loss, norelevant variations may be seen for different filled foams: for allfilled foams the second step weight loss is reduced with respect tounfilled PU foam, due to the char formation deriving from bothclays and phosphinate. Therefore, although OMLS is betterdispersed than MMT or DP (as just showed above), no significanteffect of the dispersion degree was shown on the onset decompo-sition and the weight loss pathway.

The stabilizing effect on polymer of the clays is furtherconfirmed by analysing the residue at high temperature (Table 2):synergistic effect may be inferred because the residue for foamscontaining both phosphinate and clays is higher than that inadditive case, as can be inferred from TGA curves of pure layeredsilicates (Fig. 4). Furthermore, considering that OMLS and MMTfilled foams have the same residue at 800 �C while the clayresidue is fairly different, it can be stated that the better claydispersion achieved for OMLS is more effective in promoting charformation.

3.3. Fire behaviour

The limiting oxygen index of PU foams is reported in Table 3. Theuse of phosphinate is very effective in enhancing LOI, thus showingthe flame retardant action of IPA in polyurethane system. Furtheraddition of unmodified layered silicate does not lead to any varia-tion, while the use of OMLS or DP causes a slight increase in LOI fordifferent reason. OMLS is better dispersed than MMT (as confirmed

60

70

80

90

100

110

Weig

ht (%

)

0 200 400 600 800 1000

Temperature (°C)

� DP–––––––� OMLS–––––––� MMT–––––––

Fig. 4. TGA curves under air atmosphere for clay (DP, MMT, OMLS).

by XRD) and thus probably in this case a more effective barrier layeris formed, which prevents polymer degradation. For DP theincrease of LOI may be due to a flame retardancy action of phos-phonium or its derivative such as phosphine oxide present in theclay.

The fire behaviour was also characterised by cone calorimetry. Inrecent years, dramatic improvement in cone calorimetry perfor-mance has been demonstrated for a variety of polymer nano-composites (e.g. nylon, polypropylene, ethylene–vinyl acetate),compared to virgin polymers; generally the major effect observedfor polymer nanocomposites is a drastic reduction in the peak ofheat release rate (PHRR) which is associated with the fire growth[29].

The heat release rate (HRR) curves for unfilled PU foam as wellas IPA and nanocomposite foams are reported in Fig. 5, other dataare reported in Table 4. These results are useful in order to study theflame retardancy of phosphinate as well as different clays. The claysused may have very different effect on fire behaviour of polymers,since one clay is unmodified (MMT), one clay is ammoniummodified and may be tethered to the polymer [28] throughhydroxyl group (OMLS) and the last one may have further flameretardancy effect, with respect to other clays, because of thephosphorus content (DP).

Cone calorimetric data show that the TTI (time to ignition) isalways very low (1 s) for all samples tested because of the cellularstructure of the material and the high radiant heat flux (50 kW/m2).The total mass loss (TML), or equivalently, the char yield, is fairlydifferent for the samples analysed. The residue for IPA foam is lowerthan expected, i.e. not all the IPA stays in solid phase, while thebehaviour for nanocomposites is more complex. For MMT filledfoam, the polymer residue is in agreement with the clay residue athigh temperature inferred from TGA analyses; data for OMLS showa higher char yield than the expected one, thus indicating that suchclay favours charring reaction of polymer; finally for DP filled foam,the residue is lower thus indicating that such clay has some gas-phase action.

0

50

100

150

200

250

0 50 100 150 200 250 300

Time [s]

HR

R [kW

/m

2]

REFIPA-OMLSIPA-MMTIPAIPA-DP

Fig. 5. HRR curves for unfilled PU foam (Ref), IPA filled PU foam (IPA) and PU nano-composite foams (IPA–OMLS, IPA–MMT and IPA–DP).

Table 4Cone calorimeter data for unfilled PU foam (Ref), IPA filled PU foam (IPA) and PUnanocomposite foams (IPA–OMLS, IPA–MMT and IPA–DP)

PHRR [kW/m2] THE [MJ/m2] TSR

Ref 217 26.786 14,442IPA 148 23.324 14,801IPA–OMLS 158 23.325 13,750IPA–MMT 169 22.989 12,921IPA–DP 139 21.910 13,824

THE/TML [kJ/m2g] TSR/TML [g�1] Char yield %

Ref 2076 1120 22.8IPA 1952 1239 28.0IPA–OMLS 1994 1175 33.5IPA–MMT 2034 1143 31.5IPA–DP 1857 1189 28.7

M. Modesti et al. / Polymer Degradation and Stability 93 (2008) 2166–21712170

The HRR plots obtained are typical of thick charring materials[33], i.e. the sample shows an initial increase in HRR until an effi-cient char layer is formed; a second peak in the HRR curve may arisewhen formation of cracks in the char layer takes place. The mostevident feature of HRR curves for filled foams is the dramaticdecrease of PHRR when IPA was added to PU foam, while furtheraddition of nanoclays did not lead to further reduction of PHRR,except when phosphonium modified clay (DP) was employedbecause of a possible flame retardancy action of phosphonium. Theadvantage of use of nanoclays appears in terms of reduction of thesecond peak of HRR (at about 150 s) as already observed by Bour-bigot et al. [13] for PA-6 flame retarded nanocomposites. Thelowering of the first PHRR can be assigned to the formation ofa protective charred layer, due to IPA presence, which is effective asheat and mass barrier, while the reduction of second PHRR,occurring when nanoclays are used, is related to the enhancementof integrity of the clay-reinforced char layer. The higher compact-ness of clay-reinforced char layer was proved by means of SEM(scanning electron microscope): char formed after cone calorimetertest for IPA filled foam (Fig. 6a) clearly shows a ‘‘spongy’’ structureand several holes, which are not visible when analysing charformed in IPA–clay filled foams (Fig. 6b).

The total heat evolved (THE, the integral of HRR over the wholetime of the test) which depends on the total mass loss (TML), theeffective heat of combustion and the combustion efficiency in theflame zone, shows a trend similar to HRR. It is well known that THEfor polymeric nanocomposite with respect to neat polymer isgenerally unchanged, but researchers are actually searching forsynergistic effect between traditional flame retardants and clay.Some interesting results were reported for intumescent thermo-plastic systems [30] while antisynergistic effects were proved forepoxy resins [31]. Considering the results reported here, it can beseen that use of phosphinate leads to a sensible decrease of THE(13%) while its use in conjunction to clays causes no reduction of

Fig. 6. SEM images of char layer formed after cone calo

this parameters. Only when DP clay is employed, a further reduc-tion of THE is observed, which is due to phosphorus contained inthe clay. The total heat evolved per total mass loss (THE/TML) isa measure of the heat of combustion of volatiles (which is theproduct of the effective heat of combustion of volatiles multipliedby the combustion efficiency during the cone calorimeter test [32]).Its reduction indicated a flame inhibition or fuel dilution effect [33].For PU filled foams, THE/TML decreases for both phosphinate andphosphonium containing polymers, thus indicating a gas-phasemechanism of flame retardancy. In order to study the gas-phasemechanism, the total smoke release (TSR) and the ratio TSR/TMLhave to be considered. The results reported indicate that theseparameters greatly increase for IPA filled foam, showing thena radical trapping mechanism which leads to increase in the extentof incomplete combustion and thus on smoke density [32].Otherwise, when IPA was used in conjunction with nanoclays, theTSR/TML decreases, thus showing the presence of barrier former[34], represented by the clay. Due to phosphonium, TSR/TML ratiofor DP containing foams is higher than that of MMT or OMLS filledfoams.

The results just reported may be explained considering differentflame retardancy action of phosphinate, phosphine oxide (derivingfor DP from phosphonium decomposition [26, 27]) and layeredsilicates; these results are in agreement with those already repor-ted by Braun et al. [34] for epoxy resins.

During degradation, phosphinate develops the correspondingphosphorus containing acid which catalyses dehydration reactionsleading to an accumulation of char in the solid phase. Phosphinatemay also act in gas phase, since it is mainly vaporised prior todecomposition; however its gas-phase action is lower than that ofphosphine oxide as this gas-phase effect increases with decreasingoxidation state of phosphorus, and then it is lower than that ofphosphine oxide [32]. The action of phosphinate in both gas andsolid phases may be argued from the char yield: only a part of IPAremains in the solid residue at the end of combustion, in fact theincrease in residue for IPA filled foam, with respect to neat PU, islower than the neat weight of additive. Perhaps, during thedegradation of IPA, there are competing reactions of decompositionto aluminium phosphate in solid phase and vaporisation of phos-phinate in gas phase.

The fire retardant action of the silicates analysed is verydifferent. Considering MMT and OMLS, a physical effect, rather thanthe chemical one, is responsible for potential benefits observed forlayered silicate: THE is not significantly influenced and someincreases in char yield were observed. These features indicatea barrier effect and that MMT and OMLS silicates behave like inertfiller, promoting the formation of a very compact barrier layer at thesurface which is effective in preventing or lowering down thediffusion of volatiles to gas phase or counter diffusion of oxygentoward polymer. This barrier prolongs burning times without

rimeter test for IPA and IPA–MMT filled PU foams.

M. Modesti et al. / Polymer Degradation and Stability 93 (2008) 2166–2171 2171

decreasing the total amount of combustible material. It was alsoobserved that the char yield for OMLS filled foam is higher than theexpected, thus indicating that this clay favours the charring reac-tion of polymer; nevertheless, the higher char yield is not relevantto reduce THE. The higher residue may be due to the betterdispersion degree of such clay with respect to the other ones (asconfirmed by XRD) or to the tethering effect, since OMLS may bechemically bonded to polyurethane foam [28]. Moreover, thepresence of barrier effect reduces the effectiveness of IPA withrespect to flame inhibition, in a similar way to what already shownby Braun et al. for PA 6,6 [34]. With respect to IPA filled foam, theTHE/TML ratios are higher when IPA is used in conjunction withMMT or OMLS, i.e. flame retardancy of IPA is partly inhibited, whilewhen DP is used something different occurs, due to the presence ofphosphonium which releases phosphine oxide during decomposi-tion. DP releases in fact part of the phosphorus in the gas phase,thus having different flame retardancy effect with respect to othersilicates. It was shown that phosphine oxide has gas-phase actionthrough radical trapping mechanism [32], as confirmed also bycone calorimetry data reported here: THE/TML for IPA and DP filledfoam is fairly lower than that for IPA alone filled foam and TSR/TMLis fairly higher than other nanocomposite foams. This means thatphosphine oxide deriving from phosphonium is active in gas phase.Obviously, also DP shows barrier effect, typical of layered silicates.

4. Conclusions

In this work, potential synergy between phosphinate flameretardant and nanocomposites has been studied. In particular itwas shown that phosphinate is an effective flame retardant for PUfoams and it acts in both gas and solid phases. Commonly availablelayered silicates, both unmodified and organically modified ones,act through a physical rather than a chemical mechanism as theyact in condensed phase behaving like inert filler and promoting theformation of a very compact barrier layer. Thus, they are effective indelaying the onset of thermo-oxidative degradation and improvingthermal stability to some extent, but they show no synergy withflame retardant when dealing with fire behaviour.

Only a suitably developed layered silicate, modified withphosphonium, shows both condensed and gas-phase actions,showing, like phosphinate, a radical trapping mechanism. Thesystem phosphinate and phosphonium modified clay shows thena synergy which is very effective in improving fire behaviour of

polyurethane foams; moreover this system also shows delayingeffect on thermo-oxidative degradation of the polymer.

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