Simplified structure of the condensed phase of fire retarded PA6 nanocomposites in TGA as related flammability

Fire Safety Journal 69 (2014) 69–75

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Simplified structure of the condensed phase of fire retarded PA6 nanocomposites in TGA as related flammability D. Bakirtzis, A. Ramani, J. Zhang, M.A. Delichatsios n Fire Dynamics and Materials Lab., Fire Safety Engineering Research & Technology Centre (FireSERT), School of the Built Environment, University of Ulster, Jordanstown campus, Shore Road Newtownabbey, Co. Antrim, BT37 0QB Belfast, Northern Ireland, UK

art ic l e i nf o

a b s t r a c t

Article history: Received 24 July 2013 Received in revised form 31 July 2014 Accepted 16 August 2014 Available online 14 September 2014

This paper concerns with the analysis of the nature of the mass residue of PA6 and its nanocomposites in nitrogen. To assess the structure of the condensed phase during pyrolysis, this study presents thermal (by TGA in nitrogen) and condensed phase analysis of the residue (by FTIR-ATR) of PA6 nanocomposites consisting of phosphorous based flame retardants (FR) and/or nanoparticles (based on modified Montmorillonite clay). The thermal analysis reveals that the nanoparticles do not change the pyrolysis kinetics of PA6 whereas the FR does. The FR and NC used in the polymer nanocomposites (PNC) are capable of changing the structure of the char compared with pure PA6, where the char structure consists of polycyclic aromatic hydrocarbons (PAH) whereas the PA6 does not leave any considerable amount of mass residue. This residue analysis for decomposition samples in TGA in Nitrogen of the PA6/FR/NC composites complements previously published work for gas phase analysis ( FTIR-gas) as well as cone calorimeter characterization for their flammability. The overall aim, addressed here also, is to find out to what extent microscale measurements (e.g. TGA/FTIR/DSC/ATR) can be used a priori to delineate the flammability of polymer fire retarded composites. It is shown that FTIR-gas and FTIR (ATR) residue can be used to determine the fire retardant action (solid and/or gaseous) only qualitatively. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Nanocomposite PA6 Fire retardant Flammability TGA FTIR

1. Introduction Polyamide 6 (PA6) is one of the important engineering plastics. Owing to its good mechanical property, attrition resistance, oil resistance and solvent resistance it has been widely used [1] in fibre, film, and engineering thermoplastics. However, its thermal and flammability properties need to be improved. For this purpose nanocomposites were used. The first PA6/clay nanocomposites were reported at 1976 [2]. Later the technology was further improved for the production of PA6/clay composites using in-situ polymerisation [3]. Furthermore, the addition of flame retardants to PA6/clay nanocomposites yielded improved performance for tests such as the cone calorimeter and UL-94 in comparison with pure PA6 nanocomposites [4]. However among the flame retardants, halogenated ones generate corrosive, toxic smoke and other noxious gases during pyrolysis and combustion. Hence, in this study halogen-free-flame retarded PA6 nanocomposites are studied for the structure of the condensed phase residue from TGA experiments.

This work is part of an integrated methodology to measure and characterise the flammability of a material. The following measurements are desirable: Microscale experiments


Tendency to dripping (based on rheology measurements).
Solid degradation in mg scale (using TGA/DSC and MDSC) for heat of pyrolysis and specific heat).


Solid residue analysis at different temperatures using TGA/ATR.
Gaseous products in mg scale (using TGA/FTIR/MS for toxicity and ignition kinetics). Mesoscale experiments


Tube Furnace ( toxicity).
Cone calorimeter in standard atmosphere (to assess the effec



n

Corresponding author. Tel.: þ 44 28 9036 8058. E-mail address: [email protected] (M.A. Delichatsios).

http://dx.doi.org/10.1016/j.firesaf.2014.08.006 0379-7112/& 2014 Elsevier Ltd. All rights reserved.

tiveness of nano particles and intumescent Fire Retardants and also measure heat release rates and product yields). Special calorimeter (Universal Flammability Apparatus to assess combustion in under-ventilated conditions and evaluate tendency for dripping and char strength by performing experiments in vertical orientation). The strength and melting behaviour of the char assessed in the cone calorimeter by experiments with the sample in vertical orientation.

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This integrated methodology and corresponding results for the PA6/Fire retarded composites are described in detail in [16–20] except for the results of the residue analysis (TGA/ATR) that are presented here. The present work concerns the solid residue analysis at different temperatures using TGA/ATR. In this work, the halogen free flame retardant system used is Exolit-OP1311 whereas the nanoclay used is Cloisite 30B. This study investigates the effect of Exolit-OP1311 (Flame Retardant, FR) and Cloisite 30B, (Nano Clay, NC) on PA6 where PA6, PA6 þFR, PA6 þNC and PA6 þFR þNC composites employing Thermogravimetric analysis (TGA) were investigated. The solid residues obtained at different temperatures (25, 250, 350, 450 and 550 1C) were examined using Attenuated Total Reflection infrared spectroscopy (ATR). A discussion on the usefulness of these data on characterizing the flammability of the PA6 fire retarded composites is combined with conclusions. A more detailed description of possible chemical path routes consistent with our description of decomposition in this work can be found in [21], whereas similar work using a different measurement technique has been reported in [22] for a GF þPA6 base polymer.

2.2. Methods 2.2.1. Thermogravimetric analysis Thermal analyses were carried out using a Mettler Toledo TGA/ STDA 851e measuring module, with temperature accuracy 70.5 1C and a temperature reproducibility of 70.5 1C. The samples were placed in an alumina pan (without lid) of 70.0 μl volume and were heated under dynamic linear rate of 10 1C min  1, in a 50 cm3 min  1 nitrogen flow from 25 1C to 700 1C. 2.2.2. Attenuated total reflection infrared (ATR) analysis of solid residues FTIR-ATR analyses were carried out on a NEXUS ATR, Thermo Nicolet Spectrometer, in the range of 400–4000 cm  1 with a resolution of 4 cm  1. Solid residues were collected at different stages of the thermal decomposition in the TGA and they are investigated at 25 1C, 250 1C, 350 1C, 450 1C and 550 1C and kept under nitrogen atmosphere until the sample reached the room temperature. Once the sample reached the room temperature the ATR experiments were initiated.

3. Results and discussion 2. Materials and methods

3.1. Thermal decomposition analysis

2.1. Materials

The thermal decomposition of pure PA6, PA6þFR, PA6þNC and PA6þFRþ NC were analyzed using TGA in nitrogen as shown in Figs. 2–4. Figs. 2 and 3 show the TGA mass loss normalized over the initial mass and the DTG/mass loss rate against temperature, and Fig. 4 shows the temperature differential (DTA) measurements. Figs. 2–4 show that PA6 and PA6þNC undergo a single decomposition whereas PA6þFR and PA6þFRþ NC undergo a two step decomposition. (It is noted that a secondary reaction occurs for pure PA6 after charring started which may be due to some kind of moiety.) The first step of decomposition occurs at nearly 290 1C followed by the second step roughly at 370 1C for PA6þFR and PA6þFRþNC composites. On the other hand, PA6 and PA6þNC start the decomposition at 320 1C. The degradation trends in nitrogen for PA6 and PA6þNC are similar but the residues left behind after decomposition are different. While the residue left after decomposition (4650 1C) is about 0% in the case of pure PA6, it is found to be nearly 5% for PA6þNC. On the other hand, PA6þFR gives nearly 6% and PA6þ FRþNC almost 10% residue. However to yield nearly the same amount of residue (5 to 6%) about 5% of Cloisite 30B (NC) is necessary while FR needs more amounts of loading into the PA6 matrix that is about 18%. Such a trend indicates that FR and NC are acting through different mechanisms with PA6 during pyrolysis, namely part of the Exolit-OP1311 fire retardant decomposes to gases [16–18]. From Fig. 4 it is evident that the addition of either NC or FR or the combination does not alter the melting point of PA6 which is approximately 221 1C. The temperature at which the maximum

Polyamide 6 was obtained from Rhodia Technyl, whereas nanoclay (NC) Cloisite 30B was purchased from Southern Clay. Cloisite 30 B is made from Cloisite Nas by ion exchange of pristine alkali cations with methyl, tallow, bis-2-hydroxyethyl quaternary ammonium chloride. Flame retardant Exolit-OP1311 is a mixture of aluminium diethylphosphinate and melamine polyphosphate in the ratio of 2:1 and was received from Clariant. Three different formulations were made with PA6 for this study by melt compounding. PA6 was melt-mixed with the clay using a Brabender mixer running under nitrogen at 50 rpm and at 250 1C. The clay loading (Cloisite 30B) was 5.0 wt%. The true silicate concentration used was determined by heating the dry, post-extrusion pellets in a tubular furnace at 800 1C for 12 h, and correcting for structural rearrangement by diving the ash percentage by 0.935 (the silicate rearrangement results in 6.5% loss of structural water). The three formulations are PA6 þFR (18.0 wt% of FR in PA6), PA6þ NC (contains 5.0 wt% of nanoclay Cloisite 30B in PA6) and PA6þ FR þNC (5.0 wt% of nanoclay and 18.0% of Exolit-OP 1311). After the formulation, the samples were ground in a cryogenic grinder and dried at 60 1C for 72.0 h in vacuum. The samples were then stored in sealed desiccators prior to use. The chemical structures of aluminium diethylphosphinate, melamine polyphosphate and quaternary ammonium used for the synthesis of Cloisite 30 B are given in Fig. 1.

H2N CH2CH2OH CH3

N

N N

NH2 N

(C18H35) Cl

CH2CH2OH

NH3

Methyl, octadecyl, bis-2-hydroxyethyl quaternary ammonium salt

O O

P O O P O Al3+ O O O P

P O

O n melamine polyphosphate (MPP)

aluminium diethylphosphinate (Alpi)

Fig. 1. Chemical structure of melamine polyphosphate, aluminium diethylphosphinate and the quaternary ammonium salt.

D. Bakirtzis et al. / Fire Safety Journal 69 (2014) 69–75

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Fig. 2. Thermal decomposition (pyrolysis) of PA6, PA6 þFR, PA6 þNC and PA6 þ FRþ NC heated at 10 1C min  1 in nitrogen.

Fig. 3. Mass loss rate of the thermal decomposition of PA6, PA6 þ FR, PA6 þ NC and PA6 þ FR þNC heated at 10 1C min  1 in nitrogen.

mass loss occurs is around 400 1C and the overall mass loss ends at around 510 1C. From Table 1 and Fig. 4, it is evident that up to 250 1C, the mass loss is due to the loss of water molecules that are present in the polymer matrix. PA6 is hydrophilic and hence some amount of water can be present in the polymer matrix through hydrogenbonding. Around 250–350 1C, PA6 and PA6 þNC give nearly 1.0% of mass loss whereas PA6 þFR and PA6 þFR þNC more than 10.0% of mass loss which may emerge from FR. FR is a mixture of aluminium diethylphosphinate and melamine polyphosphate in the ratio of 2:1, among them melamine polyphosphate is capable of undergoing decomposition where the melamine sublimes and contributes to the weight loss [5]. Apart from evaporating melamine, the melamine also forms melam through condensation where ammonia is getting eliminated [6–8]. On the other hand, from aluminium diethylphosphinate, ethylene gets disrupted and also by the action of melamine polyphosphate it can eliminate diethylphosphinic acid [9] thus both the components of FR contributed to the mass loss at 250–350 1C. In the temperature range of 350–450 1C, in PA6 and PA6 þFR the mass loss is due to the degradation of PA6, which yields ε-caprolactam (monomer)

and cyclic oligomers [10,11]. The monomer and the cyclic oligomers are formed via intramolecular and intermolecular aminolysis. In the case of the PA6 þNC and PA6 þFR þ NC composites along with the decomposition of PA6, the intercalated quaternary ammonium ion also undergoes degradation to yield olefins, water, 2-(diethylamino)ethanol, and carbon dioxide etc. [12], which accounts for the additional mass loss. Furthermore, the degradation of PA6 continues in all the composites from 450 to 700 1C yielding monomers and cyclic oligomers. However most of the degradation finishes around 550 1C itself. PA6 yields only few amounts of residue whereas the rest of the formulations yield considerable amounts of residue where the nature of the residue is analyzed using FTIR (ATR) as described in the following sections. The present results complement the previous work of the authors on TGA and FTIR of the pyrolysis gases reported in Refs. [16,17] and of TGA/ATR reported in Ref. [18]. 3.2. ATR analysis of PA6 Fig. 5 includes all ATR results for pure PA6 and its solid residues at 25, 250, 450 and 550 1C, with the characteristic peaks at 3291

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Fig. 4. DTA during the thermal decomposition of PA6, PA6 þFR, PA6 þ NC and PA6 þFR þ NC heated at 10 1C min  1 in nitrogen.

Table 1 Mass loss (in wt%) during thermal decomposition in nitrogen. Sample

PA6 PA6 þ NC PA6 þ FR PA6 þ FRþ NC

Mass loss (in wt%) 25– 150 1C

150– 250 1C

250– 350 1C

350– 450 1C

450– 550 1C

550– 700 1C

Total

2.35 2.26 1.92 1.95

0.25 0.22 0.24 0.45

0.90 1.09 10.61 11.60

50.37 53.47 50.00 54.08

43.00 37.16 30.83 22.49

2.67 0.81 0.27 0.47

99.54 95.01 93.87 91.04

(N–H stretch), 2930 (C–H Stretch), 2856 (C–H stretch), 1630 (CQO Stretch) and 1535 (N–H bending) of PA6. As discussed earlier, pyrolysis of PA6 is expected to proceed through the formation of monomeric εcaprolactam and other cyclic oligomers thereby it creates a mass loss by the action of heat. However the cyclic oligomers formation is not highly accountable because after 550 1C there are no cyclic oligomers found and the mass loss is very negligible, say 0.46%. In another possible mechanistic pathway it is believed that a cross-linking reaction occurs [13,14] which may yield an appreciable amount of char or residual materials, however there is no hard experimental evidence existing to support the fact of cross-linking reactions. In our findings we could not observe any cross-linking of degrading polymer, yielding residue that are observable. PA6 appears to be stable up to 350 1C, the literature reports are supportive for this fact, indicating that only partial re-crystallization of PA6 occurs till 350 1C, changing the fractions of α, γ and the amorphous phase [15]. Drastic degradation takes place by the action of heat at 350 1C onwards and after 550 1C no residue is found indicating that in PA6 depolymerization reaction releases ε-caprolactam alone.

remain implying that PA6 is not completely degraded whereas at 550 1C nearly no signals related PA6 could be seen. At 550 1C three major signals are seen and they are at 1070, 1614 and 3346 cm  1. The signal at 3346 cm  1 is assigned to melam which is a polymerized product arising from melamine of melamine polyphosphate. Thus melamine polymerizes and makes a char layer thereby it is acting in the condensed phase. The characteristic band at 1614 cm  1 is assigned to the aromatic char and the band at 1070 cm  1 to phosphate structure. Thus melamine polyphosphinate yields melamine char, aluminium diethylphosphinate gives rise to aluminium polyphosphate and the volatiles hydrocarbons evolved are converted into PAH char. 3.4. ATR analysis of PA6þNC From Fig. 7 the residual analysis of PA6 þNC nanocomposite reveals that up to 450 1C, the PA6 polymer is not completely degraded. At 550 1C all the characteristic peaks due to PA6 are absent and additional peaks at 3624, 1607 and 1008 cm  1 appear. The presence of peaks at 3624 and 1607 cm  1 indicates the formation of polycyclic aromatic structure having alkyl units attached to it and the broad band at 1008 cm  1 is assigned to Si–O–Si unit present in Cloisite 30B. As discussed earlier PA6 undergoes degradation by the action of heat, in the meantime, the intercalated ammonium ion present in the nanoclay too undergo degradation and only the Si–O unit remains from Cloisite 30B. Further the eliminated hydrocarbons from both the clay and PA6 are converted into polycyclic aromatic hydrocarbons having alkyl fragments attached to it. Thus in the solid residue at 550 1C some carbonaceous residue and montmorillonite network (Si–O) unit remains, the PA6 and the intercalated ammonium ion decomposed.

3.3. ATR analysis of PA6þ FR 3.5. ATR analysis of PA6þFR þNC In the PA6 þ FR formulation, FR contains both the aluminium diethylphosphinate and melamine polyphosphate in the ratio of 2:1 and their content is 18.0 by wt%. From Fig. 6 it is observed that both components of FR contribute to the flame retardancy in the condensed phase. Up to 450 1C all the characteristic signals of PA6

In the PA6 þFR þNC composite as seen in Fig. 8, the carbonaceous residue comprising of aromatic moieties is found at 550 1C. The peak at 1010 cm  1 accounts for both aluminium phosphinate residue and the Si–O unit of Cloisite 30B. Thus the solid residue

73

1630.43

2930.43

550 °C

2856.52

Absorbance

3291.30

769.57

D. Bakirtzis et al. / Fire Safety Journal 69 (2014) 69–75

450 °C 250 °C 25 °C

3500

3000

2500

2000

1500

1000

-1

Wavenumbers (cm )

1060.88

Absorbance

1614.10

3346.93

1060.88

Fig. 5. FTIR (ATR) spectra of PA6 and its solid residues at 25, 250, 450 and 550 1C.

550 °C

450 °C 350 °C 250 °C 25 °C 3500

3000

2500

2000

1500

1000

Wavenumbers (cm-1)

1607.33

1008.96

Fig. 6. FTIR (ATR) spectra of PA6 þFR and its solid residues at 25, 250, 350, 450 and 550 1C.

Absorbance

550 °C 450 °C 350 °C 250 °C 25 °C

3500

3000

2500

2000

1500

1000

-1

Wavenumbers (cm ) Fig. 7. FTIR (ATR) spectra of PA6 þ NC and its solid residues at 25, 250, 350, 450 and 550 1C.

D. Bakirtzis et al. / Fire Safety Journal 69 (2014) 69–75

Absorbance

3346.93

1607.33

1010.37

74

550 °C 450 °C 350 °C

250 °C 25 °C 3500

3000

2500

2000

1500

1000

Wavenumbers (cm-1) Fig. 8. FTIR (ATR) spectra of PA6 þFR þ NC and its solid residues at 25, 250, 350, 450 and 550 1C.

obtained at 550 1C for PA6 þFR þNC composite consists of char, aluminium phosphinate and Si–O unit of cloisite 30B.

4. Discussion and conclusions Combining the present results with the FTIR-gas analysis data [16,17] we can make the following observations: a. For pure PA6 (Fig. 5). PA6, being present up to 450 1C as the spectrum in the residue shows, releases after 350 1 caprolactam (its oligomer, [16,17]) and leaves negligible residue because crosslinking does not occur. Negligible residues is found after 550 1C. The heat of combustion measured in the cone calorimeter is 26 kJ/g [19,20]. For cross reference note that the spectrum of PA6 is present in the rest of the formulations as discussed next. b. For PA6þFR (Fig. 6). PA6 is seen up to 450 1C as for the pure PA6 as the ATR spectra show. At 550 1C melamine char is observed at 3346 cm  1 and aromatic char at 1614 cm  1, whereas the phosphinates stretches (e.g. 1060.88 cm  1) remain. It seems that the FR acts in the solid phase as well as in the gas phase where the heat of combustion is 21 kJ/g [19,20] namely 20% less than for pure PA6. This lower value indicates “partial” inhibition of combustion with the phosphorus gaseous products acting in the gas phase. The residue is about 6% (Table 1). c. For PA6 þNC (Fig. 7). As for the previous cases, PA6 is seen in the residue up to 450 1C and after about 550 1C the spectrum is dominated by the Si–O–Si unit of Cloisite 30B with aromatic moieties at 3624 cm  1 and1607 cm  1. The residue is about 5% (Table 1). The heat of combustion as measured in the cone calorimeter is 26 kJ/g [10,20] as in pure PA6 indicating that the main action may be in the solid phase. d. For PA6þFRþNC. As before PA6 is seen up to 450 1C, whereas the residue around 1010 cm  1 accounts for both aluminium phosphinate residue and the Si–O–Si unit of Cloisite 30B with some moieties of melamine and PAH-type char (around 1607 cm  1 and 3346 cm  1). The residue is 9% (Table 1). The heat of combustion as measured in the cone is 21 kJ/g [19,20] the same as for PA6þFR, indicating “partial” inhibition of combustion with phosphorus gaseous products acting in the gas phase. As general remarks we can state, based on the comments a, b, c, and d, that the NC and FR do not affect the decomposition of PA6,

and the main fire retarding effect of NC would be in the solid phase (by shielding and reradiation), whereas the retarding effect of the FR would be partially in the solid (by shielding and reradiation) and partially in the gas phase. We may, however, also infer that the combination of FR and NC would provide superior fire retardancy because the solid residue is larger and the FR acts also in the gas phase. The comparison is, however, more difficult if one considers the mass fraction of PA6 (95%) in the NC formulation compared to the mass fraction of PA6 (less than 72%) in the other formulations. Nevertheless, a quantitative assessment of the fire retarding effects is not possible based on these measurements alone. We note that based on the cone data for samples 6 mm thick [19,20] the maximum heat release rate for the present formulations are: for Pure PA6: 1200 kW/m2, for PA6 þFR 300 kW/m2, for PA6 þNC 410 kW/m2 and for PA6 þFR þNC 180 kW/m2. Using this information and the micro scale data (TGA/ATR/FTIR) it seems that the fire retardancy for the formulations used here is primarily due to the shielding and reradiation of a char like layer formed during combustion in the cone calorimeter (see further analysis in [19]). Our conclusion is that only microscale measurements cannot characterize quantitatively the flammability of fire retarded formulations but they are useful in combination with cone calorimeter data (or tube data for toxicity) to lead to a better understanding of the mechanisms which may help in designing new formulations until and when also advances in QC (quantum chemistry) and MD (molecular dynamics) are made. It is also safe to say that the toxicity when using NC and phosphorous based FRs (like Exolit-OP1331 in this work) is less than for brominated FRs because the heat of combustion is not as much reduced as it is for brominated FRs compared to the complete combustion of PA6.

Acknowledgements The authors would like to acknowledge the EU for financially supporting the PREDFIRE-NANO project under Grant number 013998 in the sixth Framework program. References [1] S.V. Levchik, E.D. Weil, Polym. Int. 49 (2000) 1033–1073. [2] Fujiwara S., Sakomoto T. Japanese patent application no. 109998 (1976).

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[3] A. Usuki, Y. Kojima, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurachi, et al., J. Mater. Res. 8 (1993) 1179. [4] Y. Hu, L. Song, J.Y. Xu, L. Yang, Z.Y. Chen, W.C. Fan, Colloid Polym. Sci. 279 (2001) 819–822. [5] H. Haracek, S. Piech, Polym. Int. 49 (2000) 1106. [6] H. May, J. Appl. Chem. 9 (1959) 340. [7] L. Costa, G. Camino, J. Thermal Anal. 34 (1988) 423. [8] S. Jahromi, U. Moosheimer, Macromolecules 33 (2000) 7582. [9] U. Braun, B. Schartel, M.A. Fichera, C. Jäger, Polym. Degrad. Stab. 92 (2007) 1528. [10] I. Luderwald, F. Mertz, M. Rothe, Angew. Macromol. Chem. 67 (1978) 193. [11] A. Ballestreti, D. Garozzo, M. Giufrida, G. Impallomenti, G. Montaudo, Polym. Degrad. Stab. 23 (1988) 25. [12] W. Xie, Z. Liu Gao, K. Pan, W.P. Vaia, R. Hunter, A. D. Singh, Thermal characterization of organically modified montmorillonite, Thermochim Acta 367–378 (2001) 339–350. [13] B. Kamerbeek, G.H. Kroes, W. Grolle, Thermal degradation of some Polyamides, Soc. Chem. Ind. 13 (1961) 357. [14] L.H. Peebles, M.W. Huffman, J. Polym. Sci. 9 (1971) 1807. [15] R. Schreiber, W.S. Veeman, W. Gabrielse, J. Arnauts, Macromolecules 32 (1999) 4647. [16] A. Ramani, M. Hagen, J. Hereid, J. Zhang, M.A. Delichatsios, Interaction of a phosphorous based FR, a nanoclay and PA6. Part 2 interaction of the complete PA6 polymer nanocomposites, Fire Mater. 34 (2010) 77–93.

75

[17] A. Ramani, M. Hagan, J. Hereid, J. Zhang, D. Bakirtzis, M. Delichatsios, Interaction of a phosphorus based FR, a nanoclay and PA6 part 1. Interaction of FR and nanoclay, Fire Mater. 33 (2009) 273–285. [18] D. Bakirtzis, A. Ramani, M.A. Delichatsios, J. Zhang, Structure of the condensed phase and char of fire-retarded PBT nanocomposites in N2, Fire Saf. J. 44 (2009) 1023–1029. [19] Zhang Jianping, Michael A. Delichatsios, Serge Bourbigot, . Experimental and numerical study of the effects of nanoparticles on pyrolysis of a polyamide 6 (PA6) nanocomposite in the cone calorimeter, Combust. Flame 156 (2009) 2056–2062. [20] Delichatsios, Michael A. and Jianping Zhang. Micro-to Mesoscale testing and modelling for nanocomposite polymers in fires. In: Fire Retardancy of polymeric Materials. Ed. C.A.Wilkie, A. B. Morgan, CRC, 2010 p. 509–549. [21] F. Samyn, S. Bourbigot, Thermal decomposition of flame retarded formulations PA6/aluminium phosphinate/melamine polyphosphate/organomodified clay: interactions between the constituents? Polym. Degrad. Stab. 97 (2012) 2217–2230. [22] Braun, H. Bahr, B. Schartel, Fire Retardancy Effect of Aluminium Phosphinate and Melamine Polyphosphate in Glass Fibre Reinforced Polyamide 6. e-polymers, art. no. 14, 2010.