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Polymer Degradation and Stability 94 (2009) 2241–2250

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Polymer Degradation and Stability

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

Nanoclay synergy in flame retarded/glass fibre reinforced polyamide 6

Nihat Ali Isitman a, Huseyin Ozgur Gunduz b, Cevdet Kaynak a,b,*

a Metallurgical and Materials Engineering Department, Middle East Technical University, TR-06531 Ankara, Turkeyb Department of Polymer Science and Technology, Middle East Technical University, Ankara, Turkey

a r t i c l e i n f o

Article history:Received 29 April 2009Received in revised form21 August 2009Accepted 23 August 2009Available online 29 August 2009

Keywords:Flame retardancyNanocompositeSynergyPhosphorus compoundsPolyamide

* Corresponding author at: Metallurgical and MaterMiddle East Technical University, TR-06531 Ankara, Tufax: þ90 312 210 2518.

E-mail address: ckaynak@metu.edu.tr (C. Kaynak)

0141-3910/$ – see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.polymdegradstab.2009.08.010

a b s t r a c t

Exfoliated clay nanocomposites of flame retarded/glass fibre reinforced polyamide 6 were prepared bytwin-screw extrusion compounding. A flame retardant system based on phosphorus compounds andzinc borate was used at various levels in glass fibre reinforced PA6 and nanocomposites. Thermal stabilityand combustion behaviours were evaluated by TGA, LOI, UL94 and cone calorimetry. Substitution ofa certain fraction of the flame retardant with nanoclays was found to significantly reduce the peak heatrelease rate and delay ignition in the cone calorimeter. Moreover, remarkable improvements wereobtained in LOI along with maintained UL94 ratings. Residue characterization by FTIR, XRD and SEMascribed the enhanced flame retardancy of nanocomposite formulations to the formation of a glassyboron/aluminium phosphate barrier reinforced by clay layers at the nanoscale. The physically strong andconsolidated barriers formed from nanocomposites were much more effective in impeding heat andmass transfer compared to those from conventional formulations.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Polymer-clay nanocomposites are known to possess improvedflame retardancy with an ascribed mechanism of formation ofa physical barrier which effectively impedes mass and heat transfer.The peak heat release rate (PHRR) in a cone calorimeter was foundto be significantly reduced in polyamide 6 (PA6) nanocompositesvia the enrichment of clay layers on the flaming surface [1]. Upondegradation of the clay organic modifier and polymer pyrolysis,clay layers collapse onto each other forming a barrier. The protec-tive barrier layer insulates the underlying polymer both from theexternal radiative heat and thermal feedback of the flame, andslows down the escape of flammable volatiles.

In opposition to the well-established substantial reductionsobtained in peak heat release rate under forced flaming conecalorimeter, flame retardancy obtained with polymer-clay nano-composites in terms of limiting oxygen index (LOI) and classifica-tions of the UL94 standard is still debatable [2–4]. The currentunderstanding is that nanoclays should be used in combinationwith conventional flame retardants where they act as charenhancer and stabilizer [5–7].

ials Engineering Department,rkey. Tel.: þ90 312 210 5920;

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ll rights reserved.

Of particular interest are recent studies investigating thesynergistic effect of nanoclays in flame retarded PA6. Bourbigotet al. [3] showed that LOI is remarkably increased and V-0 isreached in UL94 with the partial substitution of organophosphorusflame retardant with nanoclays. It was shown in another study thatwhen nanoclay and zinc borate are used in combination, a well-blown stable char structure is formed which results in the reduc-tion of PHRR by 70%, much larger than those brought by nanoclay orzinc borate alone [8]. Song et al. [9] prepared halogen free flameretarded PA6 nanocomposites that contain magnesium hydroxideand red phosphorus in combination with nanoclay. Substitution of2 wt% magnesium hydroxide with nanoclay was found to improveLOI from 29% to 31%, reduce the PHRR by 33% and result in a morestable char. Nanoclays were found to impart synergistic flameretardancy in PA6 containing decabromodiphenyloxide and anti-mony trioxide, further decreasing the PHRR and allowing for a UL94V-0 rating [10]. However, antagonism was also reported severaltimes when nanoclays are used in combination with melaminecyanurate which was related to the inhibition of dripping [10–12].

Polymer-clay nanocomposites often exhibit remarkablyenhanced mechanical properties [13–15]. The key to obtain effec-tive strengthening is to exfoliate and disperse the high-aspect ratio,high-modulus clay layers at the nanoscale. Molecular level entan-glements are formed between polymer chains and clay layers sincethe interlayer separation is comparable to the polymer’s radius ofgyration. These molecular level interactions serve for an efficientstress transfer from matrix to reinforcement. Through theimpressive load bearing capacity of exfoliated clay layers and

N.A. Isitman et al. / Polymer Degradation and Stability 94 (2009) 2241–22502242

satisfactory stress transfer, effective strengthening is obtained withpolymer nanocomposites having low clay loadings.

Studies on the use of nanoclays and short glass fibres together asreinforcements in thermoplastic matrices are quite limited at thetime [16–18]. And, there seems to be no publications concernedwith the synergistic flame retardancy effect of nanoclays on glassfibre reinforced PA6 with conventional flame retardants. The aim ofthe current study is to obtain enhanced flame retardancy bypreparing clay nanocomposites of flame retarded/glass fibre rein-forced PA6.

Nanocomposite morphology was characterized by TransmissionElectron Microscopy (TEM) and X-Ray Diffraction (XRD). Flamma-bility was evaluated by Limiting Oxygen Index (LOI) tests andclassifications of the UL94 standard. Bench-scale fire performanceassessment was done by Mass Loss Cone Calorimetry (MLC). Flameretardancy mechanisms were investigated using Thermogravi-metric Analysis (TGA), and by conducting Attenuated Total Reflec-tance – Fourier-Transform Infrared Spectroscopy (ATR-FTIR), X-RayDiffraction and Scanning Electron Microscopy (SEM) on solid fireresidues.

2. Experimental

2.1. Materials and specimen preparation

Materials were prepared by melt-mixing with a twin-screwextruder (Thermoprism TSE 16 TC, L/D:24) using PA6 (DSM Aku-lon), short glass fibres (Camelyaf PA1), nanoclay (Southern ClayCloisite 30B) and organophosphorus flame retardant (ClariantExolit OP1312). The organophosphorus flame retardant is a multi-component, synergistic mixture of aluminium diethylphosphinate,melamine polyphosphate and zinc borate. Compositions anddesignations of the studied materials are listed in Table 1.

Specimens for tensile and flammability testing (LOI and UL94)were produced by a lab-scale injection moulding device (DSMmicro-injector) with barrel and mould temperatures of 240 �C and80 �C. Cone Calorimeter studies were performed on compressionmoulded samples having dimensions of 100 mm� 100 mm� 4 mm.

2.2. Cone calorimeter, flammability testing and thermogravimetry

Heat release rates were determined following the procedureexplained in ISO 13927 using a Mass Loss Cone Calorimeter withattached chimney and thermopile (Fire Testing Technology, FTT).Heat release and mass loss rates from samples were measuredunder an external heat flux of 35 kW/m2 and data were recordedusing a data-acquisition system. Flammabilities were assessed bylimiting oxygen index measurements according to ISO 4589 on anFTT Oxygen index apparatus and classifications of the UL94

Table 1Compositions of the studied materials in wt%.

Materialdesignation

Short glassfibre (GF)

Nanoclaya (n) Flameretardant (OP)

PA6 – – –PA6/OP20 – – 20PA6/GF 15 – –PA6/GF-OP20 15 – 20PA6/GF-OP15 15 – 15PA6/GF-OP10 15 – 10n-PA6 – 5 –n-PA6/GF 15 5 –n-PA6/GF-OP15 15 5 15n-PA6/GF-OP10 15 5 10n-PA6/GF-OP5 15 5 5

a Nanoclay content is the nominal value instead of silicate basis.

standard (at 3.2 mm thickness) on a custom flammability meter.Thermogravimetric analyses were performed using a TG/DTAinstrument (Shimadzu DTG60H). Samples of about 15 mg weightwere heated from room temperature to 1173 K with a heating rateof 10 K/min under flowing nitrogen at 50 ml/min.

2.3. Electron Microscopy, X-Ray Diffraction and Fourier-transformInfrared Spectroscopy

Sections of 50 nm nominal thickness were ultramicrotomedusing a Leica EM UC 6 with a diamond knife operated at roomtemperature. Transmission Electron Microscopy was conductedwith a FEI TECNAI G2 F30 at an acceleration voltage of 100 kV.Scanning Electron Microscopy was done using JEOL JSM 6400 onfracture surfaces of injection moulded samples. Barrier morphol-ogies of char residues, which directly influence their ability toimpede heat and mass transfer, were also characterized by SEM.Energy-dispersive X-ray spectroscopy (EDS) showed that the flameretardant contains 10 wt% P and 17 wt% N together with certainamounts of Zn and Al. It is a synergistic combination of active flameretarding species of aluminium phosphinate, melamine poly-phosphate and zinc borate. All char residues were characterized bySEM-EDS which revealed the presence of Al and P in large fractionsas residue constituents.

X-Ray Diffraction was performed by Rigaku D-Max 2200 withCuKa radiation. The organoclay and injection moulded nano-composite samples were scanned over 1�<2q < 10� range. Residuesfrom the cone calorimeter tests were ground and powder diffrac-tions were taken over 1�<2q < 10� and 10�<2q < 80� ranges.Residue characterization was done by Attenuated Total Reflectance –Fourier-Transform Infrared Spectroscopy using a Bruker OpticsTensor Series FTIR Spectrometer at an optical resolution of 4 cm�1.

3. Results and discussion

3.1. Nanocomposite morphology

Morphological analyses of clay nanocomposites based on PA6were made by TEM and XRD.

Low magnification TEM image displayed in Fig. 1(a) reveals thehomogenous dispersion and exfoliation of nanoclay in PA6 matrix.Fig. 1(b) is a higher magnification image showing exfoliated indi-vidual clay layers preferentially oriented in the flow direction ofinjection moulding. Disrupted registry of silicate layers result in theabsence of basal reflections in XRD (Fig. 2). This suggests exfoliatedmorphologies for all specimens containing nanoclays. Exfoliation isthermodynamically favoured by the large enthalpic contribution ofthe polar and hydrogen bonding interactions between clay surfacemodifier (methyl tallow bis-2-hydroxyethyl ammonium) and PA6chains [19]. Presence of the flame retardant and/or short glassfibres has no influence on the dispersability of nanoclays, asassessed by XRD.

3.2. Thermogravimetry

All compositions possess a single decomposition step in ther-mogravimetric analysis shown in Fig. 3(a) and (b). The mainbreakdown of PA6 results in release of water, ammonia, carbonmonoxide, carbon dioxide and hydrocarbon fragments [20].Although differential thermogravimetry (DTG) peak temperatures(TDTG-Peak) of PA6/GF and n-PA6/GF are almost the same, 10% massloss temperature (T10%) is lowered by 8 K (Table 2). Additionally,n-PA6/GF-OP15 possesses about 7 K and 28 K lower temperaturescompared to PA6/GF-OP15 in terms of TDTG-Peak and T10%, respec-tively. Lower temperatures corresponding to maximal rate of

Fig. 1. TEM bright field images of PA6/clay nanocomposite with (a) low magnification, and (b) high magnification.

N.A. Isitman et al. / Polymer Degradation and Stability 94 (2009) 2241–2250 2243

decomposition and 10% mass loss are attributed to the catalyticactivity in presence of large surface area clay layers with protonicsites [9,21,22]. TDTG-Peak and T10% for PA6/GF-OP15 are 30 K and 17 Klower, respectively, compared to PA6/GF as a consequence ofdecomposition of flame retardant constituents. Decompositionof melamine polyphosphate at around 620 K [23] and evolution ofphosphinates and phosphinic acids into the vapour phase [24]result in lower degradation temperatures. In addition, it is clearfrom TDTG-Peak and T10% for n-PA6/GF with varying OP content thatthe flame retardant shifts the degradation temperatures progres-sively to smaller values, material with the highest flame retardantcontent being the earliest to degrade.

Comparing the DTG curves for PA6/GF and PA6/GF-OP15(Fig. 3(b)), flame retardant decreases the maximal rate of mass lossby interfering with the main decomposition of PA6 with degradingflame retardant constituents. Formation of flame retarded nano-composite (n-PA6/GF-OP15) further suppresses the peak of massloss rate for PA6/GF-OP15. Similarly, lower mass loss rates areobtained with n-PA6/GF compared to PA6/GF. These are attributedto the retardation of volatile evolution by means of the tortuouspathway formed by exfoliated, large aspect ratio clay layers.

It is shown in Table 2 that PA6/GF yields a residue of 13.6%corresponding solely to glass fibres. In the case of PA6/GF-OP15,

2θ (deg.)1 2 3 4 5 6 7 8

Inte

nsity

(a.u

.)

(f)

(e)

(d)

(c)

(b)

(a)

Fig. 2. XRD patterns for (a) organoclay, (b) n-PA6, (c) n-PA6/GF, (d) n-PA6/GF-OP15,(e) n-PA6/GF-OP10, and (f) n-PA6/GF-OP5.

char residue upon heating is 18.8 wt% while total concentration ofadditives is 30 wt%. Therefore it is concluded that a significantfraction of the flame retardant volatilizes and acts in the gas phase.However, it should be noted that primary flame retardancymechanism for organophosphorus flame retardant was stated to bea condensed phase action [25,26].

Fig. 3. (a) Thermogravimetric and (b) differential thermogravimetric curves of theinvestigated samples.

Table 2Results of the thermogravimetric analysis.

Specimen TDTG-Peak (K) T10% (K) % Residue

PA6/GF 728 689 13.6PA6/GF-OP15 698 672 18.8n-PA6/GF 728 681 19.6n-PA6/GF-OP5 723 670 21.6n-PA6/GF-OP10 708 657 22.2n-PA6/GF-OP15 691 644 25.1

N.A. Isitman et al. / Polymer Degradation and Stability 94 (2009) 2241–22502244

For n-PA6/GF, considering the weight fractions of nanoclay onsilicate basis (w3.5 wt%) and glass fibre residue from the referencemeasurement of PA6/GF (13.6 wt%), char yield of 19.6 wt% is higherthan the total inorganic filler content. This is indicative of certainpolymer fraction being retained as a result of charring induced inpresence of nanoclay. Particularly, comparing PA6/GF-OP15 andn-PA6/GF-OP15, 6.3 wt% higher char yield is obtained with theflame retarded nanocomposite recalling that the inorganic residuefrom nanoclay is approximately 3.5 wt%. In fact, for materials withsame nominal filler content, i.e. PA6/GF-OP15 and n-PA6/GF-OP10,char yield is still higher with nanocomposites. Further investiga-tions made by ATR-FTIR on char residues revealed the underlyingreasons behind increased amounts of char residues with nano-composites of both PA6/GF and PA6/GF-OP formulations. Lastly,considering n-PA6/GF-OP with different flame retardant contents(Table 2), it is clearly seen that the amount of char residue increaseswith the flame retardant content.

3.3. Flammability

Results of oxygen index and UL94 tests are given in Table 3. LOIis increased noticeably from 24.9% to 31.7% and a UL94 V-0 rating iseasily attained with the incorporation of the flame retardant inunreinforced PA6. It is seen that PA6/GF has a considerably lowerLOI compared to neat PA6 due to the candlewick effect which iswell-established in the literature [27,28]. Short glass fibres, havinga loading level of 15 wt% in the composite, are sufficient to stabilizethe melt in oxygen index test whereas dripping can not be pre-vented in UL94 test during which the flammable residue fell off andthe V-2 rating of PA6 is retained.

LOI is progressively improved with increasing OP content inflame retarded/glass fibre reinforced PA6’s, possessing a jump from24.9% to 29.3% when the flame retardant concentration is changedfrom 10 wt% to 15 wt%. Considering flame retarded/glass fibrereinforced PA6’s, V-0 rating is obtained in UL94, except for thelowest concentration of the flame retardant utilized here, i.e.10 wt%.

Glass fibre reinforced nanocomposite without the flame retar-dant shows only a modest enhancement in LOI compared to PA6/GF. Dripping of glass fibre reinforced PA6 is inhibited by nano-composite formation, thus, the material fails from UL94 verticalburning test. The disappointing results in LOI and UL94 have been

Table 3Results of the flammability tests.

Specimen LOI UL-94

PA6 24.9 � 0.4 V-2PA6/OP20 31.7 � 0.6 V-0PA6/GF 21.3 � 0.2 V-2PA6/GF-OP10 24.9 � 0.2 V-1PA6/GF-OP15 29.3 � 0.4 V-0PA6/GF-OP20 30.2 � 0.6 V-0n-PA6/GF 22.4 � 0.3 Failn-PA6/GF-OP5 29.1 � 0.2 V-1n-PA6/GF-OP10 30.9 � 0.4 V-0n-PA6/GF-OP15 31.7 � 0.7 V-0

the discussion of a number of studies investigating the flamma-bility of nanocomposites without conventional flame retardants[2,10].

Flame retarded/glass fibre reinforced nanocomposites haveimpressive LOI values even at a very low concentration of the flameretardant (5 wt%). Comparing PA6/GF-OP10 and n-PA6/GF-OP5having nominally the same filler concentration of 10 wt% (apartfrom glass fibre reinforcement), LOI of the nanocomposite (29.1%) isunambiguously superior to that of the conventional composite(24.9%). However, the UL94 classification of V-1 obtained by PA6/GF-OP10 is not improved by nanocomposite formation in n-PA6/GF-OP5 due to the stringent ignition conditions and geometry ofthe UL94 test. To generalize, when 5 wt% of the flame retardant isreplaced by the same amount of nanoclays, e.g. for PA6/GF-OP10and n-PA6/GF-OP5, improvements in LOI are substantial for allthree concentrations of the flame retardant utilized in this study.UL94 classifications of the flame retarded/glass fibre reinforcedmaterials are maintained as either V-1 or V-0 with the mentionedreplacements of 5 wt% of flame retardant with nanoclay, in spite ofthe degraded ratings reported previously [10,29].

3.4. Cone calorimeter

Fig. 4(a) displays the heat release rate (HRR) curves for PA6reinforced at micro-/nano-scales, i.e. with short glass fibres andnanoclays, and flame retarded PA6. PA6/GF possesses extendedtime of burning and a lower PHRR through slowing down of

Fig. 4. (a) Heat release rates and (b) mass loss rates for reinforced (both at the micro-and nanoscale) and flame retarded PA6.

Fig. 5. Heat release rate curves for flame retarded/glass fibre reinforced PA6 andnanocomposites.

N.A. Isitman et al. / Polymer Degradation and Stability 94 (2009) 2241–2250 2245

polymer transport into the pyrolysis zone. Formation of claynanocomposites of PA6/GF reduces PHRR and increases burningtime. With the incorporation of an active flame retardant in PA6,largest suppression in PHRR together with longest burning time isachieved (PA6/OP20). Longer burning times were previouslyreported when charring is promoted [8]. Mass loss rate (MLR)curves obtained in cone calorimeter, displayed in Fig. 4(b), showsthat for PA6 and PA6/GF, shapes of HRR and MLR curves are similaras an indication of a condensed phase action of glass fibres.

15 20

Peak

Hea

t Rel

ease

Rat

e (k

W/m

2 )

100

200

300

400

500

0 5 10

10 15 20

Tim

e to

Igni

tion

(s)

0

50

100

150

200

0 5

Flame Retardant (wt%)

Flame Retardant (wt%)

PA6/GF-OPn-PA6/GF-OP

PA6/GF-OPn-PA6/GF-OP

a

c

Fig. 6. Cone calorimetric data as a function of the OP flame retardant content, (a) peak heat rlines indicate equal nominal filler contents, i.e. organoclay þ flame retardant.

Particularly for n-PA6/GF, MLR curves possess an initial increase inrate of mass loss followed by a smooth gradual decrease untilextinction by the act of the established barrier. More importantly,even though n-PA6/GF and PA6/OP20 have similar mass loss rates,heat release rates are remarkably different due to the fact that PA6/OP20 shows a gas phase action.

It is inferred from Fig. 5 that burning characteristics of PA6/GFare altered such that PHRR is largely suppressed and burning timeis extended with the incorporation of the flame retardant (PA6/GF-OP15). The clay nanocomposite, n-PA6/GF-OP10, provides furtherextension of burning time compared to PA6/GF-OP15 owing to theformation of a thicker char. In fact, the initial increase of heatrelease rate followed by a steady burning plateau possessed byn-PA6/GF-OP10 is a reflection of thick charring on the heat releasebehaviour [30]. At early stages of combustion, the material burnswith the highest rate of heat release under the forced flamingconditions of cone calorimeter. Then, a plateau of lower heatrelease rate is formed upon the establishment of the protectivebarrier layer slowing down the escape of flammable volatiles.

Fig. 6 shows the evaluated cone calorimetric performances ofthe studied materials. Fig. 6(a) shows that formation of PA6/GFnanocomposite suppresses the peak heat release rate by 40% owingto the carbonaceous barrier formed on the exposed surface of thespecimen by migration and accumulation of clay layers dispersed atthe nanoscale. Considering PA6/GF-OP formulations, PHRR isreduced in proportion to the flame retardant contents of 10 wt%,15 wt%, and 20 wt% by 52%, 58% and 62%, respectively, compared toPA6/GF. In respect of nanoclay containing materials, similarreductions are obtained with flame retardant contents of 5 wt%,10 wt% and 15 wt% by 36%, 41%, 45%, respectively, compared to

Flame Retardant (wt%)10 15 20

Tota

l Hea

t Evo

lved

(MJ/

m2 )

80

100

120

140

160

0 5

10 15 20

Fire

Gro

wth

Inde

x ( P

HR

R /

t ign)

0

1

2

3

4

5

6

0 5

Flame Retardant (wt%)

PA6/GF-OPn-PA6/GF-OP

PA6/GF-OPn-PA6/GF-OP

b

d

elease rate, (b) total heat evolved, (c) time to ignition, and (d) fire growth index. Dashed

Wavenumber (cm-1)

8001000120014001600

Abso

rban

ce

(a)

(b)

(c)

930

10201080

1600

1400 680

Fig. 7. ATR-FTIR spectra of combustion residues from materials without nanoclay, (a)PA6/GF, (b) PA6/OP20, (c) PA6/GF-OP15.

N.A. Isitman et al. / Polymer Degradation and Stability 94 (2009) 2241–22502246

n-PA6/GF. When PA6/GF-OP and n-PA6/GF-OP with same nominalfiller contents (dashed lines in Fig. 6(a)) are considered, furtherPHRR reductions of 20%, 16% and 13% could be attained withnanocomposite formulations in the order of increasing total fillercontent.

Comparing PA6/GF with the nanocomposite, total heat evolved(THE) values given in Fig. 6(b) show a minor reduction causedmainly by dilution of fuel via inorganic filler content, and stabili-zation of a certain PA6 fraction by catalytic charring. THE isremarkably lowered with the addition of the flame retardant inPA6/GF and n-PA6/GF. It can be inferred from Fig. 6(b) that THE ismore or less independent of flame retardant content with PA6/GF-OP formulations, whereas a progressive reduction is obtained asa result of char enhancement and strengthening in presence of thenanoclay. Furthermore, regarding materials with same nominalfiller content (dashed lines in Fig. 6(b)), increasing flame retardantloading leads to larger reductions in THE obtained by nano-composite formulations.

Fig. 6(c) shows that n-PA6/GF ignites earlier than PA6/GF asa result of acid catalyzed degradation of polyamides [31] viaprotonic sites formed within clay galleries upon degradation of theorganic modifier [9,21,22]. However, for n-PA6/GF-OP formulations,an opposite effect is observed such that strong and consolidatedchar impedes the evolution of flammable volatiles. Consequently,ignition is delayed in nanocomposites when materials with samenominal filler content are considered, indicated by dashed lines inFig. 6(c). Time to ignition (tign) values suggest that the flameretardant content imparts no alteration in ignitability for both PA6/GF-OP and n-PA6/GF-OP formulations.

It is followed from the work of Petrella [32] that a fire growthindex (FGI) defined by PHRR/tign makes a reasonable attempt atassessing the flame spread as a fire hazard. It can be inferred fromFig. 6(d) that substitution of 5 wt% flame retardant with nanoclayssignificantly lowers FGI owing to reduced PHRR and delayedignition.

It is well established that the mechanism of flame retardancy canbe addressed considering the ratio of total heat evolved to totalmass loss (THE/TML). This ratio allows for the deduction of avail-ability of gas phase action since it is a suitable measure for theeffective heat of combustion of volatiles [30,33]. THE/TML for PA6 isreduced from 4.1 MJ/m2 g to 2.9 MJ/m2 g with the addition of theflame retardant. PA6/GF gives a THE/TML value of 3.9 MJ/m2 g,which is reduced on the average to 2.8 with PA6/GF-OP formula-tions. Therefore, it is concluded that, in addition to a dominantcondensed phase action by means of barrier formation, flameinhibition and dilution are also present as gas phase flame retardingmechanisms. As a matter of fact, Braun et al. [24] have shown thatevolution of melamine, diethyl phosphinic acid and aluminium–zinc phosphinates constitute the gas phase action of OP.

Considering nanocomposites of PA6/GF-OP formulations, nosignificant alteration is observed in THE/TML values. The averagevalue of 2.8 MJ/m2 g for PA6/GF-OP becomes 2.6 MJ/m2 g on theaverage for n-PA6/GF-OP formulations, the change being in therange of experimental error margin. Hence, combining the reduc-tions in PHRR and unaltered THE/TML values, it is concluded thatthe condensed phase action is enhanced in the presence of nano-clays, mainly owing to the formation of a stronger boron/aluminium phosphate barrier reinforced at the nanoscale (asexplained in the following section).

3.5. Residue characterization

3.5.1. Infrared spectroscopyThe ATR-FTIR spectrum for PA6/GF residue (Fig. 7(a)) includes

a diffuse band around 930 cm�1 which corresponds to the

superposition of bridging oxygen in Si–O–Si and terminal Si–Ostretching vibrations [34]. The small band around 680 cm�1 isattributed to bending vibrations of Si–O–Si bridging oxygen atoms,and an additional small band around 1400 cm�1 corresponds alsoto glass fibres [24].

The spectra recorded from PA6/OP20 residue (Fig. 7(b)) involvea sharp peak at 1080 cm�1 and a shoulder around 1020 cm�1 whichare attributed to asymmetric stretching (v3) of PO4

3- in boronphosphates [35]. Small shoulder appearing near 960 arises fromsymmetric stretching vibrations (v1) of PO4

3�. Small signal at780 cm�1 matches with the most intense signal of neat organicphosphinate (characterized by reference measurements) whichindicates that certain amount of flame retardant remains withoutdecomposing. An additional broad peak appears at 1600 cm�1

which indicates the formation of an aromatic ring breathing modeor C]C of conjugated polyene structure. It was shown by Hollandand Hay [36] that it is practically impossible to identify whether theband at 1600 cm�1 corresponds to aromatic or conjugated species.Nonetheless, aromatic or conjugated species indicates a partialstabilization of polymer degradation products in the condensedphase by the act of OP flame retardant. Residue from PA6/GF-OP15(Fig. 7(c)) contains an additional shoulder around 930 cm�1 char-acteristic of glass fibres when compared with the residue of PA6/OP20 (Fig. 7(b)).

Fig. 8 shows the spectra of the residues from n-PA6/GF-OP15,n-PA6/GF, and PA6/GF. Absorption band for Si–O stretching inmontmorillonite is at 1030 cm�1 determined by measuring refer-ence spectra and from literature [37]. Spectrum for the residue ofn-PA6/GF, given in Fig. 8(b), contains a sharp peak around1030 cm�1 characteristic of montmorillonite and a shoulder around930 cm�1 corresponding to Si–O–Si and Si–O stretching in glassfibres. Similar to previous discussions regarding materials with nonanoclays, formation of a band at 1600 cm�1 indicates the presenceof aromatic or conjugated degradation products of nylon stabilizedin the residue.

Considering the residue of n-PA6/GF-OP15 (Fig. 8(c)), an addi-tional shoulder appears around 1115 cm�1 as an evidence for theformation of aluminium phosphates such as aluminium ortho-,pyro- and poly-phosphates [24]. Here, bands for phosphateabsorption in boron phosphates are masked from the strongabsorption of montmorillonite in nanoclay containing residue. Thisrestricts further conclusions on the composition of residue in terms

Fig. 9. XRD patterns showing the amorphous structure of the char residues from(a) n-PA6/GF-OP15, (b) PA6/GF-OP15, (c) PA6/OP20 and (d) n-PA6/GF.

2θ (deg.)2 4 6 8 10

Inte

nsity

(a.u

.)

(d)

(c)

(b)

(a)

13.0 Å

20.0 Å12.3 Å

37.0 Å

Organoclay: d001= 17.9 ÅNa-MMT: d001= 11.2 Å

Fig. 10. XRD patterns showing the clay nano-morphology in char residues from(a) n-PA6/GF, (b) n-PA6/GF-OP5, (c) n-PA6/GF-OP10, and (d) n-PA6/GF-OP15.

Wavenumber (cm-1)

8001000120014001600

Abso

rban

ce

1030

1115

930

680 (a)

(b)

(c)1600

1400

Fig. 8. ATR-FTIR spectra of combustion residues from materials with nanoclay, (a) PA6/GF, (b) n-PA6/GF, and (c) n-PA6/GF-OP15.

N.A. Isitman et al. / Polymer Degradation and Stability 94 (2009) 2241–2250 2247

of the relative amounts of aluminium and boron phosphates.Nevertheless, the nanoclay alters the reactivity of aluminiumphosphinate, melamine polyphosphate and zinc borate flameretarding species in such a way that aluminium phosphates arepreferentially formed in addition to boron phosphates contrary tothe materials without the nanoclay.

The mechanism for the observed phenomenon can be explainedby considering the well-known barrier effect of nano-dispersedclay layers. Diffusion of volatile species generated at elevatedtemperatures attained in cone test is restricted by the tortuouspathway formed by the clay layer network [38]. Of particularinterest to our case, retardation of gaseous aluminium phosphinateevolution by means of the labyrinth effect of exfoliated clay layersprovides that a larger portion of aluminium phosphinate reactswith the synergist flame retardant components to form aluminiumphosphates in the char residue.

Similarly, Braun et al. [24] showed through the utilization oflarge external heat fluxes in the cone test that vaporization ofaluminium phosphinate is minimized and aluminium phosphatesare formed in addition to boron phosphates. In particular, a boron/aluminium phosphate barrier was reported to act superior asa protective layer against heat and mass transport to that composedonly of aluminium phosphates.

3.5.2. Char structure and morphologyThe diffuse reflections in the XRD patterns given in Fig. 9 indi-

cate the formation of a char residue mainly possessing amorphousstructure. The diffractograms of char residues from nano-composites differ from those of conventional composites only inthat the pattern contains crystal reflections from layered silicates asexpected. Inorganic substances, such as aluminium and boronphosphates, may exist in crystalline state of long-range crystallineorder as well as in amorphous state with short range order confinedto polyhedral coordination. In order to allow for crystallization,crystalline transformation temperatures should be reached andprolonged times are required for compositional homogenization. Inaddition, relatively slow cooling is necessary to allow for crystalli-zation on cooling. However, in the case of fire tests, reachedtemperatures are low and the time allowed is short which results inthe formation of a glassy char residue.

Interestingly, clay basal reflections from the residues of n-PA6/GF and various n-PA6/GF-OP occur either at noticeably different

angles or the pattern is silent (Fig. 10). Basal spacing of clays in theresidue of glass fibre reinforced nanocomposite attains a value of13.0 Å as a result of collapse of clay layers upon the volatilization ofintra-gallery features, i.e. polymer and organic modifier. Recallingthat neat organoclay and Na-MMT have basal spacings of 17.9 Å and11.2 Å (from reference measurements), a certain amount of stableresidue from degradation of polyamide (aromatic or conjugatedspecies detected by ATR-FTIR) can be said to have remained in theintra-gallery region.

Basal spacings of clays in the residues from n-PA6/GF-OP5 andn-PA6/GF-OP10 are 20.0 Å (12.3 Å) and 37.0 Å, whereas for thehighest flame retardant content, i.e. n-PA6/GF-OP15, clay layerregistry is still absent resembling the original state of the nano-composite (Fig. 10). It is seen that basal spacing of clays in theresidue is either larger or no layer registry exists when an effectiveflame retardant that enhances strong charring is present in glassfibre reinforced nanocomposites. An intriguing reason could be theformation of a stable residue within the clay galleries as well as inthe extra-gallery region during flaming combustion. Consequently,a double network structure develops which is an interpenetrationof clay and boron/aluminium phosphate networks. The doublenetwork forming upon the explained phenomenon possesses

Fig. 11. Appearances of char residues (whole surfaces of 100 � 100 mm are displayed) formed upon combustion in cone calorimeter, (a) n-PA6/GF, (b) PA6/GF-OP15, (c) n-PA6/GF-OP5, and (d) n-PA6/GF-OP15.

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a consolidated, crack-free and mechanically strong structure owingto the reinforcement of the flame retarding barrier at the nanoscale.

The appearances of char residues serving as barriers during conecalorimetric fire tests are given in digital images (Fig. 11) and SEMmicrographs (Fig. 12). Residue from PA6/OP20 displayed inFig. 12(a) shows intumescence. Figs. 11(a) and 12(b) show that theresidue of n-PA6/GF possesses a loose structure suffering frommajor cracks. It is once again to be noted that improved conecalorimetric performance can be obtained with the incorporationof nanoclays into glass fibre reinforced polymer (as explained in theCone Calorimeter section). However, legitimate flame retardancy isknown to be obtained if a nanofiller and a conventional flameretardant is used in combination.

Residue from PA6/GF-OP15 displayed in Figs. 11(b) and 12(c)possess intumescent natures with the carbonaceous char beingthin and mechanically weak. It is clear that the residues reinforcedat the nanoscale (Figs. 11(c, d) and 12(d)) are much more effectivebarriers in impeding mass and heat transfer when compared toresidues without the nanoclay (Figs. 11b and 12c). Fig. 11(c–d)

Fig. 12. SEM micrographs displaying the char morphologies of (a) PA

reveals that the structure of the barrier is improved with increasingflame retardant content associated with the larger basal spacing ofclays in the residue. The strong nature of the chars formed fromflame retarded/glass fibre reinforced nanocomposites provides thesignificant improvements in flame retardancy as discussed above.

3.6. Mechanical properties

Flame retardant additives are generally incompatible with thehost polymer matrix resulting in discontinuities at particle/matrixinterface, i.e. debonding (Fig. 13). Strength of a particulate rein-forced polymer is determined by the decreased effective cross-sectional area of load bearing polymer matrix. Consequently, thereis a loss in tensile strength (s) with the incorporation of flameretardant into neat PA6 as shown in Fig. 14(a). In fact, decreasingtendency of strength in proportion to flame retardant content isobserved for glass fibre reinforced PA6 (Fig. 14(b)) and nano-composites (Fig. 14(c)). Additionally, the organophosphorus flameretardant used in the current study was also shown to deteriorate

6/OP20, (b) n-PA6/GF, (c) PA6/GF-OP15, and (d) n-PA6/GF-OP15.

Fig. 13. Debonding at particulate/matrix interface as a consequence of poor adhesionin PA6/OP20.

Flame Retardant (wt%)10 15 20

Tens

ile S

treng

th (M

Pa)

60

64

68

72

Youn

g’s

Mod

ulus

(GPa

)

1.6

2.0

2.4

2.8

3.2

Flame Retardant (wt%)

0 5

0 5 10 15

Tens

ile S

treng

th (M

Pa)

55

60

65

70

75

Youn

g’s

Mod

ulus

(GPa

)

2.6

2.8

3.0

3.2

PA6 PA6/OP20 n-PA6 PA6/GF n-PA6/GF

Tens

ile S

treng

th (M

Pa)

45

50

55

60

65

70

75

Youn

g’s

Mod

ulus

(GPa

)

1.8

2.0

2.2

2.4

2.6

2.8

3.0

E

σ

σ

σ

E

E

PA6/GF-OP

n-PA6/GF-OP

a

b

c

Fig. 14. (a) Effect of particulate flame retardant and reinforcing nano-/micro-fillers onthe mechanical properties, (b) mechanical properties of flame retarded/glass fibre

N.A. Isitman et al. / Polymer Degradation and Stability 94 (2009) 2241–2250 2249

the apparent interfacial strength between glass fibres and matrix inthe case of flame retarded/glass fibre reinforced PA6. Interestingly,presence of the reinforcement at nanoscale recovers the fibre/polymer interface which will be the discussion of our forthcomingstudy involving nanocomposites of flame retarded/glass fibrereinforced PA6 [39].

On the other hand, reinforcing fillers such as nanoclays and glassfibres here provide effective strengthening (Fig. 14(a)). Recalling themuch lower volume fraction of nanoclays compared to that of shortglass fibres, reinforcement at the nanoscale is much more effectivethan that at the micro-scale in terms of composite strength. This isattributed to an order of magnitude larger average aspect ratio ofclays compared to short glass fibres. Additionally, the molecularlevel entanglements between polymer chains and nanoclays of verylarge surface area ensure highly efficient stress transfer from matrixto reinforcement. Substantial improvement in tensile modulus (E)is obtained especially with glass fibre reinforced PA6 nano-composites in comparison to neat PA6.

Tensile moduli are improved progressively with the incorpora-tion of flame retardant additive aside from poor interfacial inter-actions (Fig. 14(b and c)). At low strains encountered by the matrixin linear elastic region of modulus measurements, the mechanismof stress transfer at the particulate/matrix interface is inoperative.As a matter of fact, elastic modulus of a particulate reinforcedcomposite was shown to be dictated by the volume fraction,particle size and stiffness of the filler whereas it is not noticeablyaffected by the interfacial strength between filler and matrix [40].

reinforced PA6 and (c) its nanocomposites as a function of flame retardant content.

4. Conclusions

Exfoliated morphologies of glass fibre reinforced and flameretarded PA6 nanocomposites were revealed by TEM and XRD. Charyields of flame retarded nanocomposites in TGA are highercompared to conventionally flame retarded formulations regardingmaterials with the same nominal filler content. It is explained bythe preferential formation of aluminium phosphates as well asboron phosphates by the act of clay layers dispersed at the nano-scale, revealed by ATR-FTIR. Degradation shifts to lower tempera-tures as a result of the catalytic activity of protonic sites within claygalleries considering nanocomposite formulations of glass fibrereinforced and flame retarded PA6. Evolution of volatiles is retardedby the action of the tortuous pathway formed by exfoliated claylayers, determined from mass loss rates in TGA.

Substitution of a certain fraction of the active organophos-phorus flame retardant by nanoclay provides significant peak heatrelease rate reductions and remarkably delayed ignition in the conecalorimeter. Hence, fire growth index, as an assessment of flamespread, is lowered. Additionally, substantial improvements in LOIare obtained with nanocomposites of flame retarded/glass fibrereinforced PA6 when the nominal filler content is kept constant.The improved flammability and fire properties are attributed to theformation of a strong and consolidated barrier owing to the rein-forcement of the char at nanoscale by clay layers.

The incorporation of organophosphorus flame retardant andnanoclay in glass fibre reinforced PA6 provided improvements inelastic modulus in proportion to the level of flame retardant in

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composites and nanocomposites. Conversely, tensile strength isreduced with the incorporation of the organophosphorus flameretardant as a consequence of poor interfacial interactions andconsequent debonding at particle/matrix interface.

Acknowledgements

Partial financial support of this research as part of the Project107M347 by TUBITAK, The Scientific and Technological ResearchCouncil of Turkey, is gratefully acknowledged.

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